SLAB BREAKOFF - A MODEL OF LITHOSPHERE DETACHMENT AND ITS TEST IN THEMAGMATISM AND DEFORMATION OF COLLISIONAL OROGENS

We present a model proposing that oceanic lithosphere detaches from co ntinental lithosphere during continental collision (slab breakoff), al lowing an explanation of syn- to post-collisional magmatism and metamo rphism. Continental collisions are preceded by subduction of dense oce anic lithosphere, and followed by attempted subduction of buoyant cont inental lithosphere. This situation of opposing buoyancy forces leads to extensional deformation in the subducted slab. A narrow rifling mod e of deformation will result if there is strain localization. Slab bre akoff results. We have assessed the plausibility of this process by qu antitatively evaluating an upper bound for the strength of the lithosp here, and have compared it with the change in buoyancy force during co ntinental subduction. Whether breakoff will occur, and the depth at wh ich it will occur, is a strong function of temperature and hence the s ubduction velocity. For a subduction velocity of 1 cm/yr breakoff coul d 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 ri ft, and following breakoff it impinges on the mechanical lithosphere o f the overriding plate. The resulting conducted thermal perturbation l eads to melting of the metasomatised overriding mantle lithosphere, pr oducing basaltic magmatism that leads to granitic magmatism in the cru st. Dry asthenospheric mantle will melt only if breakoff occurs at a d epth shallower than ca. 50 km. Breakoff removes the force at the downd ip side of the continental crust, while the enhanced heating leads to a reduction of the strength of the underlying crust. Both effects faci litate the freeing of buoyant crustal sheets which can then rise towar ds the surface, leading to the rapid exhumation of eclogite facies con tinental crust. The cessation of subduction and replacement of the col d oceanic lithosphere by asthenosphere leads to rapid uplift of the or ogen. We have tested the variety of predicted expressions of slab brea koff 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 o f the orogen, demonstrating the general steepening of suture during co llision, (2) that a vague inverse correlation exists between maximum d epth of metamorphism and volume of syn-orogenic magmatism, and (3) tha t the melts can be emplaced in both compressive and extensional enviro nments. We suggest that slab breakoff is an important step in the evol ution of many orogenic belts, and it allows an explanation for the com bined presence in collisional orogens of magmatism with a mantle paren tage and ultra-high-pressure metamorphics.