Kinematic evolution of the Ama Drime detachment: Insights into orogen-parallel extension and exhumation of the Ama Drime Massif, Tibet–Nepal

The Ama Drime Massif is a north–south trending antiformal structure located on the southern margin of the Tibetan Plateau that is bound by the Ama Drime and Nyönno Ri detachments on the western and eastern sides, respectively. Detailed kinematic and vorticity analyses were combined with deformation temperature estimates on rocks from the Ama Drime detachment to document spatial and temporal patterns of deformation. Deformation temperatures estimated from quartz and feldspar microstructures, quartz [c] axis fabrics, and two-feldspar geothermometry of asymmetric strain-induced myrmekite range between ∼400 and 650 °C. Micro- and macro-kinematic indicators suggest west-directed displacement dominated over this temperature range. Mean kinematic vorticity estimates record early pure shear dominated flow (49–66% pure shear) overprinted by later simple shear (1–57% pure shear), high-strain (36–50% shortening and 57–99% down-dip extension) dominated flow during the later increments of ductile deformation. Exhumation of the massif was accommodated by at least ∼21–42 km of displacement on the Ama Drime detachment. Samples from the Nyönno Ri detachment were exhumed from similar depths. We propose that exhumation on the Nyönno Ri detachment during initiation of orogen-parallel extension (11–13 Ma) resulted in a west-dipping structural weakness in the footwall that reactivated as the Ama Drime detachment.