Journal articles on the topic 'Overprinting relationships'
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Hemingway, Jordon D., Haley Olson, Alexandra V. Turchyn, Edward T. Tipper, Mike J. Bickle, and David T. Johnston. "Triple oxygen isotope insight into terrestrial pyrite oxidation." Proceedings of the National Academy of Sciences 117, no. 14 (March 25, 2020): 7650–57. http://dx.doi.org/10.1073/pnas.1917518117.
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The mass-independent minor oxygen isotope compositions (Δ′17O) of atmospheric O2andCO2are primarily regulated by their relative partial pressures,pO2/pCO2. Pyrite oxidation during chemical weathering on land consumesO2and generates sulfate that is carried to the ocean by rivers. The Δ′17O values of marine sulfate deposits have thus been proposed to quantitatively track ancient atmospheric conditions. This proxy assumes directO2incorporation into terrestrial pyrite oxidation-derived sulfate, but a mechanistic understanding of pyrite oxidation—including oxygen sources—in weathering environments remains elusive. To address this issue, we present sulfate source estimates and Δ′17O measurements from modern rivers transecting the Annapurna Himalaya, Nepal. Sulfate in high-elevation headwaters is quantitatively sourced by pyrite oxidation, but resulting Δ′17O values imply no direct troposphericO2incorporation. Rather, our results necessitate incorporation of oxygen atoms from alternative,17O-enriched sources such as reactive oxygen species. Sulfate Δ′17O decreases significantly when moving into warm, low-elevation tributaries draining the same bedrock lithology. We interpret this to reflect overprinting of the pyrite oxidation-derived Δ′17O anomaly by microbial sulfate reduction and reoxidation, consistent with previously described major sulfur and oxygen isotope relationships. The geologic application of sulfate Δ′17O as a proxy for pastpO2/pCO2should consider both 1) alternative oxygen sources during pyrite oxidation and 2) secondary overprinting by microbial recycling.
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Williams, Harold, and M. A. J. Piasecki. "The Cold Spring Melange and a possible model for Dunnage–Gander zone interaction in central Newfoundland." Canadian Journal of Earth Sciences 27, no. 8 (August 1, 1990): 1126–34. http://dx.doi.org/10.1139/e90-117.
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Structural relationships at Cold Spring Pond and the recognition of ophiolitic melange bear on the important questions of timing and style of structural superpositioning of Dunnage Zone rocks above Gander Zone rocks in central Newfoundland. The latest models emphasize ductile shear boundaries and orogen-parallel movements. Previous models proposed west-to-east or head-on obduction of Dunnage ophiolitic rocks across the Gander Zone.At the Dunnage (Exploits Subzone) – Gander (Meelpaeg Subzone) boundary at Cold Spring Pond, discrete, outcrop-size ultramafic blocks and smaller quartzite blocks are randomly distributed, and they are surrounded by, or are embedded in, homogeneous black graphitic shale or phyllite. The ultramafic blocks are typical of nearby Early Ordovician Dunnage ophiolite suites, the quartzite blocks are typical of adjacent Early Ordovician or earlier Gander clastic rocks, and the matrix black shales are similar to those of Middle or Early Ordovician age that occur throughout central Newfoundland. This chaotic mixture of almost coeval lithologies at Cold Spring Pond is interpreted as an olistostromal melange; the Cold Spring Melange. It resembles melanges that are dated as Ordovician elsewhere in Newfoundland.The Cold Spring Melange is overprinted by the full range of structures and metamorphic effects evident in adjacent rocks of the Exploits (Dunnage) and Meelpaeg (Gander) subzones. These include the development of lineations, cleavages, schistosities, zones of ductile shearing, regional metamorphism, and contact metamorphism. The oldest of these effects are interpreted as Silurian, based on isotopic dating in southern Newfoundland.The formation of olistostromal, ophiolitic melange implies disruption of the oceanic tract (Exploits Subzone of the Dunnage Zone), and in the case of the Cold Spring example, juxtapositioning or transport of Exploits Subzone ophiolite suites against or across the supracrustal rocks of the Meelpaeg Subzone (Gander Zone). The age and provenance of Cold Spring components, lack of post-Ordovician components, overprinting structural relationships, and comparison with other Newfoundland melanges all support an Ordovician age of formation. Overprinting relationships indicate that major ductile shears at other Dunnage–Gander zone boundaries postdate initial Dunnage–Gander superpositioning.
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ALSOP, G. I., R. BRYSON, and D. H. W. HUTTON. "Ductile transpression and localization of deformation along tectonic boundaries in the Caledonides of northwestern Ireland." Geological Magazine 135, no. 5 (September 1998): 699–718. http://dx.doi.org/10.1017/s0016756898001265.
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Orogenesis is increasingly interpreted in terms of strain focusing, localization and partitioning processes. Such heterogeneous deformation is considered a consequence of the tectonic framework, with pre-existing structural and stratigraphic variability providing inherent zones of crustal weakness. Detailed structural investigation of Neoproterozoic Dalradian metasediments in the Glencolumbkille region, northwest Ireland, enables patterns of reworking and strain localization to be assessed in terms of four overprinting ductile deformation episodes. A well-defined and intricate Dalradian stratigraphy provides readily distinguishable markers which not only focus deformation along marked rheological boundaries, but also aid in the definition and identification of resultant geometries. Overall structural and stratigraphic relationships show that whilst D1 was not associated with major structures, D2 is related to north–northeast directed folding and ductile thrusting resulting in a major phase of crustal thickening and almandine-amphibolite facies metamorphism. Structures generated during D2 deformation subsequently became the locus of intense D3 strain and were reactivated in an oblique sense associated with south or southwest directed translations. Local overprinting relationships clearly demonstrate S2 fabrics being transposed by S3 resulting in a composite foliation over large areas. Similarly, the L2 mineral lineation is abruptly transposed by L3 over relatively small distances indicating high D3 strain gradients and the susceptibility of lineations to reworking. The final stage of ductile deformation (D4) which was increasingly localized and focused into earlier (D2−D3) high strain zones, is marked by a pronounced phase of sinistral transpression associated with clockwise cleavage and minor fold transection of northwest verging upright folds. Sinistral shear is strongly partitioned in to the steep limbs of mesoscopic F4 folds. The detailed investigation of structures generated within such multiply deformed and reworked zones provides evidence of both the kinematic and tectonic evolution of regional deformation systems.
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Kellett, R. L., and B. Rivard. "Characterization of the Benny deformation zone, Sudbury, Ontario." Canadian Journal of Earth Sciences 33, no. 9 (September 1, 1996): 1256–67. http://dx.doi.org/10.1139/e96-095.
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Remote sensing imagery and geophysical data are well known as valuable tools for reconnaissance mapping in unknown areas, but they can also be used to reinterpret existing regional geological maps. A combination of airborne magnetic data and synthetic aperture radar images, at both a regional and a detailed scale, have been used to identify a wrench-fault system on the Canadian Shield north of the Sudbury structure. The 3–4 km wide deformation zone comprises a set of subparallel vertical faults bounding blocks of Archean granites, Archean metavolcanics of the Benny greenstone belt, and Paleoproterozoic metasediments of the Huronian supergroup. Using high-resolution airborne radar and magnetic data, the fault zone is found to extend for 40 km along the southern margin of the Benny greenstone belt. The wrench-fault system may have been tectonically active during several episodes throughout the Proterozoic. An interpretation of these data, supported by additional field mapping, indicates that the 1240 Ma Sudbury dyke swarm has been intruded through the deformation zone after its most active period of movement. Overprinting of Sudbary impact breccia at the southern edge of the deformation zone suggests that some movement occurred on the faults postdating the 1850 Ma meteorite impact. Lineaments that correlate spatially with the wrench-fault system can be traced across the southern Superior Province and the Cobalt Embayment on the regional images. However, more high-resolution studies are required to establish the same overprinting relationships along the length of the lineaments.
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CLARKE, G. L., and R. POWELL. "Proterozoic granulite facies metamorphism in the southeastern Reynolds Range, central Australia: geological context, P?T path and overprinting relationships." Journal of Metamorphic Geology 9, no. 3 (May 1991): 267–81. http://dx.doi.org/10.1111/j.1525-1314.1991.tb00522.x.
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Xu, Wen-Liang, Fu-Ping Pei, Feng Wang, En Meng, Wei-Qiang Ji, De-Bin Yang, and Wei Wang. "Spatial–temporal relationships of Mesozoic volcanic rocks in NE China: Constraints on tectonic overprinting and transformations between multiple tectonic regimes." Journal of Asian Earth Sciences 74 (September 2013): 167–93. http://dx.doi.org/10.1016/j.jseaes.2013.04.003.
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Mancktelow, Neil, and Giorgio Pennacchioni. "Intermittent fracturing in the middle continental crust as evidence for transient switching of principal stress axes associated with the subduction zone earthquake cycle." Geology 48, no. 11 (July 21, 2020): 1072–76. http://dx.doi.org/10.1130/g47625.1.
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Abstract In the Neves area, eastern Alps, fractures that localized shear zones in middle continental crust above the Alpine megathrust are commonly oriented at a high angle to the inferred long-term shortening direction. Fractures show a segmentation geometry and, locally, a discernible offset, indicating movement opposite to the sense of subsequent ductile shear and implying a switch of principal stress axes σ1 and σ3 during fracturing. We propose that this repeated switch, demonstrated by overprinting relationships and different degrees of fracture reactivation, was due to sporadic co-seismic to early post-seismic rebound in the upper plate of the Alpine continental collision system. Fracturing occurred intermittently in the weak midcrustal rocks due to seismic stress release at high transient strain rates and pore-fluid pressures. Widespread transient fracturing in the hanging wall of the Alpine megathrust regionally controls the orientation of ductile shear zones in the middle crust, as well as the emplacement of magmatic dikes.
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Corrigan, David, Nicholas G. Culshaw, and Jim K. Mortensen. "Pre-Grenvillian evolution and Grenvillian overprinting of the Parautochthonous Belt in Key Harbour, Ontario: U–Pb and field constraints." Canadian Journal of Earth Sciences 31, no. 3 (March 1, 1994): 583–96. http://dx.doi.org/10.1139/e94-051.
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The Parautochthonous Belt in the region of Key Harbour, Ontario, is composed of Early Proterozoic migmatitic para- and orthogneiss and Mid-Proterozoic granitoids, which were reworked during the Grenville orogeny. Grenvillian deformation is localized into anastomosing arrays of high-strain shear zones enclosing elongate bands and lozenges of rock subjected to lower and near-coaxial strain. Crosscutting relationships preserved in the low-strain domains document two pre-Grenvillian plutonic and tectonometamorphic events, which are bracketed in age by U–Pb zircon geochronology. A 1694 Ma leucogranite intrudes, and provides a minimum age for, high metamorphic grade gneisses formed during an earlier tectonometamorphic event (D1–M1). The leucogranite was intruded by mafic dykes, deformed, and metamorphosed at uppermost amphibolite facies during D2–M2, before the emplacement of Mid-Proterozoic granitoids at ca. 1450 Ma. Following the emplacement of gabbro dykes and pods at ca. 1238 Ma, the area was overprinted by granulite to uppermost amphibolite facies metamorphism (Grenvillian), for which monazites provide a minimum age of ca. 1035 Ma. Titanite U–Pb ages of 1003 – 1004 Ma record cooling through 600 °C. A regionally important swarm of east–west-trending posttectonic pegmatite dykes dated by U–Pb zircon at 990 Ma provides a minimum age for Grenvillian ductile deformation. The present data support the contention that the Parautochthonous Belt in the Key Harbour area consists in part of reworked midcontinental crust of Early to Mid-Proterozoic age.
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Feng, Rui, Jianzhong Fan, and Robert Kerrich. "Noble metal abundances and characteristics of six granitic magma series, Archean Abitibi Belt, Pontiac Subprovince; relationships to metallogeny and overprinting of mesothermal gold deposits." Economic Geology 88, no. 6 (October 1, 1993): 1376–401. http://dx.doi.org/10.2113/gsecongeo.88.6.1376.
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Wilson, Matthew Coffie, Chiri G. Amedjoe, and Simon K. Y. Gawu. "STRUCTURES OF BIRIMIAN AND TARKWAIAN ROCKS AT NORTH-WEST NEW DROBO – IMPLICATION ON DEFORMATION." Malaysian Journal of Geosciences 6, no. 1 (2022): 36–44. http://dx.doi.org/10.26480/mjg.01.2022.36.44.
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This research leads to identifying the microstructures, the deformation episodes (D) and foliation grades (S) of rocks in the study area. Different geological structures such as shear zones, foliations, micro-faults, crack-seal events, etc. have been identified in the study area. Different phases of deformation episodes and foliation grades have been studied to determine the deformation environments. Senses of shear such as dextral, CS, and CS’ as well as antithetic and synthetic micro-faults and boudins may be identified at the north-west New Drobo. Three (3) main deformational (D) events and foliation grades (S) in both the Tarkwaian sandstones and Birimian volcanic rocks may be identified as D1, D2 and D3 as well as S1, S2 and S3. Recrystallization of quartz and feldspar through bulging (BLG), subgrain boundary rotation (SBR) and grain boundary migration (GBM) have been observed at the study area. The overprinting relationships in the Birimian Supergroup may be identified with three (3) different deformational phases. The first deformation (D1) defines the formation of a vertical shortening, whilst the second deformation (D2) defines oblique shortening. Moreover, the third deformation (D3) is due to high strain rate causing brittle faulting.
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Barnes, Christopher J., Jarosław Majka, Michał Bukała, Erika Nääs, and Sabine Rousku. "Detrital zircon U-Pb geochronology of a metasomatic calc-silicate in the Tsäkkok Lens, Scandinavian Caledonides." Geology, Geophysics and Environment 47, no. 1 (April 23, 2021): 21–31. http://dx.doi.org/10.7494/geol.2021.47.1.21.
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The Tsäkkok Lens of the Seve Nappe Complex in the Scandinavian Caledonides comprises eclogite bodies hosted within metasedimentary rocks. These rocks are thought to be derived from the outermost margin of Baltica along the periphery of the Iapetus Ocean, but detrital records from the sedimentary rocks are lacking.Many metasedimentary outcrops within the lens expose both well-foliated metapelitic rocks and massive calc-silicates. The contacts between these two lithologies are irregular and are observed to trend at all angles to the high-pressure foliation in the metapelites. Where folding is present in the metapelites, the calc-silicate rocks are also locally folded. These relationships suggest metasomatism of the metapelites during the Caledonian orogenesis. Zircon U-Pb geochronology was conducted on sixty-one zircon grains from a calc-silicate sample to investigate if they recorded the metasomatic event and to assess the detrital zircon populations. Zircon grains predominantly show oscillatory zoning, sometimes with thin, homogeneous rims that have embayed contacts with the oscillatory-zoned cores. The zircon cores yielded prominent early Stenian, Calymmian, and Statherian populations with a subordinate number of Tonian grains. The zircon rims exhibit dissolution-reprecipitation of the cores or new growth and provide ages that span similar time frames, indicating overprinting of successive tectonic events. Altogether, the zircon record of the calc-silicate suggests that the Tsäkkok Lens may be correlated to Neoproterozoic basins that are preserved in allochthonous positions within the northern extents of the Caledonian Orogen.
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Andersson, Joel B. H., Tobias E. Bauer, and Edward P. Lynch. "Evolution of structures and hydrothermal alteration in a Palaeoproterozoic supracrustal belt: Constraining paired deformation–fluid flow events in an Fe and Cu–Au prospective terrain in northern Sweden." Solid Earth 11, no. 2 (April 17, 2020): 547–78. http://dx.doi.org/10.5194/se-11-547-2020.
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Abstract. An approximately 90 km long Palaeoproterozoic supracrustal belt in the northwestern Norrbotten ore province (northernmost Sweden) was investigated to characterize its structural components, assess hydrothermal alteration–structural geology correlations, and constrain a paired deformation–fluid flow evolution for the belt. New geological mapping of five key areas (Eustiljåkk, Ekströmsberg, Tjårrojåkka, Kaitum West, and Fjällåsen–Allavaara) indicates two major compressional events (D1 and D2) have affected the belt, with each associated with hydrothermal alteration types typical for iron oxide–apatite and iron oxide Cu–Au systems in the region. Early D1 generated a regionally distributed, penetrative S1 foliation and oblique reverse shear zones that show a southwest-block-up sense of shear that formed in response to NE–SW crustal shortening. Peak regional metamorphism at epidote–amphibolite facies broadly overlaps with this D1 event. Based on overprinting relationships, D1 is associated with regional scapolite ± albite, magnetite + amphibole, and late calcite alteration of mafic rock types. These hydrothermal mineral associations linked to D1 structures may form part of a regionally pervasive evolving fluid flow event but are separated in this study by crosscutting relationships. During D2 deformation, folding of S0–S1 structures generated F2 folds with steeply plunging fold axes in low-strain areas. NNW-trending D1 shear zones experienced reverse dip-slip reactivation and strike-slip-dominated movements along steep, E–W-trending D2 shear zones, producing brittle-plastic structures. Hydrothermal alteration linked to D2 structures is a predominantly potassic–ferroan association comprising K-feldspar ± epidote ± quartz ± biotite ± magnetite ± sericite ± sulfides. Locally, syn- or post-tectonic calcite is the main alteration mineral in D2 shear zones that intersect mafic rocks. Our results highlight the importance of combining structural geology with the study of hydrothermal alterations at regional to belt scales to understand the temporal–spatial relationship between mineralized systems. Based on the mapping results and microstructural investigations as well as a review of earlier tectonic models presented for adjacent areas, we suggest a new structural model for this part of the northern Fennoscandian Shield. The new model emphasizes the importance of reactivation of early structures, and the model harmonizes with tectonic models presented by earlier workers based mainly on petrology of the northern Norrbotten area.
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Andrew, Joseph E. "Geologic map of central Panamint Range, California, USA." Geosphere 18, no. 2 (March 10, 2022): 730–31. http://dx.doi.org/10.1130/ges02344.1.
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Abstract This detailed geologic map and supplemental digital data set1 examine and demonstrate the complex deformational history and reactivation relationships of the Panamint Range (California, USA), from active transtension of the Walker Lane belt, Miocene extension of the Basin and Range, to multiple Mesozoic events related to subduction, and Neoproterozoic extension. This collection of map data focuses on the geometry, kinematics, and relative timing of deformation to understand the deformation history and effects of structural reactivation. A minor portion of this geologic mapping data was presented in the analysis and figures of Andrew and Walker (2009). The Neogene extension and subsequent dextral transtension deformation has created a complex network of faults via partial reactivation of Mesozoic and Neoproterozoic structures. Structural data show oblique normal slip overprinting earlier normal slip along the western range flank fault of the western Panamint Range. Jurassic and Cretaceous deformation is localized along the western range on the Goldbug fault. The hanging wall of this fault preserves migmatitic fabrics and intense deformation due to Jurassic contraction. The Goldbug fault places Paleoproterozoic to Mesoproterozoic rocks over Neoproterozoic rocks. The Jurassic contraction has top-to-the-northeast relative transport and the more discrete Cretaceous thrust faulting has top-to-the-east transport. A set of Late Cretaceous plutonic rocks and mylonitic gneisses derived from them, occur along the Goldbug fault and demonstrate the reactivated nature of this fault in the Late Cretaceous. New data for the Butte Valley fault show that this fault cuts Late Jurassic plutonic rocks and has normal slip. The Butte Valley fault ends northward at the linked sinistral slip Warm Spring Canyon fault, which was previously interpreted to be an intrusive contact. A previously unrecognized rim syncline structure occurs along the boundary of the Late Jurassic Manly Peak quartz monzonite. Neoproterozoic deformation is difficult to discern due to the overprinting deformations. Numerous Neoproterozoic deformation-related mass wasting deposits can be seen within this formation, including a set of conspicuous allochthonous deposits and clasts of older Beck Spring Dolomite that appear to be frozen in the process of breaking away from intact, normal thickness beds in the Surprise–Happy Canyons divide. This detailed geologic mapping and collection of structural data for the rocks in the central Panamint Range were created using digital in-the-field geographic information systems software running on a field-hardened laptop computer combined with an earlier set of field data that were digitized into the digital georeferenced database. This map is a simplification of detailed geologic mapping data collected at 1:2000–1:6000 scales and reduced to 1:20000 scale. Structural data include kinematic and relative timing of deformation information.
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Dutta, Upama, Ayan Kumar Sarkar, Sadhana M. Chatterjee, Anirban Manna, Alip Roy, and Subhrajyoti Das. "Petrological implications of element redistribution during metamorphism: insights from meta-granite of the South Delhi Fold Belt, Rajasthan, India." Geological Magazine 159, no. 5 (February 23, 2022): 735–60. http://dx.doi.org/10.1017/s0016756821001345.
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AbstractMeta-granites of the South Delhi Fold Belt, northwestern India, contain spectacular reaction textures formed during the metamorphic replacement of primary minerals. Textural relationships imply that amphibole was replaced sequentially in two stages. Epidote + titanite + quartz symplectite formed syn-tectonically on amphibole grain boundaries/fractures, followed by post-deformational growth of euhedral garnet overprinting amphibole grains. Besides occurring as symplectite grown during deformation, titanite in this rock also developed as a post-tectonic corona around magnetite. Parent magnetite contains exsolutions of ilmenite and/or ultrafine lamellae of Ti-rich oxide (Ti-oxd). Textures involving coronal titanite suggest their formation through a magnetite + ilmenite(/Ti-oxd) + plagioclase → titanite reaction. Compositional attributes and the calculation of the gain versus loss of components during the reaction suggest that the Mn2+ for garnet (XSpss = 0.23–0.29) that grew replacing amphibole was supplied by ilmenite (Mn2+ is 0.118–0.128 apfu) as it disintegrated to form coronal titanite. The redistribution of components between the metamorphic reaction sites connects the texturally unrelated domains and suggests that these zones were in chemical equilibrium during metamorphism. We estimated the P–T conditions of metamorphism for these post-tectonic assemblages as ∼650–700 °C from pseudosection modelling and conventional thermometry. Zircon data from this study suggest that the granitic rock crystallized at 988.8 ± 8.8 Ma. We propose that the metamorphic phases replaced the primary minerals during the mid Neoproterozoic tectonic activity reported from this terrane. The syn-tectonic symplectitic assemblage formed as the temperature increased during prograde metamorphism, and the post-tectonic minerals developed at peak conditions following the cessation of deformation.
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Mueller, Andreas G., Neal J. McNaughton, and Janet R. Muhling. "Albite ± Actinolite-Altered Porphyry Dykes in Archean Gold Deposits of the Boulder Lefroy-Golden Mile Fault System, Yilgarn Craton, Western Australia: Petrography, Chronology, and Comparison to Canadian Albitites." Minerals 11, no. 11 (November 19, 2021): 1288. http://dx.doi.org/10.3390/min11111288.
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The Boulder Lefroy-Golden Mile fault system in the Archean Yigarn Craton is the most productive gold-mineralized structure in Australia (>2300 t Au). The New Celebration deposit (51 t Au) is part of a group of hematite- and anhydrite-bearing mesothermal deposits and Fe-Cu-Au skarns associated with monzodiorite-tonalite intrusions in the strike-slip fault system. Ore-grade biotite-carbonate and late sericite-carbonate-alkali feldspar replacement is bound to the contacts of a felsic (low Cr, Ni, V) quartz-plagioclase porphyry dyke dated at 2676 ± 7 Ma. The sodic-potassic alteration of the felsic boudinaged dyke contrasts with the albite-actinolite alteration in the adjacent mafic (high Cr, Ni, V) plagioclase porphyry dated at 2662 ± 4 Ma, although both share the same sulfide-oxide assemblage: pyrite ± chalcopyrite, magnetite ± hematite. The younger porphyry locally crosscuts foliation and is bordered by post-kinematic actinolite-pyrite selvages overprinting talc-chlorite-phlogopite-dolomite schist. It contains auriferous pyrite (70 ppb Au; 610 ppb Ag) where sampled for zircon U-Pb chronology at +224 m elevation. Above the sample site, the dyke was mined as gold ore (1–6 g/t Au) at +300–350 m. Temperature estimates based on actinolite-albite pairs (300–350 °C) agree with the fluid inclusion trapping temperature of main-stage auriferous veins (330 ± 20 °C). These relationships are interpreted to indicate syn-mineralization emplacement. Gold-related albite-altered porphyry dykes (albitites) also occur in the world-class Hollinger-McIntyre (986 t Au) and Kerr Addison-Chesterville deposits (336 t Au), Abitibi greenstone belt, Canada.
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Erickson, Emerald J. "Structural and kinematic analysis of the Shagawa Lake shear zone, Superior Province, northern Minnesota: implications for the role of vertical versus horizontal tectonics in the Archean." Canadian Journal of Earth Sciences 47, no. 12 (December 2010): 1463–79. http://dx.doi.org/10.1139/e10-054.
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The steeply dipping Shagawa Lake shear zone, which strikes ENE through the Vermilion District of the Superior Province in northeastern Minnesota, is marked by zone-parallel foliation and a mineral elongation lineation (Le). Le includes (i) a broad population with pitch of 90° ± 60°, and (ii) a less abundant population with pitch of 15° ± 15°. Shallowly plunging Le is rare, and where it occurs, it overprints the pervasive steeply plunging Le. Shear sense indicators occur within the L–S tectonite motion plane, normal to foliation and parallel to Le. Microstructures define both south-side-up and north-side-up displacement domains, but no spatial patterns emerge across the shear zone. L–S tectonites with east-plunging Le indicate either south- or north-side-up shear parallel to Le, whereas L–S tectonites with west-plunging Le indicate predominantly north-side-up shear parallel to Le. Strike-slip L–S tectonites are rare, but consistently record sinistral shear. Overprinting relationships and structural-kinematic patterns can be attributed to sinking of the Vermilion District volcanic basin and relative rise of the southern region followed by rise of the northern region, as represented by the Giants Range Batholith and Vermilion Granitic Complex, respectively. Structural and kinematic evidence indicates that a process like sagduction–diapirism could explain the rising granitoids and sinking volcanic basin. The narrow width of the shear zone, the need for structural dates and radiometric dates of the surrounding plutons, and lack of documented strike-slip kinematic data make it difficult to evaluate when and how the shear zone transitioned to horizontal displacement.
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HUYSKENS, MAGDALENA H., and MICHAEL BRÖCKER. "The status of the Makrotantalon Unit (Andros, Greece) within the structural framework of the Attic-Cycladic Crystalline Belt." Geological Magazine 151, no. 3 (July 19, 2013): 430–46. http://dx.doi.org/10.1017/s0016756813000307.
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AbstractThis study focuses on the status of the Makrotantalon Unit (Andros, Greece) within the framework of the Cycladic nappe stack. We document unambiguous evidence that this unit has experienced blueschist-facies metamorphism and identify previously unknown lawsonite ± pumpellyite assemblages in glaucophane-free metasediments. The position of the presumed tectonic contact at the base of this unit is vague, but roughly outlined by serpentinites. Only a single outcrop displays a weak angular unconformity with cohesive cataclasites in the footwall. Rb–Sr geochronology was carried out on 11 samples representing various rock types collected within or close to inferred or visible fault zones. Owing to a lack of initial isotopic equilibration and/or subsequent disturbance of the Rb–Sr isotope systematics, isochron relationships are poorly developed or non-existing. In NW Andros, direct dating of distinct displacement events has not been possible, but a lower age limit of ~ 40 Ma for final thrusting is constrained by the new data. Sporadically preserved Cretaceous ages either indicate regional differences in the P–T–d history or a different duration of metamorphic overprinting, which failed to completely eliminate inherited ages. The detachment on the NE coast records a later stage of the structural evolution and accommodates extension-related deformation. Apparent ages of ~ 29–25 Ma for samples from this location are interpreted to constrain the time of a significant deformation increment. On a regional scale, the Makrotantalon Unit can be correlated with the South Evia Blueschist Belt, but assignment to a specific subunit is as yet unconfirmed.
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ALSOP, G. I., R. BRYSON, and D. H. W. HUTTON. "Tectonic and kinematic evolution within mid-crustal orogenic root zones: a case study from the Caledonides of northwestern Ireland." Geological Magazine 138, no. 2 (March 2001): 193–211. http://dx.doi.org/10.1017/s0016756801005258.
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The Slieve League Peninsula of northwest Ireland lies on the western limb of a major orogenic strike-swing in which regional foliation trends have deviated from the northeast–southwest trends typical of much of Scotland, to west–east orientations. Across-strike coastal exposures on the western tip of the peninsula through Neoproterozoic Dalradian metasediments enable a detailed examination and analysis of the structural evolution of a Caledonian orogenic root zone which has been previously correlated with the Loch Awe Syncline of southwest Scotland. Minor structural development may be evaluated in terms of regional strain profiles and overprinting relationships. Over much of the area, a composite, steep northeast–southwest-trending S2–S3 foliation containing a gently southwest-plunging quartz mineral elongation lineation is the dominant fabric at outcrop, and is associated with MP2 almandine–amphibolite facies metamorphism. F2 folds are isoclinal with curvilinear hinges and similar geometry. They typically plunge steeply towards the southwest and display variable (dextral) or north-directed vergence, whilst minor F3 fold hinges plunge moderately towards the southwest and typically verge (sinistrally) towards the south. Major, composite D1–D3 tectonic slides are developed in the Argyll Group. Structural and stratigraphic relationships indicate that D1 induced large-scale reversals in younging across tectonic slides, resulting in reversals in subsequent F2 and F3 facing patterns. Tectonic sliding is associated with an intensification of strain demonstrated by increasingly intrafolial and curvilinear folding, together with extensional crenulations, sheared quartz pods and metre-scale asymmetric boudinage of metadolerites, all of which indicate dextral (D2) and sinistral (D3) shear. After unfolding subsequent folds (F4), this corresponds to top-to-the-north (D2) and top-to-the-south (D3) translations. D4 results in regionally northwest-verging structures, with minor crenulations and the S4 cleavage transecting fold hinges in an anticlockwise sense, suggesting a dextral component of deformation. The detailed kinematic data indicate that the overall geometry of this western, deep-level arm of the root zone is not a product of the classic mushrooming fountain of nappes model, but rather major interference between consistent northerly directed D2 thrusting and a later phase of southeast-directed (D3) retrocharriage (‘back-folding’) which intensifies towards the south.
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Gasperini, Luca, Massimiliano Stucchi, Vincenzo Cedro, Mustapha Meghraoui, Gulsen Ucarkus, and Alina Polonia. "Active fault segments along the North Anatolian Fault system in the Sea of Marmara: implication for seismic hazard." Mediterranean Geoscience Reviews 3, no. 1 (March 2021): 29–44. http://dx.doi.org/10.1007/s42990-021-00048-7.
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AbstractA new analysis of high-resolution multibeam and seismic reflection data, collected during several oceanographic expeditions starting from 1999, allowed us to compile an updated morphotectonic map of the North Anatolian Fault below the Sea of Marmara. We reconstructed kinematics and geometries of individual fault segments, active at the time scale of 10 ka, an interval which includes several earthquake cycles, taking as stratigraphic marker the base of the latest marine transgression. Given the high deformation rates relative to sediment supply, most active tectonic structures have a morphological expression at the seafloor, even in presence of composite fault geometries and/or overprinting due to mass-wasting or turbidite deposits. In the frame of the right-lateral strike-slip domain characterizing the North Anatolian fault system, three types of deformation are observed: almost pure strike-slip faults, oriented mainly E–W; NE/SW-aligned axes of transpressive structures; NW/SE-oriented trans-tensional depressions. Fault segmentation occurs at different scales, but main segments develop along three major right-lateral oversteps, which delimit main fault branches, from east to west: (i) the transtensive Cinarcik segment; (ii) the Central (East and West) segments; and (iii) the westernmost Tekirdag segment. A quantitative morphometric analysis of the shallow deformation patterns observed by seafloor morphology maps and high-resolution seismic reflection profiles along the entire basin allowed to determine nature and cumulative lengths of individual fault segments. These data were used as inputs for empirical relationships, to estimate maximum expected Moment Magnitudes, obtaining values in the range of 6.8–7.4 for the Central, and 6.9–7.1 for the Cinarcik and Tekirdag segments, respectively. We discuss these findings considering analyses of historical catalogues and available paleoseismological studies for the Sea of Marmara region to formulate reliable seismic hazard scenarios.
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Abu, Clara, Christopher A. L. Jackson, and Malcolm Francis. "Strike-slip overprinting of initial co-axial shortening within the toe region of a submarine landslide and a model for basal shear surface growth: a case study from the Angoche Basin, offshore Mozambique." Journal of the Geological Society 179, no. 2 (October 18, 2021): jgs2021–032. http://dx.doi.org/10.1144/jgs2021-032.
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Submarine landslides (slides) are some of the most voluminous sediment gravity-flows on Earth and they dominate the stratigraphic record of many sedimentary basins. Their general kinematics and internal structure are relatively well-understood. However, how slides increase in volume and internally deform as they evolve, and how these processes relate, in time and space, to the growth of their basal (shear) zone, are poorly understood. We here use three high-resolution 3D seismic surveys from the Angoche Basin, offshore Mozambique to map strain within a shallowly buried, large, and thus seismically well-imaged slide (c. 530 km3). We document several key kinematic indicators, including broadly NW-trending lateral margins and longitudinal shears bounding and within the slide body, respectively, and broadly NE-trending symmetric pop-up blocks in the slide toe. Approximately 7 km downdip of the slide toe wall, thrusts and related folds also occur within otherwise undeformed slope material, with thrusts detaching downwards onto the downslope continuation of the basal shear zone underlying the slide body. Based on the style, trend and distribution of these features, and their cross-cutting relationships, we propose an emplacement model involving two distinct phases of deformation: (i) bulk shortening, parallel to the overall SE-directed emplacement direction, with contractional shear strains reaching c. 8%; and (ii) the development of broadly emplacement direction-parallel shear zones that offset the earlier-formed shortening structures. We infer that the contractional strains basinward of the slide body formed due to cryptic basinward propagation of the basal shear zone ahead of, and to accommodate updip sliding and shortening associated with, the entire slide mass. Our study demonstrates the value of using 3D seismic reflection data to reveal slide emplacement kinematics, especially the multiphase, non-coaxial nature of deformation, and the dynamics of basal shear zone growth.
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Weismüller, Christopher, Rahul Prabhakaran, Martijn Passchier, Janos L. Urai, Giovanni Bertotti, and Klaus Reicherter. "Mapping the fracture network in the Lilstock pavement, Bristol Channel, UK: manual versus automatic." Solid Earth 11, no. 5 (September 18, 2020): 1773–802. http://dx.doi.org/10.5194/se-11-1773-2020.
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Abstract. The 100 000 m2 wave-cut pavement in the Bristol Channel near Lilstock, UK, is a world-class outcrop, perfectly exposing a very large fracture network in several thin limestone layers. We present an analysis based on manual interpretation of fracture generations in selected domains and compare it with automated fracture tracing. Our dataset of high-resolution aerial photographs of the complete outcrop was acquired by an unmanned aerial vehicle, using a survey altitude optimized to resolve all fractures. We map fractures and identify fracture generations based on abutting and overprinting criteria, and we present the fracture networks of five selected representative domains. Each domain is also mapped automatically using ridge detection based on the complex shearlet transform method. The automatic fracture detection technique provides results close to the manually traced fracture networks in shorter time but with a bias towards closely spaced Y over X nodes. The assignment of fractures into generations cannot yet be done automatically, because the fracture traces extracted by the automatic method are segmented at the nodes, unlike the manual interpretation in which fractures are traced as a path from fracture tip to fracture tip and consist of several connected segments. This segmentation makes an interpretation of relative age impossible, because the identification of correct abutting relationships requires the investigation of the complete fracture trace by following a clearly defined set of rules. Generations 1 and 2 are long fractures that traverse all domains. Generation 3 is only present in the southwestern domains. Generation 4 follows an ENE–WSW striking trend, is suborthogonal to generations 1 and 2, and abuts on them and generation 3, if present. Generations 5 is the youngest fracture set with a range of orientations, creating polygonal patterns by abutting at all other fracture generations. Our mapping results show that the northeastern domains only contain four fracture generations; thus, the five generations of the outcrop identified in the southwestern domains are either not all present in each of the five domains or vary locally in their geometry, preventing the interpreter from linking the fractures to their respective generation over several spatially separate mapping domains. Fracture intensities differ between domains where the lowest is in the NE with 7.3 m−1 and the highest is in the SW with 10 m−1, coinciding with different fracture orientations and distributions of abutting relationships. Each domain has slightly different fracture network characteristics, and greater connectivity occurs where the development of later shorter fractures is not affected by the stress shadowing of pre-existing longer fractures.
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BOZKURT, ERDİN. "Extensional v. contractional origin for the southern Menderes shear zone, SW Turkey: tectonic and metamorphic implications." Geological Magazine 144, no. 1 (October 26, 2006): 191–210. http://dx.doi.org/10.1017/s0016756806002664.
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The southern Menderes Massif in southwest Turkey consists mainly of orthogneisses and overlying Palaeozoic–Middle Paleocene schists and marbles. The contact between the two distinct rock types is almost everywhere structural, herein named the southern Menderes shear zone: a S-facing, high-angle ductile shear zone that separates metamorphic rocks of differing grade. Although there is a consensus that the shear zone was associated with top-to-the-S–SSW shearing and is of Tertiary age, its origin and nature have been highly debated over the last decade. Some claim the contact is a thrust fault, while others have argued for an extensional shear zone. Integration of field and microstructural data (the identification of different fabrics, associated kinematics and overprinting relationships) with fission-track thermochronology and the P–T paths of the rocks above and below the shear zone, supports the conclusion that the southern Menderes shear zone is an extensional shear zone and not a thrust. The data are consistent with a model that the exhumation and cooling of the southern Menderes Massif occurred after a period of extensional deformation. Pervasive top-to-the-N–NNE high-temperature–medium-pressure ductile shear structures (D2 deformation) overprint an early HP event (D1 deformation). The subsequent top-to-the-S–SSW greenschist shear band foliation (D3 deformation) developed mostly around the orthogniess–schist contact and forms the most characteristic features of the massif. The top-to-the-N–NNE structures are attributed to the main Alpine constructional deformation that developed during back-thrusting of the Lycian nappes during the latest Palaeogene collision between the Sakarya continent and the Anatolide-Tauride platform across the Neotethyan Ocean. The top-to-the-S–SSW structures are interpreted to be the result of the exhumation of the massif during the activity of the southern Menderes shear zone. The presence of these two distinct fabrics with differing kinematics suggests that the southern Menderes shear zone operated as a top-to-the-N–NNE thrust fault during early Alpine contractional deformation but was later reactivated with an opposite sense of movement (top-to-the-S–SSW) during subsequent Oligocene–Miocene extensional collapse.
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Luukkonen, Erkki J. "Structural and U–Pb isotopic study of late Archaean migmatitic gneisses of the Presvecokarelides, Lylyvaara, eastern Finland." Transactions of the Royal Society of Edinburgh: Earth Sciences 76, no. 4 (1985): 401–10. http://dx.doi.org/10.1017/s0263593300010634.
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ABSTRACTThe migmatitic gneiss complex of Lylyvaara in the eastern part of Finnish Presvecokarelides of the Baltic Shield shows evidence of a polyphase deformational and metamorphic history and of the emplacement of a number of mafic and felsic igneous intrusions at various stages during this history. Sequential structural development has been established on the bases of refolding and cross-cutting relationships. U–Pb zircon and sphene isotopic data combined with structural studies indicate that the first six deformational phases took place in late Archaean (=Presvecokarelian) times. The seventh deformational phase is constrained as being early Proterozoic (=Svecokarelian) from regional considerations.The gneissic foliation in the dominant tonalitic to trondhjemitic palaeosome is parallel to lithological layering (So). Mostly it is composite S1–S2; only in F2 fold hinges can separate S1 and S2 be unequivocally distinguished. There, both of these fabrics, which were formed in amphibolite facies conditions of metamorphism during D1 and D2 have retained their identity despite extensive tectonic overprinting. Further tonalitic or granodioritic material was intruded during D3 or between D2 and D3. Effects of the third deformational phase (D3) ar6e expressed only locally as asymmetrical and polyclinal folds, which deform S1–S2 and F2. These folds now have a northeasterly axial trend and they show considerable variations in the style of their parasitic structures. F4 folds are common. They are dextral and asymmetrical, have NW—NNW-trending axes and show complex interference patterns with F2 and F3 folds. During D4, much aplogranitic neosome material was emplaced in NW—SE-trending movement zones, which correspond to the axial planes of F4 folds. Superimposition of F5 and F6 structures on previously formed patterns add to the structural complexity although they only result in minor modifications. Both are open and upright and locally have associated cleavages or healed fractures (S5, S6). D7 is expressed throughout the migmatitic complex as narrow NW—SE-trending shear zones which reactivate the S4 trend.U–Pb zircon isotopic data indicate that the metamorphism associated with gneiss formation took place 2843 ± 18 Ma ago. U–Pb sphene ages of c. 2660 Ma and 2620 Ma indicate that metamorphic conditions prevailed for a very considerable time. An aplogranitic neosome related to F4 axial planes gave a 2657 ± 32 Ma U–Pb zircon age, while granodiorite and pegmatite dykes related to D6 yielded U–Pb zircon ages of c 2670 Ma and 2640 Ma, respectively.
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Yergeau, D., P. Mercier-Langevin, B. Dubé, V. McNicoll, S. E. Jackson, M. Malo, and A. Savoie. "The Westwood Deposit, Southern Abitibi Greenstone Belt, Canada: An Archean Au-Rich Polymetallic Magmatic-Hydrothermal System—Part II. Hydrothermal Alteration, Mineralization, and Geologic Model." Economic Geology 117, no. 3 (May 1, 2022): 577–608. http://dx.doi.org/10.5382/econgeo.4879.
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Abstract The Westwood deposit, located in the Archean Doyon-Bousquet-LaRonde mining camp in the southern Archean Abitibi greenstone belt, contains 4.5 Moz (140 metric t) of gold. The deposit is hosted in the 2699–2695 Ma submarine, tholeiitic to calc-alkaline volcanic, volcaniclastic, and intrusive rocks of the Bousquet Formation. The deposit is located near the synvolcanic (ca. 2699–2696 Ma) Mooshla Intrusive Complex that hosts the Doyon epizonal intrusion-related Au ± Cu deposit, whereas several Au-rich volcanogenic massive sulfide (VMS) deposits are present east of the Westwood deposit. The Westwood deposit consists of stratigraphically stacked, contrasting, and overprinting mineralization styles that share analogies with both the intrusion-related and VMS deposits of the camp. The ore zones form three distinct, slightly discordant to stratabound corridors that are, from north (base) to south (top), the Zone 2 Extension, the North Corridor, and the Westwood Corridor. Syn- to late-main regional deformation and upper greenschist to lower amphibolite facies regional metamorphism affect the ore zones, alteration assemblages, and host rocks. The Zone 2 Extension consists of Au ± Cu sulfide (pyrite-chalcopyrite)-quartz veins and zones of disseminated to semimassive sulfides. The ore zones are spatially associated with a series of calc-alkaline felsic sills and dikes that crosscut the mafic to intermediate, tholeiitic to transitional, lower Bousquet Formation volcanic rocks. The metamorphosed proximal alteration consists of muscovite-quartz-pyrite ± gypsum-andalusite-kyanite-pyrophyllite argillic to advanced argillic-style tabular envelope that is up to a few tens of meters thick. The North Corridor consists of auriferous semimassive to massive sulfide veins, zones of sulfide stringers, and disseminated sulfides that are hosted in intermediate volcaniclastic rocks at the base of the upper Bousquet Formation. The Westwood Corridor consists of semimassive to massive sulfide lenses, veins, zones of sulfide stringers, and disseminated sulfides that are located higher in the stratigraphic sequence, at or near the contact between calc-alkaline dacite domes and overlying calc-alkaline rhyodacite of the upper Bousquet Formation. A large, semiconformable distal alteration zone that encompasses the North Corridor is present in the footwall and vicinity of the Westwood Corridor. This metamorphosed alteration zone consists of an assemblage of biotite-Mn garnet-chlorite-carbonate ± muscovite-albite. A proximal muscovite-quartz-chlorite-pyrite argillic-style alteration assemblage is associated with both corridors. The Zone 2 Extension ore zones and associated alteration are considered synvolcanic based on crosscutting relationships and U-Pb geochronology and are interpreted as being the distal expression of an epizonal magmatic-hydrothermal system that is centered on the upper part of the synvolcanic Mooshla Intrusive Complex. The North and Westwood corridors consist of bimodal-felsic Au-rich VMS-type mineralization and alteration produced by the convective circulation of modified seawater that included a magmatic contribution from the coeval epizonal Zone 2 Extension magmatic-hydrothermal system. The Westwood Au deposit represents one of the very few documented examples of an Archean magmatic-hydrothermal system—or at least of such systems formed in a subaqueous environment. The study of the Westwood deposit resulted in a better understanding of the critical role of magmatic fluid input toward the formation of Archean epizonal intrusion-related Au ± Cu and seafloor/subseafloor Au-rich VMS-type mineralization.
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Virgo, Simon, Max Arndt, Zoé Sobisch, and Janos L. Urai. "Development of fault and vein networks in a carbonate sequence near Hayl al-Shaz, Oman Mountains." GeoArabia 18, no. 2 (April 1, 2013): 99–136. http://dx.doi.org/10.2113/geoarabia180299.
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ABSTRACT We present a high-resolution structural study on the dip slope of the southern flank of Jabal Shams in the central Oman Mountains. The objectives of the study were: (1) to test existing satellite-based interpretations of structural elements in the area; (2) prepare an accurate geological map; and (3) collect an extensive structural dataset of fault and bedding planes, fault throws, veins and joints. These data are compared with existing models of tectonic evolution in the Oman Mountains and the subsurface, and used to assess the applicability of these structures as analogs for fault and fracture systems in subsurface carbonate reservoirs in Oman. The complete exposure of clean rock incised by deep wadis allowed detailed mapping of the complex fault, vein and joint system hosted by Member 3 of the Cretaceous Kahmah Group. The member was divided into eight units for mapping purposes, in about 100 m of vertical stratigraphy. The map was almost exclusively based on direct field observations. It includes measurement of fault throw in many locations and the construction of profiles, which are accurate to within a few meters. Ground-truthing of existing satellite-based interpretations of structural elements showed that faults can be mapped with high confidence using remote-sensing data. The faults range into the subseismic scale with throws as little as a few decimeters. However, the existing interpretation of lineaments as cemented fractures was shown to be incorrect: the majority of these are open fractures formed along reactivated veins. The most prominent structure in the study area is a conjugate set of ESE-striking faults with throws resolvable from several centimeters to hundreds of meters. These faults contain bundles of coarse-grained calcite veins, which may be brecciated during reactivation. We interpret these faults to be a conjugate normal- to oblique fault set, which was rotated together with bedding during the folding of the Al Jabal al-Akhdar anticline. There are many generations of calcite veins with minor offset and at high-angle-to-bedding, sometimes in en-echelon sets. Analysis of clear overprinting relationships between veins at high-angle-to-bedding is consistent with the interpretations of Holland et al. (2009a); however we interpret the anticlockwise rotation of vein strike orientation to start before and end after the normal faulting. The normal faults post-date the bedding-parallel shear veins in the study area. Thus these faults formed after the emplacement of the Semail and Hawasina Nappes. They were previously interpreted to be of the same age as the regional normal- to oblique-slip faults in the subsurface of northern Oman and the United Arab Emirates, which evolved during the early deposition of the Campanian Fiqa Formation as proposed by Filbrandt et al. (2006). We interpret them also to be coeval with the Phase I extension of Fournier et al. (2006). The reactivation of these faults and the evolution of new veins was followed by folding of the Al Jabal al-Akhdar anticline and final uplift and jointing by reactivation of pre-existing microveins. Thus the faults in the study area are of comparable kinematics and age as those in the subsurface. However they formed at much greater depth and fluid pressures, so that direct use of these structures as analogs for fault and fracture systems in subsurface reservoirs in Oman should be undertaken with care.
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Szaszak Bongartz, Ulla. "El nombre propio como modulador de vínculos sexo-afectivos y genealógicos en las novelas de Armonía Somers: una poética de la suplementariedad nominal." Catedral Tomada. Revista de crítica literaria latinoamericana 8, no. 15 (January 5, 2021): 356–94. http://dx.doi.org/10.5195/ct/2020.453.
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This article explore the potential of the proper name as a contingent identity modulator in the Uruguayan author Armonía Somers’ novels, and, specifically, of nominal supplementarity (the act of twisting, overprinting, displacing or substituting sometimes momentarily the "original" or "legal" name of the characters for another(s), as a form of nominal drift). This operation is presented as a producer of subjective transformations of the characters, as verified in certain areas of Latin American literary discourse in the 20th century. Among these are three novels by Somers: The naked woman (1950), Sólo los elefantes encuentran mandrágora (1986) and Viaje al corazón del día: elegía por un secreto amor (1986), which participate in this nominal poetics. In these texts, the nominal displacements cause the reconfiguration or even an inauguration of new forms of relationship between the characters. So the use of this thematic, structural, and subjective resource exceeds the mere change of nomenclature and reveals itself as a pole of subjective and intersubjective production.
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Stern, Nicola. "Early Hominin Activity Traces at FxJj43, a One and a Half Million Year Old Locality in the Koobi Fora Formation, in Northern Kenya: a Field Report." Proceedings of the Prehistoric Society 70 (2004): 233–58. http://dx.doi.org/10.1017/s0079497x00001183.
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FxJj43 is one of a series of Early Stone Age archaeological sites preserved in the Okote Member of the Koobi Fora Formation, in northern Kenya. It is the focus of a new research project that aims to explore the impact of time-averaging on the composition and characteristics of Early Stone Age archaeological assemblages. FxJj43 lends itself particularly well to this exercise because, unlike other sites in the Okote Member, it preserves a laterally extensive set of interlocking landforms. These include part of a sandy river channel, its southern bank, levee, and adjacent floodplain.Chipped stone artefacts and broken-up animal bones occur in clusters of varying size and density all the way along the 200 m long strip of outcrops containing the remnants of these landforms. Small-scale excavations aimed at investigating the characteristics of archaeological assemblages preserved in different palaeotopographic settings, and in clusters of varying size and density, suggest the existence of archaeological occurrences representing different amounts of overprinting. This underscores the long-term research potential of this locality for exploring the relationship between the material remains of individual behavioural events and agglomerations of debris resulting from many, often unrelated sets of activities.
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Srivastava, Deepak C., Ajanta Goswami, and Amit Sahay. "Strain-partitioned dextral transpression in the Great Boundary Fault Zone around Chittaurgarh, NW Indian Shield." Geological Magazine 158, no. 9 (March 22, 2021): 1585–99. http://dx.doi.org/10.1017/s0016756821000157.
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AbstractDelimiting the Aravalli mountain range in the east, the Great Boundary Fault (GBF) occurs as a crustal-scale tectonic lineament in the NW Indian Shield. The structural and tectonic characteristics of the GBF are, as yet, not well-understood. We attempt to fill this gap by using a combination of satellite image processing, high-resolution outcrop mapping and structural analysis around Chittaurgarh. The study area exposes the core and damage zone of the GBF. Three successive phases of folding, F1, F2 and F3, are associated with deformation in the GBF. The large-scale structural characteristics of the GBF core are: (i) a non-coaxial refolding of F1 folds by F2 folds; and (ii) the parallelism between the GBF and F2 axial traces. In addition, numerous metre-scale ductile shear zones cut through the rocks in the GBF core. The damage zone is characterized by the large-scale F1 folds and the mesoscopic-scale strike-slip faults, thrusts and brittle-ductile shear zones. Several lines of evidence, such as the inconsistent overprinting relationship between the strike-slip faults and thrusts, the occurrence of en échelon folds and the palaeostress directions suggest that the GBF is a dextral transpression fault zone. Structural geometry and kinematic indicators imply a wrench- and contraction-dominated deformation in the core and damage zone, respectively. We infer that the GBF is a strain-partitioned dextral transpression zone.
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Behrensmeyer, Anna K., C. Tristan Stayton, and Ralph E. Chapman. "Taphonomy and ecology of modern avifaunal remains from Amboseli Park, Kenya." Paleobiology 29, no. 1 (2003): 52–70. http://dx.doi.org/10.1666/0094-8373(2003)029<0052:taeoma>2.0.co;2.
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Avian skeletal remains occur in many fossil assemblages, and in spite of small sample sizes and incomplete preservation, they may be a source of valuable paleoecological information. In this paper, we examine the taphonomy of a modern avian bone assemblage and test the relationship between ecological data based on avifaunal skeletal remains and known ecological attributes of a living bird community. A total of 54 modern skeletal occurrences and a sample of 126 identifiable bones from Amboseli Park, Kenya, were analyzed for weathering features and skeletal part preservation in order to characterize preservation features and taphonomic biases. Avian remains, with the exception of ostrich, decay more rapidly than adult mammal bones and rarely reach advanced stages of weathering. Breakage and the percentage of anterior limb elements serve as indicators of taphonomic overprinting that may affect paleoecological signals. Using ecomorphic categories including body weight, diet, and habitat, we compared species in the bone assemblage with the living Amboseli avifauna. The documented bone sample is biased toward large body size, representation of open grassland habitats, and grazing or scavenging diets. In spite of this, multidimensional scaling analysis shows that the small faunal sample (16 out of 364 species) in the pre-fossil bone assemblage accurately represents general features of avian ecospace in Amboseli. This provides a measure of the potential fidelity of paleoecological reconstructions based on small samples of avian remains. In the Cenozoic, the utility of avian fossils is enhanced because bird ecomorphology is relatively well known and conservative through time, allowing back-extrapolations of habitat preferences, diet, etc. based on modern taxa.
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Su, Zhi-Kun, Xin-Fu Zhao, Xiao-Chun Li, Mei-Fu Zhou, Allen K. Kennedy, Jian-Wei Zi, Carl Spandler, and Yue-Heng Yang. "UNRAVELING MINERALIZATION AND MULTISTAGE HYDROTHERMAL OVERPRINTING HISTORIES BY INTEGRATED IN SITU U-Pb AND Sm-Nd ISOTOPES IN A PALEOPROTEROZOIC BRECCIA-HOSTED IOCG DEPOSIT, SW CHINA." Economic Geology 116, no. 7 (November 1, 2021): 1687–710. http://dx.doi.org/10.5382/econgeo.4840.
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Abstract Precambrian iron oxide copper-gold (IOCG) deposits are generally encountered with multistage hydrothermal overprints and hence have complex isotopic records. Precise dating of ore-forming and overprinting events and assessment of time-resolved metal sources are fundamental for understanding ore genesis. Here, we quantify the evolution history by integrating in situ U-Pb dating of texturally constrained allanite and Sm-Nd isotope data of ores and major rare earth element (REE) minerals in the breccia-hosted Lanniping Fe-Cu deposit in Kangdian region, southwestern China. The economically mineralized breccia in Lanniping Fe-Cu deposit is characterized by pervasive and texturally destructive replacement of polymictic clasts, including host metasedimentary packages, the intruded dolerite, and pre-ore halokinetic breccia. Ore minerals in cements are mainly composed of magnetite, chalcopyrite, bornite, and variable amounts of REE-rich minerals (e.g., apatite and allanite/epidote). Two types of allanite were identified in ores. Type I prismatic allanite texturally intergrown with magnetite has a SHRIMP U-Pb age of 1728 ± 20 Ma (1σ), which matches a zircon U-Pb age of 1713 ± 14 Ma (2σ) for the dolerite clasts and provides the direct age constraint on the Fe-Cu mineralization event. Type II anhedral allanite shows complex zoning and is spatially associated with, but texturally later than, magnetite, apatite, and chalcopyrite. This type of allanite yields significantly younger SHRIMP dates of 1015 ± 33 (1σ) and 800 ± 16 Ma (1σ) for cores and rims, respectively, which correspond to discrete regional magmatic events and hence record hydrothermal overprint/remobilization events of ore minerals in the deposit. Integrated Sm-Nd isotope compositions of type I allanite, apatite, and whole ores generally align along the reference Sm-Nd isochron of 1728 Ma, further confirming the primary ore formation at ~1.7 Ga. Corresponding εNd(1728 Ma) values ranging from –2.8 to 0.3 are significantly higher than those of the host metasedimentary rocks (–9.5 to –6.2) but comparable to those of contemporaneous igneous intrusions (–0.3 to 5.3) in the region, demonstrating that REE components of the primary ores were dominantly sourced from rocks of mantle-derived affinity. Both cores and rims of the younger type II allanite grains have Nd isotope compositions consistent with the unique time-evolved line of the ~1.7 Ga ores, implying that REEs incorporated into type II allanite were ultimately sourced from the primary ores in this deposit. The combined texture, chemical, U-Pb, and Sm-Nd isotope data thus demonstrate that REE remobilization was localized during post-ore hydrothermal overprint with negligible external inputs of REEs to the primary ores in the Lanniping deposit. In this contribution, we not only date primary ore formation but also recognize several younger allanite generations that record internal metal redistributions in response to post-ore tectonothermal events. Our study highlights the potential of ore-associated REE minerals such as allanite for resolving the age of multiple stages of hydrothermal events in complex ore deposits by ion probe, provided that careful examination of textural and paragenetic relationship of ores is conducted. Our finding of these younger allanite generations also exemplifies the significance of evaluation on time-resolved metal input for better characterizing the evolution history of the IOCG deposits.
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Saraswat, Rajeev, Thejasino Suokhrie, Dinesh K. Naik, Dharmendra P. Singh, Syed M. Saalim, Mohd Salman, Gavendra Kumar, et al. "Large freshwater-influx-induced salinity gradient and diagenetic changes in the northern Indian Ocean dominate the stable oxygen isotopic variation in Globigerinoides ruber." Earth System Science Data 15, no. 1 (January 10, 2023): 171–87. http://dx.doi.org/10.5194/essd-15-171-2023.
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Abstract. The application of stable oxygen isotopic ratio of surface-dwelling planktic foraminifera Globigerinoides ruber (white variety; δ18Oruber) to reconstruct past hydrological changes requires a precise understanding of the effect of ambient parameters on δ18Oruber. The northern Indian Ocean, with its huge freshwater influx and being a part of the Indo-Pacific Warm Pool, provides a unique setting to understand the effect of both the freshwater-influx-induced salinity and temperature on δ18Oruber. Here, we use a total of 400 surface samples (252 from this work and 148 from previous studies), covering the entire salinity end-member region, to assess the effect of freshwater-influx-induced seawater salinity and temperature on δ18Oruber in the northern Indian Ocean. The analysed surface δ18Oruber mimics the expected δ18O calcite estimated from the modern seawater parameters (temperature, salinity, and seawater δ18O) very well. We report a large diagenetic overprinting of δ18Oruber in the surface sediments, with an increase of 0.18 ‰ per kilometre increase in water depth. The freshwater-influx-induced salinity exerts the major control on δ18Oruber (R2=0.63) in the northern Indian Ocean, with an increase of 0.29 ‰ per unit increase in salinity. The relationship between temperature- and salinity-corrected δ18Oruber (δ18Oruber−δ18Osw) in the northern Indian Ocean [T=-0.59⋅(δ18Oruber-δ18Osw)+26.40] is different than reported previously, based on the global compilation of plankton tow δ18Oruber data. The revised equations will help create a better palaeoclimatic reconstruction from the northern Indian Ocean by using the stable oxygen isotopic ratio. The entire data set (newly generated and previously published) used in this work is available both as a Supplement to this article and at PANGAEA (https://doi.org/10.1594/PANGAEA.945401; Saraswat et al., 2022).
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Bhatt, S. C., and Vinod K. Singh. "Neoarchean crustal shear zones and implications of shear indicators in tectonic evolution of Bundelkhand craton, central India." Journal of Geoscience, Engineering, Environment, and Technology 4, no. 2-2 (July 25, 2019): 11. http://dx.doi.org/10.25299/jgeet.2019.4.2-2.2125.
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The gneisses and granitoids emplaced along E-W sub-vertical crustal shear zones are represented as important tectonic units in Bundelkhand craton of central India. The tonalite-trondhjemite-granodiorite (TTG) gneisses (3.5-3.2 Ga; oldest unit), and streaky to mafic gneisses structurally deformed in D 1 deformation. The metabasic, felsic, banded iron formation and metasedimentaries of greenstone complex exposed in central part, have characteristics of three sets of folding (F 1 -F 3 ). These gneisses associated with migmatite, amphibolite, quartzite, and schist were evolved in D 2 compressive phase, which are not occurring in northern part of craton. The K-rich Neoarchean granitoids (2.6-2.49 Ga) were intruded as granitic complex (D 3 magmatic phase) and the E-W strike-slip Raksa-Garhmau shear zone reported as important tectonic unit, were evolved in asyn-to post-tectonic D 3 phase. The dolerite dykes (ca. 2.0 Ga) were emplaced along NW-SE fractures in extension setting during D 4 magmatic event. The NE-SW riedel shears occupied by giant quartz veins (reefs) evolved in Paleoproterozoic during D 5 endogenic activity.
The relationship between macro and microstructural fabrics has been documented within mylonitic foliation, stretching lineation, S-C planes and rotated fabrics, reflect mesoscopic shear indicators, as noted in three types of mylonitic rocks. i) The rotated porphyroclasts of quartz, feldspars and asymmetric pressure shadows showing strong undulose extinction, deformation lamellae, and dynamic recrystallization are characteristic features of protomylonite where altered orthoclase and kinked plagioclase are noticed. ii) Mylonite, a distinct mylonitic foliation represented by parallel orientation of elongated quartz and feldspar with flakes of mica. iii) The ground matrix of recrystallized quartz with few protoliths of quartz and feldspar are observed, important features of ultramylonite. The asymmetric microstructures viz. σa and σb mantled porphyroclasts, othermicrostructures show progressively deformed by crystal plastic (non-coaxial) strain softening under low to moderate temperature conditions. The sinistral top- to- SW sense of shear movement was dominant. The microfractures/ microfaults, kinking and pull apart structures observed in K- feldspars and are indicative of overprinting of brittle deformation on ductile shearing.
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Giuntoli, Francesco, Pierre Lanari, and Martin Engi. "Deeply subducted continental fragments – Part 1: Fracturing, dissolution–precipitation, and diffusion processes recorded by garnet textures of the central Sesia Zone (western Italian Alps)." Solid Earth 9, no. 1 (February 26, 2018): 167–89. http://dx.doi.org/10.5194/se-9-167-2018.
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Abstract. Contiguous continental high-pressure terranes in orogens offer insight into deep recycling and transformation processes that occur in subduction zones. These remain poorly understood, and currently debated ideas need testing. The approach we chose is to investigate, in detail, the record in suitable rock samples that preserve textures and robust mineral assemblages that withstood overprinting during exhumation. We document complex garnet zoning in eclogitic mica schists from the Sesia Zone (western Italian Alps). These retain evidence of two orogenic cycles and provide detailed insight into resorption, growth, and diffusion processes induced by fluid pulses in high-pressure conditions. We analysed local textures and garnet compositional patterns, which turned out remarkably complex. By combining these with thermodynamic modelling, we could unravel and quantify repeated fluid–rock interaction processes. Garnet shows low-Ca porphyroclastic cores that were stable under (Permian) granulite facies conditions. The series of rims that surround these cores provide insight into the subsequent evolution: the first garnet rim that surrounds the pre-Alpine granulite facies core in one sample indicates that pre-Alpine amphibolite facies metamorphism followed the granulite facies event. In all samples documented, cores show lobate edges and preserve inner fractures, which are sealed by high-Ca garnet that reflects high-pressure Alpine conditions. These observations suggest that during early stages of subduction, before hydration of the granulites, brittle failure of garnet occurred, indicating high strain rates that may be due to seismic failure. Several Alpine rims show conspicuous textures indicative of interaction with hydrous fluid: (a) resorption-dominated textures produced lobate edges, at the expense of the outer part of the granulite core; (b) peninsulas and atoll garnet are the result of replacement reactions; and (c) spatially limited resorption and enhanced transport of elements due to the fluid phase are evident along brittle fractures and in their immediate proximity. Thermodynamic modelling shows that all of these Alpine rims formed under eclogite facies conditions. Structurally controlled samples allow these fluid–garnet interaction phenomena to be traced across a portion of the Sesia Zone, with a general decrease in fluid–garnet interaction observed towards the external, structurally lower parts of the terrane. Replacement of the Permian HT assemblages by hydrate-rich Alpine assemblages can reach nearly 100 % of the rock volume. Since we found no clear relationship between discrete deformation structures (e.g. shear zones) observed in the field and the fluid pulses that triggered the transformation to eclogite facies assemblages, we conclude that disperse fluid flow was responsible for the hydration.
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Laumonier, Bernard, Christian Marignac, and Philippe Kister. "Polymetamorphism and crustal evolution of the eastern Pyrenees during the Late Carboniferous Variscan orogenesis." Bulletin de la Société Géologique de France 181, no. 5 (September 1, 2010): 411–28. http://dx.doi.org/10.2113/gssgfbull.181.5.411.
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AbstractAbridged English version. – The Variscan Pyrenean belt (fig. 1) has been for long famous for its Late Carboniferous LP-HT metamorphism, characterised by the prograde succession, in medium grade metapelites, of biotite, cordierite, andalusite and sillimanite, together with staurolite and garnet [Guitard et al., 1996]. However, the discovery of two kyanite generations lead Azambre and Guitard [2001] to propose a polymetamorphic evolution, with an early (MI) and a late (MIII) kyanite-bearing Barrovian stage, preceding and following the main LP-HT stage (MII).Geological settingThe Variscan orogeny in the Pyrenees occurred from Namurian to Early Stephanian (c. 325-300 Ma), following the deposition of thick Ediacarian-Ordovician silico-clastites, Silurian to Early Caboniferous carbonates, and pre-orogenic Mid-Carboniferous flyschs.Two main tectonic events are recorded, each one subdivided into regionally correlated sub-events (phases) (table I), allowing a detailed correlation between tectonics, metamorphism and plutonism. The Namurian to Westphalian D1 event (c. 325-310 Ma) resulted in a S-vergent fold and thrust belt (with 100–150 km of N-S shortening) and the development of the main, sub-horizontal, Sr schistosity (D1c phase), coeval with MI. The Westphalian-Early Stephanian D2 event (310-300 Ma) was more complex. First, a syn-convergence extensional phase (N-vergent backfolds and E-W extension) resulted in the E-directed escape of the upper crust (D2a phase). Then, a renewal of the N-S shortening was marked by large upright anticlines (domes) and narrower synclines, with up to 10 km amplitudes (e.g., the Canigou anticline-Villefranche syncline pair) (D2b phase). Both D2a and D2b were coeval with MII and the emplacement of early granitoid sills and laccoliths (e.g., the Ansignan hypersthene-granite in the Agly Massif). Later on, D2 evolved into a transcurrent regime, with belt-parallel dextral transpression (D2c and D2c phases). D2c was coeval with the main stage of granite emplacement under low-grade conditions, allowing the expression of a conspicuous Mγ contact metamorphism (e.g., Mont-Louis pluton). D2d ended the D2 event, with the development of retrograde dextral-reverse mylonites. The late MIII metamorphic event encompassed D2c and D2d (and possibly D2b).The early MI Barrovian metamorphic eventThe MI Barrovian metamorphic event resulted from the crustal thickening associated with the development of the D1 intra-cratonic wedge. It was of low-grade, with a chlorite-muscovite Sr schistosity, in the part of the belt that was subsequently overprinted by the syn-MII transformation of chlorite into biotite. The only remnants of MI medium-grade conditions are found as early kyanite in the deepest domains of the Castillon, St-Barthélémy, Agly and Aston massifs, being there obliterated under high-grade MII conditions, and in the core of the Canigou anticline (Velmanya, point v in fig. 2), where a relict kyanite-staurolite-anorthite paragenesis is known, shielded by MII cordierite. The reconstructed P-T conditions at the thermal peak of MI are 5 kbar (19 km) and 575oC (fig. 2), implying the existence of a (now eroded) major D1 nappe (≥ 7 km thick).The main MII LP-HT metamorphic eventStructural domes and medium– or high-grade MII zones are broadly coincident, high-grade conditions being only encountered in the core of the Albères massif, the southern Aston Dome and the North-Pyrenean massifs (grading there up to the LP granulite facies) (fig. 1).Subdivisions of the MII eventThe prograde MII metamorphism is essentially syn-D2a, with clear syn-kinematic growth of the medium-grade minerals, and the main regional tectono-metamorphic D2a/MII structure is evidently deformed and strongly folded by the D2b phase: the D2b domes are basically post-metamorphic. However, a detailed examination of the blastesis-deformation relationships shows that staurolite is pre- to-synkinematic for D2a, whereas andalusite is strictly synkinematic (and consequently is often observed shielding the staurolite), cordierite being syn-to post-kinematic and syn-D2b in some instances. This allows a subdivision of the MII event into three stages:– MIIs, pre-to-syn-D2a, characterised by the staurolite-andalusite (And1 without cordierite) association, with development of a staurolite zone grading downwards into an andalusite (St → And1) zone. – MIIa, syn-to post-D2a (but always developed prior to D2b), characterised by the cordierite (Cord1)-andalusite (And2) association (without staurolite), with development of a thin cordierite zone grading downwards into an andalusite (Cord1 → And2) zone. – MIIb, post-D2a and syn-D2b, characterised by a large cordierite (Cord2) zone developed at the expense of an-dalusite (And → Cord2), only found in the core of the D2b anticlines (e.g., the Garonne dome).Thus, although MII is basically pre-D2b, and the MIIs and MIIa medium-grade isogrades are folded, it appears that metamorphism was still active in the cores of the ascending D2b domes (MIIb). Moreover, in the core of some domes, prograde sillimanite is also syn-kinematic of the D2b phase, and the sillimanite-muscovite isograde may obliquely overprint the MIIa isogrades, as in the Canigou dome. This is related to the syn-D2b emplacement of granite sheets (e.g., the Canigou granite) and may be interpreted as an aureola of “regional-contact” metamorphism, noted MIIγ, that was evidently coeval with MIIb, and enhanced its effects.P-T-t path of the MII eventThe P-T-t path of the MII event may be described using the petrogenetic grids of Pattison et al. [2002] and Pattison and Vogl [2005] (fig. 3). From MIIs to MIIb, it records a prograde anti-clockwise path, following a post-MI clockwise exhumation path, with ≥ 7 km eroded (fig. 2B). The MIIs pressure was close to 3 kbar (10–11 km) in the St zone and decreased to 2.5 kbar (9 km) at the MIIa stage (And2 isograde), for an estimated temperature of 540oC (based on the triple point of Holdaway [1971], the thermobarometer of Pattison et al. [2002] and independent fluid inclusion data by Kister et al. [2003]). A further pressure decrease, down to 2 kbar (7 km), and a temperature increase (up to 600oC) is registered in the MIIb cordierite zone in the core of active D2b domes. Except for the cores of the domes, MIIa remained the peak temperature event, and during MIIb pressure remained constant (or was re-increasing in the syncline cores) and temperature was constant or decreasing. At the end of the MII event (MIIb-MIIγ), extreme conditions of c. 4 kbar and 700–730oC are recorded in the deepest parts of the belt, where anatexis, succeeding to a sillimanite-K-feldspar zone, is observed, as in the Albères Massif and some North-Pyrenean Massifs.The MII metamorphism as a syn-tectonic plutono-metamorphic eventBased on the observation of the deep crust outcropping in the North Pyrenean massifs, Vielzeuf [in Guitard et al., 1996] concluded that emplacement of mafic melts in the Carboniferous lower crust was responsible for the MII metamorphism. At the beginning of the process, a regional thermal anomaly is superimposed to the middle crust (MIIs-MIIa), directly reflecting the emplacement of mafic sills in the underlying lower crust (fig. 4A). Heat is transferred conductively and, most likely, advected by the aqueous-carbonic fluids issued from the devolatilising lower crust (fluid inclusion data). Heat advection by melts characterised the end of the MII event, with development of more or less local thermal anomalies: still “regional” (MIIbγ) as in the Garonne dome, or directly liked to sheet-like granite intrusions (MIIγ) as at the bottom of the Mont-Louis pluton (fig. 4B) or at the contact of the Canigou granite (fig. 4C).The late MIII Barrovian metamorphic eventThe MIII event is mainly characterised in the eastern massifs (Albères, Cap de Creus), where a retrogressive kyanite (so-called “hysterogenic” kyanite) is overprinting high-grade assemblages. Although poorly expressed, MIII minerals in these massifs define two zones, with an external chloritoid zone and an internal kyanite-staurolite zone. A MIII chloritoid zone (sillimanite → chloritoid) is also observed in the core of the Canigou dome. Under the kyanite-staurolite equilibrium hypothesis, the peak MIII P-T conditions in the eastern massifs are estimated at 5 kbar and 575oC, that would imply a pressure increase of 1 to 1.5 kbar (4–6 km deepening) starting from the end of MII, associated with a severe temperature decrease of 150oC. Such an overpressure cannot be due to the D2d dextral-inverse mylonites. However, a fluid inclusion study [Kister et al., 2003] demonstrated that the rocks of the Villefranche syncline did register a pressure increase at the D2b stage, i.e., experienced effective downwards displacement during the syncline formation, and it may be estimated that, in the core of the syncline, a depth increase of 7–8 km could have been attained. Now, in the Cap de Creus massif, the highest MIII grade is observed in the core of the D2b Birba syncline, analogous to the Villefranche syncline. Thus, D2b deepening in the syncline cores may have contributed to the pressure increase. An additional increase may have been provided by sedimentary accumulation in an overlying (and now eroded) syn-orogenic basin (fig. 5). While such a process may explain the development of MIII associations in the D2b synclines, it remains to explain its appearance in the anticlines (Albères, Canigou). However, in the same fluid inclusion study referred to just above [Kister et al., 2003], it is demonstrated that, post-dating D2c and the late pluton emplacement, the studied area suffered a severe isobaric temperature drop, allowing the appearance of chloritoid in the Canigou core (fig. 5). A similar explanation may hold for the Albères massif, if it is accepted there that late kyanite and staurolite were not in equilibrium: starting from the peak MII conditions (c. 4 kbar and 650o–700oC), a strong isobaric cooling would have allowed the successive appearance of staurolite and kyanite.Discussion and conclusionTimingThe youngest pre-orogenic flyschs are dated (in the Axial Zone) from the Namurian-Westphalian boundary (315±5 Ma), thus setting a minimal age for D1-MI. On the other hand, in the northern Pyrenean Agly massif, the Ansignan hypersthene-granite, which is coeval with MII, is dated at around 315-305 Ma, and the associated norites, likely testifying for the mafic magmatism at the origin of the heat flux responsible for MII, are themselves dated at c. 315 Ma. Finally, the large syn-D2c (post-MII) granite plutons are all dated at 307±3 Ma (i.e., close to the Westphalian-Stephanian boundary). Taken together (with the possibility of a slight diachronism between the North Pyrenean massifs and the Axial Zone, and, within the Axial Zone, between east and west), these data indicate that the MI-MII transition and the whole D2a–c/MII development took place in a very restricted time interval (c. 10 Ma), in Westphalian to Stephanian times.Crustal rheology and orogenic developmentAt the end of the Namurian crustal subduction (D1-MI), the Pyrenean crust, that had been thickened with at least a doubling of the upper crust thickness, had begun to experience uplift and erosion. This exhumation process rapidly changed from retrograde to prograde (MIIs-MIIa) during the D2a (MII) syn-convergence extensional phase.The D2a sub-event was marked by the development of three interrelated processes: (i) isotherm upwelling, regional stratiform MII metamorphism and partial melting in the middle crust, as a result from the intrusion, in the lower crust, of mafic magmas of mantellic derivation; (ii) thinning of the thickened crust; (iii) first arrival of granite plutons in the middle crust. It is thought, according to Vielzeuf [inGuitard et al., 1996], that these processes were initiated by a lithospheric delamination process.At the end of D2a, the crustal rheology had been modified, with a partially melted middle crust that received granitic melts issued from the melting of the lower crust. This highly ductile middle crust was sandwiched between a thick (≥ 10 km) rigid upper crust and a less ductile granulitised hot lower crust (800o–900oC), thus allowing the progressive decoupling of the upper and lower crust from D2a to D2c. The buckling of the upper crust, with formation of the large upright D2b folds, became therefore possible, forcing the injection of deep anatectic melts in the anticline cores (a probable explanation of the MIIbγ thermal culmination), and creating, in the deepened syncline cores, the strong pressure increase that favoured MIII inception.However, the MII isogrades are frozen in their folded position, indicating that cooling of the belt had indeed begun since at least the end of the D2b phase. The cooling was sufficiently rapid to be expressed in the Axial Zone by a sub-isobaric temperature decrease, at the origin of the MIII Barrovian and retrograde event, coeval with the late D2c and D2d phases. In the North Pyrenean Massifs, where the D2d phase was extensive, the retrograde MIII event could not be expressed, due to both decompression and thermal effects of the extension.A summary of this complex evolution is given in figure 6. Finally, the interrelated D2 and MII events appear as the record, in the middle-upper crust, of a very short, but very intense heating event that strongly modified the rheologic behaviour of the crust inherited from the D1 crustal subduction and allowed a transitory decoupling of the upper and lower crust. The isobaric MIII event records an exceptionally rapid return to the “normal” thermal and rheologic structures of the crust.The rapidly changing tectonic and thermal conditions that characterise the Variscan Pyrenees during the D2 event may be understood if the position of the Pyrenees within the southern branch of the West European Variscan belt is considered (fig. 7).
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Taerum, Robert L. "Evidence for pre-Cenozoic extension in the eastern Main Ranges of the southern Canadian Rockies." Geosphere, March 8, 2022. http://dx.doi.org/10.1130/ges02347.1.
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The eastern Main Ranges of the southern Canadian Rocky Mountain thrust-and-fold belt include a network of normal faults (the result of apparent extensional episodes) that occur within a contractional orogen. The origin, timing, and nature of these normal faults remain unresolved. A widely accepted explanation proposes that the normal faults developed as a consequence of postcontractional transtension that occurred west of the Rocky Mountain Trench during the Paleogene Period. Detailed field mapping of deformation in the vicinity of several normal faults has provided evidence that the normal fault surfaces and adjacent strata underwent deformation during a contractional episode after the normal faults had formed. Within the study area, located in the upper Kicking Horse region of Yoho National Park, British Columbia, Canada, and within the larger region of the Rocky Mountain belt, the network of normal faults is proposed to have developed as a consequence of rifting that separated pericratonic terranes from North America and produced the Slide Mountain Ocean during the Carboniferous and Permian Periods. Overprinting from more recent tectonic episodes has obscured most of these inferred extensional faults throughout the North American Cordillera. Within the study area, however, the Cretaceous to Paleogene contraction carried the normal faults to their present location over unattenuated continental crust, without significant overprinting. This preservation of the network of normal faults allows for investigation of the relationships among the fault surfaces and the strata adjacent to each fault.
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Liu, Yihao, Yun Zhao, Chunji Xue, Liang Yu, Haixia Chu, and Xiaobo Zhao. "The Changshagou gold deposit, Eastern Tianshan, NW China: orogenic gold mineralization overprinting a porphyry gold occurrence." Geological Society, London, Special Publications, October 1, 2021, SP516–2020–248. http://dx.doi.org/10.1144/sp516-2020-248.
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AbstractThe temporal–spatial relationships of porphyry and orogenic gold mineralization in the Eastern Tianshan Orogenic Belt are ambiguous. The newly discovered Changshagou deposit in this belt contains both porphyry and orogenic gold mineralization, which are characterized by polymetallic–sulfide veinlets and quartz–pyrite veins, respectively. Fluid inclusions in the porphyry mineralization episode were trapped at 290–340°C with salinities of 3.0–8.0 wt% NaClequiv. The homogenization temperatures and salinities in the orogenic mineralization episode range from 240 to 300°C and from 1.0 to 5.0 wt% NaClequiv. Coexisting V- and L-type fluid inclusions with similar homogenization temperatures are indicative of fluid immiscibility. The δ18Ow and δDw values are in the ranges 7.6–9.1 and −70.9 to −84.0‰ in the porphyry mineralization episode, and 6.4–7.1 and −65.7 to −72.1‰ in the orogenic mineralization episode, overlapping magmatic and metamorphic ranges, respectively. The pyrite δ34S values range from 3.5 to 4.9‰, falling into the magmatic range. Pyrite in porphyry and orogenic mineralization episodes yield Re–Os isotope ages of 269.1 ± 2.9 and 257.4 ± 2.4 Ma. The porphyry and orogenic gold mineralizations are genetically associated with the quartz syenite porphyry and Kanggur strike-slip shear activity, respectively.
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Morita, Isamu, Tatsuki Tsujimori, Nelson Boniface, Kennet E. Flores, Shogo Aoki, and Kazumasa Aoki. "Neoproterozoic Eclogite-to Granulite-Facies Transition in the Ubendian Belt, Tanzania, and the Timescale of Continental Collision." Journal of Petrology 63, no. 3 (February 21, 2022). http://dx.doi.org/10.1093/petrology/egac012.
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Abstract In collision-type orogens, where high-pressure and ultrahigh-pressure (HP–UHP) metamorphism usually occurs, deeply subducted continental slabs with eclogitized mafic rocks often undergo recrystallization/overprinting with various geothermal gradients after the peak conditions at lower-to-middle-crustal levels. During the crustal stabilization, the transition from eclogite-to granulite-facies is common. We conducted metamorphic petrology and zircon geochronology on (1) bimineralic and (2) partially granulitized eclogites from the Neoproterozoic Ufipa Terrane (Southwestern Tanzania). Microtextural relationships and mineral chemistry define three metamorphic stages: eclogite metamorphism (M1), HP granulite-facies overprinting (M2), and amphibolite-facies retrogression (M3). The bimineralic eclogite has a basaltic composition and lacks M2 minerals. In contrast, the kyanite eclogite is characterized by a gabbro-dioritic whole-rock composition and contains inherited magmatic zircon. Although the matrix is highly granulitized, garnet and kyanite contain eclogite-facies mineral inclusions. Phase equilibria modeling revealed P–T conditions of 2.1–2.6 GPa and 650–860°C for the M1 stage and 1.4–1.6 GPa and 750–940°C for the M2 stage. Zircon with eclogite-facies mineral inclusions from the bimineralic eclogite lacks Eu anomaly in the REE patterns and yielded the M1 eclogite metamorphic age of 588 ± 3 Ma. Zircon overgrowths surrounding the inherited Paleoproterozoic magmatic cores in kyanite eclogite yielded 562 ± 3 Ma. A weak negative Eu anomaly in the REE patterns and the absence of eclogitic mineral inclusions suggest the zircon growths at the M2 HP granulite-facies metamorphic stage. These new data indicate an eclogite-to granulite-facies transition time of 26 ± 4 million years (Myr), suggesting a rate of HP rock exhumation toward a lower crustal level of 0.7–1.5 mm/year. Furthermore, the density evolution model indicates that buoyant host orthogneiss with low-density gabbro-dioritic eclogite plays an important role in carrying high-density basaltic eclogite. Our 2D thermomechanical modeling also suggests that a slab break-off with a lower angle subduction of <20° triggers the exhumation of the HP slab sliver with 20–30 Myr eclogite-to granulite transition time of large HP–UHP terranes in major collision zones.
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Tappe, Sebastian, Malcolm Massuyeau, Katie A. Smart, Alan B. Woodland, Nikolaus Gussone, Sarah Milne, and Andreas Stracke. "Sheared peridotite and megacryst formation beneath the Kaapvaal craton: a snapshot of tectonomagmatic processes across the lithosphere–asthenosphere transition." Journal of Petrology, May 27, 2021. http://dx.doi.org/10.1093/petrology/egab046.
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Abstract The cratonic lithosphere–asthenosphere boundary is commonly invoked as the site of sheared peridotite and megacryst formation, a well-recognized petrologic assemblage whose genetic relationships – if any – remain poorly understood. We have undertaken a comprehensive petrology and Sr-Nd-Hf-Ca isotope study of sheared peridotite xenoliths and clinopyroxene megacrysts from the ca. 1150 Ma Premier kimberlite pipe on the central Kaapvaal craton in South Africa. New textural and mineral trace element evidence suggests that strong tectonic and magmatic overprinting affected the lower cratonic mantle over a vertical distance of ≥ 50 km from the lithosphere–asthenosphere boundary located at ∼200-225 km depth. Although modification of the central Kaapvaal cratonic mantle is commonly linked to the ca. 2056 Ma Bushveld large igneous event, our thermobarometry, mantle redox, and Sr-Nd-Hf-Ca isotope data support a model in which volatile-rich low-volume melts and associated high-density fluids refertilized the lithosphere base shortly before or during asthenosphere-derived kimberlite and carbonatite magmatism at around 1150 Ma. This episode of lithospheric mantle enrichment was facilitated by exceptionally strong shear movements, as are recorded in the plastically deformed peridotites. We argue that stress-driven segregation of percolating carbonated melts contributed to megacryst formation along, or in close proximity to, shear zones within the cratonic mantle lithosphere.
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Németh, Norbert, Ferenc Kristály, Péter Gál, Ferenc Móricz, and Réka Lukács. "Metavolcanic formations in the Paraautochthonous Triassic successions of the Bükk Mts, NE Hungary." International Journal of Earth Sciences, September 4, 2022. http://dx.doi.org/10.1007/s00531-022-02246-6.
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AbstractThe Bükk Mts. in NE Hungary exposes Paleozoic and Mesozoic successions containing volcanic formations both in the oceanic crust-derived Szarvaskő Unit and in the continental crust-derived Paraautochthonous Unit. The rocks of this latter unit were subject of multiple metamorphic, also metasomatic alterations and deformation events obscuring and overprinting original petrographic and geochemical characteristics and producing a complex structure in which stratigraphic relationships are not always possible to be identified. This situation was leading to various stratigraphic hypotheses. This study aims to provide a basis for distinguishing metavolcanic formations using trace element geochemical data combined with quantitative mineralogical data based on XRD and EPMA. Our data were obtained from a wide range of samples collected in the Bükk Mts. Mineralogy and major element geochemistry reflect regional Alpine metamorphism and local alteration processes but high field strength elements remained relatively stable during most of these processes except local HFSE enrichment. Zr/TiO2, Nb/Y and Nb/Ta ratios were effectively used for classifying the rocks into three formations: Bagolyhegy Metarhyolite, Szentistvánhegy Metavolcanics and Szinva Metabasalt. Bagolyhegy Metarhyolite is a unique volcanic formation formed from highly differentiated and HFSE-depleted magma, probably in a single volcanic centre of uncertain age. Szentistvánhegy Metavolcanics comprises heterogeneous rocks of a calc-alkaline arc-type suite with wide distribution in a Ladinian chronostratigraphic horizon. Szinva Metabasalt represents within-plate-type alkaline lava flows and adjacent volcaniclastic, mostly peperitic rocks embedded in Carnian platform and basin facies limestone formations. Like the sedimentary formations of the Bükk Mts, rocks of magmatic origin can be correlated with the formations of the South Alpine and Dinaric successions with the significant difference that abundant metavolcanics are not accompanied by intrusive bodies.
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Mesquita, Maria José, Márcia Elisa Boscato Gomes, Igor de Camargo Moreira, Renata Augusta Sampaio Paes, Hevelyn Eduarda da Silva Martins, José Henrique Matos, Alberto Ruggiero, et al. "Paleoproterozoic gold deposits at Alta Floresta Mineral Province, Brazil: two overprinted mineralising events?" Geological Society, London, Special Publications, June 28, 2022, SP516–2021–64. http://dx.doi.org/10.1144/sp516-2021-64.
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AbstractLarge gold provinces commonly show complicated mineralization histories, and the Paleoproterozoic Alta Floresta, one of Brazil's most exciting Au-Cu mineral provinces, is a good example. The current models defined four deposit types, all connected to a single (1.88-1.75 Ga) magmatic-hydrothermal event. However, long Province history, diverse geodynamic environment, and older ages of Type-1mineralisation weaken the single metallogenic event and enable the hypothesis of overprinted mineral events. By scale-integrated analyses, we revise the tectonic-geological context, structural-hydrothermal alterations, and chlorite-white mica geothermobarometer and propose the type-1 as an older, granitoid-hosted orogenic mineralisation, with subsequent overprinting by the magmatic-hydrothermal event. The older orogenic gold event developed orogenic gold deposits on WNW-trending shear zones in the Peixoto de Azevedo domain granitic-gneiss rocks. Phengite, biotite, chlorite-carbonate phyllonites (3.3-6.1 kbar, 300º-420ºC) host fault-fill quartz veins (pyrite-chalcopyrite-magnetite-pyrrhotite-gold-Bi-Ag tellurides). Mg-rich chlorite-phengite is the main alteration footprint for this mineralisation type. A younger magmatic-hydrothermal event in the Juruena magmatic-arc rocks produced Fe-rich chlorite-white mica alteration zones (0.6-4.6 kbar, 120º-380ºC) and disseminated and stockwork-breccia ore (pyrite-chalcopyrite-gold-molybdenite- Ti minerals-allanite) in porphyry-epithermal deposits. Where the younger mineralisation overprints the older, phyllic alteration destroyed the phengite orogenic gold phyllonite Sn+1 foliation. The ages of two pyrite populations (1979 and 1841 Ma) in the older fault-fill veins and molybdenite in late fractures (1805-1782 Ma) or disseminated in the ca. 1.79 Ga syenogranite porphyry suggest more than two episodes of mineralisation. These two events differ in their alteration styles, P-T conditions, and structural, mineralogical, and textural ore styles. The multi-scale approach enlightens the relationships between the various mineralisation events, allowing a new explorational potential within the province.Supplementary material at https://doi.org/10.6084/m9.figshare.c.6056324
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Wu, Chen, Jie Li, Andrew V. Zuza, Peter J. Haproff, An Yin, and Lin Ding. "Paleoproterozoic–Paleozoic tectonic evolution of the Longshou Shan, western North China craton." Geosphere, April 18, 2022. http://dx.doi.org/10.1130/ges02491.1.
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The Longshou Shan of western China is the northern backstop of the Cenozoic Himalayan-Tibetan orogen and occupies a key linkage between the Tarim continent and North China craton which separate the pre-Cenozoic Tethyan orogenic system and Central Asian orogenic system. Therefore, the Paleoproterozoic–Paleozoic evolution of this region is critical to understanding the extent of overprinting Cenozoic deformation, construction of the Eurasian continent, and relationships between the pre-Cenozoic Tethyan orogenic system and Central Asian orogenic system. Here we present detailed field observations and results of geochronological and major and trace element and Sr-Nd isotope geochemical analyses of samples from the Longshou Shan to decipher its complex Paleoproterozoic–Paleozoic tectonic history. Our results show that the Paleoproterozoic basement rocks of the Longshou Shan were part of the North China craton and involved in Paleoproterozoic northern North China orogeny. A ca. 965 Ma granitoid in the Longshou Shan provides key evidence for a spatial linkage between northern Tibetan continents, the North Tarim continent, and the North China craton in the early Neoproterozoic. The presence of Early Ordovician granitoids and arc volcanic rocks in the Longshou Shan suggest that bivergent subduction of Qilian oceanic lithosphere occurred during the early Paleozoic. Crustal shortening and thickening during Ordovician–Carboniferous orogenesis are evidenced by the presence of several unconformities in the Longshou Shan. Late Carboniferous arc granites exposed in the study area are likely associated with the southward subduction of the Paleo-Asian Ocean to the north and with Permian siliciclastic strata sourced from a proximal arc-subduction system, based on detrital zircon ages. Although the tectonic history of the Long- shou Shan can be traced back to Neoproterozoic time, most of the recorded deformation and uplift of the region occurred during the early Paleozoic Qilian orogeny and late Paleozoic Central Asian orogeny. Furthermore, we interpret that the several orogenic events recorded in the Longshou Shan (i.e., northern North China, Qilian, and Central Asian orogenies) are spatially and temporally correlative along strike with those recorded in the Tarim and North China cratons.
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Li, Jie, Chen Wu, Xuanhua Chen, An Yin, Andrew V. Zuza, Peter J. Haproff, Yanfei Chen, Luojuan Wang, and Zhaogang Shao. "Tectonic setting of metamorphism and exhumation of eclogite-facies rocks in the South Beishan orogen, northwestern China." Geosphere, December 6, 2022. http://dx.doi.org/10.1130/ges02548.1.
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High-pressure metamorphic rocks occur as distinct belts along subduction zones and collisional orogens or as isolated blocks within orogens or mélanges and represent continental materials that were subducted to deep depths and subsequently exhumed to the shallow crust. Understanding the burial and exhumation processes and the sizes and shapes of the high-pressure blocks is important for providing insight into global geodynamics and plate tectonic processes. The South Beishan orogen of northwestern China is notable for the exposure of early Paleozoic high-pressure (HP), eclogite-facies metamorphic rocks, yet the tectonism associated with the HP metamorphism and mechanism of exhumation are poorly understood despite being key to understanding the tectonic evolution of the larger Central Asian Orogenic System. To address this issue, we examined the geometries, kinematics, and overprinting relationships of structures and determined the temperatures and timings of deformation and metamorphism of the HP rocks of the South Beishan orogen. Geochronological results show that the South Beishan orogen contains ca. 1.55–1.35 Ga basement metamorphic rocks and ca. 970–866 Ma granitoids generated during a regional tectono-magmatic event. Ca. 500–450 Ma crustal thickening and HP metamorphism may have been related to regional contraction in the South Beishan orogen. Ca. 900–800 Ma protoliths experienced eclogite-facies metamorphism (~1.2–2.1 GPa and ~700–800 °C) in thickened lower crust. These HP rocks were subsequently exhumed after ca. 450 Ma to mid-crustal depths in the footwall of a regional detachment fault during southeast-northwest–oriented crustal extension, possibly as the result of roll-back of a subducted oceanic slab. Prior to ca. 438 Ma, north-south–oriented contraction resulted in isoclinal folding of the detachment fault and HP rocks. Following this contractional phase in the middle Mesozoic, the South Beishan orogen experienced thrusting interpreted to be the response to the closure of the Tethyan and Paleo-Asian Ocean domains. This contractional phase was followed by late Mesozoic extension and subsequent surface erosion that controlled exhumation of the HP rocks.
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Cook, Nigel J., Cristiana L. Ciobanu, Kathy Ehrig, Ashley D. Slattery, and Sarah E. Gilbert. "Micron- to atomic-scale investigation of rare earth elements in iron oxides." Frontiers in Earth Science 10 (October 12, 2022). http://dx.doi.org/10.3389/feart.2022.967189.
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Hematite (α-Fe2O3) and magnetite (Fe3O4) readily accommodate a wide range of minor and trace elements from across the periodic table at up to wt.% concentrations. This prompts the question of whether these common minerals can also host rare earth elements (lanthanides, Y and Sc; REEs)? If so, what is the chemical and physical nature of the elements: are they incorporated into the oxide crystal structures, or do they occur as nanometer-to micron-sized inclusions of discrete REE-minerals? By combining micron-scale petrography and analysis by LA-ICP-MS with nanoscale imaging and energy-dispersive spectroscopy, the relationships between REEs and iron-oxides are addressed in samples from the world-class Olympic Dam Cu-U-Au-Ag deposit, South Australia. Spatially co-existing silician magnetite and hematite from the outer shell at Olympic Dam show stages of interconversion during which REEs are redistributed. REEs are shown to be preferentially incorporated into the magnetite structure, whereas hematite concentrates U, W, and Sn, and contains negligible structurally bound REEs. Abundant, <20 nm-sized uraninite nanoparticles (NPs) are a key host for REEs in hematite. In contrast, hematite from mineralized breccias displays co-precipitation of Fe-oxides and REE-minerals facilitated by discharge of high-volatile fluids. Variation in the shape of chondrite-normalized REE fractionation trends, ranging from concave (hematite containing uraninite NPs) to steeply downwards-sloping (included LREE-dominant phases such as monazite, florencite, fluocerite and cerianite), reflect these differences. Evolving NP mineralogy in the breccias, from in-situ NP formation to an abundance of monazite reflects successive overprinting events and remobilization of elements from solid-solution and subsequent trapping as NPs via coupled dissolution replacement reaction. REE-minerals such as florencite can host nanoinclusions of hematite. Atomic-scale observations, including defects in magnetite and an O-deficient, two-fold hematite superstructure show crystal structural accommodation during Fe-oxide interconversion and inclusion nucleation. Both silician magnetite and hematite show nanoscale muscovite forming along planar defects. Understanding the mineralogical deportment of REEs at Olympic Dam carries potential implications for investigating the viability of REE extraction opportunities from any resource in which Fe-oxides are the dominant component.
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Levy, Drew A., Andrew V. Zuza, Peter J. Haproff, and Margaret L. Odlum. "Early Permian tectonic evolution of the Last Chance thrust system: An example of induced subduction initiation along a plate boundary transform." GSA Bulletin, October 13, 2020. http://dx.doi.org/10.1130/b35752.1.
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The late Paleozoic is an important precursor stage in the development of the Mesozoic Cordilleran subduction system along the western margin of North America, but the tectonic history remains ambiguous due to complex overprinting deformation and magmatism. Determining the driving mechanism of large magnitude Permian shortening in southwest Laurentia is critical to understanding the late Paleozoic transition from transform margin to subduction zone. We investigated the driving mechanism of the Permian Last Chance thrust system in east-central California to understand this transition prior to the development of the Mesozoic Cordilleran arc. Here, we present the results of new geological mapping, detrital zircon U-Pb geochronology, and a synthesis of regional tectonics to inform a kinematic model of the Last Chance thrust system and outline the Permian−Triassic tectonic evolution of the plate boundary during induced subduction initiation. The record of subduction initiation along an inferred late Paleozoic transform fault (the California-Coahuila transform) is preserved by (1) Permian arc magmatism, (2) the onset of volcaniclastic sedimentation, and (3) the development of a regional transpressional system in present-day east-central California. The evolution of this transpressional system and subduction zone is recorded by development of the Last Chance thrust system of the Death Valley region. Geological mapping in the Last Chance Range, northern Death Valley National Park, and the Inyo Mountains reveals the east-directed Last Chance thrust system was constructed by repetitive out-of-sequence deformation consistent with transpressional strain. The Last Chance thrust system accommodated a minimum of >75 km (60%) shortening, based on cross-section restorations guided by regional stratigraphic relationships and restoration of subsequent Mesozoic deformation. Our revised model of Jurassic extensional exhumation of the Snow Lake terrane argues the Last chance thrust was not reactivated during the Mesozoic. Large-magnitude shortening along the California-Coahuila transform accommodated a significant component of the convergent plate motion as the Panthalassan crust was thrust below the continental margin before initial slab sinking. Numerical models show the forces resisting subduction are greatest before initial slab sinking takes place, and compression is transmitted in board from the plate boundary. We argue the Last Chance thrust system developed in response to this compression. Early-middle Permian plutons and late Permian detrital zircons in coeval basins suggest subduction was well established by the early Permian. Collectively, the preservation of a thrust system, early arc magmatism, and syntectonic sedimentary basins, which are features typically destroyed by subduction magmatism and deformation, allow for the evaluation of subduction initiation mechanisms based on field observations.
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Woodland, Alan B., Carolin Gräf, Theresa Sandner, Heidi E. Höfer, Hans-Michael Seitz, D. Graham Pearson, and Bruce A. Kjarsgaard. "Oxidation state and metasomatism of the lithospheric mantle beneath the Rae Craton, Canada: strong gradients reflect craton formation and evolution." Scientific Reports 11, no. 1 (February 11, 2021). http://dx.doi.org/10.1038/s41598-021-83261-6.
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AbstractWe present the first oxidation state measurements for the subcontinental lithospheric mantle (SCLM) beneath the Rae craton, northern Canada, one of the largest components of the Canadian shield. In combination with major and trace element compositions for garnet and clinopyroxene, we assess the relationship between oxidation state and metasomatic overprinting. The sample suite comprises peridotite xenoliths from the central part (Pelly Bay) and the craton margin (Somerset Island) providing insights into lateral and vertical variations in lithospheric character. Our suite contains spinel, garnet-spinel and garnet peridotites, with most samples originating from 100 to 140 km depth. Within this narrow depth range we observe strong chemical gradients, including variations in oxygen fugacity (ƒO2) of over 4 log units. Both Pelly Bay and Somerset Island peridotites reveal a change in metasomatic type with depth. Observed geochemical systematics and textural evidence support the notion that Rae SCLM developed through amalgamation of different local domains, establishing chemical gradients from the start. These gradients were subsequently modified by migrating melts that drove further development of different types of metasomatic overprinting and variable oxidation at a range of length scales. This oxidation already apparent at ~ 100 km depth could have locally destabilised any pre-existing diamond or graphite.
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Say, Michael C., and Andrew V. Zuza. "Heterogenous late Miocene extension in the northern Walker Lane (California-Nevada, USA) demonstrates vertically decoupled crustal extension." Geosphere, September 23, 2021. http://dx.doi.org/10.1130/ges02409.1.
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The spatial distribution and kinematics of intracontinental deformation provide insight into the dominant mode of continental tectonics: rigid-body motion versus continuum flow. The discrete San Andreas fault defines the western North America plate boundary, but transtensional deformation is distributed hundreds of kilometers eastward across the Walker Lane–Basin and Range provinces. In particular, distributed Basin and Range extension has been encroaching westward onto the relatively stable Sierra Nevada block since the Miocene, but the timing and style of distributed deformation overprinting the stable Sierra Nevada crust remains poorly resolved. Here we bracket the timing, magnitude, and kinematics of overprinting Walker Lane and Basin and Range deformation in the Pine Nut Mountains, Nevada (USA), which are the westernmost structural and topographic expression of the Basin and Range, with new geologic mapping and 40Ar/39Ar geochronology. Structural mapping suggests that north-striking normal faults developed during the initiation of Basin and Range extension and were later reactivated as northeast-striking oblique-slip faults following the onset of Walker Lane transtensional deformation. Conformable volcanic and sedimentary rocks, with new ages spanning ca. 14.2 Ma to 6.8 Ma, were tilted 30°–36° northwest by east-dipping normal faults. This relationship demonstrates that dip-slip deformation initiated after ca. 6.8 Ma. A retrodeformed cross section across the range suggests that the range experienced 14% extension. Subsequently, Walker Lane transtension initiated, and clockwise rotation of the Carson domain may have been accommodated by northeast-striking left-slip faults. Our work better defines strain patterns at the western extent of the Basin and Range province across an approximately 150-km-long east-west transect that reveals domains of low strain (~15%) in the Carson Range–Pine Nut Mountains and Gillis Range surrounding high-magnitude extension (~150%–180%) in the Singatse and Wassuk Ranges. There is no evidence for irregular crustal thickness variations across this same transect—either in the Mesozoic, prior to extension, or today—which suggests that strain must be accommodated differently at decoupled crustal levels to result in smooth, homogenous crustal thickness values despite the significantly heterogeneous extensional evolution. This example across an ~150 km transect demonstrates that the use of upper-crust extension estimates to constrain pre-extension crustal thickness, assuming pure shear as commonly done for the Mesozoic Nevadaplano orogenic plateau, may not be reliable.
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Massiot, Cécile, Iona McIntosh, Jeremy Deans, Sarah D. Milicich, Fabio Caratori Tontini, Cornel E. J. de Ronde, Ludmila Adam, Kannikha Kolandaivelu, and Gilles Guerin. "Petrophysical Facies and Inferences on Permeability at Brothers Volcano, Kermadec arc, Using Downhole Images and Petrophysical Data." Economic Geology, February 14, 2022. http://dx.doi.org/10.5382/econgeo.4897.
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Abstract Downhole data and cores collected during International Ocean Discovery Program (IODP) Expedition 376 at Brothers volcano, Kermadec arc, provide unprecedented, in situ views of volcanic facies and fluid pathways in an actively forming volcanogenic massive sulfide (VMS) ore deposit. Brothers volcano is a submarine caldera with extensive sea floor hydrothermal alteration. Downhole data were collected in two holes: Hole U1530A at the NW Caldera and Hole U1528D at the Upper Cone. Textural analysis of microresistivity images in Hole U1530A provides a continuous image facies record that greatly improves findings based upon sporadic and partial (18%) core recovery. Between 90 and 214 meters below sea floor (mbsf), the heterogeneous image facies with local pattern variations is consistent with the volcaniclastic facies interpreted from cores. Between 232 and 445 mbsf, a volcanic facies was not recognizable in cores because of overprinting alteration, apart from five intervals of coherent lava flows that were less altered. Based on the fairly constant petrophysical data, Vp-porosity relationship, and presence of five to six coherent image facies intervals on the microresistivity image, we propose that the apparent volcaniclastic textures observed on cores and microresistivity images beneath 232 mbsf are dominantly lava flows. The change from volcaniclastic to dominant lava flow facies occurs over a transition zone (214–232 mbsf) where all petrophysical properties gradually change. In Hole U1528D, cores and petrophysical data show a similar transition from deep coherent lava flows to shallower, largely volcaniclastic sequences at ~270 mbsf. Down to 232 mbsf in Hole U1530A and 360 mbsf in Hole U1528D, the overall first-order downward decrease in porosity is interpreted to be caused by compaction and increased alteration intensity. Volcanic facies and fractures exert a second-order local control on petrophysical properties. Beneath 232 mbsf in Hole U1530A, the prolonged hydrothermal activity is inferred to have diminished local petrophysical property variations within the proposed lava flow-dominated rock package. High downhole fluid temperatures in Hole U1528D contrast with the moderate temperatures in Hole U1530A. Permeable zones show a mix of structural (inferred fault in Hole U1530A) and lithological controls in both holes. Some low-permeability layers and/or lithological interfaces possibly focus fluids laterally in higher-permeability layers, which may act as a trap for metal-rich fluids to form stratabound massive sulfides and deposits. Matrix is likely too low in permeability to conduct fluids but provides perfect conditions for the storage of super saline brines. In Hole U1530A, located near active vents at the sea floor, the relatively low fluid temperature and the alteration overprint of moderate temperature demonstrate the high spatial and temporal variations at Brothers volcano. The implications of the new stratigraphy and controls on permeability proposed here for Brothers volcano include a better understanding of the following: 1) submarine volcanic eruption sequences, 2) permeability in active submarine volcanoes, and 3) the formation of volcanogenic massive sulfide deposits on (and near) the sea floor.