Depositional chronology and fabric of Siwalik group sediments in Central Nepal from magnetostratigraphy and magnetic anisotropy

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概要

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• Magnetostratigraphic research, undertaken within the past 15 years in the Siwaliks distributed along 400 km of the Sub-Himalaya in central Nepal, has proved that the sediments possess highly reliable hematite-based primary detrital remanent magnetization suitable to determine depositional chronology. In order to bring out the polarity sequences in a common chronological frame, all available data are newly correlated to the latest global magnetic polarity time scale of Cande and Kent (1995). Chronological data presented are referred in relation to the diverse lithological nomenclature to the formations whose ages are not constrained by isotopic or paleontologic ages. The age of the sections dated by magnetostratigraphy ranges between 14 and <2 Ma. Sediment accumulation rates average to 32-50 cm kyr^[-1]. Rock-magnetic parameters, e.g. initial susceptibility and isothermal remanent magnetization ratios, allow correlation with an accuracy of up to a few hundred meters among several kilometers thick adjacent sections. Anisotropy of magnetic susceptibility (AMS) data reveal a well-defined fabric contributed by paramagnetic (k = 10^[-5] to 3x10^[-4] SI) as well as ferromagnetic minerals (k = 3x10^[-4] to 10^[-2] SI). AMS ellipsoids are mainly oblate along with prolate ones and the degree of anisotropy is mostly low (P'<1.2). The magnetic fabric is of pre-folding origin with tilt-corrected sub-vertical magnetic foliation poles. The magnetic lineations do not show parallelism to the expected paleocurrent directions. Rather, sub-parallelism between the clusters of magnetic lineation and the fold axes/bedding strikes/thrust fronts is observed. A superimposed fabric consisting of a sedimentary-compactional and an overprint induced by mild deformation process is suggested. The latter process was active during and subsequent to the deposition in the compressive tectonic setting of the foreland basin. The magnetic lineations for Tinau Khola and Surai Khola sections cluster around N80°W and N88°W respectively, whereas N27°W trend characterizes the Amiliya-Tui area south of Dang. The peak clusters in lineations are probably orthogonal to the true shortening axes. Their variation along the Sub-Himalaya, together with the fold axes or thrust front trends, may be used for accurate tectonic reconstruction. It is especially important when orthogonality of the latter to the shortening axes may not hold true in the sectors with imbricate fold-and-thrust structures.

• Pergamonの論文

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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