TETHYAN OPHIOLITES, EMPLACEMENT MODELS AND ATLANTIC SPREADING

The relative motions between Africa and stable Europe during Mesozoic and Cenozoic time are defined by the plate circuit Africa-North America-(Greenland)-Europe based on Atlantic ocean-floor spreading . Recent changes to the geological time-scale do not significantly alter the calculated motions.

The relative motion of Africa to stable Europe is remarkably simple and at the same time rather puzzling. In the first phase, from about 175 Ma (~Bajocian) to about 140 Ma (earliest Valanginian) Afro-Arabia slides approximately SSE past western Europe. The rate obviously varies from place to place, but in the Mediterranean region it is about 40-50 mm yr^-1. In phase two from ~140 Ma to ~125 Ma, from earliest Valanginian to the Barremian/Albian boundary, the motion slows to less than 5 mm yr^-1. It then speeds up again in phase three to about 20 mm yr^-1, ending in the Campanian at ~75 Ma. Phase four is another slow interval of about 4 mm yr^-1, from ~75 Ma to ~55 Ma, or Campanian to earliest Eocene. The final phase is at about 15 mm yr^-1, slowing to less than 10 mm yr^-1 in the last 20 m.y. The motion changes direction abruptly by more than 60º from phase two to phase three, and by more than 40º from phase three to phase five.

The emplacement phase of the mid-Jurassic ophiolites of Greece and former Yugoslavia onto adjacent passive continental margins ended in latest Jurassic to early Cretaceous time. It coincided with the slow-down of phase two in the relative motion between Africa and Eurasia. The similar slow-down of phase four coincided broadly with the end of the emplacement of the late Cretaceous ophiolites in Cyprus, Syria, Turkey and Oman. There are no other major changes in the relative motions between Africa and Europe in the interval 180-0 Ma. Both sets of ophiolites have a trace element signature that indicates formation above a subduction zone, i.e. they are supra-subduction zone (SSZ) ophiolites.

The slow phases of relative motion and the timing of ophiolite emplacement can be understood in terms of the simple “roll-back” model, modified here, developed by the Open University to account for the emplacement of the Oman ophiolite and its SSZ chemistry. Ophiolite emplacement onto an adjacent continent can be regarded as the attempted subduction of a continental margin at a plate boundary, which must eventually cease. Given that the plate boundary concerned is probably the major plate boundary between Africa and Europe, it follows that the motions between Africa and Europe will, as inferred, slow down and eventually stop until a new margin is created elsewhere. The time taken for subduction that has been terminated at one location to start up at another location is estimated at about 15-20 m.y. Once a new margin between Africa and Europe has been established the rates of relative motion return to higher values. Thus the emplacement of the SSZ ophiolites seems to have influenced the spreading pattern in the Atlantic Ocean.