Porosity network of a ductile shear zone

Abstract

The permeability of ductile shear zones is assumed to be significantly higher than that of surrounding undeformed rocks, although lower than that of fracture systems. This assumption, which results indirectly from geochemical mass balance calculations, is checked in this paper by the tomography of the porosity network in a metre-wide deformation zone. Combined techniques, with different resolutions ranging between 1 mm and 1 μm, are used on oriented thin sections and drilled cores: nuclear magnetic resonance (NMR) imaging, X-ray tomodensitometry (medical scanner), mercury injection porosimetry and scanning electron microscopy (SEM). We show that porosity ranges between 1% at the margins and 8% in the median part, with throat diameters between 0.8 and 0.01 μm. The porosity network is heterogeneous, mixed (cracks and tubes) and anisotropic. Cracks are common in the less deformed samples, whereas tubes prevail in the more strained median part. The porosity network is anisotropic, especially in the high strained samples, where tubes are preferentially oriented in the c-planes. This shape duality is expected to influence the transfer properties of the studied rocks. Estimates of related permeability are about 5 × 10 −16 and 5 × 10 −15 m 2 in the shear zone, and about 10 −17 m 2 in the less deformed granite. We discussed the occurrence of such a contrast at depth between the shear zone and the granite.