P. Schmoldt, PhD - MTNet - DIAS
P. Schmoldt, PhD - MTNet - DIAS P. Schmoldt, PhD - MTNet - DIAS
10. Data inversion TM+TE Depth (km) 0 50 100 150 200 250 300 TE-only 0 50 Depth (km) 100 150 200 250 300 TM-only 0 50 Depth (km) RMS misfit: TM+TE: 3.39 TE-only: 4.01 TM-only: 2.76 Error floor: ra=10%(TM), 20%(TE) f =5%(TM), 10%(TE) ra=20% f =10% ra=10% f =5% S Campo de Montiel M.P. Loranca Basin N 100 150 200 250 300 pic020 pic019 pic011 pic009 0 20 40 60 80 100 120 140 Distance (km) pic007 pic005 pic004 pic003 pic002 pic001 (Wm) 4 Fig. 10.15.: Results of initial isotropic 2D smooth inversion for the Tajo Basin subsurface, using a starting model that contains crustal structures obtained through inversion with shorter periods and crustal geoelectric strike direction (Fig. 10.6). In the inversion for mantle structures periods greater 100 s are selected, using data from both MT modes (‘TM+TE’, uppermost plot), only from the TE mode (‘TE-only’, middle plot), and only from the TM mode (‘TM-only’, bottom plot). Shaded areas indicate regions with low resolution (see text for details) and dotted grey lines denote Niblett-Bostick depths (Sec. 6.3.1) for the longest period of each mode at the respective MT recording station. Locations of stations are indicated on the top of the figure, together with labels denoting certain regions within the Tajo Basin (M.P.: Manchega Plain). Also shown is the average RMS misfit of the stations with colours denoting values for each of the three datasets. at each station). However, significant difference in electric resistivity can also be observed for the region in the south of the PICASSO Phase I profile, for which a Niblett-Bostick depth of more than 200 km is calculated for both modes. Moreover, subsurface structures inferred by, particularly the combined and TM-only, models are implausible; i.e. the increase of resistivity at depths that are usually associated with the Earth’s asthenosphere. Implausible isotropic 2D inversion models are most likely the result of oblique geoelectric strike direction of the Tajo Basin crust and 3D effects that are not adequately taken into account during decomposition of the impedance tensor (Sec. 4.4). Hence, inversion results for mantle structures of the Tajo Basin confirm result of the synthetic model study inferring shortcomings of isotropic 2D inversions using structures from a crustal model with an oblique geoelectric strike direction (cf. Chap. 8). 250 log 10 3 2 1 0
10.2. Inversion for mantle structures Features of the inversion models shown in Figures 10.14 and 10.15 should not be overinterpreted as they are solely the result of an initial inversion sequence in order to examine contributions of the two modes. However, through this initial inversion sequence insights about the characteristics of each modes are already gained. These insights can be used to guide subsequent isotropic 2D combined-mode inversion steps; namely, (a) that TE mode data are potentially distorted by crustal or off-profile features and should be downweighted, and (b) an enhanced starting model based on the TM mode inversion model suggesting an eLAB in the depth range 100 – 150 km. Estimation of lithosphere-asthenosphere boundary depth In its original, rheological, definition the lithosphere–asthenosphere boundary (LAB) denotes the transition from a rigid to a viscously deforming region within the Earth’s mantle; i.e. a transition from a mechanical strong to a weak region 3 . The LAB depth is therefore of major importance for understanding tectonic processes in the study area. Moreover, due to the strong temperature-dependence of mantle material viscocity, a correlation of the mechanical lithosphere (or mechanical boundary layer, MBL) and the thermal LAB (tLAB, also referred to as thermal boundary layer, TBL) has been proposed [e.g. Artemieva, 2009]. The tLAB indicates the change from conductive to convective heat transport and its depth is commonly defined using an isotherm; with the isotherm value depending on the choice of the investigator, usually within the range 1200 – 1350°C [e.g. Tejero and Ruiz, 2002; Artemieva, 2006; Tesauro et al., 2009b]. The thermal estimate of the LAB is derived through thermal modelling using surface heat flow measurements with a range of assumptions regarding heat conduction and production within the lithosphere [e.g. Tejero and Ruiz, 2002] or by deducing thermal conditions from results of other methods like seismology [e.g. Artemieva, 2006; Tesauro et al., 2009b]. The LAB depth has in most cases a strong impact on the geologic setting of the regions above, as it significantly affects local T-P condition. Owing to its great depth only few methods are capable of probing the LAB location, namely seismology and magnetotellurics. These methods do not measure rheological properties directly, but yield LAB estimates in terms of their respective properties under investigation. Accordingly, these LAB estimates may be referred to as seismic LAB (sLAB), electric (eLAB). A range of different seismic approaches are used to determine parameters that are used as indicators for the LAB; i.e. change of seismic anisotropy direction or magnitude (sLABa), reduction of surface wave velocity (sLABsw), and signals in receiver functions data (sLABrf). Since these seismic approaches use different parameters as LAB indicator, depth estimates can vary between the approaches [e.g. Eaton et al., 2009, and references within] and may even be refer to features that at not related to the LAB; i.e. variation in magnitude or direction of seismic anisotropy, the top of an older solidified melt layer, the spinel-garnet transition, or presence of water or carbon dioxide 3 Over time different, and sometimes misguiding, use has been made of the term LAB by different authors and disciplines; see Section 5.2.2 and the reviews by Eaton et al. [2009] Artemieva [2009] for details. 251
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10. Data inversion<br />
TM+TE<br />
Depth (km)<br />
0<br />
50<br />
100<br />
150<br />
200<br />
250<br />
300<br />
TE-only 0<br />
50<br />
Depth (km)<br />
100<br />
150<br />
200<br />
250<br />
300<br />
TM-only 0<br />
50<br />
Depth (km)<br />
RMS misfit: TM+TE: 3.39 TE-only: 4.01 TM-only: 2.76<br />
Error floor: ra=10%(TM), 20%(TE)<br />
f =5%(TM), 10%(TE)<br />
ra=20% f =10%<br />
ra=10% f =5%<br />
S<br />
Campo de Montiel M.P.<br />
Loranca Basin<br />
N<br />
100<br />
150<br />
200<br />
250<br />
300<br />
pic020<br />
pic019<br />
pic011<br />
pic009<br />
0 20 40 60 80 100 120 140<br />
Distance (km)<br />
pic007<br />
pic005<br />
pic004<br />
pic003<br />
pic002<br />
pic001<br />
(Wm)<br />
4<br />
Fig. 10.15.: Results of initial isotropic 2D smooth inversion for the Tajo Basin subsurface, using a starting model that contains crustal<br />
structures obtained through inversion with shorter periods and crustal geoelectric strike direction (Fig. 10.6). In the inversion for<br />
mantle structures periods greater 100 s are selected, using data from both MT modes (‘TM+TE’, uppermost plot), only from the<br />
TE mode (‘TE-only’, middle plot), and only from the TM mode (‘TM-only’, bottom plot). Shaded areas indicate regions with low<br />
resolution (see text for details) and dotted grey lines denote Niblett-Bostick depths (Sec. 6.3.1) for the longest period of each mode at<br />
the respective MT recording station. Locations of stations are indicated on the top of the figure, together with labels denoting certain<br />
regions within the Tajo Basin (M.P.: Manchega Plain). Also shown is the average RMS misfit of the stations with colours denoting<br />
values for each of the three datasets.<br />
at each station). However, significant difference in electric resistivity can also be observed<br />
for the region in the south of the PICASSO Phase I profile, for which a Niblett-Bostick<br />
depth of more than 200 km is calculated for both modes. Moreover, subsurface structures<br />
inferred by, particularly the combined and TM-only, models are implausible; i.e. the increase<br />
of resistivity at depths that are usually associated with the Earth’s asthenosphere.<br />
Implausible isotropic 2D inversion models are most likely the result of oblique geoelectric<br />
strike direction of the Tajo Basin crust and 3D effects that are not adequately taken<br />
into account during decomposition of the impedance tensor (Sec. 4.4). Hence, inversion<br />
results for mantle structures of the Tajo Basin confirm result of the synthetic model study<br />
inferring shortcomings of isotropic 2D inversions using structures from a crustal model<br />
with an oblique geoelectric strike direction (cf. Chap. 8).<br />
250<br />
log 10<br />
3<br />
2<br />
1<br />
0
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