How is the European Lithosphere Imaged by Magnetotellurics?

Abstract

I review recent investigations on the electrical conductivity of the lithosphere and asthenosphere in Europe. The principal method in the reviewed studies is the magnetotelluric method, but in many cases other electromagnetic methods (e.g., magnetovariational profilings and geomagnetic depth soundings) have provided additional information on subsurface conductivity or have been the primary method. The review shows that the magnetotelluric method has been used, and is being used, in all kinds of environments and for many different processes shaping the crust and lithosphere. The crust is very heterogeneous, both with respect to the scale of conductive/resistive features and interpretations: research targets vary from Archaean palaeostructures to ongoing processes. The European database of the depth to the lithosphere-asthenosphere boundary (LAB) in Europe is updated, and a new map showing lateral variations of the depth of LAB is provided. The compilation shows that (1) the Phanerozoic European lithosphere, with considerable variations (45-150 km), is much thinner than the Precambrian European lithosphere, (2) the Trans-European Suture Zone is a major electrical border in Europe separating electrically (as well as geophysically and geologically in general) two quite different settings, (3) the thinnest lithosphere is found under the extensional Pannonian Basin (45-90 km), (4) in most of the East European Craton there are no indications of a high conductivity zone in upper mantle. In many regions there is no information at all on upper mantle conductivity, which calls for pan-European projects to operate arrays of simultaneously recording instruments with long recording periods (2-8 months) and dense spatial sampling (20-50 km).

The elusive lithosphere–asthenosphere boundary (LAB) beneath cratons

“…Jun Korenaga is thanked for providing a preprint of a paper containing new analyses of olivine creep parameters. (Li and Burke, 2006) > 150 km (Chen et al, 2007) Consistent with 250 km (Bruneton et al, 2004a) S-Receiver function~ 250-300 km (Kumar et al, 2007) Magnetotelluric (MT) 230 km (Miensopust et al, 2006) 210 km at LdG 260 km average (Jones et al, 2003) 205 km at NAT (Jones, 1999) > 300 km (Korja, 2007) . The LAB may also correlate with a downward extinction of seismic anisotropy (e.g., Gaherty and Jordan, 1995) or a change in the direction of anisotropy (e.g., Debayle and Kennett, 2000a;2000b;Sebai et al, 2006).…”

“…Lahti et al (2005) noted that although numerous highly conductive crustal bodies and conductivity contrasts produce strong distortions, a decrease in resistivity is required at depths greater than 170 km, providing a minimum depth estimate for the electrical LAB. Korja (2007) recently presented an updated map of lithospheric thickness for Scandinavia, based on a compilation of older and newer sources. His map shows significant variation within Fennoscandia from ca.…”

The first magnetotelluric image of the lithospheric-scale geological architecture in central Svalbard, Arctic Norway

“…6a). Finally, considering 100 Vm as a typical upper boundary for asthenospheric resistivity, following Karato (1990) and references therein, we set the electric lithosphereÁasthenosphere boundary (e-LAB; Jones 1999; Martinec & Wolf 2005;Korja 2007) at about 100 km below the surface. Furthermore, the resistivity geometry we presented in Fig.…”

“…Understanding the nature of the base of the lithosphere is important since the asthenosphere plays crucial roles when it comes to the geodynamics of the overlying lithosphere (O'Reilly & Griffin 1996;Jones 1999). The top of the asthenosphere can be estimated on the basis of electrical signatures retrieved from MT data (Martinec & Wolf 2005;Korja 2007). To serve such purposes, 10Á100 Vm is suggested as a realistic range of asthenospheric resistivity (Karato 1990).…”

Geomagnetically induced currents in Norway: the northernmost high-voltage power grid in the world

“…Concerning Norway, there are three different block models listed in Table 2. The ground conductivity of the uppermost Earth layers in the Nordic countries is generally smaller than elsewhere in Europe (Korja et al 2002;Korja 2007). This is especially the case in South Norway.…”