Saibal Gupta

Structural evolution across the Eastern Ghats Mobile Belt–Bastar craton boundary, India: hot over cold thrusting in an ancient collision zone

Along the western margin of the Eastern Ghats Mobile Belt (EGMB), ultrahigh-temperature granulites are thrust westward over hornblende granites and sedimentary rocks of the Bastar craton. Multiply deformed migmatitic gneisses, and porphyritic charnockite of the EGMB unit preserve thrust-related shear fabrics (S3M). In the cratonic foreland, undeformed granites show a progressive decrease in grain-size and increase in penetrative foliation to the east, evolving into orthogneiss near the mylonitized contact zone with charnockite. Foreland fabrics with a consistent top-to-the-west shear sense are conformable with S3M in charnockite and correlate with S3B in polydeformed orthogneisses of cratonic windows. S3B in the windows parallels S3M in overlying EGMB migmatitic gneisses. While S3M accompanied granulite metamorphism in the EGMB, S3B temperature estimates vary from T>700 °C in the windows to T<550 °C in the west. Decreasing temperature and later fabric formation in the west are explained by an evolving thermal profile in the cold craton, which is caused by thrusting against hot lower crustal EGMB rocks. Based on lithologic, structural and metamorphic variations across the contact, and resemblances between the EGMB and Rayner Complex, the craton–mobile belt boundary is considered a result of Indo-Antarctic collision, leading to the formation of an ancient supercontinent.

Alkaline Magmatism Versus Collision Tectonics in the Eastern Ghats Belt, India: Constraints from Structural Studies in the Koraput Complex

Abstract Linear domains of deformed alkaline rocks and carbonatites have recently been identified as representing sites of ancient suture zones. In peninsular India, the western margin of the Proterozoic Eastern Ghats Belt (EGB) is characterized by a series of alkaline plutons that are aligned close to the contact with the Archaean Craton. Most of the complexes were deformed and metamorphosed during a subsequent orogenic event. Unlike other plutons in the belt, the alkaline complex at Koraput reportedly escaped deformation and granulite facies metamorphism forming an anomalous entity within the zone. Multiply-deformed country rocks hosting this complex underwent syn-D1CR granulite facies metamorphism followed by D2CR thrusting, with pervasive shearing along a NE-SW trending foliation. A second granulite facies event followed localized D3CR shearing. Within the Koraput Complex, strain partitioning was responsible for preserving igneous textures in the gabbroic core, but aligned magmatic amphibole needles and plagioclase laths occasionally define a S1AC fabric. Along the margins, S1AC is rotated parallel to a NE-trending, east-dipping S2AC fabric in the gabbro, fringing syenodiorite and nepheline syenite bands. Locally, D3AC shearing follows D2AC deformation; S2AC and S3AC parallel S2CR and S3CR in the country rocks. High-grade metamorphism represented by recrystallization of amphibole and plagioclase, and breakdown of amphibole and biotite to garnet, pyroxene and K-feldspar in the complex follows D3AC. Unlike earlier reports, therefore, the Koraput body is also deformed and metamorphosed. The aligned alkaline complexes in the EGB probably represent deformed alkaline rocks and carbonatites formed by rifting related to an earlier episode of continental break-up that were deformed during subsequent juxtaposition of the EGB with the Archaean Craton. This supports the contention that the western margin of the EGB and its contact with the Archaean Craton is a suture zone related to the Indo-Antarctica collision event.

Deformation History of the Kunavaram Complex, Eastern Ghats Belt, India: Implications for Alkaline Magmatism Along the Indo-Antarctica Suture

Abstract The Kunavaram alkaline complex is a NE-SW trending elongate body located along a major lineament, the Sileru Shear Zone (SSZ) that is regarded as a Proterozoic suture related to Indo-Antarctica collision. The complex is hosted within migmatitic quartzofeldspathic gneisses, mafic granulites retrogressed to amphibolites, and quartzites. The structural evolution of the country rocks and the alkaline complex are similar. The first phase of deformation, D1, produces a pervasive segregation banding (S1) in all rock units within and outside the complex. A second deformation phase D2 isoclinally folded S1 along subvertical axial planes with shallow plunging axes. F2 isoclinal folds are ubiquitous in the country rocks and the eastern extremity of the complex. In the interior of the alkaline body, D2 strain decreases and S1 is commonly subhorizontal. While amphibolite to granulite facies conditions prevailed during deformation, post-D2 annealing textures testify to persisting high grade conditions. In the west, a NNE-SSW trending dextral shear zone with strike-slip sense (D3) truncates the complex. Within this shear zone, quartzofeldspathic country rocks are plastically deformed, while hornblende-K-feldspar assemblages of the complex are retrogressed to biotite and plagioclase. Warping related to D3 shears also resulted in fold interference patterns on the subhorizontal S1 foliation in low D2 strain domains. Based on its steep dip, north-easterly trend, and non-coaxial nature with dextral strike-slip sense, the D3 shear zone can be correlated with the SSZ. Since this shear zone, i.e., the SSZ, is not associated with primary igneous fabrics and resulted in solid state deformation of the complex, it cannot be considered as a conduit for alkaline magmatism, but is probably responsible for the post-tectonic disposition of the pluton.

Jarosite occurrence in the Deccan Volcanic Province of Kachchh, western India: Spectroscopic studies on a Martian analog locality

The sulfate mineral jarosite is considered a key indicator of hydrous, acidic, and oxidizing conditions on the surface of early Mars. Here we report an analog terrestrial locality hosting jarosite from Matanumadh, Kachchh, western India, using detailed spectroscopic studies on weathered basalts of the Deccan Volcanic Province and overlying tuffaceous shales and sandstones of the Matanumadh Formation. Hyperspectral data in the visible/near-infrared (350–2500 nm) to midinfrared (4000–400 cm−1) region of the electromagnetic spectrum and X-ray diffraction patterns have been acquired on samples collected from the field to detect and characterize the hydrous sulfate and phyllosilicate phases present at the studied site. Hydrous sulfates occur in association with Al-rich phyllosilicates (kaolinite) that overlie a zone of Fe/Mg smectites in altered basalts. Jarosite is found within both saprolitic clay horizons altered from the basalt and within variegated sandstone and shale/clay units overlying the saprolite; it mostly occurs as secondary veins with or without gypsum. Jarosite is also seen as coatings on kaolinite clasts of varying shapes and sizes within the tuffaceous variegated sandstone unit. We argue that the overall geological setting of the Matanumadh area, with this unusual mineral assemblage developing within altered basalts and in the overlying sedimentary sequence, mimics the geological environment of many of the identified jarosite localities on Mars and can be considered as a Martian analog from this perspective.

The Rauer-Rengali connection in the Indo-Antarctica amalgam: Evidence from structure, metamorphism and geochronology

Abstract India and East Antarctica collided during assembly of the Rodinia supercontinent at around 1 Ga. Granulites related to this orogeny are exposed in the Eastern Ghats Province (EGP) in India, and these are believed to have been contiguous with granulites of the Rayner Province in East Antarctica at that time. In the Indian segment, we describe a shear zone between the EGP and the Rengali Province to its north along which strongly foliated bands of garnetiferous quartzofeldspathic gneisses, khondalites and charnockites are intercalated. The foliation is consistently east–west trending and subvertical, with downdip intersection lineations. Maximum asymmetry in horizontal sections and textural analysis using electron backscattered diffraction (EBSD) analysis confirm that the transport vector during shearing was horizontal. The shear zone is interpreted as a dextral strike-slip fault that operated under greenschist-facies conditions, juxtaposing 1 Ga EGP granulites with 2.8 Ga cratonic granulites to the north. The corresponding region in East Antarctica is represented by the Rauer Group, where intercalations between 2.8 and 1.0 Ga, vertically orientated lithologies, are observed alongside 0.5 Ga shear zones. These features in the Rauer Group can be correlated with those in the Rengali Province, further supporting existing palaeogeographical reconstructions of Gondwana.

Alteration and submergence of basalts in Kachchh, Gujarat, India: implications for the role of the Deccan Traps in the India–Seychelles break-up

Abstract The Deccan Trap flood basalt volcanism has commonly been considered to have initiated the break-up of India from the Seychelles (c. 62.5 Ma). In Kachchh, Gujarat, western India, the sedimentary succession in the Paleocene Matanomadh Formation was deposited on highly weathered Deccan Trap basalts that were altered to kaolinite before basin formation. This contrasts with the weathering pattern on flat-topped hills of the Deccan Traps outside the Cenozoic rift basins in the Kachchh region and other parts of India, where basalt is dominantly altered to smectitic minerals. As basalts that are altered to smectite and kaolinite occur just across the faults that bound the Matanomadh Basin, the differential weathering cannot be attributed to climate. Geochemical modelling shows that kaolinite stabilizes in preference to smectite if CO2- and O2-buffered rainwater interacts with well-drained basaltic rock at high water:rock ratios. Such conditions can be accomplished by rainfall on a slope created by Cenozoic rifting that exposes the graben flank and basin floor basalts to continuously flowing water, the composition of which is buffered by equilibration with the atmosphere. As the rift post-dates both the eruption of the basalts and subsequent smectite formation, the associated extensional tectonics must be unrelated to flood basalt volcanism, and is most likely to correspond to the India–Seychelles break-up.

Laterite Covered Mafic-Ultramafic Rocks: Potential Target for Chromite Exploration - A Case Study from Southern Part of Tangarparha, Odisha

Exposed chromite deposits in the Sukinda belt, Odisha, India, have already been identified and exploited; but a large part of the area is covered by laterite and remains unexplored. As a case study to establish the feasibility of chromite exploration under laterite rocks, an integrated ground-based gravity, magnetic and very low frequency (VLF) - electromagnetic study was performed over a laterite-covered area at Tangarparha within the belt. North of the present laterite-covered area, a quartzofeldspathic gneiss contains proved chromite pods within ultramafic complexes. The gneiss-laterite contact is depicted by a transition from low to high in both gravity and magnetic anomaly maps at the northern part of the present study area. High Bouguer and residual anomalies (> 10 mGal and > 1 mGal, respectively) within the laterite-covered area indicates the existence of a high density rock in the sub-surface. The 2D models of the residual gravity anomaly depict the presence of high density (> 3570 kg/m3) layer under the laterite cover. The 2D magnetic models mostly reveal shallow surface effects of laterite covers. However, along profiles P2 and P3 high magnetic susceptibilities are detected at depths ≥ 20 m, and are likely to be caused by sub-surface chromite mineralization, as the locations are coincident with gravity highs. High current densities in VLF profiles are also recorded at the same locations confirming the presence of conducting sub-surface layer. Thus, the zone with high density, magnetic susceptibility and conductivity is most likely to be a chromitite-bearing sub-surface layer. The targeted chromite ore is distributed in east-west direction in the form of discontinuous pods of variable vertical thickness and strike lengths at the centraleastern part of profiles P2 and P3. The present study demonstrates that integrated use of ground gravity, magnetic and VLF techniques can effectively identify the target chromite deposits even under lateritic cover.

Proterozoic Orogens of India — A Critical Window to Gondwana

There are a number of descriptions, reviews and books on the various cratons that comprise the Indian shield. Most of these describe the geology of the individual cratons, with little or no reference to the processes that amalgamated these disparate units to form the shield. Recent studies indicate that the cratons were possibly amalgamated along intensely deformed and metamorphosed mobile zones that may well represent ancient equivalents of the Himalayas and Alps, the culmination of continent-continent collision processes in the Proterozoic. Strangely, these ancient orogens have not been the dedicated subject of any book, and a conscious effort does not appear to have been made to interpret the geology of these belts from a plate tectonic perspective. It is for this reason that this book by T. R. K. Chetty is an interesting and important contribution to the available literature on Proterozoic orogens of India. Chetty has enviable experience of working on precisely this aspect of the Indian shield, and much of his work has been, and still is directed towards identifying and characterizing the tectonic processes associated with these mobile belts or Proterozoic orogens. The book discusses four major Proterozoic orogens of India – the Southern Granulite Terrane, the Eastern Ghats Mobile Belt, the Central Indian Tectonic Zone and the Aravalli-Delhi Orogenic Belt, within an uncommon framework that attempts to assess their importance in the formation of the Gondwana supercontinent. The author has personally contributed substantially to the scientific dataset available from the first two orogens, but has also integrated information from a wide variety of sources along with his own personal perspective. He has also extensively covered the two orogens that were not subjects of his personal research, and has successfully integrated them into the framework constructed for the other two orogens. The book is divided into a sequence of six chapters. The book begins with an introduction that outlines the basic concept of an orogen, and then defines the terminology that is used in this book to characterize Proterozoic orogens. The introduction is engaging as well as informative, and has the requisite historical documentation of the evolution of certain geological terms. The structure of the chapter is logical, and the combination of features that are used to identify orogens in the Indian shield are listed and defined. This is followed by separate chapters on each of the four orogens mentioned above. Each chapter divides the individual orogens into units (blocks, sectors or terranes) separated by distinct structural discontinuity zones (shear zones), and discusses each unit and discontinuity from the perspective of its structure, along with available metamorphic, geochronologic and geophysical data. For each such unit or discontinuity zone, the collated information is integrated into a plausible tectonic model through a section that is described as a ‘synthesis’. This approach is perhaps what sets the book apart from others of its genre, as it emphasizes the importance of integrating information from various methods, including geophysics, if a viable tectonic mechanism is to be devised. Description of the individual orogens is adequate, and I consider it unnecessary to go into critical evaluation of the individual descriptions of the respective terranes, since aspects that may be missing are in any case available from a large body of existing, accessible literature. What sets this book apart is its approach, which concentrates on collating the information relevant to the formulation of the tectonic model. The most unique aspect of the book is the last chapter, in which the orogens are described in the context of their relevance and contiguity with other orogenic belts that were utilized to construct Gondwanaland. The Indian orogens are compared and correlated with their supposed counterparts in other Gondwanaland fragments. The comparison is effected mostly by correlating radiometric ages. Unfortunately, the structural, metamorphic and geophysical information is not fully utilized for establishing these correlations, which is a little baffling since this information has actually been collated in the four chapters describing the orogens. In all, however, this chapter is potentially very useful for those interested in the position of India in reconstructed Precambrian supercontinents. The treatment of the individual orogens is comprehensive and incorporates most recent references relevant to a tectonic interpretation. This is done largely without obvious partiality towards views that disagree with those of the author, which is commendable. However, there are instances where more classical, field-based studies appear to have been overlooked, and this could definitely be cited as one of the few short-comings of the book. An aspect that may also be construed as a weakness is the absence of hard data, apart from an occasional geological map with complete fabric orientation information. Indeed, many of the structural sections are schematic in nature, and sketches of individual outcrop sections lack supporting field structural data, which somewhat limits their usefulness. Field photographs are also lacking; these would certainly have been useful in representing key mesoscopic structures that form the bases of critical interpretations. Another aspect that is entirely missing is microstructural information. Considering that this book describes a number of major shear zones, photomicrographs and petrofabric information from these zones would have been of great value in places where they are available. Any book that deals with a topic as broad as this is bound to have lacunae, and I have pointed out some of these. However, what makes this book particularly enjoyable is its language. The book is an excellent read, and is written in an extremely lucid manner. Given the topic, this is possibly best suited to a more specialized readearship of people involved in research on mobile belts and their global tectonic significance. For a first hand reference on mobile belts in the Indian shield, I would be hard-pressed to think of an alternative to the “Proterozoic Orogens of India – a critical window to Gondwana” at the present time. I would personally thank Dr. Chetty for deciding to write a book on this topic at this juncture. Given the focus at present on supercontinent formation, and the role of mobile belts in assembling them, I am sure that I will not be the only delighted reader.

Structural data extraction from stereographic or equal area projections using image processing

Structural orientation data from the field are commonly represented on stereographic or equal area projections, and the raw data are usually not accessible to other workers for integration into larger datasets or comparative study. We have developed a technique for retrieving raw planar structural data from the above projection types using image analysis techniques. The principle involves two steps: (i) conversion of the foliation pole plots in the image into Cartesian coordinates, and (ii) inversion of the data soobtained into a geographic reference frame. The process involves either scanning of the target projection, or cropping of the projection figure from a pdf file using image processing software, followed by tracing of the data points and generation of an image file that can be subsequently input into a MATLAB supported program, “StrucExtract”. The results from the program are displayed in the form of dips and dip directions. The program has been tested with synthetic and natural examples, and may be invaluable for retrieval of structural data from older structural literature.