Coseismic ultramylonites: An investigation of nanoscale viscous flow and fault weakening during seismic slip

Pozzi, Giacomo; De Paola, Nicola; Holdsworth, Robert E; Bowen, Leon; Nielsen, Stefan B; Dempsey, Edward D

Abstract

Faults weaken during the propagation of earthquakes due to the onset of thermally-activated mechanisms, which vary depending on the rock type. Recent experimental work suggests that carbonate-hosted faults are lubricated by viscous flow in nano-granular aggregates having ultramylonitic textures. However, their frail nature has often hindered unbiased characterization of the textures and deformation mechanisms operating at such extreme conditions (strain rates as high as 10^4), which remain so far poorly investigated and understood. We explore the formation, evolution and deformation mechanisms of coseismic ultramylonites in carbonate-hosted faults generated during high velocity (1.4 m/s), displacement-controlled shear experiments in a rotary apparatus. Microstructures were analysed using integrated SEM and TEM imaging while detailed crystallographic fabrics were investigated using the electron back-scattered diffraction (EBSD) technique. Mechanical data show that the strength of the experimental fault decays dynamically with slip, according to a characteristic four stage evolution; each stage is associated with characteristic textures. Microstructural observations show that brittle processes dominate when the fault is strong (friction coefficients > 0.6). Cataclasis aided by twinning and crystal plasticity operate forming an extremely comminuted shear band (mean grain size ~ 200 nm). As the fault starts weakening, shear localizes within a well-defined principal slip zone. Here, thermally-activated grain size sensitive (GSS) and insensitive (GSI) creep mechanisms compete with brittle processes in controlling fault strength. GSI mechanisms produce strong monoclinic crystallographic preferred orientations in the slip zone, while textures and crystallographic orientations in adjacent locations do not evolve from the previous deformation stage. By the end of the transient weakening stage, the slip zone has reached a steady state thickness (30 µm) and shows a nanogranular ultramylonitic texture. The intensity of the crystallographic preferred orientation in the coseismic ultramylonite is reduced compared to the previous stage, due to grainsize sensitive creep mechanisms becoming gradually more dominant. As the experimental fault

Citation

Pozzi, G., De Paola, N., Holdsworth, R. E., Bowen, L., Nielsen, S. B., & Dempsey, E. D. (2019). Coseismic ultramylonites: An investigation of nanoscale viscous flow and fault weakening during seismic slip. Earth and planetary science letters, 516, 164-175. https://doi.org/10.1016/j.epsl.2019.03.042

• Journal Article Type: Article
• Acceptance Date: Mar 28, 2019
• Online Publication Date: Apr 19, 2019
• Publication Date: Jun 15, 2019
• Deposit Date: Apr 3, 2019
• Publicly Available Date: May 31, 2019
• Print ISSN: 0012-821X
• Publisher: Elsevier
• Peer Reviewed: Peer Reviewed
• Volume: 516
• Pages: 164-175
• DOI: https://doi.org/10.1016/j.epsl.2019.03.042
• Keywords: Earthquakes; Weakening mechanisms; Ultramylonites; Rock mechanics; Carbonates; EBSD
• Public URL: https://hull-repository.worktribe.com/output/1155122
• Publisher URL: https://www.sciencedirect.com/science/article/pii/S0012821X19301979