3-D lithospheric structure and seismotectonics of the central Himalayan region

The objective of this study is to investigate the 3-D lithospheric structure and seismotectonics of the central Himalayan region using the data of the HIMNT and other PASSCAL projects. The HIMNT project consists of 28 stations in the central Himalayan region (between eastern Nepal and southern Tibet) deployed from Sept., 2001 to Oct., 2002. Two methods including tomographic and moment tensor inversions have been employed to accomplish this objective. The crustal geometry is indirectly determined by picking Pn contours (generally for VP=7.9-8.1 km/s under continents), suggesting that the crustal thickness gradually increases from ∼60 km in the Lesser Himalaya to 70--80 km under southern Tibet. The crust begins to thicken under the High Himalaya, with no sign that the High Himalaya is underlain by a local mountain root.; A mid-crust reflector is present under the Tethys Himalaya, indicated by the strong contrast of the velocity gradients. Several seismogenic zones are identified from the tightly clustered hypocenters after the tomographic relocation. Two different focal mechanism patterns, one on each side of the High Himalaya, indicate that the high mountain may be a current strain boundary. There is no sign of an active subduction zone under the High and Tethys Himalayas. The high-VP anomaly revealed from the tomographic images suggests slab relics from an ancient subduction which took place before the closure of the Tethys ocean. The seismicity and low-angle-thrust events in the Lesser Himalaya indicate an interface between the Indian and the Eurasian plates, suggesting that the collision between the two plates is confined to the upper most crust.; The dipping Vp contours in the crust and the vertical P-axes of two normal faulting events at the border between the Lesser and High Himalayas suggest that the Indian plate is bent by the load of the Eurasian plate. Across the high mountains, a two-layer pattern of the seismicity is observed, in which the focal mechanisms consistently present as the E-W extension. No seismicity is present on the proposed decollement under the Tethys Himalaya. North of the High Himalaya, strike-slip faulting in the lower-crust/upper-mantle may be attributed to a stress accommodation between the load of the overriding Eurasian plate and the pushing force from the northward motion of the Indian plate.