Geophysical studies of the structure of the Appalachian orogen in the Atlantic borderlands of Canada

Abstract

Results of 6000 km of crustal seismic profiling are presented with gravity and aeromagnetic maps for the Appalachian orogen in eastern Canada. Wide-angle seismic surveys show that the central mobile belt of the orogen has a thinner crust than its margins. High-velocity lower crust, attributed to underplating, is found below the former Laurentian continental margin in Newfoundland and below the Magdalen basin. Potential field data are used to trace the surface zones of the orogen from the northeast Newfoundland shelf to Cape Breton, but extrapolation to New Brunswick and Quebec is unclear because of late Paleozoic basin development. The central mobile belt of the orogen is only a few tens of kilometres wide in southwest Newfoundland and Cape Breton, but broadens substantially to around 200 km elsewhere. Reflection images show a strong deep-crustal fabric that runs along the orogen, with a margin that crosses into the Avalon zone in southern Newfoundland but coincides with the Avalon-Gander zone boundary elsewhere. The fabric formed during mid-Silurian continental collision and (or) during postorogenic collapse. Variation in fabric pattern and metamorphic grade, tightening of structures towards southwest Newfoundland and Cape Breton, and voluminous plutonism in southern Newfoundland are all in accord with maximal crustal thickening followed by erosion and isostatic readjustment in southwest Newfoundland and Cape Breton, and relatively little in northeast Newfoundland and its adjacent shelf. Reflection fabrics in the upper crust appear to be detached from those in the deeper crust; this is attributed to rheological contrast across the base of a quartz-rich upper crust.Résumé : Nous présentons les résultats d'un profil sismique crustal complété sur une longueur de 6000 km et des cartes de levés gravimétriques et aéromagnétiques de l'orogène des Appalaches de l'Est au Canada. Les relevés de sismique grand-angle montrent que la ceinture mobile centrale de l'orogène possède une croûte qui est plus mince que ses marges. La vitesse élevée dans la croûte inférieure, attribuée au sous-placage, est observée sous l'ancienne marge continentale Laurentienne de Terre-Neuve et sous le bassin de la Madeleine. Les données de champ de potentiel sont utilisées pour tracer les zones de surface de l'orogène comprises entre le nord-est de la plate-forme de Terre-Neuve et le Cap Breton, quoique l'extrapolation au Nouveau-Brunswick et au Québec n'est pas claire en raison du développement tardif du bassin paléozoïque. La largeur de la ceinture mobile centrale de l'orogène n'est que de quelques dizaines de kilomètres dans le sud-ouest de Terre-Neuve et au Cap Breton, mais ailleurs elle augmente substantiellement jusqu'à environ 200 km. Les images des réflexions montrent une fabrique crustale profonde très bien développée qui s'étend le long de l'orogène, avec une marge qui traverse la zone d'Avalon dans le sud de Terre-Neuve, mais ailleurs elle coïncide avec la limite des zones de Avalon-Gander ailleurs. La fabrique s'est formée durant la c...

Comparison of lithosphere structure across the Orphan Basin–Flemish Cap and Irish Atlantic conjugate continental margins from constrained 3D gravity inversions

“…There is no evidence for significant crustal extension in these areas and 30 km is therefore regarded as the unstretched crustal thickness in the Irish margin. In contrast, the Newfoundland margin exhibits greater variability of 'unstretched' crustal thickness (30 km beneath Flemish Cap (Funck et al 2003); 36 km beneath the Grand Banks (Lau et al 2006a); 40 km for the onshore Avalon zone, 45 km for the Proterozoic Grenville province of western Newfoundland and 30-37 km for the onshore crust between those two regions (Hall et al 1998)). Although this variability does generally correlate with basement terranes and their inferred boundaries onshore Newfoundland, the contrast in crustal thickness between the Grand Banks and Flemish Cap occurs within the same inferred basement terrane, suggesting some rheological or deformational variability within terranes.…”

“…A phase of Cretaceous (Aptian-Albian) rifting was part of a major plate reorganization Hall et al (1998) and Tyrrell et al (2007). Locations of magnetic anomaly 34 (from Srivastava et al 1988Srivastava et al , 1990) are plotted as dotted line segments.…”

Geology and Hydrogeology of Faults on Cape Breton Island, Nova Scotia, Canada: an overview

“…Cape Breton Island was positioned where the St. Lawrence promontory on the western (Laurentian) margin and the Cabot promontory on the eastern (Gondwanan) margin met obliquely (Stockmal et al 1987;Lin et al 1994). As a result the Gondwanan terranes of Cape Breton Island were telescoped into a width of only tens of kilometres in comparison to around 200 km (Hall et al 1998) elsewhere (Fig. 1).…”

“…The geology of the Hollow fault -Aspy fault zone has been investigated by, for example, Gibling et al (1987), Hall et al (1998), Braid andMurphy (2006), andRehill (1996). It is an approximately 20 km wide zone of complex structure (Fig.…”

Logan Medallist 2. Geophysics and Geology: An Essential Combination Illustrated by LITHOPROBE Interpretations–Part 1, Lithospheric Examples

“…Accessing available roads, the northern profile crossed the wide part of the Central Mobile belt and the southern one crossed the narrower part. Quinlan et al (1992) presented an initial interpretation of the contractor-processed data set and Hall et al (1998) included a more comprehensive interpretation that also considered the wide-angle reflection/refraction and potential field studies that were carried out. The data quality for these interpretations was not particularly good.…”