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 vertical-axis 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.

INTRODUCTION

Progressive oroclines are thin-skinned structures that develop curvature over the course of orogen formation through local vertical-axis rotations driven by the same stress field responsible for orogen-perpendicular shortening. Secondary, or “Carey” oroclines, are thick skinned and extra-orogenic, involving strike-parallel shortening of an entire orogen in response to a regional stress field distinct from that responsible for initial orogenic shortening (Johnston et al., 2013, and references therein); see Marshak (2004) for further review of the terminology of map-view orogenic curvature.

An S-shaped pair of coupled oroclines characterizes the Iberian segment of the western European Variscan belt (Fig. 1). The more northerly and convex-toward-the-west Cantabrian orocline hosts the Variscan foreland in its core; allochthonous terranes occupy the core of the southerly, convex-toward-the-east Central Iberian orocline (Du Toit, 1937; Aerden, 2004; Martínez Catalán, 2011; Shaw et al., 2012). The hinge of the Central Iberian orocline is mostly buried beneath post-Variscan sedimentary cover; the Cantabrian orocline, in contrast, is incredibly well exposed and is hence one of the best-studied structures of its kind. Despite a wealth of research aimed at deciphering the dynamics, mechanics, kinematics, and lithospheric-scale effects of the Cantabrian orocline (e.g., Julivert, 1971; Julivert and Marcos, 1973; Brun and Burg, 1982; Pérez-Estaún et al., 1988; Ribeiro et al., 1995, 2007; Weil, 2006; Weil et al., 2000, 2001, 2010, 2013a; Gutiérrez-Alonso et al., 2004, 2008, 2011a, 2011b, 2012, 2015; Pastor-Galán et al., 2011), there remains debate with regard to the nature of its development.

Of the many distinct models for its formation, most interpret the Cantabrian orocline as progressive. Progressively developed curvature is postulated to have resulted from (1) the impingement of a rigid Gondwanan continental indenter (Ribeiro et al., 1995, 2007), (2) a gradual and continuous change in thrust transport direction (e.g., Pérez-Estaún et al., 1988), (3) sinistral transpression confined to the upper plate and driven by collision at a transform-trench triple junction (Brun and Burg, 1982), or (4) lithospheric-scale dextral transpression driven by oblique collision (Martínez Catalán, 2011). Alternatively, the Cantabrian orocline has been interpreted as secondary, formed by buckling accommodating significant orogen-parallel shortening along the length of an initially linear Variscan orogen in response to a reorientation of the regional stress field (e.g., Weil et al., 2000, 2013b; Johnston et al., 2013).

Interpretation of the Cantabrian orocline as secondary is supported by paleomagnetic and structural studies that demonstrate significant post-Variscan vertical-axis rotations that, when restored, indicate an initially linear north-south Variscan trend (Parés et al., 1994; Van der Voo et al., 1997; Weil, 2006; Weil et al., 2000, 2001, 2010, 2013a; Merino-Tomé et al., 2009; Pastor-Galán et al., 2011). Consistent paleomagnetic poles within Early Permian strata unconformably deposited above strata deformed about both the Cantabrian and Central Iberian oroclines suggest that they formed pericontemporaneously over a 10–20 m.y. period immediately following the late Carboniferous cessation of east-west Variscan shortening (in modern-day coordinates; Weil et al., 2010). Variscan porphyroblasts characterized by inclusion trails that preserve a constant north-south orientation about the axes of both the Cantabrian and Central Iberian oroclines provide further evidence in support of their pericontemporaneous formation from an initially linear north-south–trending Variscan orogen (Aerden, 2004). Magmatism, metamorphism, mineralization, and associated isotopic data suggest that orocline formation involved the entire lithosphere (Fernández-Suárez et al., 2000; Gutiérrez-Alonso et al., 2004, 2011a, 2011b; Ducea, 2011).

A model of secondary formation of the Iberian coupled oroclines implies that significant components of orogen-parallel shortening were accommodated by buckling of an initially linear Variscan orogen and makes testable predictions. Buckle folding is accommodated within the fabric being folded by a principal mechanism of flexural shear identifiable by “parasitic” folds that reside in the limbs and verge toward the hinge of the larger-scale structure. Buckle folding about a vertical axis requires existing vertical structures or fabrics that are likewise subject to modification by flexural shear (Fig. 2). A buckled orogen (secondary orocline) should therefore host, within existing fabrics that are deformed about its axis, a predicable array of mesoscale vertical-axis structures with a characteristic vergence toward the oroclinal hinge (Fig. 2). In any given orogen, existing vertical fabrics may include axial planar cleavage of upright folds, and axial planar cleavage of recumbent folds or thrust-ramp shear zones subsequently imbricated during fold-and-thrust belt propagation. In addition, buckle folding on a lithospheric scale is likely to require an orogen bound by lithospheric-scale free surfaces, i.e., plate boundaries.

We present analyses of structural data collected from steeply plunging mesoscale folds affecting steep to subvertical Variscan fabrics that bend around and define the Cantabrian orocline. Our primary aim is to assess whether these structures are of the appropriate scale and geometry to be parasitic to the Cantabrian orocline, as is predicted by an interpretation of the orocline as a secondary feature formed by orogen-parallel shortening of an initially linear Variscan orogen.

GEOLOGIC SETTING: NARCEA ANTIFORM

Ediacaran to Paleozoic strata of the Iberian Massif record the late Neoproterozoic to early Paleozoic evolution of a segment of the North African Gondwana margin and its subsequent Variscan deformation. The coupled Iberian oroclines are defined within and affect a series of tectonostratigraphic zones that are distinguished on the basis of lithology, stratigraphy, and Variscan deformational style (Shaw et al., 2012, 2014). From the core of the Cantabrian orocline outward, autochthonous Iberia is divisible into (1) a foreland domain, (2) an external hinterland domain, and (3) an internal hinterland domain (Shaw et al., 2012, and references therein). The internal hinterland domain is overthrust from the west by an exotic ophiolite-bearing allochthonous package. In southern Iberia, an additional distal Gondwanan zone, a continental allochthon, and an intervening suture (Fig. 1) are separated from the remainder of the massif by a major sinistral shear zone, the youngest motion along which is Variscan in age (e.g., Quesada and Dallmeyer, 1994).

Variscan structures predate and are deformed about the axis of the 180° Cantabrian orocline. A broad antiformal culmination in the footwall of a major nappe stack, the Narcea antiform, lies along the margin between the foreland and hinterland domains and traces deflections in structural trend along the Variscan foreland-hinterland transition at the apex of the Cantabrian orocline in the northwestern massif (Fig. 3). The core of the antiform exposes the Ediacaran Narcea Slates, the stratigraphically lowermost unit of the massif, for 150 km along strike. The Narcea Slates consist of a distal turbiditic succession of metamorphosed shales and graywackes with minor volcaniclastic layers (Gutiérrez-Alonso, 1996) and Neoproterozoic orthogneisses (Fernández-Suárez et al., 1998). The sequence is interpreted to have originated in a back-arc basin (Fernández-Suárez et al., 2013; Rubio-Ordóñez et al., 2015). In the hinterland western flank of the Narcea antiform, the Narcea Slates are characterized by a penetrative steep to vertical, rough to slaty cleavage (S1) and 2-km-wide bands of subparallel shear fabric (S2). The S1 cleavage is axial planar to Variscan structures and characteristic of rocks within which primary structures are still evident. The S2 foliation is a late Variscan fabric developed within thrust-ramp shear zones under low-grade metamorphic conditions. S2 is subparallel to but obliterates the older S1 cleavage and is phyllonitic to mylonitic, characterized by abundant kinematic indicators (primarily C-S fabric and asymmetric quartz porphyroblasts) that indicate shear up dip and toward the foreland core of the Cantabrian orocline. A 40Ar-39Ar study of S2 phyllonite yielded cooling ages of 321 Ma (Dallmeyer et al., 1997). Both the S1 and S2 foliations predate formation of the Cantabrian orocline and are therefore deflected around it.

STRUCTURAL ANALYSIS

Structural data were collected for mesoscale vertical-axis folds at ∼200 road-cut exposures within the Narcea Slates of the hinterland western flank of the Narcea antiform in northwest Iberia. Shallowly plunging folds could only be considered parasitic to the steeply plunging Cantabrian orocline if they were subsequently tilted (note: constraining the timing of postorocline deformational events was outside the scope of this study). Most of the measurements were of folds of S2 fabrics, because S2 is better developed than S1 and is more likely to be refolded. Individual folds were assessed in terms of their symmetry, and the orientations of both limbs and axial planes, where apparent, were recorded. We employed a 2:1 cutoff for fold asymmetry; the long limb of a fold had to be at least twice the length of its short limb in order to be classified as asymmetric. Fold vergence was then defined by limb-length asymmetry as directed toward the short limb; S-shaped folds were considered sinistral, and Z-shaped folds were considered dextral. The number of measurable vertical-axis folds at any given field site was variable and depended on (1) the extent and quality of outcrop, and (2) the extent of development and dip of the S1 cleavage and/or S2 shear fabric. Most sites contained five or fewer individual folds. Measured structures included more minor brittle contractional kink folds and ductile folds of variable geometries, all steeply plunging. While there was no penetrative cleavage associated with the vertical-axis folds, some folds were characterized by a local and spaced axial planar cleavage. Seventy-five percent of all measured folds were classifiable as gentle, with interlimb angles of 120° or greater. Folds ranged in size from centimeter-scale to outcrop-scale deflections. The frequency of mesoscale vertical-axis folds did not vary as a function of along-strike distance from oroclinal hinge.

A beta-plot analysis was conducted for each individually measured fold. Following a cylindrical best-fit analysis constrained by the axial plane and limb measurements, the Fisher mean vector of the calculated intersection (beta point) and π-axis was taken as the final preferred fold-axis orientation. Assuming that fold limbs were measured with greater accuracy than axial planes, the dip of the measured axial plane was adjusted about its fixed strike as necessary in order to intersect the preferred fold axis. A first-order error on the calculated fold axis is thus given as a small circle centered on the preferred fold axis with a radius equal to the dip adjustment on the measured axial plane. If no axial plane was directly measured, fold axis errors could be similarly calculated from any disagreement between axes of folds with shared limbs, including box folds. Fold axes with errors greater than 15°, interlimb angles greater than 160°, or plunges shallower than 65° were eliminated from further analysis.

Beta plots were grouped for individual field sites to assess internal consistency/variability, and then further grouped by geographic location for clarity of presentation. Final plots express regional fabric (S1/S2) as great circles and distinguish measured axial planes from calculated bisecting surfaces plotted as poles, as well as the corresponding fold axes with or without calculable error (Figs. 4–6). Final plots also include average axial plane orientations for either fold symmetry, taken as the group site mean vector for poles to bisecting surfaces and axial planes. All structural analyses were conducted in Stereonet (Allmendinger et al., 2012).

Vertical-axis folds of the slaty cleavage (S1) and shear fabric (S2) characteristic of the Narcea Slates in the hinterland western flank of the Narcea antiform are dominantly sinistral in the northern limb of the Cantabrian orocline (Fig. 4) and dominantly dextral in its southern limb (Fig. 5). Large-scale deflections in S1/S2 (most apparent in the northern limb) reveal the presence of somewhat cryptic kilometer-scale folds with the same orientations and symmetries expressed by their mesoscale counterparts. An even proportion of dextral and sinistral folds accompanied by a higher proportion of symmetric folds characterize the hinge region (Fig. 6). Dual fold symmetry in both limbs is attributable to box folding and the development of parasitic folds at diminishing scales (Fig. 7). At every diminishing scale of subsidiary fold, parasites formed on the long limb of an asymmetric host will share its sense of asymmetry and necessarily outnumber those with an opposing sense of asymmetry developed on its shorter limb. The dominant vergence within mesoscale structures is therefore reflective of the sense of shear recorded by first-order parasitic fold on the largest-scale structure (the Cantabrian orocline).

Idealized flexural buckling predicts the axial planes of parasitic folds to parallel that of the largest-scale structure. While the axial planes of vertical-axis folds of the S1 and S2 fabrics in the Narcea Slates are at moderately high angles to the axial plane of the Cantabrian orocline, the overall distribution of fold symmetry (Fig. 8) illustrates dominant shear sense toward the oroclinal hinge in both limbs, suggesting that these structures are parasitic folds formed in response to a component of flexural shear during formation of the Cantabrian orocline. A model of secondary buckling accommodating orogen-parallel shortening of an initially linear Iberian Variscan orogen is therefore supported.

DISCUSSION AND CONCLUSIONS

The structural data presented herein point to formation of the Iberian coupled oroclines by buckling accommodating significant orogen-parallel shortening along an initially linear Variscan orogen. Along-strike variations in paleomagnetic declination (e.g., Weil et al., 2010) and offshore paleocurrent flow direction (Shaw et al., 2012) about the axes of the Cantabrian and Central Iberian oroclines indicate a negligible degree of initially inherent curvature within the pre-orocline Iberian Variscan orogen. Here, we discuss the potential implications of these findings for our understanding of the processes involved in Pangean amalgamation. Palinspastic restoration of the Iberian coupled oroclines yields a 2100-km-long linear segment of the Variscan orogen (Fig. 9) and shows that >1100 km of orogen-parallel shortening occurred during orocline formation (Johnston et al., 2013; Shaw et al., 2014) over a 10–20 m.y. time interval between the late Carboniferous cessation of east-west Variscan shortening (in modern-day coordinates) and the onset of Early Permian deposition of postoroclinal terrigenous sedimentary successions (Gutiérrez-Alonso et al., 2004, 2011a, 2011b; Weil et al., 2010; Pastor-Galán et al., 2011). Formation of the Iberian coupled oroclines therefore required comparatively rapid relative plate motions of between 5.5 and 11 cm yr–1, suggesting a subduction-related driving mechanism as initially proposed by Johnston et al. (2013). The Variscan is interpreted as the European record of Pangean amalgamation (e.g., Matte, 1991, 2001; Martínez Catalán et al., 1997, 2007; Nance et al., 2010); however, the requirements for formation of the Iberian coupled oroclines, including the accommodation of >1100 km of orogen-parallel shortening at rates consistent with subduction-driven convergence, are difficult to reconcile with paleogeographic and tectonic models that place orocline formation within the continental core of the Pangean supercontinent. Thorough reviews of the existing paleotectonic models for formation of the Iberian coupled oroclines can be found in Johnston et al. (2013) and Weil et al. (2013b).

The Iberian coupled oroclines are an isoclinal continental-scale fold pair; oroclines of similarly expansive scale and tightly sinuous geometry are common features, but models invoking secondary buckling are rare. For example, formation of the Fleurieu orocline in the Adelaide fold-and-thrust belt of South Australia and the Kingston orocline of the central Appalachians is proposed to have been accommodated by the development of strike-slip shear zones along lithologic boundaries (Marshak and Tabor, 1989; Marshak and Flöttmann, 1996), whereas the Makran orocline of the Eastern Himalaya and Umbrian arc of the central Apennines are proposed to be thin-skinned features accommodated by the development or reactivation of conjugate strike-slip systems (Marshak et al., 1982; Marshak, 1988, 2004). Orocline formation by secondary buckling requires an orogen bound by free surfaces and may also require a subduction-related driving mechanism (Johnston et al., 2013). Johnston (2001) proposed that the Alaskan coupled oroclines formed by vertical-axis buckling of an originally linear Cordilleran orogen; the bends are traditionally interpreted as derived from an initially sinuous North American continental margin (e.g., Box, 1985; Dover, 1994). The Carpathian orocline is coupled with the more southerly and isoclinal Balkan orocline, and buckling of an originally linear Carpathian-Balkan belt is both spatially and temporally linked with ongoing westward tectonic escape of the Anatolian block out of the Arabian-Eurasian collision zone (Burtman, 1986; Shaw and Johnston, 2012). These eastern Alpine oroclines are most commonly interpreted as the relict of a preexisting European embayment (e.g., Burchfiel, 1980; Csontos and Vörös, 2004). Ongoing vertical-axis rotations in the actively tightening Andean oroclines are popularly explained by differential shortening driven by along-strike variations in upper-plate rheology and/or degree of coupling between the upper plate and subducting Nazca slab (e.g., Isacks, 1988; Lamb, 2001; Allmendinger et al., 2005; Medvedev et al., 2006). Johnston et al. (2013) proposed an alternative model of buckling in response to an orogen-parallel stress derived from subduction-collision at the E-W–striking (orogen-perpendicular) South American–Caribbean plate margin.

The identification of a component of flexural shear accommodated during formation of the Iberian coupled oroclines is consistent with the interpretation of the oroclines as secondary features formed by buckling of an initially linear Variscan orogen in association with a regional stress field distinct from that responsible for initial orogen-perpendicular shortening. A similar mechanism of vertical-axis buckling is suggested for oroclines that are similarly extra-orogenic (developed after initial orogenic shortening) and continental in scale.

Erratum to this article.

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

J. Shaw, S.T. Johnston, and G. Gutiérrez-Alonso | (this issue, v. 7; no. 6; p. 653–661; doi: 10.1130/L461.1)

The authors wish to correct the omission of the following relevant citation, which outlines an alternative model for formation of the Cantabrian orocline involving indentation by a continental block escaping westward from a Pyrenean zone of transpression.

Şengör, A.M.C., 2013, The Pyrenean Hercynian Keirogen and the Cantabrian Orocline as genetically coupled structures: Journal of Geodynamics, v. 65, p. 3–21, doi:10.1016/j.jog.2012.10.003.

J. Shaw gratefully acknowledges the University of Victoria Faculty of Graduate Studies Fellowship program, and the International Chapter of the P.E.O. Sisterhood for financial support provided through the Scholar Award program. Continuing support for S.T. Johnston is provided by an NSERC Canada Discovery Grant. Financial support for G. Gutiérrez-Alonso is provided by Research Project ODRE III (Oroclines and Delamination: Relations and Effects) CGL2013-46061 from the Spanish Ministry of Economy and Competitiveness. This paper is part of UNESCO IGCP Projects 574: Buckling and Bent Orogens, and Continental Ribbons and 597: Amalgamation and breakup of Pangaea: The Type Example of the Supercontinent Cycle.