Pol Guennoc

Back arc extension, tectonic inheritance, and volcanism in the Ligurian Sea, Western Mediterranean

[1] The Ligurian basin, western Mediterranean Sea, has opened from late Oligocene to early Miocene times, behind the Apulian subduction zone and partly within the western Alpine belt. We analyze the deep structures of the basin and its conjugate margins in order to describe the tectonic styles of opening and to investigate the possible contributions of forces responsible for the basin formation, especially the pulling force induced by the retreating subduction hinge and the gravitational body force from the Alpine wedge. To undertake this analysis, we combine new multichannel seismic reflection data (Malis cruise, 1995) with other geophysical data (previous multichannel and monochannel seismic sections, magnetic anomalies) and constrain them by geological sampling from two recent cruises (dredges from Marco cruise, 1995, and submersible dives from Cylice cruise, 1997). From an analysis of basement morphology and seismic facies, we refine the extent of the different domains in the Ligurian Sea: (1) the continental thinned margins, with strong changes in width and structure along strike and on both sides of the ocean; (2) the transitional domain to the basin; and (3) a narrow, atypical oceanic domain. Margin structures are characterized by few tilted blocks along the narrow margins, where inherited structures seem to control synrift sedimentation and margin segmentation. On the NW Corsican margin, extension is distributed over more than 120 km, including offshore Alpine Corsica, and several oceanward faults sole on a relatively flat reflector. We interpret them as previous Alpine thrusts reactivated during rifting as normal faults soling on a normal ductile shear zone. Using correlations between magnetic data, seismic facies, and sampling, we propose a new map of the distribution of magmatism. The oceanic domain depicts narrow, isolated magnetic anomalies and is interpreted as tholeitic volcanics settled within an unroofed upper mantle, whereas calcalkaline volcanism appears to be discontinuous but massive and has jumped in space and time, from the beginning of rifting on the Ligurian margin (� 30 Ma), toward the Corsican margin at the end of the Corsica-Sardinia block rotation (� 16 Ma). This space and time shift reveals the importance of the rollback of the Apulian slab and of the migration of the Alpine-Apennines belt front toward the E-SE for driving basin formation. We also state that initial rheological conditions and inherited crustal fabric induce important changes in the styles of deformation observed along margins and between conjugate margins. In the NE Ligurian basin the prerift Alpine crustal thickening together with slow rollback velocity likely contribute to distribute strain across the whole NW Corsican margin, whereas farther south the inherited Hercynian structural pattern combined with a faster rollback of the subducting plate tend to focus the extension at the foot of the margin, up to the Sardinian rift which ends within the SW Corsican margin. Therefore the mode of opening and the margin structures mainly depend on the balance between intrinsic, inherited crustal heterogeneity (fabric and rheological changes) and external conditions imposed by rollback of the subducting lithosphere. INDEX TERMS: 3040 Marine Geology and Geophysics: Plate tectonics (8150, 8155, 8157, 8158); 3025 Marine Geology and Geophysics: Marine seismics (0935); 8109 Tectonophysics: Continental tectonics—extensional (0905); 8159 Tectonophysics: Rheology—crust and lithosphere;

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.

Les vallees fossiles de la baie de la Vilaine; nature et evolution du prisme sedimentaire cotier du Pleistocene armoricain

The study of a dense network of high resolution seismic profiles in the bay of Vilaine, INSU-CNRS cruise Geovill, have led to the characterization of the architecture of the sediment wedge preserved between the coast and the 50 m isobath. This wedge lies on a substratum composed of three seismic units, U1, U2 and U3 respectively attributed to metamorphic and magmatic rocks, Lutetian and Ypresian sandy carbonates and post-Eocene sediments. The coastal sediment wedge comprises three major units. A basal unit (U4), dated around 600 to 300 ky BP, interpreted as braided river sandy conglomerates. A median unit (U5) corresponding to estuarine and fluvial sandstones and clays that give way to the west to mouth bar sandstones. A sommital unit (U6) attributed to marine argillites and barrier island sandstones dated from 8110+ or -200 years at the base. These three units are bounded by two major surfaces: an unconformity between U4 and U5 and a marine (wave and tidal) ravinement surface between U5 and U6. The unconformity is interpreted as a sequence boundary between two depositional sequences: a lower one with U4 seismic unit and a topmost one with U5 and U6 seismic units. Based on the available datations, the lower sequence is attributed to the Saalian and/or Elsterian glacial cycles and, the upper sequence to the Weichselian (lowstand systems tract) and to the Holocene marine transgression (transgressive systems tract). The passage from one sequence to the other corresponds however to a drastic shift in the paleoflow directions (60 degrees ) in the Bay of Vilaine closely related to the main faults orientations. The tectonic activity in Brittany during the Pleistocene, linked to intraplate stress, seems to exert a control on sediment architecture in the coastal wedge. Indeed, the tilt of the Armorican Massif during that period has caused a complete rejuvenation of the fluvial profiles in land and the separation of the paleo-Vilaine from the Paleo-Loire river courses.

Importance du volcanisme calco-alcalin miocène sur la marge sud-ouest de la Corse (campagne MARCO)

Abstract During the MARCO cruise, systematic exploration along the western and northern Corsican margins was carried out by dredging. The compositions of the dredged rocks range from basalt to amphibole-biotite bearing andesite with a broad calc-alkaline character. A Miocene age has been obtained for the amphibole-biotite andesite sample DR02 both by 40Ar-39Ar dating on hornblende (16.0 ± 0.4 My) and using fission-track method on apatite (17.2 ± 0.8 My). The south-western Corsican volcanic zone represents the direct extension of the Miocene Sardinian graben volcanism north of 42 °00 N. It could either be synchronous with or post-date the oceanic opening event. Such an arc volcanism probably results from the subduction to the north of the Liguro-Piemont ocean beneath Europe, the Provencal-Ligurian basin being in back-arc position with respect to the studied volcanic centres.

Cadomian tectonics in northern Brittany: a contribution of 3-D crustal-scale modelling

Abstract Both available and newly acquired geological and geophysical data processed in the framework of the ARMOR project have been used to construct a 3-D geometric and geological model of the Cadomian domain of northern Brittany. The major results of the data processing are presented and discussed relative to a composite profile transecting the studied domain and constructed from the surface geology, reflection seismics, magnetotelluric soundings and gravimetric data. They are then extrapolated at regional scale through a series of cross-sections compiled from the surface geology and gravimetric data and calibrated onto the composite profile. The 3-D crustal-scale models were computed according to two procedures. The first, using a STRIM Matra Datavision modeler, defines various interfaces through correlation between adjacent cross-sections and is well adapted to the 3-D representation of geological structures. The second, based on Voronoi diagrams, leads to a volumetric model of the geological units and is a powerful tool for calculating 3-D geophysical responses of the model when the physical properties of the rock units are known. The tectonic implications of the computed 3-D crustal-scale geometry of the Cadomian domain are then discussed in terms of volcanic-arc thrusting, with special emphasis being placed on the obliquity of the shortening direction by reference to the thrust fault trend. Finally, the data are used to demonstrate the reactivation of some Cadomian faults during the deposition of Ordovician sandstones and following the emplacement of the Paleozoic dolerite dyke swarm of northern Brittany.

Gravity modellings of the Cadomian active margin of northern Brittany

Abstract The geometrical relationships existing between the Saint-Brieuc, Saint-Malo and Guingamp units, which form the Cadomian active margin of northern Brittany, have been determined by gravity analyses and 2.5D modelling, correlated with other geophysical (magnetic, seismic, etc.) and geological observations. Automatic structural analyses were performed before modelling, to identify lateral gravity discontinuities. Averaged density measurements were used for 2.5D gravity modelling along six profiles across the arcuate geological and gravity shape of the Cadomian basement. This study allowed the third dimension to be obtained through correlating each profile with its neighbours. We propose that orthogneiss formations equivalent to the Port-Morvan orthogneiss, constitute the Saint-Brieuc upper-crustal unit, below the Lanvollon Formation. 2.5D gravity modelling outlines downward geological differences in the western and eastern parts of the Ploufragan Complex, as previously indicated by surface mapping. These results stress the necessity for distinguishing between the Guingamp and Saint-Malo units.