Ligurian Ocean Bottom Seismology and Tectonics Research (LOBSTER)

The LOBSTER project constitutes the offshore component of the DFG Priority Program “Mountain Building Processes in Four Dimensions” (SPP 2017, 4DMB) and aimed to expand the densely spaced AlpArray broadband seismic network to the offshore domain in the Ligurian Sea. The LOBSTER program encompassed research cruises on the French RV Pourquoi Pas? in 2017 to deploy a long-term ocean bottom seismology network that was recovered using the German RV Maria S. Merian in 2018 (Fig. 1). The LOBSTER long-term seismic network consisted of 7 French (from IPGP) and 22 German (from the DEPAS pool and from GEOMAR) stations. During the second cruise an active seismic experiment was conducted to complement the passive seismology study. The refraction seismic data acquisition was conducted along two wide-angle profiles: P01 runs from the Gulf of Lion to Corsica and P02 trends parallel to the center of the Ligurian Basin in a NE-SW direction. Both profiles were analyzed using a travel time tomography (Dannowski et al., 2020 and in prep). The combined data set in addition to high-resolution bathymetry data shed light on today’s active deformation of the Ligurian Sea (Thorwart et al., 2021). In addition, the 3-D crustal and upper mantle structure of the Ligurian Basin was inferred from surface wave tomography (Wolf et al., 2021). The main technical aim of the LOBSTER project is to provide consistent data that can be smoothly integrated with the onshore seismology data. Key features in the data pre-processing are the correct timing, determining of the orientation of the horizontal seismometer components, and the searchability and availability of the data based on FAIR data standards. LOBSTER studied the Ligurian Sea at the transition from the western Alpine orogen to the Apennine system. This complex geodynamic setting is manifested in pronounced variations in crustal thickness. Topographic gradients in the area are the largest for the entire Alpine-Mediterranean domain, rising from -2500 m in the Ligurian basin to > +3000 m in the Alpine-Apennine orogen over a distance of less than 100 km. The Ligurian Basin is a back-arc basin opened by the south-eastward trench retreat of the Apennines-Calabria-Maghrebides subduction zone, which also triggered the opening of the adjacent western Mediterranean basins. The recent deformation in the Ligurian Sea results from compression along its northern margin (0.3 - 1.5 mm/year shortening), but no significant convergence is evident from GPS data, and rates of deformation are very low. The LOBSTER data set offers a better understanding of the complex geodynamic setting of the Ligurian Sea, which is characterized by pronounced variations in crustal thickness. Based on the LOBSTER data the following conclusions were documented: - Extension in the Ligurian Basin led to stretched and very thin continental crust or exhumed, partially serpentinised mantle. - Continental crustal thinning from north to south is related to the increase of extension with increasing distance from the rotation pole of the anticlockwise rotation of the Corsica–Sardinia block. - Seafloor spreading and formation of mantle-derived oceanic crust was not initiated during the extension of the Ligurian Basin. - The Ligurian Sea is currently closing while Africa and Eurasia are converging. Part of the stresses are taken up in the basin center through re-activation of extension-related faults. Data analysis is still ongoing and further results are expected from local earthquake tomography in the area of the Alps-Ligurian Junction conducted with the data from the long-term ocean bottom seismometer deployment

Basin inversion: reactivated rift structures in the central Ligurian Sea revealed using ocean bottom seismometers

The Alpine orogen and the Apennine system are part of the complex tectonic setting in the Mediterranean Sea caused by the convergence between Africa and Eurasia. Between 30 Ma and 15 Ma, the Apennines-Calabrian-Maghrebides subduction retreated in a southeast direction pulling Corsica and Sardinia away from the Eurasian landmass, opening the Ligurian Sea. In this extensional setting, the Ligurian Sea was formed as a back-arc basin. The northern margin of the Ligurian Basin shows notable seismicity at the Alpine front, including frequent magnitude 4 events. Seismicity decreases offshore towards the basin center and Corsica, revealing a diffuse distribution of low-magnitude earthquakes. Within the framework of the AlpArray research initiative, a long-term amphibious seismological experiment was conducted in the Ligurian Sea to investigate the lithospheric structure and the seismicity in the Ligurian Basin. The passive seismic network consisted of 29 broad-band ocean bottom stations from Germany and France next to permanent and temporary broad-band land stations. The ocean bottom stations were in operation between June 2017 and February 2018. Two clusters consisting of 18 earthquakes occurred between ∼ 10 km to ∼ 16 km depth below the sea surface, within the lower crust and uppermost mantle, in the centre of the basin. Thrust faulting focal mechanisms indicate compression and tectonic inversion of the Ligurian Basin, which is an abandoned Oligocene–Miocene rift basin. The basin inversion is suggested to be related to the Africa–Europe plate convergence. The locations and focal mechanisms of seismicity suggest reactivation of pre-existing rifting-related structures. Slightly different striking directions of presumed rifting-related faults in the basin center compared to faults further east and hence away from the rift basin may reflect the counter-clockwise rotation of the Corsica–Sardinia block. Mantle refractions Pn and Sn have apparent velocities of 8.2 km/s and 4.7 km/s. The low Vp-Vs-ratio of 1.72 indicates a more brittle behavior of the mantle material. This supports the hypothesis of strengthening of crust and uppermost mantle during the Oligocene–Miocene rifting-related extension and thinning of continental crust. This project is part of the DFG Priority Program “Mountain Building Processes in Four Dimensions (4DMB)”. This research has been supported by the Deutsche Forschungsgemeinschaft (grant nos. TH_2440/1-1, KO_2961/6-1, and LA_2970/4-1) and the Agence Nationale de la Recherche (grant no. ANR-15-CE31- 0015)

Land seismic data of the ALPHA amphibious controlled source experiment - Report

Raw-, SEG-Y and other supplementary data of the landside deployment from the amphibious wide-angle seismic experiment ALPHA are presented. The aim of this project was to reveal the crustal and lithospheric structure of the subducting Adriatic plate and the external accretionary wedge in the southern Dinarides. Airgun shots from the RV Meteor were recorded along two profiles across Montenegro and northern Albania

Seismicity cluster below the Moho indicates thrust faulting in the central Ligurian Basin

International audienceThe Alpine orogen and the Apennines system are part of the complex tectonic settings in the Mediterranean Sea caused by the convergence between Africa and Eurasia. Between 30 Ma and 15 Ma, the Calabrian Subduction retreated in southeast direction pulling Corsica and Sardinia away from the Eurasian continent. In this extensional setting, the Ligurian Sea was formed as a back-arc basin. The rifting jumped 15 MA ago to the Tyrrhenian Sea leaving Corsica and Sardinia in a stable position relative to Eurasia as observed by GPS measurements.Within the framework of the AlpArray research initiative and its German component "4D Mountain building" (SPP2017 4D-MB) a long-term experiment was conducted in the Ligurian sea to investigate the lithosphere structure and the seismicity in the Ligurian basin. The passive seismic network was operated by France and Germany and consisted of 29 broad-band ocean bottom stations. It was in operation between June 2017 and February 2018. At the end of the experiment two active seismic profiles were conducted additionally.A cluster of 15 events with magnitudes lower than 2.5 occurred in the centre of the Ligurian Basin. The earthquakes are located at a depth of 20 km to 35 km, i.e. 10 - 25 km below the Moho. The cluster was not continuously active but had several active periods which lasted between 2 and 5 days.A fault plane solution could be determined of the larger events in the cluster. The mechanism is a thrust faulting. Smaller events should have a similar mechanism due to the highly coherent waveforms. A similar mechanism was observed for the Mw=4.9 earthquake on 07.07.2011 which occurred 50 km east of the cluster. Both solutions show a SW-NE striking, northwest dipping fault plane. This indicates a convergence in NW-SE direction between Corsica and Eurasia

The Fatty Acid Synthase of the Basidiomycete Omphalotus olearius Is a Single Polypeptide

Fatty acids are essential components of almost all biological membranes. Additionally, they are important in energy storage, as second messengers during signal transduction, and in post-translational protein modification. De novo synthesis of fatty acids is essential for almost all organisms, and entails the iterative elongation of the growing fatty acid chain through a set of reactions conserved in all kingdoms. During our work on the biosynthesis of secondary metabolites, a 450-kDa protein was detected by SDS-PAGE of enriched fractions from mycelial lysates from the basidiomycete Omphalotus olearius. Protein sequencing of this protein band revealed the presence of peptides with homology to both α and β subunits of the ascomycete fatty acid synthase (FAS) family. The FAS encoding gene of O. olearius was sequenced. The positions of its predicted 21 introns were verified. The gene encodes a 3931 amino acids single protein, with an equivalent of the ascomycetous β subunit at the N-terminus and the α subunit at the C-terminus. This is the first report on an FAS protein from a homobasidiomycete and also the first fungal FAS which is comprised of a single polypeptide

Kinematics and rifting processes of the Liguro-Provençal Basin, Western Mediterranean

The Liguro-Provençal Basin, situated at the junction of the Northern Apennines and the Western Alps, formed due to the rollback subduction of the Adriatic-African plate underneath Europe and the subsequent upper plate extension in the Oligocene to early Miocene times. The opening of the basin was accompanied by the counter-clockwise rotation of the Corsica-Sardinia block relative to Europe until 16 Ma, with the basin widening towards the southwest. It remains controversial if the extension ever reached seafloor spreading with the production of oceanic crust, or whether it led to anomalously thin continental crust and/or to mantle exhumation. Although considered as tectonically inactive today, the Liguro-Provençal Basin shows active seismicity, indicating compression and possible basin inversion (Thorwart et al. 2021). Thus it is crucial to better understand the opening of the basin and the tectonic inheritance due to rifting in order to interpret the present-day seismicity. To this end, we compiled existing geological and geophysical data, including recent data from the 4DMB project (“Mountain Building Processes in Four Dimensions”), to constrain the crustal and sedimentary thicknesses throughout the basin. We focus specifically on two profiles in the NE (Corsica-Provence) and SW (Sardinia-Gulf of Lion) parts of the basin, along the opening direction of the basin. For each selected profile we calculated the average velocity using the kinematic reconstructions of Le Breton et al. (2021) and the amount of extension using an aerial balancing approach. We then compared these profiles and amounts of extension with results of coupled thermo-mechanical of asymmetric rifting and surface processes modelling using Aspect and Fastscape codes from Neuharth et al. (2022). The results of the thermo-mechanical modelling fit very well the present-day geometry of the rifted continental crust, with a wider hyper-extended rifted margin on the European and a narrower rifted margin on the Corsica-Sardinia side. Rifting migrated southeastward through time and seems to not have reached oceanic spreading nor mantle exhumation in the northeast part of the basin, as observed in the most recent seismic profile A401A-SMPL obtained within the 4DMB SPP project. Towards the southwest, the model confirms the presence of exhumed mantle, as proposed in previous study (Jolivet et al. 2015). The synthesis of geophysical data and thermomechanical modelling also fits very well in the existing kinematic reconstructions from 35 to 0 Ma of the Western Mediterranean, allowing us to infer the lateral extent of oceanic crust and exhumed mantle domains within the basin. Finally, present-day compressional seismicity seems to reactivate rift-related structures

3D crustal structure of the Ligurian Basin revealed by surface wave tomography using ocean bottom seismometer data

The Liguro-Provençal basin was formed as a back-arc basin of the retreating Calabrian–Apennines subduction zone during the Oligocene and Miocene. The resulting rotation of the Corsica–Sardinia block is associated with rifting, shaping the Ligurian Basin. It is still debated whether oceanic or atypical oceanic crust was formed or if the crust is continental and experienced extreme thinning during the opening of the basin. We perform ambient noise tomography, also taking into account teleseismic events, using an amphibious network of seismic stations, including 22 broadband ocean bottom seismometers (OBSs), to investigate the lithospheric structure of the Ligurian Basin. The instruments were installed in the Ligurian Basin for 8 months between June 2017 and February 2018 as part of the AlpArray seismic network. Because of additional noise sources in the ocean, OBS data are rarely used for ambient noise studies. However, we carefully pre-processed the data, including corrections for instrument tilt and seafloor compliance and excluding higher modes of the ambient-noise Rayleigh waves. We calculate daily cross-correlation functions for the AlpArray OBS array and surrounding land stations. We also correlate short time windows that include teleseismic earthquakes, allowing us to derive surface wave group velocities for longer periods than using ambient noise only. We obtain group velocity maps by inverting Green's functions derived from the cross-correlation of ambient noise and teleseismic events. We then used the resulting 3D group velocity information to calculate 1D depth inversions for S-wave velocities. The group velocity and shear-wave velocity results compare well to existing large-scale studies that partly include the study area. We observe a high-velocity area beneath the Argentera Massif in onshore France, roughly 10 km below sea level. We interpret this as the root of the Argentera Massif. Our results add spatial resolution to known seismic velocities in the Ligurian Basin, thereby augmenting existing seismic profiles. The velocity model indicates that the southwestern and north-eastern Ligurian Basin are structurally separate (Figure 1, panel a). In agreement with existing seismic studies, our shear-wave velocity maps indicate a deepening of the Moho from 12 km at the south-western basin centre to 20–25 km at the Ligurian coast in the north-east and over 30 km at the Provençal coast. The lack of high crustal vp/vs ratios beneath the southwestern part of the Ligurian Basin precludes mantle serpentinisation there. The poster summarises the findings published in Solid Earth (Wolf et al. (2021)

Investigations of the Oligocene-Miocene opening of the Ligurian Basin using refraction seismic data

The Ligurian Basin is located north-west of Corsica at the transition from the western Alpine orogen to the Apennine system. The Back-arc basin was generated by the southeast trench retreat of the Apennines-Calabrian subduction zone. The opening took place from late Oligocene to Miocene. While the extension led to extreme continental thinning and un-roofing of mantle material little is known about the style of back-arc rifting. To shed light on the present day crustal and lithospheric architecture of the Ligurian Basin, active seismic data have been recorded on short period ocean bottom seismometers in the framework of SPP2017 4D-MB, the German component of AlpArray. An amphibious refraction seismic profile was shot across the Ligurian Basin in an E-W direction from the Gulf of Lion to Corsica. The profile extends onshore Corsica to image the necking zone of continental thinning. The majority of the refraction seismic data show mantle phases at offsets up to 70 km. The arrivals of seismic phases were picked and inverted in a travel time tomography. The results show a crust-mantle boundary in the central basin at ~12 km depth below sea surface. The mantle shows rather high velocities >7.8 km/s. The crust-mantle boundary deepens from ~12 km to ~18 km within 25 - 30 km towards Corsica. The results do not map an axial valley as expected for oceanic spreading. However, an extremely thinned continental crust indicates a long lasting rifting process that possibly does not initiated oceanic spreading before the opening of the Ligurian Basin stopped

3D crustal structure of the Ligurian Basin revealed by surface wavetomography using ocean bottom seismometer data

The Liguro-Provençal basin was formed as a back-arc basin of the retreating Calabrian-Apennines subduction zone during the Oligocene and Miocene. The resulting rotation of the Corsica-Sardinia block is associated with rifting, shaping the Ligurian Sea. It is still debated whether oceanic or atypical oceanic crust was formed or if the crust is continental and experienced extreme thinning during the opening of the basin. We invert velocity models using an amphibious network of seismic stations, including 22 broadband Ocean Bottom Seismometers (OBS) to investigate the lithospheric structure of the Ligurian sea. The instruments were installed in the Ligurian Sea for eight months between June 2017 and February 2018 as part of the AlpArray seismic network. Because of additional noise sources in the ocean, OBS data are rarely used for ambient noise studies. However, we attentively pre-process the data, including corrections for instrument tilt and seafloor compliance. We took extra care to exclude higher modes of the ambient-noise Rayleigh waves. We calculate daily cross-correlation functions for the LOBSTER array and surrounding land stations. Additionally, we correlate short time windows that include teleseismic earthquakes that allow us to derive surface wave group velocities for longer periods than using ambient noise only. Group velocity maps are obtained by inverting Green’s functions derived from the cross-correlation of ambient noise and teleseismic events, respectively. We then used the resulting 3D group velocity information to calculate 1D depth inversions for S-wave velocities. The shear-wave velocity results show a deepening of the Moho from 12 km at the southwestern basin centre to 20–25 km at the Ligurian coast in the northeast and over 30 km at the Provençal coast. We find no hint on mantle serpentinisation and no evidence for an Alpine slab, at least down to depths of 25 km. However, we see a separation of the southwestern and northeastern Ligurian Basin that coincides with the promoted prolongation of the Alpine front