Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

U. Raval ¹, D. K. Veeraswamy ²

Affiliations
1 National Geophysical Research Institute, Hyderabad - 500 007
2 National Geophysical Research Institute, Hyderabad - 500 007, IN

Abstract

The study examines the interaction between continental lithosphere (CL) and deep mantle plume. For this purpose, the Indian CL and the Reunion plume have been considered, with special emphasis on the consequent development of geophysical, tectonic and structural features. It reveals a bimodal pattern (linear and radial) of these manifestations. The radial one is correlated with the outburst of the plume-head, while the pre- and post-outburst phases follow a linear mode due to the traverse of Indian plate over the plume. It is seen that, the plume-CL interaction is facilitated by mobile belts and resisted by cratons.

Keywords

Mantle Plume, Continental Lithosphere, Bimodal Interaction, Outburst Phase, Western India.

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U. Raval ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad, India, IN

Abstract

Recent investigations of the seismic, electrical conductivity and rheological structures of the continental crust favours the presence of a fluid-rich layer at the mid and lower crustal levels. Such layers may exhibit high electrical conductivity, seismic reflectivity and ductility. In the light of this observation the results of (i) geomagnetic depth sounding (GDS) experiment carried out by means of a magnetometer array (MMA) spread over a large area along the Delhi-Aravalli belt (DA-B) and (ii) petrological investigations to recover the (P,T) regimes of the Vaikrita formation near the Main Central Thrust (MCT) are examined. It is suggested, on combining the findings of these studies. that owing to the-(a) thrusts and consequent uplifts of the crustal block and (b) intense compressive zones due to collision, the deeper crustal layer may be exposed or lie close to surface in the Himadri region. The metamorphic characteristics of the crystalline complex near the MCT and the high Himalayan topographic front corroborates this. The uparching (folding) and uplift of these (ductile) layers may, in turn release, with great force, the fluid (including free water) and volatiles. Such a thermotectonic mechanism may have significantly contributed in the development of the water regimes of the Himalayas. The mechanism involves (a) development of fluid (water) rich deep crustal layer and (b) intensely compressive regime as exists in the convergent boundary. The former may take place when the continental crust is thermally treated which may be the case, if the Reunion hotspot has carved out a trace (from Vaikrita to Cambay via DA-B) as has been recently suggested.

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U. Raval ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad-500 007, IN

Abstract

The subduction/collision processes and consequent great Himalayan uplift are studied in the light of the plausibility that large amount of aqueous fluids may accumulate at the boundary due to (a) compaction of oceanic and marginal basin sediments, (b) various metamorphic episodes and (c) meteoric water facilitated by high topography. This would imply that during the major phase transitions in the dynamics of the plate margin, transient reservoirs of fluids and attendant heat would develop along the orogenic belt. Manifestations of related tectonomagmatic and metamorphic processes along the Himalayan arc are therefore assessed. It is noticed that epicenters of the large earthquakes, and major river-sources and morphogenic peaks seem to occur over certain nodal zones along the collision boundary as also the leucogranitic plutons. We also examine the possibility of fluid expulsion from the topographic/collision front into the foredeep basins and continental interior. It is seen that linear trends of relatively 'large' activity seem to exist over the Indian platform which nearly coincide with its basement heterogeneities (BHs) which in tum form part of the mobile belts. These BHs appear to extend upto the Himalayan arc and would thus act as subterranean links between the boundary processes and intraplate region. In particular large distance migration of fluids and thermal energy may take place through focussing offlow along these BHs because during at least the major tectonic episodes the in situ hydraulic conductivity and/or permeability along the BHs could be sufficiently raised. The plausibility of the BHs acting as hot fluid channels, is quite crucial since it may play a controlling role in the development (or remobilization) of (i) ore-deposits and hydrocarbon fields, and (ii) many geophysical features of the continental crust such as brittle-ductile transition, low velocity layer (LVL), anomalously high conductive layer (HCL), deep seismic reflectors, hydrothermal circulation, high heat flow, seismicity, etc. It is shown that indeed some of these features have been inferred along the BHs. From this possibility a tectonothermal framework emerges which may be able to integrate a number of physicochemical processes and geophysical signatures having basic as well as applied significance.

Keywords

Fluid Processes, Plate Technics, Himalaya, Structural Geology.

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T. V. Sivaraman ¹, U. Raval ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad-500007, IN

Abstract

U -Pb isotopic analysis of zircons of three granitoids from Chhapoli, Kishangarh and Ajmer localities in the Delhi fold belt of Rajasthan, shows that these granitoids have been emplaced around 1700 Ma and were thermally disturbed at -70 Ma resulting in Pb loss in these zircons. Later studies by other workers on rhyolites, carbonatites and lamprophyres from Rajasthan and Gujarat, corroborate the -70 Ma event which was possibly a period of intense regional magmatic activity in this region. This Cretaceous thermal episode, when evaluated in the light of the paleogeography of the Indian plate, strongly suggests that the reactivation of this part of the subcontinent may be due to: (a) the continental breakups along the west coast at ∼80 and 64 Ma, (b) plausible passage of this part of the Indian continental lithosphere (CL), over the Reunion mantle plume between 80 to 60 Ma and (c) the earliest phase of the India - Eurasia Collision. It is also found that the existing knowledge of the geophysical anomalies (such as geothermal signatures, seismicity, gravity, magnetic and electrical conductivity) over the Delhi Aravalli mobile arm (DA-MA), Kutch, Jaisalmer- Mari- arch (JM-A), and Cambay graben - the latter two being parts of the NNW-SSE trending Madagascar-Dharwar mobile arm (MD-MA)- also support this inference.

Keywords

Geochronology, Zircons, Granites, Delhi-Aravalli Belt, Rajasthan.

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B. P. Singh ¹, U. Raval ²

Affiliations
1 Department of Geology, Banaras Hindu University, Varanasi - 221 005, IN
2 Hyderabad, IN

Abstract

No Abstract.

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U. Raval ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad, IN

Abstract

No Abstract.

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K. R. Subrahmanya ¹, K. S. Valdiya ², U. Raval ³

Affiliations
1 Department of Marine Geology, Mangalore University, Mangalagangotri - 574 199, IN
2 Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore -560 064, IN
3 National Geophysical Research Institute, Uppal Road, Hyderabad - 500 007, IN

Abstract

No Abstract.

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U. Raval

Abstract

No Abstract.

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