Thermo-mechanical modelling of arc-trench regions

Abstract

The thermal structure of the shallow part of a subduction zone, i.e. the region between the trench and the volcanic line, is calculated with a finite difference method. Published heat flow measurements, in combination with rheological arguments and the distribution of interplate thrust earthquakes, are used to constrain the thermal structure of this region and the magnitude of shear stresses acting on the plate contact. A pressure and temperature dependent rheology is used to model shear stresses. From our thermo-mechanical modelling it follows that temperatures at the plate contact and within the upper plate are determined by the subduction of cold material and by frictional heating. For models that satisfy the constraints the average shear stress at the plate contact (between the trench and the volcanic line) varies from about 10 to about 40 MPa and shear stresses during brittle deformation range from 2.5 to 7.5% of the lithostatic pressure. For a wide range of convergence velocities (4–12 cm/yr) and ages of the subducting oceanic lithosphere (30–150 Ma), shear stresses and temperatures at and above the plate contact are essentially independent of these parameters. Temperatures at the upper surface of the slab for the preferred thermal models are in good agreement with pressure-temperature conditions during high-pressure metamorphism inferred from mineral assemblages in the Franciscan Complex of California. Frictional heating has a large influence on the conditions for high-pressure metamorphism.