Syncollisional extension along the India-Asia suture zone, south-central Tibet: Implications for crustal deformation of Tibet

Crustal deformation models of the Tibetan plateau are assessed by investigating the nature of Neogene deformation along the India-Asia suture zone through geologic mapping in south-central Tibet (84 degrees 30'E). Our mapping shows that the suture zone is dominated by a system of 3 to 4 ENE-striking, south-clipping thrust faults, rather than strike-slip faults as predicted by models calling upon eastward extrusion of the Tibetan plateau. Faults along the suture zone are not active, as they are cut by a system of NNW-striking oblique slip normal faults, referred to herein as the Lopukangri fault system. Fault-slip data from the Lopukangri fault system shows that the mean slip direction of its hanging wall is N36W. We estimate the net slip oil the Lopukangri fault by restoring components of the thrust system. We estimate that the fault has accommodated similar to 7 km of right-slip and similar to 8 km of normal dip-slip, yielding a net slip of similar to 10.5 km, and 6 km of horizontal east-west extension. The Lopukallgri fault system is active and geomorphic offsets indicate Fight separations and westside-down clip-separation. The mapview curviplanar geometry and geomorphic expression of the Lopukangri fault system is similar to faults and rift basins to its east and west. These extensional faults are en echelon in map view and encompass a region that is 200 kill long (east-west) and 95 km wide (north-south). Assuming our results for the Lopukangri fault are applicable to the entire system. we estimate a maximum of 18% extension across the zone. All active faults in the system terminate southward adjacent to the India-Asia suture zone. Because the individual rift geometries are similar and Suggest a common kinematic relationship, we propose that the extensional system formed as a trailing extensional imbricate fan at the southern termination of the central Tibet conjugate fault zone. Alternatively, the extensional system may terminate to the north and represent a group of isolated crustal tears. Both kinematic interpretations imply a semi-smooth north-south variation in the magnitude of east-west extension in southern Tibet, with higher magnitudes in the north along the Bangong-Nujiang suture zone than in the south along the India-Asia suture zone. Our results from southern Tibet show that deformation between southern Tibet and the Himalayas is broad (95 kill wide) and best described as a continuum possibly since the Lite Miocene. Conversely, the Structural boundary between western Tibet and the Himalayas, which is defined by the Karakoram fault, is presently a discrete boundary, and probably has been since the Middle Miocene. We think this variation in the displacement gradient and age of these structural boundaries within the interior of Tibet is best explained by the fault patterns and strain history describing wholesale E-W stretching and N-S shortening of the Tibetan crust. (C) 2009 Elsevier B.V. All rights reserved.