P.R. Reddy

Evolution of Proterozoic Aravalli Delhi Fold Belt in the northwestern Indian Shield from seismic studies

A deep seismic reflection study across the Aravalli Delhi Fold Belt, situated in the northwestern part of the Indian Shield, has revealed a deep penetrating 25-km-wide crustal-scale thrust fault, dipping reflections from the upper crust to the Moho and a divergent reflection fabric. Paleo-subduction zones and island-arc signatures are identified from the present study. Seismic images of the crust reveal tectonics of the region with two distinct episodes of rifting, sedimentation, collision and suturing corresponding to the Aravalli and Delhi orogenies. Plate tectonic processes were responsible for the evolution of the Paleoproterozoic Aravalli and Mesoproterozoic Delhi Fold Belts with the juxtaposition of the Bundelkhand craton in the east and the Marwar craton in the west. A 50-V m electrical conductor, extending to a depth of 25 km, and a steep gradient gravity anomaly of 70 mGal, extending all along the strike of the fold belt with conspicuous lows on either side, correlate well with the deepseated dipping reflections/sutures inferred from the seismic reflection data. The present study suggests that a high-velocity, thick crust was produced in the Proterozoic orogens of the region. q 2000 Elsevier Science B.V. All rights reserved.

A Mesoproterozoic Supercontinent: Evidence from the Indian Shield

Abstract Geophysical signatures of late Paleoproterozoic collisional sutures/fold belts between the Deccan Protocontinent and Bundelkhand craton, and between the Bundelkhand and Mewar cratons of the north Indian shield are presented. The Aravalli and Satpura orogenies evolved during these continent-continent collision episodes. Both the orogenic episodes are associated with granulite facies metamorphism and metallogeny. Serpentinised ultramafic breccia in ophiolitic melange is present in both regions. Structural discontinuities and oppositely dipping reflection fabrics with high conductivity anomalies are the characteristic features of these collision zones. A major gravity high is also observed all along the strike of these fold belts to a distance of 700-1000 km. Paleoproterozoic (1.8-2.1 Ma) ages corresponding to compressional activity have also been observed in the Southern Granulite Terrain and other Gondwana fragments. Extensive granites and gneisses of ∼1900 Ma age are observed in the lower Himalayas and Nanga Parbat ranges. Late Paleoproterozoic orogenic activity is also observed in all of the earths shields. Intense orogenic activity of this period observed in various widely separated cratonic blocks is related to global plate tectonic processes and supercontinental episodes. The available geophysical evidences in various parts of the globe suggest the presence of a supercontinent, referred to as ‘Columbia’ during the early Mesoproterozoic period.

Structure and tectonics of the Proterozoic Aravalli-Delhi Fold Belt in northwestern India from deep seismic reflection studies

Abstract Seismic imaging of the crust along a 400-km-long deep seismic reflection profile across the Palaeo/Mesoproterozoic Aravalli-Delhi Fold Belt, in the northwestern Indian Shield, brings into focus its complex structure and provides clues to understand the geological processes involved in the evolution of this belt. The reflectivity pattern varies considerably for different crustal units along the profile. The deep-crustal reflection data image two sets of oppositely dipping strong reflection bands, from upper- to lower-crustal levels. These are identified as the signatures of the collision corresponding to Aravalli and Delhi orogeny. The data also exhibit a clear Moho and strong lower-crustal reflections near the collision boundaries. A stack of dipping reflections from the top of the Moho to the surface is identified as a major thrust fault indicating that the Proterozoic collision and deformation were primarily thick-skinned in nature

Deep sub-crustal features in the Bengal Basin: Seismic signatures for plume activity

The basaltic Rajmahal Traps (117 Ma), India are believed to be due to a mantle plume. The plausible plume path on the continental side has not been traced probably because of the thick sedimentary cover. These Traps are located close to the Bengal basin; therefore, a re-examination of the deep seismic velocity structure should help in tracing the plume path on the continental side and its possible relationship to plume activity. Data obtained along four seismic profiles in the Bengal basin were re-examined, which helped in identifying and modeling strong reflections from lower crust and Moho. The results indicate a 7.5 km/s velocity zone as an underplated mantle material injected by the mantle plume or hotspot at the base of the continental crust. The study also indicates the probable trace of plume in the continental region, a NNW-SSE trending path east of 87°E with an up-warp in Moho.

Velocity modelling of Bengal Basin refraction data—refinement using multiples

Abstract Free-surface multiples that are generated due to high-velocity gradients in the sedimentary layer sequence above the basement of the West Bengal Basin were modeled to constrain a 2-D velocity–depth section of the southern part of the basin. Modelling of the velocity structure utilizing free-surface multiples with the asymptotic ray method produced synthetic record sections that could replicate the observed seismic phases better than those generated by matching only the primary refractions.

Seismic Signature of Sub-Trappean Gondwana Basin in Central India

Abstract The digitized and trace normalized record sections of original analog Deep Seismic Sounding (DSS) data collected along the Multai-Pulgaon profile, have been re-interpreted to find basement and intra-basement structure in the area between the Satpura basin and the Pranhita-Godavari Gondwana graben in Central India. It is found that the data are best satisfied by a model with a low velocity zone in between the Deccan Traps and the basement. The study delineates a basement fault that could be associated with the known Gavaligarh-Salbardi lineament. Low velocity sediments (Vp = 3.7 km/s) delineated from the present study could be Gondwana sediments, which have been reported in surrounding areas. This finding indicates the possible extension of Gondwana formations of the Pranhita-Godavari graben to the Satputra basin through the Wardha valley.

Use of postcritical reflections in solving the hidden‐layer problem of seismic refraction work

In a multilayered earth system, when the thickness of a layer compared to the overlying layer is small, refraction signal from that layer may not appear as a first arrival. In such a case, the analysis of first-arrival refraction data cannot detect the layer and this leads to errors-overestimation of the thickness of the overlying layer and underestimation of depths to all underlying layers. This is known as the hidden-layer problem. In a field situation, hidden layer(s) can be identified with the help of high-energy postcritical reflections, which appear as strong later arrivals. In this paper, we describe an approach to calculate the thickness of the overlying layer and the thickness and velocity of the hidden layer based on the traveltime inversion of postcritical reflections from the top and bottom of the hidden layer. The blind-zone thickness is also calculated using the estimated velocity of the hidden layer and the thickness of the overlying layer. The applicability of the method is illustrated with the help of both synthetic and field data.

Direct calculation of thicknesses for high-velocity and underlying low-velocity layers using post-critical reflection times in a seismic refraction experiment

Abstract In a multi-layered earth system, when the velocity of a layer is lower than that of the overlying layer, the former cannot be recognized on the time-distance plot resulting in an overestimation of the thickness of the overlying layer and the depths of all subsequent deeper layers. This low-velocity layer ( lvl problem in seismic refraction work cannot be solved using traveltimes of first arrivals alone. Use of post-critical reflections (observed strongly after first arrivals on a seismogram) from the bottom of the lvl provides valuable information regarding the solution to the lvl problem. Here, we propose a layer-stripping method applied to the strongly observable post-critical reflection times from the bottom of the lvl to calculate the thickness of the overlying high-velocity layer ( hvl ) and that of the underlying lvl directly. A-priori information for the velocity of the lvl from other seismic evidence is utilized. We show in this paper that even if we use traveltimes of both first arrivals and wideangle reflections from the bottom of the lvl , we cannot calculate three parameters (i.e. the thickness of hvl and, the thickness and velocity of ( lvl ) unequivocally.