Evaluating the Detrital Record of Continental Collision in the Songpan-Ganzi Complex, Central China, the World's Largest Pre-Cenozoic Turbidite System

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The Songpan-Ganzi region, an area the size of Kansas on the NE corner of the Tibet Plateau, is underlain by a vast expanse of Triassic-age deep-marine sedimentary sediments (turbidites). The Songpan-Ganzi Complex (SGC) is the world's largest ancient turbidite system, comparable in volume to the largest modern sedimentary accumulation, the Bengal submarine fan, yet the sources of these sediment are very poorly known. Continental blocks and volcanic arc systems fringing the SGC ocean basin could have provided considerable sediment to the SGC. Alternatively, a likely source for the sediments is the nearby suture belt between the North China and South China continental blocks, which collided during the Triassic. With collision-driven uplift, this suture belt rapidly eroded to great depths in the monsoonal climate of Triassic-Jurassic east Asia, removing a volume of crust comparable to the sediment volume in the SGC. By analogy, the occurrence of the SGC turbidite sequence structurally wedged into a coeval suture between two continental blocks is reminiscent of the temporal and geometric elements of the modern Himalayan collision system, including the ultimate repositories for Himalayan detritus, the Bengal and Indus submarine fans. However, whereas the Himalayan system will continue to evolve far into the future, the long extinct North China/South China/Songpan-Ganzi system contains a full record of continental collision. This study is identifying the sources of sediment for the SGC through a program of analytical studies of new samples and extensive pre-existent collections (600 kg, 400 samples) housed at Stanford University, complemented by field investigation of sediment transport directions. The study includes conventional sandstone petrographic analyses, as well as sophisticated analytical approaches such as use of the Stanford-USGS Sensitive High Resolution Ion Microprobe (in co-operation with J.L. Wooden, USGS) and Stanford's Raman spectroscopy facility to identify minerals (e.g., zircon with microdiamond or coesite inclusions) indicative of ultrahigh-pressure deep-crustal sources, currently the focus of major research efforts in the North China/South China suture zone. If suture-derived sediment can be identified in these sediments, then it may be possible to track the record of erosion of the North China/South China suture belt in much the same way that modern Bengal and Indus fan sediments archive a record of erosion of the Himalayan collision system. Thus, comparison of data from the SGC with the Himalayan system will yield better understanding of the long-term evolution of collisional systems and their sedimentary records.