The tectonics of the sub-Himalayan fault zone in the northern Potwar region and in the Kangra district of the Punjab

Abstract

Summary The paper defines three major zones of deformation across the Nimadric (Tertiary freshwater) geosyncline between the foreland and the autochthonous folded belt in the North-West Himalaya:— (1) A zone of open folding, affected to a varying degree by fold-faulting. (2) A fault zone, displaying a number of reversed faults of steep northerly hade and great lateral extent with open folds between them in the west and, in places, gently dipping monoclines in the east. (3) A zone with closely spaced strike-faults and severely compressed folds, referred to in the Potwar area as the isoclinal zone, terminating at the main boundary fault which forms the general northern limit of Nimadric rocks. This succession of structural zones has a superficial resemblance to progressive deformation of the Jura type towards the nappe zone, and previous workers have regarded the tectonic pattern as reflecting tangential pressures from the north. The author seeks to demonstrate that such an interpretation is denied by the evidence of the structures themselves. Two areas are chosen to illustrate in adequate detail the tectonics of all three zones, and in particular the crucial fault zone. In the west, the Jhamat-Khushalgar area of western Potwar typifies the fault zone of the area between the Indus and the Jhelum rivers. In this area there is clear evidence of the steep hade of the great strike-faults at the surface, and also important evidence of their continued steep hade in depth from the results of boreholes drilled by the Attock Oil Company. Analysis of the relationship of the faults to the fold structures leads to the conclusion that the faults have no genetic relationship to the fold axes. They are considered to be fractures due to shearing stresses principally with a vertical orientation, whilst the folding reflects the subsidiary compressive force. The Nadaun Dun area of Kangra district is some 200 miles to the southeast of the Potwar area, but lies across similar structural zones and typifies the relationships of the anticlinal zone and the fault zone in the region east of the Jhelum River. In this area the Gumber fault is seen to present all the features of the great strike-faults of the fault zone in the Potwar and is an upthrust structure produced by quasi-vertical movement. A feature not present in the Potwar is the marked contrast between the gently dipping monocline to the north of the fault and the strongly folded country to the south, providing a clearer distinction between the influence of tangential and vertical forces. It appears that the fault movement provided a block-line against which tangential forces from the south were directed. Important evidence of tangential pressure from the south is provided by the Barsa fault, which typifies the most common form of fold-fault in the anticlinal zone throughout the region. Gravity collapse structures are recorded in the form of severely overturned “ flaps ” of sandstone on the south side of the Sola Singhi ridge.