Magnetic fabric variations along the fault related anticlines of Eastern Kachchh, Western India

An investigation of the anisotropy of magnetic susceptibility (AMS) along the Kachchh Mainland Fault (KMF) and South Wagad Fault (SWF) in the eastern Kachchh has been carried out to determine the variation of magnetic fabric in different anticlines. 105 cores drilled from 35 Jurassic sandstone samples collected from both forelimbs and backlimbs of the anticlines are analyzed. The various types of fabrics from the three sites indicate essentially three stages of generation of magnetic fabric. The sedimentary fabric (type I) is affected a) initially by layer parallel shortening, resulting in the development of type-III fabric with oblique to parallel bedding and magnetic foliation, b) later by bedding parallel shear associated with the rotation of limbs resulting in the tectonic fabric type-IV and c) finally by bedding parallel thrusting, dominantly in the backlimbs resulting in the imprint of higher tectonic fabric type-V/VI. About 14% of the sampled sites belong to the sedimentary type-I fabric, 54% to the type-III and 32% show tectonic fabric (type-IV and V/VI). The different stages of fabric evolution are not accompanied by linear increase in the degree of anisotropy, but show construction and destruction of magnetic fabric during the layer parallel shortening to late stage bedding parallel thrusting. The tectonic magnetic lineation orientations are mostly not consistent with the bedding strikes or fold axes. The intermediate fabric lineation direction of NNE–SSW is superimposed by later tectonic lineation with varying orientations of NW–SE in the western end to NNE–SSW to NNW–SSE along the eastern end of KMF. We attribute the variations in shortening direction essentially to the transpressional deformation of the basin in general, and individual anticlines in particular. The variation in slip/propagation ratio along individual segmented faults has resulted in the rotation of fold limbs.