Petrography of the Neogene Siwalik molasse (Arunachal Himalayan foreland basin, India): constraints on basin–mountain evolution and activation of the main boundary thrust

ABSTRACT We propose a new tectono-sedimentary evolutionary reconstruction of the Neogene Arunachal Siwalik Foreland Basin (ASFB), illustrating the interplay among episodic tectonism, sea-level fluctuation, and drainage reorganization in shaping the eastern Himalayan foreland basin. Based on integrated bulk-sandstone petrography, conglomerate-clast composition, paleocurrent pattern, and published data of detrital-zircon geochronology and depositional system, our findings reveal that the lowermost shallow marine Dafla Formation (∼ 10–7 Ma) was derived largely from erosion of the Tethyan and Greater Himalayan rocks via a south-flowing Himalayan drainage system. A southwest-flowing axial-stream system contributed a subordinate sediment input from the Trans-Himalayan arc into the Dafla Sea. The overlying Subansiri Formation (∼ 7 to > 3.5 Ma) was deposited by the west-southwest flowing axial paleo–Brahmaputra braided-river system via the rapidly exhuming Eastern Himalayan Syntaxis. The Subansiri Formation was derived predominantly from the Trans-Himalayan arc, Greater Himalayan rocks, and Abor volcanics. It was hitherto unknown that the overlying tidally influenced estuarine to fan deltaic Siji Formation (∼ 3.5 to < 2 Ma) was chiefly sourced from the Lesser Himalayan Gondwana rocks by the south-flowing Himalayan drainage. Also, there was a minor contribution from the Trans-Himalayan arc by the antecedent southwest-flowing axial drainage system. This reorganization in paleodrainage indicates exhumation of the Lesser Himalayan Sequence before the deposition of the Siji Formation and plausibly reflects activation of the Main Boundary Thrust around 5 to 4 Ma in the Arunachal Himalaya. This increased thrust loading at the basin margin, coupled with continued subduction of the Indian Plate, contributed to a greater rate of subsidence in the ASFB, creating increased accommodation space. This combined with eustatic sea-level rise led to marine transgression from ∼ 4 to 3.5 Ma. From 2 Ma onwards, southward progradation of the Lesser Himalayan gravel front and a fall in relative sea level led to the deposition of the alluvial-fan-dominated Kimin Formation.