Rafael Bartolomé

Seismic and gravity constraints on the nature of the basement in the Africa‐Eurasia plate boundary: New insights for the geodynamic evolution of the SW Iberian margin

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.

Large, deepwater slope failures: Implications for landslide-generated tsunamis

Deepwater landslides are often underestimated as potential tsunami triggers. The North Gorringe avalanche (NGA) is a large (~80 km3 and 35 km runout) newly discovered and deepwater (2900 m to 5100 m depth) mass failure located at the northern fl ank of Gorringe Bank on the southwest Iberian margin. Steep slopes and pervasive fracturing are suggested as the main preconditioning factors for the NGA, while an earthquake is the most likely trigger mechanism. Near-fi eld tsunami simulations show that a mass failure similar to the NGA could generate a wave >15 m high that would hit the south Portuguese coasts in ~30 min. This suggests that deepwater landslides require more attention in geo-hazard assessment models of southern Europe, as well as, at a global scale, in seismically active margins.

Evidence for active strike-slip faulting along the Eurasia-Africa convergence zone: Implications for seismic hazard in the southwest Iberian margin

New seismic imaging and seismotectonic data from the southwest Iberian margin, the site of the present-day boundary between the European and African plates, reveal that active strike slip is occurring along two prominent lineaments that have recently been mapped using multibeam bathymetry. Multichannel seismic and subbottom profiler images acquired across the lineaments show seafloor displacements and active faulting to depths of at least 10 km and of a minimum length of 150 km. Seismic moment tensors show predominantly WNW–ESE right-lateral strike-slip motion, i.e., oblique to the direction of plate convergence. Estimates of earthquake source depths close to the fault planes indicate upper mantle (i.e., depths of 40–60 km) seismogenesis, implying the presence of old, thick, and brittle lithosphere. The estimated fault seismic parameters indicate that the faults are capable of generating great magnitude (Mw ≥ 8.0) earthquakes. Such large events raise the concomitant possibility of slope failures that have the potential to trigger tsunamis. Consequently, our findings identify an unreported earthquake and tsunami hazard for the Iberian and north African coastal areas.

An interpretation of a prominent magnetic anomaly near the boundary between the Eurasian and African plates (Gulf of Cadiz, SW margin of Iberia)

Abstract The Gulf of Cadiz is located at the boundary between the Eurasian and African plates in the southwestern margin of Iberia, and it straddles a place of major tectonic events since the Mesozoic. The Gibraltar belt migrated westward into the Gulf of Cadiz during the Miocene, while a north–south trending margin was built upon allochthonous formations at the western end of the Betic–Rif orogen. The geological setting of the Gulf allows a study of both the Mesozoic and Cenozoic evolution of the contact between these two major plates. This study reports a regional geophysical survey with an interpretation of the magnetic results, using a 2D modelling, constrained by recently acquired multichannel seismic profiling (MCS), and gravity data. From the magnetic pattern the area can be subdivided into three sectors which may be the expression of different geological provinces. In the eastern sector we find relatively small anomalies (∼50 nT), interpreted as the magnetic expression of the morphological ridges and valleys intersecting the central Gulf of Cadiz. In the southern sector, where data are scarce, we find a smooth magnetic zone with large-wavelength anomalies. The western sector, dominated by the Guadalquivir Bank, is where the largest magnetic anomalies are observed. The Guadalquivir Bank is a structural high which represents the offshore extension of the Hercynian Iberian Massif. We have interpreted a large-amplitude magnetic anomaly, of more than 300 nT (peak-to-peak) with a very steep gradient trending northward, to be caused by an intrusive material with an average estimated susceptibility distribution of 0.035 (SI). From the model studies, we believe that the magnetic source lies southwest of the Guadalquivir Bank, and is mainly composed of crustal material. The material rises up to 4 km depth, and probably was uplifted prior to the initial opening extensional stages of the central North Atlantic during the Triassic.

Expedition gathers new data on crust beneath Mexican West Coast

During the spring of 1996, scientists explored the North American plate boundary of southern Mexico and the Gulf of California through the Crustal Offshore Research Transect by Extensive Seismic Profiling (CORTES-P96) experiment (Figure la). Through dense sampling of the plates, the new data provides images that unravel the style of deformation along and across the subduction zone and in the Gulf interior, the dimensions of the accretionary prism, and the geometry of the subduction zone, which is well constrained by the reflection and refraction records. The subduction process along the south coast of Mexico, in spite of the high seismic risk that it represents, is poorly constrained due to the lack of high-resolution data. This project is aimed at resolving the crustal architecture in a zone of confronted plates.

Strike-slip faults mediate the rise of crustal-derived fluids and mud volcanism in the deep sea

We report on newly discovered mud volcanoes located at ~4500 m water depth ~90 km west of the deformation front of the accretionary wedge of the Gulf of Cadiz, and thus outside of their typical geotectonic environment. Seismic data suggest that fluid flow is mediated by a >400-km-long strike-slip fault marking the transcurrent plate boundary between Africa and Eurasia. Geochemical data (Cl, B, Sr, 87 Sr/ 86 Sr, d 18 O, dD) reveal that fluids originate in oceanic crust older than 140 Ma. On their rise to the surface, these fluids receive strong geochemical signals from recrystallization of Upper Jurassic carbonates and clay-mineral dehydration in younger terrigeneous units. At present, reports of mud volcanoes in similar deep-sea settings are rare, but given that the large area of transform-type plate boundaries has been barely investigated, such pathways of fluid discharge may provide an important, yet unappreciated link between the deeply buried oceanic crust and the deep ocean.

Compressional tectonic inversion of the Algero-Balearic basin: Latemost Miocene to present oblique convergence at the Palomares margin (Western Mediterranean)

Interpretation of new multichannel seismic reflection profiles indicates that the Palomares margin was formed by crustal-scale extension and coeval magmatic accretion during middle to late Miocene opening of the Algero-Balearic basin. The margin formed at the transition between thinned continental crust intruded by arc volcanism and back-arc oceanic crust. Deformation produced during the later positive inversion of the margin offshore and onshore is partitioned between ~N50°E striking reverse faults and associated folds like the Sierra Cabrera and Abubacer anticlines and N10–20°E sinistral strike-slip faults like Palomares and Terreros faults. Parametric subbottom profiles and multibeam bathymetry offshore, structural analysis, available GPS geodetic displacement data, and earthquake focal mechanisms jointly indicate that tectonic inversion of the Palomares margin is currently active. The Palomares margin shows a structural pattern comparable to the north Maghrebian margins where Africa-Eurasia plate convergence is accommodated by NE-SW reverse faults, NNW-SSE sinistral faults, and WNW-ESE dextral ones. Contractive structures at this margin contribute to the general inversion of the Western Mediterranean since ~7 Ma, coeval to inversion at the Algerian margin. Shortening at the Alboran ridge and Al-Idrisi faults occurred later, since 5 Ma, indicating a westward propagation of the compressional inversion of the Western Mediterranean.

Backscattering and geophysical features of volcanic ridges offshore Santa Rosalia, Baja California Sur, Gulf of California, Mexico

Abstract Volcanic ridges formed by series of volcanic edifices are identified in the central part of the Gulf of California, between Isla Tortuga and La Reforma Caldera–Santa Rosalia region. Isla Tortuga is part of the 40-km-long Tortuga Volcanic Ridge (TVR) that trends almost perpendicular to the spreading center of the Guaymas Basin. The Rosalia Volcanic Ridge (RVR), older than TVR, is characterized by volcanic structures oriented towards 310°, following a fracture zone extension and the peninsular slope. It is interpreted that most of the aligned submarine volcanic edifices are developed on continental crust while Isla Tortuga lies on oceanic-like crust of the Guaymas Basin. From a complete Bouguer anomaly map, it is observed that the alignments of gravity highs trending 310° and 290° support the volcanic and subvolcanic origin of the bathymetric highs. Volcanic curvilinear structures, lava flows and mounds were identified from backscattering images around Isla Tortuga and over a 400-m high (Virgenes High), where the TVR and the RVR intersect. A refraction/wide-angle seismic profile crossing perpendicular to the Virgenes High, together with gravity and magnetic data indicate the presence of shallow intrusive bodies presumably of basaltic or andesitic composition. It is inferred that most volcanic edifices along the ridges have similar internal structures. We suggest that the growth of different segments of the ridges have a volcano-tectonic origin. The older RVR lies along the extension of a fracture zone and it probably is associated with Pliocene NE–SW extension.

Habitats of the Chella Bank, Eastern Alboran Sea (Western Mediterranean)

Publisher Summary This chapter provides the first characterization of the geomorphic features and benthic habitats of the Chella Bank, a flat-topped volcanic peak situated in the Eastern Alboran Sea, Western Mediterranean. The Chella Bank, also named “Seco de los Olivos,” occurs along the upper slope of the Almeria Margin, showing a subcircular shape and covering a surface area of 100 km2 within a depth range of 70–700 m. High-resolution swath bathymetric mapping reveals three main large-scale morphological features on Chella Bank: the flat subhorizontal top and two main ridges, located to the west and to the east of the bank-top. Video tracks acquired in the area showed the occurrence of macrobenthic communities such as gorgonian assemblages (Callogorgia verticillata, Viminella flagellum), small patches of living cold-water corals (Madrepora oculata), and sponges (Fakelia ventilabrum). An automatic classification of multibeam data has been tested on the area as a predictive habitat mapping method, using backscatter characteristics (intensity, texture), and depth measurements as surrogate descriptors of the habitats recognized on the Chella Bank. The Chella Bank is one of the study areas of the LIFE-INDEMARES Project, which aims to contribute to the protection and sustainable use of the marine biodiversity in the Spanish seas.

Pre-stack depth migration seismic imaging of the Coral Patch Ridge and adjacent Horseshoe and Seine Abyssal Plains (Gulf of Cadiz): tectonic implications

Los braquiopodos retzidinos son una fraccion menor de las faunas devonicas de la CordilleraCantabrica (Norte de Espana). Aparte de un par de formas raras, impublicadas, del Praguiense delDominio Palentino y del Emsiense inferior del Astur-Leones, proximas al genero Rhynchospirina, ellinaje alcanzo su maximo de diversidad en la parte superior del Emsiense, con dos especies del generoRetzia, R. adrieni y R. cf. prominula, Cooperispira subferita y, quizas, una forma impublicada dePlectospira. El grupo no es conocido en el resto del Devonico y reaparece en el Pensilvaniense con algunasformas del genero Hustedia. En este trabajo se propone un nuevo taxon de la Familia Retziidae,Argovejia n.gen., de la parte final del Emsiense superior de Asturias y Leon, constituido por su especietipo,A. talenti n.sp. y, quizas, por las formas del Emsiense superior del Macizo Armoricano (Francia)Retzia haidingeri var. armoricana y Retzia haidingeri var. dichotoma.The Ronda Depression is filled by Neogene sediments on the boundary between Subbeticreliefs, with NE-SW structural trends, and the frontal Subbetic Chaotic Complexes. The folding stylein the Subbetic Units of Western Betics is strongly controlled by the rheology of the rocks: thick andmassive beds of Jurassic limestones over Triassic marls and gypsum with plastic behaviour. Main deformationstructures in the sedimentary infill of the Ronda depression are simultaneous box folds withNNE-SSW and WNW-ESE trends that only affect its southwestern part. This distribution of folds isa consequence of the inherited fold trend that affected the basement during Early Burdigalian age.