K. L. Kaila

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.

Deep seismic sounding in the Godavari Graben and Godavari (coastal) basin, India

Abstract Deep Seismic sounding investigations along two profiles, one along and the other across the Godavari Graben delineate the northeastern extension of the Bapatla Ridge which separates the Godavari Graben from the Godavari (coastal) Basin. The Godavari Graben is filled with a maximum of 2.8 km thick Lower Gondwana (Upper Permian to Lower Triassic) sediments. It is inferred on the basis of P-wave velocities that the Upper Precambrian Pakhal Formation does not continue south of Paloncha. The Upper Gondwana (Upper Jurassic to Lower Cretaceous rocks with marine incursions appear to continue towards the southeast for about 15 km from the exposures, below the Rajahmundry sandstone (Mio-Pliocene). Two-dimensional velocity modelling of the upper crust indicates a major interface at 3.5 km depth in the upper crust where the velocity increases from 5.4–5.5 km s−1 to 6.2–6.4 km s−1. This interface shows a dpmal upwarp across the Godavari Graben which may be an indication of NE-SW crustal extension in this region. The deeper part of the crust consists of a large number of almost horizontal small reflector segments. Two-dimensional crustal velocity modelling for some correlatable wide-angle reflections yielded a three-layered structure below 10 km depth, with the Moho at about 41 to 43 km.

A crustal density model across the Cambay basin, India, and its relationship with the Aravallis

Abstract The Cambay rift basin in the northwest Indian platform shows high Bouguer gravity anomaly values as compared to the values outside the basin. The seismic results show the presence of a large thickness of sediments, which should have produced a gravity low of about 50–60 mGal. The deep-seismic sounding results along the strike of the basin show the presence of a velocity of 7.3 km/s at 23–25 km depth, followed by the Moho at a depth of 31–33 km. Two-dimensional density modelling of the seismic structure across the Cambay basin shows that the crust is thinner underneath the basin, and is associated with a high-density lower crust. A comparison of the gravity anomaly across the Cambay basin with that across the Proterozoic Aravalli/Delhi trends suggests crustal thinning in both the regions, on either side of which the Moho deepens rapidly.

Upper mantle velocity structure in the Mediterranean and surrounding regions from seismological data

Abstract Upper mantle velocity structure in the Mediterranean and surrounding regions has been studied in detail to a depth of 500 km by analysis of P- and S-wave travel-time data of 229 deep earthquakes. P- and S-wave velocities, obtained “at the depths of foci” by using Kailas (1969) analytical method, in the intermediate and deep earthquake regions of the Calabrian arc-Tyrrhenian Sea, western Mediterranean-southern Spain, western and southern margins of the Balkan peninsula and the Vrancea region of the eastern Carpathian reveal lateral velocity variations of the order of 3–4% down to 150 km depth, the P velocities in the Balkan margins being the highest. In the Calabrian arc-Tyrrhenian Sea deep earthquakes region, the P velocity increases from 8.01 km/s at 40 km to 8.14 km/s at 210 km depth. The P velocity increases rapidly at deeper depths from 210 km with a gradient of 0.005 s −1 , reaching 9.62 km/s at 500 km depth. The S velocity in this region increases from 4.58 km/s at 40 km to 4.73 km/s at 400 km depth without any indication of a zone of rapid velocity increase. However, the S velocity in the Calabrian arc-Tyrrhenian Sea deep earthquakes region is found to be about 6% higher than the average S velocity of 4.34 km/s in the top 150–200 km of the surrounding upper mantle region as deduced from surface waves dispersion studies. Similarly, the S velocities in the intermediate earthquake regions of the western Mediterranean and the Balkan margins are found to be about 5% higher than the corresponding average values in the surrounding upper mantle regions. These variations are essentially due to the presence of a well delineated sub-lid low S velocity channel extending over most of central Europe. The P velocity model for the Calabrian arc-Tyrrhenian Sea deep earthquakes region, determined in the present study, agrees remarkably well with the P velocity models EKW (England et al., 1977) and MR (Mayer-Rosa and Mueller, 1973) which are applicable for the upper mantle beneath south-central Europe. A comparison of the P velocity models in south-central Europe, including that of the present study, with the P velocity models SNE (Hurtig et al., 1979), KCA (King and Calcagnile, 1976) and MA (Masse and Alexander, 1974) which are applicable for the upper mantle beneath the east European platform reveals strong lateral velocity variations in the upper mantle beneath the two regions. The velocities in south-central Europe are about 5% lower than those in the east European platform to a depth of at least 250 km. These results are consistent with the temperature-depth functions for Europe given by Stromeyer (1978) which show 400°–500°C higher temperatures at depths in central Europe than those in the east European platform. The nearly constant S velocity of 4.6–4.7 km/s from 40 to 400 km depth in the Calabrian arc-Tyrrhenian Sea deep earthquakes region, as determined in the present study, is also consistent with the higher temperatures expected in the surrounding upper mantle region beneath central Europe. The “400 km discontinuity” has not been revealed by both P- and S-wave velocity models down to 500 km depth in the present study, possibly due to its larger depth of 520–540 km in south-central Europe, in comparison to its relatively shallow depth of 380–420 km in the east European platform as given by several models. Alternatively, the high P-wave velocity gradient from 210 to 500 km depth may imply absence of a “sharp” 400 km discontinuity which is probably replaced by a broad transition zone in this region.