THE CENTRAL ATLANTIC RIFTING - REACTIVATION OF PALEOZOIC STRUCTURES

During Late Triassic and Early Liassic times,clastic and evaporitic se quences were deposited in Morocco, eastern Canada and the U.S.A., in a ngular unconformity, upon deformed Palaeozoic rocks. This unconformabl e sedimentary cover was accompanied by tholeiitic flows, sills and dik es, In Morocco, all of these rocks show evidence of a thermal episode, isotopically dated to around 200 Ma, which represents the synrift met amorphism. The American and African Upper Triassic-Lower Liassic seque nces present several differences: -In the nature of the depositional e nvironment, lacustrine in the onshore American basins and lagoonal and marine in the offshore American basins and onshore African basins. -I n the shape of the basins which are mainly halfgrabens in America and symmetric grabens in Morocco. -In the thermal regime, at most incipien t in America, but strong enough to have favoured development of a synr ift metamorphism in Morocco, -In the volume of the emitted magnas, bei ng more abundant in the African margin. All of these differences sugge st that the Atlantic rifting was asymmetrical, probably controlled by an E-dipping detachment fault. This crustal and/or lithospheric struct ure is thought to correspond to Palaeozoic shear zones, reactivated du ring the Mesozoic extension, at the end of the post-Alleghanian lithos pheric delamination.