Plate Rheology and Mechanics

Abstract

Assessment of mechanical properties of lithosphere is of primary importance for interpretation of deformation at all spatial and timescales, from local to geodynamic spatial scale and from seismic timescale to billions of years. Depending on loading conditions and timescale, lithosphere exhibits elastic, brittle (plastic), or viscous (ductile) behavior. As can be inferred from laboratory rock mechanics data and observations, 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 timescale(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., timescales, strain rates, temperature, and loading conditions) are too far from those of real Earth. Therefore, these data cannot be reliably extended to geologic timescales and spatial scales (strain rates ~ 10 − 17 −10 − 13 s − 1 ) without additional parameterization or validation based on geologic timescale observations of large-scale deformation. For the oceanic lithosphere, the Goetze and Evans 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, T e (flexure) and T s (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 thermomechanical 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 thermomechanical models and of T e data based on the most robust variants of flexural models suggests that continental plates with T e 30–50% smaller than their theoretical mechanical thickness h m (i.e., T e = 20–60 km) should be characterized by a weak lower or intermediate crustal rheology enabling mechanical decoupling between the crust and the mantle. Older plates such as cratons are strong due to crust–mantle coupling and specific properties of the cratonic mantle lithosphere.