The Proterozoic Cuddapah Basin of south India hosts a number of unconformity-bound sequences deposited over continental crust. The Palaeoproterozoic Papaghani Group with basal polymict conglomerate unconformably overlies the Archaean Peninsular Gneiss and set the stage for Palaeoproterozoic sedimentation following a long hiatus. Two cycles of fluvial–shallow-marine psammite–carbonate–pelite sedimentation in the western part of the basin (Papaghni sub-basin) is punctuated at different levels by mafic flows, sills and dykes, and less common acidic tuffs. Each of the sedimentation cycles fosters extensive carbonate platforms with dolomitic stromatolites and algal laminites. Emplacement of shallow crustal intrusives at the end of the first cycle was possibly linked to thermal anomalies in the mantle and associated widespread crustal extension in south India. Brittle fault-slip analysis from the western Cuddapah indicates repetitive extensional to strike-slip regimes. This paper proposes a likely late Palaeoproterozoic shallow-marine sedimentary sequence of the Nallamalai Fold Belt as allochthonous, thrusted over the successions of the Papaghni sub-basin and the Kurnool Group in the western part of the basin. Inversion of the Papaghni sub-basin and the development of regional erosional unconformities is discussed in the context of Palaeoproterozoic and early Mesoproterozoic orogenic events at the SE margin of India.
The Cuddapah Supergroup succession can be divided into four unconformity-bound sequences, namely, the Papaghni, Chitravati, Srisailam and Kurnool groups and formation representing four major cycles of sedimentation. The oldest sequence, the Papaghni Group is represented by fan-delta, prodelta and shallow shelf deposits. The rifting stage of the basin evolution is attested by the immature delta succession deposited as a major fault-controlled basin evolution, and was followed by the early subsidence stage. Basement uplift and a hiatus in deposition follows the first cycle of sedimentation. The Chitravati Group, representing the second cycle of sedimentation, consists of mature sandstones separated by a heterogeneous shale-sandstone-dolomite interval. The third cycle starts with the deposition of widespread coastal fluvial to shallow marine sandstone of the Srisailam Formation, and the fourth cycle is represented by the Kurnool Group consisting of conglomerates, feldspathic sandstones, supermature quartzarenites, minor shale and carbonates. Each cycle represents a rifting phase followed by a stable subsidence stage when the basin evolved into a large epicontinental sea. The supermature Gandikota Quartzite of the Chitravati Group and the Paniam Quartzite of the Kurnool Group represent relative sea-level fall and forced regression. The siliciclastics in each of the sequences display signatures of macrotidal sedimentation pointing to open ocean connection. The sequences further display signatures of passive margin sedimentation with multiple events of carbonate-shale rhythmite deposition. Mafic flows and dykes in the Papaghni and Chitravati groups reflect thermal anomalies associated with phased crustal extension; successive extensional phases were punctuated by basin inversion. Extensive and pulsed development of epicontinental seas as recorded in the Cuddapah sequences in the south Indian craton, possibly reflect global sea-level changes associated with supercontinent (eg. Columbia in the Palaeoproterozoic) break-up and assembly.
Recent works suggest Proterozoic plate convergence along the southeastern margin of India which led to amalgamation of the high grade Eastern Ghats belt (EGB) and adjoining fold-and-thrust belts to the East Dhrawar craton. Two major thrusts namely the Vellikonda thrust at the western margin of the Nellore Schist belt (NSB) and the Maidukuru thrust at the western margin of the Nallamalai fold belt (NFB) accommodate significant upper crustal shortening, which is indicated by juxtaposition of geological terranes with distinct tectonostratigraphy, varying deformation intensity, structural styles and metamorphic grade. Kinematic analysis of structures and fabric of the fault zone rocks in these intracontinental thrust zones and the hanging wall and footwall rocks suggest spatially heterogeneous partitioning of strain into various combinations of E-W shortening, top-to-west shear on stratum parallel subhorizontal detachments or on easterly dipping thrusts, and a strike slip component. Although relatively less prominent than the other two components of the strain triangle, non-orthogonal slickenfibres associated with flexural slip folds and mylonitic foliationstretching lineation orientation geometry within the arcuate NSB and NFB indicate left lateral strike slip subparallel to the overall N-S trend. On the whole an inclined transpression is inferred to have controlled the spatially heterogeneous development of thrust related fabric in the terrane between the Eastern Ghats belt south of the Godavari graben and the East Dharwar craton.
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