We have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self-consistent set of global isochrons and associated plate reconstruction poles. The age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading. Ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models. We have constructed an age grid with error estimates for each grid cell as a function of (1) the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid, (2) the distance of a given grid cell to the nearest magnetic anomaly identification, and (3) the gradient of the age grid: i.e., larger errors are associated with high age gradients at fracture zones or other age discontinuities. Future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere, the heat loss of the Earth, ridge-push forces through time, asymmetry of spreading, and providing constraints for seismic tomography and mantle convection models.

Digital isochrons of the world's ocean floor

The convincing identification of terrestrial meteorite impact structures: What works, what doesn't, and why

Abstract Extending along the east coast of peninsular India, the Eastern Ghats expose a deep section through a composite orogenic belt that once formed part of the Proterozoic mobile belt system within East Antarctica and East India. The critical evaluation of the existing geological and isotopic data strongly suggests that this orogenic belt includes not only the granulite facies Eastern Ghats Belt but also the Nellore-Khammam Schist Belt and lower grade units at the southern margin of the Singhbhum Craton. The present authors propose its subdivision into four crustal provinces with widely different geological evolutions. The Rengali and Jeypore Provinces formed at the margin of the Bhandara Craton in the Late-Archaean. In the Krishna Province, volcanosedimentary rocks equivalent to the Cuddapah Supergroup accumulated, probably on the Dharwar Craton in the Palaeoproterozoic, and the major tectonometamorphic event took place between 1.67 and 1.55 Ga, subsequent to a short-lived igneous activity. The Eastern Ghats Province, which shows considerable similarities with the Rayner Province of East Antarctica, was strongly affected by pervasive deformation, high-grade metamorphism and crustal-derived magmatism between 1.1 and 0.9 Ga, which extensively modified the crustal structure of present eastern peninsular India. Neoproterozoic and Early Phanerozoic tectonothermal activities were largely restricted to pre-existing shear zones, but the present configuration of the composite orogenic belt may have been achieved only during the Pan-African Orogeny.

Crustal architecture and evolution of the Eastern Ghats Belt and adjacent regions of India

Precambrian crustal evolution of Peninsular India: A 3.0 billion year odyssey

Late Cretaceous charnockite with adakitic affinities from the Gangdese batholith, southeastern Tibet: Evidence for Neo-Tethyan mid-ocean ridge subduction?

The Precambrian granite-Greenstone terrain of eastern Indian shield includes Singhbhum-Orissa craton, Singhbhum Mobile Belt supracrustals (Singhbhum Group) along its northern, eastern and western margins, and Chotanagpur Gneissic Complex towards further north. Recent isotopic ages along with other geological considerations show that these three crustal units perhaps constitute a single cratonic block, which grew in sequence from -3 55 to 1 00 Ga. The center of crustal growth gradually migrated with younging from the present south to north. Prior to the onset of this crustal growth, there was a still older crust in this region at -3 60 Ga, the remnants of which are preserved only as detrital zircons at present. However, the Singhbhum-Orissa craton represented the nucleus of the presently existing cratonic block, which had formed between -3 55 and 3 12 Ga through two successive supracrustal-granite cycles, well separated by an erosional unconformity. The first cycle included Older Metamorphic Group supracrustals, Older Metamorphic Tonalite Gneiss and Singhbhum Granite, phase-I, Singhbhum Granite, phases II, Chakradharpur Granite Gneiss and Nilgiri Granite that grew in sequence from -3 55 to 3 30 Ga. The second cycle included Iron Ore Group supracrustals followed by emplacement of Bonai Granite and Singhbhum Granite, phase III, and had an evolutionary history ranging from -3 30 to 3 16 Ga or up to -3 12 Ga. The supracrustal-Granite cycles began with formation of supracrustal sequences in anorogenic setting, followed successively by major folding during following orogeny and final emplacement of orogenic granites into these deformed supracrustals. The emplacement of anorogenic Mayurbhanj Granite pluton with Mayurbhanj Gabbro along the northeastern margin of the craton at -3 09 Ga marked the stabilization of Singhbhum-Orissa craton. Emplacement of Singhbhum Granite, phase II, Singhbhum Granite, phase III and Mayurbhanj Gabbro recorded metamorphism on the older rock units of Singhbhum-Orissa craton. In the late Meso- to Neoarchaean period Singhbhum-Orissa craton grew along its northern, western and eastern margins by formation of a supracrustal sequence of syn-Rift nature including mostly clastic sediments and minor basic and acid volcanics, which followed upward by major basic volcanisms. These supracrustals included Singhbhum Group, which unconformably overlay the Singhbhum Granite, phase-III and equivalent granite basements and was subjected to two major phases of folding, before being overlain by undeformed. Simlipal volcano-Sedimentary basin occurring at the eastern part of the craton. The evolution of Singhbhum Group and Simlipal basin took place between -3 12 and 3 09 Ga, before being intruded by anorogenic Mayurbhanj Granite( -3 09 Ga) and was followed by the formation of overlying riftogenic Dalma Group and adjacent Dhanjori Group of basic volcanics and some minor acid plutonics at the marginal part of the craton at -2 80 Ga and their subsequent major metamorphism at -2 50 Ga. A critical review of present available geological and radiometric age data suggests that Chotanagpur Gneissic Complex was evolved from a supracrustal precursor, some of them were formed at least before -2 3 Ga. The Singhbhum Group supracrustals and adjacent Chotanagpur Gneissic Complex perhaps had evolved from a common sedimentary precursor along with volcanics, which were deposited in a marine basin that existed at the present north of Singhbhum-Orissa craton during late Mesoarchaean period. The syn-rift sedimentary assemblage of the Singhbhum Group was deposited along the margin of Singhbhum Orissa craton, whereas the post-rift stable shelf sedimentary precursor of the Chotanagpur Gneissic Complex was deposited towards further north. In the Neoarchaean period between - >3 09 Ga and -2 5 Ga the nature of deformation along the marginal part of Singhbhum-Orissa craton was extensional type. In the following Palaeo- to Mesoproterozoic period, the protolith of Chotanagpur Gneissic Complex grew in sequence between - > 2 3 and 1 0 Ga through two time-Separated supracrustal granite cycles where the first and second cycles ended at -1 60 and 1 00 Ga respectively. Radiometric ages suggest that Chotanagpur Gneissic Complex along with Singhbhum Group experienced major magmato-metamorphic activities during Palaeo- and Mesoproterozoic periods at -2 5 or >2 3 Ga, 1 6 Ga, 1 0 Ga and 0 9 Ga. During this period Singhbhum-Orissa craton recorded basic volcanism (Jagannathpur and possibly Malangtoli Volcanics) at -2.25 Ga, followed by deposition of clastic sediments (Kolhan Group) and finally emplacement of mafic dykes and sills (Newer Dolerite) in three successive sequences at -2.0 Ga, 1.6 Ga and 1.0 Ga, where all these phenomena took place under anorogenic setting. The Singhbhum Shear Zone, which marked a tectonic boundary between Singhbhum-Orissa craton and Singhbhum Mobile Belt to north, was reactivated several times during geological past, the oldest being at -3.09 Ga, followed by emplacement of Soda Granite pluton along this shear zone at -2.22 Ga, copper mineralization at -1.77 Ga, shearing at - 1.67-1.63 Ga, uranium mineralisation at -1.58- 1.48 Ga and the final phase of shearing at - 1.0 Ga.

Precambrian Chronostratigraphic Growth of Singhbhum-Orissa Craton, Eastern Indian Shield: An Alternative Model

Geochronological Constraints on Evolution of Singhbhum Mobile Belt and Associated Basic Volcanics of Eastern Indian Shield

The Lonar crater, India, is the only well-preserved simple crater on Earth in continental flood basalts; it is excavated in the Deccan trap basalts of Cretaceous-Tertiary age A representative set of target basalts, including the basalt flows excavated by the crater, and a variety of impact breccias and impact glasses, were analyzed for their major and trace element compositions Impact glasses and breccias were found inside and outside the crater rim in a variety of morphological forms and shapes Comparable geochemical patterns of immobile elements (eg, REEs) for glass, melt rock and basalt indicates minimal fractionation between the target rocks and the impactites We found only little indication of post-impact hydrothermal alteration in terms of volatile trace element changes No clear indication of an extraterrestrial component was found in any of our breccias and impact glasses, indicating either a low level of contamination, or a non-chondritic or otherwise iridium-poor impactor

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Target rocks, impact glasses, and melt rocks from the Lonar impact crater, India: Petrography and geochemistry

2.8 Ga Old Anorogenic Granite-Acid Volcanics Association from Western Margin of the Singhbhum-Orissa Craton, Eastern India

The Eastern Ghats granulite belt of India has traditionally been described as a Proterozoic mobile belt, with probable Archaean protoliths. However, recent findings suggest that synkinematic development of granulites took place in a compressional tectonic regime and that granulite facies metamorphism resulted from crustal thickening. The field, petrological and geochemical studies of a charnockite massif of tonalitic to trondhjemitic composition, and associated rocks, document granulite facies metamorphism and dehydration partial melting of basic rocks at lower crustal depths, with garnet granulite residues exposed as cognate xenoliths within the charnockite massif. The melting and generation of the charnockite suite under granulite facies conditions have been dated c. 3.0 Ga by Sm–Nd and Rb–Sr whole rock systematics and Pb–Pb zircon dating. Sm–Nd model dates between 3.4 and 3.5 Ga and negative epsilon values provide evidence of early Archaean continental crust in this high-grade terrain.

Saumitra Misra

Papers

Precambrian Chronostratigraphic Growth of Singhbhum-Orissa Craton, Eastern Indian Shield: An Alternative Model

Geochronological Constraints on Evolution of Singhbhum Mobile Belt and Associated Basic Volcanics of Eastern Indian Shield

Target rocks, impact glasses, and melt rocks from the Lonar impact crater, India: Petrography and geochemistry

2.8 Ga Old Anorogenic Granite-Acid Volcanics Association from Western Margin of the Singhbhum-Orissa Craton, Eastern India

Early Archaean continental crust in the Eastern Ghats granulite belt, India: isotopic evidence from a charnockite suite