Taconic Orogeny Research Articles

Lower Paleozoic foreland basins in eastern Canada: tectono-thermal events recorded by faults, fluids and hydrothermal dolomites

Abstract Dolomites in Lower and Middle Ordovician and Lower Silurian carbonates in Anticosti and Gaspe, respectively, are documented petrographically and geochemically to originate from early replacement and later void-filling phases, both derived from high temperature brines. These dolomite-rich intervals are interpreted as hydrothermal dolomites that formed during the early phases of foreland basin development along the continental margin of Laurentia; the Ordovician examples are related to the Taconian Orogeny whereas the Lower Silurian one is associated with the Salinic Orogeny. The dolomitization of the Lower and Middle Ordovician carbonates on Anticosti platform occurred shortly after the demise of shallow marine carbonate sedimentation in early Late Ordovician, at a time of significant increase of subsidence rates and associated higher geothermal gradients along the entire continental margin of Laurentia. Two geochemically different fluids are documented; the differences could be related to more pronounced rock-water interactions for the Lower Ordovician carbonates. The dolomitization of the Lower Silurian carbonates on Gaspe Peninsula is demonstrated to be an early event, pre-Late Silurian, which occurred during a significant magmatic period and likely high geothermal gradients. The Lower Silurian hydrothermal dolomites record significant exchanges between the high temperature fluid and the mafic to ultramafic basement that supplied large volumes of Mg+2 needed for this regional alteration. This paper illustrates the tectonic link between active foreland basin and hydrothermal dolomite. Moreover, the efficiency of fault-controlled circulation related to upward thermal convection of dolomitizing fluids is possibly enhanced at times of significant magmatic and thermal episodes along the continental convergent margins.

Formation of high‐pressure metabasites in the southern Appalachian Blue Ridge via Taconic continental subduction beneath the Laurentian margin

Tectonic models that account for the occurrence of eclogite, retrograde eclogite, peridotite, and migmatitic basement gneisses in the southern Blue Ridge invoke closure of a small ocean basin between the eastern Blue Ridge (EBR) and western Blue Ridge (WBR). In this “obducted ophiolite” model, Franciscan‐type, eastward subduction of oceanic crust beneath the Piedmont arc occurred prior to obduction of the arc onto the Laurentian margin. The discovery of kilometer‐scale eclogite bodies near the EBR‐WBR boundary are cited as evidence for obducted ophiolite/accreted island arc models. However, thermobarometric estimates based on mineral chemistry from HP metabasites of the western, central, and eastern Blue Ridge indicate that eclogites and granulites are medium‐temperature type (550–900°C), inconsistent with the obducted ophiolite model. Mid‐ocean ridge basalt subducted beneath continental margins and island arcs rarely reaches temperatures in excess of 550°C. Thermobarometric comparisons of the present study and a reexamination of deep seismic profiles from the Blue Ridge allow for a model of westward subduction of continental material beneath Laurentia during the Taconic orogeny that produced a megamélange now preserved in the central and eastern Blue Ridge tectonic provinces. P‐T paths determined from Blue Ridge HP metabasites are similar to P‐T paths determined previously from Caledonian and Variscan eclogites. Thermobarometric similarities imply a common tectonic history of continental collision, subduction of continental crust to depths of 50–65 km, followed by rapid eduction.

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.

Pre-Carboniferous, episodic accretion-related, orogenesis along the Laurentian margin of the northern Appalachians

Abstract During the Early to Middle Palaeozoic, prior to formation of Pangaea, the Canadian and adjacent New England Appalachians evolved as an accretionary orogen. Episodic orogenesis mainly resulted from accretion of four microcontinents or crustal ribbons: Dashwoods, Ganderia, Avalonia and Meguma. Dashwoods is peri-Laurentian, whereas Ganderia, Avalonia and Meguma have Gondwanan provenance. Accretion led to a progressive eastwards (present co-ordinates) migration of the onset of collision-related deformation, metamorphism and magmatism. Voluminous, syn-collisional felsic granitoid-dominated pulses are explained as products of slab-breakoff rather than contemporaneous slab subduction. The four phases of orogenesis associated with accretion of these microcontinents are known as the Taconic, Salinic, Acadian and Neoacadian orogenies, respectively. The Ordovician Taconic orogeny was a composite event comprising three different phases, due to involvement of three peri-Laurentian oceanic and continental terranes. The Taconic orogeny was terminated with an arc–arc collision due to the docking of the active leading edge of Ganderia, the Popelogan–Victoria arc, to an active Laurentian margin (Red Indian Lake arc) during the Late Ordovician (460–450 Ma). The Salinic orogeny was due to Late Ordovician–Early Silurian (450–423 Ma) closure of the Tetagouche–Exploits backarc basin, which separated the active leading edge of Ganderia from its trailing passive edge, the Gander margin. Salinic closure was initiated following accretion of the active leading edge of Ganderia to Laurentia and stepping back of the west-directed subduction zone behind the accreted Popelogan–Victoria arc. The Salinic orogeny was immediately followed by Late Silurian–Early Devonian accretion of Avalonia (421–400 Ma) and Middle Devonian–Early Carboniferous accretion of Meguma (395–350 Ma), which led to the Acadian and Neoacadian orogenies, respectively. Each accretion took place after stepping-back of the west-dipping subduction zone behind an earlier accreted crustal ribbon, which led to progressive outboard growth of Laurentia. The Acadian orogeny was characterized by a flat-slab setting after the onset of collision, which coincided with rapid southerly palaeolatitudinal motion of Laurentia. Acadian orogenesis preferentially started in the hot and hence, weak backarc region. Subsequently it was characterized by a time-transgressive, hinterland migrating fold-and-thrust belt antithetic to the west-dipping A–subduction zone. The Acadian deformation front appears to have been closely tracked in space by migration of the Acadian magmatic front. Syn-orogenic, Acadian magmatism is interpreted to mainly represent partial melting of subducted fore-arc material and pockets of fluid-fluxed asthenosphere above the flat-slab, in areas where Ganderian's lithosphere was thinned by extension during Silurian subduction of the Acadian oceanic slab. Final Acadian magmatism from 395– c . 375 Ma is tentatively attributed to slab-breakoff. Neoacadian accretion of Meguma was accommodated by wedging of the leading edge of Laurentia, which at this time was represented by Avalonia. The Neoacadian was devoid of any accompanying arc magmatism, probably because it was characterized by a flat-slab setting throughout its history.

Structural analysis of high‐pressure metamorphic rocks of Svalbard: Reconstructing the early stages of the Caledonian orogeny

The basement of Svalbard comprises several terranes that were assembled along NS‐trending faults during the Caledonian orogeny. The NW and NE parts of Svalbard are considered to be derived from east Greenland, while SW Svalbard has affinities with Pearya, Ellesmere Island. High‐pressure (HP) metamorphic rocks, such as eclogites and blueschists occur in west Svalbard and provide constraints on the history of the Caledonian convergence. The Motalafjella HP rocks display a geometry which is compatible with an initial dome structure sealed by Ordovician deposits. Retrogressed eclogites in Biscayarhalvøya appear in the uppermost unit of a nappe stack that formed in a constrictional deformation regime. The syncollisional exhumation of the eclogites was achieved before the deposition of Late Silurian conglomerates. The two field occurrences of HP rocks are interpreted with a detailed age review for the Svalbard terranes and a restoration of Svalbard paleogeography in the Devonian. We propose that the different tectonic events are part of a continuous Ordovician‐Silurian orogenic sequence, predating the Scandian collision between Laurentia and Baltica. The closure of an ocean between Laurentia and a microplate was responsible for the formation and exhumation of the HP rocks of Motalafjella in the Ordovician, and later followed by the collision of the microplate with Laurentia in the Silurian. This orogenic phase can be considered as an equivalent of the Taconic orogeny in the Appalachians. The relative position of the Svalbard terranes and Greenland during this orogenic episode is constrained by strain directions measured in the field.

A 40Ar/39Ar study of oceanic and continental deformation processes during an oblique collision: Taconian orogeny in the Quebec reentrant of the Canadian Appalachians

Two phases of penetrative deformation are documented in the Taconian hinterland of the Appalachian orogen in the Gaspé Peninsula. D1 is associated with the obduction of the Mont‐Albert ophiolite onto the Paleozoic Laurentian margin, whereas D2 corresponds to later transport of allochthons across the margin. In the metamorphic sole, S1 is a SE‐dipping mylonitic fabric with a downdip lineation. In underlying metabasalts, D1 is characterized by NW‐overturned and recumbent folds, and a subhorizontal S1 schistosity with an ENE‐trending orogen‐parallel lineation. D2 is characterized by a S2 steeply dipping penetrative axial‐planar crenulation cleavage and NE‐trending F2 folds. The intraoceanic thrusting of ophiolite is dated at 465 Ma (early D1) whereas emplacement of ophiolite and subsequent deformation of the margin was recorded by isotopic signatures between 459 and 456 Ma (late D1). D2 is dated at 448 Ma throughout the hinterland. Taconian transpressive deformation is related to an oblique collision within the Quebec reentrant of the Canadian Appalachians during the Ordovician.

Age, origin, and tectonic significance of Mesoproterozoic and Silurian felsic sills in the Berkshire massif, Massachusetts

Discontinuous sills of felsic gneiss in the interior and western margin of the Berkshire massif and granite sills on the eastern margin of the massif were correlated by Ratcliffe (1984a, 1984b, 1985) and Ratcliffe and Hatch (1979), and interpreted by them as syntectonic anatectic melts that intruded Taconic thrusts. We dated three felsic gneiss sills and two granite sills in an attempt to constrain the age of Taconic thrusting, but discovered that the sills are not coeval. Rather, they were intruded during two widely separated episodes, one during the Mesoproterozoic at approximately 1000 Ma and the other during the Silurian at approximately 430 to 435 Ma. The 1000 Ma sills of felsic gneiss in the interior of the massif are located in Mesoproterozoic units and many of the mapped Taconic thrusts within the massif closely follow the distribution of this unit, here informally called the felsic gneiss of Harmon Brook. These 1000 Ma sills formed during the Ottawan or Rigolet orogeny and they have no connection to the Taconic orogeny. The 430 to 435 Ma granite sills along the eastern margin of the massif, informally called the granite of Becket Quarry, are found in both Mesoproterozoic basement and the Neoproterozoic Hoosac Formation. The sills are too young to have intruded during the Ordovician Taconic orogeny, but they may have formed during later faulting near the contact between Mesoproterozoic basement and Neoproterozoic cover rocks. The Tyringham Gneiss is one of the most common Mesoproterozoic units in the Berkshire massif. Zircons from the Tyringham Gneiss contain cores with oscillatory zoning and thin unzoned rims. The weighted average of eight 206 Pb/ 238 U analyses from the cores is 1179 /- 9 Ma, whereas nine spot analyses from the rims yield an age of 1004 /- 9 Ma. We interpret these two ages to represent the crystallization of the Tyringham Gneiss protolith and a subsequent high grade metamorphism, coeval with the intrusion of the felsic gneiss of Harmon Brook. The western contact between Mesoproterozoic rocks of the Berkshire massif and underlying Early Paleozoic rocks is clearly a thrust, but there is no independent evidence that movement occurred during the Taconic orogeny; displacement may also have occurred during the Silurian Salinic or the Devonian Acadian orogeny. Many contacts mapped as Taconic thrusts within the Berkshire massif follow the distribution of the 1000 Ma felsic gneiss of Harmon Brook. The age of the sills is clearly incompatible with this interpretation, and evidence for faulting along these mapped thrusts is lacking. Instead of being deformed into an imbricate stack, the massif behaved as a rigid block during Paleozoic uplift. Finally, the age of granite sills along the eastern margin of the massif does not constrain the basement-cover contact to be a Taconic thrust, as previously interpreted. The contact may be a Silurian fault, possibly related to extension and the opening of the Connecticut Valley trough as a back-arc basin. According to this model, the magma for the granite sills was generated above a west-dipping subduction zone under the Laurentian margin, which developed after the Taconic orogeny.

Taconic Orogeny in Pennsylvania: A ∼15 –20 m.y. Apennine-style Ordovician event viewed from its Martic hinterland

In Pennsylvania, the Taconic Orogeny lasted from ∼461 to ∼443 Ma as Cambro-Ordovician slope deposits were deformed into mountains edging the Laurentian craton at the same time that materials from an adjacent deep-water basin were being transported ∼50 –70 km across a carbonate platform into foreland basins. This paper focuses on shelf-edge hinterland features, mostly the Martic Zone as a folded, stack of imbricate thrust sheets of slope materials that corresponds to Vermont's Taconic Mountains and Southern Quebec's zone of Taconic allochthons. Work of the last century is summarized, corrected, and combined with a new ∼450 Ma radiometric date and fluid inclusion data from the Pequea Mine within the Martic Zone. These and abundant new graptolite and conodont dates in the foreland paint a revised Pennsylvania picture differing from the northern Taconic areas. Differences are: (1) transport of very large allochthonous masses of deep-water material, the Dauphin Formation, far across the carbonate platform, and (2) deformation migrating progressively across that platform during a ∼15 –20 m.y. period, incorporating it and its foreland cover into alpine-scale, recumbent folds and thrusts. The scenario has many analogies to Italy's modern Apennine Mountains minus the Latian volcanics.

The petrogenesis of the Highlandcroft and Oliverian Plutonic Suites, New Hampshire: Implications for the structure of the Bronson Hill terrane

Both Laurentian and peri-Gondwanan origins for the Oliverian and Highlandcroft Plutonic Suites of New Hampshire have been proposed. A peri-Gondwanan origin places the Red Indian Line, the principle Iapetus suture, to the west of the Bronson Hill Anticlinorium and requires the plutons to be allochthonous. In contrast, a Laurentian margin emplacement setting places the Red Indian Line farther to the east in New Hampshire. The combination of ε~Nd~ and ^207^Pb/^204^Pb values are most compatible with the suites representing mixtures of arc-derived magmas and Laurentian crust, indicating that both suites were derived to the west of the Red Indian Line. The tectonic setting for the arc probably was a westerly dipping subduction zone under the Laurentian margin, resulting from a polarity flip after the collision of the Shelburne Falls arc during the Taconic orogeny. Bulk-rock major and trace elements, as well as Nd, Sr, and Pb isotopic results, indicate that the Highlandcroft and Oliverian Plutonic Suites of New Hampshire are both members of the same arc complex.

Stratigraphic and Structural Framework of Upper Middle Ordovician Rocks in the Head Lake-Burleigh Falls Area of South-Central Ontario

Field investigations in the Head Lake-Burleigh Falls area of south-central Ontario, that focused mainly on the Covey Hill(?), Shadow Lake, Gull River and Coboconk formations along the Paleozoic escarpment, provided clear evidence of faulting. Observed and inferred structural deformation, coupled with faciès changes within the Shadow Lake and lower Gull River, point to a succession of basement movements during the Phanerozoic. These range from Hadrynian-Early Cambrian, through Middle Ordovician to post-late Middle Ordovician times. Some of the earlier movements (Hadrynian-Early Cambrian to late Middle Ordovician) appear to be coincident with, and probably related to, plate tectonic events and the associated Taconian orogeny that were in progress along the southeastern margins of the North American continent. Post-Middle Ordovician block faulting, on the other hand, may have been triggered by any number of epeirogenic events related to late stage Taconian, Acadian or Alleghanian orogenies in Late Ordovician to Carboniferous times, or possibly to rifting associated with continental break-up and initiation of seafloor spreading processes in the early Mesozoic. Manuscrit révisé accepté le 5 août 1993

Distinct Taconic, Salinic, and Acadian deformation along the Iapetus suture zone, Newfoundland Appalachians

Structural mapping in central Newfoundland has identified seven distinct phases of deformation (D1 to D7), the most significant of which are D1, D2, and D4. D1 involved the formation of a Middle and Late Ordovician south-southeast-directed thrust belt and concomitant development of mylonite and phyllonite. A Late Ordovician to Early Silurian D2 thrust and fold belt overprints D1 mylonitic deformation and is the most distinctive deformation event in the study area. Late Silurian to Devonian D4 is responsible for folds and north-northwest-directed dextral thrust and reverse faults that overprint D1 to D3 structures. D4 structures in central Newfoundland include the Exploits–Gander boundary. Subsequent deformation is generally of local significance only. The arc–back-arc complexes making up the various terranes in central Newfoundland are predominantly juxtaposed along D1 shear zones, which include the Red Indian Line. Our data indicate that terrane boundaries initiated during D1 may have protracted deformation histories spanning several deformation events. This has important implications for the interpretation of terrane boundaries in Newfoundland, as D1 terrane boundaries may be interpreted as D2 or D4 shear zones depending on the intensity of overprinting or reactivation. The deformation history proposed in this paper corresponds closely to that of established Appalachian orogenic cycles. D1 is correlated with the Ordovician Taconic orogeny and involved accretion of arc–back-arc complexes to the Laurentian margin. D2 and D4 are correlated with the Ordovician–Silurian Salinic and Silurian–Devonian Acadian orogenies, which involved the subsequent accretion of the Ganderia and Avalonia microcontinents to the Laurentian margin, respectively.

K–Ar age constraints on the evolution of polydeformed fold–thrust belts: The case of the Northern Appalachians (southern Quebec)

Precise geometric and temporal constraints are essential for understanding the evolution of fold–thrust belts. The lack of isotopic dating for shallow faults in the Northern Appalachian Taconic allochthons limits the interpretation of their structural evolution. New and more precise kinematic constraints have been obtained using a structural study that combined field work with the mineralogical, morphological and K–Ar isotopic analysis of clay-rich fractions (<0.2 and 1–2 μm) from fault rocks and host lithologies in selected fault zones within the southern Quebec allochthons. The validated K–Ar data define four age groups at ca. 490, 465–450, 410 and 360 Ma, which are, respectively, referred to as D1 and D2 Taconic deformation pulses, D3 Late Silurian–Early Devonian deformation, and D4–5 Acadian deformation. The recognition of discrete contractional pulses during the Taconic Orogeny suggests that folding and thrusting were initiated during Early Ordovician time (ca. 490 Ma) and culminated with the out-of-sequence imbrication of thrust stacks during Middle (ca. 465 Ma) to Late Ordovician time (ca. 450 Ma). The Silurian–Early Devonian subsidence (ca. 410 Ma) of the Appalachian foreland along wrench and normal fault systems occurred simultaneously with D3 hinterland-directed tectonic transport at the boundary between the external and internal Humber zones. Late Devonian Acadian imbrications (D4) in the Taconic allochthons of the Quebec Appalachians are reported here for the first time. The combination of a D4 contractional pulse and Silurian–Early Devonian D3 back-thrust faulting caused the exhumation of low-grade metamorphic rocks affected by an Early Devonian thermal event. Across the Laurentian margin, the Acadian contraction evolved from Middle Devonian deformation in the internal metamorphic domain to Late Devonian deformation in the allochthons. The relaxation of Acadian tectonic stress is characterized by D5 normal faults and related hydrothermal activity. The recognition of an Acadian contraction event in the southern Quebec Appalachians challenges the traditional view of a gradual NW-directed progression of faulting in the Taconic thrust belt, and allows for a new reconstruction that bridges the Appalachians of mainland Canada and Newfoundland.

Resolving the timing of orogenesis in the Western Blue Ridge, southern Appalachians, via in situ ID-TIMS monazite geochronology

Both Taconian (ca. 450 Ma) and Acadian (ca. 380 Ma) ages of high-grade deformation and metamorphism have been advocated for the Western Blue Ridge, southern Appalachians, United States. To resolve this debate, metamorphic monazite from the Great Smoky Mountains was dated via in situ microsampling plus isotope dilution–thermal ionization mass spectrometry (ID-TIMS). U-Pb ages are 450 ± 5 Ma (±2σ error), and mean 207 Pb/ 206 Pb ages are 453 ± 10 Ma. These data indicate that peak metamorphism and high-grade deformation occurred during the Taconian orogeny, although they do allow for a low-grade (sub-amphibolite-facies) Acadian overprint. Unresolved issues remain regarding the comparability of U-Pb TIMS ages with those obtained via other methods.

Three-dimensional seismic-based definition of fault-related porosity development: Trenton�Black River interval, Saybrook, Ohio

Oil and gas reservoirs of the Ordovician Trenton–Black River interval in the Appalachian Basin are commonly associated with fault-related hydrothermal dolomites. However, relationships between porosity development and fault geometry in these fields are poorly documented. In this article, we integrate three-dimensional (3-D) seismic and wire-line data from the Trenton–Black River interval at Saybrook field in northeastern Ohio to study relationships between faulting and porosity development there. Faults were mapped using a combination of amplitude and coherency versions of the seismic data, and a 3-D porosity volume was generated for the Trenton–Black River interval by integrating attributes derived from the seismic data with log-based measures of porosity. The productive trend in the Trenton–Black River interval at Saybrook is controlled by a 3.4-mi (5.5-km)-long, northwest-southeast–oriented basement fault that was probably reactivated during the Taconic orogeny (i.e., Late Ordovician). Strike-slip movement along the fault generated en echelon synthetic shear faults that branch at least 1350 ft (411.5 m) upward into the Trenton–Black River interval. The best porosity is developed in areas between overlapping synthetic shear faults. Antithetic shear faults probably formed at these locations and, when combined with minor dip-slip movement, created conduits for subsequent porosity-generating fluids. Circular collapse structures associated with localized extension between overlapping shear faults are the primary drilling targets, and horizontal wells running parallel to the strike of the fault would have the best chances of intercepting good porosity development.

Feedback between deformation and magmatism in the Lloyds River Fault Zone: An example of episodic fault reactivation in an accretionary setting, Newfoundland Appalachians

The Lloyds River Fault Zone is a 10–15 km wide amphibolite‐grade shear zone that formed during the Ordovician Taconic Orogeny. It separates ophiolites and arc–back‐arc complexes formed in Iapetus from a peri‐Laurentian microcontinent (Dashwoods microcontinent). The Lloyds River Fault Zone comprises three high‐strain zones, dominantly composed of mylonitic amphibolites, separated by less deformed plutonic rocks. Structural, age and metamorphic data suggest the Lloyds River Fault Zone accommodated sinistral‐oblique underthrusting of ophiolites underneath the Dashwoods microcontinent prior to 471 ± 5 Ma at 800°C and 6 kbar. Plutonic rocks within the Lloyds River Fault Zone comprise two suites dated at 464 ± 2 plus 462 ± 2 and 459 ± 3 Ma, respectively. The younger age of the plutons with respect to some of the amphibolites, evidence for magmatic deformation, and the elongate nature of the plutons parallel to the Lloyds River Fault Zone suggest they were emplaced within the fault zone during deformation. Both intrusive episodes triggered renewed deformation at high temperatures (770–750°C), illustrating the positive feedback between deformation and magmatism. Offshoots of the plutons intruded undeformed ophiolitic gabbros outside the Lloyds River Fault Zone. Deformation localized within the intrusive sheets, coeval with static contact metamorphism of the host gabbros, leading to the development of new, small‐scale shear zones. This illustrates that channeling of plutons into shear zones and nucleation of shear zones in melt‐rich zones may occur simultaneously within the same fault system.

Precambrain Fault Systems as Control on Regional Differences in Relative Sea Level Along the Early Ordovician Platform of Eastern North America

Abstract This study proposes that there was regional variation in Early Ordovician subsidence histories along eastern Laurentia controlled by broad differences in the susceptibility of inherited Precambrian crustal structure to reactivation with changing ocean-basin tectonism. Two continental-scale Phanerozoic fault systems (Southern Oklahoma Fault System, Ottawa–Bonnechere Graben) extend into the craton interior from the apices of the Ouachita and Quebec embayments or reentrants, respectively. Their traces identify reactivated Proterozoic fault zones. Previous studies show that rates of subsidence for Early Ordovician sedimentation along the axis of the Southern Oklahoma Fault System are elevated compared to regional rates beyond this structural corridor. A similar history applies to Early Ordovician sedimentation within the Ottawa Embayment (eastern Ontario), along the present axis of the Ottawa–Bonnechere Graben. The Theresa Formation (lower Beekmantown Group) contains a third-order peritidal (siliciclastic-dominated) sequence. Evidence for local tectonism prior to and during its development within the embayment includes: (1) Late Cambrian (pre-Theresa) preferential uplift, differential erosion, and subsidence along the eastern structural boundary of the embayment; (2) differential intrabasinal subsidence during Theresa deposition; (3) an apparent offset in timing of peak flooding within the embayment to that along the St. Lawrence Promontory, ~ 1500 km distant; and (4) absence of a subaerial unconformity in the embayment that otherwise separates eustatic cycles on the promontory, and defines a likely sea-level fall below the shelf margin. In comparison, subsidence within the embayment appears to have matched (at least) the rate of the eustatic fall. Regional differences in Early Ordovician subsidence histories along this margin may illustrate that the Precambrian fault zones were structurally more sensitive than promontories to the developing reconfiguration of oceanic lithospheric elements that would lead to the Taconic orogeny.

Lower to Middle Ordovician evolution of peri-Laurentian arc and backarc complexes in Iapetus: Constraints from the Annieopsquotch accretionary tract, central Newfoundland

The Annieopsquotch accretionary tract in Newfoundland is composed of a series of west-dipping structural panels, each containing remnants of ophiolitic and arc-backarc complexes of Laurentian affinity formed during the Ordovician closure of Iapetus. Panels were transferred from an upper-plate to a lower-plate setting during their Middle to Late Ordovician accretion to the Laurentian margin and become progressively younger eastward. Geochronological data indicate a complex and rapid history of generation and accretion of peri-Laurentian suprasubduction zone rocks. The rapid changes in tectonic environments and the complexity of the relationships are analogous to the complex arc-backarc relationships observed in the western Pacific today. The recognition of the peri-Laurentian provenance of these units based on stratigraphy, geochronology, isotopes, and geochemistry defines the position of the Red Indian Line, the fundamental suture zone in the northern Appalachians, but more importantly enables the development of a realistic tectonic model for the Annieopsquotch accretionary tract involving both thrust and sinistral transcurrent displacements. The oldest and most inboard unit in the Annieopsquotch accretionary tract is the Annieopsquotch ophiolite belt (ca. 480 Ma), which marks the initiation of subduction outboard of the Laurentian margin. The Lloyds River ophiolite complex (ca. 473 Ma) preserves a fragment of younger, more mid-ocean-ridge–like backarc-oceanic crust than the adjacent, structurally overlying Annieopsquotch ophiolite belt. The Lloyds River ophiolite complex originated as a backarc to the Buchans Group (ca. 473 Ma) ensialic bimodal calc-alkaline arc. The panels containing the Annieopsquotch ophiolite belt and Lloyds River ophiolite complex were stitched and overlain by ensialic arc rocks of the Otter Pond Complex (ca. 468 Ma) immediately after their accretion to composite Laurentia together with the structurally underlying Buchans Group. The youngest, structurally lowest two panels comprise the elements of the Red Indian Lake group (465–460 Ma), which record the opening of a backarc basin and the subsequent establishment of a bimodal ensialic calc-alkaline arc sequence. The observed relationships indicate that the Annieopsquotch accretionary tract was generated above a single west-dipping subduction zone outboard of the Laurentian margin over ∼20 m.y. Accretion mainly took place in two stages at ca. 468 and 450 Ma, which correspond with the collision between Laurentia and the Dashwoods ribbon continent and the collision with the peri-Gondwanan Victoria Arc along the Red Indian Line, respectively. Both collisions form part of the Taconic orogeny. The latter, Late Ordovician collision terminated the relatively rapid closure of the main Iapetan tract. The proposed model is similar to the correlative tracts in the British and Irish Caledonides, and may encourage a new look at the New England Appalachians.

Syncollisional basin development in the Appalachian orogen--The Saint-Daniel Melange, southern Quebec, Canada

The Saint-Daniel Melange is an orogen-scale sedimentary basin of the Quebec Appalachians that is commonly interpreted as the remnant of a subduction complex formed during the Taconian orogeny. However, geochronological, structural, and stratigraphical data from Laurentian continental margin rocks and adjacent ophiolitic rocks in southern Quebec indicate that the Saint-Daniel Melange is an olistostromal, syncollisional piggyback basin that represents the base of the Magog Group forearc basin. The regional stratigraphic framework of the Saint-Daniel Melange and its relationships with underlying and overlying rock units have been established on the basis of six stratigraphic sections from the Thetford-Mines, Asbestos, and Mont-Orford ophiolitic complexes. Our results imply that (1) the Saint-Daniel Melange is a sedimentary sequence that unconformably overlies different structural and pseudostratigraphic levels of the southern Quebec ophiolites; (2) it is made up of four distinct and laterally discontinuous units that record the obduction of ophiolites onto Laurentia and exhumation of basement rocks, followed by subsidence and deposition of the overlying Saint-Victor Formation of the Magog Group; (3) the sedimentary rocks of the Saint-Daniel record a transition from ophiolite-dominated to continental sources, indicating progressive exhumation of both the ophiolite and the continental margin upon which it was obducted; 4 0 Ar/ 3 9 Ar analysis of muscovite from metamorphic rock fragments in debris flows of the melange yield an age of 467 ′ 2 Ma, which is within the range of Ar ages measured in metamorphic rocks that underlie the ophiolites, and indicates the uplift of metamorphosed continental deposits during or shortly after obduction; (4) the Saint-Daniel Melange is stratigraphically overlain by the Magog Group and represents the base of a syncollisional basin developed in a forearc setting during the Taconian orogeny.

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...

Darriwilian graptolites of the Hamburg succession (Dauphin Formation), Pennsylvania, and their geologic significance

Graptolites from the Dauphin Formation in the allochthonous Hamburg succession of the Appalachians in Pennsylvania, USA, are late Darriwilian (Da) 3 to early Da 4 age (Middle Ordovician); this age range constrains the timing of the latest depositional episode before the terrane was tectonically mobilized. These rocks were emplaced into the Martinsburg foreland basin of Laurentia during the Taconic orogeny in the early Caradoc (Late Ordovician). Nineteen taxa are described defining a narrow biostratigraphic interval. Among the characteristic fauna collected from of the Da 4 Zone are Pterograptus elegans Holm, Cryptograptus schaeferi Lapworth, Hustedograptus teretiusculus (Hisinger) ?, Haddingograptus oliveri (Bouček), Glossograptus hincksii (Hopkinson), Pseudophyllograptus angustifolius s.l. (J. Hall), and Archiclimacograptus cf. riddellensis (Harris). Tetragraptus cf. erectus Mu et al. found with the above suggests a level low in the Da 4 Zone and a limited occurrence of Bergstromograptus crawfordi (Harris) may indicate some strata within the Da 3 Zone. Proposed new forms include Pseudotrigonograptus ? ricardo sp. nov., and at least two reteograptids. Four examples of Kalpinograptus and Kalpinograptus ? may be new.