Exhumation of the Main Central Thrust from Lower Crustal Depths, Eastern Bhutan Himalaya

Abstract

Geothermometry and mineral assemblages show an increase of temperature structurally upwards across the Main Central Thrust (MCT); however, peak metamorphic pressures are similar across the boundary, and correspond to depths of 35–45 km. Garnet‐bearing samples from the uppermost Lesser Himalayan sequence (LHS) yield metamorphic conditions of 650–675 °C and 9–13 kbar. Staurolite‐kyanite schists, about 30 m above the MCT, yield P‐T conditions near 650 °C, 8–10 kbar. Kyanite‐bearing migmatites from the Greater Himalayan sequence (GHS) yield pressures of 10–14 kbar at 750–800 °C. Top‐to‐the‐south shearing is synchronous with, and postdates peak metamorphic mineral growth. Metamorphic monazite from a deformed and metamorphosed Proterozoic gneiss within the upper LHS yield U/Pb ages of 20–18 Ma. Staurolite‐kyanite schists within the GHS, a few metres above the MCT, yield monazite ages of c. 22 ± 1 Ma. We interpret these ages to reflect that prograde metamorphism and deformation within the Main Central Thrust Zone (MCTZ) was underway by c. 23 Ma. U/Pb crystallization ages of monazite and xenotime in a deformed kyanite‐bearing leucogranite and kyanite‐garnet migmatites about 2 km above the MCT suggest crystallization of partial melts at 18–16 Ma. Higher in the hanging wall, south‐verging shear bands filled with leucogranite and pegmatite yield U/Pb crystallization ages for monazite and xenotime of 14–15 Ma, and a 1–2 km thick leucogranite sill is 13.4 ± 0.2 Ma. Thus, metamorphism, plutonism and deformation within the GHS continued until at least 13 Ma. P‐T conditions at this time are estimated to be 500–600 °C and near 5 kbar. From these data we infer that the exhumation of the MCT zone from 35 to 45 km to around 18 km, occurred from 18 to 16 to c. 13 Ma, yielding an average exhumation rate of 3–9 mm year−1. This process of exhumation may reflect the ductile extrusion (by channel flow) of the MCTZ from between the overlying Tibetan Plateau and the underthrusting Indian plate, coupled with rapid erosion.

Testing the effects of topography, geometry, and kinematics on modeled thermochronometer cooling ages in the eastern Bhutan Himalaya

“…In addition to residence time and thermal conditions experienced by the rocks affecting argon loss, they suggested that a ±2 Ma age dispersion would be expected due to diffusive differences caused by grain size variations. MAr ages from eastern Bhutan postdate the age of south-directed shear on the MCT in this region (Stüwe and Foster, 2001;Grujic et al, 2002;Daniel et al, 2003;Kellett et al, 2009;Long et al, 2012). Thus, we interpret the age range of these four MAr samples as the window of permissible exhumation-induced cooling through the modeled closure temperatures of white mica (Ehlers, 2005;Braun, 2003).…”

“…The Greater Himalaya is divided into two structural levels: the lower unit is above the MCT but below the out-of-sequence Kakhtang Thrust (KT), while the higher unit is in the hanging wall of the KT (Grujic et al, 2002). Estimates for the initiation of motion on the MCT range from ∼ 25 to 20 Ma (e.g., Hodges et al, 1996;Daniel et al, 2003;Tobgay et al, 2012), with continued shearing in the Bhutan Himalaya through 18-16 Ma (Grujic et al, 2002;Daniel et al, 2003;Kellett et al, 2009). The age of motion on the KT is notably younger (14-8 Ma; Daniel et al, 2003;Grujic et al, 2002Grujic et al, , 2011Coutand et al, 2014).…”

“…Estimates for the initiation of motion on the MCT range from ∼ 25 to 20 Ma (e.g., Hodges et al, 1996;Daniel et al, 2003;Tobgay et al, 2012), with continued shearing in the Bhutan Himalaya through 18-16 Ma (Grujic et al, 2002;Daniel et al, 2003;Kellett et al, 2009). The age of motion on the KT is notably younger (14-8 Ma; Daniel et al, 2003;Grujic et al, 2002Grujic et al, , 2011Coutand et al, 2014). Regional-scale upright, non-cylindrical antiforms and synforms mapped throughout the Greater Himalaya are interpreted to be a result of underlying Lesser Himalayan duplex formation (Long et al, 2011b).…”

Cambrian rocks and faunas of the Wachi La, Black Mountains, Bhutan

“…These 'Grenville'-age orogens surrounded the Indian continent within Gondwanaland and include provinces from east Antarctica (Boger et al 2000;Harley & Kelly, 2007), south-central Africa (Cahen et al 1984) and Sri Lanka/south India (Fitzsimons, 2000a,b). Local potential sources for grains of these ages include magmatic rocks in various tectonic units of Bhutan, Arunachal Pradesh and the Shillong plateau (Daniel et al 2003;Richards et al 2006;Chakungal et al 2010;Yin et al 2010a,b).…”

Ductile deformation within Upper Himalaya Crystalline Sequence and geological implications, in Nyalam area, Southern Tibet

“…In the central Himalaya, most leucogranite emplacement ages are Early-Middle Miocene (~24 to ~12 Ma) [4,9,12,29,31,36,37,69]; but some leucosome yield Oligocene ages [6,23].…”