Secular trends in the thermal evolution of metamorphic terrains

Abstract

The thermal evolution of convergent orogens is sensitive to the deformation history of the mantle part of the lithosphere. If the dense mantle lithosphere thickens as a consequence of convergent deformation and remains attached during subsequent denudation of the thickened crust then high-pressure/low- to intermediate-temperature glaucophane schist assemblages will form during both thickening and denudation; the actual form of the P-T paths for a given deformation depending primarily on the initial thermal state of the lithosphere and, particularly, the ratio of the thickness of the crust and mantle lithosphere. However, if mantle lithosphere is thinned during convergence by processes such as thermal boundary layer detachment then any early formed high-pressure/low- to intermediate-temperature glaucophane schist assemblages will be obliterated during subsequent high-temperature overprints. Since the stability of the thermal boundary layer during convergent deformations is controlled by the rate of convergence and the convective vigour of the subjacent mantle, any change in the convective vigour of the mantle through geologic time in response to, for example, changing global heat production may therefore be reflected in the thermal evolution of metamorphic terrains formed during convergent deformations. Models of the thermal evolution of deforming lithosphere appropriate to an earth in which the effect of declining global heat production and convective vigour is mediated by the process of thermal boundary layer detachment suggest that the preservation of high-pressure/low- to intermediate-temperature glaucophane schist assemblages in convergent orogens has become increasingly probable through geological time; a suggestion in accord with the observed secular trend in the character of metamorphic terrains preserved in the geological record.