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UBC Theses and Dissertations

Cooling and exhumation in the footwall of the Dangardzong fault, Thakkhola graben, west central Nepal Brubacher, Alex


Late-stage E-W orogen-parallel extension is well documented in the Tibetan plateau, however, the nature and timing of extension related features remains unclear. In order to investigate the temporal development of the graben that accommodate this extension, ⁴⁰Ar/³⁹Ar and U-Th/He thermochronology were used to characterize the evolution of the Thakkhola graben in west-central Nepal, a major N-S striking, E-W extensional structure that cuts across the High Himalaya. The Mugu and Mustang granites occur in the footwall of the Dangardzong fault, which defines the western boundary of the graben. Monazite U-Th/Pb geochronology is used to constrain the minimum crystallization age of these granites to ~22-20 Ma. ⁴⁰Ar/³⁹Ar geochronology of micas date the cooling of the granites through calculated Ar migration closure temperatures (580-344 °C) to ~17 Ma. Zircon and apatite U-Th/He dating yield ages of ~15-9 Ma and ~12-4 Ma, respectively, corresponding to cooling of the granites through respective He migration closure temperatures of ~220-140 °C and ~116-44 °C. Microstructures in rocks collected in this region show that E-W extensional structures were contemporaneous with cooling through muscovite closure to Ar migration at ~17 Ma, providing a minimum age of initiation of extension. Detailed low-temperature cooling paths, based on U-Th/He data extracted from the undeformed intrusive bodies, record inflections in cooling rates at ~13-8 Ma and at ~5 Ma. These two inflections mark deceleration of cooling in the footwall, which is interpreted to reflect a reduction in the spreading rate of the graben. The slowdowns are coeval with increased graben development across the Tibetan plateau and as such are interpreted to reflect strain being partitioned into these newly generated structures. This study adds to a growing body of evidence for a major kinematic shift in the Himalaya occurring around ~13 Ma potentially linked with the flow of material eastward out of the Tibetan plateau.

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