Was the Himalayan crystalline core emplaced by extrusion or underplating?
Abstract
Models of Himalayan mountain-building are generally dominated by one of two end-member processes: extrusion and underplating. Extrusion requires exhumation of mid-crustal material between surface-breaching faults: a thrust fault below and a normal fault above. Underplating involves mid-crustal accretion of material from underthrusting India to the over-riding Himalayan orogen. There is a key structure in the Himalaya for differentiating the models - the South Tibet detachment (STD): is it a normal fault (consistent with extrusion) or a backthrust (consistent with underplating)? We test the underplating model by examining two regions in the Nepal Himalaya where the STD is proposed to merge with the fault system bounding the base of the crystalline core, i.e., the Main Central thrust (MCT). These study regions are along the northern margins of the Dadeldhura klippe and Kathmandu nappe. Field mapping, microstructural, quartz c-axis fabric, and geochronological studies were integrated to analyze transects across these two regions. Our structural mapping reveals top-north shear zones along the north margin of the Kathmandu Nappe (the Galchi shear zone) and along the Tila river (the Tila shear zone) on the northeast margin of the Dadeldhura klippe. Gneisses between these shear zones and MCT pinch out to the south. Asymmetric quartz c-axis fabrics indicate consistent top-north shearing in both shear zones. Quartz microstructures and opening angles of quartz c-axis fabrics indicate deformation temperatures of ~650 -350 °C from the base to the immediate hanging wall of the Tila shear zone. Zircon U-Pb dating of rims for crosscutting leucogranites yields age clusters ~30-17 Ma for the Tila shear zone and ~35-14 Ma for the Galchi shear zone. These ages constrain the motion of the two shear zones as Early to Middle Miocene. All characteristics of the two shear zones including the sense of shear, structural position, thermal profile, and timing of motion are consistent with those associated with the STD. Thus we interpret these two shear zones as southern occurrences of the STD and the gneisses as the crystalline core. The pinch out of the gneisses between the two shear zones and MCT indicates that a MCT-STD branch line bounds the leading edge of the crystalline core. We interpret the STD as a sub-horizontal fault on the basis of its multiple exposures that extend for >200 km from the frontal klippen in the south, to major traces along the range crest in the north, even further north to the Northern Himalayan gneiss domes. The MCT-STD branch line precludes extrusion models such as wedge extrusion and channel flow-focused denudation, but supports underplating models such as tectonic wedging. The presence of the MCT-STD branch line requires that the STD is a backthrust. Sparse work suggests that the crystalline core may have been constructed via underplating and duplexing of multiple mid-crustal horses. These findings are synthesized in a reconstruction showing that the Himalayan orogen has built up via underplating processes.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.T53F..07H
- Keywords:
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- 8011 STRUCTURAL GEOLOGY / Kinematics of crustal and mantle deformation;
- 8107 TECTONOPHYSICS / Continental neotectonics;
- 8108 TECTONOPHYSICS / Continental tectonics: compressional;
- 8159 TECTONOPHYSICS / Rheology: crust and lithosphere