In order to constrain models for Himalayan evolution three contemporaneous Neogene molasse deposits of the Siwalik Group were sampled at 1 Myr intervals for ⁴⁰Ar-³⁹Ar dating of single white micas, petrography/dense mineral analysis, and whole-rock Nd-Sr geochemistry. The sedimentary successions are exposed in three road and river sections through the Sub-Himalayan Zone of central-southern Nepal (from east to west: Tinau Khola section (12-6.3 Ma), Surai Khola section (12-1 Ma), and Karnali section (16-5 Ma)). From bottom to top the Siwaliks consist of mudstones, sandstones and conglomerates, representing distal to proximal deposition within a fluvial mega-fan environment. Laser fusion ⁴⁰Ar-³⁹Ar ages from 1415 single detrital white micas show a peak of ages between 20-15 Ma with a mode of 16.8 Ma, corresponding to the period of most extensive exhumation of the Greater Himalaya. The shortest lag-time (host sediment depositional age minus mineral cooling age) occurs in sediment deposited at 16 Ma, reflecting the 16.8 Ma mode. Lag-times of less than 5 Myr persist until 10 Ma, indicating Greater Himalayan exhumation rates of up to 2.6 mm/yr using 1 D thermal modelling. There are few micas younger than 12 Ma, no lag-times of less than 5 Myr after 10 Ma, and a significant provenance change at 12 Ma indicating erosion from the Lesser Himalaya at this time (from petrography and geochemistry data). These changes suggest that a switch in the dynamics of the orogen took place during the 12-10 Ma period whereby most strain began to be accommodated by structures within the Lesser Himalaya rather than the Main Central Thrust-South Tibetan Detachment system. The increased sediment flux from the new area of uplift above the Lesser Himalayan Duplex resulted in a rapid increase in sedimentation rates across the foreland basin and the Bay of Bengal between 11-9 Ma.