P. Schmoldt, PhD - MTNet - DIAS
P. Schmoldt, PhD - MTNet - DIAS P. Schmoldt, PhD - MTNet - DIAS
7. Geology of the Iberian Peninsula Fig. 7.19.: Location of refraction and reflection profiles used by Díaz and Gallart [2009] to derive the Moho depth with colours accounting for the different experiments (see Díaz and Gallart [2009] for respective references). Therein circles depict the location of Ocean Bottom Seismometers (OBS) and land stations of onshore-offshore profiles. Seismic tomography Seismic tomography allows, similar to magnetotellurics, investigation of deep-seated structures, thus having the potential to identify features in the region of the lithosphere– asthenosphere boundary (LAB). However, the seismically defined LAB does not necessarily always coincides with the one obtained by MT since the methods are sensitive to different material properties, namely seismic velocity and electric conductivity (cf. Sec. 5.2.2). Therefore, it is advised to refer to them as seismic LAB (sLAB) and electric LAB (eLAB) instead. Since the changes in velocity and conductivity are often due to changes of the material which are reflected in both properties, one may be used a good indicator of the other, justifying the consideration of seismic tomography as auxiliary material here. Villaseñor et al. [2007] utilised four months of recorded waveforms for an ambient noise surface wave tomography study in which Rayleigh waves at periods from 8 s to 25 s are used to map group velocity across the Iberian Peninsula shallow subsurface. Obtained Rayleigh wave group velocity maps at 10 s and 20 s (representing average shear velocities in the upper 10 km and between 15 km and 30 km, respectively) reveal a SE-NW striking 154
7.3. Tajo Basin and central Spain Fig. 7.20.: Velocity–depth distribution along a profile through the Iberian Peninsula (location of the profile is shown in the right hand side); modified after Díaz and Gallart [2009]. Results of the study are summarised in Table 7.2. ‘N Ib Margin’: North Iberian margin; CM: Cantabrian Mountains; ‘Ib M’: Iberian Massif. high-velocity anomaly for the central Spain region (Fig. 7.21). The authors associated this feature with Palaeozoic basement rocks of the central region within the Iberian Range, in contrast to the lower velocities of Mesozoic sediments in the surrounding Tertiary basins, i.e. Ebro, Duero, and Tajo Basin. The high velocity is diminished in the 20 s map, interpreted by the authors as an indicator for the presence of an igneous and/or metamorphic core and a deep crustal root in the Iberian Range. This results in relatively low velocities in comparison with the surrounding material, in particular the high velocities in the Iberian Massif to the south and west. This lateral change in seismic velocity, indicating a lateral change in geology for this region, is likely to be reflected in MT data as well. It is supposedly emerging as a geoelectric strike with an approximate SE-NW direction for data at the northern end of the PICASSO profile, related to crustal depths of 30 km and above; see Section 9.6.1 for details on the geoelectric strike estimation. Ambient noise tomography can only adequately resolve crustal structures given the small amplitude of the long period signals. Body wave and surface wave tomography studies, on the other hand, use signals generated by earthquakes, and are therefore able 155
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7.3. Tajo Basin and central Spain<br />
Fig. 7.20.: Velocity–depth distribution along a profile through the Iberian Peninsula (location of the profile is shown in the right hand<br />
side); modified after Díaz and Gallart [2009]. Results of the study are summarised in Table 7.2. ‘N Ib Margin’: North Iberian<br />
margin; CM: Cantabrian Mountains; ‘Ib M’: Iberian Massif.<br />
high-velocity anomaly for the central Spain region (Fig. 7.21). The authors associated this<br />
feature with Palaeozoic basement rocks of the central region within the Iberian Range, in<br />
contrast to the lower velocities of Mesozoic sediments in the surrounding Tertiary basins,<br />
i.e. Ebro, Duero, and Tajo Basin. The high velocity is diminished in the 20 s map, interpreted<br />
by the authors as an indicator for the presence of an igneous and/or metamorphic<br />
core and a deep crustal root in the Iberian Range. This results in relatively low velocities<br />
in comparison with the surrounding material, in particular the high velocities in the<br />
Iberian Massif to the south and west. This lateral change in seismic velocity, indicating<br />
a lateral change in geology for this region, is likely to be reflected in MT data as well. It<br />
is supposedly emerging as a geoelectric strike with an approximate SE-NW direction for<br />
data at the northern end of the PICASSO profile, related to crustal depths of 30 km and<br />
above; see Section 9.6.1 for details on the geoelectric strike estimation.<br />
Ambient noise tomography can only adequately resolve crustal structures given the<br />
small amplitude of the long period signals. Body wave and surface wave tomography<br />
studies, on the other hand, use signals generated by earthquakes, and are therefore able<br />
155
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