Evolution of the Indo-Asian Orogen: Insights from the Deformation of the Northern Tibetan Plateau, Mass Balance Calculations, and Volcanic Geochemistry.

Abstract

The Indo-Asian orogen is the world’s largest region of high topography, which affects both regional climate and mammalian speciation. How and when it achieved modern elevations and crustal thicknesses remains controversial, though there is agreement that ~40-50 Ma collision between India and Asia led to widespread crustal thickening and some amount of elevation gain since that time. Development of pre-collisional topography, the structural mechanisms of crustal thickening since collision, and the role of mantle lithosphere removal in producing elevation gain are common processes by which we interpret the topographic history of the orogen. In chapter 2, I address pre-collisional topography by reconstructing crustal volume before collision using total plate convergence estimates since 40 Ma. Results show that modern crustal volumes are 30% less than those estimated for 40 Ma using a pre-collisional crustal thickness of ~40 km (global average). I argue that this imbalance can be eliminated if the majority of Asian and Indian crust now found within the boundaries of the orogen was 23-32 km thick and consequentially at low elevations prior to collision. Chapter 3 focuses on mechanisms of recent faulting from a rare occurrence of thrusting at high elevation in the Guaizi Liang of the northern Tibetan Plateau. (~5000 m asl). We document 9 km (20%) shortening since Pliocene time and Quaternary shortening rates of 1.7 mm/yr based on balanced cross-sections and 36Cl cosmogenic radionuclide dating of a folded alluvial surface. Observed structural trends suggest that thrust faulting is related to simple shear between the Altyn Tagh and Kunlun/Manyi strike-slip faults, which need not produce crustal thickening. In chapter 4, I investigate the modification of the mantle lithosphere through a compilation of new and published geochemistry data from volcanic rocks across the plateau. I interpret slab flattening to have occurred at 40-30 Ma to drive widespread metasomatism of the Asian lithospheric mantle by melts from subducted Indian sediments. Removal of the slab at 30 Ma generated widespread volcanism across the plateau and likely thinned or removed them mantle lithosphere. Termination of volcanism in the southern Tibetan Plateau suggests renewed Indian underthrusting since 15 Ma.