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

Within the middle of the Qiangtang terrane of central Tibet is an enormous (>500-km-long and up to 100-km-wide) E-W trending belt of oceanic melange. The belt includes blueschist-facies melange which occurs structurally beneath continental margin strata, in the footwall of domal low-angle normal faults. This structural setting requires 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 (e.g., Li et al., 1995). (2) The melange consists of Songpan-Ganzi sediments and subduction-accretion rocks which have been underthrust ~200 km from the Jinsha suture to the north (Kapp et al., 2000). George Gehrels, Ding Lin, and I are conducting regional geologic mapping, structural analysis, and detrital zircon geochronology in an attempt to test these models.

Our 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 our studies 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. Given that Qiangtang melange projects directly beneath regions that have been and are being investigated by INDEPTH and HI-CLIMB, the results of our study will help constrain interpretations of geophysical data (and vice versa).