Shallow velocity structure along the Hirapur-Mandla profile using traveltime inversion of wide-angle seismic data, and its tectonic implications

In order to investigate the velocity structure, and hence shed light on the related tectonics, across the Narmada-Son lineament, traveltimes of wide-a ngle seismic data along the 240 km long Hirapur-Mandla profile in central I ndia have been inverted. A blocky, laterally heterogeneous, three-layer vel ocity model down to a depth of 10 km has been derived. The first layer show s a maximum thickness of the upper Vindhyans (4.5 km s(-1)) of about 1.35 k m and rests on top of normal crystalline basement, represented by the 5.9 k m s(-1) velocity layer. The anomalous feature of the study is the absence o f normal granitic basement in the great Vindhyan Graben, where lower Vindhy an sediments (5.3 km s(-1)) were deposited during the Precambrian on high-v elocity (6.3 km s(-1)) metamorphic rock. The block beneath the Narmada-Son lineament represents a horst feature in which high-velocity (6.5 km s(-1)) lower crustal material has risen to a depth of less than 2 km. South of the lineament, the Deccan Traps were deposited on normal basement during the u pper Cretaceous period and attained a maximum thickness of about 800 m.