THE CENTRAL ATLANTIC INITIAL OPENING

The development of the Central Atlantic is well known, for the periods that follow the initiation of the oceanic accretion, dated around 175 Ma. However, the preceeding stages of its development remain less stu died. They are considered here in the conjugate margins of the ocean. Comparison between the Triassic sedimentary and magmatic rocks, and th e contemporary extensive structures in the American and African margin s of the central Atlantic shows that the rift was asymmetrical: the se dimentary rocks are coarser and they were deposited earlier on the Ame rican side than on the African one. Similarily, the thermal and magmat ic activity, coeval with the extension, was more pronounced on the Afr ican side of the future ocean. The assymmetrical rift developed as the result of the simple shear along a detachment fault corresponding to old, Alleghanian and Hercynian, thrusts. Several mechanisms of lithosp heric thinning, following the Alleghanian continental collision, may h ave triggered the motion along the detachment fault.