Results from the SEFASILS Experiment: Evidence for Mantle Exhumation in the Ligurian basin

Description

The north Ligurian margin is a stretched continental margin located at the junction of the Western Mediterranean Sea and the Alpine belt. The back-arc Ligurian basin opened from late Oligocene to early Miocene, as a result of the Apulian slab rollback. It has then evolved next to the active Alpine orogen, in a regional compressional setting between the Corsica-Sardinia continental block and mainland Europe, inducing a significant uplift of the north margin by more than 1000 m offshore Imperia (Italy).The SEFASILS project (Seismic Exploration of Faults And Structures In the Ligurian Sea) aims to better understand the Ligurian basin -especially its North margin– to comprehend the mechanisms of the ongoing tectonic inversion of the margin and the crustal-scale tectonic structures –active or not– marking its evolution. Acquiring quality deep seismic data in the Ligurian Sea is challenging due to the complexity of structures beneath the steep margin and to the screening effect of the thick Messinian evaporitic series. To this end, joint acquisitions of deep, long-streamer multichannel seismic (MCS) reflection data and dense sea-bottom wide angle refraction data (WAS) have been carried out along a 150 km long profile offshore Nice perpendicular to the basin's axis.Based on various geophysical observables, the basin has classically been divided into three domains: a narrow stretched continental margin, a transitional domain of debated nature, and an atypical oceanic domain featuring a less than 5 km-thick crust. Our results from SEFASILS, including pre- and post-stack migrations and some first arrival travel time tomography of MCS and WAS data, confirm this subdivision but point to an absence of crust in the deep basin. In the transitional domain, the crust may also be absent or very thin, overlain by thick sedimentary filling. This apparent mantle exhumation tends to indicate that the continental break-up was not followed by oceanic accretion. It raises some fundamental questions, first on the modalities of break-up and opening, then on those of compressive reactivation of such basin and the ocation of observed seismicity and deformation in such atypical structural setting. On this latter point, SEFASILS results also provide indirect hints of the ongoing compression, even where morphological evidence is less pronounced

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