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THE IBERO-ARMORICAN ARC: A COLLISION EFFECT AGAINST AN IRREGULAR CONTINENT?

The Ibero-Armorican Arc is the main Variscan macrostructure in western Europe. Although it was recognized in the 1920s, its genesis is still debatable and the proposed models for its generation questionable. However, even if some doubts persist, it seems that most agree on the general geodynamic evolution of this virgation in both branches. In the Middle to Late Devonian, Iberia was deformed by a sinistral transpressive regime, while in the northern branch thrusting events were predominant. In the Carboniferous, a dextral transpression begins to predominate in the Armorican branch, while the southern branch was deformed by southward thrusting. In an attempt to correlate these events, we propose that during the Late Devonian a Cantabrian indentor moved northward, producing the oblique closure of the southern part of the Rheic Ocean and an almost orthogonal closure in central Europe. In the Carboniferous, the collision with the irregular margin of Laurasia induced a rotation of the indentor; the intracontinental deformation was then achieved by dextral transpression in the northern branch and thrusting in the southern one.

Constriction in a transpressive regime: an example in the Iberian branch of the Ibero-Armorican arc

Abstract Detailed strain analyses in the Armorican quartzites have been performed in various places within the Centro-Iberian Zone of western Europe. The observed strain pattern shows that in the inner Ibero-Armorican arc, as well as near the hinge zone, sinistral transpression produces constrictive ellipsoids. In the outer arc, away from the hinge zone, the deformation regime produces plane strain to oblate strain ellipsoids. Some of these data clearly contradict existent kinematical models, which associate constriction with transtension, and consider transpression zones to generate oblate strain ellipsoids. However, newly developed models show that it is possible to obtain a wide range of finite strain ellipsoids in transpression zones if one takes account of lateral escape, axial depression and volume change. The geodynamic implications of the strain analyses suggest generation of constriction in a transpressive regime, caused by lateral expulsion in front of the Cantabrian indenter that produced the Ibero-Armorican Arc. This mechanism was responsible for the Variscan wrench component in western Europe, which was mainly sinistral in Iberia and dextral in Britanny. In Iberia, the displacement by lateral escape was easily accommodated by subduction of the South Portuguese Terrane under the Ossa-Morena Zone of the Iberian Terrane.

Rheologically induced structural anomalies in transpressive regimes

Abstract Local structural anomalies are sometimes found in transpressive regimes. They can be related to strain partitioning processes but in some cases other types of heterogeneities could affect the deformation style. This paper shows that rheological heterogeneities can radically influence the geometry of faults in transpression zones and cause them to be markedly non-planar. In the Marao region of northern Portugal, a zone in which Variscan structures are SW-facing is found within the regional NE-facing geometry. This anomaly is adjacent to a complex shear zone (Mina/Ribeira das Cestas) where the fault plane is curved; however the surrounding material does not show any signs of a related folding event. Detailed field studies, mainly concerning the rheological contrasts between the deformed lithostratigraphic units, show a close relationship between the structural anomaly and the movement along rheological anisotropies. These observations, complemented by experimental deformation of multilayers composed of analogue materials, allow the construction of a generic model. Strain partitioning during the initial stage of the sinistral transpressive Variscan deformation produced the juxtaposition of lithostratigraphic units with strongly different rheological contrasts. Subsequent flattening deformation induced heterogeneous indentation phenomena, producing the non-planar geometry of the complex shear zone. The structural heterogeneities caused could have an important role in the evolution of a transpressive domain.

Transient inversion during the opening stageof the Wilson cycle “Sardic phase” in the Iberian Variscides –Stratigraphic and tectonic record

The Variscides of Iberia have a bilateral symmetry with east vergence in the eastern branch and west vergence in the western, on both sides of a Centro-Iberian Zone (CIZ), with predominant steep axial planes. All the structures curve around the Ibero-Armorican Arc (IAA). Unconformities in the sedimentary sequences of Cambrian to Early Ordovician age were ascribed to “Sardic phase” by correlation with similar tectonosedimentary events in Sardinia. Recent studies showed diachronism between these events in Sardinia and Iberia but migration of major geodynamic regime in time may be due to regional variation of major events at plate tectonic scale. We studied in detail two critical areas in the CIZ, the Marão anticline in the NE and the Amêndoa-Carvoeiro synform in the SW. Two unconformities can be put in evidence, as elsewhere in CIZ. A stronger lower unconformity of a Volcano-Sedimentary Complex of Lower Arenig (and Tremadocian?) age on top of a Cambrian clastic sequence with flysch characteristics; and a milder upper unconformity of Armorican Quartzite of Arenig age on both the Volcano-Sedimentary Complex and the Cambrian sequences. The lithostratigraphy of the studied areas is described and correlated with other areas in Iberia. The Volcano-Sedimentary Complex and coeval magmatic bodies with bimodal composition are briefly described. The Sardic event corresponds to folds with steep axial planes at high angles to Variscan structures that produce the penetrative cleavage that cut across the unconformity surfaces. Sardic thrusts are also present and can be explained by thin-skinned compressive tectonics. Sardic folds and thrusts suggest a brief period of transient inversion between a major extensional regime from Cambrian to Devonian. The obliquity of Sardic structures to Variscan compression suggests a component of transpression during the Sardic tectonic event, corresponding to a tectonically enhanced unconformity near the Cambro-Ordovician boundary. The transient Sardic inversion is interpreted in terms of a break-up unconformity related to the migration of an intracratonic rift; in the Ordovician this rift moves into the SW of Ossa Morena Zone (OMZ) and since then become the SW Iberia suture during the Variscan Wilson cycle. This migration induced transient compression and dextral strike-slip in the major boundary between CIZ and OMZ due to presence of incipient primary curvature in this segment of IAA.

Finite strain analysis in a transpressive regime (Variscan autochthon, northeast Portugal)

Abstract We apply a method of estimating finite strain that can be widely used in Armorican quartzites affected by transpressive regimes. The method proposed uses data from Skolithos , which are quite common in these rocks. The main assumption is that the orientation of the finite strain ellipsoid can be represented by the cleavage and the stretching lineation observed in the rocks. This method was tested in a macroscopic fold in the Moncorvo region (northeast Portugal).

Heterogeneous strain behaviour in competent layers during folding in transpressive regimes

The understanding of folding processes in transpression is crucial in structural geology. However, the study of folding in multilayers where adjacent strata exhibit high competence contrast is impossible if the observations are only restricted to geometrical features. Nevertheless, using detailed three dimensional finite strain analyses it is possible to constrain the solutions. Strain data from the deformed northern Portugal Ordovician quartzites show that folding mechanisms in transpression are highly variable. For the more deformed materials found, not only in the short limbs, but also in the thinner layers of the long limbs, the strain ellipsoids orientations can be explained by a combination of flexural shear (related to the folding process) and sinistral wrench component parallel to the bedding plane (a pervasive regime in the southern branch of the Ibero-Armorican Arc). In the thicker layers of the long limbs, orthogonal flexure seems to be the dominant mechanism. Concerning the strain ellipsoid shape, the constrictional forms predominate mainly in the long limbs. Strain partitioning in transpressive regimes is important at the mesoscopic fold scale, especially in asymmetric folds. In the more deformed short limb, the data indicate that transpression tends to be always an active mechanism, while in the long limb the regional wrench component its often less important.

Exhumation of a migmatite complex along a transpressive shear zone: inferences from the Variscan Juzbado–Penalva do Castelo Shear Zone (Central Iberian Zone)

High-grade metamorphic rocks associated with S-type granites are recorded in the Central Iberian Zone, Iberian Variscides. Though most of these occur as inferred metamorphic core complexes affiliated with detachment faults, others, such as the Figueira de Castelo Rodrigo–Lumbrales Anatectic Complex, crop out between low-grade metamorphic rocks separated by steeply-dipping strike-slip shear zones, such as the Juzbado–Penalva do Castelo Shear Zone. Our structural analysis has been able to constrain two major deformation stages during the Variscan D3: (a) D3a ductile deformation event, with clear sinistral kinematic criteria; and (b) D3b thrusting ductile–brittle deformation event. The petrological investigation confirmed the jump in metamorphic grade between the host rocks and the anatectic complex. P–T estimates on calc-silicate rocks interlayered with the metapelites of the anatectic complex provided minimum metamorphic peak conditions of T = 761 ± 50°C and P = 5.0 ± 1.0 kbar. However, petrological modelling results show that P–T conditions must have exceeded T > 800°C. Both structural and geothermobarometric data support a two-step model for the exhumation of the Anatectic Complex, including a 5 – 8 km vertical exhumation along a 65 – 100 km horizontal displacement due to a simple shear-dominated transpression mechanism during the Variscan D3 events. Supplementary material: The petrology data, mineral chemistry analyses of the calc-silicate units and EPMA analytical conditions, and the P–T modelling methodology can be found at https://doi.org/10.6084/m9.figshare.c.3785648

Reply to comment by M. Francisco Pereira et al. on ''Geodynamic evolution of the SW Europe Variscides''

] The interpretation reported by Ribeiro et al. [2007]favors the presence of a Cadomian basement within the SWEurope Variscides, eventually including older relics,considering as well the role of this basement in theevolution of the Variscan cycle; this particular issue wasdeveloped in a subsequent paper [Ribeiro et al., 2009]. Thepossibility of Grenville inliers is not definitely ruled out bythe arguments of Pereira et al. [2009]. Indeed, a few (nearlyconcordant) U/Pb zircon ages of 0.94 to 1.2 Ga are reportedby Linnemann et al. [2008]; several other U/Pb zircongeochronological data from Na¨gler et al. [1995], de la Rosaet al. [2002], Sola´etal.[2008], and Cordani et al. [2006](upper intercept date for sample 43B-5.1 assuming lead lossduring Variscan metamorphism at 360 Ma), although notdefinitive, also suggest the inheritance of Grenville ages inOssa-Morena Zone (OMZ) rocks. Furthermore, detritalzircons in Me´rtola Formation (South Portuguese Zone)proximal greywackes(includinghigh-grademetamorphiclith-oclastssimilar to those rocks found at the OMZ E´vora Massif)reveal a frequency age distribution peak at 0.94 Ga (R. Jorge,personal communication, 2009), consistent with derivationfrom inliers of a Greenville orogen, which is well datedbetween 1.2 and 0.95 Ga in the North American craton.Therefore, the possibility of pre-Cadomian cycles remainsdebatable (as signed by a question mark in the text of Ribeiroet al. [2007]), as well as the provenance of detrital zircons andtheconnectionsbetweenIberiaandWestAfrica,AmazoniaandLaurentia cratons prior to the assemblage of Pannotia Super-continent. Further investigations are needed to solve thisproblem within an acceptable range of uncertainty.[