Variations of the lithospheric strength and elastic thickness in North America

Abstract

AbstractWe evaluate the effect of temperature variations on strength and effective elastic thickness (Te) of the lithosphere of the North American (NA) continent. To this purpose, we use two thermal models that are corrected for compositional variations and anelasticity effects in the upper mantle. These thermal models are obtained from a joint inversion of gravity data and two recent seismic tomography models (NA07 and SL2013sv). The crustal rheology was defined using NACr14, the most recent NA crustal model. This model specifies seismic velocities and thickness for a three‐layer model of the crystalline crust. Strength in the lithosphere and in the crust has similar distributions, indicating that local geotherms play a dominant role in determining strength rather than crustal composition. A pronounced contrast is present in strength between cratonic and off‐cratonic regions. Lithospheric strength in the off‐cratonic regions is prevalently localized within the crust and Te shows low values (<20 km), while the inner part of the cratons is characterized by a strong lithosphere with large Te (>150 km). In contrast to previous results, our models indicate that Phanerozoic regions located close to the edge of the cratons, as the Appalachians, are characterized by low strength. We also find that locally weak zones exist within the cratons (e.g., beneath the intracratonic Illinois Basin and Midcontinent rift). Seismic tomography models NA07 and SL2013sv differ mainly in some peripheral parts of the cratons, as the Proterozoic Canadian Platform, the Grenville, and the western part of the Yavapai‐Mazatzal province, where the integrated strength for the model NA07 is 10 times larger than in model SL2013sv due to a temperature difference (>200°C) in the uppermost mantle. The differences in Te between the two models are less pronounced. In both models, Proterozoic regions reactivated by Meso‐Cenozoic tectonics (e.g., Rocky Mountains and the Mississippi Embayment) are characterized by a weak lithosphere due to the absence of the mechanically strong part of the mantle lithospheric layer. Intraplate earthquakes are distributed along the edges of the cratons, demonstrating that tectonic stress accumulates there, while the cores of the cratons remain undeformed. In both models, intraplate earthquakes occur in weak lithosphere (∼0.5 × 1013 Pa s, Te ∼ 15 km) or near the edges of strong cratonic blocks, characterized by pronounced contrasts of strength and Te.