Abstract:
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 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 rheological 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.
