The central Atlantic rifting: Reactivation of Palaeozoic structures?

Abstract

During Late Triassic and Early Liassic times, clastic and evaporitic sequences were deposited in Morocco, eastern Canada and the U.S.A., in angular unconformity, upon deformed Palaeozoic rocks. This unconformable sedimentary cover was accompanied by tholeiitic flows, sills and dikes. In Morocco, all of these rocks show evidence of a thermal episode, isotopically dated to around 200 Ma, which represents the synrift metamorphism. The American and African Upper Triassic-Lower Liassic sequences present several differences: — In the nature of the depositional environment, lacustrine in the onshore American basins and lagoonal and marine in the offshore American basins and onshore African basins. — In the shape of the basins which are mainly halfgrabens in America and symmetric grabens in Morocco. — In the thermal regime, at most incipient in America, but strong enough to have favoured development of a synrift metamorphism in Morocco. — In the volume of the emitted magnas, being more abundant in the African margin. All of these differences suggest that the Atlantic rifting was asymmetrical, probably controlled by an E-dipping detachment fault. This crustal and/or lithospheric structure is thought to correspond to Palaeozoic shear zones, reactivated during the Mesozoic extension, at the end of the post-Alleghanian lithospheric delamination.