The continental margin of the Mesozoic Tethys in the Western Alps

Most of the structural units of the Western Alps were derived from the European Continental margin of the Ligurian Ocean, a segment of the Mesozoic Tethys ocean. Their Mesozoic palaeotectonic and stratigraphic evolution bears witness of the following main stages: (1) Deposition of the Triassic platform carbonates, essentially prerift but nevertheless bearing the imprint of some extensional movements. (2) The Liassic-Middle Jurassic rifting corresponds to the creation of a horst-and-graben system with especially tilted blocks and hence a shoal-and-basin palaeogeography, but without a preliminary doming event. (3) The late Jurassic-Early Cretaceous opening and spreading of the Ligurian ocean began with a general collapse of the continental margin (‘thermal’ subsidence, latest Middle Jurassic and Earliest Late Jurassic).In this paper, emphasis is given to the refting-derived structures, more especially to the major tilted crustal blocks, a few tens of kilometres wide, which can be either reconstructed or directly observed. The rifting stage lasted roughly 40 m.y., with the alternation of extensional tectonic phases and of relatively ‘quiet’ periods. During the extensional phases, movements along fault-planes and related episodes of tilting were followed by sudden and rapid episodes of subsidence: the latter may be interpreted as resulting from the isostatic and thermal readjustment that follows a phase of stretching of the crust and of the lithosphere. The tectonic evolution of the margin continued, but decreased, during the late Jurassic-Early Cretaceous oceanspreading stage. Tectonic activity resumed in the late Cretaceous: this probably resulting from the beginning of contraction of both the ocean and the continental margin, leading progressively to the continental collision in the Tertiary.