Rapid transition from the salinic to Acadian orogenic cycles in the northern Appalachian Orogen: Evidence from northern New Brunswick, Canada

Abstract

Geochronological data from volcanic rocks aid in reconstructing the Silurian–Devonian evolution of the northern Appalachians of New Brunswick in the context of Salinic (Silurian) and Acadian (Devonian) orogenesis. Late Silurian to Early Devonian sedimentation, volcanism and deformation in northern New Brunswick is complex, and characterized by transgressive-regressive sedimentary cycles, local disconformities and angular unconformities, and two stages (ca. 422–419 Ma, and ca. 417–407 Ma) of typically bimodal, within-plate magmatism. These events overlapped the end of the Salinic orogenic cycle, related to the ca. 430 Ma collision of the Ganderian passive margin with composite Laurentia, and the beginning of the Acadian cycle, which is associated with subsequent (ca. 424–422 Ma) collision and underthrusting of Avalonian crust beneath composite Laurentia (now including the Gander margin). Following Ganderia−Laurentia collision, the earlier (Pridolian) stage of within-plate magmatism is interpreted to result from ca. 425 Ma detachment of the subducting Tetagouche backarc lithospheric slab and is linked to the Salinic cycle, along with local, pre-Acadian (Salinic C) deformation of Ludlow−Pridoli rocks. Break-off of the Tetagouche slab also led to rapid uplift and subsequent extensional collapse of the Salinic metamorphic core, resulting in latest Silurian-earliest Devonian D3 vertical shortening and flat belt formation in the Brunswick subduction complex. Precise ID-TIMS U-Pb (zircon) dating of volcanic rocks reveals the existence of a 2.2 myr hiatus in the stratigraphic record, corresponding to an interpreted latest Silurian–earliest Devonian erosional surface between volcanic rocks identified with the Salinic cycle (Dickie Cove Group and lower part of Tobique Group) and those associated with the Acadian cycle (Dalhousie Group). The disconformity coincides with a widespread marine regression recorded by roughly coeval deposition of shallow-water to intertidal redbeds across the northern Appalachians. Marine regression is closely followed by deposition of Lochkovian deep marine sedimentary rocks (turbidites) in the Seboomook foredeep. This regressive-transgressive sequence immediately precedes arrival of the northwest-migrating Acadian deformation front, and is interpreted as evidence of a migrating Acadian forebulge-foredeep system associated with flexural loading of Ganderian crust by the Acadian orogenic wedge situated farther southeast in southern New Brunswick and coastal Maine. Early Devonian volcanism and Acadian retro-arc foreland deformation are linked to flat-slab convergence of Avalonia after underthrusting of buoyant Avalonian continental crust began ca. 418 Ma. It is proposed that the latest Silurian−earliest Devonian disconformity and marine regression mark the beginning of the Acadian orogenic cycle, implying that, in northern New Brunswick, the effects of waning, late Silurian (Salinic) deformation temporally overlapped the early, far-field effects of Acadian orogenesis.