On the recovery of effective elastic thickness using spectral methods: Examples from synthetic data and from the Fennoscandian Shield

There is considerable controversy regarding the long‐term strength of continents ( T e ). While some authors obtain both low and high T e estimates from the Bouguer coherence and suggest that both crust and mantle contribute to lithospheric strength, others obtain estimates of only <25 km using the free‐air admittance and suggest that the mantle is weak. At the root of this controversy is how accurately T e can be recovered from coherence and admittance. We investigate this question by using synthetic topography and gravity anomaly data for which T e is known. We show that the discrepancies stem from comparison of theoretical curves to multitaper power spectral estimates of free‐air admittance. We reformulate the admittance method and show that it can recover synthetic T e estimates similar to those recovered using coherence. In light of these results, we estimate T e in Fennoscandia and obtain similar results using both techniques. T e is 20–40 km in the Caledonides, 40–60 km in the Swedish Svecofennides, 40–60 km in the Kola peninsula, and 70–100 km in southern Karelia and Svecofennian central Finland. Independent rheological modeling, using a xenolith‐controlled geotherm, predicts similar high T e in central Finland. Because T e exceeds crustal thickness in this area, the mantle must contribute significantly to the total strength. T e in Fennoscandia increases with tectonic age, seismic lithosphere thickness, and decreasing heat flow, and low T e correlates with frequent seismicity. However, in Proterozoic and Archean lithosphere the relationship of T e to age is ambiguous, suggesting that compositional variations may influence the strength of continents.