Seismic Structure of the Central Indian Crust and its Implications on the Crustal Evolution

Abstract

The crustal structures of the Narmada region in Central India bounded by fault system (Narmada- North and South faults: NNF and NSF) has been derived from deep seismic sounding (DSS) studies along the two profiles trending almost north-south direction. The wide-angle phases have been modeled kinematically and dynamically using the 2-D asymptotic ray tracing technique. The combined seismic and gravity modeling reveals a multilayer crust in the region. The crustal wide-angle reflection phases map the Moho discontinuity, where the P-wave velocity jumps from 7.2 km s-1 to 8.0-8.1 km s-1, at depth varying between 38 km and 44 km. A layer with velocity 7.2 km s-1, exists above the Moho in most parts of the profiles and is attributed to the magmatic underplating related to the Deccan volcanism (~65 Ma). The intriguing observation of the study is a zone characterized by anomalous high velocity (6.5-6.6 km s-1) within the upper crust. 2-D gravity modeling demonstrates that this anomalous layer has a density of ~2.9 gm cm-3, which is equivalent to the rocks metamorphosed to granulite/amphibolite facies. This high velocity layer probably represents the granulite enclaves within the Archaean granites/gneiss rocks and was formed during the cratonization of the Achaean crust. Importantly, this high velocity layer shows an average upward displacement of ~8.5 km within the region bounded by NNF and NSF as compared to the regions beyond it. The studies suggest that the observed displacement in the high velocity layer of the upper crust is a result of repeated reactivation of the Narmada fault system.