Structure and evolution of a passive margin in a compressive environment: Example of the south-western Alps-Ligurian basin junction during the Cenozoic

We focus on the northern Ligurian margin, at the geological junction of the subalpine domain and the Ligurian oceanic basin, in order (1) to identify the location of the southern limit of the Alpine compressive domain during the Cenozoic, and (2) to study the influence of a compressive environment on the tectonic and sedimentary evolution of a passive margin.Based on published onshore and offshore data, we first propose a chronology of the main extensional and compressional regional tectonic events.High-resolution seismic data image the margin structure down to ∼3 km below seafloor. These data support that past rifting processes control the present-day margin structure, and that 2800-4000 m of synrift sediment was deposited on this segment of the margin in two steps. First, sub-parallel reflectors indicate sediment deposition within a subsident basin showing a low amount of extension. Then, a fan-shaped sequence indicates block tilting and a higher amount of extension. We do not show any influence of the Miocene Alpine compression on the present-day margin structure at our scale of investigation, despite the southern subalpine relief formed in the close hinterland at that time. The southern front of the Miocene Alps was thus located upslope from the continental margin.Finally, a comparison with the Gulf of Lions margin suggests that the tectonic influence of the Alpine compression on the rifting processes is restrited to an increase of the subsidence related to flexure ahead of the Alpine front, explaining abnormally high synrift thicknesses in the study area. The Alpine environment, however, has probably controlled the sedimentary evolution of the margin since the rifting. Indeed, sediment supply and distribution would be mainly controlled by the permanent building of relief in the hinterland and by the steep basin morphology, rather than by sea-level fluctuations, even during the Messinian sea-level low-stand.     

Bathymay : la structure sous-marine de Mayotte révélée par l'imagerie multifaisceaux

The BATHYMAY marine survey was carried out in January 2004 around the French volcanic island of Mayotte, onboard the research vessel Marion Dufresne 2. Multibeam bathymetry revealed for the first time the morphology of the whole outer slope. Preliminary interpretations show broad canyons that deeply incise the slope, numerous volcanoes and huge landslides. Large submarine plateaus are also highlighted, bounded by steep cliffs and active normal faults. A model for Mayotte is suggested, in which eruptive activity, sometimes influenced by normal faulting, was concentrated along a N140° regional line (the Comoros archipelago axis), with landslide activity concentrated on the northeastern and southwestern flanks of this line. To cite this article: J.-C. Audru et al., C. R. Geoscience 338 (2006).     

Processes controlling very low sedimentation rates on the continental slope of the Gonone-Orosei canyon system,NE Sardinia—terrestrial and oceanic significance

The narrow shelf and upper slope immediately above the Gonone canyon head off NE Sardinia represent areas of very low sedimentation rates. Along the sides of the canyon head (1,600 m water depth), the sediment deposits are homogeneous but show alternating light-grey intervals rich in carbonate and dark-grey ones rich in organic matter, possibly related to distal turbidite processes. Deposits older than 50,000 years are already encountered at core depths of 2.50 m, the sedimentation rates varying from 6–21 cm/10³ years in the lower parts of two cores and from 1.5–3 cm/10³ years in the upper parts. At about 35,000 years BP, both cores show a simultaneous drop in sedimentation rate by a factor of 3, probably in response to local mechanisms of channel avulsion. Lithological, mineralogical and geochemical properties reveal the environmental factors which are responsible for the extremely slow sediment accumulation. The southernmost sector of the coast, and partly also of the shelf, consists of Jurassic limestones which supply only small amounts of fine-grained material transported in suspension. During the last sea-level highstand, the accumulation of the Cedrino River pro-delta remained restricted to the coast, the low siliciclastic sediment yields resulting in poor shelf sediment trapping. The present morphology of the canyon head prevented the occurrence of gravity processes in the deeper part of the canyon system, including the coring sites. Accordingly, deposition was mainly fed by hemipelagic material of planktonic origin, together with only moderate terrigenous inputs. On a wider late Pleistocene timescale, seismic data indicate the occurrence of a coarse-grained, layered turbidite facies, implying a very different architecture of the canyon drainage system probably prior to 60,000 years BP.     

Premières datations directes de défrichements protohistoriques sur les chaumes secondaires des Vosges (Rossberg,Haut-Rhin). Approche pédoanthracologique

The age of the upland grasslands of the Vosges Mountains is still not well known. On the basis of the study of historical archives, it was assumed that the forest clearings, which led to grasslands establishment, were done by the monks who colonized the Vosges valleys between the 7th and the 8th centuries. Our pedo-anthracological study raises questions about this hypothesis, based on the discovery of Juniperus communis charcoal in soils from the 2nd or 1st century BC. This plant specie is characteristic of grasslands developing into fallows. The occurrence of Juniperus communis charcoals indicates that upland grasslands did exist at least 800 years earlier than it was expected before our study, i.e. at least since the late Iron Age. To cite this article: D. Schwartz et al., C. R. Geoscience 337 (2005).