Abstract

Dextral and sinistral shear along an arcuate intracontinental transform system parallel to the present coastlines of eastern North America and western Africa is believed to have played a significant role in the decoupling of continental blocks during the extremely complex tectonic history of central Atlantic rifting. Near-pole small-circle transform systems are developed as wide zones of shear during counterclockwise and clockwise rotations of Africa and South America relative to North America about a pole of rotation in the Sahara. Transform and rifting histories are combined in a model proposed for Atlantic rifting that includes the following phases of rotation interacting with the linear belt of pre-existing Appalachian structural grain: (1) Late Permian to Middle Triassic counterclockwise rotation of Africa with dextral shear localizing an initial thermal arch and early sedimentation in southern Appalachian grabens along a reactivated structural grain; (2) dominant Late Triassic to Early Jurassic extensional reactivation in a northwest-southeast direction, with minor plate separation producing the main phase of graben sedimentation and a major 190 m.y. B.P. basalt sill and flow event; (3) Middle Jurassic clockwise rotation of Africa with sinistral shear resulting in the folding and faulting of graben sediments and the injection of basalt dikes 175 m.y. B.P. along a distinctive fan-shaped distribution of extensional structures; and (4) separation of the North American and African continents parallel to early Atlantic fracture-zone trends with the onset of plate accretion.

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