Lithospheric structure and deep earthquakes beneath India, the Himalaya and southern Tibet

This paper is concerned with the implications of earthquake depth distributions in the Himalayan-Tibetan collision zone for the general understanding of lithosphere rheology. In particular, recent studies have argued that microearthquakes in the uppermost mantle beneath Nepal and some earthquakes at 80-90 km depth, close to the Moho in SE and NW Tibet, reinforce the conventional view of the last 25 yr that the continental lithosphere is well represented by strong seismogenic layers in the upper crust and uppermost mantle, separated by a weak and aseismic lower crust. That view was recently challenged by an alternative one suggesting that the continental lithosphere contained a single strong seismogenic layer, that was either the upper crust or the whole crust, but did not involve the mantle. We re-examine the seismic structure and seismicity of the Himalayan-Tibetan collision zone, recalculating earthquake depths in velocity structures that are consistent with seismic receiver functions and surface wave dispersion studies, and calculating a geotherm for the Indian Shield consistent with kimberlite nodule geochemistry. Earthquakes occur throughout the crustal thickness of the Indian Shield, where the lower crust is thought to consist of dry granulite, responsible for its seismogenic behaviour and strength as manifested by its relatively large effective elastic thickness. The crust of the Indian Shield is thin (similar to 35 km) for an Archean shield, and this, in turn, leads to a steady-state Moho temperature that could be as low as similar to 500 degrees C. When this shield is thrust beneath the Himalaya in Nepal, the relatively low mantle temperature, together with the high strain rates associated with it adopting a 'ramp-and-flat' geometry, may be responsible for the mantle microearthquakes that accompany other earthquakes in the lower crust. Further north, the upper crust of India south of the Indus Suture Zone has been removed, the uppermost lower crust of India has heated up, and seismicity is restricted to a few earthquakes very close to the Moho at 80-90 km, where errors in Moho and earthquake depth determinations make it unclear whether these events are in the crust or mantle. A similar situation exists in NW Tibet beneath the Kunlun, where earthquakes at 80-90 km depth occur very close to the Moho. Both places are about 400 km north of the Himalayan front, and we suspect both represent the minimum distance India has underthrust Tibet, so that India underlies most of the SE and nearly all of the NW Tibetan plateau. The distribution of earthquake depths throughout the region is consistent with a generic global view of seismicity in which earthquakes occur in (1) 'wet' upper crustal material to a temperature of similar to 350 degrees C, or (2) higher temperatures in dry granulite-facies lower crust or (3) mantle that is colder than similar to 600 degrees C.