The Evolution of Eastern Himalayan deformation: Geometry and kinematics of the Himalayan Fold-thrust Belt, Eastern and Central Bhutan

Extensive geologic mapping in the Himalayan kingdom of Bhutan, in conjunction with U-Pb zircon ages, stratigraphic columns, mineral assemblages, balanced cross-sections, and microstructural and strain analyses, have allowed us to: 1) define the tectonostratigraphy of the frontal portion of the orogen, 2) quantify the magnitude of ductile flow within Greater Himalayan (GH) rocks, and 3) estimate the crustal shortening accommodated by the orogen. We also: 4) constrain deformation temperatures and internal strain magnitudes in Himalayan thrust sheets, and 5) present a new geologic map of Bhutan.;We divide Lesser Himalayan (LH) rocks into six map units, which range between Paleoproterozoic and Permian in age. LH deposition age ranges indicate that: 1) two unconformities identified in northwest India continue into the eastern Himalaya, and 2) there was time-equivalent deposition of LH units proximal to India and GH and Tethyan Himalayan (TH) units distal to India.;The distribution of metamorphic mineral assemblages and interfingering of distinct lithologies at the GH-TH contact indicate that rocks mapped as TH are in depositional contact above GH rocks in central Bhutan. Map geometries limit slip on the South Tibetan detachment to ∼20 km, and strain analysis indicates ∼23-34 km of top-to-the-north shear distributed through the GH-TH section. These data indicate that the magnitude of channel flow in central Bhutan is only ∼12-15% of the total amount of mass added to the system through shortening (359 km minimum).;Balanced cross-sections depict the geometry of deformation, and illustrate two LH duplex systems and structurally-lower and higher GH sections. Cross-section retro-deformation indicates at least 344-405 km of crustal shortening (70-75%). Shortening estimates across the orogen broadly mimic the arc-normal width of the Tibetan Plateau, indicating the plateau borders likely correspond with the leading edge of subducted Indian lower crust at depth.;The LH zone displays an inverted deformation temperature gradient, attributed primarily to stacking of foreland thrust sheets. Frontal thrust sheets exhibit layer-parallel shortening (LPS) strain, and all other thrust sheets exhibit layer-normal flattening (LNF) strain. We propose that LPS strain developed foreland-ward of the deformation front, and that LNF strain resulted from tectonic loading, and preceded thrust imbrication.