P. Schmoldt, PhD - MTNet - DIAS

11. Summary and conclusions
high misfits associated with impedance tensor decomposition for every direction indicate
invalidity of a 2D approximation for this region. Thus, further investigation was focussed
on the Tajo Basin subsurface using the novel inversion approaches developed in the course
of this thesis to cope with the issue of oblique geoelectric strike directions in the basin’s
crust and mantle regions. In addition, extensive isotropic 2D and 3D inversions were
conducted to further enhance knowledge about the geological setting.
The final Tajo Basin subsurface model is the synthesis of results from the different inversion
approaches. Crustal structures in the model were determined through isotropic
2D inversion of a dataset decomposed according to the crustal geoelectric strike direction
with the period range limited to crustal penetration depths. Owing to inadequacy
of isotropic 2D and anisotropic 1D inversion results, and uncertainties associated with
the anisotropic 2D inversion results, mantle structures were mostly deduced from the 3D
inversion model. Most striking features of models for the Tajo Basin crust are:
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• A well-defined difference in terms of electric resistivity of the crust between the
southern and northern parts of the Tajo Basin. The interface between the two regions
coincides with a significant change in seismic velocity determined in a seismic
tomography study. The southern region exhibits high electric resistivities and
high seismic velocities, whereas the northern region comprises relatively low electric
resistivities and low seismic velocities. Based on the correlation of the seismic
velocity model with the border between the so-called ‘Variscan Spain’ and ‘Alpine
Spain’ regions for most of the Iberian Peninsula, the southern high resistivity –
high velocity region was inferred to be associated with Precambrian and Palaeozoic
rocks of the Variscan Iberian Massif, whereas the northern region was attributed
to the domain that underwent significant Alpine deformation. Alpine deformation
of the northern Tajo Basin region is related to collisions of the Iberian Peninsula
with the rest of Eurasia and Africa during Late Mesozoic and Cenozoic times that
resulted in the orogeny of the Pyrenees and Betic Mountain chains. It is concluded
that the difference of the Alpine Spain and Variscan Spain region in terms of electric
resistivity values is due to compositional differences with additional temperature effects.
The Precambrian and Palaeozoic rocks of the Iberian Massif, constituting the
majority of the Variscan Spain region, are significantly older than the Alpine Spain
region (Mesozoic and Cenozoic material). Thus, the Variscan Spain region has undergone
a respectively longer cooling, a situation similar to cratonic regions when
compared with surrounding mobile belts. Compositional differences may therein
involve a higher amount of resistive olivine, pyroxene, garnet components for the
Variscan Spain region in contrast to the higher amount of metallic elements as well
as graphite and sulphide bearing oxides in the Alpine Spain region (cf. Sec. 5.2.1).
Further, higher conductivity may be indicative of an enhanced connectivity of the
conducting phases in the Alpine Spain region. The enhanced connectivity may originate
from deformation events during the more recent Alpine orogeny in contrast
to the relatively undeformed Iberian Massif. However, additional studies about the