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

小行星9614616 该项目是INDEPTH项目（西藏和喜马拉雅国际深度剖面）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。 该项目得到了NSF（大陆动力学计划）、中国国家自然科学基金、中国地质部和德国地质研究所（德国）的支持。 INDEPTH项目已经在西藏南部完成了两个实地项目，旨在阐明这一关键地区之下的岩石圈结构。 第一个项目于1992年进行，是对青藏高原深反射剖面进行可行性研究，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）。 第二个项目于1993- 1994年进行，是一个更为实质性的项目，涉及沿着喜马拉雅山峰顶以北约300公里的一条横断面（包括INDEPTH I）（INDEPTH II）获取补充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的综合结果为藏南岩石圈提供了一个精致而令人兴奋的新视角。 他们支持一些先前的假设，而必然矛盾的其他人。 INDEPTH资料表明：1）印度陆壳在藏布缝合带以北的印度-亚洲滑脱构造下盘并没有作为一个连贯的单元延伸一段相当长的距离与此相反，与上覆地壳接触的印度地幔岩石圈显然延伸到缝合线以北至少150 km;（3）藏布缝合带目前不是一个穿岩构造，而是在混合地壳中被一个较年轻的特征切断的，最后，可能具有最大的全球意义;（4）新的数据强烈地暗示在藏南之下存在着广泛的中地壳部分熔融层。 综合这些数据，我们可以得出一个青藏高原南部岩石圈目前状态的模型，其中印度大陆地壳在北喜马拉雅穹隆附近部分熔融，从而形成了一个部分熔融的有效流体中地壳层，该层位于高原北部。 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，以及我们对大陆-大陆碰撞过程中岩石圈形成和修改的概念具有重要意义。 INDEPTH III的目的是解决三个基本的，相互关联的问题的性质和该地区之下的岩石圈的演变。 这些问题是：1）青藏高原新近纪/第四纪变形的可容纳性大小和机制是什么？（2）青藏高原地壳内部是否存在一个大面积的中地壳部分熔融层，并对高原新近纪/第四纪构造演化和现今地貌起主要控制作用？（3）青藏高原地幔岩石圈特征的一级南北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地壳熔融的主要信息，并描述该区域下的南-北上地幔过渡的特征。 这些努力将通过一个适度的受控源“测试”和一个适度的野外地质计划来加强，该计划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中地壳亮斑层的存在 如果存在的话，这可能表明中地壳部分熔融区。 ***

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. ***