The GROSMarin experiment: three dimensional crustal structure of the North Ligurian margin from refraction tomography and preliminary analysis of microseismic measurements

Abstract

International audienceThe deep structure of the North Ligurian margin and its contiguous Ligurian basin as well as the seismicity recorded in these zones are neither well understood nor precisely constrained. In order to better address these questions, there is a need for offshore instrumenting, which was realised for a duration of nearly 6 months during the GROSMarin (Grand Réseau d'Observation Sous-Marin) experiment. An array of 21 ocean bottom seismometers was deployed over the most active area of the margin and was complemented on land by mobile seismological stations that densified existing permanent networks. We also realised the acquisition of deep refraction seismic shots at sea in order to get a 3D distribution of velocities along the margin through travel time tomography. We present here a preliminary analysis of the seismicity recorded during this experiment and a tomographic model of the margin structures obtained using data from the offshore network only. Our results support the existence of a high velocity zone at the base of a domain interpreted as transitional between continental and oceanic ones, on the northern part of the deep basin. A very similar pattern is observed across the neighbouring margin of the Gulf of Lions and is most likely related to serpentinisation of the underlying mantle during late rifting and continental break-up. North of this transition zone, we observe the basinward crustal thinning of the continental crust beneath the margin that seemingly narrows eastward. To the south, our results hint at transition to the oceanic domain. In contrast, our velocity distribution does not reveal a transition along strike between transitional and oceanic domains, as previous works suggest. Some microseismic activity was recorded throughout the duration of the experiment, on land and at sea. The number of detected events and precision of location were both improved by our considering French and Italian permanent networks. The detection capabilities of our dense network still need to be fully exploited

Distributed sensing of earthquakes and ocean-solid Earth interactions on seafloor telecom cables

“…3B). This interpretation is confirmed by simulations of the fundamental Scholte mode ( 22 ) using a regional 1D velocity model ( 23 ) (Fig. 3B).…”

Deep structure of the continental margin and basin off Greater Kabylia, Algeria – New insights from wide-angle seismic data modeling and multichannel seismic interpretation

“…Indeed, thin oceanic crusts with variable thickness are found in many back-arc basins, at very-slow to fast spreading rates, such as the Lau basin (Turner et al, 1999;Crawford et al, 2003), the Philippine Sea and the Parece Vela basin (Louden, 1980), the Japan Sea (Hirata et al, 1992), and all western Mediterranean basins (e.g., Hinz 1973;Vidal et al, 1998;Grevemeyer et al, 2011;Prada et al, 2014) (Figure 14C, D, E and F). For instance, crustal thicknesses found in the Liguro-Provençal basin vary from 4 to 6 km (Pascal et al, 1993;Contrucci et al, 2001;Dessa et al, 2011;Gailler et al, 2009;Afilhado et al, 2015;Moulin et al, 2015) ( Figure 15) and velocity-depth profiles are similar to those in the Algerian basin ( Figure 14D and F). Moreover, unusually low mantle temperatures due to the subduction of a cold slab have been proposed to explain the anomalously thin oceanic crust in the Philippine Sea basin (Sclater et al, 1976;Louden, 1980) and in the Provençal basin (Gailler et al, 2009).…”

Multiphased tectonic evolution of the Central Algerian margin from combined wide‐angle and reflection seismic data off Tipaza, Algeria

“…[59] The Algerian margin off Tipaza is characterized by a narrow ocean-continent transitional zone compared to other margins of the Western Mediterranean formed in a similar back-arc context. For example, the OCT extends over 80-90 km in the Gulf of Lions [Gailler et al, 2009] (Figure 12b), 30-40 km along the North Ligurian and West Sardinian margins [Rollet et al, 2002;Gailler et al, 2009;Dessa et al, 2011] (Figure 12b), and 20 km at the West Corsican margin [Contrucci et al, 2001;Rollet et al, 2002]. The Gulf of Lions and the West Sardinian Miocene margins represent two conjugate margins, formed during the back-arc opening of the Liguro-Provençal basin in the Western Mediterranean domain (Figure 1).…”