Himalayan Geology, Vol. 43 (1B), 2022, pp. 123-139., Printed in India

The Rise of the Tibetan Plateau and Removal of Some its Mantle Lithosphere in Light of Recent Observations

PETER MOLNAR

Department of Geological Sciences, Cooperative Institute for Research in Environmental Sciences (CIRES)

Benson Earth Sciences Building, Campus Box 399, Room 462C, University of Colorado at Boulder

Boulder, Colorado 80309-0399

Email: peter.molnar@colorado.edu

Abstract: Much evidence shows that before the Indian subcontinent collided with the southern edge of Eurasia, high topography comprising the Transhimalaya had already developed along the southern edge of Eurasia and, therefore, along the southern part of the region that eventually grew to become the Tibetan Plateau. Moreover, high terrain had already begun to grow within what is now the plateau, in the Jangtang of central Tibet. Since that collision, two periods characterize post-collision growth of the plateau: first, the rise of now high terrain, presumably largely by shortening and thickening of crust in a state of Airy isostasy, and second, slow horizontal extension of the Plateau and crustal thinning.  Until the past decade, constraints on the elevation history were few.  Recent paleobotanical studies, however, suggest that until late Oligocene or early Miocene time, an east-west trending belt of basins with elevations ~2000 ± 1000 m lay between the relatively high Transhimalaya in southern Tibet and the Jangtang in central Tibet, and the terrain north of the Jangtang, the Hoh Xil Basin and surrounding regions, also lay ~3000 ± 1000 m below their current mean elevation of ~4500 m. Analyses, obtained mostly in the 21st Century, of normal fault systems with thermochronological constraints on timing of exhumation show that widespread normal faulting and crustal thinning began since ~15 Ma, though presumably at different times in different places. The absence of evidence of thrust faulting younger than ~25 Ma, except on the margins of the Plateau, together with surface uplift concurs with the suggestion that mantle lithosphere was removed from at least part of the Tibetan Plateau since ~25 Ma, but before ~15 Ma. The increase in gravitational potential energy that the Tibetan lithosphere gained from removal of its mantle lithosphere then powered outward growth of the Plateau.

Keywords : Surface uplift, mantle dynamics, paleogeography