Late Cambrian deformation in the Lesser Himalaya

摘要

The Tons Valley, situated in the central-easternmost part of the Himachal Lesser Himalaya, adjoining the Garhwal Himalaya, shows geological features suggestive of a strong pre-Tertiary deformational episode. The Paleoproterozoic Dharagad Group, overlain by the Mesoproterozoic Deoban and Neoproterozoic Simla groups rest as a thrust sheet over the Middle Cambrian Chilar Formation, which occurs as windows and also as tectonic slivers within the thrust sheet designated as the Dharagad Thrust Sheet (DTS). The mineral lineation, inclination of tectonic slivers and overturned beds suggest that the DTS was translated from the NE. The westernmost and southwesternmost leading edges of the DTS are exposed at Subathu and Morni WNW and WSW respectively of the Tons Valley. The position of the leading edges of the DTS vis-a-vis the windows in the Tons Valley suggest a minimum translation of about 50 km for the DTS. The Simla Group at Subathu and the Deoban at Morni, forming parts of the DTS, constitute basement for the Thanetian-Lutetian Subathu Formation of the Himalayan Foreland Basin (HFB). This stratigraphic relationship unambiguously demonstrates that the Simla and the Deoban Groups, forming leading edges of the allochthonous DTS, were already translated and emplaced at Subathu and Morni before the creation of the HFB in which the deposition commenced with the Subathu Formation in Thanetian. It implies that the DTS was translated from the NE to the present position at Subathu and Morni in pre-Thanetian time. There is no direct evidence to constrain the age of the thrusting.In view of regional regression in Late Cambrian, a distinct angular unconformity between the Cambrian and the overlying Ordovician, Early Paleozoic metamorphism and extensive development of Early Paleozoic granites and their rapid exhumation, a Late Cambrian age is suggested for the DTS thrusting. Not only the direction of movement of the DTS is same as that of the Tertiary thrust sheets but also Cambrian folds are co-axial with the Tertiary folds. This strange coincidence shows that similar kinematic field existed during two tectonic events. A ridge, like the present Central Crystalline Axis, was elevated between the Tethyan and Lesser Himalayan basins, which contributed zircons of the Early Cambrian age to both basins.