Antonio Azor

Tectonic evolution of the boundary between the Central Iberian and Ossa-Morena zones (Variscan belt, southwest Spain)

Three tectonometamorphic units can be differentiated in the boundary between the Central Iberian and Ossa-Morena zones of the Iberian Massif (Variscan belt). The three units have been named the Northern Unit, Central Unit, and Southern Unit. The Northern Unit corresponds to the border of the Central Iberian Zone; it evolved at low-temperature and intermediate- or low-pressure metamorphic conditions and was affected by top to the SE ductile shearing. The Central Unit, placed under the Northern Unit, preserves high-pressure Silurian eclogitic assemblages retrograded by high- to medium-temperature and, finally, low-temperature ductile shearing with top to the NW sense of movement (oblique left-lateral). The Central Unit is superposed on the Southern Unit. The latter corresponds to the border of the Ossa-Morena Zone and underwent low-pressure, intermediate-temperature metamorphism synchronous with right-lateral (top to the SE) ductile shearing. The envisaged tectonic evolution is as follows: after a stage of lower Paleozoic rifting, subduction of the Central Unit under the Northern Unit took place in Silurian times. As a result of the crustal thickening, a gravitational instability developed, giving way to left-lateral extensional shearing that affected the entire Central Unit. The combined action of oblique thrusting at the front of the wedge and oblique extensional shearing at the rear caused the exhumation of the high-pressure rocks of the Central Unit. This tectonic evolution reveals that the boundary between the Central Iberian and Ossa-Morena zones is a suture of the Variscan belt.

Geomorphic indicators of active fold growth: South Mountain–Oak Ridge anticline, Ventura basin, southern California

South Mountain–Oak Ridge, near Ventura, California, is an asymmetric anticlinal uplift forming at the present time above the active, buried Oak Ridge reverse fault. Shortening along the Oak Ridge fault accumulated largely in Quaternary time and is responsible for the growth and present topography of the westernmost 15 km of the ridge during the past 0.5 m.y. Tectonic geomorphic analysis using several indices of active tectonics provides information concerning fold growth. Stream-gradient indices are relatively high in the northern, eroded fold scarp of the ridge, a pattern consistent with the existence of active, rapid slip on the Oak Ridge fault. Mountain-front sinuosity along the northern slope of the anticlinal ridge roughly decreases from ∼2 to 1 toward the westernmost 10 km of observed surface folding. Valley floor width to valley height ratios along the northern flank of the ridge generally decrease westward from ∼1.5 to 0.5. Values of the hypsometric integral along the northern flank increase significantly from ∼0.35 to 0.4 (maximum ∼0.55) from east to west. Drainage density varies from ∼4 to 6 km/km2 along both flanks of the South Mountain–Oak Ridge anticline. Entrenchment of streams into the (southern) backlimb of the fold along the westernmost 9 km of the structure decreases from ∼20 m to <1 m from east to west. Apparent backlimb rotation, as measured by dip of strata along the westernmost 7 km of the fold, decreases from east to west, from ∼35° to 20°. Fold growth of the South Mountain–Oak Ridge anticline occurred during the past 0.5 Ma following deposition of the Saugus Formation. Lateral and vertical fold growth were likely produced by westward decrease in fault slip along the buried Oak Ridge fault.

The structure of a major suture zone in the SW Iberian Massif: the Ossa-Morena/Central Iberian contact

Abstract We have investigated the stratigraphy, structure and metamorphism of the boundary between the Ossa Morena Zone (OMZ) and the Central Iberian Zone (CIZ), two significant continental portions of the Variscan Iberian Massif. The OMZ/CIZ contact is marked by a strongly deformed and metamorphosed NW–SE trending narrow band, namely, the Central Unit, in which partially retrogressed eclogites are included. During the Middle-Late Devonian the CIZ overthrust the OMZ, and in the footwall km-scale recumbent folds and thrusts developed with decoupling and underplating of the lower crust. At the same time, in the hanging wall there took place intense though localized back-folding and back-shearing. In the Early Carboniferous a transtensional tectonic regime sank the overthrust block resulting in the exhumation of eclogites. These eclogites probably came from the underthrust OMZ lower crust, and they are at present included in the suture zone (Central Unit) of this continental collision. The extension is responsible for the origin of a basin and bimodal magmatism on the southern border of the CIZ. A late episode of folding and fracturing significantly contributed to the final complex picture of this suture boundary.

CalHypso: An ArcGIS extension to calculate hypsometric curves and their statistical moments. Applications to drainage basin analysis in SE Spain

We present the GIS extension CalHypso to automatically extract multiple hypsometric curves from digital elevation models (DEM). This extension is programmed in Visual Basic 6 and uses the ArcObjects architecture of ArcGIS. It employs a new and easy methodology to extract the hypsometric curve by using the integer data-model properties of ArcGIS and summations algorithms. This provides an optimal integration within the program environment, allowing the representation and comparison of curves directly within the GIS main program. The CalHypso extension can also calculate the main statistics related to the hypsometric curve by applying polynomial fits. We have tested this tool in several basins of the eastern border of the Sierra-Nevada dome in the Betic Cordillera (SE Spain), showing important differences in the north- and south-slope hypsometric curves. These variations are probably related to the tectonically controlled drainage evolution of the southern border of Sierra Nevada. An eastward decrease in tectonic-driven erosion is also suggested by the hypsometric curves analyzed.

The amphibolites from the Ossa–Morena/Central Iberian Variscan suture (Southwestern Iberian Massif): geochemistry and tectonic interpretation

Abstract Basic rocks included in the Ossa–Morena/Central Iberian Variscan suture of SW Iberia have been studied in order to decipher the pre-orogenic evolution along this major boundary. These rocks appear as garnet-free amphibolites and garnet-bearing amphibolites. Previous geochronological data on these basic rocks suggest the existence of two different protolith ages: Upper Precambrian and Lower Palaeozoic (Ordovician). Upper Precambrian amphibolites form small-size (up to several metres thick), lense-, dike- or elongated-shaped bodies, while Lower Palaeozoic ones form elongated bodies with thicknesses up to several hectometres. The chemical compositions of the amphibolites enable us to separate cumulate rocks from those representing more basaltic magmas. As for the originally basaltic rocks, their geochemical characteristics indicate a shallow melting with different proportions of at least three end-members, namely (i) a MORB-like asthenospheric source, (ii) a more enriched (plume-derived or subcontinental lithosphere) mantle source, and (iii) a continental crust component. Moreover, Upper Precambrian and Lower Palaeozoic amphibolites have different geochemical affinities. In the former group, we envisage an advanced intra-continental rifting situation or an intra-continental back-arc setting related to the Cadomian orogeny, while for the latter, we propose an environment of oceanic crust formation predating Variscan subduction and collision. The recognition of these remains of oceanic-affinity basic rocks along the OMZ/CIZ boundary allows us to propose that some sort of ocean may well have existed between the two zones during Early Palaeozoic times, and that this ocean was presumably closed at a later stage, probably during the Devonian, by oceanic subduction under the CIZ crust.

Opposite subduction polarities connected by transform faults in the Iberian Massif and western European Variscides

The Iberian Massif has been divided into two branches of opposite vergence, thus obtaining the image of a cylindrical bivergent orogen. However, we show that the architecture of the Iberian Massif is not cylindrical, the two branches representing opposite subduction polarities connected by a transform fault. The northern branch has suture units in the allochthonous Galicia Tras-os-Montes zone; its Variscan evolution includes west-dipping subduction, obduction of oceanic-derived units and eastward propagation of the deformation. The southern branch shows southwest vergence and two sutures: (1) the boundary South Portuguese zone-Ossa-Morena zone, and (2) the boundary Ossa-Morena zone-Central Iberian zone. The Central Iberian zone is the footwall of the allochthonous terranes of the Galicia Tras-os-Montes zone in the northern branch, but it is the hanging wall of the suture-related rocks in the southern branch. The change in subduction polarity is explained by the existence of a transform fault connecting both domains, presently represented by the Porto-Tomar fault. The correlation of the Iberian sutures with other Variscan sutures in Europe has a main difficulty in the stratigraphic and paleontological similarities between the Armorican domain and the Central Iberian zone, which prevent the Massif Central ocean and the Galicia ocean to be considered as continuous. Instead, another transform would have connected these two oceanic domains. The closure of such oceanic domains bounded by transform faults, followed by rotation and escape tectonics in opposite directions, generated the Ibero-Armorican arc.

Phase diagram sections applied to amphibolites: a case study from the Ossa-Morena/Central Iberian Variscan suture (Southwestern Iberian Massif)

Abstract Amphibolites included in the Ossa-Morena/Central Iberian suture of SW Iberia offer the opportunity of investigating the subduction and collision events, followed by exhumation, which affected this major boundary during the Variscan orogeny. This suture contact is marked by a band of highly deformed and metamorphosed rocks, namely, the Central Unit. The main deformation affecting this unit is a ductile shearing that produced an intense planar–linear fabric. This shearing has two components: a dominant left lateral one and a subordinate dip-slip normal one, which can be taken to be responsible for the exhumation of this unit in Devonian–Carboniferous times. The rocks studied in the present paper are garnet-free amphibolites and garnet-bearing amphibolites intercalated between gneisses. They appear as lens-, dike- or elongated-shaped bodies, their thickness ranging between a metre and several hectometres. The investigation of their metamorphic evolution has been carried out by computing phase diagram sections (pseudosections) for specified bulk rock compositions. This method enables us to achieve a twofold goal: (i) to model the P – T path undergone by the rocks during the metamorphism in question, and (ii) to assess the influence of the fluid phase abundance on the formation and the preservation of the mineral assemblages as well as on the density decrease of the rocks during exhumation. The metabasites of the Central Unit underwent a metamorphic evolution, characterized by an initial eclogite facies event with peak pressures around 1.9 GPa and temperatures about 550 °C, during which grossular-rich garnet and jadeite-rich clinopyroxene coexisted with chlorite, glaucophane, and paragonite. The former two hydrous minerals disappeared during the subsequent stage, characterised by a slight decompression with strong heating. At the peak temperature (≈725 °C and 1.6 GPa), important amounts of water were released owing to paragonite breakdown. This water produced (i) the formation of large amounts of amphibole coexisting with jadeite-rich clinopyroxene and sodium-rich plagioclase, and (ii) a sharp density decrease of the rocks, which must have contributed to their exhumation. Fast uplift is consistent with the deduced sharp isothermal decompression (1.18 GPa). This was, in turn, followed by simultaneous strong cooling and decompression (≈455 °C and 0.3 GPa), which produced the calcium enrichment observed in plagioclase and the jadeite decrease and diopside enrichment in clinopyroxene. This final retrograde evolution occurred at fluid absent conditions, thus allowing the preservation of previously formed high-pressure assemblages.

Imaging the crustal structure of the Central Iberian Zone (Variscan Belt): The ALCUDIA deep seismic reflection transect

ALCUDIA is a 230 km long, vertical incidence deep seismic reflection transect acquired in spring 2007 across the southern Central Iberian Zone (part of the pre-Mesozoic Gondwana paleocontinent) of the Variscan Orogen of Spain. The carefully designed acquisition parameters resulted in a 20 s TWTT deep, 60–90 fold, high-resolution seismic reflection transect. The processed image shows a weakly reflective upper crust (the scarce reflectivity matching structures identified at surface), a thick, highly reflective and laminated lower crust, and a flat Moho located at 10 s TWTT (30 km depth). The transect can be divided into three segments with different structural styles in the lower crust. In the central segment, the lower crust is imaged by regular, horizontal and parallel reflectors, whereas in the northern and southern segments it displays oblique reflectors interpreted as an important thrust (north) and tectonic wedging involving the mantle (south). The ALCUDIA seismic image shows that in an intracontinental orogenic crust, far from the suture zones, the upper and lower crust may react differently to shortening in different sectors, which is taken as evidence for decoupling. The interpreted structures, as deduced from surface geology and the seismic image, show that deformation was distributed homogeneously in the upper crust, whereas it was concentrated in wedge/thrust structures at specific sectors in the lower crust. The seismic image also shows the location of late Variscan faults in spatial association with the lower crustal thickened areas.

Comparative review of the Variscan granitoids of Morocco and Iberia: proposal of a broad zonation

The present paper takes the Variscan granitoids of Morocco as the starting point for the discussion of broad correlations of magmatism along the Variscan orogenic belt. Remarkable differences stand out between the Western Moroccan Meseta (peraluminous granites) and the Eastern Moroccan Meseta (potassic to shoshonitic, calc-alkaline granitoids). In Iberia, the Variscan magmatism can be arranged into distinctive zones too, which have been compared with those in Morocco. Although two of these zones mimic the I-type and S-type subduction paired belts, the intrusion of the granitoids was mostly late-collisional. Calc-alkaline, metaluminous (I-type) potassic granitoids of the southern zone may reflect a source of enriched mantle (such an enrichment taking place during Devonian Variscan subduction or latest Precambrian subduction) and different degrees of crustal contamination. Considerable volumes of peraluminous S-type granites in the central magmatic zone reflect a widespread melting of fertile levels in the crust. The third magmatic zone is characterized by the precocity (Middle Devonian to Early Carboniferous) of its magmatism with respect to the two other zones and the abundance of basic rocks. For the latter zone, we suggest that most of the magmas were generated in the context of an intra-orogenic continental rifting, while others may be subduction-related.

Transpressional collision tectonics and mantle plume dynamics: the Variscides of southwestern Iberia

Abstract In southwestern Iberia, three continental domains (the South Portuguese Zone (SPZ), Ossa-Morena Zone (OMZ) and Central Iberian Zone (CIZ) collided in Devonian-Carboniferous time. The collision was transpressional, with left-lateral kinematics, and was interrupted by extensional tectonics during the earliest Carboniferous, when bimodal magmatism (with associated mineral deposits) and basin development were the dominant orogenic features. Transpression was renewed in Visean time, and persisted until the end of the Carboniferous. The IBERSEIS deep seismic reflection profile helps to define the 3D geometry of transpressional structures: out-of-section displacements concentrate in bands, which bound wedges of upper crust; this crustal wedging strongly modifies the geometry of the sutures between continental blocks. A mid-crustal strongly reflective thick band (the Iberseis Reflective Body, IRB) is interpreted as a huge body of basic rocks. The IRB magma trapped in the middle crust was linked to the Early Carboniferous mantle-derived magmatism that crops out in the SPZ, OMZ and CIZ. Magmatism at the surface and trapped in the crust, high thermal gradients and basin development reflect a thermal anomaly in the underlying mantle, influencing both the thermal and the stress state of the orogen at that time. A mantle plume is inferred to have existed in the Early Carboniferous, the transpressional tectonic regime dominating again after its decay.