Ancient oceans and continental margins of the Alpine- Mediterranean Tethys : deciphering clues from Mesozoic pelagic sediments and ophiolites

In contrast to Northern Europe where, in the 19th Century, a coherent lithostratigraphy and biostratigraphy was established, many of the ‘odd rocks of mountain belts’ (as christened by Alfred G. Fischer) of the Alpine- Mediterranean region defied an easy interpretation in terms of the simple English layer-cake stratigraphy mapped by William Smith. However, as early as 1862, Eduard Suess recognized close affinities between Mesozoic faunas of the Alps and those of the Himalaya. Suess also described many such Alpine sedimentary rocks as pelagic (a term discussed by Antoine Lavoisier in the context of sea-level change), and the results of the Challenger Expedition (1872 to 1876) convinced the Viennese school of geologists of their deep-water nature. Based on the composition of Jurassic–Cretaceous faunas, Melchior Neumayr postulated the existence of an equatorial ocean, the ‘Central Mediterranean’ which extended from Central America through the Alpine belt to the Himalaya and beyond, an ocean which Suess subsequently named ‘Tethys’. The question of whether or not deep-sea sediments could occur on land, as deliberately posed by Gustav Steinmann in 1925, became central to the long-lasting controversy on the permanency of continents and ocean basins. Indeed, the occurrence of deep-sea deposits in mountain belts required the disappearance of oceanic areas and a mobilist concept of orogeny that was subsequently applied to the Tethyan region by Emile Argand in the wake of the continental drift hypothesis of Alfred Wegener. ^