D. Leblanc

Role of strike-slip faults in the Betic-Rifian orogeny

Abstract A new model for the Betic-Rifian orogeny of the Western Mediterranean (Spain and North Africa) is proposed in which four strike-slip faults play an important role; the faults are not of the same age. Two faults, the left-lateral Jebha fault to the south (in Morocco and principally in the Mediterranean Sea) and the right-lateral North Betic fault (southern Spain) to the north, define the boundaries of the Alboran block (Betic and Rifian internal zones). Final movement along these faults was during the Burdigalian time. Two other faults, the left-lateral Nekor fault (North Africa) to the south of the Jebha fault and the right-lateral Crevillente fault, somewhat to the north of the North Betic fault, define a larger Alboran block (including part of the Betic and Rifian external zones) that was present during the Tortonian. The following sequence of events is proposed: 1. (a) During the Eocene and Oligocene, the African and European plates converged in a N-S sense causing the breakup and overthrusting of the Betic, Rifian and Kabyle internal zones and then the movement towards the WSW of the Alboran block by slip along the Jebha and North Betic faults. 2. (b) By the end of Burdigalian time, movement along the Jebha and North Betic faults ceased. 3. (c) With continued N-S convergence, the Nekor and Crevillente faults, which bound a larger Alboran block, were formed during the mid- and late Miocene. The Arc of Gibraltar (the zone lying between the four major faults) seems to be a result of WSW motion of a crustal block being thrust over external zones. The model proposed adds to the earlier idea that tectogenesis proceeds from the interior to the exterior of an erogenic belt. In the Betic-Rifian orogeny major strike-slip fracture zones shifted to the exterior of the orogenic belt as the orogeny progressed in order to relieve the stress caused by locking of the more internal faults.

Variscan dextral transpression in the French Pyrenees: new data from the Pic des Trois-Seigneurs granodiorite and its country rocks

Abstract New structural data from both the Trois-Seigneurs granodiorite and its country rock lead to new conclusions about the Variscan tectonic history of this part of the Pyrenees. Sedimentary rocks of the massif, that increase in metamorphic grade from north to south, are imprinted by a single major deformation responsible for both the tight, upright, E-W-trending folds with vertical to steeply N-dipping foliations and the gently W-plunging stretching lineations. Syntectonic emplacement of the Pic des Trois-Seigneurs granodiorite is demonstrated by the parallelism between structures of the pluton, as determined by magnetic fabric measurements, and the foliations and lineations imprinted in the country rocks. Large deformation of the granodiorite magma during emplacement is indicated by strong anisotropy and ubiquitous magmatic microstructures. The high thermal gradients of regional metamorphism, which is also syntectonic, are attributed to a large thermal aureole provided by deep-seated magma reservoirs. Shear-sense criteria in XZ thin sections of the metasedimentary country rocks indicate a dextral sense of the non-coaxial component of strain throughout the massif and regardless of metamorphic grade. The overall structure of the massif is concluded to have derived from a major shearing event that is equated with the D 2 main phase observed in the entire Variscan Pyrenees. As the E-W-oriented stretch and dextral shear cannot be separated from compressive structures generally considered to be characteristic of D 2 , this phase is concluded as representing regional transpression.

The maladeta granite polydiapir, Spanish Pyrenees: A detailed magnetostructural study

Abstract The Maladeta granitic complex (Spanish Central Pyrenees) has been subjected to a detailed magnetostructural study, through the systematic measurement of magnetic susceptibility and anisotropy of magnetic susceptibility at 253 regularly spaced sampling sites. Due to the dominant paramagnetic property of the granites, a first-order correlation can be derived between the susceptibility magnitude and the bulk-rock iron content, hence the petrographic nature of the rock. The Maladeta complex is shown to be composed of four principal juxtaposed plutons, each one being normally zoned, i.e. becoming more basic towards the periphery. The largest pluton, namely the Colomers unit, is itself made of a number of sub-plutons due to distinct but more-or-less coeval batches of magmas. The various magnetic data, complemented by field and microstructural observations, allow us to propose that the Maladeta polydiapir probably exploited NE- and SE-trending openings of the brittle crust for its emplacement, that accompanied or just preceded a SW-verging compressive event. A later N-S compression marked by inverse dextral, and E-W-trending shears, is preferably attributed to late Variscan rather than to Alpine tectonics.

Tectonic adaptation of the External Zones around the curved core of an orogen: the Gibraltar Arc

Abstract There is no real curvature in the External Zones of the Arc of Gibraltar. These External Zones are made up by the juxtaposition of segments with NNW-SSE- or E-W-trending structures. Juxtaposition does not result from late tectonics nor from a paleogeographic layout, but only from Miocene tectonics. During and after the late Burdigalian, relative motion between Europe and Africa was essentially a N-S convergence which accounts for the E-W structural trends. The NNW-SSE trends are the result of offset of the two main plates along sinistral strike-slip fault zones that cross-cut the previously mobile block of the Internal Zones and that shifted southward through time. To explain the sharp juxtaposition of different trending structures, two tectonic stages must be considered in each area of the External Rif. During the first one the rocks behaved plastically while during the second they could only move as a block. The Internal Zones (Alboran block) slid independently between Europe and Africa only until early Burdigalian. After this time, they were locked in the main plates and broken together with them to allow further displacements.

Syntectonic emplacement of the Maladeta granite (Pyrenees) deduced from relationships between Hercynian deformation and contact metamorphism

The relationships between Hercynian deformation and contact metamorphism in the aureole of the Aneto unit, a constituent pluton of the Maladeta granite complex (Spanish Pyrenees), provide new data relevant to the controversy between the syn- and post-tectonic models of emplacement for the Pyrenean granites. The deformation in the Aneto aureole is related to two superimposed Hercynian episodes. The first was a ‘top to the SW’ thrusting which induced SW-verging recumbent folds. The second, which was essentially compressive in the Aneto area, is represented by E–W-trending subvertical cleavage and folds with subhorizontal axes. This second episode can be assigned to the D2 main Hercynian tectonic event of the Pyrenees. The contact metamorphic assemblages grew during the D2 event and were deformed by it close to the contact. The conclusion that the pluton was emplaced during D2 is strengthened by the structural and kinematic similarities between the magmatic deformation in the southern rim of the pluton and the D2 deformation in the country rocks. This syntectonic timing is supported by new geochronological data that conflict with an existing Rb–Sr post-tectonic Permian whole-rock age for the Maladeta granite. More generally, in accordance with some very recent structural and U–Pb zircon studies, our results contradict the geodynamical models of the Pyrenees which relate the granite intrusions to a late extensional event.

Hercynian tectonics in the Pyrenees: a new view based on structural observations around the Bassie`s granite pluton

Abstract The structural study of the metasedimentary rocks surrounding the Bassie`s granite pluton allows definition of the succession and kinematics of the Hercynian tectonic events in this part of the Pyrenees, and supports a model of syntectonic emplacement of the pluton. The following sequence of events has been identified: (i) an early phase of N-S compression, essentially resulting in southwards thrusting; (ii) emplacement of the granite body along a NE-SW sinistral strike-slip shear zone which could be a late effect of the first phase; (iii) a major dextral and transpressive phase, responsible for themain fold structures trending, on average, N100 °, and following a sigmoidal pattern around the granite body. This succession of events and their kinematics could be extended to the whole Hercynian basement of the Pyrenees.

Sequential nesting of magmas in marble, southwestern Grenville Province, Québec: from fracture propagation to diapirism

Abstract In the southwestern Grenville Province (Quebec), 1090-1070 My old K-rich alkaline equidimensional plutons, such as the Kensington pluton, occur in marble-rich country rock, but coeval dykes bearing lower-crustal xenoliths are found only in a quartzofeldspathic gneiss complex. The sequential nesting of magmas, the various episodes of skarn formation and the progressively more mafic character of the intruding magmas of the Kensington pluton are interpreted in terms of magma ascent through a series of propagating fractures, cessation of upward fracturing and pooling of magma in marble-rich ductile crust, diapirism, radial shortening of wall rock, and in-situ pluton expansion. This suite provides evidence that at deep crustal levels, the country-rock rheology influences the mode of magma ascent and emplacement and that diapirism is a possible mechanism of magma transport through rocks behaving rheologically by a power-law model.

Composite-laccolith emplacement of the post-tectonic Vila Pouca de Aguiar granite pluton (northern Portugal): a combined AMS and gravity study

The Vila Pouca de Aguiar granite pluton, emplaced during the latest event of the Variscan orogeny of northern Portugal, is here subjected to a detailed study that combines magnetic fabric measurements and gravity modelling of its shape at depth. This laccolith, less than 1 km in thickness over ≈60% of its outcrop area, appears to be fed from its northern area, through narrow conduits, up to 5 km deep, belonging to a set of Y-shaped valleys that almost perfectly correspond to the local Regua-Verin fault-system identified in the geological maps. A normal petrographical zonation, already identified geologically, appears to be rather progressive, although a gradient in magnetic suceptibility magnitude in-between the two main magma types is evidenced. It is suggested that the first to be emplaced and the least evolved granite type (Vila Pouca de Aguiar Granite) upwelled from the local, NE-trending fault-zone, acting as a dyke, and formed a thin sill where NE-directed magma flow was dominant, at least close to the floor. The more evolved granite type (Pedras Salgadas Granite), located just above the main feeder zone, and deeply rooted at the intersection beween underlying faults, is at the centre of a remarkably regular concentric distribution of the foliation trajectories. They may reflect the late doming of the laccoliths northern part, coeval with a slight E-W extension of the inflating magma reservoir, as marked by the E-W-trending lineations. Along with ubiquitous magmatic to near-magmatic microstructures and particularly low anisotropy magnitudes, such patterns can be entirely explained by magma movement within its inflating reservoir. This composite laccolith, during emplacement of which no interference with the regional strain pattern can be recorded, is therefore considered as typical of post-tectonic emplacement.

Homogeneity of Granite Fabrics at the Metre and Dekametre Scales

Magnetic fabrics of biotite-bearing granites were systematically determined, at the metre and dekametre scales, in three plutons previously studied for their overall magnetic structures, in order to characterize the spatial homogeneity and variability of the fabrics. These granites, with typically magmatic microstructures, have different mean magnetic anisotropies (P%): Sidobre (southwest Massif Central of France; P%=2.3), Bassies and Trois-Seigneurs (French Pyrenees; respectively P%= 3.3 and P%=5.6). In each site, two grids of 50 oriented specimens each, respectively one and ten metres apart from each other, have been studied in detail. The directional data, especially the lineations, strongly cluster around their means and have similar orientations on both scales. In map view, the fluctuations of these data are generally gradual and tend to form sigmoids but no clearly defined pattern, such as a C/S system was observed. The magnetic anisotropy and the bulk susceptibility are homogeneous as a whole, and display spatial organizations with no simple relationships with the structures. These results confirm, however, the validity of the homogeneous structural patterns obtained from entire plutons.