The effect of tectonic environment on permeability development around faults and in the brittle crust

•The permeability tensor in the brittle crust is controlled by tectonic environment.•New deformation/permeability experiments show the effect of tectonic environment.•Effective mean stress and differential stress influence the permeability.•Permeability cannot always be viewed as a 'toggle switch' in geological processes.

Fluid flow in the brittle crust is strongly influenced by brittle deformation. The tectonic environment in which brittle deformation occurs will dictate both the orientation and magnitude of the resulting permeability tensor. The effect of the tectonic environment on permeability is considered conceptually, and triaxial deformation experiments showing microfracture permeability development in Westerly granite in the pre-failure region, prior to pervasive shear failure of the sample, are used to illustrate the concept. In the experiments, the permeability changes are greatest in a simulated extensional tectonic regime and smallest in a compressional tectonic environment. The results are most usefully considered in the context of effective mean stress (P) and differential stress (Q) space where permeability may be contoured to allow an indication of permeability development under any experimental loading path.