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

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概要

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• A molasse sequence comprising 1.5-km-thick sediments of the Lower and Middle SiwaliksGroup in the sub-Himalayan North Belt along the Tinau Khola River in west-central Nepalwas studied for the low-field magnetic susceptibility and anisotropy using single core specimensof siltstone/sandstone from 127 stratigraphic levels. The magnetic fabric comprisesprimary 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 m3 kg−1) contributed mainly byparamagnetic and diamagnetic minerals and (iii) WNW–ESE magnetic lineations subparallelto the fold axes/bedding strikes/thrust front, and hence normal to the direction of palaeotectoniccompression.Thermal demagnetization of single core specimens from 103 levels across 1120 m of thelower part revealed a characteristic remanence of the high unblocking temperature (> 600 ◦C)in hematite. Remanence ratios derived from demagnetization data allowed the first-order estimationof remanence contributions from magnetic minerals (goethite, maghemite, magnetiteand hematite), and discrimination of rockmagnetic zones correlatable with distinct lithofacies,which will facilitate objective mapping. We correlated a magnetic polarity sequence, constructedfrom normal and reverse polarity directions from 77 levels that passed the reversal testand represented primary remanences, with the standard geomagnetic polarity timescale to constrainthe 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 accumulationrate 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 withthe value of 32–50 cm kyr−1 reported from Siwaik sections in Nepal. The notable increasein accumulation rate after 12.1 Ma probably reflects the peak of the earlier phase of upliftand/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 remanencefrom the latest APWP for the Indian Plate 10–13 Ma, the best-defined mean (351.8◦/20.9◦) isrotated anticlockwise by 9.2◦ and records an inclination shallowing of 25◦. Constraining thebase of the Tinau Khola north section to 13.2 Ma (i.e. older than the Tinau Khola south by1.7Myr) should open up new horizons for multidisciplinary and multiproxy research targetinggeotectonic/climatic/environmental palaeoreconsructions of Himalaya-wide events.

著者

• Gautam Pitambar

  北海道大学理学部地質学鉱物学教室

• Gautam P

  Hokkaido Univ. Sapporo Jpn

• Gautam Pitambar

  Coe For Neo-science Of Natural History Graduate School Of Science Hokkaido University

• Gautam Pitambar

  Hokkaido Univ.

• Gautam Pitambar

  Division Of Earth And Planetary Sciences Graduate School Of Science Hokkaido University

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