Mantle weakening and strain localization: Implications for the long-term strength of the continental lithosphere

Abstract

Mechanics of the continental lithosphere require the presence of a high-strength uppermost mantle that defines the “jelly sandwich” model for lithosphere strength layering. However, in deforming regions, growing numbers of geological and geophysical data predict a sub-Moho mantle strength lower than the crustal strength, or a “crème brûlée” model. To reconcile these two opposite views of lithosphere strength layering, we account for a new olivine rheology, which could promote some weakening during dynamic grain size reduction that enhances grain boundary sliding. We performed a one-dimensional numerical model of a deforming rock in order to quantify strain localization due to this weakening rheology. Strain localization occurs at temperatures lower than 800 °C and reaches a maximum at 610 °C, increasing the strain rate from 10⁻¹⁵ to >10⁻¹³ s⁻¹. These results imply the existence of a sub-Moho ductile localizing mantle on a lithosphere scale, which occurs at Moho temperatures lower than 800 °C. Also, the localizing degree of this ductile mantle increases with decreasing Moho temperatures down to 550 °C. Such a ductile localizing mantle could therefore promote large strain localization during lithosphere deformation, like the brittle mantle that is commonly assumed in the “jelly sandwich” model. Furthermore, the long-term deformation (≥10⁶ yr) of the ductile localizing mantle could change the lithosphere strength layering from “jelly sandwich” to “crème brûlée” in response to the grain boundary sliding–induced weakening in mantle shear zones.