Antonio M. Casas-Sainz

Intraplate deformation and basin formation during the Tertiary within the northern Iberian plate: Origin and evolution of the Almazán Basin

The Tertiary Almazan Basin was a topographic high during the Mesozoic rifting at the northern part of the Iberian plate. It was bordered by two Mesozoic basins: (1) the Castilian Basin (with a maximum thickness of more than 2000 m) to the south and (2) the Cameros Basin (with a maximum thickness of 9000 m) to the north. To the east the Almazan Basin is bordered by Paleozoic rocks of the NW-SE trending Aragonian Range, which underwent little subsidence during the Mesozoic. The two Mesozoic basins show an E-W direction. During the Tertiary NNE-SSW compression (mainly late Eocene-early Miocene), both Mesozoic basins were inverted and uplifted, and the Almazan Basin, with an overall synclinal geometry, was filled with syn-tectonic terrestrial sediments, reaching a maximum thickness of 3500 m. Deformation during the Tertiary was strongly controlled by preexisting structures, both Mesozoic extensional faults and reactivated Variscan folds. The depocenter of the Tertiary basin migrated southward during the Paleogene. This migration is consistent with a piggyback transport of the whole basin northward, due to the movement on the underlying, crustal-scale north verging Cameros thrust. The geometry and sedimentary evolution of the Almazan thick-skinned piggyback basin was controlled by the uplift associated with the main crustal-scale thrust, which was the main source area, and was separated from the foreland basin since the early stages of filling (late Eocene). This makes the main difference with thin-skinned piggyback basins, controlled by the foreland thrust belt and separated from the foreland basin only at their late stages.

Granite laccolith emplacement in the Iberian arc: AMS and gravity study of the La Tojiza pluton (NW Spain)

A laccolithic geometry is inferred from geological and geophysical data for the La Tojiza granitic body emplaced in an extensional regime. Nearly circular in outcrop, this 16 km diameter pluton, one of the so-called ‘post-tectonic granodioritic plutons’ of Galicia, is located in the northern Variscan arc of Iberia, within the Mondoñedo Nappe (Western Asturian–Leonese Zone). From anisotropy of magnetic susceptibility measurements and extensive microstructural data, the magnetic fabric of this pluton has been inferred to correlate with the magmatic fabric. Magnetic foliations strike NE–SW and dip gently SE. Magnetic lineations trend NW–SE and plunge gently SE. Gravity data suggest that the pluton has the shape of a sheet with a maximum preserved thickness of 1 km and a root zone located at its eastern border. This pluton is reinterpreted as late kinematic and emplaced during an extensional tectonic regime related to the ductile Vivero normal fault. It is suggested that the emplacement of most post-tectonic plutons of this part of the Variscan orogenic belt may have been controlled by the late extensional collapse of the Iberian Massif.

Cenozoic landscape development within the Central Iberian Chain, Spain

Abstract In the Central Iberian Chain (Spain), covering an area of some 7400 km 2 , up to seven stepped erosion surfaces are determined from aerial photographs and field observations. These planation surfaces situated between 1000 and 1600 m a.s.l. are separated by erosional scarp slopes. Most parts of the erosion surfaces are developed on Mesozoic limestones and form stepped rims around the main elevations of the Iberian Chain. It is proposed that the surfaces were formed by erosional events associated with periods of uplift related to Eocene–Miocene compressional tectonic activity. The palaeogene uplift was related with syntectonic sedimentation in internal basins with up to 3500 m of accumulated thickness. During the Neogene (Middle Miocene–Pliocene) post-tectonic period, the lowered synclinal areas formed residual, internally drained basins that were filled with non-marine deposits (mainly clays and limestones). The filling of the internally drained sedimentary basins located within the Iberian Chain onlapped onto the ancient erosional surfaces, covering the lower younger levels. The recent landscape evolution of the whole area is controlled by the capture of the internal basins and the dissection of the ancient relief by the present-day fluvial network. This process is related with the transition from internal to external drainage of the Ebro basin.

Granite emplacement during contemporary shortening and normal faulting: structural and magnetic study of the Veiga Massif (NW Spain)

Abstract The Veiga Massif belongs to the calc-alkaline series of Hercynian granitic rocks of the Ibero-Armorican arc The Veiga granodiorite intruded during the Upper Carboniferous into the core of the WNW-ESE N-verging ‘Ollo de Sapo’ antiform, formed by Precambrian and Palaeozoic metasediments. Internal fabrics show that magma intrusion was contemporary with shortening. Measurements of feldspars orientations and anisotropy of magnetic susceptibility (AMS) throughout the granite are consistent and indicate a foliation striking WNW-ESE (parallel-to-folding), with a constant dip of 75–85 °N. The zonation of bulk low-field susceptibility is related to mineral content and indicates a more basic composition at the southern and western borders. The difference in elevation between outcrops (more than 600 m) allows us to infer the three-dimensional attitude of granite fabrics throughout the Massif. Syn-magmatic fabric folds are preserved in the inner part of the igneous body. The highest degree of magnetic anisotropy is observed in areas located near the bottom and top of the intrusion. At the scale of the Massif, foliation is convergent toward the bottom of the intrusion, along a line located at its northern border, where the magma source is interpreted to be located. In the western border of the Massif, the presence of C and S structures indicates that magma cooling was coeval with movement of the Chandoiro fault, a N-S striking normal fault with a N290E hanging wall displacement direction. These results indicate that emplacement of the Veiga granite is coeval with NNE-SSW shortening and with an WNW-ESE extension direction, parallel to the trend of the late folds.

Stress field and thrust kinematics: a model for the tectonic inversion of the cameros massif (Spain)

Abstract Alpine tectonics of the Cameros Thrust has been studied from several points of view: geometry (surface and sub-surface structure, three-dimensional outline of the thrust), kinematics (transport direction) and dynamics (characterization of the regional compressional stress field). All three aspects are related by means of a model based on Botts equation. Under a N to NNE horizontal compression, with an average stress ratio R = (σz − σx)/(σy − σx) = 0.2, the eastern ramp of the Cameros Thrust (oriented 155°, 30°W) underwent a strike-slip right-lateral movement with a small reverse component. This gave rise to rigid displacement of the Cameros block towards the NNW and produced a ‘guided movement’ on the western, frontal ramp (oriented 060°, 13°S).

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.

Widespread Cretaceous inversion event in northern Spain: evidence from subsurface and palaeomagnetic data

Abstract: In this paper we present geological evidence of a mid-Cretaceous inversion event in the Polientes basin (northern Iberian plate) based on geological data. Analysis of seismic profiles across the basin indicates that this tectonic event resulted in the uplift of the basin depocentre and the formation of an upright gentle anticline before Cenomanian times. The presence of a pre-Alpine, syntectonic remagnetization in the Jurassic and Lower Cretaceous rocks of the Polientes basin allows us to define the geometry of the basin at the time between this early inversion event and the main Alpine compressional period. The early inversion in the Polientes basin is consistent with a transpressional tectonic setting, contemporaneous with the left-lateral strike-slip movement of Iberia with respect to Europe. This study shows the successful use of two approaches to unravel the geodynamic scenario of the northern Iberian plate during the Cretaceous, which was later obliterated during the Tertiary Pyrenean orogeny.

Deflection of a compressional stress field by large-scale basement faults. A case study from the Tertiary Almazán basin (Spain)

Abstract Palaeostresses deduced from brittle mesostructures (more than 50 sites and 1500 structures measured) in the Mesozoic and Tertiary rocks of the Almazan basin, located in the northwestern Iberian Chain, indicate a regional NNE-SSE-directed compression that lasted during all the Tertiary shortening stage. The stress regime corresponding to this compression ranges from thrust to wrench. N-S-directed compression was the responsible for most macrostructures (folds and thrusts forming a S-vergent system with NE-SW to E-W and NW-SE orientations) and mesostructures (mainly faults, stylolytes and impressed pebbles). We propose that the stress trajectories are deflected, following Andersons model, in the western part of the Almazan basin by the NE-SW-striking left-lateral strike-slip Soria fault. This basement fault was inherited from the late Variscan fracturing stage and controlled the deposition in the Mesozoic basins as well as the location of Tertiary compressional macrostructures. In the eastern border of the Almazan basin palaeostress trajectories were deflected both by right-lateral strike-slip faults and NW-SE-trending thrusts, near which the regional N-S-directed compression becomes perpendicular to them. Three minor compression directions have also been recorded in several sites: NE-SW, E-W and NW-SE. The NE-SW-directed compression is interpreted as a deviation of the main N-S-directed compression at a regional scale, whereas the NW-SE-directed stress field corresponds to the regional field responsible for the main Tertiary shortening stage in the Spanish Central System. The age of compression ranges from Eocene to Miocene.

Reconstruction of inverted sedimentary basins from syn-tectonic remagnetizations. A methodological proposal

Abstract Syn-tectonic remagnetizations related to burial processes that occurred during their extensional stage are frequently recognized in inverted sedimentary basins. The incremental fold test is the main analytical tool used to detect these syn-tectonic remagnetizations. However this test gives spurious results when asymmetrical folding occurs (i.e. both limbs were not tilted simultaneously at the same rate). Asymmetrical folding is very common in sedimentary basins, especially at their margins, during the basinal stage and subsequent tectonic inversion. To correctly analyse syn-tectonic remagnetizations in these scenarios, we propose a method to restore palaeomagnetic vectors, which allows determining the tilting of beds at the remagnetization stage and therefore gives some hints on the geometry of sedimentary basins and/or folds at the overprint acquisition time. In this paper we consider the analysis of syn-tectonic remagnetization directions for basin reconstruction in two end-member basin models (i.e. syncline vs roll-over geometry). Finally, we compare the results obtained with several northern Iberian examples (Cameros basin in the Iberian Chain, Cabuérniga and Polientes Basin in the Pyrenées), formed during the Mesozoic as extensional basins and subsequently inverted during the Cenozoic compression. From these examples we propose generalizations for the application of syn-tectonic remagnetizations to constrain the geometry of sedimentary basins.

Palaeostresses associated with thrust sheet emplacement and related folding in the southern central Pyrenees, Huesca, Spain

Brittle mesostructures have been studied in the detached Mesozoic and Lower Tertiary cover rocks of the south-central Pyrenees (Huesca province, Spain), in order to reconstruct the state of stress within the thrust sheets during Tertiary compression. The structure of the area studied consists of NW–SE- to E–W-striking, south-verging thrust-sheets, located west of the South-Pyrenean Central Unit. The palaeostresses obtained show dominant N–S to ENE–WSW compression directions for stress tensors with horizontal σ1 axes, and E–W to NW–SE extension directions for stress tensors with vertical σl axes. The values of the stress ellipsoid ratios for stress tensors with horizontal σ1 are predominantly between 0 and 0.5 (indicating uniaxial compression to wrench regime). Regarding their relationship with macro-structures, the palaeostresses show transport-parallel compression in most sites, although thrust-parallel compression is also found. There are no differences between palaeostresses found in the hanging walls and the footwalls of thrusts. Stress tensors with vertical σ1 do not show systematic relations with macro-structures, but transport-parallel extension and thrust-parallel extension are found in some sites. The analysis of palaeostresses in areas of fold-and-thrust belts shows a high degree of complexity and the results are not easy to interpret. The continuous variation from N–S to ENE–WSW in the orientation of the compression axes is here interpreted in two ways: (1) as the result of the clockwise rotation of the Mesozoic and Tertiary cover, which was pushed in the east by the South-Pyrenean Central Unit with a constant N to NNE regional compression, and (2) due to NE–SW compression resulting from a SW-directed local transport of minor thrusts. Palaeostress analysis can, therefore, reveal important information about thrust kinematics, at high structural levels where ductile deformation is absent.