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UBC Theses and Dissertations

Cooling and exhumation of the Himalayan mid-crust, Khimti-Tamakoshi-Sindhuligadhi section, east central Nepal K C, Kumar


This thesis presents a new ⁴⁰Ar*/³⁹Ar thermochronologic dataset that constrains the cooling history of the exhumed, former mid-crust exposed in the Khimti-Tamakoshi-Sindhuligadhi section of east-central Nepal. These results are incorporated with existing geological and geochronological data to prepare a revised, internally consistent kinematic model of the geologic evolution of the study area. Except for those from the highest structural levels, biotite examined as part of this study are contaminated with excess argon and therefore yield anomalously old, geologically meaningless ages. Muscovite cooling ages, however, are more robust, and constrain the timing of exhumation of the leading edge of the Himalayan Metamorphic Core to the late Oligocene/early Miocene in advance of deeper-seated material in the orogenic hinterland. Most of the muscovite ages from rocks in the lower part of the Himalayan Metamorphic Core are partially reset, indicating that these rocks experienced only limited burial and heating prior to exhumation. The structural position and cooling ages of these partially reset rocks indicate their incorporation into the thrust system through underplating and subsequent exhumation facilitated by out-of-sequence thrusting. Cooling ages obtained from the more hinterland, structurally higher portion of the Himalayan Metamorphic Core are consistent with it evolving as a result of movement along multiple late-stage, thrust-sense structures. These faults, which facilitated cooling as young as late Miocene, occur as discrete out-of-sequence thrust sheets within the Himalayan Metamorphic Core. These out-of-sequence structures, which have not been accounted for in total shortening estimates across the Himalaya, accommodated significant convergence and helped modify the shape/geometries of the original midcrustal structures. This indicates a need to revisit existing models of the orogen that integrate the current orientation of the major structures as representative of past geometries.

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