Dilip Saha

The Purana Basins of Southern Cratonic Province of India - A Case for Mesoproterozoic Fossil Rifts

Abstract Since its cratonization in the Palaeoproterozoic, southern peninsular India witnessed the development of a number of large intracratonic sedimentary basins, traditionally referred to as Purana basins, spanning in age from late Palaeoproterozoic through Neoproterozoic. The localization of these intracratonic basins, namely Pranhita-Godavari (PG), Chattisgarh and Cuddapah basins, is apparently controlled by pre-existing sutures and/or weak zones. The PG basin and the Cuddapah basin host unconformity-bound, thick, sediment-dominated successions attesting to several cycles of fluvial-shallow marine to shelf-slope-basin sedimentation. Deposition was punctuated by block uplifts resulting in local hiatuses and/or volcanic upheavals leading to intercalation of thin but persistent basaltic flows and acid tuffs and ignimbrites. Basin-margin deep faults apparently played a role in the facies distribution in these basins. Based on these features we propose that these basins initiated as continental rifts which, however, never opened up into a full-fledged ocean basin, but links with open seaway are evident from frequent occurrence of deposits representing tidal and storm influence particularly in upper part of the Chattisgarh succession. Spatial distribution of facies and sediment thickness in the Cuddapah and PG basins suggest that an open seaway existed to the east of the south Indian cratonic province during the Mesoproterozoic, while similar criteria point to the existence of an open seaway north of the Chattisgarh basin. Development history, including nature of inversion, suggest that the southern cratonic province of India existed as a single large continental mass since the Mesoproterozoic, in spite of episodes of supercontinent build-up and fragmentation involving India and East Gondwana during the Proterozoic.

Multi-Stage Deformation in the Nallamalai Fold Belt, Cuddapah Basin, South India - Implications for Mesoproterozoic Tectonism Along Southeastern Margin of India

Abstract The common occurrence of small-scale refolded folds, deformed lineations and overprinting of foliation in the northern Nallamalai Fold Belt (NFB) within the intracratonic Cuddapah Basin, South India, suggest three phases of deformation, D 1 , D 2 and D 3 during the late Paleoproterozoic to Neoproterozoic. D 1 structures are represented by tight to isoclinal folds, a slaty cleavage and local development of mylonites. D 2 structures include NE trending tight to open folds (F 2 ) with variable plunge indicating control of large domal structures and a steep crenulation cleavage developed in phyllites and other schistose rocks. The above structures are overprinted by E-W trending D 3 folds and cleavage, which affect Mesoproterozoic rocks of the Srisailam Formation (uppermost Nallamalai Group) in the E-W trending Vami Konda range and metasediments of the Palnad area, thought to be equivalent of the Neoproterozoic Kurnool Group. Taking into account the ages of intrusive Vellaturu Granite (∼1575 Ma) which is post-D 2 deformation in the NFB and the Chelima Lamproite (∼1400 Ma) which intrudes the folded Nallamalai rocks, we suggest that the earlier episodes of multiple deformation in the NFB represent either late Paleoproterozoic or early Mesoproterozoic NW-SE compression in this part of the East Gondwana. The E-W fold-thrust structures affecting the Srisailam Quartzite and adjacent metasediments in the Palnad area represent late Mesoproterozoic and/or Neoproterozoic event. The granite gneisses and schists of the Nellore Schist Belt, bordering the NFB are affected by both early and later events and thrust over rocks of the Nallamalai Group. We propose that early deformation in the NFB lying southwest of the Eastern Ghats and roughly parallel to the present southeastern coast of India, was unrelated to, and occurred prior to the main deformation in the Eastern Ghats, which is generally correlated with the ∼1000 Ma old Grenville event. Alternatively, in view of the complex, protracted evolution of the Eastern Ghats and published geochronological data supporting pre-Grenvillian tectonism particularly in the Western Charnockite Zone of the Eastern Ghats, D 1 and D 2 deformation in the NFB may be linked to this older (1600-1400 Ma) tectonic disturbance in the Eastern Ghats.

Palaeoproterozoic sedimentation in the Cuddapah Basin, south India and regional tectonics: a review

Abstract 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.

An overview of the Palaeoproterozoic geology of Peninsular India, and key stratigraphic and tectonic issues

Abstract The Archaean–Proterozoic rock successions in India have the potential to enrich the global database on Precambrian stratigraphic development, and to offer valuable clues to global tectonic reconstructions. Built over four distinct Archaean cratonic nuclei, the major Palaeoproterozoic supracrustal belts/cover sequences include the Dhanjori Group, the Singhbhum Group (including the Dalma volcanics and the Chandil Formation in eastern India), the Cuddapah Supergroup and the adjoining Nellore Schist Belt bordering the eastern Dharwar Craton, the Aravalli Supergroup in the Aravalli–Delhi Fold Belt in NW India, the lower Vindhyan (Semri Group) and the Mahakosal/Sausar/Betul belts close to the Central Indian Tectonic Zone (CITZ), and, possibly, the Dongargarh Supergroup in Bastar. A major erosional unconformity separates the Archaean tonalite–trondjhemite gneiss basement in these cratons from the overlying volcano-sedimentary successions. An overview of Palaeoproterozoic stratigraphic development in these cratonic blocks is presented to bring out the salient features for global comparison and to highlight issues requiring further attention. Multiple metamorphic, magmatic and deformation events are recorded in the fold belts at the join of the cratons or their margins, inviting application of the plate tectonic paradigm. However, a comprehensive tectonic model for the amalgamation of the Indian Archaean nuclei is yet to emerge, and is crucial for our understanding of Palaeoproterozoic supercontinent development.

Deformation Pattern in the Kurnool and Nallarnalai Groups in the Northeastern Part (Palnad Area) of the Cuddapah Basin, South India and its Implication on Rodinia/Gondwana Tectonics

Abstract The Proterozoic basins of India adjoining the Eastern Ghats Granulite Belt (EGGB) in eastern and southern India contain both Mesproterozoic and Neoproterozoic successions. The intracratonic set-up and contractional deformation fo the Neoproterozoc successions in the Paland sub-basin in the northeastern part of Cuddapah basin and similar crustal shortening in contemporaneous successions lying west of the EGGB and Nellore Schist Belt (NSB) are considered in relation to the proposed geodynamic evolution of the the Rodinia and Gondwana supercontinents. Tectonic shortening in the Palnad sub-basin (northeast Cuddapah), partitioned into top-to-westnorthwest thrust shear, flexural folds and cleavage development under overall E-W contraction, suggests foreland style continental shortening within an intracratonic set-up. A thrust sheet containing the Nallamalai rocks and overlying the Kurnool rocks in the northeastern part of Palnad sub-basin exhibits early tight to isoclinal folds and slaty (phylllitic) cleavage, which can be correlated with early Mesoproterozoic deformation structures in the nothern Nallamalai Fold Belt (NFB). NNE-SSW trending folds and cleavage affect the Kurnool Group and overprint earlier structures in the thrust sheet. Thrusting of the Nallamalai rocks and the later structures may have been related to convergence of the Eastern Ghats terrane and the East-Dharwar-Bastar craton during Early Neoproterozoic (Greenvillian) and/or later rejuvenation related to Pan-African amalgamation of East and West Gondwana.

SPIN8: a FORTRAN 77 program for automated rotation of poles

Abstract SPIN8 is a FORTRAN 77 program for computer-assisted rotation of bedding or foliation normal, lineations, crystallographic axes, or directions of magnetization represented as geometric poles. The algorithm is based on the transformation of the axes of the reference coordinate system. The program has the capability of handling multiple rotations of up to 400 poles.

Dynamically recrystallised quartz c-axis fabrics in greenschist facies quartzites, Singhbhum shear zone and its footwall, eastern India—influence of high fluid activity

Abstract Oriented specimens of quartz tectonites from the Singhbhum shear zone (SSZ), an important tectonic element in the Precambrian eastern Indian shield, and its footwall have been analysed for c -axis fabrics of dynamically recrystallised quartz grains. In individual specimens the fabric pattern is either an asymmetric type I cross-girdle or an asymmetric, kinked single-girdle, irrespective of whether the fabric is from the SSZ or its footwall. The asymmetry of the fabric confirms a southward thrust movement of the northern block on a northerly dipping shear zone. The measured fabrics are characterised by a concentration near the Y-axis of the finite strain ellipsoid with an equally or subordinately developed concentration near the Z-axis. Although some earlier works suggest the Y-concentration of the quartz c -axis fabric as due to the influence of higher temperature (obtained in amphibolite facies), the occurrence of a similar concentration in the quartz c -axis fabric from quartz tectonites of the SSZ and its footwall, interpreted to have formed under greenschist facies condition, is explained by formation of the fabric under high fluid activity during deformation.

Chapter 18 Tectonostratigraphic evolution of the Nellore schist belt, southern India, since the Neoarchaean

Abstract Tectonostratigraphic development of the c. 300 km-long Nellore schist belt (NSB) of southern India is described in relation to the post-Neoarchaean growth of the Dharwar cratonic nucleus. Lying along the eastern margin of the Dharwar cratonic nucleus, the NSB comprises several geologically and geochemically distinct tracts of deformed Palaeoproterozoic to Mesoproterozoic volcanosedimentary successions: the Vinjamuru Group, the Kandra ophiolite complex (KOC), the Kanigiri ophiolitic melange (KOM) and the Udaigiri Group, arranged in relative order of younging. The high-grade Eastern Ghats belt occurs further to the east of the NSB with a tectonic contact. Thrust-transported oceanic crust remnants occur in the 1.9 Ga KOC, 1.34 Ga KOM, and the Vinjamuru Group, which show multiple deformation, amphibolite facies metamorphism and granitic intrusions. The available geological, geochemical and geochronological data have been examined to tentatively constrain the relative age of the different tectonostratigraphic units of the NSB, tectonic juxtaposition and implications in relation to global events in the Proterozoic. Subduction-related ocean closures outboard and east of the Dharwar Craton, evidenced by the KOC and KOM, possibly had links with the assembly of Columbia and its final dispersal, respectively.

Influence of micaceous impurity on dynamically recrystallized quartz c-axis fabric in L-S tectonites from the Singhbhum Shear Zone and its footwall, Eastern India

When temperature and strain rate remain constant the quartz c-axis fabric in deformed pure quartz aggregates, is largely dependent on deformation kinematics. Asymmetry of the fabric, e.g. in type-I asymmetric crossed girdle pattern in natural quartz tectonites, simulated fabric or experimentally deformed quartz aggregates is generally related to sense of vorticity for a non-coaxial flow. Natural quartz tectonites, however, often contain micaceous impurities. Measurement on a sample of 59 quartz tectonites with mesoscopic L-S fabric and representing low TTm, deformation under non-coaxial flow, from the Singhbhum Shear Zone and Dhanjori quartzites. Eastern India provides the basic data to quantitatively assess the influence of mica on (i) asymmetry of quartx c-axis fabric and (ii) degree of crystallographic preferred orientation, i.e. fabric intensity, taking c-axes of dynamically recrystallized quartz grains as a fabric element. A fabric intensity parameter (κ) is defined as the ratio of the greatest eigenvalue to the least eigenvalue of the orientation tensor matrix corresponding to c-axis orientations in each measured specimen. The modal percent of mica (μ) in the sample varies from 2 to 35; that of recrystallized quartz grains (ν), as opposed to relict clasts. from 45 to 98. The asymmetry of the fabrics in the above sample, measured either as the Am statistic or as the angle between the central segment of the fabric skeleton and direction of mineral elongation lineation is independent of mica content. Correlation-regression analysis of the variables κ, μ, and ν. demonstrate a negative correlation between micaceous impurity and the fabric intensity parameter. The regression equation is of the form κ = 0.13 ν1.11 μ−0.617.

Palaeoproterozoic of India

The Indian shield represents a vast repository of the Palaeoproterozoic geological record. Built over the four large amalgamated Archaean nuclei (Dharwar, Bastar, Singhbhum and Aravalli–Bundelkhand) the major and minor Palaeoproterozoic sedimentary basins and supracrustal sequences in India are comparable in scale, and perhaps also in development, to those of North America, Africa, Australia and Brazil. The deformation of these supracrustal sequences, attendant metamorphism and emplacement of plutonic bodies hold important clues to their connection with major orogenies. Research in these areas has led to investigations into global correlation, which in turn has had a direct bearing on refining models of Palaeoproterozoic supercontinent assembly and break-up. This book covers various aspects of regional geology as well as broader issues of the Indian Palaeoproterozoic geology and its global context. It is an outcome of the UNESCO-IGCP 509 Palaeoproterozoic Supercontinents and Global Evolution research project.