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Plate tectonics describes oceanic transform faults as conservative strike-slip boundaries, where lithosphere is neither created nor destroyed. Therefore, seafloor accreted at ridge-transform intersections should follow a similar subsidence trend with age as lithosphere that forms away from ridge-transform intersections. Yet, recent compilations of high-resolution bathymetry show that the seafloor is significantly deeper along transform faults than at the adjacent fracture zones. We present residual mantle Bouguer anomalies, a proxy for crustal thickness, for 11 transform fault systems across the full range of spreading rates. Our results indicate that the crust is thinner in the transform deformation zone than in either the adjacent fracture zones or the inside corner regions. Consequently, oceanic transform faulting appears not only to thin the transform valley crust but also leads to a secondary phase of magmatic addition at the transition to the passive fracture zones. These observations challenge the concept of transform faults being conservative plate boundaries.

Among the recently evolving transform faults, the Blancotransform fault system (BTFS) in the northwest off the coast of Oregon,is strongly segmented and resembles other segmented oceanic transform systems, such as the Siqueiros in the East Pacific Rise, which developed from a pre-existing transform fault that was subjected to several extensional events due to a known shift in spreading direction. However,plate tectonic reconstructions suggested that the BTFS developed from at least two large ridge offsets rather than a single transform fault, emerging from a series of ridge propagation events after the plate reorientation at ~5 Ma.We investigated the seismotectonic behavior and tectonic evolution of the BTFS using ocean-bottom seismometer data from the Blanco Transform OBS Experiment (2012–2013) as well as high-resolution multibeam bathymetry, aeromagnetic, and gravity datasets. Interestingly, we observed no evidence for the existence of either transform faults or fracture zones around the BTFS before 2 Ma, indicating that there were no pre-existing transform faults before the formation of the BTFS. Therefore, we propose that the BTFS developed from two broad transfer zones rather than pre-existing transform faults. Moreover, local seismicity shows significant along-strike variations, revealing different modes of slip in the eastern and western BTFS.Seismic slip vectors suggest that the western BTFS is an immature transform fault system since its reorganization is still taking place, but the eastern BTFS accommodating the plate motion ismature. The BTFS acts as a natural laboratory to understand the mechanisms controlling the development of oceanic transform plate boundaries., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Until now, few offshore seismic studies have acquired simultaneously P- and S- wave data to derive in detail the seismic structure of the oceanic crust. We present 2-D Vp and Vs models using wide-angle seismic data at the Indian basin adjacent to the NinetyEast Ridge. Here, an outcrop basement located at the middle of the seismic line presents uppermost crustal Poisson's ratios (ν) of 0.28–0.29 (Vp ∼ 4.2 km/s and Vs ∼ 2.3 km/s). At the flanks of the outcrop basement, the sediment cover is 200–300 m thick and ν values are similar (0.28–0.3), but Vp and Vs values are higher (4.5–4.8 and 2.4–2.6 km/s, respectively). We interpret the relatively lower Vp and Vs around the basement outcrop in terms of hydrothermal alteration, while at the flanks of the basement outcrop, hydrothermal alteration has most likely ceased by sedimentation and compaction processes. Across the seismic layer 2, the Vp–Vs trend is linear and follows a ν value of 0.28–0.29, however, at the seismic layer 2/3 transition, the Vp–Vs trend abruptly changes following a ν value of 0.25–0.26. These reduced observed ν values at the layer 2/3 transition are lower than those reported by laboratory measurements for gabbro (ν ∼ 0.293) and are interpreted in terms of epidotization at the dike-gabbro contact and/or crack-change properties around the lower part of the intrusive sheeted dike section. Key Points We obtain 2-D Vp and Vs models from active seismic data for the Indian oceanic crust The seismic models suggest hydrothermal alteration near a basement outcrop Poisson's ratios change at the layer 2/3 transition from 0.28–0.29 to 0.25–0.26

The VP/VS ratio is an important property for understanding magmatic and tectonic processes at passive continental margins as it is an indicator of the crustal composition. To classify the dominant lithologies in the Zhongsha Block, South China Sea (SCS), we present a detailed VP/VS crustal model based on the independent tomographic inversion of P wave and S wave data. The average VP/VS in the crust of the Zhongsha Block is ∼1.77, indicating an overall felsic to intermediate composition lacking remnant magmatic intrusive rocks. The VP-density relationship from gravity modeling suggests that the lower crust of the extended continental domain contains more greenschist and hence may have experienced metamorphism resulting from an elevated geotherm in the Northwest Sub-basin either during the syn-spreading or postspreading stage. The variability of the VP/VS ratio in the continental block is larger than that in the oceanic basin, showing distinct crustal properties. Several low VP/VS ratio anomalies (VP/VS < 1.7) were found near tectonic boundaries and are interpreted to either result from felsic metamorphism during an interval of rifting, or during the migration of magma along faults and cracks in the postrift period. VP/VS ratios occurring in concert with high VP anomalies in the continent-ocean transition zone support a mafic composition of metapelitic granulite, which was either formed by magmatic intrusions or contact with mantle melting that stem from the upwelling of the asthenospheric mantle during the initial break-up and onset of the seafloor spreading stage in the SCS.

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.

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 centre and Corsica, revealing a diffuse distribution of low-magnitude earthquakes. We analyse data of the amphibious AlpArray seismic network with focus on the offshore component, the AlpArray ocean bottom seismometer (OBS) network, consisting of 24 broadband OBSs deployed for 8 months, to reveal the seismicity and depth distribution of micro-earthquakes beneath the Ligurian Sea. Two clusters occurred between ∼ 10 km to ∼ 16 km depth below the sea surface, within the lower crust and uppermost mantle. Thrust faulting focal mechanisms indicate compression and an 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 rift-related structures. Slightly different striking directions of presumed rift-related faults in the basin centre compared to faults further east and hence away from the rift basin may reflect the counter-clockwise rotation of the Corsica–Sardinia block. High mantle S-wave velocities and a low Vp/Vs ratio support the hypothesis of strengthening of crust and uppermost mantle during the Oligocene–Miocene rifting-related extension and thinning of continental crust.

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 centre and Corsica, revealing a diffuse distribution of low magnitude earthquakes. We analyse data of the amphibious AlpArray seismic network with focus on the offshore component, the AlpArray OBS network, consisting of 24 broadband ocean bottom seismometers deployed for eight months, to reveal the seismicity and depth distribution of micro-earthquakes beneath the Ligurian Sea. Two clusters occurred between ~10 km to ~16 km depth below sea surface, within the lower crust and uppermost mantle. Thrust faulting focal mechanisms indicate compression and an inversion of the Ligurian Basin, which is an abandoned Oligocene 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 rift structures. Slightly different striking directions of faults in the basin centre compared to faults further east and hence away from the abandoned rift may mimic the counter-clockwise rotation of the Corsica-Sardinia block during ~20–16 Ma. The observed cluster events support the hypothesis of strengthening of crust and uppermost mantle during rifting related extension and thinning of continental crust.

International audience; 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 Calabrian subduction retreated in a southeast direction pulling Corsica and Sardinia away from the Eurasian landmass. In this extensional setting, the Ligurian Sea was formed as a back-arc basin. The rifting jumped 15 Ma ago in the Tyrrhenian Sea leaving Corsica and Sardinia in a stable position relative to Eurasia.Within the framework of the AlpArray research initiative a long-term 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. It was in operation between June 2017 and February 2018.Two seismicity clusters occurred in the centre of the Ligurian Basin. The 18 earthquakes are located in the lower crust and in the upper-most mantle at depths between 10 km and 16 km. Re-location was performed only using picks from the OBS in the centre of the Ligurian Sea to avoid artifacts from the complex 3D velocity structure of the basin. 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 behaviour of the mantle material.Fault plane solutions were determined for four events using also the data of land stations in southern France, Corsica, Sardinia and northern Italy. The focal mechanisms are thrust faulting. Fault planes strike in a NE-SW direction, coinciding with the alignment of the events and the direction of the basin axis.We interprete the two earthquake clusters related to the inversion of the Ligurian Basin where the basin"s centre is under compression and stresses are taken up by reactivated faults in the crust and uppermost mantle. The compressional forces could be caused by the convergence of Africa and Europe. In general, observations of earthquakes in continental mantle lithosphere are rare and they reveal on the one hand a strengthening of the crust and uppermost mantle during rifting and on the other hand they support the interpretation that rifting failed in the northern Ligurian 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 centre and Corsica, revealing a diffuse distribution of low magnitude earthquakes. We analyse data of the amphibious AlpArray seismic network with focus on the offshore component, the AlpArray OBS network, consisting of 24 broadband ocean bottom seismometers deployed for eight months, to reveal the seismicity and depth distribution of micro-earthquakes beneath the Ligurian Sea. Two clusters occurred between ~10 km to ~16 km depth below sea surface, within the lower crust and uppermost mantle. Thrust faulting focal mechanisms indicate compression and an inversion of the Ligurian Basin, which is an abandoned Oligocene 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 rift structures. Slightly different striking directions of faults in the basin centre compared to faults further east and hence away from the abandoned rift may mimic the counter-clockwise rotation of the Corsica-Sardinia block during ~20–16 Ma. The observed cluster events support the hypothesis of strengthening of crust and uppermost mantle during rifting related extension and thinning of continental crust.

TRANSFORMERS ; Emden, Deutschland (11.01.2021) – Emden, Deutschland (14.02.2021)

TRANSFORMERS ; Emden, Deutschland (11.01.2021) – Emden, Deutschland (14.02.2021)

The 2014 Mw 8.1 Iquique earthquake ruptured the boundary between the subducting Nazca Plate and the overriding South American Plate in the North Chilean subduction zone. The broken segment of the South American subduction zone had likely accumulated elastic strain since an M~9 earthquake in 1877 and what therefore considered a mature seismic gap. The moderate magnitude of the 2014 earthquake and its compact rupture area, which only broke the central part of the seismic gap, did not result in a significant tsunami in the Pacific Ocean. To investigate the seismo-tectonic segmentation of the North Chilean subduction zone in the region of the 2014 Iquique earthquake at the shallow seismic/aseismic transition, we combine two years of local aftershock seismicity observations from ocean bottom seismometers and long-offset seismic reflection data from the rupture area. Our study links short term deformation associated with a single seismic cycle to the permanent deformation history of an erosive convergent margin over millions of years. A high density of aftershocks following the 2014 Iquique earthquake occurred in the up-dip region of the coseismic rupture, where they form a trench parallel band. The events spread from the subducting oceanic plate across the plate boundary and into the overriding continental crust. The band of aftershock seismicity separates a pervasively fractured and likely fluid-filled marine forearc farther seaward from a less deformed section of the forearc farther landward. At the transition, active subduction erosion during the postseismic and possibly coseismic phases of the 2014 Iquique earthquake leads to basal abrasion of the upper plate and associated extensional faulting of the overlying marine forearc. Landward migration of the seismogenic up-dip limit, possibly at similar rates compared to the trench and the volcanic arc, leaves behind a heavily fractured and fluid-filled outermost forearc. This most seaward part of the subduction zone might be too weak to store sufficient elastic strain to nucleate a large megathrust earthquake.

Fracture zones were recognized to be an integral part of the seabed long before plate tectonics was established. Later, plate tectonics linked fracture zones to oceanic transform faults (OTF), suggesting that they are the inactive and hence fossil trace of transforms. Yet, scientists spend little time surveying them in much detail. Recent evidence suggests that the traditional concept of transform faults as being conservative (non-accretionary) plate boundary faults might be wrong. Instead, numerical modelling results suggest that transform faults seem to suffer from extensional tectonics below their strike-slip surface fault zone, and a global compilation of legacy bathymetric data suggest that ridge-transform intersections seem to be settings of magmatic activity, modifying the lithosphere and burying the transform valley before it passes into the fracture zone region. During M170 we tested those hypotheses by collecting a suite of new data from the Oceanographer and Hayes transform faults offsetting the Mid-Atlantic Ridge to the south of the Azores near 35°N and hence conducted a pilot study revealing the state-of-stress derived from micro-earthquakes and bathymetry as well as geological sampling to evaluate magmatic and tectonic processes shaping transform faults. Preliminary analysis of 10-days of seismicity data recorded at hydrophone stations showed 10-15 local earthquakes per day and bathymetric data supports that ridge-transform intersections support indeed a second phase of magmatic accretion.

The 2014 Mw 8.1 Iquique earthquake ruptured the boundary between the subducting Nazca Plate and the overriding South American Plate in the North Chilean subduction zone. The broken segment of the South American subduction zone had likely accumulated elastic strain since an M~9 earthquake in 1877 and what therefore considered a mature seismic gap. The moderate magnitude of the 2014 earthquake and its compact rupture area, which only broke the central part of the seismic gap, did not result in a significant tsunami in the Pacific Ocean. To investigate the seismo-tectonic segmentation of the North Chilean subduction zone in the region of the 2014 Iquique earthquake at the shallow seismic/aseismic transition, we combine two years of local aftershock seismicity observations from ocean bottom seismometers and long- offset seismic reflection data from the rupture area. Our study links short term deformation associated with a single seismic cycle to the permanent deformation history of an erosive convergent margin over millions of years. A high density of aftershocks following the 2014 Iquique earthquake occurred in the up-dip region of the coseismic rupture, where they form a trench parallel band. The events spread from the subducting oceanic plate across the plate boundary and into the overriding continental crust. The band of aftershock seismicity separates a pervasively fractured and likely fluid-filled marine forearc farther seaward from a less deformed section of the forearc farther landward. At the transition, active subduction erosion during the postseismic and possibly coseismic phases of the 2014 Iquique earthquake leads to basal abrasion of the upper plate and associated extensional faulting of the overlying marine forearc. Landward migration of the seismogenic up-dip limit, possibly at similar rates compared to the trench and the volcanic arc, leaves behind a heavily fractured and fluid-filled outermost forearc. This most seaward part of the subduction zone might be too weak to store sufficient elastic strain to nucleate a large megathrust earthquake.

The 2014 Mw 8.1 Iquique earthquake ruptured the boundary between the subducting Nazca Plate and the overriding South American Plate in the North Chilean subduction zone. The broken segment of the South American subduction zone had likely accumulated elastic strain since an M~9 earthquake in 1877 and what therefore considered a mature seismic gap. The moderate magnitude of the 2014 earthquake and its compact rupture area, which only broke the central part of the seismic gap, did not result in a significant tsunami in the Pacific Ocean. To investigate the seismo-tectonic segmentation of the North Chilean subduction zone in the region of the 2014 Iquique earthquake at the shallow seismic/aseismic transition, we combine two years of local aftershock seismicity observations from ocean bottom seismometers and long-offset seismic reflection data from the rupture area. Our study links short term deformation associated with a single seismic cycle to the permanent deformation history of an erosive convergent margin over millions of years. A high density of aftershocks following the 2014 Iquique earthquake occurred in the up-dip region of the coseismic rupture, where they form a trench parallel band. The events spread from the subducting oceanic plate across the plate boundary and into the overriding continental crust. The band of aftershock seismicity separates a pervasively fractured and likely fluid-filled marine forearc farther seaward from a less deformed section of the forearc farther landward. At the transition, active subduction erosion during the postseismic and possibly coseismic phases of the 2014 Iquique earthquake leads to basal abrasion of the upper plate and associated extensional faulting of the overlying marine forearc. Landward migration of the seismogenic up-dip limit, possibly at similar rates compared to the trench and the volcanic arc, leaves behind a heavily fractured and fluid-filled outermost forearc. This most seaward part of the subduction zone might be too weak to store sufficient elastic strain to nucleate a large megathrust earthquake.

TRANSFORMERS ; Emden, Deutschland (11.01.2021) – Emden, Deutschland (14.02.2021)

About 25% of the Earth’s mid-ocean ridges spread at ultraslow rates of less than 20 mm/yr. However, most of these ultraslow spreading ridges are located in geographically remote areas, which hamper investigation. Consequently, how the crust forms and ages at such spreading centres, which traditional models predict to be magma-starved and cold, remains poorly understood. One of the most accessible ultra-slow spreading centres is the Mid Cayman Spreading Centre (MCSC), in the Caribbean Sea, with spreading rates of ~15-17 mm/yr. CAYSEIS project was proposed to survey the Cayman Trough area in order to obtain new data that constraints the nature of the crust, tectonic structures, lithologies outcropping and hydrothermal processes taking place in this area. Understanding the sub-seabed geophysical structure of the MCSC is key to understanding not only the lithologies and structures exposed at the seabed, but more fundamentally, how they are related at depth and what role hydrothermal fluid flow plays in the geodynamics of ultraslow spreading. CAYSEIS was a joint and multidisciplinary programme of German, British and US American top tier scientists designed for the obtaining of a new high-quality dataset, including 3D Wide-Angle Seismic (WAS), magnetic, gravimetric and seismological data. During the CAYMAN project, we took leverage of the CAYSEIS dataset to invert a 3D tomographic model of the Cayman Trough lithosphere using the Tomo3D code (Meléndez et al., 2015; 2019). This is one of the first times that the Tomo3D code is used for 3D inversion of real datasets. Thus, we are checking our results comparing them with tomographic inversions of 2D lines and testing the different parameters to obtain the more accurate and higher resolution model as possible. The results of this experiment will show not only the lithospheric structure along and across the MSCS, including the exhumed Ocean Core Complexes in the surrounding areas, but the 3D lithospheric configuration of the region which is important to understand the crustal formation processes and the evolution of ultra-slow spreading settings., Poster, poster

International audience

19 pages, 8 figures, supporting information https://doi.org/10.1029/2020JB019854.-- Data Availability Statement. The WESTMED data are archived at PANGAEA repository (https://doi.pangaea.de/10.1594/PANGAEA.921252), GEBCO bathymetry grid (https://doi.org/10.5285/a29c5465‐b138‐234d‐e053‐6c86abc040b9), and SRTM bathymetry/topography grid (https://doi.org/10.5067/MEaSUREs/SRTM/SRTMGL1.003), In continental settings, seismic failure is generally restricted to crustal depth. Crustal structure is therefore an important proxy to evaluate seismic hazard of continental fault systems. Here we present a seismic velocity model across the Gibraltar Arc System, from the Eurasian Betics Range (South Iberian margin), across offshore East Alboran and Pytheas (African margin) basins, and ending onshore in North Morocco. Our results reveal the nature and configuration of the crust supporting the coexistence of three different crustal domains: the continental crust of the Betics, the continental crust of the Pytheas Basin (south Alboran Basin) and onshore Morocco, and a distinct domain formed of magmatic arc crust under the East Alboran Basin. The magmatic arc under the East Alboran Basin is characterized by a velocity structure containing a relatively high‐velocity lower crust (~7 km/s) bounded at the top and base by reflections. The lateral extension of this crust is mapped integrating a second perpendicular wide‐angle seismic profile along the Eastern Alboran basin, together with basement samples, multibeam bathymetry, and a grid of deep‐penetrating multichannel seismic profiles. The transition between crustal domains is currently unrelated to extensional and magmatic processes that formed the basin. The abrupt transition zones between the different crustal domains support that they are bounded by crustal‐scale active fault systems that reactivate inherited structures. Seismicity in the area is constrained to upper‐middle crust depths, and most earthquakes nucleate outside of the magmatic arc domain, This work is supported by the Cluster of Excellence “The Future Ocean”, within the framework of the Excellence Initiative by the Deutsche Forschungsgemeinschaft (DFG) on behalf of the German federal and state governments. Efforts benefitted from funding of the German Science Foundation (DFG Grants GR1964/12‐1, RA 925/2‐1+2‐2, and RE 873/17‐1). The TOPOMED cruise was part of the EUROMARGINS and TOPO‐EUROPE initiatives of the EUROCORES Programme of the European Science Foundation (ESF). This study benefitted from an EU Marie Skłodowska‐Curie Individual Fellowship to L. Gómez de la Peña (H2020‐MSCA‐IF‐2017 796013). The Spanish Science and Innovation Ministry funded C. R. Ranero through the project FRAME CTM2015‐71766‐R and G. Booth‐Rea through the project PID2019‐107138RB‐I00. This is a contribution of the Barcelona Center for Subsurface Imaging, Grup de Recerca 2017 SGR 1662, Generalitat de Catalunya, With the funding support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S), of the Spanish Research Agency (AEI)

In continental settings, seismic failure is generally restricted to crustal depth. Crustal structure is therefore an important proxy to evaluate seismic hazard of continental fault systems. Here we present a seismic velocity model across the Gibraltar Arc System, from the Eurasian Betics Range (South Iberian margin), across offshore East Alboran and Pytheas (African margin) basins, and ending onshore in North Morocco. Our results reveal the nature and configuration of the crust supporting the coexistence of three different crustal domains: the continental crust of the Betics, the continental crust of the Pytheas Basin (south Alboran Basin) and onshore Morocco, and a distinct domain formed of magmatic arc crust under the East Alboran Basin. The magmatic arc under the East Alboran Basin is characterized by a velocity structure containing a relatively high‐velocity lower crust (~7 km/s) bounded at the top and base by reflections. The lateral extension of this crust is mapped integrating a second perpendicular wide‐angle seismic profile along the Eastern Alboran basin, together with basement samples, multibeam bathymetry, and a grid of deep‐penetrating multichannel seismic profiles. The transition between crustal domains is currently unrelated to extensional and magmatic processes that formed the basin. The abrupt transition zones between the different crustal domains support that they are bounded by crustal‐scale active fault systems that reactivate inherited structures. Seismicity in the area is constrained to upper‐middle crust depths, and most earthquakes nucleate outside of the magmatic arc domain. Key Points New velocity model reveals the lithospheric structure under the Betics (South Iberia), the Alboran Basin and the North African margin The East Alboran Basin is floored by magmatic arc crust, while the southern area of the Alboran Basin is floored by continental crust Seismic activity is constrained to the upper‐middle continental crust. Crustal domains are likely bounded by active faults

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 the basin. Yet it remains unclear if rifting caused continental break-up and seafloor spreading. To reveal its lithospheric architecture, we acquired a 130 km long seismic refraction and wide-angle reflection profile in the Ligurian Basin. The seismic line was recorded in the framework of SPP2017 4D-MB, a Priority Programme of the German Research Foundation (DFG) and 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 were recorded on the newly developed GEOLOG recorder, designed at GEOMAR, and are dominated by sedimentary refractions and show mantle Pn arrivals at offsets of up to 70 km and a very prominent wide-angle Mohorovičić discontinuity (Moho) reflection. The main features share several characteristics (e.g. offset range, continuity) generally associated with continental settings rather than documenting oceanic crust emplaced by seafloor spreading. Seismic tomography results are complemented by gravity data and yield a ∼ 6–8 km thick sedimentary cover and the seismic Moho at 11–13 km depth below the sea surface. Our study reveals that the oceanic domain does not extend as far north as previously assumed. Whether Oligocene–Miocene extension led to extremely thinned continental crust or exhumed subcontinental mantle remains unclear. A low grade of mantle serpentinisation indicates a high rate of syn-rift sedimentation. However, rifting failed before oceanic spreading was initiated, and continental crust thickens towards the NE within the northern Ligurian Basin.

European Geosciences Union (EGU) General Assembly 2020, 4-8 May 2020, Marie Tharp (1920-2006) was a pioneer of modern oceanography. She was an American geologist and oceanographic cartographer who, together with his husband Bruce Heezen, generated the first bathymetric map of the Atlantic Ocean floor. Tharp's work revealed the detailed topography and geological landscape of the seafloor. Her work revealed the presence of a continuous rift valley along the Mid-Atlantic Ridge axis, causing a paradigm in earth sciences that led to the acceptance of plate tectonics and continental drift theories. Piecing maps together in the late 1940s and early 1950s, Marie and his partner Bruce Heezen discovered the 75.000 km underwater ridge bounding around the globe. By this finding, they laid the conclusion from geophysical data that the seafloor spreads from mid-ocean ridges and that continents are in motion with respect to one another¿a revolutionary geological theory at that time. Many years later, satellite images demonstrate that Tharp¿s maps were accurate. In this contribution, we focus on detailed bathymetric maps collected from year 1992 to today, which include bathymetric maps from diverse parts of the world. For instance, we will show a) Back-arc basins (i.e. the Bransfield Basin, Antarctica; and the North Fiji Basin, SW Pacific); b) Mid-ocean ridges and fracture zones (i.e. the MAR at the South of Azores, the MAR at the Oceanographer-Hayes, and the St. Paul Fracture Zone at the Equator), and c) Active tectonic structures from the Gulf of Cadiz and Alboran Sea, located at the Africa-Eurasia plate boundary (Gibraltar Arc). Regarding this last area, we will characterize the seafloor expression of the fault systems, as well as the subsurface structure of the faults in the Gulf of Cadiz and Alboran Sea. This zone is characterized by a moderate seismicity, mainly reverse and strike-slip focal mechanisms; although large historical (AD1755, AD1829) and instrumental earthquakes or large/great magnitude also occurred, such as the earthquakes of 1969, 1994, 2004 and 2016. In addition, the Gulf of Cadiz-Alboran Sea area is compartmentalized in different crustal domains, bounded by active strike-slip fault systems. We adopted a multi-scale approach, including morphological analysis of shipboard multibeam bathymetry, near-bottom bathymetry obtained with Autonomous Underwater Vehicles (AUVs) at a resolution of 1-2 m, and medium to deep penetration multi-channel seismic (MCS) data. Finally, we will also show a couple of videos from recent marine cruises in the Gibraltar Arc (SHAKE-2015 and INSIGHT-2018), both using state-of-the-art high-resolution marine technologies

GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany (1); Kiel University (2); Institut de physique du globe de Paris, Paris, France (3); ISTerre - Institut des Sciences de la Terre, Grenoble, France (4); AlpArray Working Group: http://www.alparray.ethz.ch (5); The Liguro-Provencal-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 debated though, 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 contribute to the debate by surveying the type of crust and lithosphere flooring the Ligurian Sea using 29 broadband Ocean Bottom Seismometer (OBS). 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 extensively pre-process the data, including corrections for instrument tilt and seafloor compliance, to improve the signal-to-noise ratio. We calculate daily cross-correlation functions for the LOBSTER array and surrounding land stations. Additionally, we correlate short time windows that include strong earthquake events 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. Our group velocity maps show strong heterogeneities for short periods (5-15 s, corresponding to shallow depths). In general, the velocities increase with depth and the velocity anomalies can be related to varying sediment thickness and magmatism. The longer periods (20-90 s) show a smoother velocity structure that reveals mantle velocities in the vicinity of the Ligurian margin, north of the basin centre. However, resolution at greater depth is limited along the Corsican margin due to less station coverage. Our results do not indicate an oceanic spreading centre, however, may hint to an asymmetric opening of the Ligurian Basin.

The Liguro-Provencal-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 at roughly 32–24 Ma is associated with rifting, shaping the Ligurian Sea. It is highly debated though, whether oceanic or atypical oceanic crust was formed or if the crust is continental and experienced extreme thinning during the opening of the basin.To investigate the velocity structure of the Ligurian Sea a network (LOBSTER) of 29 broadband Ocean Bottom Seismometer (OBS) was installed jointly by GEOMAR (Germany) and ISTerre (France). The LOBSTER array measured continuously for eight months between June 2017 and February 2018 and is part of the AlpArray seismic network. AlpArray is a European initiative to further reveal the geophysical and geological properties of the greater Alpine area.We contribute to the debate by surveying the type of crust and lithosphere flooring the Ligurian Sea.Because of additional noise sources in the ocean, causing instrument tilt or seafloor compliance, OBS data are rarely used for ambient noise studies. However, we extensively pre-process the data to improve the signal-to-noise ratio. Then, we calculate daily cross-correlation functions for the LOBSTER array and surrounding land stations. Additionally, we correlate short time windows that include strong events. The cross-correlations of these are dominated by earthquake signals and allow us to derive surface wave group velocities for longer periods than using ambient noise only. Finally, phase velocity maps are obtained by inverting Green’s functions derived from cross-correlation of ambient noise and teleseismic events, respectively. The phase velocity maps show strong heterogeneities for short periods (5-15 s, corresponding to shallow depths). Causes for these include varying sediment thickness, fault zones and magmatism. For longer periods (20-80 s) the velocity structure smoothens and reveals mantle velocities north-northwest of the basin centre. This might hint on an asymmetric opening of the basin. We do not see strong indications for an oceanic spreading centre in the Ligurian basin.

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 little is known about the style of back-arc rifting. Today, the seismic events indicate the closure of this back-arc basin. 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 seismometersin the framework of SPP2017 4D-MB, the German component of AlpArray. An amphibious refraction seismicprofile was shot acrossthe Ligurian Basininan E-W direction from the Gulf of Lion to Corsica. The profile extends onshore Corsica to image the necking zone of continental thinning. Local seismicity was recorded on broad band ocean bottom seismometers over a period of 8 month. The majority of the refraction seismic data show mantle phases at offsets up to 70km. The arrivalsof seismic phases were picked and inverted in a travel time tomography. The results show acrust-mantle boundary in the central basin at ~12km depth below seasurface. The mantle shows rather high velocities >7.8 km/s.The crust-mantle boundary deepens from ~12 km to ~18 km within 25 -30 kmtowards 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. In the centre of the Ligurian Basin, a cluster of 15 seismic events with magnitudes lower than 2.5 occurred in the upper mantle in a depth of 10-15 kmbelow the seismic Moho. The fault plane solutions indicate reverse faulting and a convergence in NW-SE direction.

European Geosciences Union (EGU) General Assembly, 7-12 April 2019, Vienna, Austria.-- 1 page, We present seismic data collected in summer 2018 of the structure of the opening of the central Atlantic in NW Africa, and the west Iberia Mesozoic rift basins and rifted margins. We obtained crustal-scale seismic images collected with a large tuned source and a 6-km-long streamer, and new coincident wide-angle seismic data with ~10-km-spaced OBSs. Our cross-sections were collected in transects in regions where previous work from the 1980 to mid 1990's collected some of the most emblematic geophysical cross-sections. New images of the Galicia Interior Basin (GIB) display prominent thinning at the basin center with an asymmetric tectonic and crustal structure, indicating that extension stopped when the basin was in the latest phase before break up. The basin is characterized by large-scale normal faults that are laterally bounded by transfer fault structures. New Vp models from the Iberia abyssal plain indicate that the crustal structure is more complex than models with lower resolution -widely used in geological interpretations- have inferred. The new data preliminarily supports that the exhumed mantle may contain magmatic bodies and that continental break up may have occurred before mantle exhumation. Comparison of observations from Iberia and Tagus Abyssal Plains display the transition from exhumed mantle to the oceanic crust across the J-anomaly ridge, providing data of on the interaction between magma and tectonics. Seismic images from NW Africa display a rather different structure, with upper Triassic evaporite diapirs extruding from fault-blocks of highly-extended possibly continental crust, with Early Jurassic (drilled) synrift deposits in tilted blocks. Here, a rather abrupt continental slope is followed by a deep-water basin underlain by perhaps as much as 100 km of highly-extended continental crust

European Geosciences Union (EGU) General Assembly, 7-12 April 2019, Vienna, Austria.-- 1 page, Deformation by bending of incoming oceanic plates prior to subduction was noted soon after the beginning of the Plate Tectonics revolution 50 years ago, however the observations available at the time did not permit any detailed scrutiny. It is only since 15 years ago that we begun to realize the intensity of the processes that modify oceanic plates at subduction trenches, and the potential implications for a broad range of subduction-related phenomena. Albeit the limited number of dedicated studies in the world subduction system, a growing body of observations supports that the physical and chemical nature of oceanic lithosphere created at spreading centres is profoundly modified during their passage of the outer rise and trench. Yet, there is neither consensus for conceptual models of the processes at work, nor robust global averages of the intensity of those changes. To understand the structure and nature of subducting slabs and the implications for material recycling and mantle evolution we need to further study the variability of those processes with new experiments and novel methods. It is likely that as methods improve and research intensifies we will serendipitously discover new still unreported features and phenomena. However, the available data are currently enough to warrant the proposal that the intensity of the physical and chemical modifications imply that incoming plates at subduction trenches are a class of geodynamic setting in their own that requires to be understood to unravel the earth evolution in terms of material and volatiles

European Geosciences Union (EGU) General Assembly, 7-12 April 2019, Vienna, Austria.-- 1 page, When continents break apart, continental crust and lithosphere is stretched until break-up occurs and seafloor spreading forms new oceanic basins. However, at settings with low magma supply the area between the oceanic and continental domain is a wide transition zone (COT) rather than a sharp boundary. This end-member type of margin has been defined as non-volcanic or magma-poor margin and type-examples are the Iberia Abyssal Plain and the conjugated Newfoundland margin. Yet, at both margins, the structure of the COT and the onset of seafloor spreading are poorly defined with available data and a matter of long debate. For instance, some authors suggested that a prominent magnetic anomaly ¿ called the J-anomaly ¿ is not marking an age-isochron, but is mimicking the onset of oceanic crust and hence seafloor spreading. Others have proposed that the COT is actually ultra-slow seafloor spreading before the J-anomaly. Here, we present results from a new seismic refraction and wide-angle profile shot in September of 2018 aboard the Spanish research vessel Sarmiento de Gamboa in the Iberia Abyssal Plain about 20 km to the south of seismic line IAM9. Thirty ocean-bottom-seismometers and hydrophones spaced at 10-12 km intervals recorded seismic shots fired along the 360 km long profile P3. The line is roughly centred at the J-anomaly, revealing the nature of crust and lithosphere on either side. Seismic travel time data were inverted to yield the P-wave seismic velocity (Vp) structure using a least-square damped tomographic inversion procedure. To assess the nature of basement, we used two features derived from the seismic velocity structure: (i) average Vp within 4 km below basement and (ii) maximum P-wave speed in the same depth range. Results show that the J-anomaly marks indeed a major change in lithology. Thus, to the west, sedimentary cover is in the order of 2 km and average crustal velocity ranges from >6 km/s to 6.7 km/s, with a maximum velocity of 6.6 to 7.5 km/s and maximum velocity ranges from 7.0 to 8.1 km/s. Therefore, the domain to the east of the J-anomaly seems to be dominated by serpentinized mantle as previously proposed, but the structure is more complex than the image obtained in previous sparser data sets. The domain of serpentinized mantle may include some bodies of magmatic crust and thus could either be interpreted as domain of un-roofed mantle, which had been affected by some magmatic activity as observed in the Tyrrhenian Sea or it may represent ultra-slow spreading lithosphere as observed in the Cayman Trough. The domain west of J may indicate magmatic crust, though it shows notable differences when compared to normal oceanic crust. The new higher resolution data set challenges the concepts built in numerous publications based on lower resolution geophysical images

Workshop. Alboran Domain and Gibraltar Arc: Geological Research and Natural Hazards - El dominio de Alborán y el Arco de Gibraltar: Investigación geológica y riesgos naturales - Le Domaine Alboran et l'Arch de Gibraltar: Recherche géologique et risques naturels, 16-18 octubre 2019, Granada..-- 1 page, Large continental faults extend for thousands of kilometres and often form the tectonic boundaries between plates, associated with prominent topographic features and can produce large earthquakes, thus implying a high seismic hazard. These paradigms are called into question in the Alboran Sea, which hosts an allegedly complex diffuse boundary between the Eurasia and Nubia plates, and where we found one of the few examples worldwide of the initial stages of these key tectonic structures. On the 25th January 2016, a magnitude Mw6.4 submarine earthquake struck the north Moroccan coast, the largest event ever recorded in the Alboran Sea. The quake was preceded by an earthquake of magnitude Mw5.1 and was followed by numerous aftershocks whose locations mainly migrated to the south. The mainshock nucleated at a releasing bend of the poorly known Al-Idrissi Fault System (AIFS). According to slip inversion and aftershock distribution, we assumed a rupture length of 18 km. Here we combine multiscale bathymetric and marine seismic reflection data with a resolution comparable to the studies on land, together with seismological data of the 2016 Mw 6.4 earthquake offshore Morocco – the largest event recorded in the area – to unveil the 3D geometry of the AIFS. Despite its subdued relief, the AIFS is a crustal-scale boundary. We show evidence of left-lateral strike-slip displacement, characterize their fault segments and demonstrate that the AIFS is the source of the 2016 seismic events. The occurrence of the Mw 6.4 earthquake and previous events of 1994 and 2004 reinforce that the AIFS is now growing through propagation and linkage of its segments, which eventually may generate a greater rupture (up to Mw 7.6), increasing the potential hazard of the structure

The Liguro-Provencal-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 at roughly 32–24 Ma is associated with rifting, shaping the Ligurian Sea. It is highly debated though, whether oceanic or atypical oceanic crust was formed or if the crust is of continental nature, which was extremely thinned during opening of the basin. In order to investigate the velocity structure of the Ligurian Sea a network (LOBSTER) of 29 broadband Ocean Bottom Seismometer (OBS) was installed jointly by GEOMAR (Germany) and ISTerre (France). The LOBSTER array measured continuously for eight months between June 2017 and February 2018 and is part of the AlpArray seismic network. AlpArray is a European initiative to further reveal the geophysical and geological properties of the greater Alpine area. We contribute to the debate by surveying the type of crust and lithosphere flooring the Ligurian Sea. Because of additional noise sources in the ocean, causing instrument tilt or seafloor compliance, OBS data are rarely used for ambient noise studies. However, we extensively pre-process the data to improve the signal-to-noise ratio. Then, we calculate daily cross-correlation functions for the LOBSTER array and surrounding land stations. We use teleseismic events by correlating short time windows that include strong events. Those cross-correlations are dominated by earthquake signals and allow to derive surface wave group velocities for longer periods than using AN. Finally, phase velocity maps are obtained by inverting Green’s functions derived from cross-correlation of ambient noise (AN) and teleseismic events. In the course of this ongoing project we target to derive 3D velocity models of the adjacent Alpine belt region and its complex subduction geometry contributing to questions like the prolongation of the Alpine from beneath the Ligurian Sea.

The Ligurian Basin is situated at the transition from the western Alpine orogeny to the Apennine system, an area where a change in subduction polarity is observed. 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 continental thinning and subsidence, oceanic spreading with unroofing of mantle material was proposed for the late opening period, 21-16 Ma. To shed light on the present day crustal and lithospheric architecture of the Ligurian Basin, active and passive seismic data have been recorded on ocean bottom seismometers of a long-term network consisting of 29 broad-band stations, installed from June 2017 to February 2018 in the framework of SPP2017 4D-MB, the German component of AlpArray. Two refraction seismic profiles were shot to serve two aspects: (1) Determine the orientation of the horizontal components of the long-term instruments and (2) estimate the velocity distribution of the upper lithosphere, to provide a velocity model for the passive seismic data analysis. Good quality data have been recorded, regional and teleseismic events as well as active shots could be detected by the network stations. The majority of the refraction seismic data show mantle phases at offsets up to 70 km and a very prominent wide-angle reflection originating at the crust mantle boundary. Its features share a number of characteristics (i.e. offset range, continuity) generally associated with continental settings rather than mimicking seafloor spreading lithosphere emplaced in back-arc basins. Based on traveltime tomography along the refraction lines, the crust-mantle boundary is determined at ~9.5 km depth below seafloor. The acoustic basement is difficult to map seismically. The transition to the crystalline basement is indicated at a depth of ~6.5 km below seafloor. The absolute seismic velocities can be interpreted as hyper-extended continental crust or serpentinised mantle. The thick sedimentary coverage allows for long lasting extension of the crust. The crustal portion interpreted from the seismic velocities thickens towards the north which is in good agreement with the anti-clockwise rotation of the Corsica-Sardinia block and an associated gradual opening of the Ligurian Basin.

European Geosciences Union (EGU) General Assembly, 7-12 April 2019, Vienna, Austria.-- 1 page, The subducting ocean lithosphere may carry large amount of chemically bound-water into the deep Earth interior, returning water to the mantle, facilitating melting and hence keeping the mantle mobile, in turn, nurturing plate tectonics. Bending-related faulting in the trench-outer rise region prior to subduction has been recognized to be an important process, governing the return flux of water into the mantle. Thus, extensional faulting in the trench-outer rise is opening pathways into the lithosphere, promoting hydration of the lithosphere, including alteration of dry peridotite to water rich serpentine. In this paper, we review and summaries evidence for supporting that bending-related faulting is indeed a key process in the global water cycle. Two features are found in a world-wide compilation of tomographic velocity models derived from seismic refraction data, indicating that ocean lithosphere is modified when approaching a deep-sea trench: (i) seismic velocities in both the lower crust and upper mantle are significantly reduced compared to the structure found in the vicinity of mid-ocean ridges and in mature crust away from subduction zones; (ii) along profiles approaching a trench, both crustal and upper mantle velocities decrease systematically with decreasing distant to the trench, highlighting an evolutionary process as velocity reduction is related to alteration and hydration. P-wave velocity anomalies suggest that serpentinization at trenches is a global feature of subducting oceanic plates older than 10-15 Ma. However, the amount of water trapped within the uppermost mantle and carried into subduction zones depends on how serpentines are distributed in the upper mantle. Thus, estimates of bulk upper mantle hydration would be reduced from 3.5 wt% of water to as low as 0.3 wt% by assuming that reduction of seismic velocitiy results from serpentinized joints. In addition to observations on the state of hydration of the incoming plate, we will evaluate the fate of subducted water carried into subduction zones and its release in the >subduction factory>.

Report of the research cruise no. MSM69 of the German research vessel Maria S. Merian into the aquatorial Atlantic, conducting seismic refraction and wide-angle measurements., MARIA S. MERIAN-Berichte

On April 2014 the Iquique Mw 8.1 earthquake ruptured the interpolate contact between the oceanic Nazca and continental South American plates offshore northern Chile between 19.5◦S to 21◦S in April 2014. This earthquake did not fully release the strain accumulated since the last great megathrust (Mw 8.8) in 1877 and had left an unbroken segment in the south. From December 2014 to November 2016, we deployed an offshore network of 15 Ocean Bottom Seismometers (OBS) that covered the rupture area and the unbroken southern segment using the Chilean Navy ship OPV Toro and R/V Sonne. That data set is supplemented by five weeks of data from 67 OBS installed for a controlled source seismic experiment during cruise MGL1610 of the R/V Marcus Langseth in late 2016. Data acquired onshore by stations from of the IPOC (Integrated Plate Boundary Observatory Chile) and CSN (Chilean Seismological Service) networks are also included. We present first results of this ongoing project, which include double-difference hypocenter relocations based on waveform cross-correlation. Most of the seismicity occurs between 19.5 and 21◦S up-dip of the patch of maximum coseismic slip during the 2014 earthquake, while the seismicity in seismogenic depths is highly concentrated forming well-defined clusters. The observed seismicity provides constraints on the structure of the marine forearc and enables us to relate the seismicity distribution to the background seismicity, seafloor morphology, and regional tectonics.

18 pages, 9 figures, supporting information https://doi.org/10.1111/bre.12211, The structural complexity of back‐arc basins is related to the evolution of the associated subduction system. Here, we present an integrated geophysical and geological study that constrains the 3D spatial variability of magmatic activity along the Tyrrhenian back‐arc basin. We use wide‐angle seismic and gravity data, acquired in 2010 within the MEDOC experiment along a ~300 km‐long NW‐SE transect that extends from SE Sardinia Island to the NW Sicily continental margin, across the Cornaglia Terrace. The geophysical transect is coincident with a seismic reflection line from the Italian CROP experiment that we have re‐processed. The geophysical results, together with available basement dredges, support a basement along the profile fundamentally composed of continental‐type rocks, locally affected by subduction‐related magmatism. The continental nature of this region contrasts with the nature of the basement inferred along two geophysical cross‐sections located to the north of the Cornaglia Terrace in which seismic velocity of the lower crust supports significant magmatic crustal accretion. The comparison of these three cross‐sections supports that the highest magmatic activity occurred in the central and most extended region of the basin, whereas it was less important in the North and practically nonexistent in the South. These observations indicate abrupt variations of magmatism during the basin formation. As in other back‐arcs, the temperature, water content and composition of the mantle might have played an important role in such variation, but they fail to explain the abruptness of it. We propose that the interaction of the overriding continental lithospheres of Adria and Africa with the Apenninic‐Calabrian subduction system caused changes in slab rollback and trench retreat dynamics, which in turn resulted in variations of back‐arc stretching and magmatism. Based on our observations, we suggest that the Cornaglia Terrace formation process might share some similarities with the formation of oceanic crust in the Red Sea, We thank the teams involved in the MEDOC project and the Complementary Action OSMART, which has been funded by the former Spanish Ministry of Science and Education, with reference CTM2007-66179/C02-01/MAR and CTM2007-66179-C02-02/MAR, and CTM 2009-07772-E/MAR respectively. The German Science Foundation (DFG grant GR 1964/14-1) supported I. Grevemeyer. This work was also supported by project CO-DOS funded by Repsol. […]M. Prada and M. G. Vendrell work were funded by the Spanish CSIC Jae-PreDoc and Spanish Ministry of Science and Education FPI grants system respectively

Congresso congiunto Società Geologica Italiana (SGI) - Società Italiana di Mineralogia e Petrologia (SIMP), Geosciences for the environment, natural hazards and cultural heritage, 12-14 September 2018, Catania.-- 1 page, The Tyrrhenian back-arc basin is a Neogenic basin that opened in response to the E-SE-ward retreat of the subducting Ionian slab under the Apennine chain. To date a large bibliography is available for this basin (Moeller et al., 2014), but un updated morpho-structural map which takes into account the latest multibeam and multichannel seismic data sets is still lacking. Indeed, the structural maps of the entire Tyrrhenian date back to ‘80/’90. In this work, we present a new morpho-tectonic map of the entire Tyrrhenian basin, from slopes to abyssal plains. This map results from the analysis of 65 multichannel seismic profiles, several sparker profiles and 200-m-grid cell size bathymetry data. All data were georeferenced and interpreted together using the kingdom package (HIS Markit). In detail, the data sets used are composed by the: CROP profiles acquired in the ’90; MS profiles acquired by the OGS during the ’60/’80 (http://snap.ogs.trieste.it/); SITHERE profiles acquired for the site survey of IODP leg 107; MEDOC profiles acquired in 2010; and ISTEGE profiles acquired in the 2010. IODP/DSDP wells (Leg 107 and 42A) and crustal velocity information (Prada et al., 2014, 2015; 2018) were used for the seismo-stratigraphic calibration allowing the mapping of three main units: pre-Messinian; Messinian including evaporites; and Plio-Quaternary. This information allowed to constrain the age of faults activity. Our work results highlight a basin dominated by extension locally overprinted by compression or by transcurrent-motion fault systems. In the western Tyrrhenian, the extension was mostly accommodated by large listric faults forming deep and elongated sedimentary basins. In its eastern (Campania) and southern (Sicily-Calabria) margins the stretching was mainly accomplished through normal faults forming irregular and smaller basins. Furthermore, the central Magnaghi/Vavilov abyssal plain is confined by normal faults that bound the magmatic crust and exhumed mantle rocks (Prada et al., 2016) to continental blocks of the Sardinia-Campania conjugate margins. The Marsili abyssal plain instead is surrounded by fewer, not well expressed, normal faults and, to north and to south, by lithospheric faults (STEP fault)

Congresso congiunto Società Geologica Italiana (SGI) - Società Italiana di Mineralogia e Petrologia (SIMP), Geosciences for the environment, natural hazards and cultural heritage, 12-14 September 2018, Catania.-- 1 page, The main object of the Proposal 927-pre: “ TyrrhenIan Magmatism & Mantle Exhumation” (TIME) is the understanding of the relationship between the exhumed mantle rocks and the overlying basalts sampled in the Tyrrhenian Basin during the previous ODP Leg 107 (Kastens et al 1988).The scientific objectives of the proposal include studying the kinematics of the opening, the nature and timing of associated magmatism and the geochemistry and deformation of the exhumed mantle section.The proposal resulted from the outcome of the MagellanPlus Workshop “TyrrhenIan Magmatism & Mantle Exhumation” (TIME) sponsored by IODP and ECORD. The workshop was held in the Institute of Marine Science (ISMAR) of the National Council of the Italian Research (CNR) in Bologna on June 5-7, 2017. The proposal 927-pre was submitted on October. 2017 and received the recommendation for the development into a full proposal by the IODP STEP Panel on February 2018.The database used to conceive the drilling project is possibly one of the best available from a rifted basin. The basement of the Tyrrhenian basin has been dredged at highs and drilled in several campaigns, and the stratigraphy is reasonably well known from three previous drilling expeditions, DSDP leg 13, DSPD leg 42 and the ODP leg 107 It is available a full-coverage, high-resolution, multibeam bathymetry of the basin as well as a large data set of vintage and modern 2D MCS reflection profiles and seven regional wide-angle seismic profiles crossing the main geological domains (Moeller et al 2014; Prada et al 2016, 2018)

18 International SEISMIX Symposium, Seismology between the Poles, 17-22 June 2018, Cracow, Poland.-- 1 page, Subduction zones are known to be key in shaping the lithospheric structure of the Earth, and consequently its surface. The dehydration of ultramafic products from the subducted oceanic lithosphere triggers the hydration of the overlying mantle wedge, resulting in the production of hydrous flush melting, and the consequent formation of new crustal material at the volcanic arc. However, the evolution of processes such as crustal construction and mantle hydration are still poorly constrained mainly because of the lack of observations from young subduction settings. In this study, we focus on the Plio-Quaternary Calabrian subduction system, located in the Central Mediterranean. This subduction zone is the result of the same slab rollback process that shaped back-arc basins in the Western Mediterranean and is characterized by the subduction of the Ionian Lithosphere under Calabria. To date, there is limited understanding of its shallow crustal structure and the transition between lithospheric domains (e.g. Arc - back-arc) due to the lack of dedicated active-source experiments. Here, we present the 2D P-wave velocity structure of the entire subduction system from travel-time tomography of wide-angle seismic data. The data were recorded during the CHIANTI amphibious experiment in 2015 along a ~600 kmlong transect that extends from the Tyrrhenian back-arc domain to the fore-arc in the Ionian Sea, across Calabria. In the back-arc system, OBS sections lack PmP-like arrivals and the velocity structure of the model shows a continuous and strong vertical velocity gradient of ~1 s-1. These results together with previous tomographic models, analysis of Vp/Vs and basement rock sampling strongly supports the exhumation of the mantle along ~300 km long section in the back-arc region. The back-arc region is also characterized by large volcanic edifices that were emplaced soon after the exhumation of the mantle. The transition between back-arc and Calabria is marked by a steep Moho geometry that shallows from SE to NW, revealing a dramatic crustal thinning along the N Calabrian margin. The lower crust of the margin has localized Vp of ~7 km/s under the submarine volcanic arc. The vertical velocity structure of this young arc (< 2Ma) reveals a thin and mafic crust, suggesting that crustal growth by magmatic addition is still at an early stage in the Calabrian volcanic arc, and that little crustal felsic-to-mafic differentiation has occurred as opposed to older well-established volcanic arcs in the Pacific. SE Calabria, the model shows a strong horizontal velocity gradient that is interpreted as the backstop of the subduction. In the Ionian, a 3-5 km thick sedimentary wedge thickens towards the NW. The frontal part of the wedge shows sub-vertical low-velocity anomalies indicating the presence of fluid-saturated large thrusts faults

Inciativa Ibérica para el Estudio de las Fallas Activas, Tercera Reunión Ibérica sobre Fallas Activas y Paleosismología IBERFAULT 2018 - Third Iberian Meeting on Active Faults and Paleoseismology, 11-13 June 2018, Alicante, Spain.-- 4 pages, 1 figure, 1 table, [EN] Although several geodetic studies reveal motion across the Alboran Basin, the low seismicity derived of the slowness of the structures has put the seismic and tsunamigenic potential of the faults on a second order problem. Due to the recent earthquakes recorded in the region (e.g. January 2016 Mw= 6.4) , with magnitudes high enough to cause considerable damage in the nearby cities and to be felt in all the coastal cities surrounding the basin, this concept is rapidly changing. Moreover, despite the low slip rate estimated for the faults at this region, their seismic potential resides on their large dimensions. Most of them coincide with crustal scale boundaries, are well oriented to the present-day stress regime and show a seafloor morphological expression that points out to their present (upper Quaternary) activity. Considering that the location of these faults seems to be controlled by the lithospheric configuration of the basin, a suitable characterization to the crustal domain is fundamental to define these tectonic boundaries and to estimate its seismic and associated tsunamigenic hazard. Within this study we propose to identify the active tectonic structures and to estimate their seismogenic potential by advancing in the deep structure characterization, [ES] Aunque los estudios geodésicos revelan movimiento en la cuenca de Alborán, la baja sismicidad registrada ha puesto el potencial sísmico y tsunamigénico de estas estructuras en un segundo plano. Debido a los recientes terremotos registrados en la zona (ej. enero 2016, Mw=6,4), con magnitud suficiente para afectar a las ciudades costeras adyacentes y causar daños, este concepto está cambiando. A pesar de la baja sismicidad registrada instrumentalmente, causada por la baja velocidad en el movimiento de estas fallas, el potencial sísmico de estas estructuras reside en sus grandes dimensiones. Muchas de estas fallas actúan como límites a escala cortical, bien orientadas en el sistema de esfuerzos actual y por lo tanto, activas. Su localización parece estar controlada por la configuración litosférica de la zona, por lo que una correcta caracterización de la estructura profunda es fundamental para definir estas fallas de grandes dimensiones y su potencial sísmico asociado, This work was supported by the Spanish Ministry of Education, Culture and Sport through the FPU fellowship 2013-2017 to L. Gómez de la Peña (AP2012-1579). Part of this project have been funded by the Cluster of Excellence ‘The Future Ocean’

European Geosciences Union (EGU) General Assembly 2018, 8-13 April 2018, Vienna, Austria.-- 1 page., The deep structure of the Alboran Basin (westernmost Mediterranean) has been under debate since the first surveys in the late 1980’s. Although the regional crustal and upper mantle structure has been extensively studied recently (i.e. global tomography, receiver function analysis or full-waveform inversion) the lithosphere underneath the Alboran Basin remains inadequately imaged, as the resolution of the crustal model is limited due to sparse data, inhibiting a precise characterization of the crust underneath the basin. The deep structure of the Alboran Basin is directly linked to the seismogenic potential of this area, as the main faults seem to be located at the edges of the crustal domains. Thus, an accurate seismic characterization of the crustal domains of the Alboran Basin and the transition between them is key to improve our understanding of the seismogenic potential of the faults in this complex area. Based on multichannel seismic reflection studies (i.e. TOPOMED project), with seafloor and basement groundtruthing through available drill and dredge samples, four different crustal domains have been defined for the area: (i) a thin continental domain (West Alboran and Malaga basins), (ii) the magmatic arc domain (East Alboran Basin), (iii) the North-African continental domain (South Alboran, Pytheas and Habibas basins) and (iv) oceanic crust (Algero-Balearic Basin). Unfortunately, basement dredges and well data are scarce, and, to a better understand of the deep structure of the Alboran Basin, wide-angle seismic deep-penetrating data are needed. Here we present the velocity and density model obtained from the inversion of profile P01 (WESTMED project), running from the south of Iberia (Almeria) to the north African margin (east of Nador). This profile includes 24 ocean bottom hydrophones and seismometers (OBH/OBS) with a spacing

European Geosciences Union (EGU) General Assembly 2018, 8-13 April 2018, Vienna, Austria.-- 1 page, The Calabrian subduction system is characterized by the subduction of the Ionian Lithosphere under Calabria. The SE migration of this system, triggered by slab rollback, caused the opening of the Tyrrhenian back-arc basin. The large-scale lithospheric structure of the subduction system is mostly imaged by regional earthquake tomography studies. The limited resolution of these studies, however, hinders the definition of smaller-scale details on the location, nature, and transition of different lithospheric domains (e.g. Arc - back-arc transition), which are crucial to understanding the geodynamic evolution of the system. Here we perform travel-time tomography of offshore and onshore active-source wide-angle seismic data to define the 2D Vp structure of the entire Calabrian subduction system. The data were acquired along a 550 km-long transect that extends from the Tyrrhenian back-arc domain to the fore-arc in the Ionian Sea, across Calabria. From NW to SE, the tomographic model shows abrupt variations of the velocity structure. In the back-arc system, particularly in the Vavilov and Marsili basins, OBS sections lack PmP-like arrivals and the velocity structure shows a continuous and strong vertical velocity gradient of ~1 s-1. These results together with previous tomographic models, analysis of Vp/Vs and basement rock sampling strongly supports the exhumation of the mantle along > 200 km long section in the back-arc region. Between the Vavilov and Marsili basins, a relatively thick, low-velocity block is interpreted to be of continental affinity. The transition between Marsili Basin and Calabria is marked by a steep Moho geometry that shallows from SE to NW, revealing a dramatic crustal thinning along the N Calabrian margin. The lower crust of the margin has localized Vp of ~7 km/s under the submarine volcanic arc. The vertical velocity structure of this young arc (