Detrital-zircon fission-track (FT) ages from the Lower Cenozoic Sub-Himalayan foreland basin reflect the progressive effects of crustal thickening and exhumation on the Himalayan source rocks as a consequence of the India-Asia collision. The oldest stratum, the transgressive marine Paleocene-Eocene Subathu Formation (57–41.5 Ma) contains ca. 50 Ma detrital-zircon P1 peak, which was derived from the Indus Tsangpo Suture Zone and the Ladakh Batholith of the Asian plate. A dominant 302.4 ± 21.9 Ma peak with a few 520 Ma grains in this formation has been derived by erosion of the zircon partial-annealing zone (ZPAZ) of 240–180 °C. As the first imprint of the collision, this zone affected the Himalayan Proterozoic basement and its Tethyan sedimentary cover.
Since the detritus in the Subathu has been derived both from the Indian and Asian plates, the possible suturing of these plates took place during the Subathu sedimentation. A sudden change in the provenance is recorded in the detrital-zircon FT cooling ages in the Oligo-Miocene Dagshai and Kasauli Formations, which have dominant 30 and 25 Ma P1 peaks, respectively. We interpret a distinct unconformity spanning ~10 m.y. between the Subathu and Dagshai Formations. Since ca. 30 Ma, molassic sedimentation coincides with shifting of the source rocks to the Himalayan metamorphic belt. This belt has sequentially undergone three distinct cooling and exhumation pulses after the ultrahigh-pressure–high-pressure (UHP-HP) metamorphism (53–50 Ma) in the extreme north and two widespread M1 and M2 metamorphisms (40–30 and 25–15 Ma) in the middle parts. These events appear to be largely responsible for the deposition of the ca. 30 Ma zircon Himalayan peak and ca. 25 and 15 Ma young Himalayan peaks, respectively; the latter appears within the Lower Siwalik Subgroup (13–11 Ma). During the Lower Siwalik deposition, pre-Himalayan peaks gradually decrease with the intensification of the Himalayan events in source rocks. In spite of uninterrupted fluvial sedimentation in the Dagshai-Kasauli–Lower Siwalik sequences since 30 Ma, breaks of ~5–7 m.y. in the zircon FT ages reveal pulsative cooling and exhumation in the well-identified source areas. Although cooling and exhumation of the Himalayan source rocks remained almost uniform during the Eocene, source heterogeneity is reflected in fluvial sedimentation since 37 Ma from Pakistan to Nepal in response to the India-Asia collision.