Transverse Faults and their Role in Himalayan Tectonics and Hazards: a Solukhumbu Example

Mary Hubbard, Montana State University
Ananta Prasad Gajurel, TriChandra Campus, Tribhuvan University
David Lageson, Montana State University

Though thrust faults and, more recently, extensional faults, have been the dominant topic in Himalayan structural geology literature, there is another breed of structure that may be equally important to the mountain building process and the geologic hazard in the Himalaya. These structures are the transverse faults that are highly oblique/perpendicular to the strike of the thrusts and the South Tibetan Detachment System (STDS). Previous workers have discussed the importance of the Yadong Fault on the western border of Bhutan as well as the Gish and Kosi faults in the Sikkim region. Further west the bounding faults of the Thakhola Graben are transverse to the range, as are the bounding faults of the Leo Pargil Dome. A number of these structures are clearly extensional, but other structures of this orientation appear to have a strike-slip sense of displacement and may serve to partition seismicity related to convergence. We present here a newly-recognized structure from eastern Nepal, the Bemkar Fault. This structure has a linear trace possibly extending from south of Salleri to the lower Khumbu glacier and possibly into Tibet. There is no topographic expression of extension as seen in the Thakkola Graben or along the Yadong Fault. The fault trace cuts a landslide deposit along the Dudh Kosi river and has possibly been partially buried by a younger landslide. The apparent length of this structure suggests that it has played a significant role in the tectonic development of the range, but the fact that it is spatially associated with landslide deposits and it may pass beneath glacial lake, Cho La, makes it an important structure in the realm of natural hazards for the area (earthquake, landslide, flooding). To the east, the Gish and Kosi faults are faults of similar length and orientation, but have yet to be studied in detail. We propose that these structures may play a role in partitioning zones of different structural (i.e. dip of thrusts) or seismic character (locked versus active).