Slab breakoff: A model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens

Abstract

We present a model proposing that oceanic lithosphere detaches from continental lithosphere during continental collision (slab breakoff), allowing an explanation of syn- to post-collisional magmatism and metamorphism. Continental collisions are preceded by subduction of dense oceanic lithosphere, and followed by attempted subduction of buoyant continental lithosphere. This situation of opposing buoyancy forces leads to extensional deformation in the subducted slab. A narrow rifting mode of deformation will result if there is strain localization. Slab breakoff results. We have assessed the plausiblity of this process by quantitatively evaluating an upper bound for the strength of the lithosphere, and have compared it with the change in buoyancy force during continental sbduction. Whether breakoff will occur, and the depth at which it will occur, is a strong function of temperature and hence the subduction velocity. For a subduction velocity of 1 cm/yr breakoff could occur at depths of between 50 and 120 km, while at higher velocities it is still likely to occur, but at deeper depths. As a result of the rifting during breakoff, the asthenosphere upwells into the narrow rift, and following breakoff it impinges on the mechanical lithosphere of the overriding plate. The resulting conducted thermal perturbation leads to melting of the metasomatised overriding mantle lithosphere, producing basaltic magmatism that leads to granitic magmatism in the crust. Dry asthenospheric mantle will melt only if breakoff occurs at a depth shallower than ca. 50 km. Breakoff removes the force at the downdip side of the continental crust, while the enhanced heating leads to a reduction of the strength of the underlying crust. Both effects facilitate the freeing of buoyant crustal sheets which can then rise towards the surface, leading to the rapid exhumation of eclogite facies continental crust. The cessation of subduction and replacement of the cold oceanic lithosphere by asthenosphere leads to rapid uplift of the orogen. We have tested the variety of predicted expressions of slab breakoff in the geological record of the Alps, the Aegean Islands, and the Dabie Shan. A comparison of the various belts highlights (1) that the magmatism and metamorphism are found near the suture and the centre of the orogen, demonstrating the general steepening of suture during collision, (2) that a vague inverse correlation exists between maximum depth of metamorphism and volume of syn-orogenic magmatism, and (3) that the melts can be emplaced in both compressive and extensional environments. We suggest that slab breakoff is an important step in the evolution of many orogenic belts, and it allows an explanation for the combined presence in collisional orogens of magmatism with a mantle parentage and ultra-high-pressure metamorphics.