Thermal model for approach of a spreading ridge to subduction zones and its implications for high-P/high-T metamorphism: Importance of subduction versus ridge approach ratio

Abstract

The approach of oceanic ridges to subduction zones finally resulting in ridge subduction is observed on the present Earth's surface. The thermal effects of the ridge approach event are studied using a two-dimensional numerical model around subduction zones with particular emphasis on the implications for the associated high-P/high-T metamorphism. The ridge approach occurs when the subduction rate (v) is larger than the half-spreading rate at the ridge. The difference between the two rates represents the ridge approach rate to the trench, u, which is newly incorporated in the present thermal model. The calculated thermal structure along the slab surface when a ridge reaches the trench generally depends only on u/v when shear heating or mantle flow is not taken into account. If u/v is small, or the ridge approach is slow compared to the subduction rate, the thermal effect of the ridge approach is large enough to explain the "hot" general P-T conditions of high-P/high-T metamorphism relative to the steady state conditions predicted by numerical simulations. In addition, when the ridge is close to the trench, the subduction stage P-T paths experienced by high-P/high-T metamorphic rocks show a strongly curved trajectory with the dP/dT progressively increasing with pressure. This suggests that the slope of petrologically determined subduction P-T paths may indicate if ridge approach contributed to the formation of the metamorphic rocks. Copyright 2005 by the American Geophysical Union.