Formation of Metamorphic Soles Underlying Ophiolites During Subduction Initiation: A Systematic Numerical Study

Abstract

AbstractThe understanding of subduction initiation (SI) remains ambiguous due to limited geological records. The metamorphic sole, generally considered to be generated by oceanic crustal metamorphism during SI, is characterized by high temperature condition (∼800°C) at shallow depths (<40 km). However, the exact tectonic setting of the metamorphic sole with such a high geothermal gradient is still controversial. The petrological and geochemical signatures of ophiolites and metamorphic soles in nature indicate three different types: (a) supra‐subduction zone (SSZ)‐type ophiolite with mid‐ocean ridge basalt (MORB)‐type metamorphic sole; (b) SSZ‐type ophiolite with SSZ‐type metamorphic sole; and (c) MORB‐type ophiolite with MORB‐type metamorphic sole. To clarify the conditions of metamorphic sole generation in different tectonic settings, a series of numerical models are conducted. The model results indicate that the SI at a (back‐arc) spreading center or spontaneous SI at a transform fault provides the favorable high‐temperature condition for formation of the metamorphic sole underlying the ophiolite. The former regime generates SSZ‐type ophiolite with SSZ‐type sole, whereas the latter generates SSZ‐type ophiolite with MORB‐type sole. The P‐T conditions of natural metamorphic soles may not represent the characteristic subduction channel condition for the majority of ophiolites, but stand for the end‐member high‐temperature regime that facilitates weakening, detachment and further exhumation of metamorphic soles. It thus illustrates the less widely distributed metamorphic soles than ophiolites in nature. The model results are further compared with three present‐day back‐arc basins on the Earth to evaluate the likelihood of future metamorphic sole generation and preservation in these basins.