Stress distribution at the transition from subduction to continental collision (northwestern and central Betic Cordillera)

International audienceWe analyze focal mechanisms of shallow-intermediate earthquakes in a NW-SE transect along the western Betic Cordillera and Alboran Sea, and deep earthquakes located in the central Betics to constrain the state of stress at the Gibraltar Arc slow convergence area. Shallow earthquakes ( 620 km) show very similar focal mechanisms, fitting the general slab behavior of resistance to further descent at the 660 km discontinuity. Seismicity features evidence the present-day stress distribution in a context of transition from subduction to continental collision

Constraints on the frontal crustal structure of a continental collision from an integrated geophysical research: The central-western Betic Cordillera (SW Spain)

International audienceMélange rocks outcrop widely in the central and western frontal sectors of the Betic Cordillera, as in many other collisional orogens where they form part of the accretionary wedges. Extrusion of Triassic plastic clays and evaporites was favored by the progressive accretion of the Betic External Zones, mixing rocks of different provenance and forming a synorogenic frontal mélange unit. MT data coupled with gravity data are a valid combined methodology to characterize the geometry of these mélange units, since the characterization of plastic rocks geometry is usually uncertain using seismic techniques. The results correlate well with known geological features (sedimentary basins, calcareous ranges, evaporitic rocks) and reveal the deep geometry. A resistive body, slightly dipping toward the SE, points to the continuity of the Iberian Massif below the Guadalquivir basin (2°) and the External Zones (6-8°). To the south, gravity models show the Iberian continental crust subducting below the Internal Zones with a roughly 20-35° slope. The main conductive bodies are related to the location of evaporitic rocks involved in the frontal mélange. They overlie the Iberian Massif and, southwards, the frontal Jurassic and Cretaceous limestone sequences of the External and Median Subbetics. In this setting, thick plastic rock units placed above the foreland could act as a lubricant facilitating continental subduction, and being progressively accreted toward a frontal mélange

Is there an active subduction beneath the Gibraltar orogenic arc? Constraints from Pliocene to present-day stress field

International audienceWe present a new set of brittle microtectonic measurements carried out in the Pliocene and Quaternary rocks outcropping in several key sectors of the western Betic and Rif orogen, the so-called Gibraltar orogenic arc. This data set, along with available earthquake focal mechanisms and borehole breakouts, allowed us to compile the Pliocene and Quaternary stress map of this area. This map provides new constraints for tectonic models and the present-day tectonic activity of the proposed active eastward subduction of oceanic lithosphere beneath the Gibraltar Arc and roll-back. The horizontal maximum compressive stress (S-Hmax) is NW-SE in the Betic Orogen and N-S/NNW-SSE in the southern Rif Cordillera. There is a significant consistency between S-Hmax and the displacement field deduced from GPS measurements with respect to the African plate: both appear to reflect the NW-SE convergence between the African and the European plates that is perturbed in the Rif. We propose that part of the eastern Rif behaves as a quasi-rigid block welded to the stable African plate. This block is bounded by important faults that localized most of the deformation disturbing the stress and surface displacement field. Pliocene to Quaternary N-S to NW-SE Africa-Europe plate convergence seem to be associated to the reorganization of the remnant Early Miocene subduction system in a continental-continental collision framework. Three-dimensional reconstruction of available seismic tomography plotted against the intermediate seismicity shows that only part of the old subduction system, whose orientation ranges from N20 degrees E to N100 degrees E, remains active: the portion ranging from N30 degrees E to N40 degrees E, orthogonal to the regional convergence. (C) 2011 Elsevier Ltd. All rights reserved

The oldest managed aquifer recharge system in Europe: New insights from the Espino recharge channel (Sierra Nevada, southern Spain)

In Sierra Nevada (southern Spain), the highest mountain range in southern Europe, the application of an ancestral Integrated Water Resources Management system (IWRM), based on the conjunctive use of groundwater and surface water, provides water resources for irrigation and supply in the driest months of the year in this semiarid mountain region. Meltwater is derived from the headwaters of the mountain streams and rivers through a set of uncoated channels excavated in the ground (locally known as acequias de careo) to infiltrate at the upper part of the valleys. Water infiltrated along the acequias de careo slowly flows down the hillsides, through the weathered zone of the hard rock aquifer and the glacial and periglacial sediments. The recharge accomplished through this Managed Aquifer Recharge technique (MAR) activates numerous springs located halfway down the hillside and increases the base flow of the rivers. In this study, focused on a careo channel located on the southern slope of Sierra Nevada called Acequia de El Espino, different archaeological, sedimentological, geophysical and hydrogeological techniques are applied to determine the age and the efficiency of this ancestral example of a MAR and IWRM system. Results suggest that the acequias de careo may be the oldest MAR system in Europe, and that this MAR technique could be applied in other high mountain alpine watersheds to mitigate the effects of climate change