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Pure and Applied Geophysics

, Volume 171, Issue 10, pp 2641–2664 | Cite as

Frictional Properties of a Low-Angle Normal Fault Under In Situ Conditions: Thermally-Activated Velocity Weakening

  • André R. Niemeijer
  • Cristiano Collettini
Article

Abstract

The Zuccale fault is a regional, low-angle, normal fault, exposed on the Isle of Elba in central Italy that accommodated a total shear displacement of 6–8 km. The fault zone structure and fault rocks formed at <8 km crustal depth. The present-day fault structure is the final product of several deformation processes superposed during the fault history. In this study, we report results from a series of rotary shear experiments performed on 1-mm thick powdered gouges made from several fault rock types obtained from the Zuccale fault. The tests were done under conditions ranging from room temperature to in situ conditions (i.e., at temperatures up to 300 °C, applied normal stresses up to 150 MPa, and fluid-saturated.) The ratio of fluid pressure to normal stress was held constant at either λ = 0.4 or λ = 0.8 to simulate an overpressurized fault. The samples were sheared at a constant sliding velocity of 10 μm/s for at least 5 mm, after which a velocity-stepping sequence from 1 to 300 μm/s was started to determine the velocity dependence of friction. This can be represented by the rate-and-state parameter (a–b), which was determined by an inversion of the data to the rate-and-state equations. Friction of the various fault rocks varies between 0.3 and 0.8, similar to values obtained in previous studies, and decreases with increasing phyllosilicate content. Friction decreases mildly with temperature, whereas normal stress and fluid pressure do not affect friction values systematically. All samples exhibited velocity strengthening, conditionally stable behavior under room temperature conditions and (ab) increased with increasing sliding velocity. In contrast, velocity weakening, accompanied by stick–slips, was observed for the strongest samples at 300 °C and sliding velocities below 10 μm/s. An increase in fluid pressure under these conditions led to a further reduction in (a–b) for all samples, so that they exhibited unstable, stick–slip behavior at low sliding velocity. The results suggest that phyllosilicate-bearing fault rocks can deform by stable, aseismic creep at low, resolved shear stress and low shear rate conditions. An increase in fluid pressure or loading of stronger portions could lead to a runaway instability. The runaway instability might be limited in size because of (1) the fault heterogeneity, (2) the observed strengthening at higher sliding velocities, and (3) a co-seismic drop in pore-fluid pressure.

Keywords

Friction sliding stability hydrothermal conditions LANF 

Notes

Acknowledgments

We wish to thank Eimert de Graaff, Gert Kastelein, Thony van de Gon-Netscher, and Peter van Krieken for technical support, and an anonymous reviewer for his/her comments that helped to improve this paper. A. Niemeijer is/was supported by the Dutch Foundation for Scientific Research (NWO) through a VENI grant (nr. 863.09.013) and a VIDI grant (nr. 854.12.011), and by the ERC starting grant SEISMIC (nr. 335915). C. Collettini is supported by the ERC Starting Grant GLASS (Grant 259256).

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© Springer Basel 2013

Authors and Affiliations

  1. 1.Faculty of Geosciences, HPT LaboratoryUtrecht UniversityUtrechtThe Netherlands
  2. 2.Dipartimento di Scienze della TerraUniversità degli Studi di Roma “La Sapienza”RomeItaly
  3. 3.Istituto Nazionale di Geofisica e VulcanologiaRomeItaly

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