| Regional metamorphism occurs in an ambiguous rheological regime between the brittle upper
crust and ductile sub-lithospheric mantle. This ambiguous position has allowed two schools
of thought to develop concerning the nature of metamorphic fluid flow. The classical school
holds that metamorphic rocks are inviscid and that any fluid generated by devolatilization is
squeezed out of rocks as rapidly as it is produced. According to this school permeability is a
dynamic property and fluid flow is upward. In contrast the modern school, selectively uses
concepts from upper crustal hydrology that presume implicitly, if not explicitly, that
rocks are rigid or, at most, brittle. For the modern school, the details of crustal
permeability determine fluid flow and as these details are poorly known almost anything is
possible.
Reality, to the extent that is reflected by field studies, offers some support to
both schools. In particular, evidence of significant lateral and chanellized fluid flow
are consistent with flow in rigid media, while evidence for short (104 - 105 y)
grain-scale fluid rock interaction during much longer metamorphic events, suggests
that reaction-generated grain-scale permeability is sealed rapidly by compaction; a
phenomenon that is also essential to prevent extensive retrograde metamorphism. These
observations provide a compelling argument for recognizing in conceptual models for
metamorphic fluid flow that rocks are neither inviscid nor rigid, but have finite
strength. The surprising result of this strength is that the steady state solutions for
fluid flow in porous compacting media require that fluid expulsion is channeled
into waves of fluid-filled porosity. The waves develop on a characteristic length
scale known as the viscous compaction length, δ, that is also the length scale for
lateral fluid flow. In this context, porosity refers to any hydraulically connected
void space present on spatial scales |