A. K. Dubey

Simultaneous development of noncylindrical folds, frontal ramps, and transfer faults in a compressional regime: Experimental investigations of Himalayan Examples

Experiments were performed with modeling clay models to understand the simultaneous development of folds, frontal ramps, and transfer faults/oblique ramps in layered sequences. The models were deformed by layer parallel contraction. At a late stage of deformation, the models were subjected to a maximum contraction parallel to the layering and perpendicular to the early axis of maximum contraction to study the effect of the superposed deformation. The experimental results reveal that a transfer fault joining two frontal thrust ramps acts as a shear zone and controls the orientation of fold hinge lines in the adjacent region. Hence two orientations of fold hinge lines may be observed: (1) oblique to the axis of maximum compression in the vicinity of the transfer fault and (2) normal to the axis and parallel to the frontal ramps away from the transfer fault. During the longitudinal fold propagation, the two hinge lines may coalesce to form a fold curvature opposite to the tectonic transport direction. The curvature varies along the transfer fault depending on the shear zone geometry. Additional fold hinge line curvatures may result during the superposed deformation both in the early and the superposed fold hinge lines. All these curvatures produce a systematic pattern which can be analyzed with the help of the experimental results. The fold hinge line orientation may also be used to establish an oblique ramp in field as demonstrated by examples from the Lesser Himalayas, India.

Superposed folding of a blind thrust and formation of klippen: results of anisotropic magnetic susceptibility studies from the Lesser Himalaya

Abstract The rocks of the Garhwal Lesser Himalaya have undergone a weak superimposed deformation, hence linear and planar structures are either absent or poorly developed. This puts a severe limitation on application of conventional methods of finite strain determination in understanding the deformation pattern. However, the geometry, orientation, and distribution of magnetic susceptibility strain ellipses clearly reveal the effects of early and superposed deformations in the area. The orientation patterns of the ellipses also help to identify reversal of displacement along an oblique fault ramp during the superposed deformation. The Hrouda double plot reveals a combination of lateral shortening and simple shear, thereby suggesting a small translation along the klippe detachment thrust. The study has important implications for understanding the structural evolution of the Lesser Himalayan klippen, because the earlier models, in the absence of the relevant data, are based on assumptions concerning thrust displacement. The present field studies and the AMS data favour an alternate model for the structural evolution of the Lesser Himalayan klippen, that lie in the core of the Mussoorie Syncline. The model explains structural features and outcrop patterns as due to a combination of fault bifurcation, back thrusts, pop-up, and subsequent superposed deformation. The klippen lie over their roots and are described as pop-up klippen.

Development of extension faults on the oblique thrust ramp hanging wall: example from the Tethys Himalaya

Abstract Inhomogeneous strain at late stages of deformation of modelling clay models with frontal and oblique thrust ramp geometries results in formation of extension faults on the hanging wall. The extension faults trend nearly parallel to the axis of maximum compression and orthogonal to the trend of the frontal ramp. Faults with similar geometries occur in the Upper Satluj River Basin of the Tethys Himachal Himalaya. The experimental results help in understanding the present day normal faulting and related active tectonics of the area.

Mesoscopic and magnetic fabrics in arcuate igneous bodies: an example from the Mandi-Karsog pluton, Himachal Lesser Himalaya

Field, microstructural and anisotropy of magnetic susceptibility (AMS) data from the Palaeozoic Mandi-Karsog pluton in the Lesser Himalayan region reveal a concordant relationship between fabric of the Proterozoic host rock and the granite. The pluton displays a prominent arcuate shape on the geological map. The margin-parallel mesoscopic and magnetic fabrics of the granite and warping of the host rock fabric around the pluton indicate that this regional curvature is either synchronous or pre-dates the emplacement of the granite body. Mesoscopic fabric, magnetic fabric and microstructures indicate that the northern part of the pluton preserves its pre-Himalayan magmatic fabric while the central and southern part shows tectonic fabric related to the Tertiary Himalayan orogeny. The presence of NW-SE-trending aplitic veins within the granite indicates a post-emplacement stretching in the NE-SW direction. Shear-sense indicators in the mylonites along the margin of the pluton suggest top-to-the-SW shearing related to the Himalayan orogeny. Based on these observations, it is envisaged that the extension that gave rise to this rift-related magmatism had a NE-SW trend, that is, normal to the trend of the aplite veins. Subsequently, during the Himalayan orogeny, compression occurred along this same NE-SW orientation. These findings imply that the regional curvature present in the Himachal Lesser Himalaya is in fact a pre-Himalayan feature and the pluton has formed by filling a major pre-Himalayan arcuate extension fracture.