The Ardèche palaeomargin of the South-East Basin of France: Mesozoic evolution of a part of the Tethyan continental margin (Géologie Profonde de la France programme)

Abstract

This paper incorporates the results of seismic and gravity investigations, borehole data and studies of sedimentology, biostratigraphy, sequence stratigraphy and structural geology, carried out in the framework of the Deep Geology of France programme. The main purpose of the paper is to define the structure and to reconstruct the evolution of the Ardèche palaeomargin of the South-East Basin of France from the Stephanian to the end of the Mesozoic. The acquisition and processing of three seismic reflection profiles, the drilling of two scientific boreholes (BA1 and MM1), and subsequent structural modelling, with the use of cross-section balancing techniques applied to both the depth-migrated seismic and geological sections, provide new geometrical constraints which lead to the following hypothesis of the geodynamic evolution of the margin: 1. (1) Late Palaeozoic extension with the development of high-angle basement faults, asymmetrical Permian half-grabens, (pre-extension) sedimentary cover. 2. (2) During the Middle Triassic, a pre-rift stage with the creation of small fault blocks and, in the Late Triassic, the formation of an E-W flexure. 3. (3) End-Lias (Early Jurassic) to Middle Jurassic formation of a major detachment in the Carboniferous coal measures, leading to complete decoupling of the Mesozoic sedimentary cover from the basement. This episode of extension, coeval with the opening of Tethys, can be related to a phase of episodic rifting between the late Sinemurian and the late Bathonian (i.e. during 30 Ma). 4. (4) Late Jurassic to Early Cretaceous thermal subsidence, with regional downwarping of the basement towards the east. 5. (5) After the Late Cretaceous (the period of maximum burial), Pyrenean and Alpine deformation in the south and east, causing progressive uplift of the Ardeche palaeomargin, as much as two kilometres of Mesozoic deposits subsequently being eroded from the section during the Tertiary.