Collaborative Research: Seismotectonics and Structure of Creeping Thrusts and Mountain Building of the Himalaya

Collisions between continental plates have been a recurring event in geologic history responsible for many of the world’s mountain chains. The most impressive collision today is in the Himalaya, where India is colliding with Asia. Along the entire Himalayan range, the Nepalese section is among the most active. Here many of the highest peaks of the range are found. Recent GPS surveys in this area have identified a zone less than about 100 km wide where some 22±2 mm/yr of the overall collision are being absorbed; this area also coincides with the epicentral region of many large earthquakes (M~8). Based on geologic mapping, several major thrust faults in the low Himalaya have been identified. These are assumed to merge into a low angle fault at depth, perhaps with a local steepening of the fault under northern Nepal. A reflection seismic profile in southern Tibet, to the north of Nepal, indicates the presence of a major reflector dipping gently toward the north and it has been interpreted to be the Main Himalayan Thrust. A model based on these observations includes a shallowly northward dipping decollement extending from the Gangetic Plain to southern Tibet along which the Indian plate subducts southern Tibet; ramps in this plane are responsible for high peaks. Other models assume a steep subduction zone. In addition, east-west normal faults have been mapped in the high Himalaya; this observation presents a paradox: how can north-south tension be compatible with the generally north-south compression generated by collision? Since Himalayan orogeny is very active, the accompanying seismicity, in response to orogenic stresses, can be used to test the above models and decipher the role of normal faulting in orogeny. Knowledge of seismicity in the region is so far somewhat limited, based on single component local network and teleseismic recordings. Our experiment includes 29 broadband seismometers in eastern Nepal and southern Tibet. The main studies and their purposes are: (1) teleseismic P and S arrival times and waveforms will be used to decipher the general crustal and upper mantle structures under the Himalaya: to determine the presence of subduction zone and other structures under the Himalaya, (2) the arrival times of hundreds of earthquakes under the orogen will be used to image the 3-D crustal structures underneath the Himalaya and relocate earthquakes: to determine the internal structure and rheology of the orogen, (3) SKS anisotropy under the Himalaya: to map upper mantle flow, (4) focal mechanisms from regional moment tensor inversion: to determine kinematics of orogenic deformation. For testing the current hypotheses we shall determine whether fault ramps, where seismic events with characteristic focal mechanism should concentrate, exist and whether the decollement can be detected through seismicity or material differences. Using SKS and teleseismic data we expect also to map features in the upper mantle underneath the orogen, such as the subducted Indian lithosphere and products of delamination. The geometry of the source zone, the nature of the seismic slip and kinematics derived from this experiment will also help constrain the interpretation of ongoing expanded geodetic surveying in the area. We aim at a better model for Himalayan tectonics in particular and mountain building in general.

PUBLICATIONS:

Monsalve et al., 2009, Himalayan flexure
Huang et al., 2009, Himalaya 3D tomography
Monsalve et al., 2008, Himalaya Crust and upper mantle structure
Sheehan et al., 2008, HIMNT Earthquakes and crustal structure
De la Torre et al., 2007, Earthquake focal mechanisms
Monsalve et al., 2006, Himalayan Seismicity
De la Torre et al., 2005, Station noise characteristics
Schulte-Pelkum et al., 2005, Subsurface fault imaging