SLAB BREAKOFF, PLATE INTERACTIONS, AND THE EXHUMATION OF HIGH- AND ULTRAHIGH-PRESSURE ROCKS

Recent studies have documented that certain high-pressure (HP) and ultrahigh-pressure (UHP) rocks at plate boundaries have been exhumed at plate tectonic rates. Several authors advocate a model involving the buoyant rise of a thin wedge of HP or UHP rocks within an active subduction zone. This model is questionable because the upward buoyancy of a thin wedge may be insufficient for rapid exhumation. Here, an alternative model suggests that sudden changes in the slab pull force due to breakoff of the dense part of the slab leads to exhumation of the entire buoyant part of the slab.

The rationale for the model begins with the idea that the slab pull force, due to the weight of the subducting slab, is an important driving force for plate tectonics. The average density of a subducting slab varies with the age of the slab and with the presence or absence of aseismic ridges, oceanic plateaus, extinct arcs, microcontinents, or continents within the subducting plate. The incipient subduction of buoyant lithosphere decreases the slab pull force and may lead to the stagnation of the subduction zone. During stagnation, the rate-dependent shearing stresses resisting subduction will dissipate, facilitating breakoff of the dense deep slab. The positive buoyancy of the shallow slab is the driving force for exhumation.

During exhumation, the buoyant slab is withdrawn from the subduction zone. Because the earth has a fixed surface area, the above change must be compensated by changes at other plate boundaries. These plate interactions enable the exhumation of HP and UHP rocks at plate tectonic velocities. Once regional isostatic equilibrium is restored, exhumation is expected to cease, and compensatory changes at other plate boundaries are again necessary.

It is argued that the above model may explain the exhumation of most, perhaps all, occurrences of high- and ultrahigh-pressure rocks at plate boundaries. In particular, the model may explain the exhumation of Pliocene HP rocks in Papua New Guinea and Mesozoic HP rocks in the Franciscan Complex of the California Coast Ranges. The UHP rocks associated with the Norwegian Caledonides may represent Paleozoic exhumation following breakoff of the subducting Laurentian plate.