NEW METHODS OF STRAIN ANALYSIS AND DEFORMATION STYLE IN PARTS OF KUMAUN LESSER HIMALAYA

Abstract

PART A: NEW METHODS OF STRAIN ANALYSIS Part A presents several new methods of strain estimation in the folds and the distorted forms of fossils. As elucidated with the help of computer-graphics software, the new techniques based on simple transformation equations, not only provide rapid and accurate strain estimates, but also retrodeform the distorted strain markers. For the first time, the application of the Mohr circle for deciphering strain in the flattened parallel folds, and that of the Wellman method for strain estimations in the folds as well as in the distorted fossil vertebrates are demonstrated. Many single distorted forms of bilaterally symmetric fossils occur in the sedimentary rocks which do not provide any clue for knowing the principal strain directions. Such situations have so far been considered as unsolvable in the context of strain estimation. Results from several hundred computer-graphic simulations show that it is possible to estimate the minimum amount of strain from the distorted single fossil forms, albeit the exact amount of strain cannot be deciphered in these situations. A new method, namely, the 'isogon intersection' method is developed for the unified geometrical classification of folds, and strain estimation in the flattened folds. The method is based on the principle that the points of tangency on the outer and inner arcs of any fold are the material points, and the line joining these material points, namely, the isogons behave as material lines during the flattening process. The 'isogon intersection' method also allows representing each fold/limb by a point on a two-dimensional plot between strain ratio and inclination of the strain ellipse. The plot is useful for representing the geometries of a large number of folds on a single graph. PART B: DEFORMATION STYLE IN PARTS OF THE KUMAUN LESSER HIMALAYA Part B aims at deciphering the structural geometry of rocks in the outer most part of the Kumaun Lesser Himalaya. This part is based on extensive fieldwork involving the geological and the structural mapping, the structural analyses of the field data and the critical synthesis of the major and minor structures. Study area For deciphering the structural style in the Kumaun Lesser Himalayan sequence, three critical sectors, namely, the Amritpur sector, the Khairna sector, and the Ramgarh sector are selected. These sectors include all the major lithounits exposed between the footwall of the Main Boundary Thrust, and the hanging-wall of the Ramgarh Thrust. Detailed geological and structural maps of each of these three sectors are prepared on the basis of extensive fieldwork. The Siwalik Group rocks, exposed on the footwall of the Main Boundary Thrust are, in general, deformed into large scale gentle folds characterized by NE trending upright axial planes and low plunging to non-plunging hinge lines. That the angle of plunge of these folds steepens in the vicinity of the Main Boundary Thrust is demonstrated by the structural analysis of the Ranibagh syncline in the Amritpur sector. Main Boundary Thrust Zone An 88 m thick tectonized zone, containing the intensely deformed Neogene rocks of the Siwalik Group and the Proterozoic rocks of the Lesser Himalaya, represents the Main Boundary Thrust zone. This zone consists of several parallel to subparallel E-W striking faults/thrusts that dip at subvertical angles. Between any two consecutive faults within the Main Boundary Thrust zone, there is a tectonic domain of characteristic structural style. Following nine tectonic domains are mapped within the Main Boundary Thrust zone: (i) pressure solved Siwalik sandstone, (ii) refolded maroon shale, (iii) the Siwalik sandstone cut by strike fractures, (iv) intricately folded and fractured Siwalik sandstone, (v) white quartzite with maroon shale (vi) sheared black shale containing isoclinally folded calcareous shale bands of the Lesser Himalayan rocks, (vii) white quartzite with sheared black shale, (viii) the mixed zone of anastomosing shear zones around the white quartzite, and (ix) the zone of kinked mafic schist of the Nagthat Formation. Detailed structural mapping and the analyses of minor structures in different tectonic domains reveal that: (i) both the Neogene Siwalik and the Proterozoic Lesser Himalayan rocks are repeatedly folded and ductilely sheared within the Main Boundary Thrust zone, (ii) the contact between the Lesser Himalayan rocks and the Siwalik rocks is a ductile shear zone that has been folded into overturned folds. The faults separating different tectonic domains parallel the subvertical limbs of these overturned folds. Amritpur granite The Proterozoic Amritpur granite, overlying the Main Boundary Thrust zone, has two mappable components: (i) the older mesocratic and porphyritic granite and, (ii) the younger leucocratic granite. The contact between these two types of granites is commonly faulted. It is the younger leucogranite that is juxtaposed mostly against the Siwalik Group of rocks along the Main Boundary Thrust. The Amritpur granite is, in turn, overlain by the Nagthat Formation along a faulted boundary. The original relationship between the Amritpur granite and the Nagthat Formation is extensively modified due to faulting along the contacts. A mega-enclave (?) of Nagthat Formation occurs within the Amritpur granite in the eastern part of the Amritpur sector. Whether the Amritpur granite is an intrusive body or a basement slice remains uncertain due to the lack of unambiguous xenoliths or enclaves, and the tectonic nature of the contact between the granite and the Nagthat Formation rocks. Comparison of structural styles in different lithounits Structural analysis of the mesoscopic fabric data coupled with the outcrop scale overprinting relationships, and the map pattern reveal a common deformation plan for the Nagthat Formation, the Ramgarh granite and the Nathuakhan Formation. All the three lithounits are folded by the two main groups of folds, Fi and Fj, which are characteristically non-coaxial. The large scale structures in all the three lithounits are the F2 folds plunging at low angles towards NNW-NW. In spite of a common deformation plan, there are subtle differences in the large scale fold geometry due to variations in intensity of ductile shearing in different lithounits. For example, the large scale F2 folds are characteristically gentle in the Nagthat Formation due to low amount of ductile shearing. By contrast, the large scale F2 folds are commonly tight to isoclinal in the intensely sheared Ramgarh granite. Geometry of thrusts and deformation within the thrust sheets Both the Ramgarh Thrust and the Chamaria Thrust strike NNW or NW but dip at variable angles ranging from subhorizontal to very steep, or subvertical towards NNE. Both the thrusts have developed along the F2 axial planes during the final stages of F2 folding and they themselves are rotated about the F2 hinge lines. The thrusting is, therefore, a syn-i<2 folding event. Besides the Ramgarh Thrust and the Chamaria Thrust, several large scale thrusts/faults occur along the contacts between different rock types within the Nagthat and Nathuakhan Formations. The shear sense indicators in most of the mesoscopic scale faults and thrusts, as well in the available outcrops of the large scale faults and thrusts imply a consistent topto- the-south sense of movement in the Lesser Himalayan sequence. The shear sense on a few mesoscopic thrusts and faults is, however, top-to-the-north due to folding of the thrust planes, or back thrusting. Analyses of folds and thrusts developed at different scales reveal that the Lesser Himalayan sequence is made up of three major thrust sheets that are successively emplaced one over other with a consistent top-to-the-south sense of motion. These thrust sheets, namely the Nagthat thrust sheet, the Ramgarh thrust sheet and the Nathuakhan thrust sheet, are deformed by at least two dominant phases of folds that are characteristically non-coaxial with respect to each other. Palaeostress states in the Lesser Himalayan sequence Dynamic analysis of the mesoscopic kink folds reveals that all the three types of tectonic regime, the thrust type, the strike slip type and the normal fault type can be recognized within the rocks of the Lesser Himalayan sequence. Amongst these, the thrust type of F2 kink bands, developed due to NE-SW directed maximum compression, are most dominant. Different rocks stretching from the Neogene molasse on the footwall of the Main Boundary Thrust to the Proterozoic Nathuakhan Formation on the hanging wall of the Ramgarh Thrust are cut by two to five sets of striated faults. The analyses of these faults by the dihedra method, the trihedra method and, the direct inversion method reveal that the thrust type of tectonic setting characterized by NE-SW directed subhorizontal maximum compression and an approximately axial extensional type of stress state was most dominant during the Himalayan orogeny.