Seismological Evidences for the Multiple Incomplete Crustal Subductions in Himalaya and Southern Tibet

SEISMOLOGICAL EVIDENCES FOR THE MULTIPLE INCOMPLETE CRUSTAL SUBDUCTIONS IN HIMALAYA AND SOUTHERN TIBET     

青藏高原的地幔结构:地幔羽、地幔剪切带及岩石圈俯冲板片的拆沉

通过横穿青藏高原近 80 0 0km长的 4条天然地震层析剖面 ,获得 4 0 0km深度以上的地壳和地幔速度图像及地震波各向异性 ,揭示了青藏高原 4 0 0km深度范围内的地壳和地幔结构特征。地幔速度图像显示 ,青藏高原腹地的深地幔中存在以大型低速异常体为特征的地幔羽 ,其可能通过热通道与大面积分布的可可西里新生代高钾碱性火山作用有成因联系 ;阿尔金、康西瓦、金沙江、嘉黎及雅鲁藏布江等走滑断裂可下延至 30 0～ 4 0 0km深度 ,显示了低速高热物质组成的垂向低速异常带特征及大型超岩石圈或地幔剪切带的产出 ;发现康西瓦、东昆仑—金沙江、班公湖—怒江和雅鲁藏布缝合带下部存在不连续的高速异常带 ,可以解释为青藏高原地体拼合及碰撞过程中可能保留的加里东、古特提斯和中特提斯大洋岩石圈“化石”残片 ,是“拆沉”的地球物理证据。印度大陆岩石圈的巨厚俯冲板片以 15～ 2 0°倾角向北插入唐古拉山下 30 0km深处 ,并被高热物质组成的地幔剪切带分开。结合新的横穿喜马拉雅及青藏高原的地幔层析资料 ,提出青藏高原碰撞动力学新模式 :青藏高原南部印度岩石圈板片的翻卷式陆内超深俯冲 ,北缘克拉通向南的陆内俯冲 ,腹地深部的地幔羽上涌 ,以及地幔范围内的高原“右旋隆升”及物质向东及北东方向运动及挤出。     

Progress in the Study of Deep Profiles of Tibet and the Himalayas (INDEPTH)

This paper introduces 8 major discoveries and new understandings with regard to the deep structure and tectonics of the Himalayas and Tibetan Plateau obtained in Project INDEPTH, They are mainly as follows. (1) The upper crust, lower crust and mantle lithosphere beneath the blocks of the plateau form a "sandwich" structure with a relatively rigid-brittle upper crust, a visco-plastic lower crust and a relatively rigid-ductile mantle lithosphere. This structure is completely different from that of monotonous, cold and more rigid oceanic plates. (2) In the process of north-directed collision-compression of the Indian subcontinent, the upper crust was attached to the foreland in the form of a gigantic foreland accretionary wedge. The interior of the accretionary wedge thickened in such tectonic manners as large-scale thrusting, backthrusting and folding, and magmatic masses and partially molten masses participated in the crustal thickening. Between the upper crust and lower crust lies a large detachment (e.g     

Pb, Sr AND Nd-ISOTOPIC COMPOSITIONS OF PALEO AND NEO-TETHYAN OCEANIC CRUSTS IN THE EASTERN TETHYAN DOMAIN: IMPLICATION FOR THE INDIAN OCEAN-TYPE ISOTOPIC SIGNATURE

Pb, Sr AND Nd-ISOTOPIC COMPOSITIONS OF PALEO AND NEO-TETHYAN OCEANIC CRUSTS IN THE EASTERN TETHYAN DOMAIN: IMPLICATION FOR THE INDIAN OCEAN-TYPE ISOTOPIC SIGNATURE     

DEVELOPMENT OF THE EARTH AND ITS TECTOGENESIS

It is assumed that the earth having been formed in a cold state was warmed up by radioactive heat. Its further evolution was determined by differentiation of its constituent material through successive smelting from the mantle of relatively light components and subsequent displacement upward. The most intensive differentiation in the upper “layer” (evidently at depths of from 100 to 200km) causes strong vertical movements of the crust. After that is exhausted slower differentiation of the deeper “layer” (evidently at depths of from 200-300 km) is manifested on the surface in slow oscillatory movements forming platforms. Further heating activates the material of much deeper layers (as deep as 900 km) and causes large masses of basalt to rise to the surface. This ascent of basic material induces post-platform activity (observed in Central and East Asia), extrusions of plateau basalts and, filially, destruction of the continental crust through melting, metasomatic replacement and dissolution into the large volume of superheated basalt. As a result of destruction of the continental crust, large grabens, mediterranean seas and ocean basins have been formed. The development of tectonic processes is profoundly influenced by 1) the formation of deep faults in the crust and upper mantle which determine the routes of material distribution and 2) by the “the -lid-on-the-kettle-with-boiling-water” phenomenon. The last involves periodical accumulation of heat at depth, expansion of the mantle, the opening of deep faults, and quick removal of heat to the surface, together with heated material, along the faults. The periodicity of tectogenesis may be connected with this mechanism. It is assumed that up to now the earth has been warming up. Traces of tension of the crust accompanying the process of formation and expansion of the oceanic deeps are therefore considered to be an expression of a more general tension of whole crus and the upper mantle of the expanding earth. The expansion process expressed in the opening of defaults and uplift of deep-seated material to the surface along faults may be nonuniform, affecting some regions earlier than others, which causes the simultaneous existence of several regions in different stages of development. The author accepts the idea of deep-seated origin of sea-water, which rises from the mantle during subsidence of the oceanic basins and destruction of the continental crust. The author divides the history of the earth into two partly overlapping stages: granitic and basaltic. — Auth English summ.     

SUBDUCTION AND ACCRETION TECTONICS OF HIMALAYAN AND KARAKORAM TERRANES AND THEIR PALAEOGEOLOGIC CONFIGURATION

he 2500km long Indus\|Tsangpo Suture has been recognized as one of the best examples of continent to continent collisional Suture Zone. It has come into existence as a result of subduction followed by continental collision (55～60Ma) between Indian (Sinha, 1989, 1997; Sinha et al., 1999) and Eurasian plates. While considering the recent palaeogeographic reconstruction of Pangea during late Palaeozoic it appears that a southern belt of Asian microcontinents stretching from Iran and Afghanistan through southern Tibet to western Thailand, Malaysia and Sumatra, comprise several continental blocks and numerous fragments that have coalesced since the Mid\|Palaeozoic along with the closure of Tethys. The origin, migration, assembly and timing of accretion of all these blocks to their present geotectonic position is not well known and there is no Permo—Triassic crust left in the present day Indian Ocean. The oldest ocean crust adjacent to the west African and Antarctic margin is of early or middle Cretaceous age (approximately 140～100Ma) (Searle, 1991). The Karakoram\|Hindukush microplate in the west and the Qiangtang\|Lhasa block in the central and eastern segment of South Asia margin are among those blocks already welded with Asian plates around 120～130Ma ago, before the collision of India (55～60Ma) with the collage of plates forming Peri\|Gondwanian microcontinents. But the reconstruction of palaeogeographic configuration remain incomplete due to paucity of authentic geologic information available from Karakoram, Pamir and Western Tibet. Prior to our discovery no early Permian plant remains and palynomorphs were ever reported from Karakoram terrane. Our discovery of Early Permian remains and late Asselian (about 280～275Ma) palynomorphs provides crucial clue regarding the palaeogeographic reconstruction of the Karakoram\|Himalayan block in the Permian time.     

P-wave tomography and dynamics of the crust and upper mantle beneath western Tibet

The continental collision between the Indian and Asian plates plays a key role in the geologic and tectonic evolution of the Tibetan plateau. In this article we present high-resolution tomographic images of the crust and upper mantle derived from a large number of high-quality seismic data from the ANTILOPE project in western Tibet. Both local and distant earthquakes were used in this study and 35,115 P-wave arrival times were manually picked from the original seismograms. Geological and geochemical results suggested that the subducting Indian plate has reached northward to the Lhasa terrane, whereas our new tomography shows that the Indian plate is currently sub-horizontal and underthrusting to the Jinsha river suture at depths of ~ 100 to ~ 250 km, suggesting that the subduction process has evolved over time. The Asian plate is also imaged clearly from the surface to a depth of ~ 100 km by our tomography, and it is located under the Tarim Basin north of the Altyn Tagh Fault. There is no obvious evidence to show that the Asian plate has subducted beneath western Tibet. The Indian and Asian plates are separated by a prominent low-velocity zone under northern Tibet. We attribute the low-velocity zone to mantle upwelling, which may account for the warm crust and upper mantle beneath that region, and thus explain the different features of magmatism between southern and northern Tibet. But the upwelling may not penetrate through the whole crust. We propose a revised geodynamic model and suggest that the high-velocity zones under Lhasa terrane may reflect a cold crust which has interrupted the crustal flow under the westernmost Tibetan plateau.     

SEISMIC STUDIES ON THE CRUST-MANTLE STRUCTURE ACROSS KARAMAY-KUCHA IN XINJIANG, CHINA

SEISMIC STUDIES ON THE CRUST-MANTLE STRUCTURE ACROSS KARAMAY-KUCHA IN XINJIANG, CHINA     

喜马拉雅西部雅鲁藏布江缝合带地壳尺度的构造叠置

陆陆碰撞过程是板块构造缺失的链条。印度板块与亚洲板块的碰撞造就了喜马拉雅造山带和青藏高原的主体。然而,人们对印度板块在大陆碰撞过程中的行为尚不了解。如大陆碰撞及其碰撞后的大陆俯冲是如何进行的、印度板块是俯冲在青藏高原之下还是回转至板块上部(喜马拉雅造山带内)以及两者比例如何,这些仍是亟待解决的问题。印度板块低角度沿喜马拉雅主逆冲断裂(MHT)俯冲在低喜马拉雅和高喜马拉雅之下已经被反射地震图像很好地揭示。然而,关于MHT如何向北延伸,前人的研究仅获得了分辨率较低的接收函数图像。因而,MHT和雅鲁藏布江缝合带之间印度板块的俯冲行为仍是一个谜。喜马拉雅造山楔增生机制,也就是印度地壳前缘的变形机制,反映出物质被临界锥形逆冲断层作用转移到板块上部,或是以韧性管道流的样式向南溢出。在本次研究中,我们给出在喜马拉雅造山带西部地区横过雅鲁藏布江缝合带的沿东经81.5°展布的高分辨率深地震反射剖面,精细揭示了地壳尺度结构构造。剖面显示,MHT以大约20°的倾斜角度延伸至大约60 km深度,接近埋深为70~75 km的Moho面。越过雅鲁藏布江缝合带运移到北面的印度地壳厚度已经不足15 km。深地震反射剖面还显示中地壳逆冲构造反射发育。我们认为,伴随着印度板块俯冲,地壳尺度的多重构造叠置作用使物质自MHT下部的板块向其上部板块转移,这一过程使印度地壳厚度减薄了,同时加厚了喜马拉雅地壳。     

Pre-Tertiary felsic magmatism of the Nepal Himalaya: recycling of continental crust

At least seven different groups of felsic magmatic rocks have been observed in the Lesser and Higher Himalayan units of Nepal. Six of them are pre-Himalayan. The Ulleri Lower Proterozoic augen gneiss extends along most of the length of the Lesser Himalaya of Nepal and represents a Precambrian felsic volcanism or plutono-volcanism, mainly recycling continental crustal material; this volcanism has contributed sediment to the lower group of formations of the Lesser Himalaya. The Ampipal alkaline gneiss is a small elongated body appearing as a window at the base of the Lesser Himalayan formations of central Nepal; it originated as a Precambrian nepheline syenite pluton, contaminated by lower continental crust. The “Lesser Himalayan” granitic belt is well represented in Nepal by nine large granitic plutons; these Cambro-Ordovician peraluminous, generally porphyritic, granites, only occur in the crystaline nappes; they were probably produced by large-scale melting of the continental crust at the northern tip of the Indian craton, during a general episode of thinning of Gondwana continent with heating and mantle injection of the crust. The Formation III augen gneisses of the Higher Himalaya, such as the augen gneiss of the Higher Himalayan crystalline nappes (Gosainkund) are coeval to the “Lesser Himalayan” granites, and their more metamorphic (lower amphibolite grade) equivalents. Limited outcrops of Cretaceous trachytic volcanism lie along the southern limb of the Lesser Himalaya and are coeval with spilitic volcanism in the Higher Himalayan sedimentary series. This volcanism foreshadows the general uplift of the Indian margin before the Himalayan collision. The predominance of felsic over basic magmatism in the 2.5 Ga-long evolution of the Himalayan domain constitutes an unique example of recycling of continental material with very limited addition of juvenile mantle products.     

喜马拉雅地区深反射地震──揭示印度大陆北缘岩石圈的复杂结构

喜马拉雅山的崛起和青藏高原的隆升被认作是印度板块和亚洲板块中、新生代以来汇聚、碰撞、挤压的结果，是典型的陆－陆碰撞地带。此文介绍了在喜马拉雅山区进行的第一次深反射地震试验的结果。试验剖面布置在北喜马拉雅地区内，从喜马拉雅山山脊南的帕里到康马南的萨马达共中１５点（ＣＭＰ）叠加剖面上表现出如下特点：①显示了在地壳中部有一强反射带，向北缓倾斜下去，延长达１００ｋｍ以上。它可能代表了一个活动的道冲断裂或是一条巨大的拆离带，印度地壳整体或下地壳沿此拆离层俯冲到藏南之下；②上部地壳的反射，显示了上地壳存在着大规模的叠瓦状结构；③下地壳的反射显示了塑性流变特征；④在测线南部莫霍反射明显，深度达７２─７５ｋｍ，发现了南部有双莫霍层的存在；⑤试验中还取得莫霍层下面３２ｓ、３８ｓ、４８ｓ等双程走时的多条反射，均向北倾斜，反射同相轴延续较长，信息丰富，反映了上地幔的成层结构。这些结果对印度大陆地壳整体或其下地壳俯冲到藏南特提斯喜马拉雅地壳之下并导致西藏南端地壳增厚的观点给予了实质性的支持。     

~(40)Ar-~(39)Ar GEOCHRONOLOGY OF THE SUTURE ZONE, LADAKH, INDIA

~(40)Ar-~(39)Ar GEOCHRONOLOGY OF THE SUTURE ZONE, LADAKH, INDIA1　TalatAhmedetal.GeochemicalJournal,1999. 2　HoneggerK ,etal.EarthandPlanetaryScienceLetters,1982 ,6 0 :2 53. 3　SearleMP ,etal.GeologicalSocietyofAmericaBulletin ,1987,98:6 78. 4　SharmaK ,K .PhysicsandChemistryoftheEarth ,1990 ,17( 2 ) :133. 5　Venkatesan ,etal.EarthandPlanetSciLett,1993,119:181.…     

Density inhomogeneity of the lithosphere in the southeastern periphery of the North Asian craton

The study addresses the space distribution of lithospheric density contrasts in 3D and 2D surface (spherical) sources of gravity anomalies to depths of 120 km below the geoid surface and their relationship with shallow deformation and Archean, Early Paleozoic, and Late Mesozoic geodynamic environments. The lithospheric section in northeastern Transbaikalia and the Upper Amur region includes two layers of low-density gradients attendant with low seismic velocities and low electrical resistivity. The lower layer at depths of 80–120 km is attributed to an asthenospheric upwarp that extends beneath the North Asian craton from the Emuershan volcanic belt and the Songliao basin. The concentric pattern of density contrasts in the middle and lower crust beneath the Upper Amur region may be produced by the activity of the Aldan-Zeya plume, which spatially correlates with the geometry of the asthenospheric upwarp as well as with the regional seismicity field, magnetic and heat flow anomalies, and stresses caused by large earthquakes and recent vertical crustal movements. The relationship between shallow and deep structures in the crust and upper mantle bears signature of horizontal displacement (subduction) of the lower crust of the Baikal-Vitim and Amur superterranes beneath the North Asian craton.     

中国大陆构造及动力学若干问题的认识

中国(东亚)大陆受特提斯、古亚洲和太平洋构造体系的制约,具有复杂的地体构架和特殊的岩石圈结构。本文从地学前沿——大陆动力学的视野出发,围绕中国大陆构造及动力学四个方面的研究,总结已有的进展并提出新的思考:①中国大陆板块下的构造和整个地幔运动的构架:地震层析资料揭示西太平洋板片向西俯冲到东亚大陆之下,其倾角逐渐减小,最后近水平地插进400～600km深度的地幔过渡带中,成为箕状几何形态的超深俯冲板片。印度岩石圈板片超深俯冲至青藏高原之下～800km的深度,在喜马拉雅西构造结部位发生双向不对称深俯冲,印度岩石圈板片向东俯冲至东构造结东侧之下300～500km的深度。②中国大陆变质基底的再活化:中国大陆的大部分陆块未受显生宙以来构造、变质和岩浆事件的改造与激活,在冈瓦纳大陆北缘的印度陆块和阿拉伯陆块北缘还发育有形成于泛非期(530～470Ma)的造山带,其影响范围至高喜马拉雅、拉萨地体和三江地区。新生代的变质活化普遍出现在喜马拉雅、南迦巴瓦、拉萨地体和三江-缅甸地区,最新的变质年龄仅2～1Ma(南迦巴瓦)。③中国主要高压-超高压变质带的大地构造背景及深俯冲-折返机制:中国及邻区含榴辉岩的高压-超高压(HP/UHP)变质带有洋壳(深)俯冲和陆壳(深)俯冲之分。青藏高原中,大部分洋壳俯冲形成的高压/超高压变质带与原-古特提斯洋盆中诸多微陆块之间的小洋盆的汇聚碰撞有关,陆壳深俯冲作用有两种机制,它们分别是大陆块之间剪式碰撞和撕裂式岩石圈舌形板片的深俯冲。④中国大陆造山带的深部物质可经3类机制挤出,即深部地壳物质"牙膏式"挤出、侧向挤出和"挤压转换式"挤出。     

初论幔柱构造成矿体系

从板块构造与板块边界矿床、超大陆旋回与大陆边界矿床、地幔热点与大陆内部矿床的角度 ,阐述了幔柱构造成矿体系的基本思想、分类、成矿特征及旋回性 ,提出了热幔柱和冷幔柱两个成矿体系和地幔热柱 -热点、地幔热柱 -大陆裂谷、地幔热柱 -大洋扩张、冷幔柱 -前寒武纪硅铝壳造山、冷幔柱 -显生宙硅铝壳 /洋壳造山等五个成矿系统 ,并初步划分了矿床成矿系列 ,例举了某些典型矿床。     

TECTONIC EVOLUTION OF THE WESTERN KUNLUN AND KARAKORAM MOUNTAINS—SOME NEW OBSERVATIONS FROM A MULTI-DISCIPLINARY GEOSCIENTIFIC TRANSECT (MGT) IN NW TIBET

TECTONIC EVOLUTION OF THE WESTERN KUNLUN AND KARAKORAM MOUNTAINS—SOME NEW OBSERVATIONS FROM A MULTI-DISCIPLINARY GEOSCIENTIFIC TRANSECT (MGT) IN NW TIBET     

Structure of the lithosphere along the deep seismic sounding profile: Tien Shan—Pamirs—Karakorum—Himalayas

During 1973–1977, as part of the International Geodynamic Project, some seismic investigations of the Earth's crust have been carried out by geotraverses of the Tien Shan—Pamirs—Karakorum—Himalayas. The seismic data obtained together with other geophysical information, allow the construction and interpretation of the lithospheric section through the Pamirs-Himalayas structure. This section includes thick crust with complex layering, supra-asthenospheric and asthenospheric layers of the upper mantle. The thickness of the Earth's crust increases from 50–55 km in the north, in the Ferghana depression (Tien Shan), to 70–75 km in the south, near the Karakul Lake (Northern Pamir). It varies within 60–65 km for the Central and Southern Pamir, Karakorum and the Inner Himalayas. Its thickness is least (35–37 km) in the south, under the outer margin of the Himalayan foredeep. Extreme gravity minima and depressions on the geoid surface correspond to the regions with maximum thickness of the Earth's crust. The centers of the disturbing masses on the geoid surface are located in the vicinity of the asthenosphere's upper layer; this determines the effect of the whole lithospheric layer, including its asthenospheric layer, at intense changes of gravity anomalies. The asthenospheric upper layer is recorded at a depth of about 120 km, its base at a depth of 200 km, in the northern and southern regions, and 300 km in its central part (Southern Pamir, Karakorum). In the middle asthenospheric layer, wave velocities decrease to 7.5 km/sec, under the base they increase to 8.4 km/sec and reach 9.4 km/sec at a depth of about 400 km. In the supra-asthenospheric layer of the upper mantle, longitudinal and shear wave-velocities slightly increase (by less than 0.1 km/sec) towards its base.     

http://dx.doi.org/10.1016/j.gsf.2016.07.005

The Hadean history of Earth is shrouded in mystery and it is considered that the planet was born dry with no water or atmosphere. The Earth-Moon system had many features in common during the birth stage. Solidification of the dry magma ocean at 4.53 Ga generated primordial continents with komatiite. We speculate that the upper crust was composed of fractionated gabbros and the middle felsic crust by anorthosite at ca. 21 km depth boundary, underlain by meta-anorthosite (grossular + kyanite + quartz) down to 50–60 km in depth. The thickness of the mafic KREEP basalt in the lower crust, separating it from the underlying upper mantle is not well-constrained and might have been up to ca. 100–200 km depending on the degree of fractionation and gravitational stability versus surrounding mantle density. The primordial continents must have been composed of the final residue of dry magma ocean and enriched in several critical elements including Ca, Mg, Fe, Mn, P, K, and Cl which were exposed on the surface of the dry Earth. Around 190 million years after the solidification of the magma ocean, “ABEL bombardment” delivered volatiles including H₂O, CO₂, N₂ as well as silicate components through the addition of icy asteroids. This event continued for 200 Myr with subordinate bombardments until 3.9 Ga, preparing the Earth for the prebiotic chemical evolution and as the cradle of first life. Due to vigorous convection arising from high mantle potential temperatures, the primordial continents disintegrated and were dragged down to the deep mantle, marking the onset of Hadean plate tectonics.     

THICKNESS OF THE CRUST AND ITS RELATIONSHIP TO RELIEF AND GRAVITY ANOMALIES

The effect of different crustal thickness on a regional gravity field may be differentiated, as a first approximation, into-three layers: 1) sedimentary, 2) granitic, and 3) basaltic. The study of complex “wave pictures” obtained in deep seismic sounding has lead to differentiation of the crust as continental, oceanic, and transitional, with a general relationship existing between the surface tectonics of the crust and its deeper structures. The crust is thickest in the mountain regions (40 km-80 km) as against an average for the platforms of about 25 km-35 km. It appears that there are two particularly conspicuous gravity and seismic discontinuities in the crust; one between the sedimentary mantle and the so-called crystalline layer and the other between the latter and the M surface. Tentative estimations of crustal thickness are as follows: the Russian Platform and the north of the western Siberian Platform; 30 km-34 km; the Black Sea about 24 km; the entire south, southeast and east of the U. S. S. R. are marked by greater depth with the Pamirs having a thickness of over 70 km; in the Caucasus the M surface lies below 45 km; in the Northern Kazakhstan the crust is 34 km-36 km thick; in the Altay thickness of around 50 km are indicated; in the Eurasian continent, Tibet has the thickest crust, the gravity minimum indicating about 85 km; in the Verkoyansk region the M surface is over 43 km. Large areas of the Arctic Ocean is occupied by the shelf with a thickness similar to that in the north of the country. This suggests that a considerable stretch of the ocean adjacent to the northern shores of the U. S. S. R. has a continental type. The crust thins rapidly to the north to about 10 km. Along the Pacific coast the M surface is about 33 km, the shelf zone is rather narrow including the Sea of Okhotsk. Toward the ocean and the Kuriles the crust thins rapidly to 10 km. -- C. E. Sears.     

冈底斯中东部底垫-俯冲转换带附近地壳上地幔结构远震P波层析成像研究

利用冈底斯中-东部197个宽频带天然地震台站记录到的数据和远震P波走时层析成像方法,获得了该区域的P波速度扰动图像。层析成像结果显示研究区地壳和上地幔地震波速度结构存在着复杂的空间变化。首先,在藏南拆离系断层(STD)以北的特提斯喜马拉雅地壳中存在着较强的低速异常,但是该低速异常的北端在远离裂谷带的地方并没有明显越过雅鲁藏布江缝合线(YZS),这与前人的观测结果略有不同;在亚东-古露(YGR)和措美-桑日(CSR)裂谷带的下方存在低速异常,但异常强度都没有前者大;在两个裂谷带之间的拉萨地块中-南部,地壳表现为强高速特征。这些结果表明,影响青藏高原地壳构造演化的"地壳通道流(Crustal Channel Flow)"在藏南主要分布在特提斯喜马拉雅地区,在雅鲁藏布江缝合线以北的冈底斯地区,可能主要局限于沿裂谷带分布。其次,被解释为印度岩石圈地幔的上地幔高速异常,在研究区西部,抵达了雅鲁藏布江缝合线以北100km或更远的地方,而在研究区东部,并没有越过雅鲁藏布江缝合线,而是停留在缝合线以南~100km的高喜马拉雅下方,印证了前人给出的印度板块俯冲角度在研究区附近存在东西向变化的层析成像结果。此外,我们的层析成像结果还印证了冈底斯东南侧的上地幔低速异常根植于上地幔底部,我们认为该现象可能与巽他块体的顺时针旋转引起向东俯冲的缅甸弧向西后撤有关。