Active continental deformation and regional metamorphism

Abstract

The vertical and horizontal distribution of present-day continental deformation is examined to see how tectonic movements may be related to large wavelength perturbations to the temperature and pressure experienced by rocks in the crust. Earthquakes are generally restricted to the upper part of the continental crust. The lower crust is usually aseismic and assumed to be weaker. The uppermost mantle beneath continental regions has minor seismic activity that does not account for much deformation, but probably indicates an important strength contrast between the lower continental crust and the upper mantle. The maximum focal depth of earthquakes in any region appears to be limited by temperature, with most restricted to material colder than 350± 100 °C in the crust and colder than 700± 100 °C in the mantle. At length scales long compared with the thickness of the brittle upper crust, the deformation in regions of continental extension or shortening appears to be continuous, even though, in reality, discontinuous movement on faults occurs. This probably indicates that the deformation is dominated by distributed flow in the ductile portion of the lithosphere and not by the behaviour of the thin brittle upper crust. The distribution of seismicity, elevation contrasts and vertical movements at the surface suggests that there is little spatial separation between the brittle deformation in the upper crust and the ductile deformation below on length scales larger than the lithosphere thickness. For this reason, and because of the short thermal time constant of the crust, long-wavelength perturbations to the thermal regime are more influenced by the behaviour of the lithosphere as a whole than by the precise geometry of deformation in the crust. Large-scale regional metamorphism in zones of shortening may result from the re-establishment of the initial geotherm in thickened crust when the lower part of the lithosphere detaches and falls into the asthenosphere. In regions of extension, an increased geothermal gradient is unlikely to result in regional metamorphism unless magmatic augmentation to the heat supply is important. However, if the stretched region is covered by thick sediments, the basement may experience a small increase in temperature and remain significantly hotter than it would be if there were no sediment cover. While unlikely to account for significant metamorphism, this effect may strongly influence the rheological behaviour of the lithosphere in extending regions. The rapid vertical movements associated with syn- or post-orogenic normal faulting in regions of large-scale crustal thickening are probably at least as important in exhuming mid-crustal metamorphosed rocks, and in disrupting patterns of isograds, as those associated with erosion.