Abstract

This paper deals with the general features of Mesozoic and Tertiary rock sequences and paleogeography in the Alps. It seeks to outline the paleotectonic significance of the rocks and to discuss the structural evolution of the Alpine geosyncline up to the main deformation, with special reference to the sector between the rivers Rhine and Durance. Accent is placed on the relative independence of Alpine structures involving the pre-Triassic basement rocks and of cover nappes consisting only of Mesozoic and Tertiary formations.

Normal shallow-water deposits of platform or miogeosynclinal type were laid down over the whole area before eugeosynclinal conditions set in. The typical eugeosynclinal sediments in the central, Penninic belt of the Alps are the Schistes lustrés and Bündnerschiefer, with sills and submarine lava flows of basic volcanic rocks (ophiolites). Before metamorphism they consisted mainly of shales and of impure arenaceous and argillaceous limestones. The bathymetric environment of radiolarian cherts and associated rocks is examined, and their deep-water origin is upheld for the Alpine occurrences. Marine polygenic breccias are characteristic of geosynclinal slopes (commonly fault scarps) and not of a particular depth zone. The Alpine Flysch is a particularly significant sediment. Flysch is a thick marine deposit of predominantly detrital rocks, in part turbidites, generally without volcanic rocks, and laid down during compressional deformation of the geosyncline. Of the many different kinds of Flysch some represent transitions to either Bündnerschiefer or Molasse. Essential differences between the early synorogenic Flysch and the later synorogenic to post-orogenic Molasse are listed.

Argand's stimulating embryotectonic theory of the evolution of the geosyncline is outlined and rejected. The Triassic corresponds to a neutral interval, between the Hercynian and Alpine cycles. Early Alpine geosynclinal history was characterized by vertical or tensional movements along normal faults that limited narrow platforms and rapidly subsiding troughs. This tensional deformation weakened in the Late Jurassic, whereupon bottom relief diminished, and “para-oceanic” conditions prevailed over a large part of the Mediterranean realm. New linear welts of compressional origin arose during the Cretaceous and developed into steep island chains limiting the Flysch basins. Gradually the central part of the Alps rose above sea level, and the geosyncline migrated to the north and west. Here it was finally filled and gave way to the foreland trough in which the Molasse was laid down.

There is no simple, direct, genetic and space relationship between Hercynian structures, fault-bordered platforms and troughs of the earlier geosynclinal phase, island chains and basins of the Flysch phase, and the later nappe structures. Paleogeographical features are commonly short-lived and migratory.

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