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 DevonianEarly 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 fluidfluxed 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 accommoda...

Evidence of Late Ediacaran Hyperextension of the Laurentian Iapetan Margin in the Birchy Complex, Baie Verte Peninsula, Northwest Newfoundland: Implications for the Opening of Iapetus, Formation of Peri-Laurentian Microcontinents and Taconic – Grampian Orogenesis

“…Although such a kinematic model is consistent with Middle Ordovician (Taconic, ca. 465 Ma) 40 Ar/ 39 Ar ages of muscovite and hornblende in the Birchy Complex mafic schists (van Staal et al 2009a;Castonguay et al 2010), metamorphic studies reveal that the Birchy Complex was buried to significantly greater depths (≥10 kb) during the Taconic than the structurally overlying Baie Verte oceanic tract (≤7 kb; Willner et al 2012). Their present tectonic juxtaposition therefore took place subsequent to peak-Taconic burial metamorphism.…”

“…; 2) the formation of microcontinents (e.g. Dashwoods) along the Newfoundland Humber margin and the Appalachian -Caledonian margin in general (van Staal et al , 2009bChew et al 2010); 3) the markedly variable and spotty preservation of radiogenic age evidence for Taconic-related metamorphism and deformation along strike (Cawood et al 1994;Castonguay et al 2001Castonguay et al , 2010van Staal et al 2009 a, b); and 4) evidence for crustal contamination in felsic rocks of the BVOT and its Early to Middle Ordovician Snooks Arm Group cover ) and other outboard terranes in the peri-Laurentian realm (e.g. Whalen et al 1997;van Staal et al 2007;Zagorevski et al 2006) Thermal Subsidence The highly thinned continental margins of studied OCT zones commonly display significant retardation of thermal subsidence.…”

“…East-directed subduction began in the Taconic seaway at ca. 490 Ma, forming the 490-483 Ma supra-subduction zone BVOT (van Staal et al 2007, 2009b. The latter subsequently became the forearc terrane to the 489-477 Ma Notre Dame arc, as indicated by ample continental arc fragments in the basal part of the Flat Water Pond/Snooks Arm Group (Bédard et al 2000;Skulski et al 2010).…”

Influence of deformation and fluids on Ar retention in white mica: Dating the Dover Fault, Newfoundland Appalachians

“…For example, the western leading edge of Ganderia was still undergoing the late stages of the Salinic orogeny at the time of the onset of Acadian-related arc magmatism and deformation at its eastern trailing edge (van Staal et al, 2009;2014).…”

“…The Ganderia microcontinent consists of Late Neoproterozoic-Ordovician arc and associated clastic metasedimentary rocks, whereas Avalonia comprises sub-greenschist facies Neoproterozoic volcanic, plutonic and sedimentary rocks overlain by Cambrian-Ordovician platform cover sequences (Hibbard et al, 2006;van Staal et al, 2009;van Staal and Barr, 2012).…”