António Ribeiro

Variscan intracontinental deformation: The Coimbra—Cordoba shear zone (SW Iberian Peninsula)

The Coimbra—Cordoba shear zone is a major lineament roughly parallel to the NW—SE Variscan structures of the southern Iberian Peninsula. Inside the shear zone evidence for a non-coaxial strain path exists from structural, microstructural and lattice-preferred orientation data on quartz. Plane strain is suggested by the study of microstructures in the (XY) and (YZ) sections in all formations occurring in the shear zone. With increasing strain, a mylonitic series develops. The deformation, contemporaneous with epizonal to mesozonal metamorphism, is responsible for: 1. (1) the regional subvertical foliation oblique to the shear zone 2. (2) the well defined subhorizontal lineation striking NW—SE 3. (3) the discrete planes of shear roughly parallel to the shear zone. The data are interpreted as resulting from a left-lateral slip motion with a displacement of at least 72 km. In the light of plate tectonics it is inferred that the Coimbra—Cordoba lineament could have been a suture zone which evolved into an intracontinental left-lateral shear zone during the Variscan orogeny of the Ibero-Armorican arc.

Geodynamic Evolution of the Iberian Massif

The Iberian Massif represents the largest continuous exposure of pre-Permian rocks within the Iberian Peninsula (Fig. 1). It was first described in terms of a coherent tectonic unit by Staub (1926) who considered it to be comprised of Archean basement sequences that had been successively reworked during Caledonian and Herycnian orogenic events. Stille (1924, 1929) contested these ideas and argued, instead, that the Iberian Massif largely records the effects of Hercynian tectonothermal events. This was later documented in more detail by Lotze (1945), who divided the massif into six zones on the basis of contrasting lithologic, structural and/or metamorphic characteristics (Fig. 1). With the advent of plate tectonic concepts, a new view of the Iberian Massif emerged, and many different geodynamic plate models were proposed. Van der Voo (1982) suggested that the Iberian Massif constituted the southwesternmost and largest piece of the so-called Armorican plate, a continental block which appears to have rifted from Gondwana in the early Paleozoic.

Stress pattern in Portugal mainland and the adjacent Atlantic region, West Iberia

The Portuguese mainland territory is located close to the Azores-Gibraltar plate boundary, in a tectonic setting responsible for significant neotectonic and seismic activities. However, few data concerning the present regional lithospheric stress field were available, as testified by recently published maps of stress indicators for the Europe and Mediterranean regions. One of the authors already presented a synthesis on this subject [Cabral, 1993], where geological and geophysical stress indicators were considered. In this paper we introduce new information, mainly a considerable amount of borehole breakout data. The updated data set comprises 32 reliable stress indicators showing a mean azimuth of 145° (standard deviation 21°) for the maximum horizontal stress direction (SHmax). On the average, the geological data are rotated clockwise and the focal mechanism data deviated anticlockwise to that azimuth, while the borehole elongation results are consistent with the mean SHmax trend. These differences in stress trend suggest a regional progressive rotation of the SHmax direction from NNW-SSE to WNW-ESE since the upper Pliocene. To estimate stress trajectories, new and published stress indicators in the adjacent Atlantic area and northern Africa were also investigated, showing a very uniform NW-SE SHmax trend in west Iberia. A high level of horizontal compressive stress acting oblique to the western Portuguese continental margin is inferred and interpreted in view of a proposed regional geodynamical model, of activation of this passive margin, with the nucleation of a subduction zone in the Atlantic SW of Iberia, at the Gorringe submarine bank, which is propagating northward along the base of the continental slope, at the transition between thinned and normal continental crust.

A review of Alpine tectonics in Portugal: Foreland detachment in basement and cover rocks

Abstract The Alpine foreland in Portugal was deformed by compressional tectonism during the Miocene. In the NNE-SSW oriented Lusitanian Basin, most folds and thrusts in the Meso-Cenozoic cover are oriented ENE-WSW, parallel to the Alpine front in the Betic Cordillera, and verge towards the north-northwest and south-southeast. The thrusts are connected by lateral ramps: most of these are oriented NNE-SSW to N-S and show sinistral movement, and some are transpressional. The lateral ramps result from reactivation of older extensional faults related to crustal thinning of the continental margin. In the E-W oriented Algarve Basin a simpler basin inversion occurred, with older E-W normal faults reactivated as essentially pure thrusts. In both basins Alpine structures formed above decollements in the Hettangian evaporite-clastic complex. Variscan basement was also deformed by ENE-WSW reverse faults during Miocene time. The similarity in orientation and style of the basement structures to those in the cover suggests that they also occurred by detachment, but their larger scale indicates that the detachment is deep and involves much of the crust. Thus, we interpret the Central Cordillera, in which basement rocks are thrust over Miocene sediments on both sides, as a “pop-up” of crustal scale, elevated above downward-flattening faults that dip towards each other and merge into a single deep detachment. Alpine structures in the Iberian foreland are therefore similar in structural style to those of the Appalachian and Laramide forelands of North America and the Alpine foreland of northwest Europe.

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.

Geodynamic models for the Azores triple junction: A contribution from tectonics

Abstract The American, Eurasian and African lithospheric plates meet at the Azores triple junction. The nature of the northern and southern branches of the junction is well known and uncontroversial, the American plate is separated from Eurasia and Africa by the Mid-Atlantic Ridge (MAR). The western group of islands of the Azores archipelago (Flores and Corvo) lies on the American plate, just west of the rift, and their tectonic features agree with that location. The nature of the third branch of the junction, to the east of the MAR, is however still controversial. Some authors believe that a ridge-ridge-ridge junction occurs in the Azores area and adopt the structure known as the “Terceira Ridge” (working as a simple rift boundary) as the third branch. Another model proposes a triangular microplate in the Azores region bounded by the Mid-Atlantic Rift to the west, the East Azores Fault Zone to the south and a fault crossing S. Miguel and the Terceira islands to the northeast; in this model the fault zones are pure dextral strike-slip boundaries. Both models would create space problems at Gloria Fault, a well-known pure dextral strike-slip structure. In the first model transtension would be necessary at Gloria Fault to accommodate plate motion, while the second would require a transpressive regime on that structure. Neotectonic and seismotectonic data in the Azores indicate a transtensile regime in the central and eastern island groups in present and recent times. This is compatible with a “leaky transform” structure acting as the third branch of the junction in the Azores area, a model which has already been presented by several authors in the past. Stress trajectories deduced from neotectonic studies in some of the islands suggest that the main plate boundary passes between the islands of S. Jorge and Pico south of S. Miguel and joins Gloria Fault east of Santa Maria. This model and boundary location is also compatible with seismotectonic and magnetic data.

Tectonics of the Beja-Acebuches Ophiolite: a major suture in the Iberian Variscan Foldbelt

The Beja-Acebuches Ophiolitic Complex (BAOC) (south Portugal/Spain) corresponds to a high grade metamorphic belt along the boundary between Ossa-Morena and the South Portuguese Zones and comprises a lithostratigraphic sequence including (from top to bottom) metabasalts, (metamorphosed) multiple dyke intrusions in gabbro, flasergabbros and metaserpentinites. It is affected by three deformation phases. D1 affects the ophiolite lower stratigraphic units and is represented by a mylonitic cleavage with a stretching lineation where shear criteria indicates the sense of shear to be towards the north-north-east; this deformation event can be related to the ophiolite emplacement above the crystalline footwall of the Serpa antiform, affecting Precambrian basement and Cambrian cover rocks. The obduction polarity ist north-eastwards, similar to the subduction polarity that generates the Beja Gabbroic Complex (BGC), implying a flake geometry. The second deformation phase, D2, is represented by an intense WNW-ESE sinistral shear event which is responsible for the shattered appearance of the suture; D2 is reactivated later by a more brittle D3 event involving thrusting to the south-west, again with a sinistral component. 40Ar/39Ar isotopic ages were obtained for (metamorphosed) multiple dyke intrusions in the BAOCs gabbro (342.6 ± 1.4 Ma), for metagabbroic cumulates (340.7 ± 1.9 Ma), and for the undeformed/unmetamorphosed BGC (341.1 ± 1.3 Ma) occurring to the north of the ophiolitic suture. These ages reflect a last regional cooling event in the area which post-dates the ophiolite emplacement and the intrusion of the BGC through this oceanic sequence.

40Ar/39Ar mineral age constraints for the tectonothermal evolution of a Variscan suture in southwest Iberia

Abstract Dallmeyer, R.D., Fonseca, P.E., Quesada, C. and Ribeiro, A., 1993. 40Ar/39Ar mineral age constraints for the tectonothermal evolution of a Variscan suture in southwest Iberia. Tectonophysics, 222: 177–194. Mafic units interpreted to represent an internally disrupted ophiolite succession are exposed along a Variscan suture situated between the South Portuguese and Ossa-Morena zones in southern Iberia. Structural characteristics of various units adjacent to the suture zone suggest maintainence of an oblique (transpressional), sinistral tectonic regime during late Paleozoic plate convergence. This involved initial, limited, north-directed subduction beneath the Ossa-Morena zone, and was followed by oblique collision of the South Portuguese Terrane. Metamorphic evolution of rocks adjacent to the suture occurred during maintainence of a moderately high geothermal regime. Five samples of amphibolite were collected within the hanging wall of the suture. These included samples from the ophiolitic terrane and from the Ossa-Morena zone. Two samples were also collected within the post-kinematic Beja Gabbro which intruded the other two tectonic units following their structural juxtaposition and penetrative ductile deformation. Hornblende concentrates from these seven samples record well-defined 40Ar/39Ar plateau ages which range between c. 342 and 335 Ma. These results imply that the regional tectonostratigraphic units were maintained at moderate crustal levels until the Visean, and that their juxtaposition and internal ductile deformation occurred in the late Devonian. Subsequent cooling was probably effected by transpressional uplift during oblique collision following closure of intervening oceanic elements.

Are subduction zones invading the Atlantic? Evidence from the southwest Iberia margin

Subduction initiation at passive margins plays a central role in the plate tectonics theory. However, the process by which a passive margin becomes active is not well understood. In this paper we use the southwest Iberia margin (SIM) in the Atlantic Ocean to study the process of passive margin reactivation. Currently there are two tectonic mechanisms operating in the SIM: migration of the Gibraltar Arc and Africa-Eurasia convergence. Based on a new tectonic map, we propose that a new subduction zone is forming at the SIM as a result of both propagation of compressive stresses from the Gibraltar Arc and stresses related to the large-scale Africa-Eurasia convergence. The Gibraltar Arc and the SIM appear to be connected and have the potential to develop into a new eastern Atlantic subduction system. Our work suggests that the formation of new subduction zones in Atlantic-type oceans may not require the spontaneous foundering of its passive margins. Instead, subduction can be seen as an invasive process that propagates from ocean to ocean.

Compressive Episodes and Faunal Isolation during Rifting, Southwest Iberia

Evidence for three short‐lived compressive episodes of late Carixian, late Callovian–early Oxfordian, and Tithonian‐Berriasian ages that lasted <5 m.yr. and occurred during the process of rifting of the Algarve Basin is presented. These tectonic‐inversion episodes are described at outcrop and cartographic scales and have been dated with the accuracy provided by the ammonoid scale. An uplift event of late Toarcian–Aalenian age of undetermined tectonic origin is also described. We show that these four tectonic episodes coincide in time with important ecological events, such as the onset of migration and/or the segregation of Boreal and Tethyan ammonite species and the confinement of the Algarve Basin. Stratigraphic and paleoecological data from the Algarve and Lusitanian Basins are compared and discussed together with eustatic and tectonic information. We propose that the tectonic‐inversion episodes that caused uplift are the origin of the Mesozoic sedimentary gaps and the intermittent opening and closure of the seaway located offshore the SW corner of Iberia between the Algarve and Lusitanian Basins (i.e., a seaway between the Boreal and Tethyan realms). Three tectonic mechanisms for the origin of these short‐lived compressive episodes are presented after comparing the tectonic setting of the Algarve Basin with other geological provinces of the world where similar phenomena also occurred.