Inmaculada Expósito

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.

Deformation of garnets in a low-grade shear zone

Elongated shapes of garnets in high-grade metamorphic rocks have been explained as a result of plastic crystal flow or anisotropic growth. In the case of low- to medium-grade metamorphic rocks, a satisfactory explanation has not been proposed yet for elongated garnets. We have studied elongated garnets grown under low- grade metamorphic conditions in a shear zone with a composite planar-linear fabric. Garnet shapes in three dimensions define oblate ellipsoids. Drawing on evidence from compositional X-ray maps, it can be deduced that growth zoning is truncated along the long borders of grains, whereas subcircular garnets show non-truncated concentric growth zoning. This fact shows that selective dissolution along planes parallel to the foliation and the C surfaces can be claimed to be the main mechanism responsible for the deformation of the garnets under examination here. More specifically, dislocation-enhanced dissolution, which occurs at low temperatures and low dislocation mobility, is arguably the mechanism responsible for partially dissolving the garnet grains. Selective dissolution is expected to yield plane-strain oblate ellipsoids (i.e. with a volume loss) as measured in the elongated garnets. However, the rock as a whole has been deformed by plastic flow in a simple-shear regime, as shown by the existence of numerous shear criteria and the strain calculations performed. 0 1997 Elsevier Science Ltd.

Diapiric emplacement in the upper crust of a granitic body: the La Bazana granite (SW Spain)

Abstract The ascent and emplacement of granites in the upper crust is a major geological phenomenon accomplished by a number of different processes. The active processes determine the final geometry of the bodies and, in some favourable cases, the inverse problem of deducing mechanisms can be undertaken by relying on the geometry of plutons. This is the case of the La Bazana granitic pluton, a small Variscan igneous body that intruded Cambrian rocks of the Ossa-Morena Zone (SW Iberian Massif) in the core of a large late upright antiform. The granite shows no appreciable solid-state deformation, but has a late magmatic foliation whose orientation, derived from field observations, defines a gentle dome. The regional attitude of the main foliation in the country rock (parallel to the axial plane of recumbent folds) is NW–SE, but just around the granite, it accommodates to the dome shape of the pluton. Flattening in the host rock on top of the granite is indicated by boudinaged and folded veins, and appears to be caused by an upward pushing of the magma during its emplacement. The dome-shaped foliation of the granite, geometrically and kinematically congruent with the flattening in the host rock, can be related in the same way to the upward pushing of the magma. The level of final emplacement was deduced from the mineral associations in the thermal aureole to be of 7–10 km in depth. Models of the gravity anomaly related to the granite body show that the granite has a teardrop–pipe shape enlarged at its top. Diapiric ascent of the magma through the lower middle crust is inferred until reaching a high viscous level, where final emplacement accompanied by lateral expansion and vertical flattening took place. This natural example suggests that diapirism may be a viable mechanism for migration and emplacement of magmas, at least up to 7–10 km in depth, and it provides natural evidence for theoretical discussion on the ability of magmatic diapirs to pierce the crust.