Michel Séranne

The Gulf of Lion continental margin (NW Mediterranean) revisited by IBS: an overview

Abstract The Gulf of Lion margin is one of the Tertiary extensional basins of the western Mediterranean that opened during convergence of Africa and Europe. This Oligocene-Aquitanian rifted margin and associated Burdigalian oceanic basin have been used as case study for stretching models of ‘Atlantic-type’ margins. However, when the Integrated Basin Study (IBS) project was initiated, several outstanding questions remained about the present structure and the geodynamic setting of the margin within the Western Mediterranean. IBS-Gulf of Lion research was based on the existing onshore and offshore, industrial and academic data, which were heterogeneous and unevenly distributed. Compilation of the stratigraphic correlations on a regional scale allowed precise calculation of the timing of rifting, and clarification of the relationships with Alpine and Mediterranean geodynamics. Reprocessing of the existing ECORS deep seismic reflection profiles shed new light on the extensional structure and mechanisms of extension of the continental margin. Structural and sedimentological studies onshore led to the definition of new tectonostratigraphic models for extensional basins. Results of structural analyses showed a partitioning of the extensional deformation processes across the continental margin. 3D gravity modelling of the margin and basin area led to the production of a new map of the Moho depth by inversion, and testing several hypotheses for the origin of the present day subsidence. Although the Gulf of Lion margin displays structural and stratigraphic features similar to ‘Atlantic-type’ margins, its structure and evolution corresponds to that of a rifted margin of a large continent formed during the opening of a marginal basin. Integration of the new results of IBS-Gulf of Lion within the geodynamic evolution of the western Mediterranean suggests that the Oligocene rifting of the Gulf of Lion represents the initial stage of a succession of rifting events and back-arc basin formation, due to continuously retreating subduction during convergence of Africa and Europe.

The Devonian basins of western Norway: tectonics and kinematics of an extending crust

Summary The Devonian basins of western Norway represent shallow to deep exposures of a synthetic extensional sedimentary basin and provide field evidence for ductile extensional deformation within the basin fill and for the evolution of a brittle low-angle fault and ductile shear zone along the basal contact. The motion along this low-angle (5–25°) detachment is synchronous with both deposition and tilting (25°) of the huge (up to 25-km thick) overlapping coarse detrital Middle Devonian series. Such a geometry requires a minimum dip-slip offset of 50 km. The structural data are consistent with fault-rock associations along the basal contact and with the prograde greenschist metamorphism observed in the southern basin: deeper and deeper levels are observed from N to S. Except along the highly sheared and retrogressed basal shear zone, the footwall remained unaffected by deformation during basin development. We discuss three crustal models for basin development and propose that the displacement along the basal contact of the basins is transformed into pervasive ductile flow within the lower crust both at some distance to the side of the basin and beneath the basin.

Structural style and evolution of the Gulf of Lion Oligo-Miocene rifting: role of the Pyrenean orogeny

Abstract The Gulf of Lion margin results from the Cligo-Aquitanian rifting and Burdigalian crustal separation between continental Europe and Corsica-Sardinia. Immediately before the onset of extension, the area of the Gulf of Lion was affected by the Pyrenean orogeny which controlled the structural style of the evolving margin. During extension, the foreland of the Pyrenean orogen was affected by extensional thin-skinned tectonics. The decollement level ramped down into the basement, in areas where the latter was thickened during orogeny. In this intermediate part, the margin was extended by several crustal-scale low-angle faults, which generated small amounts of syn-rift sedimentation compared with the accumulation of post-rift sediments. However, more than 4 km of syn-rift sediments were deposited in the Camargue basin, which is located at the transition between thin- and thick-skinned extensional systems. Kinematic restorations and stratigraphy suggest a pre-rift surface elevation above sea-level of at least 1 km in the intermediate part of the margin, which is in agreement with reduced syn-rift sedimentation. The slope area extends seaward of the North Pyrenean Fault, a terrane boundary inherited from the Pyrenean collision. This part of the margin was stretched by seaward dipping low-angle block tilting of the upper crust, and antithetic lower crustal and sub-crustal detachment. The lithospheric structures inherited from the Pyrenean orogeny exerted a strong control on the kinematics of the rifting and on the distribution and history of subsidence. Such parameters need to be integrated in the definition of pre-rift initial conditions in future basin-modelling of the Gulf of Lion.

South Atlantic continental margins of Africa: A comparison of the tectonic vs climate interplay on the evolution of equatorial west Africa and SW Africa margins

Africa displays a variety of continental margin structures, tectonic styles and sedimentary records. The comparative review of two representative segments: the equatorial western Africa and the SW Africa margins, helps in analysing the main controlling factors on the development of these margins. Early Cretaceous active rifting south of the Walvis Ridge resulted in the formation of the SW Africa volcanic margin that displays thick and wide intermediate igneous crust, adjacent to a thick unstretched continental crust. The non-volcanic mode of rifting north of the Walvis ridge, led to the formation of the equatorial western Africa margin, characterised by a wide zone of crustal stretching and thinning, and thick, extensive, syn-rift basins. Contrasting lithologies of the early post-rift (salt vs shale) determined the style of gravitational deformation, whilst periods of activity of the decollements were controlled by sedimentation rates. Regressive erosion across the prominent shoulder uplift of SW Africa accounts for high clastic sedimentation rate during Late Cretaceous to Eocene, while dominant carbonate production on equatorial western Africa shelf suggests very little erosion of a low hinterland. The early Oligocene long-term climate change had contrasted response in both margins. Emplacement of the voluminous terrigenous Congo deep-sea fan reflects increased erosion in equatorial Africa, under the influence of wet climate, whereas establishment of an arid climate over SW Africa induced a drastic decrease of denudation rate, and thus reduced sedimentation on the margin. Neogene emplacement of the African superswell beneath southern Africa was responsible for renewed onshore uplift on both margins, but it accelerated erosion only in the Congo catchment, due to wetter climatic conditions. Neogene high sedimentation rate reactivated gravitational tectonics that had remained quiescent since late Cretaceous. � 2005 Elsevier Ltd. All rights reserved.

Extension-parallel folding in the Scandinavian Caledonides: Implications for late-orogenic processes

Abstract Structural analyses of the Devonian basins and basement rocks in the Scandinavian Caledonides show the existence of large-scale E-W-oriented folds which affect the para-autochthonous Western Gneiss Region of the Baltic Craton. These folds mostly occur in areas of widespread and intense late-orogenic extension. Chronology, based on structural relationships both within the Devonian sediments and Caledonian rocks, shows that folding has been active from the first stages of the ductile extension and ended after Devonian basin sedimentation. Folding evolved through time towards crustal-scale transcurrent left-lateral faulting in the zones of maximum deformation. Such an evolution is explained by progressive unloading of the Western Gneiss Region due to extension, which involved a permutation of the maximum stress from a vertical position at the onset of extension to a horizontal position in the subsequent stages. Folding parallel to late-orogenic extension, as well as left-lateral translation on east- to northeast-trending crustal-scale faults suggest the occurrence of a regional direction of compression perpendicular to the direction of extension. This in turn argues for an external force triggering the late-orogenic extensional process in the Scandinavian Caledonides.

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°) 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.

Polyphased uplift and erosion of the Cévennes (southern France): An example of slow morphogenesis

The Cevennes are bordering the French Massif Central and the Gulf of Lion margin. The morphogenesis of this area results from an interaction between deep-seated and superficial processes, whose origin and timing is still discussed. We attempt a reconstruction of the surrection and erosion history of the area through a multidisciplinary approach including geology, geomorphology, thermochronology and geochronology. Thermochronology shows that the Cevennes basement underwent some 2 km denudation in mid-Cretaceous time. Analyses of the sediments preserved on uplifted surfaces and in peripheral sedimentary basins indicate a differential surface uplift of the Cevennes, of the surrounding calcareous plateaus, and of the coastal plain, that occurred in several stages during the Tertiary. Early Miocene rifting of the Gulf of Lion margin and opening of the NW Mediterranean drastically modified the drainage network. Geomorphology analyses of the incised rivers and karst network suggest that most of the incision results from uplift that occurred sometime in the Serravalian-Tortonian interval. U/Th dating of calcite concretions in karsts allows to chronologically bracket the formation of some fluvial terraces, and to find very low incision rates during the Pleistocene. Most of the morphogenesis predates the Quaternary. This ongoing study shows an example of polyphased and very slow morphogenesis, with present-day landscape including elements as old as Cretaceous.

Oligocene to Holocene sediment drifts and bottom currents on the slope of Gabon continental margin (west Africa): Consequences for sedimentation and southeast Atlantic upwelling

Seismic reflection profiles on the slope of the south Gabon continental margin display furrows 2 km wide and some 200 m deep, that develop normal to the margin in 500‐1500 m water depth. Furrows are characterised by an aggradation=progradation pattern which leads to margin-parallel, northwestward migration of their axes through time. These structures, previously interpreted as turbidity current channels, display the distinctive seismic image and internal organisation of sediment drifts, constructed by the activity of bottom currents. Sediment drifts were initiated above a major Oligocene unconformity, and they developed within a Oligocene to Present megasequence of general progradation of the margin, whilst they are markedly absent from the underlying Late Cretaceous‐Eocene aggradation megasequence. The presence of upslope migrating sediment waves, and the northwest migration of the sediment drifts indicate deposition by bottom current flowing upslope, under the influence of the Coriolis force. Such landwards-directed bottom currents on the slope probably represent coastal upwelling, which has been active along the west Africa margin throughout the Neogene.

Burial and exhumation history of the south-eastern Massif Central (France) constrained by apatite fission-track thermochronology

Abstract The Southeast Basin in southern France is part of the north European continental margin of the Mezosoic Tethys. The original amount and extent of the Mesozoic sedimentary cover across this basin margin (formed from the Variscan Massif Central basement) is unknown and has been the subject of much debate. We have used apatite fission-track thermochronology to better understand Mesozoic sedimentation and later erosion histories of this region. Thirty-three outcrops and borehole samples were collected from Paleozoic granitic and metamorphic basement and Paleozoic and Triassic sandstones in broadly E–W transects. Except from a restricted area (Vivarais), measured fission-track ages are

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 Ales 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 Cevennes 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 Ales 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 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 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 Ales 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 Cevennes 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 Cevennes 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.