Françoise Sage

Interplate patchiness and subduction-erosion mechanisms: Evidence from depth-migrated seismic images at the central Ecuador convergent margin

Prestack depth-migrated seismic lines provide accurate images of the convergent Ecuadorian margin, where the southern flank of the Carnegie Ridge subducts. The margin is fronted by a 5–7-km-wide compressional sediment prism. Beneath the slope, margin thinning is accomplished by gradual thinning of the lower part of the basement, accompanied by pervasive seaward-dipping normal faulting, indicating basal subduction erosion at a low-friction plate interface. Along the plate boundary, a thin subduction channel locally thickens to form low-velocity, ∼600-m-thick lenses of underthrusted fluid-rich sediment supplied by slope mass wasting. The contrast between the fluid-rich sediment and the surrounding thinner and drier sections of the subduction channel creates a three- dimensional patchiness across the plate boundary, implying variations in mechanical interplate coupling. The subduction channel patchiness modulates subduction erosion processes by alternately favoring margin basement weakening and material removal. Basement weakening would occur (1) at the base of the upper plate, where enhanced reflectivity indicates basement damage by overpressured fluids from the subduction channel, and (2) at the basement apex, where basement breakup is caused by superposition of compressional and extensional fault systems resulting from time-variable interplate mechanical coupling. The detachment of rock debris in the higher coupled sections and the subsequent dragging of the fragments into the subduction channel cause tectonic erosion. Deeper in the subduction, the subduction channel patchiness may influence processes like earthquake nucleation and rupture propagation and material recycling in the mantle.

Pliocene deformation of the north-Ligurian margin (France): consequences of a south-Alpine crustal thrust

The Oligo-Miocene extension phase of the Mediterranean basins rifting (30-25 Ma) [Jolivet and Faccenna, 2000] followed by the Ligurian basin oceanic crust formation (21-18 Ma) [Le Pichon et al., 1971 ; Rehault et al., 1984 ; Carminati et al., 1998 ; Gueguen et al., 1998] occurred during the western Alps compression phase. The deformations were characterised during the Miocene by the southwestward structuration of the Castellane Arc [Fallot and Faure-Muret, 1949 ; Laurent et al., 2000] and during the Mio-Pliocene by the southward structuration of the Nice Arc. This latter arc is bounded on its western side by a dextral strike-slip fault and on its southern side by a thrust inducing an uplift of this arc [Ritz, 1991 ; Guglielmi and Dubar, 1993 ; Clauzon et al., 1996 ; Guardia et al., 1996 ; Schroetter, 1998]. Fission tracks thermochronology data [Bigot-Cormier et al., 2000] suggest a general uplift at ~3.5 Ma of the Argentera massif. Stratigraphical [Irr, 1984 ; Hilgen, 1991 ; Hilgen and Langereis, 1988, 1993] and geomorphological studies [Clauzon et al., 1996 b ; Dubar and Guglielmi, 1997] show evidences for an uplift of the Ligurian coast increasing east of the Var river. The analysis of 70 seismic-reflection profiles allows us to better characterise and quantify the deformation from Antibes to Imperia (fig. 1). We then reconstruct vertical motions in space and time since the Messinian crisis in order to propose a deformation model of the margin related to crustal thickening.

Structure of oceanic crust adjacent to a transform margin segment: the Côte d’Ivoire–Ghana transform margin

Abstract The structure of the oceanic crust adjacent to the Côte d’Ivoire–Ghana transform margin is deduced from multichannel seismic reflection and seismic wide-angle data, showing crustal heterogeneities within oceanic basement; the oceanic crust adjacent to the transform margin is half as thick as standard Atlantic oceanic crust. Refraction data indicate a gradual velocity transition towards typical mantle velocities. Such an abnormal oceanic crustal structure appears quite similar to crustal structures known along transform faults. This crustal thinning may be related to thermal effects of the nearby continental crust, on the oceanic accretion processes. We did not find geophysical evidence for oceanic crust contamination by continental lithosphere.