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

Abstract

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 tiltcorrected 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.

Magnetic minerals and magnetic properties of the Siwalik Group sediments of the Karnali river section in Nepal

“…The magnetic fabric observed in the KR section has characteristics which are similar to those found for other Siwalik sections in central Nepal (Gautam and Rösler, 1999). It is interpreted to result from the combination of an initial sedimentary-compactional fabric and a later secondary tectonic fabric induced by a mild deformation process.…”

“…The AMS method is effective in characterizing such fabric recorded by the sediments of the Siwalik Group (Tauxe et al, 1990;Gautam and Rösler, 1999). It involved determination of the principal susceptibility directions (κ max ≥ κ int ≥ κ min ) as well as the quality and quantity of the fabric by calculating the shape factor (T ) and the corrected degree of anisotropy (P ), respectively, following Jelinek (1981):…”

“…These data are invaluable for regional chronological correlation throughout the Sub-Himalayan belt. In addition, studies on rock magnetic properties and magnetic fabrics have also been useful for our knowledge of magnetic carriers of remanences and tectonic deformation of the region (Tauxe et al, 1990;Gautam and Pant, 1996;Rösler and Appel, 1998;Gautam and Rösler, 1999). In spite of this research effort, the eastern and western sectors in Nepal remain sparsely studied and little is known about their chronology and tectonic development.…”

The record of climate and uplift in the palaeo-Ganga plain: A way to decipher the interactions between climate and tectonics

“…Our analysis mainly focuses on sections which have been accurately dated by magnetostratigraphy (Gautam and Roesler 1999): Karnali section, Surai section and Tinau section, respectively located in Western, Middle West and Central Nepal.…”

Detrital zircon U–Pb geochronology of the Siwalik Group of the Nepal Himalaya: implications for provenance analysis

“…In this regard, foreland basin sediments of the Siwalik Group have been extensively studied to reconstruct the paleogeography (Miall 1995;DeCelles and Giles 1996;Sinclair 1997;DeCelles et al 1998a), constrain the rate and timing of Himalayan exhumation (Dickinson 1985;DeCelles and Giles 1996;Mugnier et al 1999) and understand paleohydrology (Huyghe et al 2001(Huyghe et al , 2005Ulak 2005). Along the Nepal Himalaya, geochronological studies have established depositional age from 13.2 Ma spanning a period till 1 Ma (DeCelles et al 1998a, b;Ojha et al 2000;DeCelles et al 2001DeCelles et al , 2004Szulc et al 2006;Chirouze et al 2012), which corresponds to the age ranges yielded by paleomagnetic studies (Appel et al 1991;Harrison et al 1993;Gautam and Rösler 1999;Gautam et al 2012). Similarly, there are number of milestone works in the northwestern Himalaya that interlinked the erosional records of the foreland basin with tectonic history (Burbank et al 1996;Najman et al 2004;Jain et al 2009;Najman et al 2009).…”