The Ainslie Detachment occurs near the base of the Carboniferous Windsor Group, forming a regional flat-lying extensional fault distributed across 10000 km2. New mapping has delineated the structure through southwestern Cape Breton Island and into central Nova Scotia. Shearing is concentrated at the top of the basal Macumber limestone along its contact with overlying evaporites and younger allochthonous units. The highly contrasting rheologies of the formations created an anisotropic zone of weakness which acted as an upper crustal stress guide, stratigraphically controlling the trajectory of the detachment through the basin. The detachment is characterized by an approximately 3 -10 m thick calc-mylonite zone, with an intense planar fabric featuring alternating very fine grained shear planes and coarser annealed layers. Coarser layers are boudinaged into pinch and swell structures, locally producing segmented augen. Highly strained intraclasts, ooids, and peloids, recrystallized carbonate boudins, and carbonate vein segments are included in the calc-mylonite as semirigid inclusions and rotated porphyroclasts. Thick zones of fault breccia straddle portions of the detachment and overprint the mylonite, demonstrating an evolution to brittle conditions during progressive shear. Listric faults in the hanging wall of the detachment feature a ramp and flat geometry, with an upper detachment occurring along the upper contact of the Windsor Group with the overlying Namurian Mabou Group. Locally up to 2 km of the stratigraphic succession has been removed, with faults cutting downsection in a westerly direction producing rollover in the hanging wall.RCsumC : Le detachement d'Ainslie apparait prBs de la base du Groupe de Windsor, il reprksente une faille rkgionale extensive, subhorizontale, distribuke dans une aire de 10 OOO km2. Un nouveau lev6 cartographique montre que cette structure s'btend au travers la rkgion sud-ouest de l'fle du Cap-Breton et dans la partie centrale de la ~ouvelle-~cosse. Une zone de cisaillements est concentrke au sommet du calcaire basal de Macumber, le long de son contact avec les Cvaporites sus-jacentes et les unitks allochtones plus jeunes. Le contraste trBs net des propriCtCs rhkologiques des formations a crkk une zone anisotropique de faiblesse, laquelle a semi i guider les contraintes dans la croCite supkrieure, et a contr6lC stratigraphiquement la trajectoire du detachement au travers le bassin. Le dCtachement est caractCris6 par une zone de mylonite calcareuse d'une puissance approximative de 3 i 10 m, avec une forte fabrique planaire exhibant des plans de cisaillement h grain trks fin et des couches recristalliskes i grain grossier. Les couches avec le grain le plus grossier sont boudinkes, elles foment des Ctranglements produisant localement une structure oeillCe segment6e. Des intraclastes ooydes et pCloYdes intenskment dkformCs, des boudins de carbonate recristallisks et des segments de filons de carbonate sont incorporks dans la mylonite calcareuse produisant des inclusions semi-rigide...

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Field and seismic data in northern Nova Scotia, eastern Canada, demonstrate that displacement transfer from steep basement faulting to bedding-parallel detachment is necessary in the development of forced folds. Lateral translation of the strata above horsted and down-dropped blocks generates a monoclinal structure which, as the faults are kinematically linked, evolves in a manner similar to fault-bend folds in thrust-and-fold belts. In the case of partial transfer of displacement a breached drape syncline is developed. The breached syncline is characterized by steep upturned beds against the fault that truncates them. In the Nova Scotia example the detachment horizon is located at the base of a Visean evaporitic sequence, and is exposed in the study area showing shearing structures within the evaporites. Brittle fault rock types (gouge and cataclasite) and meso-to microstructures were formed, including stretching lineation, principal schistosity plane and secondary shear planes, as well as intrafolial to upright asymmetrical folds. The regionally extensive weak evaporitic layer was remarkably effective in transferring displacement between the two faults, with mechanical decoupling of the strata above the evaporitic detachment being observed in the horsted block 70 km away from the steep basement fault. Moreover, displacement was also transferred as much as 40 km onto the down-dropped block at the frontal part of the system. Forced (drape) folds develop by flexure of cover rocks (usually sediments) above an underlying dip-slip fault, with the cover rocks being deformed into a broad monoclinal structure.

Early Cretaceous clastic volcanic-arc rocks of the Gambier Group in the southern Coast Belt were deposited in estuarine and marine environments on a deeply incised unconformity exposing Jurassic plutonic and arc assemblages. The Cretaceous arc was deformed in response to Late Cretaceous oblique subduction, producing orogen-parallel and orogen-normal shortening. Supracrustal Early Cretaceous rocks are preserved, in part, within the footwalls of overthrust sheets.Basal conglomerate and transgressive clastic successions underlie the volcanic edifices, with clasts reflecting volcanicplutonic provenance. Volcanic rocks are calc-alkalic and span the complete basalt-andesite-dacite-rhyolite association typical of composite volcanoes. Extensive coarse pyroclastic deposits record an explosive volcanic environment.The Gambier Group occurs within the foreland of the major structural and metamorphic culmination of the southeastern Coast Belt. Early thin-skinned thrusting occurred to the east, repeating the Cretaceous stratigraphy. Overturned detached folds are associated with southerly directed thrusting developed during orogen-parallel shortening, likely in relation to large strike-slip fault systems. Later southwest-directed thrusting and associated large-amplitude folding occurred during Late Cretaceous arc-normal shortening, folding the earlier thrusts. To the southwest, tectonic wedging developed, with much of the Gambier Group preserved in the footwall of opposite southwest-and northeast-facing thrust systems; here southwestdirected thrusts emplaced Late Jurassic plutonic rocks, an unconformity, and lower Gambier strata over younger members, whereas concomitant or younger northeast-directed back thrusts emplaced the mid-Cretaceous plutonic roots of the arc above its volcanic derivative.Les volcanites d'arc clastiques du Groupe de Gambier, d'bge crCtacC prCcoce, dans le sud du Domaine cbtier, furent dCposCs en milieux marin et d'estuaire, sur une discordance profondCment inciske exposant des assemblages d'arc et des plutonites d'bge jurassique. La subduction oblique au CrCtacC tardif a dCformC l'arc crCtacC, crtant un raccourcissement parallkle a l'orogkne et normal l'orogkne. Les roches supracrustales du CrCtacC prCcoce sont prCservCes, en partie, sous la surface de charriage.Le conglomkrat de base et les successions clastiques transgressives sont sous-jacents aux Cdifices volcaniques, incluant des clastes qui tCmoignent d'une provenance volcanique-plutonique. Les roches volcaniques sont calco-alcalines et elles sont reprCsentCes par le spectre complet de basalte-andCsite-dacite-rhyolite, une association typique des volcans composites. De vastes dCp8ts de pyroclastites grossikres indiquent un milieu caractCrisC par un volcanisme explosif.Le Groupe de Gambier apparait a l'inttrieur de l'avant-pays de la zone paroxysmale de dtformation structurale majeure et de mCtamorphisme du sud-est du Domaine c8tier. Les anciens chevauchements de faible Cpaisseur qui se sont produits a l'est ont crek une rCpCtition de la stratigraphi...

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