Anatexis, cooling, and kinematics during orogenesis: Miocene development of the himalayan metamorphic core, east-central nepal

Abstract

The exposed mid-crustal rocks of the Himalayan orogen provide a natural laboratory for constructing the kinematic evolution of the midcrust during a large-scale continental collision. Kinematic models provide testable, geometrically valid, internally consistent, integrated solutions for diverse geological data from deformed regions. We investigated the Tama Kosi region of east-central Nepal with structural, geochemical, and geochronological methods to refine a detailed kinematic model for the Miocene Epoch, during which the mid-crust was pervasively deformed, translated southward, and progressively stacked via basal accretion. Geochemical and U-Pb zircon data demonstrate that two similar orthogneiss bodies were derived from different protoliths, one formed through vapor-absent melting at 1940 ± 16 Ma and the other via vapor-present melting at 1863 ± 14 Ma, respectively, indicating that they do not reflect structural repetition. In situ Th-Pb monazite petrochronology from the Mahabharat Range links the orogenic foreland to the exposed mid-crust of the High Himalaya via a coeval, protracted metamorphic growth-crystallization and/or recrystallization record spanning late Eocene or early Oligocene to early Miocene. Differential cooling of white mica, evidenced by 40Ar/39Ar cooling ages across the studied area, may outline a previously unrecognized out-of-sequence thrust, the occurrence of which is coincident with the location of a sharp break previously recognized from quartz crystallographic fabric deformation temperatures. Together with previous work, these data form the basis for a new, internally consistent kinematic model for rocks of the Tama Kosi region during the Miocene Epoch that tracks the transition from distributed ductile deformation in the mid-crust to deformation along discrete surfaces during their exhumation.