Phyllosilicates formation in faults rocks: Implications for dormant fault-sealing potential and fault strength in the upper crust

• Published in: Geophysical research letters Vol. 40; no. 16; pp. 4272 - 4278
• Main Authors: Cavailhes, Thibault, Soliva, Roger, Labaume, Pierre, Wibberley, Christopher, Sizun, Jean-Pierre, Gout, Claude, Charpentier, Delphine, Chauvet, Alain, Scalabrino, Bruno, Buatier, Martine
• Format: Journal Article
• Language: English
• Published: Washington, DC Blackwell Publishing Ltd 28.08.2013; American Geophysical Union; John Wiley & Sons, Inc
• Subjects: Crusts; Diagenesis; Earth Sciences; Earth, ocean, space; Environmental Sciences; Exact sciences and technology; Fault lines; Faults; Feldspars; Geochemistry; Geophysics; Global Changes; Hydraulics; micas; Mineralogy; permeability; Rocks; sandstone; Sciences of the Universe; Sealing; Seismology; Shale Gouge; Strength; Tectonics; Transformations; diagenesis; localization; strength; thickness; permeability; host rocks; reservoirs; forelands; sealing; normal faults; feldspar; crust; cuttings; upper crust; hydraulics; transformations; fault zones; silicates; alteration; deformation; microstructures; framework silicates; micas; sandstone; Shale Gouge
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1002/grl.50829

Phyllosilicate content and related permeability of fault zones form primary controls on hydraulic and mechanical behavior of the brittle crust. Hence, understanding and predicting the localization of these ubiquitous minerals is a major issue for fundamental and applied geosciences. We describe normal fault zones cutting a foreland arkosic turbiditic formation suffering high‐T diagenesis and formed under conditions (~200°C) typical of deeply buried reservoirs and common within the seismogenic interval. Microstructural analyses show a large proportion of phyllosilicates (up to 34%) in the fault rock, derived from near‐complete feldspar alteration and disaggregation during deformation. This study shows that even faults with offsets (~20 cm) much lower than bed thickness can have such large feldspar‐to‐phyllosilicate transformation ratios, implying that the origin of the phyllosilicates is purely transformation related. These results imply that the potential sealing capacity and strength of faults could be predicted from the host rock feldspar content. Where sealing capacity and fault strength can be related to phyllosilicate content, these properties can then also be inferred from the predicted phyllosilicate content: this opens up new horizons concerning the hydraulic and the mechanical behavior of the upper crust. Key Points first quntification of phyllosilicates formation Shale Gouge ratio has to be modified in deeply buried reservoir Feldspar content of host rock as a predictive proxy