P. Schmoldt, PhD - MTNet - DIAS
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P. Schmoldt, PhD - MTNet - DIAS

P. Schmoldt, PhD - MTNet - DIAS P. Schmoldt, PhD - MTNet - DIAS

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10. Data inversion in the lithospheric-mantle [e.g. Levin and Park, 1997; Stixrude and Lithgow-Bertelloni, 2005; Thybo, 2006; Abt et al., 2010]. Magnetotelluric, on the other hand, yields electric LAB (eLAB) depth estimates using a significant decrease of electric resistivity as proxy for the LAB. Electric conductivity of Earth materials is strongly temperature-dependent (cf. Sec. 5); hence, a relation between eLAB and tLAB, and ultimately with the mechanical LAB, is plausible (cf. Sec. 5.2.2). Depths of the different LAB indicators as well as difference or semblance between LAB estimates for the different parameters can yield valuable information about geological settings of the study area. For regions of the Iberian Peninsula in proximity of the PICASSO Phase I profile, lithosphere thicknesses between 110 km and 130 km have been inferred by different methods (cf. Sec. 7). In their investigations of the eastern Betic Cordillera Rosell et al. [2010] determine an eLAB depth of 110 km using magnetotellurics (the study area of Rosell et al. [2010] is shown in Figure 7.11). A 110 km depth of the tLAB beneath the Tajo Basin was derived by Tejero and Ruiz [2002] using thermal modelling with heat-flow values from Fernandez et al. [1998] and assuming a temperature of 1350°C at the tLAB. Tesauro et al. [2009a] and Tesauro et al. [2009b], on the other hand, use the seismic tomography model by Koulakov et al. [2009] to construct a thermal model of the European lithosphere and derive a tLAB between 120 km and 130 km for the same region using the 1200°C isotherm as indicator for the tLAB. Fullea et al. [2010] determined a LAB depth between 110 km and 130 km for the southern Tajo Basin region using integrated geophysical modelling; however, the focus of their work is on regions in close proximity of the Alboran Domain and characteristics of areas to the North, such as the Tajo Basin, are less constrained. The eLAB is usually associated with a significant change in electric conductivity from values of approximately 1000 Ωm in the lithospheric-mantle to values of 100 Ωm or lower in the asthenosphere (values as low as 5 Ωm have been reported for the upper asthenosphere; see Section 5.2.2 for a discussion about the electric asthenosphere anomaly). Thus, the eLAB should yield a measurable response in MT data of sufficient period length. Reduced electric resistivities and increased impedance phases of the PICASSO Phase I response data shown in Figure 9.13 (feature ‘d’) may indicate an eLAB response in the data, therefore motivating a detailed investigation of eLAB depth and characteristics beneath the Tajo Basin. First, the eLAB beneath the Tajo Basin is studied using isotropic 2D sharp-boundary inversion with conductivity interfaces (in the following referred to as ‘CI-inversion’), which is part of the 2DMTinv programme implemented in the WinGLink software package [WinGLink, 2005]. For that purpose a starting model is constructed (upper plot in Figure 10.16) that comprises three layers representing a 100 Ωm crust (depth range 0 – 33 km), a 1000 Ωm lithospheric-mantle (depth range 33 – 110 km), and a 100 Ωm asthenosphere (depths ≥ 110 km). Therein, resistivity values are based on the TM-only inversion model obtained in the previous Section 10.2.1. The starting model has a RMS misfit of 6.28 with error floors of ρa = 20% and φ = 5% (TM mode) and ρa = 40% and φ = 10% (TE mode). 252

Start Depth (km) 0 35 70 105 140 175 210 CI-fixed 0 35 Depth (km) S N Campo de Montiel M.P. Loranca Basin 70 105 140 175 210 CI-cons. 0 35 Depth (km) 70 105 140 175 210 Start: 6.28 CI-fixed: 4.14 CI-cons: 3.53 Error floor: r a=20%, f=5% (TM) r a=40%, f=10% (TE) pic020 pic019 pic011 pic009 0 15 30 45 60 75 90 105 120 135 Distance (km) pic007 10.2. Inversion for mantle structures pic005 pic004 pic003 pic002 pic001 (Wm) 5 Fig. 10.16.: Isotropic 2D sharp boundary inversion (CI-inversion) models of the Tajo Basin subsurface; top: Starting model, middle: model from CI-inversion with fixed interfaces at 30 and 110 km, bottom: model from CI-inversion with variable interfaces. Error floors used in the inversion are ρa = 20% and φ = 5% for the TM mode and ρa = 40% and φ = 10% for the TE mode, taken into account the previously inferred galvanic distortion and its higher effect on the TE mode. Dashed black lines denote the lithosphere-asthenosphere boundary (LAB); the dashed green lines indicate shallower and deeper LABs for the northern region of the profile, inferred from models with similar RMS misfit. Error floors of TE mode and apparent resistivity data are increased because of galvanic distortion, inferred during the initial inversion sequence (cf. Fig. 10.14), and its different impact on the model parameters; see Chapter 4 for a detailed description regarding distortion of MT data. As a first approach, it is tested whether a subsurface model of electric conductivity can be found that contains an entirely levelled eLAB at a depth of 110 km. For that reason, locations of conductivity interfaces are fixed during the inversion and the misfit can only be reduced through variation of electric resistivity values within the layers. Changes of electric resistivity values at crustal depth are constrained to avoid inversion artefacts due to the oblique strike direction. The resulting model (middle plot in Figure 10.16) exhibits a relatively lower resistivity in the centre-south region of the profile. However, an unacceptable misfit of the model prevails (RMS = 4.14), indicating that a levelled log 10 4 3 2 1 0 253

Start<br />

Depth (km)<br />

0<br />

35<br />

70<br />

105<br />

140<br />

175<br />

210<br />

CI-fixed 0<br />

35<br />

Depth (km)<br />

S N<br />

Campo de Montiel M.P.<br />

Loranca Basin<br />

70<br />

105<br />

140<br />

175<br />

210<br />

CI-cons. 0<br />

35<br />

Depth (km)<br />

70<br />

105<br />

140<br />

175<br />

210<br />

Start: 6.28 CI-fixed: 4.14 CI-cons: 3.53 Error floor: r a=20%, f=5% (TM)<br />

r a=40%, f=10% (TE)<br />

pic020<br />

pic019<br />

pic011<br />

pic009<br />

0 15 30 45 60 75 90 105 120 135<br />

Distance (km)<br />

pic007<br />

10.2. Inversion for mantle structures<br />

pic005<br />

pic004<br />

pic003<br />

pic002<br />

pic001<br />

(Wm)<br />

5<br />

Fig. 10.16.: Isotropic 2D sharp boundary inversion (CI-inversion) models of the Tajo Basin subsurface; top: Starting model, middle:<br />

model from CI-inversion with fixed interfaces at 30 and 110 km, bottom: model from CI-inversion with variable interfaces. Error floors<br />

used in the inversion are ρa = 20% and φ = 5% for the TM mode and ρa = 40% and φ = 10% for the TE mode, taken into account the<br />

previously inferred galvanic distortion and its higher effect on the TE mode. Dashed black lines denote the lithosphere-asthenosphere<br />

boundary (LAB); the dashed green lines indicate shallower and deeper LABs for the northern region of the profile, inferred from<br />

models with similar RMS misfit.<br />

Error floors of TE mode and apparent resistivity data are increased because of galvanic<br />

distortion, inferred during the initial inversion sequence (cf. Fig. 10.14), and its different<br />

impact on the model parameters; see Chapter 4 for a detailed description regarding<br />

distortion of MT data.<br />

As a first approach, it is tested whether a subsurface model of electric conductivity can<br />

be found that contains an entirely levelled eLAB at a depth of 110 km. For that reason,<br />

locations of conductivity interfaces are fixed during the inversion and the misfit can only<br />

be reduced through variation of electric resistivity values within the layers. Changes<br />

of electric resistivity values at crustal depth are constrained to avoid inversion artefacts<br />

due to the oblique strike direction. The resulting model (middle plot in Figure 10.16)<br />

exhibits a relatively lower resistivity in the centre-south region of the profile. However,<br />

an unacceptable misfit of the model prevails (RMS = 4.14), indicating that a levelled<br />

log 10<br />

4<br />

3<br />

2<br />

1<br />

0<br />

253

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