Juan Ramón Colmenero

Emplacement of the Cuera and Picos de Europa imbricate system at the core of the Iberian-Armorican arc (Cantabrian zone, north Spain): New precisions concerning the timing of arc closure

The final stages of the Variscan orogeny (mostly Carboniferous) in the Western European Variscan belt involved the development of the Iberian-Armorican arc, which is cored by the Cantabrian zone (NW Iberia). The Cantabrian zone is the foreland of the Western European Variscan belt, and it is interpreted to record the waning stages of the closure of the Rheic Ocean. The distalmost tectonic units within the Cantabrian zone (the Cuera Unit and the Picos de Europa Province) were the last tectonic units emplaced at the core of the Western European Variscan belt orocline. Together, they form an imbricate system and associated wedge-top basins that are key to understanding the development of the orocline. The emplacement of the Cuera Unit and the Picos de Europa Province occurred in the latest Pennsylvanian, between Moscovian and Gzhelian times. New detailed mapping together with stratigraphic, sedimentologic, and biostratigraphic data analysis of syntectonic successions and structural information constrain the timing and evolution of this imbricate system. Our analysis indicates that the thrust sheets were emplaced roughly perpendicular to previous tectonic units of the Cantabrian Variscan foreland fold-and-thrust belt, most probably during the oroclinal bending of the SW European Variscan belt that formed the Iberian-Armorican arc. The N-S–directed imbricate system was characterized by a shallow dip of the topographic surface (α 150 ± 15 km) and complex structure of the orogenic wedge are thought to result from the progressive increase in the dip of the basal decollement during bending of the underlying Gondwana lithosphere and may reflect the closure of the Iberian-Armorican arc.

Growth and demise of Late Carboniferous carbonate platforms in the eastern Cantabrian Zone, Asturias, northwestern Spain

Abstract Geometric relationships of seismic-scale stratal patterns observed on aerial photographs of Late Carboniferous carbonate platforms in the Cuera Region in Asturias, northwestern Spain, allow the identification of two discrete stages of platform development with different styles of deposition. The first phase, represented by the Bashkirian Valdeteja Formation, is characterized by progradation of more than 10 km with steep clinoforms of up to 35°, and slope heights up to 650 m. During the second phase, corresponding to the Moscovian Picos de Europa Formation, the platform aggraded mainly vertically, nearly 850 m. Both phases of platform growth were terminated by major relative sea-level lowstands in conjunction with local filling of the adjacent basins by shales and mudstones that onlap the platform flanks. Calibration with outcrops suggests a subdivision into the following lithofacies, directly related to the depositional profile: (1) platform interior, parallel, well- to thick-bedded skeletal pack- to grainstones; (2) margin, algal- and sponge-dominated bioherms with little internal structure; (3) slope, clinothems of medium- to thick-bedded margin-derived breccia beds; (4) toe-of-slope, alternating grain- and packstones with spiculites; and (5) basin, marly shales, shales, sandstones, and calcareous mudstones. Relative sea-level changes caused by regional tectonic movements are suggested to be responsible for the generation of the two platform phases; sea-level highstands triggered the nucleation and growth of the platforms, whereas major sea-level lowstands in combination with terrigenous infill of the basins terminated the platform growth. In addition, it is proposed that the termination of Late Carboniferous carbonate platforms is caused by the combined effect of a sea-level fall, subaerial exposure and erosion, and, finally, a rapid transgression that drowned and polluted the platform with shales and mudstones. Local variations in the infilling of the basins controlled the variations in the style of progradation. The subsidence rates for the vertically aggrading phase (Picos de Europa Formation) were approximately twice that for the progradational phase (Valdeteja Formation) and may explain the aggradational versus progradational character.

The submarine volcanic succession of the basal complex of Fuerteventura, Canary Islands: A model of submarine growth and emergence of tectonic volcanic islands

Three lithostratigraphic units have been distinguished in the volcanic succession of the basal complex of Fuerteventura Island. These units are, from bottom to top: the submarine volcanic group, the transitional volcanic group, and the subaerial volcanic group. These three groups record the submarine growth and emergence of the island. The volcanism is represented by ultra-alkaline and strongly alkaline igneous series. The igneous activity was due to the presence of an anomalous zone in the sublithospheric mantle, the low density of which also caused uplift of the Mesozoic oceanic crust. Two extensional phases and an intervening contractional phase developed coeval to the generation of the volcanic succession. The submarine volcanic group was deposited in the hanging wall basin of a large listric extensional detachment directed toward the SSW. The transitional volcanic group was syntectonic with respect to a late inversion of the listric detachment. Finally, the subaerial volcanic group resulted from a second episode of WNW extension. This study of the evolution of the basal complex of Fuerteventura serves as the basis for a tectonic model of submarine growth and emergence of volcanic islands.

Significance of detrital zircons in Siluro-Devonian rocks from Iberia

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.