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A new interpretation for the nature and significance of mirror-like surfaces in experimental carbonate-hosted seismic faults

Pozzi, G.; De Paola, N.; Nielsen, S.B.; Holdsworth, R.E.; Bowen, L.

A new interpretation for the nature and significance of mirror-like surfaces in experimental carbonate-hosted seismic faults Thumbnail


Authors

G. Pozzi

Leon Bowen leon.bowen@durham.ac.uk
Senior Manager (Electron Microscopy)



Abstract

Highly reflective, continuous smooth surfaces, known as "mirror-like surfaces" (MSs), have been observed in experimental carbonate-hosted faults, which were sheared at both seismic and aseismic velocities. MSs produced during high-velocity friction experiments (>0.1 m s–1) are typically interpreted to be frictional principal slip surfaces, where weakening mechanisms are activated by shear heating. We re-examined this model by performing friction experiments in a rotary shear apparatus on calcite gouge, at seismic velocities up to v = 1.4 m s–1 and normal stress σn = 25 MPa, to analyze the evolution of microstructures as displacement increases. After the onset of dynamic weakening, when the friction coefficients are low (μ << 0.6), sheared gouges consistently develop a well-defined, porosity-free principal slip zone (PSZ) of constant finite thickness (a few tens of micrometers) composed of nanometric material, which displays polygonal grain shapes. MSs occur at both boundaries of the PSZ, where they mark a sharp contrast in grain size with the sintered, much coarser material on either side of the PSZ. Our observations suggest that, with the onset of dynamic weakening, MSs partition the deformation by separating strong, sintered wall rocks from a central weak, actively deforming viscous PSZ. Therefore, the MSs do not correspond to frictional slip surfaces in the classical sense, but constitute sharp rheological boundaries, while, in the PSZ, shear is enhanced by thermal and grain-size–dependent mechanisms.

Citation

Pozzi, G., De Paola, N., Nielsen, S., Holdsworth, R., & Bowen, L. (2018). A new interpretation for the nature and significance of mirror-like surfaces in experimental carbonate-hosted seismic faults. Geology, 46(7), 583-586. https://doi.org/10.1130/g40197.1

Journal Article Type Article
Acceptance Date Apr 25, 2018
Online Publication Date May 30, 2018
Publication Date May 30, 2018
Deposit Date Apr 26, 2018
Publicly Available Date Mar 29, 2024
Journal Geology
Print ISSN 0091-7613
Electronic ISSN 1943-2682
Publisher Geological Society of America
Peer Reviewed Peer Reviewed
Volume 46
Issue 7
Pages 583-586
DOI https://doi.org/10.1130/g40197.1

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