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Geoelectric Structure of Northern Cambay Rift Basin from Magnetotelluric Data Nagarjuna Danda1, C

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Geoelectric Structure of Northern Cambay Rift Basin from Magnetotelluric Data Nagarjuna Danda1, C Danda et al. Earth, Planets and Space (2017) 69:140 DOI 10.1186/s40623-017-0725-0 FULL PAPER Open Access Geoelectric structure of northern Cambay rift basin from magnetotelluric data Nagarjuna Danda1, C. K. Rao1* and Amit Kumar2 Abstract Broadband and long-period magnetotelluric data were acquired over the northern part of the Cambay rift zone along an east–west profle ~ 200 km in length. The decomposed TE- and TM-mode data were inverted using a 2-D nonlinear conjugate gradient algorithm to obtain the lithospheric structure of the region. A highly conductive (~ 1000 S) layer was identifed within the Cambay rift zone and interpreted as thick Quaternary and Tertiary sediments. The crustal conductors found in the profle were due to fuid emplacement in the western part, and the presence of fuids and/or interconnected sulfdes caused by metamorphic phases in the eastern part. The demarcation of the Cambay rift zone is clearly delineated with a steeply dipping fault on the western margin, whereas the eastern margin of the rift zone gently dips along the NE–SW axis, representing a half-graben structure. A highly resistive body identifed outside the rift zone is interpreted as an igneous granitic intrusive complex. Moderately conductive (30–100 Ω-m) zones indicate underplating and the presence of partial melt due to plume–lithosphere interactions. Keywords: Cambay rift zone, Deccan basalt, Lithosphere, Magnetotellurics, Reunion plume Introduction of the Seychelles and India ~ 65 Ma ago. As the Indian Te western continental margin of India (WCMI) dem- plate started moving northward, the adjacent ofshore onstrates its evolution through the mid-Cretaceous areas were infuenced by the Reunion hotspot, resulting and Tertiary periods (Courtillot et al. 1988; White and in various magmatic intrusions within the stretched con- McKenzie 1989; Storey et al. 1995; Bhattacharya and tinental crust of the WCMI (Bhattacharya and Chaubey Chaubey 2001 and references therein). Te structural 2001; Chaubey et al. 2002; Royer et al. 2002). Te region architecture of the WCMI was also modulated by two was uplifted around the impingement location of the hot major geodynamic events, the activity of the Reunion mantle plume and developed a four-way radially deviat- hotspot and the collision that formed the Indo-Eurasian ing mega-fracture pattern. Te roughly N–S trending continental plate. Te interaction between the Reunion fractures produced further extensional stresses, eventu- hotspot and the Indian continental lithosphere during the ally resulting in the breakup between the Seychelles and Cretaceous–Tertiary boundary (~ 65 Ma ago) resulted in Indian plates along these fracture arms. Te western con- a huge food of basaltic eruptions, which formed the Dec- tinental margin then reactivated along the eastern parts can volcanic province (DVP) over the western and cen- of these extending fractures. Te Narmada geofracture tral Indian sub-continent (Raval and Veeraswamy 2000; zone formed along the third arm, whereas the fourth arm Veeraswamy and Raval 2004). It covers an area of more resulted in the Cambay basin, due to extensional stresses than 500,000 km2, almost one-sixth of the total surface associated with the separation and movement of the area of the Indian Peninsula. Te eruption of the DVP by Indian plate away from the Seychelles. Te second-stage the Reunion hotspot has been inferred as late syn-rift-to- rifting of the Cambay basin is considered to be coeval breakup volcanism contemporaneous with the separation with the breakup of the Indian plate and the Seychelles as well as the eruption of Deccan basalts in the Late Cre- taceous (Raval and Veeraswamy 2000; Veeraswamy and *Correspondence: [email protected]; [email protected] Raval 2004) by generating signifcant variations in vol- 1 Dr. K. S. Krishnan Geomagnetic Research Laboratory, IIG, Allahabad, India Full list of author information is available at the end of the article canic intensity as observed in the Baikal rift zone, a Rio © The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Danda et al. Earth, Planets and Space (2017) 69:140 Page 2 of 14 Grande rift system. Several geophysical studies were car- trend swings eastwards and syncs with the ENE–WSW ried out to understand the tectonics, sediment thickness, Precambrian orogenic trends. Te extension of the and rifting processes of the Cambay basin (Kaila et al. NNW–SSE trend parallel to the WCMI in the western 1990; Tewari et al. 1991, 1995; Mishra et al. 1998; Singh part of the peninsular Indian shield led to the forma- et al. 2003; Dixit et al. 2010; Kumar et al. 2016). Most tion of the Cambay basin during the Mesozoic period. authors have reported ~ 1–5-km-thick Tertiary and Qua- Tough the formation of the basin took place during the ternary sediments, inferring that lower crustal magmatic Mesozoic, the Cambay basin is named a Tertiary basin underplating occurred associated with plume interaction because of its higher rate of subsidence during the Ter- and rifting events. tiary. Te NNW–SSE-trending Cambay rift basin extends Tough various geophysical studies have been car- from Barmer in the north to the ofshore region in the ried out, information on the deeper structure across the south (Gupta 1981). Initially, the basin started rifting basin is limited. Te MT method has been successfully perpendicular to the eastern and western faults, which used in delineating deep crustal and upper mantle struc- formed parallel to the axis of the basin. Te rift is wider tures (Jones 1992; Gokarn et al. 2001; Rao et al. 2014b; on its southern side and narrow in the northern Barmer Abdul Azeez et al. 2017), the presence of partial melts and Sanchore regions. Te NE–SW Aravalli–Delhi trend (Unsworth et al. 2005) and areas of magmatic underplat- splits into three directional components in the western ing (Wannamaker et al. 2008) by studying the variations margin of India. Te main NE–SW Aravalli trend con- in the electrical resistivity of Earth’s interior. Although tinues across the Cambay basin into Saurashtra, while several MT studies have been carried out to understand the Delhi trend swings E–W by forming a series of step the resistivity structure of the Mesozoic rift basins at faults, continuing into the Kutch basin. Te third compo- the western margin of India, and to delineate areas of nent bends counterclockwise and merges with the ENE– magmatic underplating and intrusions in the rift zones WSW Satpura trend. induced by the Reunion plume (Rao et al. 2004; Nagan- Te Cambay basin is surrounded by the Precambrian janeyulu and Santosh 2010; Abdul Azeez et al. 2013; Aravalli group of formations in the NE and is bounded by Patro and Sarma 2016 and references therein), no studies discontinuous faults, including Deccan traps in the east have yet been made to image the deep resistivity struc- and west as shown in Fig. 1. Deccan volcanism dates to ture across the Cambay rift. In order to decipher the the Mesozoic era, near the Cretaceous–Tertiary bound- deeper structure and sediment thickness in the northern ary, and subdued early rifting by erupting thick Deccan part of the basin, an MT study was performed as shown basalts over the Cretaceous sediments, shaping the basin in Fig. 1. Te detailed modeling of the lithospheric geoe- basement. Subsequently, the basin was flled with Ter- lectric structure in the study region, combined with geo- tiary and Quaternary sediments consisting of sandstone, dynamic and tectonic information, establishes a plausible siltstone, claystone, and shales. Biswas (1982) categorized deep crustal resistivity structure, increasing the under- the Cambay rift as a present-day active rift that continues standing of evolutionary processes and the impact of the to be a low-lying area. Due to the large sediment thick- Reunion plume on the structure of the crust and mantle. ness, the Cambay basin has become one of the major hydrocarbon-producing sedimentary basins in India. Regional tectonic framework Te WCMI is made up of the Kutch, Cambay and Nar- Geophysical studies mada rift basins that developed during the breakup of the A large number of hydrocarbon exploration surveys have Indian continent from Gondwanaland during the Meso- been carried out resulting in geological, geophysical, zoic era. Tese basins were formed by rifting along with and well log data sets of the Cambay basin. Despite the the three major Precambrian tectonic trends of western availability of fairly good information on shallow depths India, Dharwar (NNW–SSE), Aravalli–Delhi (NE–SW) along the rift basin, not much is known about its deeper and Satpura (ENE–WSW), that controlled the tectonic structure, the sedimentary thickness over the northern style of the basins, and these rifting events were synchro- region, or the presence of Mesozoic sediments beneath nous with the major events of Indian continental drifting the Deccan basaltic basement. Initially, gravity surveys history (Biswas 1982, 1987 and references therein). Tese (Negi 1951; Qureshi 1964; Rao 1968) were carried out to trends can be seen as metamorphic belts, which repre- study the basement structure and nature of subsurface sent the three major orogenic cycles. Te Aravalli–Delhi formations. Tese studies revealed a high Bouguer grav- and Dharwar trends represent the oldest orogenic cycle ity anomaly within the basin compared to the surround- and are followed by the Satpura. Te Narmada-Son line- ing regions, inspiring a few studies (Tewari et al. 1991, ament developed along the Satpura trend by dividing the 1995) to attempt to establish a relationship with Precam- Indian shield into two blocks.
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