GEOSCIENCE CANADA Volume 41 2014 165 HAROLD WILLIAMS SERIES serpentinized continental mantle onto SOMMAIRE the seafloor. The OCT on present-day Des travaux d’imagerie sismique et des margins is difficult to sample, and so forages profonds ont montré que la much of our knowledge on the transition océan-continent (OCT) de detailed nature of OCT sequences marges continentales de divergence comes from obducted, magma-poor pauvre en magma exposée de nos OCT ophiolites such as those pre- jours, correspond à une zone d’hyper- served in the upper portions of the étirement tectonique caractérisée par Alpine fold-and-thrust belt. Allochtho- un amincissement extrême de la croûte nous, lens-shaped bodies of ultramafic continentale, qui a exhumé sur le fond rock are common in many other marin, jusqu’à la tranche la plus pro- ancient orogenic belts, such as the fonde de la croûte continentale, voire The Ocean – Continent Caledonian – Appalachian orogen, yet du manteau continental serpentinisé. Transition Zones Along the their origin and tectonic significance Parce qu’on peut difficilement échantil- remains uncertain. We summarize the lonner l’OCT sur les marges actuelles, Appalachian – Caledonian occurrences of potential ancient OCTs une grande partie de notre compréhen- Margin of Laurentia: within this orogen, commencing with sion des détails de la nature de l’OCT Examples of Large-Scale Laurentian margin sequences where an provient d’ophiolites pauvres en OCT has previously been inferred (the magma d’une OCT obduite, comme Hyperextension During the Dalradian Supergroup of Scotland and celles préservées dans les portions Opening of the Iapetus Ireland and the Birchy Complex of supérieures de la bande plissée alpine. Newfoundland). We then speculate on Des masses lenticulaires de roches Ocean the origin of isolated occurrences of ultramafiques allochtones sont com- Alpine-type peridotite within Laurent- munes dans de nombreuses autres ban- David M. Chew1 and Cees R. van ian margin sequences in Quebec – Ver- des orogéniques anciennes, comme Staal2 mont and Virginia – North Carolina, l’orogène Calédonienne-Appalaches, 1Department of Geology focusing on rift-related units of Late mais leur origine et signification tec- Trinity College Dublin Neoproterozoic age (so as to eliminate tonique reste incertaine. Nous présen- Dublin 2, Ireland a Taconic ophiolite origin). A combina- tons un sommaire des occurrences E-mail: [email protected] tion of poor exposure and pervasive d’OCT potentielles anciennes de cet Taconic deformation means that origin orogène, en commençant par des 2Natural Resources Canada and emplacement of many ultramafic séquences de la marge laurentienne, où Geological Survey of Canada bodies in the Appalachians will remain la présence d’OCT a déjà été déduites Vancouver, BC, Canada, V6B 5J3 uncertain. Nevertheless, the common (le Supergroupe Dalradien d’Écosse et occurrence of OCT-like rocks along d'Irlande, et le complexe de Birchy de SUMMARY the whole length of the Appalachian – Terre-Neuve). Nous spéculons ensuite A combination of deep seismic imag- Caledonian margin of Laurentia sug- sur l'origine de cas isolés de péridotite ing and drilling has demonstrated that gests that the opening of the Iapetus de type alpin dans des séquences de the ocean-continent transition (OCT) Ocean may have been accompanied by marge des Laurentides du Québec-Ver- of present-day, magma-poor, rifted hyperextension and the formation of mont et de la Virginie-Caroline du continental margins is a zone of hyper- magma-poor margins along many seg- Nord, en nous concentrant sur les extension characterized by extreme ments. unités de rift d'âge néoprotérozoïque thinning of the continental crust that tardif (pour éviter les ophiolites du exhumed the lowermost crust and/or Taconique). La conjonction d’affleure- Geoscience Canada, v. 41, http://dx.doi.org/10.12789/geocanj.2014.41.040 © 2014 GAC/AGC® 166 ments de piètre qualité et de la défor- the degree and nature of magmatism km, OCT sequences have only recently mation taconique omniprésente, signi- associated with hyperextension varies been recognized (e.g. the Laurentian fie que l'origine et la mise en place de (e.g. Müntener and Manatschal 2006) margin of Scotland and Ireland, Chew nombreuses masses ultramafiques dans and hyperextension is not exclusive to 2001; Henderson et al. 2009; Baltic les Appalaches demeureront incer- magma-poor margins such as Iberia. margin of Norway, Andersen et al. taines. Néanmoins, la présence For example the northeast Atlantic 2012; Laurentian margin of New- fréquente de roches de type OCT tout ‘volcanic’ margin was affected by foundland, van Staal et al. 2013). Long le long de la marge Calédonnienne- hyperextension processes in the Late linear belts of ultramafic rocks are Appalaches de Laurentia suggère que Jurassic – Early Cretaceous (Osmund- common within the Caledonian – l'ouverture de l'océan Iapetus peut sen and Ebbing 2008; Lundin and Appalachian orogen (e.g. the avoir été accompagnée d’hyper-étire- Doré 2011). Appalachian serpentinite belts of Hess ment et de la formation de marges Allochthonous, lens-shaped 1939, 1955) and the origin and tectonic pauvres en magma le long de nom- bodies of ultramafic rock are common significance of many of these isolated breux segments. in many orogenic belts, particularly the occurrences of Alpine-type peridotite Alps. Along the western boundary of remains uncertain. INTRODUCTION the Austroalpine nappes in eastern Our knowledge of the ocean-continent Switzerland, podiform ultramafic bod- CRITERIA FOR IDENTIFYING AN transition (OCT) of magma-poor pas- ies are found in close association with OCEAN-CONTINENT TRANSITION sive margins has increased significantly dolerite dykes and radiolarian chert, (OCT) IN THE GEOLOGICAL RECORD since exhumed mantle rocks were first and have long been regarded to be dredged and then drilled during ODP characteristic of the deep ocean floor The Field Relationships of OCT Leg 103 off the Western Iberian mar- (Steinmann 1905). This rock associa- Rocks gin (Boillot et al. 1980, 1987). A com- tion in the Alps, later referred to as the On present-day rifted margins, the bination of modern high-quality geo- ‘Steinmann Trinity’, has traditionally OCT is typically covered by a thick pile physical data, deep sea drilling and been regarded to represent Tethyan of sediments at abyssal depths; the comparative studies of analogue areas oceanic mantle sequences that have Iberia–Newfoundland conjugate rifted onshore (e.g. Manatschal 2004; Péron- been imbricated within ophiolite com- margin (Fig. 1) is the only location Pinvidic and Manatschal 2009) has plexes. However, the discovery of dis- where a ‘complete’ OCT sequence has shown that the OCT of magma-poor tal margin sequences directly overlying been drill-intersected across a magma- passive margins is a zone of hyperex- subcontinental mantle in many places poor rift system. Little is therefore tension characterized by extreme thin- in the Alps (see Manatschal and Mün- known about the detailed nature of ning of parts of the continental crust, tener 2009 for a historical review) sup- OCT rock types on present-day rifted resulting in exhumation of the lower- ports the idea that at least some of the margins, and the low resolution of most crust and/or serpentinized conti- ophiolites in the Alps represent ancient deep seismic imaging techniques means nental mantle onto the seafloor (e.g. OCTs, similar to the Western Iberian that the structural and intrusive rela- Iberian margin, Tucholke et al. 2007; and Newfoundland margins. The tionships of the various OCT compo- Sibuet and Tucholke 2013). Serpen- Alpine Tethyan OCT ophiolites typi- nents are difficult to ascertain (Man- tinization is facilitated by the move- cally contain only minor amounts of atschal and Müntener 2009). Hence, ment of large volumes of water from mafic igneous rocks and are character- much of our knowledge about the the surface down into the mantle along ized by blocks of ancient subcontinen- structural, magmatic, hydrothermal and major extensional structures, the tal mantle exhumed by top-down base- sedimentary record of continental largest of which is typically a concave- ment detachment faults and overlain by breakup and early seafloor spreading downwards lithosphere-scale master extensional allochthons, tectono-sedi- comes from obducted OCT ophiolites, detachment (e.g. Manatschal 2004; mentary breccias and a post-rift sedi- the best examples of which occur in Manatschal et al. 2007, 2011; Sutra and mentary sequence similar to that of the the Alpine Tethyan domain such as the Manatschal 2012). adjacent distal continental margin (e.g. Platta, Tasna and Chenaillet ophiolite Rifted continental margins Manatschal 2004; Manatschal and units (Manatschal and Müntener 2009). have been divided into two types Müntener 2009). Similar OCT The Platta and Tasna units (Fig. 2a) are depending on the amount of rift-relat- sequences have been reported in the believed to represent the OCT of the ed magmatism. A ‘volcanic’ or magma- Pyrenees (Lagabrielle and Bodinier former Adriatic and European/Bri- rich margin is characterized by sea- 2008; Lagabrielle et al. 2010) but the ançonnais conjugate rifted margins, ward-dipping reflectors typical of sub- recognition of OCT ophiolites in older whereas the Chenaillet unit (Fig. 2b) aerial lava flows which mask the rift- orogenic belts has received less atten- has an affinity closer to that of true related extensional structures, whereas tion. oceanic crust. All three units