P. Schmoldt, PhD - MTNet - DIAS

10. Data inversion
isotropic 2D layer model is not in agreement with data collected for the Tajo Basin.
Because of the prevailing increased misfit for the CI-inversion with fixed conductivity
interfaces, constrains on the interfaces are lowered in order to permit a laterally changing
LAB depth. A variety of intermediate processing steps are conducted and effects of different
sequences of apparent resistivity and impedance phase data weighing are examined;
the final model is then chosen on the base of the lowest overall misfit. The CI-inversion
model with variable interface depths exhibits a RMS misfit of 3.53 indicating that the
model is not an acceptable fit; however, the model remains an interesting entity to investigate
structures supported by the dataset. The LAB is modelled at depths between
90 km and 140 km with the shallowest region in the south-central region of the profile
and greater depths at the edges of the profile. The upwards extension of the eLAB in
the south-central region of the profile, together with the relatively lower resistivity of the
lithospheric-mantle in this region (≈ 10 3 Ωm), was also modelled in the CI-inversion with
fixed interfaces (upper plot in Figure 10.16); it is further supported by the seismic tomography
model of Amaru [2007] (Fig. 7.24). The seismic model exhibits a collocated low
velocity region that may be caused by fluids or higher temperature; the latter could be
caused by enhanced heat transport in conjunction with a shallower tLAB.
LAB depth of the northernmost region is not strongly constrained since, for the same
misfit, models with thinner as well as with thicker lithosphere could be found. Modelled
thickening of the lithosphere in the bottom plot of Figure 10.16 is based on geological
considerations. The deeper LAB in the north of the profile coincides with the Iberian
Range, an intraplate mountain ranges formed by the Mesozoic collision of the Iberian
Peninsula with the Eurasian Plate (cf. Sec. 7). The associated compressional stress
regime in the Iberian Peninsula, in conjunction with isostatic equilibrium processes, is
likely to have caused thickening of crust and lithosphere [e.g. Airy, 1855; Pratt, 1855;
Surinach and Vegas, 1998; Vegas et al., 1990; Turcotte and Schubert, 2002; Stacey and
Davis, 2008; Teixell et al., 2009]. The uppermost mantle beneath the Tajo Basin has been
inferred as a competent layer [Cloetingh et al., 2002; Tejero and Ruiz, 2002; Fernández-
Lozano et al., 2011] and is therefore likely to exhibit a vertical displacement as a result
of horizontal stress. Cases of thinner lithosphere beneath mountain ranges have been
reported for other regions of the Earth, namely the Atlas Mountains in Morocco [Teixell
et al., 2005; Fullea et al., 2007] and the southern Sierra Nevada region in California [e.g.
Jones, 1987; Zandt and Carrigan, 1993; Park et al., 1996; Zandt et al., 2004; Abt et al.,
2010]. However, lithospheric thinning under mountain ranges can, if at all, only to be
found in exceptional geological settings. Keeping the crust–mantle boundary at a fixed
depth of 30 km in order to reduce effects of the oblique crustal strike direction, rather than
increasing its depth beneath the northern region, may contribute to the uncertainty of the
LAB depth below. For the southern region of the profile, the model exhibits an eLAB
depth of approximately 110 km, which is in agreement with results by Rosell et al. [2010]
for the proximate eastern Betics region, as well as with LAB estimates of Fullea et al.
[2007] and Fullea et al. [2010] for the southern Tajo Basin.
Besides the overall increased misfit of the isotropic 2D models, isotropic 2D inversion
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