138 Volume 41 2014 HAROLD WILLIAMS SERIES were mainly deposited in molasse-style urien (vieux-grès-rouges), un magma- foreland basins in front of an east- tisme lié à une rupture de segments de propagating terminal Salinic deforma- croûte, et la déformation appalachi- tion front. New U–Pb zircon dating of enne à Terre-Neuve. La sédimentation volcanic rocks interlayered with the Sil- des couches rouges qui a débuté au urian red beds in key structural loca- début du Silurien témoigne du soulève- tions yielded ages ranging between 425 ment progressif des monts saliniques and 418 Ma, which, combined with the de l’arrière-pays tectonique de existing geochronological database, l’orogène. Les couches rouges se sont suggests that the sedimentary rocks are déposées sous forme de molasses dans progressively younger from west to des bassins d’avant-pays, à l’avant du east and overstep the accreted Gond- front de déformation salinique termi- Time-Transgressive Salinic wana-derived terranes. We propose that nale qui se déployait vers l’est. De and Acadian Orogenesis, deposition of the red beds is a good nouvelles datations U-Pb sur zircon de proxy for the time of cratonization of roches volcaniques interstratifiées avec Magmatism and Old Red the accreted terranes. Eastward migra- des couches rouges siluriennes en des Sandstone Sedimentation tion of the Salinic deformation front lieux structurels stratégiques montrent in Newfoundland was accompanied by eastward-widen- des âges qui varient entre 425 Ma et ing of a slab-breakoff-related asthenos- 418 Ma, ce qui, combiné aux bases de pheric window. The latter is interpreted données géochronologiques existantes Cees R. van Staal1, Alexandre to have formed due to a combination permet de penser que les roches sédi- Zagorevski2, Vicki J. McNicoll2 and of progressive steepening of the mentaires sont progressivement plus Neil Rogers2 down-going plate following entrance of jeunes d’ouest en est, et qu’elles sur- the leading edge of the Gander margin plombent les terranes accrétés du 1Natural Resources Canada and its eduction. Gander margin educ- Gondwana. Nous suggérons que les Geological Survey of Canada tion (reversed subduction) is proposed couches rouges sont de bons indica- Vancouver, BC, Canada, V6B 5J3 to have been instigated by the trench teurs temporels de la cratonisation des E-mail: [email protected] migration of the Acadian coastal arc terranes accrétés. La migration vers built upon the trailing edge of the l’est du front de la déformation salin- 2Natural Resources Canada Gander margin, which developed con- ique a été accompagnée par un élar- Geological Survey of Canada temporaneously with the Salinic colli- gissement vers l’est d’une fenêtre Ottawa, ON, Canada, K1A 0E8 sion. The resultant thinning of the lith- asthénosphérique liée à une rupture de SUMMARY osphere beneath the Salinic orogen, la croûte. Cette dernière aurait été built upon the leading edge of the provoquée par la combinaison de l’en- We propose an intimate relationship Gander margin immediately prior to foncement progressif de la plaque qui between Silurian terrestrial red bed the onset of the Early Devonian Aca- a suivi l’entrée du bord d’attaque de la sedimentation (Old Red Sandstone), dian orogeny, set the stage for genera- marge de Gander, et son éduction. slab breakoff-related magmatism and tion of the widespread bloom of Aca- Nous proposons que l’éduction (l’in- deformation in the Newfoundland dian magmatism. verse de la subduction) de la marge de Appalachians. Red bed sedimentation Gander a été provoquée par la migra- started during the Early Silurian, and SOMMAIRE tion de la fosse tectonique côtière aca- records the progressive rise of the Nous proposons qu’il y a eu une rela- dienne, induite par la migration du Salinic mountains in the tectonic hin- tion intime entre la sédimentation des bord d’attaque de la marge de Gander, terland of the orogen. The red beds couches rouges continentales au Sil- contemporaine de la collision salinique. Geoscience Canada, v. 41, http://dx.doi.org/10.12789/geocanj.2014.41.031 © 2014 GAC/AGC® GEOSCIENCE CANADA Volume 41 2014 139 L’amincissement de la lithosphère sous ended and the Acadian began at any distinct phase of mountain building. l’orogène salinique qui en a résulté, et given location in the Appalachian oro- Understanding of the tectonic qui s’est déployé au bord d’attaque de gen is a contentious issue and depends significance and absolute age of the la marge de Gander juste avant l’en- on whether one considers the defini- Salinic and Acadian orogenies is partic- clenchement de l’orogénie acadienne tion of orogeny from a purely absolute ularly pertinent to the origin and affili- au début du Dévonien, a préparé le ter- age perspective or from a tectonic- ation of Wenlockian to earliest Devon- rain du déploiement à grande échelle dynamic point of view (see discussion ian bimodal magmatic suites (Fig. 1). du magmatisme acadien. in van Staal et al. 2008 and below). The magmatic axes of these suites Defining the end of the Salinic oroge- become younger towards the east- INTRODUCTION ny and the start of the Acadian oroge- southeast and they are commonly spa- Beginning with his benchmark 1967 ny from an absolute age perspective tially associated with deposition of publication on the Silurian rocks of has been problematic, at least in part syn-tectonic terrestrial sedimentary Newfoundland (Williams 1967) the Sil- due to the changes in the mean rocks, such as the Botwood Group in urian Period was of special interest to absolute ages of the series and stage central Newfoundland (Fig. 1). The Harold (Hank) Williams. His highly boundaries of the Silurian and Devon- affiliation of these magmatic suites to insightful stratigraphic and structural ian (e.g. 408 ± 12 Ma [Palmer 1983] vs. either Salinic or Acadian orogenesis is studies of the superbly exposed Siluri- 419.2 ± 3.2 Ma [ICS 2013]). This led not always clear and sometimes prob- an rocks in the Notre Dame Bay of to confusion and grouping of unrelat- lematic. The standard model for late north-central Newfoundland (Williams ed events (e.g. Dunning et al. 1990; Lin syn- to post-tectonic bimodal magma- 1972, 1993a), led him to conclude that et al. 1994; O’Brien 2003; van Staal and tism invokes orogen-wide extension the Exploits subzone must have been Zagorevski 2012). In addition, defor- (Turner et al. 1992); however, Salinic internally divided into two parts sepa- mation associated with both Acadian and Acadian bimodal magmatism was rated by a significantly wide Middle (e.g. Bradley et al. 2000) and Salinic synchronous with regional shortening Ordovician to Silurian marine seaway orogenies (see below) was diachronous and uplift throughout the central oro- that is now sutured along the Dog Bay and consequently appears to overlap in genic core of the Newfoundland Line (Williams et al. 1993). Williams time and/or be continuous, which led Appalachians (e.g. Dunning et al. considered the closure of this vestige some workers to group them together 1990). In this contribution, we utilize of the Iapetus Ocean to be the tecton- (e.g. O’Brien 2003; Murphy and Kep- new and existing geochronological and ic cause of the Silurian deformation, pie 2005). However, overprinting rela- geochemical databases to integrate the although the responsible tectonic tionships and tight age constraints on magmatic and tectonic evolution of processes (i.e. subduction vs. strike-slip the structures generated by the Salinic the Newfoundland Appalachians and and transpression) were never speci- and Acadian orogenies consistently explore the possible tectonic settings fied. Williams’ work was contrary to show that the Acadian deformation and causes of this Late Silurian mag- the established pre-1985 dogma, which postdated Salinic structures in any matism, terrestrial sedimentation and divided the pre-Alleghanian deforma- given area where both are present (van associated deformation. tion in the northern Appalachians into Staal and de Roo 1995; van Staal et al. only two orogenic phases: the Ordovi- 2009; Wilson and Kamo 2012). This MIDDLE ORDOVICIAN–LATE cian Taconic and Devonian Acadian suggests that phases of Salinic and SILURIAN GEOLOGICAL orogenies (e.g. Rodgers 1970). The Acadian orogenic events may have ARCHITECTURE OF THE CENTRAL intervening Silurian period was thought overlapped in time but never in space NEWFOUNDLAND APPALACHIANS to be characterized by tectonic quies- at any time (van Staal et al. 2009). In The central Newfoundland Appalachi- cence (e.g. Colman-Sadd 1982; Bradley addition, a number of different lines of ans (Fig. 1) are underlain by Paleozoic 1983). Concurrent with Williams’ work, evidence indicate that they constitute and older rocks of the Dunnage and evidence for latest Ordovician to Siluri- two kinematically and dynamically dis- Gander tectonostratigraphic zones an, post-Taconic to pre-Acadian oroge- tinct events that followed each other (Williams 1978). Tectonostratigraphic nesis was progressively growing in the closely in time (e.g. Dunning et al. zones and subzones were principally Northern Appalachians (e.g. van Staal 1990; West et al. 1992; van der Pluijm defined on their pre-Silurian geological 1987, 1994; van der Pluijm and van et al. 1993; Hibbard 1994; Holdsworth characteristics, although it was later Staal 1988; Dunning et al. 1990; van 1994; Cawood et al. 1994; Burgess et shown that geological contrasts locally Staal et al. 1990; West et al. 1992; al. 1995; Kerr et al. 1995; D’Lemos et continued into the Silurian (Williams et Cawood et al. 1994; Hibbard 1994; van al. 1997; Schofield and D’Lemos 2000; al. 1988, 1993; Reusch and van Staal Staal and de Roo 1995). Following the O’Brien 2003; van Staal et al. 2003; 2012). The Dunnage zone mainly com- discovery by Boucot (1962) of strati- McNicoll et al.