Magnetostratigraphy of the Neogene Siwalik Group in the far eastern Himalaya: Kameng section, Arunachal Pradesh, India

Abstract : The Siwalik Group was deposited from the Mid-Miocene to the Pliocene in the foreland of the Himalaya and records the unroofing history of the mountain belt. In this study we provide the first magnetostratigraphic data for the eastern Himalayan foreland basin. We analyzed two sections of the lower to upper Siwalik Group along the Kameng River in Arunachal Pradesh, India. Magnetostratigraphic data were acquired from 395 sites within a 5.8 km-thick molassic series. Thermal demagnetization and magnetic rock-property analyses indicate a relatively low temperature (150-340 °C) Characteristic Remanent Magnetization (ChRM) yielding reliable primary directions carried by iron sulfides such as greigite. The results show local counter-clockwise rotation of the thrust sheets, which is consistent with partitioning of arc-normal and left-lateral strain along the Himalaya. Nineteen polarity zones have been identified in the two sections. Detrital fission-track data from the top and bottom of the sections are used to constrain correlations with the geomagnetic polarity time scale (GPTS) from chron C5Ar.1n to chron C2An.2n. From these results, we propose that the Siwalik Formation in Arunachal Pradesh was deposited between 13 and 2.5 Ma. The transition between the lower and middle Siwaliks is dated at about 10.5 Ma and the middle to upper Siwaliks transition is dated at 2.6 Ma. These results, coupled with sedimentological observations, suggest that the eastern Himalayan chrono-stratigraphic record is nearly synchronous with that analyzed in other parts of the Neogene Himalayan foreland basin. Nevertheless, some differences in the evolution of the sedimentation rate and in the thickness of the middle Siwaliks suggest that Indian plate flexure in the East is different from that observed in other parts of the Indian foreland basin, and could be affected by the presence of the Shillong Plateau to the south.