1. Introduction
The Proterozoic mobile belts formed during cratonic collisions were pivotal for continental growth. These belts, often identified as elongated zones of deformed and metamorphosed rocks, consist of various components like supracrustal belts, granulite belts, shear zones and fault systems that record multiple episodes of Precambrian geodynamic events (Kröner, Reference Kröner1983). Deciphering their complex deformation histories is an integral part of developing coherent tectonic models. In the Peninsular India, several Archean cratons are amalgamated by Proterozoic mobile belts, such as Aravalli-Delhi Mobile Belt, Central Indian Tectonic Zone, Eastern Ghat Mobile Belt and Singhbhum Mobile Belt (Radhakrishna & Naqvi, Reference Radhakrishna and Naqvi1986; Mandal et al. Reference Mandal, Mitra, Misra and Chakraborty2006; Bhowmik et al. Reference Bhowmik, Chattopadhyay, Gupta and Dasgupta2012b; Saha et al. Reference Saha, Bhowmik, Bose and Sajeev2016; Chetty & Kehelpannala, Reference Chetty and Kehelpannala2022). These mobile belts in Peninsular India are often associated with large sedimentary basins, commonly referred to as Purana Basins. Some of these basins have undergone deformation and low-grade metamorphism (Ramakrishnan & Vaidyanadhan, Reference Ramakrishnan and Vaidyanadhan2010).
Among all these mobile belts in India, the Central Indian Tectonic Zone (CITZ) runs east-west through the middle of the Indian subcontinent (Fig. 1a). The northern boundary of the CITZ is marked by the Son-Narmada North Fault (SNNF), located north of the Mahakoshal Supracrustal Belt (MSB), while the southern boundary is marked by the Central Indian Shear Zone (CISZ) (Fig. 1a). The CITZ formation began around 2.2–2.1 Ga with the initiation of convergence between the Bundelkhand Craton (BKC) and Bastar Craton (BC) and continued until their collision at ∼1.0 Ga, coinciding with the final assembly of the Rodinia (Sarkar et al. Reference Sarkar, Banerjee and Chakrabarty1995; Roy & Hanuma Prasad, Reference Roy and Hanuma Prasad2003; Li et al. Reference Li, Bogdanova, Collins, Davidson, De Waele, Ernst, Fitzsimons, Fuck, Gladkochub, Jacobs, Karlstrom, Lu, Natapov, Pease, Pisarevsky, Thrane and Vernikovsky2008; Bhowmik et al. Reference Bhowmik, Wilde, Bhandari, Pal and Pant2012a; Bhowmik et al. Reference Bhowmik, Chattopadhyay, Gupta and Dasgupta2012b; Bora et al. Reference Bora, Kumar, Yi, Kim and Lee2013; Khanna et al. Reference Khanna, Rao, Bizimis, Satyanarayanan, Krishna and Sai2017; Bhowmik, Reference Bhowmik2019; Chattopadhyay et al. Reference Chattopadhyay, Bhowmik and Roy2020; Bhowmik et al. Reference Bhowmik, Bose, Chattopadhyay, Karmakar and Pant2024). The evolution of CITZ is characterized by complex and multiple events of ductile deformation, greenschist to granulite facies metamorphism and late-stage brittle deformation (Acharyya & Roy, Reference Acharyya and Roy2000; Acharyya, Reference Acharyya2003; Bora et al. Reference Bora, Kumar, Yi, Kim and Lee2013; Chattopadhyay et al. Reference Chattopadhyay, Bhowmik and Roy2020; Deshmukh & Prabhakar, Reference Deshmukh and Prabhakar2020). All these deformation events are linked to the formation and breakup of supercontinents like Columbia and Rodinia (Bhowmik et al. Reference Bhowmik, Wilde, Bhandari, Pal and Pant2012a; Deshmukh et al. Reference Deshmukh, Prabhakar, Bhattacharya and Madhavan2017; Chattopadhyay et al. Reference Chattopadhyay, Bhowmik and Roy2020; Lachhana Dora et al. Reference Lachhana Dora, Meshram, Rao Baswani, Malviya, Upadhyay, Shareef, Atif Raza, Ranjan, Meshram, Kumar Patnaik and Randive2023).
Geological maps of the study area. (a) Central Indian Tectonic Zone and associated tectonic units. Modified after Acharyya & Roy (Reference Acharyya and Roy2000) and Roy et al. (Reference Roy, Hanuma Prasad and Devarajan2002b) (b) Geological map of Son Valley. Field locations, marked by numbered white circles, indicate sites where structural data were collected. The age-data of the stratigraphic formations are sourced from various published literatures, which are cited in the text. The maps are adapted from the quadrangle geological maps published by the Geological Survey of India. The dashed line boxes demarcate the regions shown in the subsequent figures.
The north of the CITZ is characterized by the sedimentary rocks of the Meso-Neoproterozoic Vindhyan Basin, with recent studies suggesting basin closure around 0.77 Ga (Ray, Reference Ray2006; Basu & Bickford, Reference Basu and Bickford2015; Kumari et al. Reference Kumari, Tandon, Tomson and Ghatak2024). The Vindhyan Basin has been variously interpreted as an intracratonic basin (Holland, Reference Holland1906), a foreland basin (Chakrabarti et al. Reference Chakrabarti, Basu and Chakrabarti2007) and a rift basin (Bose et al. Reference Bose, Sarkar, Chakrabarty and Banerjee2001; Bickford et al. Reference Bickford, Mishra, Mueller, Kamenov, Schieber and Basu2017). The Lower Vindhyan Group of the Vindhyan basin was predominantly deposited ca. 1.6 Ga (Ray, Reference Ray2006), and is overlain by a disconformity (Mishra, Reference Mishra2011). Subsequent tectonic activity within the CITZ temporally coincides with renewed sedimentation of the Upper Vindhyan Group in the Vindhyan Basin (Mishra, Reference Mishra2015) and corresponds to the convergence of the BKC and BC during the assembly of Rodinia (Bhowmik et al. 2012). The distinct demarcation between the Lower and Upper Vindhyan Groups, coupled with the intervening unconformity, provides key insights into the tectonic and sedimentary history associated with the CITZ and its broader geodynamic implications.
The Lower Vindhyan Group exhibits a variety of deformation features, including reverse faults, folds, slump folds, convolute beds, contorted cross-beds, flame structures, pinch-and-swell structures, dikes and breccias etc., which have been attributed to soft- and synsedimentary deformation processes (Sarkar et al. Reference Sarkar, Banerjee and Chakrabarty1995; Bose et al. Reference Bose, Banerjee and Sarkar1997; Singh et al. Reference Singh, Mondal, Singh, Mittal, Singh and Kanhaiya2020; Singh & Chakraborty, Reference Singh and Chakraborty2022). The prevailing hypothesis suggests that these structures were primarily induced by seismic tremors during sedimentation (Singh et al. Reference Singh, Mondal, Singh, Mittal, Singh and Kanhaiya2020) indicating a history of earthquake activity around 1.6 Ga in the central India. This period of seismicity indicates that the Vindhyan Basin experienced tectonic instability during this time frame, as evidenced by the presence of soft-sediment deformation structures (Singh et al. Reference Singh, Mondal, Singh, Mittal, Singh and Kanhaiya2020; Singh & Chakraborty, Reference Singh and Chakraborty2022). Unlike the Lower Vindhyan rocks, the Upper Vindhyan sediments remain largely undeformed, with only minor faulting events in the Cenozoic time (Malone et al. Reference Malone, Meert, Banerjee, Pandit, Tamrat, Kamenov, Pradhan and Sohl2008).
Despite evidence of compressional deformation in the Lower Vindhyan Group, its potential link to CITZ tectonics remains largely unexplored. We hypothesize that the Lower Vindhyan Group may contain signatures of late-stage compressional deformation associated with the convergence and collision events of the CITZ. Given the proximity of the CITZ to the Lower Vindhyan Group, it is plausible that these deformation patterns were transmitted northward, where they are better preserved compared to the more extensively altered structures in the older, adjacent MSB, located within the CITZ (Fig. 1a). Thus, these preserved deformation structures in the Lower Vindhyan Group are particularly important to investigate, as they offer a relatively undisturbed record of tectonic processes, compared to the extensively and multiply deformed MSB.
Deformation structures such as folds, reverse faults and fault-associated folding in foreland basins arising from adjacent convergent tectonic orogeny have been extensively studied in various sedimentary basins worldwide (Tavani et al. Reference Tavani, Storti, Lacombe, Corradetti, Muñoz and Mazzoli2015 and references therein). Some of the examples include the Zagros foreland basin in Iran (Koshnaw et al. Reference Koshnaw, Horton, Stockli, Barber, Tamar-Agha and Kendall2017), the Timor foreland basin in northern Australia (Langhi et al. Reference Langhi, Ciftci and Borel2011), the Central Apennines in Italy (Scisciani et al. Reference Scisciani, Calamita, Tavarnelli, Rusciadelli, Ori and Paltrinieri2001) and the Austral foreland basin in the Andes (Carbonell et al. Reference Carbonell, Dimieri and Martinioni2013). While the compressional deformation structures within the CITZ have been previously studied in the context of their geneses (Acharyya, Reference Acharyya2003; Roy & Hanuma Prasad, Reference Roy and Hanuma Prasad2003; Chattopadhyay & Khasdeo, Reference Chattopadhyay and Khasdeo2011; Bhowmik et al. 2012; Deshmukh et al. Reference Deshmukh, Prabhakar, Bhattacharya and Madhavan2017), a gap remains in understanding whether the deformation structures of the Vindhyan Groups have any relationship with the CITZ, particularly the MSB.
In this study, we have addressed this existing gap by systematically investigating the compressional deformation structures within the Lower Vindhyan Group (LVG) and Mahakoshal Supracrustal Belt (MSB) and establish a structural relationship between them. We studied deformation-related structural features, performed detailed geological mapping and constructed geological cross-sections in the Son Valley region covering the MSB and LVG (Fig. 1a, b) to reconstruct the tectonic history of the region. Finally, we integrate our findings with existing literature and propose a tectono-kinematic model that explains the effects of Mesoproterozoic collisional deformation within the CITZ on the Lower Vindhyan Group.
2. Geological setting of the Son Valley
The Son Valley, situated in central India, prominently features exposures of the Lower and Upper Vindhyan Groups (Fig. 1b). The southern portion of the Son Valley contains the northern margin of the Central Indian Tectonic Zone (CITZ), demarcated by the Son Narmada North Fault (SNNF). Immediately south of the SNNF lies the MSB (Fig. 1b). In this section, we will first provide a general geological overview of the MSB and the Vindhyan Basin. Subsequently, in Section 3, we will present the structural characteristics of these two lithological units.
2.a. Overview of Mahakoshal Supracrustal Belt (MSB)
The ENE-WSW-trending Mahakoshal Supracrustal Belt (MSB) extends from Hoshangabad (Madhya Pradesh) in the west to Palamau (Jharkhand) in the east and occupies the northernmost position in the Central Indian Tectonic Zone (CITZ). It is bounded by the Son Narmada South Fault (SNSF) and the Son Narmada North Fault (SNNF) to the south and north, respectively (Fig. 1a). The MSB contains the Mahakoshal Group, which is subdivided into three formations: Agori (or Sleemanabad), Parsoi and Dudhmania (Roy & Devarajan, Reference Roy and Devarajan2000; Sharma et al. Reference Sharma, Das, Chakraborty, Shiraishi and Kayama2022). Phyllite is the dominant rock type throughout these formations (Fig. 1b). Para-schists, micaceous quartzites, intrusive rocks, banded iron formations (BIFs), carbonates and amphibolites are also reported within the MSB (Acharyya, Reference Acharyya2003; Deshmukh et al. Reference Deshmukh, Prabhakar, Bhattacharya and Madhavan2017; Sharma et al. Reference Sharma, Das, Chakraborty, Shiraishi and Kayama2022).
Two prominent theories exist on the origin and tectonic evolution of the MSB: (i) the basin formed in an intracontinental rift setting (Nair et al. Reference Nair, Jain and Yedekar1995), and (ii) it originated in a back-arc rift setting (Roy & Devarajan, Reference Roy and Devarajan2000; Chattopadhyay & Khasdeo, Reference Chattopadhyay and Khasdeo2011). The Mahakoshal basin is proposed to have begun forming after ca. 2.2–2.1 Ga, based on geochronological ages including Rb-Sr dating of basement rocks (Sarkar et al. Reference Sarkar, Banerjee and Chakrabarty1995) and Lu-Hf/Sm-Nd dating of ferropicrites from the Agori (or Sleemanabad) Formation (Khanna et al. Reference Khanna, Rao, Bizimis, Satyanarayanan, Krishna and Sai2017), followed by subsequent sedimentation, multiple phases of deformation, metamorphism and magmatic activity within the MSB between 1.8 and 1.5 Ga (Sarkar et al. Reference Sarkar, Boda, Kundu and Mamgain1998). While the initial chronological ages were established using Rb-Sr whole-rock dating (Sarkar et al. Reference Sarkar, Boda, Kundu and Mamgain1998), recent U–Pb zircon methods provide more reliable ages of these events. Pyroclastic rocks from the Parsoi Formation yielded a crystallization age of 1894.3 ± 9.4 Ma (Sharma et al. Reference Sharma, Das, Chakraborty, Shiraishi and Kayama2022), documenting volcanic activity during the developmental stage of the basin. Similarly, U-Pb zircon SHRIMP dating of the Jhirgadandi pluton in the eastern MSB yielded an age of ca. 1.75 Ga (Bora et al. Reference Bora, Kumar, Yi, Kim and Lee2013), re-confirming basin evolution during this timeframe. These ages also align well with the Paleoproterozoic metamorphic ages (1.9–1.6 Ga) documented from the eastern MSB (Deshmukh et al. Reference Deshmukh, Prabhakar, Bhattacharya and Madhavan2017). Collectively, these geochronological evidences indicate that the MSB experienced multiple magmatic and metamorphic episodes between 1.9 and 1.6 Ga (Nair et al. Reference Nair, Jain and Yedekar1995; Bora et al. Reference Bora, Kumar, Yi, Kim and Lee2013; Deshmukh et al. Reference Deshmukh, Prabhakar, Bhattacharya and Madhavan2017; Sharma et al. Reference Sharma, Das, Chakraborty, Shiraishi and Kayama2022; Sharma et al. Reference Sharma, Chakraborty, Pandey and Das2024), which correlates with the collision of the Northern and Southern Indian blocks during the assembly of the Columbia Supercontinent. During this period, the MSB underwent significant deformation and metamorphism, resulting in the formation of folds, faults and ductile shear zones (Acharyya & Roy, Reference Acharyya and Roy2000; Roy & Devarajan, Reference Roy and Devarajan2000; Roy & Hanuma Prasad, Reference Roy and Hanuma Prasad2003) involving compression, extension and shearing processes. The rocks in MSB were folded with NE-SW-striking and southerly dipping axial planes, associated with reverse-slip ductile shear zone developed along SNSF (Roy & Hanuma Prasad, Reference Roy and Hanuma Prasad2003).
2.b. Overview of Vindhyan Basin
The Vindhyan Basin is one of the most extensive and well-preserved Proterozoic sedimentary basins in the world and belongs to the Purana Basins in India (Fig. 1a). With a distinctive sickle shape, it spans an area of approximately 178,000 square kilometres (Chakraborty, Reference Chakraborty2006). While the southern and southwestern parts of the basin are covered by Deccan flood basalts, around 40,000 square kilometres of the basin lie concealed beneath the Indo-Gangetic alluvium in the north (Chakraborty, Reference Chakraborty2006). The Vindhyan Supergroup is predominantly composed of sedimentary rocks with a stratigraphic division into four major Groups: Semri, Kaimur, Rewa and Bhander. The Lower Vindhyan Group, represented by the Semri Group, forms the older sequence within the basin and is primarily characterized by thick deposits of carbonates, sandstone, shale and volcanogenic sedimentary rocks (Ray, Reference Ray2006). The Upper Vindhyan, comprising the Kaimur, Rewa and Bhander Groups, represents the younger sequences (Prasad, Reference Prasad1984; Basu & Chakrabarti, Reference Basu and Chakrabarti2020; Fig. 1a, b). The rocks in the Upper Vindhyan Groups are predominantly quartzitic sandstone, shale and limestone, deposited in an environment ranging from shallow marine to fluvial, with evidence of storm-generated structures (Chakraborty & Bose, Reference Chakraborty and Bose1990; Singh & Chakraborty, Reference Singh and Chakraborty2022). Major division in the Vindhyan Supergroup is marked by a regional unconformity at the base of the Kaimur Group, which separates the Lower and Upper Vindhyan Groups (Ray, Reference Ray2006; Mandal et al. Reference Mandal, Choudhuri, Mondal, Sarkar, Chakraborty and Banerjee2019)
U-Pb zircon geochronology, for both volcanic ash beds and detrital zircons within the Vindhyan Groups, yielded ages spanning from 1.64 to 0.77 Ga (Bickford et al. Reference Bickford, Mishra, Mueller, Kamenov, Schieber and Basu2017; Kumari et al. Reference Kumari, Tandon, Tomson and Ghatak2024). The sedimentation age for the Lower Vindhyan is ∼1.6 Ga (Ray Reference Ray2006), based on the U-Pb zircon dating ages of 1642 Ma for Rhyolite (Bickford et al. Reference Bickford, Mishra, Mueller, Kamenov, Schieber and Basu2017), 1628–1631 Ma for the Porcellanite Formation (Rasmussen et al. Reference Rasmussen, Bose, Sarkar, Banerjee, Fletcher and Mcnaughton2002; Ray et al. Reference Ray, Martin, Veizer and Bowring2002) and 1599–1602 Ma for the overlying Rampur Shale (Rasmussen et al. Reference Rasmussen, Bose, Sarkar, Banerjee, Fletcher and Mcnaughton2002). The age of the Upper Vindhyan subgroup remains more contentious. The sedimentation of Upper Vindhyan began around 1.2 Ga (based on Re-Os dating of black shales by Tripathy & Singh (Reference Tripathy and Singh2015). The Pb-Pb dating ages of carbonates from the Bhander Formation suggest basin closure around 900 Ma (Gopalan et al. Reference Gopalan, Kumar, Kumar and Vijayagopal2013). However, recent U-Pb detrital zircon dating by Kumari et al. (Reference Kumari, Tandon, Tomson and Ghatak2024) suggest an age of 770 ± 12 Ma, indicating that the uppermost Bhander Group may extend into the Neoproterozoic. Alternative dating methods using Sr, C and O isotopes have suggested even younger ages (700–570 Ma) for the Bhander Group (Kumar et al. Reference Kumar, Das Sharma, Sreenivas, Dayal, Rao, Dubey and Chawla2002), but these ages carry substantially higher uncertainties.
Several hypotheses have been proposed to explain the genesis of the Vindhyan Basin and the deposition of the Lower Vindhyan sediments (Chakrabarti et al. Reference Chakrabarti, Basu and Chakrabarti2007; Bickford et al. Reference Bickford, Mishra, Mueller, Kamenov, Schieber and Basu2017). Chakrabarti et al. (Reference Chakrabarti, Basu and Chakrabarti2007) suggest that the basin formed along the edge of the Bundelkhand Craton with sedimentary contributions from a convergent margin setting to the south during the assembly of the Columbia supercontinent. Their geochemical analysis of the porcellanite within the Semri Group indicates the incorporation of rhyolitic melt and ash derived from subduction-related volcanism (Chakrabarti et al. Reference Chakrabarti, Basu and Chakrabarti2007). They proposed a foreland basin model, where the Bundelkhand Craton subducted in a southerly direction, and the basin formed against the uplifted Mahakoshal Mobile Belt (Chakrabarti et al. Reference Chakrabarti, Basu and Chakrabarti2007). Bickford et al. (Reference Bickford, Mishra, Mueller, Kamenov, Schieber and Basu2017) proposed that collisions between the Bundelkhand and Bastar cratons during the formation of the Columbia supercontinent resulted in far-field extension within the Bundelkhand Craton, leading to the formation of the Vindhyan Basin and deposition of sediments of Semri Group. The formation and evolution of the Vindhyan Basin, therefore, are complex, and multiple events, as mentioned above, either individually or in combination, may have contributed to its development.
3. Deformation in the Mahakoshal Supracrustal Belt (MSB) and Lower Vindhyan group
To decipher the regional deformation patterns, we conducted field investigations along various approximately N-S transects (Figs. 1b, 2a, 2b) within the MSB and Lower Vindhyan Group. Below, we present detailed descriptions of the deformation structures observed in the MSB, followed by those in the Lower Vindhyan rocks.
Structural deformation patterns of the Mahakoshal Supracrustal Belt (MSB). (a and b) Structural maps of the MSB in the Son Valley. Note that the S1 foliation is co-axially folded with the S0 near Renukoot in (b), producing Type-3 fold interference pattern, whereas near Churki, Type-2 fold superposition of the S1 and the S2 produced the S3 foliation. (c) Stereographic projections showing poles to S0, S1 and S2 beddings/foliations, and F1 and F2 fold axes. The stereoplots are generated using Fabrica and MTEX v5.8.0 in Matlab 2020a. The bedding pole data are contoured to multiples of uniform density (m.u.d). The numbers in brackets below each stereonet are the respective data counts. (d) Steeply dipping S1 associated with folded S0 in phyllite, (e) folded S0 giving rise to steep S1 foliation, (f) S0 and pervasive S1 in phyllites, (g) Striping lineation on the S1 foliation plane in phyllite. (h) Parallel stretching lineation with pinch-and-swell structures and quartz boudins on a sub-horizonal S1 exposure. The field locations of the photographs are marked at the bottom right corners (refer to the map in Figure 1b).
3.a. Structural Elements in the Mahakoshal Supracrustal Belt (MSB)
Field investigations in two regions of MSB (Fig. 2a, b) revealed three distinct sets of planar fabrics in the exposed rocks, (i) S0 (primary sedimentary bedding planes), (ii) S1 (1st generation continuous axial planar foliation) and (iii) S2 (2nd generation axial planar spaced cleavage). Among these, S0 and S1 are the most prominent features (Figs. 2a-h, 3a-h). Additionally, sporadic occurrences of S3 (the 3rd generation axial planar spaced cleavage) were noted in some locations within the study area. Lineations are present as interactions between the planar fabrics, together with striping lineation, stretching lineation (Fig. 2g, h), and kink and crenulation axes (Fig. 3c, d). The overprinting relationships between these planar and linear elements revealed three deformation episodes within the MSB as presented below.
Deformation structures in the Mahakoshal Supracrustal Belt (MSB). Reclined (a) and upright to inclined (b) F2 folds in phyllites. In (a) the synform is highlighted with blue colour. (c) E-W-trending asymmetric kink bands (D2) associated with north-south-trending crenulation lineation (D3) in phyllites developed on S1. (d) NE-SW and NW-SE-trending conjugate kink bands (D3, yellow arrowheads) and east-west-trending kink bands (D2, blue arrowheads) in phyllites developed on S1. (e-g) Rootless early folds, sigmoidal quartz clast, winged inclusion and synthetic C’ bands indicating sinistral shear sense in a ductile shear zone at location S38. (h) D3 deformation phase characterized by N-S-trending orogen-transverse spaced cleavages.
3.a.1. Deformation episode 1 (D1)
We found intricate folding patterns within the Agori, Parsoi and Dudhmania formations, both at outcrop and map scales (Figs. 2, 3). The overall southerly dipping S0 (Fig. 2a-c) were folded (F1) to produce the pervasive S1 in both the phyllites and quartzites. (Fig. 2d-f). F1 have diverse orientations, as they were further deformed by the D2 and D3 deformation episodes (Fig. 2c). In the eastern part of Figure 2a, the map-scale folding of the S0 fabric gave rise to the F1 folds and S1 (Fig. 2a, c). The S1 are steep, mostly southerly dipping (Fig. 2c), and stripping lineation on S1 is prominent (Fig. 2g). The S1 is also characterized by stretching lineation (Fig. 2h). Pinch-and-swell structures are also observed parallel to stretching lineation on S1 (Fig. 2h).
3.a.2. Deformation episode 2 (D2)
The D2 episode is the folding of S0 and S1 that produced distinctive approximately E-W-trending antiforms and synforms (F2) (Fig. 2a, b). The S2 axial planes of these F2 folds trend E-W (Fig. 2a-c). The F2, exposed in multiple orders at the southern part of the Parsoi formation, are mostly upright to inclined and variably plunging (Fig. 3a, b). The D2 deformation also produced asymmetric kink bands on the S1 surfaces throughout the MSB, particularly on the mica-rich planes (Fig. 3c, d). Nevertheless, some outcrops exhibit conjugate kink bands trending NE-SW and NW-SE (Fig. 3d). The axial planes of these kink bands produced by D2 deformation are mostly E-W trending, parallel to F2 (Fig. 3c, d). The asymmetric kink bands define a top to the north/north-west sense of shear movement. D2 deformation is also characterized by E-W-trending crenulation lineation on S1 surfaces (Fig. 2c).
The deformation in the southern part of the MSB is intensified by the SNSF shear zone (locations S36, S38, S52; Fig. 1b), which is synchronous with the D2. Intercalated garnetiferous phyllite and quartz-rich layers of MSB are mylonitic here (Fig. 3e-g) and separate the MSB from the CGGC (location S38; Fig. 1b). The early F1 folds appear as intrafolial folds, with their axial planes locally parallel to the mylonitic foliation (D2) of the SNSF (location S38; Fig. 3f, g). These F1 are mostly east-west-trending, plunging, isoclinal (Fig. 3f) folds. The mylonitic lineation is also parallel to the hinges of the F2 folds. Top to the west sinistral shear sense indicators, manifested by asymmetric folds, winged inclusion, sigmoids and C’ shear bands, are visible in this area (Fig. 3e-g).
3.a.3. Deformation episode 3 (D3)
The third phase of ductile deformation (D3) is weak and occurs locally. D3 is characterized by the non-coaxial refolding of F2 and the formation of N-S-trending orogen-transverse deformation structures (S3) on outcrop to terrain scale (Fig. 2b). The D3 deformation develops N-S-trending orogen-transverse crenulation lineation (Fig. 3c) and spaced cleavages (Fig. 3h).
3.b. Structural elements in the Lower Vindhyan group
In the Lower Vindhyan Group, we made three traverses along the Son Valley (i) Patauha to Sidhi, (ii) Churhat to Sidhi and (iii) Silwar-Bahri-Singrauli, and examined the deformation-related structures in the Chopan Porcellanite and Chorhat Sandstone (Fig. 1b). The rest of the Lower Vindhyan rocks along our traverse, e.g. Rohtasgarh Limestone, Rampur Shale, Kheinjua Shale, Arangi Shale and Deoland Sandstone, are either poorly exposed or soil-covered. The exposures of these rocks that we observed within our study area show no deformation signatures. In these three traverses, we have generated the litho-structural cross-sections with the collected structural data. The cross-sections are prepared following a systematic approach that includes measuring the exposure length, collection of precise structural data at close intervals and careful characterization of geologic features such as folding, faulting, etc. in the sedimentary layers.
3.b.1. Patauha-Sidhi traverse
In this N-S traverse, the best exposure was found at the north of the Son River, which exposes the Chorhat Sandstone of Kheinjua Formation (location S01, Fig. 1b). The outcrop extends for nearly 70 m along a north-south-trending road-section (Fig. 4a) and exhibits sandstone-shale intercalations. The transect consists of symmetric upright (Fig. 4b) to asymmetric inclined folds (Fig. 4c) with sub-horizontal fold axes and E-W axial planes (Fig. 4d). Asymmetric folds are abundant in this outcrop. The thinner shale layers exhibit higher orders of folding within sandstone layers and are more undulated than those of sandstone (Fig. 4c, e). The cross-section and the overall stereoplot in this area (Cross Section I, Fig. 4f) reveal E-W-striking fold limbs are moderately dipping with sub-horizontal fold axes. The axial planes dip northerly in the southern part of the outcrop and progressively become sub-vertical and then dip southerly towards the northern part (Fig. 4f).
(a) Geological map and structural data from the Patauha-Sidhi traverse. The numbers in the stereonet legend are the respective data counts. (b-e) Folded sandstone-shale interbedded sequence observed in Location S01. (f) Structural cross-section at location S01 highlighting the folds in the sedimentary strata of Chorhat Sandstone. Asymmetric folds are more pronounced than their symmetric counterparts. (g) Geological map and structural data from the Churhat-Sidhi traverse. The number in the stereonet legend is the data count. (h) A large, open and symmetric antiform in the porcellanite-shale interbedded sequence. The fold axis is sub-horizontal and trends nearly ENE-WSW. (i) Thinner shale layers separating the thicker porcellanite layers in the northerly dipping fold limb. (j) A steep normal fault exhibiting an apparent displacement of ∼50 cm. (k) A close-up view of the porcellanite-shale alterations. The porcellanite layers contain two perpendicular fracture sets that are orthogonal to the litho-contact. (l) Structural cross-section at location S03 illustrates the folded geometry of the Chopan Porcellanite and the steeply dipping normal fault in (j). The accompanying stereoplot with cross-sections illustrates the orientation of bedding planes (black girdles), fault planes (red dashed girdle) and fold axis (red triangles). The field locations of the photographs are marked at the bottom right corners (refer to the map in Figure 1b).
3.b.2. Churhat-Sidhi traverse
This NW-SE-trending traverse focused on the ‘non-foliated’ Chopan Porcellanite formation exposed to the SE of the River (location S03; Figs. 1b, 4g). Here, we examined road-cut exposures approximately 8 m thick and 170 m long, featuring intercalated porcellanite and shale layers (Fig. 4h-k). The cross-section illustrates a gently folded structure and steeply dipping normal fault that cross-cuts the beds (Cross Section II, Fig. 4l). The fold limbs dip moderately towards the northwest and southeast, and the easterly trending fold axis is nearly horizontal (Fig. 4l). The apparent displacement along a fault affecting the Porcellanite beds is about 50 cm (Fig. 4j).
3.b.3. Silwar-Bahri-Singrauli traverse
The road-cut sections along the Silwar-Bahri-Singrauli traverse preserve several exposures of the Chopan Porcellanite Formation (Fig. 5a-o). We closely investigated the deformation of the porcellanite-shale sequences, which are better exposed here than in the Churhat-Sidhi traverse. We prepared three cross-sections along this traverse. The first one is on either side of the Son River, between the Silwar and Bahri (Cross Section III, Fig. 6a). The other two are about 20 km SE from the first one and between the Bahri and Singrauli (Cross Sections IV and V, Figs 6b, c).
(a) Geological map and structural data from the Silwar-Bahri-Singrauli Traverse. The numbers in the stereonet legend are the respective data counts. (b) An outcrop-scale synform. (c) Asymmetric fold. The southern limbs are longer and dip gently whereas the northern limbs dip steeply and are shorter. Note the Class 1B (blue) and Class 3 (red) folds in porcellanite and shale layers, respectively. (d, e) Broad hinge and Chevron folds. (f) Gently north-dipping porcellanite beds containing down-dip slickenlines (inset). (g) A recumbent fold with E-W-trending fold axis. (h) A tight antiform in the Porcellanite Formation. The axial plane dips steeply towards southeast. The down-dip slickenlines (inset) near the hinge zone of the northwesterly dipping limb suggest flexural slip folding. Also note the Class 1B (blue) and Class 3 (red) folds. (i) Southerly dipping axial planes within the Porcellanite Formation. (j) Prominent kink-fold structure generated by a series of parallel, northerly dipping axial planes. Deformation features in the Lower Vindhyan Group at locations S72 and S73B. (k) Upright synform in the Porcellanite plunging sub-horizontally towards southwest. The axial plane trends in an east-west direction while dipping toward the north. (l) North-dipping reverse fault truncating the sub-horizontal porcellanite layers against steeply inclined porcellanite layers. (m, n) Kink-folds in the deformed porcellanite layers. (o) Pop-ups and low-angle faults in the Porcellanite Formation. Note that these structures terminate at the upper extent of sub-horizontal, undeformed sedimentary layers, which implies the persistence of north-south compression during the deposition of the Porcellanite Formation. The field locations of the photographs are marked at the bottom right corners (refer to the map in Figure 1b).
(a) A 110 m long cross-section from S76 illustrating the fault-bend folds and kink-folds, suggesting folding associated with a shallow detachment. (b) Structural cross-section at S73 illustrating the pop-up structure that resulted from compressional deformation. (c) Structural cross-section illustrating the observed folds, faults and pop-up structures in the Porcellanite Formation at location S73B. (d) The regional cross-section across the Semri Group and the MSB (red line AB of Figure 5a). The red curve demarcates the present-day topographic elevation. The black pins indicate the dip amount measurements of the bedding planes of the litho-units in which they occur. The locations of the pins correspond to the location of measurement in the field. Deformation structures are observed in the Chopan Porcellanite, Kheinjua Shale and Chorhat Sandstone. The underlying Kajrahat Formation (Kajrahat limestone and Arangi shale), however, is undeformed and acts as a detachment layer. The MSB is thrusted over the Lower Vindhyan Group by the southwesterly dipping SNNF (Ghosh & Singh, Reference Ghosh and Singh2013). Conspicuous large-scale detachment folding and pop-up structures in the Porcellanite Formation support presence of a shallow detachment layer within the Lower Vindhyan Group.
3.b.3.a. Section between Silwar and Bahri
We examined three outcrops along this traverse. Two of them, flanking the Son River, display folded porcellanite and shale layers. The third outcrop, located on the southern bank of the river, lacks shale layers.
At location S74 (Figs. 1b, 5a), the 2–6 m-thick and 110-m-long outcrop comprises a layered sequence composed of porcellanite with intermittent shale. The thickness of individual porcellanite layers ranges from 10 to 50 cm. This outcrop preserves outcrop-scale synform and antiform with steep axial planes (Fig. 5b-e). At S74, we observed both chevron/kink -folds (Fig. 5c, e) and folds with broad hinges (Fig. 5d). The axial planes of the kink folds dip steeply to the south or are subvertical, whereas the axial planes of the broad-hinge folds dip steeply both to the north and south. On the southern riverbank (Fig. 5f), the Porcellanite slope gently northward. Some bedding surfaces preserve slickenlines parallel to their dip directions (Fig. 5f, inset). This section preserves both recumbent E-W-trending folds (Fig. 5g) and a tight antiform with a steeply southward-dipping axial plane (Fig. 5h) was observed, featuring down-dip slickenlines on fold limbs (Fig. 5h, inset). In many of these folds, the competence contrast between the relatively weaker shale and stronger porcellanite layers developed Class 3 and Class 1B folds, respectively (Fig. 5c, h).
3.b.3.b. Sections between Bahri and Singrauli
The Chopan Porcellanite overlies the Mahakoshal rocks along this traverse (Fig. 5a). We selected the two best-exposed outcrops (locations S72 and S73) to study the deformation structures in this area. Location S73 along the Bahri-Singrauli road, comprises a layered sequence of porcellanite and intermittent shale. These exposures exhibit pop-up-like structures with trains of ramp-flat-like kink-folds, where the axial planes are dipping towards both north and south (Fig. 5i, j).
At location S72, about 200 meters south of location S73 and situated on the other side of the road, a large upright synform in the Porcellanite layers was observed (Fig. 5k). Along the same road-cut section, at location S73B, a north-dipping reverse fault was identified, where sub-horizontal Porcellanite layers were truncated by steeply dipping layers (Fig. 5l). At this location, Porcellanite layers show outcrop scale kink-folds (Fig. 5m, n). The most spectacular feature at this location is a large-scale, about 45-metre-long pop-up-like structure with low-angle faults and kinks, which terminated at the top of sub-horizontal undeformed sediment layers (Fig. 5o).
3.c. Geological cross-section along Silwar-Bahri-Singrauli traverse
We prepared geological cross-sections with the structural data (Fig. 6a-c) along the three road-cut sections, described under section 3.b.3. The cross-sections reveal asymmetric kink-folds with sub-horizontal fold axes. Overall, they represent pop-up structures and axial planes dipping in both north and south. Minor reverse faults are noticed in Figure 6c.
We created a 20-km-long geological cross-section from Silwar to Singrauli, capturing Lower Vindhyan Group deformation (Fig. 6d). Specific points (black pins in Figure 6d) were measured for structural parameters and incorporated into the cross-section. A NW-SE reference line (AB, Fig. 5a) between Silwar and Singrauli was aligned with a DEM (ASTER Global Digital Elevation Model) for topographic elevation accuracy. Structural data, including strike, dip and axial planes, were projected onto the cross-section. We used average thickness of the units reported in previous studies (Jain et al. Reference Jain, Banerjee and Kale2020; Singh & Chakraborty, Reference Singh and Chakraborty2022), and observed outcrop-scale structures (e.g., box folds, kink-folds and faults) were included. Notably, outcrop-scale structures such as box folds, kink-folds and faults (as detailed in Section 3.b.3) were integrated into the construction of the cross-section
The prepared geological cross-section, spanning across the boundary between the Lower Vindhyan Group and the MSB, explains the genesis of observed outcrop-scale deformation structures within the study area. The cross-section demonstrates folded Chopan Porcellanite and Kheinjua Formations, lying above relatively undeformed Kajrahat limestone and Arangi shale of the Deoland Formation. The details of the deformation structure and deformation mechanism of cross-section are discussed in the subsequent Section 4.c.
4. Discussion
4.a. Deformation of MSB and its relationship with the CITZ
The deformation structures observed in the MSB provide crucial insights into the tectonic movements that have shaped the tectonics of the Central Indian region. Our field investigations indicate that the MSB has experienced three episodes of deformation events, which align with the findings from previous studies (Roy & Devarajan, Reference Roy and Devarajan2000; Deshmukh et al. Reference Deshmukh, Prabhakar, Bhattacharya and Madhavan2017).
The D1 deformation phase in the MSB is characterized by E-W-trending F1 folds, S1 foliation and striping lineation on S1 surfaces (Fig. 2g). These structural features developed through the deformation of the primary bedding planes (S0). The orientation of these structures, specifically E-W-trending steeply southerly dipping regional S1 foliation (Fig. 2c), suggests they formed during N-S-directed compression associated with the early stages of continental convergence. The parallel alignment of stretching and striping lineation on the S1 foliation suggests an extensional regime oriented along the axes of the F1 folds (Fig. 2g, h). This extension is evidenced by E-W-trending pinch-and-swell structures developed within quartz veins observed on sub-horizontal S1 surfaces (Fig. 2h), suggesting an extension parallel to the E-W-trending F1 fold axes.
The D2 deformation is characterized by the folding of F1 and S1, and the development of a large-scale ductile shear zone at the southern part of MSB along SNSF, resulting in E-W-trending mylonitic foliation (Roy & Hanuma Prasad, Reference Roy and Hanuma Prasad2003; Deshmukh et al. Reference Deshmukh, Prabhakar, Bhattacharya and Madhavan2017; Deshmukh et al. Reference Deshmukh, Prabhakar and Bhattacharya2021). The diverse orientation of F1 folds, as seen in stereographic projections (Fig. 2c), indicates that the earlier F1 folds are further deformed and modified by D2 deformation, resulting in the development of F2 folds.
At location S82 near Renukoot, in close proximity to the SNSF, the granite gneiss exhibits mylonitic foliation. In certain areas, these are near sub-vertical and steeply dipping towards the south. Furthermore, these mylonitic foliations, with the similar orientation of S2, have been locally transposed by progressive shearing. For example, in Figures 3e-g, the late-stage folds nucleated by deforming earlier structures (tight isoclinal folds developed in the early stage of shear deformation) and mylonitic foliation, which develop newly formed mylonitic foliation overprinting the earlier one. The diverse orientations of F2 fold axes within the mylonite further suggest rotation of F2 folds during progressive shearing and/or subsequent deformation events (Fig. 2c). The interaction between D1 and D2 deformation produced map-scale Type-3 (coaxial) interference pattern near Renukoot through progressive buckling of the Mahakoshal metasedimentary sequence (Fig. 2b).
The late-stage D3 deformation is identified by the sporadic occurrence of N-S-trending orogen-transverse cleavage and crenulation lineation in phyllites (Fig. 3h). During this deformation episode, S0, S1 and S2 foliations were reoriented in some places, leading to the refolding of F1, F2 folds and S1, S2 foliations (Fig. 2c). For example, the D3 deformation localized near the Churki area, deformed S2 foliations, gave rise to map-scale Type-2 fold interference patterns (Fig. 2b). D3 deformation also signifies the development of late-stage orogen-transverse folds (or cross folds) in the MSB, which is similar to those reported from the Himalayan orogeny (McQuarrie et al. Reference Mcquarrie, Robinson, Long, Tobgay, Grujic, Gehrels and Ducea2008; Bose et al., Reference Bose, Mandal, Acharyya, Ghosh and Saha2014a).
As we progress southwards of SNNF, mylonitic foliations and oriented augen gneisses of the Chottanagpur Granite Gneiss Complex (CGGC) become more prominent. These structures indicate intense deformation and metamorphism within the CGGC and Southern MSB, signifying an escalation in deformation intensity compared to the northern MSB. Approximately at 1.8 Ga, granitic intrusions occurred along the SNSF, resulting in elevated pressure and temperature conditions in the southern part of the MSB and the CGGC (Roy et al. Reference Roy, Devarajan and Hanuma Prasad2002a; Roy et al. Reference Roy, Hanuma Prasad and Devarajan2002b). These intrusive events contributed to the formation of augen gneisses and biotite schists, resulting in intense deformation and metamorphism towards the southern side of SNNF.
4.b. Tectonic deformation in the Lower Vindhyan Group
Proterozoic sedimentary basins worldwide have developed through various mechanisms, including cratonic extension (e.g. Gwalior Basin, Chakraborty et al. Reference Chakraborty, Tandon, Roy, Saha and Paul2020; Shrivastava et al. Reference Shrivastava, Raza, Saha, Yi, Nasipuri and Pati2023) or along intra-/inter-cratonic tectonic lineaments (e.g. Cuddapah Basin, Saha & Tripathy, Reference Saha and Tripathy2012; Chhattisgarh Basin, George & Ray, Reference George and Ray2021; Pranhita-Godavari Basin, Amarasinghe et al. Reference Amarasinghe, Chaudhuri, Collins, Deb and Patranabis-Deb2015), and as intracratonic sag basins (e.g. Michigan Basin, Howell and van der Pluijm, 1999). In the following paragraphs, we discuss the deformation structures in the Lower Vindhyan sedimentary rocks, which we interpret as resulting from compressional tectonics affecting the Son Valley region.
Structures such as syn-depositional reverse faults, kink-folds and asymmetric folds typically form and persist at collisional plate margins (Bonini et al. Reference Bonini, Sani and Antonielli2012; Casas-Sainz et al. Reference Casas-Sainz, Cortés and Maestro2002). In the Chorhat Sandstone, both symmetric and asymmetric folds are observed (Fig. 4b-e), with a sub-horizontal, E-W-trending fold axis aligning with the regional Lower Vindhyan fold axis (Fig. 4a). Mechanical anisotropy in interbedded thin shale and thick sandstone layers has caused buckle folding (Fig. 4e). Similarly, the slickenlines on the exposed limbs of the folds suggest flexural-slip folding.
These buckle and flexural-slip folds indicate ductile deformation from layer-parallel shortening within anisotropic units over geologic timescales (Ramsay et al. Reference Ramsay, Huber and Lisle1983; Ghosh, Reference Ghosh2013; Misra and Burg, Reference Misra and Burg2012). At S74, Class 3 folds in shale are closely associated with Class-1B (parallel) folds in porcellanite. Here, the competent porcellanite forms Class 1B folds with minimal thickening, while the less competent shale shows Class 3 folds with greater thickening at the hinge (Ramsay et al. Reference Ramsay, Huber and Lisle1983). The consistent E-W trend of fold axes throughout the Lower Vindhyan Group suggests formation under sustained N-S compression, ruling out seismic liquefaction as a formation mechanism as suggested earlier (Singh et al. Reference Singh, Mondal, Singh, Mittal, Singh and Kanhaiya2020; Singh & Chakraborty, Reference Singh and Chakraborty2022). Furthermore, the porcellanite-bearing Lower Vindhyan Group is often characterized by multiple occurrences of two distinct kink-folds, oriented in opposite directions, developing pop-up structures in some places (Figs. 5d, 5m-o). This kind of pop-up structures were explicitly demonstrated by sandbox experiments with sub-horizontal decollement to explain the structural evolution of fold-and-thrust belts (Dahlen, Reference Dahlen1990). The cross-sections at location 73 (Fig. 6b) show that the porcellanite layers are deformed by one set of southerly dipping kink-folds and sequential development of northerly dipping multiple kink-folds. Although significantly smaller in scale, these kink-folds display geometric patterns similar to bi-vergent deformation structures observed in analogue experiments for simulating fold-and-thrust belts (McClay et al. Reference Mcclay, Whitehouse, Dooley and Richards2004; Bose et al. Reference Bose, Mandal, Saha, Sarkar and Lithgow-Bertelloni2014b). Fold-and-thrust belts are characterized by fault-related folds, which include detachment folds, fault-propagation folds and fault-bend folds, forming broad anticlinal structures due to horizontal shortening (Cosgrove, Reference Cosgrove2015; Butler et al. Reference Butler, Bond, Cooper and Watkins2020). A broad box-fold-like anticline was observed at location S74 (Fig. 6a), suggesting its formation due to fault-related folds. However, the precise identification of underlying faults below the sedimentary sequence remains challenging in our field area. From the geometry of folds and the undeformed nature of lower-layered rocks, it is plausible that these folds are developed through detachment folding. In Section 4.c, we discuss and describe the regional cross-section and explore the possible detachment layer in the Lower Vindhyan Group.
4.c. Deformation in the Son Valley from a geological cross-section
The field-data-based regional cross-section, presented in Figure 6d, illustrates the presence of large-scale folds both in the Lower Vindhyan Group and MSB. These folds are characterized by E-W-trending wide anticlines and narrow synclines, suggesting a N-S compressional stress regime during the deformational history of the Son Valley area.
Throughout our study area in the Son Valley Lower Vindhyan Group, we observed that folding and other deformation features are confined in the Chopan Porcellanite Formation (Porcellanite) and the overlying Kheinjua Formation (Kheinjua shale and Chorhat sandstone). However, the Kajrahat Formation, consisting of Kajrahat limestone and Arangi shale, underlying the Chopan Porcellanite Formation remains undeformed. Additionally, the folds here have box fold geometries characterized by short and steep limbs (Figs. 5o, 6a). The cross-sections along the Silwar and Bahri transects demonstrate pop-up structures (bounded by two oppositely dipping kink-folds) within the Porcellanite Formation, which overlies the undeformed limestone/shale of the Kajrahat Formation.
Analogue experiments using weak, viscous decollement demonstrate that the fold-and-thrust belts with weak detachment at the base, develop symmetrical sequential folded structures, similar to the pop-up structures observed in our study area (Costa & Vendeville, Reference Costa and Vendeville2002; Smit et al. Reference Smit, Brun and Sokoutis2003; Figs. 5d, 5o, 6a-c). The sequential development of the open box folds within some specific layers in the Lower Vindhyan indicates thin-skinned tectonic deformation, as suggested by earlier studies in fold-and-thrust belts (Gwinn, Reference Gwinn1964; Mitra, Reference Mitra2003). In fold-and-thrust belts, weak lithologies (e.g. salt, evaporites etc.) define the detachment zone (or layer) that facilitates detachment-style folding (Buxtorf, Reference Buxtorf1916; Jamison, Reference Jamison1987; Suppe, Reference Suppe2011; Butler et al. Reference Butler, Bond, Cooper and Watkins2018, Reference Butler, Maniscalco and Pinter2019). Similarly, limestone detachments have been documented in fold-and-thrust belts where carbonate layers facilitate strain partitioning (Vergés et al. Reference Vergés, Saura, Casciello, Fernandez, Villasenor, Jimenez-Munt and Garcia-Castellanos2011a; Vergés et al. Reference Vergés, Goodarzi, Emami, Karpuz, Efstathiou and Gillespie2011b). We propose that the Kajrahat formation in the Lower Vindhyan acted as a detachment layer during the large-scale detachment folding in the Lower Vindhyan Group due to N-S convergence in the CITZ (Fig. 6d).
These folds in the Lower Vindhyan Group are characterized by E-W-trending broad anticlines and relatively narrow synclines, suggesting a N-S compressional stress regime during the deformational history of the Son Valley area. Interestingly, the orientations of the F2 fold axes in the MSB are similar to those in the Lower Vindhyan Group, both trending E-W (Figs. 2c, 6a-c). However, the cross folds produced during the D3 deformation in the MSB are absent in the Lower Vindhyan Group. Our structural analysis indicates that the deformation in the Lower Vindhyan Group occurred at shallow crustal levels, whereas D2 deformation in the MSB took place under lower amphibolite facies conditions (P∼8 kbar, T∼520°C) at approximately 15 km depth (Deshmukh et al. Reference Deshmukh, Prabhakar, Bhattacharya and Madhavan2017). Therefore, despite the structural similarities between the folds in the Lower Vindhyan Group and those associated with D2 deformation in the MSB, their formation likely occurred at different times. Integrating structural observations with the available chronological data, we propose that deformation in the Lower Vindhyan Group postdates the D3 deformation event in the MSB but precedes the deposition of the Upper Vindhyan Group (1210 ± 52 Ma; Tripathy and Singh, Reference Tripathy and Singh2015).
4.d. Kinematic model for structural evolution of the Lower Vindhyan Group
Several tectonic models have been proposed to explain the complex evolutionary history of CITZ. Based on field, geochronological and geophysical studies; a group of researchers postulated that the evolution of the CITZ resulted from south-directed subduction, with the North Indian Craton subducting beneath the South Indian Craton (Yedekar et al. Reference Yedekar, Jain, Nair and Dutta1990; Acharyya, Reference Acharyya2003; Chakrabarti et al. Reference Chakrabarti, Basu and Chakrabarti2007). Conversely, some other studies proposed a north-directed subduction model (Roy & Hanuma Prasad, Reference Roy and Hanuma Prasad2003; Bhowmik & Chakraborty, Reference Bhowmik and Chakraborty2017; Chattopadhyay et al. Reference Chattopadhyay, Chatterjee, Das and Sarkar2017; Karim et al. Reference Karim, Low, Tripathi and Prasad2024). To resolve these contrasting models, some researchers proposed a subduction polarity reversal model (Bhowmik et al., Reference Bhowmik, Bose, Chattopadhyay, Karmakar and Pant2024; Deshmukh & Prabhakar, Reference Deshmukh and Prabhakar2020). In a single polarity reversal model, an initially south-directed subduction zone later transitioned to a north-directed configuration (Deshmukh and Prabhakar, Reference Deshmukh and Prabhakar2020). Bhowmik et al. (Reference Bhowmik, Bose, Chattopadhyay, Karmakar and Pant2024) proposed a model involving multiple episodes of polarity reversal within the CITZ.
Among all these, the northward subduction model accounts for the initiation of convergent tectonics, the arc-related magmatism and volcanism on Bundelkhand Craton, as well as the formation of the MSB in a back-arc setting between 2.2 and 1.8 Ga (Chattopadhyay et al. Reference Chattopadhyay, Bhowmik and Roy2020). However, the northward subduction model alone does not give an explanation for all deformation structures observed in the Lower Vindhyan Group and the evolution of CITZ.
The southward-directed subduction model attributes the formation of the Vindhyan foreland basin to the convergence between the Bundelkhand and Bastar Cratons (Chakrabarti et al. Reference Chakrabarti, Basu and Chakrabarti2007). These southward subduction models also provide a coherent explanation for the development of compressional structures within the LVG and foreland basin characteristics. However, the southward subduction model falls short of explaining the initiation of arc magmatism and the opening of the Mahakoshal back-arc basin, as arc magmatism is more consistent with a northward subduction model.
Recent studies have challenged the previously proposed model of initial southward subduction followed by northward subduction polarity (Lachhana Dora et al. Reference Lachhana Dora, Meshram, Rao Baswani, Malviya, Upadhyay, Shareef, Atif Raza, Ranjan, Meshram, Kumar Patnaik and Randive2023; Bhowmik et al. Reference Bhowmik, Bose, Chattopadhyay, Karmakar and Pant2024; Karim et al. Reference Karim, Low, Tripathi and Prasad2024). Lachhana Dora et al. (Reference Lachhana Dora, Meshram, Rao Baswani, Malviya, Upadhyay, Shareef, Atif Raza, Ranjan, Meshram, Kumar Patnaik and Randive2023) proposed a northward-directed subduction by the initiation of convergent tectonics in the region around 2.2 Ga, marked by arc-related magmatism and volcanism in the Bundelkhand Craton. Pillow lavas dated to ∼2.05 Ga in the Betul Belt and back-arc bimodal volcanism around 1.7 Ga further support this northward subduction scenario (Lachhana Dora et al. Reference Lachhana Dora, Meshram, Rao Baswani, Malviya, Upadhyay, Shareef, Atif Raza, Ranjan, Meshram, Kumar Patnaik and Randive2023). More recently, Karim et al. (Reference Karim, Low, Tripathi and Prasad2024) utilized gravity anomaly data and tectonic modelling to conclude that the MSB was formed due to back-arc volcanic activity associated with the northward subduction of the Bastar Craton beneath the Bundelkhand Craton. Their findings showed significant gravity highs, attributed to dense mafic-ultramafic rocks, which indicate the presence of a back-arc setting where volcanic and sedimentary sequences accumulated. Collectively, these recent studies support a model of northward subduction of the Bastar Craton beneath the Bundelkhand Craton during the Paleoproterozoic.
To explain our field observations, especially the deformation seen in the Lower Vindhyan Group, we propose an initial kinematic setting of CITZ that incorporates all these perspectives. In this model, the Bastar Craton initially subducted northward beneath the Bundelkhand Craton, followed by a reversal to south-directed subduction (Fig. 7).
Kinematic model of the tectonic evolution of Vindhyan Basin and the adjacent CITZ. (a) Northward subduction of Bastar Craton beneath the Bundelkhand Craton and formation of the Betul and the Mahakoshal Basins as intra-arc and back-arc basin, respectively (Roy & Hanuma Prasad, Reference Roy and Hanuma Prasad2003; Bhowmik et al. Reference Bhowmik, Bose, Chattopadhyay, Karmakar and Pant2024). (b) Cessation of northward subduction and the slab break-off beneath the Bundelkhand Craton at ∼1.8 Ga during Columbia assembly (Rogers & Santosh, Reference Rogers and Santosh2002; Bora et al. Reference Bora, Kumar, Yi, Kim and Lee2013). (c) Southward subduction of the Bundelkhand Craton beneath the Bastar Craton. (d) Continental collision resulting in the formation of the Lower Vindhyan Group as a foreland basin, and updoming of the crust towards the Bastar Craton, forming a peripheral forebulge (Chakrabarti et al. Reference Chakrabarti, Basu and Chakrabarti2007) and metamorphism of BBG (Bhowmik et al. Reference Bhowmik, Bose, Chattopadhyay, Karmakar and Pant2024). (e) Deposition of lower Vindhyan sediments in the north of Mahakoshal Basin at ∼1.6 Ga. (f) Development of detachment folding in the Lower Vindhyan Group due to far-field N-S compression. (g) Late-stage reverse reactivation of SNNF during the India-Asia collisional event. (h) Present tectonic configuration of the Lower Vindhyan Group and the Mahakoshal Basin.
In the Paleoproterozoic period, the Bastar and Bundelkhand Cratons were initially separated by a basin. Compressional tectonics triggered north-directed subduction of oceanic crust beneath the Bundelkhand Craton, initiating the development of the Mahakoshal Basin as a back-arc basin along the southern margin of the overriding Bundelkhand Craton around 2.2–2.1 Ga (Roy & Hanuma Prasad, Reference Roy and Hanuma Prasad2003; Bhowmik et al. Reference Bhowmik, Bose, Chattopadhyay, Karmakar and Pant2024; Sharma et al. Reference Sharma, Chakraborty, Pandey and Das2024; Fig. 7a). The Betul Belt (Fig. 1a) is interpreted as the volcanic arc associated with this subduction event (Chattopadhyay et al. Reference Chattopadhyay, Bhowmik and Roy2020; Abhirami & Satyanarayanan, Reference Abhirami and Satyanarayanan2023; Lachhana Dora et al. Reference Lachhana Dora, Meshram, Rao Baswani, Malviya, Upadhyay, Shareef, Atif Raza, Ranjan, Meshram, Kumar Patnaik and Randive2023). Approximately at 1.8 Ga, coinciding with the Columbia supercontinent assembly, north-directed subduction ceased through possible slab break-off of oceanic lithosphere attached to the Bastar Craton beneath the Bundelkhand Craton (Fig. 7b). Following this slab break-off event, a subduction polarity reversal occurred with south-directed subduction, where the oceanic lithosphere attached to the Bundelkhand Craton subducted beneath the Bastar Craton (Bhowmik et al. Reference Bhowmik, Bose, Chattopadhyay, Karmakar and Pant2024; Fig. 7c). This is followed by the collision of Bundelkhand and Bastar Cratons during 1.6 Ga (Fig.7d). This tectonic setting also matches with the 1.60–1.54 Ga metamorphic ages of Balaghat-Bhandara Granulites (BBG) which occurred due to southward subduction of Bundelkhand Craton (Bhowmik & Chakraborty, Reference Bhowmik and Chakraborty2017).
From here, we will focus solely on the tectonic evolution of the Vindhyan Basin and integrate our structural observations with the kinematic framework described in the preceding paragraphs (Figs. 7a-d). At 1.6 Ga, the collision between the Bastar and the Bundelkhand Cratons caused the down-warping of the Bundelkhand Craton, resulting in a foreland basin – the Vindhyan Basin (Fig. 7d-e). Concurrently, crustal uplift towards the Bastar Craton formed a peripheral forebulge (Fig. 7d). Sediments from the elevated Bastar Craton and its associated northern collisional zone were transported northward and deposited in the Vindhyan Basin (Prasad, Reference Prasad1984; Chakrabarti et al. Reference Chakrabarti, Basu and Chakrabarti2007). The age of sedimentation in the Lower Vindhyan Basin is 1.6 Ga (Ray, Reference Ray2006). Geochemical analysis and Nd-isotopic ratios indicate that the majority of sediments, particularly those in the Lower Vindhyan Group, were derived as arc sediments from Bastar Craton, with minimal contribution from the Bundelkhand Craton (Chakrabarti et al. Reference Chakrabarti, Basu and Chakrabarti2007).
Between 1.5 and 1.1 Ga, the Central Indian Tectonic Zone (CITZ) experienced crustal extension following earlier collisional events, leading to the opening of the Sausar Basin and deposition of the Sausar Group of sediments (Bhowmik, Reference Bhowmik2020; Bhowmik et al. Reference Bhowmik, Bose, Chattopadhyay, Karmakar and Pant2024). Deformation in the Lower Vindhyan Group occurred after the sedimentation of LVG and prior to the deposition of the Upper Vindhyan Group (Fig. 7f). We suggest that the detachment folding in the Lower Vindhyan Group might be related to the far-field stress produced due to the opening of the Sausar Basin during 1.5–1.1 Ga. This led to formation of compression-related thin-skinned detachment folding in the Lower Vindhyan Group (Fig. 7f). As discussed earlier, the deposition of Upper Vindhyan Group initiated at 1.2 Ga, and the rocks here are undeformed indicating that deformation in the Lower Vindhyan Group ceased before 1.2 Ga (Fig. 7f). Therefore, we bracket the timing of deformation in the Lower Vindhyan Group between 1.5 Ga and 1.2 Ga (Fig. 7f). Additionally, the deformation in the Lower Vindhyan Group predates the activation of the Son-Narmada North Fault (SNNF) and thrusting of the MSB, which occurred in the post-Tertiary period, due to N-S far-field tectonic stress during India-Asia collision (Chattopadhyay & Bhattacharjee, Reference Chattopadhyay and Bhattacharjee2019; Chattopadhyay & Khasdeo, Reference Chattopadhyay and Khasdeo2011; Karim et al. Reference Karim, Low, Tripathi and Prasad2024; Fig. 7g). Subsequent erosion led to the present tectonic configuration of the Lower Vindhyan Group and the CITZ (Fig. 7h).
Our tectonic model thus links the collision between the Bundelkhand and Bastar Cratons to the formation and deformation of the Central Indian Tectonic Zone (CITZ), including the development of folding and faulting in the Lower Vindhyan Group. The proximity of the Vindhyan Basin to the CITZ enabled the northward propagation of deformation, resulting in the preservation of tectonic compression signatures in the Lower Vindhyan Group. These processes, occurring between 1.5 and 1.2 Ga, played a pivotal role in shaping the Mesoproterozoic tectonic evolution of the Lower Vindhyan Group.
5. Conclusions
We investigated the deformation history of the Lower Vindhyan Group (LVG) and the Mahakoshal Supracrustal Belt (MSB) within the Son Valley and documented three different stages of deformation in the latter, that produced Type-2 and -3 fold interference patterns. The first phase of deformation in the MSB is identified by the development of S1 regional foliation and diversely oriented F1 folds. The second deformation phase, characterized by E-W-trending folds with steep axial planes, is more pronounced in the MSB and synchronous with the SNSF structures. The third deformation phase results in the development of cross folds locally. Deformation in the LVG was confined to the relatively stronger Chopan Porcellanite Formation (porcellanite), Kheinjua Formation (Koldaha shale and Chorhat sandstone) and exhibited upright to moderately inclined, sub-horizontal buckle folds including kink-folds and pop-up-like structures, that are often associated with fold-thrust-belts. The underlying and relatively weaker Kajrahat Formation (limestone and Arangi shale) acted as detachment layers and are thus undeformed.
Our field investigations suggest that the deformation of the Lower Vindhyan Group occurred due to N-S compressional tectonics during 1.5–1.2 Ga, i.e. after the D3 deformation phase in the MSB and before the sedimentation of Upper Vindhyan Group. Our inference challenges the previous ones that attributed the deformation in the Lower Vindhyan Group to seismic activity or soft-sediment deformation processes. With the help of the kinematic model (Fig. 7), we additionally demonstrate that the Vindhyan basin developed as a peripheral foreland basin during the southward subduction of the Bundelkhand Craton beneath the Bastar Craton. The continued late-stage convergence of Mesoproterozoic collision along the Central Indian Tectonic Zone (CITZ) triggered and maintained the deformation that propagated northward via detachment folding into the Lower Vindhyan Group.
Acknowledgements
We sincerely thank Dr Abhinaba Roy for his invaluable guidance, drawing from his decades of experience working in the CITZ. His suggestions during our fieldwork and thoughtful contributions throughout the preparation of this manuscript are greatly appreciated. We are grateful to the anonymous reviewers whose valuable feedback greatly improved the quality of this manuscript. This work is a part of TBT’s doctoral research at IIT Kanpur. The authors gratefully acknowledge IIT Kanpur for providing PhD, Postdoctoral Fellowships and a REO position to TBT, DD and PS, respectively. SM acknowledges a research grant (MOES-ES-2023506) from the Ministry of Earth Sciences (MoES), Government of India, for this work.
Author Statement
Tushar B Todkar: Conceptualization, Methodology, Formal analysis, Investigation, Data Curation, Writing - Original Draft and Writing - Review & Editing. Puspendu Saha: Conceptualization, Methodology, Formal analysis, Investigation, Data Curation, Writing - Original Draft, Writing - Review & Editing and Supervision. Dripta Dutta: Methodology, Formal analysis, Writing - Original Draft and Writing - Review & Editing. Santanu Misra: Conceptualization, Methodology, Investigation, Resources, Data Curation, Writing - Original Draft, Writing - Review & Editing, Supervision and Funding acquisition.
Competing interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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