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

E. Sanchis et M. Seranne, Structural style and tectonic evolution of a polyphase extensional basin of the Gulf of Lion passive margin: the Tertiary Ales basin, southern France, TECTONOPHYS, 322(3-4), 2000, pp. 219-242
Citations number
Categorie Soggetti
Earth Sciences
Journal title
ISSN journal
0040-1951 → ACNP
Year of publication
219 - 242
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The Ales basin is one of the Tertiary onshore basins of the Gulf of Lion ma rgin, related to Oligo-Aquitanian rifting of the NW Mediterranean. This bas in is presently exposed in the landwardmost part of the margin, along the C evennes 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 t he geometries of the Tertiary extensional fault systems and related hanging wall, and the architecture of syn-rift sediments are genetically related. T his analysis makes it possible to re-evaluate the structural style and the kinematics of the Tertiary Ales sedimentary basin. Our results indicate two distinct architectures for the late Eocene and Oli gocene syn-rift deposits, respectively, that were controlled by two differe nt extensional systems: During late Eocene, the basin was a half-graben formed along a simple listr ic fault responsible for the development of a hangingwall roll-over, and no rth-west divergence of syn-rift infill. The bordering extensional fault (th e Ales 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 Cevennes Fault that affects the Pala eozoic basement. During Oligocene, a complex ramp-flat extensional system reactivated the de ep part of the late Eocene extensional system and was responsible for the f ormation of a hangingwall syncline basin. The latter displays progressive u nconformities within synrift continental sediments onlaping the hangingwall flat. Alluvial-fan clasts are exclusively derived from the hangingwall fla t formations and were deposited along a NE-trending syncline parallel to th e basin border. During extension, depocentres migrated north-westward, as a result of ongoing extension and of erosion of the hangingwall flat. Extens ion on the Ales Fault propagated upwards and north-westwards across the Mes ozoic formations, while the low-strength marl beds of the Neocomian formed flats, and the emerging ramp was superimposed on to the Cevennes Fault at s urface level. The late Eocene ramp was then passively transported down-dip. The Oligocene tectono-sedimentary system suggests(1) an original extensiona l system where inherited high-angle faults were not reactivated, but locali zed the emergence of a decollement propagating within the Trias formations, and (2) the presence of Mesozoic (up to Neocomian) cover on the Cevennes m argin during Oligocene; consequently, the Palaeozoic basement presently exp osed in the footwall was exhumed later. The rift basins that developed alon g the Cevennes fault do not fit with the landward boundary of stretched con tinental crust, which is located tens of kilometres to the SE, the Nimes Fa ult. 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 cau tion in studies of continental passive margins. (C) 2000 Elsevier Science B .V. All rights reserved.