ACTIVE OROGEN-PARALLEL LEFT-SLIP FAULTS IN THE EASTERN HIMALAYAN RANGE, SOUTHERN TIBET: IMPLICATIONS FOR SOUTHWARD EXPANSION OF THE HIMALAYAN ARC

Active right-slip faulting sub-parallel to the Himalayan Range has been long known in the western Himalayan arc west of its symmetry point at about 85°E. These faults have been interpreted to have resulted either from eastward extrusion of the Tibetan plateau or southward expansion of the Himalayan arc. Although the latter hypothesis is supported by focal-mechanism studies indicating that the Himalayan arc is expanding radially over the Indian craton, evidence for the required left-slip faults in the eastern Himalayan arc has been elusive. To investigate this problem we conducted geologic mapping and performed a systematic analysis of satellite images in the eastern Himalayan arc between the Indus-Tsangpo suture zone in the north and the crest line of the Himalayan Range in the south. Our work reveals the presence of a series of east-trending faults that bound east-trending valleys, elongate lakes, and Quaternary basins between ~ 87.5°E and 93°E. In this general area, several east-trending faults are observed both in the field and from examination of satellite images. Some of the east-trending faults were interpreted previously as down-to-the-north normal faults, an interpretation implying the Himalayan Range is currently undergoing coeval north-south contraction extension across the range. However, our work shows that these faults are left-slip structures and are kinematically linked with north-trending rifts. We studied in detail three east-trending left-slip faults in the eastern Himalaya between the north-trending Xianza-Dingjye rift (~88°E) in the west and the north-trending Chigu Co rift in the east (~92°E). Motion on the left-slip faults has caused left-lateral drainage deflection of north-flowing rivers, offset of regressive lake shorelines, formation of small pull-apart basins, and development of subhorizontal striations in the fault zones. Our new structural discovery indicates that arc-parallel strike-slip faulting is an integral part of the contractional Himalayan orogen and their presence in the eastern Himalayan Range in particular supports the notion that the Himalayan arc has been expanding radially southward.