Significant results pertaining to the shallow and deep structural features of the Indian continental crust are achieved by extensive Deep Seismic Sounding (DSS) experiments accomplished by NGRI since 1972 in a variety of geological settings. Seismic refraction and wide-angle reflection data-sets have been acquired along 20 profiles in the southern peninsular shield, Deccan Traps covered regions, across the Narmada-Son lineament, the Himalayas and more recently in the West Bengal basin, covering more than 5000 km of DSS profiling. Interpretation of these datasets brought out for the first time, models of the shallow and deep crustal structure and the Moho configuration delineating fault-controlled crustal blocks (terranes) in various regions. These results provide new insights to a better understanding of the tectonic processes involved in the formation and evolution of the Cuddapah, Dharwar, Vindhyan, Gondwana, and the Cambay basins. The crustal depth sections in the south Indian shield along the Kavali-Udipi profile reveal that the Cuddapah basin on the east and the Dharwar sub-basin on the west were both formed by downfaulting of the Moho. Due to a major thrust movement along a low-angle fault, well delineated by the DSS data, the Eastern Ghats section comprising the Dharwars was thrust up and lies over and in juxtaposition with the upper Cuddapah sediments on the eastern margin of the basin. The large Dharwar sub-basin was subdivided into two parts, the eastern depression and the western depression by an uplifted crustal block where the Closepet granites are now exposed. Besides mapping the thickness of the Deccan Trap flows, a large Mesozoic basin, consisting of two grabens separated by an E-W trending fault, the 'Tapti graben' (with maximum sedimentary thickness of 1800 m) and the 'Narmada graben' (with maximum sedimentary thickness of 1000 m), is delineated under a relatively thin cover of the Deccan Traps in the Narmada-Tapti area from the results obtained along six DSS profiles in the region. This hidden Mesozoic basin probably formed part of a larger Mesozoic sea bordering the exposed Vindhyans, Aravallis and the Delhis. Another relatively small Gondwana basin with a maximum sedimentary thickness of 400 m, separated from this large Mesozoic basin is also delineated in the Multai-Pulgaon region, that may be an extension of the Gondwana Godavari graben, under a thin cover of the Deccan Traps. The crustal block bounded by deep East-West faults at Narsinghgarh and Katangi along the Hirapur-Mandla profile across the Narmada-Son lineament, created a graben in the crystalline basement forming the great Vindhyan basin with accumulation of a large thickness of the Vindhyan sediments. The Deccan Traps at the base of a thick sequence of the Tertiary sediments, as well as the subtrappean Mesozoic sediments in the Cambay basin are also well resolved by the DSS datasets. The high velocity lower crustal layer of velocity 7.3-7.4 km/s delineated from Billimora to Tharad throughout the Cambay basin indicates underplating of the crust due to mantle upwelling and rifting with large scale extrusion of the Deccan Traps. The region from Surat to Bombay on the west coast was also characterized by an upwarp of the Moho during the late Cretaceous period, probably representing a transitional crust and a major source for the Deccan Trap flows. The DSS datasets in the wide-angle range also yielded velocity-depth models of the Indian continental crust in various regions, revealing presence of low velocity layers, transitional nature of the Moho and other intracrustal boundaries, providing a unique geophysical framework to understand the rheological structure and intracontinental seismicity, particularly in the Deccan Traps covered Koyna region. The Moho boundary is delineated with steep dips of about 15-degrees-25-degrees from the Kashmir valley towards the Great Himalayas, with its depth reaching 65-67 km in the region of Kanzalwan in the Great Himalayas. The Moho appears to rise to shallower depths further north towards the Nangaparbat due to a local reversal of the dip across a deep fault in the region that might be associated with the uplift of the Nangaparbat massif. It was inferred that the Himalayas were formed possibly due to block uplift against steep-angle faults. The crustal depth section across the Pir Panjal range, besides delineating the Moho, also brought out the Srinagar and Jammu synclines each having a maximum sedimentary thickness of about 18 km. The deep reflection data along the long-range profile Qarrakol-Zorkol-Sopur-Tral revealed presence of two broad zones of reflectors at depths of 150-180 km and 340-365 km that may respectively correspond to the top and bottom of the Asthenosphere layer in the region of the Great Himalayas, Karakoram and Pamir ranges. Adequate facilities have been recently developed at NGRI for seismic imaging of the continental crust and the subcrustal lithosphere by deep seismic reflection profiling and stacking of near-vertical-incidence reflection data extending to about 20 s two-way-times. Experimental results of reflection profiling and stacking of reflection data in the West Bengal basin revealed promising results consistent with the coincident wide-angle reflection data to the Moho depths.
