Seismic reflection data in the Gulf of St. Lawrence were used to construct an isopach map of the Horton Group (Middle Devonian-Early Carboniferous) in the Magdalen Basin. The map shows that the Horton Group was deposited in basins that developed parallel to the offshore extension of structural trends in New Brunswick and the Gaspe Peninsula. Horton Group strata were deposited in narrow linear fault bounded basins, presently up to 8 km deep, and in broad sag basins up to 3 km deep. The sag basins may thin latterally to form sedimentary veneers less than 1 km thick. The fault bounded basins are mostly half-grabens and they developed during an early crustal extension phase. The largest and deepest basin strikes north east-southwest, almost parallel to the Moncton Basin in New Brunswick. Sag basins and thin veneers are not fault controlled and are widespread, underlying most of the Magdalen Basin. They probably formed later than the fault bounded basins in response to the onset of a regional subsidence phase.Deformation of Horton Group rocks in the Magdalen Basin is concentrated in fault zones up to 20 km wide, and mainly affected deep subbasins, where thrust faults and possible flower structures are observed. An early deformation phase oc curred towards the end of Horton Group sedimentation (Late Toumaisian-Early Visean) and a later phase occurred, with less intensity, during the deposition of Visean to ?Namurian sediments. The first phase of deformation affected most of the Magdalen Basin, whereas the second phase affected mainly the southern areas. La deformation des roches du Groupe d'Horton dans le bassin de Madeleine est concentree dans des zones de failles dont la largeur atteind 20 km, et elle a principalement affecte des sous-bassins profonds ou des failles de chevauchement et peut-etre des "flower structures" sont observees. La premiere phase de deformation a eu lieu vers la fin de la sedimentation du Groupe d 'Horton (Tournesien Tardif-Viseen precoce), puis une autre, moins intense, a eu lieu pendant le depot des sediments du Viseen au Namurien(?). La premiere phase de deformation a affecte la majeure partie du bassin de la Madeleine, tandis que la seconde en a principalement affecte les regions meridionales.[Traduit par la redaction]

We compiled industry seismic and LITHOPROBE deep seismic data in the southwestern Gulf o f St. Lawrence, and interpreted them together with gravity, magnetic and industry well data, to study the deepest sediments, which are assignable to the Horton Group o f Late Devonian-Early Carboniferous age, and the basement rocks of the northern Appalachians.Three upper crustal basement blocks with differing seismic character and orientation of geological structures were identified. South o f the Appalachian structural front and parallel to it, two blocks with a northwest regional strike were interpreted to be allochthonous on Grenvillian crust.

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The structure and tectonic evolution of an evaporite basin are investigated in this case study, which combines the interpretation of magnetic data with the more commonly applied seismic reflection and gravity methods. The Maritimes Basin contains up to 18 km of Upper Palaeozoic sedimentary rocks resting on the basement of the Acadian orogeny. Carboniferous rocks are intensely deformed to the southeast of the Magdalen Islands as a result of deformation of evaporites of the Vis ean Windsor Group. Short-wavelength (<5 km) magnetic lineations define NNE-and ENE-trending linear belts, coincident with the mapped pattern of salt structures. Magnetic models show that these lineations can be explained by the infill of subsidence troughs by high-susceptibility sediment and/or the presence of basaltic rocks, similar to those uplifted and exposed on the Magdalen Islands. Additional shallow, magnetic sources are interpreted to result from alteration mineralization in salt-impregnated, iron-rich sedimentary rocks, brecciated during salt mobilization. Magnetic susceptibility measurements of samples from the Pugwash mine confirm the presence of higher susceptibility carnallite-rich veins within salt units. Salt tectonism and basin development were influenced by the structure of the base group, the deepest regionally continuous seismic reflections (ca. 5-11 km), associated with an unconformity at the base of the Windsor Group, sampled at the Cap Rouge well. Salt structural evolution, formation of the magnetic lineations and geometry of the base group are associated with regional dextral transpression during basin development (late Carboniferous) and/or Alleghanian Orogeny (late Carboniferous to Permian). In this and similar studies, the effective use of magnetics is dependent upon the presence of rocks of high magnetic susceptibility in contrast to the low-susceptibility salt bodies. In the absence of high-susceptibility rocks, magnetic lows over the salt structures may be modelled, similar to commonly applied gravity techniques, to derive the internal structure and geometry.