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. (c) 200

Determination of directions of horizontal principal stress and identification of active faults in Kachchh (India) by electromagnetic radiation (EMR)

The Kachchh basin in the western India is known for its recent high seismicity. This study presents an application of the geogenic Electromagnetic Radiation (EMR) technique for deciphering the directions of principal horizontal stress in the eastern Kachchh. The principal direction of horizontal stress obtained from EMR differs from those obtained from earthquake focal plane solutions. The major horizontal principal stress based on the EMR study shows an azimuth of N60 degrees E +/- 10 degrees. The principal directions of EMR emissions are parallel to the acute bisector of conjugate microcracks. The azimuthal distribution of EMR signal and dimension of microcracks suggest that the EM emissions are transversely polarized. The study also deals with the first application of electromagnetic radiation emissions to identify active fracture planes in sandstones that could become potential active faults later, which might be seismogenic or nonseimogenic. This study is based on linear profiling at six different places across two major faults, the Kachchh Mainland Fault (KMF) and the South Wagad Fault (SWF) in the eastern Kachchh. Anomalously, high EMR emissions are observed in the eastern part of the KMF, indicating active surface deformation. (C) 200

[Pan-African overprint on Eburnian granitoids at the northern boundary of the West African Craton, Zenaga Inlier, central Anti-Atlas, Morocco]

The Zenaga Inlier shows a comprehensive record of the Eburnian and PanAfrican Orogenies. The Eburnian is characterised by high-temperature regional metamorphism and complex magmatism. The early (Azguemerzi) granodiorite has an isotopic mantle signature and was emplaced diapirically during the Eburnian Orogeny causing local thermal metamorphism. The foliation observed in this granitoid is a result of the interference between its primary syn-emplacement foliation and the regional foliation under amphibolite-facies conditions. The northern part of Zenaga has been intruded by the leucocratic granites of Tazenakht. These granites are cut by mylonites and phyllonites, corresponding to the PanAfrican shear zones and accompanied with sub-greenschist-facies metamorphism during the Pan-African Orogeny. The deformation was the result of a regional sinistral transpressive event. This study in the northern part of the West African Craton shows the superposition of the Pan-African on the Eburnian Orogeny and the presence of a major fault in the Anti-Atlas. ((C)) 2002 Elsevier Science Limited. All rights reserved