Passive seismological imaging of the Narmada paleo-rift, central India

Precambrian Research 270 (2015) 155–164

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Passive seismological imaging of the Narmada paleo-rift, central India M. Ravi Kumar a,∗ , Arun Singh b , Narendra Kumar c , Dipankar Sarkar a a

National Geophysical Research Institute (Council of Scientific and Industrial Research), Hyderabad, India Department of Geology and Geophysics, Indian Institute of Technology Kharagpur, India c Wadia Institute of Himalayan Geology, Dehradun, India b

a r t i c l e

i n f o

Article history: Received 2 July 2015 Received in revised form 22 August 2015 Accepted 9 September 2015 Available online 21 September 2015 Keywords: Narmada-Son lineament Indian shield Crustal structure Magmatic underplating Crustal anisotropy Receiver functions

a b s t r a c t The Narmada-Son lineament (NSL) is a prominent, nearly E–W trending tectonic feature dividing India into the peninsular and extra-peninsular regions. It marks the possible suture between two contrasting geological settings termed proto-continents lying to the North and South of it. Various views prevail about the nature of the rift zone. The present study delineates the seismic structure of the crust using P wave receiver functions at 10 broadband seismic stations installed along a profile across the NSL. The results from Common Conversion Point (CCP) stacking reveal a near continuous image of the Moho boundary, with the amplitudes of the Ps conversions from the Moho being weaker at stations within the rift. Modelling the receiver functions employing nearest neighborhood algorithm approach brings out a distinct (15 km) thickening of the crust and a complex nature of the Moho within the rifted zone. We find that the backazimuthal variations of the SV and SH receiver functions at the permanent station Jabalpur can be explained better by introducing azimuthal anisotropy in the middle and lower crust with the fast axes perpendicular to the rift axis. Also, we find substantial evidence of post rifting magmatic underplating in the lower crust, which could be reminiscence of Deccan volcanism and/or Reunion plume path. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The Narmada-Son lineament (NSL) with a total strike length of nearly 1200 km extending in the E–W direction from 72.5◦ E to 82.5◦ E is a prominent tectonic feature in the Indian subcontinent. It is believed to mark the suture between two contrasting geological settings namely the Bundelkhand proto-continent to the north and the Dharwar proto-continent to the south (Naqvi and Rogers, 1987). In a broader perspective, the Central Indian Tectonic Zone (of which NSL is a part) is envisaged as a collision zone involving the amalgamation of Singhbhum, Bastar and Dharwar cratons in the south and the Bundelkhand craton in the north during the late Archaean (Yedekar et al., 1990; Jain et al., 1995). Also, it is argued that the NSL is a continental rift zone that has been reactivated a few times since the Proterozoic (Choubey, 1971). Various views prevail about the nature of the rift zone. While some workers designate the NSL as a rift valley, others have described it as a fault zone, a major thrust zone or a rift valley with boundary faults (Yellur, 1968). From the available geophysical information, Mishra (1977) concluded that

∗ Corresponding author. Tel.: +91 4027012897. E-mail address: [email protected] (M.R. Kumar). http://dx.doi.org/10.1016/j.precamres.2015.09.013 0301-9268/© 2015 Elsevier B.V. All rights reserved.

parts of the NSL represent a typical rift structure and the lineament extends into the Arabian Sea up to the Murray ridge (Fig. 1). Crawford (1978) compared similar structures and traced the extension of the lineament into Madagascar, Somalia and Ethiopia. From geological evidence, Biswas and Deshpande (1983) considered this rift to be complementary to the East African rifts. It is argued that the stratigraphy of the region is akin to a horst type feature, delimited by the Son-Narmada fault in the north and the Tapti fault to the south (Qureshy, 1982). The regional gravity anomaly map shows the Tapti-Narmada-Son zone to be a broad region of gravity “high” in which the NSL appears as a narrow “low” (Qureshy, 1982). Ghosh (1976) state that the Narmada-Son lineament represents an erosional post-Deccan Trap Narmada valley formed at the crest of a domal upwarp with tensional features and probably shallow depressions along the crest. Their observations are based upon the fact that the faults bounding the Narmada zone played a significant role in the deposition of Meso-Neoproterozoic Vindhyan sediments on the northern side and the Permo-Carboniferous Gondwana sediments on the southern side. Ghosh (1976) further opined that this lineament represented by the upwarp may form part of a major regional megalineament active periodically since the Archaean, and extending probably through the northern side of Chotanagpur plateau, the Shillong massif and the so-called syntaxial area of

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Fig. 1. Major rifts basins located in the Indian shield. The study region is shown by a rectangle. EDC: Eastern Dharwar Craton; WDC: Western Dharwar Craton.

the northeastern Himalaya. This zone also finds expression in the gravity anomalies, and may thus be a mega-lineament with deep roots. The NSL is bounded by two major seismically active faults namely the Narmada south fault (NSF) and the Narmada north fault (NNF) (Fig. 2), whose origin is traced back to the middle to late Archaean period (Choubey, 1971; Jain et al., 1995). The region between the two fault systems is covered with alluvial deposits and late Archean greenstone belts. To the north, the NSL is bounded by Precambrian terrains like the 750–1721 Ma Vindhyan basin and the 2.5 Ga Bundelkhand craton, and in the south by the ∼65 Ma old Deccan volcanics (Meert et al., 2010; Malone et al., 2008) (Fig. 2)). Recent studies constrain the age of the Chattisgarh supergroup rocks located towards the south of rift zone as ∼1 Ga, much older than the previously envisaged age of ∼500 Ma (Bickford et al., 2011; Patranabis-Deb et al., 2007). During the last 50 years, eight moderate size earthquakes occurred in the stable continental region of India, a majority of them being spatially coincident with paleo-rifts. For example, the Mw 6.0, 1997 Jabalpur earthquake is associated with the Narmada rift that is well expressed by the tectonic signatures and is host to moderate seismicity (Rajendran, 2000). Occurrence of lower crustal earthquakes that are typical of the Narmada rift region is attributed to the presence of ultrabasic and alkaline intrusives along these rifts (Rao et al., 2002). The high Bouguer gravity anomalies in the region are explained by the possible presence of copious high density material in the lower crust as a result of large scale asthenospheric upwelling and intrusion along the Moho and within the crust (Qureshy, 1964). The Moho temperature in the Jabalpur earthquake region is estimated between 500 and 580 ◦ C indicating that the lower crust in this region is cool and brittle (Manglik and Singh, 2002; Rai and Thiagarajan, 116). Reddy

and Rao (2003) suggested that the lower crust is underplated by material from asthenospheric depths or mantle derivatives added through fractures ascending along deep crustal faults. As summarized by Murty et al. (2010), the models that explain the occurrence of earthquakes in the mid-continental regions ascribe various reasons like fault reactivation (Sykes, 1978), stress concentration at high density materials present in the faults (Campbell, 1978) and strain localization (Zoback et al., 1985). The high seismicity associated with the NSL has been attributed to (i) reactivation and stress concentration, amplification of horizontal compressive stresses due to the effect of a rift pillow as an active element (Rajendran and Rajendran, 1998), (ii) stress accumulation due to horizontally elongated or elliptical, possibly serpentinized mafic intrusives in the lower crust (Rao and Reddy, 1998), (iii) high pore pressure, low frictional coefficient and high strain rates that promote brittle failure in the lower crustal segments of the NSL (Gahalaut et al., 2004), (iv) unsteadiness of the crustal blocks due to crustal inhomogeneity, presence of high pore fluid pressure, diffusion of pore-pressure relaxation and high strain rate (Rao and Rao, 2006). Prior to this study, teleseismic receiver functions were computed using data over a period of 6 months recorded at nine short period stations installed along a 250 km long profile across the NSL (Rai et al., 2005). Their results suggest a Moho downwarp to 52 km across the width of the lineament, compared to an average depth of 40 km elsewhere. In addition, the crust beneath the NSL is found to have a higher Vp/Vs of 1.84 compared to 1.73 in the surrounding regions. This is suggestive of a high density mafic mass at depth that compensates the crustal root, also supported by a small topographic variation (200 m) across the lineament. They concluded that presence of such an anomalous mass in the deep crust may lead to

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Fig. 2. Tectonic map of the study region together with prominent faults and seismic stations (inverted triangles). Inset shows it’s position in India. NSF: Narmada South Fault; NNF: Narmada North Fault; TF: Tapti Fault; GF: Gavilgarh Fault; TS: Tan shear zone. Green filled circles represent the locations of (1) May 22, 1997 Jabalpur earthquake (Mw 6.0) (Rao et al., 2002), (2) May 17, 1903 Bargi earthquake (Ms 5.0), (3) April 18, 1987 Pauner Earthquake (Mb 4.9), (4) August 25, 1957 Lalburra earthquake (Ms 5.5), (5) October 16, 2000 Kundam earthquake (Mb 4.4) and (6) June 2, 1927 Umaria earthquake (Mw 6.4) (source for 2, 3, 4, 5, 6: ISC catalog). Available ages of the different tectonic regions are also shown (source, (Malone et al., 2008). Crosses denote piercing points of individual rays at a depth of 40 km. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

gravitationally induced stresses in the lower crust that contribute to the failure of rock along the pre-existing Narmada-Son fault leading to the earthquakes in deep crust. The Jabalpur earthquake has thus been associated with the Narmada South Fault characterized as a thrust. The present study illuminates the seismic structure of the crust across the Narmada rift using Common Conversion Point (CCP) imaging and nonlinear inversion of teleseismic receiver functions computed using data retrieved from an array of broadband seismic stations. 2. Data and method The National Geophysical Research Institute has operated a network of 10 broadband seismograph units across the NSL over a period of 3 years from May, 2008 (Fig. 2). These are a combination of CMG3T and KS2000M sensors connected to REFTEK-130 digitizers. Data accrued at these stations are augmented by those from the permanent station at Jabalpur (JBP) which is being operated continuously since 2004. This passive experiment across the Narmada region has been carried out along a profile close to that of Rai et al. (2005). Here, we have the advantage of utilizing a larger quantity data registered by seismometers of much broader bandwidth over a period that is about five times longer than the earlier experiment. For receiver function analysis, we extracted the teleseismic earthquake waveforms in the distance range of 30–100◦ and quality checked using vertical components, to retain the good quality ones (SNR ≥ 2.5). Although the backazimuthal distribution of the earthquake sources is skewed towards eastern azimuths, three years of operation of the stations enabled recording of a good number of events from the western azimuths also (Fig. 3). The computation of receiver functions is performed by first rotating the Z, N, E waveforms to decompose the wavefield into P(L), SV(Q) and SH(T) components. The converted phases are then isolated from the

Fig. 3. Earthquakes in the distance range of 30–100◦ whose waveforms are used in this study. The study region is indicated as a filled (black) rectangle.

P-coda by deconvolving the P from the SV and SH components by simple spectral division using a water level stabilization. This exercise resulted in a total of 1419 receiver functions. 3. Shear wave structure of the crust In order to qualitatively assess the layering beneath each station, the receiver functions in the slowness range of 4.4–8.8 s/◦ are sorted in 50 bins and stacked. Prior to stacking, the IASP91 velocity model has been used to correct the traces for the move out arising due

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Fig. 4. SV (top panel) and SH (bottom panel) receiver functions at stations, stacked in narrow slowness bins. Stations are sorted from south to north. Positive amplitudes are represented by red and negative ones by blue. A total of 1463 receiver functions are used to construct this image. Station KMR with only six traces is not plotted. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

to geometry of the ray paths corresponding to different epicentral distances. The RFs are reduced to a fixed slowness of 6.4 s/◦ , corresponding to an epicentral distance of 67◦ . In case of isotropic crustal layers with a gentle dip, such a moveout correction would make the Ps conversion times at different slowness values comparable. The slowness images of stations juxtaposed along a S–N profile traversing the NSL reveal that the P-to-s conversion from the Moho (Pms) is clearly seen at all the stations with its arrival time varying from 4 s to 6 s (Fig. 4). Further, sorting of the receiver functions based on the regions sampled by the piercing points at the Moho reveals that the Moho conversions within the rift are weaker compared to those from the adjacent DVP and the Vindhyan basin (Fig. 5). 3.1. Common conversion point imaging We generated CCP images of the receiver functions to qualitatively assess differences in crustal thickness values within and

outside the Narmada rift and examine the nature of the Moho boundary. The common conversion point (CCP) imaging using 1419 P receiver functions is done by transforming the receiver functions into the depth domain by back projecting the energy to their corresponding conversion points along their paths, using the IASP91 standard velocity model by treating all the energy as Ps conversions at the discontinuities. To enhance the spatial coherence of the converted phases, the back projected amplitudes are binned into 2 × 2 km grids corresponding to different depth offsets and subsequently stacked and projected onto a 2D reference plane whose strike coincides with the seismic profile. The back projected amplitudes of the RFs falling in a spatial grid are plotted using a colour scheme where red represents a positive polarity and blue indicates a negative one. As can be seen from the CCP image (Fig. 6) the crustal thickness varies between 40 km outside the rift to more than 52 km within the rift.

Fig. 5. Slowness sections of SV receiver functions whose piercing points at the Moho sample the Deccan Volcanics, rift zone and the Vindhyan basin. Positive amplitudes are represented by red and negative ones by blue. The top frame in each section contains two summation traces. The bottom one corresponds to the stack of the RFs moveout corrected for converted phase and the top one corresponds to the stack of the RFs moveout corrected for multiples. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 6. Common conversion point image of the SV component P wave receiver functions along a S–N prole (22.5◦ , 81◦ ; 23.7◦ , 79◦ ). Star denotes location of the Jabalpur earthquake.

3.2. Modelling for crustal structure We perform nonlinear inversion of receiver functions to obtain the average shear velocity and Moho depth estimates and attempt to characterize crustal anisotropy in this rifted region using data from the permanent station JBP, which afforded adequate backazimuthal coverage to model for dip and anisotropic effects. Most seismological studies on anisotropy have targeted the bulk anisotropy of the upper mantle utilizing the core refracted phases like the SKS and SKKS. Passive seismic studies offer an elegant way to determine the crustal contribution to this bulk anisotropy. Presence of energy on the transverse component of receiver functions (Fig. 4) is indicative of dipping effects and/or anisotropy. When clear energy is seen on the transverse component, we can model the subsurface in terms of dip and anisotropy. For modelling, we sort the receiver functions first in 36 back-azimuthal bins. Further, we sort the traces in each back-azimuthal bin in 4 slowness bins and stack them. However, when the energy on the transverse component is either absent, weak or noisy, the data can only be modelled for 1D structure. To quantify the crustal structure beneath the stations, we adopted the neighbourhood inversion scheme (Frederiksen et al., 2003), an efficient approach to model the complex crustal structure involving dip and anisotropic effects. The forward modelling approach of Frederiksen and Bostock (2000) is used to generate the synthetics. The model parameter space comprises of layer thicknesses, P and S velocities in each layer and additionally the strike and dip of the layer and azimuth and plunge of anisotropy within a particular layer, if required by the data. The values of the P and S

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velocities in the mantle are fixed at 8.1 km/s and 4.65 km/s, respectively during the inversion process. The suite of models that best explain the radial RFs are obtained by iteratively matching the observed and synthetic traces. As described by Frederiksen et al. (2003), random points are selected in a given multi-parameter model space and the remaining unsampled space is divided into neighbourhoods, which are regions of points that are closer to one sampled point than to any other. Since the ray theoretical approach used by us to generate synthetics is more sensitive to travel times of phases compared to their amplitudes, the misfit function is based on the correlation coefficient between synthetic and real traces. The misfit function is determined for each sampled point, and regions with the lowest misfit are further considered. In each iteration, a set of N models within the specified parameter limits is pseudorandomly generated. Forward modelling is then performed for each model and the misfit with respect to the real data is determined. The best M models are then retained from the entire set determined in the run, and their neighbourhoods are used for the next iteration. The only inversion parameters required are the total number of iterations, and numbers N and M, whose ratio determines the efficiency of the search. A high M/N ratio indicates a highly explorative, slowly converging search that covers the model space widely and a low ratio means that the search will converge quickly. In principle, backazimuthal variations of the arrival times, together with the periodicities in the polarity of amplitudes of the conversions permit extraction of dip and anisotropy of the subsurface layers (Cassidy, 1992; Frederiksen and Bostock, 2000). Although dipping and anisotropic interfaces (assuming hexagonal symmetry) produce a 360◦ and 180◦ periodicities on the transverse component receiver functions, their combined effect coupled with deviations from hexagonal symmetry, non-uniform backazimuthal distribution and more importantly, low quality of the transverse components of receiver functions, pose a challenge to unambiguously separate their effects. In the present study, most of the stations except JBP, are modelled for 1D structure. The inversions are performed using a four layer model for JBP, invoking anisotropy, dip and their combined effect (Figs. 7 and 8). Invoking dip and anisotropy for a layer between 12 and 25 km depth clearly fits the observed data better (Fig. 7). The parameters that best explain the data at station JBP are listed in Table 1. As evident from Fig. 9, the strike, dip of the upper crustal layer and trend of anisotropy are well constrained, in contrast to the strength of anisotropy. The best fit models reveal a thickened crust (57 km) beneath this station. For stations CHR, LLP, GMR, SGR, KDM and BJD, the inversions

Fig. 7. Real (color filled) and synthetic (black wiggles) data generated by the best fit model (red color in central panel) for the upper crust beneath the Jabalpur station obtained through inversion. Left panel: Fits obtained by modelling only for dip of the layer. Right panel: Fits obtained by incorporating both anisotropy and dip. Invoking anisotropy along with dip clearly improves the fits. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 8. Real (color) and synthetic data generated by the best fit model (central panel) for the whole crust beneath Jabalpur station, obtained through inversion.

Table 1 Crustal parameters at stations across the NSL region obtained from 1D inversion of receiver functions except for station JBP where the data are inverted for 2D structure including dip and anisotropy. The dip and anisotropy parameters for the layers beneath JBP are listed in the lower part. Stat.

Lat. (◦ )

Lon. (◦ )

Moho depth (km)

Avg. crustal Vs (km/s)

KDM SGR LLP BJD JBP* GMR CHR

22.54 22.70 22.78 22.98 23.12 23.36 23.58

80.41 80.36 80.27 80.15 79.87 80.02 79.14

40 40 43 55 57 45 43

3.8 3.7 3.8 3.9 3.5 3.6 3.8

JBP Layer (km)

Geometry ◦

12–25 40–57

Anisotropy ◦

Strike ( )

Dip ( )

Trend (◦ )

Plunge (◦ )

P&S (%)

60 –

10 –

149 18

15 5

20 12

.

Fig. 9. Tradeoff of crustal parameters for Jabalpur station obtained through inversion using the neighborhood algorithm. Parameters that yield the lowest misfit are shown by cross.

are performed using 1 or 2 layer models. The estimated values of crustal thickness and average crustal shear velocity listed in Table 1 provide reasonable fits to the observed data (Fig. 10). Due to poor quality, the data at stations JMA and KMR did not permit modelling. 4. Discussion This study presents new results of the seismic structure of the crust across the Narmada rift (Fig. 11) and furthers the previous results of the shear velocity structure obtained using a small data set of short period receiver functions (Rai et al., 2005). While our

Common Conversion Point (CCP) images of the receiver functions reiterate distinct thickening of the crust, duplex nature of the Moho and magmatic underplating within and in the proximity of the rift zone, results of nonlinear inversion of receiver functions yield the average crustal shear velocity and Moho depth estimates. The 1997 Jabalpur earthquake that occurred in the lower crust seems to be associated with a high velocity zone representing a mafic intrusive (Fig. 6). Our results complement the findings from active seismics that clearly show that the crustal velocity structure indicates significant magmatic underplating, starting from the western flank through middle and eastern segments of the NSL (Kaila et al., 1987;

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Fig. 10. Real (color) and synthetic data generated by the best fit models (central panel) for the other stations obtained through 1D inversion. The average velocity and crustal thickness estimates are shown in the bottom-right panel. The seismic stations with no fill are the stations where data is either insufficient or the phases are not clear to perform a meaningful inversion.

Fig. 11. A summary plot of the results from the present and previous active and passive seismological studies (Rai et al., 2005; Jagadeesh and Rai, 2008; Juli’a et al., 2009) (gray filled square).

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Murty et al., 2005). Importantly, using data from station JBP, we demonstrate that the middle and lower crust within the rift are distinctly anisotropic in nature. Our results suggest that the average crustal thickness in the vicinity of the NSL is 40 km, deepening to ∼55 km within the lineament zone. The high Vp/Vs ratios (1.84) associated with the thicker crust were previously interpreted in terms of a mafic to ultra-mafic composition of the lower crust (Rai et al., 2005). High Vp/Vs ratios (1.9) are also reported from two broadband stations within the NSL, which are 70 km to the west of the Hirapur-Mandla prole (Rai et al., 2005; Jagadeesh and Rai, 2008). Interestingly, these data reveal a complex nature of the Moho, with two distinct conversions from layers at depths of 35 km and 45 km (Rai et al., 2005). Through an analysis of data digitized from analog Deep Seismic Sounding (DSS) data along the Hirapur-Mandla profile (Fig. 11), Murthy et al. (1998) found an unusually high P velocity (6.7 km/s) layer at a very shallow (∼2 km) depth and a Moho upwarp below the NSL, in conformity with previous results (Kaila et al., 1987). Subsequently, a new crustal velocity model derived from travel time inversion of wide-angle reflections reconfirmed an uplifted crustal block and Moho up-warp beneath the NSL, possibly indicating that the NSF and NNF are deeply penetrating faults, through which high velocity mafic materials had intruded into the upper crust from the upper mantle (Murty et al., 2008). The depth to the Moho boundary along this profile was found to be 41–46 km similar to the value of 40–44 km determined earlier (Murty et al., 2004). The P-velocity of 6.95 km/s found at the base of the crust in some parts of this profile suggests the possibility of an underplated layer like that seen in other profiles in the western part of the NSL (Tewari and Kumar, 2003; Singh and Meissner, 1995). However, our results from inversion of receiver functions bring out a thicker crust and a rather complex nature of the Moho particularly underneath the NSL. The crust is found to be distinctly thicker within the rift zone compared to that outside on either side and in the adjacent Archean cratons like the Bastar and Bundelkhand where the crustal thickness values inferred from receiver function studies are in the range of 30–40 km (Singh et al., 2015). Interestingly, Rajesh et al. (2009) provide evidence for an undeformed 3.6 Ga true granite in the Bastar craton and argue that its age and composition imply that the continental crust of this craton attained sufficient thickness to permit intracrustal melting in the early Archean. Rai et al. (2005) also observed a Moho downwrap to 52 km across the width of the NSL, compared to an average depth of 40 km elsewhere. Later, these results were interpretated by Jagadeesh and Rai (2008) in terms of a duplex Moho character beneath NSL at depths 31–38 km and at 45 km, which is highly mafic in nature (Poisson’s ratio ∼1.9). Gravity studies in the Narmada-Tapti region provide further evidence for magmatic underplating due to Deccan flood basalt eruptions in terms of an anomalous density of 3.02 g/cm3 at the base of the crust (Singh and Meissner, 1995). Here, we find substantial evidence for post rift magmatic underplating in the lower crust. The complicated nature of the Moho below the NSL is attributed to magmatic underplating expected in a rift environment. In continental flood basaltic provinces, magmatic underplating is recognized as an important mechanism of crustal growth and evolution (Thybo and Nielsen, 2009; Thybo and Artemieva, 2013). When the ascent rates of the partially molten mantle material are not sufficiently high, most of it cools and gets frozen in the lithosphere without reaching the Earth’s surface. Trapped basaltic magmas are located near the Moho or within the crust beneath continental areas. Seismic studies and other geophysical means have provided evidence for underplated bodies beneath the Hawaiian Islands, La Reunion, Ninety East ridge, and the Deccan Traps (McNutt and Caress, 2007; Leahy et al., 2010; Lenat et al., 2012; Grevemeyer et al., 2001; Holbrook, 1995; Minshull et al., 2008;

Rao et al., 2015). Evidence for magmatic underplating was also presented for other rifts in India, like the Godavari, Cambay and Kutch. In the Kutch rift region, existence of a thick (∼45 km) and highly reflective crust at the hypocentral zone of the 2001 Kutch earthquake (M 7.7) could either be attributable to magmatic intrusions that date back to Mesozoic rifting associated with the breakup of Gondwanaland or indicative of crustal thickening due to the compressive regime over the past 55 Ma (Sarkar et al., 2007). The Cambay rift basin in the northwest Indian platform shows high Bouguer gravity anomaly values compared to the values outside the basin. The deep seismic sounding results along the strike of the basin show the presence of a high P-velocity (7.3 km/s) layer at a depth of 23–25 km, followed by the Moho at a depth of 31–33 km. Two dimensional density modelling of the seismic structure by (Tewari et al., 1991) across the Cambay basin shows that the crust is thinner underneath the basin, but is associated with a high-density lower crust (rift pillow). Modelling of receiver functions at 6 stations in the Godavari rift revealed a 40 km thick crust within the rift compared to a 33 km thick crust in the adjacent Dharwar craton (Saul et al., 2000). Also, the lower crustal character was found to be distinctly different, with a faster (mafic) lower crust beneath the rift. It was hypothesized by Singh et al. (2012) that a weak Moho beneath the rift could be due to magmatic underplating. Magmatic underplating within the crust is possibly indicative of rift formation due to lithospheric stretching. In the Mahanadi rift region, active refraction studies delineated a ∼10 km thick high P velocity (7.5 km/s) and high-density (3.05 g/cm3 ) layer at the base of a 35–37 km thick crust (Behera et al., 2004). They opined that Moho upwarping or crustal thinning in the rift zone and emplacement of thick, high-velocity material at the base of the crust strongly suggests basaltic underplating probably due to the Kerguelen hot spot activity responsible for the 130 Ma old Rajmahal Traps exposed in northeast India. Another aspect of the present study is crustal anisotropy. Our results from the Jabalpur station located within the rift show that for both the largely anisotropic mid- and the less anisotropic lower crust, the fast axes are nearly orthogonal to the rift axis. The patterns of azimuthal upper mantle anisotropy observed in rift zones from across the world suggest that the fast polarization directions are either parallel to the extension direction or to the orientations of the rifts. Providing a unique explanation for the observed anisotropy is ridden with uncertainties, since many factors can contribute to anisotropy in the crust. Anisotropy within the lithosphere originates primarily from lattice-preferred orientation (LPO) and shape preferred orientation (SPO) of the constituent materials. The latter mechanism results from presence of oriented fluid-filled cracks in the crust or melt-filled cracks or lenses in the mantle, and due to laminated solid materials with contrasting elastic properties (e.g., Crampin, 1984; Walker et al., 2004). The SPO due to decompression melting may also produce seismic anisotropy in continental rift zones. Liu and Niu (2012) found that the observed fast polarization direction follows the direction of the maximum horizontal tensile stress, suggesting that the observed seismic anisotropy is likely caused by mineral alignment in the lower crust. Anisotropy in the crust can also be caused by strain-induced Earth structures such as aligned macro-scale fractures and faults related to regional tectonics, fault-zone fabrics and aligned minerals and/or grains, which cause the S-waves to be polarized with the fast direction in the plane of the feature. The presence of aligned melt within magmatic structures such as dykes, commonly used to explain patterns of mantle anisotropy in magma-rich extensional settings, also provides an efficient means of generating seismic anisotropy in the crust. In regions like western Tibet anisotropy in the mid to lower crustal depths is explained in terms of mineral alignment due to lateral flow (Sherrington et al., 2004). Considering the evidence for magmatism in the NSL region, the frozen anisotropy in the crust

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could be due to mineral alignment related to flow across the rift axis. 5. Conclusions 1. This study presents for the first time, a nearly continuous image of the Moho and the overlying crust along a seismic profile across the Narmada rift. The image reveals a duplex nature of the Moho with the crust being distinctly thicker (>52 km) within the rift zone compared to that on either side (40 km). 2. The Common Conversion Point image of receiver functions illustrates a complex nature of the Moho and the overlying lower crust below the stations within the NSL. 3. Qualitative evidence for high velocity magmatic underplating within the rift zone comes from the weak nature of the converted phases from the Moho. 4. The results of nonlinear inversion of receiver functions show that the average crustal shear velocity (3.5–3.6 km/s) for seismic stations within the NSL is lower compared to that found for stations at the margins (3.8 km/s). 5. Modelling the back azimuthal variations of the receiver function amplitudes yields an anisotropic middle and lower crust with the fast axes oriented perpendicular to the rift axis. This could represent anisotropy frozen in the crust due to mineral alignment caused by magmatism across the rift axis. Acknowledgements We sincerely thank two anonymous reviewers and the Associate Editor H.M. Rajesh for their comments. Financial support for the experiment was provided by the Seismology division of the Department of Science and Technology (now in MoES), India. We sincerely thank Vijaya Raghavan, Satish Saha and M. Kousalya for providing excellent field support. This work is performed under the GENIAS project of CSIR-NGRI. References Behera, L., Sain, K., Reddy, P.R., 2004. Evidence of underplating from seismic and gravity studies in the Mahanadi delta of eastern India and its tectonic significance. J. Geophys. Res. 109, http://dx.doi.org/10.1029/2003JB002764. Bickford, M.E., Basu, A., Patranabis-Deb, S., Dhang, P.C., Schieber, J., 2011. Depositional history of the Chhattisgarh basin, central India: constraints from new SHRIMP zircon ages. J. Geol. 119, 33–50. Biswas, S.K., Deshpande, S.V., 1983. Geology and hydrocarbon prospects of Kutch, Saurastra and Narmada basins. Petrol Asia J. 6 (4), 111–126. Campbell, D.L., 1978. Investigation of the stress concentration mechanism for intraplate earthquakes. Geophys. Res. Lett. 5, 477–479. Cassidy, J.F., 1992. Numerical experiments in broadband receiver function analysis. Bull. Seism. Soc. Am. 82, 1453–1474. Choubey, V.D., 1971. Narmada-Son lineament, India. Nat. Phys. Sci. 232, 38–40. Crampin, S., 1984. Effective anisotropic elastic constants for wave propagation through cracked solids. Geophys. J. R. Astron. Soc. 76, 135–145. Crawford, A.R., 1978. Narmada-Son lineament of India traced into Madagascar. J. Geol. Soc. India 19, 144–153. Frederiksen, A.W., Bostock, M.G., 2000. Modelling teleseismic waves in dipping anisotropic structures. Geophys. J. Int. 141, 401–412. Frederiksen, A.W., Folsom, H., Zandt, G., 2003. Neighborhood inversion of teleseismic Ps conversions for anisotropy and layer dip. Geophys. J. Int. 155, 200–212. Gahalaut, V.K., Rao, V.K., Tewari, H.C., 2004. On the mechanism and source parameters of the deep crustal Jabalpur earthquake, India, of 1997 May 21: constraints from aftershocks and changes in static stress. Geophys. J. Int. 156, 345–351. Ghosh, D.B., 1976. The nature of Narmada-Son lineament. In: Seminar Volume on Tectonics and Metallurgy of South and East Asia, vol. 34. Geol, Surv. India Misc. Publ., pp. 119–132. Grevemeyer, I., Flueh, E.R., Reichert, C., Bialas, J., Klschen, D., Kopp, C., 2001. Crustal architecture and deep structure of the Ninetyeast Ridge hotspot trail from activesource ocean bottom seismology. Geophys. J. Int. 144 (2), 414–431, http://dx.doi. org/10.1046/j.0956-540X.2000.0133. Holbrook, W.S., 1995. Underplating over hotspots. Nature 373, 559, http://dx.doi. org/10.1038/373559a0. Jagadeesh, S., Rai, S., 2008. Thickness, composition, and evolution of the Indian Precambrian crust inferred from broadband seismological measurements. Precambarian Res. 162, 4–15, http://dx.doi.org/10.1016/j.precamres.2007.07.009.

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