Antonio Teixell

Crustal structure and orogenic material budget in the west central Pyrenees

Surface and subsurface data are combined to construct a crustal-scale cross section of the western central Pyrenees (France and Spain) at the boundary between the European and Iberian plates. The position of Moho reflections in the ECORS-Arzacq reflection profile suggests a tectonic wedge of European crust and upper mantle had indented the Iberian plate at lower crustal levels. The European wedge is overlain by an upper, Iberian wedge thus constituting a double (stacked) wedge geometry. The upper wedge was delaminated and deformed giving rise to the Pyrenean orogenic prism, manifested as a bivergent fan in upper crustal levels. The underthrust lower Iberian plate has been imaged to depths of 55–60 km, but crustal budget considerations based on a palinspastic reconstruction require that this continental root subducted to depths up to 90 km. Total orogenic contraction calculated from surface structures is about 75–80 km, which was accomplished between the latest Cretaceous and the early Miocene at an averaged rate of 1.2 mm/yr. Consistent with these moderate values, exhumation of the orogen is much less than in the more shortened eastern parts of the range. Paleozoic basement and preorogenic Mesozoic rocks are little eroded, and much of the exhumation involved cannibalization of early foreland basins, which once covered the entire, poorly emergent orogen at this transect. This resulted in a continuous process of sediment recycling and, coupled with a considerable lateral arrival of material, a bulk negative erosion-sedimentation budget.

The Ansó transect of the southern Pyrenees: basement and cover thrust geometries

A balanced geological section through the western central Pyrenees allows the Alpine geometry of Hercynian basement to be determined. This shows marked differences with other transects of the southern part of the chain. Basement-involved thrust sheets do not form an antiformal stack, but are imbricated and define steps separated by frontal culmination limbs. Basement thrusts propagate upwards and have formed distinct thrust systems in the cover. In this transect, the North Pyrenean Zone is bounded to the south by gently dipping thrusts of large displacement, and the Axial Zone massif, at its western termination area, is a flat-topped culmination caused by a single thrust fault, the Gavarnie thrust. This thrust emerges to the south in the interior of the syntectonic Jaca Basin, the geometry of which is also controlled by underlying basement thrusts. Various generations of structures can be correlated with distinct episodes of syntectonic sedimentation. A sequential restoration of the section illustrates how successive foreland sedimentary wedges were generated and progressively involved in deformation. Tectonic shortening is 48 km, almost half of that estimated further to the east for the southern Pyrenees. The structure deduced at the ECORS-Pyrenees seismic section cannot be translated to this transect.

Epicenter distribution and magnitude of earthquakes in fold‐thrust belts: Insights from Sandbox Models

Scaled analogue models are used to illustratethe eect of basal friction and erosion on fault activity andhence on epicentre distribution and magnitude of earth-quakes in the sedimentary cover of active fold-thrust belts.Model results suggest that in fold-thrust belts shortenedabove low-friction ductile decollements (rock salt or over-pressured mudstone), low- to moderate-magnitude earth-quakes (Mw=5:3−5:6), distributed over a wide area, occuralong several long-lived thrust faults. Conversely, in areasshortened above high-friction decollements large-magnitudeearthquakes (Mw=6:6−6:8), distributed over a narrowzone are likely to occur along few short- lived thrust ramps.Calculated magnitude of earthquakes from models and theirdistribution are in agreement with recorded earthquake pat-tern from the Zagros mountain belt, which is partially short-ened above a ductile decollement of Hormuz salt formation.Model results also showed that erosion reactivates older in-active thrusts and promotes formation of out-of-sequencethrusts.

Platform sedimentation and collapse in a carbonate-dominated margin of a foreland basin (Jaca basin, Eocene, southern Pyrenees)

The Eocene Jaca basin is a foreland basin with well-exposed carbonate platforms in the distal margin. This margin underwent alternating periods of stable platform growth and platform drowning. Periods of drowning were accompanied by large-scale collapses with generation of shelf-edge truncations and resedimentation of carbonate megabreccias into a terrigenous turbiditic trough. These features caused a stepped retreat of the platforms and are related to episodic variations of tectonic loading in the hinterland and correlative flexural bending of the distal basin.

Stress and fluid control on décollement within competent limestone

Abstract The Larra thrust of the Pyrenees is a bedding-parallel decollement located within a competent limestone unit. It forms the floor of a thrust system of hectometric-scale imbrications developed beneath a synorogenic basin. The fault rock at the decollement is a dense stack of mainly bedding-parallel calcite veins with variable internal deformation by twinning and recrystallization. Veins developed as extension fractures parallel to a horizontal maximum compressive stress, cemented by cavity-type crystals. Conditions during vein formation are interpreted in terms of a compressional model where crack-arrays develop at applied stresses approaching the shear strength of the rock and at fluid pressures equal to or less than the overburden pressure. The cracks developed in response to high differential stress, which was channelled in the strong limestone, and high fluid pressure in or below the thrust plane. Ductile deformation, although conspicuous, cannot account for the kilometric displacement of the thrust, which was mostly accommodated by slip on water sills constituted by open cracks. A model of cyclic differential brittle contraction, stress reorientation, slip and ductile relaxation at a rheological step in the limestone is proposed as a mechanism for episodic decollement movement. The model accounts for the peculiar microstructural character of the fault zone, for alternating sequences of bedding-parallel shortening (leading to crack dilation) and bedding-parallel shear (leading to decollement slip) and for hanging wall imbrication consequent upon decollement slip.

Crustal thickness and velocity structure across the Moroccan Atlas from long offset wide‐angle reflection seismic data: The SIMA experiment

The crustal structure and topography of the Moho boundary beneath the Atlas Mountains of Morocco has been constrained by a controlled source, wide-angle seismic reflection transect: the SIMA experiment. This paper presents the first results of this project, consisting of an almost 700 km long, high-resolution seismic profile acquired from the Sahara craton across the High and the Middle Atlas and the Rif Mountains. The interpretation of this seismic data set is based on forward modeling by raytracing, and has resulted in a detailed crustal structure and velocity model for the Atlas Mountains. Results indicate that the High Atlas features a moderate crustal thickness, with the Moho located at a minimum depth of 35 km to the S and at around 31 km to the N, in the Middle Atlas. Upper crustal shortening is resolved at depth through a crustal root where the Saharan crust underthrusts the northern Moroccan crust. This feature defines a lower crust imbrication that, locally, places the Moho boundary at ∼40–41 km depth in the northern part of the High Atlas. The P-wave velocity model is characterized by relatively low velocities, mostly in the lower crust and upper mantle, when compared to other active orogens and continental regions. These low deep crustal velocities together with other geophysical observables such as conductivity estimates derived from MT measurements, moderate Bouguer gravity anomaly, high heat flow, and surface exposures of recent alkaline volcanism lead to a model where partial melts are currently emplaced at deep crustal levels and in the upper mantle. The resulting model supports the existence of a mantle upwelling as mechanism that would contribute significantly to sustain the High Atlas topography. However, the detailed Moho geometry deduced in this work should lead to a revision of the exact geometry and position of this mantle feature and will require new modeling efforts.

Depositional model of a Cenomanian-Turonian extensional basin (Sopeira Basin, NE Spain): interplay between tectonics, eustasy and biological productivity

Abstract The Sopeira Basin is a small extensional basin with a continuous record of sedimentation from the Late Albian to the Late Turonian. In its early stages (Late Albian-Middle Cenomanian), it formed a half-graben, bounded to the south by a low-angle normal fault. During this time, sedimentation was limited to the downfaulted block until the emerged footwall was flooded, resulting in the deposition of shallow-water limestones over a vast platform. The platform was exposed during the Middle-Late Cenomanian by a relative sea-level fall. The sedimentation up to this point was controlled by local factors (extensional tectonics), and influenced by eustatic sea-level change. Subsequently, the entire area was flooded by the Late Cenomanian transgression. During the Cenomanian-Turonian boundary interval, the basinal sedimentation was characterized by calcisphaerid limestones with black cherts and organic-rich shales, while on the platform a hiatus was produced. These sedimentary features are interpreted as having been caused by global mechanisms: changes in atmospheric CO 2 and temperature, along with an increased rate of precipitation that caused an intense primary productivity at the water surface. In the basinal sea, eutrophication led initially to an unusual development of calcisphaerids and later, to the development of opportunistic planktonic foraminifera, whereas in the platform, it affected the life of carbonate-producing organisms dramatically and sedimentation ceased. Eutrophication of marine water is thus interpreted to be the main cause of the extinction of K-strategist foraminifera and associated fauna at the end of the Cenomanian.

Timing and nature of Quaternary fluvial incision in the Ouarzazate foreland basin, Morocco

The history of alluvial fan and terrace formation within a stretch of the Ouarzazate basin along the southern margin of the Central High Atlas is reconstructed using geomorphological and 10Be terrestrial cosmogenic nuclide (TCN) methods. Alluvial fan and terrace incision was controlled partially by a drop in base level during the Pliocene or early Pleistocene as the outlet channel, the Draa river, progressively cut through the Anti-Atlas to the south of the Ouarzazate foreland basin, the drainage of which started to become external after a long period of internal drained conditions. The alluvial fans and terrace surfaces have abandonment ages that date to at least the past four glacial cycles. Their formation was strongly modulated by climate on glacial–interglacial time scales as base level dropped. This demonstrates a strong climatic control on sediment transfer and landscape denudation during the Quaternary and provides a model for understanding sediment transfer in other intracontinental mountain belts. Furthermore, these data show that mean rates of fluvial incision in this region range between 0.3 and 1.0 mm a−1 for the latter part of the Quaternary. This study provides the first comprehensive TCN chronology for the Atlas Mountains, and it illustrates the applicability and limitations of TCN methods.

Forelimb deformation in some natural examples of fault-propagation folds

An analysis of two natural examples of fault-propagation folds from the Pyrenees and the Cantabrian zone (N Spain) is presented. These folds possess common features despite their different geological settings, i.e. related to thrust tectonics in the Pyrenean example and to strike-slip faulting in the Cantabrian zone. Transition from fault-bend to fault-propagation folding along individual thrusts is related to changes in lithology. The fault-propagation folds studied are tight and markedly asymmetric with respect to limb thickness and shape. The forelimbs are curvilinear and display progressive thinning of beds towards the fault surface, which can be interpreted in terms of a characteristic heterogeneous strain superimposed onto flexural shear, and restricted to them. This strain has been factorized by means of grid construction, and is found to be a combination of thrust-parallel heterogeneous simple shear and pure shear, attributed here to a shear zone with diverging walls and extrusion. In contrast with previous models, it is shown that thrust-parallel simple shear is not necessarily distributed above the entire fault surface but may instead be restricted to regions where motion was inhibited such as tip zones or ramps.

Longitudinal to transverse drainage network evolution in the High Atlas (Morocco): The role of tectonics

The High Atlas of Morocco is a still-active, linear intracontinental mountain chain in the NW African plate, which results from weak crustal thickening associated with rift inversion during the Cenozoic and from uplift related to mantle thermal doming. A striking morphological feature of the High Atlas is the occurrence of both transverse and longitudinal (i.e., strike-parallel) drainage characterized by deep fluvial incision of more than 1000 m in low-relief topography of the axial zone of the chain. Most of the transverse component of the drainage appears to postdate the longitudinal component as indicated by recent or incipient captures and wind gaps. The longitudinal drainage is inherited from an early stage of fluvial organization controlled by the tectonic structures developed during upper crustal folding and thrusting in the post-Paleozoic cover. Amplification of N-S regional slope in the western High Atlas by continued crustal shortening and thickening triggered: (i) higher erosion rates in transverse than in longitudinal catchments and (ii) captures of longitudinal streams by transverse ones, creating a new organization of the drainage system toward the regional slope. Such evolution from a longitudinal to a transverse-dominated drainage may represent a common mechanism of fluvial network development in mountain belts where the amplification of the regional slope results from long-lived lithospheric convergence.