Slab breakoff: A critical appraisal of a geological theory as applied in space and time

The idea that prominent geological phenomena observed in the crust and at the surface may be caused by the detachment of subducting dense lithosphere has been anticipated by tectonic models in the Sixties and Seventies, and imaged with improved geophysical techniques in the Eighties. In the mid-Nineties, the model of slab breakoff was defined formally by Davies and von Blanckenburg. Initially proposed as a thought-provoking working hypothesis, the theory rapidly received wide a priori acceptance and was applied to virtually every mountain range around the world, and even in orogens as old as Paleoproterozoic and Archean, to explain a range of different phenomena. These include magmatic flare-ups, rapid topographic uplift, increasing sediment supply, fast exhumation of metamorphic rocks, ore mineralizations, anomalous distribution of seismicity, changes in stress regimes, and inversion of subduction polarity. Even multiple breakoff events were assumed to have occurred at different times in the same orogenic belt or subduction zone. In the last 20 years, slab breakoff has been invoked in so many settings and time frames that it could have hardly taken place in each and every case in which it was called upon.

This article does not critically examine the theory, but critically examines its use. The extensive review of the vast literature presented here on the subject reveals how the model has been often applied to provide ad hoc explanations for a range of poorly understood observations based on incomplete evidence of deep-seated processes. Our aim is to illustrate a paradigmatic example of how earth scientists, in the face of evidence that challenges our capacity of understanding, often recur to hypotheses based on other hypotheses. Such an approach may induce researchers to look for confirmation in the absence of compelling constraints, or even in the face of conflicting evidence. The faith in models should not lead us to confuse speculative theories with axiomatic truths, and to build upon them theoretical edifices that are vulnerable and exposed to the risk of circular reasoning.