Tectonic Framework, Structure and Tectonic Evolution of Kutch Basin, Western India

Abstract

The Kutch basin is a western margin pericratonic Mesozoic rift basin. The rift basin originated during break up of Eastern Gondwanaland. The rift basin is bound by Nagar Parkar uplift on the north and Kathiawar uplift (Saurashtra horst) on the south respectively along Nagar Parkar and North Kathiawar faults (NPF&NKF). The rift is styled by three main uplifts along three master faults, (from north to south) Island Belt, Kutch Mainland and Wagad uplifts, with intervening grabens and half-grabens. The uplifts are upthrust basement blocks tilted along sub-vertical faults with initial normal separation. The NKF is the principal master fault along which the rift subsided most. The structure is thus styled by tilted blocks and half-grabens within a south tilted asymmetric rift basin. Blanketing carapace of sediment over the basement drape folded over the tilted edges of the basement blocks as marginal flexures. The flexures are knee-bend folds with narrow deformation zones along the crest. The deformation zone encloses complicated folds, locally much faulted and intruded by igneous rocks. In the western part the uplifts are tilted to the south with flexures draped over the faulted up northern edges. In the eastern part a large uplift, Wagad Uplift, occurs between the Mainland and Island Belt uplifts. It is tilted towards the north along the South Wagad Fault (SWF), opposite to other uplifts, with a narrow deformation zone along the faulted up southern edge. The back slope ends up against Bela horst of the Island Belt uplift along Gedi Fault. A subsurface basement ridge - Median High, crosses the basin in the middle at right angle to its axis. Acting as a hinge it divides the basin into a deeper western part and a shallower and more tectonised eastern part. The rift is terminated in the east against a transverse subsurface basement ridge, Radhanpur-Barmer Ridge, which is the western shoulder of the adjacent N-S oriented Cambay rift. To the west, the rift merges with offshore shelf. The Kutch basin is the earliest pericratonic rift basin to form in the western volcanic passive margin of the Indian craton during the Late Triassic break up of Gondwanaland. The rift evolution with syn-rift sedimentation continued through Jurassic till Early Cretaceous as Indian plate separated from Africa and drifted northward. The rift expanded from north to south by successive reactivation of primordial faults of Mid-Proterozoic Delhi fold belt. The faults strike E-W but eastward the strike swings to NE-SW merging with the Delhi- Aravalli strike. The rifting was aborted by the trailing edge uplift during Late Cretaceous pre-collision stage of the Indian plate. The uplift caused structural inversion during rift-drift transition stage when most of the uplifts with drape folding over the edges came into existence by upthrusting along the master faults. The motion during the drift stage of the plate induced horizontal stress and the near vertical normal faults, which were reactivated as reverse faults during initiation of inversion cycle, became strike-slip faults involving divergent oblique-slip movements. The present structural style evolved by right lateral slip, which shifted the uplifts progressively eastward relative to each other from south to north. This resulted in the present en echelon positioning of the uplifts with respect to Kutch Mainland uplift. The strike slip related structuring modified the linear flexures breaking them into individual folds at the restraining and releasing bends. Narrow deformation zones are complicated by conjugate reidel faults formed along the master faults modifying the initial drape folds. Upward migration of basement faults through ductile sediment drape and the accompanying fault propagated folds further complicated flexural deformation zone. Syntectonic intrusions modified the shape of the structures. The structure of the SWF deformation zone demonstrate a typical strike-slip fault related structural style with two intertwining faults and related folds. Igneous rocks extensively intruded the Mesozoic sediments. Synrift plutonic activity and post rift hotspot related Deccan volcanism are two major magmatic events. Studies on the intrusive bodies and seismological data suggest the presence of an ultramafic magmatic body close to the crust-mantle boundary. This suggests mantle stretching, thinning and rupturing during rifting which led to deep crustal melting and formation of magma that intruded during inversion stage.