The effect of oceanic crustal structure on phase changes and subduction

Abstract

Recent numerical models for the thermal regime of subduction zones are in disagreement with observations of depth to formation of eclogite, which require higher temperatures than those found in the models. This discrepancy may be reconciled if more accurate geometric depictions of the subducted slab are incorporated in the modelling studies. The transition from 15° dip along the thrust plane to 30° along the Benioff zone starts at depths of about 40 km. Dehydration reactions and the gabbro-eclogite phase change are presumed to increase the average density of the lithosphere and to cause the steepening of the subducted lithosphere. The Benioff-zone seismicity, which starts at about 40 km depth, may itself be a result of stresses caused by the phase changes. Since the gabbro-eclogite transformation occurs in gabbroic sections of the crust, thickened crustal sections beneath oceanic plateaus will affect subduction. First, the width of the thrust zone is reduced due to the increased dip angle because of early transformation of lower and warmer crustal sections; second, incomplete conversion of the entire crustal thickness results in an average density intermediate between unsubducted oceanic lithosphere and fully transformed “normal” lithosphere. The first effect leads to decreased thrust-mechanism seismicity because the coupling between the plates is reduced. The second effect leads to flat-lying subcontinental subduction or arcward advance of the trench in suboceanic subduction.