Deformation Fabric Analysis using AMS and EBSD in the Precambrian Rocks of Dharwar Craton (South India) - Implications for Kinematic Studies

Kinematic analysis for many structural geological studies is routinely carried out in XZ section of the strain ellipsoid, i.e., section parallel to stretching lineation and perpendicular to the foliation. However, not all deformed rocks contain foliation and/or stretching lineation e.g., the Precambrian metavolcanic rocks of Hutti-Maski Greenstone Belt (HMGB) within the Dharwar Craton (South India), which is the area investigated here. The region is replete with auriferous quartz veins but the absence of visible planar and linear fabrics in most samples makes kinematic analysis challenging. Hence, the authors use a combination of Anisotropy of Magnetic Susceptibility (AMS) and SEM-EBSD studies. AMS analysis reveals that the mean magnetic foliation is NW-SE striking with very steep westerly dip, and the mean magnetic lineation is steeply plunging towards NW. The magnetic fabric recorded in the massive rocks of Hutti region is inferred to be a manifestation of regional D1/D2 deformation that affected the Dharwar Craton. The orientations of three principal axes of AMS ellipsoid (K1>K2>K3) are equated with the principal axes of strain ellipsoid (X>Y>Z). Rock thin sections are prepared parallel to K1K3 plane of AMS ellipsoid, which is equivalent to XZ section of strain ellipsoid. Petrographic studies of these sections is done for quartz grain shape determination and vein geometry analysis. Crystallographic preferred orientation of quartz is analyzed using SEM-EBSD method. The study reveals down-dip sense of movement as well as strain partitioning within veins. Past studies have advocated an important role of strike-slip movement in the formation of quartz veins. However, the present investigation indicates that an extensional stress was important during their formation, which led to down-dip sense of shear. Thus, this study demonstrates the use of integrating AMS and SEM-EBSD data in kinematic analysis of naturally deformed rocks.