The long-term strength of Europe and its implications for plate-forming processes

Field-based geological studies show that continental deformation preferentially occurs in young tectonic provinces rather than in old cratons(1). This partitioning of deformation suggests that the cratons are stronger than surrounding younger Phanerozoic provinces. However, although Archaean and Phanerozoic lithosphere differ in their thickness(2-4) and composition(4,5), their relative strength is a matter of much debate. One proxy of strength is the effective elastic thickness of the lithosphere, T-e. Unfortunately, spatial variations in T-e are not well understood, as different methods yield different results. The differences are most apparent in cratons, where the 'Bouguer coherence' method yields large T-e values (> 60 km)(6-9) whereas the 'free-air admittance' method yields low values (< 25 km)(10). Here we present estimates of the variability of T-e in Europe using both methods. We show that when they are consistently formulated(11), both methods yield comparable T-e values that correlate with geology, and that the strength of old lithosphere (>= 1.5 Gyr old) is much larger ( mean T-e > 60 km) than that of younger lithosphere ( mean T-e < 30 km). We propose that this strength difference reflects changes in lithospheric plate structure ( thickness, geothermal gradient and composition) that result from mantle temperature and volatile content decrease through Earth's history.