G. de Vicente

Soft-sediment deformation structures interpreted as seismites in lacustrine sediments of the Prebetic Zone, SE Spain, and their potential use as indicators of earthquake magnitudes during the Late Miocene

Abstract Lacustrine formations of Late Miocene age in the Prebetic area, SE Spain, show several types of deformational structures that are interpreted as seismites. They are present in both marginal and deep lacustrine facies. Seismites formed in marginal lake environments comprise sand dikes, pillows and intruded and fractured gravels. In deep lacustrine facies, the seismites are represented by pseudonodules, mushroom-like silts protruding into laminites, mixed layers, disturbed varved lamination and loop bedding. The measured orientations of these structures are consistent with the orientations of the main faults limiting the basins, showing that their origin is clearly related to the tectonic stress field that prevailed in the region during the Late Miocene. The magnitudes of the earthquakes that deformed the sediments have been estimated after published data from both ancient and recent lake deposits accumulated in tectonic active regions elsewhere. A rank of earthquake magnitudes with two end-members, i.e. the lowest magnitudes recorded by loop bedding in laminites and the highest magnitudes represented by intruded and fractured gravels, is proposed.

The recent (upper Miocene to Quaternary) and present tectonic stress distributions in the Iberian Peninsula

A general synthesis of the recent and present stress situation and evolution in the Iberian Peninsula was obtained from microstructural and seismological analysis. The stress evolution was deduced from (1) fault population analysis (FPA) from 409 sites distributed throughout the Iberian Peninsula, (2) paleostress indicators given by 324 stations taken from the bibliography, and (3) seismic data corresponding to 161 focal mechanisms evenly spread in the studied region. The application of FPA together with the determination of stress tensors and focal mechanisms for the whole Iberian microplate has provided two main results: (1) the Iberian Peninsula is undergoing a NW-SE oriented compression, except for the northeastern part Pyrenees,Ebro Basin, and Iberian Chain), where it is N-S to NE-SW, and the Gulf of Cadiz, where it seems to be E-W, and (2) the main trends of the stress field have remained almost constant since the upper Miocene. The analysis performed by zones suggests the presence of local heterogeneities in the stress field.

Lithospheric folding in Iberia

Integration of stress indicator data, gravity data, crustal kinematics data, and analysis of topography and recent vertical motions demonstrates the occurrence of consistently oriented spatial patterns of large-scale Alpine to recent intraplate deformation in Iberia. The inferred upper crustal and lithospheric deformation patterns and the timing of the associated expressions at or near the surface support the existence of a close coupling with plate boundary processes operating at the margins of Iberia. Patterns of lithosphere and upper crustal folds are oriented perpendicular to the main axis of present-day intraplate compression in Iberia inferred from structural analysis of stress indicator data and focal mechanism solutions. These findings suggest the presence of lithospheric folds, with wavelengths compatible with theoretical predictions of folding wavelengths of Variscan lithosphere. Stress-induced intraplate deformation set up by plate interactions is compatible with indications for the absence of present-day deep mantle-lithosphere interactions inferred from seismic tomography.

Local stress fields and intraplate deformation of Iberia: variations in spatial and temporal interplay of regional stress sources

Tertiary to present deformation in the interior of the Iberian Peninsula reflects spatial and temporal variations of the activity of the plate boundaries. Local deformation patterns observed in many of the numerous intraplate Tertiary basins and their borders, such as the Madrid Basin and the Sierra de Altomira, are at first sight incompatible with the regional stress field under which they were formed. We demonstrate, however, that they can be explained as the effect of several stress fields that acted on the Iberian Peninsula from earliest Tertiary onward. Data on local deformation can constrain both magnitudes and directions of forces acting on the plate boundaries of Iberia, enabling us to estimate the relative importance of the different plate driving and deforming mechanisms providing further constraints on the tectonic evolution of Iberia.

Determination of present-day stress tensor and neotectonic interval in the Spanish Central System and Madrid Basin, central Spain

A brittle deformation tectonic analysis was performed in central Spain (Spanish Central System and Madrid Basin) in order to decipher and understand the deformation processes that take place in a typical intracontinental zone. 1174 fault slickensides obtained in materials with ages between Late Cretaceous and Quaternary have been analyzed by means of fault population analysis methods to reconstruct paleostress tensors. Nine earthquake focal mechanisms have been determined, with magnitudes ranging between 3 and 4.1. With regard to regional structural features and sedimentary record data, the characteristics of present-day and neotectonic stress fields have been figured out, which determine the neotectonic period for this region. Thus, we have established that the intraplate zone represented by central Spain has been subjected to a stress field from the Middle Miocene until the present-day with a largest horizontal shortening direction (SHMAX) located between N130E and N160E. Finally, three paleostress maps with the main active structures are presented for: (a) Middle Miocene to Late Miocene, the period when the Spanish Central System was mainly formed, (b) Late Miocene to Quaternary, and (c) the present-day stress field, deduced from earthquake focal mechanisms.

The role of pre-existing faults in basin evolution: constraints from 2D finite element and 3D flexure models

Abstract The lithospheric driving forces which cause intraplate basin deformation are relatively constant over large areas. Consequently, lateral variations in deformation and stress and strain concentrations seem to be primarily caused by (pre-existing) heterogenities in the rheological signature of the continental lithosphere underlying the sedimentary basins. In this paper, we explore the weak character of upper crustal faults and their control on basin shape for a number of case studies on intraplate Phanerozoic basin settings, using a 2D finite element and a 3D flexure model. Of key importance is the integration of seismic data and field observations with the tectonic modelling, allowing the investigation of deformation processes and their expressions on different scales, operating on different levels of the lithosphere. Finite element models for sub-basin scales, incorporating weak upper crustal faults, predict strong control of these weak zones on local stress distributions and subsequent deformation, in agreement with observed deformation patterns. Slip along upper crustal faults control stress distribution and subsequent faulting in overlying sedimentary rocks. The effect of weak upper crustal fault movements on basin-wide (regional) upper crustal flexure is looked at in two case studies on: (1) extensional tectonics in the Lake Tanganyika Rift Zone (East Africa); and (2) compressional tectonics in the Central System and Tajo Basin (Central Spain). Both settings indicate that basement warpings are controlled by large amounts of slip along so-called weak crustal-scale border faults, which are mostly planar. Adopting border fault displacements in the 3D flexure model, the results indicate low effective elastic thickness (EET) values in a range of 3–7 km for induced basement deflection patterns in accordance with observations. The low EET values most likely reflect a (partly) decoupling of upper crustal and subcrustal deformation, facilitated by the weak lower crust, and in agreement with standard rheological assumptions for Phanerozoic lithosphere. In contrast, the inferred weakness of upper crustal faults relative to surrounding rock is not evident from uniform rheological assumptions. However, observations of reactivations of faults which are not preferably aligned with the stress field, and reactivations of basin deformation on long time-scales are in support of this feature.

Exceptional river gorge formation from unexceptional floods.

An understanding of rates and mechanisms of incision and knickpoint retreat in bedrock rivers is fundamental to perceptions of landscape response to external drivers, yet only sparse field data are available. Here we present eye witness accounts and quantitative surveys of rapid, amphitheatre-headed gorge formation in unweathered granite from the overtopping of a rock-cut dam spillway by small-moderate floods (∼100-1,500 m(3) s(-1)). The amount of erosion demonstrates no relationship with flood magnitude or bedload availability. Instead, structural pattern of the bedrock through faults and joints appears to be the primary control on landscape change. These discontinuities facilitate rapid erosion (>270 m headward retreat; ∼100 m incision; and ∼160 m widening over 6 years) principally through fluvial plucking and block topple. The example demonstrates the potential for extremely rapid transient bedrock erosion even when rocks are mechanically strong and flood discharges are moderate. These observations are relevant to perceived models of gorge formation and knickpoint retreat.

Seismotectonics of the Sierra Albarrana area (southern Spain). Constraints for a regional model of the Sierra Morena-Guadalquivir Basin limit

Abstract A new approach to the present day stress field of the Sierra Albarrana area (southern Spain) has been inferred from the application of fault population analysis (FPA) to a set of fault/striae pairs and a short sample of earthquakes and from the simultaneous determination of stress tensor and fault plane solutions for this last set. The first population of data concerned 226 fault and striae pairs obtained in 8 field outcrops. In their turn, seismic data correspond to 45 polarities of P-wave arrival and take-off angles for a set of source-station pairs. These data were obtained in a microseismicity survey of 30 months carried out in an area of 50 × 40 km 2 . Four FPA methods have been chosen: the slip model, the right dihedral method, a 3D random search of the stress tensor and the stress inversion method. In addition, fault planes have been selected by using the symmetries established in the slip model, and the methodology of FPA has also been applied to seismic data. In this way, it has been possible to take advantage of two sources of information too small to be considered independently. The use of complementary methods based on the FPA has allowed the possible solutions for the fault planes to be constrained and has yielded a picture of the tectonic evolution of the area. The analysis of the seismic information and the comparison of the results obtained independently with FPA methods provide a reliable interpretation of the present-day stress distribution in the zone. The regional stress tensor shows a compression close to uniaxial with its compressive axis, σ y (or σ Hmax ), oriented N132E and plunging 0°–19°. Focal mechanisms correspond to either reverse or normal faults, some of them with a noticeable strike-slip component. The reverse faults show a NW dip and are deeper than the normal ones. A regional deformation model including a crustal flexure with axis parallel to Guadalquivir River has also been sketched.

An application of the slip model of brittle deformation to focal mechanism analysis in three different plate tectonic situations

Abstract The “slip model” of brittle deformation ( Reches, 1983 ) imposes a set of constraints on the angular relationships between faults, their striations on slickensides and the type and orientation of the deformation ellipsoid. In focal mechanisms of earthquakes, these relationships can be used to determine which of the two nodal planes is the fault plane, and then to obtain the strain ellipsoid of regional tectonic deformation controlling seismicity. In this approach it is convenient to use pitch/dip diagrams ( De Vicente, 1988 ) and P/T diagrams ( Angelier and Mechler, 1977 ), both developed for fault population analysis. In this paper three regions in different plate-tectonic situations are analysed using these methods. These regions are a zone of intracontinental seismicity in the eastern Peruvian Andes, a zone of continental compressional deformation in the Iranian Plateau, and the Fenwei Graben of China. Nodal planes of focal mechanisms, published in different papers about seismicity in these regions, have been used, the following conclusions being reached. In the analysed zone of the Peruvian Andes, three regional deformation types appear, causing fault movements that range from slightly reverse strike-slip to reverse strike-slip. In addition, exclusively at depths in excess of 18 km, there is a deformation of an uniaxial shortening type. In all of these deformations the horizontal shortening axis is oriented from N87° to N93°. In the Iranian Plateau two horizontal shortening directions appear, N151° and N61°, even showing faults that imply an uniaxial deformation ellipsoid the shortening axis of which is at N61°. In the Fenwei Graben the ellipsoid of regional deformation has a stretching principal axis at N145°/150° and a shortening axis at N55°/60°, the strike-slip movement along faults being as important as the normal one.

The role of lithospheric heterogeneities in the location of the Cenozoic intraplate deformation of Iberia from finite element modeling

Received 31 May 2011; revised 28 November 2011; accepted 12 December 2011; published 9 February 2012. [1] The Cenozoic evolution of the Central System basement uplift and of the continental Duero and Madrid basins on the Iberian Peninsula occurred in a compressive context triggered by the Alpine orogeny. Although several models of deformation have been suggested, the factors that led to the development of these have not been analyzed to date. We present here finite element modeling of the intraplate deformation of the Iberian lithosphere during the Cenozoic. This modeling has taken into account the role played by (1) the tectonic shortening originating from the Pyrenean and Betic collisions, (2) the weight of Tertiary continental sediments, and (3) thermomechanical heterogeneities present in the lithosphere. The numerical model represents a lithospheric section, with a creep-type mechanical behavior, perpendicular to the Central System, and the two sedimentary basins. Both initial conditions and the results obtained have been specified based on the available geophysical and geological data. This study has enabled us to simulate the present lithospheric structure of the central Iberian peninsula: (a) the formation of the Central System relief and its horizontal shortening, (b) the basement folding in the Duero and Madrid basins, (c) the development of the root of the mountain range, and (d) the depth of the lithosphere-asthenosphere border in the interior of Iberia. The deformations occurred through a large-scale folding and thickening mechanism, by the shortening of a lithosphere that was initially thinner, and with lateral contrasts in temperatures and composition.