Articles de revues : « Sedimentary rocks – Indiana »

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Thomka, James R., Thomas E. Bantel et Marissa J. Tomin. « Unusual preservation of the trace fossil Conostichus in middle Silurian carbonate facies of Indiana, USA ». Canadian Journal of Earth Sciences 53, n^o 8 (août 2016) : 802–7. http://dx.doi.org/10.1139/cjes-2015-0198.

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The long-ranging, plug-shaped ichnofossil Conostichus, attributed to solitary polypoid cnidarians, is most commonly described in the Paleozoic from fine-grained siliciclastic sediments, with few descriptions from carbonate settings. The few described examples of putative Conostichus preserved in carbonate sedimentary rocks of Paleozoic age are essentially conical masses lacking recognizable external radial or longitudinal structures. Herein, we describe the occurrence of well-preserved examples of this ichnogenus within middle Silurian (Wenlock: Sheinwoodian) carbonate sedimentary rocks of the Massie Formation from southeastern Indiana, USA. These specimens represent isolated apical discs of Conostichus with prominent radiating physal impressions displaying duodecimal symmetry. Interestingly, well-preserved specimens co-occur with relatively poorly preserved (“typical carbonate”) specimens. The factors responsible for this unusual taphonomic state are unclear, but the most likely explanation is that exceptionally preserved specimens represent burrows that were somehow infilled, at least in their apical terminations, with fine-grained carbonate sediment, enhancing preservational fidelity. In contrast, other burrows had their apical regions passively filled with larger carbonate particles that could not preserve fine details. This occurrence indicates that early Paleozoic carbonate sediments are capable of preserving Conostichus, and potentially other ichnofossils, in similar modes to later Paleozoic siliciclastic deposits.

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Sexton, John L., L. W. Braile, W. J. Hinze et M. J. Campbell. « Seismic reflection profiling studies of a buried Precambrian rift beneath the Wabash Valley fault zone ». GEOPHYSICS 51, n^o 3 (mars 1986) : 640–60. http://dx.doi.org/10.1190/1.1442118.

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Sixty‐eight kilometers of 12-fold seismic reflection data were collected in the Wabash River Valley of southwestern Indiana and southeastern Illinois to investigate the configuration of a basement structure inferred from regional gravity and magnetic anomaly data. The seismic profiles were also positioned to cross faults of the Wabash Valley fault system in a number of locations. Interpretation of the seismic reflection profiles and detailed gravity and magnetic profile data provides evidence for a series of northeasterly trending grabens in the basement. These grabens are filled with pre‐Mt. Simon layered rocks and are overlain by Paleozoic sedimentary rocks of the Illinois basin. Beneath the Wabash River near Grayville, Illinois, an interpreted graben (the Grayville graben) is approximately 15 km wide and contains about 3 km of fill. Individual boundary faults for the graben cut prominent reflectors within pre‐Mt. Simon rocks and display offsets of up to 500 m. The interpreted configuration of basement faults and thickness of pre‐Mt. Simon layered rocks provide evidence of a late Precambrian rift inferred to be one arm of the New Madrid rift complex. Post‐Pennsylvanian faulting of the Wabash Valley fault system is visible on the seismic reflection record sections as small offsets (less than 100 m) on steeply dipping normal faults. The downward projection of these faults intersects the older large‐offset faults within the pre‐Mt. Simon rocks suggesting that the Wabash Valley faults represent a post‐Pennsylvanian reactivation of the rift system.

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Kassab, Christine M., Samantha L. Brickles, Kathy J. Licht et G. William Monaghan. « Exploring the use of zircon geochronology as an indicator of Laurentide Ice Sheet till provenance, Indiana, USA ». Quaternary Research 88, n^o 3 (20 septembre 2017) : 525–36. http://dx.doi.org/10.1017/qua.2017.71.

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AbstractA pilot study was designed to evaluate the potential of zircon geochronology as a provenance indicator of till from the Lake Michigan, Saginaw, and Huron-Erie Lobes of the Laurentide Ice Sheet. Based on existing ice flow-path models, we hypothesized that till from each lobe would have different zircon age population distributions because the lobes originated from regions of the Canadian Shield with different bedrock ages. After correcting for zircon fertility, the majority of grains in all till samples are 1600–950 Ma, with ~30 % of ages >2500 Ma. This similarity means that till from the three lobes cannot be clearly differentiated based on their zircon populations. The dominant ages found and the homogeneity of distributions in the till indicates a non-Shield source and, instead, reflect an origin from some combination of underlying till and sedimentary bedrock in the Great Lakes region. Even though the datasets are small, the tills have similarities to zircon distributions in Michigan Basin rocks. This implies that a substantial fraction of zircon in till was not transported long distances from the Canadian Shield. Although zircon ages are not distinct between tills, the method provides a novel application to understand Laurentide Ice Sheet glacial erosion and transport.

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Saja, David B., et Joseph T. Hannibal. « Quarrying History and Use of the Buena Vista Freestone, South-Central Ohio : Understanding the 19th Century Industrial Development of a Geological Resource ». Ohio Journal of Science 117, n^o 2 (29 juin 2017) : 35. http://dx.doi.org/10.18061/ojs.v117i2.5498.

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The Buena Vista Member of the Mississippian Cuyahoga Formation is an economically valuable freestone that is homogeneous with almost no sedimentary structures. The Buena Vista was one of the earliest clastic rocks quarried in Ohio. Early quarries dating at least back to 1814 were located in the hills on the north bank of the Ohio River near the village of Buena Vista, south-central Ohio. By the 1830s, quarries had also opened up along the route of the Ohio & Erie Canal in the Portsmouth area to the east; followed by quarries that opened along a railway line that ran north up the Scioto River valley. Waterways transported the Buena Vista to many cities and towns, including Cincinnati, Ohio, Louisville, Kentucky, and Evansville, Indiana, on the Ohio River, New Orleans on the Mississippi River, and Dayton and Columbus on the Ohio canal system. Later railways transported this stone further afield to Illinois, Wisconsin, and Alberta. Census reports, industry magazines, and other historical accounts document the use of this stone across much of the eastern US and into Canada. Historically, it has been used for a variety of items, including entire buildings, canal structures, fence posts, and laundry tubs. Some 19th-century structures built with this stone remain in cities where it was once commonly used. Literature reviews, field observations, and lab analyses are here compiled as a useful reference to both the urban and field geologist in the identification of the Buena Vista Member, a historically important building stone, in buildings and outcrops, respectively.

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Xu, Hengchao, Xiaotong Peng, Shun Chen, Jiwei Li, Shamik Dasgupta, Kaiwen Ta et Mengran Du. « Macrofaunal burrowing enhances deep-sea carbonate lithification on the Southwest Indian Ridge ». Biogeosciences 15, n^o 21 (30 octobre 2018) : 6387–97. http://dx.doi.org/10.5194/bg-15-6387-2018.

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Abstract. Deep-sea carbonates represent an important type of sedimentary rock due to their effect on the composition of the upper oceanic crust and their contribution to deep-sea geochemical cycles. However, the role of deep-sea macrofauna in carbonate lithification remains poorly understood. A large lithified carbonate area, characterized by thriving benthic faunas and a tremendous amount of burrows, was discovered in 2008, blanketing the seafloor of the ultraslowly spreading Southwest Indian Ridge (SWIR). Benthic inhabitants – including echinoids, polychaetes, gastropods and crustaceans – are abundant in this carbonate lithified area. The burrowing features within these carbonate rocks, as well as the factors that may influence deep-sea carbonate lithification, were examined. We suggest that burrowing in these carbonate rocks enhances deep-sea carbonate lithification. We propose that active bioturbation may trigger the dissolution of the original calcite and thus accelerate deep-sea carbonate lithification on mid-ocean ridges. Macrofaunal burrowing provides a novel driving force for deep-sea carbonate lithification at the seafloor, illuminating the geological and biological importance of bioturbation in global deep-sea carbonate rocks.

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Shukla, Shradha. « Provenance Characterization and Palaeoenvironmental Analysis of the Meta-Sedimentary Rocks of Sonaghati Formation, Betul District, Madhya Pradesh Using Geochemical Approach ». Journal of The Indian Association of Sedimentologists 38, n^o 1 (30 juin 2021) : 75–92. http://dx.doi.org/10.51710/jias.v38i1.181.

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Betul belt, ENE-WSW trending, 135 km long, prominent litho-tectonic unit exposed in the central part of Central Indian Tectonic Zone (CITZ) is composed of meta-sedimentary & meta-volcanic rocks intruded by mafic-ultramafic and granitic suite of rocks, belonging to Palaeoproterozoic to Neoproterozoic age. This belt is traversed by several ENE-WSW trending, sub-vertical ductite shear zones. The meta-sedimentary rocks of Sonaghati Formation were geochemically characterized and their geochemical composition was interpreted for provenance characterization and paleo-environmental assessment. The weathering indices including Chemical index of Alteration, Chemical index of Weathering, Plagioclase Index of Alteration and Weathering Index of Parker indicate that theses meta-sedimentary rocks have witnessed the substantial amount of weathering at the source without any evidence of potash metasomatism. The Bivariate plots using the major and trace element composition show co-linear trends, which reflect that all these samples belong to co-genetic population and the visible compositional variation could be attributed to chemical, mineralogical and textural maturity. The Sonaghati metasedimentary rocks are enriched in REE with negative Eu anomaly. The LREE enrichment varies from 122 to 174 times and that of the HREE enrichment ranges from 12 to 31 times of Chondrite indicating highly varied protoliths. The provenance characterization was attempted using the large ion lithophile elements and high field strength elements. The results show that the precursor for these meta-sedimentary litho-units are mixed source with the major contributor being felsic to intermediate and minor contribution has come from the mafic end members. These meta-sedimentary rocks were deposited in the overall semi arid climate with a sequential transition, suggesting the variable climatic conditions ranging from semi-arid to arid. The Cu/Zn, V/Cr ratios, and presence of pyrites dissemination and stringers eventually indicate the prevalence of reducing environmental conditions during the deposition of these meta-sediments.

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Zagorevski, A., et V. McNicoll. « Evidence for seamount accretion to a peri-Laurentian arc during closure of Iapetus 1This article is one of a series of papers published in CJES Special Issue : In honour of Ward Neale on the theme of Appalachian and Grenvillian geology.2 Geological Survey of Canada Contribution 20100465. » Canadian Journal of Earth Sciences 49, n^o 1 (janvier 2012) : 147–65. http://dx.doi.org/10.1139/e11-016.

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The Red Indian Line is the fundamental Iapetus suture zone in the Newfoundland Appalchians along which the main tract of the Iapetus Ocean was consumed. Despite being the site of the closure of a wide ocean, few vestiges of the Iapetus plate have been accreted along Red Indian Line. Ordovician rocks in the Notre Dame Bay area preserve the only evidence for accretion of a seamount in Newfoundland. The seamount is characterized by alkali basalt and hypabyssal rocks that are juxtaposed with Darriwilian peri-Laurentian volcanic arc rocks (466 ± 4 and 467 ± 4 Ma) along a major mylonite zone. The mylonite zone lacks sedimentary rocks suggesting that the seamount was accreted to the arc along a sediment-starved interface and that significant subduction erosion took place along the Laurentian margin. Identification of subduction erosion indicates that an accretionary prism did not exist outboard of Laurentia in Newfoundland, in contrast to the well developed accretionary prisms of the Caledonides.

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Jnawali, Bharat Mani. « Tectonic setting of the Nepal Himalaya and its potential for hydrocarbon exploration ». Journal of Nepal Geological Society 39 (25 septembre 2009) : 77–84. http://dx.doi.org/10.3126/jngs.v39i0.31490.

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Nepal lies at the collision zone between the Indian subcontinent and the Tibetan Plateau of the Eurasian continent. It is made up of enormous tectonic stacking of sedimentary and metamorphic rocks with granite intrusions that resulted from the collision and under-plating of the Indian Craton with the Lhasa block of Tibet. The five major tectonic zones separated from each other by thrust contacts from south to north are the Terai, Siwalik or Sub Himalaya, Lesser Himalaya, Higher Himalaya and Tibetan Tethys. On the northern margin of the Indian subcontinent, foreland sedimentary basins began to develop immediately after the terminal collision between the northward drifting Indian Plate and relatively passive Eurasian Plate in Late Eocene time. The southern part of Nepal known as the Terai and Siwalik foothill, lies in the northern margin of the Ganga Basin and Purnea Basin that extend from India. Such basins with thick accumulation of sediments are considered as the potential area for petroleum exploration. Regional scale seismic reflection, gravity and magnetic data combined with surface mapping and basin analysis have established the subsurface framework of southern Nepal. Geological settings potential for hydrocarbon prospects recognized in Nepal include structural traps related to normal faulting involving pre-Siwalik formation and thrusting involving Siwaliks, structural traps associated with frontal blind thrusts, anticlines and thrust-faults, basement controlled structures and stratigraphic pinchouts. Drilling data consists of only one well drilled in the eastern part of Nepal. Oil and gas seeps have been observed in Dailekh area emanating through deep faults. Geochemical analyses of these seep samples indicate that these oil and gas have geologic origin from mature source rocks. Various outcrop samples from different parts of the country have been found rich in organic carbon. Source-rock maturity basin modeling constructed for various sections indicates that the level of thermal maturity is within oil and gas generating window. The Potwar Basin to the west in Pakistan and Assam Basin to the east in India having similar geologic setting to that of Nepal are producing oil and gas for a long time. In the Indo-Gangetic Plain across the border on Indian side, many deep wells have recorded the presence of gas and high content of organic carbon. Assessment of the available data acquired so far indicate that there is a fairly good possibility of discovering petroleum resource in Nepal.

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Levchenko, O. V., Yu G. Marinova, M. V. Portnyagin, R. Werner et L. I. Lobkovsky. « New data on geology of the Osborn Knoll, Indian ocean ». Доклады Академии наук 489, n^o 6 (23 décembre 2019) : 631–36. http://dx.doi.org/10.31857/s0869-56524896631-636.

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The Osborne Plateau is a large intraplate elevation in the eastern part of the Indian Ocean, which has been poorly studied by geological and geophysical methods. In cruise SO258/1 on RV Sonne, were collected new data with Parasound seismic proﬁling and multi-beam echo sounder survey. Fractures in the sedimentary cover, which extend to the bottom surface, indicate on high neotectonic activity in the area of the Osborne Plateau. It can continue up to the present, as well as in the adjacent segment of the Ninetyeast Ridge, where strong earthquakes are recorded. Two reflectors in the upper part of the sedimentary cover mark global regressive changes in the World Ocean level at the boundary of the Miocene / Pliocene and Pliocene / Pleistocene. The reflector in the sediments at the boundary of the Lower / Upper Pliocene is associated with a change in the regional hydrodynamic regime at the time in the eastern Indian Ocean. The rocks dragged on the Osborn Knoll are identical to volcanic rocks of the Ninetyeast Ridge, confirming their assumed genetic similarity, but they are more identical to basalts of the Kerguelen plateau. Extremely modified vitroclastic tuffs appear to have been formed as a result of explosive volcanic eruptions of alkaline basalts or foidites in subaeral or relatively shallow water conditions and represent the most recent eruption in the region.

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Treloar, Peter J., Richard M. Palin et Michael P. Searle. « Towards resolving the metamorphic enigma of the Indian Plate in the NW Himalaya of Pakistan ». Geological Society, London, Special Publications 483, n^o 1 (2019) : 255–79. http://dx.doi.org/10.1144/sp483-2019-22.

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AbstractThe Pakistan part of the Himalaya has major differences in tectonic evolution compared with the main Himalayan range to the east of the Nanga Parbat syntaxis. There is no equivalent of the Tethyan Himalaya sedimentary sequence south of the Indus–Tsangpo suture zone, no equivalent of the Main Central Thrust, and no Miocene metamorphism and leucogranite emplacement. The Kohistan Arc was thrust southward onto the leading edge of continental India. All rocks exposed to the south of the arc in the footwall of the Main Mantle Thrust preserve metamorphic histories. However, these do not all record Cenozoic metamorphism. Basement rocks record Paleo-Proterozoic metamorphism with no Cenozoic heating; Neo-Proterozoic through Cambrian sediments record Ordovician ages for peak kyanite and sillimanite grade metamorphism, although Ar–Ar data indicate a Cenozoic thermal imprint which did not reset the peak metamorphic assemblages. The only rocks that clearly record Cenozoic metamorphism are Upper Paleozoic through Mesozoic cover sediments. Thermobarometric data suggest burial of these rocks along a clockwise pressure–temperature path to pressure–temperature conditions of c. 10–11 kbar and c. 700°C. Resolving this enigma is challenging but implies downward heating into the Indian plate, coupled with later development of unconformity parallel shear zones that detach Upper Paleozoic–Cenozoic cover rocks from Neoproterozoic to Paleozoic basement rocks and also detach those rocks from the Paleoproterozoic basement.

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Bharali, Bubul, Pradip Borgohain, Devojit Bezbaruah, V. Vanthangliana, Parakh Protim Phukan et Raghupratim Rakshit. « A geological study on Upper Bhuban Formation in parts of Surma Basin, Aizawl, Mizoram ». Science Vision 17, n^o 3 (30 septembre 2017) : 128–47. http://dx.doi.org/10.33493/scivis.17.03.02.

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Tertiary sediments are thickly deposited in most part of the northeast India attaining a maximum thickness of ±7 km sedimentary succession. Surma basin located in the eastern proximity of India is also characterized by a thick sedimentary column which can be considered as the northeastern extension of Greater Bengal basin. This basin was initiated due to the mutual collision between Indian and Burmese Plate. Due to this collision, the bed rocks have undergone folding which are oriented N-S trending hill ranges. The basin was also cut by a number of parallel to sub-parallel transverse faults and thrusts. The litho association is consisting of sandstone, siltstone, shale and their various proportions. The present study focused on the provenance of the sediments, tectonic settings of the basin and various paleoclimatic conditions prevailing during the time of deposition by using petrography, granulometric and heavy mineral analysis of representative rock samples which were collected from various parts of Aizawl district of Mizoram belonging to the Upper Bhuban Formation. Based on the various proxies it was confirmed that the sediments were primarily derived from surrounding orogens and deposited in a shallow marine basin under the influence of fluvial-deltaic conditions which were basically sourced from felsic provenance. The sediments were moderately weathered under semi-humid to humid climatic condition before they deposited into Surma basin. Sandstones samples are litharenite and wacke type which were deposited in an active continental margin to recycled orogen settings.

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Wilson, Reginald A., Elliott T. Burden, Rudolf Bertrand, Esther Asselin et Alexander D. McCracken. « Stratigraphy and tectono-sedimentary evolution of the Late Ordovician to Middle Devonian Gaspé Belt in northern New Brunswick : evidence from the Restigouche area ». Canadian Journal of Earth Sciences 41, n^o 5 (1 mai 2004) : 527–51. http://dx.doi.org/10.1139/e04-011.

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The Gaspé Belt in the Restigouche area comprises three successions separated by a Late Silurian (Salinic) disconformity and an Early Devonian angular unconformity. The lower, Upper Ordovician to Lower Silurian sequence consists of siliciclastic turbidites of the Boland Brook and Whites Brook formations (Grog Brook Group), overlain by calcareous turbidites of the Pabos and White Head formations (Matapédia Group), and slope and shelf deposits of the Upsalquitch and Limestone Point formations (lower Chaleurs Group). Above the Salinic disconformity, the upper Chaleurs Group and the Dalhousie Group record a transgressiveregressive cycle. The former comprises Pridolian carbonate rocks of the West Point Formation and overlying Pridolian to Lochkovian sedimentary rocks of the Indian Point Formation. The Chaleurs Group is conformably overlain by Lochkovian to early Emsian subaerial volcanic rocks of the Dalhousie Group (Val d'Amour Formation), which is unconformably overlain by alluviallacustrine deposits of the late Emsian Campbellton Formation. Acadian orogenesis began during the Emsian and is characterized by open to closed folding, heterogenous cleavage development, and reverse and strike-slip faults. The Salinic orogeny is manifested in extensional block faulting, within-plate volcanism, and uplift and deep erosion of Early Silurian strata. Early Devonian high-level intrusion of the Matapédia Group, White Head clasts in Indian Point conglomerate, and thermal maturation data all indicate an extended period of Late Silurian Early Devonian uplift in parts of the Restigouche area. Thermal maturities of West Point and Indian Point strata are within the oil and condensate windows and suggest potential for hydrocarbons in the study area.

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Haque, Md Masidul, et Mrinal Kanti Roy. « Geology and sedimentary environment of the Surma Group of rocks, Bandarban anticline, Bandarban, Bangladesh ». Journal of Nepal Geological Society 62 (14 septembre 2021) : 88–106. http://dx.doi.org/10.3126/jngs.v62i0.38697.

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The study illustrates the effect of tectonics, climate, and relative sea-level change on the depositional process of the Miocene Bhuban and Boka Bil Formation of Bengal Basin. Outcrop sediments of five transverse sections exposed along the axial zone of Bandarban anticline were studied. Twelve lithofacies such as Gm, Gms, Sm, ST, Sp, Sr, Sl, Sf, Sll, Fw, Fl and Fm have been identified within the successions and grouped into (i) turbidite generated, (ii) outer fan distal lobe basin plain and (iii) tide-influenced facies association. The analyses reveal that the Bhuban Formation was turbidite- generated that deposited below the continental shelf-slope environment. The Lower Bhuban Member consists of gray to brownish-gray calcareous sandstone with shale deposited under the channelized lobe of submarine fan. The Middle Bhuban Member dominated by black shale-siltstone deposited in distal turbidite lobe due to change the flow regime. The Upper Bhuban Member consists of yellow to yellowish gray, coarse to medium-grained sandstone-siltstone with black shale that deposited under channelized to nonchannelized lobes of submarine fan. The increasing sedimentation during the formation of the Upper Bhuban Member can be caused by increased the intensity of the Asian Monsoon that carried huge sediment from the Himalaya. The Boka Bil Formation was deposited under estuary to tidal flat environment. The area was uplifted during and/or after subduction of the Indian Plate beneath the Burmese Plate. The monsoonal intensity enhances sedimentation that moved prograding delta towards the south. These processes shifted depositional environment from continental shelf-slope to marginal shallow marine during deposition of the Boka Bil Formation. The continental slope aligned east-west direction and sediments likely derived from the Himalaya and Trans-Himalaya in the present geographical setup during deposition of the sediments.

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Singh, D. D. « Quasi-continental oceanic structure beneath the Arabian Fan sediments from observed surface-wave dispersion studies ». Bulletin of the Seismological Society of America 78, n^o 4 (1 août 1988) : 1510–21. http://dx.doi.org/10.1785/bssa0780041510.

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Abstract The fundamental and higher modes of surface waves generated by 31 earthquakes and recorded at seismographic stations along the western margins of India and Pakistan (Trivandrum, Kodaikanal, Goa, Bombay, Poona, New Delhi, Nillore, and Quetta) are used to estimate the crustal structure beneath the Arabian Fan sediments. The sedimentary thickness is determined from the observed higher mode data. The observed dispersion data suggest an increase in crustal thickness northward, from an approximately 16 km crustal thickness at the southern tip of India (Trivandrum) to an approximately 28 km crustal thickness at the regions of 20°N and above latitude, with an overlying 6 km sedimentary thickness. This gradual increase in crustal thickness in the northward direction and the attaining of quasi-continental oceanic (transition from continent to ocean) structure beneath the Arabian Fan sediments suggest that the Mohorovičić discontinuity may have resulted from a change in crystal structure due to increase of pressure and not a phase change. The same material exists beneath the Moho, and it does not represent the boundary between two different materials. The transition has given rise to crustal thickening in the northward direction. Another possible explanation is that the increase in hydrostatic pressure due to the load exerted by a large sedimentary column together with horizontal pressure caused by the collision of Indian and Eurasian plates has given rise to an increase in temperature near the Moho. Because of the thermal blanketing effect of this large sedimentary column, an inferred rise in temperature may have either changed the upper mantle into material with crustal-like velocity or may have given rise to metamorphism of earlier existing sedimentary rocks. An inferred high temperature near the Moho depth beneath the Arabian Fan sediments is in close agreement with the high attenuating zone at the shallow depth (30 to 45 km) as determined by Singh (1987).

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Panezai, Muhammad, Muhammad Ishaq Kakar, Umar Farooq, Nisar Ahmed et Khawar Sohail. « Petrography and Mapping of the Gwal Melange of Khanozai Region, Balochistan, Pakistan ». Pakistan Journal of Geology 4, n^o 1 (1 juin 2020) : 1–11. http://dx.doi.org/10.2478/pjg-2020-0001.

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AbstractThe Gwal mélange is mapped on a large scale and is divided into the lithological units such as ultramafic, mafic, volcanic, volcanoclastic rocks, pelagic sediments and ophicarbonates. Petrographically, the mapped rocks are classified as harzburgite, dunite, wehrlite, serpentinite, gabbro, basalt, and andesite. These rocks are quite deformed and altered into the secondary minerals. Harzburgite is a layered mantle peridotite consists of olivine and orthopyroxene while dunite lacks the presence of any pyroxene. Serpentinite is the secondary product after peridotite is the product of post magmatic stages. The mesh structure is usually observed when olivine is completely altered to serpentine. The volcanic rocks are structurally sheeted and pillow type while the volcanoclastic rocks are essentially hyaloclastites associated with pelagic sediments. The Ophicarbonate is composed of serpentinite fragments and carbonate minerals, most probably calcite. Minor to trace amounts of opaque minerals are also present in association with major components. The gabbros may be a fragment of the main crustal rocks and have been formed in a magma chamber by fraction crystallization. The origin of ophicarbonate may be due to gas seeps originated by mantle or as the surficial process where ultramafic rocks and carbonates are mixed through processes of gravity, tectonic crushing and sedimentary reworking. The Gwal mélange may the southern extension of Bagh Complex found beneath the Muslim Bagh Ophiolite. The mantle peridotite of the mélange is much like that of the Khanozai peridotite and may represent its detached blocks. Volcanic and volcanoclastic rocks may be the representatives of the uppermost part of ophiolite crust which might have trimmed off from subducting slab and are, now, part of the Gwal accretionary wedge. The mélange may have tectonically emplacement over the Indian platform sediments along with overlying the ophiolite sheet during the Late Cretaceous.

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IMMENHAUSER, ADRIAN, GUIDO SCHREURS, EDWIN GNOS, HEIKO W. OTERDOOM et BERNHARD HARTMANN. « Late Palaeozoic to Neogene geodynamic evolution of the northeastern Oman margin ». Geological Magazine 137, n^o 1 (janvier 2000) : 1–18. http://dx.doi.org/10.1017/s0016756800003526.

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When the highlands of Arabia were still covered with an ice shield in the latest Carboniferous/Early Permian period, separation of Gondwana started. This led to the creation of the Batain basin (part of the early Indian Ocean), off the northeastern margin of Oman. The rifting reactivated an Infra-Cambrian rift shoulder along the northeastern Oman margin and detritus from this high was shed into the interior Oman basin. Whereas carbonate platform deposits became widespread along the margin of the Neo-Tethys (northern rim of Oman), drifting and oceanization of the Batain basin started only in Late Jurassic/Early Cretaceous time. Extensional tectonics was followed in the Late Cretaceous by contraction caused by the northward drift of Greater India and Afro-Arabia. This resulted in the collision of Afro-Arabia with an intra-oceanic trench and obduction of the Semail ophiolite and the Hawasina nappes south to southwestward onto the northern Oman margin ∼80 m.y. ago. During the middle Cretaceous, the oceanic lithosphere (including the future eastern ophiolites of Oman) drifted northwards as part of the Indian plate. At the Cretaceous–Palaeogene transition (∼65 Ma), oblique convergence between Greater India and Afro-Arabia caused fragments of the early Indian Ocean to be thrust onto the Batain basin. Subsequently, the Lower Permian to uppermost Maastrichtian sediments and volcanic rocks of the Batain basin, along with fragments of Indian Ocean floor (eastern ophiolites), were obducted northwestward onto the northeastern margin of Oman. Palaeogene neo-autochtonous sedimentary rocks subsequently covered the nappe pile. Tertiary extensional tectonics related to Red Sea rifting in the Late Eocene was followed by Miocene shortening, associated with the collision of Arabia and Eurasia and the formation of the Oman Mountains.

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Wild, Toban J., et Jeffrey D. Stilwell. « First Cretaceous (Albian) invertebrate fossil assemblage from Batavia Knoll, Perth Abyssal Plain, eastern Indian Ocean : taxonomy and paleoecological significance ». Journal of Paleontology 90, n^o 5 (septembre 2016) : 959–80. http://dx.doi.org/10.1017/jpa.2016.76.

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AbstractThe first sedimentary rocks from Batavia Knoll, on the western edge of the Perth Abyssal Plain, eastern Indian Ocean, have been recovered, yielding an assemblage of invertebrate fossils hitherto undocumented from this part of the world. The fauna consists of 22 species of Mollusca, including new gastropods, a calliotropidPlanolateralus acanthanodusn. sp.; a margaritidIgonoia levimargaritan. sp.; a procerithiidProcerithium arenacollicolan. sp.; and aporrhaidsDrepanocheilus bataviensisn. sp. andAnchura pelsaertin. sp. In addition, pleurotomariid, ringiculid, and architectonicid gastropod taxa were recovered. Bivalves are represented by members of the Nuculanidae, Inoceramidae, Pinnidae, Buchiidae, Lucinidae, Veneridae, and Hiatellidae. Scaphopods (Dentaliidae) and ammonites (two taxa, of Desmoceratidae and Hamitidae) are also present. Further recovered were one species of Serpulidae (Polychaeta), two of Trachyleberididae (Ostracoda), and a probable echinoid fragment. The fossil assemblage was dominated by shallow marine suspension-feeding taxa (39% of the suite). Detritivorous and herbivorous taxa comprised 22% and 9%, respectively, with nektic and epifaunal carnivores amounting to 30%. Taphonomic analyses of these fossils and their host sedimentary facies revealed the Batavia Knoll sandstone was deposited in a shallow marine environment during a mass-flow event. Biostratigraphic range data of the preserved macro- and microfossil assemblages imply an age of latest Albian, contemporaneous with the rifting of Batavia Knoll from Greater India during the broader India–Australia–Antarctica breakup in the mid-Cretaceous.

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Garzanti, Eduardo. « The Himalayan Foreland Basin from collision onset to the present : a sedimentary–petrology perspective ». Geological Society, London, Special Publications 483, n^o 1 (2019) : 65–122. http://dx.doi.org/10.1144/sp483.17.

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AbstractThis chapter summarizes the available stratigraphic, petrographical and mineralogical evidence from sediments and sedimentary rocks on the evolution of the Himalayan belt and its associated foreland basin. The use of compositional signatures of modern sediments to unravel provenance changes and palaeodrainage evolution through time is hampered by a poor match with detrital modes of ancient strata markedly affected by selective chemical dissolution of unstable minerals during diagenesis. Only semi-quantitative diagnoses can thus be attempted. Volcanic detritus derived from Transhimalayan arcs since India–Asia collision onset at c. 60 Ma was deposited onto the Indian lower plate throughout the Protohimalayan stage, with the exception of the Tansen region of Nepal that is characterized by quartz-arenites yielding orogen-derived zircon grains. During the Eohimalayan stage, begun in the late Eocene when most sedimentation ceased in the Tethys Himalayan domain, low-rank metasedimentary detritus was overwhelming in the central foreland basin, where a widespread unconformity developed spanning locally as much as 20 myr. Volcanic detritus from Transhimalayan arcs remained significant in northern Pakistan. Arrival of higher-rank metamorphic detritus since the earliest Miocene, and the successive occurrence of garnet, staurolite, kyanite and finally sillimanite, characterized the Neohimalayan stage, when repeated compositional changes in the foreland-basin succession document the stepwise propagation of crustal deformation across the Indian Plate margin and widening of the thrust belt with exhumation of progressively more external tectonic units. The correspondence in time between the activity of major thrusts and petrofacies changes indicates a promising approach to accurately reconstruct the geological evolution of the coupled orogen–basin system. Conversely, a poor conceptual framework and the general reliance on ad hoc mechanisms to explain phenomena unpredicted by simplified models represent major factors limiting the robustness of palaeotectonic interpretations. Improved knowledge requires taking into full account the dynamic role played by still poorly understood subduction processes – rather than exclusively the effect of passive loading – as well as the role played by the presence of inherited structures on the downgoing Indian Plate, which control both lateral variability of orogenic deformation and the location of depocentres in the foreland basin.

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Gobeil, Jean-Philippe, Georgia Pe-Piper et David JW Piper. « The West Indian Road pit, central Nova Scotia : key to the Early Cretaceous Chaswood Formation ». Canadian Journal of Earth Sciences 43, n^o 3 (1 mars 2006) : 391–403. http://dx.doi.org/10.1139/e06-032.

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The West Indian Road pit is the only large outcrop in Nova Scotia of the Chaswood Formation, the terrestrial equivalent of the offshore Missisauga and Logan Canyon formations. It provides outcrop information on sedimentology, gravel petrology, and structures for a formation that is otherwise known from a few small overgrown pits and from boreholes. The Chaswood Formation in the pit is > 60 m thick and consists principally of sorted sand and gravel with three thinner clay units. Successions of sedimentary structures indicate deposition from a coarse-bedload river flowing to the east-southeast. Gravel consists principally of vein quartz, quartz arenite, and subarkose, together with minor igneous lithologies that can be matched to sources in the Cobequid Highlands to the north. Quartz arenite and subarkose appear derived from Carboniferous Horton Group. Single-crystal 40Ar/39Ar dates of detrital muscovite are a little older than the muscovite ages for the South Mountain batholith, interpreted to mean that the muscovite is second cycle from the Horton Group, which records the earliest unroofing of the batholith. The Chaswood Formation accumulated during progressive tectonic deformation along NNE-trending strike-slip faults in basement rocks, resulting in syn-sedimentary faulting and local unconformities. Sedimentation kept pace with the creation of accommodation. Unrelated younger deformation folded the Chaswood Formation at the pit into an eastwest-trending syncline. The Early Cretaceous paleogeography of the Maritime Provinces is interpreted to have consisted of fault-bound horsts shedding coarse detritus surrounded by an interconnected series of basins that accumulated fluvial sands and gravels and overbank muds with well-developed paleosols.

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Dyck, Brendan, Marc St-Onge, Michael P. Searle, Nicole Rayner, David Waters et Owen M. Weller. « Protolith lithostratigraphy of the Greater Himalayan Series in Langtang, Nepal : implications for the architecture of the northern Indian margin ». Geological Society, London, Special Publications 483, n^o 1 (20 septembre 2018) : 281–304. http://dx.doi.org/10.1144/sp483.9.

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AbstractReconstruction of the protolith lithostratigraphy of amphibolite-facies metasedimentary rocks of the Greater Himalayan Series (GHS) in Nepal documents a single, long-lived passive-margin succession that was deposited along the northern margin of the Indian Craton. In the Langtang area, Paleoproterozoic gneisses are unconformably overlain by a succession of upper Neoproterozoic–Ordovician fluvio-deltaic quartzite, basinal pelite and psammitic beds that grade upsection into micaceous semipelite and pelite. U–Pb zircon geochronology yields maximum depositional ages between c. 815 and 460 Ma for the GHS in Langtang. Regional variations in the composition and thickness of the GHS along the length of the Himalaya are attributed to siliciclastic depocentres centred on Zanskar in northern India, Langtang and Everest in central to western Nepal, which contrast with coeval marine carbonate shelf deposition in the Annapurna region. The protolith lithostratigraphy documented for Langtang provides a coherent framework for interpreting subsequent Cenozoic Himalayan deformation, specifically the homogeneously distributed layer-normal shortening (i.e. flattening) and layer-parallel stretching (i.e. transport-parallel stretching) that characterizes the GHS. Within the context of a single protracted northern Indian marginal sedimentary succession, the distinction between the Lesser, Greater and Tethyan Himalaya is structural rather than lithostratigraphic in origin.

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Raza, Mahshar, A. M. Dayal, Abdullah Khan, V. R. Bhardwaj et Sarwar Rais. « Geochemistry of lower Vindhyan clastic sedimentary rocks of Northwestern Indian shield : Implications for composition and weathering history of Proterozoic continental crust ». Journal of Asian Earth Sciences 39, n^o 1-2 (juillet 2010) : 51–61. http://dx.doi.org/10.1016/j.jseaes.2010.02.007.

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Lucas, Spencer G., William A. DiMichele, Karl Krainer, James E. Barrick, Daniel Vachard, Michael P. Donovan, Cindy Looy, Hans Kerp et Dan S. Chaney. « The Pennsylvanian System in the Sacramento Mountains, New Mexico, USA ». Smithsonian Contributions to Paleobiology, n^o 104 (22 février 2021) : iv—215. http://dx.doi.org/10.5479/si.14079809.v1.

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Pennsylvanian sedimentary rocks in the Sacramento Mountains, New Mexico, comprise an ~1 km thick stratigraphic section. The Morrowan-Desmoinesian Gobbler Formation was deposited by shallow marine processes in and near the Alamo clastic trough. In this trough, the Desmoinesian-Missourian Gray Mesa Formation (Bug Scuffle Member, Gobbler Formation) is a relatively thin unit (Space History Member) representing the glacioeustatic Amado event. The Missourian-Virgilian Beeman Formation includes the lower, siliciclastic Indian Wells Canyon Member and overlying, carbonate-rich Horse Ridge Member. The Virgilian Holder Formation consists of algal bioherms (Little Dry Canyon Member) overlain by the mixed carbonate-siliciclastic Mill Ridge Member. The Virgilian-Wolfcampian Bursum Formation is mixed siliciclastic-carbonate strata that represent shallow marine and nonmarine paleoenvironments. Animal and plant remains occur throughout the section. Unit age determinations are primarily based on conodont faunas recovered from the Gobbler, Gray Mesa, and Beeman Formations. Many conodont faunas correlate with Midcontinent cyclothems. Extensive algal and foraminiferal fossils also were identified in limestones from the section and contributed to age determinations. The Beeman Formation in particular contains an extensive Missourian macroflora. The macroflora is of “mixed” composition, containing typical wetland elements intimately intermixed with taxa indicative of seasonally dry habitats. A seasonally wet-dry background climate is indicated. It is unlikely that droughttolerant plants were transported exclusively from “uplands.” Some plant remains have arthropod-feeding evidence. Previous analyses identified late Paleozoic ice-age glacioeustasy as the primary depositional driver of Pennsylvanian sedimentation in the Sacramento Mountains. We question this because of problems with those analyses and because of ample evidence of local tectonics and microclimate changes as important drivers of sedimentation in this area. Three Pennsylvanian Ancestral Rocky Mountain orogeny tectonic pulses can be identified in the Sacramento Mountains: Morrowan-Atokan, Missourian, and late Virgilian-Wolfcampian.

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Li, Juan, Xiumian Hu, Eduardo Garzanti, Santanu Banerjee et Marcelle BouDagher-Fadel. « Late Cretaceous topographic doming caused by initial upwelling of Deccan magmas : Stratigraphic and sedimentological evidence ». GSA Bulletin 132, n^o 3-4 (14 août 2019) : 835–49. http://dx.doi.org/10.1130/b35133.1.

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Abstract This study focused on uppermost Cretaceous sedimentary rocks deposited in the Himalayan region and around the core of peninsular India just before the eruption of the Deccan Traps. Detailed stratigraphic and sedimentological analysis of Late Cretaceous successions in the Himalayan Range together with literature data from the Kirthar fold-and-thrust belt and central to southeastern India document a marked shallowing-upward depositional trend that took place in the Campanian–Maastrichtian before the Deccan magmatic outburst around the Cretaceous-Tertiary boundary. Topographic uplift of the Indian peninsula began in Campanian time and is held responsible for thick sediment accumulation associated with shorter periods of nondeposition in peripheral areas (Himalayan Range, Kirthar fold belt, and Krishna-Godavari Basin) than in the central part of the Deccan Province. Surface uplift preceding Deccan volcanism took place at warm-humid equatorial latitudes, which may have led to an acceleration of silicate weathering, lowered atmospheric pCO2, and climate cooling starting in the Campanian–Maastrichtian. The radial centrifugal fluvial drainage in India that is still observed today was established at that time.

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Colman-Sadd, S. P., P. Stone, H. S. Swinden et R. P. Barnes. « Parallel geological development in the Dunnage Zone of Newfoundland and the Lower Palaeozoic terranes of southern Scotland : an assessment ». Transactions of the Royal Society of Edinburgh : Earth Sciences 83, n^o 3 (1992) : 571–94. http://dx.doi.org/10.1017/s0263593300005885.

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AbstractThe Notre Dame and Exploits subzones of Newfoundland's Dunnage Zone are correlated with the Midland Valley and Southern Uplands of Scotland, using detailed comparisons of two key Lower Palaeozoic successions which record similar histories of extension and compression. It follows that the Baie Verte Line, Red Indian Line and Dover Fault are equivalent to the Highland Boundary Fault, Southern Upland Fault and Solway Line, respectively.The Betts Cove Complex and overlying Snooks Arm Group of the Notre Dame Subzone are analogous to the Ballantrae Complex of the Midland Valley, both recording the Arenig evolution and subsequent obduction of an arc and back-arc system. The Early Ordovician to Silurian sequence unconformably overlying the Ballantrae Complex is poorly represented in the Notre Dame Subzone but important similarities can still be detected suggesting corresponding histories of continental margin subsidence and marine transgression.In the Exploits Subzone, Early Ordovician back-arc volcanic rocks are overlain by Llandeilo mudstones and Late Ordovician to Early Silurian turbidites. A similar stratigraphy occurs in the Northern and Central Belts of the Southern Uplands and both areas have matching transpressive structural histories. Deeper erosion in the Exploits Subzone reveals Cambrian and Early Ordovician volcano-sedimentary sequences structurally emplaced on the Gander Zone, and such rocks are probably present beneath the Southern Uplands. Combined data from the Notre Dame Subzone and Midland Valley suggest an Arenig southeast-dipping subduction zone. Early Ordovician volcanic rocks in the Exploits Subzone and Southern Uplands have back-arc basin geochemistry and support the model of the Southern Uplands as a transition from back-arc to foreland basin. Preferential emergence of the Dunnage Zone and contrasts between Exploits Subzone and Southern Uplands turbidite basins are attributed to collision of Newfoundland with a Laurentian promontory and Scotland with a re-entrant. This hypothesis also explains the transpressive structural regime common to both areas.

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NADERLOO, REZA, et MICHAEL TÜRKAY. « Decapod crustaceans of the littoral and shallow sublittoral Iranian coast of the Persian Gulf : Faunistics, Biodiversity and Zoogeography ». Zootaxa 3374, n^o 1 (4 juillet 2012) : 1. http://dx.doi.org/10.11646/zootaxa.3374.1.1.

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A total of 150 species belonging to five infraorders (Caridea, Stenopodidea, Thalassinidea, Anomura and Brachyura) are recorded from the Iranian coast of the Persian Gulf. About 17% (26 species) are new records for the area. The infraorder Brachyura shows the highest species richness with 83 species. The Anomura and Caridea, with 29 and 27 species, respectively, follow. The Thalassinidea is represented by 10 species, whereas Stenopodidea only by one. The species with the highest occurence are ranked as follows: Nanosesarma sarii Naderloo & Türkay, 2009 (Brachyura), Alpheus lobidens De Haan, 1850 (Caridea), Metopograpsus messor (Forskål, 1775) (Brachyura), Coenobita scaevola (Forskål, 1775) (Anomura), Diogenes avarus Heller, 1865 (Anomura), Pilumnopeus convexus Maccagno, 1936 (Brachyura) and Portunus segnis (Forskål, 1775) (Brachyura). Among the surveyed habitats, mixed rocky/cobble show the highest diversity with 99 (66%) recorded species, followed by sedimentary muddy-sandy substrates and mangroves with 40 (26%) and 24 species (16%), respectively. The least diverse habitat is sandy beaches with 6 species. The decapod fauna shows a predominance of wide ranging Indo-West Pacific species (43%) and a small degree of endemism (5%). A considerable number of species is restricted to the northwestern Indian Ocean (27%), supporting the presence of a zoogeographical subregion within the western Indian Ocean Province.

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Pang, Zhonghe, Jie Li et Jiao Tian. « Noble gas geochemistry and chronology of groundwater in an active rift basin in central China ». E3S Web of Conferences 98 (2019) : 01040. http://dx.doi.org/10.1051/e3sconf/20199801040.

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Stable noble gas isotopes are excellent groundwater tracers. Radioactive noble gases are emerging new tools in the study of groundwater circulation dynamics. Among these, the 85Kr and 81Kr, and 39Ar have advanced very fast in recent years and exhibit strong potential in the reconstruction of the history of groundwater recharge and evolution in sedimentary basins at different scales. Here, we report the findings in groundwater circulation dynamics as relative to intensive water-rock interactions, heat transfer and He gas flux in Guanzhong Basin located in Xi’an, the geographical centre of China, which is a rift basin created by collision between the Eurasia and Indian plates, with active neotectonic activities. The recent technological breakthrough in noble gas isotope measurements, i.e. the atomic trap trace analysis (ATTA) techniques on Kr and Ar gas radionuclei, has revolutionized groundwater dating. Noble gas samples from shallow and deep wells to 3000 m depth have been collected to study isotope variations to reconstruct the history of groundwater recharge and understand the water-rock interaction processes. Stable isotopes of water show strong water-rock interaction in the formation, creating a strong positive O-isotope shift up to 10 ‰, a phenomenon that is rarely seen in a fairly low temperature environment. Analysis of 85Kr and 81Kr show groundwater ages up to 1.3 million years old along both North-South and a West-East cross sections, which offers strong evidence about the slow moving flow, strong water-rock interaction, rich geothermal resources as well as He gas resources.

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Zhang, Zhen Guo, Chang Shui Liu, Lian Feng Gao, Ying Zhang, Guo Yuan Shi et Peng Zhang. « Causative Mechanism of the Continental Margin Polymetallic Nodules from the South China Sea and its Resource Effects ». Advanced Materials Research 524-527 (mai 2012) : 408–12. http://dx.doi.org/10.4028/www.scientific.net/amr.524-527.408.

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Polymetallic nodules are one of the most important sedimentary mineral resources in the ocean, in which iron, manganese, copper, cobalt, nickel and other metals are rich, and rare earth elements are rich, too. The samples are collected from the northwest continental margin of South China Sea (SCS). Their model show the similar appearance to the oceanic nodules which collected from the Pacific and Indian Ocean. They are big, regular shape and clear layers. But their geochemical characteristics show distinct difference with oceanic nodules.The samples formed by multiple millimeter-thick layers of Fe and Mn oxyhydroxides surrounding the nucleus composed of plastic marl and sediment. Massive, laminated, detrital and mottled to dendritic textural features were developed by the Fe and Mn oxyhydroxide layers.Based on the detailed study of the geochemistry and growth rate, the nodules may represent new-type ones which grow fastly in high sediment rates environment from the northwest continental margin of the SCS. The reason of the fast growth may be affected by the environmental fluctuations and the change of terrigenous sediments. Elements correlation of Mn-Fe-(Cu+Ni) suggests that the origin of the sample may be of hydrogenic. It may be show that these nodules are dominative of the special environment of the marginal sea which includes the geographical condition and the oceanic environmental factors. The average content of Rare Earth Elements (REEs) in these samples are much higher than those recorded in Earth’ crust and sedimentary rocks. The enrichment of rare earth elements is controlled by iron and manganese oxides and clay minerals in nodules, which could absorb rare earth elements from seawater and terrigenous sediment. Ce elements are highly enriched, making polymetallic nodules become the first used rare earth elements in oceanic mineral development.

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Ashwal, L. D. « Wandering continents of the Indian Ocean ». South African Journal of Geology 122, n^o 4 (1 décembre 2019) : 397–420. http://dx.doi.org/10.25131/sajg.122.0040.

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Abstract On the last page of his 1937 book “Our Wandering Continents” Alex Du Toit advised the geological community to develop the field of “comparative geology”, which he defined as “the study of continental fragments”. This is precisely the theme of this paper, which outlines my research activities for the past 28 years, on the continental fragments of the Indian Ocean. In the early 1990s, my colleagues and I were working in Madagascar, and we recognized the need to appreciate the excellent geological mapping (pioneered in the 1950s by Henri Besairie) in a more modern geodynamic context, by applying new ideas and analytical techniques, to a large and understudied piece of continental crust. One result of this work was the identification of a 700 to 800 Ma belt of plutons and volcanic equivalents, about 450 km long, which we suggested might represent an Andean-type arc, produced by Neoproterozoic subduction. We wondered if similar examples of this magmatic belt might be present elsewhere, and we began working in the Seychelles, where late Precambrian granites are exposed on about 40 of the >100 islands in the archipelago. Based on our new petrological, geochemical and geochronological measurements, we built a case that these ~750 Ma rocks also represent an Andean-type arc, coeval with and equivalent to the one present in Madagascar. By using similar types of approaches, we tracked this arc even further, into the Malani Igneous Province of Rajasthan, in northwest India. Our paleomagnetic data place these three entities adjacent to each other at ~750 Ma, and were positioned at the margins, rather than in the central parts of the Rodinia supercontinent, further supporting their formation in a subduction-related continental arc. A widespread view is that in the Neoproterozoic, Rodinia began to break apart, and the more familiar Gondwana supercontinent was assembled by Pan-African (~500 to 600 Ma) continental collisions, marked by the highly deformed and metamorphosed rocks of the East African Orogen. It was my mentor, Kevin Burke, who suggested that the present-day locations of Alkaline Rocks and Carbonatites (called “ARCs”) and their Deformed equivalents (called “DARCs”), might mark the outlines of two well-defined parts of the Wilson cycle. We can be confident that ARCs formed originally in intracontinental rift settings, and we postulated that DARCs represent suture zones, where vanished oceans have closed. We also found that the isotopic record of these events can be preserved in DARC minerals. In a nepheline syenite gneiss from Malawi, the U-Pb age of zircons is 730 Ma (marking the rifting of Rodinia), and that of monazites is 522 Ma (marking the collisional construction of Gondwana). A general outline of how and when Gondwana broke apart into the current configuration of continental entities, starting at about 165 Ma, has been known for some time, because this record is preserved in the magnetic properties of ocean-floor basalts, which can be precisely dated. A current topic of active research is the role that deep mantle plumes may have played in initiating, or assisting, continental fragmentation. I am part of a group of colleagues and students who are applying complementary datasets to understand how the Karoo (182 Ma), Etendeka (132 Ma), Marion (90 Ma) and Réunion (65 Ma) plumes influenced the break-up of Gondwana and the development of the Indian Ocean. Shortly after the impingement of the Karoo plume at 182 Ma, Gondwana fragmentation began as Madagascar + India + Antarctica separated from Africa, and drifted southward. Only after 90 Ma, when Madagascar was blanketed by lavas of the Marion plume, did India begin to rift, and rapidly drifted northward, assisted by the Marion and Deccan (65 Ma) plumes, eventually colliding with Asia to produce the Himalayas. It is interesting that a record of these plate kinematics is preserved in the large Permian – Eocene sedimentary basins of western Madagascar: transtensional pull-apart structures are dextral in Jurassic rocks (recording initial southward drift with respect to Africa), but change to sinistral in the Eocene, recording India’s northward drift. Our latest work has begun to reveal that small continental fragments are present in unexpected places. In the young (max. 9 Ma) plume-related, volcanic island of Mauritius, we found Precambrian zircons with ages between 660 and 3000 Ma, in beach sands and trachytic lavas. This can only mean that a fragment of ancient continent must exist beneath the young volcanoes there, and that the old zircons were picked up by ascending magmas on their way to surface eruption sites. We speculate, based on gravity inversion modelling, that continental fragments may also be present beneath the Nazareth, Saya de Malha and Chagos Banks, as well as the Maldives and Laccadives. These were once joined together in a microcontinent we called “Mauritia”, and became scattered across the Indian Ocean during Gondwana break-up, probably by mid-ocean ridge “jumps”. This work, widely reported in international news media, allows a more refined reconstruction of Gondwana, suggests that continental break-up is far more complex than previously perceived, and has important implications for regional geological correlations and exploration models. Our results, as interesting as they may be, are merely follow-ups that build upon the prescient and pioneering ideas of Alex Du Toit, whose legacy I appreciatively acknowledge.

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Chen, Xian, Xiaoming Sun, Zhongwei Wu, Yan Wang, Xiao Lin et Hongjun Chen. « Mineralogy and Geochemistry of Deep-Sea Sediments from the Ultraslow-Spreading Southwest Indian Ridge : Implications for Hydrothermal Input and Igneous Host Rock ». Minerals 11, n^o 2 (29 janvier 2021) : 138. http://dx.doi.org/10.3390/min11020138.

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Detailed mineralogical and geochemical characteristics of typical surface sediments and hydrothermal deposits collected from the ultraslow-spreading Southwest Indian Ridge (SWIR) were studied by high-resolution XRD, SEM-EDS, XRF, and ICP-MS. The SWIR marine samples can be generally classified into two main categories: surface sediment (biogenic, volcanic) and hydrothermal-derived deposit; moreover, the surface sediment can be further classified into metalliferous and non-metalliferous based on the metalliferous sediment index (MSI). The chemical composition of biogenic sediment (mainly biogenic calcite) was characterized by elevated contents of Ca, Ba, Rb, Sr, Th, and light rare earth elements (LREE), while volcanic sediment (mainly volcanogenic debris) was relatively enriched in Mn, Mg, Al, Si, Ni, Cr, and high field strength elements (HFSEs). By contrast, the hydrothermal-derived deposit (mainly pyrite-marcasite, chalcopyrite-isocubanite, and low-temperature cherts) contained significantly higher contents of Fe, Cu, Zn, Pb, Mn, Co, Mo, Ag, and U. In addition, the metalliferous surface sediment contained a higher content of Cu, Mn, Fe, Co, Mo, Ba, and As. Compared with their different host (source) rock, the basalt-hosted marine sediments contained higher contents of Ti–Al–Zr–Sc–Hf and/or Mo–Ba–Ag; In contrast, the peridotite-hosted marine sediments were typically characterized by elevated concentrations of Mg–Cu–Ni–Cr and/or Co–Sn–Au. The differences in element enrichment and mineral composition between these sediment types were closely related to their sedimentary environments (e.g., near/far away from the vent sites) and inherited from their host (source) rock. Together with combinations of certain characteristic elements (such as Al–Fe–Mn and Si–Al–Mg), relict hydrothermal products, and diagnostic mineral tracers (e.g., nontronite, SiO2(bio), olivine, serpentine, talc, sepiolite, pyroxene, zeolite, etc.), it would be more effective to differentiate the host rock of deep-sea sediments and to detect a possible hydrothermal input.

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Wani, H., et M. E. A. Mondal. « Petrological and geochemical evidence of the Paleoproterozoic and the Meso-Neoproterozoic sedimentary rocks of the Bastar craton, Indian Peninsula : Implications on paleoweathering and Proterozoic crustal evolution ». Journal of Asian Earth Sciences 38, n^o 5 (mai 2010) : 220–32. http://dx.doi.org/10.1016/j.jseaes.2010.01.003.

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Khan, Tavheed, D. Srinivasa Sarma et M. Shamim Khan. « Geochemical study of the Neoproterozoic clastic sedimentary rocks of the Khambal Formation (Sindreth Basin), Aravalli Craton, NW Indian Shield : Implications for paleoweathering, provenance, and geodynamic evolution ». Geochemistry 80, n^o 1 (avril 2020) : 125596. http://dx.doi.org/10.1016/j.chemer.2019.125596.

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ANTONIETTO, LUCAS SILVEIRA, CLÁUDIA PINTO MACHADO, DERMEVAL APARECIDO DO CARMO et JOSÉ WILSON CORREA ROSA. « Recent Ostracoda (Arthropoda, Crustacea) from São Pedro-São Paulo Archipelago, Brazil : a preliminary approach ». Zootaxa 3335, n^o 1 (6 juin 2012) : 29. http://dx.doi.org/10.11646/zootaxa.3335.1.2.

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The present study analyses ostracods from sedimentary samples collected in the São Pedro-São Paulo Archipelago, a small set of remote rock islands located Northeastern to the Brazilian coast. Thirteen species were identified, and their zoogeographical distribution was studied. An emendation for Keijcyoidea praecipua (Bold, 1963) is proposed in this paper. The distribution of the species which occur in the archipelago varies significantly: Triebelina sertata Triebel, 1948, is a cosmopolitan species; Loxoconcha (Loxocorniculum) tricornata is assumed to occur from the Caribbean Sea to the tropical portion of the Brazilian coast and Western Africa. Keijcyoidea praecipua is recorded through the Pacific and Atlantic coasts of Central America and Northeastern Brazil. Xestoleberis toni? Wouters, 2003 and Triebelina cf. intermedia Witte, 1993 are known from the African coast. Neonesidea tenera? (Brady, 1886) emend. Maddocks, 1969 is found along the Indian and Pacific oceans. The species Pontocypris (Ekpontocypris) pirifera? (Müller, 1894) is also present in the western European coast and the Mediterranean Sea. Six species are probably new and have not been observed elsewhere: Aurila sp. 1, Paradoxostoma sp. 1, Paradoxostoma sp. 2, Xestoleberis sp. 1, Xestoleberis sp. 2 and Xestoleberis? sp. 3.

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Jena, Ratiranjan, Biswajeet Pradhan et Abdullah M. Alamri. « Susceptibility to Seismic Amplification and Earthquake Probability Estimation Using Recurrent Neural Network (RNN) Model in Odisha, India ». Applied Sciences 10, n^o 15 (3 août 2020) : 5355. http://dx.doi.org/10.3390/app10155355.

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The eastern region of India, including the coastal state of Odisha, is a moderately seismic-prone area under seismic zones II and III. However, no major studies have been conducted on earthquake probability (EPA) and hazard assessment (EHA) in Odisha. This paper had two main objectives: (1) to assess the susceptibility of seismic wave amplification (SSA) and (2) to estimate EPA in Odisha. In total, 12 indicators were employed to assess the SSA and EPA. Firstly, using the historical earthquake catalog, the peak ground acceleration (PGA) and intensity variation was observed for the Indian subcontinent. We identified high amplitude and frequency locations for estimated PGA and the periodograms were plotted. Secondly, several indicators such as slope, elevation, curvature, and amplification values of rocks were used to generate SSA using predefined weights of layers. Thirdly, 10 indicators were implemented in a developed recurrent neural network (RNN) model to create an earthquake probability map (EPM). According to the results, recent to quaternary unconsolidated sedimentary rocks and alluvial deposits have great potential to amplify earthquake intensity and consequently lead to acute ground motion. High intensity was observed in coastal and central parts of the state. Complicated morphometric structures along with high intensity variation could be other parameters that influence deposits in the Mahanadi River and its delta with high potential. The RNN model was employed to create a probability map (EPM) for the state. Results show that the Mahanadi basin has dominant structural control on earthquakes that could be found in the western parts of the state. Major faults were pointed towards a direction of WNW–ESE, NE–SW, and NNW–SSE, which may lead to isoseismic patterns. Results also show that the western part is highly probable for events while the eastern coastal part is highly susceptible to seismic amplification. The RNN model achieved an accuracy of 0.94, precision (0.94), recall (0.97), F1 score (0.96), critical success index (CSI) (0.92), and a Fowlkes–Mallows index (FM) (0.95).

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Palmer, Sarah E., John W. F. Waldron et D. M. Skilliter. « Post-Taconian shortening, inversion and strike slip in the Stephenville area, western Newfoundland Appalachians ». Canadian Journal of Earth Sciences 39, n^o 9 (1 septembre 2002) : 1393–410. http://dx.doi.org/10.1139/e02-041.

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Deformed terrigenous and carbonate sedimentary rocks representing the early Paleozoic Laurentian continental margin form a series of elongate, fault-bounded blocks that plunge north beneath the Humber Arm Allochthon in the Stephenville area, west Newfoundland Appalachians. The continental shelf succession was folded and thrust-faulted after emplacement of the Humber Arm Allochthon. In the west of the area, Table Mountain is cut by a "pop-up" structure bounded by downward-converging reverse faults. Structures at the east margin of Table Mountain indicate both dextral and reverse slip. The Phillips Brook Structure, farther east, contains multiple, fault-bounded carbonate slices; one slice, carried by the West Blanche Brook fault, is thrust over the Humber Arm Allochthon. The western edge of the Indian Head massif, consisting of Grenville basement, is also a thrust contact. These reverse faults and thrusts, which cut the carbonate succession and postdate emplacement of the Humber Arm Allochthon, are in turn overprinted by structures formed during dextral strike-slip motion. Unconformable Early Carboniferous cover postdates most of the deformation. Cross-sections indicate shortening of a few kilometres, and basement was clearly involved in the deformation. The Port au Port Peninsula, immediately to the west, records a history of Acadian inversion of Taconian basins. The prevalence of pop-up structures and mappable variations within the stratigraphic units forming the top of the carbonate succession indicate that a complex geometry of Taconian horsts and grabens was inverted during post-Taconian (?Acadian) shortening and dextral strike-slip motion. These relationships suggest a variety of attractive targets in petroleum exploration.

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Willner, Arne P., Axel Gerdes, Hans-Joachim Massonne, Cees R. Van Staal et Alexandre Zagorevski. « Crustal Evolution of the Northeast Laurentian Margin and the Peri-Gondwanan Microcontinent Ganderia Prior to and During Closure of the Iapetus Ocean : Detrital Zircon U–Pb and Hf Isotope Evidence from Newfoundland ». Geoscience Canada 41, n^o 3 (29 août 2014) : 345. http://dx.doi.org/10.12789/geocanj.2014.41.046.

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Detrital zircon populations in sedimentary rocks from the Laurentian margin and the accreted microcontinent Ganderia on both sides of the main Iapetus suture (Red Indian Line) in central Newfoundland have been studied by combined U–Pb and Lu–Hf isotope analyses. Variation in εHf(t) values with age of zircon populations of distal provenance (>900 Ma) reflect the crustal evolution within the source continents: in zircon derived from Laurentia, episodes of juvenile magma production in the source could be detected at 1.00 – 1.65 and 2.55 – 3.00 Ga, and mixing of juvenile and recycled crust in continental magmatic arcs occurred at 0.95 – 1.40, 1.45 – 1.60, 1.65 – 2.05 and 2.55 – 2.75 Ga. These ages are consistent with the crustal history of northeastern Laurentia. Similarly, zircon of distal provenance from Ganderia reveals times of juvenile magma production in the source at 0.70 – 0.90, 1.40 – 1.75, 1.85 – 2.40 and 2.7 – 3.5 Ga, and episodes of mixing juvenile and recycled crust at 0.95 – 1.35, 1.45 – 1.60, 1.70 – 2.15 and 2.6 – 2.8 Ga. These data reflect the crustal evolution in the present northern part of Amazonia, its likely source craton. The evolution of magmatic arcs at the margins of both continents can be studied in a similar way using detrital zircon having a proximal provenance (<900 Ma). In contrast to the Laurentian margin, Ganderia is characterized by development of Neoproterozoic – Cambrian continental arcs (ca. 500 – 670 Ma) that were built on the margin of Gondwana. εHf(t) values indicate recycling of Neo- and Mesoproterozoic crust. During and following accretion of the various elements of Ganderia to Laurentia, the syn-tectonic Late Ordovician to Silurian sedimentary rocks deposited on the upper plate (composite Laurentia) continued showing only detritus derived from Laurentia. These sedimentary rocks contain detrital zircon from Iapetan juvenile, continental and successor arcs that were active between ca. 440 and 550 Ma, and from continuing magmatic activity until 423 Ma. Arrival of the first Laurentian detritus at the outermost part of Ganderia indicates that the Iapetus ocean was closed at ca. 452 Ma. The magmatic arcs along the former Laurentian margin in Newfoundland evolved differently. In the northwestern part, εHf(t) values point to recycling of Mesoproterozoic and Paleoproterozoic crust. In the southwest, εHf(t) values indicate addition of juvenile crust, recycling of Mesoproterozoic crust and mixing with juvenile magma. SOMMAIRELes populations de zircons détritiques des roches sédimentaires issus de la marge laurentienne et du microcontinent d’accrétion de Ganderia, des deux côtés de la principale suture Iapetus (linéation de Red Indian) dans le centre de Terre-Neuve, ont été étudiés par analyses combinées U–Pb et Lu–Hf. Les variations des valeurs εHf(t) en fonction de l’âge des populations de zircons distaux (>900 Ma) reflètent l’évolution de la croûte des continents sources : les zircons de Laurentie ont permis de détecter des épisodes magmatiques juvéniles dans la source entre 1,00 - 1,5, et 2,55 - 3,00 Ga, ainsi que des épisodes de mélange de croûte juvénile avec des croûtes d’arcs magmatiques continentaux recyclés entre 0,95 – 1,40, 1,45 – 1,60, 1,65 – 2,05, et 2,55 – 2,75 Ga. Ces datations correspondent bien à l’histoire de la croûte de la portion nord-est de la Laurentie. De même, le zircon distal de Ganderia révèle des épisodes de production de magmas juvéniles dans la source entre 0,70 - 0,90, 1,40 - 1,75, 1,85 - 2,40, et 2,7 - 3,5 Ga, ainsi que des épisodes de mélanges de matériaux juvéniles et de croûtes recyclés entre 0,95 - 1,35, 1,45 - 1,60, 1,70 - 2,15, et 2,6 - 2,8 Ga. Ces données reflètent l’évolution de la croûte dans la portion nord actuelle de l’Amazonie, son craton source probable. L’évolution des arcs magmatiques à la marge de ces deux continents peuvent être étudiées de la même manière en utilisant le zircon détritique proximal (<900 Ma). Contrairement à la marge laurentienne, celle de Ganderia est caractérisée par le développement d’arcs continentaux Néoprotéozoïque-Cambrien (env. 500 – 670 Ma) qui se sont constitués à la marge du Gondvana. Les valeurs de εHf(t) indiquent un recyclage de la croûte au Néoprotérozoïque et au Mésoprotérozoïque. Durant et après l’accrétion des divers éléments de Ganderia et de la Laurentie, les roches sédimentaires syntectoniques de la fin de l’Ordovicien et du Silurien qui se sont déposées sur la portion supérieure de la plaque (Laurentie composite) ne montrent toujours que des débris provenant de la Laurentie. Ces roches sédimentaires renferment des zircons détritiques juvéniles iapétiques, et d’arcs continentaux et d’arcs subséquents, qui ont été actifs entreentre (env. 440 et 550 Ma) et d’une activité magmatique continue jusqu’à 423 Ma. L’apport des premiers débris à la marge extrême de Ganderia indique que l’océan s’est fermée il y a env. 452 Ma. Les arcs magmatiques le long de l'ancienne marge laurentienne à Terre-Neuve ont évolué différemment. Dans la portion nord-ouest, les valeurs de εHf(t) indiquent un recyclage de la croûte au Mésoprotérozoïque et au Paléoprotérozoïque. Dans la portion sud-ouest, les valeurs de εHf(t) indiquent l’ajout d’une croûte juvénile, un recyclage de la croûte mésoprotérozoïque et un mélange avec un magma juvénile.

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Ojha, Maheswar, et Ranjana Ghosh. « Assessment of gas hydrate using prestack seismic inversion in the Mahanadi Basin, offshore eastern India ». Interpretation 9, n^o 2 (11 mars 2021) : SD15—SD26. http://dx.doi.org/10.1190/int-2020-0139.1.

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The Indian National Gas Hydrate Program Expedition-01 in 2006 has discovered gas hydrate in the Mahanadi offshore basin along the eastern Indian margin. However, well-log analysis, pressure core measurements, and infrared anomalies reveal that gas hydrates are distributed as disseminated within the fine-grained sediment, unlike massive gas hydrate deposits in the Krishna-Godavari Basin. The 2D multichannel seismic section, which crosses holes NGHP-01-9A and 19B located at approximately 24 km apart, indicates a continuous bottom-simulating reflector (BSR) along it. We aim to investigate the prospect of gas hydrate accumulation in this area by integrating well-log analysis and seismic methods with rock-physics modeling. First, we estimate gas hydrate saturation at these two holes from the observed impedance using the three-phase Biot-type equation. Then, we establish a linear relationship between the gas hydrate saturation and the impedance contrast with respect to the water-saturated sediment. Using this established relation and impedance obtained from prestack inversion of seismic data, we produce a 2D gas hydrate-distribution image over the entire seismic section. The gas hydrate saturation estimated from resistivity and sonic data at well locations varies within 0%–15%, which agrees well with the available pressure core measurements at hole 19. However, the 2D map of gas hydrate distribution obtained from our method indicates that the maximum gas hydrate saturation is approximately 40% just above the BSR between the common-depth points of 1450 and 2850. The presence of gas-charged sediments below the BSR is one of the reasons for the strong BSR observed in the seismic section, which is depicted as low impedance in the inverted impedance section. Closed sedimentary structures above the BSR are probably obstructing the movements of free-gas upslope, for which we do not see the presence of gas hydrate throughout the seismic section above the BSR.

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Raza, Mahshar, Abdullah Khan, V. R. Bhardwaj et Sarwar Rais. « Geochemistry of Mesoproterozoic sedimentary rocks of upper Vindhyan Group, southeastern Rajasthan and implications for weathering history, composition and tectonic setting of continental crust in the northern part of Indian shield ». Journal of Asian Earth Sciences 48 (avril 2012) : 160–72. http://dx.doi.org/10.1016/j.jseaes.2011.11.012.

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Bhardwaj, Priyanka, Mainak Mitra et Pranab Kumar Saha. « Interpreter's Corner ». Leading Edge 40, n^o 8 (août 2021) : 619–25. http://dx.doi.org/10.1190/tle40080619.1.

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Basaltic reservoirs have produced hydrocarbon from Yurihara Field in Japan, Quiko Depression in China, and Padra Field of Cambay Basin and Western Offshore Basin in India. The availability of fractured, altered, and vesicular basalts contributes to reservoir development in this stratigraphic unit. This study is conducted in the Kutch-Saurashtra Basin, located at the western continental margin of the Indian subcontinent wherein, the Deccan basalt, with a thickness range of 200–2500 m, overlies Mesozoic sediments. The Jurassic and Cretaceous sediments constitute the main source rock in the area. Several wells have been drilled through the entire basalt section, and some are hydrocarbon bearing in basalt. The entire basalt section in the study area has been classified into four major units using gamma-ray logs. These units have been further subdivided into individual flows and correlated all over the basin. Analysis shows that the base of an individual basalt flow is massive, and the top is differentially altered. Crossplot analysis of P-impedance and VP/VS ratio carried out on logs delineates a zone of moderately weathered/altered basalt, which is due to spheroidal weathering and calibrated with sidewall cores. These moderately altered zones between two successive flows of basalt are the probable reservoir facies for hydrocarbon accumulation, provided that there is an overlying seal in the form of massive or completely altered basalt. Three-dimensional seismic data in the area show an alternating reflection pattern in the basalt section due to the alternation of massive and weathered basalt. The seismic signature of basalt in the area is similar to that of a sedimentary sequence in any given area. Continuity of the identified individual flows in seismic scale has been propagated over the entire seismic, and subsequent inversion has facilitated the deciphering of the probable hydrocarbon-bearing locales within basalt.

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Xue, Er-Kun, Wei Wang, Mei-Fu Zhou, Manoj K. Pandit, Si-Fang Huang et Gui-Mei Lu. « Late Neoproterozoic–early Paleozoic basin evolution in the Cathaysia Block, South China : Implications of spatio-temporal provenance changes on the paleogeographic reconstructions in supercontinent cycles ». GSA Bulletin 133, n^o 3-4 (4 août 2020) : 717–39. http://dx.doi.org/10.1130/b35588.1.

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Abstract The role of the Cathaysia Block, South China, and its linkage with orogenesis in the Rodinia and Gondwana supercontinents remain unresolved because of uncertainties in its paleoposition in supercontinental reconstructions. The lithostratigraphic, geochronologic, geochemical (including isotopic), and paleocurrent data on late Neoproterozoic to early Paleozoic sub-basins in the Cathaysia Block reveal spatio-temporal, tectono-sedimentary, and provenance diversity that show linkages with previously adjacent terranes and orogens in East Gondwana. An abundance of siliceous and conglomeratic rocks, local unconformities, and pinch-out in certain layers indicate a late Cryogenian proximal deposition, late Ediacaran transgression, and Middle Cambrian uplift. Cryogenian to early Ordovician strata contain predominantly 1000–900 Ma (late Grenvillian age) detrital zircon population, whereas 1300–1050 Ma (early Grenvillian age) zircons are only dominant in strata earlier than late Ediacaran. Besides, 850–700 Ma zircons are the most prominent group in the Middle Cambrian strata along with an occasional increase in the 650–500 Ma (Pan-African age) zircons. The Grenvillian age zircon groups exhibit significant sediment input from the eastern Indian terrane (990–950 Ma) and western Australia (1300–1050 Ma), underlining the fact that the Cathaysia Block was located between these two terranes in the northern part of East Gondwana. The diminishing contribution of early Grenvillian components in the late Ediacaran strata can be linked to the Pinjarra Orogeny (550–520 Ma), which led to uplift that blocked the transport of detritus from Australia. Middle Cambrian provenance variation with high abundance of 850–700 Ma components indicates the presence of intrinsic sediment from the Wuyishan terrane of South China. Given that the Cathaysia Block was a passive continental margin, this change was caused by the uplift of the southeastern Cathaysia Block, which was related to the far-field stress effects of the late phase of the Kuunga Orogeny (530–480 Ma). The decrease in abundance of early Grenvillian and Pan-African zircons in space implies that they were transported into the basins through independent drainage systems. This is consistent with the local and temporal variations in paleocurrent orientations during the Cambrian, further implying diverse and complex drainage systems in the southwestern Cathaysia Block during this period.

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Lee, Jeffrey, Andrew K. R. Hoxey, Andrew Calvert et Peter Dubyoski. « Plate boundary trench retreat and dextral shear drive intracontinental fault-slip histories : Neogene dextral faulting across the Gabbs Valley and Gillis Ranges, Central Walker Lane, Nevada ». Geosphere 16, n^o 5 (31 juillet 2020) : 1249–75. http://dx.doi.org/10.1130/ges02240.1.

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Abstract The spatial-temporal evolution of intracontinental faults and the forces that drive their style, orientation, and timing are central to understanding tectonic processes. Intracontinental NW-striking dextral faults in the Gabbs Valley–Gillis Ranges (hereafter referred to as the GVGR), Nevada, define a structural domain known as the eastern Central Walker Lane located east of the western margin of the North American plate. To consider how changes in boundary type along the western margin of the North American plate influenced both the initiation and continued dextral fault slip to the present day in the GVGR, we combine our new detailed geologic mapping, structural studies, and 40Ar/39Ar geochronology with published geologic maps to calculate early to middle Miocene dextral fault-slip rates. In the GVGR, Mesozoic basement is nonconformably overlain by a late Oligocene to Miocene sequence dominated by tuffs, lavas, and sedimentary rocks. These rocks are cut and offset by four primary NW-striking dextral faults, from east to west the Petrified Spring, Benton Spring, Gumdrop Hills, and Agai Pah Hills–Indian Head faults. A range of geologic markers, including tuff- and lava-filled paleovalleys, the southern extent of lava flows, and a normal fault, show average dextral offset magnitudes of 9.6 ± 1.1 km, 7.0 ± 1.7 km, 9.7 ± 1.0 km, and 4.9 ± 1.1 km across the four faults, respectively. Cumulative dextral offset across the GVGR is 31.2 ± 2.3 km. Initiation of slip along the Petrified Spring fault is tightly bracketed between 15.99 ± 0.05 Ma and 15.71 ± 0.03 Ma, whereas slip along the other faults initiated after 24.30 ± 0.05 Ma to 20.14 ± 0.26 Ma. Assuming that slip along all four faults initiated at the same time as the Petrified Spring fault yields calculated dextral fault-slip rates of 0.4 ± 0.1–0.6 ± 0.1 mm/yr, 0.4 ± 0.1–0.5 ± 0.1 mm/yr, 0.6 ± 0.1 mm/yr, and 0.3 ± 0.1 mm/yr on the four faults, respectively. Middle Miocene initiation of dextral fault slip across the GVGR overlaps with the onset of normal slip along range-bounding faults in the western Basin and Range to the north and the northern Eastern California shear zone to the south. Based on this spatial-temporal relationship, we propose that dextral fault slip across the GVGR defines a kinematic link or accommodation zone between the two regions of extension. At the time of initiation of dextral slip across the GVGR, the plate-boundary setting to the west was characterized by subduction of the Farallon plate beneath the North American plate. To account for the middle Miocene onset of extension across the Basin and Range and dextral slip in the GVGR, we hypothesize that middle Miocene trench retreat drove westward motion of the Sierra Nevada and behind it, crustal extension across the Basin and Range and NW-dextral shear within the GVGR. During the Pliocene, the plate boundary to the west changed to NW-dextral shear between the Pacific and North American plates, which drove continued dextral slip along the same faults within the GVGR because they were fortuitously aligned subparallel to plate boundary motion.

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Zou, B., C. S. McCool, D. W. Green et G. P. Willhite. « A Study of the Chemical Interactions Between Brine Solutions and Dolomite ». SPE Reservoir Evaluation & ; Engineering 3, n^o 03 (1 juin 2000) : 209–15. http://dx.doi.org/10.2118/64536-pa.

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Summary Application of gelled polymer treatments to change the flow characteristics of a reservoir is a viable improved oil recovery technique. Many gel systems are affected by the solution pH in that gel time is pH dependent. The treatment of carbonate reservoir rock is of particular concern because (1) fluid-rock interaction can alter the pH of the injected solution from the design value and (2) dissolution of carbonate can increase the divalent ion concentration, which can also affect gelation behavior. In this study, the interaction between injected potassium chloride brine and dolomite was investigated through displacement experiments in dolomite cores and mathematical modeling based on equilibrium thermodynamics. Parameters varied were pH of the injected solution, flow rate, and the common ion effect through variation of the calcium and magnesium ion concentrations in the injected solution. Core effluent values of pH and concentrations were measured. Experiments at different flow rates established conditions in which equilibrium was achieved in the core. Equilibrium values of pH were almost constant at a value of 10 when the injected pH was varied between 4 and 10. Results indicate that equilibrium conditions exist for most field conditions. A geochemical model was used to predict equilibrium pH and concentrations, as well as the amount of dolomite dissolved. The model accurately predicted effluent pH of experimental displacement data for the conditions wherein equilibrium was achieved. All model parameters were obtained from literature equilibrium data and were not dependent on curve fitting of the experimental data. Introduction Systems utilizing Cr(III) as the polymer crosslinker are probably the most frequently studied and used gelled polymer systems for water conformance treatments. Both xanthan and partially hydrolyzed polyacylamide form gels with Cr(III). Both of these gel systems are affected by the pH of the solution in at least two ways. One is that the gelation time decreases significantly with increasing pH.1 The other is that Cr(III) is subject to precipitation in solutions with pH over 5.5, and the precipitation is aggravated with increasing pH.2 The treatment of carbonate reservoir rock is of particular concern because of the fluid-rock interaction, which can alter the pH of the injected solution. Previous studies3–5 show that change of pH of the injected solution inhibits the propagation of Cr(III) in carbonate cores or sandstone cores containing carbonate as the solution flows through porous rock. Seright3 studied the propagation of chromium acetate or chromium chloride through Indiana limestone cores. Cr(III) concentration in the effluent never reached the injected concentration after injecting about 10 pore volumes of chromium solution for any case studied. Chromium propagated more rapidly when the counterion was acetate as opposed to chloride. No chromium was detected in the effluent after injecting 10 pore volumes of chromium chloride solution through a limestone core. Stavland et al.4 studied the retention of chromium(III) in Brent and Berea sandstone cores (with about 2% carbonate content). The authors found precipitation was the most important reason for chromium retention in cores. Precipitation was caused by the dissolution of carbonate minerals that increased the pH of the injected solution. Their experiments also revealed that the retention rate of Cr(III) was lower with less carbonate present in the cores. McCool et al.5 studied the interaction between a dolomite core and a xanthan-Cr(III) gel system. Significant amounts of Cr(III) precipitated because the pH in the injected solution increased due to the dissolution of dolomite. Equilibrium relations and the dissolution kinetics in dolomite-carbonic acid-water systems have been studied for such purposes as soil science, the study of secondary changes in sedimentary deposits, the neutralization of acid mine drainage, and the acidizing of petroleum wells. 6 Most previous studies were conducted in agitated batch reactors, rotating disk, or fluidized bed reactor systems in the laboratory by using relatively pure dolomitic rock or synthetic dolomite. A few investigators have studied the dissolution reaction using flow through packed-bed reactors or consolidated rock cores.7,8 Many investigators recognize the complexity of the dissolution of dolomite. Most have attempted to simplify the modeling by delineating the rate-limiting steps. It is apparent that in making assumptions and interpretations of data most investigators have often been limited by the type of apparatus, the size and origin of dolomite used, and the experimental conditions. Consequently, the kinetic equations and mechanisms proposed by different investigators explain their experimental data, but they are not easily generalized. The rate equations derived by Plummer and Busenberg9 and those from Chou et al.10 are often used to predict the dolomite dissolution rates within a 2- to 100-fold difference, depending on the reaction conditions. It is believed that the dissolution rate is controlled by surface reaction, reactant or product diffusion, or a combination depending on reaction conditions. Also, the dissolution of dolomite reaches equilibrium much faster in a closed-to-atmosphere system than in an open system in which the transport of carbon dioxide from the atmosphere is involved in the reactions. The dissolution rate is affected by the rock lithology such as impurity content, crystal size, rock texture, and Ca/Mg ratio in the rock and the injected solution chemistry such as the pH and composition. The effect of the rock formation on dolomite dissolution was the dominant interest in many previous investigations. Rauch and White7 investigated the effect of lithology on carbonate dissolution rate. The authors found that the dissolution rate decreased as the percentage of dolomite and disseminated insolubles increased. The general chemistry of dolomite dissolution has been studied extensively, though some dissolution mechanisms are still under debate. As a salt of a weak acid, dolomite dissolves in strong acid, carbonated water, and water by different mechanisms. In water, the general chemistry is C a M g ( C O 3 ) 2 = C a 2 + + M g 2 + + 2 C O 3 2 − . ( 1 ) When carbon dioxide is present, the dissolution has the reaction C a M g ( C O 3 ) 2 + 2 H 2 C O 3 = C a 2 + + M g 2 + + 4 H C O 3 − . ( 2 ) When in strong acidic solution, the following reaction occurs:6 C a M g ( C O 3 ) 2 + 4 H + = C a 2 + + M g 2 + + 2 C O 2 ( a q ) + 2 H 2 O . ( 3 )

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Castro, Ana Paula O., Eliane Da Costa Alves et Guilherme F. Vasquez. « A STUDY OF THE ELASTIC BEHAVIOUR PRESENTED BY DIFFERENT TYPES OF SEDIMENTARY ROCKS ». Revista Brasileira de Geofísica 33, n^o 2 (1 juin 2015). http://dx.doi.org/10.22564/rbgf.v33i2.721.

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ABSTRACT. The purpose of this paper is to study the elastic behavior of the different kinds of sedimentary rocks from outcrops of the Middle-West region of the United States of America, among those, Berea sandstone from Bedford formation (Ohio), Indiana limestone from Salem formation (Indiana) and Silurian dolomite from Thornton formation (Illinois). To do so, it has been made in the Laboratório de Física de Rochas from Cenpes (Centro de Pesquisas e Desenvolvimento da Petrobras ), measurements of porosity, density and elastic wave propagation velocity presented by each type of studied rock. The wave propagation velocities were estimated by measuring the transit time ultrasonic pulses transmitted through the samples. From the results obtained, it was possible to compare the measured velocities with predictions from theoretical models, as well as to observe correlations between the petrophysical properties of rocks and its seismic behavior. Understanding these correlations helps to improve the interpretation of geophysical measurements and to improve the seismic method, which in turn has a direct impact on exploration and development activities reservoirs.Keywords: seismic velocities, elastic properties, sedimentary rocks.RESUMO. O objetivo deste trabalho é estudar o comportamento elástico de diferentes tipos de rochas sedimentares provenientes de afloramentos da região Centro-Oeste dos Estados Unidos da América, entres estas, o arenito Berea da formação Bedford (Ohio), o calcário Indiana da formação Salem (Indiana) e o dolomito Silurian da formação Thornton (Illinois). Para tanto, foram realizadas, no Laboratório de Física de Rochas do Cenpes (Centro de Pesquisas e Desenvolvimento da Petrobras), medidas de porosidade, densidade e de velocidades de propagação das ondas elásticas apresentadas por tipo de rocha estudada. As velocidades de propagação das ondas foram determinadas medindo-se o tempo de trânsito de um pulso ultrassônico de alta frequência transmitido através das amostras. A partir dos resultados obtidos, foi possível comparar as velocidades medidas com as previsões feitas a partir de modelos teóricos, assim como foi possível observar correlações existentes entre as propriedades petrofísicas das rochas e o seu comportamento sísmico. O entendimento dessas correlações contribui para a melhoria da interpretação de medidas geofísicas e para o aprimoramento do método sísmico, que por sua vez, tem impacto direto nas atividades de exploração e desenvolvimento de reservatórios.Palavras-chave: velocidades sísmicas, propriedades elásticas, rochas sedimentares.

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Hassanpour, S. « Lahroud, a Paleo-Tethys Remnant in Northwestern Iran : Implications for Geochemistry, Radioisotope Geochronology, and Tectonic Setting ». Russian Geology and Geophysics, 17 mars 2021. http://dx.doi.org/10.2113/rgg20194116.

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Abstract —The Lahroud Ophiolite in northwestern Iran contains extensive zones of Paleozoic ophiolite as remnants of the Paleo-Tethys oceanic crust. The principal rock units are gabbro overlain by pillow basalt, which is intruded by granites and interbedded with pelagic sedimentary units including radiolarian cherts. Geochemistry and radioisotope studies, supported by Nd, Sm, Sr, and Pb isotope data, indicate that the Lahroud Ophiolite originates from a within-plate basaltic mantle source. The isotope studies show that the basalts are derived from Indian-type oceanic mantle sources. The radiogenic data indicate the involvement of subduction-related terrigenous materials in the source magma. All the rocks are geochemically cogenetic and were generated by fractionation of a melt with a composition of average E-MORB with a calc-alkaline signature. Two 40Ar/39Ar ages, 343 ± 3 Ma for muscovite minerals and 187.7 ± 7.7 Ma for glasses, suggest that metamorphic and basaltic rocks formed during the Late Paleozoic to Early Jurassic, respectively. Microfossil studies show the presence of Paleozoic biostratigraphy. The crystallization process and rifting into the oceanic crust in the Lahroud Ophiolite probably began in the Carboniferous, with volcanic activity continuing during the Late Triassic.

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li, xue, Guo-Sheng Sun, Gen-Yi Liu, Huan Zhou, Zi-Ling Shan, Jiu-Da Sun et Guang-Wei Wang. « Crustal thickening and uplift of the Qiangtang Terrane, Tibetan Plateau during the Late Cretaceous to Early Palaeocene : geochronology and geochemistry of the Saiduopugangri granite ». Canadian Journal of Earth Sciences, 16 octobre 2020. http://dx.doi.org/10.1139/cjes-2020-0057.

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There continues to be debate regarding the timing of the collision between the Indian and Eurasian plates and the uplift of the Tibetan Plateau. This study presents zircon U–Pb geochronology, whole-rock geochemistry, and Lu–Hf isotopic data for the Saiduopugangri granite of the Qiangtang Terrane, located within the core of the Tibetan Plateau. These data provide the basis for the geodynamic setting, petrogenesis, and characteristics of its magma source. Zircons from the Saiduopugangri granite yield a weighted-mean 206Pb/238U age of 62.72 ± 0.06 Ma, indicating that these rocks formed during the early Palaeocene. The rocks are members of the highly calc-alkaline to shoshonitic series, with weak peraluminous characteristics. Trace elements are characterised by high Sr (483–616ppm), and low Y (6–10ppm) and Yb (1ppm) content, typical of a high Sr and low Yb granite. The εHf(t) of zircon range from −2.14 to 2.35, with two-stage Hf model ages (TDM2) ranging from 1182 to 895Ma. These data suggest that the Saiduopugangri granite magma was derived from the melting of lower-crustal clastic meta-sedimentary rocks and mantle-derived basalts. The high Sr and low Yb granite characteristics and experimental results indicate that melting occurred at >1.2 GPa and >750 °C, consistent with a crustal thickness greater than 50km. Magmatism occurred from the Late Cretaceous to the early Palaeogene and is broadly synchronous with the collision timing between the Indian and Eurasian plates. The Saiduopugangri granite provides evidence of crustal thickening of the Tibetan Plateau and its age and petrogenesis constrain the timing of the initial uplift.

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Khan, T., M. A. Khan, M. Q. Jan et M. Latif. « The Kohistan between Gilgit and Chilas, northern Pakistan : regional tectonic implications ». Journal of Nepal Geological Society 14 (1 novembre 1996). http://dx.doi.org/10.3126/jngs.v14i0.32317.

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In this paper, we present geological description of an area located between Gilgit and Chilas within the Kohistan terrane. This terrane has been considered an intra-oceanic island arc, formed due to northward subduction of the Neo-tethyan lithospheric plate. At present, it is squeezed between the Karakoram Asian and Indian continental plates. Both the contacts are marked by suture zones, that is, Shyok (MKT) in the north and Indus (MMT) sutures in the south, respectively. The investigated area consists of plutonic, metamorphosed volcanic and sedimentary rocks, the Chilas Complex, and the Kamila Amphibolite. The metamorphosed volcanic and sedimentary rocks are packaged into the Jagfot Group. This group comprises basal turbiditic sediments, intercalated with amphibolites and calc-silicates (the Gilgit Formation), followed upward by the Gashu-Confluence Volcanics = Chait Volcanic Group, and finally the Thelichi Formation = Yasin Group of Aptian-Albian age. The Thelichi Formation comprises a volcanic base (Majne volcanics) and overlying turbidites, local intercalation of marbles, volcaniclastics and lava flows. Greenschist and amphibolite facies are common in the Jaglot Group, and particularly the sillimanite in the Gilgit Formation. A pair of anticline (the Gilgit anticline) and syncline (the Jaglot syncline) make up the structural scenario. On the basis of field geology, we conclude that the entire Jaglot Group and its equivalents, the Yasin Group, Chait Volcanic Group in Kohistan, and Burjila Formation, Bauma Harel Formation and Katzarah Formation in Ladakh show intra-oceanic back-arc basin rather than island arc affinities as suggested in the past.

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Arak Sangsomphong, Dhiti Tulyatid, Thanop Thitimakorn et Punya Charusiri. « Tectonic blocks and suture zones of eastern Thailand : evidence from enhanced airborne geophysical analysis ». Annals of Geophysics 56, n^o 1 (18 avril 2013). http://dx.doi.org/10.4401/ag-5547.

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<p>Airborne geophysical data were used to analyze the complex structures of eastern Thailand. For visual interpretation, the magnetic data were enhanced by the analytical signal, and we used reduction to the pole (RTP) and vertical derivative (VD) grid methods, while the radiometric data were enhanced by false-colored composites and rectification. The main regional structure of this area trends roughly in northwest-southeast direction, with sinistral faulting movements. These are the result of compression tectonics (sigma<span style="font-size: 8px;">_1</span> in an east-west direction) that generated strike-slip movement during the pre Indian-Asian collision. These faults are cross-cut by the northeast-southwest-running sinistral fault and the northwest-southeast dextral fault, which occurred following the Indian-Asian collision, from the transpession sinistral shear in the northwest-southeast direction. Three distinct geophysical domains are discernible; the Northern, Central and Southern Domains. These three domains correspond very well with the established geotectonic units, as the Northern Domain with the Indochina block, the Central Domain with the Nakhonthai block, the Upper Southern Sub-domain with the Lampang-Chaing Rai block, and the Lower Southern Sub-domain with the Shan Thai block. The Indochina block is a single unit with moderate radiometric intensities and a high magnetic signature. The direction of the east-west lineament pattern is underlain by Mesozoic non-marine sedimentary rock, with mafic igneous bodies beneath this. The Nakhonthai block has a strong magnetic signature and a very weak radiometric intensity, with Late Paleozoic-Early Mesozoic volcanic rock and mélange zones that are largely covered by Cenozoic sediments. The boundaries of this block are the southern extension of the Mae Ping Faults and are oriented in the northwest-southeast direction. The Lampang-Chaing Rai and Shan Thai blocks, with very weak to moderate magnetic signatures and moderate to very strong radiometric intensities are dominated by marine clastic and igneous rocks or a northwest-southeast trending deformation zone of inferred Precambrian complexes, respectively. It is suggested that these tectonic plates collided against one another in a west-east direction.</p>

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Bhattacharya, Gourab, Delores M. Robinson et Matthew M. Wielicki. « Detrital zircon provenance of the Indus Group, Ladakh, NW India : Implications for the timing of the India-Asia collision and other syn-orogenic processes ». GSA Bulletin, 5 octobre 2020. http://dx.doi.org/10.1130/b35624.1.

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The timing of the India-Asia collision is greatly debated and is critical for elucidating early orogenic processes. This study documents, for the first time, evidence of India-Asia detrital mixing in the continental sedimentary rocks of the India-Asia collision zone of NW India at ca. 50 Ma and presents the largest detrital zircon (DZ) U-Pb age data set (n = 1225) from the region. Our DZ U-Pb age spectra from the early Eocene−late Oligocene continental Indus Group reveal a hybrid India-Asia provenance. The dominant Mesozoic−Cenozoic DZ peaks are ca. 107 Ma, 100−80 Ma, 60−50 Ma, 40 Ma, and 26 Ma, and they are mostly derived from Asia. The primary Precambrian DZ peaks are ca. 2.5 Ga, 1.2−0.95 Ga, 0.78−0.63 Ga, and 0.55 Ga and are representative of Tethyan Himalayan rocks on Greater India. Maximum depositional ages (MDAs) for four key Indus Group units, the Nurla, Hemis, Basgo, and Temesgam Formations, support syn-orogenic deposition in the Indus Basin from early Eocene to at least late Oligocene time. The Nurla Formation, with an MDA of ca. 50 Ma, records the first arrival of Greater Indian zircons on the Asian plate, thereby indicating uplift and erosion along the subducting Indian plate and collision of India with Asia by ca. 50 Ma. The ca. 27−26 Ma zircons in the younger late Oligocene Basgo and Temesgam Formations were contributed by the Lhasa terrane in south Tibet, which implies that the Indus River flowed from east to west across NW India at least by ca. 27 Ma.

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« Folding and imbrication of the Indian crust during Himalayan collision ». Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences 326, n^o 1589 (septembre 1988) : 89–116. http://dx.doi.org/10.1098/rsta.1988.0081.

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India collided with a northern Kohistan-Asian Plate at about 50 Ma ago, the time of ocean closure being fairly accurately defined from syntectonic sediments as well as the effect on magnetic stripes on the Indian Ocean floor. Since collision, Asia has over-ridden India, developing a wide range of thrust scrapings at the top of the Indian Plate. Sections through the imbricated sedimentary cover suggest a minimum displacement of over 500 km during Eocene to recent plate convergence. This requires the Kohistan region to the north to be underlain by underthrusted middle to lower Indian crust, deformed by ductile shears and recumbent folds. These structures are well seen in the gneisses immediately south of the suture, where they are uplifted in the Indus and Nanga Parbat syntaxes. Here there are several phases of thrust-related small-scale folding and the development of a large folded thrust stack involving basement rocks, the imbrication of metamorphic zones and the local development of large backfolds. Some of the important local structures: the large late backfolds, the Salt Ranges and the Peshawar Basin, can all be related to the necessary changes in thrust wedge shape as it climbs through the crust and the three dimensional nature of the thrust movements associated with interference between the Kohistan and western Himalayan trends.

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West, R. M. « The Cenozoic of Nepal : mountain elevation and vertebrate evolution ». Journal of Nepal Geological Society 14 (1 novembre 1996). http://dx.doi.org/10.3126/jngs.v14i0.32318.

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The Cenozoic of Nepal was a time of great activity, in terms of both the establishment and uplift of the Himalaya and the development of a vertebrate fauna which changed through time in response to the environmental events caused by the elevation of the mountains. Field work conducted over the past twenty years has generated a body of data which brings together palaeontological, ecological, and tectonic interpretations of the Cenozoic history of Nepal Himalaya. Palaeontological data from Nepal are geographically limited. At this time, the early Cenozoic is represented by modest marine and terrestrial mammal remains found near Tansen. The middle and late Cenozoic is also documented from abundant materials found in the Siwaliks in a broad band of Sub-Himalayan sedimentary rocks between Butwal and Nepalganj and north of Jaleswar. Ice Age in Nepal may be interpreted from several Pleistocene localities in the Kathmandu Valley. Nepal's Cenozoic palaeoenvironments are interpreted in large from the fossils found in the areas mentioned above, by analogy to India and Pakistan, and by study of the sedimentology of the enclosing rocks. It is possible to document the arrival of the Indian tectonic plate in South Asia in the early Cenozoic using palaeontologic, sedimentologic and tectonic data. At this time the broad open seaway (the remnant of Tethys) which occupied much of Nepal until the early Cenozoic closed and terrestrial communication with other areas became possible. By the middle Cenozoic, Nepal was the site of major erosional deposition from the rising Himalaya. This palaeoenvironment is indicated by both the terrestrial elastic sedimentary rocks which dominate the Nepal middle and late Cenozoic sequences as well as by the particular vertebrate taxa which have been recovered from the Siwaliks in western Nepal. Nepal's Pleistocene was a time of cool and dry environments; Kathmandu Valley deposits have yielded vertebrate remains which are indicative of this environment. Of particular interest are efforts to relate Himalayan Cenozoic tectonics to the palaeobiological record of Nepalese environments. There are strong indications that the primary elevation of the Himalaya was a mid to late Cenozoic event; this correlates well with the environmental evidence from the fossil assemblages. This paper is devoted to review of palaeontologic, sedimentologic and tectonic data which are used to interpret the Cenozoic history of Nepal Himalaya.

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Valdiya, K. S. « Three terrane-defining thrusts of the Kumaun Himalaya : controversies on position and nomenclature ». Journal of Nepal Geological Society 30 (1 décembre 2004). http://dx.doi.org/10.3126/jngs.v30i0.31673.

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Three of the five thrust fault systems defining the boundaries of four lithotectonic terranes of the Himalaya and involved in controversies related to their positions and nomenclature, are the objects of discussion in this paper. Youngest of the five terrane-defining faults, the Himalayan Frontal Fault (HFF) is a series of reverse faults that demarcates the boundary of the Siwalik front of the Himalayan province with the alluvial expanse of the Indo-Gangetic Plains. Over large tracts, it is either concealed under younger sediments or has as yet not reached the ground surface and is therefore a blind fault. The nature of this frontal fault varies along its length. Where the hidden ridges of the Indo-Gangetic basement impinge the Himalaya, the mountain front is ruptured and the HFF is repeatedly reactivated. In the sectors intervening these ridges, it is not expressed on the surface, but the ground of the adjoining Indo-Gangetic Plain is sinking, the rivers are shifting their courses and large tracts of land are waterlogged and characterized by The Vaikrita Thrust is the plane that marks pronounced metamorphic break and abrupt change in style and orientation of structures within the succession of crystalline rocks that build the bulk of the snowy ranges in the Kumaun Himalaya. Not only is there a jump of pressure of the order of 4 kb and a temperature rise of >200 oC, but also is there a conspicuous change of neodymium isotope value across the tectonic plane that separates the low-grade metarnorphics in the lower part from the high-grade metamorphic rocks of the upper part of the Great Himalayan succession. The Vaikrita Thrust is therefore recognised as the Main Central Thrust (MCT). While the basal low-grade metamorphic assemblage comprises 1900±100 Ma old highly tectonised porphyritic granite characterised by low initial strontium isotope ratio, the upper high -grade metamorphic group is intruded by 20±1 Ma old anatectic granites characterised by garnet, kyanite, sillimanite and cordierite, and a high but variable value of strontium isotope ratio. Moreover, the anatectic Lower Miocene granites are singularly absent in the succession of the Lesser Himalayan nappes. Probably it is this thrust that has flexed downwards the plane of decoupling and displacement between the under thrusting Indian plate and the overlying Himalayan mass. The terrane-defining fault between the high-grade metamorphics with granitic rocks of the Himadri (i.e., Great Himalaya) and the Tethyan sedimentary pile was recognised as the Malari Thrust Fault in the northern Kumaun Himalaya in the early seventies, as the South Tibetan Detachment System in southern Tibet adjoining north-eastern Nepal in the early eighties and as the Zanskar Shear Zone in north-western Himachal Pradesh in the late eighties. Not only is there an abrupt change in the metamorphic grade across the tectonic plane, but also an attenuation and wholesale elimination of some lithostratigraphic formations of the hanging wall besides the difference in the style of deformation. Exhibiting predominant dip-slip movement in the central sector of the Himalayan arc, the Trans-Himadri Fault (T-HF) was formed as a consequence of the Tethyan sedimentary cover detaching from its rigid foundation of the basement complex that was squeezed up following blocking or slowing down of tectonic movements related to India-Asia convergence. The sedimentary cover lagging behind the thrust up basement complex slid down and toppled over northward and gave rise to back-folds and back-thrusts. In Kashmir, western Himachal Pradesh, central Nepal and north-western Bhutan, in sharp contrast, there was very strong compression, so that the Tethyan sedimentary rocks-along with a slice of the low-grade metamorphic basement in the hanging wall advanced southwards across the Himadri and were emplaced as nappes and klippen south of the Main Central Thrust. The T-HF movement occurred approximately around 20.9 Ma, although the movement had started quite earlier in some places. Quaternary reactivation resulted in river ponding and development of huge lakes such as the Garbyang palaeolake in the Kali valley and the ~ 40,000 year-old Goting palaeolake in the Western Dhauli Valley.

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