A. Gil-Imaz

Characterizing the Mesozoic extension direction in the northern Iberian plate margin by anisotropy of magnetic susceptibility (AMS)

We present the results of anisotropy of magnetic susceptibility (AMS) analyses carried out on weakly deformed Triassic red beds, Jurassic and Lower Cretaceous limestones, sandstones and shales from several northern Iberian Mesozoic basins located near the northern Iberian plate margin (Santander, Cabuérniga and Polientes basins in the western part of the Basque–Cantabrian basin, and Bilbao and Aralar basins in the central–eastern part; Cameros basin in the northernmost part of the Iberian chain). A well-defined magnetic lineation of tectonic origin is found at most of the sampled sites, and is interpreted as the stretching direction resulting from the extensional deformation characterizing this period. During the Mesozoic, the northern margin of the Iberian plate underwent two rifting stages, the Triassic and Late Jurassic–Early Cretaceous (Kimmeridgian–Albian) rifting events. Both the Triassic and the period of the Late Jurassic–Early Cretaceous rifting event with maximum subsidence rates (i.e. until the Barremian) are characterized by a NE–SW extensional direction in the main subsidence areas. This indicates that NW–SE master faults controlled basin subsidence and sedimentation through these periods in the northern Iberia plate margin basins. Conversely, during Jurassic and Aptian times a lower activity of NW–SE faults, together with more important extension along a NW–SE direction can be inferred from AMS analysis. This study indicates the power of AMS analysis in basin analysis and geodynamic reconstructions even in subsequently inverted basins.

Integration of surface and subsurface data, paleomagnetism and analogue modelling to reconstruct the extensional geometry of the Cameros basin (N Spain)

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.

40 Ar/ 39 Ar chronological constraints on syn- and post-Variscan biotite porphyroblasts from the Iberian Chains, NE Spain

AbstractWe perform a multidisciplinary study of biotite porphyroblasts and veinlet infills hosted in Cambrian strata of the hanging walls from the NW-SE-trending Datos, Jarque and Daroca thrusts (Iberian Chains). Stratigraphic and microstructural crosscutting features indicate that a biotite isograd runs parallel to the southeastern transects of the three thrusts. The metamorphic grade reached in the southeastern edge of the Iberian Chains is clearly distinct from both the Cadomian epizonal metamorphism, exclusively recorded in the Ediacaran–basal Cambrian Paracuellos Group, and the Variscan anchizone metamorphism recorded throughout Cambrian–Devonian strata. During post–Variscan negative inversion tectonics, renewed K-metasomatism along the same thrusts led to crystallisation of geochemically similar biotites in parallel fissures and veins. 40Ar/39Ar ages are (i) latest Westphalian–to–Guadalupian (285.82 ± 23.75 Ma) for metamorphic biotite porphyroblasts affected by diffusion loss of Ar, including the Carboniferous–Permian transition within error; and (ii) Early Triassic (246.87 ± 5.36 Ma) for biotite occlusion in post–Variscan veinlets. The Iberian Chains represent the southeastern prolongation of the Cantabrian and West Asturian-Leonese zones. The latter displays a uniform eastward decrease in metamorphic grade, whereas the Iberian Chains exhibits a heterogeneous distribution of low-grade metamorphic conditions (chlorite zone). The local presence of the biotite isograd is linked to the higher tectonic gradients associated with the Daroca, Jarque and Datos thrusts, quite similar to the metamorphic isogrades recognized both in the northeastern edge of the Demanda Massif and the Novellana-Pola de Allande-Degaña Belt of the West Asturian-Leonese Zone. This distribution allows the identification of a broad NW-SE belt of biotite-in isograd-related synkinematic metamorphism, following the western contact of the Narcea Antiform, the Anguiano Thrust and the southeastern edges of the Daroca, Jarque and Datos Thrusts in the Iberian Chains. Based on Variscan structural style and metamorphic grades, the Datos Thrust links with the contact that separates the Cantabrian and West Asturian-Leonese zones of the Iberian Massif.ResumenSe presenta en este trabajo un análisis multidisciplinar de biotitas metamórficas (porfiroblastos) y metasomáticas (de relleno fisural) contenidas en pizarras y mármoles cámbricos que forman el bloque superior de los cabalgamientos de Daroca, Jarque y Datos en las Cadenas Ibéricas. Su contexto estratigráfico y microtectónico permite reconocer una isograda de la biotita situada paralelamente a la traza de los tres frentes de cabalgamiento, que representan tres estructuras variscas de primer orden con orientación NO-SE. La isograda de biotita, alcanzada en el extremo sudoriental de las Cadenas Ibéricas, es claramente diferenciable del metamorfismo epizonal cadomiense, exclusivamente reconocido en el Grupo de Paracuellos (Ediacárico–Cámbrico basal) y del grado regional de anquizona varisco registrado en el Cámbrico–Devónico. Los cabalgamientos citados experimentaron una inversión tectónica negativa post–varisca, a la que se asoció un metasomatismo potásico, responsable de la precipitación de biotitas geoquímicamente similares a las variscas, como relleno de fisuras y venas. Las edades de los porfiroblastos y rellenos fisurales de biotita, determinadas mediante el método de 40Ar/39Ar, son de 285.82 ± 23.75 Ma (Westfaliense a Guadalupiense, incluyendo el margen de error el tránsito Carbonífero–Pérmico) y 246.87 ± 5.36 Ma (Triásico temprano), respectivamente. Se considera que las Cadenas Ibéricas representan la prolongación sudoriental de las zonas Cantábrica y Asturoccidental Leonesa del macizo Ibérico. La isograda varisca reconocida en las Cadenas Ibéricas representa la prolongación de las isogradas contemporáneas reconocidas en el extremo nororiental de la Sierra de la Demanda y el cinturón de Novellana-Pola de Allande-Degaña de la Zona Asturoccidental Leonesa. Esta distribución permite identificar un cinturón de metamorfismo sincinemático, a lo largo de una orientación actual NO-SE, a través del Antiforme del Narcea, el cabalgamiento de Anguiano y los cabalgamientos de Daroca, Jarque y Datos. Considerando los estilos de deformación y los grados de metamorfismo variscos, el cabalgamiento de Datos representa la prolongación del contacto que separa las zonas Cantábrica y Asturoccidental Leonesa en las Cadenas Ibéricas.