of strongly contrasting geodynamical environments (Fig. 2; e.g. Ribeiro et al., 1990; Martínez Catalán et al., 1997). These nappes have been displaced over a major thrust which, in northern Portugal, is known as the Main Trás-os-Montes Thrust. In the traverse between the two sectors described in this paper (Marão and Murça), this thrust seems to be a layer parallel décollement controlled by the particular rheological properties of Silurian autochthonous black shales in which it is rooted. Further E the Main Trás-os-Montes Thrust becomes a D2 out-of-sequence thrust inducing metamorphic jumps and cutting across lower thrust in the vicinity of Moncorvo (Ribeiro, 1974; Dias, 1994; Rodrigues et al, 2003).
While for the upper nappes a far distance source is inescapable, the close similarities between the lithostratigraphy of the lower unit and the upper part of the Centro-Iberian autochthon succession points to a shorter displacement (only some tens of kilometers; Ribeiro et al., 1990). Due to these affinities between both units, the lower allochthonous complex is considered as a parautochthonous unit (Iglésias et al., 1983; Ribeiro et al., 1990). In spite of the similarities, when considering the first and main Variscan structural deformation (D1) a deep contrast is found.
Figure 2. Cross-section
Northern Portugal geological cross section emphasizing the spatial relations between major thrusted units (adapt. from Ribeiro et al., 1990) ; see Figure 1 for location.
Due to the sinistral transpressive deformation typical of the Ibero-Armorican Arc southern branch (Dias and Ribeiro, 1995; Ribeiro et al., 1995), major D1 left lateral wrench faults are pervasive in the northern domains of the autochthonous (Dias, 1994; Dias and Ribeiro, 1994; Coke, 2000). Although NW-SE is the predominant orientation of these structures, they swing from N-S close to the Atlantic coast, to E-W close to the Spanish boarder, depicting the expected major arcuate pattern. These shear zones bound sectors where the main structures are macrofolds with an axial plane subparallel to the coeval boundary faults. This unusual spatial relation between the axial planes and the wrench faults, have been interpreted (Dias, 1994, 1998) as due to a strong strain partitioning: the simple shear component is concentrated in discrete fault zones while the pure shear predominates in the blocks between them (pure shear dominated transpression of Fossen et al., 1994 and Tikoff and Greene, 1997). These folds have low dipping hinge zones (usually less than 10º) that are subparallel to the intersection lineation between the bedding and the cleavage (L1), a geometrical relation emphasizing the generalized absence of transection in the main Variscan folding event. The coeval stretching lineation (X1), well marked in the interbedded pelitic sequences, is always close to the horizontal being subparallel to the kinematic b axis.
Concerning the facing of this major tectonic event an heterogeneous behavior is found at the Centro-Iberian autochthon scale (Dias, 1998; Dias et al., this volume). While in the southern sectors subvertical structures predominate, in the northern ones an asymmetric flower-structure (Dias, 1998) was induced by the Malpica-Lamego major basement anisotropy (Llana-Fúnez and Marcos 1998, 2001; Coke et al., 2000); around a narrow sub vertical domain a short branch with SW facing and a long northern one with a NW facing has been developed. The NW facing predominates adjacent to the southern boundary of the parautochthon close to the studied domains.
As previously mentioned, when considering the parautochthonous, the geometry and kinematics of the major Variscan structures are strongly different. Until recently, no clear distinction is made concerning the existence of different structural styles inside the lowermost nappe of NW Iberia. In several works (e.g. Pereira, 1987; Clavijo, 1997) this nappe has been described as an imbricated thrust system, but there are other works with references to recumbent macrofolds with kilometric inverted limbs (Ribeiro, 1974; Ribeiro et al., 1990). Recent detailed geological mapping of the displaced units in the region of Murça allows the distinction inside the parautochthon of two main structural domains (Rodrigues et al., 2003) separated by a major thrust plane of regional extent, the Palheiros – Vila Flor Thrust:
- The Lower Parautochthonous Structural Domain or Subautochthonous Domain, characterized as an imbricate thrust system;
- The Upper Parautochthonous Structural Domain characterized by a fold-nappe structure.
The main reason to consider this structural subdivision in the Parautochthon is not a stratigraphic one, or even the regional extent of the Palheiros – Vila Flor Thrust. In fact the establishment of a hierarchy of thrust planes is difficult using only outcropping data. Features like lithologic types, metamorphic grade or units age are frequently used to distinguish first order thrusts from minor internal thrust planes (Merle, 1994). In this case, macrogeometries of the D1 Variscan deformation phase are the main reason to consider the Palheiros – Vila Flor Thrust as a first order thrust plane and, consequently to separate two structural domains in the Parautochthon.
The Lower Parautochthonous (Meireles et al., 1995) or Subautochthonous (Meireles, 2000; Pereira, 2000) domain was individualized in order to include a set of lithostratigraphic units tectonically imbricated by reverse faults. Some works (e. g. Pereira, 1987; Clavijo 1997) give fully descriptions of the D1 Variscan structural style and orientation in this structural domain. From these works, as well from some outcrops in Murça sector several main features can be emphasized:
- the imbricate pattern of internal reverse faults;
- the asymptotic vertical profile of these faults (second and third order thrust planes) with coalescence in deep with the Main Trás-os-Montes Thrust or lateral equivalents;
- the inexistence in the Lower Parautochthonous Domain of units older than the Silurian;
- the predominance of normal stratigraphic successions except in some minor inverted limbs, usually in the vicinity of thrusts;
- the asymptotic vertical profile of the S1 cleavage, passing from a sub vertical slaty cleavage to the close parallelism with the subhorizontal thrust planes, underlined by phyllonites.
Concerning the Upper Parautochthonous it was also firstly named in Meireles et al. (1995) in order to include the set of stratigraphic units of the Peritransmontano Domain of Ribeiro (1974). In the present work, the name Upper Parautochthonous Domain specifically refers to the Paleozoic metassedimentary sequence between the Palheiros – Vila Flor Thrust and the Lower Allochthonous basal thrust. Concerning the predominant D1 Variscan structures, several characteristics could be emphasized:
- the rootless fold-nappes with subhorizontal axial planes parallel to the basal thrust;
- the curved geometry of the hinges underlining an arcuate pattern;
- a deformation regime far from plane strain as shown by the arcuate fold hinge pattern;
- the heterogeneous nucleation of shear zones rooted in reverse limbs of D1 macrofolds is common.
The field recognition and mapping of the main regional thrust planes are based on the occurrence of fault rocks. In the case of the Main Trás-os-Montes Thrust, a zone of variable thickness (usually some decimeters) of cataclasis and fault gouge can be easily distinguished (Ribeiro et al., 1990). The Palheiros – Vila Flor Thrust is essentially characterized by a strip of tens of meters of intense phyllonitization.
A second folding phase affects previous Variscan structures with variable intensity. The mesoscopic structures show, in most cases, the progressive folding of S1. This progressive behavior of the D2 structures is clearly shown by the correspondence between the tightening of D2 folds and the orientation of the fold hinges. In cases of very tight folds the hinge direction is close to the transport direction of the D2 thrusts; the progression of this process can originate sheath folds.