⁴⁰ Ar/ ³⁹ Ar thermochronological constraints on the cooling and exhumation history of the South Tibetan Detachment System, Nyalam area, southern Tibet

The Nyalam detachment is part of the east-west striking South Tibetan Detachment System exposed in the Nyalam area, southern Tibet. Seventeen muscovite and biotite 40 Ar/ 39 Ar age spectra and three K-feldspar multidiffusion domain modelling and cooling ages are presented for metamorphic rocks, leucogranite, granite and mylonite, collected from the Nyalam detachment and surrounding areas. The majority of the 40 Ar/ 39 Ar results are cooling ages related to exhumation, which therefore place important constraints on formation of the Nyalam detachment and exhumation history of the region. Muscovite 40 Ar/ 39 Ar ages from mylonite within the normal fault system and from the footwall of the fault are 16.1–15.2 Ma. Biotite 40 Ar/ 39 Ar ages from the same samples are 15.6–14.8 Ma, slightly younger than the muscovite cooling ages. K-feldspar multidiffusion domain modelling suggests that samples collected from both mylonite on the fault surface and from footwall rocks underwent rapid cooling between 16.1 Ma and 11.7 Ma. Ages and cooling histories in the Nyalam detachment and Greater Himalayan metamorphic sequence have similar characteristics and time constraints: the K-feldspar modelling indicates a sudden change in cooling rates for these regions during 15.5–14.0 Ma and c. 12 Ma, respectively. Taking the regional thermal history into account, cooling could be associated with significant northward surface movement triggered by detachment normal faulting in the Nyalam area. The Nyalam detachment and Greater Himalayan metamorphic sequence experienced similar cooling and exhumation histories during c. 17.0–11.7 Ma. Formation of the Nyalam detachment may have accompanied the southward extrusion of the Greater Himalaya zone along shear zones formed in response to underthrusting of the Indian plate beneath southern Tibet.