THE GEOLOGIC RECORD OF SLAB BREAK-OFF IN CONTINENTAL COLLISIONS

Slab break-off occurs when a slab detaches from a subducting plate and sinks into the mantle. Break-off is a transient process that exerts a first-order control on the geodynamic evolution of a collisional orogen by fundamentally altering the force balance along the convergent margin, thereby triggering profound shifts in exhumation, deformation, metamorphism, and magmatism. Break-off is a key aspect of the transition from oceanic subduction to continental collision and presumably has operated since the onset of plate tectonics on Earth. However, because break-off is a transient phenomenon, it has proven difficult to study in natural settings. Here we review geologic records in regions of known or inferred break-off, together with numerical models of the break-off process, in an attempt to identify uniquely diagnostic indicators of slab break-off during continental collision.

A number of geologically observable effects have been attributed to slab break-off, including reduction in convergence rate, accelerated rock and surface uplift, changes in foreland-basin sedimentation, metamorphism, and post-collisional magmatism. Numerical modeling studies have largely focused on the conditions under which break-off occurs and how slabs detach during break-off; investigations of the geologic and geodynamic effects of break-off within collisional orogens have received less attention. While current models make specific predictions regarding geological observables, all geological processes attributed to slab break-off are non-unique and can be produced by mechanisms other than break-off. Because, there does not appear to be a single, uniquely diagnostic geologic indicator of slab break-off, the key to establishing past break-off events may be to identify co-existing suites of indices that collectively attest to slab break-off. Work focused on comparing crustal shortening histories with the spatial patterns and temporal evolution of rock and surface uplift during break-off appears to be a fruitful direction for future research focused on identifying past episodes of slab break-off.