Janardan G. Negi

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.

Three-dimensional model of the Koyna area of Maharashtra State (India) based on the spectral analysis of aeromagnetic data

The estimation of thickness of trap rocks in the earthquake‐affected Koyna area is an important parameter for revealing the topography that existed before the Deccan volcanism. In the present work, a case history is presented delineating a three‐dimensional block model for the Koyna area by the spectral analysis of aeromagnetic data. The thickness in the area was found to vary from 700 to 2200 m, which correlates well with the results of other geophysical investigations.

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.

Signals of degeneration of the sub-crustal part of the Indian lithosphere since the break-up of Gondwanaland

Abstract Currently, the continental fragments of the erstwhile Gondwanaland supercontinent are widely separated. A relative comparison of Magsat anomalies of these fragments reveals a striking similarity across all the rifted continental margins except at the western and southern margins of India. It reflects that magnetized crust has largely remained unchanged elsewhere since the break-up of Gondwanaland. A similar analysis of satellite derived free-air gravity anomalies also indicate a similar continuity across other rifted margins except those of India. These anomalies characterise the existing differences in the crustal and upper-mantle structures and causal dependence on the relative mobilities of the Gondwanic continents. It is evident that after the break-up, only the Indian continental structure has suffered a dramatic change, possibly due to its super northward drift from Antarctica during Late Cretaceous—Early Tertiary.

High Moho temperature in the Indian shield

Abstract Moho temperatures in the Indian shield region are estimated using step, exponential and power-law models of the depth variation of radiogenic heat, with temperature-dependent thermal conductivity. The Moho temperatures and heat flows are around 550–600°C and 0.8 HFU (33 mWm−2) in the southern part of the Indian shield and 850–900°C and 0.95 HFU (40 mWm−2) in its northern part. It is argued that such high temperatures and their lateral variations may contain signatures of Deccan Trap volcanism and Himalayan orogeny. The implications of the temperature estimates on some related crustal parameters viz., P n - wave velocity, Curie isotherm depth and melting point temperature at Moho depth, are also indicated.

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.

ELECTROMAGNETIC SCREENING DUE TO A DISSEMINATED SPHERICAL ZONE OVER A CONDUCTING SPHERE

The electromagnetic field response under the influence of an alternating magnetic field is investigated for an isometric ore body covered by a disseminated spherical zone. The embedding medium is homogeneous and a relatively poor electrical conductor. The formal solution of the diffraction problem is obtained when the overlying non‐uniform conducting shell has a general spherical symmetry. More complete analysis is made for three important cases where the screening shell is assumed to have: 1. a power law conductivity distribution, 2. propagation constant decreasing linearly outside, and 3. a homogeneous electrical behaviour. The computational procedure for the evaluation of the in‐phase and quadrature components of the screening factor for the last case is presented along with some relevant curves for parametric values suitable in the exploration of massive sulfide deposits. The computed results establish a paradoxical phenomenon of negative screening, meaning that under favorable conditions an ore body ...

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.

Periodicities of palaeomagnetic intensity and palaeoclimatic variations: a Walsh spectral approach

Abstract The Walsh power spectral technique has been applied to the non-sinusoidal palaeoclimatic (theGloborotalia menardii climatic curve) and palaeomagnetic intensity variation records of the last 1.2 Myr. The analysis reveals, in the case of palaeomagnetic variations, statistically significant peaks (at 90% confidence level) centered around 150, 60 and 24 kyr. The palaeoclimatic spectrum, however, indicates only two significant peaks of 60 and 24 kyr. The identical periodicities in both the phenomena may suggest a causal link between the geomagnetic intensity variations and climatic changes.