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

The rocks of the Garhwal Lesser Himalaya have undergone a weak superimposed deformation, hence linear and planar structures are either absent or poorl y developed. This puts a severe limitation on application of conventional m ethods of finite strain determination in understanding the deformation patt ern. However, the geometry, orientation, and distribution of magnetic susce ptibility strain ellipses clearly reveal the effects of early and superpose d deformations in the area. The orientation patterns of the ellipses also h elp to identify reversal of displacement along an oblique fault ramp during the superposed deformation. The Hrouda double plot reveals a combination o f lateral shortening and simple shear, thereby suggesting a small translati on 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 base d 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 Synclin e. The model explains structural features and outcrop patterns as due to a combination of fault bifurcation, back thrusts, pop-up, and subsequent supe rposed deformation. The klippen lie over their roots and are described as p op-up klippen. (C) 2001 Elsevier Science Ltd. All rights reserved.