Seismological evidence for shallow crustal melt beneath the Garhwal High Himalaya, India: implications for the Himalayan channel flow

Abstract

We investigate the seismic attenuation along a 160 km profile from the Lower to the High Himalaya in the Garhwal region, India, through the analysis of Lg waveforms from regional earthquakes recorded on 18 broadband seismographs with interstation spacing of 7-10 km. Lateral variability in attenuation is derived through the inversion of 36 two-station Q₀ (Lg Q at ~1 Hz) measurements using a global optimization scheme. We observe a contrasting attenuation property in the two Himalayan belts: the Lower Himalaya has Q₀ of 742±235 similar to those observed in the Indian shield, whereas the High Himalaya is characterized by an unusually low Q₀ value of 30-60. The seismic attenuation is also well correlated with the P-wave teleseismic traveltime residual pattern: faster arrival (approximately 0.2 s) at stations in the Lower Himalaya as compared to azimuthally independent time delay of ~0.75 s for the High Himalayan stations. The high attenuation and low velocity in the High Himalaya suggests that low viscosity and partial melt in the crust can be correlated to the presence of Miocene leucogranite plutons in this Himalayan belt, a magmatic product of the Indo-Asian collision and presumably evidence of a partial melting event. This shallow low viscosity channel possibly connects the mid-crustal channel beneath Tibet.