Crustal density structure across the Central Indian Shear Zone from gravity data

The Central Indian Shear Zone is a distinct tectonic feature in the central part of India that separates the Central Indian Tectonic Zone from the Bastar Craton. The complete Bouguer anomaly map of the region encompassing Central Indian Shear Zone is characterised by a broad relative gravity high over Central Indian Tectonic Zone as compared to the Bastar Craton. The paired gravity anomaly across the two crustal domains signifies that the Central Indian Shear Zone is a locus of density discontinuity along which the two crustal domains accreted. Horizontal gravity gradient analysis further demonstrates the northward subduction of the Bastar Craton beneath the Bundelkhand Craton. 2½D gravity modelling along Mungwani–Rajnandgaon profile, constrained by Seoni–Kalimati seismic section, delineates a thick crust beneath the Bastar Craton as compared to the Central Indian Tectonic Zone. Northward dipping contact of the two crustal domains when projected on the surface coincides with the Central Indian Shear Zone. With a well defined Moho offset and crustal density discontinuity, Central Indian Shear Zone represents a suture zone that separates the Bastar Craton from the Central Indian Tectonic Zone. A low-velocity (6.4 km/s)/density (2.90 g/cm 3 ) layer at the base of the crust and relatively lower density (3.21 g/cm 3 ) subcrustal mantle may be the imprint of thermal remobilization beneath the Central Indian Tectonic Zone. Keywords Central Indian Shear Zone Central Indian Tectonic Zone Bouguer gravity anomaly Horizontal gravity gradient Crustal reflectivity pattern Wide angle reflection 1 Introduction Evolutionary history of continents and supercontinents require an understanding of crustal growth processes through time and space (e.g., Santosh, 2010 ). Proterozoic shields are often viewed as wide orogenic domains of amalgamated terranes but with smaller plates and longer ocean ridge lengths ( de Wit, 1998 ). The long-lived shear systems between amalgamated terranes are commonly assumed to represent boundaries of Proterozoic continental collision (e.g., Santosh and Yoshida, 1996; Santosh et al., 2009 ). Imprints of subsequent tectonic signatures lead to very different geological signatures in terms of tectonometamorphic evolution. Unravelling those superimposed signatures and delineation of ancient cryptic sutures is of seminal importance to understand the crustal growth process that shaped the Proterozoic continents and supercontinents. The Indian shield in general and its central part in particular is one of the critical examples in the current debate ( Desai et al., 2010; Meert et al., 2010, 2011; Naganjaneyulu and Santosh, 2010; Chatterjee and Ghose, 2011 ) ( Fig. 1 ). The crustal architecture of Central India comprises large Archaean cratons (Bastar, Singhbhum and Bundelkhand) well separated by crustal scale Proterozoic Satpura orogenic belt (also known as Central Indian Tectonic Zone: CITZ) ( CRUMANSONATA, 1995 and references therein). The CITZ has been interpreted as a Proterozoic mobile belt, a tectonic suture ( Naqvi et al., 1974 ), and an active Precambrian rift ( Nayak, 1990 ). According to Radhakrishna and Naqvi (1986) the collage of Dharwar, Bastar and Singhbhum cratons collided with a northerly block consisting of the Bundelkhand Craton during the Palaeoproterozoic period. Central Indian Shear Zone (CISZ), southern border of the CITZ, is the one regarded to have acted principally as the suture between the Bundelkhand Craton and the Bastar Craton ( Yedekar et al., 1990, 2003; Jain et al., 1991 ). Crustal structure and geodynamic processes across the CISZ have been studied earlier covering a wide spectrum of geomorphology, geology, and geophysics (e.g., CRUMANSONATA, 1995; Divakara Rao et al., 1998 ). Yedekar et al. (1990) provided the first plate tectonic model for evolution of the CISZ, by invoking southerly dipping subduction of the Bundelkhand Craton below the Bastar Craton. In contrast, later tectonic models invoked a north-directed subduction of the oceanic crust of the Bastar Craton below the Bundelkhand Craton ( Bhowmik et al., 1999; Acharyya and Roy, 2000 ). With equivocal direction of subduction, CISZ is perhaps one of the less understood regions of crustal growth processes and its role in East Gondwana reconstruction during late Mesoproterozoic period ( Acharyya, 2003; Bhandari et al., 2010; Mohanty, 2010 ). Several geophysical studies employing active seismology (e.g., Sain et al., 2000; Tewari and Kumar, 2003; Murty et al., 2008 ), potential field data (e.g., Mishra, 1992; Verma and Banerjee, 1992; Singh and Meissner, 1995; Singh, 1998; Rajaram and Anand, 2003 ) and magnetotelluric (e.g., Gokarn et al., 2001; Patro et al., 2005; Naganjaneyulu, 2010; Naganjaneyulu and Santosh, 2010; Naganjaneyulu et al., 2010 ) provide deep insights into the crustal structure and tectonic evolution of the CITZ. In contrast the only coincident seismic reflection and refraction/wide-angle reflection study along Seoni–Kalimati profile provided some vital information about its crustal configuration ( Reddy et al., 1996 ). A diffused reflectivity over a 20–30 km wide zone however, indicates that the structure around CISZ is indistinct ( Mall et al., 2008 ). Through the integration of gravity data along the available seismic section ( Reddy et al., 1996 ) a better constrained crustal structure across the CISZ ( Mishra et al., 2000 ) was provided; an opposite direction of subduction along the CISZ is however proposed, recently ( Mall et al., 2008 ). To ascertain the deep crustal structure and unequivocal direction of subduction across the CISZ, gravity signature is analyzed along with the recent seismic information. The horizontal gravity gradient technique ( Cordell and Grauch, 1985 ) was employed to ascertain the lateral extension of the CISZ and possible direction of subduction. Mungwani–Rajnandgaon gravity profile ( Fig. 2 ), integrating constraints from Seoni–Kalimati seismic profile ( Mall et al., 2008 ), is modelled using 2½D forward modelling algorithm to reconstruct the deep crustal structure across the CISZ. The geometric constraints thus obtained from the derived crustal density model are utilised to infer the likely tectonic domains that shaped the CISZ. 2 Geological setting A synthesis of the available geological information ( Fig. 2 ) reveals that the southern part of the CITZ is mostly occupied by Mesoproterozoic Sausar metasediments and Tirodi gneisses. Ramakona–Katangi granulite (RKG) occurs discontinuously over a strike length of 240 km in the north of the Sausar supracrustal belt. Similarly, Balaghat–Bhandara granulite (BBG) belt is exposed over a strike length of 190 km near southern margin of the CITZ. A pile of basaltic lava flows of Deccan Traps forms another important litho-suite with occasional thin intertrappean beds. Laterite forming the cap over the basalt is preserved in the Seoni area and to the southeast. Gondwana sediments of Mahanadi Basin cover the eastern part of the CITZ. The volcanic and volcanoclastics of Sakoli, Nandgaon, Khairagarh and Chilpi Groups, Malanjkhand and Dongargarh granite plutons and Chattisgarh shelf sediments lie over the Bastar Craton ( GSI, 1998 ). The ENE–WSW trending divide line (CISZ), a 0.2–4 km wide and 500 km long ductile shear zone extending from southeast of Nagpur to south of Korba, is characterised by mylonites and phyllites with minor amounts of cataclasites. Other notable shear zones of the region are the Southeast Sakoli and Tan Shear. The tectonic trends in Bastar Craton vary from WNW–ESE to N–S, in contrast to ENE–WSW to E–W in the Bundelkhand Craton. The regional structural grain on the CITZ is thus concordant with the CISZ while that of Bastar Craton is discordant with it. CISZ has therefore been suggested as a suture that separates the two distinct Precambrian crustal domains ( Yedekar et al., 1990; Jain et al., 1991; Roy and Prasad, 2003 ). According to Yedekar et al. (1990) , the southward subduction of Bundelkhand Craton below Bastar Craton led to the development of rift basins and arc related intrusions (e.g. Dongargarh and Malanjkhand granites) in the Bastar Craton. This subduction system culminated with the continent–continent collision, during which the passive margin Sausar sediments were metamorphosed. This model considers Bhandara–Balaghat granulite belt to be the obducted granulitic oceanic crust, which was exhumed during collisional orogeny. In contrast, the Mahakoshal rift basin is considered as a back-arc rift primarily related to north-directed subduction of the Bastar Craton below the Bundelkhand Craton ( Bhowmik et al., 1999; Acharyya and Roy, 2000 ). The northerly dipping structural grain along the RKG belt further supports the south-directed thrusting during the collision ( Acharyya, 2003; Roy and Prasad, 2003 ). This collision resulted in anomalous crustal thickening and attending migmatization and lower crustal melting. This stage was followed by rapid decompression, during which ∼15 km thick crust was removed and granulites were brought to middle crustal levels ( Bhowmik et al., 1999 ). 3 The data 3.1 Topography The topographic image in Fig. 3 is based on the original version of the Shuttle Radar Topography Mission (SRTM) 90 M elevation data ( ftp://edcsgs9.cr.usgs.gov/pub/data/srtm ). The most prominent topographic feature of the region is the Satpura Mobile Belt/CITZ with a maximum elevation of about 1100 m. Around Amarkantak, the two rivers Narmada and Son flow towards the west and the east, respectively. Down south, a relatively low lying Bastar Craton is characterised by 100–500 m high moderate topography. The low lying region associated with Wianganga River towards southwest finally merges with the river Godavari whereas the low level landform of Bilaspur region towards east is the catchment area of Mahanadi River. Many consider the Satpura mountain range as a consequence of Proterozoic continental collision when all the oceanic lithosphere between the converging continents is subducted (e.g., Mohanty, 2010; Naganjaneyulu and Santosh, 2010 ) while others suggest Mesozoic tectonic uplift responsible for the lofty heights and youthful stage of the ranges ( Qureshy, 1971; Verma and Banerjee, 1992 ). Apparently, the present day landscape of the CITZ is a coupled response to the kinematics of the plate convergence, material properties of the lithosphere, surface erosion, isostatic adjustment of the over thickened crust and intense Mesozoic tectonic activity. 3.2 Complete Bouguer anomaly Majority of the gravity data used in this study was collected under various scientific programmes ( NGRI, 1978; Mishra et al., 2000, 2002 ) ( Fig. 4 ). To fill the gap areas, 1244 gravity measurements were made along two profiles (1 and 2) across CISZ and in the SE corner of the study region leading to a gravity data set with a station spacing of about 5 km along the accessible roads. The total data set using different reference frames was reduced to the IGSN 71 gravity datum. The theoretical gravity values are calculated using the GRS 80 formula ( RGMI, 2006 ). Bouguer gravity was evaluated using the sea-level datum and density of 2.67 g/cm 3 for both the slab and topographic reduction ( GEOSOFT ). Terrain correction was computed by approximating the topographic masses with polyhedrons within a Hayford-Zone (circular segment of terrain with 167 km diameter) using a high resolution digital elevation data SRTM 90 M. Lack of accuracy of elevations is often a source of error and due care was taken to maintain the accuracy of about ±1.5 mGal. The terrain corrected Bouguer anomaly map, also known as complete Bouguer anomaly map, of the region shows the gravity trends that are in conformity with the structural trends of the protocontinental blocks ( Fig. 4 ). The most significant feature of the map is a relatively higher gravity field over the western half of the CITZ than the regional background over the Bastar Craton. Though a major part of this high may be attributed to Deccan traps of 0.5–1.0 km thickness, it could not explain this gravity high completely ( Verma and Banerjee, 1992 ). It is also not appropriate to explain this gravity high to be due to Satpura Mountains, since the high topography should yield a Bouguer low due to the isostatic compensation. Evidently, it may be indicative of under compensation caused by a horst type structure ( Qureshy, 1971 ) and/or accretion of mantle material into the crust ( Mishra, 1992; Verma and Banerjee, 1992 ). The eastern part of the CITZ is characterised by a relatively lower Bouguer anomaly partly because of the Mesozoic deposition in the Mahanadi basin. Assuming that only the lower sequence of Gondwana sediments is present in this region, the gravity low could also be due to deep seated source associated with the Mahanadi tectonics. The discordance in the trends of gravity contours along a line from Nagpur eastwards up to Korba is otherwise quite distinct. On the northern side of this divide line, the gravity trends are seen paralleling the outline of CITZ while in the southern part, gravity trends are almost perpendicular to those in the north. The line demarcating these two domains coincides well with the CISZ. At locations such as ancient sutures, density contrasts caused by the juxtaposition of domains with different petrophysical properties result in paired (positive and negative) gravity anomalies. Such paired gravity anomalies encountered across dissimilar structural provinces worldwide ( Gibb and Thomas, 1976; Fountain and Salisbury, 1981; Thomas, 1992 ) were used as markers to define the ancient suture zones of the Peninsular India ( Subrahmanyam and Verma, 1986; Singh and Mishra, 2002; Singh et al., 2004, 2006 ). To understand the nature of domain boundary between the Bastar Craton and CITZ, five gravity profiles (1–5) across the CISZ are plotted along with a profile (6) based on average value of five profiles across the Dharwar Craton-Eastern Ghats Mobile Belt ( Subrahmanyam and Verma, 1986 ) ( Fig. 5 ). The gravity anomaly along the two profiles (3 and 4) decreases from the Bastar Craton towards the CITZ and then rises sharply with a steep gradient across the contact of the two provinces. It is further noted that similar characteristic feature is seen at Southeast Sakoli shear in Profile-5 indicating it to be another domain boundary in this region. The two easternmost profiles (1 and 2) are masked heavily by Mahanadi tectonics restricting their utility for delineation of Proterozoic domain tectonics of the region. Particularly, since the CISZ (in Profile 1 and 2) is characterised by a typical gravity signature of block faulted type structure, one expects steep negative anomaly over the basin bounded by normal fault. 4 Horizontal gravity gradients The complete Bouguer anomaly map is further augmented by edge enhancement analysis that typically involves derivatives of the gravity field, such as the horizontal gravity gradient, maxima coinciding with the edges of lateral density contrasts ( Cordell and Grauch, 1985; Blakely and Simpson, 1986 ). The edges or contacts so derived may be analyzed for trends and patterns which characteristically resemble structural fabrics. The extensions of the maxima lines beyond exposed contacts and the places where such lines deviate from mapped geology often yield useful information about the extrapolations of the surface geology into the subsurface ( Thomas et al., 1988; Niraj Kumar et al., 2009a,b; Dhanunjaya Naidu et al., 2011 ). The computed horizontal gradient of the gravity field ( Fig. 6 ) of the study region together with the established structural domain boundaries may be regarded as a pseudo structural image with each linear segment of gravity gradient representing the boundary between rock packages with contrasting densities. On the basis of orientation, continuity, magnitude and pattern of the gradient features, the map can be subdivided into two possible gravity domains. The Gravity domain-I is mostly dominated by ENE–WSW trends of CITZ. The Gravity domain-II, corresponding to the Bastar Craton, is characterised by NS trends almost orthogonal to the CISZ trend. 5 2½D gravity modelling along Seoni–Rajnandgaon profile In the absence of a priori information, 2½D gravity modelling is inherently non-unique particularly on crustal scale where our knowledge is inadequate. In order to control the geometry and density, two vital parameters of the gravitating bodies, the earlier results of Seoni–Kalimati seismic experiment ( Reddy et al., 1996; Mall et al., 2008 ) were used in this study as far as possible. Though the seismic section provides constraints on the crustal structure, the CISZ is not imaged as a sharp boundary probably due to the tectonically disturbed crust over a 20–30 km wide zone ( Mall et al., 2008 ). The low reflection amplitude due to geometrical spreading, seismic attenuation and scattering of energy due to disturbed reflectors perhaps restricted its efficacy to resolve this highly deformed deep vertical structure. Despite that the Mungwani–Rajnandgaon profile across the CISZ has been selected for 2½D gravity modelling because of the seismic constraints available over a part of the profile. It would also be ideal to carry out gravity modelling along this line while other profiles (1 and 2) on the eastern side of the region are characterised by thick Mesozoic overburden and Profile 5 on the western side is crossed by Southeast Sakoli shear. Additional constraints along the Mungwani–Rajnandgaon profile were drawn from other geophysical experiments ( Mishra et al., 2000; Rai et al., 2005; Murty et al., 2008 ). 5.1 Seismic constraints to initial gravity model As a first step towards gravity modelling, the available velocity model of Mall et al. (2008) was converted into an initial density model. The 55-km long Mungwani–Katangi section of the available seismic profile runs towards the northwest of the CISZ whereas the 76 km long Katangi–Kalimati section crosses the CISZ towards the southeast ( Figs. 2 and 4 ). The high-amplitude band of reflections at 8 and 9 s TWT (labelled A in Fig. 7 ) at the northern end of the profile can be traced southward to Katangi at around 4–5 s TWT, and consists of predominantly northward dipping reflectors ( Fig. 3 a and b of Mall et al., 2008 ). The digitized line drawing of the reflection profile to the northwest of CISZ shows strong horizons around 5 s, 10 s and 13 s TWT (labelled B, C and D in Fig. 7 ) dip south ward, whereas to the southeast of the CISZ, these horizons dip to the north (labelled P, Q, and R in Fig. 7 ). Termination of crustal reflectivity at approximately 14 s TWT (∼44 km) is interpreted as the crust-mantle boundary (Moho). The Moho is revealed at a depth corresponding to 13.5–15.5 s TWT on either side of the Katangi. Further south, the Moho lies at 14.5–15 s TWT corresponding to a varying depth of 43–46 km, but with relatively lower amplitude. A close look at the line drawing ( Fig. 7 ) indicates doming up of the entire crustal column with the apex close to Katangi. Overall, the reflectivity northwest and southeast of the CISZ, over a length of approximately 30 km, is poor. Thus, it was interpreted that the CISZ is a disturbed zone affecting the continuity of reflectors ( Mall et al., 2008 ). To have velocity control for stacking, two refraction shot points, SP 0 and SP 100 ( Fig. 2 ) were recorded from Mungwani to Kalimati. Subtle details of seismic data acquisition and processing were discussed by Reddy et al. (2000) and Mall et al. (2008) . The modified depth section over Reddy et al. (1996) from the refraction data ( Fig. 8 ) provides constraints on the depths of various reflectors derived from reflection profile. Subsurface coverage by reflected ray path from SP 100 through the derived model based on the analysis of wide angle reflection and refraction data is presented in Fig. 8a , while Fig. 8b shows a match between observed (vertical bars) and theoretical travel time (continuous lines). For further refinement of the model, amplitude modelling was carried out. Relative amplitudes of reflection phases from different boundaries can be seen in observed seismogram of SP 100 ( Fig. 8c ), which can be compared with the synthetic seismogram corresponding to the derived model ( Fig. 8d ). The geometric constraints provided by the seismic common depth point data ( Fig. 7 ) together with the depth and velocity from refraction and wide angle data ( Fig. 8 ) provided an initial density model comprising five bodies. To construct an acceptable geological model, the density value for each seismic layer was obtained converting P-wave velocities to densities using the conversion function shown in Fig. 9 . This function integrates a compilation of global sample sets and is characterised by two correlations. The first correlation called Nafe–Drake curve is for the P-wave velocity conversion for sedimentary rocks ( Ludwig et al., 1970 ), while the second one relates velocities and densities for igneous and metamorphic rocks ( Christensen and Mooney, 1995 ). The average density finally selected for the causative bodies range from 2.85 g/cm 3 in the uppermost part of the crust to 3.30 g/cm 3 in the mantle. This density converted structure of the velocity model was inserted into a programme “GMSYS” which is interactive, graphical computer system for the interpretation of potential field data in 2½Dimension (GEOSOFT). Gravity response of this initial gravity model was calculated and compared with the observed Bouguer anomaly ( Fig. 10 a). 5.2 Density and geometry changes made It is well known that neither the average velocities can be unambiguously translated into density values nor there exists a one-to-one correlation between highly reflective seismic layer and density layer boundaries (e.g., Holliger and Kissling, 1992 ). Holding the configuration of the constrained crustal geometry based on the most trustworthy seismic layer boundaries, the average density of the individual layers was inverted within the logical limits of velocity to match the calculated with the observed gravity anomalies. Though the velocity–density conversion allows a range of densities for the respective velocity, it is not possible to obtain a fit alone by changing the densities along the profile. Assuming that the CISZ is a geo-suture and the two domains have different petrophysical properties, a relatively lower density for all the three crustal columns of the Bastar Craton is also considered for the modelling. By reducing the average density of individual crustal layers, a better fit between the observed and calculated gravity response of the initial gravity model is achieved ( Fig. 10 ). Yet, a clear mass deficit occurs across the granulite belt of CITZ towards the NW revealed by an anomaly of significant width and amplitude. Furthermore, the amplitude of the calculated gravity response over CITZ is offset by >30 mGal of the observed anomaly necessitating to alter the geometry of the initial density model to yield a better fit with the observed gravity. Keeping the average density of individual layers of best fit model fixed, geometry of the various crustal layers was then changed within the limits provided by seismic model to match the calculated and observed gravity anomalies. In spite of several iterations with changing geometry of the crustal bodies, a reasonably good match could not be found between the observed and calculated gravity. Hence, the initial crustal geometry was modified by introducing an additional upper crustal layer and smaller bodies associated with exposed geology in the uppermost crust ( Fig. 10 ). Staring from the north, first body represents high-density (2.90 g/cm 3 ) basalt of the Deccan traps followed by a body associated with the low-density (2.65 g/cm 3 ) Tirodi gneisses ( Table 1 ). About 2 km thick low-density (2.5 g/cm 3 ) body to the north of the Tan shear and the CISZ apparently represents a high conductive body which was also inferred from the magneto-telluric investigations ( Sarma et al., 1996 ). Bands of low- and high-density Amgaon gneiss, Pitepani (Nandgaon), Khairagarh metasediments, Dongargrah granite and Bijili rhyolite (Nandgaon) and Chattishgarh sediments characterise the Bastar Craton. Bulk density of the exposed rock formations was taken from the earlier study ( Mishra et al., 2000 ). 5.3 Final gravity model The final gravity model constructed by heuristic process is shown in Fig. 10 b. This model initially used the width and density of the exposed geological units while the faults and sub-crustal layers are derived from the seismic reflection model ( Mall et al., 2008 ). Values of thickness, density and depth of various crustal layers were varied up to 10% of the initial parameters during the modelling process. The expected mass deficit was matched with a density lower than the normal subcrustal density beneath the granulite belt of CITZ. The notable changes incorporated into the initial crustal model are the induction of exposed geological formations and distributed density contrast across the CISZ. The projected contact of the Bundelkhand and Bastar crustal blocks on the surface coincides well with the CISZ. A low density (2.90 g/cm 3 ) layer at the base of the crust and lower than the normal density (3.21 g/cm 3 ) upper mantle beneath the CITZ are two most significant features of our derived density model which may have a bearing on mantle crust connections in the CITZ region. 6 Discussion Based on the geomorphic character, P/T conditions of regional metamorphism and geological/geochemical observations, the Archaean–Palaeoproterozoic rocks of the study region was subdivided into two geo-tectonic domains. It was proposed that the crustal scale CISZ bounding these domains has allowed the juxtaposition of Bundelkhand and Bastar cratons during Palaeo- to Mesoproterozoic period ( Yedekar et al., 1990; Bhowmik et al., 1999; Roy and Prasad, 2003 ). Though the complimentary geophysical studies addressed the nature of deep crustal structure across the CISZ ( Mishra et al., 2000, 2002; Rajaram and Anand, 2003; Mall et al., 2008 ), the proposed evolutionary models are still under debate (e.g., Naganjaneyulu and Santosh, 2010 ). Identification of ancient sutures between amalgamated terranes and associated direction of subduction is one of the most difficult challenges in the understanding of Proterozoic tectonics. The paired gravity anomaly, a characteristic feature across two distinct structural domains ( Gibb and Thomas, 1976; Fountain and Salisbury, 1981 ) clearly indicates variation in crustal density or depth to the Moho in both the terranes ( Fig. 5 ). The largest horizontal gradient within the areas of Precambrian shield further define boundaries between two structural provinces ( Thomas et al., 1988 ). In the present context, the most prominent feature of the horizontal gradient map coincides with the locus of CISZ and/or Tan Shear ( Fig. 6 ). This linear belt of gradient maxima extends discontinuously up to Korba towards east and may be attributed to a boundary between two crustal domains (CITZ and Bastar Craton) having basic differences in the nature of the crust. Discontinuity in this first order gravity gradient along the CISZ/Tan Shear may be accounted for by the loss of the detail resulting from the sparse nature of the data set in a rugged terrain. According to Thomas et al. (1988) , an active margin involved in collision will retain its inherent pre-collision pattern of structure and, thus, gravity trends. On the other hand, the passive margin will be characterised by structures with resultant gravity expression, parallel or sub-parallel to the collision boundary. The regional E–W gravity gradient and structural grain on the CITZ is concordant with the CISZ while the N–S gravity gradient and structural grain of the Bastar Craton is discordant with it. The aeromagnetic map of the Central India also shows that the contours of CITZ are oriented in E–W direction in contrast to the NW–SE Dharwarian trend which appears to dominate the Bastar Craton ( Rajaram and Anand, 2003 ). Gradient analysis of gravity data thus validates the contention that CISZ separates two distinct Precambrian crustal domains, which have been brought in juxtaposition due to northward subduction of the Bastar Craton below the Bundelkhand Craton ( Bhowmik et al., 1999; Acharyya and Roy, 2000; Roy and Prasad, 2003; Roy et al., 2006; Mall et al., 2008 ). The subsequent crustal shortening of the passive continental crust must have initiated the crustal uplift along the Satpura orogenic belt ( Mohanty, 2010; Naganjaneyulu and Santosh, 2010 ). Our 2½D forward modelling of the gravity data, constrained by the Seoni–Kalimati seismic section, clearly shows that all the crustal layers of Bastar Craton consistently dip towards north nearing the CISZ as if the crust of the Bastar Craton has subducted beneath the CITZ ( Fig. 10 b). This logic is in agreement with Acharyya (2003) and Roy and Prasad (2003) who noted that the shear zone dips northward at the surface, and the Sausar Belt rocks over-thrust from north to south offsetting the Moho across the CISZ. Besides, the general trend of bulk density is low to high from southeast to northwest, respectively across the CISZ, as indicated by the gravity modelling ( Fig. 10 b). This is consistent with the geological section of the Bastar Craton varying from more pelite-dominated gneisses in the southeast, to denser, mafic granulites of CITZ to the northwest. We interpret this as indicative of the exhumation of the successively deeper levels of the Bundelkhand Craton crust in the southeast-directed imbricated thrust stack. As the active continental margin is partially subducted, the passive continental margin would be obducted. Subsequent uplift and erosion may expose the deeper-level crustal rocks along the suture ( Fountain and Salisbury, 1981; Thomas, 1992 ). Interestingly, the upper crust in a region north of Jabalpur is almost missing ( Kaila et al., 1989; Gokarn et al., 2001 ) and the exhumed lower crustal granulites are overlain directly by late Proterozoic Vindhyan sediments. Integrating these data with the exposed geology, we arrived at a model for the crust beneath the CITZ wherein the lower crustal granulites are brought up as slivers and slices that show a listric disposition along the CISZ ( Mall et al., 2008; Desai et al., 2010 ). Geochronological studies of two granulite belts (RKG and BBG) in the CITZ developed a tectonic model for the region whereby the Bastar Craton was juxtaposed with the Bundelkhand Craton during the younger Sausar orogenic cycle as part of the larger assembly of Rodinia ( Roy et al., 2006 ). The contention was contested by Stein et al. (2004) who argued that the juxtaposition between the two cratons along the CITZ took place along the Sausar belt during the earliest Palaeoproterozoic. However, they surmised that the region underwent significant reworking around 1100–1000 Ma. The reflectivity pattern near Katangi dips away from each other as well ( Fig. 7 ) and their continuity throughout the depth section supports the upward (domal) intrusive nature of the magmatic body. Desai et al. (2010) proposed that the crustal growth in CITZ region occurred predominantly by the intrusion of mafic and ultramafic rocks derived from basaltic magmas originating from the mantle. Naganjaneyulu and Santosh (2010) opined that the lower crust in this region is composed of metamorphosed subducted oceanic lithosphere, together with mantle wedge and arc magma components. This ancient crustal structure was later intruded by mafic/ultramafic bodies and layered bodies derived from under-plated basaltic magmas probably associated with the Deccan volcanic regime. Thus, the present-day crust beneath this zone is composed of Archaean/Palaeoproterozoic lithosphere and Cretaceous magmatic products ( Naganjaneyulu and Santosh, 2010 ). According to Desai et al. (2010) , the underplated nature of the deep crust and the subjacent mantle rendering them transitional and the Moho is located within this transition zone. Using the receiver function measurements in the adjoining regions Rai et al. (2005) presented evidences for a duplex Moho with highly mafic ( V p / V s = 1.84) underplated lower crust. A high heat flow indicates that the thermal structure of the region has not yet completely stabilised despite the fact that the region experienced magmatic activity some 65 Ma ago. The low seismic velocity/density in the lower crust may thus be due to high temperatures that maintain the minerals at near melting point conditions ( Desai et al., 2010 ). However, the role of fluid inclusion and metasomatism in lowering the seismic velocity cannot be ruled out ( Desai et al., 2010 ). We postulate that the delineated lower velocity/density layer at the base of the crust represents the same lower crustal emplacement of ultramafic lithologies that caused the up warp above this body. Uplift of the CITZ through movement and incorporation of mafic and ultramafic material from the upper mantle into the crust also explains the departure from isostatic compensation ( Qureshy, 1971 ). The sill-like intrusive body makes it a potential candidate as lower crustal secondary magma chambers and crustal magma path-ways for the Deccan Trap basalts ( Desai et al., 2010 ). Out of the many major earthquakes led by epeirogenic uplifts in Satpura mobile belt, the focal depth of only two are reported at a depth of more than 35 km ( Mall et al., 2005 ). They, however, supported the idea of weak inclusion in the lower crust that is capable of stress concentration. Rao et al. (2002) and Rao and Rao (2006) inferred that the most probable composition of the lower crust could be serpentinized ultramafic rocks formed by dehydration of molten mantle material. Serpentinization of the mafic and ultramafic rocks or the presence of grain boundary graphites was also proposed for the anomalous conductance of more than 1200 S ( Gokarn et al., 2001 ). According to Durrheim and Mooney (1994) , another factor that could have some effect on conductivity is the fluid inclusion. The xenolith data however, lends support to ultramafic rocks that could be responsible for the high conductance observed along the magnetotelluric profile ( Desai et al., 2010 ). A mechanically strong lower crust overlying a relatively weak sub-Moho layer is further suggested to enhance the stress concentration in the hypocentral region, implying a weaker mantle in comparison to the lower crust for this region of central India ( Manglik et al., 2008 ). The Pn velocity, a major source of information on the petrologic nature of the upper mantle, is unfortunately not discernable along Seoni–Kalimati seismic profile. Moreover, a Pn velocity of 7.8 km/s in the adjoining region ( Murty et al., 2008 ) is lower than the seismic velocity of 8.0–8.1 km/s representative of a normal continental mantle lithosphere is reflected in our derived density model ( Fig. 10 b) where the requisite mass deficiency is balanced by a lower density (3.21 g/cm 3 ) in the subcrustal mantle beneath the Mungwani–Katangi region. It has been hypothesized that through underplating and fractionation of basaltic magma at the base of the crust, the ultra mafic root of the crust became stronger. It would now be imperative to search for imprints of major tectonic event that made the upper mantle comparatively weak. Recently, a similar upper mantle density beneath the Southern Granulite Terrain is hypothesized as a highly remobilised lithosphere due to crustal delamination ( Singh et al., 2006, in press; Niraj Kumar et al., 2009a,b, 2010 ). Chemical stratification of the Indian lithospheric mantle with the upper lithosphere (up to 80 km) comprising of the spinel peridotite (of lower density) and the lower lithosphere with that of garnet peridotite composition (of higher density) may be the other distinct possibility ( Jagadeesh and Rai, 2008 ). A detailed and high resolution integrated geophysical mapping of the region is therefore necessary to determine the nature of the upper mantle more conclusively. 7 Conclusions Our study brought out new constraints on deep crustal structure and possible models for the geodynamic evolution of the CISZ. Paired gravity anomaly, horizontal gravity gradient and derived 2½D density modelling along Seoni–Rajnandgaon profile clearly demonstrate that the CISZ is a locus of crustal density discontinuity. With a well defined Moho offset and a crustal scale density discontinuity, the model implies CISZ likely to represent a suture zone that separates different Palaeoproterozoic terranes. The gravity gradient pattern across this curvilinear crustal boundary together with our derived 2½D model implies a convergence of Bastar Craton beneath the Bundelkhand Craton, with CITZ being an element of the resulting accretionary zone. A mechanically strong lower crust and relatively weak sub-crustal lithosphere is postulated to understand the cause and effect of the crust-mantle interaction beneath the Mungwani–Katangi region. Acknowledgements The authors thank Director, NGRI, Hyderabad for his encouragement and permission to publish this work. 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