Description: We use seismic oceanography to document and analyze oceanic thermohaline fine structure across the Tyrrhenian Sea. Multichannel seismic (MCS) reflection data were acquired during the MEDiterranean OCcidental survey in April–May 2010. We deployed along‐track expendable bathythermograph probes simultaneous with MCS acquisition. At nearby locations we gathered conductivity‐temperature‐depth data. An autonomous glider survey added in situ measurements of oceanic properties. The seismic reflectivity clearly delineates thermohaline fine structure in the upper 2,000 m of the water column, indicating the interfaces between Atlantic Water/Winter Intermediate Water, Levantine Intermediate Water, and Tyrrhenian Deep Water. We observe the Northern Tyrrhenian Anticyclone, a near‐surface mesoscale eddy, plus laterally and vertically extensive thermohaline staircases. Using MCS, we are able to fully image the anticyclone to a depth of 800 m and to confirm the horizontal continuity of the thermohaline staircases of more than 200 km. The staircases show the clearest step‐like gradients in the center of the basin while they become more diffuse toward the periphery and bottom, where impedance gradients become too small to be detected by MCS. We quantify the internal wave field and find it to be weak in the region of the eddy and in the center of the staircases, while it is stronger near the coastlines. Our results indicate this is because of the influence of the boundary currents, which disrupt the formation of staircases by preventing diffusive convection. In the interior of the basin, the staircases are clearer and the internal wave field weaker, suggesting that other mixing processes such as double diffusion prevail.

Description: We use seismic oceanography to document and analyze oceanic thermohaline finestructure across the Tyrrhenian Sea. Multichannel seismic (MCS) reflection data were acquired during the MEDiterranean OCcidental survey in April-May 2010. We deployed along-track expendable bathythermograph probes simultaneous with MCS acquisition. At nearby locations we gathered conductivity-temperature-depth data. An autonomous glider survey added in-situ measurements of oceanic properties. The seismic reflectivity clearly delineates thermohaline finestructure in the upper 2,000 m of the water column, indicating the interfaces between Atlantic Water/Winter Intermediate Water, Levantine Intermediate Water, and Tyrrhenian Deep Water. We observe the Northern Tyrrhenian Anticyclone, a near-surface meso-scale eddy, plus laterally and vertically extensive thermohaline staircases. Using MCS we are able to fully image the anticyclone to a depth of 800 m and to confirm the horizontal continuity of the thermohaline staircases of more than 200 km. The staircases show the clearest step-like gradients in the center of the basin while they become more diffuse towards the periphery and bottom, where impedance gradients become too small to be detected by MCS. We quantify the internal wave field and find it to be weak in the region of the eddy and in the center of the staircases, while it is stronger near the coastlines. Our results indicate this is because of the influence of the boundary currents, which disrupt the formation of staircases by preventing diffusive convection. In the interior of the basin the staircases are clearer and the internal wave field weaker, suggesting that other mixing processes such as double-diffusion prevail. Synopsis We studied the internal temperature and salinity structure of the Tyrrhenian Sea (Mediterranean) using the multichannel seismic reflection method (the same used in the hydrocarbon industry). Low frequency sound (seismic) waves are produced at the surface with an explosive air source and recorded by a towed cable containing hydrophones (underwater microphones). The data are processed to reveal 'stratigraphy' that result from contrasts in density that are themselves caused by changes in temperature and salinity. In this way we can map ocean circulation in two-dimensions. We also deployed in situ oceanographic probes to measure temperature and salinity in order to corroborate and optimize the processing of the seismic data. We then quantified the internal gravity wave field by tracking the peaks of seismic trace wavelets. Our results show that the interior of the Tyrrhenian Sea is largely isolated from internal waves that are generated by a large cyclonic boundary current that contains waters from the Atlantic ocean and other parts of the Mediterranean. This isolation allows the thermohaline finestructure to form, where small scale vertical mixing processes are at play. Understanding these mixing processes will aid researchers study global ocean circulation and to add constraints that can help improve climate models.

Description: The Gulf of Cadiz seismicity is characterized by persistent low to intermediate magnitude earthquakes, occasionally punctuated by high magnitude events such as the M ~ 8.7 1755 Great Lisbon earthquake and the M = 7.9 event of February 28th, 1969. Micro-seismicity was recorded during 11 months by a temporary network of 25 ocean bottom seismometers (OBSs) in an area of high seismic activity, encompassing the potential source areas of the mentioned large magnitude earthquakes. We combined micro-seismicity analysis with processing and interpretation of deep crustal seismic reflection profiles and available refraction data to investigate the possible tectonic control of the seismicity in the Gulf of Cadiz area. Three controlling mechanisms are explored: i) active tectonic structures, ii) transitions between different lithospheric domains and inherited Mesozoic structures, and iii) fault weakening mechanisms. Our results show that micro-seismicity is mostly located in the upper mantle and is associated with tectonic inversion of extensional rift structures and to the transition between different lithospheric/rheological domains. Even though the crustal structure is well imaged in the seismic profiles and in the bathymetry, crustal faults show low to negligible seismic activity. A possible explanation for this is that the crustal thrusts are thin-skinned structures rooting in relatively shallow sub-horizontal décollements associated with (aseismic) serpentinization levels at the top of the lithospheric mantle. Therefore, co-seismic slip along crustal thrusts may only occur during large magnitude events, while for most of the inter-seismic cycle these thrusts remain locked, or slip aseismically. We further speculate that high magnitude earthquake's ruptures may only nucleate in the lithospheric mantle and then propagate into the crust across the serpentinized layers.

Description: We investigate the crustal structure of the SW Iberian margin along a 340 km-long refraction and wide-angle reflection seismic profile crossing from the central Gulf of Cadiz to the Variscan continental margin in the Algarve, Southern Portugal. The seismic velocity and crustal geometry model obtained by joint refraction and reflection travel-time inversion reveal three distinct crustal domains: the 28–30 km-thick Variscan crust in the north, a 60 km-wide transition zone offshore, where the crust abruptly thins ~ 20 km, and finally a ~ 7 km-thick and ~ 150 km-wide crustal section that appears to be oceanic in nature. The oceanic crust is overlain by a 1–3 km-thick section of Mesozoic to Eocene sediments, with an additional 3–4 km of low-velocity, unconsolidated sediments on top belonging to the Miocene age, Gulf of Cadiz imbricated wedge. The sharp transition between continental and oceanic crust is best explained by an initial rifting setting as a transform margin during the Early Jurassic that followed the continental break-up in the Central Atlantic. The narrow oceanic basin would have formed during an oblique rifting and seafloor spreading episode between Iberia and Africa that started shortly thereafter (Bajocian) and lasted up to the initiation of oceanic spreading in the North Atlantic at the Tithonian (late Jurassic-earliest Cretaceous). The velocity model displays four wide, prominent, south-dipping low-velocity anomalies, which seem to be related with the presence of crustal-scale faults previously identified in the area, some of which could well be extensional faults generated during this rifting episode. We propose that this oceanic plate segment is the last remnant of an oceanic corridor that once connected the Alpine-Tethys with the Atlantic ocean, so it is, in turn, one of the oldest oceanic crustal fragments currently preserved on Earth. The presence of oceanic crust in the central Gulf of Cadiz is consistent with geodynamic models suggesting the existence of a narrow, westward retreating oceanic slab beneath the Gibraltar arc-Alboran basin system.

Description: Recently acquired high-resolution multichannel seismic profiles together with bathymetric and sub-bottom profiler data from the external part of the Gulf of Cadiz (Iberia-Africa plate boundary) reveal active deformation involving old (Mesozoic) oceanic lithosphere. This area is located 180 km offshore the SW Iberian Peninsula and embraces the prominent NE-SW trending Coral Patch Ridge, and part of the surrounding deep Horseshoe and Seine abyssal plains. E-W trending dextral strike-slip faults showing surface deformation of flower-like structures predominate in the Horseshoe Abyssal Plain, whereas NE-SW trending compressive structures prevail in the Coral Patch Ridge and Seine Hills. Although the Coral Patch Ridge region is characterized by subdued seismic activity, the area is not free from seismic hazard. Most of the newly mapped faults correspond to active blind thrusts and strike-slip faults that are able to generate large magnitude earthquakes (Mw 7.2-8.4). This may represent a significant earthquake and tsunami hazard that has been overlooked so far.

Description: We present a new classification of geological domains at the Africa-Eurasia plate boundary off SW Iberia, together with a regional geodynamic reconstruction spanning from the Mesozoic extension to the Neogene-to-present-day convergence. It is based on seismic velocity and density models along a new transect running from the Horseshoe to the Seine abyssal plains, which is combined with previously available geophysical models from the region. The basement velocity structure at the Seine Abyssal Plain indicates the presence of a highly heterogeneous, thin oceanic crust with local high-velocity anomalies possibly representing zones related to the presence of ultramafic rocks. The integration of this model with previous ones reveals the presence of three oceanic domains offshore SW Iberia: (1) the Seine Abyssal Plain domain, generated during the first stages of slow seafloor spreading in the NE Central Atlantic (Early Jurassic); (2) the Gulf of Cadiz domain, made of oceanic crust generated in the Alpine-Tethys spreading system between Iberia and Africa, which was coeval with the formation of the Seine Abyssal Plain domain and lasted up to the North Atlantic continental breakup (Late Jurassic); and (3) the Gorringe Bank domain, made of exhumed mantle rocks, which formed during the first stages of North Atlantic opening. Our models suggest that the Seine Abyssal Plain and Gulf of Cadiz domains are separated by the Lineament South strike-slip fault, whereas the Gulf of Cadiz and Gorringe Bank domains appear to be limited by a deep thrust fault located at the center of the Horseshoe Abyssal Plain.

Description: The Gorringe Bank is a gigantic seamount that separates the Horseshoe and Tagus abyssal plains offshore SW Iberia, in a zone that hosts the convergent boundary between the Africa and Eurasia plates. Although the region has been the focus of numerous investigations since the early 1970s, the lack of appropriate geophysical data makes the nature of the basement, and thus the origin of the structures, still debated. In this work, we present combined P-wave seismic velocity and gravity models along a transect that crosses the Gorringe Bank from the Tagus to the Horseshoe abyssal plains. The P-wave velocity structure of the basement is similar in the Tagus and Horseshoe plains. It shows a 2.5–3.0 km-thick top layer with a velocity gradient twice stronger than oceanic Layer 2 and an abrupt change to an underlying layer with a five-fold weaker gradient. Velocity and density is lower beneath the Gorringe Bank probably due to enhanced fracturing, that have led to rock disaggregation in the sediment-starved northern flank. In contrast to previous velocity models of this region, there is no evidence of a sharp crust–mantle boundary in any of the record sections. The modelling results indicate that the sediment overlays directly serpentinite rock, exhumed from the mantle with a degree of serpentinization decreasing from a maximum of 70–80% under the top of Gorringe Bank to less than 5% at a depth of ∼20 km. We propose that the three domains were originally part of a single serpentine rock band, of nature and possibly origin similar to the Iberia Abyssal Plain ocean–continent transition, which was probably generated during the earliest phase of the North Atlantic opening that followed continental crust breakup (Early Cretaceous). During the Miocene, the NW–SE trending Eurasia–Africa convergence resulted in thrusting of the southeastern segment of the exhumed serpentinite band over the northwestern one, forming the Gorringe Bank. The local deformation associated to plate convergence and uplift could have promoted pervasive rock fracturing of the overriding plate, leading eventually to rock disaggregation in the northern flank of the GB, which could be now a potential source of rock avalanches and tsunamis.

Description: The Tyrrhenian basin has been formed by extension of overriding continental lithosphere driven by roll back of the Ionian slab across the mantle. The basin is not actively extending but the tectonic structure provides information of the processes that controlled rifting and formation of conjugate margins. The basin opened from west to east, with rifting stopping after progressively larger stretching factors from north to south. The northern region stopped opening at extension factors about 1.8. Towards the south extension continued until full crustal separation that produced first intense magmatism that subsequently was followed by mantle exhumation. The final structure displays two conjugate margins with structures that evolved from symmetric to asymmetric as extension rates increase and a complex tectonic structure in between. The basin provides a natural laboratory to investigate a full rift system with variable amounts of extension. We present observations from a two-ship wide-angle (WAS) and multichannel reflection seismic (MCS) experiment that took place in spring 2010. The experiment took place on two legs: The first leg with Spanish R/V Sarmiento de Gamboa and Italian R/V Urania collected five WAS profiles striking E-W across the entire basin recorded on ocean bottom seismic stations and land stations with a 4800 c.i. G-II gun array as source. The second leg with R/V Sarmiento de Gamboa collected 16 MCS profiles (about 1500 km) using a 3.75 km-long streamer and a 3100 c.i. G-II gun array as source. MCS profiles were shot coincident with WAS profiles. WAS – MCS transects were located in regions with different amount of extension the study the full structure including the two conjugate margins. Additional MCS lines were shot concentrated in the region where mantle exhumation has taken place. The seismic information is placed in a 3D context with the integration of the multibeam bathymetry that covers the entire basin. We present the interpretation of the tectonic structure from MCS images and bathymetry and the calibrated stratigraphy of the basin that gives information of timing, duration, and amount of the tectonic extension for the different transects. We compare those results with the final P-wave velocity models from the five WAS profiles that supply information on the nature of the crust. Each transect provides information of the relationships among extension rates, crustal thickness, nature of the crust, and style of deformation. This information allows to interpret mechanisms of deformation, to infer the importance of magmatism in the rifting process, and to interpret the changes leading of mantle exhumation. Furthermore, the data provide insight in the process of formation of the structure conjugated margins.