Tectonic framework of the Cabo Ortegal Complex: A slab of lower crust exhumed in the Variscan orogen (northwestern Iberian Peninsula)

Abstract

The Cabo Ortegal Complex is a composite allochthonous terrane that was thrust onto the western edge of Gondwana during the Variscan orogeny. It is formed of two main tectonic units: the Upper Tectonic unit, comprising rocks affected by highpressure (P)-high-temperature (T) metamorphism, and the Lower Tectonic unit, which represents the resulting suture of the Variscan collision. The suture preserves remnants of strongly deformed and metamorphosed ophiolitic rocks overriding the parautochthon, and the lower Paleozoic sequence of the Ollo de Sapo antiform, regarded as the autochthonous sequence of the Iberian plate. The Upper Tectonic unit is formed by layered ultramaµc, maµc, and quartzo-felspathic rocks that were buried at levels in excess of 50 km (~1.56 GPa) before the Variscan collision in a convergent plate boundary within the Rheic ocean domain ca. 490-480 Ma (Early Ordovician). They have been interpreted either (1) as an earlier thinned continental crust, underlain by a lithospheric mantle and oceanic spreading, or (2) as independent terranes, formed in different geodynamic settings (island arc, oceanic). Most structures observed in these rocks are ductile and are associated to their exhumation process. It started with the development of a persistent horizontal foliation in granulite facies conditions, which equilibrated in amphibolite facies conditions ca. 385 Ma, and ended in higher crustal levels with the progressive development of noncoaxial structures, such as eastverging asymmetrical isoclinal folds and thrusts, leading to the emplacement of the Upper Tectonic unit over the Lower Tectonic unit ca. 365 Ma (Late Devonian).

Insights on high-grade deformation in quartzo-feldspathic gneisses during the early Variscan exhumation of the Cabo Ortegal nappe, NW Iberia

“…1b and d; Marcos et al, 1984Marcos et al, , 2002. The subsequent tectonic evolution was controlled by the progressive localization of strain and the deformation along thrusts and the final emplacement of the HP-HT Cabo Ortegal nappe.…”

“…This complex is exposed along a coastal section with cliffs up to 600 m above sea-level, and contains some of the best exposures of HP-HT rocks of the Variscan orogen. Internally, it is divided into two main tectonic units: the Cabo Ortegal nappe and the lower tectonic unit (Marcos et al, 2002). The Cabo Ortegal nappe (Fig.…”

“…These units are interpreted to outcrop largely in the shape of an early recumbent synform ( Fig. 1d; Marcos et al, 2002). Overall, the whole lithostratigraphic sequence has been used as a proxy for the continental crust-mantle transition (Brown et al, 2009); and according to Ábalos et al (2003), it represents a stack of crustal and mantle units assembled in a subduction channel, i.e.…”

“…Ábalos et al, 2003); even though only the fabric elements of the retrograde Pressure-Temperature-time (P-T-t) path have been unequivocally recognized Fernández, 1997;Galán and Marcos, 2000). All units of the high-grade nappe show a pervasive blastomylonitic tectonic fabric, which locally is highly heterogeneously developed (Fernández, 1997;Marcos et al, 2002). This main tectonic fabric and associated structures define a regionally recognizable deformation episode (labelled D 2 ), formed during the exhumation from high-pressure conditions.…”

“…Ábalos et al, 2003), or were due to the superposition of structures during their exhumation from high-pressure conditions (e.g. Marcos et al, 2002). Regarding the initial exhumation from highpressure conditions to mid-crustal depths, several mechanisms have been proposed, in relation to extensional tectonics (e.g.…”