P. Schmoldt, PhD - MTNet - DIAS

1. Introduction
thermal modelling, which are inherently non-unique, requiring a range of assumptions
regarding values and distribution of related parameters within the Earth. Accordingly,
MT is steadily gaining popularity among geoscientist since its development by Rikitake
[1948], Tikhonov [1950] and Cagniard [1953] in the middle of the last century and has
been used to study various aspects in different regions of the world.
For this investigation, MT data were acquired in the Iberian Peninsula during the first
phase of the multinational, multi-disciplinary PICASSO (acronym for Program to Investigate
the Convective Alboran Sea System Overturn) program, which studies tectonic
processes and internal structure of the western Mediterranean lithosphere and surrounding
regions. In order to derive structures of the Iberian Peninsula subsurface, MT recordings
were carried out along the approximately 400 km long, north-south oriented PICASSO
Phase I profile situated in the Spanish Tajo Basin and Betic Cordillera regions. In the
Tajo Basin, comprising the northern half of the PICASSO Phase I profile, a difference
of approximately 70 degrees is determined for the geoelectric strike directions of the
crust (≈N41W) and lithospheric-mantle (≈N29E) regions. The different strike directions
are most likely related to the different tectonic events forming the approximately NW-
SE stretching Pyrenees in the northeast of the peninsula and the approximately NE-SW
stretching Betics in the south of the peninsula.
Oblique geoelectric strike directions for different subsurface regions, e.g. at crust and
mantle depths, are a known problem in MT investigation [e.g. Marquis et al., 1995; Eaton
et al., 2004; Miensopust et al., 2011]. Crustal structures can usually be recovered in a
straightforward manner by confining the modelled frequency range to crustal penetration
depths; recovery of mantle structures, on the other, hand is more challenging when
the structures have a different strike from the overlying crust. Commonly employed 2D
inversion approaches are likely to yield models with inversion artefacts due to misrepresentation
of the strike direction in at least one of the regions. Therefore, Miensopust
et al. [2011] conducted separate inversions for regions with different geoelectric strike
directions along their profile using datasets adapted to meet the strike characteristics in
the respective regions. Ultimately, the authors decomposed their impedance vectors according
to a N35E strike direction for most parts of their profile and used a N55E strike
direction for a subset. For more oblique geoelectric strike directions, however, inversion
artefacts in the mantle model will prevail, owing to effects of the significantly erroneous
decomposition of the impedance tensor at crustal depth. Hence, a simple ‘stitching’ of
inversion models from different strike directions does not adequately recover structures
in the deeper regions. Three-dimensional (3D) inversion of MT data, capable of dealing
with more complex subsurface structures like oblique strike direction, is computationally
expensive, which usually permits detailed inversion of a region with the size of the Tajo
Basin.
The problem of oblique geoelectric strike directions in two-dimensional (2D) inversion,
previously requiring costly 3D inversion, motivated development of a novel 2D inversion
approach. This inversion approach uses electric anisotropy to image 2D structures, enabling
the investigator to derive a subsurface model with oblique geoelectric strike di-
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