Strain, deformation temperatures and vorticity of flow at the top of the Greater Himalayan Slab, Everest Massif, Tibet

Abstract

This paper presents quantitative data on strain, deformation temperatures and vorticity of flow at the top of the Greater Himalayan Slab. The data were collected from the Tibetan side of the Everest Massif where two low-angle normal faults bound the upper surface of the Greater Himalayan Slab, the earlier and structurally lower Lhotse Detachment and the later and structurally higher Qomolangma Detachment. Greenschist- to sillimanite-grade quartz-rich metasedimentary rocks exposed in the Rongbuk to North Col region of the Everest Massif are characterized by cross-girdle quartz c -axis fabrics indicating approximate plane strain conditions. Fabric opening angles progressively increase with depth beneath the overlying Lhotse Detachment, and indicate progressively rising deformation temperatures of 525–625 ± 50 °C at depths of 300–600 m beneath the detachment. Deformation temperatures of c . 450 °C are indicated by fabric opening angles in epidote amphibolite-facies mylonites located closer to the overlying detachment. A top down-to-the-north (normal) shear sense is indicated by the asymmetry of microstructures and c -axis fabrics, but the degree of asymmetry is low at distances greater than 400 m beneath the detachment, and sillimanite grains are drawn into adjacent conjugate shear bands but still appear pristine, indicating that deformation occurred at close to peak metamorphic temperatures. These ‘quenched’ fabrics and microstructures indicate rapid exhumation in agreement with previous isotopic dating studies. Mean kinematic vorticity numbers ( W m ) were independently calculated by three analytical methods. Calculated W m values range between 0.67 and 0.98, and indicate that although a simple shear component is generally dominant, particularly in greenschist-facies mylonites located between the Lhotse and overlying Qomolangma detachments, there is also a major component of pure shear in samples located at 400–600 m beneath the Lhotse Detachment (pure and simple shear make equal contributions at W k =0.71). Our integrated strain and vorticity data indicate a shortening of 10–30% perpendicular to the upper surface of the Greater Himalayan Slab and confirm that the upper surface of the slab is a ‘stretching fault’ with estimated down-dip stretches of 10–40% (assuming plane strain deformation) measured parallel to the flow plane–transport direction.