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

Abstract

Abstract Introduction.– 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 ; Réhault 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. Morpho-structural and sedimentary characteristics of the margin. – The morphology of the margin results both from the Oligocene rifting and the Messinian crisis (5.8–5.3 Ma) characterised by a sea level fall of ~1500 m. At the surface, the margin, with a steep mean slope of 6–8o near Antibes [Réhault, 1981] to 12o near Imperia [Savoye and Piper, 1991], is cut by several canyons. At depth, there are two or three tilted blocks covered by Mesozoic sediments and in the Imperia area by the Helminthoïd Flyschs [Sosson et al., 1998]. In the basin, above the Miocene units, we observe some lower evaporites at the bottom, the Messinian salt in the middle and upper evaporites (E) marking the end of the low sea level 5.32 Ma ago [Ryan et al., 1973 ; Mauffret et al., 1973 ; Réhault 1981 ; Savoye and Piper, 1991]. The sedimentary series ends with 1500 m thick of Plio-Quaternary units [Gennesseaux and Le Calvez, 1960 ; Sosson et al., 1998]. At the top of the margin, we observe an erosion surface while toward the basin, two units are evidenced : the Messinian fan (CYL 30–05, fig. 2) unconformity, covered by a seismic facies similar to the one of the upper evaporites in the basin. The “M” surface, that relates the erosional surface of the margin and the upper evaporites of the basin, has a regular slope toward the basin (fig. 3). We will use this surface as a stratigraphic and structural reference for this work. Acquisition and methodology. – We analysed 12 profiles from the MALIGU cruise (1993–1994) [Chaumillon et al., 1994] and ~ 60 from several cruises (1992–2001) with the “Tethys” oceanographic ship to synthesize all stratigraphic and structural observations along the margin. In this paper, we only present 12 of them. We quantify the deformation at the margin/basin limit with a velocity gradient [Le Douaran et al., 1984 ; Rollet, 1999 ; Contrucci et al., 2001] on 50 profiles and we propose a deformation chronology using the “M” surface. Evidence for Pliocene deformation – Between Antibes and the east of Nice : there is no deformation of the “M” surface (fig. 3). – Between the east of Nice and the west of Menton : we observe a deformation at the top of the margin characterised by tilted seismic reflectors (fig. 4A). According to the micro-paleontology study, this deformation is dated at the Lower-Upper Pliocene limit. – From the west of Menton to San Remo : the deformation, observed in the middle of the margin, is characterised by a tilted Messinian fan and the formation of small basins (fig. 4A,B). We note that this deformation increases when the margin strikes ENE-WSW. – From San Remo to Imperia : the deformation increases from the middle to the base of the margin (fig. 4B). The apparent normal throw estimated at ~ 500 m near Antibes increases up to more than 2000 m near Imperia since ~ 5 Ma (fig. 5). This deformation induced (i) the formation of a piggy-back basin located near Imperia, (ii) a decrease of the “M” surface slope with at places a slope inversion compared with the Antibes area (fig. 4B). Space and time reconstitution of vertical motions. – In order to better visualise the geometry of the structure of the margin, we drew seismic profiles with no exaggeration. We interpret the observations seen above and the fact that normal faults on the rifted tilted blocks show a slope between 45–30o as the occurrence of a blind thrust (fig. 6). Motion along the thrusting plane induces the rotation of tilted blocks and is responsible for the margin uplift during the Lower-Upper Pliocene limit following a book-shelf mechanism [Mandl, 1987 ; Jackson and McKenzie, 1983]. At the bottom of the margin, we therefore interpret the apparent normal fault as a gravitary sliding (fig. 7) which enhances the front of the thrust vanishing in the Messinian salt unit. Discussion – Comparison between this model and others previously proposed : contrary to the model proposed by Chaumillon et al. [1994], we can explain the uplift of the margin and the presence of the “normal” faults at the limit margin/basin, only with one mechanism of crustal compression. This mechanism clearly comes from onshore and not from offshore as suggested by Béthoux et al. [1992]. The thrust, dipping toward the continent, can be observed on multichanel seismic reflection profiles [Rollet, 1999] (fig. 8). – The compression of the margin since the end of the lower-Pliocene : our results are chronologically and geometrically in agreement with reversal faults dipping toward the continent, observed along Cap Mele (fig. 1) at the bottom of the Pliocene units [Réhault, 1981]. Both, the important thickness of the Plio-Quaternary sediments near Imperia, far away from the Var river, and the many salt diapirs in the NE area, are consistent with a thrust motion. – The area Argentera massif-Ligurian margin : according to fission track data [Bigot-Cormier et al., 2000] a major uplift was detected at ~3.5 Ma. At the same time, the Ligurian margin recorded a compressive phase with a structural geometry consistent with the deformation onshore (fig. 9). The deformation on the thrust front is the most important at the axis of the main structures of the Argentera. This thrust front is located at the base of the margin near Imperia and propagated toward the top close to the western edge of the Nice arc (fig. 10). Our observations suggest that the deformations propagate offshore in relation with the advance of the Alpine front toward the south. Conclusion. – The analysis of 70 seismic reflection profiles based on stratigraphic and structural studies allows us to quantify and date the deformation of the Ligurian margin increasing eastward. This deformation dated at the Lower-Upper Pliocene limit is due to the propagation of a blind thrust front consistent with the basement tectonic deformation of this period reactivating the Oligocene rifting structures.