Nova Terra 52

foliation (S C ) observed in the hanging wall to the San Pedro thrust, both (foliation and mechanical contacts) defining the same NE-SW trending folds (Figure 4(a)). Strain and metamorphic recrystallization is heteroge- neous, but the metamorphic grade varies vertically across structure. The mineral assemblages of the main foliation (S C ) in the hanging wall to the San Pedro thrust defines an overall normal metamorphic gradient. As a reference, the main foliation (S C ) in the metasedimen- tary rocks of the Upper Schist-Metagranitoid Unit (Figure 6(b)) may include quartz, plagioclase, white mica, biotite, and minor (secondary?) chlorite, which make a typical greenschist facies fabric (probably in the biotite zone). The main foliation (S C ) in the mafic rocks of that unit (Figure 6(d)) includes plagioclase, fine-grained green amphibole, zoisite, epidote, titanite and chlorite, an assemblage also compatible with greenschist facies con- ditions. The main foliation in the mafic rocks of the Mérida Ophiolite (S C ) (Figure 5(c–e)) is defined by plagi- oclase, brown-green amphibole (hornblende), titanite, opaques, and minor rutile. Some exposures include large garnet porphyroblasts, and altogether define a typical assemblage for amphibolite facies conditions (grain-size for the meta-mafic rocks in this unit is signifi- cantly larger than in the Upper Schist-Metagranitoid Unit). PT conditions for this assemblage were estimated at 1.2 GPa and 750°C (Bandrés et al . 2000). The main foliation in the Lower Gneiss Unit (S C ) includes recrystal- lized plagioclase, K-feldspar, green amphibole, biotite and quartz ribbons with granoblastic polygonal texture. This fabric is occasionally accompanied by patches of melt crystallized along bands parallel to the main folia- tion. This piece of crust characterized by a regional nor- mal metamorphic gradient is juxtaposed onto Ordovician strata, which show a penetrative slaty clea- vage (S V ) formed by quartz, white mica, chlorite, sericite and opaques (chlorite zone). The crenulation cleavage (S V ) that is locally superimposed to the main foliation (S C ) in the metasedimentary rocks at the hanging wall to the San Pedro thrust (Figure 6(b)) consists of similar mineral assemblage. 5 . Discussion and conc l usions Alpine deformation in the Mérida Massif is limited and mostly restricted to high-angle thrusts that reworked its crystalline basement and faulted its Cenozoic cover (at least during the Pliocene). The main shortening direction is NE-SW and dominant tectonic transport for thrusts is to the SW. Given the distal position to Cenozoic plate boundaries, this deformation can be framed as an intra- plate setting for the Iberian micro-plate (e.g. de Vicente and Vegas 2009). The NW-SE trending folds that affect the basement of the Mérida Massif represent the first deformation for the Ordovician strata, but are at least the second pulse of deformation in pre-Ordovician rocks (note their main foliation and ductile shear zones are affected by these folds). This indicates that these folds, their local axial plane foliation (S V ), and the faults they are cut by (San Pedro and Barranca thrusts) are Variscan in age, and that the main foliation (S C ) and ductile shear zones in the pre- Ordovician rocks are pre-Variscan and responsible for the layered structure of most of the tectonostratigraphic units. This pre-Variscan age of deformation is supported by Sm-Nd dating of garnet in the foliated metabasites of the Mérida Ophiolite (555 Ma; Bandrés et al . 2004), and because Early Cambrian rocks from other sectors of the northern Ossa-Morena Complex rest unconformably on metagranitoids and metasedimentary rocks bearing foliations similar to those in the study area (Bandrés 2001). As a consequence, the current contacts between tectonostratigraphic units in the hanging wall to the San Pedro thrust could be interpreted as Cadomian struc- tures. Normal metamorphic gradient within and around their juxtaposed tectonic blocks, together with their crustal-scale bearing (they juxtaposed sections with dif- ferent metamorphic grades), suggest they represent large-scale extensional shear zones. This is also sup- ported by the cross-cutting and subtractive geometry of the Trujillanos detachment relative to the ultramafic layers of the ophiolite and the boundaries of orthogneiss massifs. The current folded structure of these shear zones favours a primary flat-lying geometry for all of them, and their trace is sub-parallel (yet slightly oblique) to lithological contacts within tectonostratigraphic units, so they should be referred to as extensional detach- ments. The Carija detachment affects Cambrian strata, so the functioning of some of them may be restricted to the Cambrian and perhaps Ordovician period, which is a time interval dominated by extensional tectonics in SW Iberia (Expósito et al . 2003; Simancas et al . 2004; Díez Fernández et al . 2015). The similar trend of stretching lineations within extensional shear zones and their hanging wall and footwall blocks and the continuity between planar fabrics suggest these Cadomian struc- tures were probably developed during the same defor- mation event. Opposed kinematics between the Trujillanos and Carija detachments, and the dome struc- ture in the Lower Gneiss Unit favour a model of biver- gent extension dominated by NW-SE lateral flow. The dome-like structure cored by the Lower Gneiss Unit should result from the interference between two fold sets. The younger set explains the elongate NW-SE shape of the dome. It is Variscan and related to contrac- tion, as its axial plane parallels the Variscan axial plane 12 R. D. FERNÁNDEZ ET AL. &KDSWHU