Alpine Deformation and the Oceanic Areas of the Tethys, Mediterranean, and Atlantic

Abstract

A preliminary least-squares fit of the 1,000-m contour of the continental fragments around the Mediterranean is presented. It extends the Atlantic reassembly of Bullard and others to the Balearic Islands, Corsica, Sardinia, Italy, Yugoslavia, Greece, and Turkey. Geological and geophysical data suggest that the reconstruction applies to Permo-Triassic time. The large gap between southern Eurasia and the northern and eastern parts of the reassembled fragments shows the probable shape of the Tethyan ocean of this period. During Permo-Triassic time the Mediterranean is thought not to have existed, and therefore it should not be regarded as a remnant of the Tethys. As in several other syntheses, the formation of the Mediterranean, together with that of the Alpine orogenic belts around it, are attributed principally to the relative movements between Africa and Eurasia. Unlike these syntheses, the nature of this motion has been estimated much more precisely by constructing a simple sea-floor spreading model of the central and northern Atlantic Ocean. The model permits the relative movement hypothesis to be tested more rigorously than previously. Available spreading data show that at least three phases of relative movement must have taken place. Relative to Eurasia, the opening model shows the earliest phase consists of an eastward movement of Africa in Lower Jurassic to early Upper Cretaceous time, followed by a westward movement in later Upper Cretaceous to late Eocene time, finally terminating with latest Eocene to present-day northward movement. Anomalous Early Permian and Cretaceous paleomagnetic data, the contrasting age differences between ophiolite-chert sequences in the eastern and western Mediterranean region, and the positions of the Alpine orogenic belts are all shown to be consistent with general features of the reassembly and the opening model. The reassembly and the model also permit a self-consistent plate tectonic interpretation to be made of Lower Jurassic geology, which suggests the existence of a single, irregular plate margin between Africa and Eurasia, whose rotation pole was close to the best-fit pole of Africa and North America on the Atlantic reassembly. It is believed to link such diverse features as central Atlantic opening, sea-floor spreading in Yugoslavia and Albania, transcurrent faulting in the eastern Alps, and compressional features in the Crimea and the Great Caucasus. Although Lower Jurassic geology is consistent with a single irregular plate margin, it is shown that, for the reassembly and opening model adopted, all the larger fragments (Spain, Corsica-Sardinia, Italy, and Yugoslavia-Greece-Turkey) must at times have belonged to a plate or plates independent of Africa and Eurasia. Because the Atlantic spreading pattern has been grossly simplified, and also because verifiable reassemblies of later times cannot yet be made, plate tectonic interpretations of such periods are premature, although partial interpretations are still possible. A possible interpretation is given of a phase of movement of the south Atlas fault and of the relationship of the ophiolite-chert sequences in the Oman Mountains and the Zagros Range of Iran to Atlantic sea-floor spreading. A number of fundamental problems emerge, which include the necessity to postulate different mechanisms for eliminating oceans at compressive plate margins, and the conclusion that present interpretations of structures within orogenic belts provide poor estimates of the relative displacement vectors that have produced them.