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Broad-band and long period magnetotelluric measurements made at 63 locations along ∼500 km long Chikmagalur-Kavali profile, that cut across the Dharwar craton (DC) and Eastern Ghat Mobile Belt (EGMB) in south India, is modelled to examine the lithosphere architecture of the cratonic domain and define tectonic boundaries. The 2-D resistivity model shows moderately conductive features that intersperse a highly resistive background of crystalline rocks and spatially connect to the exposed schist belts or granitic intrusions in the DC. These features are therefore interpreted as images of fossil pathways of the volcanic emplacements associated with the greenstone belt and granite suite formation exposed in the region. A near vertical conductive feature in the upper mantle under the Chitradurga Shear Zone represents the Archean suture between the western and eastern blocks of DC. Although thick (∼200 km) cratonic (highly resistive) lithosphere is preserved, significant part of the cratonic lithosphere below the western DC is modified due to plume-continental lithosphere interactions during the Cretaceous–Tertiary period. A west-verging moderately conductive feature imaged beneath EGMB lithosphere is interpreted as the remnant of the Proterozoic collision process between the Indian land mass and East Antarctica. Thin (∼120 km) lithosphere is seen below the EGMB, which form the exterior margin of the India shield subsequent to its separation from East Antarctica through rifting and opening of the Indian Ocean in the Cretaceous. Image 1 • Broadband and long period MT study across the Dhawar Craton and bordering EGMB. • Two-dimensional resistivity model shows mantle suture zone between WDC and EDC. • Plumed induced modifications in the lithosphere beneath the Dharwar Craton. • Preserved signature of Proterozoic collision between Indian and East Antarctica in EGMB lithosphere. [ABSTRACT FROM AUTHOR]

Abstract Saurashtra region forms the Western part of the Deccan volcanic province (DVP) in India and is bounded by the Cambay rift basin (CRB) in the east, Narmada rift basin (NRB) in the South and Kachchh rift basin (KRB) in the north. This region is uplifted due to different stages of inter-continental breakup (Madagascar-India & Seychelles-India), different stages of rifting, reactivation of fault zones and Deccan volcanism. The major portion of Saurashtra region is covered by Deccan basalts underlain by Mesozoic sediments. Detailed magnetotelluric (MT) and long period magnetotelluric (LMT) soundings have been carried out in the northern part of Saurashtra with an aim to bring out the geoelectrical configuration of different basins (Jamnagar, Jasdan, and Cambay) along the profile. 2D inversion carried out for different modes: transverse electric (TE), transverse magnetic mode (TM) and TE + TM independently. The inversion of different modes shows distinct electrical signatures. Due to the 3D nature of the data, we have considered the TM mode of interpretation because it is less prone to 3D distortions. The final model represents large-scale heterogeneities in the crust due to the presence of different resistive and conductive blocks. High conductive lithosphere denotes the thermal influx of the mantle material to the base of the crust. Exchange of heat with the lower crust during basaltic magmatic underplating released carbonate fluids and/or dehydration of minerals resulting in high conductivity anomalies. A prominent conductive feature on the east side of the profile at upper crustal depths may be due to partial melts associated with mafic/ultra-mafic intrusions related to the Reunion plume activity. Different resistive blocks in conjunction with seismic tomography studies represent recrystallized older Precambrian crust that forms the uplifted blocks of Jamnagar, Jasdan and western part of Cambay basins. Graphical abstract Cartoon illustration of structure (Northern part of Saurashtra region) brings out three different basins beneath Deccan traps from magnetotelluric studies. Crustal inhomogeneities are brought out by different resistivity and conductivity zones. R1 and R2 forms the uplifted blocks of Jamnagar basin, R2, R3 forms the uplifted blocks of Jasdan basin and R4 forms the western fringe of Cambay basin. Conductivity anomalies C2 and C3 denote the trapped carbonate fluids expelled from recrystallized resistivity blocks whereas C4 denotes conductive dyke that may be the source for Deccan volcanism. Conductivity anomaly (A) may represents interconnected melts. Unlabelled Image Highlights • Three different basins Jamnagar, Jasdan and Cambay have been delineated in the northern part of Saurashtra region by MT and LMT studies. • Resistive blocks (R1-R4) represent the older Precambrian crust that have been recrystallalised due to mafic/ultra mafic intrusions and forms the uplifted blocks of Jamnagar, Jasdan and the western part of Cambay basin. • The older Precambrian crust representing resistive segments have been recrystallized due to mafic/ultra-mafic intrusions and forms the uplifted blocks of Jamnagar, Jasdan and the western part of Cambay basin. [ABSTRACT FROM AUTHOR]

Magnetotelluric (MT) method is one of the powerful tools to investigate the deep crustal image of mountainous regions such as Himalayas. Topographic variations due to irregular surface terrain distort the resistivity curves and hence may not give accurate interpretation of magnetotelluric data. The two-dimensional (2-D) topographic effects in Transverse Magnetic (TM) mode is only galvanic whereas inductive in Transverse Electric (TE) mode, thus TM mode responses is much more important than TE mode responses in 2-D. In three-dimensional (3-D), the topography effect is both galvanic and inductive in each element of impedance tensor and hence the interpretation is complicated. In the present work, we investigate the effects of three-dimensional (3-D) topography for a hill model. This paper presents the impedance tensor correction algorithm to reduce the topographic effects in MT data. The distortion caused by surface topography effectively decreases by using homogeneous background resistivity in impedance correction method. In this study, we analyze the response of ramp, distance from topographic edges, conductive and resistive dykes. The new correction method is applied to the real data from Sikkim Himalayas, which brought out the true nature of the basement in this region. [ABSTRACT FROM AUTHOR]

The Central Indian Tectonic Zone (CITZ), a major tectonic feature extending across the Indian sub-continent, was formed by the accretion of the cratonic domains in the north and south blocks of peninsular India during the Proterozoic. It is characterized with prominent shear/fault zones and granulite belts that carry the evolutionary history of the central Indian region. This study presents the two-dimensional resistivity structure of the hitherto least studied eastern segment of the mega lineament in the Indian subcontinent. Magnetotelluric (MT) data along the Rajnandgaon-Bareli profile, which extends from the southern margin (Central Indian Shear-CIS) to the northern limit (Narmada-Son Lineament-NSL) of the CITZ, were used to investigate the resistivity character of the crust and shallow upper mantle. The study imaged conductive to moderately conductive structures, which show correlation to the various shear/faults and granulitic belts mapped in the region, interspersed in a resistive background. An upper crustal isolated conductor was observed below the Bhandra – Balaghat Granulite belt, which is exposed along the CIS. A vertical moderately conductive structure noticed in the middle to upper mantle depths under the Ramakona-Katangi Granulite (RKG) belt and this is interpreted as fault/shear zone that acted as the channel/pathway for the emplacement of parental magma of the RKG rocks. A more interesting feature in the MT model is a north dipping conductive horizon extending from the upper crust to the Moho. This feature shows further extension into the upper mantle as a subvertical moderately conductive zone and can be traced onto the surface mark of the Tan Shear, which suggests suture status to this prominent shear zone within the CITZ. Resistivity section also shows mid-lower crustal conductive/moderately conductive horizons under the Deccan trap and NSL, which are produced due to the large-scale magma intrusions into the crust during the Cretaceous-Tertiary Deccan volcanism through the pre-existing weak zones in the CITZ. The conductive to moderately conductive features of the MT model are indicative of the complex tectonic history of the region that range from accretion processes started in the Paleoproterozoic, followed by multiple metamorphic and deformation cycles in different geological times, to the Late Cretaceous volcanism. [ABSTRACT FROM AUTHOR]

The spatial variation of Deccan Traps thickness in and around the Koyna-Warna Seismic Zone (KWSZ), India, is crucial for the study of reservoir triggered seismicity in the region. Due to the increased trap thickness, the increment of pressure on the basement directly affects the stress accumulation within the basement fault system. Such a stressed fault system within the pressurized basement is prone to the release of strain due to the fluid infiltration. Moreover, the knowledge of the trap thickness variation aids in selecting a proper place for exploratory borehole drilling and install a seismometer to effectively eliminate the noise generated by the overlying trap. The dimensionality analysis of the magnetotelluric (MT) data acquired in KWSZ in the grid shows one dimensional (1D) characteristics in the shorter period (0.001 s – 1 s), which signifies that the layered Deccan basalts on the top is followed by the granitic basement in KWSZ. Therefore, 1D inversion is applied to delineate the trap thickness and create the detailed configuration of Deccan Traps in and around KWSZ. The result shows that the trap thickness mainly varies along E -W direction with an average thickness of ∼800 m. The base of the trap is almost flat and dipping (∼1 deg) towards the west. The flat bottom and the varying thickness signify that the trap thickness is controlled by the topographic variation, providing information for future drilling of exploratory boreholes. The high trap thickness regions, associated with a higher average overburden pressure of 25.6 MPa, coincide with seismicity distribution in KWSZ, which provides the critical inputs for pressure-seismicity relation. • Trap thickness varies from <500 m to >1100 m with an average thickness of ∼800 m. • Thickness varies primarily in E-W direction and controls by topographic variation. • Base of the trap is dipping ∼1 deg. to west without any abrupt variation in the base. • Trap exerts an average pressure of 25.6 and 17.6 MPa on middle and both sided basement. • The high pressure at the middle part increases the stress within the existing fault system. [ABSTRACT FROM AUTHOR]

Recent increase in the application of 3-D inversion of magnetotelluric (MT) data is facilitated by the availability of several 3-D inversion codes, which led to improved interpretation of MT data. However, still the galvanic effects continue to pose problems in interpretation of the MT data. We have addressed this problem using the phase tensor (PT) concept and developed a scheme based on a modification of the sensitivity matrix in the 3-D inversion code of Siripunvaraporn et al., which enables us to directly invert the phase tensor elements. We have used this modified code for PT inversion of MT data and evaluated its efficacy in reducing the galvanic effects through a few examples of inversion of synthetic data and its application on real data. The synthetic model study suggests that the prior model (mo) setting is important in retrieving the true model. Comparison of results obtained from conventional impedance inversion and the proposed PT inversion method suggests that, even in the presence of the galvanic distortion (due to near-surface checkerboard anomalies in our case), the new inversion algorithm reliably retrieves the regional conductivity structure when the prior model or regional resistivity value level can be estimated with sufficient accuracy. Data errors were propagated to the PT elements using delta method while inverting the real data from USArray. The PT results compare very well with those from full tensor inversion published earlier, signifying the efficacy of this new inversion scheme. [ABSTRACT FROM PUBLISHER]

The central-western region of the Indian subcontinent hosts the vast geological records of its evolution from the Archean to the Recent, including the youngest (∼65 Ma) episode of the Réunion mantle plume activity that produced a large igneous province, the Deccan Volcanic Province (DVP). A three-dimensional lithospheric resistivity image of central-western India is obtained to understand the lithospheric architecture and map any major eruption channels of the Deccan volcanism as no explicit geophysical revelation of such pathways of magma ascend has yet been made. Two high conductivity (< 30 Ωm) pipe-like geometric features originating from a common deep mantle conductive zone under the Malwa plateau (northernmost lobe of the DVP) and its proximity are detected in the resistivity model. These are interpreted to be remnants of the hitherto unknown primary lithospheric pathways of magma ascent from the deep mantle melt-rich zone related to the Reunion mantle plume upwelling under central-western India. This study gives first compelling geophysical evidence of key eruptive centers of the massive Deccan volcanism in central-western India at a locale not anticipated earlier. High to moderate conductivity crustal zones and weak to moderate lithospheric mantle resistivity in most parts of the study region represent an intense and multiphase tectono-magmatic evolution of the region spanning from the Neoproterozoic to the Cretaceous-Paleogene boundary. [Display omitted] • 3D lithospheric resistivity model of the central-western India. • Conductive pipes originating from a common deep mantle conductive zone. • Remnant mantle pathways of Reunion mantle plume upwelling. • Geophysical evidence of key eruptive centers of Deccan volcanism in India. • Support for the mantle plume theory for the Deccan volcanism. [ABSTRACT FROM AUTHOR]