105 GSA Special Nepal (Gorkha) Earthquake Session

Monday, 2 November 2015
8:00 AM - 12:00 PM
Room 345/346

A GREAT GREATER INDIA: A PALEOMAGNETIC PERSPECTIVE ON THE AMOUNT OF CENOZOIC SUBDUCTION AND UNDERTHRUSTING WITHIN THE CENTRAL HIMALAYA

Paper number: 105-2
Program: Special Session
Time: 8:05 AM – 8:20 AM

Authors

Abstract

The 2015 Mw 7.8 Gorkha earthquake is a devastating reminder of the long history and great magnitude of plate convergence between India and Asia. A bevy of geological, plate kinematic, and paleomagnetic observations consistently indicate that the northern, leading edge of continental units of Indian affinity collided with southern Tibet by 52 Ma. This collision age requires a minimum of 3600±35 km of plate convergence, but how and where this plate convergence has accumulated within Asian and Indian lithosphere remains disputed. Here we test pre-collision paleogeographic reconstructions of the India-Asia collision zone within a paleomagnetic and plate kinematic reference frame with a particular focus on estimating the amount of Indian lithosphere that has subducted beneath Tibet since collision began (i.e., Greater India). Our analysis, which focuses on the longitude of Nepal where geological and geophysical observations are most abundant, indicates that: 1) India-Asia convergence was accommodated within a single southward-stepping subduction zone at ~20°N; fragments of oceanic crust previously thought to have formed above a second, equatorial intra-oceanic subduction zone are shown instead to have formed adjacent to Southern Tibet. 2) Initial collision of Greater India with Asia by 52 Ma and at 20°N requires >2220 km of Greater Indian subduction beneath Tibet; earlier collisions at the same latitude require an even larger Greater India. Thus, the India-Asia collision is a subduction-dominated system with modest amounts of plate convergence partitioned into upper plate shortening, much like Cordilleran orogenic systems in North and South America. The implied magnitude of subduction requires a reappraisal of Greater Indian paleogeography and suggests that the Indian promontory was either always much larger than originally inferred from Gondwanan reconstructions or that it extended during Cretaceous time. Limited, but robust paleomagnetic data from Greater India are consistent with extension and highlight the importance of additional geologic and geophysical studies of the Greater Himalaya and Tibetan Himalaya to reveal Greater India’s poorly understood Cretaceous-Eocene history and the early record of plate convergence within the Himalayan ranges.