EVOLUTION OF TRANS-ARAVALLI TERRAIN; A TECTONIC STUDY OF PUNAGARH BASIN, RAJASTHAN, INDIA

The Trans-Aravalli terrain of north-western Indian shield has been extensively intruded by older Erinpura Granites (830 Ma) and later by Malani Igneous Suites (750 Ma). The time interval between these igneous suites has witnessed the development of Punagarh basin. Delhi Supergroup represented by Sojat Formation underlies the Punagarh sequence with a basal unconformity. The Sojat Formation consists of micaceous quartzites, slates and phyllites with sporadic intrusions of Erinpura Granites. Sojat rocks are deformed, metamorphosed and sheared at several places. The schistocity shows NE-SW trend and marked by crystallization of mica. However, phyllite carrying feldspar aggregates set in micaceous matrix are present. The shear zone trends N40⁰E/70⁰NW. The mylonitic fabric in the shear zone is well developed in micaceous quartzite where stretched quartz grains are observed in micaceous matrix. The Punagarh rocks are deposited in fault bounded grabens marked by normal faults. The normal faults in Sojat Formation strike NE-SW and are listric in nature, and produce roll over antiforms. Therefore the Punagarh rocks show a gentle dip. Punagarh Group is sub divided into Bambholai Formation represented by basalt, quartzite and shale; Khamal Formation represented by quartzite and shale and Kanawas Formation represented by dacite and quartzite. Basaltic dykes intrude Kanawas Formation. While the Kanawas Formation shows westerly dip, the Khamal and Bambholai show easterly dip. This variation has been interpreted as due to horst and graben structures and rotation of the faulted blocks. Further, conjugate sets of en-echelon quartz veins are present with trend of N100⁰ and N135⁰.These are strike slip shear zones showing extension in NNE-SSW and compression in NNW-SSE. The angle of en-echelon fissure veins to the shear zone is less than 45⁰ suggesting positive dilation. Kinematic analysis of conjugate shear planes for the orientation of principal stress axes and the ratio of the principal stress differences (R=0.7) indicates transtensional tectonic regime. Thus it has been suggested that the Punagarh basin evolved through extension when sediments were deposited and later underwent transtension when the en-echelon veins were emplaced.