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Phase transformation and nanometric flow cause extreme weakening during fault slip


Earthquake instability requires fault weakening during slip. The mechanism of this weakening is central to understanding earthquake sliding and, in many cases, has been attributed to fluids. It is also unclear why major faults such as the San Andreas Fault do not exhibit significant thermal anomalies due to shear heating during sliding and whether or not fault rocks that have been melted—pseudotachylytes—are rare. High-speed friction experiments on a wide variety of rock types have shown that they all exhibit extreme weakening and that the sliding surface is nanometric and contains phases not present at the start. Here we use electron microscopy to examine these two key observations in high-speed friction experiments and compare them with high-pressure faulting experiments. We show that phase transformations occur in both cases and that they are associated with profound weakening. However, fluid is not necessary for such weakening; the nanometric fault filling is inherently weak at seismic sliding rates and it flows by grain boundary sliding. These observations suggest that pseudotachylytes are rare in nature because shear-heating-induced endothermic reactions in fault zones prevent temperature rise to melting. Microstructures preserved in the Punchbowl Fault, an ancestral branch of the San Andreas Fault, suggest similar processes during natural faulting and offer an explanation for the lack of a thermal aureole around major faults.

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Figure 1: Kasota dolomite sliding experiment.
Figure 2: Composition of a sliding surface and TEM microstructures of a cross-section from the high-speed dolomite experiment.
Figure 3: Fault in Mg2GeO4 olivine (1.3 GPa, 1,200 K).
Figure 4: Punchbowl Fault.


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Discussions with D. Lockner and N. Beeler over several years provided important suggestions that significantly contributed to the evolving ideas now presented here. J. Zhang contributed helpful comments on experimental techniques. We also thank FEI Corporation for cutting FIB foils and for assistance with the highest-resolution scanning electron microscopy. In particular, Fig. 3d was obtained on the Magellan microscope at the FEI research facility in Portland, Oregon. F.S. acknowledges the China University of Geosciences and China Scholarship Council for a fellowship to pursue his Ph.D research at UC Riverside. Formal reviews by D. Moore and T. Tullis greatly improved the manuscript. This paper is based on work supported by the National Science Foundation under Grant #1247951 to H.W.G. II and Z.R. and #1015264 to H.W.G. II. The study was also supported by the NSF Geosciences, Equipment and Facilities, Grant No. 0732715, and partial support of NSF, Geosciences, Geophysics, Grant No. 1045414, both to Z.R.

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H.W.G. II conceived the project, contributed the primary ideas and wrote the manuscript. F.S. conducted the specific high-pressure faulting experiment and succeeded in preserving fault contents intact (Fig. 3). K.B contributed critical SEM and TEM imaging and analysis. G.X participated in electron microscopy (Supplementary Fig. 3c) and in the hunt for critical images of the Punchbowl Fault. Z.R. conducted the high-speed experiments (Figs 1 and 2 and Supplementary Fig. 3a, b) and contributed to the development of the ideas. All authors contributed to manuscript preparation.

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Correspondence to H. W. Green II.

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Green II, H., Shi, F., Bozhilov, K. et al. Phase transformation and nanometric flow cause extreme weakening during fault slip. Nature Geosci 8, 484–489 (2015).

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