J.M. Tubía

Eclogites of the Ojén nappe: a record of subduction in the Alpujárride complex (Betic Cordilleras, southern Spain)

Eclogites interlayered with amphibolites and migmatites occur in an upper unit (OjCn nappe) of the Alpujarride complex (Betic Cor- dilleras). Estimates of P-T conditions reveal a retrograde meta-morphic path from an edogitic stage at >15 kbar and c. 730 °C, to amphibolite-facies conditions at 5-8 kbar or less and c. 650 °C. The Ronda peridotites belonging to the uppermost Alpujarride nappe (Los Reales nappe) structurally overlie the eclogite-bearing unit. It is suggested that eclogitic metamorphism and overthrusting of the peridotites reflect dfierent stages of the same convergent episode.

Sequential development of the metamorphic aureole beneath the Ronda peridotites and its bearing on the tectonic evolution of the Betic Cordillera

Abstract At Sierra Alpujata (western Betic Cordillera, Spain), the Ronda peridotites overlay a metamorphic sole of felsic composition formed during the hot overthrusting of the ultrabasic rocks over the Ojen nappe. The metamorphic sole evolved from an initial eclogite facies metamorphism ( P ≥ 1.7 GPa; T = 790°C) to low-pressure ( P ≤ 300 MPa) high-temperature ( T ∼ 600°C) conditions. The active movement zone migrated downwards in the metamorphic sole as temperatures decreased during this exhumation process: high temperatures led to strain localization along the 200-m-thick ductile shear zone that forms the uppermost level of the metamorphic sole, and brittle to ductile deformations achieved at temperatures below 500°C are concentrated in the basal contact of the metamorphic sole. The maintenance of a common kinematics, with ENE-trending stretching lineations and top-to-the-ENE shearing, along the high-temperature and the low-temperature shear zones suggests that the main part of the exhumation process was performed before the overall north-south convergence of the Iberian and African plates. The subduction of the Ojen nappe, a wedge of subcontinental lithosphere, below the orogenic lherzolites of Ronda probably gave rise to the syn-collisional exhumation of the metamorphic sole and then to the slab breakoff and sinking of the subducted lithospheric mantle.

Tectonic evolution of the Alpujárride Complex (Betic Cordillera, southern Spain)

Abstract The Alpujarride Complex, belonging to the Internal Zones of the Betic Cordillera, is an E-W elongated domain of metamorphic nappes, which extends over more than 400 km in southern Spain. The Alpujarride Complex suffered two shearing episodes defined by different kinematic and physical conditions. The first one involved ductile thrusting from the south-west towards the north-east, related to the regional metamorphism of the Alpujarride nappes. This process led to crustal thickening, related to which the hot thrusting of the Ronda peridotites occurred. The second shearing episode, directed northwards, occurred after the metamorphism. It corresponds to a deformation in the brittle-ductile transition that led to the thinning of the crust thickened in the previous episode. Previous tectonic models of the Betic Cordillera have only considered the northwards shearing, which reflect the N-S convergence of Europe and Africa from the Oligocene onwards. The present work clearly points to the existence of previous episodes of ductile thrusting towards the north-east that reflect an earlier process of continental collision and that in future should be considered in tectonic models of the Betic orogen.

The Ronda peridotites (Los Reales nappe): An example of the relationship between lithospheric thickening by oblique tectonics and late extensional deformation within the Betic Cordillera (Spain)

Abstract The Internal (or metamorphic) Zone of the Betic Cordillera (southern Spain) is a 400 km long and ENE-elongated tectonic domain characterized by a widespread record of alternating compressional and extensional processes. In the westernmost part of the Internal Zone several orogenic Iherzolite massifs—the Ronda peridotites—were also involved in such processes. The Ronda peridotites form the lower portion of the Los Reales nappe and are overlain by a metamorphic sequence which varies from granulite- to greenschist-facies conditions. The tectono-metamorphic history of the Los Reales nappe involves two main synmetamorphic steps. A first extensional event at lithospheric scale is preserved within the attenuated continental crust which overlies the Ronda peridotites. It was related to the upwelling of an asthenospheric diapir below the thinned lithosphere, and led to the development of low-pressure-high-temperature metamorphism and of ductile shear zones with N110°E-trending stretching lineations. A subsequent compressional event is represented by the hot thrusting of the Ronda peridotites from the west-southwest to the east-northeast. Finally, late extensional deformations in a dominant N-S direction developed in response to the lithospheric thickening achieved during the former compressional event. The kinematics of the synmetamorphic deformations can be integrated into a model of sinistrally oblique tectonics compatible with the overall strike-slip motion of the African and Iberian plates from Jurassic to Tertiary times, whereas the later deformations should reflect the convergence between Africa and Iberia from Tertiary time onwards.

A revised Aquitanian age for the emplacement of the Ronda peridotites (Betic Cordilleras, southern Spain)

The hot emplacement of the Ronda peridotites (Betic Cordilleras) developed a dynamothermal aureole and partial melts that led to the intrusion of granite dykes in the peridotites. Previous geochronological data place rather broad limits for this event between 22 and 19 Ma. Analyses of neocrystalline zircon rims from large zircon populations yield a U–Pb SHRIMP age of 22.3±0.7 Ma for the dynamothermal aureole formation, and intrusion ages of granite dykes between 22.6±1.8 and 21.5±3.8 support that conclusion. Therefore, these new ages provide a more robust constraint on the hot emplacement of the Ronda peridotites at middle crustal levels.

The Maláguide-Los Reales Nappe: an example of crustal thinning related to the emplacement of the Ronda peridotites (Betic Cordillera)

Abstract The Malaga region in southern Spain represents a cross-section through the continental crust and subcontinental mantle of the westernmost portion of the Internal Zone of the Betic Cordillera. Recent petrological and structural studies carried out in this region show a link between the Malaguide Complex and the underlying Los Reales Nappe, where the Ronda peridotites were incorporated as allochthonous slabs. The Ronda peridotites are covered by the metamorphic sequence of the Los Reales Nappe, which consists of acidic granulites, migmatites and schists. The tectonometamorphic evolution of these granulite facies rocks involves a first high-temperature (800°C)-high-pressure (10 kbar) metamorphism replaced by high-temperature (750°C)-low-pressure (3.5 kbar) conditions. This strong thermal event, which is also recognized in biotite-bearing schists of the Los Reales Nappe and even in the overlying Malaguide Complex, developed during an extensional deformation related to the diapiric stage of the Ronda peridotite emplacement. Stretching lineations from the mylonites related to this extensional event are N 110°E in orientation and kinematic criteria show that low-grade metamorphic rocks (Malaguide Complex) move down on to higher grade ones (Los Reales Nappe) from west to east. Consequently, we interpret that the Malaguide Complex and Los Reales Nappe form a single domain of continental crust which was thinned during the emplacement of the Ronda peridotites. Stretching lineations are subparallel to the trend of the Betic Cordillera and can be integrated in a model of lithospheric transcurrent thinning.

Granite laccolith emplacement in the Iberian arc: AMS and gravity study of the La Tojiza pluton (NW Spain)

A laccolithic geometry is inferred from geological and geophysical data for the La Tojiza granitic body emplaced in an extensional regime. Nearly circular in outcrop, this 16 km diameter pluton, one of the so-called ‘post-tectonic granodioritic plutons’ of Galicia, is located in the northern Variscan arc of Iberia, within the Mondoñedo Nappe (Western Asturian–Leonese Zone). From anisotropy of magnetic susceptibility measurements and extensive microstructural data, the magnetic fabric of this pluton has been inferred to correlate with the magmatic fabric. Magnetic foliations strike NE–SW and dip gently SE. Magnetic lineations trend NW–SE and plunge gently SE. Gravity data suggest that the pluton has the shape of a sheet with a maximum preserved thickness of 1 km and a root zone located at its eastern border. This pluton is reinterpreted as late kinematic and emplaced during an extensional tectonic regime related to the ductile Vivero normal fault. It is suggested that the emplacement of most post-tectonic plutons of this part of the Variscan orogenic belt may have been controlled by the late extensional collapse of the Iberian Massif.

Tectonic implications of the granite dyke swarm in the Ronda peridotites (Betic Cordilleras, Southern Spain)

This paper deals with the origin and structural relationships of the leucocratic dykes that intruded the Ronda peridotites. Plagiogranites are frequent in ophiolitic sequences, but similar Si-rich melts are scarce in orogenic lherzolites. The Ronda peridotites could be an excellent target in which to look for Si-rich melts of mantle origin, as they form the largest mass of orogenic lherzolites in the world and contain many granite dykes. However, petrographic and geochemical data provide evidence that the dykes come from felsic magmas originated from partial melting of the continental crust. Structural studies of the deformed and undeformed dykes reveal that they are non-Andersonian dykes that exploited pre-existing fractures. Undeformed dykes display N15°E strike, whereas the deformed ones show N50°E or N100°E trends and sinistral and dextral shear senses, respectively. These structural data point to reactivation of pre-existing fractures either as tensional cracks (undeformed dykes) or transtensional shear zones (deformed dykes) during the ENE-directed thrusting of the Ronda peridotites. Consequently, the dominant N15°E trend of the undeformed dykes can be used to infer an ENE-thrusting direction for the buried basal shear zone of the Ronda peridotites.

Structural evidence for the relationship between thrusts, extensional faults and granite intrusions in the Variscan belt of Galicia (Spain)

Abstract New structural and petrofabric data are presented from the Hombreiro pluton, a synkinematically emplaced granite located in the northern part of the Variscan belt of Spain. This pluton shows the imprint of two plastic deformations: the first is related to the motion of the Mondonedo Nappe, with a top-to-the-east shear sense, and the later one to the Vivero Fault, an extensional shear zone with a top-to-the-west motion. These deformations initiated at high temperatures, close to the granite solidus as shown by the preservation of ⋎c⋎-slip fabrics in quartz. This fact indicates that extensional faulting was activated soon after the thrusting of the Mondonedo Nappe during which the Hombreiro Granite was emplaced. We propose that the position of the Vivero Fault was controlled by a crustal instability induced by the intrusion of a number of plutons that delineate the footwall block of this fault.

Preliminary palaeomagnetic results on Oligocene–early Miocene mafic dykes from southern Spain

Abstract A preliminary palaeomagnetic study has been carried out on Oligocene–early Miocene mafic dykes intruding the Malaguide and Alpujarride complexes of the Internal Betic Zone (southern Spain). The aims of the study were to ascertain if relative perpendicular orientations (NE–SW and NW–SE) shown by different dyke families are of primary or secondary origin and to find out if palaeomagnetic rotations can be detected in that area. Ten sites with dykes showing both orientations were sampled. In addition to palaeomagnetic analysis, susceptibility-versus-temperature curves of several representative samples were measured and ore microscopic studies were carried out. Palaeomagnetic behaviour of samples was characterised by low intensities of magnetisation and alterations occurring during the demagnetisation procedure, and often also by overlapping of palaeomagnetic components. Besides an initial viscous component, two different palaeomagnetic components, named M and H were recognised. Component M is carried by pyrrothite and magnetite, and seems to record a primary palaeomagnetic direction. Component H is difficult to isolate and seems to be related to the presence of magnetite and hematite. Nine of ten studied sites offer consistent palaeomagnetic results, although in two cases statistical parameters are relatively weak. Strong clockwise rotations (approximately 90–140°) are observed. These rotations are in accordance with other block rotations observed in the Internal and External Zones of the central and western Betics, although their magnitudes are much stronger. On the other hand, both dyke families show similar rotations, independently of their original direction. Thus, their relative perpendicular orientations seem to be of primary, intrusive origin.