Orogen-parallel, active left-slip faults in the Eastern Himalaya: Implications for the growth mechanism of the Himalayan Arc

One of the key issues about the evolution of the Himalayan orogen is how its map-view curvature has changed with time. Some researchers propose that the arc curvature has decreased due to arc-perpendicular rifting while others suggest that it has increased due to arc-parallel strike-slip faulting. To quantify this problem we conducted field mapping, geomorphologic analysis of active structures, and dating of Pliocene-Quaternary sedimentary units in southeastern Tibet. This study reveals the existence of a similar to 100-km wide and >500-km long, east-striking left-slip fault zone in the eastern Himalaya. The left-slip faults initiated prior to 3-4 Ma and have a total left-slip rate of 4-8 mm/yr across the fault zone. Although the left-slip rate in the eastern Himalaya is broadly comparable to the right-slip rate across the western Himalayan arc, the distributed and short-segmented geometry of left-slip faulting in the eastern Himalaya contrasts sharply to the discrete geometry Of Fight-slip faulting in the western Himalaya. The different geometry may have resulted from an earlier initiation and a greater magnitude of fault motion on the right-slip faults, which implies that asymmetric eastward extrusion of western Tibet across the Himalayan arc was a dominant process in the earlier Himalayan history. This was replaced by oroclinal bending since 4 Ma, producing symmetric right-slip and left-slip faulting in the western and eastern Himalaya, respectively. (C) 2008 Elsevier B.V. All rights reserved.