P. Schmoldt, PhD - MTNet - DIAS

10. Data inversion
owing to availability of a state-of-the-art algorithm and high-end computational facilities,
an extensive 3D inversion study was conducted to enhance information about the Tajo
Basin mantle region. The most striking feature therein is the low resistivity region in the
lithospheric-mantle below the centre of the PICASSO Phase I profile, coinciding with a
low velocity region determined in seismic tomography studies by Hoernle et al. [1995],
Bijwaard et al. [1998], Villaseñor et al. [2003], and Amaru [2007]. Low resistivity – low
velocity regions at mantle depth are indicative of increased temperatures, interconnected
partial melt or hydrous phases [e.g. Goes et al., 2000; Nover, 2005; Yoshino, 2010, and
reference therein]. In consideration of the reported extensive HIMU-like 7 reservoir beneath
central and western Europe [Cebriá and Wilson, 1995; Hoernle et al., 1995], partial
melt was concluded as most likely source for the lithospheric-mantle anomaly beneath
the Tajo Basin. An enhancement of the feature’s anomalous seismic velocity and electric
conductivity properties by a hydrous phase is possible, with dehydration processes of
the subducting slab beneath Alboran Domain and Betic Cordillera identified as potential
source for the hydrous phase.
Exceptionally low electric resistivity of the region associated with the lithosphericmantle
beneath the Tajo Basin in the 3D inversion, hence the less distinct contrast in resistivity
between lithosphere and asthenosphere, aggravates estimation of the LAB depth
and its characteristics. Based on forward modelling results it can be concluded that a
conductive uppermost asthenosphere anomaly as well as a uniformly conductive asthenosphere
in the order of 20 Ωm, observed in long-period EM induction studies of other
regions (cf. Sec. 5.2.2), are unlikely for the Tajo Basin subsurface. However, due to the
low signal-to-noise ration of, in particular, long-period data, asthenospheric properties are
not strongly constrained by PICASSO Phase I results.
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