EXHUMATION OF THE UPPER MUSTANG MASSIF, THAKKHOLA GRABEN, CENTRAL NEPAL HIMALAYA

The Thakkhola graben intersects the South Tibetan fault (STF) system and bounds the Upper Mustang massif (UMM), one of the North Himalayan gneiss domes. The UMM comprises a half-domical mountain range cored by amphibolite-facies metasedimentary rocks and an igneous complex including two plutons, the Mugu and Mustang granites. The eastern margin of the UMM is defined by the main bounding structure of the Thakkhola graben – the Dangardzong fault – so it is clear that development of the graben has greatly affected the exhumational history of the UMM. We report structural and geochronologic data from transects across the Dangardzong fault into the southeastern part of the UMM. Several generations of granitic magamtism and five generations of tectonite fabrics record the progressive structural evolution of the UMM from a metamorphic core complex setting to a rift setting. U-Pb analyses yield the first constraint on the age of the Mustang granite (23.35 ± 0.17 Ma), provide a new age constraint for the Mugu intrusive complex (20.76 ± 0.67 Ma), and demonstrate that Mugu magmatism was protracted, spanning a period of at least 3 Myr. U-Pb and 40Ar/39Ar cooling dates also place constraints on the deformational history, indicating that ductile east-west extension in the region began between ca. 23.4 Ma and ca. 20.8 Ma and that top-to-the east brittle-ductile normal faulting was ongoing by ca. 18.3-17.5 Ma. Thermochronologic data show that rapid cooling and denudation of the UMM in the Dangardzong fault footwall occurred between ca. 18.3-17.5 Ma and ca. 15.5-12 Ma. We infer that this period of rapid tectonic denudation reflects the transition from a core complex mode to a rift mode of extension in the northern Thakkhola graben by ca. 18.3-17.5 Ma. We propose a kinematic model for the evolution of upper Mustang which ties Thakkhola graben extension to the structural evolution of the UMM and activity on the STF system. Our model predicts that the material being exhumed in the UMM originates from below the STF system and that the low angle detachment active during the core complex phase was an up-warped portion of the STF basal detachment.