Reply to Discussion on ‘The sole of an ophiolite: the Ordovician Bay of Islands Complex, Newfoundland’ Journal , 170, 2013, pp. 715–722

Abstract

Zagorevski & van Staal (2014) question our model (Dewey & Casey 2013) in which we invoked forearc suprasubduction zone (SSZ) spreading to form the Bay of Islands Ophiolite Complex (BOIC) as part of the Notre Dame arc of the Appalachian–Caledonian Orogen. We interpret the formation of the BOIC to have occurred by spreading proximal to a ridge–trench–trench (R–TR–TR) triple junction. Our model explains (1) the formation of the BOIC within 15–20 myr of obduction, (2) the associated subcretion of the metamorphic sole shortly after formation during subduction of the ocean lithosphere bordering Laurentia, (3) subsequent oblique ophiolite encroachment to the Laurentian passive margin above a west-facing subduction zone as part of the Notre Dame forearc–arc system, and (4) the final obduction with the high-angle palaeo-ridge geometry relative to the Laurentian margin, with the emplacement initiated in the Late Arenig (Dapingian) during the Taconic Orogeny. We respond to the statements and questions posed by Zagorevski and van Staal that contest our model, which we further clarify and develop. Zagorevski and van Staal conclude, mistakenly, that we stated that the formation of the c . 489–484 Ma BOIC was associated with ‘intra-oceanic subduction initiation’ by the conversion of a ‘ridge–transform boundary’ in the Early Ordovician. They imply that we proposed a typical mid-ocean ‘intra-oceanic’ spreading centre environment for the origin of the BOIC that is linked to subduction initiation on a transform. On the contrary, in our fig. 1, p. 716, we proposed that the BOIC formed by sea-floor spreading adjacent to a pre-existing older lithosphere (Karson & Dewey 1978; Casey & Dewey 1984; Cawood & Suhr 1992; Kurth-Velz et al . 2004), represented by the earlier Middle–Late Cambrian Coastal Complex (CC) and Lush’s Bight (LB) oceanic tract, that we interpret as SSZ forearc. The CC–LB …