J. A. Pulgar

Seismic image of the Cantabrian Mountains in the western extension of the Pyrenees from integrated ESCIN reflection and refraction data

Abstract Integrated analysis of normal-incidence and large-aperture seismic reflection data collected in 1992 and 1993 within the Spanish ESCIN and complementary projects provide a first complete NS crustal transect across the Northern Iberian Peninsula and continental margin. Images of the crustal structure of the Cantabrian Mountains and their transition to the Duero basin and to the Cantabrian margin are obtained from: (a) a 65-km-long vertical reflection profile ESCIN-2 on land; (b) a 200-km-long reversed refraction profile; and (c) wide-angle recordings of the marine ESCIN-4 profile. Consistent results between reflectivity pattern and velocity-depth distribution reveal important lateral variations in the deep structure. The reflective crust imaged in the ESCIN-2 profile changes its attitude from sub-horizontal beneath the Duero basin to north-dipping beyond the Mountain front. Basement thrusts are observed in the upper crust merging into a detachment at 6 s (TWT) and may have triggered the Alpine uplift of the range. The Moho is identified at the bottom of the reflective lower crust and deepens from 12 to 15 s at the northern end of the profile, about 35 km inland. Modelling of the refraction data laterally extends the seismic image and provides evidence for Variscan crustal features beneath the Duero basin. Northwards, the velocity in the lower crust decreases and the Moho, constrained by the wide-angle data from profile ESCIN-2, deepens to about 60 km ending abruptly at the shoreline. The velocity-depth model is constrained along the Asturian platform up to the continental slope, where the crust-mantle boundary is located at 24 km depth. This ‘margin Moho’ shows a progressive deepening southwards, and extends to the coast where it is found at 30 km depth. The present seismic data support an important Alpine reworking and thickening of the crust under the Cantabrian Mountains. The onshore/offshore transition is marked by an imbrication of two crusts of very different thicknesses. This signature offers a strong parallelism with the one previously observed further east across the Pyrenees in the ECORS seismic profile.

Crustal structure of the external variscides in northwest spain from deep seismic reflection profiling

Abstract Recent deep-crustal seismic reflection profiling escicantabrica-1 provides a detailed image of the external parts of the Variscan Belt in northwest Spain. These seismic data together with surface geological information provide constraints on the crustal structure of the foreland thrust and fold belt (Cantabrian Zone) and the transition to the hinterland areas (Narcea Antiform area). The image in the Cantabrian Zone includes strong inclined reflections in the upper 5 s that correspond to mappable Paleozoic rocks at the surface which have been deformed by thrusting and folding during the Carboniferous. Some of the major thrust faults imaged show a ramp-flat geometry at depth and merge into a highly reflective band between 5 and 6 s that corresponds to the Cantabrian Zone detachment. The Precambrian basement is almost transparent underneath the Cantabrian Zone detachment. There are a few subhorizontal discontinuous reflections at 11 s that may correspond to the reflection Moho. At the transition to the hinterland, the seismic profile includes short inclined reflections grouped in several reflective bands in the upper 6 s. These correspond to the Narcea Antiform, which comprises at the surface a stack of thrust sheets containing Precambrian rocks. The Narcea Antiform is placed above a crustal-scale ramp imaged by strong W-dipping reflections between 6 and 9 s. These reflections merge upwards into the Cantabrian Zone detachment at 6 s, and downwards into a reflective lower-crustal zone located between 9 and 12 s. These midcrustal reflectors are interpreted as a major Variscan shear zone that marks the transition between the thin-skinned tectonics of the external areas and the thick-skinned tectonics of the hinterland areas. The lower crust is highly reflective below the Narcea Antiform area and reflectivity fades out to the east beneath the foreland thrust and fold belt. The different seismic responses at middle- and lower-crustal levels in the foreland and hinterland areas suggest that Variscan tectonics strongly affected deep-crustal levels in the hinterland areas, whereas they remained undeformed beneath the foreland.

Crustal transition between continental and oceanic domains along the North Iberian Margin from wide angle seismic and gravity data

Deep crustal features of the transition between the North Iberian mainland and the Bay of Biscay are constrained from new wide-angle seismic and gravity data. Velocity-depth models are derived from far-offset recordings onshore of two N-S marine profiles, complemented with new refraction data inland and tested against models that fit the gravity anomalies. Important lateral variations are inferred along the North Iberian margin. In the western transect, a Variscan crust is documented from the mainland across the continental shelf, and a rapid crustal thinning beneath the slope marks the transition to the oceanic domain. The eastern transect shows an outstanding crustal thickening beneath the coastline (Moho depths around 50 km) and a gradual thinning across the shelf. Differences are related to the V-shape of the Bay of Biscay that causes a W-E transition along the margin, from a short-lived oceanic subduction to continental collision during the Tertiary oblique convergence between Europe and Iberia.

SEISMIC STRUCTURE OF THE NORTHERN CONTINENTAL MARGIN OF SPAIN FROM ESCIN DEEP SEISMIC PROFILES

Abstract By the end of the Carboniferous, the crust of the continental shelf in northwestern Spain was made up of deeply rooted structures related to the Variscan collision. From Permian to Triassic times the tectonic setting had changed to mainly extensional and the northern Iberian continental margin underwent rifting during Late Jurassic-Early Cretaceous times, along with sea-floor spreading and the opening of the Bay of Biscay until the Late Cretaceous. Subsequently, the northern Iberian margin was active during the north-south convergence of Eurasia and Iberia in the Tertiary. A multichannel seismic experiment, consisting of two profiles, one north-south (ESCIN-4) crossing the platform margin offshore Asturias, and another (ESCIN-3) crossing the platform margin to the northwest of Galicia, was designed to study the structure of the northern Iberian margin. The ESCIN-4 stacked section reveals inverted structures in the upper crust within the Le Danois Basin. North of the steep continental slope, ESCIN-4 shows a thick sedimentary package from 6 to 9.5 s, two-way travel time (TWT). Within this latter package, a 40-km-long, north-tapering wedge of inclined, mainly south-dipping reflections is thought to represent a buried, Alpine-age accretionary prism. In the north western part of the ESCIN-3 (ESCIN-3-1) stacked section, horizontal reflections from 6.5 to 8.5 s correspond to an undisturbed package of sediments lying above oceanic-type basement. In this part of the line, a few kilometres long, strong horizontal reflection at 11.2 s within the basement may represent an oceanic Moho reflection. Also, a band of reflections dips gently towards the southeast, from the base of the gently dipping continental slope. The part of ESCIN-3 line that runs parallel to the NW-Galicia coast (ESCIN-3-2), is characterized by bright, continuous lower crustal reflections from 8 to 10 s. Beneath the lower crustal reflectivity, a band of strong reflections dips gently toward the southwest from 10 to 13.5 s. The part of ESCIN-3 that parallels the northern margin (ESCIN 3-3), shows good reflectivity in all levels. Upper crustal reflections image the sedimentary fill of probable Mesozoic to recent basins. Mid-crustal reflectivity is characterized by dipping reflections until 8 s that are probably related to compressional Variscan features. The lower crustal level shows ‘layered’ reflections between 8 and 12 s. Dipping reflections are found below the continental Moho.

Structural, metamorphic and magmatic history of the Mondon̄edo nappe (Hercynian belt, NW Spain)

Abstract The Mondonedo nappe is one of the main units in the internal zone of the Hercynian belt in the Iberian Peninsula. It consists of low to medium grade metasediments, late Precambrian and Palaeozoic in age, folded by large D 1 E-verging recumbent structures, and thrusted several tens of km towards the east during a D 2 deformation episode. At the base of the thrust sheet, a shear zone of up to 3 km thick was developed. Granitoid bodies, which were intruded after D 1 and before or during D 2 , were deformed in this basal zone. The geometry of D 1 and D 2 structures and that of the isograds are described, as well as the relationships between metamorphism and deformation. A structural evolution is inferred which shows an episode of generalized ductile deformation ( D 1 ) followed by another in which the deformation tended to concentrate in a subhorizontal ductile shear zone ( D 2 ) and culminated with the thrusting along an important fracture. The shear zone originated in a thermally softened level which, according to metamorphic data, was at temperatures in the range 550–600°C. The structural, metamorphic and magmatic features of the Mondonedo nappe suggest a relationship with a crustal scale overthrusting of the continental mass to which the nappe belongs. This could be a consequence of the collision against another continental mass to the west.

Seismic signature of Variscan and Alpine tectonics in NW Iberia: Crustal structure of the Cantabrian Mountains and Duero basin

The seismic structure of the crust of the NW Iberian Peninsula, along the Cantabrian Mountains and their southern foreland, the Duero basin, is investigated from the first data set available from refraction and wide-angle reflection profiles. The velocity-depth distributions obtained along E-W and N-S transects evidence lateral variations in crustal structure that can be associated to Variscan and Alpine tectonic episodes. The western part of the range, toward the Variscan hinterland zones, as well as the Duero basin below the sediments, show a crustal structure similar to that of the Variscan belt elsewere in Europe. The total crustal thickness is 30–32 km, and three main layers (upper, middle, and lower crust with average velocities 6.0, 6.25, and 6.8 km/s) are resolved. Some Paleozoic structures such as the basal detachment of the Cantabrian Zone are expressed in the seismic models. The crust appears to vary eastward in the external zones. The most outstanding feature is a crustal root beneath the highest Cantabrian summits where the Moho is found at 47 km depth. A prominent Alpine reworking of the crust in the eastern part of the Cantabrian Zone is thus revealed, and the N-S seismic transects delineate a geometry similar to the one found across the central Pyrenees in the Etude Continentale et Oceanique par Reflexion et Refraction Sismique (ECORS) profile, revealing the importance of the Alpine deformations in the northern part of the Iberian plate.

Estudio Sísmico de la Corteza Ibérica Norte 3.3: A seismic image of the Variscan crust in the hinterland of the NW Iberian Massif

An offshore vertical incidence reflection seismic study with simultaneous on-land wide-angle recording has been conducted, as part of the Estudio Sismico de la Corteza Iberica Norte (ESCIN) Project ...

Growth and propagation of buckle folds determined from syntectonic sediments (the Ubierna Fold Belt, Cantabrian Mountains, N Spain)

Abstract The geometry of syntectonic Tertiary sediments in the Ubierna Fold Belt can be explained using the buckling folding theory. The geometric features of the fold train in the pre-orogenic succession also agree with a buckling process. Each fold grew with progressive and coeval rotation of both limbs, although rotation was asymmetric and greater on the forelimb than the backlimb. The diachroneity of synrotational sequences (progressive unconformities) through different folds indicates that fold propagation migrated towards the foreland; however, the initial folds were tightened during a later deformation stage synchronous with the development of younger folds.

2-D seismic modeling of the Variscan foreland thrust and fold belt crust in NW Spain from ESCIN-1 deep seismic reflection data

Abstract The foreland thrust and fold belt (Cantabrian Zone) of the Variscan Belt in NW Spain and the transition to the hinterland (Westasturian–Leonese Zone) was the location of a seismic experiment in 1991. The seismic reflection profile (ESCIN-1) is 140 km long and runs in an E–W direction. The interpretation and seismic modeling of the main reflective interfaces in the profile were made integrating available geological and geophysical data including surface geology, deep seismic reflection data from ESCIN-1, transmission velocities from a borehole, refraction and laboratory data. The geological and velocity model of the crust was tested obtaining synthetic seismograms and can be correlated with surface geology. The velocity model images the main crustal structures interpreted from ESCIN-1. The basal detachment of the foreland thrust and fold belt dips gently from 12 km in the easternmost part of the profile to 16 km in the transition to the hinterland to the west. A new interpretation is proposed for the structure above the basal detachment in the eastern end of the profile, where the basal detachment, at a depth of 12 km, is duplicated at 6.5 km by a N-dipping Alpine thrust that also duplicates part of the basement. In the western part, two deep reflective bands dip westward and the most conspicuous one can be followed from 16–27 km depth. The two bands, previously interpreted as crustal ramps, join a reflective lower crust that is located between 25 and 29 km and fades westwards. A reflective Moho interpreted at the base of the reflective lower crust dips and fades in the same direction from 30–34 km.

The Fossil Accretionary Wedge of the Bay of Biscay: Critical Wedge Analysis on Depth-Migrated Seismic Sections and Geodynamical Implications

The accretionary wedge of the Bay of Biscay is an east-west compressive belt buried under recent sediments of the abyssal plain at the north Iberian margin. This structure formed through the partial closure of the previously extended Biscay basin during the Cenozoic north-south collision between Europe and Iberia, the same collision that produced the Cantabrian-Pyrenean range on land. Three north-south seismic sections have been prestack depth migrated, showing a narrow-tapered wedge (7°–8°) whose internal structure corresponds to a set of south-dipping thrusts converging toward a basal decollement. There are differences along strike within the wedge: thrust spacing, the dip of the basal thrust, and the thickness of the sediments at the trough augment toward the east, increasing its overall size. The two-dimensional velocity models obtained through migration analysis reflect values between 2000 km/s at the sea floor (4500 m) and 5000 km/s at 12-km depth. The syntectonic package thickness varies from 1.5 to 3 km, while the posttectonic cover attains 1.5–2 km. A simple analysis based on critical wedge theory approaches suggests that the Biscay wedge formed in similar conditions to active submarine wedges, the strength of the decollement being lower than the strength of the wedge itself. Further comparison with other examples indicates high basal stress, which could be an added factor in the convergence stopping at this margin. The eastward size increase is attributed to the provision of extra sediments by the coetaneous rising of the cordillera on land. This weight steepens the basal angle without affecting the overall taper. Surprisingly, the eastward change from an oceanic to a transitional basement does not seem to be crucial in its geometry.