Fault zone fabric and fault weakness

Abstract

Geological and geophysical evidence suggests that some crustal faults are weak1,2,3,4,5,6 compared to laboratory measurements of frictional strength7. Explanations for fault weakness include the presence of weak minerals4, high fluid pressures within the fault core8,9 and dynamic processes such as normal stress reduction10, acoustic fluidization11 or extreme weakening at high slip velocity12,13,14. Dynamic weakening mechanisms can explain some observations; however, creep and aseismic slip are thought to occur on weak faults, and quasi-static weakening mechanisms are required to initiate frictional slip on mis-oriented faults, at high angles to the tectonic stress field. Moreover, the maintenance of high fluid pressures requires specialized conditions15 and weak mineral phases are not present in sufficient abundance to satisfy weak fault models16, so weak faults remain largely unexplained. Here we provide laboratory evidence for a brittle, frictional weakening mechanism based on common fault zone fabrics. We report on the frictional strength of intact fault rocks sheared in their in situ geometry. Samples with well-developed foliation are extremely weak compared to their powdered equivalents. Micro- and nano-structural studies show that frictional sliding occurs along very fine-grained foliations composed of phyllosilicates (talc and smectite). When the same rocks are powdered, frictional strength is high, consistent with cataclastic processes. Our data show that fault weakness can occur in cases where weak mineral phases constitute only a small percentage of the total fault rock and that low friction results from slip on a network of weak phyllosilicate-rich surfaces that define the rock fabric. The widespread documentation of foliated fault rocks along mature faults in different tectonic settings and from many different protoliths4,17,18,19 suggests that this mechanism could be a viable explanation for fault weakening in the brittle crust.

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Figure 1: Example of a foliated low-angle normal fault.
Figure 2: Friction experiments.
Figure 3: Frictional properties of fault rocks and powders made from them.
Figure 4: Comparison between solid-foliated and powder sliding surfaces in L3.

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Acknowledgements

We thank I. Faoro for cutting the samples and J. P. Ampuero, D. Faulkner, R. Holdsworth and S. Smith for discussions. This research was motivated in part by stimulating discussions with P. Montone, M. Barchi and M. Cocco. We gratefully acknowledge funding by NSF grants OCE-0196462 EAR-0510182 and an INGV-DPC S5 M. Barchi grant. A.N. was supported in part by the ERC St. G. Nr.205175 USEMS project.

Author Contributions C.C., A.N. and C.M. designed the study. A.N. and C.C. carried out the experiments. A.N., C.C. and C.M. conducted the data analysis. C.C. and C.V. carried out the microstructural studies. C.V. did TEM and mineralogical characterization. All the authors contributed to the writing.

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Correspondence to Cristiano Collettini.

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Collettini, C., Niemeijer, A., Viti, C. et al. Fault zone fabric and fault weakness. Nature 462, 907–910 (2009). https://doi.org/10.1038/nature08585

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