P. Schmoldt, PhD - MTNet - DIAS
P. Schmoldt, PhD - MTNet - DIAS P. Schmoldt, PhD - MTNet - DIAS
5. Earth’s properties observable with magnetotellurics sLAB eLAB Depth (km) Fig. 5.8.: Depth of the lithosphere-asthenosphere boundary (LAB) beneath Europe, defined in terms of seismic anisotropy observed with teleseismic body waves (sLAB) and in terms of electric conductivity observed with magnetotellurics (eLAB). Figure taken from Jones [2009] using seismic data from Babuska and Plomerová [2006] and magnetotelluric data from Korja [2007]. the upper mantle at depth usually within the range 50 to 160 km; in cratonic regions the LAB can reach significantly greater depth, as much as 250 km [Eaton et al., 2009]. Depth estimates of the LAB for the same region may vary between different geophysical methods. Different LAB depths have been reported for example for Europe by Babuska and Plomerová [2006] using teleseismic body waves and by Korja [2007] using magnetotellurics (Fig. 5.8). This discrepancy might originate from different definitions of the LAB in terms of the related property, i.e. changes in mechanical properties, electric conductivity, seismic velocity, temperature gradient, or anisotropy (seismic and electromagnetic) [e.g. Eaton et al., 2009, and references therein] (Fig. 5.9). The discussion about the LAB thickness evoke the question of whether the LAB is indeed a sharp boundary or rather a smooth transition zone with considerable vertical extent [e.g. Cavaliere and Jones, 1984; Praus et al., 1990; Jones, 1999; Artemieva, 2009; Eaton et al., 2009; Jones, 2009; Meier et al., 2009]. Discrepancies between the depth estimates of the LAB from different methods might therefore result from their varying sensitivity to different properties, which are located at the top, bottom, or within the LAB. In EM induction studies (presuming that the data possess an adequate period range) the LAB can be identified as a significant reduction in resistivity, i.e. from values between 10 3 – 10 4 Ωm to values as low as 5 – 25 Ωm [Eaton et al., 2009]. However, such low resistivity values are not in agreement with predictions by integrated petrophysical modelling [e.g. Fullea et al., 2011], which propose a relatively smooth transition from lithospheric mantle values (10 3 − 10 4 Ωm) to values around 100 Ωm that are related to the asthenosphere (Fig. 5.10). Three groups of explanations for the discrepancy between EM induction studies and laboratory studies, which 92
5.2. Variation of electric conductivity with depth Fig. 5.9.: Definition of the lithosphere and common proxies used to estimate its thickness, i.e. the depth of the lithosphere– asthenosphere boundary (LAB); from Eaton et al. [2009]. The lithosphere forms, in the classical meaning, a mechanical boundary layer with the LAB defined as the top of a zone of decoupling between the lithosphere and asthenosphere, marked by an increased strain rate. The thermal boundary layer (TBL), containing a conductive lid and a transition layer, represents a near-surface region where temperature deviates from adiabatic behaviour. A zone of low seismic shear-wave velocity (Vs) is sometimes detected beneath a high velocity lid whereby various definitions have been used to correlate this zone with the LAB. The LAB may also correlate with a downward extinction of seismic anisotropy or a change in the direction of anisotropy. A significant reduction in electric resistivity at the electrical LAB is inferred from EM induction studies [Eaton et al., 2009]. are the base for the integrated petrophysical modelling, can be conceived: hypotheses A: results of EM induction studies are erroneous, hypotheses B: results of laboratory studies are erroneous, hypotheses C: results of EM induction as well as laboratory studies are correct; the discrepancy is due special characteristics of a layer in the uppermost asthenosphere. A description of causes for each of the three hypotheses is given in the paragraphs below. Hypotheses A: the asthenosphere exhibits a high degree of electric anisotropy due to relative motion between lithosphere and asthenosphere, “dragging along” and aligning material at the LAB (cf. Sec. 4.1.3). The electric anisotropy causes a misinterpretation of responses from EM induction studies that did not adequately consider its effects. Hypotheses B: laboratory studies, base of the petrophysical modelling, are carried out in the very most cases on single crystal samples. The contribution of surface conduction along grain boundaries may increase the bulk conductivity for the respective regions, 93
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5.2. Variation of electric conductivity with depth<br />
Fig. 5.9.: Definition of the lithosphere and common proxies used to estimate its thickness, i.e. the depth of the lithosphere–<br />
asthenosphere boundary (LAB); from Eaton et al. [2009]. The lithosphere forms, in the classical meaning, a mechanical boundary<br />
layer with the LAB defined as the top of a zone of decoupling between the lithosphere and asthenosphere, marked by an increased<br />
strain rate. The thermal boundary layer (TBL), containing a conductive lid and a transition layer, represents a near-surface region<br />
where temperature deviates from adiabatic behaviour. A zone of low seismic shear-wave velocity (Vs) is sometimes detected beneath<br />
a high velocity lid whereby various definitions have been used to correlate this zone with the LAB. The LAB may also correlate with<br />
a downward extinction of seismic anisotropy or a change in the direction of anisotropy. A significant reduction in electric resistivity<br />
at the electrical LAB is inferred from EM induction studies [Eaton et al., 2009].<br />
are the base for the integrated petrophysical modelling, can be conceived:<br />
hypotheses A: results of EM induction studies are erroneous,<br />
hypotheses B: results of laboratory studies are erroneous,<br />
hypotheses C: results of EM induction as well as laboratory studies are correct; the<br />
discrepancy is due special characteristics of a layer in the uppermost asthenosphere.<br />
A description of causes for each of the three hypotheses is given in the paragraphs below.<br />
Hypotheses A: the asthenosphere exhibits a high degree of electric anisotropy due to<br />
relative motion between lithosphere and asthenosphere, “dragging along” and aligning<br />
material at the LAB (cf. Sec. 4.1.3). The electric anisotropy causes a misinterpretation of<br />
responses from EM induction studies that did not adequately consider its effects.<br />
Hypotheses B: laboratory studies, base of the petrophysical modelling, are carried out<br />
in the very most cases on single crystal samples. The contribution of surface conduction<br />
along grain boundaries may increase the bulk conductivity for the respective regions,<br />
93
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