Time-Transgressive Salinic and Acadian Orogenesis, Magmatism and Old Red Sandstone Sedimentation in Newfoundland

Abstract

We propose an intimate relationship between Silurian terrestrial red bed sedimentation (Old Red Sandstone), slab breakoff-related magmatism and deformation in the Newfoundland Appalachians. Red bed sedimentation started during the Early Silurian, and records the progressive rise of the Salinic mountains in the tectonic hinterland of the orogen. The red beds were mainly deposited in molasse-style foreland basins in front of an east-propagating terminal Salinic deformation front. New U–Pb zircon dating of volcanic rocks interlayered with the Silurian red beds in key structural locations yielded ages ranging between 425 and 418 Ma, which, combined with the existing geochronological database, suggests that the sedimentary rocks are progressively younger from west to east and overstep the accreted Gondwana-derived terranes. We propose that deposition of the red beds is a good proxy for the time of cratonization of the accreted terranes. Eastward migration of the Salinic deformation front was accompanied by eastward-widening of a slab-breakoff-related asthenospheric window. The latter is interpreted to have formed due to a combination of progressive steepening of the down-going plate following entrance of the leading edge of the Gander margin and its eduction. Gander margin eduction (reversed subduction) is proposed to have been instigated by the trench migration of the Acadian coastal arc built upon the trailing edge of the Gander margin, which developed contemporaneously with the Salinic collision. The resultant thinning of the lithosphere beneath the Salinic orogen, built upon the leading edge of the Gander margin immediately prior to the onset of the Early Devonian Acadian orogeny, set the stage for generation of the widespread bloom of Acadian magmatism.SOMMAIRENous proposons qu’il y a eu une relation intime entre la sédimentation des couches rouges continentales au Silurien (vieux-grès-rouges), un magmatisme lié à une rupture de segments de croûte, et la déformation appalachienne à Terre-Neuve. La sédimentation des couches rouges qui a débuté au début du Silurien témoigne du soulèvement progressif des monts saliniques de l’arrière-pays tectonique de l’orogène. Les couches rouges se sont déposées sous forme de molasses dans des bassins d’avant-pays, à l’avant du front de déformation salinique terminale qui se déployait vers l’est. De nouvelles datations U-Pb sur zircon de roches volcaniques interstratifiées avec des couches rouges siluriennes en des lieux structurels stratégiques montrent des âges qui varient entre 425 Ma et 418 Ma, ce qui, combiné aux bases de données géochronologiques existantes permet de penser que les roches sédimentaires sont progressivement plus jeunes d’ouest en est, et qu’elles surplombent les terranes accrétés du Gondwana. Nous suggérons que les couches rouges sont de bons indicateurs temporels de la cratonisation des terranes accrétés. La migration vers l’est du front de la déformation salinique a été accompagnée par un élargissement vers l’est d’une fenêtre asthénosphérique liée à une rupture de la croûte. Cette dernière aurait été provoquée par la combinaison de l’enfoncement progressif de la plaque qui a suivi l’entrée du bord d’attaque de la marge de Gander, et son éduction. Nous proposons que l’éduction (l’inverse de la subduction) de la marge de Gander a été provoquée par la migration de la fosse tectonique côtière acadienne, induite par la migration du bord d’attaque de la marge de Gander, contemporaine de la collision salinique. L’amincissement de la lithosphère sous l’orogène salinique qui en a résulté, et qui s’est déployé au bord d’attaque de la marge de Gander juste avant l’enclenchement de l’orogénie acadienne au début du Dévonien, a préparé le terrain du déploiement à grande échelle du magmatisme acadien.

Crustal Evolution of the Northeast Laurentian Margin and the Peri-Gondwanan Microcontinent Ganderia Prior to and During Closure of the Iapetus Ocean: Detrital Zircon U–Pb and Hf Isotope Evidence from Newfoundland

“…Following arc-arc collision and cessation of arc magmatism, the Popelogan -Victoria arc rocks were overlain by a blanket of black shale and turbiditic sedimentary rocks of the Upper Ordovician to Lower Silurian Badger Group. These sedimentary rocks represent the first Laurentia-derived detritus deposited on the accreted Ganderian rocks (see previous; Waldron et al 2012), representing an accretionary forearc basin (Zagorevski et al 2008;van Staal et al 2014). In the upper section of the black shale horizon an early Katian (Dicranograptus Clingani zone) graptolite fauna was detected (Williams 1991); the absolute age of this faunal zone is 453 -450 Ma (Cohen et al 2013).…”

“…The Rogersons Lake conglomerate unconformably overlies Ganderian Neoproterozoic basement and Cambro-Ordovician volcanic rocks, and displays zircon provenance of both Laurentian and Gondwanan provenance (Pollock et al 2007). Hence the Ganderian sources were only exhumed to the surface following the WenlockLudlow closure of the TetagoucheExploits back arc basin (van Staal 1994;van Staal et al 2009van Staal et al , 2014 Evolution of the Peri-Laurentian Arcs at the Composite Margin of Laurentia The 'younger' detrital zircon population detected in our samples has the least degree of rounding and likely represents first-cycle detritus of local provenance. It mainly represents proximal detritus of magmatic arcs that developed on the leading edges of composite Laurentia and Ganderia .…”

“…The Tetagouche -Exploits back-arc basin was closed during Early Silurian time, once again bringing the various fragments of Ganderia back together (440 -430 Ma;van Staal 1994;van Staal et al 2003;Pollock et al 2007). The associated principal suture in Newfoundland is the Dog Bay Line (Williams et al 1993;van Staal et al 2009van Staal et al , 2014.…”

“…This microcontinent, which also extended towards the British Isles and further south to Nova Scotia, New Brunswick and New England, mainly comprises a collage of arc-related Neoproterozoic -Early Cambrian volcano-sedimentary belts unconformably overlain by Middle Cambrian -Early Ordovician shalerich platformal sedimentary rocks. Collision of Avalonia and subsequent deformation and metamorphism occurred during the Acadian orogeny between about 415 and 390 Ma (Murphy et al 1999;van Staal et al 2009van Staal et al , 2014.…”

Discussion of “The northern Appalachian terrane wreck model”

“…The eastern Exploits Subzone and Gander Zone contain Cambrian to Silurian passive margin sedimentary rocks and rare 470-466 Ma rift-related volcanic rocks (Valverde-Vaquero et al 2006); autochthonous Cambrian to Tremadocian volcanic rocks are absent. Backarc passive margin development (Colman-Sadd 1980; van Staal et al 2021b) was interrupted by ophiolite obduction (485-478 Ma Penobscot orogeny, Colman-Sadd et al 1992;Williams and Piasecki 1990) and terminated by Silurian closure of the Exploits basin and emplacement of Middle Ordovician backarc rocks along the Dog Bay Line (Williams et al 1993;Valverde-Vaquero et al 2006; van Staal et al 2014). New Brunswick and Maine preserve a similar history, with allochthonous Penobscot rocks also solely occurring east of the Dog Bay Line (Johnson et al 2012;Reusch and van Staal 2012).…”