|
Part II Processes |
Laboratoire de Tectonique (u.a. CNRS 266), Université des Sciences et Techniques du Languedoc, Place E. Bataillon, 34060 Montpelier Cedex, France
The Autapie Nappe (French Alps) consists of a Late Cretaceous ‘Helminthoid Flysch’ turbiditic succession. It was emplaced in the external Alps during the Late Eocene under the form of a gravity submarine nappe sliding in a turbiditic foreland basin. The syn-diagenetic gravity spreading of the Helminthoid Flysch succession resulted in a pervasive extensional disruption of the sandstone and mudstone layers related to decollements in the marlstone and claystone layers. The deformation regimes and mechanisms testify to: (i) the incomplete and inhomogeneous lithification of the sediments, and (ii) the occurrence of pore pressure gradients and, possibly, excess pore pressure during the deformation. In particular, the critical state of lithification of the sandstone allowed the occurrence of an unusual chronology of deformation mechanisms, the ‘brecciation’ resulting from a partial loss of cohesion of the sandstone in high speed escaping pore-water (ductile, ‘soft-sediment’ behaviour) post-dating calcite-filled veins (brittle, ‘rock’ behaviour). Similar deformation style might be expected to occur at modern convergent margins.
