Detrital thermochronology and sediment petrology of the middle Siwaliks along the Muksar Khola section in eastern Nepal

Sediment derived from erosion of the Himalaya during the Miocene–Pliocene was deposited in the Himalayan foreland basin and is now exposed in the Siwalik Formation between the Main Boundary Thrust and Main Frontal Thrust. These sediments hold important information on orogenic exhumation during this time. In this preliminary study we sampled the middle Siwalik Formation in eastern Nepal along the Muksar Khola section for thermochronologic and sediment petrologic analysis. Detrital zircon fission-track thermochronology shows that the age signal of the middle Siwalik Formation of eastern Nepal has some similarities but also differences with published data for western-central Nepal. The Muksar Khola section samples show two static peaks at about 14 and >140 Ma, which is similar to the Siwalik Formation of western-central Nepal. In contrast, a signal of consistently fast exhumation was not observed in the upper middle Siwaliks in eastern Nepal. This could be related to a position of the Main Central Thrust further south, to overall less erosional exhumation of Higher Himalayan rocks, or to a reduced exposure of the underlying Lesser Himalayan units in comparison with western-central Nepal. Based on our apatite fission-track data we propose that above the section dated by Ojha et al. (2009) with magnetostratigraphy about 1000 m of overlying upper Siwalik Formation exist.

Research highlights
► One fast moving peak in zircon fission-track data.
► Two static peaks in zircon fission-track data.
► Partial resetting of apatite fission-track ages in Siwalik sediments.
► Slow exhumation rate before 4 Ma.