Variations of the lithospheric strength and elastic thickness in North America

Abstract

We evaluate the effect of temperature variations on strength and effective elastic thickness (T e ) 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 T e shows low values (<20 km), while the inner part of the cratons is characterized by a strong lithosphere with large T e (>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 (>2008C) in the uppermost mantle. The differences in T e between the two models are less pronounced. In both models, Proterozoic regions reactivated by MesoCenozoic 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 3 10 13 Pa s, T e $ 15 km) or near the edges of strong cratonic blocks, characterized by pronounced contrasts of strength and T e .

Weakened Lithosphere Beneath Greenland Inferred From Effective Elastic Thickness: A Hot Spot Effect?

“…The area in north-central Greenland toward the Nares Strait and Ellesmere Island has high T e (∼60-70 km; Figure 2a) and a shallow Moho (∼30 km; Figure 1d). An increased T e for the northern part of Ellesmere Island is also observed by Tesauro et al (2015). An increased T e for the northern part of Ellesmere Island is also observed by Tesauro et al (2015).…”

“…The low T e values indicate that this area is the remnant of the failed Atlantic mid-ocean ridge in the Baffin Bay. Kirby and Swain (2009) and Tesauro et al (2015) have observed a similar feature, suggesting that the southern part of the Greenland craton is connected to the cratons of Eastern Canada. Kirby and Swain (2009) and Tesauro et al (2015) have observed a similar feature, suggesting that the southern part of the Greenland craton is connected to the cratons of Eastern Canada.…”

“…Low lithospheric temperatures generally result in mechanical coupling of the crust and uppermost mantle and high T e values, which then correspond to the base of the mechanical lithosphere, for Moho depth lower than 35-40 km (Burov & Diament, 1995). The combination of T e and Moho depth therefore indicates the state of mechanical coupling (e.g., Burov, 2011;Burov & Diament, 1995;Cloetingh & Burov, 2011;Tesauro et al, 2012Tesauro et al, , 2015. At intermediate temperatures, crustal thickness values greater than ∼40 km can result in mechanical decoupling due to weak lower crustal rheology as flexural stresses cannot propagate through the lithosphere, with estimated T e values up to 80 km (see Figure 4b in Burov & Diament, 1995).…”

Full‐Wave Seismic Tomography in the Northeastern United States: New Insights Into the Uplift Mechanism of the Adirondack Mountains

“…However, edge-driven convection alone would produce low-velocity anomalies along the entire Laurentian margin. The lithosphere beneath the Appalachian orogeny appears to be relatively weaker in comparison with the cratonic lithosphere (Tesauro et al, 2015), making lithosphere erosion possible beneath the New England region. In addition, Figure 3b shows that the upper limit of the SNE-ENY anomaly is at the depth of 70 km, which is shallower than the average depth (~90-110 km) of the lithosphere-asthenosphere boundary beneath the Appalachians Rychert et al, 2005Rychert et al, , 2007.…”

Density structure and isostasy of the lithosphere in Egypt and their relation to seismicity

“…9 (profile 2) that this area corresponds to a dense and likely strong mantle lithosphere extending to the bottom of the crust. It has been demonstrated that seismicity occurs at the boundaries of rigid lithospheric blocks in similar situations (Tesauro et al, 2015). In the case of the Nile Delta, the weak crust easily accommodates relatively small deformations, which in contrast to the Red Sea are limited by the strong lithosphere beneath.…”