Congratulations to Petr Yakovlev who successfully defended his dissertation on June 19, 2015.
Advisor: Marin Clark
Abstract: The Indo-Asian orogen is the world’s largest region of high topography in modern time. However, how and when the region achieved modern elevations and crustal thicknesses remains controversial, with numerous geodynamic mechanisms proposed over the last half century. In chapter 2, I address the mass balance of the Indo-Asian orogen by considering proposed areas of India and Asia at the time of collision, and modern crustal volumes. Results show that a volume of crustal material equal to the modern volume of the orogen needs to be removed if input areas had crustal thicknesses equal to the global average. I argue that this imbalance can be eliminated if input areas were 23-32 km thick prior to collision as is compatible with sedimentalogic, geomorphic, and paloclimatic records. Low crustal shortening in Asia requires the transfer of Indian material beneath Tibet, most likely through underplating, and flow of a lower crustal channel. Chapter 3 focuses on the~5000 m high Kumkuli Basin of northern Tibet. A palnispastic reconstruction, and changes in sedimentary facies suggest that the basin has undergone 9 km or 20% shortening since Pliocene time. 36Cl cosmogenic radionuclide dating of an uplifted alluvial surface, and local geomorphic reconstruction yield 3.4 mm/yr uplift rates, and 1.7 mm/yr shortening rates. Deformation is likely generated by simple shear between the Altyn Tagh and Manyi faults, which form a broad lithospheric shear zone. Initiation of shortening in the Kumkuli Basin at ~5 Ma then provides a maximum age of the Manyi fault. In chapter 4 I investigate the temporal and chemical evolution of volcanic rocks across the Tibetan plateau using a compilation of new, and previously published data. Results are linked with major shifts in deformation mechanisms to create a coherent geodynamic description of the evolution of the Indo-Asian orogen. This hypothesis includes slab flattening or thinning of the mantle wedge at 40-30 Ma to drive widespread metasomatism of the Asian lithospheric mantle. Removal of the Tethyain slab then led to widespread volcanism across the plateau. Indian underthrusting then led to the termination of volcanism and initiation of normal faulting in southern Tibet by 10 Ma.