BREAKING THE CRATONIC EQUATORIAL ATLANTIC BRIDGE: WHY THERE IS NO SAHARAN OCEAN

In the Mid Cretaceous, transtensional shear in the Equatorial Atlantic domain caused the ultimate breakup between South American and African lithospheres. Rifting along the present-day conjugate South Atlantic margins up into the Benue Trough as well as in the Central and West African Rift systems is initiated in the early Cretaceous (~145 Ma). Progressive crustal thinning and subsidence in these intracontinental rifts continue for about 20-25 Myrs when geological data points to an onset of strike-slip related deformation and transtensional basin formation along the present-day conjugate Equatorial Atlantic margins.

We investigate the succession of events by integrating quantified plate reconstructions with 3d thermomechanical modelling using the elasto-visco-plastic finite element code SLIM3D. Our modelling shows that oblique extensional deformation requires less tectonic force than normal extension. Hence, of two competing rift zones with different orientation but otherwise identical properties, the one with higher obliquity is energetically preferred. Applying a constant force at the extensional model boundary together with nonlinear localization effects in the rift zone induces continental break-up only after a distinct rift period.

We argue that after about 10-15 Myrs of normal extension in the South Atlantic and West African rifts at relatively slow velocities, deformation localised in the Equatorial Atlantic domain. This effectively terminated major lithospheric extension related to the South Atlantic opening north of the Benue region in Africa. The modelling further shows that at constant tectonic force, extensional velocities in an established rift will increase significantly, hence resulting in an increase of plate velocities of South America relative to Africa, implying a multi-velocity extensional history of the conjugate South Atlantic passive margins.