Thin magnetic crust in southern granulite terrain

Abstract

Aeromagnetic anomaly map of India covering the region between 8° to 17° north is prepared using data re-digitized at 6 minutes interval. The map distinctly brings out several tectonic features. Analytical signal of this map identifies the sources as charnockites in the Southern Granulite Terrain (SGT) to the south of the orthopyroxene isograd and essentially the iron ore belts and intrusives in the Dharwar craton to the north. As the source field of the magnetic anomaly within the SGT and the Dharwar craton are distinctly different, the data are subjected to high pass filtering process and the depths are calculated from the spectral plots, in order to peep into the source depth of the magnetic anomaly in the two regions. The results are constrained by the filtered Bouguer gravity anomalies. From the filtered gravity data, we find that gravity residuals are reproduced at a filtered wavelength of 180 km in both SOT and the Dharwar craton; wh6reas, for the magnetic data, the source distribution is a function of the area under consideration. In Dharwar craton, the magnetic sources extend up to a depth of around 36 km, possibly reflecting the average Moho depth. In the SGT. the magnetic crust is thin and is confined to within the first 22 km, with the deeper levels not contributing significantly to the magnetic signatures. Kuppain Palani seismic reflection / refraction profiles Also depict a change in velocity around this depth in The SGT. We conclude that below the exhumed crust Grille SGT, there is a lithological / mineralogical change at around 22 km depth caused possibly by magnetic under plating or the Curie isotherm is shallow when compared to the Dharwar craton.