S. Martínez-Loriente

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.

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.

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.

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.