THRUST OROGEN CROSS FAULTS: NEW DATA ON THE HIMALAYAN BENKAR FAULT ZONE

The Benkar Fault Zone (BFZ) is a NNE-striking, dextral, normal, cross-fault (strikes at a high-angle to the orogen) originally mapped in the Greater Himalaya in the Everest region of the eastern Nepal Himalayas. Recent mapping towards the south from Lukla, across the Main Central Thrust (MCT, thrust zone between the Greater and Lesser Himalayan Sequences) and in and around the Okhaldhunga Window, has revealed multiple exposures of cross-faults segments, which are roughly along-strike with the BFZ. These faults cut both the Greater Himalayan rocks and the underlying Lesser Himalayan formations in the core of the Okhaldhunga Anticlinorium. About 45km SSW of Lukla, towards the NW from the town of Okhaldhunga, there is a ~10m thick gouge zone that cuts into EW-striking quartzites at a high angle (030˚/62˚NW). 7 km towards the south, we found a ~150m wide brecciated zone containing NE-striking slip surfaces. Across the ridge to the south, ‘the Nourpoul Quartzites’ along the Ketuke-Dhuseni road section has multiple brittle faults with deformation lineations plunging 40˚ towards 170˚ on the NNE-striking, southeast dipping, normal, dextral, fault surfaces. South of the window, augen-gneiss (Greater Himalayas) is overriding kyanite-bearing quartzites along a brittle, narrow, cross-fault (214˚/63˚NW). About 80km SSW of Lukla, klippe rocks of the Greater Himalaya have been dragged into and offset by a ~250m wide, 229˚/74˚NW striking, semi-brittle fault zone. We interpret these brittle cross fault structures to be a southern continuation of the Benkar fault zone. Within the Lesser Himalayan thrust sheets, we have also documented a few EW-stiking, north-dipping normal faults. Future work is aimed at using low-temperature thermochronology to constrain the timing of movement(s) and possibly amount of offset along the BFZ in order to better understand the tectonic significance of this structure.