Lithospheric structure of Southern Indian shield and adjoining oceans: integrated modelling of topography, gravity, geoid and heat flow data
Résumé
For the present 2-D lithospheric density modelling, we selected three geotransects of more than 1000 km in length each crossing the southern Indian shield, south of 16 degrees N, in N-S and E-W directions. The model is based on the assumption of local isostatic equilibrium and is constrained by the topography, gravity and geoid anomalies, by geothermal data, and where available by seismic data. Our integrated modelling approach reveals a crustal configuration with the Moho depth varying from similar to 40 km beneath the Dharwar Craton, and similar to 39 km beneath the Southern Granulite Terrane to about 15-20 km beneath the adjoining oceans. The lithospheric thickness varies significantly along the three profiles from similar to 70-100 km under the adjoining oceans to similar to 130-135 km under the southern block of Southern Granulite Terrane including Sri Lanka and increasing gradually to similar to 165-180 km beneath the northern block of Southern Granulite Terrane and the Dharwar Craton. This step-like lithosphere-asthenosphere boundary (LAB) structure indicates a normal lithospheric thickness beneath the adjoining oceans, the northern block of Southern Granulite Terrane and the Dharwar Craton. The thin lithosphere below the southern block of Southern Granulite Terrane including Sri Lanka is, however, atypical considering its age. Our results suggest that the southern Indian shield as a whole cannot be supported isostatically only by thickened crust; a thin and hot lithosphere beneath the southern block of Southern Granulite Terrane including Sri Lanka is required to explain the high topography, gravity, geoid and crustal temperatures. The widespread thermal perturbation during Pan-African (550 Ma) metamorphism and the breakup of Gondwana during late Cretaceous are proposed as twin cause mechanism for the stretching and/or convective removal of the lower part of lithospheric mantle and its replacement by hotter and lighter asthenosphere in the southern block of Southern Granulite Terrane including Sri Lanka. Unusually thinned LAB beneath the region near Bangalore apparently indicates the preserved tectonocompositional effect of late Proterozoic rifted margin of Dharwar Craton.