This Experiment Cried Out for a Different Interpretation
Abstract
The results of Niemeijer et al. (2010) are said to highlight the importance of hydrothermal processes in tectonic faulting. Without replicating any experimental condition, and without any hydrothermal process, we obtained identical results in an experiment shearing granulated rock salt in Kyoto University's Ring-Shear Apparatus DPRI-6 (Sassa et al. 2004). We consolidated a 7-kg sample of room-dry granulated rock salt (3.7 mm nominal grain size) at a normal load of 400 kPa for 10 minutes, before shearing it dry at 0.05 m/s for a total distance of 10 m in three about-equal increments (all at 400 kPa). The nominal frictional contact area was 0.188 m2. With onset of shearing, the sample consolidated rapidly, indicating cataclasis. Consolidation slowed exponentially with distance of shear, but never ceased. As the sample sheared for almost 4 m, the apparent friction coefficient dropped from 0.88 to 0.50 and DPRI-6 began to emit a shrill squeal. Shear was stopped, then restarted at the same normal load and shear rate; briefly there was further consolidation (cataclasis) before the same shrill squeal emerged. The apparent friction coefficient dropped to a near-constant but slightly lower value (0.415) than before. At 6.5 m total shear, the apparatus was again stopped. On restarting, there was a briefer period of consolidation and a lower peak strength, before the high-pitched squeal emerged, and a near-constant, but slightly lower friction coefficient (0.387) was obtained. The apparatus was stopped and opened at 10 m displacement. A cataclasite about 15 mm thick was consolidated enough to examine gently in hand specimen. Shear had localised on a small number of smooth, grooved shear surfaces (Y shears), but the sample parted only on what was surmised to be the last active one. There were more Y-shear surfaces than there had been shear episodes. Under SEM magnification, the last Y-shear surface was a layer of tightly packed, uniform-sized (~ 2.4 micron) crystal fragments and fragments of crystal-fragment agglomerates. In section perpendicular to Y-shear surfaces and parallel to shear direction, the cataclastic zone appeared to be a repeating series of R2 Riedel shears between repeating discontinuous Y-shears. Given the deformation required to open R2-shears, it was apparent that Y-shears would be ephemeral under conditions for continuing R2-shear generation. We did not measure the squeal pitch, but felt aural discomfort before any sound was heard. Our experiment cried out to us that weakening can come from microseismic (acoustic) lubrication of the Y-shear interface, from the vibration that resonates to make DPRI-6 squeal. Niemeijer, A; Marone, C; Elsworth, D. 2010. Frictional strength and strain weakening in simulated fault gouge: competition between geometrical weakening and chemical strengthening. J. Geophys. Res. 115, B10207, doi:10.1029/2009JB000838, 2010 Sassa, K.; Fukuoka, H.; Wang, G; Ishikawa, N. 2004. Undrained dynamic-loading ring-shear apparatus and its application to landslide dynamics. Landslides 1: 7-19.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.T12C..08M
- Keywords:
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- 8118 TECTONOPHYSICS / Dynamics and mechanics of faulting;
- 8163 TECTONOPHYSICS / Rheology and friction of fault zones