CROSS-FAULTS AND THEIR ROLE IN HIMALAYAN STRUCTURAL EVOLUTION

The Himalaya began its rise to lofty heights, following the collision of the Indian continent with the Asian continent. The primary faults responsible for building this mountain range are found separating north-dipping slivers of the former leading edge of the Indian plate. The traces of these faults are parallel to the mountain range and, with the exception of the South Tibetan Detachment System, they generally have a thrust sense of shear. In several locations along the Himalaya, there are faults that are highly oblique to the strike of the range and we refer to these as cross faults. While a few of these structures are associated with extension, such as those bounding the Thakkhola Graben, there are others, some of them newly recognized, that do not have the topographic expression of a graben. Several of these structures are located at salient-recess transitions such as the Yamuna tear fault and the Ganga tear fault in NW Inda. Three of these unique faults are located in eastern Nepal and Sikkim. The Gish fault in Sikkim is a northeast-striking structure that is on-strike with the extensional Yadong fault, yet has left-lateral offsets of sub-Himalayan units. The Kosi fault of eastern Nepal also corresponds to left-lateral offsets at the range front. We have found a third fault of similar strike in the Khumbu region of Nepal. This structure may have had some ductile dip slip movement along its trace, but it has also had a right-lateral sense of shear in the semi-brittle to brittle regime. So far we have mapped this structure along a 30 km length in Greater Himalayan rocks. The Sikkim region, including the Kosi and Gish faults has active strike-slip seismicity that has been attributed to segmentation of the Himalayan thrust system and possibly related to slight rotation of blocks within the range. Segmentation into salients and recesses may impact the along-strike earthquake rupture propagation, thus we are interested in further understanding the structural history of these faults. Other possible causes of cross faults include irregularities in the underthrusting Indian crust or re-activation of structures related to extension within the range.