LATE MIOCENE STRIKE-SLIP RELATED EXHUMATION OF THE HIGHER AND LESSER HIMALAYA, WESTERN NEPAL

Geologic mapping within the central portion of the Himalayan arc, northwestern Nepal (82-81°E longitude) at a scale of 1:50,000 reveals a previously unrecognized 1 to 3-km-wide zone of right-lateral shear that extends southeast approximately 120 km from the Tethyan fold-thrust belt in southwest Tibet into the Lesser Himalaya of western Nepal. The fault system cuts obliquely across the South Tibetan fault system, the Main Central thrust zone, and south-directed reverse faults in the Lesser Himalaya. The mean shear sense direction along the entire mapped length of the fault system is 280°+10°. The fault system consists of WNW-striking, SSW-dipping segments and NNW-striking, WSW-dipping segments, creating two largeg releasing bends. The western releasing bend is approximately 40 km-long and is associated with exhumation of a >3 km-thick sequence of amphibolite-facies metasedimentary rocks and syn-extensional granite bodies and migmatites in its footwall. These rocks are exposed as a 120 km x 40 km dome. Along the southern margin of the dome, a high-strain, subhorizontal WNW-trending stretching lineation overprints an older N-trending stretching lineation associated with top-to-the-south contraction along the Main Central thrust zone. Slip along the eastern releasing bend (~20 km-long) has resulted in several tens of kilometers offset of the South Tibetan fault system, Main Central thrust zone, and south-directed reverse faults involving the Lesser Himalayan sequence. ²⁰⁸Pb/²³²Th ion-microprobe monazite ages of deformed leucogranite sills range from 14 to 11.4 Ma, while undeformed leucogranite dikes yield ages between 10 and 7 Ma. The geologic history of the north underscores the observation that an early orogen-normal contractional event was rapidly followed by orogen-parallel extension during the Late Miocene. This deformation history is consistent with a model involving foreland propagating structural systems facilitating orogen-normal contraction in the orogenic front and orogen-parallel extension in the hinterland.