The Beja Acebuches Ophiolitic Complex (Figure 1, sector A) is a continuous (about 130 km long) thin amphibolite serpentinite metamorphic belt (about 1500 m thick) extending from the surroundings of Torrão - Alcácer do Sal (Southwest Portugal) to Acebuches (Aracena: Southwest Spain) (Crespo Blanc, 1989; Quesada et al., 1994; Fonseca, 1995; Fonseca et al., 1999). This ophiolite complex is tectonically superposed over Paleozoic sequences of OMZ by obduction phenomena’s, along the southern boundary of the Iberian Autochthon, and is bounded towards the North, mainly by latter variscan intrusions of Beja Igneous Complex or Beja Massif (BIC/BM). This limit is frequently reworked by semifragile sinistral thrusts. Towards the south, the Ferreira-Ficalho thrust separates the ophiolitic sequence from the Pulo do Lobo metassedimentary deformed sequence. The metamorphic grade in BAOC range from greenschist-amphibolitic facies through amphibolitic facies in metabasalts, to low-pressure granulitic facies in metagabbros (Quesada et al., 1994; Fonseca, 1995).
Along the Guadiana Valley, the best BAOC exposure, from S to N (and top to bottom), we can recognise, fine-grained greenschists (locally pillowed metabasalts containing cherty intercalations (Figure 2), amphibolites s.l. (corresponding locally to sheeted dyke or multiple dyke intrusions in flasergabbros), flaser gabbros (metagabbroic cumulates with minor meta-trondhjemitic intercalations), and scattered serpentinite bodies (Fonseca, 1995; Fonseca et al, 1999, fig.3).
Towards the north, the OMZ also include klippens of several internal ophiolitic sequences (Araújo et al., 1993) and high-pressure metamorphic rocks (eclogite/blueschist) (Fonseca et al., 1993; 1999; Fonseca, 1995; Pedro, 2004). These klippens were together emplaced, contemporaneously whit the obduction of BAOC, and occur as dismembered tectonic slices imbricated within the OMZ volcano-sedimentary sequences (Araújo et al., 1993; Fonseca et al., 1993; Araújo, 1995; Fonseca, 1995; Pedro, 1996; 2004; Moita, 1997; Pedro et al., 1998; Fonseca et al., 1999).
All these tectonic units were intruded by the BIC (Fonseca, 1995); the BIC hosts xenoliths (Figure 4) of several regional tectonic units, including previously deformed metamorphic rocks from the BAOC (Fonseca and Ribeiro, 1993) and the BIC emplacement is clearly latter then D1 unconformity (Andrade, 1979,1983 and Fonseca, 1995).
Figure 3. General 3D schematic
General 3D schematic cross-section along the Guadiana river, with the pseudo stratigraphic sequence.
Pulo do Lobo metassediments;
Beja Igneous Complex-BIC;
very deformed gneissic lithologies;
complex carbonates (listwaenitic like);
leucogabbros and metaplagiogranite lithologies ;
metabasalts, locally pillowed;
amphibolites s.l.;
metagabbros and flaser gabbros;
granulitic gabbros;
mafic-ultramafic cumulates;
serpentinezed peridotites.
Figure 4. Outcrop of xenoliths
Outcrop of xenoliths in the Beja Igneous Complex. The xenoliths are essentially very deformed amphibolites from the BAOC.
The BIC is a typical layered gabbroic complex grading upwards from a peridotite-gabbro assemblage to an anorthosite-gabbro assemblage. On top of this sequence one can find dolerites and basalts. Poor exposure prevents the observation of the contact between these rocks and the BIC.
In what concerns the structure, the igneous rocks of the BAOC underwent intense Variscan deformation and metamorphism (Bard, 1969; Munhá et al., 1986; Fonseca, 1995, Fonseca et al., 1999). Original textures and mineralogy have been generally almost completely destroyed and replaced. These facts simultaneously with the disruption of the original lithological sequence, makes it very difficult to recognise the original ophiolitic lithostratigraphic sequence (Figure 3).
The metamorphic grade within the ophiolite (s.s) is variable, increasing very fast towards the north but also along strike in the ESE direction, towards the central part of the belt (Fonseca et al., 1999; Quesada et al., 1994). Metamorphic textures are dominantly nematoblastic and reflect the deformational history of the BAOC; three variscan phases of deformation are recorded and preserved in the footwall rocks of the BAOC (Fonseca, 1995; Fonseca et al., 1999). These three phases are responsible for the present dismembered configuration sketch of the suture between OMZ and SPZ. Structural mapping of a cross section along the Guadiana River shown an imbricated and folded sequence (Figure 5, map A)
The first phase of deformation (D1), which is well identified at all scales, is associated with the basal rock units of the ophiolitic sequence [including granulite/amphibolite facies dikes in gabbro, flaser gabbros with associated plagiogranitos/leucogabbros and (serpentinized) ultramafics]. In thin section, sigma asymmetric tails on retrograded recrystallized grains of brown hornblende from dikes in gabbro indicate thrust shear to the north or NNE (Fonseca, 1995). This agrees with the presence of a macroscopic mylonitic cleavage with an associated stretching lineation and related shear criteria, indicating top towards N-NNE. D1 thrusts are mainly sub-horizontal or gently dipping to the S in the SW. This deformation phase is related to the obduction and ophiolite emplacement upon the crystalline footwall of the OMZ (Fonseca, 1995; Fonseca et al., 1999).
Crossing the Ferreira-Ficalho Thrust, the sedimentary units associated with the Pulo do Lobo sequence show the same deformation as the ophiolite (s.s) (Fonseca and Ribeiro 1993; Fonseca et al., 1999, Figure 6) The most important deformation feature of the Pulo do Lobo Formation is a penetrative cleavage that contains a stretching lineation D1 (Fonseca, 1995). This stretching lineation and shear criteria indicate sense of shear towards S-SSE, with sheath folds indicating the same sense of movement (Fonseca et al., 1999: Fonseca 1995). Moreover, microscopic examination of felsic metavolcanic rocks in D1 thrust faults shows bookshelf sliding of feldspars in a ductile matrix; the displaced hard grain in a ductile matrix implies a shear zone with sense of shear to the south. This advice that the Pulo do Lobo metasedimentary deformed sequence corresponds to an accrecionary prism affected by SSE vergent structures, (synthetic to subduction), synchronous with the obduction of the BAOC to the NNE (antithetic to subduction). The Pulo do Lobo very deformed sequence is discordantly covered by a flysh (Santa Iria Unit) with latte Devonian age (Giese et al, 1989; 1994).
Figure 6. Outcrop of the Pulo do Lobo unit
Outcrop of the Pulo do Lobo unit, showing at the least tree main deformation phases verging to the South.
The second phase of deformation (D2) can be identified essentially on the upper units of the ophiolitic sequence, such as metabasalts in the greenschist facies and amphibolites (s.l.). Kinematic indicators, which include shear bands and rotated porphyroclasts of hornblende are very abundant in these units and are especially well developed in the amphibolitic unit, where well-preserved shear bands show clearly the sinistral component of shear with thrust movement towards NW (Figure 7). In the serpentinized peridotite unit it is also possible to see C-S band structures at the outcrop as well as at thin-section scales showing sense of shear to NW (Fonseca 1995; Fonseca et al., 1999). These rocks (serpentinites) show a dominant stretching lineation of relict minerals associated with a ductile sinistral shearing of NW-SE direction. In the metabasalts unit this shearing is related to a strong mylonitic cleavage generally sub-horizontal or gently dipping to ESE and to a stretching lineation, usually associated to sheath folds, indicating a movement towards NW which is also detectable at thin-section scale (Fonseca and Ribeiro, 1993; Fonseca et al., 1999). These facts indicate the same sense of shear. We consider that these structures represent the second phase of the ophiolite emplacement (Fonseca, 1995).
Figure 7. Outcrop of a very deformed sector of metabasalts unit
It is possible to observe the tree main impulses of D2 deformation event.
mylonitic foliation_ Sm
folding of D2 mylonitic foliation;
Generation of thrust faulting to the WNW following the folding.
Some latter D2 WNW-ESE sinistral faults, presently filled with hydrothermal complex carbonates - listwaenite (representing metassomatic carbonatized serpentinites) (Fonseca et al., 1999; Mateus et al., 1999), dismembered and pulled apart the original structures. These faults are responsible for the presence, side by side, of rock types corresponding to very different metamorphic grades.
These faults have been reactivated during D3, usually as sinistral thrusts towards SW with a cleavage WNW-ESE strongly dipping to NNE, and, along the Guadiana Valley, they become more brittle with thrusting to SW with a sinistral component (this movement is well represented in the Ferreira-Ficalho ductile-brittle shear zone).
The Guadiana traverse, crossing all Évora-Beja Domain (EBD) and the southernmost part of Estremoz-Barrancos Domain (EZBD), was mapped by Araújo (1995) at 1/5000 scale. All the area covered by this work is presented on Figure 8 (map B) at 1/50000 scale. This map only includes the main geological subdivisions defined by this author. The detailed geological mapping of the areas indicated as sheet 1, sheet 2 and sheet 3 is shown respectively in the maps B1, B2 and B3 (figs. 9, 10 and 11), at 1/20.000 scale, where detailed cross sections at 1/10000 scale are also presented.
The structure of the EBD is characterized by multiple sub-horizontal earlier thrusting, often associated to kilometric recumbent folds, frequently refolded by a latter deformation event. These structural complexities always make difficult the correlation among different geological units. Nevertheless, along the Guadiana Valley the tectonostratigraphic sequence could be resumed as shown in Figure 12.
Figure 12. Tectonostratigraphic sequence
Tectonostratigraphic sequence of the Évora-Beja Domain (adapted from Oliveira et al., 1991 and Mateus et al., 2005):
Moura Phylonitic Complex (alochthonous), schists and psamites (SP), lidites (L), blueschists (Bs), ophiolitic slices (Oph), mafic (B) and felsic (f) metavolcanics;
Moura-Ficalho Volcanic-sedimentary Complex (Middle Cambrian – Silurian?), calcitic marbles (M), schists (S); mafic (B) and felsic (f) metavolcanics;
Dolomitic Formation (Lower Cambrian), dolomites (D, locally silicified), felsic metavolcanics (v) and basal conglomerates;
Águas de Peixe Formation, black schists (BS), greywackes (G), black cherts (black); 5 – Escoural Formation, micaschists (m); felsic metavolcanics (v), amphibolites and gneisses (ga).
The upper tectonic unit (Moura Phyllonitic Complex - MPC) is mainly represented by micaschists and corresponds to a strongly imbricated complex, involving several layers of autochthonous sequences (mainly volcano-sedimentary rocks), but it also includes dismembered and scattered slices of ophiolites. The widespread greenschists facies overprint an earlier high-pressure metamorphic event (blueschists in the Guadiana Valley and eclogites in the western sector of EBD, Fonseca et al., 1993). According to the geochemical signature, MPC includes amphibolites ranging from N-MORB to T/P-MORB (ophiolitic slices, Araújo et al., 1993, Fonseca et al., 1999, Pedro, 2004) and mafic alkaline and peralkaline metavolcanics (autochthonous slices).
Macrocospic analysis, at the all EBD scale, indicates that the MPC truncates the units that are geometrically beneath: to the East, in Moura-Ficalho region, the MPC overlaps formations with Silurian or Lower Devonian ages (Piçarra, 2000); to the West, in Viana do Alentejo-Alvito region, it even overlaps Precambrian formations. This cartographic pattern is considered as an evidence of a major first phase overthrust, that limits the MPC from the underlying autochthonous sequences.
The overall evidences (tectonic, metamorphic and geochemical) leads to the interpretation of MPC as an obduction related “mélange” (Araújo et al., 2005), induced by the first deformation phase, between the southern boundary of the OMZ and the ophiolitic nappes (Figure 13).
Figure 13. Interpretative section of major D1 structures
Interpretative section of major D1 structures, at full EBD scale (after Araújo et al. in prep.):
Precambrian basement;
Cambrian to Silurian/Lower Devonian autochthonous sequences;
Moura Philonitic Complex; a – autochthonous fragments; b - Ophiolitic fragments; c – High pressure rocks;
Ophiolitic napes.
The first deformation event (D1) is preserved along the southernmost sector of the EBD and is correlated to D1 in the BAOC. This deformation phase is expressed by a mylonitic foliation, usually sub-horizontal. The related stretching develops a mineral lineation with NNE-SSW trending, and frequent shear criteria indicating top towards the north at meso and microscopic scales. The structures of this phase are considered a direct evidence of the ophiolitic obduction over the EBD. D1 is generally well preserved in the ophiolitic slices and in the upper tectonic levels of the MPC. The first deformation phase, although often overprinted by latter phases, is recognised until 30 kms north of the suture line materialized by the BAOC. The D1 deformation event is responsible by the main imbrication of allochthonous (ophiolites) and parautochthonous units, including high-pressure rocks (blueschists and eclogites).
The second deformation event corresponds to a thrust and recumbent fold system trending NS or NNW-SSE, with W facing and a penetrative axial plane cleavage. The stretching lineation is generally oblique or almost parallel to the fold axes and the shear criteria indicates top to the NNW. The geometry and kinematics of this phase result from a tangential transpressive regime (Araújo & Ribeiro, 1995). The D2 deformation is responsible for the accentuation of the structural complexity of the MPC. Some D1 thrusts show evidence of reactivation during this deformation event.
A third deformation phase induces a NW-SE folding with an associated steeply dipping crenulation cleavage, with vergence towards the SW. This cleavage sometimes is not axial planar and transect the coeval folds, showing the existence of a left lateral shear component related with the regional flattening. This event is clearly more brittle than the previous ones, and overprints the regional metamorphic peak in greenschists facies. The overall region is still cut by late variscan faults in several directions (mainly strike-sleep faults). Some of these faults were retaken during the alpine cycle.
The EZBD and the Elvas -Alter do Chão Domain (EAC, Figure 1) are both located in the central sector of OMZ and have a relatively well established litostratigraphic sequence. The region was mostly affected only by a singular low grade metamorphic event (low greenschist facies - chlorite zone). In these domains detailed biostratigraphic data helped to constrain the OMZ autochthonous stratigraphy (liñan et al., 1990) and hence the regional structural-tectonic framework.
The nature of the boundary between EZBD and EAC still remain controversial. According to several authors (Gonçalves, 1971; Ribeiro et al., 1979; Araújo et al., 1994) it corresponds to a major tectonic structure, the Juromenha overthrust. Silva (1997) minimizes the meaning of this structure, considering it only a discrete thrust zone. A different interpretation is proposed by Oliveira (1984) and Oliveira et al., (1991); for these authors this discontinuity is only a Cambrian -Ordovician unconformity. In this work we propose that the EZBD northeast limit is an overthrust, although we are aware that this interpretation needs to be supported by detailed complementary studies.
The Southwest EZBD boundary is established with the EBD, by a major polyphasic shear zone, the Santo Aleixo da Restauração shear zone (SAR-SZ). This main structure was considered a major overthrust (Araújo, 1995 and Araújo et al. 1995) with a sinistral shear component.
A detailed geological and structural mapping was produced in the EZBD (Transect Sector C of Figure 1) in the vicinity of the Guadiana River valley, during 2001 to 2005 (map C and related interpretative cross-sections, figs. 14 to 17). This field work has provided an extensive review of the regional geological knowledge and led to some reinterpretation of the region, partially supported by recent biostratigraphic data (Piçarra, 2000). An almost complete and continuous Silurian-lower Devonian fossil record in the EZBD has been put in evidence, which is an important constraint to the tectono-stratigraphic and geodynamic evolution of OMZ; a lithostratigraphic sequence for all the region could than be proposed (Figure 18).
The Santo Aleixo da Restauração metabasites, with alkaline signature (Carvalho, 1987; L. Ribeiro et al., 1992), correlates this rocks with the initial Cambrian continental rift phase (Quesada, 1990) , followed by a general Ordovician-Devonian passive margin stage (unites 2 to 7).
The synorogenic unit 9 (and probably also unit 8) is a foreland flysch type succession, ca. 2 km thick infilling basin (Early Devonian), which display only one penetrative deformation episode, the regional D2. Towards the East, the unit 8 is a lateral equivalent of this flysch unit; it corresponds to a distal siliclastic platform facies, of the same sedimentary basin (Piçarra, 2000).
From a structural point of view the EZBD was affected by two main penetrative and pervasive deformation phases - D1 and D2 (corresponding to D2 and D3 on the southwest sectors previously described). Both phases were generated under low grade metamorphism. The D1 event creates recumbent folds with the development of a co genetic sub horizontal axial plane slaty cleavage (S1). The D1 fold axes are scarcely recognized on a macroscopic scale, but very widespread in the outcrop scale. The trending orientation of these D1 axes is variably, but statistically there is a cluster nearby the N-S direction. This clear predominance of N-S direction locally shifts to NW-SE and could be the result of the superposition of the D2 shearing.
In some restricted areas the D1 folds show that the regional facing is towards the northern quadrant (inferred using sedimentary polarity control “way-up” criteria). Geometrically, L1 define metric folds with arcuate shape axes (sheath folds like?), mainly verging to NE, N and/or NW. The observed and mapped folds imply a structural kinematics during regional D1, that are not in accordance with some previous works (Simancas et al., 2001; Poyatos, 2002). The described scenario, approaching the Terena Syncline, is quite different; on SW limb the D1 small folds are scarce and mainly absent in the Northern limb.
The D2 deformation event is also pervasive and has developed NW-SE folded structures observed at all scales, from kilometric folds to crenulation cleavage (Figure 17). D2 main structures includes some major folds as the Terena Syncline and Zebro Synform (Araújo, 1989, and Araújo, 1995), with steep axial planes striking NW-SE.
Figure 18. Simplified litostratigraphic sequence
Simplified litostratigraphic sequence of the Estremoz-Barrancos Domain in the studied area (partially adapted from Piçarra, 2000):
Moura Phylonitic Complex (alochthonous): schists and psammites;
Santo Aleixo da Restauração amphibolite schist and metabasites (Middle Cambrian?);
Cabanas da Choupana Formation (Middle Cambrian ?), 3a- Psammite Member, greenish quartzwackes and very impure quartzwackes, 3b- Pelite Member, grey-blue to pink-brown slates with pink-brown siltstones intercalations;
Xistos de Barrancos Formation (Ordovician ?): 4a- Inferior Member, violet to grey and green slates and intercalated siltstones, 4b- Superior Member, green to grey slates and silts;
São Marcos do Campo Volcano-Sedimentary Complex (Ordovician ?), Volcaniclastic violet and green sediments (ranging in grain size from gravel to lutite, but predominantly sandy);
Colorada Formation (basal Silurian), quartzites and impure quartzites;
Xistos com Nódulos Formation (Silurian), euxinic black shales with minor black cherts;
Xistos Raiados Formation (Lower Devonian), grey shales and white siltstones ;
Terena Formation (Lower Devonian), mainly shales - greywackes alternations displaying turbidite characteristics (some deep sea fan facies may be recognized) , defining a Flysh sequence.
The variation in the minor folds axes dip (L2), suggests the actuation of a sinistral slip parallel to the D2 trending contemporaneous with the main shortening and suggesting the actuation of a sinistral transpressive regime. A similar model where proposed for other places in the EBZD, e.g. Lopes, 2003, for the Estremoz Anticline region (Figure 19).
Figure 19. Conceptual block diagram
Conceptual block diagram of D2 Variscan deformation in the EZBD, along the Guadiana River transverse.
To the south of the Zebro Syncline and towards the SARSZ there’s a progressive increment of the D1 deformation, with the local development of a stretching lineation Lx; in the vicinity of the SARSZ, Lx affects the previous D1 folds axes (materialized by the intersection lineation L1), and locally develops a S1b cleavage (Figure 17).