Nevado-Filabride Complex Research Articles

Using U-Pb detrital zircon systematics to constrain the architecture and provenance of the Ronda peridotites’ crustal hanging wall, S Spain

Detrital zircon geochronology is frequently used for sediment provenance analysis in orogenic belts, but it can also be applied to locate the boundary between basement and sedimentary cover in successions and tectonometamorphic units where the paleontological record is missing, and primary stratigraphic relationships have been erased by intense deformation and/or metamorphism. This is critical to differentiate metamorphic mineral associations, P-T conditions and deformational events associated with superposed orogenic cycles. In this work, detrital zircon geochronology has been applied to the Ronda peridotites’ crustal hanging wall (hinterland of the Alpine Betic Cordillera, southern Spain), apparently made up of two major tectonic units – the Jubrique unit and the Maláguide complex – separated by a major Alpine tectonic contact camouflaged in between low-grade metamorphic rocks.Our results show that pre-Paleozoic peaks (c. 650, 1000, 2000, and 2600 Ma) are common along the entire studied crustal sequence, and Paleozoic ages are only recorded in the upper part of the Jubrique unit (c. 290, 350, and 475 Ma) and in the Maláguide complex (c. 350 and 475 Ma). Thus, the Paleozoic rocks of the Maláguide complex, constituting the highest crustal unit overlying the Ronda peridotites, represent syn-to-late orogenic Variscan sediments deformed and slightly metamorphosed prior to the Alpine cycle. In contrast, the underlying Jubrique unit is made of a post-Variscan sedimentary cover deposited on a Variscan basement, with the contact between both (hidden unconformity?) located within low- to medium-grade schists. These data support the presence of a major tectonic contact between the Maláguide complex (Late Carboniferous maximum depositional ages) and the Jubrique unit (Early Permian maximum depositional ages), i.e. the Maláguide complex would have overthrust the Jubrique unit during the Alpine orogeny. These results indicate a Permian or younger exhumation/emplacement up to crustal levels of the Ronda peridotites subcontinental mantle slab.Altogether, these detrital zircon age spectra support a relatively, but not entirely local derivation of the studied crustal sequences (fragments of the Iberian Massif) and suggest some Mesozoic drifting with respect to its present position. Nonetheless, the different age distribution of the Paleozoic populations between the Jubrique (Alpujárride complex) and Maláguide samples suggests that the former deposited relatively close to other samples from the eastern-central Alpujárride complex and Nevado-Filábride complex, which, in turn, show strong similarities with the Cantabrian zone and NE Iberia (Iberian Massif). In contrast, the Maláguide samples were probably deposited close to the Ossa-Morena Zone (Iberian Massif) . Thus, Alpujárride/Nevado-Filábride and Maláguide complexes would have been juxtaposed during the Triassic-Cretaceous Neotethys rifting/drifting and/or the subsequent Alpine collisional events.

Refined Tectonic Evolution of the Betic‐Rif Orogen Through Integrated 3‐D Microstructural Analysis and Sm‐Nd Dating of Garnet Porphyroblasts

AbstractHigh‐resolution microstructural analysis of porphyroblast inclusion trails integrated with Sm‐Nd garnet geochronology has provided new insight into the tectonic history of the Betic‐Rif orogen. Three principal age groups of porphyroblasts are demonstrated with distinctly oriented inclusion‐trails. Inclusion‐trail curvature axes or “FIA” (Foliation Inflexion/Intersection Axes) are shown to represent “fossilized” crenulation axes from which a succession of different crustal shortening directions can be deduced. The regional consistency of microstructural orientations and their geometric relationship with multiple sets of macroscopic folds reveal the composite character of the Gibraltar Arc formed by a superposition of different folding directions and associated lineations. Bulk‐garnet ages of 35–22 Ma obtained from five micaschist samples of the Alpujarride‐Sebtide complex (ASC) and of 35–13 Ma from four micaschists of the Nevado‐Filabride complex (NFC) allow to deduce NNE‐SSW directed shortening in the Late Eocene changing to NW‐SE shortening in the early Oligocene, alternating with suborthogonal NE‐SW shortening during the Miocene. These directions can be related to a major swing in the direction of relative Africa‐Iberia plate‐motion known from kinematic modeling of magnetic seafloor anomalies, and subsequent dynamic interference between plate convergence and suborthogonal “tectonic escape” of the Alboran Domain. Coupled to previously established P‐T‐t paths, the new garnet ages support a common tectono‐metamorphic evolution of the ASC and NFC as laterally equivalent orogenic domains until, in the Miocene, the second became re‐buried under the first.

SHRIMP U–Pb detrital zircon dating to check subdivisions in metamorphic complexes: a case of study in the Nevado–Filábride complex (Betic Cordillera, Spain)

U–Pb dating on inherited detrital zircons has been applied to obtain the probable maximum age of deposition of the detrital protolith of the Nevado–Filabride complex (Betic Cordillera, Spain). Five of eight samples correspond to the lower part of the lithologic sequence of this complex, where radiometric dating of metasediments has not been presented till the present. The youngest age populations in the majority of samples are Carboniferous. The estimation of the maximum age of deposition in the lower and upper units is 349.1 ± 1.6 and 334.6 ± 2.9 Ma, respectively. In addition, samples show common age populations at ca. 490–630 and ca. 910–1010 Ma. Observations agree with the Carboniferous to early Permian U–Pb ages previously obtained in orthogneisses levels which are situated in the upper part of the complex. Combination of the minimum age of deposition deducible from the orthogneisses studies and the maximum ages of deposition obtained from the detrital zircons of this work, allow establishing the deposition of de studied lithological succession comprised between ca. 282 and 349 Ma or a shorter period.

A STEP fault in Central Betics, associated with lateral lithospheric tearing at the northern edge of the Gibraltar arc subduction system

We study the crustal and lithospheric mantle structure under central Betics in the westernmost Mediterranean region by migrating P-receiver functions along a dense seismic profile (∼2 km interstation distance). The profile, North–South oriented, probes the crustal structure of different geological units, from the Alboran domain in the south with metamorphic rocks, through the External Zones with sedimentary rocks to the Variscan terrains of the Iberian Massif in the north. From north to south, the Moho depth increases from ∼30 km to ∼46 km underneath the Guadix basin, due to the underthrusting of the Iberian crust below the Alboran crust, and suddenly shallows to ∼30 km underneath the Internal Zones with a step of 17 km. This sharp Moho step correlates well with a lithospheric step of ∼40 km, where the thickness of the lithosphere changes abruptly from ∼100 km in the north to ∼50 km in the south. We interpret this sharp and prominent lithospheric step as the termination of the Iberian lithosphere caused by a near-vertical STEP (Subduction-Transform-Edge-Propagator) fault that continues towards the surface as a positive flower tectonic structure of crustal scale. This STEP fault is located at the northern edge of the narrow Westernmost Mediterranean subduction system facilitating the slab rollback motion towards the west. The sharp termination of the Iberian lithosphere occurs under the contact between the Alpujarride and the Nevado-Filabride complexes of the Alboran domain in an ENE-WSW right-lateral transpressive shear zone. The thickest crust and lithosphere do not correlate with the highest topography along the profile suggesting that this high topography is a combined effect of the positive flower structure, and the push up of the asthenosphere produced by the removal of the Iberian lithosphere.

Transposition of foliations and superposition of lineations during polyphase deformation in the Nevado-Filabride complex: tectonic implications

Detailed structural analysis in a ca. 80 km2 area of the western Nevado-Filabride complex (Betic Cordillera) reveals a heterogeneous internal structure characterized by multiple cross-cutting foliations and lineations that locally transpose earlier ones. The large-scale geometry of these fabrics conflicts with continuous westward to south-westward tectonic transport related to thrusting or crustal extension, and mismatches a previously inferred extensional detachment in the area. Multiple crenulation lineations can be distinguished in the field and correlated with five foliation intersection axes (FIA1–5) preserved in garnet and plagioclase porphyroblasts of the western Sierra Nevada. These indicate crustal shortening in different directions associated with vertical foliation development and intermitted stages of gravitational collapse producing horizontal foliations. The large spread of lineation- and fold-axes trends in the Nevado-Filabride complex results from the mixed presence of multiple generations of these structures whose distinction is critical for tectonic models. The five principal FIA trends remarkably match successive vectors of relative Africa-Iberia plate motion since the Eocene, suggesting that deformation of the Nevado-Filabride took place during this period, although peak metamorphism in at least some of its parts was reached as late as the Middle Miocene.

Early Miocene subduction in the western Mediterranean: Constraints from Rb-Sr multimineral isochron geochronology

The Betic Cordillera of southern Spain is a complex orogen formed in the context of convergence between Africa and Iberia from the Mesozoic to the present. The internal zone of the orogen includes three tectonic complexes, two of which have been subducted to high-pressure conditions, then exhumed back to the surface during subsequent extension. Subduction in the structurally lower complex, known as the Nevado-Filabride Complex (NFC), has been a topic of debate for several years due to conflicting geochronological data. Here we use multimineral isochron 87Rb/86Sr dating on carefully selected mineral samples from high-pressure metamorphic rocks in the NFC to better constrain the timing of high-pressure metamorphism and subduction in the region. Out of five samples analyzed, statistically valid multimineral isochrons were obtained for one eclogite and two schists, yielding ages of 20.1 ± 1.1, 16.0 ± 0.3, and 13.3 ± 1.3 Ma, respectively. Despite that the other two eclogite samples appeared to preserve prograde mineral assemblages, low 87Rb/86Sr ratios in white mica precluded precise age calculations. These new ages are in close agreement with previously published Lu-Hf ages on garnet and U-Pb ages on metamorphic zircon overgrowths for the same rocks, but are substantially younger than published data from the 40Ar/39Ar technique. Combined with recently published tomographic images of slab structure beneath the Alboran Sea, the new ages support a tectonic model in which subduction occurred both prior to the Miocene and during the early to mid-Miocene, but that it was punctuated in time by a pulse of extensional exhumation in the early Miocene associated with lithospheric delamination and/or slab tearing.

Petrology and Thermobarometric Estimates For Metasediments, Orthogneisses, and Eclogites From the Nevado-Filábride Complex In the Western Sierra Nevada (Betic Cordillera, Spain)

Using PERPLE_X and garnet isopleth thermobarometry we tested whether Permian metasediments, orthogneisses, and amphibolitized eclogites from the Nevado-Filabride Complex in the western Sierra Nevada (Betics, Spain) underwent the same high-pressure metamorphism, or whether the different lithologies were tectonically emplaced after metamorphism as is commonly assumed. Our results highlight the role of some accessory minerals, especially small inclusions of Ti-bearing phases (mainly titanite and rutile) in garnet, in the thermobarometric estimates and indicate the presence of several populations of garnets in the orthogneisses—recording different stages of the prograde and retrogressive Alpine metamorphic path—in addition to pre-Alpine garnets. The schists also contain two populations of garnet: rare detrital grains and metamorphic crystals. Despite the scarcity of key phases proving that the orthogneisses and metasediments experienced high-pressure conditions, this study yields a number of lines of evidence suggesting that both rock types followed a clockwise pressure-temperature path similar to the eclogites, reaching peak pressures of 2.0–2.2 GPa for temperatures of 550–600 °C, and that the lithologies were all in contact prior to the high-pressure Alpine event.

Continental subduction, intracrustal shortening, and coeval upper-crustal extension: P-T evolution of subducted south Iberian paleomargin metapelites (Betics, SE Spain)

The Nevado–Filabride complex represents the basement and Permo-Triassic cover of the south Iberian passive margin, metamorphosed during Miocene continental subduction under the Alboran crustal domain. It forms a nappe stack with the thick Calar-Alto unit (6km thick) sandwiched between the Ragua unit below and the Bédar-Macael unit above. The nappe stacking occurred by ductile flow along basal syn-metamorphic shear zones. Structural analysis of the Calar-Alto unit together with thermobarometric data of samples from both the bottom and top of the unit served us to understand the tectonic significance of this episode of intracrustal shortening in the context of a strongly extended orogen. Multiequilibrium thermobarometry shows that the Calar-Alto schist underwent a prograde P-T path at mantle depths from 1.6GPa at 450–500°C to 1.0GPa at 550–570°C during the growth of the schistosity (S1), parallel to the compositional layering. After detachment from the downgoing slab and underplating, the unit underwent nearly isobaric cooling by up to 250°C from peak metamorphic conditions. The main crenulation cleavage (S2) and the mylonitic foliation (Sm) developed within the crustal nappe stack during decompression and heating. Sm at the base of the unit grew between 0.8GPa at 400°C and 0.4GPa at 500°C, while S2 at the top of the unit developed between 0.6GPa at 300°C and 0.2GPa at 400°C. These shortening structures developed simultaneously to the activity of overlying middle Miocene extensional detachments. Intracrustal shortening and coeval upper-crustal extension resulted in a mode of exhumation where the material rise entailed an important horizontal component of displacement at rates of approximately 15mm/year, being exhumed along an S-shaped P-T path after peak metamorphic conditions and its detachment from the subducting slab.

SURVIVING METAMORPHISM: TAPHONOMY OF FOSSIL ASSEMBLAGES IN MARBLE AND CALC-SILICATE SCHIST

Devonian massive graphitic marble and calc-silicate schist, belonging to the Lower Formation of the Nevado-Filabride Complex of the Internal Zones of the Betic Cordillera, crop out extensively near Aguilas (Murcia, SE Spain). These rocks contain a rich fossil record even though they have undergone Alpine metamorphism and deformation (350–480°C; 2 Kb). In this paper, we focus on the taphonomic characteristics of the fossil assemblages to infer the depositional processes that can be reconstructed even after metamorphism. The most abundant fossils are crinoids, followed by phacelloid colonial rugose corals, brachiopods, cephalopods (ammonoids and orthoceratids), and possible gastropods and benthic foraminifers. Crinoids occur as isolated columnal ossicles, as well as articulated portions of columns (pluricolumnals). One complete calyx was also found. Taphonomically delicate crinoids are preserved in the calc-silicate schist beds. In the massive marble beds, crinoids occur mostly as disarticulated ossicles, but they still bear delicate ornamental features. Some corals preserve external walls. The low degree of abrasion indicates disarticulation of the crinoids due to exposure in the taphonomic active zone (TAZ), but under low-energy conditions. Articulated crinoids are preserved due to catastrophic burial events. The preservation of the fauna allows the differentiation of background versus event biostratinomic processes even after being highly altered by metamorphism.

Evaluation of a combined HRXCT/EBSD method for detecting epitaxial nucleation of garnet porphyroblasts

The combination of high-resolution X-ray computed tomography (HRXCT) with electron backscattered diffraction (EBSD) analysis has the potential to reveal evidence for epitaxial nucleation of garnet porphyroblasts containing an included fabric. HRXCT measures the fabric orientation near the inferred nucleation site at the crystal centre, whereas EBSD establishes the crystallographic orientation of the porphyroblast relative to the attitude of the included fabric. Using the orientation of the central fabric as a proxy for the attitudes of the basal planes of micas on which the garnet may have nucleated, one can determine if low-index planes in garnet—(100), (110), or (111)—are parallel to (001) of mica, which would correspond to known epitaxial relationships between the two minerals. We test the feasibility of this approach with a set of exploratory measurements for six garnet porphyroblasts from Passo del Sole (Switzerland) and five from the Nevado Filabride Complex (Spain); we also report unsuccessful attempts at such measurements in rocks from Harpswell Neck (USA). The results do not deviate appreciably from those expected of a set of measurements drawn from a parent population with a random distribution of orientations. However, considering the size of the present measurement uncertainties in relation to the precision required to recognize an epitaxial relationship unambiguously, it is not possible in this data set to exclude epitaxy as a nucleation mechanism. The principal limitation to this approach is the need for high precision in determining the attitude of the included fabric at the site of nucleation, which depends largely upon the choice of suitable samples. Crystals ideal for such studies would contain a non-crenulated, planar (not helicitic) foliation, defined by numerous small, well-aligned inclusions. With careful sample selection, this approach could be a viable technique for evaluating the prevalence in nature of epitaxial nucleation of garnet porphyroblasts.

Heterogeneous extension and the role of transfer faults in the development of the southeastern Betic basins (SE Spain)

Large strike-slip faults in the eastern Betics are interpreted to have developed in a transcurrent setting in response to 4–6 mm/yr of Africa-Iberia NW-SE convergence. However, here we show that some of these faults are transfer faults accommodating heterogeneous late Miocene extension. The North Cabrera dextral fault and other E-W to NE-SW strike-slip faults in the Sorbas basin were transfer faults produced under SW-NE extension. These faults together with related normal faults form the main boundaries of two sedimentary depocenters active between the Serravallian and the Tortonian. The older North Cabrera depocenter extended between the Serravallian and the early Tortonian (approximately 13.8 to 9 Ma), while the younger Gacia depocenter formed in response to late Tortonian extension (approximately 9 to 7.5 Ma). The latter formed to the west of the North Cabrera depocenter by a listric fan of normal faults with SW directed transport that are linked by dextral and sinistral transfer fault segments. These faults root on a low-angle detachment cutting into the exhumed high-pressure Nevado-Filabride complex rocks at ~0.8 km depth. The present work reveals that (1) this extension was partially coeval with and kinematically linked to sinistral displacement along the Carboneras fault farther south in the Nijar basin; (2) this westward directed extension produced elongated core complexes and tilted blocks to the north of the Carboneras fault and magmatic accretion upon thinned continental crust to the south, probably in response to slab tearing or detachment and associated edge delamination of the Iberian continental lithospheric mantle beneath the Betics.

Borings in gneiss boulders in the Miocene (Upper Tortonian) of the Sorbas Basin, SE Spain

AbstractMarine invertebrate borings are very rare in crystalline rocks, providing evidence of particular strategies producers use to colonise these unfavourable substrates. In the Sorbas Basin (Almería, southern Spain), Upper Miocene transgressive successions contain blocks of metamorphic and igneous rocks of the Nevado–Filabride Complex of the Betic Cordillera. Ichnological analysis of the embedded blocks shows the presence of two types of macroborings located in gneiss boulders, revealed to be an extraordinary case worldwide. The most abundant are regular hemispherical depressions ascribed reservedly to the well-known, mostly bivalve boringGastrochaenolites. The second one is a pouch-like depression, tapering downward, elliptical in outline, and clearly different to other non-circular-in-outline, pouch-shaped macroborings. Thus, a new ichnogenus and ichnospeciesCuenulites sorbasensishas been defined. According to the overall shape, an endolithic or semi-endolithic bivalve using chemical means to bore is suggested as the tracemaker. Colonisation could be determined by sea-level coastal dynamics, with decreasing energy during advancing transgression allowing boring, which was then stopped due to supply of fine-grained sediment that killed the borers.

Rheological evolution of a Mediterranean subduction complex

We use field and microstructural observations, coupled to previously published P-T-time histories, to track the rheological evolution of an intracontinental subduction complex exposed in the Betic Cordillera in the western Mediterranean region. The body of rock we focus on, known as the Nevado-Filabride Complex (NFC), was originally part of the upper crust of the Iberian margin. It was subducted into hot asthenospheric mantle, then exhumed back toward the surface in two stages: an early stage of fast exhumation along the top of the subducting slab in a subduction channel, and a late stage of slower exhumation resulting from capture by a low-angle detachment fault rooted at the brittle-ductile transition. Each stage of deformation in the NFC was punctuated by changes in the dominant deformation mechanism. Deformation during initial subduction of the complex was accommodated by pressure-solution creep in the presence of a fluid phase – the grain sizes, stress magnitudes, and estimated strain rates for this stage are most consistent with a thin-film model for pressure solution in which the diffusion length scale is controlled by the grain size. During the early stages of exhumation within the subduction channel, deformation transitioned from pressure solution to dislocation creep due to increases in temperature, which resulted in increases in both water fugacity and grain size, each of which favor the dislocation creep mechanism. Differential stress magnitudes for this stage were ∼10 MPa, and are consistent with simple models of buoyancy-driven channel flow. With continuing subduction-channel exhumation, deformation remained within the dislocation creep field because sequestration of free water into hydrous, retrogressive minerals suppressed the pressure-solution mechanism. Differential stresses progressively increased to ∼100 MPa near the mouth of the channel during cooling as the rocks moved into mid-crustal levels. During the final, core-complex stage of exhumation, deformation was progressively concentrated into a narrow zone of highly localized strain beneath a mid-crustal detachment fault. Localization was promoted by a transition from dislocation creep to dislocation-creep-accommodated grain boundary sliding at temperatures of ∼350–380 °C, grain sizes of ∼4 μm and differential stress magnitudes of ∼200 MPa. Peak differential stress magnitudes of ∼200 MPa recorded just below the brittle-ductile transition are consistent with Byerlee's law in the upper crust assuming a vertical maximum principal stress and near-hydrostatic pore fluid pressures. Overall, the distribution of stress with temperature, coupled to independent constraints on strain rate from field observations and geochronology, indicate that the naturally calibrated Hirth et al. (2001) flow law for wet quartzite accurately predicts the rheological behavior of mid-crustal rocks deforming by dislocation creep.

Kinematic and thermal evolution during two‐stage exhumation of a Mediterranean subduction complex

We examine the kinematic and thermal evolution of the Nevado‐Filabride Complex (NFC), an Early to Middle Miocene subduction complex within the Betic Cordillera of southern Spain. Thermobarometry from the NFC in the Sierra Alhamilla reveals an inverted geothermal gradient, with peak temperatures and pressures of 560°C and 13 kbar reached in the uppermost tectonic unit. The NFC cooled during exhumation, defining a linear PT path that remained within the stability field of kyanite until the aluminosilicate breakdown reaction. Geo‐ and thermochronology suggest the NFC was subducted and exhumed to the surface within ∼10 m.y., recording an early stage of fast cooling from peak T through ∼250°C, followed by a late stage of slower cooling as the rocks approached the surface. The exhumation is associated with a progressive change in kinematics, with early co‐axial fabrics showing NNE‐SSW‐trending stretching lineations, and late non‐coaxial fabrics showing SSW‐trending lineations. A new tectonic model is proposed to explain these data, in which rocks at the leading edge of the Iberian margin were subducted southeastward beneath the thin, hot, Alboran lithosphere to form the NFC. The rocks at the top of the slab began to be exhumed once they reached 48–67 km depth. They were initially exhumed rapidly along the top of the subducting slab, in a subduction channel geometry; then upon reaching the middle crust, were captured by a low‐angle detachment fault with a WSW shear sense and final exhumation was accommodated more slowly in a core‐complex geometry.

Metamorphic Record of High-pressure Dehydration of Antigorite Serpentinite to Chlorite Harzburgite in a Subduction Setting (Cerro del Almirez, Nevado-Filabride Complex, Southern Spain)

Metamorphic Record of High-pressure Dehydration of Antigorite Serpentinite to Chlorite Harzburgite in a Subduction Setting (Cerro del Almirez, Nevado-Filabride Complex, Southern Spain)

Hydrological conceptual model characterisation of an abandoned mine site in semiarid climate: the Sierra de Cartagena-La Unión (SE Spain)

A comprehensive study at Sierra de Cartagena-La Union (SE Spain) abandoned mine site was carried out to characterise the regime and water quality of the groundwater system after the mine closure. The system consists of five geologic fractured blocks belonging to the Alpujarride and Nevado-Filabride complexes. The aquifer units are composed of limestone and dolostone materials. Recharge is mainly controlled by the N-130 fault system, man-made induced fractures, open-pits and underground workings. Discharge is indicated from open pit lakes by the proximal dome-shaped groundwater level contours. Aquifer natural recharge, assessed by fracture density maps and chloride mass balance, provided consistent results. The water hydrochemical facies show a marked sulphate concentration and acidic pH (average pH of 2.53-6.30). A maximum concentration of 4,100 mg/L of Zn and 40,000 mg/L of sulphate was observed in open-pit lakes. Springs present the lowest residence time and are low mineralised with an average pH of 7.6. Geochemical modelling based on the PHREEQCI code indicates water undersaturation with respect to almost all related mineral species and anoxic conditions prevail in the system. Although an adequate understanding of the regional system is provided, a further detailed hydrochemical study is necessary to assess the undergoing geochemical changes.

Geological relationships and U-Pb zircon and 40 Ar/ 39Ar tourmaline geochronology of gneisses and tourmalinites from the Nevado–Filabride complex (western Sierra Nevada, Spain): Tectonic implications

The Nevado–Filabride complex, the lowest complex in the Betic hinterland, forms a stack of Alpine nappes. The tectonic units consist of metasedimentary sequences whose ages are not well constrained. Gneiss bodies included in the sequences have been one of the few sources of geochronological data in this metamorphic complex. New radiometric data from U-Pb zircon dating on gneisses from the western Sierra Nevada confirm the presence of late Carboniferous intrusive rocks in the Betic hinterland. These results, combined with available data from the literature and a detailed structural analysis, suggest that the gneisses represent a single late Variscan magmatic event. Evidence for a close genetic relation between gneisses and tourmalinites is provided by field and petrographic observations, in conjunction with geochemical data, U-Pb zircon (314 ± 7; 304 ± 23 Ma) and 40Ar/ 39Ar tourmaline (319.85 ± 5.81; 317.85 ± 3.67 Ma) geochronology. A pre-late Carboniferous age for the basal formation of the Nevado–Filabride sequence can be inferred. The gneiss protolith and the graphite schist are considered to be the boron source and the precursor of the tourmalinites, respectively. Superposed tectonic units rather than a continuous Palaeozoic sequence is supported by the occurrence of Palaeozoic rocks at the top of the Nevado–Filabride complex. The nappe tectonics, as evidenced from the contractional character of the unit boundaries and the superposition of higher-grade on lower-grade metamorphic rocks, is additionally supported by the superposition of older on younger rocks as revealed from radiometric data.

Active tectonics in the Sierra Nevada (Betic Cordillera, SE Spain): Insights from geomorphic indexes and drainage pattern analysis

The Sierra Nevada of the central Betic Cordillera is a 3000 m-high mountain range surrounded by Neogene–Quaternary sedimentary basins, having been uplifted since Late Miocene times. The southern and western mountain fronts of the Sierra Nevada are fault-bounded, while the northern one is an unconformity between the Neogene–Quaternary sediments of the Guadix–Baza basin and the metamorphic rocks of the Nevado–Filabride complex. We have carried out a geomorphic study by examining drainage patterns and characteristics of mountain fronts in order to reveal areal variations and styles of rock uplift. Mountain front sinuosity ( S mf), area–altitude relations (hypsometric curves), and valley floor entrenchment differ significantly between the northern, western, and southern mountain fronts. The lack of important faults along the northern Sierra Nevada mountain front, together with the elevated topographic position of the Guadix–Baza basin (average altitude is around 1100 m), points to similar uplift of both geomorphic units (sierra and basin) in a single large-scale crustal block. The asymmetry factors show systematic asymmetries at both sides of the Lanjarón River, probably due to the presence of an active NNE–SSW oriented antiform in the western Sierra Nevada. Finally, river profiles indicate maximal river entrenchment in the western part of the Sierra Nevada, probably related to the uplift of the footwall of the Padul–Nigüelas fault-system. Therefore, our geomorphic analysis suggests that the western part of the Sierra Nevada is tectonically active by means of a combination of normal faults along the mountain front and NNE–SSW oriented active folds, which, in turn, likely have a gravitational origin related to the exhumation of the footwall of the normal fault-system.

An experimental investigation of antigorite dehydration in natural silica-enriched serpentinite

Piston cylinder experiments were performed to constrain the pressure and temperature conditions for two high-pressure antigorite dehydration reactions found in silica-enriched serpentinites from Cerro del Almirez (Nevado–Filabride Complex, Betic Cordillera, southern Spain). At 630–660°C and pressures greater than 1.6 GPa, antigorite first reacts with talc to form orthopyroxene ± chlorite + fluid. We show that orthopyroxene + antigorite is restricted to high-pressure metamorphism of silica-enriched serpentinite. This uncommon assemblage is helpful in constraining metamorphic conditions in cold subduction environments, where antigorite serpentinites have no diagnostic assemblages over a large pressure and temperature range. The second dehydration reaction leads to the breakdown of antigorite to olivine + orthopyroxene + chlorite + fluid. The maximum stability of antigorite is found at 680°C at 1.9 GPa, which also corresponds to the maximum pressure limit for tremolite coexisting with olivine + orthopyroxene. The high aluminium (3.70 wt% Al2O3) and chromium contents (0.59 wt% Cr2O3) of antigorite in the investigated starting material is responsible for the expansion of the serpentinite stability to 60–70°C higher temperatures at 1.8 GPa than the antigorite stability calculated in the Al-free system. The antigorite from our study has the highest Al–Cr contents among all experimental studies and therefore likely constraints the maximum stability of antigorite in natural systems. Comparison of experimental results with olivine–orthopyroxene–chlorite–tremolite assemblages outcropping in Cerro del Almirez indicates that peak metamorphic conditions were 680–710°C and 1.6–1.9 GPa.

From Adria- to Africa-driven orogenesis: Evidence from porphyroblasts in the Betic Cordillera, Spain

In order to unravel the kinematic evolution of the Betic–Rif orogen, we investigated the geometry of porphyroblast inclusion trails in the Nevado-Filabride Complex, a high-pressure unit situated at the base of the Betic nappe stack. A large number of quantitative orientation data for these microstructures, and their mutual overprinting relationships, reveal two age groups of inclusion trails in the study area with distinctive geographic trends. Coupled to a comprehensive compilation of field data in the study area, these microstructures are shown to coherently witness the development of early NE–SW trending structures and fabrics, followed by younger NW–SE trending ones, whereby the former experienced clockwise reorientations. A major change in the direction of bulk crustal shortening from NW–SE to NE–SW is inferred from these data, and tentatively linked to the plate-tectonic history of the Mediterranean region. Paleomagnetic data indicate that, prior to about 35 Ma, NW–SE convergence between Adria, Eurasia and Iberia produced orogenesis along a belt bordering “Greater Iberia” (including Sardinian and Corsica) and probably continuous in the Alps (Briançonnais domain). The roughly NE–SW trend of this precursor appears to be preserved within porphyroblasts that grew during this period, presumably due to the partitioning of deformation around them. Africa–Iberia relative motion had been mainly transcurrent until 35 Ma (Azores transform), but then rapidly changed to head-on NNE–SSW convergence. This gave rise to a second set of NW–SE trending structures, which accompanied deformation of the precursor orogen into several oroclines (Western Alps, Gibraltar and Calabria Arcs). Orogenic collapse and crustal extension initiated shortly after this transition in tectonic regime suggesting a causal relationship. We propose that the waning of compressive stresses transmitted by Adria and their substitution by a near-perpendicular maximum stress direction resulting from Africa–Eurasia convergence could have enhanced the sinking of a thick lithospheric root or roll-back of a subducted slab, which in turn would have provoked crustal extension and basin formation.