Quantifying internal strain and deformation temperature in the eastern Himalaya, Bhutan: Implications for the evolution of strain in thrust sheets

Quartz microstructural analysis together with quantification of 3-D crystal–plastic strain of quartz clasts (Normalized Fry and Rf-φ methods) constrain deformation temperature ranges and internal strain magnitude/orientation within Himalayan thrust sheets in Bhutan. Lesser Himalayan (LH) thrust sheets display an inverted deformation temperature gradient, from ∼250 to 310 °C at the Main Boundary Thrust to ∼500–670 °C at the Main Central Thrust (MCT), attributed primarily to stacking of LH thrust sheets deformed at progressively higher temperatures toward the hinterland. The ‘hot-iron’ effect of the MCT hanging wall only affects the upper few hundred meters of hinterland LH thrust sheets. Frontal thrust sheets exhibit ∼7% layer-parallel shortening (LPS) strain. All other thrust sheets exhibit layer-normal flattening (LNF) strain, with 1.9:1.8:1.0 (LH rocks) and 2.1:1.8:1.0 (Greater and Tethyan Himalayan rocks) median ellipsoids (X parallel to lineation). We propose that LPS strain developed foreland-ward of the thrust deformation front, and that LNF strain resulted from later tectonic loading. The LPS to LNF transition occurs at minimum temperatures for quartz plasticity (ca. 250–270 °C). LNF ellipsoids are folded along with strata within thrust sheets, indicating that internal strain preceded thrust imbrication and translation. At the scale of 10’s of meters structural distance, strain magnitude of hanging wall rocks does not increase near thrusts, which supports the existence of discrete faults with large translations, as generally depicted in balanced cross-sections Strain magnitudes in low-grade Greater and Tethyan Himalayan rocks in central Bhutan indicate that the top-to-the-north component of channel-flow extrusion is most likely less than 19 km.