HR: 1340h
AN: T43C-1338    [Abstracts]
TI: An integrated approach for determination of nature of the crust in the Laxmi Basin, western continental margin of India
AU: * Krishna, K S
EM: krishna@darya.nio.org
AF: National Institute of Oceanography, Dona Paula, Panaji, Goa 403004 India
AU: Gopala Rao, D
EM: drgopalarao@yahoo.com
AF: Department of Geology, Osmania University, Hyderabad, 500007 India
AU: Sar, D
AF: KDMIPE, Oil and Natural Gas Corporation Limited, 9, Kaulagarh Road, Dehradun, 246195 India
AB: The nature of the crust in the Laxmi Basin, northwest continental margin of India is an uncertain issue and more importantly this is limiting our understanding of the evolution of the lithosphere in the Arabian Sea. The aspect has become a key issue principally from the point of paleogeographic reconstructions of the western Indian Ocean. In order to determine the nature of the crust below the Laxmi Basin we have analysed two new geophysical datasets in combination with previously published datasets, thereafter gravity and magnetic anomalies were modelled with the constraints from seismic reflection and refraction results. The sediments derived mostly from the Indus cone are generally thicker on west of the Laxmi Ridge than that in the Laxmi Basin. The basement in the Laxmi Basin includes numerous intrusive structures and faulted blocks, thus the basement topography becomes highly irregular and shallower compared to that of the Western Basin. The ship-borne and satellite gravity anomalies had enabled to map the regional extent of the structures in the basin especially the gravity lows associated with the Laxmi and Panikkar ridges. Intrusive structures mapped in the Laxmi Basin are seen coinciding with significant magnetic anomalies, but these anomalies were earlier considered as pre-Tertiary spreading-related anomalies. Free-air gravity anomalies of the Laxmi Ridge and Laxmi Basin are in general anomalous as they are associated with negative and positive gravity anomalies respectively. Within the basin the gravity anomaly is gradually rising toward north, reaches 40 mGal more, indicating the accretion of additional magmatic material toward north within crust. The velocity structures of the Laxmi Ridge, Laxmi Basin, Western Basin, Seychelles Bank and Indian continental shield reveal the Moho boundary only below the Seychelles Bank, Western Basin and Indian subcontinent. An anomalous velocity layer, 7.15 to 7.19 km/s is observed in the lower crust of the Laxmi Basin and Laxmi Ridge. The velocities are contrastingly deviating from the normal velocity structure of both continental and oceanic crust. On close observation of velocity structure we found that 6.2 - 6.3 km/s layer is conspicuously present in the upper crust of all the geological provinces except below the Western Basin. Instead the Western Basin bears about 6.7 km/s velocity layer, which is considered as a normal velocity for layer 3 of the oceanic crust. As the velocity layer 6.2 - 6.3 km/s in general represents the upper crustal rocks (granitic gneisses) of the continental crust, the similar velocity layer below the Laxmi Basin may be attributed to upper crustal rocks of the continental crust. The gravity and magnetic model studies have revealed that the Laxmi Basin consists of 11-14 km stretched continental crust, in which magmatic bodies have been emplaced, while the Panikkar Ridge in the middle of the basin has relatively less disturbed stretched continental crust. The seaward dipping reflectors (SDRs) on western margin of the Laxmi Ridge and sharp changes in regional gravity and magnetic fields across it lead to place the ocean-continent boundary on west of the Laxmi Ridge. The Reunion hotspot, when the basin was passing over it, had emplaced the volcanic material in the form of dykes and sills within existing stretched continental crust. We, therefore, believe that the Laxmi Basin basically consists of continental crust, which subsequently got modified by extensive stretching and volcanic outpourings of the Reunion hotspot.
DE: 3025 Marine seismics (0935)
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 1219 Local gravity anomalies and crustal structure
DE: 1234 Regional and global gravity anomalies and Earth structure
DE: 1517 Magnetic anomaly modeling
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting