New Views on Alpine Tethys Evolution Based on Joides Results: ABSTRACT

Abstract

In the western central Atlantic, deep-sea drilling during Leg XI has revealed Late Jurassic to Cretaceous pelagic sediments that closely resemble Mesozoic sedimentary rocks exposed in the circum-Mediterranean orogens. This discovery is to be expected according to the concepts of plate tectonics because Alpine-type mountain ranges originate from the elimination and deformation of ocean basins and continental margins. In this context, the ophiolite belts of the central Mediterranean area are interpreted as remnants of oceanic crust and lithosphere which have been emplaced tectonically on the continental margins during Alpine orogeny. More specifically, close time correlations between the central Atlantic and Tethyan associations and palinspastic reconstructions suggest that the central Atlantic and the Alpine-Mediterranean Tethys had, during their early evolution, a parallel paleotectonic development and also were, at least from the Late Jurassic onward, connected directly to each other. In the Alpine-Mediterranean region Mesozoic pelagic facies are present in both true oceanic and continental-margin settings. In the latter, these facies overlie Bahamian-type platform carbonate rocks lying ultimately on a continental crust. The oldest, Triassic, pelagic sediments were deposited in basins resulting from early rifting. Ophiolites of possible Late Triassic age reported from Turkey suggest that a small ocean was formed in the eastern Mediterranean area at that time. During the Early Jurassic, the continental margins of the evolving Tethys were affected by block faulting and many of the former shallow-water sites were submerged; in these regions platform sedimentation was replaced by pelagic and turbiditic carbonate sedimentation and generally is analogous to the Cretaceous-to-recent geologic history in the Blake Plateau and the deeper troughs of the Bahamian region. Joides results and the magnetic-anomaly pattern suggest that this phase of block faulting 180 m.y. ago coincided with early rifting in the central Atlantic. The opening of this part of the Atlantic obviously implies transnational movements between Africa and Eurasia and, in fact, a more or less synchronous opening in the central Mediterranean Tethys is confirmed by radiometric data an the age of the oldest sediments. Transform movements are suggested by the local abundance of tectonic breccias in the oceanic realm (ophicalcites) and by indications of compressive movements as early as Late Jurassic. The oldest sediments recovered from the central Atlantic are greenish-gray and red, slightly nodular limestones of Late Jurassic age. These limestones are very similar to the Tethyan Ammonitico Rosso. However, the Ammonitico Rosso occurs in deeply submerged continental margin settings--different from what is observed generally; the first sediments deposited on the newly formed oceanic crust in the central Mediterranean were not pelagic limestones but radiolarites. As these radiolarites encroach locally on the deeply submerged continental margin, they may reflect a different current system resulting in a higher position of the calcite-compensation depth. From the Tithonian onward, white and gray chalky limestones and nannoplankton oozes were deposited in the western Atlantic and most of the deep parts of the Tethys. The transitional contact with overlying black clays indicates that compensation depth was reached in the truly oceanic realm during the middle Cretaceous. However, the occurrence of ophiolitic olistoliths in the Mediterranean association indicates the tectonic emplacement of large ophiolite nappes in the late Early Cretaceous and a diverging history of central Atlantic and Tethys from that time forward. A quantitative evaluation of the different plate movements in the Tethyan realm is still far from being reached, but comparisons of deformed and undeformed continental margins and ocean basins may help to provide some of the premises for such an attempt. End_of_Article - Last_Page 2215------------