Diffuse continental deformation: length scales, rates and metamorphic evolution

Abstract

In contrast with the oceanic portions of the plates, continents may deform hundreds of kilometres away from any plate boundary. Calculations that treat the continents as continuous media suggest that the across-strike length scale of deformation associated with a convergent boundary is proportional to the along-strike length of the boundary, the constant of proportionality depending on the rheology of the lithosphere. The dependence on convergent boundary length implies that, for fixed velocity of convergence, strain rates decrease with increasing length of the orogen. The dependence on rheology is less straightforward, but it appears from laboratory determination of flow laws that the scale length of compressional deformation decreases as the fraction of the lithospheric strength supported by friction on faults increases; that fraction depends on the thermal profile of the continental lithosphere and will change during the evolution of an orogenic belt. The thermal development of a diffusely deforming compressional belt is nearly independent of its strain history unless the strain rates are less than about 10 -15 s -1 . If erosion terminates the heating phase of such an orogenic belt, the dominant control on the metamorphic history is the heat supply to the continental lithosphere. In contrast, if extension of the thickened crust ends metamorphism, peak metamorphic conditions depend only on the rheological properties of the lithosphere. Metamorphism in crust that is subject to major extension following compressional orogenesis should be nearly independent of the initial thermal conditions of the crust, and may be distinguished from metamorphism terminated by erosion by a final stage of isobaric cooling from temperatures close to the maximum experienced.