Taconic Orogen Research Articles

Ordovician–Silurian back-arc silicic magmatism in the southernmost Appalachians

Abstract Ordovician and Silurian siliciclastic, volcanic, and plutonic rocks in the southern Appalachian eastern Blue Ridge and western Inner Piedmont, formerly interpreted as components of an obducted Taconic arc terrane, are recognized as parts of an Ordovician paired arc/back-arc system that formed on the seaward edge of the Laurentian plate. New crystallization ages from metamorphosed plutonic-volcanic components intruding and intercalated with metasedimentary back-arc basin lithologies of the Wedowee-Emuckfaw-Dahlonega back-arc basin confirm rapid deposition (hundreds of meters per million years) in an Ordovician–Silurian extensional basin that received input from continentally derived Mesoproterozoic crust and bimodal volcanic components. U-Pb (206Pb/238U) zircon ages from 12 samples of Zana Granite yield minimum crystallization ages between 459 Ma and 430 Ma, while zircon grains from six samples of Kowaliga Gneiss have minimum crystallization ages between 452 Ma and 435 Ma. Zircons from a probable metavolcanic unit within the Wedowee Group yield a 206Pb/238U age of 454 ± 3 Ma. Mesoproterozoic depleted mantle model ages and negative initial εhf values of zircons from the Ordovician–Silurian metaigneous rocks suggest a significant Mesoproterozoic component in their genesis. Formation of these silicic plutons was likely associated with the intrusion of mantle-derived mafic melts and decompression melting of Mesoproterozoic lower crust as part of a tectonically thickened crustal section with a contribution from sub-continental mantle lithosphere during backarc extension. Silicic magma intruding thick successions of bimodal volcanic rocks and intercalated sedimentary rocks shortly after deposition is typical of back-arc extension and contraction phases above B-type subduction zones (i.e., Benioff oceanic subduction) in Lachlan-style orogens, which is consistent with the accretionary orogenic nature of the southern Appalachian Taconic orogen.

Platform drowning leading to cool-water carbonate deposition: evolution of a Late Ordovician (Turinian–Chatfieldian) mixed-sediment platform within the Taconic orogen (Long Point Group, Newfoundland Appalachians)

Late Ordovician (Turinian–Chatfieldian) drowning of a mixed carbonate–siliciclastic platform within the Taconic Orogen (Newfoundland Appalachians) is recorded by net deepening of an initial warm, shallow-water platform succession (Lourdes Formation) culminating in a metre-scale thick condensed interval that characterizes a drowning succession punctuated by storm deposits. Composition of transported material suggests that seaward drowning was coupled with back-stepping of a high-energy carbonate factory related to hinterland uplift and erosion that would eventually lead to drowning of the outer platform beneath marine-transported siliciclastic sediments (Winterhouse Formation). In the new offshore shelf setting, a sparse reciprocal stratigraphy of fine- to very coarse-grained phosphatic carbonate and mixed sediment is interpreted to document gravity-flow deposition downgradient from either a sustained or episodically developed high-energy cool-water carbonate source along the inner shelf. Transported carbonate was cemented rapidly at temperatures no warmer than 16 °C–23 °C, possibly within a seasonal oceanic thermocline. An upsection decrease in abundance of carbonate by the early Edenian is associated with a dramatic increase in siliciclastic supply. The Turinian–Edenian succession of platform drowning, oceanographic transition to cool-water carbonate production, and, later, its termination by increased siliciclastic supply reflects a first-order tectonic control proximal to uplift within the Taconic Orogen. Similar structural and oceanographic changes along the contemporary distal Laurentian margin provides the basis, with improved biostratigraphic control, for future analysis of the significance of proximal–distal stratigraphies in response to regional foreland tectonism.

Bridging the gap between the foreland and hinterland I: Geochronology and plate tectonic geometry of Ordovician magmatism and terrane accretion on the Laurentian margin of New England

U-Pb dates on magmatic and detrital zircon from samples in the hinterland of the Taconic orogen place new constraints on the timing and plate tectonic geometry of terrane accretion and magmatic arc activity. The Moretown terrane, a Gondwanan-derived exotic block, extends from the Rowe Schist-Moretown Formation contact in the west to the Bronson Hill arc in the east. Arc-related plutonic and volcanic rocks formed above an east-dipping subduction zone under the western leading edge of the Moretown terrane from approximately 500 to 475 Ma, until collision with hyperextended distal fragments of Laurentia, represented by the Rowe Schist, at 475 Ma. Magmatic arc rocks formed during this interval are primarily located in the Shelburne Falls arc, although some are also located in the Bronson Hill arc to the east. Metasedimentary rocks in the Shelburne Falls arc contain detrital zircon derived from mixing of Gondwanan, Laurentian, and arc sources, suggesting that the Moretown terrane was proximal to Laurentia by 475 Ma. Explosive eruptions at 466 to 464 Ma preserved in the Barnard Volcanic Member of the Missisquoi Formation in Vermont and as ash beds in the Indian River Formation in the Taconic allochthons may record slab-breakoff of subducted lithosphere following collision of the Moretown terrane with distal Laurentian crustal fragments. Between 466 and 455 Ma a reversal in subduction polarity lead to a west-dipping subduction zone under Laurentia and the newly accreted Moretown terrane. Magmatic arc rocks in the Bronson Hill arc formed above this west-dipping subduction zone along the eastern trailing edge of the Moretown terrane at approximately 455 to 440 Ma. The western boundary of Ganderia in New England is east of the Bronson Hill arc, buried beneath Silurian and Devonian rocks deformed during the Acadian orogeny.

Taconian orogenesis, sedimentation and magmatism in the southern Quebec-northern Vermont Appalachians: Stratigraphic and detrital mineral record of Iapetan suturing

Cambrian-Ordovician units of the southern Quebec Appalachians oceanic domain include obducted ophiolites and an overlying syncollisional sedimentary basin represented by the Saint-Daniel Mélange and flysch units of the Magog Group. These terrains were thrust onto the Laurentian continental margin during the Taconian orogeny and are unconformably overlain by Silurian-Devonian successor basins. This contribution presents and discusses new stratigraphic data acquired in the oceanic domain, as well as U-Pb and ⁴⁰Ar/³⁹Ar geochronological data on detrital zircons and muscovites sampled at various stratigraphic levels, from the base of the sedimentary units of the oceanic domain and the Silurian-Devonian units of the Gaspé Belt. This work, combined with the compilation of existing data for southern Quebec and western New England allows a better definition of the tectono-stratigraphic evolution of the area, and are used to propose correlations with different rock units of the northern New England Appalachians, which represents a collage of Laurentian margin and oceanic units. It is shown that Cambrian-Ordovician rocks of southern Quebec and northern Vermont have undergone a similar tectonic evolution. The ophiolites were obducted onto the Laurentian margin as a more or less coherent slab of oceanic lithosphere. Both the ophiolites and continental margin rocks were uplifted, eroded and unconformably overlain by Middle to Late Ordovician units of the Saint-Daniel Mélange and Magog Group in a forearc setting. The presence of mafic and felsic volcanic flows that are interstratified within the Saint-Daniel Mélange can be correlated with an episode of delamination and slab breakoff during the final stages of obduction and exhumation of the orogenic wedge. The Magog Group was deposited during the emplacement of the Taconian nappes, as the outboard volcanic arc was still active but progressively shutting down. The provenances suggested by the age of the dated detrital minerals can be entirely reconciled with the peri-Laurentian context of the studied rock units and testify of the tectonic evolution and sedimentary recycling of the Taconian orogen, from ophiolite obduction onto the Laurentian margin, to the collapse of the orogenic belt and the formation of successor basins.

Early to Middle Ordovician back-arc basin in the southern Appalachian Blue Ridge: Characteristics, extent, and tectonic significance

Fault-dismembered segments of a distinctive, extensive, highly allochthonous, and tectonically significant Ordovician (ca. 480–460 Ma) basin, which contains suites of bimodal metavolcanic rocks, associated base metal deposits, and thick immature deep-water (turbiditic) metasediments, occur in parts of the southern Appalachian Talladega belt, eastern Blue Ridge, and Inner Piedmont of Alabama, Georgia, and North and South Carolina. The basin’s predominantly metasedimentary strata display geochemical and isotopic evidence of a mixed provenance, including an adjacent active volcanic arc and a provenance of mica (clay)-rich sedimentary and felsic plutonic rocks consistent with Laurentian (Grenvillian) upper-crustal continental rocks and their passive-margin cover sequences. Geochemical characteristics of the subordinate intercalated bimodal metavolcanic rocks indicate formation in a suprasubduction environment, most likely a back-arc basin, whereas characteristics of metasedimentary units suggest deposition above Neoproterozoic rift and outer-margin lower Paleozoic slope and rise sediments within a marginal basin along Ordovician Laurentia’s Iapetus margin. This tectonic setting indicates that southernmost Appalachian Ordovician orogenesis (Taconic orogeny) began as an extensional accretionary orogen along the outer margin of Laurentia, rather than in an exotic (non-Laurentian) arc collisional setting. B-type subduction polarity requires that the associated arc-trench system formed southeast of the palinspastic position of the back-arc basin. This scenario can explain several unique features of the southern Appalachian Taconic orogen, including: the palinspastic geographic ordering of key tectonic elements (i.e., back-arc, arc, etc.), and a lack of (1) an obducted arc sensu stricto on the Laurentian margin, (2) widespread Ordovician regional metamorphism, and (3) Taconic klippen to supply detritus to the Taconic foreland basin.

The Role of Regional and Local Structure in a Late Ordovician (Edenian) Foreland Platform-to-Basin Succession Inboard of the Taconic Orogen, Central Canada

The Upper Ordovician (Edenian) Lindsay Formation of the Ottawa Embayment represents the final stage of carbonate platform development in the Taconic foreland periphery inboard of the northern Appalachian orogen. The succession overlies a narrow (~60 km) axis of a Neoproterozoic Laurentian rift extending across the Grenville orogen. The Lindsay Formation consists of a lower heavily bioturbated skeletal limestone that represents a warm-water shoal facies following an underlying outer ramp stratigraphy, and an upper division of renewed deep-water deposition with organic-rich shale and fossiliferous lime mudstone. Pyritic deep-water black shale of the westerly advancing Taconic foreland basin disconformably overlies this platform succession. Stratigraphic correlation through the central embayment identifies likely synsedimentary faults and seaward-directed erosion bounding the Lindsay Formation in a region of older Ordovician faults and a change in the lithotectonic character of the crystalline basement. The Late Ordovician shallowing and localization of structural/erosional features are interpreted to record a structural hinge: a local accommodation to, first, foreland periphery uplift, then rapid subsidence related to westerly diachronous foreland subsidence through the platform interior. Spatial association of structures of differing ages suggests that reactivation of inherited weakened crust influenced Late Ordovician sedimentary patterns.

Quartz arenites of the Cambro-Ordovician Kamouraska Formation, Quebec Appalachians, Canada: I. Deep-water depositional processes in a continental-slope environment

The Kamouraska Formation is an uppermost Cambrian – lowermost Ordovician quartz-arenite-dominated unit of controversial origin deposited on the southeastern slope of Laurentia bordering the Iapetus Ocean. It is exposed in the Quebec Appalachians on the south shore of the St. Lawrence Estuary. The formation consists of basal polymictic conglomerate and overlying massive sheet-like quartz arenite. The conglomerate beds are reversely and reversely to normally graded. The quartz arenite beds are generally massive, although they may show coarse-tail grading. Beds containing full or partial Bouma sequences are rare. Paleoflow directions from ripple-cross lamination, ripple marks on bed surfaces, and sole marks point towards southeast, south, and southwest. The clastic sediments of the Kamouraska were transported into the deep sea by sediment gravity flows that evolved from hyperconcentrated to concentrated density flows, and then to turbidity currents. The depositional environment is interpreted to have been a southwest-trending meandering submarine canyon. The exposed part of the canyon deposits is slightly oblique to the strike of slope. If correct, our interpretation establishes the preservation of continental-slope deposits in more distal deep-water siliciclastic sedimentary rocks of the Taconian orogen in Quebec, which traditionally have been interpreted as submarine-fan and (or) basin-plain deposits. The orientation of a canyon near parallel-to strike of the slope may have been controlled by syn-depositional growth faults. The coarsest hyperconcentrated flows, which deposited the conglomerate, were restricted to the deepest parts of the canyon during its early stages of development, whereas the concentrated density flows that deposited the massive quartz-arenite beds covered a wider area.

Arc–continent collision and the formation of continental crust: a new geochemical and isotopic record from the Ordovician Tyrone Igneous Complex, Ireland

Abstract: Collisions between oceanic island-arc terranes and passive continental margins are thought to have been important in the formation of continental crust throughout much of Earth's history. Magmatic evolution during this stage of the plate-tectonic cycle is evident in several areas of the Ordovician Grampian–Taconic orogen, as we demonstrate in the first detailed geochemical study of the Tyrone Igneous Complex, Ireland. New U–Pb zircon dating yields ages of 493 ± 2 Ma from a primitive mafic intrusion, indicating intra-oceanic subduction in Tremadoc time, and 475 ± 10 Ma from a light rare earth element (LREE)-enriched tonalite intrusion that incorporated Laurentian continental material by early Arenig time (Early Ordovician, Stage 2) during arc–continent collision. Notably, LREE enrichment in volcanism and silicic intrusions of the Tyrone Igneous Complex exceeds that of average Dalradian (Laurentian) continental material that would have been thrust under the colliding forearc and potentially recycled into arc magmatism. This implies that crystal fractionation, in addition to magmatic mixing and assimilation, was important to the formation of new crust in the Grampian–Taconic orogeny. Because similar super-enrichment of orogenic melts occurred elsewhere in the Caledonides in the British Isles and Newfoundland, the addition of new, highly enriched melt to this accreted arc terrane was apparently widespread spatially and temporally. Such super-enrichment of magmatism, especially if accompanied by loss of corresponding lower crustal residues, supports the theory that arc–continent collision plays an important role in altering bulk crustal composition toward typical values for ancient continental crust.

Facies development of a Late Ordovician mixed carbonate-siliciclastic ramp proximal to the developing Taconic orogen: Lourdes Formation, Newfoundland, Canada

The Upper Ordovician (late Whiterockian to Mohawkian) Lourdes Formation represents a narrow (tens of kilometers), short-lived [∼5–7 million years (my)], open-ocean (high-energy) mixed siliciclastic-carbonate ramp that onlapped allochthonous strata along the orogen side of the local Taconic foreland basin. Platform development followed a 6–8 my hiatus during which weathering had concentrated chemically mature siliciclastics that were admixed with initial carbonate sediments. A cross-platform facies gradient contains paleokarst and peritidal carbonates and sandstones, shallow-ramp carbonate bioherms and skeletal shoals, and deeper ramp calcareous shales. Transgressive systems tracts are marked by ramp-wide sheets and shoals of skeletal grainstone and low accumulation rates, and highstand systems tracts are marked by significant admixture and interbedding of siliciclastics with cross-ramp carbonate facies. Platform demise coincides with increased siliciclastic input, which is likely tectonically influenced. The Lourdes platform is equivalent to epicontinental foreland ramps along eastern Laurentia, but its narrower width precluded formation of oceanographically restricted platform-interior facies.

Dating polyphase deformation across low-grade metamorphic belts: An example based on 40Ar/39Ar muscovite age constraints from the southern Quebec Appalachians, Canada

Research Article| July 01, 2007 Dating polyphase deformation across low-grade metamorphic belts: An example based on 40Ar/39Ar muscovite age constraints from the southern Quebec Appalachians, Canada Sébastien Castonguay; Sébastien Castonguay 1Geological Survey of Canada, GSC-Québec, 490 rue de la Couronne, Québec G1K 9A9, Canada Search for other works by this author on: GSW Google Scholar Gilles Ruffet; Gilles Ruffet 2Geosciences Rennes, CNRS-Université de Rennes 1, Campus de Beaulieu, Avenue Général Leclerc, Rennes Cedex 35042, France Search for other works by this author on: GSW Google Scholar Alain Tremblay Alain Tremblay 3Département des Sciences de la Terre et de l'Atmosphère, Université du Québec à Montréal, C.P. 8888, Succursale Centre-Ville, Montréal, Québec H3C 3P8, Canada Search for other works by this author on: GSW Google Scholar GSA Bulletin (2007) 119 (7-8): 978–992. https://doi.org/10.1130/B26046.1 Article history received: 25 May 2006 rev-recd: 01 Feb 2007 accepted: 02 Feb 2007 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Sébastien Castonguay, Gilles Ruffet, Alain Tremblay; Dating polyphase deformation across low-grade metamorphic belts: An example based on 40Ar/39Ar muscovite age constraints from the southern Quebec Appalachians, Canada. GSA Bulletin 2007;; 119 (7-8): 978–992. doi: https://doi.org/10.1130/B26046.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Geochronologic 40Ar/39Ar data of fabric-forming metamorphic minerals, in conjunction with structural and metamorphic studies, are being increasingly used to constrain the deformation and tectonometamorphic evolution of polyphase low-grade orogens. Careful data interpretation is needed to extract meaningful age constraints from neo- and recrystallized minerals affected by isotopic disturbances. In the southern Quebec Appalachians, the Sutton Mountains anticlinorium exposes the metamorphic core of the early Paleozoic continental margin of Laurentia. Orogenesis in this part of the Appalachians was the result of tectonic events that have been classically attributed to the combined effects of the Middle to Late Ordovician Taconian and the Middle Devonian Acadian orogenies; however, evidence of separate and distinct Silurian–Early Devonian tectonism has also been recently documented. Laser step-heating 40Ar/39Ar data on single-grains of muscovite from polydeformed greenschist facies samples of the Sutton Mountains anticlinorium indicate that these tectonometamorphic events are heterogeneously preserved as a prograde Taconian event at ca. 456 Ma and an Acadian overprint at ca. 390 Ma. The integration of 40Ar/39Ar age spectra analyses with structural relationships provides precise age constraints on the duration, propagation, and evolution of Silurian–Early Devonian hinterland-directed deformation as it migrated across the anticlinorium, including back thrusting from ca. 433 Ma to ca. 420 Ma and extensional faulting from ca. 417 Ma to ca. 405 Ma. Along the Laurentian margin of the northern Appalachians, such Silurian–Early Devonian tectonism is currently attributed either to the collapse of the Taconian orogen triggered by the delamination of the subducted slab or to the outboard accretion of peri-Gondwanan terranes during the Salinic orogeny. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Diagenesis and provenance of Silurian quartz arenites in south-eastern New York State

Combined scanning electron microscopy — cathodoluminescence and optical microscopy (SEM-CL/OM) of single quartz grains was used to determine the diagenetic history and provenance of framework grains in quartz arenites of the Silurian Shawangunk Conglomerate and Binnewater Sandstone in south-eastern New York State. Most sand-sized sedimentary rocks of these two units experienced compaction and quartz cementation after deposition, except for those with high matrix or pseudomatrix content. No primary porosity is left today and secondary porosity is insignificant, leaving the Shawangunk Conglomerate and Binnewater Sandstone with poor reservoir qualities. The medium to coarse sand fraction of the Shawangunk Conglomerate and Binnewater Sandstone is dominated by quartz of plutonic origin. Of these usually sub-rounded to rounded grains, 10–15% carry remnants of inherited quartz cement, indicating recycling from a sedimentary source such as quartz-rich sedimentary rocks of the former Iapetus passive margin. In contrast, polycrystalline vein quartz pebbles in the Shawangunk Conglomerate were most likely derived from a proximal crystalline source in the Taconic orogen to the east. Formation of the Shawangunk Conglomerate and Binnewater Sandstone quartz arenites was likely supported by subtropical climatic conditions, given the latitudinal position of eastern North America during the Silurian, with relatively high mean annual temperatures and high precipitation rates, which accelerated the breakdown of unstable grains.

Late Shortening and Extensional Structures and Veins in the Western Margin of the Taconic Orogen (New York to Vermont)

Abstract Along the western margin of the Taconic orogen in New York and Vermont, undeformed quartz‐calcite veins commonly occur in the belt of melange that formed beneath the westward‐advancing Taconic Allochthon during the Middle‐Late Ordovician Taconic orogeny. The veins have mineral slickenfibers recording either reverse or normal slip. In New York, the reverse‐motion veins recording the latest phase of shortening are crosscut by the normal‐motion veins and faults. The shortening indicated by the reverse‐motion veins is correlated with the convergence along the Champlain thrust in Vermont, which is also crosscut by a significant, strike‐parallel normal fault. Fluid inclusion data from the veins, complemented by stable isotope data, lead to a reconstruction of the sequence of events in the context of a cooling of the fluids, which is consistent with crosscutting relationships among the veins. Following cessation of the convergence, there was regional extension of the western margin of the Taconic orogen...

Petrology and potential tectonic significance of a K-bentonite in a Taconian shale basin (eastern Ontario, Canada), northern Appalachians

A 6 cm thick K-bentonite, herein defined as the Russell Bed, occurs in an Upper Ordovician deep-basin shale succession in eastern Ontario, Canada, forming part of the distal Taconic foreland in eastern North America. The bed lies within the pygmaeus graptolite Biozone, which is about 451 to 452 Ma in age. Although some bentonites are reported from this interval in eastern North America, we are reporting the first set of compositional data for a bentonite of this age. Gamma-log correlation identifies a potential minimum distribution area of <2×105 km2 for the K-bentonite, covering part of southern Quebec, New York State and eastern Ontario. The deposit coincides with the first influx of distal turbdites into this shale basin, associated with Taconic flysch, and simultaneous abrupt ventilation of the once anoxic deep-water basin, which had formed initially after foundering of the Upper Ordovician carbonate platform. Concurrent intrabasinal extinction of several graptolite species suggests that change in sedimentation, palaeoceanography and volcanism were linked to a regional external process. Compositionally, the bentonite is distinct from the older Ordovician platform deposits in eastern North America. The deposit contains abundant titaniferous phlogopite with 1.6% BaO, fluoroapatite with 2.5% F, and dynamically shaped glass spherules now altered to clay. The spherules and clay matrix constitute 45% of the bed and, compositionally, define an illite (>90%)–smectite (I/S) structure with about 7.5% K2O%. Age-dating by Ar–Ar analysis of the phlogopite crystals yielded a younger than expected (440–445 Ma) age. This difference, along with evidence of localized chloritization of phlogopite, likely reflects known post-Ordovician hydrothermal activity within the basin. On the basis of several geochemical proxies, the magmatic source of the Russell K-bentonite falls within the trachyandesite field and was Ba-enriched. Comparison of geochemistry and mineralogy with older, Middle to Late Ordovician and younger Early Silurian K-bentonites within the Taconic orogen along eastern Laurentia and Baltica reveals that the potential source magma for the Russell Bed was more mafic, more alkaline, and less fractionated than sources typical of older (platform) bentonites. Instead, it is more similar to the younger Llandovery bentonites of Scandinavia and Scotland. It remains uncertain if it signals local or regional compositional change in volcanic source in the northern Appalachians.

A new stratigraphic framework for the Gaspé Belt in southern Quebec: implications for the pre-Acadian Appalachians of eastern Canada

The post-Taconian units in the Quebec and northern New Brunswick Appalachians constitute the Gaspé Belt and geological studies have mostly focussed on its eastern Quebec segment. Biostratigraphic data indicate that the succession in southern Quebec is no older than Late Silurian and extends into the Early Devonian. Two distinct stratigraphic assemblages are present. The first assemblage (Saint-Luc, Cranbourne, and Lac Aylmer formations, and Glenbrooke Group) unconformably overlies the Humber and Dunnage zones. The units show a basal alluvial conglomerate that passes progressively to deeper marine facies upsection, which have recorded a post-Late Silurian transgressive event. The second assemblage (Saint-Francis Group and Frontenac Formation) is faulted against either Dunnage units or autochthonous post-Taconian units. It locally unconformably overlies units of the Dunnage Zone; the succession shows progressively deeper marine conditions upsection and also has recorded a post-Late Silurian transgressive event. The biostratigraphic framework suggests that some of the units that were assumed to be vertically stacked are rather laterally equivalent. Independant evidence supports the hypothesis that the Gaspé Belt in southern Quebec formed after the collapse of the Taconian orogen in Late Silurian time. This event is ascribed to the Salinian Orogeny. The framework from southern Quebec is incorporated in a regional scenario. The Gaspé Belt experienced a Pridolian–Lochkovian sea-level rise. In Pragian time, shallower marine conditions were established in southern Quebec, whereas in the Gaspé Peninsula, the shallower conditions only occurred in early Emsian time.

Reconstruction of Taconian and Acadian paleostress regimes in the Quebec and northern New Brunswick Appalachians

A numerical analysis of fault populations was applied in the Quebec and northern New Brunswick Appalachians to characterize paleostress fields attributed to the Taconian and Acadian orogenies. The first brittle deformation documented in the external domain of the Humber Zone is associated with the thin-skinned tectonics of the Taconian orogen. The Taconian brittle deformation, characterized by north–south to NE–SW reverse conjugated brittle faults, evolved under a pure shear compressional regime (vertical σ3 axes) with σ1 axes oriented ESE–WNW. The ENE–WSW dextral and NW-SE sinistral faults that are observed in the St.Lawrence Lowlands are also attributed to Taconian compression. The first brittle deformation recognized in the Dunnage Zone and the Gaspé Belt is attributed to an ESE–WNW compression during the Acadian Orogeny. In the Dunnage Zone of southern Quebec, the Acadian paleostress axes are similar to the directions of σ1 axes and to the pure shear compressional regime computed for the Silurian and Devonian rocks of the Gaspé Belt. In the Gaspé Peninsula, the Acadian stresses are oriented ESE–WNW and are mechanically compatible with transpressional regimes. Reverse brittle faults and pure shear stress are identified along regional and secondary sets of NE–SW brittle faults, whereas a strike-slip stress regime is reconstructed along major east–west faults. In the Humber Zone of the Quebec Appalachians, a second compression is identified. It is characterized by conjugate ENE–WSW dextral and NW–SE sinistral strike-slip faults that crosscut Taconian thrust faults. This late deformation event is attributed to a distal expression of the Acadian Orogeny.

Tectonic setting of outer trench slope volcanism: pillow basalt and limestone in the Taconian orogen of eastern New York

The only pillow basalt in synorogenic sedimentary rocks at the exterior margin of the Taconic orogen in eastern North America is at Stark's Knob in eastern New York. Earlier reported as extrusive into allochthonous Ordovician slope and rise facies, this small lens (ca. 125+ m long, 39 m thick) is a fault-bounded block in Upper Ordovician melange under the Taconian frontal thrust. Its N-MORB (normal mid-ocean ridge basalt) basalt geochemistry and spinel composition are characteristic of oceanic ridge settings at a water depth of 2 km or more. Abundant limestone lenses on pillows and lava shelves within pillows yielded a middle Late Ordovician gastropod. The limestones are reconciled with this extrusion depth and with limited early Paleozoic pelagic carbonate production by lime mud transport from the Laurentian platform or abiotic carbonate precipitation with sea-water heating during basalt extrusion. A genetic relationship between the parautochthonous Stark's Knob basalts and the allochthonous Jonestown volcanics in slope and rise facies of the Hamburg klippe, eastern Pennsylvania, is likely. Both are Ordovician MORB basalts that reflect volcanism on the subducting outer trench slope prior to the Taconic arc – Laurentia collision. Taconic orogenesis may have led to basalt production on the subducting plate by (1) the setting up of orogen-parallel, predominantly strike-slip motion on the subducting slab with MORB basalt generated at offsets in a setting analogous to the Gulf of California or (2) development of faults in a flexure-induced extensional regime. By either process, mafic volcanism appears to be a rare but tectonically significant process on outer trench slopes as continental margins or oceanic plates enter subduction zones.

143Nd/ 144Nd and Sm/Nd stratigraphy of Upper Ordovician epeiric sea carbonates

Stratigraphic changes in the ε Nd of epeiric sea carbonates from central North America track the submergence history of the interior craton during the Late Ordovician. Fluctuations in sea level changed the Nd isotope balance of the epeiric sea by modifying the flux of Nd weathered from the highlands of the Taconic Orogen (ε Nd = −6 to −9) and from the low relief Precambrian basement (ε Nd = −22 to −15) of the Transcontinental Arch and Canadian Shield. Transgressions over the Arch and Shield, which diminished the weathering flux of Nd from the Precambrian basement, are recorded as positive shifts in the ε Nd profiles of carbonates. Negative ε Nd shifts reflect regression and reexposure of the Precambrian basement to erosion. Correlation of Upper Ordovician carbonates by use of the ε Nd profiles demonstrates the potential for Nd isotope stratigraphy. Comparison of stratigraphic variations in carbonate Sm/Nd ratios with sea level curves, conodont paleoecology, and the ε Nd profiles suggest that our observed variations in Sm/Nd ratios are related to changes in depth. Increasing Sm/Nd ratios correlate with increasing depth, whereas decreasing Sm/Nd ratios correlate with decreasing depth. This relationship between Sm/Nd ratios and depth suggests Sm/Nd profiles have potentially wide applications in understanding the paleoceanography of ancient epeiric seas.

Geochemistry and provenance of the Middle Ordovician Austin Glen Member (Normanskill Formation) and the Taconian Orogeny in New England

The Austin Glen Member of the upper Middle Ordovician Normanskill Formation is a sandstone‐shale flysch succession deposited in the foreland of the Taconian Orogen. Petrographic, major and trace element, and Nd–Pb isotopic data provide substantial constraints on its provenance. Lack of K‐feldspar and paucity of plagioclase, in addition to the dominance of sedimentary rock fragments, indicate that the source was dominated by recycled, sedimentary components. Major and trace element data support this conclusion and indicate that the provenance of both shales and sandstones was the same. No evidence of an ophiolitic or volcanic component was observed. Interpretation of Nd isotopic characteristics are complicated by a partial resetting of the Nd isotope system at about the time of sedimentation but indicate that the provenance of the Austin Glen Member had a long‐term history of light rare earth element (LREE) enrichment (average TDM = 1·8 Ga). Furthermore, Nd isotopic compositions are extremely homogeneous (ɛNd = –8·1 ± 0·6; 1 s.d.; n = 23) at 450 Ma, the approximate depositional age, indicating either a single source or very well‐mixed sources. 207Pb/204Pb ratios are variable but within the range of Pb isotopic compositions typically described as Grenvillian. The range of 207Pb/204Pb is greater than expected for the range of 206Pb/204Pb and suggests an additional component of Pb, possibly introduced during diagenesis. The immediate source of the Austin Glen Member may have been the accretionary prism that developed as older sediments of the Laurentian margin were scraped off the basin floor, incorporated within the accretionary prism and shed into the basin. No evidence indicating the arrival of an undifferentiated island arc or continental fragment during the Taconian Orogeny has been found. The data acquired during this study can be explained almost exclusively by Grenville Province source components but with possible additional contributions from older Laurentian terranes and Late Proterozoic rift volcanics that are not readily quantified but likely to have been minor. Accordingly, we conclude that the Taconian Orogeny in New England involved either: (1) a continental arc that involved exclusively Laurentia; (2) collision of a continental block with identical geochemical characteristics as Laurentia; or (3) essentially no detritus from any exotic colliding block (island arc or continental fragment) reached the foreland basin at the time of Austin Glen deposition.

Interpretation of ages of arc magmatism, metamorphism, and collisional tectonics in the Taconian Orogen of western New England

Interpretation of ages of arc magmatism, metamorphism, and collisional tectonics in the Taconian Orogen of western New England

Distribution and characteristics of Taconian and Acadian deformation, southern Québec Appalachians

The regional structure of the southern Quebec Appalachians is redefined in terms of superposed Taconian and Acadian deformation events. Taconian structures and metamorphism dominate in Cambro-Ordovician rocks west of the Baie Verte-Brompton Line (BBL). The Taconian orogen is subdivided into a northwestern external zone, composed of fault-imbricated continental rocks displaying single-phase foreland structures, and a south-eastern internal zone of polydeformed and metamorphosed continental and oceanic rocks. The structural relationships of Cambro-Ordovician rocks astride the BBL demonstrate that the continent-ocean contact was deformed by both southeast-verging late Taconian and upright Acadian folds. East of the BBL, syn- and post-Taconian rocks were mainly affected by Acadian deformation. The Acadian external zone consists of Cambro-Ordovician and post-Ordovician rock units characterized by a single phase of folding. The zone is bounded to the southeast by the Riviere Victoria fault. The Acadian internal zone displays polyphase deformation in the easternmost part of the Quebec Appalachians. The Acadian deformation shows a significant overlap with Taconian structures and metamorphism, and structural windows of the Taconian orogen are found within both the external and internal zones of the Acadian orogen.