Plate Rheology and Mechanics

  • Burov E
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Depending on loading conditions and timescale, 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. At seismic timescale(s), the entire lithosphere should respond in the elastic or brittle-elastic regime. However, rock mechanics experiments are conducted on simple rocks/minerals at simplified conditions and high strain rates (∼10-8-10-4s-1). These data cannot be reliably extended to geological time and spatial scales (strain rates ∼10-17-10-13s-1) without additional parametrization or validation based on geological timescale 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. For continents, the uncertainties of flexural models and of other data sources are much stronger 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 the 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, Te(flexure) and Ts(earthquake) data, and long-term observations such as folding or 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 to test 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 downgoing slab in collisional systems. Also there is certainly no single rheology model for continents: the 'jelly-sandwich' is a useful first-order model with which to parametrize the long-term strength of the lithosphere. It is concluded that dry olivine rheology laws seem to well represent long-term behavior of the continental and oceanic mantle lithosphere. As to the crustal rheology, analysis of the results of thermomechanical models and of Tedata based on robust variants of flexural models suggests that continental plates with Te30-50% smaller than their theoretical mechanical thickness hm(i.e., Te=20-60km) should be characterized by a weak lower or intermediate crustal rheology enabling mechanical decoupling between the crust and the mantle. The older plates such as cratons are strong due to crust-mantle coupling. It is shown also that Tsdata may reflect current intraplate stress level but cannot be decoded in terms of long-term rheology. © 2007 Elsevier B.V. All rights reserved.

Author-supplied keywords

  • Elastic thickness
  • Mechanics of the lithosphere
  • Rheology
  • Yield strength envelopes

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  • E. B. Burov

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