Multidimensional isotropic and anisotropic investigation of the Tajo Basin subsurface - A novel anisotropic inversion approach for subsurface cases with oblique geoelectric strike directions

Abstract

The Tajo Basin and Betic Cordillera are the areas investigated by the first phase of the PICASSO (Program to Investigate the Convective Alboran Sea System Overturn) project in south-central Spain. The magnetotelluric (MT) component of PICASSO Phase I provides information about the electric conductivity distribution in previously unprobed subsurface regions, as well as enhancing results of prior geological and geophysical investigations, thereby enabling the devise of a petrological subsurface model and a comprehensive understanding about the tectonic setting. During analysis of the MT data, oblique geoelectric strike directions were determined for the Tajo Basin subsurface, which causes severe artefacts in models from commonly applied isotropic two-dimensional (2D) inversion, or requires computationally expensive three-dimensional (3D) inversion. A novel approach is developed in this thesis, which utilises electric anisotropy in one-dimensional (1D) and 2D inversions to image oblique 2D subsurface structures. Computational expense of this approach is considerably lower and its performance is successfully tested in a synthetic model study and subsequently employed to derive a model of the Tajo Basin subsurface. In addition to the novel anisotropic approach, isotropic 2D and 3D inversions are used to obtain enhanced insight into Iberian subsurface geology. The most striking features of the model are (i) a distinct vertical interface in the centre of the Tajo Basin that is associated with the "Alpine Spain" - "Variscan Spain" boundary; (ii) a mid- to lower crustal anomaly that is related to remnants of asthenospheric intrusion in connection with Pliocene volcanic events in the Calatrava Volcanic Province; (iii) a low resistivity - low velocity feature in the Tajo Basin lithospheric-mantle, related to the extensive reservoir responsible for volcanic events throughout Europe and Morocco. For the latter two, effects of hydrous phases are inferred that may originate from dehydration processes at the subducting slab beneath Alboran Domain and Betics Cordillera.