Rheology and strength of the lithosphere

Mechanical properties of lithosphere are of primary importance for interpretation of deformation at all spatial and time scales, from local scale to large-scale geodynamics and from seismic time scale to billions of years. Depending on loading conditions and time scale, lithosphere exhibits elastic, brittle (plastic) or viscous (ductile) properties. As can be inferred from rock mechanics data, a large part of the long-term lithospheric strength is supported in the ductile or ductile–elastic regime, while it also maintains important brittle strength. Yet, at short seismic time scale (s), the entire lithosphere responds in elastic/brittle–elastic regime. Even though rock mechanics experiments provide important insights into the rheological properties of the lithosphere, their conditions (e.g., time scales, strain rates, temperature and loading conditions) are too far from those of real Earth. Therefore, these data cannot be reliably extended to geological time- and spatial scales (strain rates ∼10−17 to 10−13 s−1) without additional parameterization or validation based on geological time scale observations of large-scale deformation. For the oceanic lithosphere, the Goetze and Evan’s brittle–elastic–ductile yield strength envelopes (YSEs) were validated by geodynamic scale observations such as the observations of plate flexure. However, oceanic lithosphere behavior in subduction zones and passive continental margins is strongly conditioned by the properties of the continental counterpart, whose rheology is less well understood. For continents and continental margins, the uncertainties of available data sources are greater due to the complex structure and history of continental plates. For example, in a common continental rheology model, dubbed “jelly sandwich”, the strength mainly resides in crust and mantle, while in some alternative models the mantle is weak and the strength is limited to the upper crust. We address the problems related to lithosphere rheology and mechanics by first reviewing the rock mechanics data, Te (flexure) and Ts (earthquake) data and long-term observations such as folding and subsidence data, and then by examining the physical plausibility of various rheological models. For the latter, we review the results of thermo-mechanical numerical experiments aimed at testing the possible tectonic implications of different rheology models. In particular, it appears that irrespective of the actual crustal strength, the models implying weak mantle are unable to explain either the persistence of mountain ranges for long periods of time or the integrity of the subducting slabs. Although there is certainly no single rheology model for continents, the “jelly sandwich” is a useful first-order model with which to parameterize the long-term strength of the lithosphere. It is concluded that dry olivine rheology laws seem to represent well the long-term behavior of mantle lithosphere in oceans, margins and continents. As to the continent and margin crust rheology, analysis of the results of thermo-mechanical models and of Te data based on the most robust variants of flexural models, suggests that continental plates with Te 30–50% smaller than their theoretical mechanical thickness hm (i.e. Te = 20–60 km) should be characterized by a weak lower or intermediate crustal rheology enabling mechanical decoupling between crust and mantle. Older plates such as cratons are strong due to crust–mantle coupling and specific properties of the cratonic mantle lithosphere.

► We review key questions concerning long-term rheology, mechanics and strength of the lithosphere.
► Rock mechanics data has to be parameterized to be used for geological time scales.
► Parameterization is based on long-term deformation data (e.g. lithospheric flexure) and models.
► There is no direct link between seismic and post-seismic deformation and the long-term strength.
► Continents are rheologically stratified; their strength is mainly concentrated in mantle layer.