LOW-ANGLE CRUSTAL RAMP AND BASIN GEOMETRY IN THE GULF OF LION PASSIVEMARGIN - OLIGOCENE-AQUITANIAN VISTRENQUE GRABEN, SE FRANCE

With more than 4000 m of Oligocene-Aquitanian sediments, the Vistrenqu e 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 Nim es fault, which bounds the graben to the NW, forms at depth a low-angl e (25 degrees) crustal ramp. Along-strike changes of hangingwall geome try 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 grab en, while a pseudo-rollover and compensation graben result from a two- segments planar geometry of the fault in the northern part. Preexistin g structures inherited from Mesozoic extension and Late Cretaceous-Eoc ene Pyrenean thickening controlled the location of the Nimes fault and the transfer zones which divide the graben into different compartment s. Since both hangingwall and fault profile are well constrained, rest oration 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 o f the basement faulted domain of the margin. Extensional deformation w as restricted to this domain during most of the rifting interval. Smal l amounts of extension were transmitted landward to Mesozoic cover dec ollement rooted in the Nimes fault, only during short episodes, probab ly resulting from gravitational instability during margin collapse. Th e Nimes low-angle crustal ramp, as well as the other crustal ramps of the margin of similar orientation, are probably newly formed extension al structures rather than reactivated Pyrenean thrusts. Their activati on at a low-angle may have been allowed by crustal weakening resulting from the previous Pyrenean thickening. Upper crustal extension corres ponding to the graben formation was transmitted basinward through an i ntracrustal detachment, or/and distributed in the lower crust across t he 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 episod e because of its location at the front of the Pyrenean orogen.