Significance of Underplated Oceanic Melange in the Tectonic Evolution and Crustal Structure of Central Tibet

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Within the middle of the Qiangtang terrane of central Tibet is an enormous (greater than 500-km-long and up to 100-km-wide) east-west trending belt of blueschist-bearing melange. This belt occurs structurally beneath continental margin strata, in the footwall of early Mesozoic domal low-angle normal faults, requiring the melange to have underplated continental margin strata prior to exhumation. The tectonic significance of the melange is highly uncertain, with two leading hypotheses proposed to date: (1) the melange marks the location of a suture zone separating formerly unrelated crustal fragments to the north and south. (2) the melange consists of Songpan-Ganzi sedimentary and subduction-accretion rocks which have been underthrust approximately 200 km from the Jinsha suture to the north. In an effort to test these hypotheses, this three-year investigation is conducting (1) regional and detailed geologic mapping and structural analysis along two N-S traverses across the Qiangtang terrane, (2) detailed structural analyses and thermochronologic and thermobarometric studies within the Qiangtang melange, (3) comparative studies of detrital zircon ages from the melange, Songpan-Ganzi strata, and northern and southern Qiangtang region strata, and (4) biostratigraphic studies, led by colleagues from the collaborating Institute of Tibetan Plateau Research in Beijing. Project findings will have a major impact on understanding the tectonic assembly and crustal structure of central Tibet and the general tectonic processes by which melange is incorporated into continental crust. If project results indicate that Qiangtang melange has been underthrust from the Jinsha suture to the north, then much of the deeper central Tibetan crust should include underplated melange. This relationship would also provide an unparalleled opportunity to directly examine processes of low-angle subduction which are rarely observed in the geologic record. Conversely, the conclusion of a major oceanic suture in the central Qiangtang region would require significant revisions to conventional thought regarding the tectonic framework and development of central Tibet. The project involves one Ph.D. student and is engaging a group of undergraduate students to assist in the collection and interpretation of detrital zircon data. In addition, a Tibetan Tectonics" seminar is being organized that will bring together University of Arizona faculty, graduate and undergraduate students, and researchers from Chinese and other U.S. institutions.