Lateral variation in geometry of thrust planes and its significance, as studied in the Shumar allochthon, Lesser Himalayas, eastern Bhutan

Abstract

The features mapped indicate a SSE-striking displacement vector for the Shumar thrust of eastern Bhutan. Sections parallel to this direction show that the frontal “flat” segment of the thrust plane, along with the subthrust and the hanging-wall rocks, has been folded into a synform. A longitudinal section, however, shows a cuspate-lobate shape (broad concave-upward curves meeting along sharp crests) of the thrust plane, which laterally cuts up through the subthrust rocks. The broad salients and sharp re-entrants defining the multiple lobate pattern of the thrust trace is due to the cuspate-lobate lateral curvature, which is a primary feature of the thrust plane. Each cusp, combined with the forward or backward inclination of the thrust plane imparts a scoop-like shape, several of which are joined by transverse ridges parallel to the displacement vector. The S-surfaces, transposed parallel to the limbs and axial surfaces of the isoclinal “early” folds in the allochthon, have been progressively reoriented parallel to the curviplanar thrust surface. “Later” post-thrusting folds superposed on these initially curved surfaces have curved hinges which trend at high angles to the displacement direction. As a result, the bent hinges are congruent to the laterally curved thrust surface below. The evolution of the Shumar thrust can be explained by lateral propagation along concave-upward trajectories. Thrust fronts propagating from multiple nuclei subsequently coalesced to form a single thrust surface. The ridges along which the thrusts coalesce have the form of lateral ramps.