The remarkable history of diamond trading in India is well-documented along the mighty Krishna River, which traverses the stable cratonic regions of central and southern India. The granite-greenstone terrain of the Dharwar Craton (DC), located in Peninsular India, is one of the world’s most renowned classical geological regions. It hosts a diverse range of mantle-derived mafic-ultramafic rocks, including pyroxenites, diabase dyke swarms, kimberlites, lamproites, and lamprophyres ( Mishra & Prajapati, 2003 ; Chadwick et al., 2007 ; Ramakrishnan & Vaidyanadhan, 2008 ; Chalapathi Rao et al., 2013 ; Kumar et al., 2013 ; Sushel et al., 2016 ; Pandey & Chalapathi Rao, 2019 ; Giri et al., 2019 ; Jayananda et al., 2020 ; Mohan et al., 2020 ; Singh et al., 2020 ; Mukherjee et al., 2021b ; Sreehari et al., 2021 ).

Regional geological and geophysical studies indicate a favorable environment for the intrusion of deep-seated mantle-derived magmas, such as kimberlites and lamproites, which are emplaced in the eastern part of the DC in southern India ( Chalapathi Rao et al., 2013 ; Raghuvanshi et al., 2019 ; Kusham et al., 2021 ). Similarly, semi-detailed and detailed studies highlight the role of structural features in identifying kimberlite and lamproite bodies in the cratonic regions of the Eastern Dharwar Craton (EDC) ( Vasanthi & Mallick, 2005 ; Chalapathi Rao et al., 2013 ; Ananda Reddy, 2014 ; Sushel et al., 2016 ). Numerous lamprophyre dykes have also been extensively studied along the eastern margin of the Paleo-Mesoproterozoic Cuddapah Basin (CB) ( Lakshminarayana & Bhattacharjee, 2000 ; Lakshminarayana et al., 2001 ; Shivanna et al., 2002 ; Anand et al., 2003 ; Tripathy & Saha, 2013 ; Pandey et al., 2018 ; Raghuvanshi et al., 2019 ; Pankaj et al., 2020 ). These studies emphasize the importance of detailed investigations along the western and northwestern margins of the CB to demarcate potential target areas for kimberlite and lamproite bodies.

The present study focuses on the Proterozoic sedimentary cover outlier of the Cuddapah Basin (CB), situated on the northern bank of the Krishna River in the Kollapur region, northwest CB ( Figure 1 ). This region is surrounded by known kimberlite and lamproite occurrences in the EDC, including the Wajrakarur Kimberlite Field in the southwest; the Raichur and Tungabhadra Kimberlite fields in the west; the Narayanpet Kimberlite Field in the northwest; the Krishna Lamproite Field in the southeast; and the Ramadugu Lamproites in the north ( Figure 1 ). Regionally, the Kollapur area comprises granite-gneissic basement rocks of the Peninsular Gneissic Complex-II (PGC-II), which are unconformably overlain by the basal sedimentary units of the Cuddapah and Kurnool formations ( Figure 2 ). Furthermore, the northwestern margin of the CB is known for numerous kimberlite and lamproite occurrences ( Meshram et al., 2012 ; Ananda Reddy, 2014 ; Giri et al., 2019 ; Nandhagopal et al., 2015 ). These Kimberlites/lamproites are particularly controlled by E-W to ENE-WSW deep seated fault/fracture zone, their intersection with NW-SE trend linear parallel tectonic fabric and NE-SW to N-S faults ( Vasanthi & Mallick, 2005 ; Veeraiah et al., 2009 ; Vani et al., 2013 ; Ananda Reddy, 2014 ; Jogu et al., 2022 ).

The Kollapur region, the focus of this study, hosts alkaline and sub-alkaline rocks (lamproites and lamprophyres) within the basement rocks at Thellarlapalle and Ankiraopally, as well as in the cover sediments at Amaragiri and Somsila ( Figure 2 ). Some of these occurrences align along the WNW–ESE-trending dykes, which form an important structural domain in the region. This study emphasizes a detailed analysis of structural features derived from gravity data and their potential role in controlling the emplacement of alkaline and sub-alkaline rocks in the basement and covered areas. Detailed investigations are also aimed at identifying potential target areas in the Kollapur region. The structural features were derived from gravity data, which were processed and interpreted using qualitative and quantitative analyses techniques. Additionally, a Two-Dimensional (2D) model was constructed to understand the subsurface structural framework, particularly in the covered areas along the northern bank of the Krishna River.

The study area, located in northern bank of Krishna River, exposed with Archaean grey biotite granite basement rocks of PGC-II in northern, central parts, and the Proterozoic cover sediments of Cuddapa and Kurnool groups are occupied in southern and south-western parts ( Figure 2 ). The angular and erosional unconformable contact relationship is well in between basement and cover sediments in all along the western margin of CB ( Lakshminarayana et al., 2001 ). The amphibolite schist associated with BIF (Banded Iron Formation) is noticed to the west of Mollachintapally, and in the north of Narlapur and to the eastern margin of Yerragutta quartz reef. According to Sushel et al. (2016) the amphibolite schist may have potential to host gold mineralization and to a strike in length is 10 km and with 500 m width.

The cover sediments of Cuddapah Supergroup are represented by Gulcheru quartzite and Vempalle formation of Papaghni Group (A-B geological cross-section in Figure 2(b) ). The Gulcheru group rocks represented with arkose conglomerates and quartzite, along with gritty arkoses, are found on the eastern margin of the study area. The Vempalle formation rests over Gulcheru rocks, represented by dolomites, chert, and mudstone with basic sill and intrusive rocks. The sills are Vempalle formations are low dipping and interstratified with other sediments. The dolomites are exposed along the banks of Krishna River, in the Kollapur, Somasila, and Amargiri areas.

The Kurnool formations are well exposed in the south, and southwestern region along the Krishna River with alternate arenaceous and argillaceous beds. The Banaganapalle quartzite is the basal of Kurnool group unconformably rest over the sediments Cuddapah Supergroup of rocks after a prolonged gap in deposition. It is resting over Vempalle dolomite with a thin conglomerate horizon is known for its distinct diamond bearing. The basal conglomerate has polymictic sediments and consists of a variety of cherts, jasper, and vein quartz pebbles followed by grit and quarzites. The exposures of the Banganapalle quartzite with conglomerate were noticed in the west of Samptraopalle, south of Pentlavalli, and in the southeast of Somasila and Amaragiri villages. Numerous dykes of WNW-ESE, NNW-SSE and NE-SW, NW-SE trends cut-across the basement rocks in the north and northwestern part. The quartz reef/veins occur in the basement rocks along the weak zones and the major quartz reefs/veins are noticed near the Thellarlapalle, Kethapally, and west of Mollachintapally area with an NNW-SSE, NW-SE trend. A major fault from Yaparla to Jatprole, in the E-W direction is a contact of cover sediments and basement rocks (A-B geological cross-section in Figure 2(b) ).

The lamproite bodies are noticed around Somasila and Amaragiri villages. Somasila is located on the northern bank of Krishna River. During fieldwork, it is observed that, the intrusive-contact relationship is noted for the lamproites bodies within the dolomite rocks of Vempalle formation, at Somasila area. In the contest old working areas were brought to cultivable and the ground is leveled, and still, there are somerelics of old dumps left as paved stone and organized gravel. The village Amaragiri is currently submerged under reservoir, the old workings are visible during summer months as and when reservoir water recedes are traceable as a small mound formed by dumping of sorted gravels, well sorted and rounded, oblate, oval shapes observed during field visits. Further, lamprophyre dykes are seen at the Ankiraopally, Kethapally and Thellarlapalle areas ( Meshram et al., 2012 ; Sushel et al., 2016 ; Giri et al., 2019 ). All the lamprophyre dykes are intruded into the granite basement rocks of Peninsular Gneissic Complex.

The gravity survey was conducted along accessible roads and pathways to maintain consistent station sampling across the study area. However, the uniform station distribution was occasionally disrupted due to the inaccessibility of conserved forests and the challenging hilly terrain. A total of 725 gravity stations were established using a CG-6 Scintrex gravimeter ( Figure 3 ). The surveys were designed to maintain a uniform station interval of 300 meters throughout the study area. To minimize cultural noise, gravity data was acquired away from tall buildings, power lines, bridges, pipelines, and other infrastructure. The gravity observation locations were superimposed on the elevation map ( Figure 3 ), with white dots indicating station positions. The elevation within the study area varies by 252 meters, ranging from 263 to 515 meters. The overall gradient of elevation decreases toward the south, transitioning from the basement to the covered Proterozoic basin region.

The International Gravity Statin Network-IGSN 1971 Gravity base was established in the Kollapur RDO office (old R&B Building, 16˚06'26.02''N and 78˚18'47.07''E). It was tied with the known IGSN-1971 gravity base at Gadwal railway station, Gadwal, Telangana, that was established by Ramadass et al. (2001) . The CG-6 gravimeter has inbuilt software for taking care of tidal correction. After applying drift correction, the entire data have also been subjected to Free Air and Bouguer corrections. Bouguer correction has been accomplished assuming the standard crustal density of 2.67 gm/cc and International Gravity Formula 1980 has been used for reduction of Latitude correction effect. The differential global positioning system (DGPS-Trimble R8s) was used for station location and the reduced level of the observation point. These observations were made at the same stations to verify the accuracy. The gravity data was processed using Geosoft software and the projection method UTM zone-43N with datum WGS-84 for special distribution of the observed data ( Figure 4 ). The gravity and elevation data were gridded with the Minimum Curvature method by keeping a grid size of 250 meters ( Figure 3 and Figure 4 ). The gravity observation points covered all the geological units, to obtain a meaningful interpretation, to generate a data base, and prepare Bouguer anomaly map of study area to understand the density distribution in corroboration with the existing geological and structural inferences.

The present study focuses on delineating the structural features of the basement, both exposed and covered, along the northern bank of the Krishna River at the northwestern margin of the Cuddapah Basin (CB). The Bouguer gravity map highlights the density distribution across the area ( Figure 4 ). The density distribution (BA) has a direct relationship with density property lithological units; hence, a correlation can be established to integrate gravity data with geological inferences ( Table 1 ). The overall gravity intensity decreasing towards south (regional trend in Figure 5 ), indicates the overall basement disposition dipping from north to south direction ( Figure 5 ). In the northeastern part, the high-gravity zone compared to the southern part is resulted due to high-density source rocks at shallow and deeper levels in the granite-gneissic rocks and as well as the basement fault running along the basin in the Kollapur region ( Figure 4 ). It is further corroborating with causative source depth inferences derived from Euler 3D depth showing deeper causative source depth of >3 km in the northeastern part of the area ( Figure 9 ). The near-surface density distribution map (Residual gravity in Figure 6 ) shows residual high anomalies, located at Narlapur, Kalwakole and Yelur in northern part, indicate the disposition of amphibolite schist enclaves within the overall granite-gneissic terrain of PGC-II. Sushel et al. (2016) have opined that these enclaves have potential trap for the gold mineralization in the north-western margin of CB. Similarly, the wide low-gravity zone (GL) in the southern and southwestern parts, around the Peddamaru-Vellatur area, is recorded over the sedimentary cover of the Cuddapah Basin (CB), where the thickness of the cover sediments is relatively higher compared to the covered areas in the southeastern part at Somasila and Amaragiri areas. In low-gravity zone (GL), shallow causative source depths of about 1 km have been derived using the Euler 3D convolution technique ( Figure 9 ).

It is interesting to note that majority of estimated depths are mainly confined along the major structural features (Gr1, Gr2, and Gr3) trending in NW-SE and E-W directions ( Figure 9 ). The distribution of shallow to intermediate depth range (red and yellow dots) along Gr2 and Gr3 represents the limited depth extension of these structural features up to 3000 m. In the central part (Bollaram), a major gravity low (nosing) anomaly differentiated as a concealed younger granite pluton within the granite-gneiss. The presence of a concealed body in a nosing anomaly is clearly indicated by the regional gravity anomaly map ( Figure 6 ). In the Bollaram area, intersection of NW-SE, N-S and ENE-WSW gravity anomalies forms a potential zone for the emplacement of younger intrusives such as kimberlite/lamproite etc. The NW-SE and E-W gravity gradients (Gr1, Gr2 and Gr3) are delineated as major structural features correlate well with total horizontal derivative map ( Figure 7 ). The E-W steep gravity gradients in the southern part define a major boundary fault (Gr3) along the northern contact of the Kurnool sub-basin is clearly demarcated from Bouguer anomaly map ( Figure 4 ) and 2D gravity model ( Figure 10 ). It is interesting to note that, the NW-SE, E-W and ENE-WSW lineament intersections and circular features of the Bouguer anomaly contours are noticeable as potential zones for emplacement of kimberlite/lamproite bodies in Bollaram, Chinthapally and Kollapur in this area along the structural features of Gr1 and Gr4. It is further observed that, the most of known kimberlite occurrences in and around the study area are mainly confined to these intersection zone ( Rao et al., 1999 ; Vasanthi & Mallick, 2005 ; Nayak et al., 2005 ; Ramadass et al., 2006 ; Veeraiah et al., 2009 ; Vani et al., 2013 ; Smith et al., 2017 ; Pal & Kumar, 2019 ; Mukherjee et al., 2021a ; Kumar et al., 2022 ). The residual gravity high and low anomalies within the sedimentary basin represent an undulating basement configuration.

The 2D gravity model depicts an unconformably overlain sedimentary layer (upper Kurnool and lower Cuddapah sediments) (1.5 km average thickness) on the granite-gneissic basement and provided new understandings on the nature and geometry of sedimentary rocks in south part of the area. Based on the 2D gravity modelling, a maximum basement depth of ~1.5 km is estimated in the southern part. The previous geophysical model derived from gravity and seismic data across CB revealed homogeneous nature of Cuddapah sediments ( Singh & Mishra, 2002 ; Chandrakala et al., 2015 ) in terms of density and seismic velocity. The modelling of gravity data in combination with seismic velocity shows a density of Cuddapah sediments to vary from 2.43 to 2.58 g/cc ( Pandey et al., 2018 ). The seismic study by Chandrakala et al. (2015) indicates a maximum thickness of the Cuddapah sediments as ~4.0 km. The present study with gravity modelling reveals that the nature of Cuddapah sediments varies in terms of density as well as shows considerably low thickness in particularly, northwestern margin of the CB. The basement depth considerably from ~1.5 km.

Based on the detailed gravity study in northern bank of Krishna River at Kollapur region, the litho-structural and features are derived in basement exposed and covered area. The highlights of detailed gravity work summarised below.

1) The detailed gravity study in Kollapur region brought out several anomalies, characterized as steep gradients, highs, lows, and nosing reflect deposition lithological and structural features in basement exposed and covered region in the northwestern part of CB.

2) A Prominent high-gravity zone (GH) in the northeastern is resulted due to due to high-density causative source units at shallow and deeper depth levels, where the high-density rocks of amphibolite schist and BIF bands are occurred as enclaves within the granite-gneissic rocks.

3) A low-gravity low (GL) in south and southwestern parts corresponds to cover sedimentary rocks having thickness of about 1 km.

4) A gravity low at Bollaram is interpreted as a concealed younger granite pluton within the granite-gneissic terrain.

5) An E-W steep gravity gradient (Gr3) along the northern margin of Cuddapah and Kurnool group of sediments marks a deep-rooted fault (~2 km depth), with similar faults near Somasila along the Papaghni Group.

6) The gravity gradients and nosing (Gr1, Gr2, Gr4, Gr5 and Gn1, Gn2) of NW-SE, NE-SW, and N-S trends are interpreted as major structural features, which are corroborated with geological inferences.

7) The two N-S 2D gravity models across the cover sediments show an overall thickness of ~1.5 km bounded by the fault contact.

8) The gravity gradient intersections at Bollaram, Chinthapally and Kollapur are interpreted as loci for kimberlite/lamprophyre bodies emplacement, supported by associated lamprophyre bodies. The known lamprophyre bodies reported in this area are mainly confined to such intersection zones.

The authors are very much grateful to Head, Department of Geophysics for providing the necessary facilities to carry out the investigations.

Linga Swamy Jogu: Carried out the geophysical field work and collected rock samples, data interpretation and final draft preparation. Udaya Laxmi Gakka: Guided the entire work, contributed to the write-up of the final version, and overall supervised the work.