Anomalous crustal and lithospheric mantle structure ...

Journal of Asian Earth Sciences xxx (2013) xxx–xxx

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Anomalous crustal and lithospheric mantle structure of southern part of the Vindhyan Basin and its geodynamic implications O.P. Pandey ⇑, R.P. Srivastava, N. Vedanti, S. Dutta, V.P. Dimri CSIR-National Geophysical Research Institute, Uppal Road, Hyderabad 500007, India

a r t i c l e

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Article history: Available online xxxx Keywords: Vindhyan Basin Crustal evolution Geodynamics Heat flow Lithospheric thickness Gravity data inversion Seismic velocities

a b s t r a c t Tectonically active Vindhyan intracratonic basin situated in central India, forms one of the largest Proterozoic sedimentary basins of the world. Possibility of hydrocarbon occurrences in thick sediments of the southern part of this basin, has led to surge in geological and geophysical investigations by various agencies. An attempt to synthesize such multiparametric data in an integrated manner, has provided a new understanding to the prevailing crustal configuration, thermal regime and nature of its geodynamic evolution. Apparently, this region has been subjected to sustained uplift, erosion and magmatism followed by crustal extension, rifting and subsidence due to episodic thermal interaction of the crust with the hot underlying mantle. Almost 5–6 km thick sedimentation took place in the deep faulted Jabera Basin, either directly over the Bijawar/Mahakoshal group of mafic rocks or high velocity-high density exhumed middle part of the crust. Detailed gravity observations indicate further extension of the basin probably beyond NSL rift in the south. A high heat flow of about 78 mW/m2 has also been estimated for this basin, which is characterized by extremely high Moho temperatures (exceeding 1000 °C) and mantle heat flow (56 mW/m2) besides a very thin lithospheric lid of only about 50 km. Many areas of this terrain are thickly underplated by infused magmas and from some segments, granitic–gneissic upper crust has either been completely eroded or now only a thin veneer of such rocks exists due to sustained exhumation of deep seated rocks. A 5–8 km thick retrogressed metasomatized zone, with significantly reduced velocities, has also been identified around mid to lower crustal transition. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Mid to late Proterozoic period is characterized by the development of several intracratonic basins all over the continental shields and platforms (Windley, 1977; Condie, 1989). It includes four major sedimentary basins on the Indian subcontinent, which are Vindhyan Basin, Cuddapah Basin, Pranhita–Godavari Basin, and Chhattisgarh Basin. All of these basins came into existence due to continued rifting and subsidence of the central part of the Indian peninsular shield. Out of these four basins, the sickle shaped Vindhyan Basin, situated in the central India, is considered the largest (Chakraborty, 2006; Ramakrishnan and Vaidyanadhan, 2008). It spreads in east–west direction along Rajasthan, Bundelkhand and Son valley sectors, encircling the Bundelkhand granitic–gneissic massif, which sits in the middle of Bundelkhand craton (Fig. 1). This basin covers an exposed area of only about 60,000 sq. km while a much larger portion (about 162,000 sq. km) is concealed below the Indo-Gangetic alluvium plain in north and thick 65 Ma Deccan volcanic lavas in south. Currently, this basin is in lime light, due to its hydrocarbon potential which is considered as moderate to good, specially the ⇑ Corresponding author. Tel.: +91 40 27012818; fax: +91 40 23434651. E-mail address: [email protected] (O.P. Pandey).

southern part, where sediment thickness is maximum and there have been reports of surface gas shows. In order to assess its subsurface information, Oil and Natural Gas Corporation has drilled three exploratory wells, Jabera-1, Damoh-1 and Kharkhari-1, in the southern part of the basin (W1, W2, W3 in Fig. 2), which has also been studied in great detail by 1500 new gravity observations in and around Jabera–Damoh–Katangi regions utilizing a novel fractal based gridding approach (Srivastava, 2006; Srivastava et al., 2007, 2009). Besides, the Vindhyan Basin has been mapped in detail by seismic investigations along three deep seismic sounding profiles which cover its southern parts (Fig. 1). The updated seismic sections along these profiles (Tewari et al., 2002; Murty et al., 2004) indicate an anomalous crustal structure underneath, which is also supported by subsequent magnetotelluric (MT) studies (Gokarn et al., 2001). Besides, crustal S-wave velocity distribution, derived by the receiver function technique, has also now become available for five broadband seismic stations, out of which two stations are located over the thick Vindhyan strata (Julia et al., 2009; Vijay Kumar et al., 2012). Geothermally, this basin has not been studied well. A sole heat flow reported for Shivpuri, located in upper Vindhyans of central India ranges from 45 to 61 mW/m2 (Nagaraju et al., 2012). Being covered by thick sequences of Deccan volcanics and Indo-Gangetic alluvium, as mentioned earlier, very little is understood about the evolutionary and paleo-geodynamic

1367-9120/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jseaes.2013.11.015

Please cite this article in press as: Pandey, O.P., et al. Anomalous crustal and lithospheric mantle structure of southern part of the Vindhyan Basin and its geodynamic implications. Journal of Asian Earth Sciences (2013), http://dx.doi.org/10.1016/j.jseaes.2013.11.015

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O.P. Pandey et al. / Journal of Asian Earth Sciences xxx (2013) xxx–xxx

Fig. 1. Simplified geological map of the Vindhyan Basin, central India (Azmi et al., 2007; Bengtson et al., 2009). Solid rectangles refer to location of broadband seismic stations. GBF is great boundary fault. AB, CD, EF are the respective locations of three DSS profiles, Hirapur–Mandla, Khajuriakalan–Rahatgaon and Ujjain–Mahan.

Fig. 2. Gravity bases distribution map of Jabera–Damoh area in Vindhyan Basin. Locations of bore holes W1, W2 and W3 drilled by ONGC are also shown.

nature of this basin. Present study makes an attempt to provide a new insight into the nature and evolution of the crust and uppermost mantle beneath the southern part of the Vindhyan Basin, using recently acquired deep geological and geophysical data. 2. Geotectonic and geologic features Geotectonically, the Vindhyan Basin evolved on a rifted crust apparently as a foreland basin. It is bounded by a prominent NE– SW trending great boundary fault on its west, which virtually separates it from the Aravalli-Delhi Mobile Belt (ADMB). Similarly, the

Son-Narmada-Tapti rift (SONATA), which has been associated with complex thermo-tectonic history, demarcates its southern boundary. This rift structure divides the Indian subcontinent into two major geotectonic segments exhibiting distinct geological and geophysical signatures on either side (Pandey and Agrawal, 1999). The depositional history of this basin is said to be intimately related to the evolution of this rift valley. Reportedly, the Vindhyan rocks are totally absent south of this rift valley. The Bundelkhand Archean granitic–gneissic massif (Fig. 1) divides this basin into two parts; Chambal valley Vindhyans to the west and Son valley Vindhyans to the east. Sedimentation in this basin seems to have begun somewhere around 1.7 Ga (Ray, 2006) and continued till the end of Proterozoic era. Lithostratgraphically, Vindhyan Supergroup, comprises mainly un-metamorphosed and mildly deformed sandstone, shale and limestones with a few volcanoclastic and conglomerate beds (Chakraborty and Bhattacharya, 1996). They are usually considered as deposits of shallow marine environment with maximum sedimentary thicknesses in its southern part near Jabera region (Kaila et al., 1989; Srivastava et al., 2007, 2009). These sediments have been classified into four individual groups, Semri, Kaimur, Rewa and Bhander (Chakraborty, 2006). The Semri Group, which comprises lower Vindhyan rocks, is the oldest as well as thickest and forms bottom of the Vindhyan sediments, which in turn rests unconformably on a variety of rocks like Archean Bundelkhand granite-gneisses and Bijawar Group of metamorphics and Mahakoshal group of rocks belonging to the Paleoproterozoic age. Southern part of the basin hosts a prominent domal structure (Jabera dome) which is located about 40 km NW of Jabalpur and extends in ENE–WSW direction. Detailed geological and geotectonic information of this region can be found in Srivastava (2006), Srivastava et al. (2007, 2009) and references therein. 3. Gravity investigations Southern part of the Vindhyan Basin is very well studied by mounting a close spaced gravity survey network designed with


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74 mGal. Gravity anomalies are particularly low in the southern part of the study area near Jabera and further south of it. In order to quantitatively estimate subsurface structure, the inverse modeling of gravity data along two profiles AA1 and BB1 (Fig. 3) were attempted, which cut across major geotectonic features of this region. Subsurface lithostratigraphy is reasonably well known through two exploration wells (W1, W2 shown in Fig. 3), drilled by ONGC near Jabera and Damoh. Besides, a Deep Seismic Sounding (DSS) profile from Hirapur-Damoh-Mandla runs across this region (Kaila et al., 1989). Katangi–Narsinghgarh section of this profile falls over our study area. Initial modeling constraints were thus taken from the available seismic information as well as well log data over the study area (Kaila et al., 1989; Das et al., 1999; Murty et al., 2004; Srivastava et al., 2007), apart from the measured density on the borehole core samples of different formations (Table 1).

Fig. 3. Bouguer gravity anomaly image map of Jabera–Damoh area of the Vindhyan Basin obtained using fractal method. Locations of the modeled gravity Profiles AA1, BB1 (as shown in Figs. 4 and 5), as well as boreholes W1, W2 drilled by ONGC, are also shown. H1, H2 and L1, L2 represents delineated gravity highs and lows in the study area.

optimum fractal dimension, to delineate subsurface structure below 112 100 km surface area covering parts of Katni, Damoh, and Panna districts of Madhya Pradesh in Central India. In this survey, as many as 1500 stations were covered and 40 new gravity bases were established (Srivastava et al., 2013) at about 10–15 km spacing (Fig. 2). Gravity measurements were made along all the accessible roads and tracks at a varying interval of 0.5 km to 2.0 km using Lacoste-Romberg G-type gravimeters (LRG), having a precision of 0.01 mGal. Bouguer gravity anomaly map, prepared using fractal based gridding method is shown in Fig. 3. The anomaly map prepared by the fractal method look smoother and free from aliasing errors, hence produced the best representation of the subsurface features of the study area. The new findings of this gravity map are two gravity highs H1 and H2 and two gravity low regions L1 and L2, which were not known earlier. High Bouguer anomaly observed towards NE of Damoh (H2) may be related to the presence of high density Mahakoshal group of rocks, which are exposed near Panna region (24°430 N, 80°120 E). The incomplete pattern of this high Bouguer gravity anomaly (pink color in Fig. 3), clearly indicates further extension of this anomaly in the NW direction. H1 represents the extent of this high gravity anomaly in the easterly direction. It is likely that the 1.1 Ga kimberlitic event which affected Panna, Majhgaon (both proven diamondiferous regions) and its surroundings, caused upliftment of basement comprising of Mahakoshal group of rocks besides numerous kimberlitic and lamproitic intrusions, resulting into high gravity anomaly. We feel some kimberlite pipes may be hidden below Mahakoshal group of rocks also apart from Panna and Majhgaon areas. Similarly, this study clearly outlined an anomalous rifted structure, which is bounded by parallel faults on either side in the Jabera region. This structure is marked as L2 in Fig. 3. A moderate gravity low, shown as L1 near Damoh, possibly characterizes shallow sequences of lower Vindhyans. 3.1. Inverse modeling of gravity data Although, the study area is small (about 1° 1°), it shows significant variations in Bouguer gravity anomaly from 25 to

3.1.1. Profile AA1 This profile is selected in order to infer a tentative geological cross section across the conspicuous gravity low observed in the southern part of the study area. DSS studies (Kaila et al., 1989; Murty et al., 2004) indicate maximum thickness of sediments in this part of the Vindhyan Basin. The gravity anomaly drawn along this profile shows a typical anomaly pattern of a sedimentary basin faulted on its both margins (Fig. 4). The preliminary depth estimates of the major interfaces were obtained from the 2D scaling spectral analysis of the gravity data. The inverse modeling results (Fig. 4) suggests that this gravity anomaly corresponds to a deep faulted basin in the crystalline basement, in which the layer with 2.46 g/cc density corresponds to the upper Vindhyan rocks. This layer is underlain by a thick layer (1.0–6.5 km) of lower Vindhyan sediments, which has a gentle slope from NW to SE direction. As much as 6.5 km thick sediments seems to have been deposited directly over the high density Bijawar/Mahakoshal group of rocks. It was hard to fit the observed anomaly pattern without introducing this high density (2.80 g/cc) layer. In absence of this layer, the thickness of sediments would have increased to such an extent that it would have not matched

Table 1 Density of rock types from the core sampling in the study area (Srivastava, 2006). Sample no.

Rock type

Density

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Lameta sandstone Lameta sandstone Lameta limestone/sandstone Lameta limestone/sandstone Lameta sandstone Lameta sandstone Gondwana sandstone Gondwana sandstone Basalt Basalt Gondwana sandstone Gondwana shale Gondwana shale Granite Granite and quartzite gneiss Phyllite (with or without quartz) Phyllite (with or without quartz) Phyllite (with or without quartz) Phyllite (with or without quartz) Lameta sandstone Gondwana sandstone Granite Marble Lameta sandstone/limestone Lameta sandstone/limestone Marble (amphibole patch)

2.66 2.57 2.61 2.65 2.50 2.51 2.39 2.40 2.93 2.92 2.25 2.52 2.72 2.62 2.50 2.73 2.62 2.69 2.56 2.42 2.39 2.62 2.82 2.47 2.47 2.91


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Fig. 4. Subsurface structural model as obtained from the inversion of gravity data along the profile AA1.

Fig. 5. Subsurface structural model as obtained from the inversion of gravity data along the profile BB1.

with the known seismic results. It appears that in the major part of the basin, the Bijawar/Mahakoshal groups of rocks were deposited directly over the high velocity (6.5 km/s) and high density (2.93 g/ cc) exhumed mid crustal segment having an intermediate composition. Seismic studies (Murty et al., 2004) support this conjecture. Gokarn et al. (2001), even suggested that the Vindhyan sedimentation took place directly over the lower crust. As per our model (Fig. 4), granitic–gneissic layer is missing in the right margin of the faulted basin, whereas there may be only a thin patch on its left side. Small undulations in the anomaly are attributed to the disturbance in the top of upper Vindhyan layer, faulting and folding. Since this area lies in the vicinity of Jabera, we term it as Jabera basin gravity low.

information on velocity–depth distribution, which helps in elucidating petrological and compositional nature of the underlying crust. This basin has been seismically imaged along three DSS profiles running through the southern part of the Vindhyan Basin (Fig. 1). These include (i) Hirapur–Mandla, (ii) Khajuriakalan–Pulgaon, and (iii) Ujjain–Mahan profiles, along which both shallow and deep seismic refraction and wide-angle reflection data have been acquired earlier in analog form. This data was later digitized to produce composite record sections, which provided first hand crustal structure and the Moho configuration (Kaila and Krishna, 1992). The updated seismic crustal sections (Tewari et al., 2002; Murty et al., 2004) along these profiles are shown in Figs. 6 and 7, which indicate anomalous nature of the crust underneath.

3.1.2. Profile BB1 The gravity model presented in Fig. 5 shows the section below profile BB1. The model reveals a prominent fault structure nearly 10 km east of Hardua (Fig. 2). In this area, Bijawar/Mahakoshal group of rocks, overlying granitic–gneissic basement may either be absent or very thin. From the trend of gravity high (H2) in the anomaly map (Fig. 3), it is obvious that it extends further north-west much beyond the yellow line. As mentioned earlier, this high possibly corresponds to nearby exposed Mahakoshal rocks and kimberlite/lamproite intrusives. Thickness of granitic– gneissic layer overlying the mid-crustal rocks may only be around 1–3 km in this region.

4.1.1. Hirapur–Mandla (profile 1) Detailed crustal seismic section along this 235 km long profile (Murty et al., 2004) which runs between the southern border of Bundelkhand granitic–gneissic massif (Hirapur) to Mandla, covering almost entire stretch of the exposed Vindhyan rocks north of the SONATA rift, is shown in Fig. 6. Two major faults delineated at Narsinghgarh and Katangi, conform to a graben structure, filled with thick pile of lower and upper Vindhyan sediments. The region further south, bounded between Katangi and Jabalpur faults, corresponds to a horst feature coinciding with the SONATA rift. Five layered crust is delineated between Damoh and Katangi regions, where sediment thicknesses are maximum (Fig. 6) It is envisaged that as much as about 5 km thick sequence of the Vindhyan sediments sit directly over an extremely thin granitic –gneissic upper crust, which may even be virtually absent from this region, conforming with the gravity observations (Figs. 4 and 5), as well as magnetotelluric studies (Gokarn et al., 2001). These sediments are characterize by two distinct P-wave velocities; Upper Vindhyans with a velocity of 4.5 km/s, compared to a much higher velocity of 5.3–5.4 km/s for the lower Vindhyans (Murty et al., 2004), which would mean that the lower Vindhyans are denser and much more compact, compared to the upper Vindhyan sediments. Presence of high velocity crust (6.5 km/s or even more) is observed immediately below the thin wafer of granitic–gneissic layer, at the depths between 6 and 20 km. This layer is characterized by

4. Crustal seismic structure Our knowledge of the seismic structure of this basin is based on three DSS profiles and 5 bulk receiver function estimates in the southern part of the basin (Fig. 1). Our inferences on the nature of the crust and the structural disposition of different layers, as revealed by both kinds of studies, are detailed below. 4.1. Deep seismic sounding (DSS) studies DSS studies are considered an important tool to infer the nature of the deep seated crust as well as uppermost mantle. It provides



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Fig. 6. 2-D crustal velocity model along Hirapur–Mandla seismic profile (after Murty et al., 2004).

Fig. 7. 2-D deep crustal velocity model along (i) Khajuriakalan–Rahatgaon (top) and Ujjain–Mahan (bottom) seismic profiles (after Tewari et al., 2002).

two different velocity patterns, 6.5 km/s at the top, followed by 6.35 km/s at the bottom (Fig. 6). Magnitude of the velocity drop in the bottom layer situated at the mid to lower crustal transition may indicate mass influxing and infiltration of mantle fluids and gases. Such conditions cause metasomatic alteration/replacement of the in situ rocks by hydrothermal fluids that operate under high

stress and strain conditions, which in turn lead to significant lowering of velocities due to processes like, biotitisation, saussuritisation and iron enrichment, etc., (Tripathi et al., 2012a,b). This metasomatically altered layer is further underlain by a 20–22 km thick typical lower crust, characterized by velocity 6.8 km/s. Moho boundary is detected between 41 and 44 km depth.


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Surprisingly however, no underplated magma layer above the Moho is reported from this region, which has been conspicuously found along other two profiles, as mentioned below. 4.1.2. Khajuriakalan–Rahatgaon (profile 2) Along this profile (Fig. 7), only a couple of hundred meters thick Proterozoic Vindhyan and Bijawar sediments are present over a 5–7 km thick upper (granitic–gneissic) crust. Below this layer, a 13–14 km thick high velocity (6.5–6.6 km/s) layer is present. Bottom half of this layer too appears metasomatised and thus has a much lower velocity (6.3 km/s), similar to that found below the Hirapur–Mandla profile (Fig. 6). Below this region, lower crust is again about 20 km thick, having a typical lower crustal velocity of 6.7 km/s in top 10 km, followed by a more than 10 km thick high velocity (7.2 km/s) magmatic layer just above the Moho. As mentioned earlier, no such magmatic layer is reported from the Hirapur–Mandla profile. Along this profile, on an average, the crust is about 41 km thick. 4.1.3. Ujjain–Mahan (profile 3) Cumulative thickness of the Deccan volcanics, Lameta beds, Vindhyans and Bijawar group of rocks, above the crystalline basement, are only about 600 m (Kaila et al., 1985) in the Vindhyan segment of the profile (Fig. 7). Below the central part of this region, the seismic crustal structure is somewhat similar to that found below the Khajuriakalan–Rahatgaon profile. Granitic–gneissic layer is again moderately thick (7–8 km), which rests over 16 km thick mid-level crust (Fig. 7). First half of this layer is associated with a reasonably high mid crustal velocity of about 6.6 km/ s while the metasomatised bottom layer, has a much lower velocity of 6.3 km/s. In this region, lower crust is about 17 km thick that contains 10 km thick underplated mafic magma (7.2 km/s) at the bottom. Moho is detected at about 41 km depth. 4.2. S-wave velocity structure Apart from the DSS studies, the S-wave velocity structure of this region has been investigated in detail by Kumar et al. (2007), Julia et al. (2009) and Vijay Kumar et al. (2012) at 5 individual sites (Fig. 1). They used receiver function technique to obtain S-velocity variations, Moho depth and Vp/Vs ratios at these stations. Distribution of S-wave velocity variation with depth, as obtained from their study is shown in Fig. 8, which suggests narrow variation in Moho depth from 38 to 44 km, with a mean of around 41 km. Julia et al. (2009) however, indicated a much deeper Moho (52.5 km) at BPL station (Fig. 1), which may primarily be due to presence of a thick high velocity-high density underplated magmatic layer in which Vs gradually increases from about 4.03 km/s at 35 km to 4.43 km/s at 52.5 km. We feel, out of this, only about 3 km thick magma layer lies above the Moho, while the rest appear to be stacked below it, indicating strong thermal integration between the lower crust and the underlying mantle. Gradual increase in velocity would conform to its differential nature. Incidentally, Kumar et al. (2007) too reported a 38 km thick crust below this station. In Fig. 9, we have combined crustal Vs averages versus depth for all the broadband seismic stations, from which some interesting conclusions can be drawn. On an average, the Vindhyan sediments are about 5 km thick in the southern part of the basin conforming to the gravity and DSS studies. It is followed by a combined thickness of only 10 km for the upper (granitic–gneissic) and middle crust, in which velocities gradually increase from 3.35 km/s to 3.71 km/s. It is difficult to demarcate the two layers based on in situ velocities. Such situations usually reflect assimilation and alteration of the fabric of the upper and middle crust during exhumation of the lower crust to shallow level (in this case 15 km

Fig. 8. S-wave velocity variations with depth below five broadband seismic stations, located in the southern part of the Vindhyan Basin (Julia et al., 2009; Vijay kumar et al., 2012).

depth). Here, the lower crust is extremely thick (27 km) and can be divided into two distinct parts. One between 15 and 35 km depth, which corresponds to lower crust and the other between 35 and 42 km depth, indicating stacked magmatic crust in which, shear velocities gradually increase from 4.01 km/s to 4.31 km/s This suggests that the stacked magmatic body is differentiated and stratified, as would be expected in the case of thermal remobilization of the lower crust and uppermost mantle. Fig. 9 would further indicate that the lower crust beneath this region may have been exhumed by as much as 10–13 km. To this effect, a clear cut velocity enhancement (0.2 km/s), compared to the global shields and platforms, can be seen at the depths levels around 12–25 km. Further, we observe that the average shear wave velocity in the 20 km thick lower crust (between 15 and 35 km) is slightly lower at about 3.87 km/s. On an average in the lower crust, Vs hoovers around 3.9 ± 0.1 km/s. It would possibly indicate metasomatic alteration of some form or other, as seen in DSS sections also (Figs. 6 and 7). From this study too, Moho is found at the depth of 42 km below this region.

5. Magnetotelluric studies Gokarn et al. (2001) carried out magnetotelluric studies between Damoh and Anjaneya, which cuts across SONATA rift. Part of this profile, between Damoh and Jabalpur, coincides with the Hirapur–Mandla DSS profile (Fig. 6). Their investigation confirmed the results obtained through seismic studies, indicating presence of a 5 km thick conductive (6–30 ohm meter) Vindhyan sediments between Damoh and Katangi region, which rest directly over the lower crust, characterized by 200 ohm meter resistivity, and 6.5 km/s P-wave velocity. This study also indicated that the upper crust from this region may have been completely eroded during the uplift and erosional process. Apart from this, another



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(Nagaraju et al., 2012). Since the boreholes are not very deep, we adopt the heat flow of 61 mW/m2 for Shivpuri which comes from the deepest borehole. 6.1. Heat flow from bottomhoIe temperature measurements in Jabera Basin

Fig. 9. Averaged S-wave velocity distribution beneath southern part of the Vindhyan Basin, based on S-wave velocity distribution shown in Fig. 8. For comparison, S-wave velocity distribution beneath global shields and platforms (shown as solid dots and obtained by converting Vp values from Christensen and Mooney, 1995 into Vs using Vp/Vs = 1.75), is also included. Consequent to exhumation of deeper mafic crust, significantly higher velocities can be seen between 13 and 25 km depth.

conductive feature (conductivity less than 3 ohm meter) at the depths of 15–20 km below Damoh was delineated by Arora et al. (1995), based on geomagnetic depth sounding studies. This depth range coincides with the metasomatised low velocity (6.35 km/s) mid to lower crustal transitional layer. As mentioned earlier, such processes involve mantle derived fluids, which may have been trapped in this layer thereby causing high conductivity. In that case, a strong thermal anomaly must have persisted below this region during early to mid-Proterozoic period, causing large-scale uplift and erosion, followed by rifting, thermal subsidence and eventually thick sedimentation. Such thermal anomalies may even be persisting now. Inferred heat flow for this rifted region provides some clues to this effect.

Besides conventional temperature measurements to shallow depths at Shivpuri, southern part of the Vindhyan Basin has been pierced by three deep boreholes, two of them reaching more than 3.5 km depth. Bottom hole temperatures were available at various depths for one of the borehole (Jabera-1, shown as W1 in Fig. 3). Such measured temperatures are usually recorded after 4–8 h from the cessation of drilling at the depths of about 5–10 m above the bottom where the thermal disturbances are expected to be the least. In this borehole, one such temperature measurement was carried out after 8.5 h. Bottom hole temperatures, in general, do not correspond to static formation temperatures because of cooling effects of the circulating fluid, and drilling disturbances. Stabilized conditions are rarely reached by the time various logs are run and therefore they are typically lower than the static temperatures depending mainly on the properties of the borehole fluid and surrounding rocks, drilling history, and the natural temperature regime. Such temperatures therefore, need to be corrected accordingly. For the present study, we use the correction curve (Fig. 10) prepared for hydrocarbon bearing deep (up to 5 km) sedimentary basins of New Zealand, which is based on the measured static and formation-test temperatures, for which corresponding time sequential bottomhole temperatures had also been recorded previously (Pandey, 1981). Similar curves are sometimes used by petroleum companies. For 8 h since cessation of drilling, the measured temperature needs to be corrected upwards by 15%. For comparison sake, we have also used Kahle correction equation and SMU-Harrison correction equation (Blackwell et al., 2010) to the measured bottom hole temperatures. Corrected temperatures based on Fig. 10, conform very well with those obtained from Kahle correction equation at all the depths. However, SMU-Harrisson equation did not hold good at shallow depth and matched well only after 2 km depth, so we do not use it. Calculated temperature gradients (after the required correction) between 620 and 3607 m depth interval (Fig. 11) comes to about 25.1 °C/km using Fig. 10 and 26.6 °C/km, using Kahle correction equation. We adopt a mean temperature gradient of 25.9 °C/km for heat flow estimation.

6. Heat flow studies Heat flow studies provide useful insight on the thermal state of the earth’s crust and uppermost mantle. It is closely related to deeper temperature regime and thus considered an important parameter for regional geotectonic and geodynamic interpretations. Although heat flow coverage is fairly reasonable over the major part of the Indian terrain, but only one conventionally measured heat flow value is available over the Vindhyan Basin. This comes from the cluster of seven heat flow sites at Shivpuri (central India), spread in an area of about 15 10 km2 (Fig. 1) and located within the upper Vindhyan sedimentary sequences of Kaimur and Rewa group of rocks. The Bundelkhand granitic–gneissic complex, situated east of Shivpuri, apparently forms the crystalline basement. Combining thermal conductivity and temperature gradient measurements, heat flow varying from 45 to 61 mW/m2 has been reported among seven sites, together with a mean of 52 mW/m2

Fig. 10. Bottom hole temperature correction curve used in the present study (Pandey, 1981).


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Lithology of this borehole is fairly well known based on drill hole information. It is dominated mainly by shale together with sandstone, siltstone, limestone glauconite, porcellanite, chert and quartz. The thermal conductivity of the Vindhyan sediments have been measured by Nagaraju et al. (2012). Estimated conductivities are 5.6 W/m °C for sandstone, 2.0 W/m °C for shale and 2.6 W/m °C for siltstone. Conductivity of limestone is taken as 2.6 W/m °C from Mildren and Sandiford (1995), which is based on the compilation of Clark (1966) and Kappelmeyer and Haenel (1974). Conductivities of quartz, glauconite, chert and porcellanite is taken same as sandstone. Weighted mean conductivity of 3.01 W/m °C is obtained for the adopted temperature depth range of 620–3607 m. When we combined this thermal conductivity with the mean temperature gradient of 25.9 °C/km, we get a heat flow of 78 mW/m2 for the Jabera Basin. 6.2. Lithospheric thermal regime To elucidate deep thermal regime, we require knowledge of crustal radioactive heat production, in situ thermal conductivity and thickness of different crustal layers, as discussed below. 6.2.1. Radioactive heat production Radioactive elemental concentrations, measured by gamma ray spectrometer on the granitic–gneissic basement rocks exposed in the vicinity of Shivpuri area, indicate mean concentrations of U at 3.8 ± 1.2 ppm, Th at 19.2 ± 7.1 ppm, and K at 2.90 ± 0.63% (Nagaraju et al., 2012). This results into a radioactive heat production of 2.7 ± 0.8 lW/m3 for the crystalline basement rocks. Similarly radioactivity of the Vindhyan sediments has been measured by them at around 0.5 lW/m3. We have measured the heat production for the mid-crustal amphibolite to granulite facies rocks, encountered in KLR-1 borehole drilled at Killari region of the Deccan Traps (Pandey et al., 2009; Tripathi et al., 2012a,b),

Fig. 11. Corrected bottom hole temperatures in the ONGC deep borehole near Jabera (marked with W1 in Figs. 2 and 3), located in the southern part of the Vindhyan Basin. It exhibits an average geothermal gradient of 25.9 °C/km between the depths 620 and 3607 m. Temperatures corrected by Kahle correction equation are shown by solid triangles, while that using Fig. 10, are shown by solid circles.

which comes to about 0.78 lW m3. Besides, the radioactivity of the granulitic lower crust is also well resolved, varying in a narrow range between 0.14 and 0.20 lW/m3 with a mean of. 0.16 ± .078 lW/m3 (Ray et al., 2003). Heat production for the underplated crust and lithospheric mantle is conventionally taken as 0.02 and 0.01 lW/m3 respectively (Liu and Zoback, 1997). The latter value conforms to peridotitic composition.

6.2.2. Thermal conductivity Upper crustal thermal conductivity in Indian Archean–Proterozoic terrain usually fall between 3.0 and 3.5 W/m °C (Roy and Rao, 2003). Similarly, crustal granulites exhibit a conductivity 2.5 W/ m °C (Ray et al., 2003). Therefore, we chose a conductivity of 3.0 W/m °C for the upper crust, considering temperature–pressure dependence of thermal conductivity with depth. Since there is not much effect of temperature over the conductivity of mafic rocks (Kappelmeyer and Haenel, 1974) and it is also partially compensated by increasing pressure, we chose a value of 2.5 W/m °C for the granulitic crust and 3.0 W/m °C for the underlying mantle. 6.2.3. Temperature–depth distribution As mentioned earlier, detailed vertical section of the continental crust beneath the Vindhyan Basin is now available through three DSS profiles (Figs. 6 and 7) and receiver function studies at 5 broadband seismic location (Figs. 8 and 9). Based on these seismic images and other information, we choose two crustal heat production models (Fig. 12), for calculating lithospheric geotherms, one representing the Jabera Basin and the other, Shivpuri. Since heat flow and heat generation data is reasonably constrained, we use layered crustal model, with uniform distribution of radiogenic elements in each of these layers, corresponding to representative rock types. Adopted geothermal parameters are summarized in Table 2 and the calculated steady state geotherms for both Shivpuri and Jabera, are shown in Fig. 13.

6.2.4. Thickness of the lithosphere and mantle heat flow We adopt base of the lithosphere as the intersection point of the calculated geotherm with the peridotite incipient melting point curve (Gass et al., 1978), which results into a lithospheric thickness of about 50 km below Jabera rifted basin where heat flow is high at 78 mW/m2 and radioactively rich granitic–gneissic layer is almost absent. The absence of radioactive layer would mean that only a small amount of heat is generated in the crust and most of the heat flow contribution would come from the mantle. We do observe a very high contribution of mantle heat flow (56 mW/m2) for this region. Moho temperatures are also extremely high at about 1030 °C. Its thermal regime can be compared with the hydrocarbon bearing Tertiary Cambay graben of Gujarat. Usually in Proterozoic terrains, such anomalous thermal regime are seldom observed. In contrast to Jabera Basin, however, we obtain a lithospheric thickness of about 105 km for the Shivpuri region. Based on the receiver function studies almost a similar thickness of the lithosphere (95 km) has been reported below BPL seismic station (Bhopal) (Fig. 14), situated on the southwestern flank of the concealed Vindhyan Basin (Kumar et al., 2007). Since we know the lithosphere–asthenosphere boundary at BPL station, we have estimated surface heat flow for this location using inverse recurrence method (Vedanti et al., 2011) which comes to about 71 mW/m2. Lithospheric thickness obtained for both at the BPL seismic station and Shivpuri, are quite close to that obtained for hot Indian shield (Negi et al., 1986, 1987; Pandey and Agrawal, 1999; Kumar et al., 2007), but much lesser than global shields which have 200– 350 km thick lithosphere (Polet and Anderson, 1995; Pandey and Agrawal, 1999; Artemieva and Mooney, 2001).


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Fig. 12. Representative heat flow and crustal heat production models for Shivpuri (northern part) and Jabera (southern part) areas of Vindhyan Basin. Figures within the layered segments represent heat production in lW/m3 together with thermal conductivity (W/m °C). Values of heat flow, shown at the bottom of the models, represent estimated contribution of heat flow from mantle. High mantle heat flow for Jabera region can be noted.

Fig. 13. Estimated temperature–depth profile for the Shivpuri (northern part) and Jabera (southern part) regions of the Vindhyan Basin. Mantle solidus cuts at an extremely shallow depth of about 50 km below the Jabera region, indicating melting conditions at shallow depths.

7. Discussions Deep drilling in stable platforms, especially among the older terrains, have now confirmed that the bulk of continental crust may be made up of metamorphic rocks, deformed repeatedly by underlying mantle processes. In these areas, deeper crustal segments have exhumed and subsequently eroded, and as such their original igneous texture and composition no longer remain the same. The Indian shield in particular is no exception. It had a dynamic past and contains highly radioactive, enriched, fertile and juvenile mantle underneath (Rogers and Callahan, 1987; Chalpathi Rao and Lehmann, 2011). Arguably, its warm lithosphere has progressively become thin, sheared and degenerated (Negi et al., 1986, 1987; Polet and Anderson, 1995; Pandey and Agrawal, 1999; Priestly and Mckenzie, 2006; Kumar et al., 2007; Rychert and Shearer, 2009) consequent to episodic thermal turbulence below the lithospheric mantle. Even the Vindhyan Basin lithosphere which forms a major geotectonic segment of the Bundelkhand craton, could not resist such turbulences, as our present integration of newly acquired multiparametric geological and geophysical data would indicate. It does give a glimpse of the role played by the thermotectonic processes in restructuring the ancient continental crust under the southern part of the Vindhyan Basin, which appears heavily influenced by the geodynamic processes associated with the evolution of SONATA rift and its surroundings.

Fig. 14. Stack S-receiver function trace of broad-band seismic station Bhopal (Kumar et al., 2007). The Moho and LAB (i.e., lithosphere–asthenosphere boundary) are marked.

7.1. Thick sedimentation over the rifted mafic crust and extent of Vindhyan Basin We feel that the crustal extension and rifting over this region possibly started around mid-Proterozoic time and continued till the end of the era (Verma, 1996) thus covering a span of nearly 800 Ma (Venkatchala et al., 1996). Almost 5–6 km thick lower and upper Vindhyan sediments deposited directly over the Mahakoshal–Bijawar group of rocks in the rifted Jabera basin (Fig. 4). At some places, sedimentation took place directly over the thin veneer of granitic–gneissic basement (Figs. 4 and 6). However, in the southwestern part of the Vindhyan Basin, through which two deep seismic sounding profiles run, sediment thickness is much smaller around a few hundred meters only, indicating that this region was unaffected during the Proterozoic rifting episode. It is pertinent to mention here that only a part of the rifted Jabera basin has been mapped gravimetrically and therefore, the

Table 2 Adopted geothermal parameters for the crust and underlying mantle lithosphere beneath Vindhyan Basin. Standard values of Vs for different segments of the crust are also included.

Sediments (Vs 3.8 km/s; equivalent Vp 6.6 km/s) occurs at an average depth of about 15 km in the southern part of the Vindhyan Basin. Thus here, the upper and middle crust may be quite thin, which goes against the very common belief that the Archean–Proterozoic terrains have a thick granitic–gneissic crust. 7.3. High velocity thick lower crust and magma underplating Sustained exhumation brought high velocity mafic crust to much shallower levels. Fig. 9 reveals that as much as 13 km exhumation may have taken place during this period. In this figure, velocities in the depth range between 15 and 25 km, are higher by about 0.2 km/s compared to that expected in shields and platforms at similar depths (Christensen and Mooney, 1995). Consequently, in contrast to a combined thickness of 10 km for the upper and mid crust together, lower crust is as thick as 27 km, which is characterized by Vs greater than 3.8 km/s. It can be divided into two distinct segments, one between the depths 15 and 35 km and the other between 35 and 42 km depth. The first one characterizes the typical lower crust in which Vs varies between 3.81 and 3.97 km/s. It is then followed by a 7 km thick solidified emplaced magmatic layer above the Moho. This layer is characterized by gradational increase in Vs from 4.01 to 4.31 km/s indicating segregated and differentiated nature of the magmas. This could be the reason for causing density and velocity stratification from top to bottom. Such situations are often seen in rift zones or tectonothermally induced active terrains which underwent strong thermal interaction between the lower crust and the hot, enriched and buoyant asthenosphere. That induces sub crustal erosion consequent to rise in mantle solidus to shallow depths. In such areas, mafic or ultramafic magma underplating becomes a major process

responsible for the growth and the thickening of the lower crust and consequently deepening of the Moho. 7.4. Metasomatised low velocity layer at mid to lower crustal transition All the three DSS profiles (Figs. 6 and 7) reveal presence of 5–8 km thick low velocity zone coinciding with the exhumed mid to lower crustal transition. In this layer Vp ranges from 6.3 to 6.4 km/s, compared to 6.5–6.7 km/s at the top and 6.7–6.8 km/ s at the bottom, indicating a drop in velocity of around 6%. It would point to significant metasomatic modification (or alteration) of the crust, due to interaction with hydrothermal and other mantle fluids, associated with thermal and magmatic influx from below. It usually happens during retrograde metamorphic conditions, wherein prevailing P&T conditions shifts to lower grade due to exhumation of the mafic crust. Such phenomena are quite prevalent in the Deccan volcanic region (Desai et al., 2004; Sen et al., 2009; Tripathi et al., 2012a,b). Earlier, it was believed that the metasomatised layer constitutes part of the mid crust (Murty et al., 2010) in this region. However, based on current broadband seismic information, the metasomatic layer transgresses from the mid crust to lower crust. Laboratory measurements, geological, geochemical and petrological studies (Tripathi et al., 2012b), indicate a drop of up to 15% in measured velocities on metasomatised mid crustal rock samples recovered from the KLR-1 borehole drilled in the 1993 Latur earthquake region. There is a possibility that the observed Vs in the lower crust may have been higher initially, but for the metasomatic influences. Presence of thick underplated magmatic layer at the bottom of the crust would support metasomatic alteration. Moho temperature and mantle heat flow are high and the asthenosphere is shallow below these areas (specially in Jabera basin), as mentioned earlier. This indicates that these areas are subjected to much higher in situ temperature than usually recorded in Proterozoic terrains elsewhere. 7.5. 1.1 Ga super plume interaction and geotectonic implications High Moho temperatures and high heat flow input from the mantle consequent to rise in mantle isotherms, as found for the Jabera Basin, is in conformity with the processes like sustained uplift, exhumation, crustal rifting and magmatic extrusions. Such thermal anomalies are usually manifested through dyke swarm activity like those seen in the Bundelkhand granitic–gneissic massif (Rao et al., 2005) and possibily beyond under the thick sediment cover. Presence of high order positive gravity anomalies (Mishra and Rajasekhar, 2008; also in Fig. 3) as well as magnetic highs, specially in areas of basic kimberlitic and lamproitic intrusions like Panna, Majhgaon, etc. would support it. There is a possibility that this region was under the influence of a super mantle plume for a long time, which culminated at around 1.1 Ga (Anil Kumar et al., 1993; Mall et al., 2008; Chandrakala et al., 2010). It rifted and magmatically infused the Indian crust at several places in the forms of magmatic extrusions, kimberlitic/lamproitic intrusions and dyke swarms and also led to destruction of cratonic roots. Part of one such super plume has been seismically imaged below the southwestern part of the Cuddapah Basin near Parnapalle (Mall et al., 2008). Such plume infected regions have been getting reactivated time and again, for example, Gondwana and Deccan eruptive periods. Consequently, beneath them, mantle heat flow input is still higher. In fact, Vindhyan Basin is surrounded by many high heat flow regions like Cambay graben, Aravalli-Delhi mobile Belt, Damodar graben and SONATA rift (Fig. 15). All of them have reasonably thin lithosphere and as such will be prone to multiple tectonic reactivations.



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Fig. 15. Heat flow (in mW/m2) distribution in and around Vindhyan Basin (Data source: Annual Report, 1969–1970; Gupta and Rao, 1970; Verma and Gupta, 1975; Rao et al., 1978; Rao and Rao, 1980, 1983; Gupta, 1981; Gupta and Gaur, 1984; Gupta et al., 1988, 1993; Sunder et al., 1990; Pollack et al., 1991; Roy and Rao, 2000; Nagaraju et al., 2012). Presence of reasonably high heat flow around Vindhyan Basin can easily be seen. ADMB: Aravalli-Delhi Mobile Belt, CG: Cambay graben, DG: Damodar graben, NSL: Narmada-Son-Lineament (or SONATA rift).

Fig. 16. Inferred crustal cross section beneath southern part of the Vindhyan Basin based on averaged S-wave velocity distribution.

Further, since the receiver function derived crustal seismic structure below the Bundelkhand granitic–gneissic massif, differs considerably to that found beneath the Vindhyan Basin (latter being faster than the former), Vijay Kumar et al. (2012) suggested that the Bundelkhand block may not be extending below the Vindhyan sediments. Our studies would support it at least for the southern part of the Vindhyans.

8. Conclusions This paper deals with first detailed synthesis of recently acquired multiparametric deep geological and geophysical data over southern part of the Vindhyan Basin, which provided a new

understanding of the crust–mantle configuration, prevailing lithospheric thermal regime and the nature of geodynamic evolution of this region. In particular, the entire studied area appears to have been subjected to sustained uplift, erosion and magmatism, followed by crustal extension, magma underplating, rifting and subsidence due to episodic thermal remobilization of the lower crust and uppermost mantle that continued till the end of Proterozoic era. Almost 5–6 km thick sedimentation took place in the deep faulted Jabera Basin, directly over the high velocity mafic crust, brought closer to the surface by sustained exhumation to the tune of almost 10–13 km. Consequently, a substantial thickness of granitic–gneissic upper crust has been eroded from many segments. In some areas like Jabera rifted basin, this layer may even be almost absent, where a very thin lithosphere of only about 50 km


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has been obtained. In this rifted basin (i) estimated heat flow is high at about 78 mW/m2, (ii) Moho temperature exceeds 1000 °C, and (iii) the heat flow input from the mantle is also very high at around 56 mW/m2. Usually in Proterozoic terrains, such anomalous thermal conditions are rarely observed. An inferred crustal cross section based on averaged S-wave velocity distribution is shown in Fig. 16. Apart from the above, we also observed significant metasomatic modification of mid to lower crust beneath this part of basin, which is normally caused due to its interaction with hydrothermal and other mantle fluids, associated with thermal and magmatic influx from below during the course of interaction between the lower crust and the hot, enriched and buoyant asthenosphere. It can cause significant drop in velocity (up to almost 15%) which often happens during retrograde metamorphic conditions. Presence of thick underplated magma layer at the bottom part of the crust, which appear segregated and differentiated, would support this finding. It would further appears that the Bundelkhand granitic block may not be extending below the Vindhyan sediments. Even the lateral extent of the Jabera basin may be much larger than what is known today, as it seems to extend further south beyond the SONATA rift under the thick pile of Deccan volcanics. This can be considered a new finding, as there are no such reports yet. There is also a fair possibility that this region may have been under the influence of a super mantle plume for a long time, which culminated at around 1.1 Ga. Acknowledgements We thank Drs. A.S.N. Murthy, Sandip Gupta, D.M. Mall and U. Raval for many helpful discussions and suggestions. O.P. Pandey is extremely thankful to CSIR (New Delhi) for granting Emeritus Scientist position at CSIR-NGRI, Hyderabad. R.P. Srivastava wishes to thank Department of Science and Technology, Govt. of India for financial support to carryout gravity studies in and around Damoh-Jabera basin and ONGC for Jabera-1 well data. We would also like to thank Prof. John J.W. Rogers and anonymous reviewer for their useful comments and suggestions. Permission accorded by the Director, CSIR-National Geophysical Research Institute, Hyderabad to publish this work is gratefully acknowledged. References Anil, Kumar, 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. Precambr. Res. 62, 227–237. Annual Report, 1969–70. National Geophysical Research Institute, Hyderabad, India. Arora, B.R., Waghmare, S.Y., Mahashabde, M.V., 1995. Geomagnetic depth sounding along the Hirapur–Mandla–Bhandara profile, central India. Mem. Geol. Soc. India 31, 519–535. Artemieva, I.M., Mooney, W.D., 2001. Thermal thickness and evolution of Precambrian lithosphere: a global study. J. Geophys. Res. 106, 16387–16414. Azmi, R.J., Joshi, D., Tewari, B.N., Joshi, M.N., Mohan, K., Srivastava, S.S., 2007. Age of the Vindhyan super group of central India: an exposition of biochronology vs radiochronology. In: Sinha, D.K. (Ed.), Micropaleontology: Application in Stratigraphy and Paleooceanography. Narosa Publishing House, New Delhi, pp. 29–62. Bengtson, S., Belivanova, V., Rasmussen, B., whitehouse, M., 2009. The controversial ‘‘Cambrian’’ fossils of the Vindhyan are real but more than a billion years older. Proc. Natl. Acad. Sci. USA 106, 7729–7734. Blackwell, D., Richards, M., Stepp, P., 2010. Final Report, Texas Geothermal Assessment for the 135 Corridor East, for Texas State Energy Conservation Office Contract CM709. SMU Geothermal Laboratory, Dallas, Texas, USA, 78 pp. Chakraborty, C., 2006. Proterozoic intracontinental basin: the Vindhyan example. J. Earth Syst. Sci. 115, 3–22. Chakraborty, C., Bhattacharya, A., 1996. The Vindhyan basin: an overview in the light of current perspectives. Mem. Geol. Soc. India 36, 301–312. Chalpathi Rao, N.V., Lehmann, B., 2011. Kimberlites, flood basalts and mantle plumes: new insights from the Deccan large igneous province. Earth-Sci. Rev. 107, 315–324. 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. J. Geol. Soc. India 76, 565–572.

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