Aeromagnetic Evidence of Crustal Structure in the Granulite Terrane of Tamil Nadu-Kerala

As conventional mapping of near-surface geology has been completed already on a certain scale for most part of the country, the primary objective of the on-going National Aeromagnetic Survey seems to lie in uncovering the structural configurations at depth by producing relief maps at one or more levels within the crustal column, although such maps cannot but be tentative and open to revision in the light of new data.

This exercise has been attempted for the peninsular segment south of 12°N latitude resulting in the production of a map of a relatively shallow magnetic basement. A filtered (low-pass) anomaly map is also presented that apparently reflects a deeper simatic layer and, therefore, the Moho, by implication. The area seems to be composed of a mosaic of independent crustal blocks involved in relative vertical movements. A graben is indicated sub-parallel to the west coast extending into the Gulf of Mannar and possibly beyond into Sri Lanka. The Palghat-TiruchirapalJi region is also downfaulted suggesting a junction of profound structural dislocation between the granulite terrane and the Karnataka craton. An overall increase in crustal thickness towards north and east is also inferred. The gravity rise towards the Tamil Nadu coast (in striking contrast with the uneventful fatI in magnetic values) may have to be explained in terms of lateral changes in crustal and subcrustal parameters rather than in terms of a simile rise of the Moho.

These results should hopefully encourage the generation of subsurface maps of structure even for the rest of the country based on aeromagnetic and other ground geophysical data, thus, ushering in a new chapter of Indian geology in three dimensions.