Deformation and seismic structure of the upper lithosphere beneath the Himalayan collision

Arrival times of local and regional earthquakes recorded at the Himalayan collision zone suggest that the upper mantle of the continental Indian plate yields by brittle failure and that an eclogitic body exists at around-Moho depths beneath southernmost Tibet. Records of local earthquakes at a 28- seismic station temporary network were used to collect P and S arrival times, to invert for earthquake location, 1-D and 2-D seismic speed structure. Inversion results and various tests with arrival times of seismic events at near-Moho depths beneath the High Himalaya and south Tibet indicate that these events are located in the upper mantle. Some earthquakes beneath the southern Himalaya and the foreland also originate in the upper mantle. Finite element modeling of the flexural response of the Indian plate to loading supports the occurrence of brittle failure in the uppermost mantle of the continental lithosphere. An increase of P-wave speeds in the uppermost mantle from the Himalayas of Nepal to southern Tibet of more than 0.2 km/s, in conjunction with the absence of evidence for upper mantle seismic anisotropy in the study area, suggest the presence of an eclogitic body in the Indian plate beneath the collision zone. The eclogite most likely formed as a result of the temperature-pressure increase in the Indian plate as it underthrusts the Himalayas and Southern Tibet.