Orocline formation at the core of Pangea: A structural study of the Cantabrian orocline, NW Iberian Massif

Abstract

The Variscan orogen provides the European record of the late Paleozoic continental collisions that culminated with formation of the supercontinent Pangea. An S-shaped pair of isoclinal coupled oroclines characterizes the Variscan orogen of the Iberian Massif. Though oroclines are common features of the world's orogenic belts, the mechanisms that drive oroclinal formation, and the manner in which these continental-scale vertical-axis folds of orogens are accommodated are poorly understood. The northerly Cantabrian and the southerly Central Iberian oroclines are structurally continuous and pericontemporaneous, suggesting that they formed in the same fashion. Exposures of the Ediacaran Narcea Slates within the so-called Narcea antiform trace a 150-km-long arcuate belt around the 180° Cantabrian orocline. In the western flank of the Narcea antiform, the Narcea Slates are characterized by a penetrative steep to vertical, rough to slaty cleavage (S1) and subparallel 2-km-wide reverse shear zones with a penetrative fabric (S2) that are postdated by asymmetric meso-to outcrop-scale verticalaxis folds (plunge >65°) with a dominant vergence toward the oroclinal hinge; i.e., fold geometry is dominantly dextral (Z-shaped) in the southern limb of the Cantabrian orocline and dominantly sinistral (S-shaped) in its northern limb. Axial planes are consistently steeply dipping, but they are typically oriented at a high angle to S1/S2 and are therefore variable in strike about the orocline hinge. Vertical-axis folds affecting the Narcea Slates are of the appropriate scale and geometry to be interpreted as parasitic structures developed in response to a component of flexural shear within the limbs of the forming Cantabrian orocline. A model of formation of the Iberian coupled oroclines by buckling accommodating significant orogen-parallel shortening along an initially linear Iberian Variscan belt is therefore supported, providing new insight into the complexities associated with the final stages of Pangean amalgamation.

The enigmatic curvature of Central Iberia and its puzzling kinematics

“…Hirt et al, 1992;Parés et al 1994;Stewart, 1995;van de Voo et al, 1997;Weil, 2006;Weil et al, 2000;2001;, along with important contributions from structural (e.g. Gutiérrez-Alonso 1992; Kollmeier et al, 2000;Merino-Tomé et al, 2009;Pastor-Galán et al, 2011;2014;Shaw et al, 2015) and geochronological studies (e.g., Tohver et al, 2008;Gutiérrez-Alonso et al, 2015). The more southern Central Iberian curve has a similar magnitude, but opposite curvature compared to the Cantabrian Orocline ( Fig.…”

“…Within the inner core a variety of structures record non-coaxial strain, which produced complex interference folds and rotated thrust sheets (e.g. Julivert and Marcos, 1973;Julivert and Arboleya, 1984;Pérez-Estaún et al, 1988;Aller and Gallastegui, 1995: Weil, 2006Pastor-Galán et al, 2012b;Shaw et al, 2015;2016a;Del Greco et al, 2016). In contrast, the outer arc shows a ca.…”

“…The pioneering works in the last decade that resurrected the idea of a Central Iberian curve, speculated that both the Cantabrian and Central Iberian zones buckled together as secondary oroclines (Fig. 12;Shaw et al, 2012Shaw et al, , 2014Shaw and Johnston, 2016;Carreras and Druguet, 2014). Later, Martínez Catalan et al (2014) and Díez Fernández and Pereira (2017) reformulated Martínez-Catalán's 2011 hypothesis and proposed that the Central Iberian curve formed as an orocline between 315 and 305 Ma, and assigning the Cantabrian Orocline a time frame between 305 and 295 Ma (Fig.…”

Wrench‐Related Dome Formation and Subsequent Orogenic Syntax Bending in a Hot Orogen (Variscan Ibero‐Armorican Arc, the Ouessant Island, France)

“…The West European Variscan belt exposes many large strike‐slip shear zones, revealing wrenching that largely contributed to orogenic construction (e.g., Arthaud & Matte, ; Gapais & Le Corre, ). These shear zones are interpreted as the result of (1) the accommodation of the lateral component of oblique convergence between Gondwana and Laurasia plates (Braid et al, ; Martínez Catalán, ; Pastor‐Galán et al, ; Shelley & Bossière, ), (2) the accommodation of indentation of an irregular pre‐Variscan Gondwanan margin (e.g., Braid et al, ; Brun & Burg, ; Casas & Murphy, ; Dias & Ribeiro, ; Matte & Ribeiro, ; Matte, ; Kroner & Romer, ; Llana‐Fúnez & Marco, ; Murphy et al, ; Perroud & Bonhommet, ; Quesada, ; Ribeiro et al, , ), and/or (3) the accommodation of oroclinal buckling leading to the Cantabrian orocline/Ibero‐Armorican Arc (Casas & Murphy, ; Cochelin et al, ; Edel et al, ; Gutiérrez‐Alonso et al, ; Llana‐Fúnez & Marco, ; Martínez Catalán, ; Murphy et al, ; Pastor‐Galán et al, ; Ribeiro et al, ; Shaw et al, ; Figure a). Many of these strike‐slip shear zones bound gneiss domes and were active during exhumation of their cores with different tectonic scenarios proposed: transfer zones (Augier et al, ; Gapais et al, ), pull‐apart bounding structures (Echtler & Malavieille, ; Roger et al, ), or transpressional zone boundaries (Cochelin et al, ; Denèle et al, , ; Rabin et al, ).…”

An evaporite‐bearing accretionary complex in the northern front of the Betic‐Rif orogen

“…Highly arcuate orogens throughout the Earth have puzzled researchers regarding the involved lithosphere mechanisms [e.g., Ghiglione and Cristallini , ; Copley , ; Boutelier and Cruden , ; Sippl et al ., ; Moresi et al ., ; Hodges and Miller , ] and mode of deformation and strain partitioning in the upper crust [ Egydio‐Silva et al ., ; Murphy and Copeland , ; Del Ben et al ., ; Rosenbaum , ; Shaw et al ., ]. The Betic‐Rif orogen configure an extremely tight orogenic arc, namely, the Gibraltar Arc, superimposed to a converging Africa and Iberia plate boundary [ Platt et al ., ].…”