A paper published in the latest edition of Nature describes a process called "creep cavitation" that the authors say is the first direct observation of fluids moving through the mid-crustal shear zone.
"We are seeing the direct evidence for one of the processes that got ore forming fluids moving up from the mantle to the shallow crust to form the ore deposits we mine today, it is also one of the mechanisms that can lead to earthquakes in the middle crust," co-author Dr Rob Hough said.[right, a rock sample of less than 1 mm that was deformed in the middle crust. Porosity is highlighted in red. Credit CSIRO]
Ref: F. Fusseis, K. Regenauer-Lieb1,J. Liu, R. M. Hough & F. De Carlo,
Creep cavitation can establish a dynamic granular fluid pump in ductile shear zones
Nature 459, 974-977 (18 June 2009) | doi:10.1038/nature08051
[taken in part from the news release on Eurekalert]