Oligocene-Miocene extension led to mantle exhumation in the central Ligurian Basin, Western Alpine Domain

Abstract

The Ligurian Basin is located in the Mediterranean Sea to the north-west of Corsica at the transition from the western Alpine orogen to the Apennine system and was generated by the south-eastward trench retreat of the Apennines-Calabrian subduction zone. Late Oligocene to Miocene rifting caused continental extension and subsidence, leading to the opening of 10 the basin. Yet, it still remains enigmatic if rifting caused continental break-up and seafloor spreading. To reveal its lithospheric architecture, we acquired a state of the art seismic refraction and wide-angle reflection profile in the Ligurian Basin. The seismic line was recorded in the framework of SPP2017 4D-MB, the German component of the European AlpArray initiative, and trends in a NE-SW direction at the centre of the Ligurian Basin, roughly parallel to the French coastline. The seismic data recorded on the newly developed GEOLOG recorder, designed at GEOMAR, are dominated by sedimentary 15 refractions and show mantle Pn arrivals at offsets of up to 70 km and a very prominent wide-angle Moho reflection. The main features share several characteristics (i.e. offset range, continuity) generally associated with continental settings rather than documenting oceanic crust emplaced by seafloor spreading. Seismic tomography results are augmented by gravity data and yield a 7.5-8 km thick sedimentary cover which is directly underlain by serpentinised mantle material at the south-western end of the profile. The acoustic basement at the north-eastern termination is interpreted to be continental crust, thickening towards 20 the NE.Our study reveals that the oceanic domain does not extend as far north as previously assumed and that extension led to extreme continental thinning and exhumation of sub-continental mantle which eventually became serpentinised.

Seismic Exploration of the Deep Structure and Seismogenic Faults in the Ligurian Sea by Joint Multi Channel and Ocean Bottom Seismic Acquisitions: Preliminary Results of the SEFASILS Cruise

“…Geosciences 2020, 10, 108 3 of 11 well as the relationship between the two fault systems, is critical in assessing the seismogenic potential and to advancing our understanding of inversion processes (structural inheritance, possible partitioning over margin and basin fault systems). The nature and structure of basement domains formed during the Oligo-Miocene opening of the Ligurian basin remain enigmatic, between continental, oceanic, or transitional, possibly including exhumed subcontinental mantle, partly serpentinised e.g., [9,[11][12][13][14]. The second objective of SEFASILS is to clarify the boundaries and nature of said domains ( Figure 3) in order to decipher the modalities of back-arc opening of the basin [15] and the influence of the strong heterogeneity resulting from structural inheritance (also including Alpine orogenesis) on the current inversion.…”

“…Three crustal domains (the continental, transitional, and central oceanic domains) were previously defined in the Ligurian Basin, based on available geophysical and geological data [9,11] and references therein (Figures 3 and 5a). Some correspondence can be found in the SEFASILS morphostructural observables, although not implying first order causality: (1) the continental slope is part of the continental domain made of thinned continental crust; (2) the Var canyon/channel and ridge/levee are part of the transitional domain, whose nature is discussed, possibly including exhumed lithospheric mantle [10]; and (3) the distal basin is part of the central domain where sparse wide-angle data indicate a basement made of anomalously thin (~4 km) oceanic crust, classically labelled "atypical" [9] or, alternatively, hyperextended (less than 3 km-thick) continental crust, possibly to the point of having outright exhumed serpentinised continental mantle beneath basinal sediments [14]. In this latter view, Dannovski et al [14] conclude that no oceanic spreading would have taken place in the Ligurian Sea.…”