Magnetostratigraphic dating of the prime‐time sedimentary record of Himalayan tectonics and climate: new age constraints (13–10 Ma) from the Siwaliks of the Tinau Khola north section, Nepal

SUMMARY A molasse sequence comprising 1.5‐km‐thick sediments of the Lower and Middle Siwaliks Group in the sub‐Himalayan North Belt along the Tinau Khola River in west‐central Nepal was studied for the low‐field magnetic susceptibility and anisotropy using single core specimens of siltstone/sandstone from 127 stratigraphic levels. The magnetic fabric comprises primary sedimentary‐compactional and secondary tectonic components. It is characterized by (i) predominantly oblate magnetic susceptibility anisotropy ellipsoids, (ii) low anisotropy ( P ′ mostly <1.1) and low magnetic susceptibility (< ca. 10 −7 m 3 kg −1 ) contributed mainly by paramagnetic and diamagnetic minerals and (iii) WNW–ESE magnetic lineations subparallel to the fold axes/bedding strikes/thrust front, and hence normal to the direction of palaeotectonic compression. Thermal demagnetization of single core specimens from 103 levels across 1120 m of the lower part revealed a characteristic remanence of the high unblocking temperature (> 600 °C) in hematite. Remanence ratios derived from demagnetization data allowed the first‐order estimation of remanence contributions from magnetic minerals (goethite, maghemite, magnetite and hematite), and discrimination of rockmagnetic zones correlatable with distinct lithofacies, which will facilitate objective mapping. We correlated a magnetic polarity sequence, constructed from normal and reverse polarity directions from 77 levels that passed the reversal test and represented primary remanences, with the standard geomagnetic polarity timescale to constrain the depositional age between ca. 13.2 Ma (base of Chron C5AAN, 13.015–13.183 Ma) and the middle of Chron C5n.2n (9.987–11.040 Ma). We calculated the sediment accumulation rate for polarity zones from the chronologically better constrained part below Chron C5n.2n (i.e. below 11.040 Ma) to be 25–61 cm kyr −1 (average, 39 cm kyr −1 ), which is consistent with the value of 32–50 cm kyr −1 reported from Siwaik sections in Nepal. The notable increase in accumulation rate after 12.1 Ma probably reflects the peak of the earlier phase of uplift and/or unroofing of the Himalayan source region followed by rapid accumulation in the foredeep, and a link to monsoon initiation/intensification. Compared to the expected remanence from the latest APWP for the Indian Plate 10–13 Ma, the best‐defined mean (351.8°/20.9°) is rotated anticlockwise by 9.2° and records an inclination shallowing of 25°. Constraining the base of the Tinau Khola north section to 13.2 Ma (i.e. older than the Tinau Khola south by 1.7 Myr) should open up new horizons for multidisciplinary and multiproxy research targeting geotectonic/climatic/environmental palaeoreconsructions of Himalaya‐wide events.