30 research outputs found

    Deformation of the Cape Chignecto Pluton, Cobequid Highlands, Nova Scotia: thrusting at the Meguma-Avalon boundary

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    The Early Carboniferous Cape Chignecto pluton, at the western end of the Cobequid Highlands in the Avalon Terrane of Nova Scotia, consists predominantly of two-feldspar biotite granite. Intrusion of smaller diorite bodies locally melted and hybridized the granite. Diabase and microgranite dykes cut the plutons. Much of the pluton has a flat-lying mylonitic foliation, with a mineral elongation lineation trending between north and west. Quartz is dynamically recrystallized, but feldspars show predominantly brittle deformation. The phiton was probably deformed under greenschist facies conditions, after cooling. C-S fabrics observed in the field, and tails on porphyroclasts seen in thin section, indicate northward overthrusting. The Hadrynian Jeffers Group is thrust over the south edge of the phiton in the Fowler Brook inlier. At its north edge, the pluton is probably thrust over volcanic rocks of die Fountain Lake Group. The deformation of the pluton is constrained by stratigraphic relationships and isotopic dates to a Namurian or possibly early Westphalian age. Deformation within the pluton is correlated with Namurian uplift of the Cobequid and Caledonia Highlands. It records either transpression resulting from the curvature of the Meguma-Avalon boundary or a major component of convergent movement between the Meguma and Avalon terranes. RÉSUMÉ Le pluton de Cape Chignecto, daté du début du Carbonifere et situé à l'extremité occidentale des Monts Cobequid (Lanière d'Avalon, Nouvelle-Écosse), est formé en majeure partie de granite à biotite à deux feldspaths. Le granite subit une fonte et une hybridation locales lore de lÔÇÖ intrusion de batis dioritiques de plus faibles dimensions. Des dykes de diabase et de microgranite recoupent les plutons. La majeure partie du pluton présente une foliation mylonitique horizontale, avec une linétion d'éirement orientée entre le nord et lÔÇÖouesL On note une recristallisation dynamique du quartz mais la déformation des feldspaths est surtout fragile. Le pluton fut probablement déformé sous des conditions appartenant au faciès des schistes verts, et ce après son refroidissement. Des fabriques planaires C-S, observées sur le terrain, et des ombres de pression dissymdtriques, vues en lame mince, indiquent un charriage vers le nord. Le Groupe hadrynien de Jeffers chevauche l'extrémité méridionale du pluton dans la fenetre de Fowler Brook. A son extrémité septentrionale, le pluton chevauche probablement les volcanites du Groupe de Fountain Lake. Les relations stratigraphiqueset des datations radiométriques limitent l'âge de la déformation du pluton au Namurien ou possiblement au début du Westphalien. On corrèle la déformation au sein du pluton avec la surrection namurienne des monts Cobequid et Caledonia. Cette déformation enregistre soil une transpression résultant de la cambrure de la jonction Meguma-Avalon, soil une forte composanle de mouvement convergent entre les lanières de Meguma et d* Avalon. [Traduit par le journal

    How was the Iapetus Ocean infected with subduction?

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    Because subduction in the Iapetus Ocean began only Ôł╝35 m.y. after the end of rifting, spontaneous foundering of mature passive margins is an unlikely subduction-initiation mechanism. Subduction is more likely to have entered the Iapetus from the boundary with the external paleo-Pacific, similar to the incursion of the Scotia, Caribbean, and Gibraltar arcs into the modern Atlantic. The subduction zone probably became sinuous, entraining fragments of the Gondwanan margin along its complex sinistral southern boundary where oblique collision caused Monian-Penobscottian deformation. Following Taconian-Grampian collision of part of the subduction system with Laurentia, remaining parts of the Iapetus were progressively infected with subduction, leading to Silurian closure

    Highly depleted isotopic compositions evident in Iapetus and Rheic Ocean basalts: implications for crustal generation and preservation

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    Subduction of both the Iapetus and Rheic oceans began relatively soon after their opening. Vestiges of both the Iapetan and Rheic oceanic lithospheres are preserved as supra-subduction ophiolites and related mafic complexes in the AppalachianÔÇôCaledonian and Variscan orogens. However, available SmÔÇôNd isotopic data indicate that the mantle source of these complexes was highly depleted as a result of an earlier history of magmatism that occurred prior to initiation of the Iapetus and Rheic oceans. We propose two alternative models for this feature: either the highly depleted mantle was preserved in a long-lived oceanic plateau within the Paleopacific realm or the source for the basalt crust was been recycled from a previously depleted mantle and was brought to an ocean spreading centre during return flow, without significant re-enrichment en-route. Data from present-day oceans suggest that such return flow was more likely to have occurred in the Paleopacific than in new mid-ocean ridges produced in the opening of the Iapetus and Rheic oceans. Variation in crustal density produced by Fe partitioning rendered the lithosphere derived from previously depleted mantle more buoyant than the surrounding asthenosphere, facilitating its preservation. The buoyant oceanic lithosphere was captured from the adjacent Paleopacific, in a manner analogous to the MesozoicÔÇôCenozoic ÔÇťcaptureÔÇŁ in the Atlantic realm of the Caribbean plate. This mechanism of ÔÇťplate captureÔÇŁ may explain the premature closing of the oceans, and the distribution of collisional events and peri-Gondwanan terranes in the AppalachianÔÇôCaledonian and Variscan orogens

    Terrane history of the Iapetus Ocean as preserved in the northern Appalachians and western Caledonides

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    The Iapetus Ocean was the first ancient ocean to be identified following the development of plate tectonics; its history has been fundamental in relating orogenesis and plate motion. The ocean probably formed following 3-way rifting between Laurentia, Baltica, and Amazonia ÔÇô West Africa (a block that became incorporated in Gondwana). Closure of the ocean trapped numerous terranes during the development of the AppalachianÔÇôCaledonide Orogen. Subsequent deformation, including late Paleozoic strike slip, transpression, and transtension, and Mesozoic stretching during Pangea breakup, must be taken into account in models for orogen development. Traditional analyses of Iapetan terranes have focussed on Cambrian sedimentary successions, and on isotopic criteria, to classify terranes into larger domains: Ganderia, Avalonia and Megumia. Detrital zircon data show that these domains did not cross the Iapetus as single entities, while paleomagnetic data reveal significant vertical-axis terrane rotations. We here review and interpret 17 paleomagnetic poles and >350 published detrital zircon data sets from the northern Appalachians and western Caledonides, using consistent and rigorous criteria for the selection and presentation of data. We place these data on an integrated stratigraphic chart to show timing relations and to seek constraints on the provenance and travel of terranes in the Iapetus Ocean. We distinguish groups of terranes that likely travelled together as terrane assemblages. In the Taconian/Grampian Orogeny, Furongian to Katian continentÔÇôarc collision involved off-margin blocks along the hyperextended Laurentian margin. In New England, early Taconian collision by 475 Ma involved the Gondwana-derived Moretown assemblage. An assemblage of the Bronson and Popelogan arc terranes probably arrived at the main Laurentian margin 25-30 Myr later. Subduction polarity reversal then led to the progressive accretion of additional terrane assemblages (Salinian Orogeny). The MiramichiÔÇôVictoria assemblage arrived close to the OrdovicianÔÇôSilurian boundary. The Miramichi terrane underwent partial subduction in the Qu├ębec re-entrant, whereas the Victoria terrane was juxtaposed with the Newfoundland promontory without major metamorphism. In mid-Silurian time, an assemblage including the Gander terrane of Newfoundland and related portions of Britain and Ireland was accreted to Laurentia, along with Baltica (Scandian Orogeny). The St. Croix ÔÇô La Poile assemblage may have been accreted slightly later, but is distinguished by the development of a Silurian arcÔÇôbackarc system (coastal igneous belt) above a northwest-dipping subduction zone. The AvalonÔÇôBrookville assemblage encountered this system in P┼Ö├şdol├ş to Middle Devonian time (Acadian Orogeny), leading to the collapse of the backarc basin and northwest-vergent thrust emplacement onto Laurentia during sinistral transpression in the Appalachian Orogen. Acadian deformation involved mainly sinistral strike slip in Britain and Ireland. Several of the terranes that were accreted to the Laurentian margin carried internal records of earlier deformation that took place near Amazonia ÔÇô West Africa in Early Ordovician time and earlier (Monian/Penobscottian Orogeny). The Iapetus Ocean thus contained a complex array of terranes, small ocean basins, arcs, and previously emplaced ophiolites analogous to modern southeast Asia. It closed to form a complex array of sutures in an orogen within which no single Iapetus suture can be clearly identified

    Temporal evolution of shallow marine diagenetic environments:Insights from carbonate concretions

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    Early diagenesis of marine organic matter dramatically impacts EarthÔÇÖs surface chemistry by changing the burial potential of carbon and promoting the formation of authigenic mineral phases including carbonate concretions. Marine sediment-hosted carbonate concretions tend to form as a result of microbial anaerobic diagenetic reactions that degrade organic matter and methane, some of which require an external oxidant. Thus, temporal changes in the oxidation state of EarthÔÇÖs oceans may impart a first-order control on concretion authigenesis mechanisms through time. Statistically significant variability in concretion carbonate carbon isotope compositions indicates changes in shallow marine sediment diagenesis associated with EarthÔÇÖs evolving redox landscape. This variability manifests itself as an expansion in carbon isotope composition range broadly characterized by an increase in maximum and decrease in minimum isotope values through time. Reaction transport modelling helps to constrain the potential impacts of shifting redox chemistry and highlights the importance of organic carbon delivery to the seafloor, marine sulfate concentrations, methane production and external methane influx. The first appearance of conclusively anaerobic oxidation of methane-derived concretions occurs in the Carboniferous and coincides with a Paleozoic rise in marine sulfate. The muted variability recognized in older concretions (and in particular for Precambrian concretions) likely reflects impacts of a smaller marine sulfate reservoir and perhaps elevated marine dissolved inorganic carbon concentrations. Causes of the increase in carbon isotope maximum values through time are more confounding, but may be related to isotopic equilibration of dissolved inorganic carbon with externally derived methane. Ultimately the concretion isotope record in part reflects changes in organic matter availability and marine oxidation state, highlighting connections with the subsurface biosphere and diagenesis throughout geologic time

    ANTALYA KARMA┼×I─×I KUZEYDO─×U UZANIMININ ─░SPARTA B├ľLGESiNDEKi STRAT─░GRAF─░S─░ VE SED─░MANTER EVR─░M─░

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    Antalya karma┼č─▒─č─▒ (Antalya naplar─▒) Bat─▒ Toroslar'─▒n ─░sparta dirse─čine yerle┼čmi┼č olan ve ba┼čl─▒ca Mesozoyik ya┼čl─▒ kayalardan olu┼čan allokton bir topluluktur. Bu karma┼č─▒─č─▒n kuzeydo─ču b├Âl├╝m├╝nde, E─čridir g├Âl├╝ do─čusunda iki grup tan─▒mlanm─▒┼čt─▒r. Bunlardan Pazark├Ây grubu, mafik lavlar, radyolaritler, ├çamurta┼člar─▒, t├╝rbiditik kumta┼člar─▒, t├╝rbiditik kire├žta┼člar─▒ ve pelajik kire├žta┼člar─▒n─▒ i├žeren on formasyondan olu┼čur. Yuval─▒ grubu ise tamamen s─▒─č deniz karbonatlar─▒n─▒ kapsayan iki formasyondan olu┼čur,inceleme alan─▒nda bunlardan ba┼čka harzburgitler ve olduk├ža geni┼č yay─▒l─▒ml─▒ melanj ve megabre┼čler bulunur. Antalya karma┼č─▒─č─▒n─▒n kuzeydo─ču uzan─▒m─▒, Mesozoyik ya┼čl─▒ bir k─▒ta kenar─▒n─▒n karma┼č─▒k paleoco─črafyas─▒n─▒ yans─▒tan bir y├Âreyi simgeler. Devaml─▒ karbonat banklar─▒ (Yuval─▒ grubu) derin deniz ├ž├Âkel ortamlanyle (Pazark├Ây grubu) ├ževrilmi┼čtir. Bu b├Âlge ├ťst Kretase zaman─▒nda kuzeydo─ču y├Ânl├╝ itki faylar─▒yle deformasyona u─čram─▒┼čt─▒r. B├Âlgede Tersiyer ya┼čl─▒ deformasyonun etkileri de g├Âzlenmi┼čtir

    Paleolatitude and Tectonic Rotations of the Early Carboniferous Fountain Lake Group, Cobequid Highlands, Nova Scotia, Canada

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    The ca. 355 Ma Fountain Lake Group, in the Cobequid Highlands of Nova Scotia, is part of the transtensional basin fill which formed during dextral strike-slip motion between Avalonia and the Meguma terranes following the Acadian Orogeny. Paleomagnetic analysis of the Fountain Lake Group offers a paleolatitude estimate for the Laurentian accretionary margin in the Early Carboniferous and locality-specific paleomagnetic directions which indicate clockwise-sense block rotations during dextral strike-slip motion along the Cobequid Fault zone. Stepwise demagnetization of 142 specimens from 20 sites in three Fountain Lake Group localities across the Cobequid Highlands (Squally Point, West Moose River, and Wentworth exposures) reveals remanence consisting of an easily removed component of probable recent origin, and more persistent components carried by magnetite and hematite, which in petrographic and electron beam analysis appear to be of primary igneous and volcanic oxidation origins, respectively. Sites from all three localities carry stable characteristic remanent magnetization (ChRM) directions that assume similar moderate downward inclinations when tilt-corrected. A Block Rotation Fisher analysis inclination-only fold test demonstrated best agreement at 90% unfolding, showing that remanence acquisition pre-dates Alleghenian deformation in the Late Carboniferous and is most likely of primary 355 Ma age. Paleomagnetic results for the Squally Point, West Moose River and Wentworth localities show relative rotations between the blocks that are variously clockwise-rotated compared with a Laurentia cratonic reference frame. Inclinations at all three localities imply a subtropics paleolatitude for the margin (at Squally Point, 27.2 9.4; N= 7 sites), directly supporting the depicted location of Laurentia and its Appalachian accretionary margin in most Devonian to Early Carboniferous reconstructions.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

    Diachronous Paleozoic accretion of peri-Gondwanan terranes at the Laurentian margin

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    In the original Wilson cycle, the northern AppalachianÔÇôCaledonide orogen resulted from the collision of two continental masses separated by a single ocean. One of these corresponds to the modern concept of Laurentia, but the colliding continent to the east has been variously subdivided into many smaller terranes and domains, including Ganderia, Avalonia and Megumia. Using published stratigraphic evidence and detrital zircon provenance data from units of known depositional age, the timing of arrival of these units at the Laurentian margin between the Early Ordovician and Early Devonian can be constrained. Several of the accreted terranes do not extend over the entire length of the orogen, with the result that the lines separating them change character along strike from terrane-bounding sutures to simple accretionary faults. The Ganderia domain consists of at least four separate terranes that share a common origin on the continental margin of Gondwana, but were separated by back-arc oceanic crust as they crossed the Iapetus Ocean and collided diachronously with the Laurentian margin

    Peri-Gondwanan terrane interactions recorded in the CambrianÔÇôOrdovician detrital zircon geochronology of North Wales

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    Precambrian to Ordovician sedimentary basins of Wales display contrasting histories across major NE-striking fault systems. Northeast of the Menai Strait Fault System the Monian Supergroup is a deformed succession of mainly metaclastic rocks. Within the fault system the Arfon Basin contains sandstone and slate overlying Neoproterozoic volcanic rocks. South of the fault system, in the Harlech Dome of the Welsh Basin, a distinctive succession of clastic rocks has been correlated with those of the Meguma Terrane in the northern Appalachian orogen, together comprising the domain Megumia. A detrital zircon analysis from Cambrian sandstone in the Harlech Dome is consistent with this correlation. However, the Early Ordovician Dol-cyn-afon Formation higher in the succession and the overlying Late Ordovician Conway Castle Sandstone show a more diverse provenance, consistent with derivation from the Monian Supergroup. Cambrian sandstone from the Llanberis Slates Formation in the Arfon Basin shows a distinct provenance dominated by a Neoproterozoic source. None of the samples analyzed contains Laurentian detritus. These results suggest that the Welsh Basin was juxtaposed with the Monian Supergroup and its Precambrian substrate along the Menai Strait Fault System by the Tremadocian, and indicate that the Iapetus Ocean remained open at least until the Silurian. The Cambrian detrital zircon record from the Arfon Basin does not show clear similarity to the Monian Supergroup, nor to the Welsh Basin and adjacent Midland Platform, indicating that the basin was isolated from these sedimentary sources within the fault system. The juxtaposition of these terranes probably took place during strike-slip to transpressional tectonism close to the northern margin of Gondwana during the Early Ordovician
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