ALBERT

Description: Seven samples of Siluro-Devonian sedimentary rocks from the Cantabrian and Central Iberian zones of the Iberian Variscan belt have been investigated for provenance and contain four main age populations in variable relative proportion: Ediacaran–Cryogenian ( c . 0.55–0.8 Ga), Tonian–Stenian (0.85–1.2 Ga), Palaeoproterozoic ( c . 1.8–2.2 Ga) and Archaean ( c . 2.5–3.3 Ga). Five samples contain very minor Palaeozoic (Cambrian) zircons and six samples contain minor but significant zircons of Middle and Early Mesoproterozoic (Ectasian–Calymmian, 1.6–1.8) age. These data highlight the transition from an arc environment to a stable platform following the opening of the Rheic Ocean. Variations in detrital zircon populations in Middle–Late Devonian times reflect the onset of Variscan convergence between Laurussia and Gondwana. The presence of a high proportion of zircons of Tonian–Stenian age in Devonian sedimentary rocks may be interpreted as (1) the existence of a large Tonian–Stenian arc terrane exposed in the NE African realm (in or around the Arabian–Nubian Shield), (2) the participation, from the Ordovician time, of a more easterly alongshore provenance of Tonian–Stenian zircons, and (3) an increase in the relative proportion of Tonian–Stenian zircons with respect to the Ediacaran–Cryogenian population owing to the drift of the Avalonian–Cadomian ribbon continent, or the progressive burial of Ediacaran–Cryogenian rocks coeval with the denudation of older source rocks from the craton interior. Supplementary material: Tables with the analytical data and the geochronological results are available at http://www.geolsoc.org.uk/SUP18812 .

Description: Detrital zircon laser ablation–inductively coupled plasma–mass spectrometry U-Pb age data from the Lower Ordovician Armorican Quartzite (deformed passive margin strata of Gondwanan affinity) of the Iberian Massif are presented herein. The S -shaped coupled Iberian oroclines defined within these zones palinspastically restore to a 2300 km linear Variscan orogen with a paleomagnetically constrained Late Carboniferous north-south trend. Detrital zircons are used to assess paleogeography and interpreted geometry of the Iberian portion of the Gondwana passive margin. A common signature is identified by (1) Neoproterozoic (ca. 500–850 Ma), (2) Stenian–Tonian (ca. 0.9–1.1 Ga), and lesser (3) Paleoproterozoic and (4) Archean populations (ca. 1.8–2.15 and 2.5–2.7 Ga, respectively). Minor site-to-site variation in relative proportion of widely ranging age groups suggests near-uniform distribution of a highly varied detrital input. Provenance analysis reveals strong correlations with Cambro-Ordovician clastic rocks from northeast African realms. Similarity with underlying sequences suggests a common paleogeography from the Ediacaran through early Paleozoic and persistence of a provenance distinction within the autochthonous Iberian Massif. Consistent northward paleoflow within widespread northeast African lower Paleozoic sedimentary cover suggests long-distance sedimentary transport across a North African peneplain from outlying basement terranes. We propose that the 2300-km-long Cantabrian–Central Iberian portion of the early Paleozoic Gondwana margin stretched east-west along the northern limits of the then low-lying Saharan Metacraton and Arabian-Nubian Shield. Accepting paleomagnetic constraints, a 90° counterclockwise rotation is required to reorient the Iberian portion to a pre-oroclinal (Late Carboniferous) north-south trend. The mechanisms for accommodating such a rotation are unclear.

Description: An orocline is a thrust belt or orogen that is curved in map-view due to it having been bent or buckled about a vertical axis of rotation. Two distinct types of oroclines are recognized: progressive and secondary. Progressive oroclines are restricted to the scale of a thrust sheet to thrust belt, are thin-skinned, and develop during thrust sheet emplacement. Secondary oroclines are larger, occurring at the scale of an orogen, and are plate-scale features that affect crust and lithospheric mantle. Unlike progressive oroclines, which develop during initial orogenesis and in response to the same orogen-perpendicular stress responsible for thrust sheet emplacement, secondary oroclines are extra-orogenic, developing after initial orogenesis and in response to an orogen-parallel principal compressive stress that is oriented at a high angle to the stress responsible for orogen development. We present case studies of the Wyoming Salient, a progressive orocline that characterizes the Sevier thrust belt of the western United States, and the coupled Cantabrian and Central Iberian oroclines, which are linked secondary oroclines affecting the Variscan orogen of Iberia. The vertical-axis rotations involved in progressive and secondary orocline formation are most readily quantified through paleomagnetic analysis. Detailed three-dimensional palinspastic restoration that incorporates translation rotation and strain can distinguish the role, if any, of primary curvature in progressive oroclines. The use of tectonic vectors, such as paleocurrent directions, provides a means of recognizing and characterizing the initial geometry of secondary oroclines. Because secondary oroclines involve the entire lithosphere, detailed studies of coeval metamorphism and magmatism provide a means of constraining the fate of the mantle lithosphere during oroclinal buckling.

Description: Seven centimetre-thick volcanic ash-fall layers interbedded within the thick Carboniferous successions of the Cantabrian Zone in northern Spain were dated by U–Pb zircon laser ablation inductively coupled plasma mass spectrometry across an interval ranging from Visean to Kasimovian, thus covering most of the Carboniferous period. All these ash layers occur in fossiliferous successions, allowing us to insert the radiometric data within a well-constrained biostratigraphic framework. Considering the analytical uncertainty, the obtained ages match the ages inferred from the conodont biostratigraphy established in the Mississippian succession (which hosts the oldest two ash layers, Visean in age), and the fusuline and mega- and microflora data from the strata hosting the Moscovian and Kasimovian (Westphalian–Stephanian) tonsteins. The age of a Langsettian tonstein along with data provided by several papers stating that in the Cantabrian Zone Langsettian floras were contemporaneous with lowermost Moscovian fusulines suggest that Langsettian floras could have been younger in Spain than in other areas. Our absolute ages provide new constraints not only for the correlation of the Carboniferous successions of the Cantabrian Zone with time-equivalent reference successions in other parts of the world but also for calibrating the Carboniferous global chronostratigraphic units based on marine fossils with the West European regional units. Supplementary material: Stratigraphic position and biostratigraphic information on the studied ash-fall layers, analytical methods and geochronological results are available at https://doi.org/10.6084/m9.figshare.c.3768701

Description: 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.

Description: The origin of the Cantabrian orocline of the Variscan orogen in NW Iberia remains a topic of debate. We present a structural study of the Ponga Unit, a Cambrian to Carboniferous tectonostratigraphic package within the West European Variscan belt foreland fold-and-thrust belt that lies within the core region of the orocline. Our primary goal was to determine if W-plunging folds of the fold-and-thrust belt are attributable to formation of the Cantabrian orocline, or if they reflect lateral ramps in the underlying Variscan thrust faults. The major lithologic units of the Ponga Unit are the rheologically competent Lower Ordovician Barrios quartzite, and the less-competent, Carboniferous Barcaliente limestone and Beleño shale and sandstone formation. Our mapping and structural analysis within the Ponga Unit focused on the Laviana, Rioseco, and Campo de Caso thrust sheets, and associated bounding thrusts. Over 800 structural orientation measurements were collected across the study area. These data, coupled with data compiled from regional geological maps, allow for analysis of the crustal structure. West-plunging folds of the Laviana, Rioseco, and Campo de Caso thrust sheets form kilometer-scale anticline-syncline pairs, producing a complex fold interference pattern that is characteristic of the Ponga Unit. Our analysis shows that: (1) the geometry of the W-plunging folds is inconsistent with a lateral ramp model; (2) the map pattern defines a mushroom-type fold interference pattern, indicating two distinct deformational events characterized by principal compressive stresses oriented at a high angle (perpendicular) to one another; and (3) paleomagnetic data from the study area are consistent with the secondary model of orocline formation and indicate that there was a short window of time between the end of Variscan orogenesis and the onset of oroclinal buckling. Our results indicate that early N-S–trending folds, which resulted from Variscan orogenesis, were refolded during a post-Variscan orogen-parallel compression event attributable to formation of the Cantabrian orocline.