Neil Rogers

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 Ganderians 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.

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.

Pressure-temperature paths and exhumation of Late Ordovician–Early Silurian blueschists and associated metamorphic nappes of the Salinic Brunswick subduction complex, northern Appalachians

The Late Ordovician–Early Silurian Brunswick subduction complex in northern New Brunswick preserves a nearly intact southeast-facing forearc terrane with clear links among Silurian subduction, under-plating, exhumation, and forearc sedimentation. The forearc terrane grew over time due to successive accretion and underplating of seamounts and isolated ribbons made up of continental and transitional crust concomitant with foreland migration of the trench. A sliver of strongly deformed epidote-blueschists derived from an accreted seamount was stacked together with slightly lower-pressure winchite-bearing tectonites and medium- to high-pressure greenschists of the Fournier, California Lake, and Tetagouche blocks into a series of D 1 metamorphic nappes during Salinic closure of the Tetagouche-Exploits backarc basin. The blueschists (~375 °C, 7.2 kbar) locally preserve mineralogical (zoning from actinolite or barroisite core to a rim of glaucophane) and microstructural evidence for a counterclockwise pressure-temperature ( P-T ) path ascribed to underthrusting beneath the young oceanic lithosphere of the Fournier block shortly after the inception of subduction at ca. 450 Ma. The next phase of accretion is represented by underplating of the California Lake block at ca. 442 Ma. This may have caused emplacement of the blueschists above the slightly lower metamorphic grade Canoe Landing Lake winchite-bearing nappe (350 °C, 6.5 kbar). Underplating of the Tetagouche block at ca. 435 Ma extruded the blueschist sliver and underlying nappes of the California Lake block immediately above greenschist-facies nappes (~5.8 kbar) and beneath lower-pressure (4–5 kbar) greenschist-facies basalt and gabbro of the ophiolitic Fournier block, which was deformed and imbricated earlier, probably during accretion of the California Lake block. Shear sense indicators confirm that the hanging-wall shear zone of the blue-schist sliver accommodated Early Silurian normal motion. D 1 is generally time-trans-gressive due to down-stepping of the under-plating-related deformation. Metamorphic relicts, as well as zoning in amphibole and epidote, indicate that the Canoe Landing Lake and other nappes underwent clockwise P-T paths related to subduction of the cool oceanic lithosphere and/or accretion of the California Lake and Tetagouche blocks.

Upper cambrian to upper ordovician peri-gondwanan island ARC activity in the victoria lake supergroup, central newfoundland : Tectonic development of the Northern Ganderian Margin

The Exploits Subzone of the Newfoundland Appalachians comprises remnants of Cambro-Ordovician peri-Gondwanan arc and back-arc complexes that formed within the Iapetus Ocean. The Exploits Subzone experienced at least two accretionary events as a result of the rapid closure of the main portion of the Iapetus tract: the Penobscot orogeny (c. 480 Ma), which juxtaposed the Penobscot Arc (c. 513 –486 Ma) with the Gander margin, and c. 450 Ma collision of the Victoria Arc (c. 473 –454 Ma) with the Annieopsquotch Accretionary Tract that juxtaposed the peri-Laurentian and peri-Gondwanan elements along the Red Indian Line. The newly recognized Pats Pond Group forms a temporal equivalent to other Lower Ordovician intra-oceanic complexes of the Penobscot Arc. The Pats Pond Group (c. 487 Ma) has a geochemical stratigraphy that is consistent with rifting of a volcanic arc. An ensialic setting is indicated by low εNd values (εNd 0.3 to -0.5) near the stratigraphic base and its abundant zircon inheritance (c. 560 Ma and 0.9 –1.2 Ga). The spatial distribution of Tremadocian arc –back-arc complexes indicates that the Penobscot arc is best explained in terms of a single east-dipping subduction zone. This model is favored over west dipping models, in that it explains the distribution of the Penobscot arc elements, continental arc magmatism, and the obduction of back-arc Penobscot ophiolites without requiring subduction of the Gander margin or subduction reversal. The newly recognized Wigwam Brook Group (c. 454 Ma) disconformably overlies the Pats Pond Group and records the youngest known phase of ensialic arc volcanism (εNd –4.1) in the Victoria Arc, which is also related to east-dipping subduction. Thus the Penobscot and the overlying Victoria Arc are reinterpreted in terms of a single, relatively long-lived east-dipping subduction zone beneath the peri-Gondwanan microcontinent of Ganderia. The cessation of arc volcanism towards the top of the Wigwam Brook Group and the subsequent syn-tectonic sedimentation in the Badger Group constrain the arrival of the leading edge of Ganderia with the ensialic arc complexes to the Laurentian margin to c. 454 Ma.

The age of salinic deformation constrained by 40Ar/39Ar dating of multiple cleavage domains: Bathurst Supergroup, New Brunswick Appalachians

In the New Brunswick Appalachians, polydeformed felsic volcanic rocks of the Bathurst Supergroup record four cleavage-forming tectonic events, of which D1 and D2 record subduction-related underplating of buoyant elements of the Tetagouche backarc basin and subsequent collision between composite Laurentia and the Gander margin, respectively, during the Salinic orogenic cycle. Here we present an integrated approach to dating multiple cleavage domains in which we performed step heat 40Ar/39Ar analyses, microstructural observations and mineral-chemistry analysis on a suite of samples, as well as in situ 40Ar/39Ar analysis on two samples from the suite with clear S1 and S2 cleavage relationships. We use this dataset to characterize the complex relationships between S1 and S2 white mica between samples at different structural settings across the supergroup. We refine the timing of S1 white mica growth, and hence M1-D1 of the Salinic cycle in the Bathurst Supergroup to ca. 452 to 437 Ma, and S2 white mica growth (also D2) to ca. 427 to 418 Ma. New and published data indicate local thermal resetting of white mica at ca. 411 Ma, which we interpret to indicate a buried intrusion, likely a component of the Central plutonic belt. Delineation of the two white mica age components, particularly within a single sample, was informed by both spatially-controlled analysis (in situ 40Ar/39Ar), and the higher age precision of step heat 40Ar/39Ar analyses.