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 ∼ 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 right-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.