The metamorphism and exhumation of the Himalayan metamorphic core, eastern Garhwal region, India

Abstract

[1] Geothermobarometric together with micro-and macro-structural data indicate ductile flow in the metamorphic core of the Himalaya in the Garhwal region of India. Peak metamorphic pressure and temperature increase dramatically across the Main Central Thrust (MCT) from $5 kbar and $550°C in the Lesser Himalayan Crystalline Sequence (LHCS) to $14 kbar and $850°C at $3 km above the MCT in the Greater Himalayan Sequence (GHS). Pressures within the GHS then decrease upsection to $8 kbar while temperatures remain nearly constant at $850°C up to the structurally overlying South Tibetan Detachment (STD). The GHS exhibits sheath fold geometries are indicative of high degrees of ductile flow. Overprinting ductile structures are two populations of extensional conjugate fractures and normal faults oriented both parallel and perpendicular to the orogen. These fractures crosscut major tectonic boundaries in the region such as the MCT and STD, and are found throughout the LHCS, GHS, and Tethyan Sedimentary Sequence (TSS). The thermobarometric and metamorphic observations are consistent with a form of channel flow. However, channel flow does not account for exhumational structures that formed above the brittle-ductile transition. To explain all of the features seen in the metamorphic core of the Garhwal region of the Himalaya, both the theories of channel flow and critical taper must be taken into account. Channel flow can explain the exhumation of the GHS from the middle crust to the brittle-ductile transition. The most recent extensional deformation is consistent with a supercritical wedge.Citation: Spencer, C. J., R. A. Harris, and M. J. Dorais (2012), The metamorphism and exhumation of the Himalayan metamorphic core, eastern Garhwal region, India, Tectonics, 31, TC1007,

Pressure‐Temperature‐Deformation‐Time Constraints on the South Tibetan Detachment System in the Garhwal Himalaya (NW India)

“…According to Jain et al () the general architecture of the study valleys could be simplified as N‐NE dipping imbrications of rock packages with the main tectonic foliation trending NW‐SE and dipping gently toward the NE. The LHS, present in the SW portion of the area (Figure b), is composed of garnet‐bearing metapelite, limestone, quartzite, calcsilicate, and amphibolite layers (Ahmad et al, ; Jain et al, ; Spencer et al, ). Their metamorphic peak temperature never exceeded 480–520°C (Célérier et al, ).…”

“…(a) Geological map of the Himalaya, after Weinberg (). (b) Geological sketch of the Alaknanda‐Dhauli Ganga valleys (Garhwal Himalaya, NW India) after Spencer et al (, ) and Jain et al (). (c) Detailed geological map in the area around the village of Malari.…”

“…This formation still preserves sedimentary structures like bedding (S 0 ; Figure f) and cross lamination (Jain et al, ; Spencer et al, ). The main foliation in the Martoli Formation, related to northeast verging folds (Spencer et al, ), is defined by the SPO of phyllosilicates, which are mainly fine‐grained white mica and chlorite (Figure g). Detrital large flakes of white mica were also detected (Figure g).…”

“…The study area is located in the Garhwal Himalaya (Figure 1a), NW India, along the Alaknanda-Dauli Ganga valleys (Figure 1b), where a complete structural transect of the Himalaya is exposed (Gururajan & Choudhuri, 1999;Hodges & Silverberg, 1988;Jain et al, 2013Jain et al, , 2014Shreshtha et al, 2015;Spencer, Harris, & Dorais, 2012a, 2012bThakur, Patel, & Singh, 2015). According to Jain et al (2014) the general architecture of the study valleys could be simplified as N-NE dipping imbrications of rock packages with the main tectonic foliation trending NW-SE and dipping gently toward the NE.…”

“…Five different types of leucosomes/melt patches have been described by Jain et al (2013) from the migmatite, reflecting different stages of structurally controlled melting and melt migration in the Bhapkund Formation. P-T data by Spencer et al (2012b) suggest an increase of P-T conditions from the Joshimath (~700-800°C, 1.0-1.4 GPa) to the Suraithota (~800-860°C,~1.40 GPa) Formations, whereas similar T for the Suraithota Formation, but lower equilibration P (~0.85-1.00 GPa) were reported for the Bhapkund Formation (Spencer et al, 2012b). The P-T estimates by the latter authors are significantly higher than those reported by Hodges and Silverberg (1988) for the GHS rocks along the Alaknanda-Dauli Ganga valleys (530-680°C, 0.30-1.00 GPa).…”