Tectonic evolution of the Himalaya constrained by detrital 40Ar–39Ar, Sm–Nd and petrographic data from the Siwalik foreland basin succession, SW Nepal

Abstract : 40Ar–39Ar dating of detrital white micas, petrography and heavy mineral analysis and whole-rock geochemistry has been applied to three time-equivalent sections through the Siwalik Group molasse in SW Nepal [Tinau Khola section (12–6 Ma), Surai Khola section (12–1 Ma) and Karnali section (16–5 Ma)]. 40Ar–39Ar ages from 1415 single detrital white micas show a peak of ages between 20 and 15 Ma for all the three sections, corresponding to the period of most extensive exhumation of the Greater Himalaya. Lag times of less than 5 Myr persist until 10 Ma, indicating Greater Himalayan exhumation rates of up to 2.6 mm year−1, using one-dimensional thermal modelling. There are few micas younger than 12 Ma, no lag times of less than 6 Myr after 10 Ma and whole-rock geochemistry and petrography show a significant provenance change at 12 Ma indicating erosion from the Lesser Himalaya at this time. These changes suggest a switch in the dynamics of the orogen that took place during the 12–10 Ma period whereby most strain began to be accommodated by structures within the Lesser Himalaya as opposed to the Greater Himalaya. Consistent data from all three Siwalik sections suggest a lateral continuity in tectonic evolution for the central Himalayas.