David J. Martínez Poyatos

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.

Relationships between very low‐grade metamorphism and tectonic deformation: examples from the southern Central Iberian Zone (Iberian Massif, Variscan Belt)

We have studied the syn‐kinematic very low‐grade metamorphism in a polyphase Variscan deformed region using X‐ray diffraction techniques. Two phases of regional metamorphism are related to their respective episodes of penetrative deformation in the southern Central Iberian Zone. The data obtained suggest that the rocks did not reach metamorphic equilibrium, but strain favoured the progress of mineral reactions in the more deformed parts. The first deformation is Devonian in age and consists in a heterogeneous ductile shearing coeval with large‐scale recumbent folding that were produced under high‐anchizone to epizone metamorphic conditions. The heterogeneity of the shearing originated strain gradients that can be said to enhance the growth of new minerals and the illite polytype transformation in the highly strained overturned limb of the preserved pile of recumbent folds, but illite crystallinity remained constant throughout the structure. The second deformation is Mid‐Carboniferous in age and consists in an upright folding that took place under late diagenesis to low‐anchizone metamorphic conditions. The distribution of mineral parageneses and illite crystallinity across one of the upright folds suggests that strain gradients favoured the metamorphic reaction progress from the hinge (low strain) towards the limbs (high strain). Other characteristics of the region such as a metamorphic gap associated with an unconformity at the base of the Lower Carboniferous rocks, or cryptic contacts aureoles surrounding volcanic intercalations and a large granitic batholith, are also studied.

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.

Lithospheric velocity model across the Southern Central Iberian Zone (Variscan Iberian Massif): The ALCUDIA wide‐angle seismic reflection transect

A P wave seismic velocity model has been obtained for the Central Iberian Zone, the largest continental fragment of the Iberian Variscan Belt. The spatially dense, high-resolution, wide-angle seismic reflection experiment, ALCUDIA-WA, was acquired in 2012 across central Iberia, aiming to constrain the lithospheric structure and resolve the physical properties of the crust and upper mantle. The seismic transect, ~310 km long, crossed the Central Iberian Zone from its suture with the Ossa-Morena Zone to the southern limit of the Central System mountain range. The energy generated by five shots was recorded by ~900 seismic stations. High-amplitude phases were identified in every shot gather for the upper crust (Pg and PiP) and Moho (PmP and Pn). In the upper crust, the P wave velocities increase beneath the Cenozoic Tajo Basin. The base of the upper crust varies from ~13 km to ~20 km between the southernmost Central Iberian Zone and the Tajo Basin. Lower crustal velocities are more homogeneous. From SW-NE, the traveltime of PmP arrivals varies from ~10.5 s to ~11.8 s, indicating lateral variations in the P wave velocity and the crustal thickness, reflecting an increase toward the north related with alpine tectonics and the isostatic response of the crust to the orogenic load. The results suggest that the high velocities of the upper crust near the Central System might correspond to igneous rocks and/or high-grade metamorphic rocks. The contrasting lithologies and the increase in the Moho depth to the north evidence differences in the Variscan evolution.