P.K. Agrawal

Super-mobility of hot Indian lithosphere

Negi. J.G., Pandey. O.P. and Agrawal, P.K.. 1986. Super-mobility of hot Indian lithosphere. Tectonoph.vsics. 131: 147-156. The great mobility of the Indian subcontinent during the last 180 million years ( - 9000 km of south-north motion), with an anticlockwise rotation of about 60 O, has long been a major puzzle for theorists of plate tectonics. A relative analysis of the available heat flow data, the estimated temperature-depth regime of India and the gravity anomalies, reveals that the Indian lithosphere does not have the characteristics of a typical shield zone and that it has higher radioactivity, smaller viscous drag (decidedly low viscosity of - 10” P). lower density and a well-defined low-velocity zone as compared to other shield areas of the world. These factors appear to contribute significantly to the faster northward movement of the thin Indian lithosphere as compared to the almost immobile African plate.

Madagascar: A continental fragment of the paleo-super Dharwar craton of India

The morphological kinship of Madagascar to its immediate neighbors on the west (African continent) and east (Indian subcontinent) during the Early and middle Cretaceous has been debated for the past two decades on the basis of available geologic, tectonic, and paleomagnetic information. Most of the paleoreconstructions of Madagascar have shown its attachment to the east African continent. We present magnetic satelite and gravity data, and morphological, geophysical, and geotectonic similarities to hypothesize that in the period before the breakup of Gondwana, Madagascar was a continental fragment of the paleo-super Dharwar craton of India.

Lithospheric structure beneath Laxmi Ridge and late Cretaceous geodynamic events

The geophysically unusual Laxmi Ridge (eastern basin, Arabian Sea) is associated with a prominent elongated negative gravity anomaly. A seismically and geodynamically constrained detailed 2D gravity modeling suggests an 11-km-thick normal oceanic crust and an asthenospheric upwarp to a depth of 35 km. We attribute the apparent thickening of the crust to a possible emplacement of an anomalous subcrustal low-density layer between 11 and 19 km depth. We hypothesize that a K-T boundary bolide impact near the Bombay offshore led to several geological events, including eruption of Deccan flood basalts. The spreading Carlsberg Ridge in the Indian Ocean and rifting associated with Deccan volcanism generated the compressive regime, which perhaps originated the Laxmi Ridge.

A possible K-T boundary bolide impact site offshore near Bombay and triggering of rapid Deccan volcanism

Abstract The temporal coincidence of a major biological mass extinction (including dinosaurs), the well-known iridium excess anomaly at the Cretaceous-Tertiary (K-T) boundary and the eruption of Deccan flood basalts at about 65 Ma has aroused global interest among geologists and biologists. It is widely debated whether the mass extinction and iridium anomaly are due to an asteroid impact or the massive outpouring of extensive Deccan volcanism. An oval shaped unusual positive gravity anomaly (10 000 km 2 in area) near Bombay has attracted our attention during a search for an impact site near Deccan basalts. A detailed gravity interpretation indicates the presence of a fossil conduit structure of 12 km height extending from a shallow crust-mantle boundary (at 18 km) to an approximate depth of 6 km from the surface. The conduit structure, with a maximum diameter of about 35 km at its base, may originate from cracking of a weak pre-Deccan trap shallow upwarped mantle. The structure may have been caused by a bolide impact which triggered the eruption of massive flood basalts (Deccan traps) on the western margin of the fast-moving Indian plate. An impact in this locality can explain the sudden detachment of the arcuate Seychelles block from India as well as the large-scale reorganisation of plate boundaries in the Indian Ocean. Our hypothesis of impact-triggered volcanism at 65 Ma advocates a bimodal cause for the mass extinction at the K-T boundary. Extraordinary geothermal and structural conditions of the nearby region are also discussed as circumstantial evidence to support the twin-cause mechanism by weakened features and the presence of partial melt at subcrustal depth.

Evidence of low density sub-crustal underplating beneath western continental region of India and adjacent Arabian Sea: Geodynamical considerations

The known high mobility of the Indian subcontinent during the period from 80 to 53 Ma has evoked considerable interest in recent times. It appears to have played an important role in shaping the subcontinental structures of western India and the adjoining Arabian Sea. During this period, a major catastrophic event took place in the form of Deccan volcanism, which coincides with the biological mass extinction at the K-T boundary, including the death of dinosaurs. The origin of Deccan volcanism is still being debated. Geophysically, western India and its offshore regions exhibit numerous prominent anomalies which testify to the abnormal nature of the underlying crust-lithosphere. In this work, we develop a two-dimensional structural model of these areas along two long profiles extending from the eastern basin of the Arabian Sea to about 1000 km inland. The model, derived from the available gravity data in the oceanic and continental regions, is constrained by seismic and other relevant information in the area, and suggests, for the first time, the presence of an extensive low-density (2.95–3.05 g/cm3) sub-crustal underplating. Such a layer is found to occur between depths of 11 and 20 km in the eastern basin of the Arabian Sea, and betweeen 45 and 60 km in the continental region where it is sandwiched in the lower lithosphere. The low density may have been caused as a result of serpentinization or fractionation of magma by a process related in some way to the Deccan volcanic event. Substantial depletion of both oceanic and continental lithosphere is indicated. We hypothesize that the present anatomy of the deformed lithosphere of the region at the K-T boundary is the result of substantial melt generated owing to frictional heat possibly giving rise to a hot cell like condition at the base of the lithosphere, resulting from the rapid movement of the Indian subcontinent between 80 and 53 Ma.

Crustal magnetisation-model of the Indian subcontinent through inversion of satellite data

Abstract An apparent magnetisation-model of the Indian subcontinent and the adjoining regions has been obtained through the inversion of MAGSAT total field (scalar) and vertical field data. A staggered 4° × 4° grid pattern with a constant continental crustal thickness of 40 km yielded magnetisation values over a 2° × 2° grid. Over the major part of the Indian subcontinent, the magnetisation picture brings out a prominent “high” sandwiched between the Eastern Ghats and the main boundary fault of the Himalayas. In contrast, the identity of the Western Ghats, comprising mainly Deccan Traps, is missing. While a distinct magnetisation “low” is observed over the known hot-spot region of the Saurashtra Peninsula, two magnetisation “highs”—one over the Shillong Massif and the other over the Central part of India covering Aravallis and the Bundelkhand Massif—are obtained. Interestingly, even at a height of 400 km, the well known Narmada-Son lineament is clearly reflected in the total intensity map as well as in the corresponding magnetisation map. The Bay of Bengal and the Himalayas ara characterised by magnetisation “lows”.

Thermal regime, hydrocarbon maturation and geodynamic events along the western margin of India since late Cretaceous

Abstract The passive continental margin of western India and the adjacent offshore region are associated with a transitional type thinned crust. It contains several sedimentary basins where substantial recoverable oil/gas reserves exist. The northern Cambay graben, northern and eastern parts of the Bombay offshore and the Konkan coast region that are situated close to western margin exhibit reasonably high heat flow and geothermal gradients beneath which the asthenosphere is upwarped to a depth of 30–70 km. Temperatures at the depth of 3 km are estimated to be in the range of 105–260°C. Curie depth analysis from MAGSAT studies in an area between latitudes 11°N and 19°N and longitudes 65°E and 73°E also indicates a high geothermal gradient of about 30°C/km within the upper crustal column. We suggest that the occurrence of oil and gas in these areas may be due to catastrophic and geodynamic events which took place in the last 130 Ma. India’s super-mobility, continental breakups, possible bolide impact and Deccan volcanic episode at the western margin resulted in substantial lithospheric heating, accompanied by subcrustal melting and rise of isotherms, to eventually enhance the hydrocarbon maturation process. The study indicates that all other sedimentary basins situated on the western margin are also thermally mature and may have high potential for the occurrence of hydrocarbons.

MAGSAT data and Curie-depth below Deccan flood basalts (India)

Ground and airborne magnetic data are severely disturbed due to random susceptibility variations in Deccan flood basalts. However, Magnetic Satellite (MAGSAT) data over the Deccan flood basaltic region of the Indian subcontinent exhibit filtering of surficial noise. Three passes over Deccan traps show a “low” at about 20°N latitude and a “high” at about 23°N latitude. Spectral analysis of these passes and an available 2-D MAGSAT vertical intensity map indicate a deep (40±4 km) magnetic interface. It is interesting to note that the determination of Curie-depth from MAGSAT matches and confirms the geothermal data model. The estimates correspond to the Moho depth derived from gravity and deep seismic sounding studies. The study suggests a continental shield-like geothermal gradient of about 14°C/km below the area.

Relevance of hot underlying asthenosphere to the occurrence of Latur earthquake and Indian peninsular shield seismicity

Abstract The cause of the highly destructive Latur earthquake of September 30, 1993, which occurred below the flood basaltic region of the Indian peninsular shield killing more than 10,000 people, is still not well understood despite several geoscientific investigations carried out after the main event. In the present work, we have examined in detail multiparametric geophysical data to understand its origin in particular and the seismicity of the Indian shield in general. Our study suggests that the Indian peninsular region is characterised by large variation in asthenospheric depths from 31 km to 186 km, depending on tectonic segments. The unusual seismic activity thus appears to stem from hot and upwarped underlying asthenosphere, which causes continuous build-up of localised stresses due to differential isothermal rise, and large lateral tem- perature differences on a regional scale beneath the highly fragmented Indian shield. The shield appears to be undergoing large scale rejuvenation and has become much more unstable than other global shields.

Can depression of the core-mantle interface cause coincident Magsat and geoidal ‘lows’ of the Central Indian Ocean?

Abstract According to Hide, undulations at the core-mantle interface distort geopotential fields that can be observed in lower order harmonics. Hide and Malin obtained a significant correlation between long wavelength gravity and non-dipole magnetic fields, when the latter is displaced in longitude by 160°. The Central Indian Ocean is characterised by the largest geoidal low, which correlates well with the Magsat component low without any longitudinal displacement. An integrated interpretation of geoidal and magnetic anomalies suggests that the source of these anomalies should be the same and may originate at the depression of the core-mantle boundary. The shallow Curie-isotherm, the matching of the long wavelength of the geoidal anomaly with the vertical Magsat anomaly and the identical location of the centres of the anomalies, provide an excellent confirmation of Hides hypothesis, but contradicts earlier geoidal low interpretations of mass deficiency in the Earths crust and mantle.