Encarnación García-Navarro

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.

Tectonic style and structural analysis of the Puebla de Guzmán Antiform (Iberian Pyrite Belt, South Portuguese Zone, SW Spain)

Abstract In the South Portuguese Zone (Iberian Massif), thin-skinned tectonics linked to the collision with the Ossa-Morena Zone produced the inversion of previous extensional basins in Carboniferous times. Its central domain, namely the Iberian Pyrite Belt, underwent two deformation phases at mostly low-grade metamorphic conditions linked to a progressive deformation migrating upwards from a basal detachment and from north to south. The Puebla de Guzmán Antiform is one of the most outstanding cartographic structures in the Iberian Pyrite Belt, representing a imbricate fan thrust system developed during the second regional deformation phase. In the Puebla de Guzmán Antiform, the first deformation phase gave rise to a penetrative slaty cleavage (S1), which is also recognized in the whole Iberian Pyrite Belt and constitutes the main foliation all over the region. Its genesis is possibly linked to the coetaneous development of thrusts at deeper crustal levels and SSW-vergent folds at all levels above these thrusts. First phase structures were deformed by large-scale imbricated thrust systems with lateral (NNE–SSW) and frontal (WNW–ESE) ramps, which constitute the most relevant regional cartographic structures. This second deformation phase generated thrusts, two set of folds with WNW–ESE and NNE–SSW-oriented axes, as well as two related axial plane crenulation cleavages. These relatively brittle to ductile-brittle second phase structures have been identified in many areas of the Iberian Pyrite Belt, and especially in the southern limb of the Puebla de Guzmán Antiform. The second phase thrusts reported from the Puebla de Guzmán Antiform have not been folded according to both the geological map of the area and the analysis of maximum shortening and stretching axes.

Deformation of the substratum of a large shield volcano: Triggering factor for past flank collapses in the old volcanic edifice of La Gomera, Canary Islands

The Basal Complex of La Gomera (Canary Islands) has been analyzed with the aim of deciphering its structural evolution, spanning the middle to late Miocene. Detailed structural maps and cross sections, and the results of a systematic measurement of dike orientation and fault-slip data are presented in this work. The main structural features found in the Basal Complex of La Gomera are four dike sets and two large, normal faults (Guillama and Montana de Alcala faults). Several extensional episodes have been identified. The main deformation phase corresponds to a NNW-SSE– to NW-SE–directed extension that generated first a dense swarm of mafic dikes, and then a large-scale collapse of the Basal Complex with displacement along the Guillama and Montana de Alcala faults and associated rotation of large blocks of La Gomera basement. It is interpreted that the supposed staircase geometry of the Guillama fault is responsible for the observed arrangement of rotated dikes due to development of kilometric-scale fault-bend folds with rounded hinges. Deformation of the Basal Complex shows a complete geometric, kinematic, and chronologic consistency with the large volcanic flank collapses that affected the units belonging to the early growth stages of La Gomera subaerial shield volcano. The results of this work support the models that invoke the importance of the large-scale geodynamic setting on volcano destabilization. More attention should be paid to the structural and geophysical characteristics of volcano basements in order to better evaluate the danger of large, catastrophic volcanic landslide events.

Mesozoic tectonic evolution of the southwest continental Iberian Margin

An extensional event affected the southwest Margin of Iberia during Late Triassic to Early Cretaceous times, giving place to the Algarve basin. This basin was subjected to tectonic instability and it became infilled with siliciclastic and carbonate sequences with abundant interspersed volcanic rocks. Normal and strike-slip faults accommodated the deformation in the Algarve basin. The presence of a single flat or listric detachment surface is inferred from the study of hanging-wall structures. The dynamic and kinematic analyses of fault systems in the Spanish exposure of the Algarve basin allow us to establish three extensional phases. 1) A Late Triassic to Hettangian NE-SW directed extension associated with the initial breaking of Pangea and the opening of the Tethys in the eastern Mediterranean. 2) NW-SE extension from the Sinemurian to the Callovian, interpreted as a result of the activity as a sinistral fault of the Azores-Gibraltar transform boundary. 3) Finally, E-W extension during the Late Jurassic and Cretaceous, related to the North Atlantic rifting process.