Crustal structures and active deformation in the westernmost Betic cordillera and its foreland

Resumen

The westernmost Betic Cordillera is located along the northwestern Gibraltar Arc, which constitutes a curved orogen formed by the interaction of the Eurasian and African plates and the Alborán continental domain in between. This is a key region for understanding the alpine evolution of the western Mediterranean. This Ph.D. Thesis examines the crustal structure in the westernmost Betics and its foreland, also determining the present-day activity of the mountain front in this area. In addition, this research provides new data that contribute to discussion of the main models of the evolution of the Gibraltar Arc proposed to date. Integration of geological, geophysical and geodetic data in the region proved crucial for these purposes, given the complex regional structure and the lack of quality outcrops. The analysis of magnetic anomalies along several profiles crossing the main magnetic dipole in the southwesternmost Iberian Peninsula helps constrain the geometry of the Monchique Alkaline Complex. This body crops out in southwestern Portugal and extends in depth eastward to the Betic Cordillera. A sharp step in the anomalous body coincides precisely with the southern channel of the Guadiana River, revealing the presence of a deep N-S crustal fault unknown to date, the Guadiana Fault. Gravity and geological observations support these results. Long period magnetotelluric data are useful to elucidate the conductivity of deep crust and upper mantle, whereby it is possible to infer the lithospheric and asthenosphere structure. Sea water constitutes the main conductive body at shallow crustal levels and its influence can mask 3D geological structures. 3D forward modelling studies have been developed to constrain the bathymetry and irregular coastline effect on phase tensor and tipper vectors in the Gibraltar area and appraise the influence of the narrow Gibraltar Strait. Long period magnetovariational observations in the westernmost Betic Cordillera, considering the sea influence in the area, highlight the presence of a major conductive body (0.05 ¿·m) within the basement of the southern Guadalquivir foreland basin: the Villafranca body. Magnetic anomaly data also evidence this major structure and help constrain its intermediate or basic igneous nature, with high sulphide mineralization. Its origin is discussed in the framework of the regional geological setting. Although previous research efforts held the western Betic Cordillera frontal area to be inactive, new GPS data suggest a very consistent westward motion with respect to the foreland, reaching maximum displacement values in the Gibraltar Strait area. The rectilinear character of the northwestern mountain front, together with the southeastward increasing thickness of the Guadalquivir basin infill, support the activity of the westernmost Betics. Deformation is mainly accommodated by active folds showing a roughly WNW-ESE to W-E shortening, at least since the Pliocene. The displacement pattern is in agreement with the present-day clockwise rotation of the tectonic units in the westernmost Betic Cordillera. All these data support active lithospheric subduction and slab-rollback as the main model of tectonic evolution of the Gibraltar Arc.