THE EAST JHOMOLARI FAULT SYSTEM, NW BHUTAN: ONSET, DISPLACEMENT, AND MOST RECENT ACTIVITY OF A YOUNG LITTLE KNOWN FUALT ZONE AND ITS REGIONAL SIGNIFICANCE

The East Jhomolari Fault System (EJFS) in northwestern Bhutan is a zone of young, subparallel, normal faults striking 30^o-45^o and dipping 60^o-80^o to the east-southeast. Parts of its southern and northernmost segments have been discontinuously mapped previously, however the main central segment connecting the two has not been mapped before. The addition of this central segment demonstrates that the EJFS is a through going structure of greater, regional significance than previously recognized. Field observations of increasing offset from south to north suggest that the EJFS has a scissor like geometry with the hinge near its southern termination. The central segment of the EJFS cuts the upper South Tibetan Detachment (STDu) at the Zha Goy Baksa valley providing piercing points for determining slip across the fault zone. Field observations suggest that there is at least two kilometers of slip on this segment. Fault scarps in Quaternary fluvial-glacial sediments, including disrupted strata in moraines along strike of the EJFS suggest that this has been a recently active structure, potentially in Holocene time. There are numerous glacial lakes in the immediate footwall of the EJFS, some of which have produced glacial lake outburst floods (GLOFs) in the past. These lakes are perched above a series of villages in the Mo Chu river valley. The threat of earthquake induced GLOFs makes determining the state of activity on the EJFS of particular relevance to the inhabitants of northwestern Bhutan. In addition to contributing to the knowledge of local seismic hazards this study will use thermochronology, paleoseismic analysis, as well as field mapping and observations to estimate the onset, displacement, rate of motion, and most recent activity of the EJFS.