José Miguel Azañón

Alternating contractional and extensional events in the Alpujarride nappes of the Alboran Domain (Betics, Gibraltar Arc)

In the western Alpine system, Neogene extensional tectonics triggered the development of marine basins on the concave side of tight orogenic arcs, as happened within the Alboran Crustal Domain, the hinterland of the Gibraltar Arc. A detailed analysis of the structural and metamorphic records of one of the main Alboran Domain complexes, however, plainly reveals a complex tectonic evolution prior to the development of the Miocene arc/back arc system, which includes a major intraorogenic extensional event. This large-scale subvertical shortening, that can be assessed from the PT paths of representative tectonic units, was subsequent to the continental crust subduction inferred from high pressure-low temperature mineral asssemblages. The crustal section was thinned in nearly isothermal conditions, its thickness being reducted to at least 1/3 of the initial value. Yet still before the Miocene, a second contractional event led to the overthrusting of high-grade metamorphic rocks over other low-grade rocks, accompanied by subordinate overturning of the metamorphic zones. Since migration of the Gibraltar Arc is roughly balanced by crustal spreading in the back arc, available data concerning Miocene extension suggest that the Alboran Domain can be restored to its appropriate position several hundred kilometers to the east. Thus a collision belt that underwent significant intraorogenic extension must have existed in what is now the western South-Balearic basin.

Back arc extension and denudation of Mediterranean eclogites

Eclogite-facies rocks exposed in Mediterranean back arcs are delimited from above by low-angle normal faults and detachments. Nevertheless, our work demonstrates that these extensional structures associated with back arc extension played only a limited role in removing the overburden from above the eclogites. Extension in Mediterranean back arcs began in the late Oligocene or early Miocene, but the pressure - temperature - time (P-T-t) paths of eclogite-facies rocks exposed in these areas indicate that a major part of the overburden, several tens of kilometers, has been removed from above these rocks prior to the Oligo-Miocene. We show that the time period bracketed between the peak of eclogite metamorphism (Eocene in the central Aegean, probably Upper Cretaceous in Corsica and the Betics) and the onset of back arc extension in the Oligo-Miocene was characterized by thrust faulting. In the central Aegean, Corsica, and the Betics, eclogite-bearing units were partly unroofed and then overthrusted lower-pressure units. We emphasize that, with one exception (Tinos island, Greece), the entire inventory of extensional contacts operated subsequently to the overthrusting of the eclogites above the lower-grade sequences. Thus Mediterranean back arc extension lags behind a major part of the denudation process, and is superposed on orogenic wedges that contain eclogite-facies rocks at relatively shallow structural levels. We emphasize that the mode of occurrence of eclogites in Mediterranean back arc regions involves a continuum of in-situ crustal accretion below the eclogites, widespread P-T paths that show cooling or isothermal decompression, and lower-grade rocks at the bottom of the structural pile. Thus instead of reflecting whole - crust back arc extension, the tectonic style associated with the denudation of Mediterranean eclogites better fits an active accretionary-wedge setting. This is similar to the mode of occurrence of eclogite-facies rocks in mountain belts, such as the western Alps, where decompression was synorogenic and back arc extension played no role.

Uppermost Tortonian to Quaternary depocentre migration related with segmentation of the strike-slip Palomares Fault Zone, Vera Basin (SE Spain)

The Palomares Fault Zone (PFZ) is one of the main strike-slip brittle shear zones found in the Betics. It is segmented in several faults that have been active between the Upper Tortonian and present day. Data from drill cores in the Palomares area have permitted us to define the geometry and location of sedimentary depocentres related with the PFZ. These data show an eastward displacement between the Upper Tortonian to Messinian and the Pliocene–Quaternary sedimentary depocentres, towards the presently active Arteal fault, which bounds the western mountain front of Sierra Almagrera, showing that deformation along this fault zone has migrated towards the east, from the Palomares segment, with its main activity during the Upper Tortonian and Messinian, towards the Arteal fault, active during the Pliocene and Quaternary. To cite this article: G. Booth-Rea et al.,

A ‘core-complex-like structure’ formed by superimposed extension, folding and high-angle normal faulting. The Santi Petri dome (western Betics, Spain)

The Santi Petri dome (western Betics, southern Spain) shows a core-complex-like structure, where migmatitic gneisses and schists outcrop below low-grade slates and phyllites, all of which form the basement of the Neogene Malaga basin. The migmatites and schists suffered a coaxial-flattening event during isothermal decompression and were later exhumed by ductile ESE non-coaxial stretching. Further exhumation was achieved by W- to SW-transport brittle low-angle normal faulting. Subsequently these extensional structures were gently folded in the core of a NE/SW-oriented antiform during the Tortonian. Finally the Santi Petri domal geometry was accentuated by the interference of orthogonal high-angle faults with ENE–WSW and NNW–SSE orientation. This core-complex-like structure, formed by superposition of extensional and compressive tectonic events, does not represent a classical, purely extensional core complex, which shows that metamorphic structure and geometry are not decisive criteria to define a core-complex.Abstract The Santi Petri dome (western Betics, southern Spain) shows a core-complex-like structure, where migmatitic gneisses and schists outcrop below low-grade slates and phyllites, all of which form the basement of the Neogene Malaga basin. The migmatites and schists suffered a coaxial-flattening event during isothermal decompression and were later exhumed by ductile ESE non-coaxial stretching. Further exhumation was achieved by W- to SW-transport brittle low-angle normal faulting. Subsequently these extensional structures were gently folded in the core of a NE/SW-oriented antiform during the Tortonian. Finally the Santi Petri domal geometry was accentuated by the interference of orthogonal high-angle faults with ENE–WSW and NNW–SSE orientation. This core-complex-like structure, formed by superposition of extensional and compressive tectonic events, does not represent a classical, purely extensional core complex, which shows that metamorphic structure and geometry are not decisive criteria to define a core-complex.

Evaluation of the SBAS InSAR Service of the European Space Agency’s Geohazard Exploitation Platform (GEP)

The analysis of remote sensing data to assess geohazards is being improved by web-based platforms and collaborative projects, such as the Geohazard Exploitation Platform (GEP) of the European Space Agency (ESA). This paper presents the evaluation of a surface velocity map that is generated by this platform. The map was produced through an unsupervised Multi-temporal InSAR (MTI) analysis applying the Parallel-SBAS (P-SBAS) algorithm to 25 ENVISAT satellite images from the South of Spain that were acquired between 2003 and 2008. This analysis was carried out using a service implemented in the GEP called “SBAS InSAR”. Thanks to the map that was generated by the SBAS InSAR service, we identified processes not documented so far; provided new monitoring data in places affected by known ground instabilities; defined the area affected by these instabilities; and, studied a case where GEP could have been able to help in the forecast of a slope movement reactivation. This amply demonstrates the reliability and usefulness of the GEP, and shows how web-based platforms may enhance the capacity to identify, monitor, and assess hazards that are associated to geological processes.

A multi-method approach for the characterization of landslides in an intramontane basin in the Andes (Loja, Ecuador)

In the last several decades, population growth in the cities of the Andes has caused urban areas to expand into landslide-prone areas. Fatal landslides affecting urban settlements are especially frequent in cities located in the Neogene intramontane basins of the Andes. These basins have similar situations and include geographical and geological features that frequently generate ground instabilities. We studied the characteristics of the mass movements observed in these basins by carrying out a detailed analysis of four landslides that have occurred in the Loja Basin (Ecuador). This multi-method study integrated geophysical, geotechnical methods, mineralogical studies and analyses of precipitation time series. Our study characterizes the slope movements as active, slow-moving, complex earthslide earthflows. According to Differential GPS measurements, these landslides move at velocities of up to several metres per year. Electrical resistivity tomography profiles show that most of the landslides are mainly surficial. Time-series analyses of precipitation reveal that rainfall events that are not exceptionally intensive can reactivate these landslides. This characteristic and the development of these landslides on low-gradient slopes are explained using the results obtained from the geotechnical and mineralogical analyses. We find that the smectite clay minerals detected in the mobilized geological formations, combined with the tropical climate of the northern Andean region, induce the observed weak slope stability conditions. The conceptual model for the studied landslides may aid in assessing landslide-prone areas in Loja and other Neogene intramontane basins of the Andes and can help to mitigate the associated risks.

GIS-Based Deterministic and Statistical Modelling of Rainfall-Induced Landslides: A Comparative Study

Open image in new window In this paper three different approaches for landslide susceptibility modeling—Shallow Landslide Stability model (SHALSTAB), Likelihood Ratio (LR) and Generalized Additive Model (GAM)—are compared. They are based on deterministic and statistical methods, respectively. These methods were tested in the Pogliaschina catchment (25 km2 wide; Northern Apennines, Eastern Liguria, Italy), heavily hit by an intense rainfall on 25 October 2011, that caused hundreds of shallow landslides, human losses and severe damage to infrastructure and buildings. The paper focuses on the assessment of the predictive performance of the three methods through a two-fold cross-validation technique and prediction rate curves (PRCs) analysis. The preliminary results have revealed that statistical methods have a higher predictive capability than the deterministic one.

Deterministic and Probabilistic Slope Stability Models Forecast Performance at ~1:5000-Scale

Open image in new window Deterministic methods are appropriate for analyzing specific slopes at site-scale where geotechnical parameters are better known. Probabilistic techniques provide better results than deterministic methods at regional scales (1:10,000–1:50,000). However, the performances of deterministic and probabilistic methods at large scales (e.g. 1:5000-scale) are not well-known. We applied GIS-based deterministic (WEDGEFAIL, SAFETYFACTOR, SHALSTAB) and probabilistic (Likelihood ratio) methods to a mountain road of 14 km in the Alpujarras region (S Spain) to investigate the behavior of these models at detailed scales. The studied road stretch was affected by 111 landslides (7–8 landslides/km) during the 2009–2010 winter in a period of high precipitation. These landslides cut off the road in several points and disconnected the central region of Alpujarras from the main transport infrastructures. We delimited a small study area with only 4 km2 restricted to the slopes that cross the road where we gathered as much data as possible. Our results show that deterministic methods have less prediction capability at ~1:5000-scale than probabilistic methods and it seems that the needed effort to improve their results is not worthwhile. However, it must take into account that probabilistic methods need an inventory and they could not have been applied before the analyzed landslide event. As our results indicate, the deterministic methods, such as the SHALSTAB model, are reliable tools to make an evaluation of the stability of cut slopes in a roadway at project-scale.

Effectiveness of Deep Drainage Wells as a Slope Stabilization Measure: The Reactivation of the Diezma Landslide (Southern Spain)

The effectiveness of the stabilization measures, particularly deep drainage wells, have been analysed in a complex landslide affecting a major motorway in Southern Spain. A complete failure analysis of this instability, named as the Diezma landslide, has been performed considering different steps: (1) The slope before and after the A-92 motorway construction; (2) The slope during the 2001 Diezma landslide; (3) The slope after the stabilisation measures; (4) The slope at the 2010 reactivation; (5) Possible future reactivation. The good performance of the drainage wells was verified successfully by means of electrical resistivity tomography cross-sections. The future stability of the Diezma landslide depends on the correct performance of the drainage systems after periods of heavy rain. In addition, the reactivation of the Diezma landslide is expected in the case that a low magnitude earthquake (Mw 4.0–5.0), relatively common in the study area, occurs close to the landslide location.

Relationships Between Slope Instabilities, Active Tectonics and Drainage Systems: The Dúdar Landslide Case (Granada, Southern Spain)

A geomorphologic description of the Dudar landslide (Granada, S Spain) has been carried out using a high-resolution digital elevation model derived from LIDAR (Light Detection and Ranging) data. We have analysed the significant changes that the landslide caused in the drainage system of the Aguas Blancas and Darro rivers, which in turn are the consequence of the tectonic activity of the north-eastern border of the Granada Basin. These modifications comprise river diversions and active incision within the body of the landslide, making it susceptible to future reactivations. A stability back-analysis of the landslide has been performed to identify the mechanism of failure and the most-likely triggering factors. This analysis shows that a low-to-moderate magnitude earthquake (Mw 5.0–6.5) related to the active faults in the Granada Basin seems to be the main triggering factor of the Dudar landslide.