2.3.6. Lithospheric Strength

Based on knowledge of the thickness and thermal structure of the lithosphere (Fig. 20), strength profiles and effective elastic thicknesses can be calculated (Fig. 21). Over the last decade this has been done for a number of locations in Europe (e.g. Cloetingh and Burov, 1996). Most of these strength profiles and estimates of integrated strength were calculated along available deep seismic crustal cross sections, such as the European Geotraverse (Cloetingh and Banda, 1992) and the TransAlp deep seismic profile (Willingshofer and Cloetingh, 2003). Until recently, lithospheric strength maps have been calculated for restricted areas of Europe only, including the Pannonian Basin-Carpathian region (Lankreijer et al., 1999) and the Baltic Shield (Moisio et al., 2000), but were not yet available on a regional scale for intraplate Europe.

Fig. 21. From crustal thickness (top left) and thermal structure (top right) to lithospheric strength (bottom): conceptual thermal structure and composition of the lithosphere, adopted for the calculation of 3D strength models (after Cloetingh et al., 2006a).

Cloetingh et al. (2006a; 2006b) constructed a 3-dimensional strength map for the lithosphere of a large part of Europe. Existing models are based on a 3D multi-layer composition of the lithosphere, including one upper mantle layer, two to three crustal layers and a sedimentary cover layer (e.g. Hardebol et al., 2003). The seismic tomography data used to infer the temperature structure of the lithosphere below Europe (Goes et al., 2000a; 2000b) has, however, only limited resolution in the mechanically strong part of the lithosphere.

Figure 22 shows the integrated compressional strength of the entire lithosphere of Western and Central Europe. As evident from this figure, Europe’s lithosphere is characterized by major lateral mechanical strength variations, with a pronounced contrast between the strong lithosphere of the Proterozoic East-European Platform east of the Teisseyre-Tornquist line and the relatively weak Phanerozoic lithosphere of Western Europe. A clear strength contrast occurs also at the transition from strong oceanic lithosphere of the Atlantic to the relatively weak continental lithosphere of Western Europe. Within the Alpine foreland, a pronounced northwest—southeast trending weak zone is evident that coincides with the Mesozoic Sole Pit and West Netherlands Basins, the Cenozoic Rhine Rift System and the south-western margin of the Bohemian Massif. Furthermore, a broad zone of weak lithosphere characterizes the Massif Central and surrounding areas, as well as the Alps. Higher-strength zones are associated with the central parts of the North German Basin, the British Isles and parts of the Armorican and Bohemian Massifs, all of which are characterized by moderate seismicity.

Fig. 22. Integrated strength map for intraplate Europe (after Cloetingh et al., 2005b), showing main structural features (after Ziegler, 1988; Dèzes et al., 2004). Colours represent the integrated compressional strength of the total lithosphere. Adopted composition for upper crust, lower crust and mantle is based on a wet quartzite, diorite and dry olivine composition, respectively. Rheological rock parameters are from Carter and Tsenn (1987). The adopted bulk strain-rate is 10^−16/s.

The presence of thickened crust in the area of the Teisseyre-Tornquist suture zone gives rise to a pronounced mechanical weakening of the crustal parts of the lithosphere, whereas the lithospheric mantle retains a moderate strength. Whereas the lithosphere of Fennoscandia is characterized by relatively high strengths, the North Sea rift system corresponds to a zone of weakened lithosphere. A pronounced strength contrast is evident between the strong Adriatic indenter and the weak Pannonian Basin, the Apennines and the Alps.

The lateral strength variations of Europe’s intraplate lithosphere are primarily caused by variations in the mechanical strength of the lithospheric mantle (MSML), whereas the contribution from crustal strength variations appears to be more modest (Cloetingh et al., 2005b). The variations in MSML are primarily related to variations in the thermal structure of the lithosphere, reflecting upper mantle thermal perturbations imaged by seismic tomography, with lateral changes in crustal thickness playing a secondary role, apart from Alpine domains that are characterized by deep crustal roots. For instance, the strong lithosphere of the East-European Platform, the Bohemian Massif, the London-Brabant Massif, and the Fennoscandian Shield can be explained by the presence of old, cold lithosphere, whereas the European Cenozoic Rift System coincides with a major axis of weakened lithosphere within the Northwest European Platform. Similarly, weakening of the lithosphere of southern France can be attributed to the presence of a tomographically imaged plume rising up under the Massif Central (Granet et al., 1995; Wilson and Patterson, 2001). Linking mantle flow properties, controlling plate scale deformation patterns, with smaller scale basin deformation and near-surface expression of (neo)tectonics, requires extensive knowledge on crustal rheological properties, and particularly on the mechanical properties of faults. Researchers bundled in TOPO-EUROPE provide extensive expertise and knowledge on this topic.