The application of anisotropy of magnetic susceptibility (AMS) to decipher superposed deformation and kinematics in massive rocks lacking visible foliation and/or lineation is demonstrated in this study. This is done by discussing the case of Precambrian metavolcanic and granitic rocks of the Raichur region (Eastern Dharwar Craton, southern India). The metavolcanic rocks have a mean magnetic foliation that is NNW-SSE striking, on which the magnetic lineations are doubly plunging towards NNW and SSE. The granites show two distinct mean magnetic foliation orientations at high angles to one another (a) NNW-SSE striking magnetic foliation with magnetic lineations doubly plunging towards NNW and SSE (similar to the metavolcanics), (b) ENE-WSW striking magnetic foliation with magnetic lineations doubly plunging towards ENE and WSW. It is concluded that this is a manifestation of Type-1 fold interference (dome and basin) pattern due to superposed deformation associated with early regional D1/D2 deformation (NE-SW shortening) followed by D3 (NNW-SSE shortening) in the Dharwar Craton. A two stage model is envisaged to explain the development of magnetic fabric and its orientation distribution in the granite. Kinematic and microstructural investigations (SEM-EBSD of oblique quartz grain shape fabric) in a metavolcanic rock establish (i) fabric development under general shear (kinematic vorticity number, 0.766) (ii) top-to-NW sense of shear at the mesoscopic scale. This supports the large scale kinematics reported in the Dharwar Craton and implies regional deformation under a transpressional regime.
Quartz c-axis fabric opening angle (OA) thermometry has been widely used to determine deformation temperatures of quartz-rich tectonites. Empirical relationships between OA and the deformation temperature of quartz-rich tectonites have been proposed with generally accepted uncertainties of about ±50°C. However, there is no standardized methodology to construct pole figures (PFs) for use in OA thermometry, and possible variations resulting from different approaches represent unknown sources of uncertainty.In this study, we show that OA is affected by changes in: i) grain selection, ii) PF contour smoothness, iii) misorientation angle threshold, and iv) spatial resolution. We propose a protocol for constructing PFs from EBSD data using MTEX software, which can be applied both to existing calibrations of the OA thermometer and to constructing new OA thermometers. A unified approach to constructing quartz PFs is essential to minimize associated uncertainties and can be of assistance in evaluating the effects of water fugacity or strain rate on crystallographic preferred orientation (CPO) patterns.
