Sheng-qing Xiong

a China Aero Geophysical Survey and Remote Sensing Center for Natural Resources, China Geological Survey, Beijing 100083, China

b Key Laboratory of Airborne Geophysics and Remote Sensing Geology, Ministry of Natural Resources, Beijing 100083, China

Keywords:Aeromagnetic survey Tectonic framework Suture zone Fault Magnetic rocks Uplift Oil and gas resource assessment Ore prospecting Qinghai-Tibet Plateau

ABSTRACT The Qinghai-Tibet Plateau (also referred to as the Plateau) has long received much attention from the community of geoscience due to its unique geographical location and rich mineral resources. This paper reviews the aeromagnetic surveys in the Plateau in the past 60 years and summarizes relevant research achievements, which mainly include the followings. (1) The boundaries between the Plateau and its surrounding regions have been clarified. In detail, its western boundary is restricted by West Kunlun-Altyn Tagh arc-shaped magnetic anomaly zone forming due to the arc-shaped connection of the Altyn Tagh and Kangxiwa faults and its eastern boundary consists of the boundaries among different magnetic fields along the Longnan (Wudu)-Kangding Fault. Meanwhile, the fault on the northern margin of the Northern Qilian Mountains serves as its northern boundary. (2) The Plateau is mainly composed of four orogens that were stitched together, namely East Kunlun-Qilian, Hoh-Xil-Songpan, Chamdo-Southwestern Sanjiang(Nujiang, Lancang, and Jinsha rivers in southeastern China), and Gangdese-Himalaya orogens. (3) The basement of the Plateau is dominated by weakly magnetic Proterozoic metamorphic rocks and lacks strongly magnetic Archean crystalline basement of stable continents such as the Tarim and Sichuan blocks. Therefore, it exhibits the characteristics of unstable orogenic basement. (4) The Yarlung-Zangbo suture zone forming due to continent-continent collisions since the Cenozoic shows double aeromagnetic anomaly zones. Therefore, it can be inferred that the Yarlung-Zangbo suture zone formed from the Indian Plate subducting towards and colliding with the Eurasian Plate twice. (5) A huge negative aeromagnetic anomaly in nearly SN trending has been discovered in the middle part of the Plateau, indicating a giant deep thermal-tectonic zone. (6) A dual-layer magnetic structure has been revealed in the Plateau. It consists of shallow magnetic anomaly zones in nearly EW and NW trending and deep magnetic anomaly zones in nearly SN trending. They overlap vertically and cross horizontally, showing the flyover-type geological structure of the Plateau. (7) A group of NW-trending faults occur in eastern Tibet, which is intersected rather than connected by the nearly EW trending that develop in middle-west Tibet. (8) As for the central uplift zone that occurs through the Qiangtang Basin, its metamorphic basement tends to gradually descend from west to east, showing the form of steps. The Qiangtang Basin is divided into the northern and southern part by the central uplift zone in it. The basement in the Qiangtang Basin is deep in the north and west and shallow in the south and west. The basement in the northern Qiangtang Basin is deep and relatively stable and thus is more favorable for the generation and preservation of oil and gas. Up to now, 19 favorable tectonic regions of oil and gas have been determined in the Qiangtang Basin. (9) A total of 21 prospecting areas of mineral resources have been delineated and thousands of ore-bearing (or mineralization) anomalies have been discovered. Additionally, the formation and uplift mechanism of the Plateau are briefly discussed in this paper.

1. Introduction

The Plateau is known as “the Roof of the World” and “the Third Pole of the World”. It plays an extremely important role in global geotectonic research and thus serves as a hot spot and frontier that has attracted a high level of concern of the international community of geoscience. It is located in the Tethys-Himalaya metallogenic domain, one of the three major metallogenic domains in the world and boasts rich mineral resources and great prospecting potential. However, it suffers a low exploration level since it is difficult to carry out ground exploration in the harsh natural conditions of the Plateau. In contrast, airborne geophysical exploration is a quick method and enjoys favorable application effects since the data collection by this method is less affected by ground conditions. Therefore, it is one of the main effective ways to accelerate the basic geological research and mineral resource surveys in the Plateau.

Over the past 60 years, the aeromagnetic surveys have covered the whole Plateau with constant efforts of several generations. Through these surveys, a series of aeromagnetic maps have been plotted, and the most systematic regional geophysical data involving the widest area of the Plateau have been prepared. All these have drawn a high level of attention of the community of geoscience and have been widely applied in practice. Based on the in-depth analysis of the aeromagnetic data, significant advances have been made in basic geological research and the assessment of energy resources. Furthermore, the aeromagnetic data greatly contribute to the understanding of a series of scientific problems of the Plateau such as the basement properties,tectonic framework, distribution of suture zones and faults,concealed magmatic rocks, deep structures, geothermal resources, and the formation and uplift of the Plateau, to which the community of geoscience has attached high importance. All these play an important role in promoting the mineral resource exploration and geoscientific research of the Plateau.

The aeromagnetic surveys in the Plateau started in the 1960s and generally experienced three stages as follows. (1) The medium- and low-precision surveys on medium and small scales during 1960-1995. These aeromagnetic surveys were roughly concentrated in the central and eastern parts of Tibet,most areas of Qinghai Province, western Sichuan Province,and northern Yunnan Province, with the main purpose of seeking minerals urgently needed by the country such as chromite. They covered an area of 1500000 km2and were mainly on the scales of 1∶200000-1∶1000000, including the surveys of the central part of Tibet on a scale of 1∶500000. During 1994-1995, the 1∶200000-scaled oil and gas aeromagnetic survey was completed in Qiangtang area in northern Tibet, covering an area of about 220000 km2. (2)Small-scale high-precision aeromagnetic surveys during 1998-2000. At this stage, the 1∶1000000-scaled highprecision aeromagnetic surveys were carried out in the central and western parts of the Plateau, covering an area of 1140000 km2. As a result, the aeromagnetic surveys basically covered the Plateau and other lands in China, filling in the greatest blank in Chinese terrestrial aeromagnetic surveys. (3) Largeand medium-scale high-precision surveys since the beginning of the 21st century. At this stage, 1∶200000-scaled aeromagnetic surveys were successively completed in the Yarlung Zangbo River, Lhasa River, and Nianchu River in the Plateau and in the areas along Qinghai-Tibet Railway in the Plateau, covering an area of 554800 km2. Meanwhile, the aeromagnetic surveys on scales of 1∶200000-1∶100000 were performed in the northern part of the Southwestern Sanjiang area, covering an area of 100000 km2, and the aeromagnetic surveys on a large scale of 1∶50000 were completed in East Kunlun, West Kunlun, Altyn Tagh Mountains, and Qimantage areas on the northern margin of the Plateau, covering an area of about 200000 km2.Furthermore, 1∶200000-scaled high-precision airborne gravity and magnetic combined surveys were completed in Qiangtang Basin in 2018, covering an area of about 300000 km2.

Relevant data obtained from the aeromagnetic surveys have been widely applied to geoscientific research and geological prospecting by researchers and geological exploration institutes around the world. As a result, a series of significant outcomes have been obtained, followed by the discovery of a series of mineral deposits. Moreover, the aeromagnetic research results have been continuously confirmed by subsequent geological surveys and research. As of February 24, 2021, 3768 papers that refer to or adopt the aeromagnetic survey results of the Plateau can be retrieved from the China National Knowledge Infrastructure (CNKI)(http://www.cnki.net) in a search on “aeromagnetic data of the Plateau (in Chinese)”.

In 2020, the 1∶1000000-scaled aeromagnetic anomalies(ΔT) contour maps of the Plateau were newly prepared according to all high-precision aeromagnetic data as of the end of 2019. Meanwhile, potential field conversions such as reduction to the pole (RTP), first vertical derivative, and upward continuation were conducted on the aeromagnetic data (Xiong SQ et al., 2021), thus determining the distribution scope of magnetic sources at different depths and the features of magnetic anomalies such as their morphologies, amplitude,and gradient changes. The aeromagnetic anomaly data were further interpreted based on these results as well as the geological and geophysical understanding, gaining many new insights and outcomes. This paper summarizes the survey and research achievements of aeromagnetic surveys for sixty years in the Plateau based on the interpretation results of the latest aeromagnetic maps. The purpose is to boost the intersection,convergence, and combined integration of aeromagnetics with other disciplines and provide scientific bases for the formation and evolution, resource development, and environmental management of the Plateau and its surrounding regions, so as to jointly promote the geological mineral exploration and geoscientific research of the Plateau.

2. Tectonic framework of the Qinghai-Tibet Plateau

The Plateau lies at the intersection of the paleocontinent Gondwanaland and Eurasia continent, covering the main part of the Eastern Tethys tectonic domain and the key part where the two continents collided and were stitched together. It is a complex region formed from constant back-arc spreading,disintegration, and following splice and interlocking of the Laurisia and the western margin of Pan-Cathaysian Block in the north and the northern margin of the Gondwanaland in the south after the extinction of the Paleo-Tethys Ocean. As a result, it is comprised of multiple EW-trending zones of different scales and island arcs or continental masses that are stitched together. The former was the arc-arc and arccontinent collision-junction zones and the latter involved in the collision and junction (Pan GT et al., 2002). After that, the continental masses and combined continental masses overlapped, dislocated, were squeezed out of and moved far away from their original locations driven by large shear zones and large faults formed in the process of intensive continentcontinent collisions since the Cenozoic (Xu ZQ et al., 2006).This laid a foundation for the complex tectonic and landform features of the current Plateau. It is significant to research the distribution characteristics of the anomalous aeromagnetic fields and their relationships with the lithospheric architecture in the Plateau and its adjacent areas and to establish the tectonic system in the Plateau since they serve as important tectonic indicators for understanding the tectonic framework,lithospheric architecture and its evolution, metallogenic geological setting, and the continental dynamic process in the Plateau.

2.1. Characteristics of the magnetic field in the Qinghai-Tibet Plateau

The magnetic anomalies are weak in the Plateau and are strong in its surrounding areas, with the boundaries between the magnetic anomalies basically consistent with the regional tectonic boundaries (Xiong SQ et al., 2001a, 2001b; Teng JW et al., 2019). As shown in the latest 2020 aeromagnetic map(Fig. 1), the ΔTanomalies in the Plateau are totally different from those in its surrounding regions. In detail, ΔTmagnetic field in the Plateau is pretty gentle, which is in sharp contrast to the strong, broad, and gentle positive magnetic anomalies exhibited the Tarim block to the west, the Sichuan block in the east, and the Indian subcontinent in the south. In terms of the boundaries of the ΔTmagnetic field in the Plateau, the western boundary is restricted by West Kunlun-Altyn Tagh arc-shaped magnetic anomaly zone, the northern boundary is comprised of the boundaries between different magnetic fields on the northern margin of the Northern Qilian Mountains, and the eastern boundary consists of the boundaries among different magnetic fields along the Longnan-Kangding area.These boundaries show the scope of the Plateau and the magnetic and geological tectonic characteristics of the Plateau that differ from those of its surrounding blocks. Besides, the map also shows the tectonic framework of the Plateau that consists of four parts overall (i.e., East Kunlun-Qilian area with varying magnetic anomalies, Hoh-Xil-Songpan area with stable magnetic anomalies, Chamdo-Southwestern Sanjiang magnetic anomaly area, and Gangdese-Himalaya magnetic anomaly area with positive and negative anomalies intensively changing), and the tectonic locations of six deep fault zones (or suture zones) among the four parts.

The main part of the Plateau is an area of negative magnetic anomalous field with a strength of -10--80 nT,indicating that the basement of the Plateau is mainly comprised of weakly magnetic metamorphic rock series.Therefore, the basement features high plasticity and is a type of unstable basement. It was possibly formed in the Proterozoic (possibly in the Early Paleozoic partially). It is considered by Zhang XQ (2020) that the metamorphic basement in the Gangdese and Himalaya blocks is Pan-African basement formed 550 Ma ago, which is equivalent to the Neoproterozoic. This set of metamorphic rock series has been exposed in the northern slope of Himalaya, Gangdese,Qiangtang, Chamdo, Kunlun, and Qilian Mountains areas and the outcrops in these areas are known as Nyalam Group,Nyainqentanglha Group, Pre-Sinian System, Ningduo Group,Jinshuikou Group, and Yemananshan Group, respectively. In addition, blocky and banded positive anomalies are also developed in the Plateau. They are mainly induced by Paleoproterozoic metamorphic rock series and a large scale of magmatic rocks. For instance, blocky positive anomalies develop in the southern Qaidam Basin, Qilian-Menyuan area,and Ruoergai area, with amplitude of 10-60 nT. According to data on the magnetism in rocks (Xiong SQ, 2016), the Precambrian magnet-bearing metamorphic rock series in Baiyin area (average susceptibility: 8770×10-5SI), Proterozoic metamorphic rock series in Qilian Mountains area (average susceptibility: 9800×10-5SI), and Paleoproterozoic Jinshuikou Group in Qaidam Basin (average susceptibility:1080×10-5SI) are all magnetic and can induce positive anomalies in magnetic field. It can be seen that the Archean high-grade metamorphic crystalline basement is lacked and a small scale of strongly magnetic Paleoproterozoic crystalline blocks remain in the deep basement of the Plateau. These are consistent with the regional tectonic characteristics of the Plateau that it consists of multiple terranes that are stitched together (Chang CF et al., 1986; Pan GT et al., 2002).

2.2. Division of geotectonic framework

Owing to the unique and complex geological tectonics in the Plateau, Chinese geologists have put forward different geotectonic division schemes of the Plateau by exploring its geotectonic development regularity from different angles using different methods. In the early days, the geologists divided the Plateau into different geotectonic units by orogenic systems and belts according to tradition and polycyclic theory (Ren JS et al., 1997, 2004). Meanwhile, the Earth was further understood as the theory of plate tectonics was introduced into China. In this case, some geologists divided the Plateau by terranes on the principle of determining tectonic units by geological tectonic marks and terranes (Jiang FZ et al., 1994; Teng JW et al., 1996; Wang ZT, 2017). Later,based on the knowledge that the formation of archipelagic arc-basin systems serves as the mark of oceans evolving and transforming towards the tectonic regime of continental lithosphere, some geologists proposed the scheme of dividing the Plateau by arc-arc, arc-continent, and continent-continent collision-junction zones as well as the continental blocks and magmatic arcs among the zones (Pan GT et al., 2002). After that, other geologists divided the Plateau into tectonic units by blocks based on its current geodynamic characteristics (Cui JW et al., 2006).

In this paper, the Plateau is divided into tectonic units based on the aeromagnetic, gravity, and geological data and the depth of magnetic basement. To this end, orogenic belts are considered as the first-order tectonic units. Meanwhile, the names of the second-order tectonic units used in geotectonic zoning put forward by Pan GT et al. (2015) are further adopted, namely arc-basin system and block. In addition, the names of the third-order tectonic units include basin,depression area (zone) and uplift zone (Fig. 2; Table 1).

The four first-order tectonic units shown in Fig. 2 are East Kunlun-Qilian, Hoh-Xil-Songpan, Gangdese-Himalaya, and Chamdo-Southwestern Sanjiang orogens. Among them, the former three are extensions of adjacent Tarim and Sino-Korean plates, South China-Southeast Asia Plate, and Indian Plate, respectively, while the last formed due to the development and convergence of discrete parts of South China-Southeast Asia and Indian plates (Liu ZK et al., 1990).They are all bounded by deep faults with characteristics of plate sutures.

East Kunlun-Qilian Orogen (I). It lies in the East Kunlun and Qilian mountain area and is a part of Kunlun-Qilian-Qinling Orogenic System. It is bounded by the Altyn Tagh Fault in the west, the fault on the northern margin of the northern Qilian Mountain in the north, and the Kunzhong Fault in the south. It is in NW trending and displays the shape of a wedge narrowing eastwards in general. The main part of the orogen developed and evolved from the original archipelagic Tethys Ocean into a complex geological structure consisting of suture zones (or ophiolitic melange belts), arcbasin systems, and the blocks involved after different eras of tectonic shifting and evolution (Pan GT et al., 2015). The most ancient isotopic age in the orogen is 2300-2500 Ma. In the Late Paleoproterozoic (2000-1800 Ma), the tectothermal events in the orogen mainly included anatexis,metamorphism, and magmatic intrusion, which are the records of early orogeny. In the Middle Protecrozoic, the Zhulongguan and Jingtieshan groups in the northern Qilian Mountains constituted the basement of Zoulangnan Mountains, and the Huangyuan Group served as the rock series of the basement in the central Qilian Mountains.Meanwhile, the Paleoproterozoic Maxianshan and Yemananshan groups constitute the most ancient crystalline basement in Qilian Mountains area. As shown by the magnetic field in the orogen, the regional negative magnetic field is mainly produced by Middle Neoproterozoic weakly magnetic metamorphic rock series that constitute the basement and the blocky positive anomalies are mainly a result of the residual Paleoproterozoic metamorphic rock series. Meanwhile, local magnetic anomalies are induced by the magmatic rocks spreading along faults. The orogen features much active structures, with magmatic rocks developing. According to aeromagnetic surveys, the depth of the basement in the orogen varies greatly. It is 3000-15000 m in the basins and 500-2500 m in the uplifts, and the sediment layers formed during the Upper Paleozoic and Mesozoic.

Fig. 1. Aeromagnetic ΔT contour and magnetic-field zoning map of the Qinghai-Tibet Plateau.

Fig. 2. Regional tectonic map of aeromagnetic-based zoning of the Qinghai-Tibet Plateau (See Table 1 for details of the numbers and names of the tectonic units in the map).

Table 1. Tectonic units of the Qinghai-Tibet Plateau determined based on aeromagnetic data.

Hoh-Xil-Songpan Orogen (II). It stands in Hoh-Xil,Bayan Har Mountains, and Songpan areas, with the Kunzhong, Longnan-Kangding, and Lazhulong (Wulanwula)-Yushu faults as its northern, eastern, and southern boundaries,respectively. Meanwhile, it is bounded by the Karakoram Mountain pass-Qiongmuzitage-Sichaxuefeng area in the west.In general, it is in NWW trending. The main body of the orogen is composed of sedimentary depressions, covering an area of up to 340000 km2. The magnetic anomalies in the orogen can be divided into the two parts (western and eastern)by the Maqên-Dari area. The former is weak and gently distributed, with average amplitude of -40 nT. They reflect the weakly magnetic basement composed of the metamorphic rock series of the Middle Proterozoic Tianshuihai Group. The eastern positive magnetic anomalies with amplitude of 40 nT are possibly related to the Paleoproterozoic strongly magnetic metamorphic rock series. They substantially reflect the Ruoergai and Zhongzan microcontinents that are stitched together. The aeromagnetic data indicate stable basement,small tectonic shift, and thick sediment layers with a thickness of 2000-13000 m in the orogen. Furthermore, the Triassic strata are fully developed in the orogen. They are called Bayan Har Group in Qinghai Province and were estimated to be 11300-23000 m thick (Qinghai Bureau of Geology and Mineral Resources, 1991), which is worth discussing according to the depth of the magnetic basement. Meanwhile,they are called Xikang Group in Ngawa area, with a thickness of 4500-6800 m. According to aeromagnetic data, there is no large-scale magmatic activity in the orogen.

Chamdo-Southwestern Sanjiang Orogen (III). It is located in the basins of the Jinsha, Lancang, and Nujiang rivers and is a newly determined orogen. It is bounded by Dengqen-Nujiang Fault in the south and the Lazhulong-Yushu Fault in the north. Its eastern boundary is irregular and roughly runs along Muli-Lijiang-Honghe River area. The orogen is in the NW trending in general. The author believes that the Jinsha,Lancang, and Nujiang faults do not turn westward but continue extending in the NW direction to Buruocuo and Xuelian Lake areas after extending from SE- to NW trending till eastern Tibet. They are a set of NW-trending fault system independently existing in the Tethys fault system and are not affiliated to but intersected the deep faults in nearly EW trending inside Tibet (Xiong SQ et al., 2012a). These are different from the geological zoning results based on geotectonic features (Ren JS et al., 1999; Pan GT et al., 2015).The orogen is an independent one under the control of the NNW-NW-trending faults. The regional negative magnetic field in the orogen is a result of weakly magnetic metamorphic rock series of Middle Proterozoic Ningduo and Gaoligongshan groups, which constitute the weakly magnetic basement of the orogen. Furthermore, magmatic rocks are developed and magma-tectonic activities are frequent along the fault strike, and their genesis and tectonic implications are to be further studied.

Gangdese-Himalaya Orogen (IV). It lies to the south of Chalukou-Shuanghu-Riwoqe area, with the western section of Lazhulong (Wulanwula)-Yushu Fault as its northern boundary, the northwestern section of Dengqen-Nujiang Fault as its eastern boundary, and relevant national borders as its western and southern boundaries. Meanwhile, it is generally in EW trending. Two famous suture zones span across the orogen, namely the Yarlung-Zangbo and Bangong-Nujiang suture zones. The background magnetic field in the orogen is stable, weak, and gently distributed, with a strength of-20-200 nT. It is overlapped by the linear, cloddy, and moniliform anomaly zones in nearly EW trending with amplitude of 20-800 nT. Among them, the weak and gently distributed negative magnetic fields are mainly produced by the weakly magnetic metamorphic rock series of the Middle Neoproterozoic Nyainqentanglha and Nyalam groups that constitutes the basement. Meanwhile, the intensively varying magnetic anomalies are induced by the Paleogene volcanics and the regular local anomalies are produced by intrusive rocks distributed along faults. The orogen features frequent magmatic activities and underwent complex island-arc orogeny. Furthermore, it is characterized by a large tectonic shift, and the burial depth of its metamorphic basement is 3000-15000 m in basins and is generally less than 2500 m in the uplifts. Meanwhile, the sediment layers in the orogen were formed during the Middle Cenozoic and Paleozoic.

In addition, the magnetic fields in the Plateau are banded in the south and north (shallow parts) and are blocky in the east and west (deep parts). It was by Teng JW et al. (1999,2011) that the Plateau is basically composed of three blocks(i.e., the western, central, and western parts) with the nearly SW-trending deep negative aeromagnetic anomaly (about 90°E) as their central position. This indicates the tectonic framework of the Plateau in which western and eastern parts can be divided into different blocks. This may be due to the tectonic deformation of the Plateau as a whole during the Indian subcontinent subducted northwards, which provides a new idea of further research on the uplift and new tectonic evolution of the Plateau since the Cenozoic.

3. Basement depth and deep structures

3.1. Depth of magnetic basements

The magnetic basement depth refers to the burial depth of the top of the Precambrian metamorphic basement and magnetic intrusions formed due to tectothermal events. The magnetic basement depth map of the Plateau (Fig. 3) was plotted using the depth values obtained by calculating the geologic blocks producing magnetic anomalies by the methods including correction coefficient tangent for oblique magnetization, Vacquier inflection-point gradient method, and the Euler deconvolution (Vacquier V et al., 1963; Guo ZH et al., 2003; Guan ZN, 2005; Marello et al., 2010; Zhu Y, 2012;Liu TY, 2013). It reflects the fluctuations of the Precambrian metamorphic basement, the thickness of sediment layers, and the distribution scope and tectonic characteristics of basins in the Plateau. As jointly shown by the aeromagnetic anomaly map (Fig. 1) and the magnetic basement depth map (Fig. 3),the depth of the basement in the Plateau is mainly 5-15 km(Xiong SQ et al., 2014), which is comparable to the thicknesses of Paleozoic and Meso-Cenozoic strata reflected by the Yadong-Golmud geosciences transect (Wu GJ et al.,1991). The structures, properties, and depth of the basement are different among various subregions (Table 1).

East Kunlun-Qilian Mountains area. The basement in this area is banded in shape in EW trending primarily and blocky locally. The Paleoproterozoic groups, Mesoproterozoic Xiaomiao Group, and Meso-Neoproterozoic epimetamorphic rock series that constitute the magnetic basement in the area have been exposed. Among them, the Paleoproterozoic groups include the Jinshuikou, Yemananshan, Beidahe, and Maxianshan groups. In the Qaidam Basin, the magnetic basement is 3-13 km deep. In the East Kunlun Mountains, the basements consist of dominant basement uplifts and the basement depression in only the Ayakekumu Lake area. The metamorphic bedrocks in the basement uplifts have been exposed or buried very shallowly, while the basement depressions has a burial depth of 2-3 km. In the Qilian Mountains, the metamorphic bedrocks have been exposed or buried very shallowly in many areas except for the greatly fluctuating Longxi Basin, where the basement in areas with deep depression is buried at a depth of 3-5 km. Two depression belts of the basement develop in NW trending on the southern side of the Northern Qilian Mountains. Among them, one is distributed along Hala Lake-Qinghai Lake area,with a basement depth of 2-5 km, while the other spreads along Delhi-Gonghe area, with a basement depth of 3-7 km.

Hoh-Xil-Songpan area. It has complex basement structures. In detail, the basement in Hoh-Xil area in the west is distributed in banded shapes in nearly EW trending, the basement in Madoi and Chenduo areas in the west spreads in blocky shapes in nearly SN trending, while the basements in Yushu and Songpan areas are distributed in banded shapes in NW trending and in blocky shapes in NNE trending,respectively. The Precambrian basement has a relatively high crystallinity degree and is highly magnetic in its eastern part,existing in the form of relatively stable blocks. In this area,the basement is deeply buried in general and the depressions are wide. Specifically, the burial depth of the basement is mostly 5-11 km in Songpan-Ganzi area and is mostly 5-9 km in Hoh-Xil area.

The basement in the West Kunlun region has a single structure and is distributed in banded shapes in NW trending.According to the analysis of magnetic fields, the basements in its northwestern areas have a high degree of crystallinity and are strongly magnetic, and thus are rigid basements. They are possibly composed of Paleoproterozoic Qaraqash Group, with properties comparable to those of the Tarim Basin. In contrast, the basements in the southeastern areas have a low degree of crystallinity and are weakly magnetic, and thus are flexible basements. They are possibly composed of Mesoproterozoic Jiazitage and Alamasi groups and their properties are comparable to those of basements in main body of the Plateau. The basements in the area are very shallow,and the burial depth of the basement in areas to the north of the Karakoram Mountain pass is only 1-3 km.

Fig. 3. Magnetic basement depth map of the Qinghai-Tibet Plateau.

Chamdo-Southwestern Sanjiang area. The basements in this area are distributed in banded and blocky shapes in NW trending. The Sinian and Mesoproterozoic strata that constitute the basements in the area have been exposed in the drainage basins of Lancang and Nujiang rivers. Among them,the Sinian strata include metamorphic rock series and Ningduo Group, and the Mesoproterozoic strata include the Lancang and Gaoligongshan groups. The burial depth of basement uplifts are mainly 5-3 km in Yanshiping-Chamdo Basin and is 7-15 km in the Northern Qiangtang Basin.

Gangdese-Himalaya area. The basements in this area are mainly in banded shapes in nearly EW trending. The basement on the northern slope of Himalaya is composed of Meso-Neoproterozoic Nyalam Group, and the basement in the Gangdese area consists of the Precambrian Nyainqentanglha Group, which has been exposed. The two sets of metamorphic rock series are weakly magnetic and have high plasticity, and are unstable orogenic basements greatly fluctuating. The basement on the southern slope of Himalaya has a burial depth of 0.5-11 km. Meanwhile, a basement uplift zone in nearly EW trending is distributed along Kangma-Sajia-Burang area, with a basement depth of 0.5-3 km. Basement depression zones are distributed on the southern and northern sides of the basement uplift zone, with a basement depth of 5-11 km. Furthermore, two large-scale basement uplift zones are developed in the Gangdese area, which are distributed along the northern sides of Yarlung-Zangbo and Shiquanhe-Shenzha fault zones separately. Both of them have high uplift amplitude, with a basement depth of 0.5-1 km. Basement depression faults are distributed along the southern sides of Bangong-Nyima and Shiquanhe-Shenzha fault zones, with a depression depth of up to 7-15 km. Moreover, the burial depth of the basement is mostly 5-13 km in the Longmucuo-Meimacuo area and is mostly 5-13 km in the Southern Qiangtang Basin.

3.2. SN-trending negative magnetic anomaly zone and SNtrending structures

A major discovery from the 1∶1000000-scaled aeromagnetic survey of the Plateau is the deep-source aeromagnetic negative anomaly zone (area) in nearly SN trending that vertically span across the central part of the Plateau (Xiong SQ et al., 2001a, 2001b). The negative magnetic anomaly zone is located in the hinterland of the Plateau and distributed in the shape of an NNE-trending trapezoid, with an east-west width of up to 300-400 km. In the vertical direction, it spans across the whole plateau,achieving a south-north length of more than 1200 km. It is bounded by the Muzitage-Kangnuo-Daxiong area in the west and the Kangtong Lake-Tanggula Mountains-Maqing area in the east. Meanwhile, its northern boundary is close to Muzitage-Buluntai area and its southern boundary reaches the south of the Shenzha-Coqen area. Therefore, the negative magnetic anomaly zone basically occupies most of the central part of the Plateau, roughly belonging to the central part of the three SN-trending blocks of the Plateau (i.e., the western,central, and western parts) (Zhang X et al., 2003; Teng JW et al., 2011). In terms of the negative anomaly areas of the Plateau, only indistinctly negative background magnetic field is shown on the aeromagnetic ΔTmap. However, on the RTP maps with upward continuation heights of 20 km and 50 km,the scope (82°-92°E, 23°-38°N) and the nearly NNE-trending long axis of the aeromagnetic negative anomaly zone are more clearly presented (Xiong SQ, 2001a; Wang XW, 2008).According to the satellite-borne Asian total-intensity magnetic anomaly map, Pan GT (2004) thought that a negative magnetic anomaly zone largely appears in the southern part of Bangong-Nujiang suture zone, with the center ’s altitude of-9.9 nT. Yong YY (2012) considered that there are trails of SN-trending structures from the Northern Qiangtang-southern Qinghai area to Himalaya. He RZ et al. (2007) also revealed an NNE-trending negative anomaly zone by separating regional magnetic fields using the matched filtering method.They thought that a huge lowly magnetic physical body exists in the deep crust of the central hinterland in the Plateau. It reflects a huge SN-trending extensional tectonic zone on the ground and reflects the inhomogeneous zoning features in the deep and superficial crust. Meanwhile, it has reached the bottom of the crust and thus is subject to the differentiation,adjustment, and migration of materials in the upper mantle.Some researchers believed that the negative magnetic anomaly zone is in the same direction as the main stress due to the collision between Indian-Eurasian continents and is tangent to the suture zones where terranes are stitched together, showing a huge deep thermal-tectonic zone (Xiong SQ et al., 2001a; Yao ZX et al., 2002; Zhou FH et al., 2002;He RZ et al., 2007; Hou ZQ, 2006; Li YL et al., 2006; Teng JW et al., 2011). According to the research on gravity and magnetic data, large-sized magnetic anomalies in nearly SN trending exist in the Plateau and they clearly correspond to the grabens on the ground surface. Zhang J et al. (2004) discussed the segment division of the western, central, and eastern parts in the Plateau and their implications and discovered that there distinctly exist two major structures in SN trending or nearly SN trending as well as some minor SN-trending structures.

A large number of researchers have analyzed and studied the magnetic structures and genesis of the negative magnetic anomaly zone (Xiong SQ et al., 2001a, 2007; Zhou FH et al.,2002; Hou ZQ et al., 2006; Xue DJ et al., 2006; He RZ et al.,2007; Teng JW et al., 2011). They discovered that a deepsource magnetic anomaly zone in nearly SN trending was vertically overlapped by and horizontally intersected by a series of shallow magnetic anomaly zones in nearly EW trending horizontally cross them, forming a flyover-type duallayer magnetic anomaly structure. As for the genesis of the structure, they unanimously agreed that the deep thermal flow rose in the NNE direction due to the changes in the deepsource magnetic structures. As a result, the local magma melted and thus the geotemperature of the lower part of the upper crust was raised, which led to the demagnetization of the bottom of magnetic layers. In this way, the flyover-type dual-layer magnetic anomaly structure was formed.Accordingly, they proposed the geophysical evidence of the deep thermal demagnetization-thermal uplift mechanism in the Plateau, which provides significant constraints on the study of crust-mantle structure and tectonics and lithospheric deep processes in the Plateau. In terms of the dynamic genesis of the structure, they thought that the flyover-type dual-layer magnetic anomaly structure is due to the continual northward extrusion of the Indian Plate. The initial extrusion drove all blocks to shorten in SN trending and lengthen in EW trending until the blocks cannot be further shortened in the SN direction. Then the extrusion might cause the deep part to split along SN trending or the formation of deep interlayer shear zones. Then deep thermal flow rose along the tectonic pathways and local magma melted, which raised the geotemperature of the upper crust. Various data indicate that the geotemperature of the deep negative magnetic anomaly zone is higher than that of its eastern and western sides (Zhou FH, 2002). Pan GT (2004) considered that the deep negative magnetic anomalies are caused by that the temperature and intensity of the deep materials in the southern part of the Plateau feature are lower than those of the materials in surrounding areas of the Plateau. Moreover, the southern part is characterized by thick crust and thin mantle and a series of unique tectonic thermogenic processes (such as self-heating from upper-crustal extension, lithospheric delamination, crust remelting, and hydrothermal solution forming due to increase in the thickness of the lower crust). In addition, the materials in the asthenosphere obviously upwelled in this part. All these thermal effects led to the demagnetization of rocks and further induced negative magnetic anomalies.

The occurrence of the negative magnetic anomaly zone in the Plateau indicates that the SN-trending faults on the ground surface in southern Tibet have extended northwards. In other words, there exist structures in nearly SN trending in the deep part and lithosphere in the plateau hinterland. The faults in nearly SN trending formed based on the overall EW-trending tectonics of the Plateau, and are significant for the understanding of the geological tectonics, seismic activities,magmatic activities, and metallogenesis in the Plateau.

Much attention should be paid to the roles of the SNtrending tectonics including its ore controlling and implications for metallogenic zoning, the formation mechanism of extensional and compressional tectonics, the interactive relationships between the SN- and EW-trending tectonics in the same stress field framework, as well as the relationship between the SN-trending tectonics and the uplift of the Plateau, and all these should be further explored (Yong YY, 2012).

3.3. Seismic tectonic setting

The Plateau is a region that has the most earthquakes with the highest intensity in mainland of China, which is related to the faults formed due to the Indian subcontinent pushing northwards and then intensively colliding and continually extruding the Eurasian continent (Lai XL et al., 2016). The epicenter distribution of earthquakes withMS＞ 6.1 in the Plateau is closely related to the faults in NE, NW, and NE trending. For example, theMS8.6 earthquake that happened in Zayu on August 15, 1950 (the largest earthquake in the Plateau) is located on Pengcuo-Jiali fault zone, which is the largest NW-trending right-lateral strike-slip fault in the southeastern part of the Plateau. The left-lateral strike-slip Xianshuihe fault zone in the east is the most active seismic zone. Furthermore, there are several left-lateral strike-slip faults in the eastern part of the Plateau such as Kunlun,Jinshajiang, and Yanshiping-Chamdo faults, all of which are seismic active zones. There are a series of SN-trending rift zones inside the Plateau, which reflect the typical tectonics of neotectonic activities inside the Plateau. Multiple intensive earthquakes have occurred in the rift zone, especially in the Yadong-Anduo (Yadong-Gulu) rift (Teng JW et al., 1997),which is present as multiple NNE-trending pinnate fault zones from Yadong to Anduo on the aeromagnetic map. Another example is the right-lateral strike-slip Longnan-Kangding fault, which is closely related to the Beichuan earthquake in 2008 (Ding YY et al., 2009).

Lai XL et al. (2016) processed the aeromagnetic data of the Plateau using wavelet analysis and obtained a multi-scale aeromagnetic anomaly distribution map. Based on this, they studied the deep tectonic background of theMS8.1 earthquakes occurring in Nepal and threeMS7 strong aftershocks, and thought that the third-order wavelet analysis results can reflect the tectonic characteristics of the deep crust. As shown in their study results, severalMS8 earthquakes are mainly distributed in the NNE direction,which is mainly consistent with the location of the deep negative magnetic anomaly zone. This reflects that the deep negative magnetic anomaly zone and the strong seismic activities are controlled by the unified force source and direction of the collisions between Indian and Eurasian plates.In addition, the frequent strong seismic activities in the negative magnetic anomaly zone are closely related to the tectonic activities caused by the rising of deep thermal flow.The hypocenter depth of theMS8.1 earthquake in Nepal is 20 km, and the gestation and outbreak of the earthquake are closely related to the morphology of the Curie surface and the low-velocity zone. Meanwhile, the deep tectonic setting of theMS8.1 earthquake and its multiple strongMS7 aftershocks specifically includes the intersections between the shallow Himalaya tectonic zone in nearly EW trending and the deep structures in nearly SN trending obtained from aeromagnetic anomalies. Besides, it also includes the dislocation and break of the low-velocity zone and Moho in the crust of the southern part obtained from seismic profiles, and the rise of deep thermal flows. All these indicate that high-magnitude earthquakes mainly occurred in the NNE direction, which is consistent with the location of the deep negative magnetic anomaly zone (i.e., the seismic activities are closely tied to the deep NNE-trending structures). Meanwhile, the NNE-trending negative magnetic anomaly zone reflects the exact scope of the NNE-trending structures. Therefore, it provides references for using aeromagnetic data to analyze where earthquakes may occur on the Plateau. Therefore, the aeromagnetic data can provide valuable data for understanding and studying the magnetic tectonics and structures of crusts and exploring the formation mechanism of earthquakes in the Plateau (Lai XL et al., 2016; Yan YF et al., 2016). Zhang X et al. (2003)determined that the depth of the Curie surface in the south of the Plateau is 20-23 km according to inversion using aeromagnetic data. They considered that earthquakes tend to occur above the Curie surface since the crust below the Curie surface is at a high temperature and the rocks in the crust hardly fracture owing to their high plasticity.

4. Suture zones and deep faults

Suture zones and deep faults are developed in the Plateau since it lies in a key region of the collision orogen between the paleocontinent Gondwanaland and Eurasia continent.Subducted oceanic crusts represented by ophiolite suites develop on both sides of continental collision zones and suture zones, and there are residual marginal-sea spreading ridges represented by ophiolite suites in subduction zones.Meanwhile, strongly magnetic ophiolite suites are bound to produce linear and moniliform magnetic anomaly zones in the Plateau where bedrocks are widely exposed. Therefore,aeromagnetic anomaly maps can be used to accurately determine the trails of continental collision zones and paleosubduction zones (Yang WC, 2014).

Ren JS (2004) considered that there are two types of suture zones in the Plateau, namely huge suture zones and small suture zones. The former refers to the Indus-Yarlung-Zangbo suture zone, which is the main suture zone of the Tethys Ocean. The latter includes Kunlun-Qinling (since the Caledonian) polycyclic suture zone, and Kangxiwa-Maqên,Xijir Ulan-Jinshajiang, Longmucuo-Shuanghu-Lancang, and Bangong-Nujiang suture zones. They both show different magnetic field characteristics. In detail, the former is shown as strong linear anomaly zones, while the latter is exhibited as different magnetic fields or linearly distributed moniliform anomalies. Currently, special attention is mainly paid to the Yarlung-Zangbo, Bangong-Nujiang, Longmucuo-Shuanghu-Lancang, and Xijir Ulan-Jinshajiang suture zones.

4.1. Yarlung-Zangbo suture zone

The aeromagnetic survey in 1998-2000 (Xiong SQ et al.,2001a) firstly revealed two nearly parallel linear strong magnetic anomaly zones along the Yarlung-Zangbo River,namely the northern zone and the southern zone. They are more than 1500 km long from east to west, with a distance of about 25-30 km between them. They show distinctly different characteristics of magnetic anomalies. In detail, the positive magnetic anomaly zones in the northern zone are intact and continuous and extend throughout the whole northern zone. In contrast, the positive magnetic anomaly zones in the southern zone are merely distributed in the middle section of the southern zone. They extend from Bailang to Zhongba in the west, with a length of less than 600 km. Form the positive magnetic anomaly zones to the western end of the northern zone exist magnetic anomalies discontinuously distributed.According to analysis and research, both the northern and the southern zones are induced by ophiolite-type basic-ultrabasic rocks, reflecting two ophiolite zones. They jointly constitute the Yarlung-Zangbo River suture zone. It is proposed that Yarlung-Zangbo River suture zone is the evolutionary result of the India Plate subducting towards and colliding with the Eurasian Plate twice. The northern zone of Yarlung-Zangbo River suture zone is not only the largest ophiolite-type basicultrabasic rock zone in China but also is possibly the most promising metallogenic zone of chromite (Xiong SQ et al.,2001a, 2007; Yao ZX et al., 2001, 2002; Zhou HF et al.,2001).

Most importantly, in the western aeromagnetic anomaly zone in the northern zone (from Dazhuka to the western end),the outcrops include Yanshanian-Himalayan granites and intermediate-basic volcanics and Meso-Cenozoic sedimentary rocks (such as Cretaceous and Tertiary sedimentary rocks),with no ophiolite previously visible. Furthermore, the granites and volcanics exposed are magnetic to some degree.Therefore, the genesis of the aeromagnetic anomaly zone is still in dispute.

As revealed by field geological surveys and systemic petrological studies in recent years (Gao YF et al., 2003; Mo XX et al., 2005; Dong GC et al., 2008; Hou ZQ et al., 2012),a basic-ultrabasic complex belt is exposed along Dazhuka-Xietongmen area around the north bank of the Yarlung-Zangbo River (i.e., the southern margin of the magmatic rock zone in the Gangdese magmatic arc), corresponding to the northern zone. Furthermore, it spatially spreads in the shape of a narrow belt along the boundary faults on the northern side of the Yarlung-Zangbo suture zone, with a length of about 330 km and a width of 3-6 km generally and up to 10 km locally. The complex belt is sporadically and separately distributed along the northern bank of the Yarlung-Zangbo River in general. However, it is exposed in groups in Luodui in Nimu and Dongga in Shigatse. Furthermore, 14 small gabbro rockmasses are exposed in Dongga area, Shigatse. Part of the complex belt is concealed in the southern margin of the batholith of Gangdese granites, and its residual rocks are mainly composed of gabbro bosses that are sporadically and separately exposed; the ultramafic cumulates, diabase dyke,and basalt that are dismembered by tectonics, and deep-water sediments containing strips of siliceous rocks. It occurs in the form of a series of tectonic slices due to thrust nappe from north to south and possibly connects with the ophiolitic melange belt in the western Gangdese magmatic arc. It formed closely due to continental collision (Dong GC et al.,2008) and also may result from magma underplating (Mo XX et al., 2005, 2020).

Zhang WP et al. (2011) held that ophiolite in the Yarlung-Zangbo suture zone can be largely divided into the eastern,central, and western parts in the east-west direction, which show clear difference in spatial-temporal evolution. (1) In descending order of spatial distribution, they are the eastern,central, and western parts. (2) In terms of the distribution of rock sequences of the ophiolite suites, the rock sequences are completely exposed in Shigatse in the central part and Luobusha in the eastern part, while the ophiolites in other parts are dismembered and cannot form complete ophiolite sections. (3) The formation and emplacement ages of the ophliolites in the three parts also differ. The western and eastern parts mainly formed during the Late Jurassic-Early Cretaceous, while the central part formed earlier from the Middle Triassic to Early Cretaceous and mostly during the Late Jurassic-Early Cretaceous. The western part (from Saga in the east to the Sino-Yindian border in the west) is divided into two ophiolite belts. Among them, the northern belt is exposed along the main ridges of Mayou Mula Mountain,Mount Kailash, and Mount Ayila. The rock masses in the outcrops are small and most of them are in elongated and lensoid shapes parallel to the regional tectonic line or occur along fault zones. Most of them invade into Triassic-Jurassic shale, limestone, and basalt, and some of them are in fault contact with Cretaceous and Paleogene clastic rocks. They mainly consist of the rock masses such as Mayou Mula,Yugu, Menshi, and Dajiweng ones. In terms of lithology, the northern belt is mainly comprised of serpentinized harzburgites and peridotites, with basalt and radiolarian silicalite only visible in a few places. Most of them have distinct magnetism, which basically agrees with that of the rock masses in the northern zone of the Yarlung-Zangbo suture zone.

4.2. Bangong-Nujiang suture zone

Linearly arranged moniliform anomalies are distributed in the Bangong-Nujiang ophiolite melange belt. They are characterized by the association of negative and positive anomalies with peak amplitude of 100-300 nT. They are mainly induced by ophiolites and intermediate-basic volcanics(Xiong SQ et al., 2001a). The magnetic fields on the southern and northern sides of the linear anomaly zone vary greatly. In the Gangdese-Nyainqentanglha area on the southern side,NWW-trending and nearly-EW-trending magnetic anomalies with high amplitudes and sharp change in negative and positive attributes are distributed. As a result, a series of zones with moniliform alternate positive and negative magnetic anomalies form. Their amplitude is -100-200 nT generally,with a maximum of more than 1200 nT and gradients of 30-50 nT/km. In contrast, in the Qiangtang area on the northern side, the regional magnetic anomalies feature significantly lower amplitude and show wide, gentle, and stable magnetic background. Meanwhile, this area shows NEtrending local magnetic anomalies and the lump anomalies with amplitude of 100-200 nT. The Shuanghu-Tanggula area in the east shows gentle negative anomalies with amplitude of-20-50 nT, while the Yanshiping area exhibits some NWtrending and EW-trending local positive anomalies with amplitude of 100-200 nT again. By taking the difference of the magnetic field characteristics between both sides of the Bangong-Nujiang suture zone as one piece of powerful evidence, Pan GT et al. (2004) inferred that the Bangong-Nujiang suture zone is a deep fault running across the lithosphere.

4.3. Longmucuo-Shuanghu-Lancang suture zone

The Longmucuo-Shuanghu-Lancang suture zone was proposed by Li C in 1987 for the first time, followed by longterm field surveys and research (Li C, 1987, 1995, 2008). It is believed that the suture zone is not only a Paleo-Tethys suture zone but also serves as an important paleobiogeographic boundary. In other words, the Cretaceous-Permian glaciomarine strata in Gondwanaland passed over the Bangong-Nujiang suture zone but did not enter into the northern Qiangtang. Therefore, it is believed that the northern and southern Qiangtang belong to two tectonic units that are totally different in evolutionary mechanisms and the scale and structure of the lithosphere (Li C et al., 2006, 2008). This academic opinion has been accepted by more and more geologists as the discovery of blueschist (Deng XG, 2002; Lu JP, 2006) and ophiolite (Zhai QG, 2007) in the zone.However, there is still a lack of critical geophysical evidence of the presence and location of the suture zone due to the harsh natural and geographical conditions and the development of ground cover. Most especially, specific strike and location of the suture zone in the regions such as Shuanghu-Yaqu-Gangni are always “unsolved mysteries”.

The existence of the Longmucuo-Shuanghu-Lancang suture zone is also in great dispute from the perspective of geophysics. Yao ZX et al. (2002) considered that the Longmucuo-Shuanghu-Lancang suture zone lacks magnetic anomalous indicators of a suture zone since no aeromagnetic anomalies similar to those in the Yarlung-Zangbo suture zone have been found in the suture zone. Zhao WJ et al. (2006) and Xiong SQ et al. (2013) thought there are no central uplift zones in the eastern part of Shuanghu area according to the 1∶1000000-scaled aeromagnetic survey maps and ground gravity maps. He RZ at al. (2007, 2007) performed magnetic upward continuation by 15 km and discovered that the Longmucuo-Shuanghu-Lancang suture zone showed magnetic anomalies with only weak characteristics. Zheng HW et al.(2012) held that the southern and northern sides of the tectonic zone differ in the characteristics of magnetic basements. Zou CQ et al. (2012) obtained relatively clear deep characteristics of a subduction zone by using the teleseismic-body-wave travel-time tomography. Zeng ZF et al. (2016) analyzed and demonstrated the geophysical characteristics of the suture zone according to regional gravity, magnetic, and magnetotelluric data. They found that the calculated Mohorovicic discontinuity of the southern and northern sides of the suture zone greatly differs in depth.Therefore, the gravity data and their processing results proved that the Longmucuo-Shuanghu-Lancang suture zone possibly exists. The reversion results of electrical resistivity based on magnetotelluric data of Shuanghu indicated that there are significant electrical differences between the southern and northern sides of the suture zone. Furthermore, recent 1∶200000-scaled high-precision aeromagnetic and airborne gravity data of the Qiangtang Basin proved the existence of the Longmucuo-Shuanghu suture zone as the latest geophysical evidence. Meanwhile, they further confirmed that the suture zone serves as the boundary between Gondwanaland and Pan-Cathaysian continent from the angle of deep geological structures, and the details are as follows.(1) In the middle part of the Qiangtang basin exist the boundaries between different gravity fields that run through the basin from east to west, large linear anomaly gradient zones, moniliform aeromagnetic positive anomaly zones that intermittently extend, and dislocation lines of different magnetic anomalies. (2) The northern side shows low Bouguer gravity and high magnetism, while the southern side exhibits high Bouguer gravity and low magnetism. Xiong SQ et al. (2020) interpreted these characteristics as a (concealed)tectonic (ophiolite) melange belt and considered that the magnetic basements of the suture zone are characterized by stepwise segmentation.

4.4. Xijir Ulan-Jinshajiang suture zone

The Xijir Ulan-Jinshajiang suture zone is composed of three parts, namely the western Xijir Ulan melange belt, the central Jinsha River ophiolite melange belt, and the eastern ophiolite melange belt (Pan GT et al., 2015). The western part is shown as NNW-trending moniliform and linear magnetic anomaly zones with anomaly amplitude of 50-200 nT generally in the magnetic anomaly map (Fig. 1) and as NNWtrending gravity anomaly zones in the Bouguer gravity anomaly map (Li ZK et al., 2010). Furthermore, these characteristics are more distinctly exhibited in Xijir Ulan-Yushu part. This part is shown as strong linear anomaly zones, moniliform anomaly zones, and boundaries between different magnetic anomaly fields and different gravity fields.This part is present as a magmatite uplift zone on the ground,which is composed of dozens of Variscan, Indosinian, and Yanshanian intermediate-acidic, intermediate-basic, and ultrabasic small rock masses that are intermittently distributed like bands. According to aeromagnetic forward calculation, it is a thrust fault inclining northwards with a sharp dip angle.

Most researchers consider that Xijir Ulan-Jinshajiang suture zone is a suture. However, the Xijir Ulan-Wudaoliang part of the suture zone is only reflected as low-amplitude positive magnetic anomalies in the magnetic field, which can only form moniliform magnetic anomaly zones and the boundaries between different magnetic fields instead of highamplitude linear magnetic anomaly zones. Despite the discovery of ophiolite suites, this part lacks linear positive magnetic anomaly zones similar to those in the Yarlung-Zangbo suture zone. Therefore, the author considers that, as a suture zone, the Xijir Ulan-Wudaoliang part of the suture zone was under the tectonic environment of a neritic shelf at most when it broke into two blocks to become an ocean.When the two blocks were sutured together again, no largescaled magmatic activities occurred, leading to indistinct magnetic anomalies. Given that aeromagnetic characteristic lines reflect moniliform anomalies and the boundaries between different magnetic fields, whether the Xijir Ulan-Wudaoliang part constitutes a suture or extends westwards deserves further discussion.

4.5. Deep faults determined by aeromagnetic anomalies

It is believed that fault activity will inevitably destroy the continuity of a geological body. For example, magmatic activities will occur along faults, or cap rocks or the basements will suffer vertical or horizontal displacement due to fault cutting. As a result, the originally unified geophysical fields or ground strata will change. This can be mostly reflected by huge linear anomaly zones, moniliform anomaly zones, linear gradient zones, or boundaries between different magnetic fields on magnetic field maps and as gravity anomaly gradient zones on gravity field maps. Such fault zones generally extend over a long distance and are associated with intensive magmatic activities on the ground. It must be noted that, although perspective information is provided on aeromagnetic maps, it is difficult to use aeromagnetic maps to further determine the classification and downward-cutting depth of deep faults. Instead, they can only indicate the existence and exact locations of deep faults. Furthermore, the locations of faults delineated based on aeromagnetic maps sometimes do not completely coincide with their tectonic locations determined based on ground information. The most likely reason is that aeromagnetic data reflect the faults in medium-depth or deep pats. Deep faults develop in the Plateau. Their distribution characteristics are clearly reflected on the magnetic field maps and can still be clearly displayed in the case of magnetic upward continuation by 10 km and 20 km. Based on the above-mentioned magnetic anomaly indicators of faults, 12 deep faults were delineated in the Plateau (Fig. 4), which are mainly in NW and EW trending.

Jinshajiang-Honghe deep fault zone. The fault zone is distributed along the drainage basin of the Jinsha River in NW-NNW trending and extends to the eastern Yushu area in the north, with a length of about 1300 km. On the magnetic field map, it is shown as NNW-trending moniliform and linear magnetic anomaly zones with anomaly amplitude of 50-200 nT. Meanwhile, the NW-trending parts of the magnetic anomaly zones obviously cut the NW-trending Zhiduo-Yushu-Queershan magnetic anomaly zone, resulting in an offset distance of up to 50 km (in Yushu area). On the Bouguer gravity anomaly map, it is displayed as the NNWtrending gravity anomaly zones, which also cut and dislocate the NW-trending Zhiduo-Yushu-Queershan gravity anomaly zone in Yushu area. Therefore, the Jinshajiang-Honghe fault is in NNW trending. It ends in Yushu area and does not extend westward to Xijir Ulan area (Xiong SQ et al., 2012).

Xuelianhu-Lancang deep fault zone. The trending of this fault zone after it extends in NW trending is highly controversial. Some researchers believe that it is a part of the Longmucuo-Shuanghu-Lancang fault zone (Li C, 2008). It can be divided into two parts with Chamdo as the boundary,namely the northern and southern parts. The southern part is roughly distributed along the drainage basin of the Lancang River and extends northwards along Riwoqe and Geladandong Snow Mountain and ends in Xuelian Lake area.The author named it Xuelianhu-Lancang fault. It is in NW strike in general and about 1900 km long. On the magnetic field map, it is displayed as linear and moniliform magnetic anomaly zones, with magnetic anomaly amplitude of 10-150 nT.The magnetic anomalies are induced by concealed magnetic basic and intermediate-acidic rocks that are distributed along the fault zone. On the Bouguer gravity anomaly map, it is displayed as high-amplitude gravity anomaly zones and gravity anomaly gradient zones. According to comprehensive analyses, the Lancang fault zone does not turn westward but continues extending northwestwards to Xuelian Lake area after extending northwestward to Anduo area (Xiong SQ et al., 2012).

Dengqen-Nujiang deep fault zone. This fault zone is distributed along the drainage basin of Nujiang River in general. It continues to extend northwestwards along Zogang and Suoxian County to the northern Anduo area after arriving at Meili Snow Mountain. It possibly runs through the Qiangtang Basin and extends to Buruocuo area along Shuanghu. The author named it Dengqen-Nujiang fault. It is in NW strike in general and about 1400 km long. In the magnetic field, it is displayed as NNW-trending moniliform anomaly zones and linear gradient zones, with the magnetic anomaly amplitude of 20-150 nT. The magnetic anomalies on its sides are completely different due to its cutting. Those on the northeastern side are in NW trending, while those on the southwestern side are in nearly EW trending. These magnetic anomalies with increase in amplitude are produced by known and concealed rock masses that are distributed along the fault and thus serve as powerful evidence of the fault’s location. In terms of gravity fields, it is displayed as NNW-trending gravity anomaly zones and linear gradient zones and form boundaries between different gravity anomaly areas. As clearly reflected by the gravity and magnetic fields, the Dengqen-Nujiang deep fault zone does not turn westwards after extending north-westward through Suoxian County to the northern Anduo area. This indicates that it is in NW direction in general but does not turn westwards in Suoxian Country and connects a nearly EW-trending fault in Tibet to jointly form an arc fault zone protruding northeastwards(Bangon-Nujiang fault zone), as considered by previous researchers (He RZ et al., 2007; Xiong SQ et al., 2012).

Fig. 4. Distribution characteristics of deep fault zones in the Qinghai-Tibet Plateau reflected by the verticle derivative ofaeromagnetic data.

Longnan (Wudu)-Kangding deep fault zone. This fault zone was identified using aeromagnetic data (Ding YY et al.,2009; Xiong SQ et al., 2016). It spreads northeastward from Kangding through Lixian County and Longnan (Wudu) to the eastern Tianshui area. It is in NE strike in general and about 650 km in length. It shows clear magnetic characteristics and is displayed as a boundary between different magnetic fields.The magnetic field on the southeastern side are blocky magnetic anomaly areas with magnetic anomaly amplitude of-180-200 nT. Meanwhile, most of the magnetic anomalies are in NE strike. The magnetic field on its northwestern side are stable magnetic anomaly areas with magnetic anomaly amplitude of -40-40 nT, and most of the magnetic anomalies are in NW strike. As jointly shown by magnetic anomalies,the two sides of the fault are significantly different in properties and burial depth of basements, the development degree of cap rock, and tectonic shift. The Longnan (Wudu)-Kangding deep fault zone controls the evolution of the geological structures on both sides. On the northwestern side,the basement greatly sinks, hugely thick Triassic sediments are deposited, and the structures are in NW strike. In contrast,on the southeastern side, the Middle Cenozoic strata have always been uplifting, and Precambrian paleo strata and Lower Paleozoic strata are widely exposed, without Mesozoic-Cenozoic sediments. All these indicate that the Longnan (Wudu-Kangding deep fault zone is a deep fault that cuts through basements and experienced intensive activities during the Middle Cenozoic. Meanwhile, it is a strike-slip fault with a scale similar to the Altyn Tagh fault—a fault in the northwest border of the Plateau—and constitutes the eastern border of the Plateau. Affected by push and extrusion of the Indian block, left-lateral strike-slip dislocation might have happened along the Longnan (Wudu)-Kangding deep fault zone.

Lazhulong-Yushu deep fault zone. This fault zone is located in northern Tibet and is distributed along Lazhulong-Bairuobucuo-Xijir Ulan-Yushu-Ganzi area. Therefore, its location is similar to that of the northern part of previously identified Lazhulong-Jinshajiang fault. However, it does not turn southeastwards after passing by Ganzi. It possibly joins the northwestern part of the Xianshuihe fault and continues to extend eastwards to intersect with the Longnan (Wudu)-Kangding fault, but does not connect with the Jinshajiang fault. It is in EW-NW strike and about 2300 km in length. In the magnetic field, the Lazhulong-Yushu deep fault zone is exhibited as linear and moniliform anomaly zones and boundaries between different magnetic fields. These magnetic characteristics are still clear after the fault zone passes through Yushu, except for the decreased magnetic anomaly amplitude. In the gravity field, the Lazhulong-Yushu deep fault zone is shown as boundaries between different gravity fields. According to the aeromagnetic forward calculation, the fault is a thrust fault inclining northwards with a sharp dip angle. On the ground along the fault zone, it is present as a magmatite uplift zone on the ground, which is composed of dozens of Variscan, Indosinian, and Yanshanian intermediateacidic, intermediate-basic, and ultrabasic small rock masses that are intermittently distributed in banded shapes.

Bangong-Nyima deep fault zone. This fault zone is the western part of the previously identified Bangong-Nujiang fault. This study holds that the Nujiang fault extends northwestwards but does not turn westwards. Therefore, the Bangong-Nyima deep fault zone is an independent fault in nearly EW trending and intersects relation with the Dengqen-Nujiang fault. This is different from the viewpoint of previous researchers. On the aeromagnetic map, it is present as a magnetic anomaly zone with a width of 20-30 km,boundaries between different magnetic fields, and moniliform anomaly zones. Meanwhile, in the gravity field, it is shown as wide and gentle gravity anomaly gradient zones and anomaly zones. Meanwhile, it is in nearly EW strike and about 1500 km long. It lacks high-amplitude magnetic anomalies similar to those in the Yarlung-Zangbo fault zone, indicating non intensive magmatic activities along the fault. Ultrabasic rocks are distributed along the fault and most of them are exposed as ophiolite melange with complete basic associations of ophiolites. Besides, there are relatively developed basaltic andesites and andesite clastic rocks. All of these represent the typical characteristics of rock associations in island-arc-backarc basins (Pan YS et al., 1999). According to aeromagnetic and geological data, it is possible that the Bangong-Nyima fault is a back-arc fault zone and some depressions or basins that develop along the fault are back-arc basins. However,only partial sections have a history of becoming an ocean.According to deep seismic sounding (Zhao WJ et al., 2012),the Bangong-Nyima deep fault zone is a thrust fault that slightly inclines northwards, with a width of about 22 km, and meanwhile, it was intensively active during the Middle Cenozoic.

Additionally, the Yarlung-Zangbo deep fault has been stated above. The Shiquanhe-Shenzha, Kunzhong, and West Kunlun-Altyn Tagh deep fault zones as well as the deep fault on the northern margin of northern Qilian Mountains are all clearly reflected in magnetic fields. However, they are not repeated here due to the limitation on paper length.

5. Magmatic rocks

The records of magmatic rocks in and around the Plateau serve as not only the imprint of terrain stitching and tectonic evolution of the Plateau but also important indicators of major structure-magma-metallogenic belts. The magmatic activity is closely related to structures and is strictly controlled by plate movement. The formation of rock belts is controlled by huge fault zones or suture zones. For example, intermediate-acidic intrusions, especially the magmatic rock belts forming since the Variscan, tend to be distributed parallel to each other along huge fault zones or suture zones. Meanwhile, magmatic rocks show an intrusion sequence from old to new ones due to the evolutionary process of the Eurasian Plate (i.e., accretion from north to south). The magmatic activities frequently occurred in the Plateau, leading to the formation of the basicintermediate-acidic lava resulting from the eruption of the mantle-derived magma; emplaced basic, ultra-basic,intermediate-acid, and alkaline rocks, and granites originating from continental crust transformation. Various magmatic rocks are exposed in the Plateau, including intermediateacidic rocks primarily and basic-ultrabasic rocks secondarily in terms of outcrop number. The magmatic rocks are mostly magnetic and can cause magnetic anomalies of varying amplitude in the magnetic field. Based on this, the magmatic rocks can be classified. Generally, ultrabasic rocks are strongly magnetic and cause magnetic anomalies with high amplitude, steep gradient, and regular morphology, and thus they can be easily identified in the magnetic field. Xiong SQ(2001a) and Zhang CG et al. (2020) studied the distribution characteristics of magmatic rocks in the Plateau based on aeromagnetic data and investigated the temporal-spatial evolutionary pattern of magmatic rocks. By referring to the new aeromagnetic map (Fig. 4), 141 new ultrabasic rock masses have been delineated, the scope of 53 known ultrabasic rock masses expanded, 551 new intermediate-acidic rock masses delineated, and the scope of 90 known intermediate-acidic rock masses expanded (Fig. 5).

5.1. Basic-ultrabasic rock belts

Ultrabasic rocks include ultrabasic rocks, basic-ultrabasic rocks, and ophiolites. They are mainly distributed along and on the two sides of deep and large fault zones. They vary in scale, rock types, and emplacement time. Chromite is closely related to ultrabasic rocks, and therefore, the delineation of concealed ultrabasic rocks provides bases for surveys of chromite.

Yarlung-Zangbo ultrabasic rock belt. This rock belt starts from Gar in the west, passes through Zhongba, Ngamring, and Shigatse, and extends to Langxian County in the east,covering a distance of about 1500 km. Along it, a total of 74 ultrabasic rock masses have been discovered (including 32 known rock masses, 10 ophiolites, and 32 inferred rock masses) and the scope of 15 known rock masses has been expanded. The rock masses generally have a large scale of 400 km2mostly or even up to 1500 km2. They are basically distributed along the Yarlung-Zangbo fault zone. The rock belt is composed of two belts, i.e., the northern and southern belts. They cause two parallel magnetic anomaly zones in nearly EW trending in the magnetic field. The northern belt is linearly distributed and is characterized by the association of positive and negative anomalies with amplitude of 300-1000 nT. As for the southern belt, its eastern section is shown as the linear magnetic anomalies and its western section is present as moniliform magnetic anomalies with amplitude of 100-900 nT. The known ophiolites cause high-amplitude magnetic anomalies in the magnetic field. The on-site measurement shows that these ophiolites are the highest magnetic in the Plateau, and it is inferred that the highamplitude magnetic anomalies along the Yarlung-Zangbo ultrabasic rock belt are related to these ophiolites. A small EW-trending gabbro-diabase-pyroxenite belt is intermittently exposed on the southernmost margin of the Gangdese granite belt, especially at the intersection of the southern margin of the Amuxiong complex in the middle section of the Gangdese granite belt and an SN-trending rift zone. This basically coincides with the northern belt in space and is considered as the product of magma underplating (Dong GC et al., 2008;Mo XX et al., 2020; Xia WJ et al., 2020). According to magnetic field source depth imaging (Wang XW, 2008) and three-dimensional inversion and visual modeling (Xue DJ et al., 2006; Hu B et al., 2019; Wang J et al., 2020) of the middle section of the northern belt, it is believed that the Gangdese batholith has higher magnetization intensity than the exposed granite, and in its deep part exist ultrabasic rock masses, part of which are in a plate form inclining southward. All these indicate that the Shigatse ophiolites remain intact in deep parts (about 2 km).

Bangong-Naqu ultrabasic rock belt. This rock belt is generally distributed along Bangong Lake-Gerze-Nyima-Baingoin-Naqu-Dengqen area, extending over a distance of about 1600 km and in a nearly EW trending. Along and on both sides of the Bangong-Nyima fault zone, a total of 119 basic-ultrabasic rock masses have been delineated (including 36 known rock masses, 29 ophiolites, and 54 inferred rock masses) and the scope of 19 known rock masses has been expanded. Nevertheless, these rock masses are only exposed in small areas. They formed mainly due to the control of the Bangong-Nyima fault zone and are distributed in a relatively wide range with poor continuity. They are centralized in Rutog, Gerze, Baingoin, Naqu, and Dengqen areas. The size of concealed rock masses inferred based on aeromagnetic data is generally about 200 km2, and can reach 1200 km2at maximum. They are reflected in the magnetic field map as the intermittently extending aeromagnetic anomaly zones composed of a series of small-scale local anomalies.Compared with known rock masses, the exposed basicultrabasic rocks (or ophiolites) correspond well to magnetic anomalies. Meanwhile, the anomalies they caused have regular shapes and are characterized by the association of positive and negative anomalies. Most of them feature anomaly amplitude of 100-700 nT and a small scale.

According to geological data, the ophiolites are dismembered but cumulates are relatively developed in this rock belt. Wang XB et al. (1987) believed that the ophiolite belts developed in a back-arc basin environment on the southern margin of the Eurasian continent during the Middle-Late Mesozoic. After short-term expansion and rapid closure of the back-arc basin, the ophiolites were emplaced during the Late Jurassic-Early Cretaceous and are the back-arc basintype oceanic crust fragments that are incompletely developed.Meanwhile, the emplacement of the rock massed was dominated by diapir rise.

Fig. 5. Distribution of magmatic rocks in the Qinghai-Tibet Plateau.

West Kunlun-Altyn Tagh Mountains ultrabasic rock belt.This rock belt is located in the area from the West Kunlun Mountains to the Altyn Tagh Mountains. It starts from Jigen in the west and extends to Aksai in the east along Burun-Jiandao Peak-Yeyike-Suwuqi-Mangya area. Along it, a total of 60 basic-ultrabasic rock masses have been delineated(including four known rock masses, 31 ophiolites and 25 inferred rock masses) and the scope of eight known rock masses has been expanded. These rock masses are distributed in groups, with scales of about 200 km2. They intermittently extend over a distance of 1900 km shows the strike of an NWNE-trending arc. The basic-ultrabasic rock masses are shown as an arc-shaped magnetic anomaly zone from the West Kunlun Mountains to the Altyn Tagh Mountains in the magnetic field. Meanwhile, the known rock masses correspond well to the elevated magnetic anomalies.Therefore, the magnetic anomalies with regular morphology,large amplitude, and steep gradient in the arc-shaped magnetic anomaly zone are caused by ultrabasic rocks. The anomalies consist of positive anomalies primarily and some negative anomalies on the northern side, with anomaly amplitude of 100-900 nT. The rock masses in this rock belt are generally distributed along the northern margin of the West Kunlun,Kangxiwa, and Altyn Tagh fault zones.

According to geological data, the rock masses in this belt are ophiolites. They are widely serpentinized and some of them have even evolved into serpentines. They formed in different ears such as the Proterozoic and the Early Permian and are located at the junctions of paleo plates (Wang GP,1993). Based on the occurrence and distribution characteristics of ophiolites, the suture zones and their product ophiolites formed due to oceanic crust closure in different periods can be inferred. However, they were squeezed and superimposed due to the strong tectonics of the southern and northern ancient continents and thus were completely destroyed and transformed. As a result, they lack the characteristics of continuous distribution of general suture zones.

Qilian Mountains ultrabasic rock belt. This rock belt is distributed along Qiqing-Qilian-Gonghe-the northern margin of the Qaidam Basin. It extends in NW trending, with a length of about 650 km. Along the rock belt, a total of 55 ultrabasic rock masses have been delineated (including 10 known rock masses, 27 ophiolites, and 18 inferred rock masses) and the scope of 10 known rock masses has been expanded. These rock masses are distributed between two fault zones and on the southern sides of the two faults, namely the fault zone on the northern margin of the northern Qilian Mountains and the fault zone on the southern margin of the central Qilian Mountains. Their formation was mainly controlled by the two fault zones. In the magnetic field, the ultrabasic rock masses are located in the NW-trending magnetic anomaly zone in the northern Qilian Mountains, along which separated elevated magnetic anomalies are intermittently distributed. The elevated magnetic anomalies feature regular morphology(circular or elliptical mostly), steep gradients, and magnetic anomaly amplitude of 50-150 nT. Moreover, they correspond to the exposed ultrabasic rocks, indicating that these magnetic anomalies are induced by basic-ultrabasic rocks. Based on the characteristics of magnetic anomalies, concealed rock masses have been inferred and the scope of known concealed rock masses has been expanded. Their scales vary between 370 km2and 125 km2and they are mainly comprised of pyroxene peridotites and bears some feldspathic peridotites.

East Kunlun ultrabasic rock belt. This rock belt is distributed along the southern margin of the East Kunlun Mountains. It extends in EW trending, with a length of about 1800 km. Along the rock belt, a total of 26 rock masses have been delineated (including 14 ophiolites and 12 inferred rock masses) and the scope of one known rock mass has been expanded. These rock masses are located within and on the southern side of the Kunzhong fault zone, along which separated elevated magnetic anomalies are distributed. The elevated magnetic anomalies feature regular morphology(circular or elliptical mostly), steep gradients, and magnetic anomaly amplitude of 50-100 nT. Furthermore, they correspond to the exposed ophiolites, indicating that these magnetic anomalies are caused by ophiolites or basicultrabasic rocks. The scales of these rock masses vary between 300 km2and 100 km2.

5.2. Intermediate-acidic rocks

Magmatic activities are intensive in the Plateau, leading to the formation of intermediate-acidic intrusions and volcanic rock series of different types of rock associations. The most frequent magmatic activities occurred during the Paleozoic and Mesozoic and they are the most intensive during the Late Paleozoic and Middle Cenozoic. As a result, a large area of intermediate-acidic rock masses formed. Among them, the magnetic intermediate-acidic intrusions forming during the Paleozoic have mainly been inferred from aeromagnetic data.Since a lot of endogenetic metallic minerals are directly related to the intermediate-acidic rocks, the delineation of concealed rock masses using aeromagnetic data will provide important clues for expanding the prospecting areas of metallic minerals.

West Kunlun intermediate-acidic rock belt. This rock belt is located in the West Kunlun region. It extends in NW trending as a whole, with a length of about 750 km and a width of about 80 km. Granodiorites and granites are exposed along it. They formed during the Paleozoic primarily and the Proterozoic secondarily, with very few forming during the Mesozoic. Among them, the magnetic rock masses cause magnetic anomalies with amplitude of 50-300 nT. They are regular in morphology and vary gently. Based on these magnetic anomaly characteristics, seven concealed intermediate-acidic rock masses have been inferred and the scope of seven known rock masses has been expanded. Most of these rock masses are distributed along NW-trending fault zones and their formation is controlled by these fault zones.

East Kunlun intermediate-acidic rock belt. This rock belt is distributed along the NW-trending fault zone in the East Kunlun Mountains. It extends in NWW trending, with a length of about 1100 km and a width of 100 km. The outcrops along it include Proterozoic, Late Paleozoic, and Mesozoic granodiorites and granites, which are dominated by Late Paleozoic rocks. They induce magnetic anomalies with amplitude of 50-250 nT in the magnetic field. Among them,the magnetic anomalies that are relatively regular in morphology, vary gently, and feature amplitude of 30-200 nT are mainly caused by intermediate-acidic intrusions. Based on this, 39 concealed intermediate-acidic rock masses have been delineated and the scope of eight known rock masses has been expanded.

Qilian Mountain intermediate-acidic rock belt. This rock belt is mainly distributed along the NW-trending fault in the Qilian Mountains. It extends in NW trending as a whole, with a length of about 1300 km and a width of about 400 km.Diorites, granodiorites, and granites are exposed along it and they mainly formed during the Paleozoic. Among them, the magnetic rock masses cause magnetic anomalies with amplitude of 50-300 nT. The magnetic anomalies that are relatively regular in morphology, vary gently, and have amplitude of 50-250 nT are mainly induced by intermediateacidic intrusions. Based on this, 96 concealed intermediateacidic rock masses have been inferred and the scope of 20 known rock masses has been expanded.

Longmucuo-Shuanghu intermediate-acidic rock belt. This rock belt is located in the Shuanghu-Dinggou-Longmucuo area. It is generally distributed along the fault zones in nearly EW trending, with a length of about 1200 km and a width of 200 km. Middle Cenozoic granodiorites and granites are sporadically exposed along it. Some of them are weakly magnetic, with insignificant or no reflection in the magnetic field, while some are magnetic and induce elliptical and banded anomalies that are regular and relatively regular in morphology and vary gently, with amplitude of 50-250 nT.According to the anomaly morphology, 152 concealed intermediate-acid intrusions have been delineated and the scope of three known rock masses has been expanded.

Gangdese intermediate-acidic rock belt. This rock belt is distributed in the Lhorong-Biru-Gongbo’gyamda-Zole-Lhasa area and generally extends along the faults in nearly EW trending, with a length of about 1800 km and a width of 400 km. It serves as the most important magmatic rock belt in the Plateau. The outcrops in this rock belt mainly include composite batholith consisting of adamellites, granodiorites,and quartz diorites. They cause the elliptical and banded magnetic anomalies with amplitude of 50-300 nT in the magnetic field. According to the magnetic anomaly morphology, 109 concealed intermediate-acidic intrusions have been inferred and the scope of 36 known rock masses has been expanded. The intermediate-acidic intrusions that widely are distributed in the rock belt vary significantly in aeromagnetic anomalies. This may be caused by weakly magnetic S-type granites and strongly magnetic I-type granites. The S-type granites are related to the continental collision environment and formed due to partial melting and crystallization of the crust-derived sediments as source rocks,while the I-type granites are related to a subduction environment and formed due to the mixing of crust-derived magma and mantle-derived magma (Xiong SQ et al., 2001a).

Many researchers have studied the rock belt in terms of geology, geochemistry, isotopic ages, continental collisionrelated mineralization (Lu SW, 2004; Mo XX et al., 2005,2009; Ge LS et al., 2006; Dong GC et al., 2008; Hou ZQ,2012; Qiu JS et al., 2015; Meng YK et al., 2018; Hu B, 2018;Xia WJ et al., 2020). They consider that the Gangdese granite belt can be divided into three sub-belts from north to south.The types of them are type-I in the early age, transition type in the middle age, and type S in the late age, respectively and they formed under the tectonic conditions of plate subduction,suturing, and collision, respectively. Yao P et al. (2006)believed that the volcanic-magmatic-arc belt on the southern margin of Gangdese is located between the Yarlung-Zangbo suture zone and the Gangdese landmass and it was distributed as two parallel northern and southern sub-belts spatially in its early and late ages. Meanwhile, they considered that the formation of the volcanic-magmatic-arc belt is closely related to subduction of the Yarlung-Zangbo oceanic crust as well as the early and late subduction beneath the southern margin of the Gangdese landmass.

Yanshiping-Chamdo intermediate-acidic rock belt. This rock belt is distributed in the Yanshiping-Chamdo-Baiyu-Zhongdian-Muli area in the Southwestern Sanjiang area. It extends in NW-NNW-NW trending, with a length of about 1800 km and a width of about 300 km. Most of the rock masses along it are distributed along NW-trending faults and their formation is mainly controlled by the NW-trending and NW-trending faults. The outcrops include Mesozoic diorites and granites. The well-known Xinlong-Daocheng granite rock mass and Lincang granite rock mass are located in this rock belt, but they are not reflected in the magnetic field.Furthermore, most of the granites are also not shown in the magnetic field. These indicate that Mesozoic granites are not magnetic. Besides, many anomalies are visible in areas with Triassic strata developing. They are relatively regular in morphology are banded or elliptical and feature gentle gradients and magnetic anomaly amplitude of 40-200 nT.This is mainly related to intermediate-acidic intrusions. Based on this, 148 concealed intermediate-acidic rock masses have been delineated and the scope of 16 known rock masses has been expanded.

6. Structural characteristics of the petroliferous Qiangtang Basin

The Qiangtang Basin is located in the northern part of the Plateau. It is the largest Mesozoic marine-facies sedimentary basin and one of the important oil and gas strategic areas in China. The boundaries and range of the basin have been determined according to aeromagnetic data, especially the latest 1∶200000-scaled high-precision aeromagnetic data(Fig. 6) and airborne gravity survey data. Meanwhile, the basin has been studied in terms of the structure, properties,and the variation characteristics of basement depth, structural pattern, and the thickness of cap rocks based on the preparation of the maps of the basin including the depth map of metamorphic basements, the map of structural areas, and the thickness map of the Mesozoic strata (Xiong SQ et al.,2001a, 2013, 2020). Other researchers have also analyzed the structural characteristics of the Qiangtang Basin by combing aeromagnetic data (Jiang MZ et al., 2001; Zhao WJ et al.,2006; Tan FW et al., 2016; Zeng ZF et al., 2016).

6.1. Boundaries and central uplift zone

As shown by the characteristics of airborne gravity and magnetic fields, the Qiangtang Basin is bounded by the fault zone on the southern margin of the Xijir Ulan-Jinshajiang suture zone in the north and the fault zone on the northern margin of the Bangong-Nujiang suture zone in the south and is intercepted by the Kaixinling fault in the east. It is divided into two parts by the central uplift zone that runs through the basin from east to west. The northern and southern sides of the central uplift zone are controlled by two deep faults and their spatial spans (width) gradually converge from west to east. As a result, they form a scissor shape that opens westwards. The central uplift zone, together with faults in the northern, southern, and eastern boundaries of the basin, jointly controls the distribution characteristics of the Qiangtang Basin. Previous gravity and magnetic surveys failed to clearly characterize the gravity and magnetic anomalies in the central uplift zone due to constraints on measurement scales and instrument accuracy, leading to different views on the existence of the central uplift zone to the east of the Shuanghu area (Zhao WJ et al., 2006; He RZ et al., 2007; Xiong SQ et al., 2013, 2020). The latest 1∶200000-scaled high-precision and comprehensive airborne gravity and magnetic surveys provide clear information of the gravity and magnetic anomalies in the central uplift zone. Based on this, the central uplift zone is reflected as a large-scale linear anomaly zone with rich information, good continuity, and clear boundaries,which serves as an important boundary of the regional gravity and magnetic fields.

Fig. 6. First vertical derivative map of RTP aeromagnetic anomaly ΔT of the Qiangtang Basin (scale: 1∶200000).

With the gravity and magnetic anomaly zone of the central uplift zone as the boundary, the gravity and magnetic anomalies of the northern and southern Qiangtang Basin have significantly different characteristics. The anomaly area of the northern Qiangtang Basin generally features low Bouguer gravity and high magnetism, while the anomaly area of the southern Qiangtang Basin is characterized by high Bouguer gravity and low magnetism on the contrary. As for the anomaly area of the northern Qiangtang Basin, there are large numbers of massive cloddy gravity and magnetic anomalies.In this area, the Bouguer gravity mostly ranges from -590×10-5m/s2to -570×10-5m/s2, with the minimum values occurring in the areas such as Purog Kangri and Geladandong.Among them, the Bouguer gravity in Purog Kangri is -604 ×10-5m/s2, which is the lowest in the entire Qiangtang Basin.Meanwhile, this area features high aeromagnetic anomaly background values, with large numbers of strongly elevated local anomalies developing. For example, the aeromagnetic magnetic anomalies in the areas such as Purog Kangri and Geladandong are large in scale and show distinct characteristics, with magnetic anomaly amplitude of greater than 200 nT. As for the anomaly area of the southern Qiangtang Basin, it features a large gravity anomaly gradient zone that is gradually elevated southwards. The Bouguer gravity in this area is significantly higher than that in the northern anomaly area and falls in the range of -560×10-5-545 × 10-5m/s2mostly. The magnetic field in this area is generally weaker than that in the northern anomaly area.Meanwhile, the local aeromagnetic anomalies in this area are also significantly smaller in scale, with amplitude mostly ranging from 50 nT to 100 nT. As proved by the quantitative inversion of deep gravity and magnetic data, a central uplift zone still exists from Shuanghu in the west to Gangni in the east and it runs through the Qiangtang Basin from east to west. Inversion results reveal that the depth of the magnetic basement is 3-5 km to the west of Shuanghu, 5-7 km in Shuanghu-Yaqu, and 7-9 km in Yaqu-Gonni. The central uplift zone gradually descends from west to east, showing the form of steps. Meanwhile, the amplitude and distribution scope of the central uplift zone are controlled by the faults in nearly SN trending and can be divided into distinct segments.In combination with geophysical analyses, it is considered that the main body of the central uplift zone is the (concealed)(ophiolitic) melange belt in the Lungmucuo-Shuanghu suture zone (Xiong SQ et al., 2020), thus confirming the key geophysical field evidence for the existence of in the Lungmucuo-Shuanghu suture zone.

6.2. Basement properties and deep structure

As revealed by high-precision airborne gravity and magnetic data, the Qiangtang Basin lacks large-scale, banded,and strongly elevated magnetic anomaly zones that exist in stable cratonic basins such as the Tarim Basin, Sichuan Basin,and Ordos Basin. The strongly magnetic blocks developing within the Qiangtang Basin are caused by various intrusion complexes instead of the Pre-Sinian crystalline basement.Meanwhile, there is no Archean ancient continental nucleus in the Qiangtang area, which serves as one of the reasons in terms of deep structures for the generally gentle magnetic field and the extremely developed faults in the Qiangtang Basin. As indicated by the airborne gravity and magnetic fields, the northern Qiangtang Basin features widely distributed moderately-strongly magnetic bedrocks, deeply buried basement, and thick sedimentary cap rock. Meanwhile,the morphology of the magnetic field in the northern Qiangtang Basin mainly forms due to high-grade metamorphic crystalline basement, which can be compared with that in Ningduo Group and Jitang Group regionally(comprised of schist, gneiss, granulite, and migmatite). In contrast, the southern Qiangtang Basin is characterized by widely distributed weakly-moderately magnetic bedrocks and regional gently varying magnetic field in the case of magnetic upward continuation by different heights. This is mainly caused by epimetamorphic basement rock series, which can be compared with that in Nyalam Group (comprised of quartzite, schist, phyllite, and gneiss). Therefore, with the central uplift zone as the boundary, the northern and southern Qiangtang Basin completely differ in gravity and magnetic field characteristics, indicating differences in basement properties, structural patterns, and burial depths.

As also indicated by the latest airborne gravity and magnetic research results, the basement is deeply buried in both southern and northern Qiangtang Basin in general, with a burial depth of 5-15 km mostly. In contrast, the basement is shallowly buried in the Bangong-Nujiang suture zone, Xijir Ulan-Jinshajiang suture zone, and the western section of the central uplift zone in general, with a burial depth of 3-5 km mostly, showing the distinct characteristics of basement uplift(Xiong SQ et al., 2020). With the central uplift zone as the boundary, the southern and northern Qiangtang Basin differ greatly in basement depth. Specifically, the basement in the northern Qiangtang Basin is deeper than that in the southern Qiangtang Basin and their general burial depths are 7-15 km and 5-13 km, respectively. Multiple depression centers exist in both southern and northern Qiangtang Basin and their burial depth varies from 11 km to 15 km. Among them, six depression centers lie in the northern Qiangtang Basin. They are mainly distributed along Changliang Mountain-Wucun Village, Sewu Town-Yuegai area, with a basement depth of greater than 13 km in general and up to a maximum of 15 km in the northeastern part of Changliang Mountain. Four depression centers exist in the southern Qiangtang Basin.They are separated by a series of NS-trending salient belts,with a depth of 9-11 km in general and up to a maximum of more than 13 km in the western Yisang area. The basements are shallow in the east and deep in the west in both the southern and northern Qingtang basins, and the major depression areas in the two areas are all located in the west.This roughly reflects the characteristics of changes in the sedimentary thickness of the Paleozoic and Mesozoic strata in both southern and northern Qiangtang Basin.

6.3. Characteristics of tectono-magmatic activities

Spatially, strong tectonic deformation and most faults all occurred in the large boundary faults on the two sides and the central uplift zone of the Qiangtang Basin and the Tuotuo River area in the northeastern part of the basin. Contrarily, the central and western parts of the Qiangtang Basin feature relatively weak tectonic deformation and fewer and smaller faults. As indicated by high-precision airborne gravity and magnetic data, the northern and southern Qiangtang Basin also differ in the transformation superposed late. The northern Qiangtang Basin experienced both strong NS-trending extrusion and NW-trending shear, and thus the structures in this area are mainly in NW trending. These two types of tectonics are more obvious and are accompanied by largescale strong magmatic activities in the eastern part of the northern Qiangtang Basin. In contrast, the southern Qiangtang Basin mainly suffered NS-trending extrusion and the structures in this area are nearly in EW trending. Compared with the northern Qiangtang Basin, the southern Qiangtang Basin is closer to the orogenic front of the Plateau and experienced more obvious extrusion and uplift, and the nappe structures are more developed on the ground. Although the northern Qiangtang Basin also suffered strong extrusion, the blocky rigid basement protected the sedimentary cap rock to a certain extend. Meanwhile, the depth and range of impacts imposed by the nappe structures are far smaller in the northern Qiangtang Basin than in the southern Qiangtang Basin.

The northern Qiangtang Basin underwent active Late Paleozoic marine-facies volcanic activities and Cenozoic continental-facies volcanic eruptions. Most especially, the extensive and intensive Cenozoic volcanic eruptions in the northern Qiangtang Basin resulted in a large potassic basaltbearing volcanic rock belt that intermittently extends over a distance of 1800 km from Hoh Xil in the west and Zaduo in the east. In comparison, the volcanic activities in the southern Qiangtang Basin started from the Late Triassic and weak volcanic eruption of sodic basic basalts occurred during the Cenozoic. According to the airborne gravity and magnetic field characteristics of volcanic rocks as well as geological data, 262 volcanic rock areas exist in the Qiangtang Basin,including 167 exposed ones and 95 concealed (shallowly buried) ones. The volcanic rocks are mainly distributed in the suture zones, the central uplift zone, and the eastern part of the northern Qiangtang Depression. They can be roughly divided into the sodic basic basalt series in the southern Qiangtang Basin (60-45 Ma), high-K calc-alkaline volcanic rock series in the northern Qiangtang Basin (44-20 Ma), and Hoh Xil-Kunzhong potassic basalt series (19-7 Ma) from south to north (Li GM, 2000; Lai SC et al., 2003; Chi XG et al., 1999; Li YG et al., 2005; Liu HY et al., 2004).

6.4. Oil and gas preservation conditions

Some researchers believe that the Qiangtang Basin was strongly affected by the uplift of the Plateau, and as a result,the faults developing within the basin, the plateau uplift, and later tectonic activities possibly greatly damaged the oil reservoirs in the basin (Zhao ZZ et al., 2002). Others hold that the Plateau remained stable as a whole and the Qiangtang Basin was more stable during the uplift of the Plateau, and Cenozoic tectonic activities only slightly impacted on the oil and gas preservation conditions in the Qiangtang Basin(Wang CS et al., 1996, 2006; Nan ZB et al., 2008; Song CY,2012; Liu CY et al., 2016; Zhao WJ et al., 2006; Wu ZH et al., 2009, 2014; Wang J et al., 2009, 2020; Fu XG et al.,2020). The focus of the arguments is whether the oil and gas in the Qiangtang Basin are well preserved after multiple stages of tectonic transformation.

Airborne gravity and magnetic research reveals that,compared with the southern Qiangtang Basin, the northern Qiangtang Basic features lower uplift amplitude and are more stable overall and thus is more favorable for the formation and preservation of oil and gas. Meanwhile, the southern Qiangtang Basin is close to the orogenic front of the Plateau and thus features a low burial depth of basements and more developed shallow thrust nappe structures (Xiong SQ et al.,2020). As confirmed by Well Qiangke-1, the first well drilled in the Qiangtang Basin for oil and gas scientific research, the stratigraphic sequences are continuous and intact and the deep structures were not damaged in the Bandaohu area in the northern Qiangtang Depression. Furthermore, source rocks were discovered in Quemocuo Formation for the first time, in which 13 layers of gas logging anomalies were found. Based on this, Jurassic Quemocuo Formation and Xiali Formation were identified, which are two suites of important cap rocks comprised of anhydrite and gypsum-bearing argillaceous rocks. Oil and gas manifestations were discovered below the cap rocks. Geological and geophysical data reveal that these cap rocks laterally extend within a certain range in a continuous and stab way, showing the characteristics of regional distribution. Therefore, it is believed that the Qiangtang Basin boasts favorable oil and gas sealing and preservation conditions (Wang J et al., 2020; Fu XG et al.,2020).

7. Minerals and oil and gas resources

The Plateau is located in the middle section of the Tethyan tectonic metallogenic domain, one of the three major giant tectonic metallogenic domains in the world. It is extremely rich in energy and metal mineral resources and serves as the most potential enrichment area and a national substitute and reserve base of mineral resources in China. Over the years,aeromagnetic data have played an important role in the study of regional metallogenic regularity, analyses of metallogenic geological setting, the discovery of favorable local magnetic anomalies for ore prospecting, the delineation of prospecting areas, selection of optimal prospecting targets, and the prospecting of endogenous metal deposits. Besides, they also play a critical role in the study of the fluctuation of magnetic basements, the delineation of the scope of petroliferous basins, the determination of the thickness of sedimentary cap rocks, the identification of favorable areas for oil and gas accumulation, and the evaluation of exploration prospect of oil and gas resources in basins.

A total of 21 metal deposits have been discovered based on the 1∶1000000-scaled aeromagnetic survey results of the central part of the Plateau (Xiong SQ et al., 2001a), which effectively supported the work deployment related to geology and mineral resources in the Plateau. Furthermore, based on the above-mentioned aeromagnetic survey results, multiple large-scale iron deposits such as Nixiong iron deposit in Tibet and Zankan and Laobing iron deposits in Xinjiang have been discovered through the follow-up work (Hu JW et al., 2010)and meanwhile, it has been pointed out that Qiangtang,Coqen, and Biru basins are petroliferous basins with great prospects for oil and gas exploration (Xiong SQ et al., 2001a).

7.1. Study of metallogenic regularity

The endogenous deposits in the Plateau feature a large scale, late formation era, multiple deposit types, and favorable preservation conditions and thus serve as important enrichment regions of mineral resources in China. In this case,it is particularly important to study the metallogenic regularity of the deposits. The formation and accumulation of endogenous deposits are closely related to magmatic activities, which have a close relationship with tectonic movement. The formation and distribution of metal deposits in south Tibet are closely related to collision zones, which can be reflected by the diagenesis and mineralization of porphyry copper deposits forming due to the tectonic evolution of the deep structures in the continent-continent collision zones in south Tibet. Actually, the mineralization in the collision zone of south Tibet occurred based on the metallogenesis of the Gangdese magmatic arc forming from the subduction of the lithosphere of the Neo-Tethys ocean formed, and it is the result of intensive transformation caused by continentcontinent collision and extrusion. It will be an important clue for prospecting in the Plateau to deepen the understanding of the metallogenesis under continent-continent collision conditions in the Plateau and to identify continent-continent collision zones from the perspective of tectonic evolution(Zhao WJ, 2016).

Regional aeromagnetic anomalies tend to reflect the areas with intensive tectonic-magmatic activities and thus can clearly reflect the metallogenic geological setting in the Plateau (Xiong SQ et al., 2001a). Many researchers have applied aeromagnetic data to discuss the magmatite distribution and metallogenic regularity of the Gangdese tectonic zone, Bangong-Nujiang ophiolite melange belt, and Yarlung-Zangbo suture zone (Wang QH et al., 2002; Jiang M et al., 2013; Song Y et al., 2019). As can be seen from the aeromagnetic characteristics of the Plateau, linear banded magnetic anomalies in nearly EW and NW trending are highly developed along the Yarlung-Zangbo suture zone, Gangdese collision zone, and Southwestern Sanjiang area. This indicates that magmatic rocks are highly developed. Therefore, these areas serve as major prospecting areas of endogenous metal minerals in the Plateau. Prospecting periods can be shortened by establishing regional assessment models and then adopting the models to determine possible development areas of different minerals (Hou ZQ et al., 2012). The regional assessment models can be built according to geological conditions and aeromagnetic anomalies and consist of volcanic-magmatic arc zones + marginal thrust fault zones +island-arc granitic diorites + marine-facies carbonate suites +skarnization zones + ferritization zones + high-amplitude magnetic anomaly zones.

Yang H et al. (1988, 1991) interpreted the characteristics of geophysical fields (such as the magnetic field) of the eastern part of the Plateau by applying the theory of plate tectonics. Accordingly, they pointed out that there are five metallogenic belts (areas) with different characteristics in the Plateau, such as the Yarlung-Zangbo paleo-oceanic ridge metallogenic belt, Kangdian rift metallogenic belt, and Longmenshan paleo-island arc metallogenic belt. According to the analyses of the aeromagnetic anomaly characteristics and metallogenic geological conditions in the midwestern areas of the Plateau, Xiong SQ et al. (2001a) believed that the areas along the Yarlung-Zangbo River (especially the northern aeromagnetic anomaly zone) are the key prospecting areas of chromite deposits, Gangdese is a key prospecting area of polymetallic deposits, and the West Kunlun region and Qimantage in middle-east Kunlun area are the prospecting areas of iron and copper polymetallic deposits. Hou ZQ(2006) found that the Gangdese metallogenic belt can be clearly sectioned from east to west and zoned from south to north. Specifically, it is divided into three sections by the NNE-trending negative magnetic anomaly zone that runs through the hinterland of the Plateau. Among them, the central section is represented by the negative magnetic anomaly zone with a width of about 300 km. The structures intensively developing in this section include the rifts and normal fault systems in nearly SN trending, Paleocene Linzong volcanic rock systems, and Miocene ultrapotassic volcanic rock systems develop. They are associated with porphyry-type copper-molybdenum ores and gold-copper ores with unique genesis, among which typical deposits include Xiongcun and Dongga large-scale copper-gold deposits and Zemoduola copper-gold deposits. According to the research by Song Y et al. (2019), the Duobuza large-scale porphyry copper deposit and Tiegelongnan ultra-large-scale copper deposit in the Duolong Ore Concentration Area are shown in the gradient zones of positive/negative aeromagnetic anomalies, with magnetic anomaly amplitude of -1000 nT.Meanwhile, it has been verified that the high-amplitude magnetic anomalies are induced by the concealed magnetitebearing biotite granites and the copper deposits occur on the edges of the rock masses.

7.2. Metal minerals

Aeromagnetic anomalies have direct implications for ferromagnetic minerals and can be used for the prospecting of magnetite and polymetallic minerals with associated magnetite. Therefore, aeromagnetic data can be used to indicate prospecting targets. Many geological exploration organizations and researchers have performed prospecting prediction of chromite in the Plateau based on aeromagnetic anomalies (Wang DF et al., 2007; He LF et al., 2014; Jiang M et al., 2015). According to aeromagnetic survey results, five strongly magnetic anomaly zones and two strongly magnetic anomaly groups have been identified in the middle-south Tibet, where seven ultrabasic rock masses, five chromite ore occurrences, and four magnetite ore occurrences have been discovered. Therefore, they constitute an important concentration area of chromite and magnetite mineralization in the northern Tibet Plateau.

The Nixiong iron deposit has been found in the iron ore zone at the northern foot of the Gangdese Mountains based on aeromagnetic data. It is dominated by skarn magnetite. The resources preliminarily estimated and prospective resources of its high-grade iron ore are 130×106t and up to 300×106-500×106t, respectively. Furthermore, the 1∶200000-scaled aeromagnetic surveys along the Qinghai-Tibet Railway revealed six iron anomaly zones, namely Xiongma-Xaitongmoin, Bangduo-Xiongmei-Lhari, Saina-Xiaqiuka,Tanggula pass-Canglaila, Tuotuohe-Chagang, and the southern margin of East Kunlun area, providing rich clues for the mineral resource evaluation and subsequent prospecting breakthroughs along the Qinghai-Tibet Railway (Wang DF et al., 2007). Besides, the 1∶50000 high-precision aeromagnetic survey in west Kunlun and Qimantage, Xinjiang(Zhang HR et al., 2012) discovered 1494 new aeromagnetic anomalies, among which 181 anomalies were inferred to be related to minerals. Then 11 iron deposits and seven polymetallic deposits (mineralized points) were found through verification by ground surveys and their estimated iron ore resources are 4.272×109t. In this way, the significant prospecting results of performing aeromagnetic survey,verification, and ore-discovery in the same year were achieved.

The above examples demonstrated that aeromagnetic data have important implications for indirect prospecting of endogenous polymetallic minerals and direct prospecting of iron minerals.

7.3. Oil and gas resources

The original basins within the Plateau have been transformed to different extents due to long-term Gondwana rifting and opening-closure-orogeny of the Tethys Ocean since the Paleozoic, especially the intensive collisions between Indian and Eurasian plates and collision-induced orogenesis as well as the uplift of the Plateau, making the evaluation and exploration of oil and gas resources in the basins a worldwide challenge (Wang CS et al., 1996, 2006).Aeromagnetic surveys can provide critical bases for studying the deep geological structures of the basins and exploring the tectonic transformation processes of the basins and oil and gas exploration. Through the reconnaissance surveys in the middle-west part of the Plateau, the preliminary evaluation of the oil and gas resource prospect in the part was performed and 11 sedimentary basins were identified. Based on this, as well as the regularity of the generation, reservation, cap rock formation, and accumulation of oil and gas in the Plateau, it is believed that the Qiangtang Basin and Yanghu-Xuejinhu Basin are the most promising basins in terms of oil and gas prospect in this part, and 12 favorable local targets for further exploration have been pointed out (Xiong SQ et al., 2001a,2012b).

Based on the latest research of combined airborne gravity and magnetic data of the Qiangtang Basin, 238 local tectonic anomalies related to oil and gas have been delineated,including 98 ones of bedrock buldging (Class A), 13 ones of intermediate-basic intrusive rocks (Class B), 54 ones of acidic and intermediate-acidic intrusive rocks (Class C), 15 ones of volcanic rocks (Class D), and 58 ones related to sediment layers (Class E). Meanwhile, 24 favorable tectonic areas of oil and gas have been identified, including six first-order ones,eight second-order ones, and ten third-order ones. Most of them are located in the middle-west area of the northern Qiangtang Depression, the western part of the southern Qiangtang Depression, and the Cenozoic basins in the Bangong-Nujiang suture zone. All these indicate great potential of regional oil and gas resources. Overall, the prospect for regional oil and gas exploration in the north and west is better than that in the south and east, respectively in general. For instance, it is higher in the northern Qiangtang Depression than in the southern Qiangtang Depression, and it is higher in the western part than in the eastern part in each depression. It is believed that most of the oil and gas resources in the Qiangtang Basin are directly controlled by Mesozoic depressions, which boast thick Mesozoic sediments,relatively stable and continuous sediment layers, developed multiple suites of hydrocarbon source rocks, and excellent reservoirs and cap rocks. Furthermore, the periphery of the depressions is also favorable for oil and gas migration and accumulation. Most especially, many Mesozoic depressions in the Qiangtang Basin mostly inherited and developed based on large basement depressions. They are favorable areas for oil and gas generation, migration, and accumulation, among which major oil and gas generation and transportation are obviously affected by basement depressions.

Besides, the Coqen, Biru, and Yanshiping-Chamdo basins also have a good prospect for oil and gas exploration. The Coqen Basin covers an area of about 120000 km2and enjoys a stable basement, developed depressions, thick sediment layers, and thick Cenozoic strata. The Biru Basin covers an area of about 59000 km2, including 45000 km2of wide depressions. Meanwhile, it has sediment layers with a thickness of 5000-11000 m, relatively stable structures, weak magmatic activities, and thick cap rocks. Additionally, the salients in the basin are low and covered by cap rocks with a large thickness of about 3000-5000 m. As for the Yanshiping-Chamdo Basin, the sediment layers formed during the Paleozoic and Middle Cenozoic, with a thickness of 5000-15000 m.

8. Discussion of the formation and uplift of the Qinghai-Tibet Plateau

The formation and overall uplift of the Plateau are a global hot spot that is mostly researched and of great concern.At present, the Plateau is still undergoing tectonic movement in its deep and shallow parts. There are various opinions on its formation and evolution, including the models of thrustimbrication, folding-subduction, dual crust, and collisionextrusion-induced shrinkage, as well as theories of isostatic adjustment and thermodynamic uplift (Dewey JF et al., 1973;Allegre CJ et al., 1984; Molnar P et al., 1987; Zhao WL et al.,1987; Windley BF, 1988; Zeng RS et al., 1995; Tapponnier P et al., 1997; England PC et al., 1982; Wu GJ et al., 1991; Li TD, 1995; Zhong DL et al., 1996; Pan YS, 1998; Hacker BR,et al., 2000; Chemenda AI et al., 2000; Xu ZQ et al., 2006;Wang CS, et al., 2008; Sanchez VI, et al., 2013; Pan GT and Xiao QH, 2015). However, regardless of the model, most researchers agree that the formation of the Plateau is related to the Indian Plate subducting northwards and then colliding with the Eurasian Plate and that the formation and uplift of the Plateau are temporally and spatially uniform (Pan YS, 1999),showing the characteristics of “multiple levels, multiple stages, and multiple factors” (Zhong Dl et al., 1996; Xiao XC et al., 1998).

8.1. Formation of the Qinghai-Tibet Plateau

The Plateau is not homogeneous as a whole. As shown by the magnetic field, it can be divided into blocks and sections obviously and have clear characteristics of regional anomalies(Fig. 1). Its basements are composed of Proterozoic epimetamorphic folded basements with weak magnetism (Fig.3), which are different from the high-grade metamorphic crystalline basements with strong magnetism in the northern Tarim Basin, eastern Sichuan Basin, and southern Indian Continent. According to their structural characteristics, they formed from the stitching of multiple blocks with different development histories (Chang CF et al., 1986; Pan GT et al.,2012). Given the fact that the structures and magmatic activities (including the emplacement eras of ophiolites in suture zones) in the Plateau become newer and larger in scale from north to south, the blocks were possibly stitched together gradually from north to south, which is also the process that the Plateau continued accreting. As for stitching between different blocks, the magnetic anomalies indicate there are subduction and collisions between blocks on both sides of the areas such as West Kunlun, Yarlung-Zangbo, and Bangong-Nujiang River. In contrast, the stitching of different blocks within the Plateau is unnecessarily related to large-scale subduction and collisions. Therefore, the Plateau is a unique tectonic region and has its own formation and development process.

As for the genesis of the Plateau, it can be inferred from the aeromagnetic anomaly characteristics as well as previous results that, between two northern and southern paleocontinents represented by the Tarim and the Indian blocks, there possibly existed an archipelagic sea area dominated by continental crust, which received the accumulation of clastic materials from the two paleocontinents. According to the opinion that the Kegang ophiolite in West Kunlun formed during the Middle Proterozoic (Wang GP, 1993), the continental crustal differentiation was enhanced from the northern margin of the Plateau (i.e., the southern margin of the Tarim Block) since the Middle Proterozoic, resulting in the emergence and several opening and closure of oceanic crust. As a result,ophiolite belts of different eras formed, such as the Proterozoic Hongliugou, Sulamtage, and Kegang rock belts,Carboniferous Kudi rock belt, Permian rock belt, and Permian Zaiyikele rock belt, which were successively stitched onto the Tarim block. The opening and closure processes of the oceanic crust may have continued till the Carboniferous-Permian. Owing to the long-term dynamic effects of deep tectonics induced by the contact between Indian and Tarim blocks, various suture zones were completely damaged and transformed and the ophiolite fragments were thrust and overlapped on the southern margin of the Tarim block.Thereafter, as the Plateau expanded, the ocean basins gradually migrated southwards until the Neo-Tethys Ocean closed. Then the Eurasian continent collided with the Indian continent, leading to the generation of the Yarlung-Zangbo suture zone. In this way, the Plateau formed. According to the comparison of the development of ophiolites and volcanic magmatic arcs among all of the suture zones, ocean basins in these suture zones generally have a short development history and small scales, and some suture zones closed even before the presence of oceanic crust. For instance, ophiolites are invisible in the Longmucuo-Jinshajiang section. Contrarily,the Yarlung-Zangbo ophiolite belt boasts large scale, good continuity, and complete trench-basin-arc tectonic pattern,indicating that the Neo-Tethys Ocean developed for a long time and in a large area and possibly opened and closed twice(Xiong SQ et al., 2001a; Zhou FH et al., 2001). Meanwhile,the Yarlung-Zangbo ophiolite belt formed late and experienced a few tectonic shifts and thus has been well preserved.

The Plateau and its adjacent areas are shown as a complete negative magnetic anomaly with amplitude of greater than -10 nT and scope of 700×800 km2(larger than the aeromagnetic negative anomaly) on the satellite-borne magnetic anomaly map. The negative magnetic anomaly is consistent with the Plateau in term of range. According to the estimation based on anomaly wavelength, the burial depth of the anomaly source is greater than 100 km, meaning that the materials inducing such negative anomaly should include the lower part of the lithosphere or the upper mantle. The reason is that the upward migration of hot deep thermal materials led to the uplift of the Curie surface, the decrease in the thickness of the magnetic layer of the crustal, and the decrease in the background values of the large regional magnetic field, and thus the negative long-wave negative anomaly formed (Yang H et al., 1988).

In summary, the formation history of the Plateau is a process that the crust suffered continuous differentiation,rifted to form oceans, collided to form continents, and gradually accreted based on the Tethys Ocean. This process has a very clear direction and can be divided into stages. In detail, the oceanic basins opened and closed from early to late and from north to south, and the Plateau gradually accreted from north to south. Moreover, one opening and closure process possibly occurred northwards after another opening and closure process, and the processes did not repeat or simultaneously occurred in general.

8.2. Uplift of the Qinghai-Tibet Plateau

The research on geophysical data indicates the crustal thickness of the Plateau is up to 50-70 km (Teng JW et al.,1980). The main factors affecting the formation, increase in thickness, and the latest rapid overall uplift are stated as follows.

(i) Due to long-term continuous extrusion of the Indian Plate towards the Eurasian Plate and blockage by the rigid Tarim, Upper Yangtze, and Erdos blocks, the comparatively plastic blocks inside the Plateau suffered lateral compression and vertical stretching and thus vertically thickened (Xiao XC et al., 1988). As shown in the aeromagnetic anomaly map(Fig. 1), the blocks in the Plateau are all distributed in banded shapes in NWW-nearly EW and NW trending, obviously indicating that they formed due to long-term SN-trending extrusion (Xiong SQ et al., 2001a; Zhou FH et al., 2001).

(ii) The materials in the crust and mantle of the Indian Plate were continuously added to the internal crust of the Plateau during the former subducted towards and inserted into the Eurasian Plate, which may be the second reason for the increase in the thickness of the crust in the Plateau (Xiao XC et al., 1988). For example, the dramatically varying aeromagnetic anomalies in the Gangdese area reflect the large-scale and multi-stage intrusion and eruption caused by the re-melt of crust-mantle materials in the deep parts of the subduction zones and the rise of magma during the Neo-Tethys ocean subducted northwards. As a result, the Gangdese volcanic-magmatic arc formed and meanwhile, the crust in Plateau further thickened and rose.

(iii) The isostatic compensation of gravity at the mountain feet in the Plateau is another cause for the modern rapid uplift of the Plateau, and the main force driving the uplift is gravity isostasy (Teng JW et al., 1997; Xiao XC et al., 1988; Yuan XC et al., 2006).

(iv) The thermal source inside the Plateau serves as the most important cause for the latest rapid uplift of the Plateau(Teng JW et al., 1997). As shown by the results of the aeromagnetic upward continuation by different heights, the central part of the Plateau is present as a deep NNE-trending negative magnetic anomaly zone. This indicates that an NNEtrending heat source body possibly existed in the deep part of the crust, which is an important factor for the uplift of the crust subjected to thermal expansion and is another cause for the increase in the thickness of the crust in the Plateau (Xiong SQ et al., 2001a; Zhou FH et al., 2002; Pan GT, 2004, 2012).

Xu ZQ et al. (1996) studied the Cenozoic alkali volcanic activities in Hoh Xil in the northern part of the Plateau and considered that they were caused by mantle diapirs. Based on this, they proposed that the main cause for the uplift of the plateau hinterland is related to the deep thermal driving force inside the continent, i.e., mantle diapirs. According to aeromagnetic anomalies, the local magnetic anomalies with low amplitude but dramatically varying gradients reflected by Cenozoic volcanic rocks in Hoh Xil can extend southwards to the Yanshipin area. Therefore, the deep mantle diapirs reflected by the Cenozoic volcanic rocks possibly exist in a larger area and are more likely distributed along the NNWstrike deep negative magnetic anomaly zone. In this way, they can contribute to the overall NE-direction uplift of the Plateau.

According to the huge thickness, high temperature and low density, and obvious thermal expansion of the crust in the Plateau, low-velocity and high-resistivity layers and strongreflection bright spots exist at a depth of 15-20 km(corresponding to the depth of Curie interface obtained from aeromagnetic inversion, i.e., 20-23 km). This roughly reflects the presence of the partially melted layer. The partially melted layer may become the seismogenic zones of frequent earthquakes in the Plateau. Most especially, it serves as the source area of abundant geothermal resources in the Plateau and also the most important reason for the latest rapid uplift of the Plateau (Zhang X et al., 2003).

In summary, the heat sources at the bottom of the upper crust reflected by aeromagnetic anomalies correspond to the low-velocity and high-resistivity layers in the crust reflected by seismic surveys and magnetotelluric sounding. Meanwhile,they may also reflect the interlayer shear fractured zones at different horizons of the crust in the Plateau that were produced from the Indian Plate extruding towards. The shear fractured zones provided smooth pathways for the rise of deep magma and thermal flow, resulting in a high geotemperature of the upper crust. Meanwhile, the extension in the EW direction leads to the decrease in the thickness of the crust and the uplift of the mantle, which also accelerates the overall uplift of the Plateau.

9. Eight issues worthy of concern

(i) As can be seen from the terrestrial aeromagnetic map of China, the Plateau is distinguished from the western and eastern parts of Chinese Mainland due to its unique magnetic field, showing a “tripartite” pattern together with the two parts. This indicates that the Plateau is an independent geological body and its magnetic anomaly field characteristics, basement structure and nature, and tectonic characteristics are completely different from those of its surrounding geological bodies. Furthermore, geographically,it is flat in relief with an average elevation of 4500-5000 m but not long and narrow mountains. Therefore, it is worth considering whether it should be classified as an independent tectonic unit and collectively called the Plateau Block together with the Southwestern Sanjiang orogen.

(ii) The Plateau moved northwards along the Altyn Tagh fault and Longnan (Wudu)-Kangding fault when it was extruded northwards by Indian subcontinent. Therefore, it is to be further verified whether the Longnan-Kangding fault should be considered to be an important strike-slip fault.

(iii) The Altyn Tagh Mountains is very closely related to the Plateau in terms of tectonics. However, the Plateau moved northwards along the Altyn Tagh fault when it was extruded northwards by the Indian subcontinent. Then the Altyn Tagh Mountains moved southwards along the Chira-Weiya fault that spreads along the eastern edge of the Tarim block due to the blockage of the Tarim block. Therefore, it should be considered whether the EW-trending distribution of Altyn Tagh Mountains and its structures is the result of the clamping and countermovement of the two faults.

(iv) The aeromagnetic anomaly characteristics of different types of suture zones need to be studied in depth. For example, the Bangong-Nujiang, Longmucuo-Shuanghu-Lancang, and Xijir Ulan-Jinshajiang suture zones induce linear magnetic anomaly zones, moniliform magnetic anomalies, and boundaries between different magnetic fields instead of strongly magnetic and continuous linear anomaly zones similar to those of like Yarlung-Zangbo River.Therefore, their extension needs to be further studied.

(v) Deep faults (or sutures) play an obvious role in controlling the development of geological structures,magmatic activities, and the formation of mineral resources in the Plateau. There are frequent magmatic activities along the deep faults and their peripheral areas, and the magmatic rocks exposed or concealed will serve as the areas favorable for the accumulation of endogenous polymetallic deposits. Most especially, the deep structures of the plate collision zones deserve high attention and in-depth study.

(vi) Aeromagnetic data can clearly reflect the scope of basins and the thickness of sediment layers. Meanwhile, the formation and evolution of basins are closely related to the tectonic evolution of suture zones or deep faults. Therefore,the effects of the suture zones or deep faults on sedimentary suites as well as the migration, reservoir formation, and preservation of oil and gas in the basins are another critical issue in the evaluation of petroliferous basins in the Plateau.

(vii) The aeromagnetic data, in combination with regional geophysical data such as Pn wave, seismic Q values, and magnetotelluric sounding data, can be used to analyze the correlation among geophysical anomalies of different areas and further to explore the geological issues as hot spots including seismogenesis, crustal flow, and uplift of the Plateau.

(viii) The aero-geophysical exploration in Plateau is still at the lowest level compared with that in other regions in China. Therefore, it is recommended to deploy the 1∶200000-scaled combined airborne gravity and magnetic surveys in areas not surveyed in the whole plateau and to conduct 1∶50000-scaled airborne gravity and magnetic surveys or combined airborne electromagnetic, magnetic, and radiometric surveys in key depression areas such as Baiyunhu,Sanjiaohu and Bandaohu depressions in the Qiangtang Basin and important metallogenic belts such as the Gangdese metallogenic belt as soon as possible. The purposes are to provide more detailed and accurate geophysical information and scientific bases for achieving breakthroughs in the prospecting of oil and gas resources and strategic minerals in the Plateau more quickly and for geoscientific research of the Plateau.

10. Conclusions

(i) The basement of the Plateau is mainly comprised of weakly magnetic metamorphic rock series since the main part of the Plateau is an area of a negative magnetic anomalous field with a strength of -10--80 nT. It lacks Archean highgrade metamorphic crystalline basement and thus is unstable.Meanwhile, a small scale of strongly magnetic Paleoproterozoic crystalline blocks remain in the deep basement, and thus the Plateau consists of multiple terranes that are stitched together.

(ii) The Plateau is divided into four first-order tectonic units (orogens), namely East Kunlun-Qilian, Hoh-Xil-Songpan, Gangdese-Himalaya, and Chamdo-Southwestern Sanjiang orogens. Among them, the former three ones are the extensions of adjacent Tarim and Sino-Korean plates, South China-Southeast Asia Plate, and Indian Plate, respectively,while the last formed due to the development and convergence of discrete parts of South China-Southeast Asia and Indian plates (Liu ZK et al., 1990). They are all bounded by deep faults with the features of plate sutures.

(iii) In the East Kunlun-Qilian Orogen, the burial depth of the basement varies greatly and is 3000-15000 m in basins.The sediment layers in this orogen formed during the Upper Paleozoic and Mesozoic. The Hoh-Xil-Songpan Orogen features a stable basement, small tectonic shift, and thick sediment layers with a thickness of 2000-13000 m. The aeromagnetic data indicate that there is no large-scale magmatic activity in the orogen. The Chamdo-Southwestern Sanjiang Orogen is newly determined and is controlled by NNW-NW-trending faults. Among them, the NW-trending faults are a set of fault systems independently existing in the Tethys fault system. They are not related nor affiliated to but intersected the deep faults in nearly EW trending inside Tibet.The Gangdese-Himalaya Orogen is characterized by a large tectonic shift, and the burial depth of its metamorphic basement is 3000-15000 m. Meanwhile, the sediment layers in the orogen formed during the Middle Cenozoic and Paleozoic.

(iv) There is a giant SN-trending negative magnetic anomaly zone in the hinterland of the Plateau. It is bounded by the Muzitage-Kangnuo-Daxiong area in the west and the Kangtong Lake-Tanggula Mountains-Maqing area in the east.Meanwhile, its northern boundary is close to the Muzitage-Buluntai area and its southern boundary reaches the south of the Shenzha-Coqen area. Therefore, it roughly occupies most of the central part of the Plateau and reflects a deep thermaltectonic zone. Meanwhile, it was vertically overlapped by and horizontally intersected by a series of shallow tectonic zones in nearly EW and NW trending, forming a flyover-type geological structure in the Plateau.

(v) The Yarlung-Zangbo suture zone forming due to continent-continent collisions since the Cenozoic shows double aeromagnetic anomaly zones. Therefore, it can be inferred that the Yarlung-Zangbo suture zone formed from the Indian Plate subducting towards and colliding with the Eurasian Plate twice.

(vi) A set of NW-trending faults occur in eastern Tibet and they are intersected by without joining the faults in nearly EW trending that develop in the middle-west Tibet. As reflected by the magnetic field, the Nujiang, Lancang, and Jinshajiang faults do not turn westwards but continue extending northwestwards to Buruocuo, Xijir Ulan, and Dege after extending northwestwards through Dengqen, Riwoqe,and Dege areas. Therefore, they are renamed Dengqen-Nujiang, Xuelianhu-Lancang, and Jinshajiang-Honghe faults.Meanwhile, the western part of the Bangong-Nujiang fault determined previously is intersected by the Dengqen-Nujiang fault and thus is named Nujiang-Nyima fault. The original Lazhulong (or Xijir Ulan)-Jinshajiang fault in the north is renamed Lazhulong (or Xijir Ulan)-Yushu fault.

(vii) The central uplift zone in the Qiangtang Basin gradually descends from west to east, showing the form of steps. Meanwhile, it can be divided into distinct segments.The basement of the central uplift zone obviously differs between the eastern and western sections with the Shuanghu area as the boundary. In detail, there is large-scale basement uplift in the western section, while Mesozoic uplift instead of large-scale basement uplift occurs in the deep part of the eastern section (concealed low uplift). The Qiangtang Basin is divided into southern and northern Qiangtang by the central uplift zone and there is no uniform basement in the basin. The basement of the northern Qiangtang (depression) is clearly more strongly magnetic than that of the southern Qiangtang(depression), indicating southern and northern Qiangtang differ in basement properties. Furthermore, the basement in the Qiangtang Basin is deep in the north and west and shallow in the south and west. Specifically, the basement in the southern Qiangtang Basin is shallow and experienced intensive structural transformation since the Mesozoic. In contrast, the basement in the northern Qiangtang Basin is deep and relatively stable and thus is more favorable for the generation and preservation of oil and gas. Up to now, 19 favorable tectonic regions of oil and gas have been determined in the Qiangtang Basin. Meanwhile, it has been proposed that in the Qiangtang Basin, the prospect for oil and gas exploration in the north and west is higher than that in the south and east, respectively in general.

(viii) A total of 21 prospecting areas of mineral resources have been delineated, thus providing scientific bases for selecting strategic areas of mineral exploration and promoting breakthroughs to be made in follow-up exploration assessment in the Plateau. Meanwhile, 11 sedimentary basins have been identified. Based on this, as well as the regularity of the generation, reservation, cap rock formation, and accumulation of oil and gas in the Plateau, it is believed that the Qiangtang Basin and Yanghu-Xuejinhu Basin are the most promising basins in terms of oil and gas prospect in this plateau and 12 favorable local targets for further exploration have been pointed out.

CRediT authorship contribution statement

Sheng-qing Xiong conceived of the presented idea,collected the data and finished the final manuscript.

Declaration of competing interest

The authors declare no conflicts of interest.

Acknowledgment

The author would like to express his sincere gratitude to experts including Dao-qing Zhou, Yan-yun Ding, Zhan-kui Li, Bao-bao Cao, Bao-di Wang, Xue Yang, Hong-yu Yan, Hai Yang, Xiu-he Gao, and Si-yuan Sun for their massive data collection, collation, and research and to Academician Ji-wen Teng, Academician Zeng-qian Hou, Researcher Gui-tang Pan,and Researcher Xue-yi Xu for their guidance. Thanks also go to relevant institutes, experts, and researchers involved in the references and to the reviewers of this paper. Due to the limitation on the author’s knowledge and paper length, this paper is far from a complete introduction to the aeromagnetic research results of the Qinghai-Tibet Plateau. The author will appreciate any comments and corrections to any omissions and errors.

This research was funded by the National Key Research and Development Project (2017YFC0602200) and China Geological Survey (DD20160065, DD20190025).