Introduction
Allochthonous units in the NW Iberian Massif
In the NW of the Iberian Massif, an allochthonous nappe stack crops out in five complexes. The Órdenes complex is the largest one, where a complete pile of units can be identified (Gómez Barreiro et al., 2007; Martínez Catalán et al., 2009). Units have a distinct tectonometamorphic imprint and are separated by tectonic accidents. From bottom to top, in structural order they are: Basal, Ophiolitic, and Upper units, the last one subdivided into High Pressure – High Temperature (HP-HT) units below, and Intermediate – Pressure (IP) units above (Fig. 1 A).
Figure 1. Geology of the Órdenes complex
(A) Geological map of the Órdenes complex and its location in the Iberian Massif (NW Spain). (B) Geological map and representative sections of the NW of the Órdenes complex, where the Bazar ophiolite unit and its relationship with Upper and Basal units are depicted. The inset shows the location of Figure 2. Based on Abati et al., (1999).
Basal units are considered the outermost margin of Gondwana by the time of Variscan orogen (Martínez Catalán et al., 2009). They include terrigenous and metaigneous rocks which recorded the subduction under the acrectionary pile during Late Devonian (Abati et al., 2010). The exhumation of the Basal units was driven by ductile eastward thrusting, with the development of regional recumbent folds .
Ophiolitic units represent different oceanic realms existing between the colliding continents Laurrussia and Gondwana at that time (Rheic ocean; Arenas et al., 2007). Differences in lithological association, chemical signature and age, led to a subdivision into Upper and Lower ophiolitic units (Fig.1 A; Arenas et al. 2007, Sánchez Martínez et al. 2009, 2011, and references therein). This assemblage appears tectonically dismembered, and defines the suture of the Varican orogen in this sector. In some units it is recognized a pre-collisional structure, represented by high-T shear zones with a top-to-the ENE sense of shear, probably initiated in an intraoceanic subductive system, and followed by the building of the accretionary wedge (Díaz García et al, 1999; Gómez Barreiro et al., 2010a). The emplacement of the suture and overriding units occurs through a system of out-of-sequence thrusts, which led to an imbricate architecture. A top-to-the SE shear sense is commonly associated with this event, but it has not been widely confirmed to a regional scale (Martínez Catalán et al. 2002).
Upper units include terrigenous metasediments and metaigneous rocks with arc affinity. HP-HT units consist of paragneisses, basic and ultrabasic metaigneous rocks, metamorphosed under HP granulite to eclogite facies conditions (Martínez Catalán et al. 2002; Gómez Barreiro et al., 2006). IP units include terrigenous metasediments and bodies of amphibolites, gabbros and orthogneisses. Metamorphic grade ranges from granulite facies in the lower parts to greenschist facies on top. A polyorogenic evolution is proposed for the Upper units, where several metamorphic cycles and related structures have been identified and dated (Martínez Catalán et al., 2002; Gómez Barreiro et al. 2006, 2007a).
The structural evolution of the Variscan suture
Sutures are exceptional witnesses of the orogenesis. Their role as a boundary between colliding continents gives suture units a puzzle nature, and, in many cases, record critical events of pre-collisional and collisional stages (Gómez Barreiro et al., 2010a; Díez Fernández et al. in press).
In this paper we are focusing on the Variscan suture that appears in the NW of the Órdenes complex (Fig. 1). In particular we will examine the contentious contact between the IP Upper units and the Ophiolites, named respectively, after local places, the Monte Castelo and Bazar units (Abati, et al., 1999). Regional correlation suggests that this contact could be part of the out-of-sequence thrusts system (Martínez Catalán, 2002). However, reported structural data in the literature indicate a top-to-the E sense of shear (Díaz García, 1990), and may indicate that an older, in-sequence, architecture could be preserved there.