The enigmatic curvature of Central Iberia and its puzzling kinematics

The collision between Gondwana and Laurussia that formed the latest supercontinent, Pangea, occurred during Devonian to Early Permian times and resulted in large-scale orogeny that today transects Europe, northwest Africa and eastern North America. This orogen is characterized by an 'S' shape corrugated geometry in Iberia. The northern curve of the corrugation is the well known and studied Cantabrian (or Ibero-Armorican) Orocline and is convex to the east and towards the hinterland. Largely ignored for decades, the geometry and kinematics of the southern curvature, known as the Central Iberian curve, are still ambiguous and hotly debated. Despite the paucity of data, the enigmatic Central Iberian curvature has inspired a variety of kinematic models that attempt to explain its formation with little consensus. This paper presents the advances and milestones in our understanding of the geometry and kinematics of the Central Iberian curve from the last decade, with particular attention to structural and paleomagnetic studies. When combined, the currently available datasets suggest that the Central Iberian curve did not undergo regional differential vertical-axis rotations during or after the latest stages of the Variscan orogeny, and did not form as the consequence of a single process. Instead, its core is likely a primary curve (i.e. inherited from previous physiographic features of the crust) whereas the curvature in areas outside the core are dominated by folding interference during the Variscan orogeny or more recent Cenozoic (Alpine) tectonics.

inspired a variety of kinematic models that attempt to explain its formation with little consensus. This paper presents the advances and milestones in our understanding of the geometry and kinematics of the Central Iberian curve from the last decade, with particular attention to structural and paleomagnetic studies.
When combined, the currently available datasets suggest that the Central Iberian curve did not undergo regional differential vertical-axis rotations during or after the latest stages of the Variscan orogeny, and did not form as the consequence of a single process. Instead, its core is likely a primary curve (i.e. inherited from previous physiographic features of the crust) whereas the curvature in areas outside the core are dominated by folding interference during the Variscan orogeny or more recent Cenozoic (Alpine) tectonics. Mountain belt systems are the most striking product of plate tectonics. In addition to their astonishing visual effect, marking the locations where ancient and modern plates collided, orogenic belts often preserve a variety of rocks that have the potential to illuminate the entirety of the systems pre-and syn-orogenic history. One of the most striking characteristics of the majority of Earth's orogens are their curvature in plan-view (e.g. van der Voo, 2004;Marshak, 2004;Rosenbaum, 2014). The degree of orogenic curvature may range from a few degrees of deflection in structural trend (e.g. Kopet Dag, Iran), to 180 ̊ of arc curvature (e.g. Kazakhstan arc and the Carpathians). The kinematics, structural and geodynamic implications of these systems are as varied as their geometries (Marshak, 2004;Weil and Sussman, 2004;Johnston et al., 2013). For example, some orogenic curvatures are hypothesized to be the consequence of physiographic features of the basement that pre-date orogen formation, such as irregular basin architectures or plate margin salients and recesses (e.g. Jura mountains, Hindle et al., 2000), which then control the growth geometry of the ensuing orogen. These systems are known as primary arcs and reflect pre-orogenic geometries and show no significant or systematic verticalaxis rotations along their structural length. On the other hand, oroclines, as classically defined by Carey in 1956, involve systematic differential vertical-axis rotations subsequent to initial orogenic shortening: different sectors of an orogen rotate with variable magnitudes or in opposite directions (e.g. Li et al., 2012). Rotations in Oroclines may occur at a range of scales, from thrust emplacement at upper crustal levels (e.g. Izquierdo-Llavall et al., 2018), up to a lithospheric-scale vertical-axis folding (e.g. Li et al., 2018). They can occur as single curves (e.g.
Overall the system transitioned from a relatively isolated Early Cambrian continental, to a restricted marine basin, to development of an open marine platform that was locally punctuated by magmatism (e.g. Gutiérrez-Alonso et al., 2008b;Palero-Fernández, 2015). The Ossa Morena zone represents the outermost platform, followed by an intermediate platform characterized by an asymmetric horst (Central Iberian Zone) and graben (West-Asturian Leonese Zone), which ends in the innermost shelf environment of the Cantabrian zone (Fig. 3;e.g. Gutiérrez-Marco et al., 2019). The differences between the West Asturian-Leonese and Central Iberian Zone are mainly deeper vs. shallower sedimentary facies (respectively) and a local Lower Ordovician unconformity in the Central Iberian Zone (Toledanian, e.g. Álvaro et al., 2018) that places Lower Ordovician strata atop pre-Cambrian to Cambrian rocks (Fig. 3 The Galicia Tras-os-Montes Zone (Farias et al., 1987) is a complex structural stack including a basal schistose unit (Parautochthon; Dias da Silva et al., in press) structurally overlain by mafic rocks with an oceanic-like signature and other far-traveled rocks under highpressure metamorphism (e.g. López-Carmona et al., 2014;. The oceanic rocks of this zone are classically interpreted as a Rheic Ocean suture (e.g. Martínez . Recent interpretations support its origin as a minor oceanic basin or seaway within the realm of Gondwana (e.g. Pin et al., 2002;Arenas et al., 2016).
This belt potentially represents dismembered relics of the Rheic ocean and/or a subsidiary seaway that opened during a Variscan transtension event in SW Iberia (e.g. Pérez-Cáceres et al., 2015;Quesada et al., 2019).
Finally, Paleozoic rocks occur sporadically within the Alpine Betic chain. Their lithological monotony, paucity of fossils, and the intensity of deformation and metamorphism during Alpine orogeny, make recognizing the original features of the different successions challenging (e.g. Martín-Algarra et al., 2019). Some faunal and detrital zircon studies suggest that the Paleozoic outcrops in the Betics may be similar to the most seaward realms of the Gondwanan platform (i.e., the Cantabrian Zone; e.g. Rodríguez-Cañero et al., 2018;Jabaloy-Sánchez et al., 2018). The Variscan orogen in Iberia shows multiple deformation, metamorphic, and magmatic events (e.g. Azor et al., 2019;Fig. 2) that evolved diachronously from the suture towards the external zones (Dalmeyer et al., 1997): (1) An initial continentcontinent collision began ca. 370-365 Ma, which produced high pressure metamorphism (e.g. Lopez-Carmona et al. 2014). (2) Between 360 and 330 Ma a protracted shortening phase occurred, frequently divided into main phases C1 and C2, that were accompanied by Barrovian type metamorphism (e.g. Dias da Silva et al., in press) and plutonism at ~340 Ma (e.g. Gutiérrez-Alonso et al., 2018). (3) An extensional collapse event, so-called E1, occurred at ~333-317 Ma, which formed core-complexes and granitic domes in the Central Iberian and West Asturian-Leonese zones (Fig. 2C;e.g. Alcock et al., 2009;Díez-Fernández and Pereira, 2016;López-Moro et al., 2018). This event is coeval and genetically linked to the formation of the foreland fold-and-thrust-belt of the Cantabrian Zone (e.g. Gutiérrez-Alonso, 1996). 3 Synthesis on the Geometry and Kinematics of the Cantabrian Orocline Understanding the geometry, kinematic evolution and mechanics of curved mountain systems is crucial to developing paleogeographic and tectonic reconstructions (e.g. Marshak, 2004;Van der Voo, 2004;Li et al., 2012;van Hinsbergen et al., 2020). Introduced by Carey (1955 p.257), an orocline (from Greek ορος, mountain, and κλινο, bend) is "...an orogenic system, which has been flexed in plan to a horse-shoe or elbow shape." Although sometimes used in the literature as a geometric description of any orogenic curvature, herein orocline is strictly used as a the term for map-scale bends that underwent vertical-axis rotations (Weil and Sussman, 2004;Johnston et al., 2013;Pastor-Galán et al., 2017a). The kinematic classification of curved mountain belts (Weil and Susman, 2004;Johnston et al, 2013) distinguishes two end members: (1) Primary orogenic curves, which describe those systems in which curvature is a primary feature of the orogen and formed without significant or systematic vertical-axis rotations, and (2) Secondary oroclines, where orogenic curvature was acquired due to vertical-axis rotations subsequent to primary orogenic building. Those systems whose curvature is the The orocline test (or strike test), evaluates the relationship between changes in regional structural trend (relative to a reference trend for an orogen) and the orientations of a given geologic fabric element or magnetization (relative to a reference direction). In terms of evaluating developmental kinematics, the most relevant geologic marker is paleomagnetic declination, which can be used to quantitatively evaluate total and systematic rotations as a function of along-strike variability. Once acquired, data is plotted on Cartesian coordinate axes with the strike (S) of the orogen (relative to a reference) along the horizontal axis, and the fabric azimuth (F, relative to a reference) along the vertical axis. The test originally used a basic leastsquares (OLS) regression (Schwartz and Van der Voo, 1983) to estimate the slope (coded m in formulas), ideally between 0 and 1, which then is interpreted with respect to vertical-axis kinematics. More recently, Yonkee and Weil (2010b) and Pastor- Galán et al. (2017a) introduced more robust statistics to estimate the slope and its uncertainty, considering and propagating errors of the input data. Primary orogenic bends show no change of paleomagnetic declination orientations with varying structural trend, and therefore the slope is expected to be 0. In progressive oroclines, the declination variation records some fraction of the total observed orogenic strike variability, and thus the slope would range between 0 and 1, depending on the amount of primary curvature. Secondary oroclines are those in which the paleomagnetic vectors record 100% of the rotation, yielding slopes of 1, meaning that the orogenic system must have started as a roughly linear system that then underwent secondary vertical-axis rotations until its present-day curvature was acquired. The slope obtained with the orocline test can only be confidently interpreted when the chronology of fabric formation is well known.
The trend of the Variscan belt in Iberia follows a sinuous "S" shape that is especially prominent in the northwest region of the Iberian Peninsula, and then becomes more subtle due to the predominance of younger cover sequences in the central and eastern regions of the peninsula ( Fig. 1 and 2). This dramatic geometry has stimulated a century long scientific debate as to its origin (e.g. Suess, 1892;Staub, 1926;Martínez Catalán et al., 2015). To the north and convex to the west is the Cantabrian Orocline, and to the center-south and convex to the east is the Central Iberian curve. The overall trend of the Cantabrian Orocline starts in Brittany (France) and southern England and then curves through the Bay of Biscay and then south into central north Iberia (Fig. 1, 2 and 4). The Cantabrian Orocline (also known as Ibero-Armorican Orocline/ Arc, Asturian Arc or Cantabrian-Asturias Arc) is arguably the first curved orogen that was  Barrois, 1882, Suess, 1892. The Cantabrian Orocline traces an arc with a curvature close to 180˚ within the central Cantabrian Zone (the Gondwanan foreland in Iberia, fig. 2), and opens to approximately 150˚ as one moves to the outer arc reaches (Fig. 1). At the crustal-scale, the Cantabrian Orocline represents a first order vertical-axis buckle fold in planview that refolds pre-existing Variscan structures (e.g. Julivert and Marcos, 1973;Weil et al., 2001). The inner arc of the orocline, or the Cantabrian Zone is characterized by tectonic transport towards the core of the orocline, i.e., the orocline has a contractional core, where low finite strain values and locally developed cleavage occur (Pérez-Estaún et al., 1988;Gutiérrez-Alonso, 1996;Pastor-Galán et al., 2009). 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. 150˚ interlimb angle vertical-axis fold that was accommodated by significant shearing, both dextral and, in lesser amounts, sinistral penecontemporaneous to vertical-axis rotation (Gutiérrez- . Weil et al. (2013Weil et al. ( , 2019 extensively review the geometry of the Cantabrian Orocline. All kinematic data studied so far support a model in which the Cantabrian Orocline formed due to secondary vertical-axis rotation in a period of time younger than 315 Ma and older than 290 Ma. Overall, the southern limb of the orocline rotated counterclockwise (CCW) and the northern limb clockwise (CW; Fig. 4). Orocline formation happened subsequent to the main shortening phases of the orogen (C1 and C2) and late-stage orogenic collapse (E1), and therefore, it is an ideal example of a secondary orocline in the strictest sense. Development of the Cantabrian Orocline requires the existence of a roughly linear orogenic belt during early Variscan closure of the Rheic Ocean (with a roughly N-S orientation in present-day coordinates), which was subsequently bent in plan-view into an orocline during late stages of Pangea amalgamation. Such interpretation is grounded in paleomagnetic studies (e.g. 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. 1 and 2B). This structure has been referred to as the Central Iberian curve, arc, bend or orocline. In this paper we use 'Central Iberian curve'. The other aforementioned terms involve still unknown parameters or are misleading: orocline imply kinematics (Weil and Sussman, 2004); bend refers to a mechanism of formation (e.g. Fossen, 2016); and arc could be ambiguous, since the term is commonly used for volcanic chains. This curvature was first described by Staub (1926) and was termed the Castilian bend.
Continental drift pioneers paid some attention to Staub's description (e.g. Holmes, 1929;Du Toit, 1937), but the curved structure remained largely ignored for multiple decades (e.g.
Martínez . The hypothesis of a large-scale curvature in Central Iberia made a comeback at the beginning of the 21st century with a study of Variscan porphyroblast kinematics across Iberia by Aerden in 2004. Since then, several attempts to unveil its geometry and kinematics have been made with contrasting results.
The elusive nature of the Central Iberian curve resides in the poor exposure of its putative hinge (Fig. 2). The hinge of the Cantabrian orocline crops out extensively and the changes in thrust and fold axes trend are observable at high-resolution from aerial photographs and are readily mapped using outcrop-scale observations. In contrast, the alleged hinge of the Central Iberian curve is largely covered by Mesozoic and Cenozoic basins (Fig. 2). The curvature is most recognizable at the boundary between the Galicia-Tras os Montes and Central Iberian zones (Fig. 2A;Aereden, 2004;. The thrust fault that bounds those areas traces close to a 180˚ of curvature and marks the emplacement of the most distal units. Before the revival of Staub's curved geometry along the entire Central Iberian Zone, there were several attempts to explain the curved shape of the Galicia Tras-os-Montes Zone. Some consider the Galicia Tras-os-Montes Zone a block that escaped during an early Variscan (C1) non-cylindrical collision, forming a extrusion wedge towards the areas undergoing lesser amount of shortening , Dias da Silva, 2015 in press); or alternatively a klippe of a larger allochthonous thrust sheet, product of an interference pattern between C2, E1 and C3 structures (e.g. Ries and Shackleton, 1971;Martínez Catalán et al., 2002;Rubio Pascual et al., 2013;Díez-Fernández et al., 2015). Finally, Shaw et al. (2012) studied the orientation of paleocurrents in Ordovician Armorican Quartzite (e.g. Aramburu, 2002), which is one of the most prominent rocks exposed in Iberia (Fig. 3). The authors found that paleocurrents fanned outward with respect to the Cantabrian Orocline curve and are approximately perpendicular to the structural trend throughout the peninsula (Fig. 3). Shaw et al. (2012)  5 Move over once, move over twice: Kinematic constraints Late Variscan kinematic data (315-290 Ma; C3, E2, C4 phases) in the Central Iberian curve were scarce prior to revival of Staub's Central Iberian curve (e.g. Vergés, 1983;Julivert et al., 1983;Parés and van der Voo, 1992). More recently, a wealth of studies have been published on the kinematics of forming the Central Iberian curve (Fig. 2B), which are reviewed below.

Structural Geology and Geochronology
Curved orogens that result from differential vertical-axis rotations develop remarkable structures within their hinges where compressive and extensive radial structures often develop in combination with tangential shear structures (e.g. Li et al., 2012;Eichelberger and McQuarrie, 2015). With the re-emergence of the Central Iberian curve debate, several studies have reevaluated the well-documented structures from the Central Iberian Zone to constrain the origin and kinematics of curvature. The majority of studies focused on the hinge zone of the curve in the area surrounding Galicia Tras-os-Montes (e.g. Dias da Silva et al., 2014;Jacques et al., 2018a), but some explored more outer-arc areas (e.g. Palero-Fernández et al., 2015;Gutiérrez-Alonso et al., 2015). The following paragraphs synthesize the findings of new field, structural, and geochronological analyses from around the hinge of the Central Iberian curve and its surrounding regions. The reviewed studies identify several deformation events that are linked to regional Variscan deformation phases ( Fig. 2A).

Paleomagnetism
Paleomagnetism investigates the record of the Earth's ancient magnetic field as it is recorded in the rock record. Among other features, rocks can record the orientation of the magnetic field at the time of magnetization (e.g. Tauxe, 2010). The recorded magnetic vector can be geometrically defined by two components: inclination, which is a function of the paleolatitude (being 90˚ at the poles and 0˚ at the equator) at the time of magnetization acquisition; and declination, which is a measure of the horizontal angular difference between the of magnetization acquisition. Paleomagnetism is the best tool to quantify vertical-axis rotations in orogens due to the independence of the magnetic field from the orogen deformation and evolution (e.g. Butler, 1998).
Despite its uniqueness to study paleolatitudes and vertical-axis rotations, paleomagnetism is not flawless. Paleomagnetic data can yield spurious rotations when the local and regional structures are not properly studied and their geometries and kinematic histories not adequately corrected for (e.g. Pueyo et al., 2016). In addition, the age of magnetization acquisition is not necessarily equivalent to the age of the sampled rock. Remagnetizations are ubiquitous, especially in orogens (Weil and van der Voo, 2002;Pueyo et al., 2007;Huang et al., 2017). In remagnetized rocks, the primary magnetization is replaced or overprinted due to a set of geologic processes acting alone or in concert -usually represented by a combination of thermal or chemical reactions (Jackson, 1990). Nevertheless, remagnetizations can be useful for interpreting deformation history if the relative timing of the overprint can be established and a well-constrained reference direction for that age is known (e.g. Weil et al., 2001;Izquierdo-Llavall et al. 2015;Calvín et al., 2017).
In addition to knowing the structural geology and the timing of magnetization of the studied rocks, understanding and quantifying local and regional vertical-axis rotations require a paleomagnetic reference pole for comparison. Permian and Mesozoic paleomagnetic studies in Iberia indicate that Iberia was a relatively stable plate from at least Guadalupian times (ca. 270 Ma) to the opening of the Bay of Biscay in the Cretaceous (e.g. Gong et al., 2008;Vissers et al., 2016).  calculated the most modern Early Permian pole for stable Iberia, which will be used herein as a reference for any vertical-axis rotation analysis (hereafter, eP pole or  (Fig. 6-5). The authors found an original component in E1 grantites supported by a positive reversal test in both domes (Fig. 7). The magnetization has an inclination (Inc.) = 15˚ (paleolatitude (λ) = -7.6˚) and declination (Dec.) = 81˚ (Fig. 7), which imply a northward movement of 700 km and a ~70˚ CCW rotation with respect to the C3 granites that showed an eP component (Dec. Cantabrian Orocline (Fig. 4;Weil et al., 2013).
At the putative outer arc of the Central Iberian curve, the Iberian Ranges (Fig. 2), paleomagnetic and structural studies of Devonian and Permian rocks (Pastor-  revealed that the eP component from Permian rocks had rotated ~22° CW during the Cenozoic (Fig. 8;cf. Pastor-Galán et al., 2018). The Permian and Mesozoic rocks from the Iberian Ranges show a consistent ~22˚ CW rotation with respect to the Apparent Polar Wander Path for Iberia (e.g. Pastor- ). This rotation likely happened during the Alpine orogeny, in which the northern area of the Iberian Range underwent more shortening than the southern part, resulting in a regional CW vertical-axis rotation (Izquierdo-Llavall et al., 2019).
After restoring the Cenozoic rotation, the Devonian rocks show a positive reversal and fold-test with inclinations that are steeper than expected from the eP component (Dec. = 85.3˚, Inc. = 12.7˚, λ = -6.4). This component is statistically indistinguishable from that of the E1 granites and the southern branch of the Cantabrian Zone, showing the same 70° CCW rotation from the time they were remagnetization (estimated in 318 Ma) to the timing of the eP component (Fig. 8;Pastor-Galán et al., 2018). Once Cenozoic rotation is corrected for, the structural and paleomagnetic trends of the Iberian ranges become parallel to those in the southern limb of the Cantabrian Orocline, ruling out a Variscan or older origin for the outer Central Iberian curve (Fig. 8).
The remaining paleomagnetic works published on Central and SW Iberia rocks all reveal a ubiquitous late Carboniferous to Early Permian remagnetizations during the Kiaman superchron Pastor-Galán et al., 2015a;2017b;Leite Mendes, in press). The authors of these papers calculated the expected declination for each site as if they were part of the Cantabrian Orocline (Fig. 9A) Finally, two previous studies identified an earlier magnetization in the Almadén syncline region of the SE Central Iberian Zone Pares & Van der Voo, 1992).
However, Leite Mendes et al. (in press) argue that these studies are likely misinterpreted. Perroud et al. (1991), applied a complicated structural correction restoring a putative plunge of the regional structural axis to all sites, including those where the syncline axis does not plunge.
Leite Mendes et al. (in press) re-sampled the syncline where its axis is sub-horizontal and obtained a negative fold test, implying that the magnetization is not primary as previously interpreted. Their results, however, are similar in orientation to those components published from previous studies prior to any structural correction van der Voo, 1992).
Two additional studies sampled Laurussian margin sequences that are today adjacent to the Cantabrian Orocline region (Fig. 10). To the north, the SW area of Ireland preserves a Late Paleozoic basin filled with Devonian red sandstone and Carboniferous limestone and siltstone, which was sampled by Pastor- . To the south is the aforementioned results

The implications of not being a secondary orocline
The most relevant new data regarding the kinematics of the Central Iberian curve is the paleomagnetic study from the Iberian Ranges Pastor-Galán et al., 2018).
These results confirm that the present-day variation in trend of the tectonostratigraphic units, generally attributed to Variscan tectonics (e.g. Weil et al., 2013;Shaw et al., 2012;2014), is From a structural geology point of view, the Central Iberian curve does not display the classic geometries and structural interference patterns as found in other established oroclines (i.e., those systems that involve differential vertical-axis rotations, e.g. Li et al., 2012;van der Boon et al., 2018;Meijers et al., 2017;Rezaeian et al., in press). The geometry and structural behaviour of oroclines should resemble, at the crustal-scale, a regional vertical-axis fold preserved in plan-view, either formed by buckling (e.g. Johnston et al., 2001) or bending (e.g. Cifelli et al., 2008) mechanisms. In oroclines, pre-existing structures tend to follow fold trends around the curvature (e.g. Rosenbaum, 2014;Li et al., 2018). In addition, orocline cores tend to preserve radial structures and shortening patterns in the inner arc and orocline parallel shear zones and extension structures in their outer arc (e.g. Ries and Shackleton, 1976;Eichelberger and McQuarrie, 2015), similar to what is observed in multilayer folds (e.g. Fossen, 2016).
The structural geometry of the Central Iberian curve lacks such patterns. fold patterns in Fig. 2A) and nowhere are the expected inner and outer arc-related structures preserved (e.g. Dias da Silva et al. in press).

Paleomagnetism from the Iberian Ranges indicate that the Cantabrian and West Asturian
The curved shape of C1 fold axes in the Central Iberian zone is better explained by fold interference patterns than vertical-axis rotations (e.g. Pastor- Galán et al., 2019b). Moreover, the curved shape of the Galicia Tras-os-Montes allochthonous nappe, which was emplaced orogen parallel, shows no evidence of vertical-axis rotation related structures ( Fig. 2A; Dias da Silva et al., in press). Other authors describe the changes in trend around the Central Iberian curve expressed by C1 folds (Fig. 2A) as the product of fold interference patterns (e.g. Gutiérrez- Palero-Fernández et al., 2015;Jacques et al., 2018b;Dias da Silva et al., in press). Pastor- Galán et al. (2019b) showed that curved C1 folds in the Central Iberian Zone around the Galicia Tras-os-Montes boundary ( Fig. 2A) are coaxial with C3 folds after restoring the effects of C2 and E1 deformation phases (Fig. 11A). Both C1 and C3 formed under similar shortening directions. In the same area, Jacques et al. (2018b) found similar fold interference patterns, in addition they described kinematic incompatibility with the expected CW rotations that would have occurred if the Central Iberian curve was an orocline. In other areas of the Central Iberian Zone, the curved shape of C1 folds has been described as an interference between C1 structures and their reorientation caused by C3 shear zones (Fig. 2A;e.g. Palero-Fernández et al., 2015;Dias et al., 2016), or alternatively the interference between C1, C3 and the E2 structures (Fig. 2A;Arango et al., 2013;Rubio Pascual et al. 2013).
Overall, new geometric and kinematic data favor the interpretation that the Central Iberian curve is not a structure formed by differential vertical-axis rotation as was the Cantabrian Orocline, but one formed as a consequence of several competing processes. It is clear from the current data that a combination of several deformation events caused the orientation of structures that today delineate the shape of the Central Iberia curve. These include: (1) the northern part of the outer-arc is the product of an Alpine rigid block rotation instead of Variscan differential vertical-axis rotation ; (2) the curvature of the Galicia Trasos-Montes allochthonous nappe reflects its original shape and could be defined as a primary curve (see Weil and Sussman, 2004), since it was emplaced orogen parallel and shows no sign of vertical-axis rotations at any time ( fig. 2A; Dias da Silva et al., in press); (3) Structural analysis shows that fold interference patterns explain the geometry of the curved trends of Central Iberian Zone's C1 folds (Pastor-Galán et al., 2019b), whose kinematics are incompatible with the required CW rotations expected if the curve is an orocline (Jacques et al, 2018b). 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. 12). The proposed tectonic mechanisms to support these early kinematic models are varied: (1) buckling of a ribbon 'Armorican' continent (Fig. 12A;Shaw et al., 2014;; (2) buckling of a completely formed Variscan orogen during a putative 'Pangea B' to 'Pangea A' transition in the late Carboniferous The reviewed data in sections 4 and 5 contradict the aforementioned hypotheses.
Paleomagnetism and structural patterns (section 5; Fig. 6-11) disagree with the necessary CW rotations required to support a late Carboniferous orocline origin for the Central Iberian curve (Models in Fig. 12A and B). In addition, the sense and magnitude of the vertical-axis rotations observed in SW Iberia (Fig. 10) imply that the South Portuguese (Avalonian segment) and Ossa Morena zones moved together with the southern limb of the Cantabrian Orocline during the Pennsylvanian and Early Permian. This means that the South Portuguese Zone was already parallel to the general trend of the Variscan orogen prior to Cantabrian Orocline formation, implying the lack of a Laurussian rigid indenter into Gondwana (e.g. Simancas et al., 2013). This discrepancy leaves orogen-parallel terrane transport as a possible explanation to the kinematics observed in Ossa Morena and South-Portuguese Zones (e.g. Pérez-Cáceres et al., 2016). At the same time, paleomagnetism from SW Iberia backs the hypothesis of a Greater Cantabrian Orocline extended into both Gondwana and Laurussia in its northern and southern limbs (Fig. 10;Pastor-Galán et al., 2015b).
In spite of the kinematic constraints and structural patterns, which do not support a vertical-axis origin for the Central Iberian curve in Late Carboniferous time, other geometric constraints remain challenging. The curved shape of the aeromagnetic and gravity anomalies of Iberia are real (Fig. 5). These striking patterns could be due to Variscan-Alpine structural         Gutiérrez- Arenas et al., 2016). The irregular shape of the margin and the younging westwards deformation front (e.g. Daleyer et al., 1997) resulted in tectonic escape