On the basis of thermal and mechanical modeling, it is concluded that the subduction of continental lithosphere can lead to its breakup and the formation of a new plate contact within the middle or lower crust. As a result, (part of) the subducting continental crust is transferred to the upper plate. Breakup is caused by the resistive forces acting upon subducting continental crust, due to the buoyancy of crustal material and to friction at the plate contact, as well as the decrease in strength of the subducting crust once it has been subducted to a depth of a few tens of kilometers. The crustal thickness and the thermal and compositional structure of the continental crust just before the onset of subduction have a large influence on the depth to which continental crust can be subducted (prior to its breakup). The depth to which continental crust can be subducted coherently decreases as the surface heat flow or the crustal thickness of the subducting continental plate increases. In many cases, breakup is found to occur at a time when the upper surface of the continental plate has been subducted to a depth of 25–50 km. The subduction of a cold continental shield or of continental lithosphere with a relatively small crustal thickness, on the other hand, may lead to the subduction of both upper and lower crust to mantle depths.

Van den Beukel, J. and Wortel, R., 1988. Thermo-mechanical modelling of arc-trench regions. , 154:177-193. Tectonophysics

The temperature structure of the upper part of a subduction zone, i.e. the region between the trench and the volcanic line, has been investigated by means of numerical modelling. Published heat flow measurements are used to constrain the thermal structure of this region and the shear stresses at the plate contact. A pressure and temperature dependent rheology is used to model shear stresses. It is shown that temperatures at the plate contact and within the upper plate are primarily determined by frictional heating. Except for the subduction of very young oceanic lithosphere (less than 30 Myr old), these temperatures, as well as shear stresses, are essentially independent of the age of the subducting oceanic lithosphere and the convergence velocity between the two plates. Pressure‐temperature conditions at the upper surface of the slab for the proposed thermal model are in good agreement with conditions inferred from high‐pressure metamorphic material of regional extent in the Franciscan Complex of California.

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.

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