P. Schmoldt, PhD - MTNet - DIAS

Depth (km)
30
60
90
120
150
180
210
240
270
300
Electric resistivity ρ (Ωm)
10 100 1000
10.2. Inversion for mantle structures
LM77
LM90
LM110
LM133
LM160
sidelobes
Fig. 10.30.: Smoothed resistivity – depth profiles from models of the Tajo Basin mantle used for forward modelling; at crustal depth all
models comprise the resistivity distribution of the model labelled ‘a’ in Figure 10.27. Profiles for the different models are colour-coded
and solid lines denote models with a transition from an exponential decrease of resistivity to a 100 Ωm halfspace at the lithosphere–
asthenosphere boundary (LAB), dashed lines are related to models with an entirely electrically more conductive 20 Ωm asthenosphere,
and dotted line denote models with an thin, electrically conductive asthenospheric layer (modelled by two rows of 20 Ωm); numbers
in model names indicate the LAB depth (for the ‘sidelobes’ models a 110 km thick lithosphere is used). Markers denote resistivity
values in the middle of the related row within a model. Further details about model characteristics and creation are given in the text.
is concluded that the relatively low resistivity of the lithospheric-mantle beneath the Tajo
Basin is required by the PICASSO Phase I data. Structures in the asthenosphere are not
strongly constrained by the dataset, but a highly conductive asthenosphere anomaly in
the order of 20 Ωm (cf. Sec. 5.2.2) is regarded as unlikely for the Tajo Basin subsurface
considering the increased misfit of responses for the model comprising a 20 Ωm layer at
110 km (labelled ‘053081+ea’ in Figure 10.31 and ‘Minimum misfit model +ea’ in Figure
10.32).
The area of relatively low electric resistivity in the lithospheric-mantle beneath the
Tajo Basin (Fig. 10.33) coincides with a region of low velocity derived by Hoernle et al.
[1995], Bijwaard et al. [1998], Villaseñor et al. [2003], and Amaru [2007] (cf. Sec.
7.3.2). Potential sources of correlated seismic velocity decrease and electric conductivity
increase in the mantle are increased temperature, or the presence of partial melt or water.
Alternative sources of increased conductivity, such as interconnected sulphide or graphite
phases, are unlikely to exhibit a low velocity response. Temperature variations have a
significant effect on electric conductivity given the exponential ρ-T relationship (cf. Sec.
5.1.1) as well as on seismic velocity (i.e. a decrease of 0.5 -– 2% in Vp for an increase
of 100°C [Goes et al., 2000]). Corresponding surface expressions of an increased mantle
temperature in terms of an increased surface heat flow are not observed; surface heat flow
values of the Tajo Basin (65 – 70 mW/m 2 , cf. Sec. 7.3.2) are equal or above the global
continental average (65 mW/m 2 ) but lower than surface heat flow of regions with a thin
lithosphere (≈80 mW/m 2 ) [Pollack et al., 1993]. However, characteristics of the overlying
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