Thermokinematic evolution of the Annapurna-Dhaulagiri Himalaya, central Nepal: The composite orogenic system

The Himalayan orogen represents a ‘‘Composite Orogenic System’’ in which channel flow, wedge extrusion, and thrust stacking operate in separate ‘‘Orogenic Domains’’ with distinct rheologies and crustal positions. We analyze 104 samples from the metamorphic core (Greater Himalayan Sequence, GHS) and bounding units of the Annapurna-Dhaulagiri Himalaya, central Nepal. Optical microscopy and electron backscatter diffraction (EBSD) analyses provide a record of deformation microstructures and an indication of active crystal slip systems, strain geometries, and deformation temperatures. These data, combined with existing thermobarometry and geochronology data are used to construct detailed deformation temperature profiles for the GHS. The profiles define a three-stage thermokinematic evolution from midcrustal channel flow (Stage 1, >7008C to 550–6508C), to rigid wedge extrusion (Stage 2, 400–6008C) and duplexing (Stage 3, <280–4008C). These tectonic processes are not mutually exclusive, but are confined to separate rheologically distinct Orogenic Domains that form the modular components of a Composite Orogenic System. These Orogenic Domains may be active at the same time at different depths/positions within the orogen. The thermokinematic evolution of the Annapurna-Dhaulagiri Himalaya describes the migration of the GHS through these Orogenic Domains and reflects the spatial and temporal variability in rheological boundary conditions that govern orogenic systems

Tracking the Growth of the Himalayan Fold‐and‐Thrust Belt From Lower Miocene Foreland Basin Strata: Dumri Formation, Western Nepal

New data from the lower Miocene Dumri Formation of western Nepal document exhumation of the Himalayan fold-thrust belt and provenance of the Neogene foreland basin system. We employ U-Pb zircon, Th-Pb monazite, Ar-40/Ar-39 white mica, and zircon fission track chronometers to detrital minerals to constrain provenance, timing, and rate of exhumation of Himalayan source regions. Clusters of Proterozoic-early Paleozoic (900-400 Ma) Th-Pb monazite and Ar-40/Ar-39 white mica detrital ages provide evidence for erosion of a Greater Himalayan sequence protolith unaffected by high-grade Eohimalayan metamorphism. A small population of similar to 40 Ma cooling ages in detrital white mica grains shows exhumation of low-grade metamorphic Tethyan Himalayan sequence through the similar to 350 degrees C closure temperature along the Tethyan Frontal thrust (proto-South Tibetan detachment) during the late Eocene. Dumri Formation detritus shows a similar to 12 Myr time difference between cooling of its source rocks through the similar to 350 and similar to 240 degrees C closure temperatures as recorded by similar to 40-38 Ma youngest peak cooling ages in Ar-40/Ar-39 detrital white mica and similar to 28-24 Ma youngest populations in detrital zircon fission track. Exhumation between circa 40 and 28 Ma is consistent with slip and exhumation along the Main Central Thrust. Combined with similar data from northwestern India, our study suggests west-to-east spatially variable exhumation rates along strike of the Main Central Thrust. Our data also show an increase in exhumation during middle Miocene-Pliocene time, which is consistent with growth of the Lesser Himalaya duplex.U.S. National Science Foundation (NSF) [Tectonics-EAR-1140068]; Conoco Fellowship scholarship (2016); Chevron Texaco Geology Fellowship (2016); University of Arizona Galileo Circle Scholarship (2017); U.S. National Science Foundation (NSF) [NSF EAR-1338583, EAR1649254]6 month embargo; published online: 29 August 2019This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]