J. Duncan Keppie

FAULT REACTIVATION WITHIN AVALONIA : PLATE MARGIN TO CONTINENTAL INTERIOR DEFORMATION

Abstract Lithotectonic terranes commonly have faults with movement histories that reflect their original tectonic setting and their subsequent re-activation during terrane accretion and post-accretionary dispersal. Since later movements tend to overprint evidence of earlier motions, documenting fault reactivation can be a difficult task. Avalonia, the largest terrane within the Canadian Appalachians preserves evidence for repeated episodes of movement along NE-trending fault zones in a variety of tectonic settings between the late Neoproterozoic and late Paleozoic. Evidence of Neoproterozoic motion is preserved in pre-final crystallization deformation fabrics in arc-related igneous complexes which intrude shear zones. These record strike-slip motion related to oblique subduction along the continental margin of Gondwana. In the Paleozoic, Avalonia migrated from its original peri-Gondwanan setting. Mid-Ordovician to earliest Silurian deformation and magmatism is attributed to the sinistral accretion of Avalonia to Laurentia. This reactivated Neoproterozoic shear zones and resulted in basin inversion in mainland Nova Scotia and the formation of mylonites and injection of dikes into brittle fractures in southern New Brunswick. Sinistral motion was accompanied by terrane dispersal and was followed in the Late Silurian and Early Devonian by further reactivation in the form of dextral strike-slip that reflects convergence between Laurentia and Gondwana. Following the accretion and dispersal of Avalonia, the fault zones became stranded within the continental interior. Subsequently, the fault zones accommodated local stresses which were far-field responses to collisional tectonics associated with the mid-Late Devonian Acadian and the Late Carboniferous Alleghanian orogenies. Hence, the generation of these fault systems along plate margins in the late Neoproterozoic and early Paleozoic profoundly influenced the tectonic evolution of late Paleozoic intra-continental deformation. Late Carboniferous deformation is recorded in (a) the mylonitic and cataclastic fabrics of the shear zones themselves, (b) the orientations of spatially related fold structures, (c) local controls on basin formation and sedimentary facies and (d) offsets in stratigraphy. This motion was predominantly dextral and records continued convergence between Laurentia and Gondwana during the amalgamation of Pangea.

Observations on the Ichnology of the Meguma Group (? Cambro-Ordovician) of Nova Scotia

The trace fossils Arenicolites variabalis Cirauliahnis montanus and Paleodictyon (Glenodictyum) cf. imperfection from the ?Cambro-Ordovician Meguma Group of Nova Scotia are described in detail. The significance of the occurrence of each of these ichnospecies is also noted. The environmental and stratigraphic ranges of A. variabalis are extended respectively to deep water and the Ordovician; the stratigraphic range of C. montanus is extended to the Ordovician and its presence in the Meguma Group to date represents the first and only recording of the species in the western hemisphere; P. cf, irnperfectum provides additional evidence that at least part of the Goldenville Formation is Ordovician. n nRESUME nOn decrit en detail les traces fossiles Arenicolites variabalis, Circulichnis montanus et Paleodiotyon (Glenodictyum) cf, imperfection provenant du groupe Meguma (?Cambro-Ordovicien) de la Nouvelle-Ecosse. De plus, lon met en evidence la signification de la presence de chacune de ces ichnoespeces. On etend lenvironnement dA. variabilis jusquen eau profonde et sa portee stratigraphique jusqua lOrdovicien; la portee stratigraphique de C. montanus est etendue a lOrdovicien et jusqua ce jour, le groupe Meguma repreaente le seul endroit dans lhemisphere ouest ou sa presence est notee; P. cf. imperfectum fournlt une preuve de plus quau moins une partie de la formation de Goldenville date de lOrdovicien. n n[Traduit par le journal]

Correlation of Neoproterozoic III sequences in the Avalon Composite Terrane of mainland Nova Scotia: tectonic implications

Neoproterozoic III volcanic and sedimentary rocks in the Antigonish and Cobequid Highlands have very similar lithostratigraphy, depositional age, volcanic geochemistry, and structural and metamorphic histories. Both regions are cut into several fault blocks; from north to south these are termed Georgeville. Maple Ridge, Clydesdale and Keppoch in the Antigonish Highlands, and Jeffers and Bass River in the Cobequid Highlands. North to south facies variations allow lithostratigraphic correlation between the Keppoch and Jeffers blocks, and the Clydesdale and Bass River blocks. The maximum depositional age of these rocks is provided by U-Pb detrital zircon ages of 613 ± 5 and 605 ± 5 Ma. Three geochemically distinct types of volcanic rocks occur in both regions: continental tholeiites, calc-alkalic basaltic andesites, and volcanic arc rhyolites. Neoproterozoic III deformation varies from intense (polyphase folding and thrusting) in the Cobequid Highlands and northern Antigonish Highlands to mild and single phase in the southern Antigonish Highlands. Post-tectonic plutonic rocks common to both regions include volcanic-arc granodi-orite and hornblende-rich gabbro with ages between 620 ± 5 and 609 ± 4 Ma. These closely constrain the time of deposition, deformation and intrusion to between 618 and 615 Ma in the Antigonish Highlands and 610 and 608 Ma in the Cobequid Highlands. n nThe stratigraphic record and the volcanic geochemistry indicate deposition in an ensialic volcanic arc rift. The Neoproterozoic III deformation is attributed to basin closure. The correlations between the Cobequid and Antigonish Highlands suggest that they form part of one Neoproterozoic III terrane and that Paleozoic fault movement was limited. n n RESUME n Les roches volcaniques et sedimentaires du neoproterozoique III dans les hautes-terres dAntigonish et de Cobequid presentent de grandes similarites aux points de vue lithostratigraphie, âge de deposition, geochimie volcanique et histoires structurale et metamorphique. Les deux regions sont decoupees en plusieurs blocs de faille; ils sont nommes, du nord au sud: Georgeville, crete Maple, Clydesdale et Keppoch dans les hautes-terres dAntigonish et Jeffers et riviere Bass dans les hautes-terres de Cobequid. Les variations de facies du nord au sud permettent des correlations lithostratigraphiques entre les blocs de Keppoch et de Jeffers et entre ceux de Clydesdale et de la riviere Bass. Lege de deposition maximal de ces roches est donne par les datations U-Pb sur les zircons detriliques a 613 ± 5 Ma et 605 ± 5 Ma. Trois types geochimiques distincts de volcanites se retrouvent dans les deux regions: des tholeiites continentales, des andesites basaltiques calco-alcalines et des rhyolites darc volcaniques. La deformation neoproterozoique III va dintense (plissements et chevauchemenls multiples) dans les hautes-terres de Cobequid et le nord des hautes-terres dAntigonish a faible et dune seule phase dans le sud des hautes-terres dAntigonish. Les roches plutoniques post-tectoniques communes aux deux regions comprennent les gabbros riches en hornblende et les granodiorites darc volcanique avec des ages entre 620 ± 5 et 609 ± 4 Ma. Ceux-ci limitent etroitement les âge de deposition, de deformation et dintrusion entre 618 et 615 Ma dans les hautes-terres dAntigonish et entre 610 et 608 Ma dans les hautes-terres de Cobequid. n nLes donnees stratigraphiques et la geochimie des volcanites indiquent une deposition dans un arc volcanique ensialique. La deformation neoproterozoique III est attribute a la fermeture du bassin. Les correlations entre les hautes-terres de Cobequid et celles dAntigonish suggerent quelles font partie dun meme terrain neoproterozoique et que les mouvements de failles paleozol’ques etaient restreints. n n[Traduit par le journal]

Anatomy of a telescoped pull-apart basin: an example from the Cambro-Ordovician rocks in the Antigonish Highlands of Nova Scotia

Cambrian-Lower Ordovician rocks outcrop in the northernmost Antigonish Highlands where they unconformably overlie the Precambrian Georgeville Group. They are divided into two groups that are interpreted to be lateral fades equivalents. The Iron Brook Group is predominantly sedimentary and consists of red clastic sedimentary rocks, pink limestones containing late Early Cambrian fossils, quartzites, calcareous tuffs and ironstones containing Late Cambrian-Ordovician inarticulate brachi-opods. The McDonalds Brook Group consists of red clastic sedimentary rocks at the base overlain by bimodal, alkalic, continental rift volcanic rocks with minor interlayered red clastic rocks and late Early Cambrian fossiliferous pink limestones. These rocks are inferred to have been deposited in a pull-apart basin produced in the overlap zone between two dextral NE-trending faults. Connection between the pull-apart basin and a larger ocean is indicated by cosmopolitan, open-ocean. Early Cambrian trilobites. n nCambro-Ordovician rocks in the central part of the basin were subsequently thrusted (?)northeastwards and then back thrusted. Associated NW-SE folds verge away from a central zone. This complex structure dies out towards the NE and S W ends of the basin, where the lowest Cambrian units are relatively mildly deformed and the basal unconformity with Precambrian rocks is preserved. These structures are inferred to have been produced by sinistral motion on the NE-SW bounding faults with compression concentrated locally in the pull-apart basin. This movement expelled Cambro-Ordovician rocks lying on attenuated continental lithosphere from the basin centre onto die basin margins thereby accounting for the limited extent of the deformation. These structures are truncated by die sub-Silurian unconformity, thereby constraining the time of deformation to the Ordovician. Both the Cambro-Ordovician and Siluro-Devonian were deformed during the Devonian by upright NE-SW folds that are preferentially developed adjacent to the major NE-SW faults. The folds are consistently oriented slightly anticlockwise of the faults thereby indicating renewed dextral movements on the NE-SW faults. n n RESUME n Des roches du Cambrien a lOrdovicien inferieur affleurent dans la region la plus au nord des Monts Antigonish et recouvrent en discordance le Groupe precambrien de Georgeville. Elles constituent deux groupes interprets comme des equivalents lateraux de facies. Le Groupe dIron Brook est surtout de nature sedimentaire et se compose de roches sedimentaires clastiques rouges, de calcaires roses renfermant des fossiles du debut de lEocambrien, des quartzites, des tufs calcaires et des roches ferrugineuses contenant des brachiopodes inarticules tardicambriens a ordoviciens. Le Groupe de McDonalds Brook comprend des roches sedimentaires clastiques rouges en sa base, surmontees par des volcanites bimodales alcalines, de type rift continental et englobant quelquesniveaux a roches clastiques rouges et calcaires roses a fossiles de la finde lEocambrien. Ces depots se seraient accumules a l’inferieur d’un basin rhomboedrique sur decrochement installe dans un ilot en distension forme par l’intersection de deux failes dextres orientee NE. La presence de trilobites cosmopolites et franchement marins du debut du Cambrien prouve que ce bassin etait en large communication avec la haute mer. n nDans la partie centrale du bassin, les roches cambro-ordoviciermes furent par la suite chevauchees (?vers le nord-est) puis retrochevauchees. Sy associent des plis NO-SE dont la vergence seioigne dune zone centrale. Cette structure complexe sestompe vers les extremiles NE et SO du bassin, la ou la deformation des roches cambriennes initiales est relativement modeste et la discordance basale avec les roches precambriennes est preservee. Ces structures seraient le produit dun jeu senestre des accidents bordiers NE-SO, conjugue a une concentration locale du serage dans le basin de decrochement. Ce movement expulsa des roches cambro-ordoviciennes, sises sur une lithosphere continentale amincie, du centre vers les abords de la cuvette, ce qui explique leur faible deformation. Le recoupement de ces memes structures par la discordance angulaire antesilurienne cantonne la deformation a l’Ordovicien. Tant le Cambro-Ordovicien que le Siluro-Devonien furent deformes lors du Devonien par des plis droits NE-SO developpes de facon preterentielle au voisinage des failles majeures NE-SO. Lorientation de ces plis sinscrit, conformenient, selon une faible rotation dans le sens inverse des aiguilles dune montre par rapport aux failles, ce qui suggere un rejeu dextre des failles NE-SO. n n[Traduit par le journal]

Two Rb-Sr Whole Rock Isochrons from Plutons In the Cobequid Highlands, Nova Scotia, Canada

Two Rb-Sr Hhole rock isochrons have been obtained from plutons In the Cobequid Highlands of Nova Scotia: the Debert River and Hart Lake-Byers Lake Plutons. The undeformed, unfoilated Debert River Pluton yielded an age of 596± 70 Ma and an Initial 87Sr/88Sr ratio of 0.7059 ± 0.0007. This age places an upper age limit on the detonation recorded In the Folly River Schist (youngest unit of the Bass River Complex), the d I or I tic Frog Lake Pluton, and the sedimentary rocks immediately north and east of the Debert River Pluton. n nThe Hart Lake-Byers Lake Pluton yielded anageof348±5Maandan Initial 87Sr/88Sr ratio of 0.7046 ± 0.0008. The similarity of this age to that of the adjacent felsic volcanics of the Byers Brook For nation suggests that they are comagmatic and confine that a Carboniferous igneous event forms an Important part of the geological history of the Cobequid Highlands. The Initial 87Sr/88Sr ratio and the blotite-hornblende content of the two granites suggest a mafic Igneous loner crustal source or possible untie component for the magma. n n RESUME n Les plutons de Debert River et Hart Lake-Byers Lake dans les monts Cobequld en Nouvel le-Ecosse ont livre deux isochrones Rb-Sr de roche globale. Il deforme ni felle, le pluton de Debert River a donne un âge de 596± 70 Ma et un rapport 87Sr/88Sr initial de 0.7059 ± 0.0007. Ceci place une limite superieure sur l’âge de la deformation enregistree dans le schiste de Folly River (L’unite la plus jeune du complexe de Bass River), le pluton diorltique de Frog Lake et dans les roches sedimentaires Jouxtant le pluton de Debert River au nord et a l’est. n nLe pluton de Hart Lake-Byers Lake a produit un âge de 348 ± 5 Ma et un rapport isotopique 87Sr/88Sr initial de 0.7046 ± 0.008. Cet âge, similaire a ceux des volcanites felslques adjacentes de la formation de Byers Brook, suggere qulis sent comagnatiques, signe evident quun episode igne carbonifere forme l’un des faits majeurs de L’histoire geologique des monts Cobequid. Le rapport 87Sr/88Sr initial et la teneur en blotite et hornbIende de ces deux granites suggerent une genese crustale profonde du magma avec une possible contribution mantellique. n n[Traduit par le journal]

Rb-Sr Isotopic Data from Three Suites of Igneous Rocks, Cape Breton Island, Nova Scotia

Rb-Sr isotopic data is presented for three suites of Igneous rocks: the Fourchu Group; volcanic rocks from Ingonish Island; and the Gulch Brook pluton. Results from the Late Precambrlan metavolcanic rocks of the Fourchu Group show considerable scatter due to varying degrees of resetting of the Rb-Sr system. Seven of the sight samples yield an age of 407 ± 46 Ma which is believed to reflect the effects of the Acadian Orogeny in this area. A two-point whole-rock Isochron from the two freshest samples yield a c. 640 Ma age which could approximate the extrusive age. The redistribution of strontlus and rubidus Isotopes suggests that the Initial 87Sr/88Sr ratios in the range 0.703-0.705 should be viewed with caution. n nA whole-rock Rb-Sr isochron for volcanic rocks froe Ingonlsh Island yield an age of 412 ± 15 Ma, Interpreted to closely date the time of extrusion and crystallization. An Initial 87Sr/88Sr ratio of 0.706 suggests the lower crust as one possible source region. n nThe 413 ± 10 Ma Rb-Sr whole-rock Isochron obtained from samples of the weakly foliated Gulch Brook pluton In northern Cape Breton Island probably represents the age of crystallization. Micas, which define the weak foliation In the pluton, have previously yielded a Rb-Sr mineral Isochron of 320 Ma or Individually calculated ages of 363 Ma for muscovlte and 350 Ma for blotite (Cormier, 1980; written con. 1984) and are believed to represent resetting by a later thermal event during the Late Devonian-Early Carboniferous. The initial 87Sr/88Sr ratio of 0.7045 ± 0.0004 suggests a lower crustal source for the magma, possibly with a mantle component as wall. The similarity of the ages and Initial 87Sr/88Sr ratios of the Ingonlsh Island volcanic rocks and the Gulch Brook Pluton suggest that they nay be volcanic/subvolcanic equivalents of one another. n n RESUME n On presents les signatures isotoplques Rb-Sr de trois series de roches Ignees: le groupe de Fourchu, des volcanites de L’ile dIngonlsh et le pluton de Gulch Brook. Lanalyse des metavolcanites tardiprecanbriennes du groupe de Fourchu montre une tres grande dispersion des valours imputable au reajustement a divers degres du systeme Rb-Sr. Sept des huit echantillons ont donne un âge de 407 ± 46 Ma qui semble traduire les effets de L’orogenie acadianne dans ce secteur. Un Isochrone de roche globale a deux points, provenant des deux echantiIIons les moins aIteres, a prodult un âge autour de 640 Ma qui pourrait approximer L’âge de L’epanchement. La redistribution des isotopes de strontium et rubidium rend discutables les rapports 87Sr/88Sr initiaux aux environs de 0.703-0.705. n nUn Isochrone Rb-Sr de roche globale pour les volcanites de L’ile dIngonlsh a rendu un âge de 412 ± 15 Ma qui correspondralt de tres pres au temps de L’extrusion et la cristalllzatlon. Le rapport 87Sr/88Sr initial de 0.706 evoque une genese crustale profonde. n nDes echantiIIons provenant du pluton tres peu folie de Gulch Brook, dans L’ile du Cap-Breton septentrionale, ont produit un Isochrone Rb-Sr de roche globale de 413 ± 10 Ma qui date probablement L’âge de sa cristalllzatlon. Une etude anterleure des micas (qui impriment au pluton sa faible foliation) a deja donne un Isochrons Rb-Sr de mineral de 320 Ma ou des âges calcules de facon individualle de 363 Ma pour la muscovite et 350 Ha pour la blotIte (Confer, 1980; com. ecrlte, 1984). On pense y deviner un reajustement lie a un episode theraique ulterieur tardidevonlen ou eocarbonifere. Le rapport 87Sr/88Sr initial de 0.7045 ± 0.0004 suggere we participation de la create continentals inferieure avec une possible contribution aantellique. La similitude des âges et des rapports 87Sr/88Sr initiaux des volcanites de L’ile dlngonlsh ainsi que du pluton da Gulch Brook nous invite a considerer leur consangulnlte. n n[Traduit par le journal]