THE AMA DRIME DETACHMENT, TIBET-NEPAL: INSIGHTS INTO OROGEN-PARALLEL EXTENSION AND EXHUMATION OF THE AMA DRIME MASSIF

The Ama Drime Massif is a north-south trending antiformal structure located ~50 km northeast of Mt. Everest that is bounded by the Ama Drime and Nyönno Ri detachments on the western and eastern sides, respectively. New detailed microstructural, kinematic, and vorticity analyses were combined with deformation temperature estimates for rocks exposed within the Ama Drime detachment in order to document the spatial and temporal patterns of deformation during movement along the detachment and exhumation of the Ama Drime Massif. Micro- and macro-scale kinematic indicators suggest that top-to-the-west displacement was dominant at a range of deformation temperatures. Deformation temperatures estimated from quartz and feldspar microstructures, quartz [c] axis opening angle fabrics and two-feldspar geothermometry of asymmetric strain-induced myrmekite range between ~350–650 °C. Mean kinematic vorticity estimates based on rotated rigid porphyroclasts range between ~0.50–0.70 (~50–66% pure shear) and those based on oblique grain shape fabrics in quartz range between ~0.70–1.0 (~0–50% pure shear) suggesting a temporal variation in deformation that progressed from dominantly pure shear to dominantly simple shear (i.e. an accelerating strain path). In addition, these data suggest that exhumation of the Ama Drime Massif during orogen-parallel extension was accommodated by significant vertical thinning and horizontal extension that resulted in at least ~17 km of displacement on the Ama Drime detachment providing new insights into the role of strain partitioning and magnitude of displacement during orogen-parallel extension in the Everest region.