Breakup of young oceanic lithosphere in the upper part of a subduction zone: Implications for the emplacement of ophiolites

Abstract

It is investigated, by means of numerical modelling, whether the interaction between young oceanic lithosphere and a subduction zone can lead to the breakup of a young and thin oceanic plate. First, a thermal model of the upper part of a subduction zone (the region between the trench and the volcanic arc) is presented. Temperatures are modelled for situations in which the age of the subducting oceanic lithosphere gradually decreases, culminating in the arrival of a spreading ridge at the trench. Temperatures are used to infer the strength of material within the subducting plate from a pressure and temperature dependent rheology. Next, it is investigated by means of finite element modelling whether the forces acting upon young subducting oceanic lithosphere, recently created at a spreading center in the vicinity of a subduction zone, can lead to its breakup. Our modelling results show that very young oceanic lithosphere may indeed break up during the early phase of its subduction. Whether breakup occurs depends upon the spreading velocity and the length of the ridge segment that interacts with the trench. If breakup occurs, some young oceanic lithosphere situated in the upper part of the subduction zone and in the region between the ridge and the trench will be detached from the other parts of the subducting plate. The detached sheet of thin oceanic lithosphere will not be subducted to mantle depths. Instead, subduction of the trailing plate, at the opposite side of the ridge, can lead to the incorporation of this sheet into the forearc region. Our modelling provides a mechanism to incorporate thin oceanic lithosphere into an arc‐trench region and eventually, upon closure of the ocean basin, into an orogenic belt. It is shown that a large subset of ophiolites (those with a harzburgite‐dominated mantle section) has properties in good agreement with an emplacement history in which the breakup of young oceanic lithosphere in the upper part of a subduction zone is the first phase.