THE ACTIVE TECTONICS OF THE EASTERN HIMALAYAN SYNTAXIS AND SURROUNDING REGIONS

Source parameters of 53 moderate-sized earthquakes, obtained from the joint inversion of regional and teleseismic distance long-period body waves, provide the data set for an analysis of the style of deformation and kinematics in the region of the Eastern Himalayan Syntaxis. Focal mechanisms of Eastern Himalayan events show oblique thrust, consistent with the N-NE directed movement of the Indian plate as it underthrusts a boundary that strikes at an oblique angle to the direction of convergence. Earthquakes near the Sagaing fault show strike-slip mechanisms with right-lateral slip. Earthquakes on its northern splays, however, indicate predominant thrusting, evidence that the dextral motion on the Sagaing fault, which accommodates a portion of the lateral motion between India and southeast Asia, terminates in a zone of thrust faulting at the Eastern Himalayan Syntaxia. Remaining motion between India and southeast Asia is accommodated in a zone of distributed shear in east Burma and Yunnan, manifested by strike-slip and oblique normal faulting, east-west extension, crustal thinning, and clockwise rotation of crustal blocks. We determined strain rates throughout the region with a moment tensor summation using 25 years (modern) and 85 years (modern and historic) of earthquake data. We matched the observed strains with a fifth-order polynomial function, and from this we determined both the velocity field and rotations with respect to a specified region. Velocities calculated relative to south China stationary show that the entire area, extending from 20-degrees-N-36-degrees-N, within deforming Asia (Yunnan, western Sichuan, and east Tibet), constitutes a distributed dextral shear zone with clockwise rotations up to 1.7-degrees/m.y., maximum in the region of the Eastern Syntaxis proper. Integrated strains across this zone, relative to south China stationary, show 38 mm/yr +/- 12 mm/yr of north-directed motion at the Himalaya. Remaining plate motion, relative to south China fixed, must be taken up by the underthrusting of India beneath the lesser Himalaya, strike-slip motion on the Sagaing fault, and intraplate NE directed shortening within NE India as well as NE directed shortening within the Eastern Syntaxis proper. 10 mm/yr +/- 2 mm/yr of relative right-lateral motion between India and southeast Asia is absorbed in the region between the Sagaing and Red River faults (94-degrees-E-100-degrees-E). It is the clockwise vorticity (relative to south China) associated with the deformation in Yunnan, east Tibet, and western Sichuan that provides the relative north-directed motion of 38 +/- 12 mm/yr at the Himalaya. Not all of the deformation is accommodated in right-lateral shear between India and south China and between east Tibet and south China; velocity gradients exist that are parallel to the trend of the shear zone. Relative to a point within western Sichuan (32-degrees-N, 100-degrees-E), the velocity field shows that the Yunnan crust is moving S-SE at rates of 8-10 mm/yr. Relative to south China, there is no eastward explusion of crustal material beyond the eastern margin of the Tibetan plateau.