Collaborative Research: Structure of the Crust and Upper Mantle Beneath the Tibetan Plateau Interior (INDEPTH III)

9614616该项目是INDEPTH项目(西藏和喜马拉雅的国际深部剖面)的第三阶段，该项目是一个合作的、多国的、多学科的项目，旨在调查喜马拉雅/西藏高原碰撞带下的岩石圈。支持由NSF(大陆动力学计划)、中国国家自然科学基金、中国地质部和德国Forschungs双轴(德国)提供。INDEPTH项目已经在西藏南部完成了两个实地项目，旨在阐明这一关键区域下的岩石圈结构。第一项是1992年开展的青藏高原高海拔地区深反射剖面可行性研究，其另一个科学目标是追踪喜马拉雅山下的印度板块顶部(INDEPTH I)。第二个项目是1993-1994年进行的，是一个更为实质性的项目，涉及沿喜马拉雅山顶以北约300公里(INDEPTH II)的一条横断面(包括INDEPTH II)获取互补的CMP反射、广角反射、宽带地震、大地电磁和地表地质数据。INDEPTH I-II的综合结果使人们对藏南岩石圈有了一个精致和令人兴奋的新视角。他们支持之前的一些假设，但也必然与其他假设相矛盾。此外，INDEPTH的数据还暗示：1)印度大陆地壳并没有作为一个连贯的单元在印度-亚洲滑脱带的下盘延伸到沧波缝合线以北的相当一段距离(而且可能不会延伸那么远)；2)相反，与上覆地壳接触的印度地幔岩石圈显然确实延伸到缝合线以北至少150公里处；3)沧坡缝目前不是一个岩石圈穿透结构，而是被一个较年轻的地物切断在斑驳的地壳中，最后可能是最具全球意义的；4)新资料强烈暗示在藏南地区存在广泛的中地壳部分熔融层。综合这些数据，得出了青藏高原南部岩石圈当前行为的模型，在该模型中，推覆青藏高原南缘的印度大陆地壳在北喜马拉雅穹隆附近部分熔融，从而促成了位于高原以北的部分熔融的有效流体的中地壳。这些结果对喜马拉雅/青藏高原作为一个整体是如何演化的，以及我们关于大陆-大陆碰撞期间岩石圈形成和改造的概念具有重要的意义。INDEPTH III的目的是解决关于该区域岩石圈性质和演化的三个相互关联的基本问题。它们是：1)新近纪/第四纪变形在青藏高原内的调节强度和机制(S)？2)青藏地壳内是否存在区域上广泛的中地壳部分熔融层，它对青藏高原新近纪/第四纪构造演化和现今地貌起到了初步控制作用？3)所报道的高原下地幔岩石圈性质由南向北的一级变化的性质和原因是什么？该计划包括大地电磁剖面和宽带地震记录，以评估该地区下方地壳和上地幔的体电阻率和速度结构。这些数据将提供关于高原内部是否存在地震减弱的地壳熔融的基本信息，并表征该区域由南向北的上地幔过渡。这些努力将通过适度的可控源“测试”和适度的野外地质计划来加强，旨在圈定上地壳结构的几何形状、新近纪/第四纪缩短的分布以及地壳中部明亮的S锅状地平线的存在，如果存在的话，可能表明地壳中部部分熔融带。***

9614616 Ni This project is the third phase of Project INDEPTH (INternational DEep Profiling of Tibet and the Himalaya), a collaborateve, multinational, multidisciplinary program to investigate the lithosphere beneath the Himalaya/Tibet Plateau collision zone. Support is provided by NSF (Continental Dynamics Program), the Chinese National Natural Science Foundation, the Chinese Ministry of Geology and the DeutschesForschungs Gemneinshaft (Germany). Project INDEPTH has completed two field Programs in southern Tibet aimed at elucidating the structure of the lithosphere beneath this key region. The first, undertaken in 1992, was a feasibility study of deep reflection profiling on the high Tibetan Plateau that had the additional scientific goal of tracking the top of the Indian plate underthrusting the Himalaya (INDEPTH I). The second, undertaken in 1993-94, was a much more substantial program involving the acquisition of complementary CMP reflection, wide- angle reflection, broadband earthquake, magnetotelluric, and surface geological data along a transect (including INDEPTH I) extending approximately 300 km north of the crest of the Himalaya (INDEPTH II). The combined INDEPTH I-II results yeild a refined and exciting new view of the lithosphere beneath southern Tibet. They support some previous hypotheses while necessarily contradicting others. Among other things, the INDEPTH data imply that: 1)Indian continental crust does not extend as a coherent unit in the footwall of the India-Asia decollement for a significan distance north of the Tsangpo suture (and may not extend that far); 2) in contrast, Indian mantle lithosphere in contact with the overlying crust apparently does extend at least 150 km north of the suture; 3) the Tsangpo suture is not currently a lithosphere-penetrating structure, but rather is cut off in the mikkle crust by a younger feature, and finally, perhaps of greatest global significance; 4) the new data strongly imply the existence of a widespread mid- crustal partial melt layer beneath southern Tibet. Taken together the data lead to a model for the current behavior of the southern Tibetan Plateau lithosphere wherein the Indian continental crust underthrusting the southern margin of the Plateau partially melts in the vicinity of the North Himalayan domes, thereby contributing to a partially molten effectively fluid, mid-crustal layer that underlies the Plateau to the north. These results have important implications for how the Himalaya/Tibetan Plateau as a whole has evolved, and for our concepts of lithosphere formation and modification during continent-continent collisions in general. The purpose of INDEPTH III is to address three fundamental, interrelated issues about the nature and evolution of the lithosphere beneath the region. These are: 1) What is the magnitude and mechanism(s) of accommodation of Neogene/Quaternary deformation within the Tibetan Plateau? 2)Is there an areally extensive mid-crustal partial melt layer within the Tibetan crust that has exerted a primary control on the Neogene/Quaternary structural evolution and present morphology of the Plateau? and 3) What is the nature and cause of the reported first-order south to north change in the character of the mantle lithosphere beneath the Plateau? The program includes magnetotelluric profiling and broadband earthquake recording to assess the bulk electrical resistivity and velocity structure of the crust and upper mantle beneath the region. These data will provide primary information on the presence or absence of seismically attenuating crustal melt within the interior of the Plateau, and characterize the south-to- north upper mantle transition beneath the region. These efforts will be augmented by a modest controlled-source "test", and a modest field geology program aimed at delineating the geometry of upper crustal structure, the distribution of Neogene/Quaternary shortening, and the existence of a mid-crustal bright-s pot horizon which, if present, may indicate a mid-crustal partial melt zone. ***