Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

H. C. Tewari ¹, B. Rajendra Prasad ¹, V. Vijaya Rao ¹, P. R. Reddy ¹, M. M. Dixit ¹, N. Madhava Rao ¹

Affiliations
1 National Geophysical Research Institute, Uppal Road, Hyderabad - 500 007, IN

Abstract

Deep Continental Reflection Studies along the Nagaur-Jhalawar profile across the Aravalli-Delhi Fold Belt indicate that in most of the units comprising the seismic profile the lower crust (LC) lies towards west of their present surface exposures. Based on the reflectivity characteristics of the LC. which are in general agreement with the globally accepted norms, the Nagaur-Jhalawal profile can be broadly divided into five zones: 1) the moderately reflective Marwar Basin (MB), 2) the highly reflective Delhi Fold Belt (DFB), 3) the poorly reflective Bhilwara Granite Complex (BGC), 4) the highly reflective Hindoli Group (HG) including a thrust zone, and 5) the moderately reflective Vindhyan Basin (VB). The most commonly accepted reasons for high lower crustal reflectively are: 1) presence of free fluids. 2) crustal-scale ductile shear zones and 3) mafic intrusions and underplating with partial melts derived from the upper mantle.

High reflectivity of LC in the ADFB can possibly be assigned to primary lithologic variation in the metamorphic facies layers and Precambrian shear zones that were formed during compressional phase of orogeny. The poor reflectivity of LC in the BGC. comprising the Sandmata and Mangalwar Complexes, may be due to vertically oriented igneous intrusions which have disturbed its lamellar character. The high reflectivity in the thrust zone and LC in the HG suggests that it is due to a totally different composition than that of BGC and may be related to metamorphic layering.

Keywords

Geophysics, Deep Reflection Profiling, Aravalli-Delhi Fold Belt, Seismic Profile, Rajasthan.

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A. R. Sridhar ¹, H. C. Tewari ¹, V. Vijaya Rao ¹, N. Satyavani ¹, N. K. Thakur ¹

Affiliations
1 National Geophysical Research Institute, Uppal Road, Hyderabad - 500 007, IN

Abstract

A five-layered crustal model with 5.9-6.2, 6.0, 6.3, 6.6 and 7.0 kms^-1velocities is derived with seismic refraction/wide-angle reflection data along the Thuadara-Sendhwa-Sindad profile in central India. The profile traverses across the western part of the Narmada-Son lineament in the N-S direction. The data are subjected to 2-D forward modelling using both travel times and amplitudes. Refracted waves (P_n from the Moho observed as first arrivals at a distance of about 200 km are used to derive the upper mantle velocity of 8.1 kms-'. The Moho is well constrained both from the P_n and wide-angle reflections from four shot points. The main features of the velocity structure is the delineation of a low velocity layer (6.0 kms^-1) in the upper crust and a 12-16 km thick high velocity (7.0 kms^-1) layer at the base of the crust. The high velocity lower crustal layer, representing the magmatic underplating in the region, may be related to the formation of the Narmada Basin and the Deccan volcanic episode. The Deccan volcanism may be a consequence of the Reunion mantle plume activity. The crustal thickness varies between 37 and 43 km along the profile and the thickest crust is found between the Narmada and Tapti rivers. The gravity mode1 constrained from the seismic velocity structure corroborates the crustal structure. Deep-seated faults responsible for the evolution of Narmada Basin are inferred from the present study.

Keywords

Underplating, Deccan volcanism, Wide-Angle reflection, Low velocity layers, Moho, Narmada-Son lineament, Central India.

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