Low-angle crustal ramp and basin geometry in the Gulf of Lion passive margin: Oligocene-Aquitanian Vistrenque graben, SE France

With more than 4000 m of Oligocene-Aquitanian sediments, the Vistrenque graben (SE France) is the deepest synrift depocenter of the Gulf of Lion passive margin, NW Mediterranean. Detailed analysis of industrial seismic reflection profiles and borehole data show that the steep Nimes fault, which bounds the graben to the NW, forms at depth a low-angle (25 degrees) crustal ramp. Along-strike changes of hangingwall geometry allow us to infer along-strike changes of fault shape: A rollover structure and divergent Oligocene-Aquitanian basin fill are associated with a listric geometry of the fault in the southern part of the graben, while a pseudo-rollover and compensation graben result from a two-segments planar geometry of the fault in the northern part. Preexisting structures inherited from Mesozoic extension and Late Cretaceous-Eocene Pyrenean thickening controlled the location of the Nimes fault and the transfer zones which divide the graben into different compartments. Since both hangingwall and fault profile are well constrained, restoration techniques can be used to estimate the prerift topography. The Vistrenque graben was formed close to sealevel, but at the front of a > 1 km-high elevated area resulting from the Pyrenean orogeny, In the studied transect, the Nimes fault formed the landward (NW) boundary of the basement faulted domain of the margin. Extensional deformation was restricted to this domain during most of the rifting interval. Small amounts of extension were transmitted landward to Mesozoic cover decollement rooted in the Nimes fault, only during short episodes, probably resulting from gravitational instability during margin collapse. The Nimes low-angle crustal ramp, as well as the other crustal ramps of the margin of similar orientation, are probably newly formed extensional structures rather than reactivated Pyrenean thrusts. Their activation at a low-angle may have been allowed by crustal weakening resulting from the previous Pyrenean thickening. Upper crustal extension corresponding to the graben formation was transmitted basinward through an intracrustal detachment, or/and distributed in the lower crust across the margin. In contrast to the more stretched areas of the margin which do not display thick synrift series due to their initial high surface elevation, the Vistrenque basin fill records the whole rifting episode because of its location at the front of the Pyrenean orogen.