serie NOVA TERRA nº 49

72 allochthonous complexes. The three deformation events are described below, starting with the most recent. The youngest deformation event (D 3 ) consists of metre- to decametre-scale folding developed throughout the entire unit and comprising close, upright structures with axes plunging gently N20°E. Centimetre-wide greywacke layers show fl attened parallel folds, sometimes developing cuspate – lobate forms. A near-vertical axial planar crenulation cleavage (S 3 ) is always present and is better developed in metapelitic beds. These beds sometimes show strong differentiation with a spaced, non-penetrative crenulation cleavage that is less evident, discontinuous or absent, in the metagreywackes. In the coastal section ( Figs. 1 and 2 ) meter-scale D 3 folds show opposite asymmetry, de fi ning a major D 3 synform, the enveloping surface of which is shown in cross-section in Fig. 2 b and can be followed inland, where it de fi nes the major Órdenes synform ( Fig. 1 ). This major synform belongs to a widespread Variscan refolding phase in the Órdenes Complex that affected the stacking of allochthonous units and their bounding shear zones (e.g., ( Martínez Catalán et al., 1996 )). Structures belonging to the older D 2 deformational event affect almost all the upper units, but in the study area an upper deformation front can be traced parallel to the “ biotite in ” boundary ( Fig. 2 ). The most impor- tant structure of this event is a regional S 2 foliation that shows various microstructures corresponding to early and intermediate stages in the morphological evolution of a crenulation cleavage under increasing de- formation and temperature, as proposed by Passchier and Trouw (2005) . Although strongly affected by upright folding, it is assumed that S 2 was initially subhorizontal. The foliation is axial planar of minor folds that are dextral in asymmetry, and sometimes non-cylindrical with the intenselyshearedshortlimbs.Thisfeature,togetherwiththepresenceof a stretching lineation parallel to the fold axes, suggests top-to-the-north sense of shear. At deeper structural levels the S 2 foliation is a highly evolved compositional foliation, the development of which was coeval with amphibolite facies metamorphism ( Castiñeiras, 2005 ). Towards the base of the intermediate pressure upper units, partial melting of the metasediments took place during the fi nal stages of S 2 development, which shows a metatexitic appearance and is locally disaggregated by diatexite. U – Pb analysis of monazites frommetatexites has yielded an age of 490 Ma ( Abati et al., 1999 ). This apparently structural continuity is interrupted by the development of a major extensional shear zone, the PonteCarreiradetachment( MartínezCatalánetal.,2002;GomezBarreiro, 2007 )(PCDin Fig.2 ),whichledtothethinningofthegarnetandstaurolite zones, strong retrogression and phyllonitization, and the formation of C ′ shear bands ( Berthé et al., 1979 ). The mineral stretching lineation parallel to D 2 fold axes is well de fi ned in these rocks where it is aligned north – south and indicates top-to-the-north sense of shear. The fi rst deformation event (D 1 ) was probably present throughout the intermediate pressure upper units, although at lower structural levels no major structures have been identi fi ed, and D 1 is represented only by an S 1 cleavage strongly overprinted by S 2 . In fact, it is only recognizable in relict D 2 fold hinges or as oriented inclusion trails in garnet or staurolite porphyroblasts. The low-grade turbiditic rocks on the coastal section near Ares, however, are devoid of D 2 deformation and so are suitable for the study of minor D 1 fold asymmetries. Combining this with the angular relationships between the existing foliations (S 1 and S 3 ) and bedding (S 0 ), and the younging direction of the strata, allows the position of the fold hinges to be located and provides a means of determining fold facing vs. vergence in polyphase folded terrains. Metre-scale folds developed in alternating grey- wackes and pelites are tight with chevron and similar morphologies. Their axial planes have varying orientations as a consequence of late D 3 folding, but their axes plunge gently towards N20°E. The axial planar S 1 cleavage is a pervasive slaty cleavage at a low angle to the bedding and is well developed in the metapelitic tops of the turbiditic strata. In thin section, S 1 is a continuous foliation ( sensu ( Passchier and Trouw, 2005 )) mainly de fi ned by fl attened and elongate quartz grains and the non-domainal homogeneous distribution of platy mineral grains with a preferred orientation. The younging direction deduced from graded bedding in greywacke layers shows frequent changes, from which it can be inferred that the largest D 1 reverse limbs never exceed 1 km in wavelength. Decametre- and centimeter-scale, upward facing D 1 folds can be observed in the western limb of the major Órdenes D3 synform. From these the sinistral asymmetry of a major west-vergent fold can be deduced. In the eastern limb of the Órdenes D 3 synform, suitable minor folds have not been found, but microscopic analysis of oriented samples of S 1 , S 3 and S 0 angular relationships suggests a west vergence for this fi rst deformation event ( Fig. 3 ). Thus, the structural and younging criteria discussed above, together withthemainstratigraphicfeatures of the area permit the reconstruction of the cross section shown in Fig. 2 b. Here, the structural architecture is depicted by a set of west-vergent D 1 folds that have reverse limbs less than 2 km in wavelength and are Fig. 3. Microstructural aspects of the different types of foliation in the low-grade part of the upper units and their angular relationships that, together with the presence of graded bedding, suggest a west vergence for the D 1 fold nappe. (a) Minor “ Z shaped ” folds developed in the reverse limb of a west-vergent D 1 fold that was later affected by a crenulation cleavage (S 3 ). (b) Angular relationship between bedding (S 0 ) and slaty cleavage (S 1 ) within the normal limb of a minor downward facing D 1 fold. Fig. 2. (a) Geological map of the Ares-A Coruña – Betanzos region showing the main lithologies, the sectors with reverse stratigraphic polarity, and the asymmetry of minor folds of the different deformation events. (b) Interpretive cross sections accounting for the presence of several sectors with reverse stratigraphic polarity and S 0 /S 1 angular relationships. The structural level with abundant of conglomerates and thick greywacke layers has been highlighted. The location of the dated diabase dyke is also shown. 355 F. Díaz García et al. / Gondwana Research 17 (2010) 352 – 362