Preliminary model for an early transform history in central Atlantic rifting

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.