P. Schmoldt, PhD - MTNet - DIAS

10. Data inversion
Shtrikman upper bounds and the MBLM formulation (Equation 10.1) are valid for melt
amounts above 10 vol%. If results by Partzsch et al. [2000] can be applied to the Tajo
Basin subsurface case, partial melt conductivity of 4 S/m, solid phase conductivity of
5 ∗ 10 −4 S/m, and melt fraction between the minimum of 1% and the value of 10% could
facilitate the electric conductivity anomaly of feature ‘f’.
In the calculation of required melt fraction, discussed in the paragraph above, rocks are
assumed dry and a contribution of water to the composite conductivity is not considered.
For inferred P-T conditions of the intermediate crustal Tajo Basin subsurface (≈ 0.3 GPa
and 400°C [Tejero and Ruiz, 2002]) partial melting is implausible [e.g. Thompson and
Connolly, 1995; Gaillard, 2004; Nover, 2005]. Water would be required to reduce the
solidus of local rocks in order to facilitate partial melting. In addition, water increases ion
mobility, hence conductivity, of the melt phase [Gaillard, 2004] and, in case of interconnected
saline fluids, can further increase conductivity by adding electrolytic conduction
effects (cf. Sec. 5.1.1). Therefore, if water is present in the area of the feature ‘f’, a lower
amount of melt is required to produce the electric conductivity anomaly.
Water in the lower and intermediate Tajo Basin crust could originate from a deeper
source region, e.g. from dehydration processes in the slab subducting under the Alboran
Domain and the Betic Cordillera (cf. Section 7.2, and Figures 7.11 and 7.24). Respective
hydrous phases may have migrated upwards into the lithosphere that was weakened by
Pliocene indentation events (cf. Sec. 7.3.1), and accumulated at the bottom of an impermeable
upper crustal layer. Upward migration of fluids originating from dehydration of
a subducting slab was, for example, reported by Wannamaker et al. [2009] as cause for
an lower crustal conductor in Marlborough, New Zealand. Hence, the increased electric
conductivity in the intermediate and lower crust beneath the Campo de Montiel could be
due to a combined contribution of water an partial melt.
With the current dataset it is not possible to distinguish between these two potential
contributions and it is concluded that a combination of fluid and melting is a likely cause
of the crustal electric anomaly ‘f’ in Figure 10.6 and the low velocity region in Figure
7.21. Inversion results of the PICASSO Phase I dataset for mantle structures will provide
further insight about the contribution of water and its possible sources since in case of
upward migrations of fluids a corresponding increase in conductivity for mantle regions
between the dehydration region of the slab and the conductive feature in the crust is likely
(cf. Sec. 10.2).
10.2. Inversion for mantle structures
In the previous Section 10.1 crustal structures of the Tajo Basin were investigated using
short period response data from PICASSO Phase I stations. This Section is concerned
with structures at mantle depths beneath the Tajo Basin; thus, longer period data are
utilised. Separate investigation of crust and mantle structures is motivated by significantly
different geoelectric strike directions of the two depth ranges and related issues of 2D
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