Temperature and strain gradients through Lesser Himalayan rocks and across the Main Central thrust, south central Bhutan: Implications for transport‐parallel stretching and inverted metamorphism

Abstract

In order to understand mass and heat transfer processes that operated during Himalayan orogenesis, we collected temperature, finite and incremental strain, and kinematic vorticity data through a 5 km thickness of Lesser and Greater Himalayan rocks in southern Bhutan. This transect crosses two major shear zones, the Main Central thrust (MCT) and Shumar thrust (ST). Raman spectroscopy on carbonaceous material and garnet‐biotite thermometry are integrated with deformation temperatures from quartz petrofabrics. These data define inverted field gradients that correspond in structural position with the MCT and ST, which are separated by sections in which temperatures remain essentially constant. Temperatures increase from ~400–500°C to ~700–750°C between 675 m below and 200 m above the MCT. This defines a 269 ± 44°C/km inverted gradient, interpreted to have formed via high‐magnitude (~100–250 km) shearing on a discrete MCT zone delineated by the limits of inverted metamorphism. Temperatures increase from ~300–400°C to ~400–530°C across the ST, which is attributed to differences in maximum burial depth of hanging wall and footwall rocks. Strain and vorticity data indicate that Lesser and Greater Himalayan rocks were deformed by layer‐normal flattening. Transport‐parallel lengthening and foliation‐normal shortening increase from 38–71% to 36–49%, respectively, between 2.3 and 1.0 km below the MCT. The MCT acted as a “stretching fault,” with translation on the order of hundreds of kilometers accompanied by transport‐parallel stretching of footwall and hanging wall rocks on the order of tens of kilometers. This demonstrates that stretching accommodated between major shear zones can make a significant contribution to cumulative mass transfer.

The Influence of Foreland Structures on Hinterland Cooling: Evaluating the Drivers of Exhumation in the Eastern Bhutan Himalaya

“…Where different interpretations of dip domains were possible (e.g., for the southernmost three LH thrust sheets), varying cross‐section interpretations highlight these differences (Figure ). No attempt was made to incorporate outcrop‐scale folding or to account for penetrative ductile deformation in rocks proximal to the MCT (e.g., Long, McQuarrie, Tobgay, & Hawthorne, , ).…”

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

“…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). Both Greater and Lesser Himalayan rocks preserve pervasive ductile deformation above and below the MCT (Grujic et al, 1996;Long et al, 2011cLong et al, , 2016 that cannot be replicated with kinematics that only account for fault displacement. However, during initial emplacement of the MCT and active displacement on the MHT, ductile processes at depth transition to brittle processes as thrust and shear systems approach the surface, with a transition temperature of ∼ 350 • C (Avouac, 2007).…”

“…These cooler processes, friction on brittle faults, and erosional exhumation control modeled fault rates (Beaumont et al, 2001;Jamieson et al, 2004;Avouac, 2007). Although our approach does not capture ductile deformation at depth, it does capture the displacement and cooling below 350-400 • C. Even at temperatures below 350-400 • C, almost all of the rocks in Bhutan have undergone some component of granular-scale strain (Grujic et al, 1996;Long et al, 2011cLong et al, , 2016. In the models we present here, all thrust sheets are treated as rigid bodies that were translated by discrete structures.…”

Deformational Temperatures Across the Lesser Himalayan Sequence in Eastern Bhutan and Their Implications for the Deformation History of the Main Central Thrust

“…In the LHS of eastern Bhutan, three metamorphic isograds were mapped by Gansser (1983): the “lower greenschist facies” isograd stretching along the MBT, the “higher greenschist facies” isograd in the upper Baxa Group, and the “biotite porphyroblast facies” isograd, which closely follows the trace of the MCT (Figure 1). At the structural level of the last isograd, within the Jaishidanda Formation, peak metamorphic mineral assemblages are interpreted to be garnet + biotite + muscovite + plagioclase + quartz, with peak metamorphic P ‐ T conditions estimated in eastern Bhutan at ~650 ± 25 °C and 11–12 kbar (Daniel et al, 2003) and in central Bhutan at ~620 ± 25 °C (Long et al, 2016).…”

“…This microstructure is overprinted by progressive subgrain rotation (SGR) at deformation temperatures between 400 and 500 °C (Stipp et al, 2002) and ~450–550 °C (Law, 2014). Farther south, structurally down, Baxa Group quartzite often contains evidence for bulging recrystallization (Long, McQuarrie, Tobgay, & Hawthorne, 2011, Long et al, 2016) that corresponds to deformation temperatures between ~280 and 400 °C (Stipp et al, 2002) and 350–450 °C (Law, 2014). Active quartz slip systems determined from a and c axis distributions (Grujic et al, 1996) indicate both crossed girdle and single girdle fabrics and display the greatest concentrations of c axis orientations at the division points of the c axis cross girdles and lesser numbers at the center of the c axis stereogram.…”