The 2-D crustal velocity model along the Hirapur-Mandla DSS profile across the Narmada-Son lineament in central India (MURTY et al., 1998) has been updated based on the analysis of some short and discontinuous seismic wide-angle reflection phases. Three layers, with seismic velocities of 6.5-6.7, 6.35-6.40 and 6.8 km s )1 , and upper boundaries located approximately at 8, 17 and 22 km depth respectively, have been identified between the basement (velocity 5.9 km s )1 ) and the uppermost mantle (velocity 7.8 km s )1 ). The layer with 6.5-6.7 km s )1 velocity is thin and is less than 2-km deep between the Narmada north (at Katangi) and south (at Jabalpur) faults. The upper crust shows a horst feature between these faults, which indicates that the Narmada zone acts as a ridge between two pockets of mafic intrusion in the upper crust. The Moho boundary, at 40-44 km depth and the intra-crustal layers exhibit an upwarp suggesting that the Narmada faults have deep origins, involving deep-seated tectonics. A smaller intrusive thickness between the Narmada faults, as compared to those beyond these faults, suggests that the intrusive activities on the two sides are independent. This further suggests that the two Narmada faults may have been active at different geological times. The seismic model is constrained by 2-D gravity modeling. The gravity highs on either side of the Narmada zone are due to the effect of the high velocity/high density mafic intrusion at upper crustal level.

The seismic velocity structure of the upper and lower crusts along Hirapur-Mandla profile was obtained by 2-D forward modeling of the refraction and wide-angle reflection data. Digitized record sections of the analog data were utilized for delineation of the crustal velocity model across the Vindhyan basin and the Narmada-Son lineament by travel-time modeling and qualitatively matching the amplitudes of synthetic seismograms with seismic record sections. An interface at a depth of 8 km has been traced from wide-angle reflection data of shot points 160 and 235 between Jabalpur and Mandla. The wide-angle reflection phase from crust-mantle boundary is very prominent in all the seismic record sections of shot points 0, 40, 80, 120 and 235. The Moho horizon along the entire study area was continuously traced, depicting an upwarp between Katangi and Jabalpur below the Narmada-Son lineament. The three faults inferred near Narsinghgarh, Katangi and Jabalpur divide the crust into four blocks. The number of faults and displacement patterns along them, depicted along the profile, indicates that block tectonics appear to be active in this region throughout the geological history.

No abstract

In order to investigate the velocity structure, and hence shed light on the related tectonics, across the Narmada–Son lineament, traveltimes of wide‐angle seismic data along the 240 km long Hirapur–Mandla profile in central India have been inverted. A blocky, laterally heterogeneous, three‐layer velocity model down to a depth of 10 km has been derived. The first layer shows a maximum thickness of the upper Vindhyans (4.5 km s−1 ) of about 1.35 km and rests on top of normal crystalline basement, represented by the 5.9 km s−1 velocity layer. The anomalous feature of the study is the absence of normal granitic basement in the great Vindhyan Graben, where lower Vindhyan sediments (5.3 km s−1 ) were deposited during the Precambrian on high‐velocity (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 upper Cretaceous period and attained a maximum thickness of about 800 m.