Kinematic, Metamorphic, and Geochronologic Constraints on the Evolution of the Lhagoi Kangri Gneiss Dome, Southern Tibet: Insight into Mid-Crustal Processes During Himalayan Orogenesis

The north Himalayan gneiss domes are a series of isolated structures in southern-central Tibet that expose middle crust and record an early history of deformation, metamorphism, and partial melting. The domes are windows into the processes and physical conditions that promoted growth and uplift of the Himalaya during the early stage of collision (Eocene to Miocene) between India and Asia. Mechanisms responsible for creating the north Himalayan gneiss domes are crucial to understanding the early tectonic evolution of large orogens, such as the Himalaya, particularly with respect to crustal rheology and how middle crust is exhumed in collisional settings. Models for the formation and exhumation of these domes have been previously proposed and are based on both field evidence and thermomechanical modeling of the India-Asia collision. Each model has a set of predictable kinematic, metamorphic, and magmatic characteristics. This project focuses on a previously unstudied north Himalayan gneiss dome, Lhagoi Kangri, in order to (1) identify patterns of strain and metamorphism in the Himalayan middle to lower crust, (2) test previously developed models for north Himalayan gneiss domes, and (3) provide further constraints on the degree of commonality between the domes. Lhagoi Kangri is located ~100 km northeast of Mt. Everest. It comprises a cover of deformed, upper greenschist and amphibolite facies metasedimentary rocks and a core of orthogneiss and leucogranite. Results of this study indicate that rocks near the core-cover contact experienced Barrovian metamorphism c. 45–40 Ma followed by a prolonged period (≥11 My) of high temperatures (≥550 °C) during which pervasive ductile deformation resulted in the development of a distributed shear zone coeval with nearly isothermal decompression, the magnitude of which corresponds to ≥12 km of exhumation. The prolonged high temperatures, nearly isothermal decompression, and distributed ductile deformation demonstrate that there was structural continuity between the Lhagoi Kangri dome and other north Himalayan gneiss domes. Furthermore, these observations suggest that the middle crust was sufficiently weak to promote lateral flow that, in the Lhagoi Kangri dome, was accompanied by vertical attenuation

Timing of extension in the Pioneer metamorphic core complex with implications for the spatial-temporal pattern of Cenozoic extension and exhumation in the northern U.S. Cordillera

The Pioneer core complex (PCC) in central Idaho lies along a transition between Early Eocene and ca. 40 Ma core complexes to the north and south, respectively. Thus, the age of extensional development of the PCC is important in understanding the spatial-temporal patterns of core-complex development in the North American Cordillera. New results, including structural observations and U-Pb zircon (SHRIMP and ICPMS) geochronology, constrain the early extensional history of the footwall for the first time. High-temperature strain with a top-WNW shear-sense is pervasive throughout metamorphic rocks of the northwestern footwall. An isoclinally folded dike yields a crystallization age of ∼48-47 Ma, whereas a crosscutting dike yielded an age of 46 Ma. Metamorphic rocks are also intruded by the ∼50-48 Ma Pioneer intrusive suite (PIS), a W-dipping granodiorite sheet displaying a magmatic fabric. Northwest-trending lineations are locally visible and also defined by anisotropy of magnetic susceptibility, indicating that during emplacement, the PIS was undergoing similarly oriented extensional strain as the enclosing metamorphic rocks. Therefore, WNW-directed extension spanning this structural section occurred between ∼50 and 46 Ma. Following emplacement of crosscutting 46 Ma dikes, deformation was partitioned into the WNW-directed Wildhorse detachment. Motion on the detachment occurred between ∼38 and 33 Ma, as documented by previous 40Ar/ 39Ar thermochronology. It is not clear, however, whether extension was continuous through the interval between these two time periods. Although Early Eocene extension in the PCC was synchronous with extension in core complexes to the north, rates of footwall exhumation in central Idaho were much lower. This southward slowing is compatible with N-S differences in inferred subduction zone geometry/kinematics and in the internal character of the orogenic wedge