STRUCTURAL ANALYSIS OF THE BENKAR CROSS STRUCTURE IN THE HIGHER HIMALAYAN OF THE KHUMBU REGION, EASTERN NEPAL

The Himalaya are dominated by laterally continuous, range parallel thrust faults and some extensional structures such as the South Tibetan Detachment System. Recent discovery of range-perpendicular strike-slip and extensional fault zones (cross structures) in the Himalaya has raised questions regarding the significance of these structures in the collisional process. Previous workers have discussed the significance of the Kosi, Gish, and Yadong faults in the region of Sikkim and Bhutan. In the central and western Himalaya, the bounding faults of the Thakkhola graben, Leo Pargil dome, and Ama Drime Massif have been documented as extensional structures that are also orthogonal to the range. Further west, the Western Nepal Fault System, the Ganga tear fault, and the Yamuna tear fault are examples of young or active faults that cut obliquely across the range. We have identified a new cross structure in the Khumbu region of eastern Nepal, the Benkar lineament. We are investigating the structural nature of this lineament and its tectonic role in the Himalayan system. Preliminary results show that the Benkar structure has consistent steeply, southeast-dipping foliation (schistosity, gneissic banding, and shear fabric) that strikes NNE-SSW. We have mapped this structure from the lower Khumbu glacier southward, along the Dudh Kosi valley to the village of Benkar. Its extent to the north or south of this mapped area is not yet known. The deformation zone is roughly 6 km wide in the north, but narrows to 2-3 km in the south. In some locations lineations are steeply plunging with an extensional shear sense, though in other locations mineral fiber lineations are sub-horizontal. For some of the sub-horizontal lineations, fabric asymmetries determined in thin-section, suggest a semi-brittle to ductile, dextral shear sense. Future work will focus on determining: 1) the lateral extent of this structure to the northeast and southwest; 2) offset along the structure; 3) timing of deformation; and 4) how this structure relates to similar features in the western and eastern Himalaya and what role they may have played in the formation of the mountain belt.