Stretching over 2,000 km along the crest of the Himalaya, the South Tibetan detachment system (STDS) features a series of low-angle, north dipping, extensional faults and shear zones that played a major role in the Cenozoic tectonic evolution of the Himalayan-Tibetan orogenic system (Burchfiel et al., 1992;Hodges, 2000;Kellett et al., 2018). At some fundamental levels, the STDS marks a late Oligocene-middle Miocene decoupling horizon between the high-temperature metamorphic infrastructure of the orogen below and a more weakly metamorphosed to an unmetamorphosed sedimentary superstructure above (Hodges, 2016). Its trace also corresponds to an important strain transition in the orogen. To the south, Oligocene-Recent thrust and fold structures accommodate ∼N-S shortening. To the north, structures of the same age are predominately normal faults that accommodate E-W extension. In order to know whether this spatial correlation is just a coincidence or has a deeper tectonic significance, we must understand better the age of STDS deformation relative to the age of contractional and extensional deformation to the south and north, respectively. New geochronologic and thermochronologic data reported here provide important constraints on the age of the principal, basal structures of the STDS in an area of the Himalaya where the south-to-north strain transition is especially abrupt and well exposed: the Kali Gandaki valley of central Nepal (Figure 1). Regional GeologyThe bedrock geology of the region of Nepal that includes the 8,000-m peaks Manaslu, Annapurna, and Dhaulagiri has been studied at various levels of detail by many researchers since the foundational work of French geologists in the 1970s (Bordet et al., 1971;Le Fort, 1975). One of the most dramatic geologic transects through the Himalaya is provided by the trans-Himalayan Kali Gandaki river, which carves one of the deepest valleys on Earth between the Annapurna and Dhaulagiri massifs. North of these massifs, the river gradient is low as it flows along the ∼NNE-SSW axis of the Cenozoic Thakkhola graben (

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