Physical state of Himalayan crust and uppermost mantle: Constraints from seismic attenuation and velocity tomography

Abstract

AbstractWe jointly interpret P and S wave seismic attenuation (1/Q) along with previously published seismic velocity results for the crust and uppermost mantle of eastern Nepal and the southern Tibetan Plateau. Seismic attenuation measurements can provide information complementary to seismic velocity estimates and can help distinguish between compositional and thermal mechanisms for observed anomalies. In addition to a dramatic change in seismic velocity observed between the crust of Nepal and southern Tibet, we find a large increase in seismic attenuation from high Q (low attenuation) in eastern Nepal to low Q (high attenuation) in the crust beneath southern Tibet for both P waves and S waves. We interpret the broad zone of low Q values in the southern Tibetan crust as thermal in origin, requiring an elevated geotherm (warm) relative to Nepal, with low VP/VS corresponding to a dominantly felsic middle and upper crust beneath southern Tibet. We find a sharply bounded region with enhanced low Q and high VP/VS at 45–50 km depth beneath southern Tibet, which we suggest may be due to trapped fluid beneath an impermeable cap associated with the crustal alpha‐beta quartz transition. Using calibrations from mineral physics, the alpha‐beta quartz transition suggests a temperature of 930–960°C at 45 km depth (50 km beneath the surface) beneath the southern Tibetan Plateau. High values of QP and QS throughout the uppermost mantle in the region are consistent with cool temperatures in the underthrusting Indian Plate, contributing to brittle conditions and earthquakes in the uppermost mantle.