Ediacaran Obduction of a Fore‐Arc Ophiolite in SW Iberia: A Turning Point in the Evolving Geodynamic Setting of Peri‐Gondwana

Abstract

The Calzadilla Ophiolite is an ensemble of mafic and ultramafic rocks that represents the transition between lower crust and upper mantle of a Cadomian (peri-Gondwanan) fore arc. Mapping and structural analysis of the ophiolite demonstrates that it was obducted in latest Ediacaran times, because the Ediacaran-Early Cambrian sedimentary series (Malcocinado Formation) discordantly covers it. The ophiolite and emplacement-related structures are affected by Variscan deformation (Devonian-Carboniferous), which includes SW verging overturned folds (D 1 ) and thrusts (D 2 ), upright folds (D 3 ), extensional faults (D 4 ), and later faults (D 5 ). These phases of deformation are explained in the context of Variscan tectonics as the result of the progressive collision between Gondwana and Laurussia. Qualitative unstraining of Variscan deformation reveals the primary geometry of Ediacaran-Cambrian structures and uncovers the generation of east verging thrusts as responsible for the primary obduction of the Calzadilla Ophiolite. Restoration of planar and linear structures associated with this event indicates an Ediacaran, east directed obduction of the ophiolite, that is, emplacement of the Cadomian fore arc onto inner sections of the northern margin of Gondwana. According to regional data, the obduction separates two extension-dominated stages in the tectonic evolution of the African margin of northern Gondwana preserved in southern Europe. Preobduction extension brought about the onset and widening of fore-arc and back-arc basins in the external part of the continent, while postobduction extension facilitated the formation of extensional migmatitic domes, an oceanward migration of back-arc spreading centers across peri-Gondwana, and the eventual opening of a major basin such as the Rheic Ocean. Citation: Díez Fernández, R., Jiménez-Díaz, A., Arenas, R., Pereira, M. F., & Fernández-Suárez, J. (2019). Ediacaran obduction of a fore-arc ophiolite in SW Iberia: A turning point in the evolving geodynamic setting of peri-Gondwana. Tectonics, 38, 95-119. https://doi.

On the Rootless Nature of a Devonian Suture in SW Iberia (Ossa‐Morena Complex, Variscan Orogen): Geometry and Kinematics of the Azuaga Fault

“…The Iberian Massif hosts several suture zone exposures. Some of them have been identified as Cadomian (Neoproterozoic-Cambrian in age) (Arenas et al, 2018;Díez Fernández et al, 2019, while others are Variscan (Abati et al, 2018;Arenas et al, 2016;Azor et al, 2008;Moita et al, 2005;Pereira, Apraiz, et al, 2010;Rosas et al, 2008). A classical view for the Variscan examples is that almost all suture zone exposures are interpreted as recording separate and distinct subduction zones.…”

“…of Devonian to late Carboniferous sedimentary and igneous rocks. Variscan deformation is dominated by SW-verging folds and thrusts (which affect both preorogenic and synorogenic successions), followed by normal and strike-slip faults, and coeval upright folds (Díez Fernández et al, 2019;Eguiluz et al, 1990;Expósito et al, 2002). Variscan metamorphism ranges between low-grade (especially in the green section of Figure 3;Ramos, 2005) and medium-grade (some parts in the yellow section of Figure 3, Azor & Ballèvre, 1997;Garrote, 1976;González del Tánago & Arenas, 1991).…”

Tectonostratigraphy of the Mérida Massif reveals a new suture zone exposure in SW Iberia

“…The sequence and broad timing of tectonic events recognized for the building of Mérida Massif fit well into the regional geology of SW Iberia. The Cadomian suture zone and nappe structure identified in the Mérida Massif add to the evidence of Cadomian tectonics in southern Europe (Eguíluz et al, 2001;Díez Fernández et al, 2019;Quesada, 1990;Simancas et al, 2004;Pereira et al, 2012;Díaz García, 2006;Pieren et al, 1987;Bandres et al, 2002), which is tightly connected to subduction-accretion processes in the periphery of the African margin of Gondwana (Orejana et al, 2015;Fuenlabrada et al, 2012;Rojo-Pérez et al, 2019;Arenas et al, 2018;Sánchez Lorda et al, 2014;Linnemann et al, 2008;Bandrés et al, 2004). Variscan deformation in the Mérida Massif reworked but did not reactivate major Cadomian structures such as accretionary faults and extensional detachments, which are now observed as folded planes cut at high-angle by Variscan major faults.…”

“…The resulting tectonostratigraphy provides light into a nappe structure that had remained unnoticed for the region, despite large scale thrusting events in SW Iberia have been claimed for Variscan models (Azor et al, 1994;Díez Fernández and Arenas, 2015;Ribeiro et al, 2010) and Cadomian tectonics (Díez Fernández et al, 2019;Abalos et al, 1991). It also adds another ophiolite and suture zone exposure to the set that features the Iberian Massif, thus strengthening the notion of the Iberian domain as a region built after numerous suturing processes throughout geological history.…”

“…Yet, the current structure of the Iberian Massif is the result of three orogenic cycles. Gondwana was affected by long-lived subduction under its periphery during the Neoproterozoic and Lower Paleozoic (Linnemann et al, 2007;Quesada, 1990;Pereira et al, 2007;Eguíluz et al, 2000;D'Lemos et al, 1990;Nance et al, 1991;Chantraine et al, 2001), its northern paleomargin bearing abundant evidence of arc-related magmatism (Bandrés et al, 2004;Henriques et al, 2015;Dorr et al, 2002;Drost et al, 2004;Rubio-Ordóñez et al, 2015), basin development in active settings (Rojo-Pérez et al, 2019;Fuenlabrada et al, 2012;Fuenlabrada et al, 2016;Linnemann et al, 2000;Linnemann et al, 2007;Fernández-Suárez et al, 2013;Pereira, 2015), and contractional and extensional deformation (Díez Fernández et al, 2019;Expósito et al, 2003;Simancas et al, 2004;Eguíluz et al, 2000;Bandres et al, 2002;Kröner et al, 2000;Strachan and Taylor, 1990;Díaz García, 2006;Balé and Brun, 1989), all of which are collectively referred to as Cadomian Orogeny (cycle). It is well-established that the external section of Gondwana facing such subduction was involved in the Variscan cycle, whose onset and culmination may correspond to an extensional event that led to the opening of oceanic basins (e.g., Rheic Ocean; Nance et al, 2010;Linnemann et al, 2007), and the raise of the Variscan Orogen after their suturing (Matte, 1991;Martínez Catalán et al, 2009;Ballèvre et al, 2009;Franke, 2000), respectively.…”