Shear localization in a mature mylonitic rock analog during fast slip

• Published in: Geochemistry, geophysics, geosystems : G3 Vol. 18; no. 2; pp. 513 - 530
• Main Authors: Takahashi, M., van den Ende, M. P. A., Niemeijer, A. R., Spiers, C. J.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 01.02.2017
• Subjects: Analogs; Brines; brittle‐ductile transition; Critical velocity; Deformation; direct effect; Earthquakes; Energy; Evolution; Faults; Geophysics; Halite; Localization; Microstructure; Muscovite; mylonitic microstructure; Nucleation; Plate tectonics; Propagation; Rock; Rocks; Seismic activity; Seismic velocities; Seismology; Shear; Shear strength; Shear zone; Tests; Velocity; Viscous flow
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1002/2016GC006687

Highly localized slip zones developed within ductile shear zones, such as pseudotachylyte bands occurring within mylonitic fabric rocks, are frequently interpreted as evidence for earthquake nucleation and/or propagation within the ductile regime. To understand brittle/frictional shear localization processes in ductile shear zones and to relate these to earthquake nucleation and propagation, we performed tests with large changes in velocity on a brine‐saturated, 80:20 (wt %) mixture of halite and muscovite gouge after forming a mature mylonitic structure through frictional‐viscous flow. The direct effect a on shear strength that occurs in response to an instantaneous upward velocity‐step is an important parameter in determining the nature of seismic rupture nucleation and propagation. We obtained reproducible results regarding low‐velocity mechanical behavior compared with previous work, but also obtained new insights into effects of sudden increases in slip velocity on localization and strength evolution, at velocities above a critical velocity Vc (∼20 μm/s). We found that once a ductile, mylonitic structure has developed in a shear zone, subsequent cataclastic deformation is consistently localized in a narrow zone. This switch to localized deformation is controlled by the imposed velocity and becomes most apparent at velocities above Vc. In addition, the direct effect drops rapidly when the velocity exceeds Vc. This implies that slip can accelerate toward seismic velocities almost instantly and without much loss of fracture energy, once Vc is exceeded. Obtaining a measure for Vc in natural faults is therefore of key importance for understanding earthquake nucleation and propagation in the brittle‐ductile transitional regime. Key Points We investigated the frictional response of analog gouge with a mature mylonitic microstructure The direct response to a velocity step decreases strongly when the poststep velocity exceeds 20 μm/s Once a mylonitic structure has developed, deformation at high velocity tends to be localized