Progression from South-Directed Extrusion to Orogen-Parallel Extension in the Southern Margin of the Tibetan Plateau, Mount Everest Region, Tibet

Abstract

In the South Tibetan Himalaya, major detachment systems exhumed midcrustal rocks from different structural levels that are exposed in the Ama Drime and Mount Everest massifs. The South Tibetan detachment system (STDS) accommodated exhumation of the Greater Himalayan Series (GHS) until the Middle Miocene. Field- and laboratory-based structural data presented here indicate that early melt-present deformation in the footwall of the STDS accommodated large-scale flow of a low-viscosity middle crust. Decompression-related anatexis and emplacement of leucogranites in the structurally highest positions of the GHS (∼16–18 Ma) mark the final stages of south-directed extrusion within a relatively narrow solid-state mylonite zone that progressed into a brittle detachment. Estimates of mean kinematic vorticity and deformation temperatures record a progression in deformation conditions, from ultra-mylonitic leucogranite (∼400°–500°C; 51%–67% pure shear) to mylonitic marble (>300°C; 41%–55% pure shear) to moderately foliated marble ∼200°–300°C (46%–59% pure shear). Previous investigations demonstrated that the cessation of movement on the STDS at ∼16–13 Ma was followed by anatexis in the lower portion of the middle crust (∼12–13 Ma; Ama Drime Massif). The Ama Drime and Nyönno Ri detachments (∼8–12 Ma; Dinggyê graben) exhumed rocks from the deepest structural position in the central Himalaya during orogen-parallel extension. Brittle faulting dissected the upper crust (<4 Ma; Tingri–Kung Co graben) in a setting that was kinematically linked to extension in the interior of the Tibetan plateau.