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

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

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

Testing models that link climate and solid Earth tectonics in mountain belts 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 combine sediment geochemistry, 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 reflects a mixture of Tethyan and Greater Himalayan sources from 21 to 11 Ma, with influx from the Inner Lesser Himalaya starting after 11 Ma, and a strong increase in Crystalline Inner Lesser Himalayan erosion after 8 Ma. This distinct shift in provenance most likely reflects exhumation of the Kullu-Rampur Window, as well as the northward motion of the Jawalamukhi section towards the Himalayas, drainage reorganization in the foreland, and/or tectonically driven drainage capture in the mountains. Prior to 10.5 Ma sediment came from a large river whose sources were Greater Himalaya and Haimanta dominated, likely a paleo-Sutlej, while after 8 Ma the source river was dominated by a more local drainage. Our work is consistent with Nd isotope and mica Ar-Ar constraints from the same sections that demonstrate initial Inner Lesser Himalayan unroofing in this region from 11 Ma, earlier than the 2 Ma implied from the marine record and during a period of summer monsoon weakening when fission track data indicate very rapid cooling and erosion of the Lesser Himalaya sources from no later than10 Ma. Tectonically driven rock uplift coupled with southerly migration of the maximum rainfall belt during a time of drying, may have focused erosion over the Lesser Himalayan Duplex and created the Kullu-Rampur Window

Asian summer monsoon influence on chemical weathering and sediment provenance determined by clay mineral analysis from the Indus Submarine Canyon

© Copyright University of Washington. Published by Cambridge University Press, 2019. Clay minerals from the Indus Canyon and eastern clinoform since ∼12 ka are uniformly rich in smectite and illite, similar to those from the Holocene Indus flood plains. A systematic enrichment of smectite in the proximal delta compared to the canyon and eastern clinoform argues for preferential capture of smectite close to the river mouth since ∼12 ka. There is a rapid shift to a more smectite-rich assemblage in the canyon and eastern clinoform after ∼5 ka. This change is probably caused by a change in sediment source, with less direct flux from the Himalaya and more erosion of older, weathered, smectite-rich sediment from the Indus River flood plains, driven by incision of the Indus and its tributaries into the floodplain as summer monsoon rains weakened. This influx of smectite is consistent with lower kaolinite/smectite values since ∼5 ka. The onset of large-scale agricultural activities since ∼5 ka, especially starting with the Harappan Civilization, may also have enhanced incision and erosion of floodplain sediments over the same time period. This study reports for the first time how monsoon strength variations since ∼12 ka affected the clay mineral assemblages and sediment provenance in a major submarine canyon