Connecting Cape Breton Island and Newfoundland, Canada: Geophysical Modeling of pre-Carboniferous 'Basement' Rocks in the Cabot Strait Area

Magnetic and gravity data from northeastern Cape Breton Island, southwestern Newfoundland, and the intervening Cabot Strait area were compiled and used to generate a series of maps displaying magnetic (filtered total field, first and second derivative) and gravity (Bouguer anomaly onshore, free-air anomaly offshore) information to enhance the anomaly pattern associated with regional geology. With further constraints from previously published seismic reflection interpretations and detailed maps of onshore geology, five two-dimensional subsurface models were generated.  Potential field anomalies in the offshore can be correlated with onshore faults, rock units, and pre-Carboniferous terranes.  In Newfoundland, the Cabot – Long Range Fault separates Grenvillian basement to the northwest from peri-Gondwanan Port aux Basques subzone basement in the southeast and can be traced to the Wilkie Brook Fault on Cape Breton Island.  The Cape Ray Fault/Red Indian Line merges offshore with the Cabot – Long Range Fault so that Notre Dame subzone rocks do not extend across the Cabot Strait area.  The Port aux Basques – Exploits subzone boundary crosses the strait but is likely buried by younger rocks onshore in Cape Breton Island.  Magnetic halos in the Exploits subzone are probably caused by Silurian – Devonian plutons like those in the Burgeo Intrusive Suite. The Exploits – Bras d’Or terrane boundary is located within the Ingonish magnetic anomaly, which was resolved into four overlapping components representing basement sources intruded into metasedimentary rocks and dioritic and granodioritic plutons of the Bras d’Or terrane.  The Bras d’Or terrane can be traced to the Cinq-Cerf block and Grey River areas in southern Newfoundland.  The interpretations suggest that Bras d’Or terrane ‘basement’ may underlie all of Exploits subzone, and that the Aspy terrane of Cape Breton Island is part of that subzone. SOMMAIRELes données magnétométriques et gravimétriques du nord-est de l’île du Cap-Breton, dans le sud-ouest de Terre-Neuve, et de la région du détroit de Cabot contigu, ont été compilées et utilisées pour produire une série de cartes affichant les particularités magnétiques (champ total filtré, dérivé première et seconde) et gravimétriques (anomalie de Bouguer de la côte, anomalie à l’air libre extracôtière) pour ajouter à la compréhension des motifs d’anomalie de la géologie régionale.  En tenant compte des limitations imposées par les interprétations de données de levés de sismique réflexion déjà publiées et de cartes détaillées de géologie continentale, cinq modèles 2D du sous-sol ont été produits.  Des anomalies de champ potentiel en zone extracôtière peuvent être corrélées avec des failles, des unités lithologiques et des terranes pré-carbonifères sur la côte.   Sur l’île de Terre-Neuve, la faille de Cabot-Long Range qui sépare le socle grenvillien au nord-ouest de la sous-zone de socle péri-gondwanienne, de Port-aux- Basques au sud-est, peut être reliée à la faille de Wilkie Brook sur l’île du Cap-Breton.  La faille du Cap Ray et la linéation de Red Indian se fondent au large avec la faille de Cabot – Long  Range, ce qui signifie que les roches de la sous-zone de Notre-Dame ne traversent pas la région du détroit de Cabot.  La limite de la sous-zone de Port aux Basques-Exploits traverse le détroit, mais elle est vraisemblablement enfouie sous des roches plus jeunes sur l’île du Cap-Breton.  Les halos magnétiques dans la sous-zone Exploits sont probablement causés par des plutons siluro-dévoniens comme c’est le cas de ceux de la séquence intrusive de Burgeo.  La limite du terrane Exploits-Bras d’Or est située dans l’anomalie magnétique Ingonish, laquelle s’est révélée être constituée de quatre composantes superposées représentant des sources de socle engoncées dans des roches métasédimentaires, et dans des plutons dioritiques et granodioritiques du terrane de Bras d’Or.  On peut suivre le terrane de Bras d’Or jusque dans les régions du bloc de Cinq-Cerf et de Grey River dans le sud de Terre-Neuve.  Les interprétations permettent de penser que le « socle » du terrane de Bras d’Or pourrait constituer l’assise rocheuse de la sous-zone Exploits, et que le terrane Aspy de l’île du Cap-Breton ferait partie de cette sous-zone

Tectonics of Atlantic Canada

The tectonic history of Atlantic Canada is summarized according to a model of multiple ocean opening-closing cycles. The modern North Atlantic Ocean is in the opening phase of its cycle. It was preceded by an early Paleozoic lapetus Ocean whose cycle led to formation of the Appalachian Orogen. lapetus was preceded by the Neoproterozoic Uranus Ocean whose cycle led to formation of the Grenville Orogen. The phenomenon of coincident, or almost coincident orogens and modern continental margins that relate to repeated ocean opening-closing cycles is called the Accordion Effect. An understanding of the North Atlantic Ocean and its continental margins provides insights into the nature of lapetus and the evolution of the Appalachian Orogen. Likewise, an understanding of lapetus and the Appalachian Orogen raises questions about Uranus and the development of the Grenville Orogen. Modern tectonic patterns in the North Atlantic may have been determined by events that began before 1000 m.y. Résumé L'histoire tectonique de la portion atlantique du Canada est présenté comme la résultante d'une série d'ouvertures et de fermetures océaniques. Selon ce modèle tectonique, l'Atlantique nord moderne serait actuellement dans sa phase d'ouverture. Au début du Paléozoïque, le cycle précédent de l'océan lapétus a engendré l'orogène des Appalaches. L'océan lapétus a été précédé au Néoprotérozoïque par l'océan Uranus, dont le cycle d'ouverture-fermeture a engendré l'orogène de Grenville. Le phénomène de coïncidence ou quasi-coïncidence du profil des diverses orogènes et des marges continentale modernes qui correspond aux multiples cycles d'ouvertures-fermetures se nomme l'effet accordéon. La connaissance de l'océan Atlantique nord et de ses marges continentales permet d'appréhender certaines caractéristiques de la nature de l'océan lapétus et de l'orogène appalachîen. De même, une connaissance de l'océan lapétus et de l'orogène appalachîen suscite des pistes de questionnement sur l'océan Uranus et l'orogène de Grenville. Les profils de l'océan Atlantique nord actuelle pourrait bien être le résultat d'événements qui auraient débuté il y a environ 1 000 Ma

A seismic refraction study of the Queen Charlotte fault zone

The margin between the continental North American and oceanic Pacific plates west of the Queen Charlotte Islands is uniquely marked by an active transform fault zone. The region is the locus of oblique convergence between the two lithospheric plates. West of the fault zone the absent continental shelf is replaced by a 25 km wide scarp-bounded terrace at 2 km depth which separates the oceanic and continental crust. An onshore-offshore seismic refraction survey was carried out in 1983 across the Queen Charlotte Islands region. Thirty-two explosive charges and several airgun lines were recorded on eleven land-based and six ocean-bottom instruments. A subset of the resulting data set was chosen to study the structure of the Queen Charlotte Fault zone and adjacent terrace. Two-dimensional ray tracing and synthetic seismogram modelling produced a velocity structural model of the fault region. Underlying the deformed terrace sediments is an upper 3 km thick faulted unit with velocity 3.8 km/s and a high gradient. The lower crustal region is on average 10 km thick and has velocity 5.3 km/s and a slightly lower gradient. Beneath this unit the Moho increases in dip from 5° to 19° eastward. The terrace velocities are anomalously low compared to the adjacent oceanic and continental crustal structures. Velocities of the oceanic crust are consistent with those observed in ophiolite sequences. The velocity structure of the continental crust is not well-defined; however, vertical offset of 1.1 km is seismically observed on the Rennell-Louscoone Fault on Moresby Island. Two tectonic mechanisms are proposed to explain the anomalous terrace structure. Subduction of the oceanic lithosphere beneath the terrace would accrete sediments to the seaward edge of the terrace and subduct material beneath it. Upthrusting of terrace material along near-vertical fault planes would result from compression at the transform fault above an inactive subduction zone. A combination or alternation of the two mechanisms would explain the observed fault zone structure.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat

Integrated geophysical modelling of the northern Cascadia subduction zone

The northern Cascadia subduction zone involves convergence of the Explorer Plate and northern part of the Juan de Fuca Plate with the North American Plate along a margin lying west of Vancouver Island, Canada. A wide accretionary complex which underlies the continental slope and shelf has been formed. Two allochthonous terranes, the Crescent Terrane of Eocene oceanic crustal volcanics and the Pacific Rim Terrane of Mesozoic melange sedimentary rocks and volcanics, lie against the Wrangellia Terrane backstop beneath the west coast of Vancouver Island and outcrop on the southern tip of the island. The intrusive Coast Plutonic Complex underlies the westernmost part of the British Columbia mainland east of Vancouver Island and marks the location of the historic and modern volcanic arcs. An integrated interpretation of geophysical and geological data has been conducted for the northern Cascadia subduction zone. Regionally extensive gravity and magnetic anomaly data have formed the basis of the interpretation, while surface geology, physical properties, and seismic reflection, refraction, heat flow, borehole, magnetotelluric, and seismicity data have provided constraints on structure and composition. Horizontal gradient and vertical derivative maps of the potential field data were calculated to provide additional control on the locations of major faults and lithologic boundaries. Iterative forward modelling of the gravity and magnetic anomaly data was conducted along three offshore multichannel seismic reflection lines and their onshore extensions. The two-and-a-half-dimensional (2.5-D) models extended from the ocean basin across the accretionary complex and Vancouver Island to the mainland along lines perpendicular to the major structural trends of the margin and revealed lateral changes in the location of several structural components along the length of the margin. The interpretations were extended laterally by moving the original models to adjacent parallel positions and perturbing them to satisfy the new anomaly profile data and other constraints. The models thus formed were moved to the next position and the process repeated until a total of eleven models was developed across the margin. A twelfth line across a gravity anomaly high on southern Vancouver Island was independently modelled to examine the source of this feature. An average density model for the southern half of the convergent margin was constructed by averaging the models and profiles for seven lines at 10 km spacings. This process removed anomalies due to small source bodies and concentrated on the larger features. Finally, a regional density structural model was developed by linearly interpolating between all eleven cross-margin lines to construct a block model which could then be 'sliced' open to examine the internal structure of the margin at any location. The final models allow the Pacific Rim and Crescent Terrane positions to be extended along the offshore margin from their mapped locations. The Pacific Rim Terrane appears to be continuous and close to the coastline along the length of Vancouver Island, while the Crescent Terrane either terminates halfway along the margin or is buried at a depth great enough to suppress its magnetic signature. The location of the Westcoast Fault, separating the Pacific Rim and Wrangellia Terranes, has been interpreted to lie west of Barkley Sound at a position 15 km west of its previously interpreted position. Beneath southern Vancouver Island and Juan de Fuca Strait, the Crescent Terrane appears to have been uplifted into an anticlinal structure, bringing high density lower crustal or upper mantle material close to the surface and thereby causing the observed gravity anomaly high. The western part of the Coast Plutonic Complex has been interpreted as a thin lower density layer extending from its surface contact with Wrangellia to a position 20 to 30 km further east where the unit rapidly thickens and represents the main bulk of the batholith. The complexity of the thermal regime and its effects on density in this region allows for other interpretations. Finally, a comparison of the models along the length of the margin reveals that the crust of Vancouver Island appears to thin toward the north above the shallower Explorer Plate and the complex low - high density banding used in the southern Vancouver Island models is replaced with a single high density unit on the northernmost line.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat

The oceanic crustal structure at the extinct, slow to ultraslow Labrador Sea spreading center

Two seismic refraction lines were acquired along and across the extinct Labrador Sea spreading center during the Seismic Investigations off Greenland, Newfoundland and Labrador 2009 cruise. We derived two P?wave velocity models using both forward modeling (RAYINVR) and traveltime tomography inversion (Tomo2D) with good ray coverage down to the mantle. Slow-spreading Paleocene oceanic crust has a thickness of 5?km, while the Eocene crust created by ultraslow spreading is as thin as 3.5?km. The upper crustal velocity is affected by fracturation due to a dominant tectonic extension during the waning stage of spreading, with a velocity drop of 0.5 to 1?km/s when compared to Paleocene upper crustal velocities (5.2–6.0?km/s). The overall crustal structure is similar to active ultraslow-spreading centers like the Mohns Ridge or the South West Indian Ridge with lower crustal velocities of 6.0–7.0?km/s. An oceanic core complex is imaged on a 50?km long segment of the ridge perpendicular line with serpentinized peridotites (7.3–7.9?km/s) found 1.5?km below the basement. The second, ridge-parallel line also shows extremely thin crust in the extinct axial valley, where 8?km/s mantle velocity is imaged just 1.5?km below the basement. This thin crust is interpreted as crust formed by ultraslow spreading, which was thinned by tectonic extension

Labrador Sea and Baffin Bay from a deep seismic perspective

Separation of Greenland from the North American plate resulted in the formation of two oceanic basins comprising the Labrador Sea and Baffin Bay. These two basins are linked by a bathymetric high in the Davis Strait region. Three different continental margin styles are encountered in this region. Non-volcanic margins with serpentinized mantle in the continental-ocean transition zone characterize the margins in the southern Labrador Sea as well as the northern Baffin Bay. In contrast, seaward-dipping reflection sequences and high-velocity lower crust are found along the volcanic margins of southern Baffin Bay and northern Labrador Sea. The transform margin in Davis Strait is influenced by widespread magmatic activity that resulted in the formation of thick series of lava flows, intrusions and addition of mafic material to the lower crust as imaged by high-velocity lower crustal layers. Increased interest in the region over the last ten years has resulted in the acquisition of a substantial amount of seismic data by both academia and industry. These new data have increased our understanding of the complex tectonic development of the Labrador Sea and Baffin Bay. Two recent seismic experiments are of particular interest. The DAVIS GATE project in 2008 collected coincident reflection and refraction seismic data along three lines in Baffin Bay and Davis Strait. This experiment brought the first unequivocal evidence for oceanic crust in southern Baffin Bay where no clear magnetic spreading anomalies are found. The Greenland continental margin in southern Baffin Bay is also heavily influenced by volcanism. Seaward dipping reflectors and basalts are extending up to at least 74° N. A north-south striking zone with high P-wave velocities can be correlated through the entire Davis Strait, indicating either mafic intrusions or the formation of new igneous crust along the Ungava transform system. The SIGNAL 2009 experiment collected refraction seismic data in southern Labrador Sea along pre-existing reflection seismic lines. One focus area was the Eirik Ridge off South Greenland where the data image the non-volcanic SW Greenland continental margin that was overprinted by magma associated with the later break-up of the volcanic Southeast Greenland margin. Volcanic sequences up to 4 km thick are identified on the ridge, while an up to 8-km-thick high-velocity lower crust is observed beneath the ridge. The reflection seismic data indicate the presence of inner and outer seaward dipping reflection sequences separated by an outer basement high. This suggests a subaerial formation of the lavas on the ridge. Cooling and loss of dynamic support by the Iceland plume resulted in the subsidence of the ridge. Data in the vicinity of the extinct Labrador Sea spreading axis indicate a possible oceanic core complex and exhumation of mantle