Metamorphic P-T profile and P-T path discontinuity across the far-eastern Nepal Himalaya: investigation of channel flow models

Thermobarometric data and compositional zoning of garnet show the discontinuities of both metamorphic pressure conditions at peak-T and P-T paths across the Main Central Thrust (MCT), which juxtaposes the high-grade Higher Himalayan Crystalline Sequences (HHCS) over the low-grade Lesser Himalaya Sequences (LHS) in far-eastern Nepal. Maximum recorded pressure conditions occur just above the MCT (similar to 11 kbar), and decrease southward to similar to 6 kbar in the garnet zone and northward to similar to 7 kbar in the kyanite +/- staurolite zone. The inferred nearly isothermal loading path for the LHS in the staurolite zone may have resulted from the underthrusting of the LHS beneath the HHCS. In contrast, the increasing temperature path during both loading and decompression (i.e. clockwise path) from the lowermost HHCS in the staurolite to kyanite +/- staurolite transitional zone indicates that the rocks were fairly rapidly buried and exhumed. Exhumation of the lowermost HHCS from deeper crustal depths than the flanking regions, recording a high field pressure gradient (similar to 1.2-1.6 kbar km-1) near the MCT, is perhaps caused by ductile extrusion along the MCT, not the emplacement along a single thrust, resulting in the P-T path discontinuities. These observations are consistent with the overall scheme of the model of channel flow, in which the outward flowing 'HHCS' and inward flowing 'LHS' are juxtaposed against each other and are rapidly extruded together along the 'MCT'. A rapid exhumation by channel flow in this area is also suggested by a nearly isothermal decompression path inferred from cordierite corona surrounding garnet in gneiss of the upper HHCS. However, peak metamorphic temperatures show a progressive increase of temperature structurally upward (similar to 570-740 degrees C) near the MCT and roughly isothermal conditions (similar to 710-810 degrees C) in the upper structural levels of the HHCS. The observed field temperature gradient is much lower than those predicted in channel flow models. However, the discrepancy could be resolved by taking into account heat advection by melt and/or fluid migration, as these can produce low or nearly no field temperature gradient in the exhumed midcrust, as observed in nature.