Pr. Pant et al., Significant inferences on deep crustal structure of Deccan Trap region from spectral analyses of Bouguer anomalies, J GEOL S IN, 53(3), 1999, pp. 315-328
Qualitative interpretation of the regional Bouguer anomaly map of the Decca
n Trap region of peninsular India, based on the anomaly size, its gradient
and minor contribution from the shallow crust assessed by the results of ot
her geophysical methods, suggests that the major part of the these anomalie
s have deep seated origin with 'lows' corresponding to the zones of subside
nce and 'highs' to the uplifts involving the crust. Deep faults along the w
est coast and segmented faults demarcating the Narmada rift valley and Narm
ada-Son Lineament (NSL) have been delineated. Spectral analysis, matched fi
lter technique for deconvolving gravity effect of sources at a particular d
epth and a single interface gravimetric inversion scheme have been utilised
to delineate density discontinuities and their 3D configurations. The deco
nvolved gravity effect due to the deepest density interface at a mean depth
of 117 km seems to reflect lithospheric variations and is used to distingu
ish the stable crust from the metastable one. The Moho relief map, apart fr
om 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, wh
ich are typical of stable shield crust, in the southern part whereas the no
rthern blocks associated with rift tectonics exhibit considerably, thin upp
er 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 cru
stal layer i.e., top of the lower crustal layer comprising lower mantle der
ivatives. These inferences are based on the correlation between Bouguer ano
maly and upper crustal thickness and layered velocity structure brought out
by DSS data across the NSL. Filtered Bouguer anomaly maps have further fac
ilitated the interpretation and better demarcation of structural features i
nferred from qualitative interpretation.