[1] Beneath the brittle-ductile transition of the Earth's crust, the
dilational deformation necessary to create fluid pathways requires fluid
pressure that is near to rock confining pressure. Although the
deformation may be brittle, it is rate limited by the ductile compaction
process necessary to maintain elevated fluid pressure; thus the
direction of fluid expulsion is dictated by the mean stress gradient.
The paradox posed by the conditions required to maintain high fluid
pressure simultaneously with lower crustal rock strength can be
explained by a model whereby fluids are localized within
self-propagating hydraulic domains. Such domains would behave as weak
inclusions imbedded within adjacent fluid-poor rocks. Because the mean
stress gradient in a weak inclusion depends on its orientation with
respect to far-field stress, the direction of fluid flow in such domains
is sensitive to tectonic forcing. In compressional tectonic settings,
this model implies that fluid flow may be directed downward to a depth
of tectonically induced neutral buoyancy. In combination with dynamic
propagation of the brittle-ductile transition, this phenomenon provides
a mechanism by which upper crustal fluids may be swept into the lower
crust. The depth of neutral buoyancy would also act as a barrier to
upward fluid flow within vertically oriented structural features that
are normally the most favorable means of accommodating fluid expulsion.
Elementary analysis based on the seismogenic zone depth and experimental
theological constraints indicates that tectonically induced buoyancy
would cause fluids to accumulate in an approximately kilometer thick
horizon 2-4 km below the brittle-ductile transition, an explanation for
anomalous midcrustal seismic reflectivity.