Catastrophic pressure drops in subduction zones: an unsuspected process recorded in metamorphic rocks
Résumé
When deeply buried in subduction zones, rocks undergo mineral transformations that record the increase of pressure
and temperature. The fact that high-pressure metamorphic parageneses are found at the Earth’s surface proves
that rock burial is followed by exhumation. Here we use analysis of available data sets from high-pressure metamorphic
rocks worldwide to show that the peak pressure is proportional to the subsequent decompression occurring
during the initial stage of retrogression. We propose, using a simple mechanical analysis, that this linear relationship
can 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 drop
prior to actual rock ascent. Therefore, peak pressures are not necessarily, as commonly believed, directly dependent
on the maximum burial depth, but can also reflect a change of tectonic regime. Our results, which are in agreement
with natural data, have significant implications for rock rheology, subduction zone seismicity, and the magnitudes
of tectonic pressures sustained by rocks at the subduction interface. These implications for subduction dynamics
will be discussed.