The feedback between fluid migration and rock deformation inmid-crustal shear zones is acknowledged as being critical forearthquake nucleation, the initiation of subduction zones andthe formation of mineral deposits1–3. The importance of thispoorly understood feedback is further highlighted by evidencefor shear-zone-controlled advective flow of fluids in the ductilelower crust4 and the recognition that deformation-induced grainscaleporosity is a key to large-scale geodynamics5,6. Fluid migrationin the middle crust cannot be explained in terms of classicalconcepts. The environment is considered too hot for a dynamicfracture-sustained permeability as in the upper crust7, and fluidpathways are generally too deformed to be controlled by equilibriumwetting angles that apply to hotter, deeper environments8–10.Here we present evidence that mechanical and chemical potentialscontrol a syndeformational porosity generation in mid-crustalshear zones. High-resolution synchrotron X-ray tomographyand scanning electron microscopy observations allow us to formulatea model for fluid migration in shear zones where a permeableporosity is dynamically created by viscous grain-boundarysliding, creep cavitation, dissolution and precipitation. We proposethat syndeformational fluid migration in our ‘granular fluidpump’ model is a self-sustained process controlled by the explicitrole of the rate of entropy production of the underlying irreversiblemechanical and chemical microprocesses. The model explainsfluid transfer through the middle crust, where strain localizationin the creep regime is required for plate tectonics, the formation ofgiant ore deposits, mantle degassing and earthquake nucleation.Our findings provide a key component for the understanding ofcreep instabilities in the middle crust.
Fusseis, F., Regenauer-Lieb, K., Liu, J., Hough, R. M., & De Carlo, F. (2009). Creep cavitation can establish a dynamic granular fluid pump in ductile shear zones. Nature, 459(7249), 974-977. https://doi.org/10.1038/nature08051