Crustal deformation in Volcanic covered area as inferred from magnetotelluric studies: An example from India

2D and 3D inversions of the MT data from Saurashtra peninsula delineated the sub-surface electrical resistivity structure down to 30 km. 2D inversion carried using uniform grid Laplacian (UGL) operator demonstrated similarity with the 3D model in shallow subsurface. It was possible due to the uncomplicated nature of the shallow subsurface and horizontal smoothening of the model is controlled by the UGL operator during the 2D inversion. In this way, the Deccan basalt layer and sub- basaltic Mesozoic sediments are successfully imaged. The lateral extent of Deccan basalt is correlating with the surface geological infor- mation and basalt observed to be thick in the southern Saurashtra with maximum thickness of 800 m near Amreli. A chunk of sub-basaltic Mesozoic sediments with an average thickness of 500 m is imaged, and is observed to be connected to the exposed Cretaceous rocks in the northern part of Saurashtra.

Fig.1: (a) Topographic map of the study area. Blue solid line is the profile in which the 2D inversion is performed. NJF: North Junagadh fault, SF: Shihor Fault, UF: Umrethi fault, SJF: South Junagadh fault, SKF: Saverkindala fault, RF: Rajula fault, SSF: Saurashtra South Fault, NSF: Narmada Son Fault, TNF: Tapti North fault. NTDS: Narmada – Tapti dyke swarm, NPDS: Nasik – Pune dyke swarm, CDS: Coastal dyke swarm. NSZ: Narmada seismic zone, GSZ: Girnar seismic zone, CSZ: Cambay seismic zone, PSZ: Panvel seismic zone. Left bottom inset: Key map of the study area. DVP: Deccan Volcanic Province (Green coloured region), NSL: Narmada - Son Lineament (Dashed black line). (b) Geological map of the study area.

In deeper segment, highly resistive (>4000 Ω.m.) granitic basement extends to 20− 25 km depth. The lower crustal segment demonstrates moderately enhanced conductivity due to the combined effect of hy- drothermal alteration and underplating of the enriched mafic material introduced by Deccan volcanic event. Fault zones are identified within the crust which are in spatial correlation with Rajula fault (RF) and Saverkindala fault (SKF). These faults have deeper extension and it is particularly observed that the RF and SKF are having the same trend of Narmada – Son Lineament (NSL) and similarly fashioned as Narmada South Fault (NSF) and Tapti North Fault (TNF). Analysing the other geological and seismological studies, we infer the extension of NSL to Saurashtra and these fault systems have played an important role in the transport of the magma to the surface in Saurashtra during the Deccan volcanic event.

Fig.2: Deeper section derived using 3D inversion of the data. RF: Rajula Fault, SKF: Saverkindala Fault, SJF: South Junagadh Fault, NJF: North Junagadh Fault. P1 and P2 are the fault zones. VE - Vertical Exaggeration.

K.S. Ajithabha,b, Prasanta K. Patroa,*

Crustal deformation in Volcanic covered area as inferred from magnetotelluric studies: An example from India