Catastrophic pressure drops in subduction zones: an unsuspected processrecorded in metamorphic rocks

International audienceWhen deeply buried in subduction zones, rocks undergo mineral transformations that record the increase of pressureand temperature. The fact that high-pressure metamorphic parageneses are found at the Earth’s surface provesthat rock burial is followed by exhumation. Here we use analysis of available data sets from high-pressure metamorphicrocks worldwide to show that the peak pressure is proportional to the subsequent decompression occurringduring the initial stage of retrogression. We propose, using a simple mechanical analysis, that this linear relationshipcan be explained by the transition from burial-related compression to extension at the onset of exhumation.This major switch in orientation and magnitude of principal tectonic stresses leads to a catastrophic pressure dropprior to actual rock ascent. Therefore, peak pressures are not necessarily, as commonly believed, directly dependenton the maximum burial depth, but can also reflect a change of tectonic regime. Our results, which are in agreementwith natural data, have significant implications for rock rheology, subduction zone seismicity, and the magnitudesof tectonic pressures sustained by rocks at the subduction interface. These implications for subduction dynamicswill be discussed