The Gulf of Mexico and Caribbean Sea are a zone of north-south extension and left-lateral shear opened between the Americas as those continents moved westward from Africa. The movements are related to ocean-floor spreading from the mid-Atlantic ridge. To accommodate spreading, the ridge itself migrates westward from Africa. Ridge migration is radial outward from Africa and results in opening triangular sheared grabens with apexes against Africa. A new ridge segment extends across these openings. Spreading rates vary and the migrating and extending ridge is sheared on fracture zones in response to these variations.

The currently popular related concepts of plate tectonics and transform faults are inconsistent with ridge migration and shear because those concepts do not allow for shear on fracture zones beyond ridge offsets and in the directional sense indicated by the position of ridge segments. Ridge migration and shear are a

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necessary complication of the spreading hypothesis. T-intersections of ridges are explained as intersections between a spreading and migrating ridge with a shear. The shear is only active on the side of the ridge toward which the migration is taking place. The junction of the mid-Atlantic ridge with the Azores-Gibraltar ridge is an example of such a feature.

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