Structural analysis of the Benkar Fault Zone, a cross structure in the higher Himalaya of the Khumbu Region, eastern Nepal

Abstract

The Himalaya are dominated by laterally continuous, range-parallel thrust faults and less frequent 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. I have analyzed a newly-recognized cross structure in the Khumbu region of eastern Nepal, the Benkar Fault Zone. Structural mapping, petrographic observation, and analytical evidence reveals a zone of consistently NE-striking, SE-dipping metamorphic foliation from the villages of Phakding to Gorak Shep along the Dudh Kosi valley. Deformation within this zone is restricted to foliation-parallel, nonpenetrative, anastomosing sillimanite- and mica-bearing shear zones that wrap around poorly deformed quartz and feldspar enclaves. Kinematics of these shear zones from thin sections and outcrop observations are consistent with normal, right-lateral oblique slip and some local zones of purely extensional displacement. Quartz crystallographic fabric orientation suggests crystal plastic deformation under upper greenschist to lower amphibolite facies conditions. Outcrop observations show evidence for isolated brittle-ductile deformation, suggesting short-lived brittle events that punctuated a history of predominantly plastic deformation. Benkar Fault Zone fabric crosscuts older, thrust-related foliation and suggests that deformation within the zone postdates peak metamorphism and occurred while rocks were on the retrograde path. Crosscutting relationships between Benkar Fault Zone fabric and leucogranites constraints the timing of deformation to be younger than 20.5-21.3 Ma. The Benkar Fault Zone is interpreted to represent a tear fault within the Greater Himalayan sequence associated with differential displacement along a structurally deeper thrust.