Deep Seismic Sounding Studies In India And Its Tectonic Implications

Abstract

Deep Seismic Sounding (DSS) studies of the Indian continental crust were initiated in 1972 by the NGRI under an Indo-Soviet program. Subsequently, DSS experiments to delineate the shallow and deep structural features have been conducted in various parts of the country. Seismic refraction and wide-angle reflection data sets were acquired along more than 5000 km of profiles, including about 800 km of deep reflection profiles, in a variety of geologic and tectonic settings. Most of the studies were concentrated in the Archaean-Proterozoic peninsular shield, the southern granulitic terrain, the late Cretaceous Deccan Trap cover region of western India, across Narmada-Son lineament, the sedimentary basins of Cambay, West Bengal, Mahanadi & Godavari and the Kashmir Himalayan region. Since 1985 the data are being recorded in the digital form. The basic outcome of these studies is the determination of the velocity depth structures upto the crust-mantle boundary (Moho), in the above provinces and its imprint on the tectonics and evolution of various terrains. The amalgamation of the crustal seismic data with geologic and other geophysical data have provided new insight to a better understanding of the tectonic processes involved in the dynamics and evolution of the Indian crust. In this paper we have focused on the results after 1992 only. The deep reflection profiling studies carried out across the Paleo-Meso-proterozoic Aravalli Delhi Fold Belt and the Archaean Bhilwara Gneissic Complex in the northwest Indian platform suggest a zone of Proterozoic collision. The Delhi Fold Belt appears to be a zone of thick (45-50 km) crust where the lower crust has doubled in width. The dynamics of the central Indian region is controlled by the east-west trending deep-seated faults systems viz. the Narmada North Fault and Narmada South Fault. Another NE-SW trending fault, the Barwani-Sukta fault, divides the upper crust in two parts, the eastern part being a basement uplift and the western a sedimentary graben under the Deccan Traps. This fault appears to be the northern limit of the Deccan Trap covered sedimentary graben. A mafic intrusion in the upper crust, represented by high velocity/density body to the east of the Barwani-Sukta fault, seems to have played significant role in shaping the present structural trends of the Narmada region. The Moho depth in the central Indian region varies between 38 and 44 km. In the West Bengal basin the crustal thickness is ~30 km. Further studies indicate the probable trace of a plume in the continental region, NNW-SSE trending path east of 87° E with an up-warp in Moho. A four-layered crustal velocity structure, with a low velocity zone at depths of 7-15 km is seen on the north-south profile over the south Indian granulite terrain. The crustal thickness varies between 41-45 km. INTRODUCTION The present day configuration of continental crust is mostly an outcome of lithospheric evolution and crust-mantle interaction. To understand the mechanism of crustal evolution and its tectonic implications, it is essential to know the characteristic features of the lower crust. With this objective the Deep Seismic Sounding (DSS) studies were started in India in the year 1972, as an Indo-Soviet collaboration project for three years. Under this project, and later on independently by the National Geophysical Research Institute, Hyderabad, India, a number of geological and tectonic provinces have been covered by these studies. DSS studies by controlled source seismic observations constitute the most definitive geophysical technique for exploring the structure of the Earthʼs crust and uppermost mantle. Both shallow and deep structures can be resolved by suitable data sets, including narrow-and wide-angle reflection and refraction phases. Recent advances, both in the areas of seimsic data acquisition as well as processing and modeling techniques, offer wide-ranging possibilities to explore complex subsurface structures that may be both heterogeneous and anisotropic. It has been realized in recent times that coincident reflection/refraction experiments on selected geo-transects provide the most reliable seismic images of the deep crust and uppermost mantle as the two techniques are complementary to each other, together resolving the structural and physical property variations. The deep seismic images of the continental crust, revealed by coincident reflection/refraction profiling across various geological settings, provide necessary clues for understanding the complex geodynamic processes that might be operative during geological evolution. In tectonically active regions accurate mapping of the intracrustal boundaries, including the crust-mantle boundary (Moho), and delineation of deep penetrating steep/low angle crustal faults/fracture zones reveal various blocks that may have been relatively displaced due to movements Tewari and Kumar 2003. Journal of the Virtual Explorer 12. 30-54. 31 along these faults. Further, the DSS data sets, especially in the wide-angle range, provide viable models of the velocity distribution required to infer the petrological composition, grade of metamorphism and material properties such as brittle/ductile regimes. These may lead to consistent interpretations of reflectivity structures observed by vertical-incidence reflection data at deep crustal depths. Seismic refraction and wide-angle reflection data sets have been acquired in different geological and tectonic provinces in Himalaya, Aravalli-Delhi Fold Belt, Narmada–Son Lineament, Southern peninsular shield, Southern Granulite Provinces and Cambay, West Bengal, Mahanadi, Godavari basins. Interpretation of the DSS data along various profiles has brought out the crustal structure and the Moho configuration, leading to a better understanding of the evolutionary processes involved in the formation of the terrain. A review of the studies carried out under the DSS program was done by Kaila and Krishna, (1992). Since then a lot of new studies, including deep reflection profiling, have been added and some of the older data sets have been reinterpreted to extract more information through the use of dynamic forward modeling and inversion programs. In this review we will only restrict ourselves to those studies, which have not been covered by Kaila and Krishna (1992) and also the reinterpretation of some data sets, which provide newer insights. DSS RESULTS IN THE CENTRAL INDIAN REGION The central part of the Indian sub-continent (between 20° N to 24° N and 70° E to 82° E) consists of several important geologic and tectonic features, the main among them being the east-west trending Narmada-Son lineament (NSL) and the Central Indian Suture (CIS). The NarmadaTapti region is dissected by a series of E-W/ENE-WSW trending faults. The faulting is generally of step type without any tilt of the blocks and with successive down throw towards south of the Narmada River. Major tectonic elements of the Narmada region are shown in figure 1 and the geological map of the region in the figure 2.