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

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9614890 Nelson 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 brig ht-spot horizon which, if present, may indicate a mid-crustal partial melt zone. ***