THERMAL-MECHANICAL EVOLUTION OF THE UPPER-SECTION OF THE GREATER HIMALAYAN SLAB, EVEREST-LHOTSE MASSIF, NEPAL

Structural analysis, geothermometry, and U-Pb geochronology conducted on samples from the Everest-Lhotse Massif, Nepal, refine existing thermal-mechanical models for the top of the Greater Himalayan Slab (GHS). Here, the south Tibetan detachment system (STDS) comprises two detachments; the upper brittle Qomolangma detachment (QD) and lower ductile Lhotse detachment (LD). The location of the QD immediately below the summit rocks is confirmed by an abrupt transition in metamorphic grade and strain between the slightly deformed Tethyan limestone (summit of Everest; 8848m) and the calc-silicate band below. Microstructures within the structurally lower interlayered Lhotse calc-silicate bands and Everest Series includes calcite grains that preserve a CPO with Type III e-twins (>200°C) and Regime 2-3 recrystallized qtz. This structural position projects south to the crest of the inaccessible 3-km-high Lhotse wall below which, float samples of grt + st ± crd schist record two metamorphic events (M1: 525-550°C and M2: crd rims on st) and Regime 3 recrystallized qtz. The base of the Lhotse wall contains grt + bt + ms schist (~550°C) with NE-plunging extension lineations that are parallel to crenulation fold axes. Crenulations mark the third of at least four phases of deformation responsible for the architecture of the massif. The grt + bt + ms schist structurally overlies, and is in gradational contact with, grt + sill gneisses (~650°-750°C) containing annealed quartz and feldspar grains which typify the microstructure of the core of the GHS. The LD appears to mark the upper limit to the high-grade core of the GHS, but further work is required to establish if this boundary is discrete or gradational. The km-scale Nuptse granite was emplaced along the LD (possibly “expanding” the section) during exhumation-related high-T, low-P metamorphism (M2). Lack of solid-state fabrics in the Nuptse granite implies that it post-dates deformation or remained partially liquid during fabric development. These data support our interpretation of the thermal-mechanical structure at the top of the GHS as a decrease in deformation temperatures, strain, and metamorphic grade towards structurally higher positions: essentially a mirror image of the Main Central thrust zone along the base of the GHS.