Agnès Maillard

Tectonics and deep structure of the north-western Mediterranean Basin

Crustal data, derived from geophysical experiments in the Valencia Trough and the Provencal Basin, are compared and integrated in the framework of the north-western Mediterranean Basin. This basin is a Neogene extensional rift, located within a region of compression between the European and African plates. The northern margin of the Valencia Trough and the Provencal Basin can be compared with other Atlantic-type margins, whereas the southern margin is affected by calc-alkaline volcanism related to subduction. Thinned continental crust, covered by a thick volcanic layer in the deep Catalan margin, is widespread in the Valencia Trough and on both sides of the Provencal Basin, whereas oceanic-type crust is restricted to a narrow domain. The Provencal Basin may have been formed by a simple shear process behind a subduction zone. A geodynamic consequence of this simple shear model is overlap of the thin crust of the Provencal Basin by the thick crust of the Sardinia margin.

Influence of differential compaction above basement steps on salt tectonics in the Ligurian-Provencal Basin, northwest Mediterranean

Detailed mapping of the Ligurian-Provencal Basin (northwestern Mediterranean) indicates that salt diapirs in the deep basin are restricted to an area whose upslope boundary forms reentrants located above deep crustal transfer zones associated with the opening of the basin in Oligo -Miocene times. Because these basement faults were no longer active in Messinian and post-Messinian times, the geographic correlation between diapirs and basement faults cannot be attributed to slip along these basement faults coeval with salt tectonics. Using three physical experiments, we examine how dormant (i.e. inactive) basement steps can affect the development of the overlying salt structures during combined gravity-driven gliding and spreading. Where a basement step trends obliquely with respect to the direction of the slope and initially offsets the base salt, grabens and salt ridges form above and downdip from the basement step, in turn forming a reentrant pointing upslope. Where the basement step is buried under pre-Messinian compactable sediments, loading by Messinian and post-Messinian sediments causes differential subsidence above the step, forcing grabens and salt ridges to form above and updip of the basement step. There, too, the salt structures are confined in a triangular, reentrant-shaped area pointing upslope. The combination of these two mechanisms with passive diapir growth during Plio-Pleistocene times explains the striking geographic correlation between salt diapirs and basement structures in the Gulf of Lion. q 2003 Elsevier Science Ltd. All rights reserved.

Mass Movements in a Transform Margin Setting: The Example of the Eastern Demerara Rise

The eastern Demerara Rise located offshore French Guiana was surveyed in 2003 (GUYAPLAC cruise, part of the French EXTRAPLAC program) using multibeam bathymetry and imagery, 6-channel seismic data and 3–5 kHz echosounding. Analysis of seismic data shows that the flank of the Demerara Rise endures repetitive sliding of its Paleogene to Neogene sedimentary cover towards the ocean. Fluid escapes seem to be closely associated with the activity of those slides and deep faults seems to impact the location of the main headscarp. We suspect fluid overpressures and the specific architecture of transform boundaries (“free border”) to be key parameters in the development of wide MTD’s retrogressively eroding the eastern Demerara Rise.

Une coupe de la province volcanique Caraı̈be : premiers résultats de la campagne sismique Casis 2

The eastwards motion of the Caribbean plate is supposed to be related to a Cretaceous flip of subduction from eastwards to westwards vergence. However, we do not observe on the seismic profiles recorded during the CASIS 2 cruise any evidence of subduction beneath the Aves Ridge and Nicaragua Rise. Aves volcanic arc has been probably formed after the collision of the Caribbean volcanic plateau as shown by a wedge of volcanic-clastic sediments imaged by the seismic cruise CASIS 2. A recent left-lateral transtensional tectonics is observed in the lower Nicaragua Rise; the Colombia basin might have a motion towards the northeast relative to the rise.

Slope Instability on the French Guiana Transform Margin from Swath-Bathymetry and 3.5 kHz Echograms

Although transform margins represent ~30% of rifted margins around the world, few studies have investigated mass-movement processes in such areas and their links with this specific structural context. The French Guiana transform margin and adjacent Demerara abyssal plain have been surveyed during the GUYAPLAC cruise, collecting multibeam bathymetric data, backscatter imagery, 3.5 kHz echograms and 6-channel seismic profiles. The study area is divided into three domains: the shallow Demerara plateau, the Guiana slope and rise, and the Demerara abyssal plain. The Demerara plateau displays multi-scale slope instabilities from huge deep-seated collapses of the whole margin to surficial creeping folds and recent slumps. Giant elongated pockmarks have been also observed for the first time in this area. Fluid escape is common everywhere on the plateau and probably enhances slope instability. On the Guiana slope and rise, large stacked lobate masses have been identified testifying to repetitive failure events. Fluid escape is also ubiquitous there, suggesting a dewatering of debris flows due to sediment loading. Two main types of sedimentary structures are observed on the Demerara Abyssal Plain: small meandering channels of the Amazon Fan at its eastern edge and sediment waves at its western edge, along the foot of Demerara continental slope.

Terrestrial colonization of the Balearic Islands: New evidence for the Mediterranean sea-level drawdown during the Messinian Salinity Crisis

Guillem Mas1*, Agnès Maillard2, Josep A. Alcover3, Joan J. Fornós1, Pere Bover3, and Enric Torres-Roig3 1Grup de Recerca Ciències de la Terra, Universitat de les Illes Balears, E-07122 Palma (Mallorca), Spain 2Géosciences Environnement Toulouse–Observatoire Midi-Pyrénées (GET-OMP), Université Paul Sabatier, F-31400 Toulouse, France 3Departament de Biodiversitat Animal i Microbiana, IMEDEA (CSIC-UIB), E-07190 Esporles (Mallorca), Spain