Seismic imaging of the Proterozoic Cuddapah basin, south India and regional geodynamics

Precambrian Research 231 (2013) 277–289

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Seismic imaging of the Proterozoic Cuddapah basin, south India and regional geodynamics K. Chandrakala, D.M. Mall ∗ , Dipankar Sarkar, O.P. Pandey CSIR-National Geophysical Research Institute, Uppal Road, Hyderabad 500 007, India

a r t i c l e

i n f o

Article history: Received 12 December 2012 Received in revised form 8 February 2013 Accepted 18 March 2013 Available online 29 March 2013 Keywords: Eastern Dharwar craton Proterozoic Cuddapah basin Seismic imaging Sediment thickness Inter continental collision Foreland basin

a b s t r a c t Cuddapah basin has been believed to be one of the largest intra-cratonic Proterozoic sedimentary basins of India situated in the eastern part of the Dharwar craton of the south Indian shield, which is magmatically infested and contain thick column of sediments. Its overall sedimentary thickness as well as the nature of crustal evolution has remained enigmatic. Based on deep seismic sounding and other geological studies in the past, it was perceived that this basin may contain as much as 10–12 km thick sediments. The results of our present analysis derived up to a depth of ∼12 km reveals five layered upper crust associated with velocities (i) 4.50 km/s, (ii) 5.20–5.30 km/s, (iii) 5.50–5.80 km/s, (iv) 5.85–6.00 km/s, and (v) 6.40 km/s, out of which second and third layers correspond to upper and lower Cuddapah sediments. The results suggest the presence of only 4.0 km thick sediments in the deepest part of the basin below the Nallamalai fold belt, which has its implications in the developmental history of the basin. A thermal driving force was invoked by earlier workers to account for estimated 10–12 km thick sediments. However, the present estimate of only 4 km basement depth in the Cuddapah basin shows that the role of the thermal driving force may be marginal, particularly in the deeper eastern Cuddapah, as isostatic subsidence due to sedimentary accumulation alone is enough to explain the basin depth. Further, a basement sag of about 10 km would have logically needed lateral extension of the order of several hundreds of kilometers. However, as our present estimate of the sediment thickness gets reduced from earlier 10 km to only 4 km, the size of the basin (44,500 km2 ) would be in conformity with isostatic subsidence due to sedimentary accumulation hypothesis. The structural features derived from present analysis like maximum depth observed near the thrust/suture on the basin margin from where it decreases away from it, its association of shallow marine sediments, the arcuate shape of the basin along with its areal dimension resembles foreland basin between continent–continent collisions. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Indian peninsula hosts a number of Proterozoic basins such as Vindhyan, Cuddapah, Chattisgarh, Bastar, Bhima and Kaladgi basin, which are traditionally referred as the Purana basins (Holland, 1906; Radhakrishnan, 1987). Among these basins, the intra-cratonic Cuddapah Basin has generated considerable interest as it forms one of the largest Purana basins of the Peninsular India hosting wealth of important economic minerals. This basin has been studied extensively keeping in view that many of the world’s intra-cratonic basins are important repositories of mineral and hydrocarbon source rocks. The crescent shaped, easterly concave and N–S trending Cuddapah basin, located between latitudes 13◦ 30 N and 17◦ N and longitudes 78◦ E and 80◦ E, covers an area of almost 44,500 km2 . It extends for a length of about 450 km along the

∗ Corresponding author. Fax: +91 4023434651. E-mail addresses: [email protected], [email protected] (D.M. Mall). 0301-9268/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.precamres.2013.03.017

arcute eastern margin, with a mean width of about 150 km (Fig. 1). The Proterozoic Cuddapah basin is considered as an important constituent of eastern Dharwar craton (southern Indian shield), which is known for episodic magmatism and remobilization of crustal blocks during entire Proterozoic period (Harish Kumar et al., 2003; Ramakrishnan, 2009) and is known for history of cyclic sedimentation and interaction with mafic magmatism over a prolonged period of time. To study its subsurface and deeper structural frame work, a large number of geological and geophysical studies were carried out in the past (Nagaraja Rao et al., 1987; Anand et al., 2003 and references there in) besides a detailed deep seismic sounding (DSS) study (Kaila et al., 1979) from Kavali on the east coast to Udipi on the west coast. The profile covered all the important geotectonic units of the southern Indian shield, like Eastern Ghat Mobile Belt (EGMB), Nellore Khammam Schist Belt (NKSB), Cuddapah Basin, Closepet granite and both the eastern and western parts of the Dharwar craton. The observed seismic data along this profile were processed by several workers (Kaila et al., 1979; Reddy et al., 2000, 2004;

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Fig. 1. Geological map of the Cuddapah basin and adjoining regions (after GSI, 1998), showing the DSS profiles. Profile I is the eastern part of the Kavali-Udipi profile (Kaila et al., 1979) and profile II is the Alampur-Koniki profile (Kaila and Tewari, 1985). Solid circles represent shot point locations with corresponding numbers. NKSB: Nellore Khammam Schist Belt; EGMB: Eastern Ghat Mobile Belt.

Chandrakala, 2007; Mall et al., 2008; Chandrakala et al., 2010). Seismic studies reported huge thickness of sediments (∼10 km) in the Cuddapah basin (Kaila et al., 1979; Kaila and Tewari, 1985). Kaila et al. (1979) did not decipher sedimentary layers, but identified the underlying Archean basement characterized by velocity of 6.40 km/s. According to Kaila et al. (1979), around ∼10–12 km thick sediments overlie the presumed extremely high velocity basement. Such velocities are unusual for any granitic-gneissic basement, and in all likelihood; they characterize mid-crustal rocks (Rudnick and Fountain, 1995; Christensen and Mooney, 1995). In fact, the normal range for the uppermost crustal velocity is 5.80–6.20 km/s (Reddy et al., 1999; Rudnick and Fountain, 1995). Above all, recent studies (Chandrakala, 2007; Mall et al., 2008; Mall and Rao, 2006) do indicate presence of a 6.00 km/s basement velocity layer on either side of the Cuddapah basin. Whether the velocities around 5.80 km/s, as observed by earlier workers (say, Kaila and Tewari, 1985) in the same basin correspond to granitic-gneissic basement or to the deeper metamorphosed Cuddapah sediments, need to be investigated. If 5.80 km/s is considered as basement then the estimated thickness of the sediments drastically reduce to 4 km, and it should have ramification on the models of tectonic evolution. This encouraged us to re-look into the existing seismic data, which provided new insights into the shallow seismic structure of this basin. 2. Geology and tectonics Cuddapah basin constitutes a prominent geotectonic feature in the eastern part of the Dharwar craton. It is bounded on its east by two major tectonic features; NKSB and EGMB, while the western front borders the peninsular granites and gneisses (Fig. 1). The evolutionary history of the basin have been a subject of debate; whether it evolved through (i) vertical tectonic movement (Drury and Holt, 1980), (ii) extensional stretching (Anand et al., 2003), (iii) a large meteoritic impact (Krishna Brahmam and Dutt, 1985), (iv) rift setting (Chaudhuri et al., 2002) or (iv) even by a deep seated mantle plume (Mall et al., 2008; Chandrakala et al., 2010). This

region was associated with sustained magmatic activity since at least mid Proterozoic period. As many as six phases of igneous activities are recorded here (Nagaraja Rao et al., 1987), which include lava flows, sills, dykes and 1.1 Ga old kimberlitic magmatism (Hou et al., 2008; Kumar et al., 1993). The basin is also pierced by numerous linear schist belts running roughly north to south (Naqvi and Rogers, 1987). The sedimentological features of the Cuddapah basin comprising ripple-marks, mud-cracks, stromatolites algal mats and phosphorite in carbonates, cross-bedding, etc. show an overall shallow marine shelf environment (Ramam and Murty, 1997). Detailed stratigraphy of the Cuddapah Basin (Table 1), shows the Cuddapah supergroup can be divided into sub groups like Papaghni, Chitravati and Nallamalai. Each of the subgroups starts with quartzite and ends with a shale unit, representing cyclic behavior in the deposition of quartzite and shale sequences during transgression and regression in a sinking basin (Ramam and Murty, 1997). The Papaghni, Chitravati and Nallamalai groups are together further divided into Gulcheru quartzite, Vempalle formation, Pulivendla quartzite; Tadpatri formation and Gandikota quartzite; Bairenkonda quartzite, Cumbum (Pullampet) formation from bottom to top. Similarly, the Kurnool group, which overlies the Cuddapah super group, is divided into formations, like Banganapalli quartzite, Narji limestone, Owk shale, Paniam quartzite, Koilkuntla limestone and Nandyal shale (Ramam and Murty, 1997). 3. Seismic data During 1972–1976, a deep seismic sounding profile was shot in southern India from Kavali on the east coast to Udipi on the west coast which passed through the southern part of the Cuddapah Basin (Fig. 1). The shot points were kept 20 km apart with geophone spacing at 100 m in the basinal part of the profile. The detailed information about data acquisition, field procedure and recording techniques has been reported by Kaila et al. (1979). Initially, the refractors were determined by them from shallow data

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Table 1 Stratigraphic section of the Cuddapah supergroup and Kurnool group (after Murthy, 1981).

Stratigraphic Units

Kurnool group

Cuddapah Super group

Thickness(meters) (order of mangnitude)

Nandyal Shale 50–100 Koilkuntla Limestone 15–50 10–35 Paniam Quartzite ………Regression or local disconfirmity ……………………. Auk Sha le 10–15 Narjii Limestone 100–200 Banaganapalli Quartzite 10–57 ( and conglomerate ) …… …… . Unconformity…………………… Srisailam Quartzite 620(+) At srisailam ……………Unconformity…………………… Cumbum Formation 2000 (+) (includes Pullampet Sh, Irlakonda Qz, Kolamnala Sh) Shale, Phyllite, quartzite, dolomite, Nallamalai Bairenkonda Quartzite, 1200–1500 group (includes Nagari Quartzite (At Tirumala) Quartzite, shale 4000 (+) in Iswarakuppam area ………………………. …… Unconfirmity………………… Gandikota Quartzite 1200 Tadipa tri Formation 4600 Chitravati (Shale, tuffs, lava flows sills) group Pulivendla quartzite 1–75 6700(+) ………….……………………Disconfirmity………………… depth to Vempalle Formation 1500 basement (Dolomite, chert, shale Muddanuru Papaghni Lava flows,sills) group Gulcheru Quartzite 20–250 ………………………………………….Nonconfirmity………………… Granites, gneisses, schists, metavolcanics, dolerite dykes, etc.

by interpolating the depths obtained from intercept time method between the shot points. In the present study, using the delay time method (Barry, 1967), we have revisited the eastern segment of the profile covering Cuddapah basin and EGMB, utilizing the data from shot points SP 240, 180, 165, 140, 120, 100, 80, 40 and 0 (Fig. 1, Profile 1). This approach facilitates mapping of refractors at every geophone location. In order to validate the velocity model further, a ray tracing forward modeling (Zelt and Smith, 1992) has been performed. First arrivals with identifiable amplitudes have been observed on field seismograms. Some of the specimen seismograms of shot point SP 120 are shown in Figs. 2 and 3. The high velocity phase of ∼6.40 km/s is clearly recorded at ∼30–40 km from the shot location, which is preceded by refraction phases of lower apparent velocities of 4.5, 5.2, 5.6 and 6.0 km/s. These velocities are only indicative velocities as the seismograms are arranged according to the distance along the profile (picket numbers) and not with true (straight) distances. The refracted first arrivals were picked from these seismograms (monitor records), and travel time plots were generated with respect to the straight distances, which eliminates the effects of road bends that are observed on the field seismograms. Travel time picks from these seismograms were plotted in Figs. 4–6. All the corresponding play back seismograms of shot point 120 (Figs. 2 and 3) were also digitized, and re-plotted with reduction velocity of 6.0 km/s (Fig. 7). 3.1. Phase identification and methodology Several established techniques are available for seismic refraction data interpretation. The depth of seismic interfaces can be calculated using classical seismic methods like intercept-time

method, reciprocal or delay-time method (Redpath, 1973) and ray based inversion tools (Zelt and Smith, 1992) and tomography method (Zelt and Barton, 1998). Travel-time diagrams were created by picking the first breaks from field (monitor) seismograms, and have been used to compute the depth and velocity applying intercept time method as well as reciprocal (delay-time) methods. The intercept time is the quickest method, but provides only a basic layered velocity model. The method gives the average velocity and depth of the refractor below the shot points. The depth computed from this method represents the depth of the refracting surface at an offset but projected back to the shot point. In situations where the shots are located at far distances, continuous mapping of the refractors by this method has severe limitations over undulating surfaces and the depths of the refractor between shot points are grossly averaged. Due to such limitations of intercept time method, we have utilized delay-time method to map refractor between the shot points, and mapped the continuous refractor at each geophone position. This method is based on reciprocal time, and as such is best suitable for irregular and undulating boundaries. In order to effectively utilize the reciprocal method, significant overlap of refracted arrivals is needed in the travel-time data. The spread in this survey did contain significant overlap (Figs. 4–6), therefore, we considered this method useful to delineate velocity structure, using the delay times at the geophone locations and mapping the refractors in between the shot points. We have used program “RAYINVR” (Zelt and Smith, 1992) to model the travel time data for checking the validity of the velocity structure and further refining the model. This inversion method is based on model parameterization and method of ray-tracing suited to the forward step of the inverse approach. It was parameterized

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Fig. 2. Example of field seismograms of shot point SP 120 at picket position 118.4, which is recorded to west of the shot location, showing the refraction phases from different layers (sedimentary layer, basement) up to a distance of ∼35 km.

by linear interpolation between an irregular grid of boundary nodes and upper and lower layer velocity nodes. A smooth layer boundary simulation is used to avoid scattering and focusing of ray paths and to stabilize the inversion. The travel times and their partial derivatives with respect to velocity and boundary nodes are calculated during the ray tracing that uses an efficient numerical solution (Zelt and Ellis, 1988) of the ray tracing equations (Cerveny et al., 1977). The calculated model response is compared with the observed data, and then subsequently, the model parameters are updated (Zelt and Smith, 1992; Zelt, 1999). The process is repeated until we achieve a satisfactory fit between observed and computed travel times, as shown in figures (Figs. 4–6). These figures also illustrate the degree of confidence in our best estimated model of the velocity from refraction data as they also have reverse coverage from different boundaries. The reflection travel times were generated from the resulting velocity model and superimposed on the digitized seismogram (Fig. 7).

(iii) 5.50–5.80 km/s, (iv) 5.85–6.00 km/s, and (v) 6.40 km/s, down to a depth of 12 km. A layer having a velocity of 4.50 km/s, localized between SP 40 and SP 80, is well resolved which pinches out near SP 80, as shown in ray diagram (Fig. 4). This layer is only about 300 m thick and possibly represents Gondwana equivalents, which are found in small patches in the east of Cuddapah basin (GSI, 1979). Similarly, the second layer with a velocity variations from 5.20 km/s to 5.40 km/s, represents the exposed upper Cuddapah sediments (Fig. 10a). Its depth varies from about ∼200 m on western part of the basin to a maximum of about ∼800 m at SP 120. Near the shot

Table 2 Direct wave velocities on different rock exposures in the basin. DSS profile

Location

Rock exposures

Velocity (km/s)

Parnapalle-Kavali

SP0 SP 40 SP 80 SP 100 SP 120 SP 165 SP 240

Weathered gneissic complex Mica Schist (NKSB) Phyllite &Chlorite Schist (NKSB) Cumbum shale (upper Cuddapah) Cumbum shale (upper Cuddapah) Nandyal Shale (Kurnool group) Archean unclassified

5.30 4.50 5.80 5.30 5.20 5.20 5.90

Alampur-Koniki

SP 0 SP 60 SP 100 SP 140 SP 180 SP 220

Granite, Gneiss Nandyal Shale (Kurnool group) Cumbum Shale (upper Cuddapah) Cumbum Shale Granite, Gneiss Alluvium

6.00 5.10 5.35 5.35 6.00 2.70

3.2. Final velocity model The final velocity model obtained using the approach discussed above is shown in Figs. 4–6. Table 2 summarizes the velocities of various formations that are exposed at various shot point locations. These velocities, being measured from the direct wave arrival times, do unambiguously represent the corresponding formation velocities, thus helping us identifying the layers at depth with similar range of velocities. The derived model reveals a five layered upper crust associated with velocities (i) 4.50 km/s, (ii) 5.20–5.30 km/s,

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Fig. 3. Example of field seismograms of shot point SP 120, which is recorded to east of the shot location, showing the refraction phases from different layers (sedimentary layer, basement and mid crustal layer) up to a distance of ∼40 km.

Fig. 4. Upper panel: Ray diagram showing the subsurface coverage by direct and refracted rays through the final velocity model for the sedimentary layers (velocities 4.50, 5.20–5.30 and 5.60 km/s). Lower panel: corresponding travel time match between the observed (vertical bars) and computed (continuous lines) ones. D stands for off-set distance (km). All the distances marked on x-axis are with reference to the ‘zero’ position of the model.

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Fig. 5. Upper panel: Ray diagram showing the subsurface coverage by refracted rays through the final velocity model for the basement layer (velocity 5.9 km/s). Lower panel: corresponding travel time match between the observed (vertical bars) and computed (continuous lines) ones. D stands for off-set distance (km). All the distances marked on x-axis are with reference to the ‘zero’ position of the model.

Fig. 6. Upper panel: Ray diagram showing the subsurface coverage by refracted rays through the final velocity model for the mid crustal layer. Lower panel: corresponding travel time match between the observed (vertical bars) and computed (continuous lines) ones. D stands for off-set distance. All the distances marked on x-axis are with reference to the ‘zero’ position of the model.

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Fig. 7. Observed seismograms of shot point SP120 (Figs. 2 and 3) digitized and plotted in reduced travel time, showing refracted phases (P5.3 , P5.6 , P6.0 ) and reflected phases from the top of the basement (P6.0 ) and mid crustal layer (P6.4 ). Subscripts and super scripts represent the layer velocity. All the distances marked on x-axis are from ‘zero’ position of the model.

point SP 0, about 1 km thick sediments of velocity 5.30 km/s have also been delineated. Interestingly, sediments with similar velocities are exposed east of NKSB, however, as expected, it is absent below NKSB. The third layer has a lateral variation in velocity from 5.50 km/s to 5.60 km/s with an exception of 5.80 km/s on the east of the basin margin near SP 80. This layer with a velocity of 5.60 km/s in the basinal part possibly represents lower Cuddapah sediments between SP 100 and SP 180 and a higher velocity of 5.80 km/s, observed between SP 40 and SP 80 may correspond to NKSB. This layer is underlain by another layer (fourth layer) with velocities 5.85–6.00 km/s (equivalent to typical granitic-gneissic basement velocity), which is delineated all along the basin. It is only at a depth of few hundred meters on the western margin below Parnapalle (also found by Mall et al., 2008), to a maximum of ∼4.0 km near SP 120 (Fig. 5). This layer lies at a depth of only ∼1 km below NKSB (Fig. 5). Such layer is in fact exposed near shot point SP 240, which is situated outside the basin on its west. If we consider this layer as granitic basement, then we estimate only 4.0 km thick sediments in the Cuddapah basin, in contrast to earlier geological and geophysical studies where more than 10 km thick sediments have been reported. Our estimates are significantly different, thus there is a need to relook into earlier estimates. Tau-P analysis of refraction data along Kavali-Parnapalle in an earlier study has yielded a steep gradient in the velocity in top 3–4 km depth due to change in exposed sediment velocity of 5.30–5.40 km/s to the granitic velocity of about 6.00 km/s (Tewari and Rao, 1990). Probably due to this reason, this layer has been considered as a single layer with an averaged velocity 5.85 km/s by Kaila and Tewari (1985), who referred this layer as lower Cuddpah sediments, overlying a high velocity (6.20 km/s) basement. In our current analysis, this high velocity layer is the fifth layer within which velocity ranges from 6.30 to 6.50 km/s along the stretch of the profile-I. It is seen almost at the surface near Parnapalle (Fig. 6), situated on the western fringe of Cuddapah Basin (Mall et al., 2008). However, in the central part of the basin, it is present at the depth of almost 9 km around SP 120. This particular layer has been considered as a basement by earlier workers (Kaila et al., 1979; Kaila and Tewari, 1985). 4. Discussions The present estimate of the maximum thickness of sediments is only 4 km in the Cuddapah basin between SP 100 and

SP 120. Our present analysis indicated five layers with velocities 4.50 km/s, 5.10–5.40 km/s, 5.60–5.80 km/s, 5.85–6.00 km/s and 6.30–6.50 km/s up to a depth of 12 km. Similarly, when we reanalyzed the refraction data along the profile-II (Alampur-Koniki), situated in the northern part of this basin, where the reported sediment thicknesses (Kaila and Tewari, 1985) varies from 200 m near Atmakur to about 6–7 km below Iswarkuppam dome (Fig. 8b), similar velocities have been observed. Along this profile too, the travel time fits into three layers of velocities 5.30, 5.60 and 6.00 km/s (Fig. 9) as against a single layer with high velocity gradient. These velocities are similar to that observed along Parnapalle-Kavali DSS profile. It was found that the shot point SP 0, which is situated over the Archean gneissic exposures, has a direct wave velocity of ∼6.0 km/s (Fig. 9a). Similarly, another shot point SP140, which lies on the eastern end of Cuddapah basin, exhibits a characteristic sediment velocity of 5.35 km/s, followed by the basement velocity of ∼6.0 km/s (Fig. 9b). Apart from these, the other shot points too, like SP 60 and 100, which are situated in the middle part of the Cuddapah basin, exhibit similar order of velocities (5.35 km/s and 5.60 km/s) as observed along profile-I, followed by a velocity of ∼6.0 km/s underlain by a layer with velocity 6.30 km/s (Fig. 9d and e). The key issues of identification of the correct basement velocity and thereby re-estimation of the sedimentary thickness is being discussed below. 4.1. Characteristic basement velocity The derived velocity model (Fig. 10a) shows velocities of 5.80 km/s and 6.30–6.50 km/s apart from the sedimentary layers having velocities of 4.50, 5.30, and 5.60 km/s. Kaila et al., 1979 estimated the basement depth of the Cuddapah basin to be ∼10 km in the deepest part since they considered the apparent velocity of 6.4 km/s as that of the crystalline basement, in the absence of any other prominent refractor at shallower depths (Fig. 8a). However, Krishna Swamy (1981) raised doubt about these reflectors to be truly representing the Archean basement by not including the infolded remnants of Dharwarian metasediments. They opined that difference in geological and geophysical estimates of the sedimentary column, requires a reasonable explanation. While reprocessing the Kavali-Udipi DSS data, Mall et al. (2008) and Chandrakala et al. (2010) observed apparent velocities in the range of 5.8–6.0 km/s for the exposed basement rocks just west of the Cuddapah basin. As per Chekunov et al. (1984), iso-velocity line 6.0 km/s represents the top of the crystalline basement underneath the Cuddapah

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Fig. 8. Crustal depth sections along (a) Kavali-Parnapalle part of Kavali-Udipi DSS profile (after Kaila et al., 1979) and (b) Alampur-Koniki across the northern part of the Cuddapah Basin (after Kaila and Tewari, 1985). The basements inferred by them are indicated here by arrows.

basin. Also, Kaila et al. (1987), based on 2-D travel-time modeling, reported a velocity of 6.10 km/s for the Cuddapah basement along the Alampur-Koniki profile, located just about 100 km north of the Kavali-Udipi profile. Global compilations of seismic velocities indicate that in the uppermost continental crust (granitic-gneissic), velocities usually vary between 5.80 and 6.30 km/s (Christensen and Mooney, 1995; Rudnick and Fountain, 1995). Velocities in excess of this, represent intermediate to mafic crust as per current understanding of the earth’s crustal composition and its velocity and density structure (Christensen and Mooney, 1995; Rudnick and Fountain, 1995; Gao et al., 1998; Tripathi et al., 2012b). In fact, the normal range for the upper crustal velocity is from 5.80 km/s to 6.20 km/s only (Rudnick and Fountain, 1995). The observed 5.8 km/s velocity in the deeper part of the basin from the present study (Fig. 10a) cannot therefore be considered as the sedimentary velocity, as the velocities in known exposures along the profile do not exceed 5.30 km/s (Table 2), which represent Kurnool and Cuddapah Groups of sediments (Table 1). Even due to increased pressure–temperature conditions at such (∼5 km) depths, the sedimentary velocity cannot exceed 5.6 km/s. In fact, velocities in sedimentary rocks rarely touch 5.85 km/s with possible exceptions of dolomite and limestone. In that sense, the 5.90–6.00 km/s velocity observed by us (Fig. 10a), as well as ∼6.00 km/s velocity observed on either side of the basin (Chandrakala, 2007; Mall and Rao, 2006; Mall et al., 2008), would correspond to the granitic-gneissic basement. In such a case, the maximum thickness of sediments would be only 4.0 km in the eastern part of the basin, between shot point SP 140 and SP 100. 4.2. Mid crustal velocities In Cuddapah Basin, Kaila et al. (1979) identified a strong shallow reflector at 10 km depth near SP 100 (Fig. 8a) which was marked as basement by them. This boundary is characterized by a velocity of 6.40 km/s in Block III (Fig. 8a). In our opinion, this should be mid crustal velocity, which ranges between 6.30 and 6.60 km/s globally, conforming to amphibolite and granulite facies intermediate

to mafic crustal rocks (Christensen and Mooney, 1995; Rudnick and Fountain, 1995). Velocity of 6.40 km/s, considered as the basement velocity by Kaila et al. (1979), fall in this range. Experimental studies also do not support this conjecture as a detailed study (laboratory measurements) of the elastic properties on fresh cored samples of amphibolite and granulite facies mid crustal rocks, encountered in the 617 m deep KLR-1 borehole in the eastern part of the Dharwar craton, India (Tripathi et al., 2012a,b), indicated an average P-wave velocity of 6.20 km/s. In view of this, a much higher velocity of 6.40 km/s (as obtained by Kaila et al., 1979) is not likely to represent granitic-gneissic basement and hence, the basement depth has to be shallower than the ∼10 km as against inferred by Kaila et al. (1979). Therefore, the velocity of 6.4 km/s observed at a shallow depth (Figs. 4 and 5) would characterize the middle crust. 4.3. Sediment thickness Since King (1872) and Ball (1877), a number of investigations have been made on the classification of Cuddapah succession and its stratigraphy (Sen and Narasimha Rao, 1968; Meijerink et al., 1984; Nagaraja Rao et al., 1987; Ramam and Murty, 1997; Chaudhuri et al., 2002; Patranabis et al., 2012 and references there in). In earlier studies (King, 1872; Glennie, 1951; Murthy et al., 1978), it was reported that the sediment thickness in Cuddapah basin is only around 4.8–5.9 km. However, once the deep seismic sounding results were published by Kaila et al. (1979), estimates were drastically revised upwards. For example, the thickness of the Tadpatri formation, which was quoted to be 300–800 m thick (Murthy et al., 1978), was revised later to 4600 m (Murthy, 1981). Apparently, seismically derived thickness was taken as benchmark for subsequent studies, in which 6.4 km/s velocity was taken as basement. Also, Nagaraja Rao et al. (1987) based on geological studies reported a maximum cumulative thickness of Cuddapah sediments to be around 12 km. Deep seismic studies carried out along two profiles, (i) KavaliUdipi (Kaila et al., 1979) and (ii) Alampur-Koniki (Kaila and Tewari, 1985) have reported thicknesses from ∼7 to 10 km. The former

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Fig. 9. Reduced travel-time (reduction velocity 6 km/s) versus distance curves for the first arrival data along the Alampur-Koniki profile and observed travel times for (a) SP 0; (b) SP 140, (d) SP 60 and (e) SP 100 along with the geological map strip. The direct refracted wave from shot point, located on different geological exposures.

study reported presence of ∼10–12 km thick Cuddapah sediments between faults 7 and 4 (Fig. 8a) along the Kavali-Udipi profile. Based on surface mapping and air photo interpretation Meijerink et al. (1984) favored similar sedimentary thickness as reported by Kaila et al. (1979). Earlier Glennie (1951) used geological data to contour the bottom of the Cuddapah Basin, which also show eastward dip and attains a maximum depth of 18,000 ft (5.5 km) in eastern margin of the basin, a feature consistent with the gravity modeling result computed by them. Based on magnetotelluric studies along the DSS profile between Kavali and Ananatapur, Naganjaneyulu and Harinarayana (2004) inferred that sediment thicknesses increase from west to east, but they did not specify either the thicknesses or the resistivity of different sedimentary layers within the basin. It is reiterated here that the pioneering results of Kaila et al. (1979) has provided a deeper crustal structure rather than sedimentary configuration of the Cuddapah basin. The ‘basement refractor’ at a depth of ∼10 km (Kaila et al., 1979, Fig. 8a) does not continue in the western part of the basin and the refractor has been extrapolated toward western part of the basin by subsequent workers (Kaila and Bhatia, 1981; Singh and Mishra, 2002; Singh et al., 2004) in their studies.

In contrast to earlier studies, present study shows maximum of 4 km thick sediments. It was, however, not possible to delineate the velocities of each of the Cuddapah sedimentary sequences, but there are at least two groups of velocities, 5.20–5.40 km/s and 5.50–5.60 km/s, identified along the profile. Upper Cuddapah sediment sequences can be grouped into 5.20–5.40 km/s and the lower Cuddapah sediments into 5.50–5.60 km/s. The thicknesses of these two sequences vary widely within the basin. In the present study it is delineated only around one kilometer thick in the western part between SP 180 and SP 140 and then jumps to almost 4.0 km at SP 140, which can be interpreted as a major fault and coincides with the basement fault 7 of Kaila et al. (1979) and seems to have played a major role in tectonic evolution of this basin (Figs. 8a and 10a). Further, the depth of sediments decreases drastically again at SP 100, this location too is interpreted as fault F4 (Figs. 8a and 10a), where eastern block has gone up with respect to adjacent western block. It becomes apparent that the subsidence took place along the major faults F4 (Velikonda) and F7 (Maidakuru) (Figs. 8 and 10a), which runs in NS direction as major lineaments. Movement along these faults has produced maximum subsidence where thickest sediments have been delineated. However, Kaila et al. (1979) have shown the eastern block along fault 7

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Fig. 10. (a) Final velocity model derived from the present study. The vertical scale is exaggerated 7 times for clarity. Numbers in the model are average P-wave velocities (km/s) at various depths. Inverted triangles at top of the model show the location of shot points. F1, F2, F4 and F7 correspond to the faults 1, 2, 4 and 7 respectively (Fig. 8a). UC: Upper Cuddapah; LC: Lower Cuddapah: NKSB: Nellore Khamam schist belt; EGMB: Eastern ghat mobile belt. (b) Schematic cartoon (not to scale) illustrating the key elements of the tectonic geometry produced by continent–continent collision process in the formation of peripheral foreland basin (modified from Southard, 2008).

is thrusted upwards, where as in the present study we find thicker sediment between F7 and F4 in the form of graben like structure. Since there is abrupt difference in sedimentary thickness on eastern side of this fault (F7), it has been interpreted that the eastern block has gone down, opposite to the sense of movement suggested by Kaila et al. (1979). Since this fault is also a contact place between Kurnool and Nallamalai formations, where Nallamalai formations seem thrusted upwards, and the sense of such movement would suggest an upward movement of eastern block is a post depositional phenomenon. Thus the movement of crustal blocks along this fault has signature of both the sense of movements at different geological times, as also suggested by Meijerink et al. (1984). As per geological studies (Murthy, 1981), the basin has many depocenters for different group of rocks like Papaghni, Chitravati, Nallamalai and Kurnool. The maximum thickness reported by geological studies for different groups are at different places (Table 1), whereas our profile does not pass through all those locations. Therefore, the maximum thickness of various groups of Cuddapah sedimentary sequences as given in stratigraphic section by earlier workers cannot be added to arrive at a total thickness of sedimentary column at a particular place along the DSS profile as seems to have been done (Table 1). Moreover the basin is not of uniform depth and dips from west to east and hence adding the thickness of various sedimentary groups reported at various locations in the basin would not be justified. Further, the affect of erosion of sediments cannot be ruled out as it is integral part of basin development and there was hiatus in the deposition as evidenced by unconformities (Table 1). The Indian subcontinent is known to have passed through an extremely dynamic phase (Rogers and Callahan, 1987; Pandey

and Agrawal, 1999). Available 40 Ar–39 Ar ages would confirm periodic crustal dilation and thermal resetting at different periods, characterize by wide spread magmatic activity which begun as early as 1.9–1.8 Ga and continued episodically till about 1.0 Ga (Mallikarjuna Rao et al., 1995). The prominent among them was 1.1 Ga kimberlitic plume activity (Kumar et al., 1993; Mall et al., 2008; Chandrakala et al., 2010). These activities caused large crustal blocks to move upwards relative to each other and subsequent erosion (Fig. 10a). Consequent to sustained and episodic thermal perturbations and magmatism around the Cuddapah basin throughout the Proterozoics, may have resulted into massive removal of rocks from this region. The current thickness of the Cuddapah sediments could be the resultant of these tectonic activities. 4.4. Tectonic implications The results of the present study have major tectonic implications in the basin development. Sedimentary basins generally result from one or combination of following processes: marginal continental shelf type basin, intracratonic platform or interior basin and fault controlled basin (graben) (Bhattacharji, 1981). The evolution of the Cuddapah basin based on the geological and geophysical studies have remained inconclusive. A variety of structural settings and tectonomagmatic events associated with Cuddapah basin makes it difficult to arrive at a single satisfactory mechanism for the evolution of the basin. As discussed in earlier sections, there have been different schools of thoughts about the basin formation and subsequent modifications. Development of Cuddapah basin was invoked as a broad trough on the Archean basement created in an extensional regime caused

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by early Proterozoic thermal activities, as evidenced by mafic sills and flows (Anand et al., 2003). Loading of sediments causes Airy-type subsidence. As a result, depth of the sediments becomes greater than initial depth of topography low after the isostatic equilibrium is attained. A thermo-mechanical model was presented by Bhattacharji and Singh (1984) incorporating crustal doming, erosion and subsidence. They inferred that the total accumulated sediments and emplaced igneous material load cannot produce more than 7.2 km subsidence in the Cuddapah basin. Thereby they had to invoke a thermal driving force to account for additional ∼2 km of depression in order to explain ∼10 km thick basin depth as estimated by Kaila et al. (1979). They commented that the driving force for the initial subsidence of the Cuddapah Basin was possibly underlying the crust near the southwest margin of the basin, where a positive gravity anomaly attributed to asthenosphere upwelling, was observed. However, the present estimate of only 4 km basement depth in the eastern Cuddapah basin indicates that the role of the thermal driving force may be marginal in the deeper eastern Cuddapah, as isostatic subsidence due to sedimentary accumulation alone is enough to explain the present depth of the basin. A polycyclic evolution of the Cuddapah Basin as indicated by multiple transgression and regression cycles was explained by thermally driven crustal sagging alternating with extensional stretching (Ramam and Murty, 1997). However, basement sag of about 10 km would have needed lateral extension of the order of several hundreds of kilometers. Therefore, they proposed retro arc setting behind collision belt for the basin evolution. However, as our present estimate of the sediment thickness is only 4 km, the size of the basin (44,500 km2 ) does not necessarily contradict their thermally driven crustal sagging hypothesis. The crustal sagging in the relatively deeper eastern part of the basin (between fault F4 and F7; Fig. 10), possibly provided suitable conditions for not-so-thick (4 km, instead of 10 km as has been considered earlier) sedimentary deposition. The circular gravity high closures in the southwestern part of the basin were considered due to presence of an igneous body emplaced through meteorite impact (Krishnabrahmam, 1992). The impact structures are usually associated with circular gravity lows and characterized by reduced seismic velocities (Pilkington and Grieve, 1992). Our present study has delineated the thickest sediments in the eastern part of the basin (∼4 km) between SP 140 and SP 100 (between faults F4 and F7) and is associated with the N–S trending gravity low that is far east from their proposed meteoric impact site. Moreover, Bhattacharji (1987) interpreted the gravity high closures in southwestern part of Cuddpah basin as manifestation of intrusion in the upper crust. Peripheral foreland basins are formed after continent–continent collisions by loading over the continental crust of the subducted plate by development of thrust sheets in the subducted plate directed back away from the subduction zone. They tend to migrate away from the arc or suture zone with time. They are filled by sediments derived from the mountainous terrain associated with the compression and thrusting (Southard, 2008). Eastern margin of the Cuddapah basin is characterized by east ward dipping thrust faults where the EGMB thrusted upward against Cuddapah basin (Fault 2, Fig. 8a). The derived basement geometry of the Cuddapah basin shows gradual increase of depth toward east and reaches maximum near the thrust zone between Cuddapah basin and EGMB akin to peripheral foreland basin (Fig. 10). The shape and dimension of the Cuddapah basin too resembles with the foreland basin like tens of kilometer wide and hundreds of kilometer long commonly arcuate, reflecting the geometry of the subduction (Mitchell and Reading, 1986; Leggett, 1982). The sedimentary succession in Cuddapah Basin shows an overall shallow marine shelf-environment with cyclic transgression and regression. These features (shallow marine sedimentary succession) are

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generally observed in foreland basins (Mitchell and Reading, 1986; Leggett, 1982). Even the sedimentary fill of the basin like thinning away from the arc/suture passing into shallow marine sedimentary succession and the order of thickness also resembles the foreland basins. Fig. 10b illustrates schematically the tectonic features produced by the collision and suturing of the EDC and the EGMB. The obducted block EGMB is characterized by sills and dykes, with minor proportion of volcanic rocks of the middle to late Proterozoic period known as Kandra volcanic (Leelanandam, 1990). This along with association of Nellore greenstone belt, the Archean metamorphosed anorthositic complex near Kanigiri and exposures of high grade charnokite and khondalites along the EGMB indicates a tectonic setting of Proterozoic continent collision, where passive margin of eastern part of EDC would be partly subducted and the lower crustal rocks of obducted block (EGMB) would uplift and subsequent erosion would then expose the lower crustal rocks Khondalite, Charnokites (Granulites) along the thrust. According to Fountain and Salisbury (1981), terranes with charnokites and khondalites along ancient sutures present exposures of lower continental crust on obducted block due to erosion after continental collision. The curvilinear contact zone together with the cresent shape of the Cuddapah basin gets further support to continental collision by palaeomagnetic investigation (Prasad et al., 1987). With signatures of obducted EGMB and foreland Cuddapah basin, the juxtaposition of these two geologically distinct terrains favors a Proterozoic collision as suggested for the Proterozoic continental growth (Kroner, 1991). The Cuddapah basin being on the passive margin block could therefore be a peripheral foreland basin evolved through Proterozoic continent–continental collision. Our results favor foreland basin development of the Cuddapah basin due to collision of cratons of India and Antarctica during middle to late Proterozoic (Chetty and Murthy, 1994). A peripheral foreland origin for the Cuddapah basin development get support from Singh and Mishra (2002) whereby eastward subduction of the Dharwar craton was invoked for the collision and resultant deformation within the Nallamalai fold belt.

5. Conclusions Based on revisit of the seismic refraction data over the Proterozoic Cuddapah basin, following conclusions are made.

(1) Only 4.0 km thick sediments are present in the deepest part of the basin below Nallamalai fold belt, characterized by two distinct layers with velocities 5.20–5.30 km/s (upper Cuddapahs) and 5.50–5.60 km/s (lower Cuddapahs), overlaying the granitic gneissic basement characterized by velocity of 5.85–6.00 km/s. (2) It appears that subsidence took place along the major gravity faults F4 and F7, which runs in NS direction as major lineaments. Movement along these faults apparently produced maximum subsidence, where thickest sediments have been delineated. (3) The mid crustal layer with velocity 6.40 km/s found at a depth of ∼10 km in the deepest (eastern) part of the basin is found almost exposed in the southwestern part of the basin, possibly indicating exhumation of mid crustal layer as a result of erosion of the upper crust. (4) Our results delineating the asymmetric basinal structural features like maximum depth observed near the thrust/suture on the basin margin from where it decreases away from it, its association of shallow marine sediments, the arcuate shape of the basin along with its areal dimension resembles foreland basin related to continent–continent collisions.

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Acknowledgements KC thanks DST for Woman Scientist Scheme project(WOS-A). DMM, DS and OPP thank CSIR for their Emeritus Scientist positions. We thank P. Swamy for help in preparing figures. Permission accorded by the Director, CSIR-National Geophysical Research Institute, Hyderabad to publish this work is gratefully acknowledged.

References Anand, M., Gibson, S.A., Subba Rao, K.V., Kelley, S.P., Dickin, A.P., 2003. Early Proterozoic melt generation processes beneath the intra-cratonic Cuddapah basin, Southern India. Journal Petrology 44, 2139–2171. Ball, V., 1877. On the geology of Mahanadi Basin and its vicinity. Geological Survey of India Records 10, 167–186. Barry, K.M., 1967. Delay time and its application to refraction profile interpretation. In: Musgrave, A.W. (Ed.), Seismic Refraction Prospecting: Society Exploration Geophysics. , pp. 348–361. Bhattacharji, S., 1981. Evolution of Intracratonic Proterozoic Basin. Institute of Indian Peninsular Geology (IIPG), Hyderabad, pp. 7–28. Bhattacharji, S., Singh, R.N., 1984. Thermo-mechanical structure of the Southern part of the Indian shield and its relevance to Precambrian Basin. Tectonophysics 105, 103–120. Bhattacharji, S., 1987. Lineaments and igneous episodes in the evolution of intracratonic Proterozoic basins on the Indian Shield. In: Saha, A.K. (Ed.), Geological Evolution of Peninsular India – Petrological and Structural Aspects. Recent Researches in Geology 131, 1–15. Cerveny, V., Molotkov, I.A., Psencik, I., 1977. Ray Method in Seismology. University of Karlova, Prague214. Chandrakala, K., 2007. Lateral and vertical crustal velocity structure of Cuddapah Basin and adjoining eastern and western Dharwar craton and seismicity of segments of peninsular India. Unpublished Thesis. Chandrakala, K., Pandey, O.P., Mall, D.M., Sarkar, D., 2010. Seismic signatures of a Proterozoic thermal plume below southwestern part of the Cuddapah Basin, Dharwar craton, India. Journal Geological Society of India 76, 565–572. Chaudhuri, A.K., Saha, D., Deb, G.K., Patranabis Deb, S.P.D., Mukherjee, M.K., Gosh, G., 2002. The Purana basins of southern cratonic province of India – a case for Mesoproterozoic fossil rifts. Gondwana Research 5, 23–33. Chekunov, A.V., Sollugub, V.B., Starostenko, V.I., Kharechkko, G.E., Rusakov, Y.G., Kostyukovich, A.S., 1984. Structure of the Earth’s crust and upper mantle below Hindustan and the northern part of the Indian Ocean from geophysical data. Tectonophysics 101, 63–73. Chetty, T.R.K., Murthy, D.S.N., 1994. Collision tectonics in the late Precambrian Eastern Ghats mobile belt: mesoscopic to satellite-scale structural observations. Terra Nova 6, 72–81. Christensen, N.I., Mooney, W.D., 1995. Seismic velocity structure and composition of the continental crust: a global view. Journal of Geophysical Research 100, 9761–9788. Drury, S.A., Holt, R.W., 1980. The tectonic framework of the South Indian craton: a reconnaissance involving LANDSAT imagery. Tectonophysics 65, T1–T5. Fountain, D.M., Salisbury, M.H., 1981. Exposed cross-sections through the continental crust: implications for crustal structure, petrology, and evolution. Earth Planetery Science Letters 56, 263–277. Gao, S., Luo, T.C., Zhnag, B.R., Zhang, H.F., Han, Y.W., Zhao, Z.D., Hu, Y.K., 1998. Chemical composition of the continental crust as revealed by studies in East China. Geochimica et Cosmochimica Acta 62, 1959–1975. Glennie, E.A., 1951. The Cuddapah basin in India and crustal warping. Geophysical Journal International 6, 168–176. GSI, 1979. Geological Survey of India, Geological map of Cuddapah Basin (revised), Salient features, 3rd Peninsular Geology, Hyderabad, pp. 1–21. GSI, 1998. Geological Map of India. Geological Society of India, Calcutta, India. Harish Kumar, S.B., Jayananda, M., Kano, T., Shadakshara Swamy, N., Mahabaleswar, B., 2003. Late Archean juvenile magmetic accretion process in the eastern Dharwar craton: Kuppam–Karimangalam area. Memoir of Geological Survey of India 50, 375–408. Holland, T.H., 1906. Classification of the Indian Strata. Trasn. Min. Metam. Soc. India, pp. 1–17. Hou, G., Santosh, M., Qian, X., Lister, G.S., Li, J., 2008. Configuration of the Late Paleoproterozoic supercontinent Columbia: insights from radiating mafic dyke swarms. Gondwana Research 14, 395–409. Kaila, K.L., Roy Chowdhury, K., Reddy, P.R., Krishna, V.G., Hari Narain, Subbotin, S.I., Sollogub, V.B., Chekunov, A.V., Kharetchko, G.E., Lazarenko, M.A., Ilchenko, T.V., 1979. Crustal structure along Kavali-Udipi profile in the Indian peninsular shield from deep seismic sounding. Journal Geological Society of India 20, 307–333. Kaila, K.L., Bhatia, S.C., 1981. Gravity study along the Kavali-Udipi deep seismic sounding profile in the Indian peninsular shield: some inferences about the origin of anorthosite and Eastern Ghat orogeny. Tectonophysics 79, 129–143. Kaila, K.L., Tewari, H.C., 1985. Structural trends in the Cuddapah basin from deep seismic soundings (DSS) and their tectonic implication. Tectonophysics 115, 69–86.

Kaila, K.L., Tewari, H.C., Chowdhury, K.R., Rao, V.K., Sridhar, A.R., Mall, D.M., 1987. Crustal structure of the northern part of the Proterozoic Cuddapah basin of India from deep seismic soundings and gravity data. Tectonophysics 140, 1–12. King, W., 1872. The Kadapah and Karnul formations in Madras presidency. Memoir of Geological Survey of India 8, l–l346. Krishna Brahmam, N., Dutt, N.V.B.S., 1985. Possible relationship between initiation of Papaghni basin and meteoritic impact, in Workshop on Purana Basins (middle to later Proterozoic) of Peninsular India, Hyderabad, December 29–31, pp. 48–49. Krishna Swamy, V.S., 1981. A Bird’s eye view of some recent geological and geophysical work in the Cuddapah basin and related conceptualizations. Fourth work shop on Status, Problems, and Programmes in Cuddapah Basin Institute of Indian Peninsular Geology, pp. 1–33. Krishnabrahmam, N., 1992. A meteoritic impact theory for the initiation of the Cuddapah (Proterozoic) basin of India. Bulletin Indian Geological Association 25, 43–60. Kroner, A., 1991. Tectonic evolution in the Archean and Proterozoic. Tectonophysics 187, 393–410. Kumar, A., Padma Kumari, V.M., Dayal, A.M., Murthy, D.S.N., Gopalan, K., 1993. Rb–Sr ages of Proterozoic kimberlites of India, evidence for Contemporaneous emplacement. Precambrian Research 62, 227–237. Leelanandam, C., 1990. The Kandra volcanics in Andhra Pradesh: possible ophiolite? Current Science 59, 785–788. Leggett, J.K. (Ed.), 1982. Trench–Forearc Geology; Sedimentation and Tectonics on Modern and Ancient Active Plate Margins. Geological Society of London, Special Publication 10, 576. Mall, D.M., Rao, V.K., 2006. Exhumation of lower crust in the Pre Cambrian terrain of Indian shield. Advances in Geosciences, 167–178. Mall, D.M., Pandey, O.P., Chandrakala, K., Reddy, P.R., 2008. Imprints of a Proterozoic tectonothermal anomaly below the 1.1 Ga kimberlitic province of Southwest Cuddapah Basin, Dharwar Craton (Southern India). Geophysical Journal International 172, 422–438. Mallikarjuna Rao, J., Bhattacharji, S., Rao, M.N., Hermes, O.D., 1995. 40 Ar–39 Ar ages and geochemical characteristics of the dolerite dykes around Proterozoic Cuddapah basin, South India. Memoir of Geological Society of India 33, 307–328. Meijerink, A.M.J., Rao, D.P., Rupke, J., 1984. Stratigraphic and structural development of the Precambrian Cuddapah basin, SE India. Precambrian Research 26, 57–104. Mitchell, A.H.G., Reading, H.G., 1986. Sedimentation and tectonics. In: Reading, H.G. (Ed.), Sedimentary Environments and Facies. , second ed. Blackwell Sci. Publ., Oxford, pp. 471–519. Murthy, Y.G.K., Nagaraja Rao, B.K., Ramalinga Swamy, G., 1978. Status of Cudddapah basin geology with special reference to the D.S.S. profile strip. First workshop on Status, Problems and Programmes in Cuddapah basin, Institute of Indian Peninsular Geology, Hyderabad held on 7th and 18th March 1978, p. 41. Murthy, Y.G.K., 1981. The Cuddapah Basin – A Review of Basin Development and Basement Framework Relations. Fourth workshop on Status, Problems and Programmes in Cuddapah basin, Institute of Indian Peninsular Geology, Hyderabad held on 22nd and 23rd, pp. 51–73. Naganjaneyulu, K., Harinarayana, T., 2004. Deep crustal electrical signatures of Eastern Dharwar Craton, India. Gondwana Research 7, 951–960. Nagaraja Rao, B.K., Rajurkar, S.T., Ramalingaswamy, G., Ravindra Babu, B., 1987. Stratigraphy, structure and evolution of the Cuddapah basin. Purana basins of peninsular India. Memoir of Geological Society of India 6, 33–86. Naqvi, S.M., Rogers, J.J.W., 1987. Precambrian Geology of India. Oxford University Press, New York223. Pandey, O.P., Agrawal, P.K., 1999. Lithospheric mantle deformation beneath the Indian cratons. Journal of Geology 107, 683–692. Patranabis, D.S., Saha, D., Tripathy, V., 2012. Basin stratigraphy, sea-level fluctuations and their global tectonic connections—evidence from the Proterozoic Cuddapah Basin. Geological Journal 47, 263–283. Pilkington, M., Grieve, R.A.F., 1992. The geophysical signature of terrestrial impact craters. Reviews of Geophysics 30, 161–181. Prasad, C.V.R.K., Pulla Reddy, V., Subbarao, K.V., Radhakrishna Murthy, C., 1987. Palaeomagnetism and the crescent shape of the Cuddapah basin. Memoir of Geological Society of India 6, 331–348. Radhakrishnan, B.P., 1987. Purana basins of Peninsular India. Memoir of Geological Society of India 6, 518. Ramakrishnan, M., 2009. Precambrian mafic magmatism in the western Dharwar craton, southern India. Journal of Geological Society of India 73, 101–116, http://dx.doi.org/10.1007/s12594-009-0006-z. Ramam, P.K., Murty, V.N., 1997. Geology of Andhra Pradesh. Geological Society of India Publication, Bangalore, India. Reddy, P.R., Venkateswarlu, N., Rao, P.K., Prasad, A.S., 1999. Crustal Structure of Peninsular shield, India from DSS studies. Current Science 77, 1606–1611. Reddy, P.R., Chandrakala, K., Sridhar, A.R., 2000. Crustal velocity structure of the Dharwar Craton, India. Journal of Geological Society of India 55, 381–386. Reddy, P.R., Chandrakala, K., Prasad, A.S.S.S.R.S., Rao, Ch.R., 2004. Lateral and vertical crustal velocity and density variations in the southwestern Cuddapah basin and adjoining eastern Dharwar craton. Current Science 87, 1607–1614. Redpath, B.B., 1973. Seismic refraction exploration for engineering site investigations, U.S. Army Engineer Waterways Experiment Station Explosive Excavation Research Laboratory, Technical Report E-73-4, Livermore, California, p. 51. Rogers, J.J.W., Callahan, E.J., 1987. Radioactivity, heat flow and rifting of the Indian continental crust. Journal Geology 95, 829–836.

K. Chandrakala et al. / Precambrian Research 231 (2013) 277–289 Rudnick, R.L., Fountain, D.M., 1995. Nature and composition of the continental crust: a lower crustal perspective. Reviews of Geophysics 33, 267–309. Sen, S.N., Narasimha Rao, Ch.,1968. Igneous activity in Cuddapah basin and adjacent areas and suggestions on the paleo-geography of the basin. In: Proceeding of Symposium; Upper Mantle Project 8. GRB & NGRI Publication, Hyderabad, pp. 261–285. Singh, A.P., Mishra, D.C., 2002. Tectonosedimentary evolution of Cuddapah Basin and Eastern Ghats Mobile Belt (India) as Proterozoic collision: gravity, seismic and geodynamic constraints. Journal Geodynamics 33, 249–267. Singh, A.P., Mishra, D.C., Gupta, S.B., Prabhakar Rao, M.R.K., 2004. Crustal structure and domain tectonics of the Dharwar craton (India): insight from new gravity data. Journal Asian Earth Science 23, 141–152. Southard, J., 2008. Sedimentary Geology. http://ocw.mit.edu/courses/earthatmospheric-and-planetary-sciences/12-110-sedimentary-geology-spring2007/ Tewari, H.C., Rao, V.K., 1990. Crustal velocity model in the eastern part of the Indian peninsular shield. Journal Geological Society of India 36, 475–483. Tripathi, P., Pandey, O.P., Rao, M.V.M.S., Koti Reddy, G., 2012a. Elastic properties of amphibolite and granulite facies mid-crustal basement rocks of the Deccan

289

volcanic covered 1993 Latur-Killari earthquake region, Maharashtra (India) and mantle metasomatism. Tectonophysics 554–557, 159–168. Tripathi, P., Parthasarathy, G., Masood Ahmed, S.M., Pandey, O.P., 2012b. Mantlederived fluids in the basement of the Deccan trap: evidence from stable carbon and oxygen isotopes of carbonates from the Killari borehole basement, Maharashtra, India. International Journal of Earth Sciences 101, 1385–1395. Zelt, C.A., Ellis, R.M., 1988. Practical and efficient ray tracing in two dimensional media for rapid travel time and amplitude forward modeling. Canadian Journal of Exploration Geophysics 24, 16–31. Zelt, C.A., Smith, R.B., 1992. Seismic travel time inversion for 2-D crustal velocity structure. Geophysical Journal International 108, 16–34, http://dx.doi.org/10.1111/j.1365 246X.1992.tb00836.x. Zelt, C.A., 1999. Modeling strategies and model assessment for wide-angle seismic travel time data. Geophysical Journal International 139, 183–204. Zelt, C.A., Barton, P.J., 1998. 3D seismic refraction tomography: a comparison of two methods applied to data from the Faeroe Basin. Journal of Geophysical Research 103, 7187–7210.