Abstract Avalonia, defined by its distinctive uppermost Ediacaran–Ordovician overstep sequence, extends from New England through Atlantic Canada to Wales. It unconformably overlies: (1) parts of one cratonic Neoproterozoic arc that which records several pulses at: 760–730 Ma, 680–600 Ma and 580–540 Ma; (2) an 800–760 Ma passive margin sequence; and (3) c. 976 Ma isolated plutons, possibly basement. Comparisons with modern arc dimensions suggest the dip of the Benioff Zone ranged from c. 22° W in Newfoundland to c. 52–67° elsewhere. A 600–580 Ma hiatus in arc magmatism in Cape Breton Island is attributed to overriding an oceanic plateau, leading to a 15° decrease in the dip of the Benioff Zone. The Collector magnetic anomaly along the Grand Banks and the Minas Fault is inferred to mark the Neoproterozoic southern margin of the Avalon Plate consisting of leaky transform faults and trench segments characterized by magnetite serpentinite mantle wedge beneath forearcs. The Minas Fault/Collector Anomaly connects similar arc units in Cape Breton Island and southern New Brunswick, suggesting that they were already offset by the Minas transform fault in the late Neoproterozoic. Similar tectonic, palaeomagnetic and isotopic data in the Timan Orogen of Baltica suggest that Avalonia may correlate with the Kipchak arc.

Abstract Dashwoods is a composite peri-Laurentian terrane in Newfoundland and forms the basement to the Early Ordovician to Silurian Notre Dame arc. The southern part of Dashwoods is characterized by paragneiss that is intruded by Early Ordovician to Late Silurian plutons and affected by polyphase Taconic to Salinic deformation and high-grade metamorphism. The crystalline basement of Dashwoods is not exposed and pre-Middle Ordovician paragneiss is investigated herein to constrain the provenance of Dashwoods. SHRIMP U–Pb zircon analysis of the paragneiss yielded metamorphic rims ranging from c. 500 to 395 Ma and abundant detrital grain cores ranging from c. 1853 to 546 Ma. The presence of abundant Tonian dates differentiates Dashwoods from the adjacent Humber Margin in Newfoundland, and Hebridean and Grampian terranes in the British Isles. The detrital provenance of Dashwoods is most similar to the Baie Verte Margin in Newfoundland, and Tyrone Complex and Dalradian Supergroup in Ireland. These data suggest that Dashwoods and the Baie Verte Margin originated near the Rockall promontory and were subsequently emplaced outboard of the Humber Margin by Ordovician to Carboniferous motion along the Baie Verte–Brompton Line.

Abstract The pre-accretionary shapes of cratonic margins form successions of promontories and re-entrants inherited from the rifting of supercontinents. In accretionary orogens, the extent of deformation related to a collision with a continent characterized by an irregular margin is obfuscated through the partitioning of deformation along pre-existing structures. In the Northern Appalachians, the extent of the deformation related to the oblique collision of the Meguma terrane with the composite Laurentian margin is disputed. Herein, we use a framework based on modern collisional settings to investigate the Late Devonian to Mississippian deformation inboard of the Avalonia–Meguma boundary and evaluate the regional tectonic setting. We combine published shear zone kinematic interpretations, deformation ages and regional 40 Ar/ 39 Ar cooling ages with structural interpretation of aeromagnetic and gravimetric depth slices covering the Northern Appalachians. We find that the deformation related to the collision of the Meguma terrane, attributed to the Neoacadian orogeny, has a larger structural footprint than previously documented. While this deformation is partitioned in multiple structures in the Canadian Appalachians, northern New England is characterized by rapid crustal deformation, high palaeoelevation and fast erosional exhumation, similar to modern syntaxis structures.

Abstract The Taconian–Grampian tract was characterized by a diachronous collision of a north-facing oceanic arc–forearc terrane and associated backarc basins with an irregular Laurentian margin with hyperextended segments. Hyperextension produced outboard continental terranes, separated by exhumed subcontinental mantle from the inboard margin. The exhumed mantle facilitated continued subduction of the extended margin after it had entered the trench. Enhanced slab-rollback resulted in spreading in lenticular backarc basins, which gradually transitioned along-strike into extensional arcs where rollback was less. Obduction of the oceanic elements onto the irregular Laurentian margin was followed by diachronous slab breakoff and a subduction polarity reversal, such that south- and north-dipping subduction zones locally were coeval along-strike. The polarity flip changed the convergence obliquity from dextral to sinistral and was accompanied by shallowing of the subducting slab near the end of the Middle Ordovician. Strike-slip movements locally juxtaposed segments where tectonic events occurred at different times, producing conflicting relationships. Slab breakoff produced punctuated magmatism, largely driven by mantle-derived melts, and drove and/or enhanced metamorphism in the overlying and enveloping crustal rocks. Boninite was generated episodically over a time span of 32 myr; the oldest Cambrian phase in the Lushs Bight Oceanic Tract (LBOT) and correlatives was associated with subduction initiation.