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  • 1
    Publication Date: 2019-02-01
    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.
    Type: Article , PeerReviewed
    Format: text
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  • 2
    Publication Date: 2017-11-19
    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.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 3
    Publication Date: 2012-02-23
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 4
    Publication Date: 2013-01-28
    Description: We use coincident wide-angle (WAS), multichannel seismic reflection (MCS) images and gravity data acquired with the MEDOC cruise in 2010 to characterize the crustal domains and tectonic structure across the Tyrrhenian basin. We present a ~450 km-long, E-W-trending transect, which crosses the entire basin, from Sardinia (40N), across Sardina basin, the Cornaglia Terrace and the deep Magnaghi and Vavilov basins, to the Campanian margin (Italy). The joint interpretation of the WAS model and time-migrated MCS profile give information to understand the rifting phases leading to continental break-up and mantle exhumation. The WAS data , recorded on 26 OBH/S (Ocean bottom hydrophones/ seismometers) and 5 land stations, were modelled to obtain a P-wave velocity model of the basin and the geometry of the crust-mantle boundary by joint refraction and reflection travel-time tomography. The statistical uncertainty of the model parameters has been estimated following a Monte Carlo-like approach. Subsequently, a velocity-derived density model using existing relationships for different rocks was used to infer the composition of domains that fit gravity data being consistent with the velocity model. The model display abrupt lateral heterogeneity, showing four crustal domains based on velocity gradients. From West to East, the first domain consist of a ~23 ±2 km-thick continental crust beneath Sardinia and its shelf, with a mean velocity of 6.5 ±0.3 km/s. Eastwards, the crust thins from 22 ±2 km to 12 ±1 km in ~140 km below the Sardinia basin. This second domain is interpreted as a highly extended continental crust, containing numerous faults imaged in the coincident MCS profile. The third domain, in the central part of the profile, includes basins under the deepest water depth, and is interpreted as floored by exhumed mantle. In this domain, no crust-mantle reflections are identified, neither in the WAS data nor in the MCS images. Here, the velocity increases rapidly from 2.6 ±0,1 km/s at the sea-floor to ~7.8 ±0,15 km/s at ~5 km below. The vertical velocity gradient is twice larger than typical for oceanic Layer 2, and consistent with that observed in regions of mantle exhumation like the West Iberian Margin. In this third domain, we find three conspicuous velocity anomalies located under large volcanic seamount, formed by Upper Pliocene and Middle Pleistocene extension-related magmatism of the Magnaghi seamount, D’Ancona Ridge and Vavilov seamount, respectively. In the Eastern segment of the profile, beneath the Campanian margin, there are well-defined crust-mantle reflections in both WAS data and MCS profile, displaying a progressive thickening of continental crust towards mainland. The velocity gradient in this fourth domain is similar to that of the highly extended continental crust of the second domain, which approximately corresponds to its conjugate margin. Based on these seismic observations we conclude that, in this part of the Tyrrhenian basin, extension occurred slowly enough to exhume mantle rocks without producing significant synchronous magmatism that generated well-defined oceanic crust.
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 5
    Publication Date: 2014-04-24
    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.
    Type: Article , PeerReviewed
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  • 6
    Publication Date: 2014-04-24
    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.
    Type: Article , PeerReviewed
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  • 7
    Publication Date: 2015-07-24
    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.
    Type: Article , PeerReviewed
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  • 8
    Publication Date: 2016-05-02
    Description: In this work we present first results of two wide-angle seismic transects acquired in the Southern Tyrrhenian basin and Northwestern Ionian during the CHIANTI experiment (July 2015). The first transect runs NW to SE starting in the Vavilov basin, crossing the Marsili basin, the currently active volcanic arc of the Aeolian Islands and the Calabrian arc, ending in the accretionary prism of the NW Ionian. This transect is 〉500 km long and includes 46 OBS and 5 landstations. The second transect crosses the Vavilov basin from N to S at a longitude of 12.5ºE. This one is 180 km long and includes 15 OBS. The preliminary interpretation of the OBS data clearly shows that the crustal structure is very similar in the Marsili and Vavilov basins. They show no crust-mantle boundary reflections and high apparent velocities of up to 8 km/s a few kms below the top of the basement. These results are in good agreement with previous ones obtained in the central Tyrrhenian during the MEDOC-2010 experiment, in which a transition from extended continental crust to magmatically-affected back-arc crust to exhumed mantle that challenges current conceptual models of back-arc extension, has been interpreted. The combination of the results of these two experiments is providing a new view of the nature and configuration of the geological domains in the whole Tyrrhenian basin, giving first order constraints on the processes that have controlled its geodynamic evolution.
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 9
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    In:  [Poster] In: EGU General Assembly 2016, 17.-22.04.2016, Vienna, Austria .
    Publication Date: 2016-05-03
    Description: The main objective of the CHIANTI cruise was to collect geophysical marine data to determine the deep crustal structure and plate geometry across the subduction system of the Ionian and Tyrrhenian Seas, from the frontal wedge to the arc and back-arc. The goal is to study the processes that operated during the subduction of the Ionian slab of oceanic crust under Calabria, which lead to the development of the Aeolian volcanic arc, and the subsequent opening of the Tyrrhenian basin, and are responsible of the geological hazards that threaten the region. The CHIANTI cruise onboard the Spanish R/V BO Sarmiento de Gamboa started in Barcelona (Spain) on July 12, and finished in Catania (Italy), on August 28, 2015. It consisted of four legs devoted to acquisition of data with different seismic/acoustic techniques in the Tyrrhenian and Ionian Seas. Leg 1 and 2 were focused on the acquisition of deep penetrating Wide-Angle Reflection and Refraction Seismic (WAS) data, Leg 3 on Multichannel Seismic (MCS) Reflection data and finally Leg 4 was devoted to sidescan imaging, coring and single channel seismic acquisition. During the entire cruise, complementary acoustic data (i.e. multibeam bathymetry and sub-bottom profiler) were acquired simultaneously. In this presentation we focus on the seafloor mapping and processed multichannel seismic reflection grid collected on the IONIAN prism. The data show abundant evidence of ongoing widespread deformation across the entire region from the deformation front to the uppermost slope and extending into the Calabrian emerged region. The seafloor mapping shows numerous mud volcanoes associated to fault activity. The seismic images display deformational features active across the entire prims at different locations extending the definition of structures described in previous works of the region with fewer areal coverage. The data show a prism tectonic structure that is distinct from the structure of prism in other subduction systems worldwide.
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 10
    Publication Date: 2018-09-27
    Type: Dataset
    Format: text/tab-separated-values, 243198 data points
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