The remanent magnetization of siltstones and sandstones sampled at 476 levels/sites throughout a 3560 m thick molasse sequence belonging to the Siwalik Group (0-2015 m: Lower; 2015-3560 m: Middle) has been studied by stepwise thermal demagnetization. This section is exposed along the Karnali River in Nepal. The natural remanent magnetization (NRM) usually consists of two components: a viscous or thermoviscous component of recent field origin, and an ancient characteristic component (ChRM). The former component is of normal polarity and resides either in goethite (unblocking temperature < 150 ℃; resistant to AFD up to 150 mT) or in maghemite (unblocking temperature 150-400 ℃). Goethite contributes up to 90 per cent of the total intensity in the finer variegated muddy samples belonging to the lower half of the section. Maghemite content is significant in the grey mud-free lithologies from the upper half of the section. The main component, unblocked in the high-temperature range (commonly 610-680 ℃) and believed to reside in haematite, presumably of mostly detrital origin, represents a characteristic remanence (ChRM). The tilt-corrected ChRM directions at individual sites show antipodal clusters (ratio of normal- to reverse-polarity sites: 0.62), and yield mean inclinations recording significant inclination shallowing--a feature well recorded in the Siwaliks. This ChRM is interpreted to represent a largely primary detrital remanence. The ChRM data from 430 sites yield the Karnali River magnetic polarity sequence, whose correlation with the geomagnetic polarity timescale (Cande & Kent 1995) suggests a depositional age of 16 Ma (younger than chron C5Cn.1n) to 5.2 Ma (around the top of chron C3r) for the 3560 m section sampled. Hence, the Karnali River exposes the oldest part of the Siwalik Group in Nepal. Estimates of the sediment accumulation rate (SAR) average to 32.9 cm kyr^[-1] for the 10.8 Myr time span of deposition

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.

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S U M M A R Y Remanent magnetization of sandstones sampled at 127 levels/sites throughout a 1710m thick Siwaliks molasse sequence exposed along the Tinau Khola river in Nepal has been studied using thermal demagnetization. The magnetic remanence consists of: a secondary low-temperature component of normal polarity, unblocked mostly below 400-500 "C close to the present-day field, and a high-temperature characteristic remanence (ChRM) unblocked mostly between 600 and 685 "C, represented by both normal and reverse polarities. Demagnetization behaviour and isothermal remanence acquisition indicate that the secondary component resides on goethite and maghemite/magnetite whereas specular haematite carries ChRM. Anisotropy of magnetic susceptibility data reveals primary depositional magnetic fabric as judged by oblate ellipsoids and subvertical tilt-corrected minimum susceptibility axes.A magnetic polarity sequence established using tilt-corrected ChRM directions from 124 levels reveals more details not found in the polarity stratigraphy worked out by Munthe et af. (1983). Comparison of the sequence with a standard polarity time-scale (Harland er al. 1989) suggests a depositional time range between c. 5.9Ma (older than chron 3r) and 11Ma (younger than chron 5r-2) for the section considered. According to new data, the horizon of Sivapirhecus punjabicus falls close to the reversal boundary at c. 8.54Ma (the lower age limit of the normal polarity chron 4Ar-1). Hence, the hominoid should be regarded as of c. 8.5Ma rather than 9.0-9.5 Ma as suggested by Munthe et al. (1983).

The Kathmandu Valley is a bowl-shaped intermontane basin, which occupies an area of 583 km 2 in the heart of the Himalayas, with its floor at ca. 1400 m and the surrounding mountains attaining a height of 2000 -2800 m. It is inhabited by ca. 1.5 million people, concentrated mostly in three cities, Kathmandu, Patan and Bhaktapur. Due to rapid but uncontrolled urbanization and factors such as traffic movement, emissions from brick-kilns, cement factory, other industrial activities, waste disposal and biomass burning, environmental pollution has been constantly increasing adversely affecting land, water, air and biological systems.In order to quantify the degree of environmental pollution using magnetic methods, magnetic susceptibility of soils, sediments and roadside material, in and outside the Kathmandu urban areas, has been measured. In areas far from roads or industry, median susceptibility is (3 -35) x 10 -5 SI which is similar to that observed in the valley-filling clastic sediments being consistent with geologic or pedogenic origin. In traverses across roads, a 5-m wide zone situated at either sides of the asphalt-paved road exhibits a susceptibility enhancement zone with maximum susceptibility of 240 -850 x 10 -5 SI occurring 0.5 -2.5 m from the road edge. In urban recreational areas, it varies within a broad range (3 to >100 x 10 -5 SI) with the lowest values occurring about 50 m from any surrounding roads in areas least disturbed by human activity. A systematic increase in susceptibility towards the roads or industrial sites is observed. Within urban areas, in the 2 vicinity of heavy traffic or industrial sites, the upper 30 -50 cm of soil profiles exhibit frequent enhancement in susceptibility, of one or two orders of magnitude higher than those expected from geologic input. Such enhancements are attributed to input from anthropogenic or industrial sources. Magneto-mineralogical analyses and scanning electron microscopy on magnetic extracts, grain size fractions or bulk samples of road dust and soils suggest lithogenic magnetite-like minerals and anthropogenic magnetic spherules to be the dominant contributors to the susceptibility signal.As the soils, sediments and roadside material exhibit significant susceptibility contrasts, which are most effective in identifying traffic-related pollution "hotspots", it is highly desirable that the potential of susceptibility maps of the entire area affected by urbanization, be fully explored to assess the status of environmental degradation.