Paleoclimate and vegetation records from the Himalayan foreland basin during the Cenozoic Himalayan exhumation

Abstract The Siwalik Group is a key laboratory for the study of the Neogene environment. The lower unit of the Siwalik Group was deposited in a meandering river system (between ~ 16 and 10 Ma) that changed to braided during the deposition of the Middle Siwalik (at ~ 10 Ma). The vegetation shifted from C3 to C4 in the central Himalaya, Nepal at ~ 7.4 Ma (Surai Khola) and ~ 6.6–5.9 Ma (Bakiya Khola), in contrast to that in the western Himalaya at ~ 8.8–8.6 Ma (Potwar Basin, Pakistan). Additionally, the Indian Summer Monsoon (ISM) was active in the central Himalaya as early as ~ 10.7 Ma, remaining steady until 9.5 Ma, and then decreased by 7.5 Ma, i.e., 0.5 million years later than in the western Himalaya. Not only the uplift of the Himalaya due to the activation of the Main Boundary Thrust, but the rise of the proto-Tibetan Plateau could be a possible reason behind the intensification of the Asian Summer Monsoon (ASM) at ~ 10 Ma. There was an enrichment of δ18O, and a decrease in thickness of the leaching zones, which is most likely associated with the intensification of the monsoon. The rate of weathering and erosion was also higher during 10–8 Ma, coinciding with the rapid sedimentation rate in the Ganga Basin at ~ 10 Ma. Furthermore, geochronological studies indicate that the Tethyan Himalayan and Greater Himalayan sediments were a prominent source of sediments till ~ 10 Ma, while subsequent detritus was sourced from the Lesser Himalaya. However, in the eastern Himalaya, the presence of dominant numbers of the Cretaceous and younger detritus highlights the uneven pattern of Himalayan exhumation. The condition and formation of paleosols in Lower Siwalik in the central Himalaya indicate a decrease in ASM. The present study compares and summarizes the climatic conditions in the western and eastern Himalaya during the Neogene.