D. M. Mall

The evolution of the Vindhyan basin vis-à-vis the Narmada-Son lineament, central India, from deep seismic soundings

Abstract The evolution of the Vindhyan basin has been studied from deep seismic soundings. Block tectonics appear to be active throughout the geological history in this region. The crust is divided into four major blocks by deep faults at Narsinghgarh, Katangi and Jabalpur. The crustal block between Narsinghgarh and Katangi forms a graben in the crystalline basement, giving rise to the great Vindhyan basin in which Precambrian Vindhyan sediments of about 5.5 km thickness are deposited. The crustal block between Katangi and Jabalpur is a horst block which represents the Narmada-Son lineament. A crustal depth section obtained from wide-angle reflections shows a large number of reflector segments up to the Mono boundary, indicating crustal heterogeneity. The boundary faults at the northern and the southern ends of the Vindhyan basin extend up to the Moho. The crustal thickness varies from 39.5 to 45 km.

Crustal structure of the northern part of the Proterozoic Cuddapah basin of India from deep seismic soundings and gravity data

Abstract A crustal depth section was obtained from Deep Seismic Soundings (DSS) along the Alampur-Koniki-Ganapeshwaram profile, cutting across the northern part of the Proterozoic Cuddapah basin, India, running just south of latitude 16° N and between longitude 78° E and 81°E. The existence of a low-angle thrust fault at the eastern margin of the Cuddapah basin (Kaila et al., 1979) was confirmed along a second profile. Another low-angle thrust, along which charnockites with the granitic basement are upthrust against the Dharwars was delineated further east. The contact of the khondalites (lower Precambrian) with quaternary sediments near the east coast of India seems to be a fault boundary, which may be responsible for the thick sedimentary accumulation in the adjoining offshore region. The basement in the western part of the Cuddapah basin is very shallow and is gently downdipping eastward, to a depth of 1.7 km about 20 km west of Atmakur. It attains a depth of about 4.5 km in the deepest part of the Kurnool sub-basin, around Atmakur. Under the Nallamalai ranges its depth varies between 3.5 and 6.5 km, with an easterly dip. In the region north of the Iswarkuppam dome, the basement is at a depth of about 5.0 km, to about 6.8 km in the eastern part of the Cuddapah basin. Outside the eastern margin of the basin, the depth of the basement is about 1.8 km and further eastwards it is exposed. A fault at the contact of the khondalites with quaternary sediments near the east coast brings the basement down to a depth of approximately 1.3 km. In the Kurnool sub-basin the depth to the Moho discontinuity varies from 35 km under Atmakur to 39 km under the Nallamalai hills. In the region of the Iswarkuppam dome it is at a depth of about 36 km, deepening to about 39 km before rising to 37 km towards the east. Two-dimensional velocity modelling using ray-tracing techniques tends to confirm these results. Gravity modelling of the crustal structure, utilizing a four-layer crustal model in most parts along this profile, conforms to the observed gravity values. A weak zone in the eastern part of the profile where high-density material (density 3.05 g/cm3) has been found seems to be responsible for the gravity high in that part.

Crustal accretion beneath the Koyna coastal region (India) and Late Cretaceous geodynamics

Abstract In 1967 a major earthquake in the Koyna region attracted attention to the hitherto considered stable Indian shield. The region is covered by a thick pile of Deccan lava flows and characterized by several hidden tectonic features and complex geophysical signatures. Although deep seismic sounding studies have provided vital information regarding the crustal structure of the Koyna region, much remains unknown. The two available DSS profiles in the region have been combined along the trend of Bouguer gravity anomalies. Unified 2-D density modelling of the Koyna crust/mantle suggests a ca. 3 km thick and 40 km wide high velocity/high density anomalous layer at the base of the crust along the coastline. The thickness of this anomalous layer decreases gradually towards the east and ahead of the Koyna gravity low the layer ceases to be visible. Based on the seismic and gravity data interpretation in the geodynamical/rheological boundary conditions the anomalous layer is attributed to igneous crustal accretion at the base of the crust. It is suggested that the underplated layer is the imprint of the magmatism caused by the deep mantle plume when the northward migrating Indian plate passed over the Reunion hotspot.

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.