EFFECT OF PACKING DENSITY ON THE FRY METHOD, AND DEFORMATION HISTORY OF THE SOUTH ALMORA THRUST AND THE SOUTH TIBETAN DETACHMENT SYSTEM

Abstract

Part A: Effect of packing density on Fry method What is the optimum level of packing density for strain estimation by the Fry method? This part answers the question by using simulated samples of various degrees of packing density, distortion of these samples by known strains, estimation of strain by the Fry method in the distorted samples and error estimation by comparing the known strain and the strain given by the Fry method. A MATLAB program, GEOSAMP, is developed for simulation of synthetic samples of varying packing density and their distortion by various strain ratios. The statistical errors in the strain estimates, obtained by the Fry method, are found to grow larger with the decrease in the packing density irrespective of the level of distortion. It is demonstrated that a progressive decrease in the packing density resulted in both, an increase in the degree of anticlustering and anisotropy, hence the larger errors. Tests on the natural samples validate the results obtained from the synthetic samples, Samples with >30% packing density give satisfactory strain estimates by the Fry method. Part B: Structural and tectonics of the South Almora Thrust and South Tibetan Detachment The South Almora Thrust zone Chocolate tablet boudins and the associated structures are used to understand the deformation history of the SAT zone in the Kumaun Lesser Himalaya. Several lines of evidence from mesoscopic structures suggest that the chocolate-tablet boudins were developed as a consequence of orthogonal layer-parallel stretching in two successive phases of the regional scale shearing. The early phase of top-to-NW shearing, a probable pre-Himalayan deformation, resulted into the development of early boundins. These boudins accommodated the maximum strain ratio (Rs) of the order of 1.7. This deformation phase was superposed by a late phase of top-to-SW Himalayan shearing, during which the late boudins were developed. In the late boudins, the strain ratio is of the order of 2.1. The interference between these early and late boudins, directed orthogonally in the same layer, has resulted in the development of the chocolate-boudins in the SAT. Evidence from fluid inclusion micorthermometry and microstructures suggests that the chocolate boudins and associated veins developed at approximately 300 0C temperature with fluctuating fluid pressure conditions. 40Ar/39Ar geochronological data implies that the last tectonic activity in the SAT zone occurred during the Oligocene Period (25-28 Ma). This corresponds to the late event of top-to-SW shearing in the SAT. The South Tibetan Detachment Zone One of the basic tenets of the existing tectonic evolutionary model, such as the tectonic wedge model and the nappe model, is the occurrence of a large scale recumbent fold, the Phojal fold, in the Himachal Himalaya. Detailed remapping, done in this study, reveals that the Phojal fold is not a recumbent fold. This structure is found to be a refolded fold that folds both the Tethyan metasediments and the Greater Himalayan Crystallines. This inference is supported by cleavage vergence, fold vergence and microstructures. Deformation sequencing and structural mapping implies following order of development of the large scale structures in the Himachal Himalaya; ii (i) recumbent folding in a compressive regime, (ii) development of the South Tibetan Detachment in an extensional regime and (iii) refolding the pre-existing recumbent fold in a compressional regime. Observations strongly suggest mode switch in the NW Himalayan orogeny. This finding calls for a revaluation of the existing tectonic models in the Himachal Himalaya.