Radiolarite ages in Alpine-Mediterranean ophiolites: Constraints on the oceanic spreading and the Tethys-Atlantic connection

Abstract

The history of continental breakup and oceanic spreading of the Alpine Tethys is defined by a revision of isotopic and biochronologic ages of 65 stratigraphic sections located in the Alps, Apennines, Betic Cordillera, Rif, and central Atlantic and a reinterpretation of the stratigraphic sequences of surpraophiolitic radiolarites. The biochronology of radiolarites is revised by using the deterministic approach known as the unitary association method. During the early Bajocian (unitary association zone, UAZ 3) radiolarite sedimentation began at the continental margin. Biochronologic ages determined in the lowermost radiolarites in basinal sequences of Tethyan margins are synchronous and mark a regional change in sedimentation regime in the Alpine Tethys. The onset of oceanic spreading of the Alpine Tethys is dated by isotopic methods as Bajocian, and is consistent with the timing of the structural evolution of the continental margins. The earliest fragments of Tethyan oceanic crust are characterized by the associations of ophiolites with deep-sea sediments, and coarse reworked sediments including platform and continental basement fragments. The earliest ophiolites also show geochemical affinities with synrift and transitional mid-oceanic-ridge basalts. The oldest radiolarites on oceanic crust are so far dated as Bathonian (UAZ 6) and are located in the Gets nappe (western Alps), in the Balagne nappe (Corsica), and in the central Atlantic (Deep Sea Drilling Project [DSDP] Site 534A). The oldest remnants of Alpine Tethyan crust have been identified in weakly metamorphosed cover nappes that occupy an external tectonic position in the Alpine orogenic belts, as compared to the main ophiolitic sutures. Thus, the older relics of oceanic lithosphere were the first to be accreted and transported onto the foreland during the collision. Siliceous sedimentation during the early Bajocian is correlated with westward deep-water circulation in the Alpine Tethys related to the opening of deep seaways between Laurasia and Gondwana. In the central Atlantic no radiolarites, but thin radiolarian-rich layers, were deposited during the earliest Bathonian (UAZ 6). The similarity between radiolarian faunal assemblages and ages in the Northern Alps, Gets nappe, Betic Cordillera, and Site 534 (DSDP Leg 76) suggest a Middle Jurassic connection between the Alpine Tethys and central Atlantic. Biochronologic and isotopic ages currently indicate that oceanic spreading of the Alpine Tethys began during the Bajocian and continued until the Kimmeridgian.