Detrital Zircon U–Pb Age and Petrography of the Neogene Sedimentary Rocks along the Northern Margin of the Indian Plate: Implication of India–Burma Collision

Abstract

The northward drift of the Indian Plate from Gondwana and its subsequent collision with the Eurasian Plate led to the exhumation of rock sequences and the formation of the Himalayas. However, the timing, provenance, and exhumation history of Neogene−Pleistocene sediments in the easternmost Himalayan segment, particularly the Upper Assam Basin (UAB), located between the Himalayan and Naga orogenic belts, remain poorly constrained. To address this gap, this study integrates petrography, detailed field observations, detrital zircon U–Pb geochronology, and Hf isotopic analysis to constrain sediment sources, exhumation, erosion, and depositional processes in the UAB. The Upper Assam stratigraphic sequence lies unconformably above the granitic basement, followed by different layers of sandstones, shales, and mudstones, and the Neogene−Pleistocene rock sequences. The Tipam Sandstones and Girujan Clays contain detrital zircon grains younger than 100 Ma, suggesting input from trans-Himalayan batholiths, Tethys, and Greater Himalayan units. The overlying Pleistocene Dihing Group lacks such young grains and exhibits consistently negative εHf(t) values (down to −57.31), reflecting a shift to older, more proximal sources. This provenance shift is temporally consistent with the late Miocene–early Pliocene (~7 to 5 Ma) reorganization of the Paleo-Brahmaputra River system associated with uplift of the Shillong Plateau, which likely disrupted sediment supply from the Himalaya. Similarly, the presence of Indo-Burma Range (IBR) sediments within this Miocene basin supports the notion that the IBR was upraised due to the continuous collision between the Indian Plate and West Burma Terrane. Post-collisional compression led to the propagation of numerous thrust/fault systems within the IBR, which increased the flexural load on the Burma Plate. All of this evidence suggests that the India−Burma interaction was initiated by the early Eocene, whereas the exhumation of the Indo-Burma Range continued from the Miocene to the present.