Blueschist has a mineral assemblage such that it must have formed at high pressures and low temperatures. It must therefore form in subducting slabs (and I believe this correlates with the locations in which it's found). However, that leaves the issue of how to *get* it to the surface rapidly enough that it doesn't heat up sufficient to reequilibrate and lose the characteristic mineral assemblage.

What are the currently favoured explanations/processes for such rapid exhumation from depth?