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

• Published in: Geophysical journal international 2000-07, Vol.142 (1), p.95-107
• Main Authors: Kumar, Prakash, Tewari, H. C., Khandekar, G.
• Format: Article
• Language: English
• Subjects: basement; Deccan Traps; high velocity; Narmada; ray tracing; upper crust
• ISSN: 0956-540X; 1365-246X
• DOI: 10.1046/j.1365-246x.2000.00132.x

The Narmada zone in central India is a zone of weakness that separates the region of Vindhyan (Meso-Neoproterozoic) deposition to the north from Gondwana (Permo-Carboniferous–lower Cretaceous) deposits to the south. The reinterpretation 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 Narmada North and Narmada South faults is a region of basement uplift. A layer of anomalously high velocity (6.5–6.7 km s−1 ) at depths between 1.5 and 9.0 km appears to be present in the entire region. Within the Narmada zone this layer occurs at shallower depths than outside the Narmada zone. At two 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 this case it should have produced large positive gravity anomalies at the shallowest parts. Instead, these parts correspond to Bouguer gravity lows. Furthermore, lower crust at such shallow depths has not been reported from any other part of the Indian shield. Therefore, this layer is likely to represent the top of a high-velocity mafic body that has different thicknesses in different places.