P. Schmoldt, PhD - MTNet - DIAS

Depth (km)
Depth (km)
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(b)
S N
Campo de Montiel M.P.
Loranca Basin
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0 16 32 48 64 80 96 112 128 144
Distance (km)
0 1.1 2.2
log10 (Wm)
3.3
10.2. Inversion for mantle structures
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pic004
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Fig. 10.20.: Results of the ρYY component from the anisotropic 2D inversion approach for structures of the Tajo Basin subsurface.
Models are obtained using the MT2Dinv inversion algorithm [Baba et al., 2006] with electric resistivity gradient regularisation, smoothing
parameters α = 3, β = 1, τ = 6, and starting models shown in Figure 10.19 (with the same letters). Inversion models obtained
using either starting model are virtually identical and exhibit implausible high electric conductivity values for mantle regions. As
concluded in a synthetic model study (Sec. 8), reliable subsurface structures are to inferred from the ρXX model (Fig. 10.21) instead;
see text for details.
tive asthenosphere anomaly with only a few tens of Ωm’s, determined for some regions
of the Earth (cf. Sec. 5.2.2), is not strongly demanded by the data. For the lithosphericmantle,
resistivity values between 200 Ωm and 500 Ωm are derived with higher values
located to the south, contrary to observations of isotropic 2D inversion 4 . Note that, due
to regularisation of the inversion process, models depict the minimum resistivity values
supported by the data. This means that higher resistivities are possible, particularly in the
lithospheric-mantle region, but are restrained by smoothing constraints in the inversion
process.
In order to infer the LAB depth beneath the Tajo Basin from anisotropic 2D inversion,
ρ – depth profiles are created by laterally averaging electric resistivity values along
the PICASSO Phase I profile. In addition, average ρ – depth profiles are determined for
the southern and northern region of the PICASSO Phase I to examine lateral changes of
electric resistivity in more detail. Resulting ρ – depth profiles (Fig. 10.22) are compared
with each other as well as with reference profiles from Muller et al. [2009]. The latter are
calculated using laboratory electrical resistivity versus temperature and pressure measurements
for dry olivine and pyroxene [Constable et al., 1992; Xu and Shankland, 1999; Xu
et al., 2000b] with hypothetical mantle geotherms for different lithospheric thicknesses.
Note that the reference profiles only consider semiconduction of dry mineral as electric
4 Horizontal dimensions differ between isotropic and anisotropic inversion models due different projection,
as stations are projected onto profiles that are orthogonal to either the mantle strike direction (isotropic
inversion) or the crustal strike direction (anisotropic inversion, approach 1; see Section 8.3.3 for details
of the approach)
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