Significant Inferences on Deep Crustal Structure of Deccan Trap Region from Spectral Analyses of Bouguer Anomalies

Qualitative interpretation of the regional Bouguer anomaly map of the Deccan Trap region of peninsular India, based on the anomaly size, its gradient and minor contribution from the shallow crust assessed by the results of other geophysical methods, suggests that the major part of the these anomalies have deep seated origin with 'lows' corresponding to the zones of subsidence and 'highs' to the uplifts involving the crust. Deep faults along the west coast and segmented faults demarcating the Narmada rift valley and Narmada-Son Lineament (NSL) have been delineated. Spectral analysis, matched filter technique for deconvolving gravity effect of sources at a particular depth and a single interface gravimetric inversion scheme have been utilised to delineate density discontinuities and their 3D configurations. The deconvolved gravity effect due to the deepest density interface at a mean depth of 117 km seems to reflect lithospheric variations and is used to distinguish the stable crust from the metastable one. The Moho relief map, apart from structural features, exhibits a good agreement with the results by Deep Seismic Sounding (DSS) in Koyna area and across NSL.

The block-wise analyses reveal higher average depths to Moho and Conrad, which are typical of stable shield crust, in the southern part whereas the northern blocks associated with rift tectonics exhibit considerably thin upper crust (5.8 to 12.6 km) with another crustal interface at a depth in the range of 16.3 to 26.8 km. The latter depth forms the base of the middle crustal layer i.e., top of the lower crustal layer comprising lower mantle derivatives. These inferences are based on the correlation between Bouguer anomaly and upper crustal thickness and layered velocity structure brought out by DSS data across the NSL. Filtered Bouguer anomaly maps have further facilitated the interpretation and better demarcation of structural features inferred from qualitative interpretation.