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

Abstract

The 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.

Hot versus cold subduction initiation

“…The soles discovered in the present Earth are mostly generated under a low pressure of 0.5–1.5 GPa and high temperature of 700–900°C [ 5 ]. Such a critical condition generally requires heating from the asthenosphere, by means of asthenospheric upwelling or greatly thinned overriding lithosphere [ 6 ]. The related numerical models indicate that induced SI beneath a rather thin overriding plate contributes to the generation of pressure-temperature conditions of most metamorphic soles [ 7 ].…”

“…The high temperature condition at shallow depths generally requires heating from the asthenosphere. The SI driven by dominant vertical forces is more favorable for obtaining such conditions, although the horizontally forcing SI is also applicable when the overriding lithosphere is rather thin [ 6 , 7 ].…”

“…As a typical example, the tectonic reversal of a spreading ridge may lead to the occurrence of SI. Due to the general buoyancy of the ridge, its SI could be driven by the horizontal compression; but I think such type of SI should be categorized into the hot SI regime, because the incipient subduction channel is hot and can generate the metamorphic sole under high temperature conditions [ 6 ]. On the other hand, the new cold SI regime favors horizontal driving, but does not exclude the vertical driving cases.…”