An anomalous high-velocity layer at shallow crustal depths in the Narmada zone, India

The Narmada zone in central India is a zone of weakness that separates the region of Vindhyan (Meso-Neoproterozoic) deposition to the north from Gondw ana (Permo-Carboniferous-lower Cretaceous) deposits to the south. The reint erpretation of analogue seismic refraction data, acquired during the early 1980s, using 2-D ray-tracing techniques reveals a basement (velocity 5.8-6. 0 km s(-1) ) topography suggesting that the Narmada zone, bounded by the Na rmada North and Narmada South faults is a region of basement uplift. A laye r of anomalously high velocity (6.5-6.7 km s(-1) ) at depths between 1.5 an d 9.0 km appears to be present in the entire region. Within the Narmada zon e this layer occurs at shallower depths than outside the Narmada zone. At t wo places within the Narmada zone this layer is at a depth of about 1.5 km. This layer cannot be considered as the top of the lower crust because in t his case it should have produced large positive gravity anomalies at the sh allowest parts. Instead, these parts correspond to Bouguer gravity lows. Fu rthermore, lower crust at such shallow depths has not been reported from an y other part of the Indian shield. Therefore, this layer is likely to repre sent the top of a high-velocity mafic body that has different thicknesses i n different places.