Nova Terra 52

Márquez et al . 2003), affects the Miocene deposits and the overriding crystalline basement (Figure 7(d)), sug- gesting a post-Miocene and pre-Pleistocene age (prob- ably Pliocene) for some Alpine thrusts in the region. The Ordovician and pre-Ordovician rocks of the study area are separated by a SW-dipping fault, the San Pedro thrust (RP-3; Figure 3). Kinematics of this fault is top-to- the- NE and includes a left-lateral component that makes it an oblique-slip thrust (Gonzalo 1987, 1989; Bandrés 2001). The hanging- and footwall blocks of this fault show different structural records. The internal structure of the footwall is dominated by Ordovician strata affected by NW-SE trending, upright to NE-verging over- turned folds (Cornalvo synform), with a single main axial plane foliation (S V ; Figures 4(a), 7(c) and 8(a)). Strata is duplicated and folds are cut by SW-dipping thrusts with moderate offset, which probably represent minor fault imbricates of the San Pedro thrust. The internal structure of the hanging wall to the San Pedro thrust is defined by rocks showing a main foliation affected by NW-SE trend- ing, upright to either NE- or SW-verging overturned folds (Figure 3, 4(a), 6(b–d), and 8(b)). Fold axes vary from dominantly NW-plunging in the E and SE section to SE- plunging in the W section, as indicated by periclinal fold closures of lithological contacts and fabrics (Figure 3) and by its projection in a stereoplot (Figure 8(b)). NE- verging folds are locally accompanied by left-lateral thrusts that are sub-parallel to a poorly developed cre- nulation cleavage (S V ) and which cut across fold fore- limbs (Figure 6(b)). This relationship reproduces the major fold and thrust structures of the region (Figure 4 (a)). SW-verging folds are scarcer and tend to occur around a NE-dipping thrust that cuts across the intern- ally folded structure of the hanging wall block to the San Pedro fault; this NE-dipping thrust is referred to as the Barranca back-thrust (RP-4; Figure 3). The main foliation (S C ) parallels the boundaries of the metagranitoids observed in the Upper Schist-Metagranitoid Unit (Figure 3). The Lower Gneiss Unit crops out in the core of a dome-like fold, the Magdalena antiform or dome (RP-5; Figure 3), while the Cambrian marbles occupy the core of a NW-plunging synform (Figure 8(c)) paired with the Magdalena antiform, here referred to as the Carija synform (RP-6; Figure 3). Using the main foliation (S C ) as reference, the structure of the pre-Ordovician units com- prises a tectonic pile with the Lower Gneiss Unit resting below the Mérida Ophiolite, which is covered by the Schist-Metagranitoid Unit (Ediacaran Serie Negra Group) and then the Carija Unit (Cambrian marbles) (cross-section in Figure 4(b)). The internal structure of the Mafic-ultramafic Unit (Mérida Ophiolite) is defined by variably strained domains. Field observations suggest that strain is more intense around exposures of ultramafic rocks (Figure 5 (d)), being moderate away from them (Figure 5(e)), and low and heterogeneous over distal sections (Figure 5(c)). The development of the main foliation (S C ) in this unit is related to non-coaxial deformation, as indicated by the development of asymmetric microstructures and minor shear zones (Figure 5(c)). A map of the Mafic-ultramafic Unit (Figure 3) shows sections with relatively more strain, which show more effective metamorphic recrystalliza- tion of the mafic protoliths, and are accompanied by tabular-shaped bodies of metaperidorites, which can reach a minimum of 2 km in lateral continuity and up to 200 metres in thickness. We cannot rule out the possibility that there are more sections within this unit with similar features, but with the data at hand, the internal structure of this tectonostratigraphic unit con- sists of at least three tectonic slices, each of which is separated from the rest by a layer containing Figure 8. Stereoplots showing the orientation of planar fabrics of the Mérida Massif. (a) Bedding and Variscan axial plane foliation in the Ordovician folded series, with calculation of fold axis trend. (b) Folded Cadomian foliations with indication of the NW-SE trend of Variscan folds. (c) Bedding in the Carija Unit and calculation of Variscan fold axis trend. 10 R. D. FERNÁNDEZ ET AL. &KDSWHU