P. Schmoldt, PhD - MTNet - DIAS

7.3. Tajo Basin and central Spain
approximately 110 km, on the other hand, most likely represents a real geological feature.
The anomaly is potentially due to subduction or delamination of lithospheric material,
being replaced by warmer, and therefore less dense, mantle material [e.g. Torres-Roldan
et al., 1986; Platt and Vissers, 1989; Seber et al., 1996; Calvert et al., 2000; Gutscher
et al., 2002; Amaru, 2007]. The descending lithospheric material is most likely represented
by (c) the observed relatively high velocity region in the asthenosphere beneath
Alboran Sea and Betic Cordillera. A slab subducting beneath Alboran Sea and Betics
Cordillera has been modelled, for example, in seismic tomography studies by Bijwaard
et al. [1998]; Spakman and Wortel [2004]; Amaru [2007]. (d) Below the Tajo Basin the
seismic LAB (sLAB) may be located at a depth between 90 and 120 km depth, as indicated
by the change from relatively high velocity to low values with respect to the AK135
reference model 1 . However, due to the inherent spatial smoothing of seismic tomography
no solid conclusions can be drawn about the LAB from this method. The feature in the
seismic tomography model most interesting for the PICASSO Phase I investigation is certainly
(e) the northward-dipping, low velocity region beneath the Tajo Basin, extending
closest to the surface beneath the southern boundary of the Tajo Basin. This deep-seated
anomaly is in agreement with previous work by Hoernle et al. [1995] and results by Villaseñor
et al. [2003] deriving a structure of similar location and depth extent with an
ENE-WSW orientation (Fig. 7.23). According to its location, this anomaly could potentially
be a related to the Betic Cordillera – Iberian Peninsula collision in Miocene times
(Sec. 7.2) or to the subducting lithosphere beneath Alboran Sea and Betic Cordillera
(feature ‘c’ in Figure 7.24); however, its vast depth extent is somehow puzzling.
Potential candidates for the lowered velocity of feature ‘e’ are increased temperature,
different chemistry (e.g. a more iron-rich, fertile mantle), and presence of fluids (partial
melt or water). The latter could originate from dehydration of sinking lithospheric materials
beneath the Alboran Sea, in particular, if the material stems from a slab of oceanic
crust as indicated by seismic tomography models of Bijwaard et al. [1998], Spakman and
Wortel [2004], and Amaru [2007, p. 111]. Sensitivity of seismic velocity to various parameters
are calculated by Goes et al. [2000] for depths of 50 km and 200 km beneath
Europe, deducing that a 2% decrease of compressional wave velocity at this depth would
require a minimum temperature increase of 100°C or a melt fraction of up to 4%. The
authors also state that a minimum of 2% melt is required before an effect on seismic velocity
is observable; cf. Sato et al. [1989]. The effective velocity effect of fluids is therein
strongly dependent on the geometry of the melt distribution and interconnection of the
melt pockets, i.e.
attenuation(films) > attenuation(tubes) > attenuation(spheres)
1 The AK135 contains only a marginal jump in velocity at 80 km and a smooth transition from continuous
to increasing velocity at 120 km in the depth range that is usually associated with the LAB (cf. left-hand
side plot in Figure 7.24)
159