Late Miocene Unroofing of the Inner Lesser Himalaya Recorded in the NW Himalaya Foreland Basin

Abstract

Testing models that link climate and solid Earth tectonics requires independent erosional, structural and climatic histories. Two well preserved stratigraphic sections of the Himalayan foreland basin are exposed in NW India. The Jawalamukhi (13–5 Ma) and Joginder Nagar sections (21–13 Ma) are dated by magnetostratigraphy and span a period of significant climate change and tectonic evolution. We use detrital zircon U-Pb dating and apatite fission track analyses to reconstruct changes in the patterns of erosion and exhumation in this area from the Early Miocene to Pliocene. The provenance of the foreland sediments shows that since at least ~21 Ma the deposits are mostly derived from the Greater Himalaya Series (GHS) and/or Tethyan Himalayan Sequence (THS) (Haimanta Group). Appreciable erosion from the Inner Lesser Himalaya (ILH) initiated around 11 Ma and accelerated progressively after 8 Ma. After 8 Ma the supply from the GHS was first cut off, followed by supply from the Haimanta Group at 6 Ma. While these sections were initially located in the floodplains of a basin axial river whose catchment was eroding both Lesser Himalayan (LH) rocks and GHS-THS sources, likely a paleo-Yamuna or Sutlej River, the Jawalamukhi section since evolved into being more LH- dominated. Input from the ILH starting at 11 Ma is younger than that reported along strike in Garhwal and Nepal but somewhat earlier than the inferred regional exposure derived from data from the Indus Fan. Our inferred timing of ILH unroofing is consistent with timing of movement on the frontal thrust, as well as with the onset of ILH duplexing. Tectonically driven rock and surface uplift coupled with southerly migration of the maximum rainfall belt during a time of drying in the Late Miocene focused erosion over the LH Duplex and created the Kullu-Rampur Window.