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

Abstract

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 (11 kbar), and decrease southward to 6 kbar in the garnet zone and northward 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 (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 ÔHHCSÕ and inward flowing ÔLHSÕ are juxtaposed against each other and are rapidly extruded together along the ÔMCTÕ. 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 (570-740°C) near the MCT and roughly isothermal conditions (710-810°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.

Implications of channel flow analogue models for extrusion of the Higher Himalayan Shear Zone with special reference to the out-of-sequence thrusting

“…The High Himal Thrust dips 20-40°to the E/ENE/NW, and exhibits mylonitization of garnet, sillimanite and feldspar bearing rocks, northward plunging stretching lineations, shear bands, and a very high strain ratio (R) of between 30 and 45 (Goscombe et al 2006). In contrast to all other places, the OOST in Nepal (where it is recognized as High Himal Thrust) underwent a late phase of extensional faulting (Goscombe et al 2006; also referred to in Imayama et al 2010).…”

Lateral extrusion, underplating, and out-of-sequence thrusting within the Himalayan metamorphic core, Kanchenjunga, Nepal

“…Rocks of the Himalayan metamorphic core form an inverted metamorphic sequence from Ms + Bt + Grt grade at the base to Kfs + Sil grade in the upper portion within the field area (Goscombe et al, 2006;Imayama et al, 2010). In the studied area, the Himalayan metamorphic core is divided into five geologic units (Fig.…”

“…There is a broad zone of deformation associated with the Main Central thrust, and, as such, interpreting where it should be mapped is difficult and has typically varied with the preference of the researcher along the orogen (for a summary of problems associated with mapping the Main Central thrust, see Searle et al, 2008). Previous studies have variably mapped the Main Central thrust at the base (Goscombe et al, 2006), top (Schelling, 1992), and just above (Imayama et al, 2010) the mylonitic orthogneiss in the lowest unit of the Himalayan metamorphic core. In this study, the Main Central thrust is mapped just below the mylonitic orthogneiss (Fig.…”

“…Until the recognition of these structures, most models for burial and exhumation have focused on the two fault systems that bound the Himalayan metamorphic core: the South Tibetan detachment system above, and the Main Central thrust below (Beaumont et al, 2001Jamieson et al, 2004;Webb et al, 2007Webb et al, , 2011. Increasing chronologic and thermobarometric data from across the Himalaya, however, indicate that significant amounts of horizontal shortening and vertical thickening were accommodated along structures within the Himalayan metamorphic core (e.g., Jain and Manickavasagam, 1993;Goscombe et al, 2006;Groppo et al, 2009;Carosi et al, 2010;Imayama et al, 2010;Larson et al, 2013;Larson and Cottle, 2014;Mottram et al, 2014;Warren et al, 2014). These structures have only a cryptic surface expression and are often recognized by abrupt spatial breaks in P- T-t(-deformation [D]) paths (e.g., overall shape, timing, peak conditions).…”

Tectonics of the Himalayas

“…4a, 5a -MCT MCT Kohn et al, 2001;Imayama et al, 2010MCT -Harris et al, 2004Imayama et al, 2010Imayama et al, , 2012 Khaghn valley Nanga Parbat syntaxis Tso Morari Fig Pressure-temperature (P-T) diagram with equilibrium curves for fluid-saturated melting and mica dehydration reactions. The solid and dashed arrows represent the P-T paths from migmatites in the HHC in Sikkim, India (Harris et al, 2004) and in Eastern Nepal (Groppo et al, 2012) Aitchison et al, 2007;Zhang et al, 2012 15 11 Ma Najman et al, 2001, 2002Najman et al, 2002Bhainskati Sakai, 1983Matsumaru and Sakai, 1989 Dumri Dharamsala Fig.…”