Structural style and tectonic evolution of a polyphase extensional basin of the Gulf of Lion passive margin: the Tertiary Alès basin, southern France

Abstract

The Alès basin is one of the Tertiary onshore basins of the Gulf of Lion margin, related to Oligo-Aquitanian rifting of the NW Mediterranean. This basin is presently exposed in the landwardmost part of the margin, along the Cévennes Fault, a major structural feature that was active during Variscan (Carboniferous) orogeny, Tethyan (Lias) rifting, and Pyrenean (early–middle Tertiary) compression. Combined field studies and seismic reflection profiles analyses show that the geometries of the Tertiary extensional fault systems and related hangingwall, and the architecture of syn-rift sediments are genetically related. This analysis makes it possible to re-evaluate the structural style and the kinematics of the Tertiary Alès sedimentary basin. Our results indicate two distinct architectures for the late Eocene and Oligocene syn-rift deposits, respectively, that were controlled by two different extensional systems: • During late Eocene, the basin was a half-graben formed along a simple listric fault responsible for the development of a hangingwall roll-over, and north-west divergence of syn-rift infill. The bordering extensional fault (the Alès Fault) is a listric fault passing at depth to a SE-dipping low-angle ramp corresponding to the Triassic marl and evaporite beds. This fault is distinct from the inherited high-angle Cévennes Fault that affects the Palaeozoic basement. • During Oligocene, a complex ramp-flat extensional system reactivated the deep part of the late Eocene extensional system and was responsible for the formation of a hangingwall syncline basin. The latter displays progressive unconformities within synrift continental sediments onlaping the hangingwall flat. Alluvial-fan clasts are exclusively derived from the hangingwall flat formations and were deposited along a NE-trending syncline parallel to the basin border. During extension, depocentres migrated north-westward, as a result of ongoing extension and of erosion of the hangingwall flat. Extension on the Alès Fault propagated upwards and north-westwards across the Mesozoic formations, while the low-strength marl beds of the Neocomian formed flats, and the emerging ramp was superimposed on to the Cévennes Fault at surface level. The late Eocene ramp was then passively transported down-dip. The Oligocene tectono-sedimentary system suggests (1) an original extensional system where inherited high-angle faults were not reactivated, but localized the emergence of a decollement propagating within the Trias formations, and (2) the presence of Mesozoic (up to Neocomian) cover on the Cévennes margin during Oligocene; consequently, the Palaeozoic basement presently exposed in the footwall was exhumed later. The rift basins that developed along the Cevennes fault do not fit with the landward boundary of stretched continental crust, which is located tens of kilometres to the SE, the Nimes Fault. Although they are often well exposed for field studies, rift basins in the hinterland of continental margins are not always representative of the tectonics of the whole margin, and should therefore be integrated with caution in studies of continental passive margins.