Fault zone permeability structure evolution in basalts

• Published in: Geology
• Main Authors: Walker, Richard James, Holdsworth, Robert E., Armitage, Peter J., Faulkner, Daniel R.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Geological Society of America 2013
• Subjects: QE Geology; Faroe Islands
• Online Access: https://orca.cardiff.ac.uk/id/eprint/31339/; https://doi.org/10.1130/G33508.1; https://orca.cardiff.ac.uk/id/eprint/31339/1/Walker%20etal%20ORCA.pdf

A combination of field, microstructural and experimental static permeability characterization is used to determine fault permeability structure evolution in upper crustal basalt-hosted fault zones in the Faroe Islands. The faults comprise lower strain fracture networks, to higher strain breccias that form tabular volumes around a principal slip zone hosting gouge or cataclasite. Samples representative of these fault zone components are used for static experimental permeability measurement. Results indicate that within the appropriate effective pressure (depth) range (10–90 MPa: ~0.3 to ~3.0 km), basalt-hosted faults evolve from low strain (< 1m displacement), relatively low-permeability (<10–17 m2) structures, to high strain (≥ 1m displacement), relatively high-permeability (>10–17 m2) structures. Sample analyses reveal that static permeability is controlled by the development of: a) fault-parallel clay alteration (decreasing permeability); and b) porous zeolite vein connectivity due to hydrofracture (increasing permeability). Fault-parallel permeability is increased relative to the host rock, while fault-normal permeability is low throughout fault rock evolution. This configuration will tend to promote across-fault compartmentalization and along-fault fluid flow, facilitating migration between relatively high-permeability horizons (e.g. vesicular flow unit tops and siliciclastic horizons), bypassing the bulk of the stratigraphy.