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

这个项目是INDEPTH项目的第三阶段，INDEPTH项目是一个多国、多学科合作的项目，旨在研究喜马拉雅/青藏高原碰撞带下的岩石圈。本研究由美国国家科学基金（大陆动力学计划）、中国国家自然科学基金、中国地质部和德国科学基金（德国）资助。INDEPTH项目在藏南完成了两个野外项目，旨在阐明这一关键区域下的岩石圈结构。第一次是在1992年进行的，是在青藏高原进行深度反射剖面的可行性研究，其附加的科学目标是跟踪在喜马拉雅逆冲之下的印度板块顶部（INDEPTH I）。第二个项目于1993- 1994年进行，是一个更为实质性的项目，涉及沿喜马拉雅峰顶以北约300公里（INDEPTH II）的样带（包括INDEPTH I）采集互补CMP反射、广角反射、宽带地震、大地电磁和地表地质数据。deepi - ii的综合结果为西藏南部岩石圈提供了一个精细而令人兴奋的新视图。它们支持先前的一些假设，同时必然与其他假设相矛盾。除此之外，INDEPTH数据表明：1)印度大陆地壳并没有作为一个连贯的单元在印度-亚洲滑脱的下盘向藏布缝合带以北延伸很长一段距离（也可能不会延伸那么远）；2)相比之下，与上覆地壳接触的印度地幔岩石圈明显向北延伸了至少150公里；3)目前，藏布缝合带并不是一个穿过岩石圈的构造，而是被一个更年轻的特征切断在乳白色地壳中，最终可能具有最大的全球意义；4)新资料强烈暗示藏南地下存在广泛的中地壳部分熔体层。综合这些数据，可以得出青藏高原南部岩石圈当前行为的一个模型，在这个模型中，青藏高原南缘下方的印度大陆地壳在北喜马拉雅圆顶附近部分融化，从而形成了一个部分熔融的有效流体中地壳层，位于青藏高原北部的下方。这些结果对喜马拉雅/青藏高原作为一个整体是如何演变的，以及我们在大陆-大陆碰撞过程中岩石圈形成和改造的概念具有重要意义。第三次深研的目的是处理有关该区域下面岩石圈的性质和演化的三个相互关联的基本问题。这些问题是：1)青藏高原新近纪/第四纪变形调节的幅度和机制是什么？2)西藏地壳内部是否存在广泛的中地壳部分熔融层，对青藏高原新近纪/第四纪构造演化和现今形态起主要控制作用？3)高原下地幔岩石圈特征南向北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，以评估该地区地壳和上地幔的总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衰减性地壳融化的初步信息，并描述该地区下南向北的上地幔过渡。这些努力将通过适度的可控源“试验”和适度的野外地质项目得到加强，这些项目旨在描绘上地壳结构的几何形状，新近纪/第四纪缩短的分布，以及地壳中部亮斑层的存在，如果存在，可能表明地壳中部有部分熔融带。＊＊＊

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. ***