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

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.