María Luisa Arboleya

Paleomagnetic results in support of a model for the origin of the Asturian arc

Abstract Paleomagnetic directions have been determined at 17 Cambrian and 28 Carboniferous limestone sites distributed along the Asturian arc of the Hercynian fold belt in northwestern Spain. The stable vectors are carried by hematite in all cases, and also by magnetite in some grey Carboniferous limestones. A secondary component of magnetization is usually removed well below 400°C, and the higher-temperature component is taken to be the characteristic direction. The structure at each site was carefully evaluated in order to make optimum local tectonic corrections. In addition the data must be corrected for rotations about a vertical axis to allow for a phase of radial folding superimposed on earlier structures. The corrected paleomagnetic declinations are found to vary systematically along the arc of the fold belt. Consequently, paleomagnetic data from the Asturian arc should not be included in compilations used for the construction of a Paleozoic apparent polar wander path. The paleomagnetic data allow us to distinguish between different tectonic models for the evolution of the Asturian arc. A two-stage model for the development of the present curvature is favored. Part of the curvature appears to be primary, preceding the Hercynian deformation. The first stage of the model involved rotations of thrust sheets during their emplacement, producing a more tightly curved arc than the original form. In a second stage, the development of radial folds further tightened the curvature of the arc. Both stages result in clockwise rotations in the north and anticlockwise rotations in the south.

A geometrical and kinematical approach to the nappe structure in an arcuate fold belt: the Cantabrian nappes (Hercynian chain, NW Spain)

Abstract In northwest Spain thrust sheets occur in an arcuate fold belt. The fault style consists of an array of thrusts, merging downdip into a single decollement surface. Most of the thrust sheets were initiated as thrusts cutting across flat lying beds. Folds above the hanging-wall ramps and some minor structures indicate that the body of the nappes has been subjected to an inhomogeneous simple shear parallel to bedding (y = 1.15), with slip concentrated along bedding planes. This allows the rocks forming the nappe to remain unstrained. At the base of the nappes a thin zone of deformed rock exists. The thrust sheets die out laterally against an anticline-syncline couple, oblique to the thrust direction. A geometrical analysis shows that if anticline and syncline axes are oblique, the thrust sheet was emplaced with a rotational movement, which can be evaluated. As deformation progressed two sets of folds were formed: a circumferential set, following the arc, and a radial set. An arcuate trace of the thrust structures remains after unfolding the radial folds. With a rotational emplacement, the displacement vector for successive points has a progressively greater length, and forms a progressively lower angle with the thrust. The main thrust units are broken into several slices with rotational movements, so that each unit was curved as it was being emplaced, producing a first tightening of the arc. Later folding increased the arc curvature to its present shape. The palaeomagnetic data available support the above conclusions.

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.

Areal balancing and estimate of areal reduction in a thin-skinned fold-and-thrust belt (Cantabrian zone, NW Spain): constraints on its emplacement mechanism

Abstract The Cantabrian zone is a curved thrust-and-fold belt forming the frontal part of the Hercynian orogen in northwestern Spain. The present day structure is the result of interference between different kinds of structures describing the arc and a set of cross folds, showing a radial pattern. Some of the structures can be traced all around the arc, while some others are of limited lateral extent, so that the displacement they produce is transferred to some other kind of structure. Taking into account geometrical data and field evidence on the direction of thrust motion an area-balanced model has been obtained. This model permits the restoration of the Cantabrian zone to its pre-deformed stage, and shows that the areal reduction has been about 50%. Globally considered, the structure of the Cantabrian zone corresponds to a detached and deformed wedge of sediments which was being thrusted and folded as it was being subjected to translation. The wedge before deformation was flat-topped and had a basal backslope, but as it was being deformed a surface foreslope was generated. Slope and thickness evaluations fulfil the required theoretical conditions for the decollement to be possible under compressional conditions. Lack of tectonic denudation at the back, persistence of the basal backslope and the general structure of the belt indicate compression and crustal shortening. Only in the most frontal part of the belt, gravitational gliding of rock slabs took place and chaotic mixtures were formed, contributing to the infilling of the Carboniferous basin in the very core of the arc.

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.

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.

Structural and metamorphic evidence of local extension along the Vivero fault coeval with bulk crustal shortening in the Variscan chain (NW Spain)

Abstract The Vivero fault is a W-dipping, N-S-striking ductile shear zone separating the Ollo de Sapo antiform in its western hangingwall and the Lugo dome in its eastern footwall. Two stages of deformation ( F 1 and F 2 ) produced nearly coaxial folds with sub-horizontal axes. A crenulation cleavage S 2 transposes an older S 1 . Three sets of shear bands in the hangingwall define a pervasive fabric consistent with an E-W bulk shortening perpendicular to a composite S 1–2 foliation and NNE-stretching parallel to L 2 . The Vivero fault zone is marked by a mylonitic foliation with a steeply NW-plunging stretching lineation and extensional crenulation cleavage (ECC) indicating normal slip. In the vicinity of the fault, sub-horizontal NNE-trending F 3 folds, with a crenulation cleavage S 3 , deform earlier-formed fabrics, including a mylonitic foliation. Pressure-temperature conditions obtained from mineral assemblages on both sides of the Vivero fault yield a minimum throw of 5.5 km. Andalusite-bearing pelite in the hangingwall was infolded by an F 2 synform into the kyanite field at 450–500°C. The eastern edge of these rocks was later accreted to the footwall and heated to andalusite-staurolite conditions at ∼600°C. Slip on the Vivero and Valdovino faults is kinematically related. East-west shortening during F 2 involved folding and sinistral strike-slip on the Valdovino fault which induced local extension along the newly generated Vivero fault. Synkinematic emplacement of granitoids along the Vivero fault is favoured by extension. Coeval slip on both faults took place during the later stages of F 2 folding. Geometrical constraints caused northwards escape of the crustal block bounded by the Valdovino and Vivero faults, recorded by NNE-stretching defined by L 2 .

Provenance, age, and tectonic evolution of Variscan flysch, southeastern France and northeastern Spain, based on zircon geochronology

Small basins filled with Early Carboniferous (Mississippian) flysch in the Variscan chain in southwestern Europe formed as ocean basins between Laurussia and Gondwana closed and deformation was transferred into Gondwanan crust. To identify flysch source areas and the spatial distribution and timing of deformation as continental collision progressed, sensitive high-resolution ion microprobe (SHRIMP) U-Pb ages were determined for (1) detrital zircon and zircon in orthogneiss cobbles from flysch sediments in basins along a 300 km transect from the Montagne Noire in southeastern France across the Catalonian Massif to Minorca in northeastern Spain; (2) monazite from a potential source area; and (3) undeformed granite that crosscuts flysch sediments. A remarkable feature of flysch conglomerates all along the transect south of the Montagne Noire is that they contain pebbles and cobbles of deformed leucocratic granite, gneiss, and pegmatite, felsic volcanic porphyritic rocks, schist, and slate that resemble rocks exposed in massifs adjacent to the flysch basins. Age distributions of detrital zircon populations from 13 basins show that most grains are Neoproterozoic (younger than 850 Ma), Cambrian–Ordovician, and Early Carboniferous (Mississippian). Fewer Devonian zircons are all post-Emsian, and most are Frasnian–Famennian (Late Devonian); i.e., between ca. 359 and 385 Ma. A younger zircon population is composed mostly of igneous grains that are Early Carboniferous, in the range 360–325 Ma. In several basins, the youngest detrital zircon age group is Tournaisian; in others, it is Visean (345–330 Ma), only a few million years older than the age of flysch deposition based on biostratigraphy. The youngest zircon age group in flysch from the Canoves Basin along the Catalonian Massif has an age of 327 ± 4 Ma; this age overlaps the biostratigraphic age of Canoves flysch and coincides with monazite ages determined from a high-grade pelitic gneiss exposed in the Guilleries massif ~30 km away along strike. An orthogneiss cobble from Canoves has a crystallization age of 489 ± 5 Ma, matching the age of bedrock orthogneiss from Guilleries. These data suggest that Variscan deformation of Gondwanan crust began during the late Visean and that individual flysch basins developed in front of a series of uplifted thrust wedges where midcrustal igneous and metamorphic rocks were rapidly exhumed and eroded. A slaty cleavage is developed in all of the flysch basins, and undeformed granites that intrude four flysch basins have ages that range between 305 and 295 Ma.

Structural control on present-day topography of a basement massif: the Central and Eastern Anti-Atlas (Morocco)

The Anti-Atlas basement massif extends South of the High Atlas, and, despite a very mild Cenozoic deformation, its altitude exceeds 1500m in large areas, reaching 3305m in Jbel Sirwa. Structural contours of the present elevation of a polygenic planation surface (the High Erosional surface) and of the base of Cretaceous and Neogene inliers have been performed to characterize the major tectonic structures. Gentle Cenozoic WSW-ENE- and N-Strending folds, of 60 to100km wavelength, reactivate Variscan structures, being the major contributors to the local topography of the Anti-Atlas. Reactivated thrusts of decakilometric to kilometric-scale and E-W trend involving the Neogene rocks exhibit a steep attitude and a small displacement, but they also produce a marked topographic expression. The resulting Cenozoic horizontal shortening along N-S sections across the Anti-Atlas is about 1%. The position of the major anticlinal hinges determines the location of the fluvial divides of the Warzazat basin and the Anti-Atlas, and a structural depression on one of these hinges (Jbel Saghro anticline) allowed the formerly endorheic Warzazat basin to drain southwards. The first Cenozoic structures generating local topography are of pre-mid Miocene age (postdated by 6.7Ma volcanic rocks at the Jbel Saghro), whereas the youngest thrust movements postdate the Pliocene sedimentary and volcanic rocks (involving 2.1Ma volcanic rocks at Jbel Sirwa). In addition to these features, the mean elevation of the Anti-Atlas at the regional scale is also the result of a mantle thermal anomaly reported in previous works for the entire Atlas system.

Reactivated lithospheric-scale discontinuities localize dynamic uplift of the Moroccan Atlas Mountains: COMMENT

The Atlas Mountains of Morocco have received considerable attention in the past decade regarding the lithospheric structure and the uplift mechanisms of this structurally simple mountain belt. Miller and Becker (2014) have recently added to the discussion with new results from receiver functions and shear-wave splitting analysis. These results confirm the mantle upwelling hypothesis for the Atlas uplift that was postulated 10 years ago and we now see highlighted in several forums. The relationship between modest tectonic shortening and high topography in the Atlas was first addressed by Teixell et al. (2003). They concluded that crustal thickness is insufficient to account for the observed elevations, and proposed a mantle-driven (upwelling) component of uplift, supported by low seismic velocities in the Atlas mantle reported already by Seber et al. (1996). Subsequent efforts focused on modeling of gravity, the geoid, and topography that indicated a prominent lithospheric thinning, placing the lithosphere-asthenosphere boundary (LAB) at 70–90 km beneath the Atlas Mountains (e.g., Teixell et al., 2005; Zeyen et al., 2005; Missenard et al., 2006; Fullea et al., 2010). New contributions on the basis of active and passive seismology, partly under the Picasso project frame, are on their way (e.g., Bezada et al., 2013; Carbonell et al. 2013; Palomeras et al., 2013; Miller and Becker, 2014). Miller and Becker present a receiver functions profile that also shows a thinned crust and lithosphere beneath the Atlas Mountains. However, the thinned domain is bounded by abrupt steps coinciding with the location of the South Atlas thrust front; the authors propose offsets of 9 km of the Moho and 26 km of the LAB across the boundary thrust fault at Errachidia (Miller and Becker, 2014, their figure 2), arguing for the important role of lithosphericscale discontinuities in the Atlas mountain building. As for the crust, this is intriguing because early refraction studies by Wigger et al. (1992) and a more dense wide-angle survey by Carbonell et al. (2013) failed to detect a crustal step in this position, but especially because there is ample evidence that the South Atlas fault is a low-angle thrust zone (Saint Bezar et al. 1988; Teixell et al., 2003). Moreover, Miller and Becker imply that the same fault discontinuity goes down to offset the LAB, which means that the South Atlas thrust projects vertically to a depth of 90 km. This has doubtful geological sense even assuming that the present-day thrusts derive from the inversion of earlier extensional faults: those would also have a dip, and at a depth of 90 km they would be off the vertical projection of the fault surface exposure. Miller and Becker recognize that previous authors document shallowly dipping thrust faults from outcrop and seismic reflection studies, but they do not further discuss this issue. We are left questioning whether there is an enigmatic geological feature in the Atlas that escapes our understanding, or an issue of seismic data resolution. Miller and Becker compare the detected position of the LAB with a modeled position on the basis of the residual topography and on the assumption of isostasy, resulting in a large discrepancy, as the modeled LAB is even shallower than the actual base of the crust. This indicates a strong contribution of dynamic topography due to active mantle upwelling. We do not question the validity of dynamic topography for the Atlas, but we miss a discussion of the earlier potential field models which, on the basis of isostatic equilibrium, fitted the residual topography and the geoid with a greater lithospheric thickness, which would be in accord with the new receiver function results (see references above). Finally, we wish to conclude with a formal comment on the referencing to earlier work in the Atlas. Miller and Becker report a small orogenic shortening (15%–25%) inconsistent with Airy isostatic support and cite four papers, but none of them resulting in these small values of shortening nor do they address relationships between shortening and topography. Fullea et al. (2010) are not the primary reference for the distribution of Quaternary basalts, Frizon de Lamotte et al. (2009) do not provide cross sections to demonstrate shallowly dipping thrust faults, Sebrier et al. (2006) do not provide regional tomography models, and Beauchamp et al. (1999) do not suggest a mantle thermal anomaly as the source of volcanism in the Middle Atlas. Although these citations by Miller and Becker do not alter the main point of their paper, we wish to raise a word of caution for the persistence of incorrect citations.