Lithospheric structure of the western betic cordillera and its forelandimplications in the recent tectonic evolution

Resumo

The western Betic Cordillera is a region of great interest for recent tectonic studies due to its location in the westernmost part of the Gibraltar Arc, a key area for understanding the evolution of the westernmost Mediterranean in the framework of Eurasian-African plate interaction. This study surrounds the analysis of two complete transects crossing the main geological domains (from the Internal Zones, across the External Zones and the Guadalquivir Basin, up to the Iberian Massif foreland) together with additional detailed research of the mountain front (Morón de la Frontera-Puebla de Cazalla area), the largest intramontane outcrop of neogene sediments (Ronda Basin), and other seismogenic regions (Malaga intermediate seismicity and Cañete la Real area). Geological and geophysical research efforts have been combined to determine the geometry of lithospheric structures and their recent activity. Long-period magnetotelluric results provide the first electrical anisotropy data of the western Mediterranean mantle. Geoelectrical analysis of the uppermost mantle establishes nearly orthogonal strikes below the Betic Cordillera (N-S) and the Iberian Massif (E-W). However, at deeper mantle levels, the strike remains constant in a N-S direction for all the stations in both geological domains. Broadband magnetotelluric data afford additional constraints to the lithospheric structure. The crustal resistivity image reveals a deep conductive southward dipping body at midcrustal levels beneath the southern part of the profile. The shape of the body points to a continuation of the continental Iberian Massif crust below the western Betic Cordillera and its prolongation below the northern Alborán Sea, as supported by gravity data and seismicity distribution. Furthermore, analysis of the earthquake focal mechanisms provides information about the present day stress in the area and its relation to the main structures, which may be complemented by the study of brittle deformations. There is a regional NW-SE maximum compressional stress field, with variable plunge due to the locally dominant structures: northwestward in deeper levels of mountain front, then becoming southeastward along the intermediate seismogenic zone of the Alborán Sea, parallel to the continental subducting slab. Earthquake thrust fault focal mechanisms evidence that the northwesternmost mountain front is the only one to remain active in the arched Betic Cordillera, owing to its favourable orientation with respect to the present-day convergence of the Eurasian-African plates. The active tectonic structures are partitioned, thus indicating NW-SE oriented compression and thrusting at depth, extension at surface, and the presence of transfer faults with opposite kinematics at the northeastern boundary. They produce local perturbation of the stress field in terms of NE-SW compression or apparently inconsistent NNE-SSW and WNW-ESE. In this context, the Neogene Ronda basin underwent recent uplift and northwestward transport and may represent a piggy-back basin. The basin was slightly deformed since the Late Miocene by folds with nearly orthogonal axes, no dominant vergence and box geometries, related to the location of plastic low-density Triassic rocks. Although many geodynamic models have been proposed for the recent evolution of this region, the new data gathered together here would seem to favour the presence of an eastward dipping subduction zone-rollback model occurring during the Early-Middle Miocene evolution of the Betic-Rif-Alborán area. At present, deformation at mountain front propagates southwards up to the continental subducting slab in the Alborán Sea, which may represent the last active remaining segment of the inherited Miocene subduction.