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...

In 1986, 1181 km of marine seismic reflection data was collected to 18–20 s of two-way traveltime in the Gulf of St. Lawrence area. The seismic profiles sample all major surface tectono-stratigraphic zones of the Canadian Appalachians. They complement the 1984 deep reflection survey northeast of Newfoundland. Together, the seismic profiles reveal the regional three-dimensional geometry of the orogen.Three lower crustal blocks are distinguished on the seismic data. They are referred to as the Grenville, Central, and Avalon blocks, from west to east. The Grenville block is wedge shaped in section, and its subsurface edge follows the form of the Appalachian structural front. The Grenville block abuts the Central block at mid-crustal to mantle depths. The Avalon block meets the Central block at a steep junction that penetrates the entire crust.Consistent differences in the seismic character of the Moho help identify boundaries of the deep crustal blocks. The Moho signature varies from uniform over extended distances to irregular with abrupt depth changes. In places the Moho is offset by steep reflections that cut the lower crust and upper mantle. In other places, the change in Moho elevation is gradual, with lower crustal reflections following its form. In all three blocks the crust is generally highly reflective, with no distinction between a transparent upper crust and reflective lower crust.In general, Carboniferous and Mesozoic basins crossed by the seismic profiles overlie thinner crust. However, a deep Moho is found at some places beneath the Carboniferous Magdalen Basin.The Grenville block belongs to the Grenville Craton; the Humber Zone is thrust over its dipping southwestern edge. The Dunnage Zone is allochthonous above the opposing Grenville and Central blocks. The Gander Zone may be the surface expression of the Central block or may be allochthonous itself. There is a spatial analogy between the Avalon block and the Avalon Zone. Our profile across the Meguma Zone is too short to seismically distinguish this zone from the Avalon Zone.

Three deep-penetration seismic reflection profiles were collected off southwest Nova Scotia to determine the crustal structure and geometry beneath the Avalon and Meguma zones of the Appalachian Orogen in Canada. Onshore geological features have been traced seawards using new gravity and magnetic anomaly maps. The seismic data can also be correlated with the previous United States Geological Survey profile in the central Gulf of Maine.Two seismically distinct lower crustal blocks are identified: the Avalon and Sable lower crustal blocks, separated by a major north-dipping reflection zone that cuts the entire crust. The recognition of the Sable block adds a fourth block to the three already identified in the Canadian Appalachians. The Sable block is overlain by the Meguma Zone. The Avalon Zone overlies at least the northern part of the Avalon lower crustal block. Although offshore extension of geological features is not unequivocal, it appears that a north-dipping reflection zone southwest of Nova Scotia marks the site of Devonian thrusting of Avalon Zone over Meguma Zone. In the Bay of Fundy to the north, two south-dipping reflection zones are interpreted as major thrusts, possibly placing Avalon lower crust over a unit with different tectonic affinities. The Fundy Fault is a Carboniferous thrust within the Avalon block along the coast of New Brunswick; this was reactivated during Mesozoic extension as a transtensional fault. Extensional displacement farther southwest was probably accommodated along east-west-trending faults and small rift basins associated with them.

Three seismically defined lower crustal blocks (LCBs) have been recognized to underlie the familiar tectonic‐stratigraphic zones of the Canadian Appalachians. Tectonic development of the orogen is inferred to have been strongly influenced by collision of the two outboard LCBs (Central and Avalon LCBs) against the sharply irregular North American craton (Grenville LCB), as evidenced by an abrupt offset in the trend of the orogen localized at this irregular edge. The LCBs are restored to a precollisional configuration by (1) removing Carboniferous strike‐slip motion on major faults linking through the Magdalen pull‐apart basin; and (2) removing Devonian(?) offset along a hypothesized fault through the present‐day Strait of Canso (Canso fault) to realign the Central and Avalon LCBs, presumed to have been continuous prior to accretion to North America. The restoration leads to (1) a qualitative explanation for deep seismic reflection observations across the Magdalen basin which suggest that portions of the underlying crust do not correspond to any of the three principal LCBs; and (2) a plate tectonic model for the orogen which suggests the backarc basin separating the Taconian (Ordovician) arc from the eastern margin of the early Paleozoic Iapetus ocean was at least as wide (>450 km) as the offset in the irregular Grenville LCB, inherited from rifting of Iapetus. The present‐day Southwest Newfoundland Transform Margin may be a segment of the Canso fault reactivated during Mesozoic opening of the Atlantic ocean.

High-resolution seismic and sediment core data from the ‘Grand Lac' basin of Lake Geneva reveal traces of repeated slope instabilities with one main slide-evolved mass-flow (minimum volume 0.13 km3) that originated from the northern lateral slope of the lake near the city of Lausanne. Radiocarbon dating of organic remains sampled from the top of the main deposit gives an age interval of 1865–1608 BC. This date coincides with the age interval for a mass movement event described in the ‘Petit Lac' basin of Lake Geneva (1872–1622 BC). Because multiple mass movements took place at the same time in different parts of the lake, we consider the most likely trigger mechanism to be a strong earthquake (Mw 6) that occurred in the period between 1872 and 1608 BC. Based on numerical simulations, we show the major deposit near Lausanne would have generated a tsunami with local wave heights of up to 6 m. The combined effects of the earthquake and the following tsunami provide a possible explanation for a gap in lake dwellers occupation along the shores of Lake Geneva revealed by dendrochronological dating of two palafitte archaeological site

Vibroseis seismic reflection data have been recorded to 18 s two-way traveltime along three transects across the island of Newfoundland. The upper crust has both steep and subhorizontal reflectors consistent with a ramp–flat style of deformation, whereas the middle and lower crust are largely free of regional flats. Reflectors descend through ca. 20 km of vertical section in the middle and lower crust to flatten into the Moho or perhaps cut through it in places. The Moho is interpreted to be no younger than the dipping reflectors. Reflection fabrics, interpreted to be indicators of dominantly Mid-Ordovician to Mid-Silurian strain, show consistent orientations among the transects and divide the crust into two blocks. A northwestern block is characterized by upper and middle crustal reflectors dipping mostly southeast at variable angles. This block is underlain to the southeast by supposedly younger and dominantly northwesterly dipping reflectors that define a northwest-tapering, wedge-shaped block floored by the Moho. This latter block is cut by isolated southeast-dipping, upper crustal reflectors near the southeast ends of the seismic transects. One of these reflectors is spatially correlated with the Bay d'Est Fault, on which the last ductile motion was south over north thrusting of Mid-Silurian age. The two crustal blocks are proposed to represent the Laurentian and Gondwanan plates juxtaposed during closure of the Iapetus Ocean. The Gondwanan plate appears to be underthrust westward beneath the Laurentian plate, perhaps by as much as 200 km.

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Reflection profiles offshore of Baffin Island, south of Lancaster Sound, reveal coast-parallel half-grabens containing flatlying sedimentary sections typical of a rifted margin. In northernmost Baffin Bay, adjacent to Devon and southern Ellesmere islands, four sedimentary basins are identified. In these basins the sedimentary sections are faulted, folded, and laterally discontinuous. The basement shows linear trends, steep faults, and abrupt variations in depth. In one basin a 50 km long and 10 km wide "flower structure" is observed. A transpressive tectonic regime is suggested to explain the uplifted sedimentary and basement section and the change in depth to basement across it. A 150 km long paleoshelf, buried by a thickening sedimentary wedge, is present offshore of Devon Island. Because strike-slip and compressional features are observed on seismic profiles north of Lancaster Sound and extensional features are observed to the south, differences in the plate tectonic development are inferred. The interpretations of the seismic reflection records are shown to be compatible with plate motions determined by matching magnetic anomalies in the North Atlantic, the Labrador Sea, the Norwegian and Greenland seas, and the Eurasia Basin. In addition, the onshore geology of Baffin Island, Labrador, and Greenland is consistent with the predrift position of the plates.Les profils de sismique rkflexion dans la rkgion extracBtikre de l'ile de Baffin, au sud du dCtroit de Lancaster, dCvoilent l'existence de demi-grabens parallkles h la cBte et incluant des coupes skdimentaires horizontales typiques d'une marge en distension. Dans la rkgion extreme nord de la baie de Baffin, adjacente aux iles Devon et Ellesmere sud, quatre bassins sedimentaires ont kt6 identifiks. Dans ces bassins, les coupes skdimentaires sont fragmentees par des failles, elles sont plisskes et discontinues latkralement. Le socle exhibe des directions linkaires, des failles subverticales et des variations abruptes de profondeur. Dans un des bassins, on observe une grande u structure en fleur de 50 km de long et 10 km de large. Un regime tectonique de transpression est invoquk pour expliquer la coupe soulevke du socle et des sCdiments et les changements de profondeur jusqu'au socle en traversant la coupe. Une ancienne plate-forme d'une longueur de 150 km, recouverte par un prisme skdimentaire nourri, est prksente au large de l'ile Devon. Le fait que l'on observe des structures de dkcrochement et de compression dans les profils sismiques au nord du detroit de Lancaster et des structures d'extension au sud, implique un dkveloppement de la tectonique des plaques avec des styles diffkrents. Les interprktations des enregistrements de sismique rkflexion sont compatibles avec les mouvements de plaques dktermints par la concordance des anomalies magnktiques dans 1'Atlantique Nord, la mer du Labrador, les mers de Norvkge et du Groenland et le bassin d'Eurasie. En outre, la gkologie des terrains de l'ile de Baffin, du Labrador et du Groenland est conciliable avec la po...