F. González-Lodeiro

Progressive extensional shear structures in a detachment contact in the Western Sierra Nevada (Betic Cordilleras, Spain)

AbstractThe present contact caused by the superposition of the Alpujarride complex over that of the Nevado-Filabride in the western area of Sierra Nevada and Sierra de Filabres corresponds to a detachment. The deformation in the footwall associated with this contact, produced mylonitic fabrics with a significant stretching-lineation, over which brittle structures are superimposed. The deformation in the hanging wall associated with this contact is, on the other hand, essentially brittle. These deformations are subsequent to a series of syn-to post-metamorphic structures related to thrust phases.The micro- and meso-structures indicate that the hanging wall has moved towards the west-south-west.Other brittle structures, which began during the same extensional regime, are superimposed on the detachment and have continued to develop up to the present time. These structures were produced in an extensional regime with a non-coaxial deformation component and suggest the possibility of a tectonic evolution simila...

Stress and palaeostress in the Betic-Rif cordilleras (Miocene to the present)

Palaeostress orientation in the Betic-Rif Cordilleras can be determined for the Miocene to the Quaternary by analysis of brittle microstructures (faults, joints, striated pebbles and stylolites). In the central and northern South Iberian Domain, σ1 was subhorizontal and trended NW-SE, from the Early Miocene to the present. In the northern Alboran Domain, from the Burdigalian to the Serravallian, palaeostress ellipsoids were oblate, with σ3 subhorizontal and trending ENE-WSW. In the southern part of the South Iberian Domain and in the northern part of the Alboran Domain (Betic Cordillera) from the Tortonian to the Quaternary, palaeostress ellipsoids were variable: in the Granada region, they were prolate with σ1 subvertical; but in the Almeria region they were triaxial with σ2 subvertical and σ3 subhorizontal and trending NE-SW to E-W. In the Rif, the Alboran and African-Maghrebian domains were affected by palaeostresses with σ1 subhorizontal, varying from NE-SW in Tortonian times to N-S in the Plio-Quaternary. A study of earthquake focal mechanisms by the right-dihedra method indicates that the present-day general stress field around the Betic-Rif cordilleras is compressional, with σ1 subhorizontal and trending NW-SE. In the Betic-Rif Cordilleras, this general stress field is modified and the present-day orientations of σ1, σ2 and σ3 are nearly the same as those of the palaeostresses in the Pliocene and Quaternary. The distribution and evolution of stresses in the Betic-Rif cordilleras, from the Miocene to the present, can be correlated with the main Neogene deformations: extensional structures in the central and northern Alboran Domain and the southern part of the South Iberian Domain, and compressional structures in the South Iberian and African-Maghrebian domains and in the southern part of the Alboran Domain.

The Alpujárride-Nevado-Fibábride extensional shear zone, Betic Cordillera, SE Spain

In the Betic Cordillera, the initial thickening of the Alboran Domain produced an Alpine high pressure-low temperature (HP-LT) metamorphism in the Alpujarride and Nevado-Filabride complexes. After 19 Ma this thickening process evolved to an extensional stage with uplift rates of between 1 and 2 mm year−1. The extensional stage generated structures in a heterogeneous shear regime with a top-to-the-west sense of movement. The present-day Alpujarride-Nevado-Filabride contact is a detachment fault generated in this shearing regime. In the footwall, the first structure developed in this extensional stage is a planar-linear fabric characterized by constrictive strain, that is axial planar to kilometric recumbent tight to isoclinal folds. The planar-linear fabric is folded by inclined close to open folds with axes parallel to the stretching lineation. The planar-linear fabric and folds are deformed by a late extensional crenulation cleavage near the detachment surface. Finally, brittle deformation was generated within the same kinematic framework as that of the ductile structures. In the hanging wall, meanwhile, deformation had a brittle character producing faults and joints.

Recent and present-day stresses in the Granada Basin (Betic Cordilleras): Example of a late Miocene-present-day extensional basin in a convergent plate boundary

The diffuse convergent boundary between the Eurasian and African plates in the western Mediterranean is associated with a seismicity zone more than 300 km wide. Although the two plates are converging NW–SE, the Betic and Rif Cordilleras contain extensional structures that have been active since the Miocene. The extensional tectonics in the region, which occurred simultaneously with the uplift of the cordillera, have been analyzed in the southeastern sector of the late Miocene to recent Granada Basin, using earthquake focal mechanisms, the determination of paleostresses from the study of the orientation and kinematics of microfaults, and the study of the major structures. Both the geological surface data and the focal mechanisms indicate present-day regional conditions of NE–SW extension, with triaxial to prolate stress ellipsoids. However, the stress field is heterogeneous, with local variations in stress over time, with different stresses sometimes even acting simultaneously in adjacent areas. The most frequent changes consist of pluridirectional or NE–SW extension, favored by the prolate character of the stress ellipsoids, and NW–SE subhorizontal compression, favored by the regional tectonic setting. Strike-slip faults are scarce even though they are the most likely structure to be expected in a region with SW–NE extension and NW–SE compression. Seismicity is concentrated in the upper crust and may correspond to the activity of low- to high-angle normal faults similar to the surface faults, although they can not be correlated with them. The lower cutoff of this seismicity probably coincides with the 300°C isotherm and suggests a low thermal gradient for the region. Present-day regional stresses have σ1 vertical at the surface but in depth plunge toward the SW.

Active faulting in the internal zones of the central Betic Cordilleras (SE, Spain)

The internal zones of the Betic Cordilleras show a present-day relief that is mainly controlled by kilo- metre-size, symmetrical or north-vergent folds which developed mostly since Middle Miocene times. The Sierra Nevada, Sierra Alhamilla, Sierra de Los Filabres, Sierra Tejeda and Sierra de Gador, among others, are roughly E-W trending high mountain ranges, corresponding to antiforms where metamorphic rocks crop out. The surrounding depressions are located in synforms, where Neogene rocks are preserved from erosion. Field evidence shows that the growth of the folds is coeval with fault development, and that at least three of them, i.e. the Padul Fault, the Zafarraya Fault, and the Balanegra Fault, may be considered to be active seismogenetic structures. The Zafarraya Fault, in particular, is thought to be responsible for the 1884 Andalucia Earthquake. The fault is located at the northern limb of the Sierra Tejeda antiform, and could be interpreted as a collapse structure developed along the external arch of the uplifted fold. The Padul and Balanegra faults are located at the southeastern border of the Granada Basin and south of the Sierra de Gador, respectively. They belong to a set of NW-SE oriented faults that are mainly normal in character and indicate NE-SW extension. The set up, since 1999, of a GPS network within and around the Granada Basin and the planed installation of a new network in the Sierra Tejeda, will give us new insights on the present-day deformation behaviour of both folds and faults in the area.

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.

Crustal structure of the central sector of the Betic Cordillera (SE Spain)

Deep seismic reflection, gravimetric, and magnetometric data allow the main features of the deep structure of the central sector of the Betic Cordillera to be established. The Moho is horizontal or dips slightly toward the S below the mountain chain, and its morphology has no direct relationship with the regions topography. The deep reflectors are not deformed by the Neogene kilometric-scale folds that produced the main topographic features, probably due to the existence of detachment levels in the crust. The crust is slightly thickened in the Betic Cordillera (almost 35 km) and has an abrupt transition to the thin crust of the Alboran Sea (15 km in the central Alboran Sea) along an E-W oriented narrow band, subparallel to the coast line, where the Moho dips more than 60°N. The Neogene evolution of the mountain chain caused compressive deformation in the External Zones. In the Internal Zones, pre-Tortonian low-angle normal faults developed after the alpine metamorphism, and later high- and low-angle normal faults and strike slip faults occurred up to the Quaternary. Geophysical and field geology data can be used to establish a geological model for the recent evolution of the crustal deformation. In this model, the recent evolution of the region is probably determined by thrusting of the thin continental crust of the Alboran Sea toward the NW over the southern margin of the Iberian Massif. This deformation could be a consequence of the oblique convergent movement between the African and Eurasian plates, which may have uplifted the entire region, causing regressions in the sedimentary basins and the formation of folds and extensional structures in the upper part of the thickened crust.

The Mecina Extensional System: Its relation with the post-Aquitanian piggy-back Basins and the paleostresses evolution (Betic Cordilleras, Spain)

The Alboran Domain has undergone a westsouthwestern-east-northeastern post-Aquitanian minimal extension of 104% in two stages. In the first one (Burdigalian-early Tortonian), the Mecina Extensional System was active, the Neogene Basins were piggy-back ones, and stress ellipsoids were oblate. In the second stage (Tortonian-Present), stress ellipsoids were prolate, and the original shape of the outcropping Neogene Basins was nearly the same as their present morphology. An orogenic wedge model is proposed in which the westward strain of the wedge has been produced by variations in the spreading rates of the Ligurian-Balearic and Tyrrhenian basin oceanic crusts.

Palaeostress evolution of the Iberian Peninsula (Late Carboniferous to present-day)

Abstract Three main stages can be differentiated in Iberia from the Upper Carboniferous to the Present: the end of the Variscan orogeny (Late Carboniferous), the Permian–Early Cretaceous rifting and the Alpine orogeny (Late Cretaceous to Present). In this contribution, the main features of the stress fields determined in each of these stages are compiled and revised from the analysis of the brittle deformations, mainly microfault population. Stresses are related to the development of the principal structures of each stage. In the Late Variscan fracturing stage, sinistral strike–slip faults prevailed in a stress field with NE–SW subhorizontal compression. In the Permian-to-Early Cretaceous rifting stage, the extensional trends varied in each sector. During the Alpine orogeny, primarily the margins of the Iberian Massif (the Betic and Pyrenee mountain chains) were deformed. However, numerous deformations are also identified in its interior, where large mountain chains such as the Central System and the Iberian Chain developed. These deformations occurred in a complex stress field where zones of NW–SE to NNE–SSW horizontal compression prevailed, albeit predominantly extensional zones existed as well. At present, the stress field in most of Iberia has a NW–SE compressive trend, associated with active deformations in the Betic Cordillera, where a continental subduction developed. However, simultaneously, there are zones in which extensional settings predominate, such as the Internal Zones of the Betic Cordillera and the Gulf of Valencia region, making for quite a complex current stress field.

Continuous deformation, ductile thrusting and backfolding of cover and basement in the Sierra de Guadarrama, Hercynian orogen of central Spain

Abstract Hercynian deformation has affected two different crustal ensembles, a crystalline basement and a metasedimentary cover, in the Sierra de Guadarrama. The rheological basement is a medium- to high-grade metamorphic complex composed mostly of granitic orthogneisses, with minor amounts of sediments. The cover consists of a thick sequence of low- to high-grade metasediments, with minor gneissic bodies, ranging in age from Late Proterozoic to Devonian. The deformation in the cover was analysed mainly in the eastern part of the Sierra de Guadarrama, whereas the crystalline basement was analysed in the Segovia area, to the west. Both cover and basement deformation started with a stage (D 1 ) of continuous, penetrative deformation. The deformational regime has a strong component of subhorizontal simple shear, with east vergence. A second phase (D 2 ) of shear deformation gave rise to a broad, subhorizontal shear zone and a thrust in the cover, and to narrow ductile shear zones in the basement. The transition between the two first phases of deformation seems to have been gradual. The third phase (D 3 ) is a generalized backfolding episode, which induced an intense and penetrative strain in broad areas of both ensembles. Finally, large open folds with steep axial surfaces developed in two mutually perpendicular systems, giving rise to “dome and basin” interference patterns, and some subvertical ductile shear zones and faults were also created. The structural evolution of the basement and cover points to a thicker-skinned model of deformation, giving rise to an important crustal thickening. A salient feature of the model arose during the second phase, when a thick detachment level developed in the lower part of the cover. The generalized existence of this level may account for the scarcity of basement outcrops in the Spanish Hercynian belt.