Proterozoic Cuddapah Basin Research Articles

Earthquakes Induced by Rapid Loading of Faults During Pulichintala Reservoir Impoundment in the Stable Continental Region of India

AbstractThe first stage impoundment of the Pulichintala reservoir in the Proterozoic Cuddapah basin in the southeastern Indian shield began in August 2014. The second stage of impoundment started in 2019 when the height of the dam was increased and the filling of the reservoir was rapid. Soon after earthquakes started occurring which continued for a few months with a maximum magnitude of ML 4.6 on 25 January 2020. Two distinct clusters within the shallow crust were identified, which follow the faults/shear zones in the region. The estimated focal mechanisms with predominant strike slip motion on steep planes and the derived stress state are consistent with the regional stress and plate motion of the Indian plate. We simulated the influence of reservoir impoundment on the inferred seismogenic faults and found that the stress and pore pressure due to reservoir load indeed promoted failure, thus inducing earthquakes. We ascribe the induced earthquakes to rapid filling, longer and higher retention of high‐water levels during 2019–2020, which caused a stress change of 4–8 kPa. This stress change was at least two orders of magnitude higher than the typical tectonic loading rate in the stable continental region of the Indian Shield.

A Field Based Perspective of the Volcanism in Tadpatri Formation of Proterozoic Cuddapah Basin, India: An Analog of the Deccan Traps?

Abstract Field studies around Pulivendla (14º33'36"N: 78º19'48"E) belonging to the Tadpatri Formation of Proterozoic Cuddapah Basin document three distinct lava flows (based on internal layering), associated sediments (shale and sandstone) and intrusive dyke-sill system. The presence of previously undocumented internal layering namely Lower Vesicular Zone (LVZ), Lower Colonnade Zone (LCZ), Entablature Zone (EZ), Upper Colonnade Zone (UCZ) and Upper Vesicular Zone (UVZ) helps to delineate three lava flows in the study area. The associated sedimentary beds either correspond to lava cessation-period or dynamic lava-sediment interaction due to lava-cascading. The present study in Tadpatri Formation reveals volcano-sedimentological features analogous to that of Deccan Traps. It is postulated that magmatism within Tadpatri Formation might have been largely controlled by Proterozoic syn-rift plume activity (needs further corroboration by high precision major/trace/isotopic analyses).

India-Antarctica collisional ridge structure below Nellore Schist and Eastern Ghats Belts as revealed by seismic studies

In Columbia, Rodinia and Gondwana supercontinental assembly periods, eastern margin of India lay quite close to the east Antarctica. During these periods, this region underwent prolonged sedimentation, magmatism, oceanic subduction, multiple accretion and continent-continent collision which have left their imprints in the geological and geophysical observations. In the present study, we have analyzed crustal seismic velocity structure along the DSS profiles located in the eastern part of the Indian shield that cuts across the Proterozoic Cuddapah Basin, Godavari and Mahanadi Gondwana grabens. The study revealed presence of a prominent high velocity (6.2-6.5 km/s) hidden ridge structure, which is 30 to 50 km wide and located at a shallow subsurface depth beneath the Nellore Schist Belt and Eastern Ghats Belt. It underlies below Kaligiri-Duttaluru along the Kavali-Parnapalle profile, Darsi-Addanki along the Alampur-Koniki-Ganapeswaram profile and Sattupalli-Ashwaraopet in Kallur-Polavaram profile. This ridge structure is conspicuously characterized by high gravity anomaly gradients from -90 to -15 mGal. Gravity patterns over these regions further indicate that this ridge which appears to continue from the north of Chennai to Bhavanipatna region in Bastar Craton, may have been formed due to the collision of East Antarctic terrain with eastern Indian Shield after the cessation of supra-subduction at the end of Mesoproterozoic. Further, this ridge structure is flanked by Proterozoic sediments on either side; in Cuddapah Basin on its western side and East Coast Sedimentary Basin on its eastern side, which together with the possible occurrence of Proterozoic sediments in other basins, would indicate that the extent of Proterozoic sedimentation was much larger than hitherto known. We suggest that this region was rifting during Paleo-Mesoproterozoic period and possibly a shallow marine basin existed, when it was an integral part of the Columbia supercontinent assembly. It underwent another rifting phase and denudation during the Gondwana period as indicated by thin patches of Gondwana sediments which are lined up all along the east coast. Quite likely, the rifting between India and Antarctica during the Gondwana super assembly period initiated in the Permo-Triassic Gondwana period itself, with final separation taking place much later during early Cretaceous period.

Hydrogeochemical characterization and evaluation of subsurface water quality in the Proterozoic Cuddapah Basin, Andhra Pradesh, India.

In the current survey, different hydrogeochemical processes governing the geochemistry of aquifers, the usefulness of groundwater for regular consumption, and agricultural purposes were evaluated around the Tummalapalle area. One hundred forty-four borehole locations were chosen to characterize the major physicochemical components of the aquifer water. The analysis results of pH inferred that the groundwater is nominally acidic to basic, and pH ranged from 6.6 to 8.4. The average concentrations of TDS, Ca2+, Mg2+, total hardness (TH), HCO3-, and total alkalinity (TA) are within the allowable limits of potable water quality as prescribed by the Bureau of Indian Standards (BIS) and WHO. However, the average concentrations of Na+, K+, Cl-, and SO42- were all below the permissible limit. All samples were analyzed with the help of Piper and Chadha charts to determine the dominant hydrogeochemical components of groundwater. The dominance of cations in groundwater in this region is in the sequence of Ca2+ > Na+ > Mg2+ > K+, followed by anions HCO3- > Cl- > SO42-. The Gibbs plot analysis suggested the predominance of rock aquifer interaction as the major hydrogeochemical process governing groundwater geochemistry in this region.The water quality index (WQI) of all groundwater samples in the Tummalapalle region was estimated, with 55% of the samples being potable grade. The different irrigation indices were analyzed for the groundwater samples to estimate their desirability for agriculture. The maximum number of water samples was found to be well-suited for cultivation.

Geochemical and carbon isotopic studies of carbonaceous phyllites from Dharwar craton, India – Reconstruction of Precambrian depositional environment

Carbonaceous shales formed in anoxic, deep marine environment are depocenters of prolific microbial life and rich sources of organic matter. In the present study, geochemical and organic carbon isotopic (δ13Corg) signatures of carbonaceous phyllites from Archean Chitradurga, Gadag, Sandur, Shimoga greenstone belts and Proterozoic Cuddapah basin of Dharwar Craton were studied to understand the Precambrian depositional environment by delineating the similarities and differences in the Archean and Proterozoic samples. Both Archean and Proterozoic carbonaceous phyllites are fine grained siliceous in nature with depleted CaO, MgO and alkalies. The REE concentrations in most of the Archean samples are characterized by conspicuous positive Europium anomalies while the Proterozoic counterparts display feebly positive ratios indicating hydrothermal source. The Archean samples possess elevated Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW), Plagioclase Index of Weathering (PIA) values reflecting extreme weathering conditions while the Proterozoic carbonaceous phyllites are confined to moderate weathering conditions. Cr/Th, La/Sc, Th/Sc and Co/Th indicate mafic provenance and the Cr-Zr relationship suggests deep sea depositional setting for both Archean and Proterozoic carbonaceous phyllites. The Archean carbonaceous phyllites indicate humid to semi-humid paleoclimate conditions while the Proterozoic samples are restricted to humid palaeoclimatic conditions which is evident from Sr/Cu ratios. V/(V + Ni) ratios indicate fluctuating dysoxic to euxinic redox conditions for the Archean samples while the Proterozoic samples display euxinic conditions. The predominantly negative δ13Corg values of Chitradurga (−38.5‰ to −14.2‰ VPDB), Gadag (−24.3‰ to −8.06‰ VPDB), Sandur (−24.8‰ to −29.2‰ VPDB), Shimoga (−24.6‰ to −11.8‰ VPDB) and Cuddapah (−30.19 to −13.98‰ VPDB) suggest the role of primitive life forms through biological processes during the deposition of organic matter.

Metallogenesis and depositional environment of the Archean-Proterozoic carbonaceous phyllites from the Dharwar Craton, India

Organic carbon rich meta-sedimentary rocks are important repositories of gold, PGE and vestiges of the biogeochemical cycles that prevailed during the Precambrian time. In this study, we report gold, PGE and δ34S concentrations of carbonaceous phyllites from Neoarchean Chitradurga, Gadag greenstone belts of western Dharwar Craton and Mangampeta of Proterozoic Cuddapah basin, eastern Dharwar Craton to understand their metallogenesis and depositional environments. Chitradurga phyllites (avg. Au = 180 ppb) show elevated concentrations of refractory gold compared to Gadag (avg. Au = 38 ppb) and Mangampeta shales (avg. Au = 23 ppb). The Archean carbonaceous phyllites display three generations of pyrites and their mineral chemistry suggests hydrothermal imprints while Proterozoic shales have two generations of diagenetic pyrites. Gadag has high PGE concentration (ΣPGE = 55 ppb) compared to Chitradurga (ΣPGE = 37 ppb) and Mangampeta (ΣPGE = 27 ppb). The PGE geochemistry is characterized by enrichment of PPGE over IPGE with Au > Pt > Pd and the Au/Pd, Pd/Pt, Ru/Ir, Ir/Pd ratios coupled with negative Pt (Pt/Pt*) anomalies indicate hydrothermal source. The Chitradurga and Gadag samples yielded low sulphur isotopic signatures (δ34S = 1.8–2 and 1.3–2‰ respectively) whereas the Proterozoic Mangampeta samples have comparatively higher values (δ34S = 8.4–34.3‰) suggesting hydrothermal and sea water sulphate sources respectively. Sulphate present in Archean and Proterozoic sea water was converted to sulphur by bacterial sulphate reduction which led to the formation of pyrites. In the Archean phyllites, the metal bearing bisulphide complexes were adsorbed into the pyrites while the Proterozoic shales are characterized by the partitioning of organo-metallic complexes into the pyrite during the diagenesis. Strong positive correlations of transition metals, redox sensitive elements with PGE indicate hydrothermal source and V/(V + Ni), U/Th, Th/U, authigenic U attest euxinic paleo-redox conditions. Thus, biogenically mediated redox depositional environments which prevailed during Archean and Proterozoic times have enhanced the metal sequestration processes that subsequently resulted in the ore formation in these carbonaceous phyllites.

Sedimentation history and depositional model of Palaeo-Mesoproterozoic Tadpatri Formation, Cuddapah Basin, India

The Tadpatri Formation belonging to the proterozoic intracratonic Cuddapah Basin, in an around Utakonda (15° 16′ 08.80″ N, 77° 52′ 01.77″ E), Shidhanagar-II (15° 13′ 38.72″ N, 77° 50′ 30.84″ E), and Pacharu Mekala Palli (15° 04′ 27.23″ N, 77° 50′ 29.44″ E) area, Andhra Pradesh, Southern India records well-developed successions of shale with lesser amount of stromatolitic dolomite, chert, sandstone, and felsic as well as mafic sill bodies. Three stratigraphic sections are measured. Based on sedimentological studies, 11 facies are identified from the studied area. The array of facies of Tadpatri Formation may be grouped into following facies associations: (a) inner ramp, (b) mid-ramp, and (c) outer ramp. Facies associations along with associated sedimentary structures and stromatolite morphology demonstrate that the Tadpatri Formation in the study area is deposited in a mud-dominant mixed clastic–carbonate ramp setting. Further comparison with age-equivalent formations suggests that the Tadpatri Formation bears many of the classic signatures of proterozoic mixed clastic–carbonate deposits.

Paleoproterozoic ($$\sim$$1.88–1.89 Ga) ultramafic–mafic sills, Cuddapah basin, India—revisited: Implications for interaction between mantle plume and metasomatized subcontinental lithospheric mantle

A number of mafic–ultramafic sills, which are supposed to be part of the widespread ~1.88–1.89 Ga Hampi–Bastanar Large Igneous Province of the Indian shield, are reported to intrude into sedimentary/meta-sedimentary rocks of the Proterozoic Cuddapah basin. Mineral chemistry of chrome-spinel, olivine and pyroxene from the cumulate ultramafic rocks of the Pulivendla sills, emplaced at the base of Tadpatri Formation of the Cuddpaha basin, is presented for better understanding on their nature and genesis. The intermediate Cr#, low Mg#, high Fe3+# and Ti (apfu) coupled with relatively low Al (apfu) content of studied spinel indicate their Alaskan-type nature. Moreover, the Ca-rich clinopyroxene together with classic variations of Al2O3, TiO2 and SiO2 follow the trend of arc type of intrusions. The calculated Al2O3 content of parental melt of the studied rocks also suggests arc-type characteristics. Whole-rock geochemistry show fractionated chondrite-normalized REE patterns, higher Ba/Nb and low Nb/La that suggest contribution from an enriched mantle source, whereas higher Th/Yb and negative Ta–Nb–Ti anomaly on the primitive mantle-normalized multi-element spidergrams emphasize the involvement of a subducted component in the lithospheric mantle source. Although, the mineral chemistry and geochemistry is akin to Alaskan-type intrusion, lack of concentric zoning of lithologies (olivine-rich ultramafic rocks in centre surrounded by mafic rocks), rarity of primary hornblende and abundance of orthopyroxene contradict it to be an Alaskan-type intrusion. It is suggested that the mantle source region of the Pulivendla ultramafic–mafic rocks was modified by a fluid-induced metasomatism during an ancient subduction event. Further, Al-in-olivine thermometry suggests crystallization temperature between 1410° and 1484°C, which is 260°C higher than the average temperature of MORB. Similarly, the estimated mantle potential temperature is also significantly higher (>1600°C) than the different secular cooling models of the earth at ~1.88–1.89 Ga and consistent with the thermal regime of a mantle plume. The existence of mantle plume during 1.88–1.90 Ga, which has played an important role in the genesis of mafic–ultramafic sills of the Cuddapah basin, is also well supported by a radiating mafic dyke swarm, domal uplift and magmatic underplating.

Extension Related Magmatism and Basin Evolution in Northwestern Part of the Cuddapah Rift, India

Spatial distribution and field relationship of igneous intrusives viz. basic dykes, pink granite-leucogranite-tourmaline pegmatites and quartz reefs across interface between the Neoarchean Gadwal schist belt (Basement) and the Proterozoic Cuddapah basin have been documented. The Cuddapah basin is a half graben (rift) developed at right angle to the basement and displays two sets of faults: (1) NE-SW longitudinal and (2) NW-SE transverse. The longitudinal faults oriented parallel to basin margin have controlled sedimentation, subsidence and uplift. The transverse faults extending from rift shoulder (basement) right into central part of the basin are post depositional faults and imparted en-echelon pattern to the basement-basin interface. Sedimentary facies, depositional environments and basin setting indicate that the basin fill was deposited in a rift setting over the northwestern part of the epeiric sea embayment that flooded on to the Dharwar batholith of the Eastern Dharwar craton. Basin evolution stages have been outlined. Basin uplift/inversion is controlled by reverse faults and is viewed as a cause and effect to the post Cuddapah extension related magmatism which is attributed to magma underplating in the environs of the northwestern part of the Cuddapah rift at crustal levels.

Petrology and geochemistry of mafic dyke and sills in Cumbum Formation, of the Proterozoic Nallamalai fold belt, Rajampet area, Andhra Pradesh, India

Intensely deformed Proterozoic Nallamalai fold belt (NFB) occupies the eastern part of undeformed, Proterozoic Cuddapah basin and is separated from Cuddapah basin by easterly dipping Rudravaram thrust line. Igneous activity in NFB is represented by intrusive syenite and lamproite dykes. This study brings out the petrological and geochemical character of unreported intrusive dolerite dykes and sills within the Cumbum Formation of NFB. The dykes are undeformed, whereas the sills suffered D1 phase of deformation, although both are petrologically and geochemically similar. The rocks are highly fractionated sub-alkaline tholeiite, consist mainly of clino-pyroxene and plagioclase. Chondrite-normalised rare earth element (REE) plot shows enriched light rare earth element (LREE) and flat heavy rare earth element (HREE). A primitive mantle-normalised multi-element spider diagram shows troughs in Nb, Ta, Ti and Zr indicating subduction zone character. Tectonic discrimination plots indicate both within-plate and subduction zone environment of formation. Geochemical modelling also indicates enriched spinel peridotite as a probable source for dykes and sills. We suggest that the lithospheric mantle beneath NFB got enriched by subduction-derived fluid around 1.6 Ga, and in later stage, these dykes and sills were generated by extension of the enriched lithosphere at different phases in turn inheriting the subduction zone geochemical characters.

Detrital zircon U-Pb geochronology of stromatolitic carbonates from the greenstone belts of Dharwar Craton and Cuddapah basin of Peninsular India

Oldest rocks are sparsely distributed within the Dharwar Craton and little is known about their involvement in the sedimentary sequences which are present in the Archean greenstone successions and the Proterozoic Cuddapah basin. Stromatolitic carbonates are well preserved in the Neoarchean greenstone belts of Dharwar Craton and Cuddapah Basin of Peninsular India displaying varied morphological and geochemical characteristics. In this study, we report results from U-Pb geochronology and trace element composition of the detrital zircons from stromatolitic carbonates present within the Dharwar Craton and Cuddapah basin to understand the provenance and time of accretion and deposition. The U-Pb ages of the detrital zircons from the Bhimasamudra and Marikanve stromatolites of the Chitradurga greenstone belt of Dharwar Craton display ages of 3426 ± 26 Ma to 2650 ± 38 Ma whereas the Sandur stromatolites gave an age of 3508 ± 29 Ma to 2926 ± 36 Ma suggesting Paleo- to Neoarchean provenance. The U-Pb detrital zircons of the Tadpatri stromatolites gave an age of 2761 ± 31 Ma to 1672 ± 38 Ma suggesting Neoarchean to Mesoproterozoic provenance. The Rare Earth Element (REE) patterns of the studied detrital zircons from Archean Dharwar Craton and Proterozoic Cuddapah basin display depletion in light rare earth elements (LREE) and enrichment in heavy rare earth elements (HREE) with pronounced positive Ce and negative Eu anomalies, typical of magmatic zircons. The trace element composition and their relationship collectively indicate a mixed granitoid and mafic source for both the Dharwar and Cuddapah stromatolites. The 3508 ± 29 Ma age of the detrital zircons support the existence of 3.5 Ga crust in the Western Dharwar Craton. The overall detrital zircon ages (3.5–2.7 Ga) obtained from the stromatolitic carbonates of Archean greenstone belts and Proterozoic Cuddapah basin (2.7–1.6 Ga) collectively reflect on ∼800–900 Ma duration for the Precambrian stromatolite deposition in the Dharwar Craton.

Geochemistry of the Palaeoproterozoic quartzites of Lower Cuddapah Supergroup, South India: Implications for the palaeoweathering, provenance, and crustal evolution

The lower part of the Proterozoic Cuddapah Basin exposes three quartzite sequences, namely, Gulcheru, Pulivendla, and Gandikota formations. Gulcheru Quartzites show angular to subangular texture, whereas Pulivendla and Gandikota quartzites contain rounded to subrounded detrital grains. Mineralogically, all quartzites are predominantly composed of quartz with subordinate amount of feldspar, mica, and heavy minerals. Clay and silt are present as a matrix while silica and carbonate act as a cementing material. Based on detrital modes and major element geochemistry, these sediments are classified as arkose to subarkose. The petrological and geochemical attributes of the basal formations of the Cuddapah quartzite indicate cyclic weathering under variable humid subtropical climate. The CIA values of these quartzites suggest moderate to intense chemical weathering for the Gulcheru and Gandikota quartzites and low to moderate chemical weathering for Pulivendla Quartzites. The major element ratios (SiO2/Al2O, K2O/Na2O), trace element ratios, chondrite‐normalised REE patterns and UCC normalised multi element diagrams suggest that these quartzites were predominantly derived from the felsic sources, with subordinate contribution from mafic sources. Provenance studies and mixing models, in conjunction with palaeocurrent data suggest that these quartzites of the Lower Cuddapah Supergroup received sediments from various sources including Eastern Dharwar Craton as well as from more juvenile crust.

Phanerozoic magmatism in the Proterozoic Cuddapah Basin and its connection with the Pangean supercontinent

Magmatic pulses in intraplate sedimentary basins are windows to understand the tectonomagmatic evolution and paleaoposition of the Basin. The present study reports the U–Pb zircon ages of mafic flows from the Cuddapah Basin and link these magmatic events with the Pangean evolution during late Carboniferous–Triassic/Phanerozoic timeframe. Zircon U–Pb geochronology for the basaltic lava flows from Vempalle Formation, Cuddapah Basin suggests two distinct Phanerozoic magmatic events coinciding with the amalgamation and dispersal stages of Pangea at 300 Ma (Late Carboniferous) and 227 Ma (Triassic). Further, these flows are characterized by analogous geochemical and geochronological signatures with Phanerozoic counterparts from Siberian, Panjal Traps, Emeishan and Tarim LIPs possibly suggesting their coeval and cogenetic nature. During the Phanerozoic Eon, the Indian subcontinent including the Cuddapah Basin was juxtaposed with the Pangean LIPs which led to the emplacement of these pulses of magmatism in the Basin coinciding with the assemblage of Pangea and its subsequent breakup between 400 Ma and 200 Ma.

Complex upper mantle deformation beneath the Dharwar craton inferred from high density splitting measurements: Distinct lateral variation from west to east

Upper mantle anisotropy investigated using 172 core-refracted (SKS, SKKS) seismic phases along a ~660 km long profile at 10 to 20 km intervals from the west to the east coast of South India reveals significant lateral variations in its magnitude and direction. This profile, with 38 broadband seismic stations, covers mid-Archean Western Dharwar craton (WDC), late-Archean Eastern Dharwar Craton (EDC), Proterozoic Cuddapah Basin (CB) and the passive continental margins along the west and east coast. The observed fast polarization directions (FPDs) show lateral variability: NW50o to NW5o beneath the WDC, NW40o to NE30o beneath the EDC and N5o to N85o beneath the CB and further east. The delay time varies between 0.4 and 2.0 s with an average of 1 s. However, we are unable to fit a two layers anisotropy model for the region due to sparse azimuth coverage. Beneath the WDC, the direction of the fast axis follows trends of shear zones and faults, suggesting “frozen-in” anisotropy in the lithosphere, possibly established during the lithospheric evolution in mid-late Archean. In the EDC, the fast axis does not only follow the plate motion direction but it deviates, manifesting late Archean to Proterozoic deformation may still be present as fossil mantle anisotropy. The splitting trend beneath the CB and Eastern Ghat (EG) follows the strike of the rift along with plate motion direction, indicating anisotropy is influenced by the combination of “frozen” anisotropy due to continental rifting along the eastern margin of Indian plate and active asthenospheric flow.

Seismology in India

The controlled source seismology (CSS), where artificial sources are used and near-vertical reflections, refractions and wide-angle reflections are recorded, has been a powerful tool for delineating shallow/deep crustal structures and subsurface features. Significant results, brought out from the vintage and new CSS data in the 21st century, include geological/tectonic aspects of the Dharwar craton, Delhi-Aravalli fold belt, Central Indian Tectonic Zone, Southern Granulite Terrain, Proterozoic Cuddapah basin, Kangra fold-thrust belt, Hazara syntaxis in NW Himalaya, Chambal-valley Vindhya basin, Mahanadi delta and Bengal basins. These studies also provide useful inputs in understanding the genesis of 1997 Jabalpur and 2001 Bhuj earthquakes. Imaging sub-volcanic Mesozoic sediments, in which more than 50% of global oil is found, has been a challenge by routine geophysical methods. This problem has been alleviated by CSS experiments, and Mesozoic sediments below the Deccan volcanic rocks have been delineated in the Saurashtra peninsula, Kutch peninsula, Tapti-graben and Kerala-Konkan offshore regions, and Gondwana sediments below the Rajmahal Traps have been mapped in the Mahanadi and Bengal basin. Several approaches have been proposed for the detection, characterization and assessment of gashydrates from shallow seismic data. These have been applied to field data that has resulted into identifying gas-hydrates, a major future energy resource of India, in the Krishna-Godavari, Mahanadi and Andaman offshore regions. Gas-hydrates have been subsequently recovered by drilling & coring under India National Gas Hydrates Program. The prognosticated amount of methane stored as gas-hydrates within Indian Exclusive Economic Zone is so huge that only 10% production may suffice our vast energy requirement for the next 100 years.

Facies analysis of tuffaceous volcaniclastics and felsic volcanics of Tadpatri Formation, Cuddapah basin, Andhra Pradesh, India

The felsic volcanics, tuff and volcaniclastic rocks within the Tadpatri Formation of Proterozoic Cuddapah basin are not extensively studied so far. It is necessary to evaluate the extrusive environment of felsic lavas with associated ash fall tuffs and define the resedimented volcaniclastic components. The spatial and temporal bimodal association were addressed, but geochemical and petrographic studies of mafic volcanics are paid more attention so far. The limited exposures of eroded felsic volcanics and tuffaceous volcaniclastic components in this terrain are highly altered and that is the challenge of the present facies analysis. Based on field observation and mapping of different lithounits a number of facies are categorized. Unbiased lithogeochemical sampling have provided major and selective trace element data to characterize facies types. Thin-section studies are also carried out to interpret different syn- and post- volcanic features. The facies analysis are used to prepare a representative facies model to visualize the entire phenomenon with reference to the basin evolution. Different devitrification features and other textural as well as structural attributes typical of flow, surge and ash fall deposits are manifested in the middle, lower and upper stratigraphic levels. Spatial and temporal correlation of lithologs are also supportive of bimodal volcanism. Felsic and mafic lavas are interpreted to have erupted through the ~ N-S trending rift-associated fissures due to lithospheric stretching during late Palaeoproterozoic. It is also established from the facies model that the volcaniclastics were deposited in the deeper part of the basin in the east. The rifting and associated pressure release must have provided suitable condition of decompression melting at shallow depth with high geothermal gradient and this partial melting of mantle derived material at lower crust must have produced mafic magmas. Such upwelling into cold crust also caused partial heat transfer and associated melting of the surrounding shallow crustal rocks to generate felsic magmas.

An approach of understanding acid volcanics and tuffaceous volcaniclastics from field studies: A case from Tadpatri Formation, Proterozoic Cuddapah basin, Andhra Pradesh, India

The lower stratigraphic part of the Cuddapah basin is marked by mafic and felsic volcanism. Tadpatri Formation consists of a greater variety of rock types due to bimodal volcanism in the upper part. Presence of bimodal volcanism is an indication of continental rift setting. Various genetic processes involved in the formation of such volcanic sequence result in original textures which are classified into volcaniclastic and coherent categories. Detailed and systematic field works in Tadpatri–Tonduru transect of SW Cuddapah basin have provided information on the physical processes producing this diversity of rock types. Felsic volcanism is manifested here with features as finger print of past rhyolite-dacite eruptions. Acid volcanics, tuffs and associated shale of Tadpatri Formation are studied and mapped in the field. With supporting subordinate studies on geochemistry, mineralogy and petrogenesis of the volcanics to validate field features accurately, it is understood that volcanism was associated with rifting and shallow marine environmental condition. Four facies (i.e., surge, flow, fall and resedimented volcaniclastic) are demarcated to describe stratigraphic units and volcanic history of the mapped area. The present contribution focuses on the fundamental characterization and categorization of field-based features diagnostic of silica-rich volcanic activities in the Tadpatri Formation.

Geochemical and stable isotope signatures of Proterozoic stromatolitic carbonates from the Vempalle and Tadpatri Formations, Cuddapah Supergroup, India: Implications on paleoenvironment and depositional conditions

Spectacular stromatolitic dolomites with varied morphological features are preserved in the Vempalle and Tadpatri Formations of the Proterozoic Cuddapah basin, eastern Dharwar Craton. They exhibit stratifera, columnar, conical, domal and spheroidal/lobate morphologies in which columns are branched at some places (in Tadpatri Formation). Based on their trace elements such as REY and stable isotope compositions (carbon and oxygen) the Vempalle stromatolites have been classified as three types. Vempalle I stromatolites exhibit pronounced positive Eu anomalies with small scale positive Ce anomalies whereas Vempalle II counterparts have slight negative to positive Eu, feeble positive Ce and positive Gd anomalies. Vemaplle III stromatolites display positive Ce, Eu and Gd anomalies. The Tadpatri Formation stromatolites have pronounced Gd anomalies with low order positive Ce and Eu anomalies. Significant depletion in δ18O is observed in Vempalle ranging from – 10.70‰ to −7.07‰ and 20.73‰ to −11.87‰ in Tadpatri Formations. The Tadpatri stromatolites display negative δ13C varying from −4. 56‰ to −1.26‰. Contrarily, the Vempalle stromatolites recorded both positive and negative δ13C ranging from −0.24‰ to 2.00‰ thereby reflecting their deviation from the popular belief of the boring billion concept of displaying Midproterozoic δ13C stasis. The flat REE patterns along with Ce, Eu and Gd anomalies, chondritic to superchondritic Y/Ho ratios collectively reflect on marine water conditions for their deposition with variable inputs of siliciclastic/terrigenous materials and hydrothermal signatures. These cherty dolomitic stromatolites were deposited in subtidal-intertidal-supratidal zones of a shallow shelf of Proterozoic open ocean under warm, alkaline and anoxic conditions.

Seismically Derived Gondwana and Proterozoic Sediments East of Cuddapah Basin, South Indian Shield and Its Possible Geotectonic Implications

The study area situated east of the intracratonic Proterozoic Cuddapah basin, that covers Nellore Schist belt, Ongole domain of the Eastern Ghats Belt and adjacent East Coast Terrain, forms a geodynamically important segment of the Dharwar craton. This region came into focus due to proposition that during Paleoproterozoic to Mesoproterozoic era, it underwent persistent oceanic subduction, continent–continent collision and multi-stage accretional growth. In view of this, we have reprocessed in detail the deep seismic sounding data over this region and integrated the obtained results with other geophysical studies to decipher the deep subsurface crustal structure underneath. The derived shallow seismic section across this region showed the presence of a thin layer (~250 m) of Gondwana sediments with a distinct velocity of 4.20 km/s and a lateral dimension of about 40 km around Vinjamuru region of the Nellore Schist Belt. Another important finding has been the depiction of 5.3 and 5.5 km/s velocity sediments (comparable to upper and lower Cuddapah sediments) between Nellore Schist Belt and the east coast, indicating the possible presence of an additional Proterozoic sedimentary terrain (named here as East Coast Sedimentary Basin), overlying the concealed Archean crystalline basement. The presence of this sedimentary terrain, which is separated from the Cuddapah basin by an exhumed horst-like feature, situated below Nellore Schist Belt, would indicate Proterozoic sedimentation in a much larger area during Columbia and Rodinia supercontinental assembly periods than hitherto known. Importantly, no seismic trace of the Eastern Ghats Belt has been found between Nellore Schist Belt and the east coast. It appears that during the Gondwana period, entire eastern segment of the east Dharwar craton has been persistently rifting, uplifting and eroding together with other Gondwana grabens in India, thereby leaving behind only a thin veneer of Gondwana sediments at many places. In view of the presence of number of Gondwana occurrences all along the east coast, it is felt that the rifting during the Gondwana period may have been a prelude to India–Antarctica breakup, with final separation between the two taking place much later during early Cretaceous.

Possible magmatic underplating beneath the west coast of India and adjoining Dharwar craton: Imprint from Archean crustal evolution to breakup of India and Madagascar

The shear wave velocity of the crust along a ∼660 km profile from the west to the east coast of South India is mapped through the joint inversion of receiver functions and Rayleigh wave group velocity. The profile, consisting of 38 broadband seismic stations, covers the Archean Dharwar craton, Proterozoic Cuddapah basin, and rifted margin and escarpment. The Moho is mapped at a depth of ∼40 km beneath the mid-Archean Western Dharwar Craton (WDC), Cuddapah Basin (CB), and the west and east coasts formed through the rifting process. This is in contrast with a thin (∼35 km) crust beneath the late-Archean Eastern Dharwar Craton (EDC). Along the profile, the average thickness of the upper, middle and lower crust is ∼4 km, 12±4 km and 24±4 km respectively. Above the Moho, we observe a high-velocity layer (HVL, Vs > 4 km/s) of variable thickness increasing from 3±1 km beneath the EDC to 11±3 km beneath the WDC and the CB, and 18±2 km beneath the west coast of India. The seismic wave velocity in this layer is greater than typical oceanic lower crust. We interpret the high-velocity layer as a signature of magmatic underplating due to past tectonic processes. Its significant thinning beneath the EDC may be attributed to crustal delamination or relamination at 2.5 Ga. These results demonstrate the dual signature of the Archean Dharwar crust. The change in the geochemical character of the crust possibly occurred at the end of Archean when Komatiite volcanism ceased. The unusually thick HVL beneath the west coast of India and the adjoining region may represent underplated material formed due to India–Madagascar rifting, which is supported by the presence of seaward dipping reflectors and a 85–90 Ma mafic dyke in the adjoining island.