Ocean floor in the Caribbean was formed mainly during the Jurassic and Cretaceous. Subduction of the young Caribbean ocean floor started by Late Jurassic times and persisted until the Eocene on its northern and southern borders. By the late Eocene the Caribbean area was occupied by a buoyant mass of abnormally thick and shallow oceanic crust modified from more normal crust by the intrusion of great thickness of Late Cretaceous sills. At the beginning of the Oligocene a cordon of extinct arcs surrounded the buoyant oceanic area, and transform motion replaced subduction along the northern and southern edges. Subduction was restricted to boundaries with the American and Cocos plates that were being consumed on the east and west. Convergence at about 1 cm/yr between the North and the South American plate since 38 m.y. ago appears to have been accommodated largely by internal deformation of the Caribbean plate producing a pattern of faults with an intriguing, though unexplained, resemblance to the slip lines in a modified Prandtl cell.