To investigate differences in the frictional behavior between initially bare rock surfaces of serpentinite and powdered serpentinite (“gouge”) at subseismic to seismic slip rates, we conducted single-velocity step and multiple-velocity step friction experiments on an antigorite-rich and lizardite-rich serpentinite at slip rates (V) from 0.003 m/s to 6.5 m/s, sliding displacements up to 1.6 m, and normal stresses (σn) up to 22 MPa for gouge and 97 MPa for bare surfaces. Nominal steady state friction values (μnss) in gouge at V = 1 m/s are larger than in bare surfaces for all σn tested and demonstrate a strong σn dependence; μnss decreased from 0.51 at 4.0 MPa to 0.39 at 22.4 MPa. Conversely, μnss values for bare surfaces remained ∼0.1 with increasing σn and V. Additionally, the velocity at the onset of frictional weakening and the amount of slip prior to weakening were orders of magnitude larger in gouge than in bare surfaces. Extrapolation of the normal stress dependence for μnss suggests that the behavior of antigorite gouge approaches that of bare surfaces at σn ≥ 60 MPa. X-ray diffraction revealed dehydration reaction products in samples that frictionally weakened. Microstructural analysis revealed highly localized slip zones with melt-like textures in some cases gouge experiments and in all bare surfaces experiments for V ≥ 1 m/s. One-dimensional thermal modeling indicates that flash heating causes frictional weakening in both bare surfaces and gouge. Friction values for gouge decrease at higher velocities and after longer displacements than bare surfaces because strain is more distributed.Key Points class="unordered" style="list-style-type:disc">Gouge friction approaches that of bare surfaces at high normal stressDehydration reactions and bulk melting in serpentinite in < 1 m of slipFlash heating causes dynamic frictional weakening in gouge and bare surfaces class="kwd-title">Keywords: high-velocity friction, serpentinite, flash heating, dynamic weakening, pseudotachylyte, rapid metamorphism class="head no_bottom_margin" id="__sec2title">1. IntroductionOur understanding of the frictional behavior of faults at seismic slip velocities (>0.1 m/s) has significantly improved over the last 15 years with experiments performed on initially bare rock surfaces [e.g., Di Toro et al., ; Goldsby and Tullis, ; Han et al., ; Hirose and Shimamoto, ; Tsutsumi and Shimamoto, ] and gouges [e.g., Brantut et al., ; Han et al., ; Kitajima et al., ; Mizoguchi et al., ; Reches and Lockner, ]. In general, these studies of high-velocity friction (HVF) demonstrate that rock friction coefficients decrease dramatically from ∼0.7 to as low as 0.1 as slip velocities approach seismic rates and (in most cases) increase rapidly as velocities decelerate; this general behavior is nominally independent of rock composition [Di Toro et al., ; Goldsby and Tullis, ]. Such dynamic fault-weakening behavior revealed in laboratory experiments is consistent with several earthquake-related observations retrieved from the following: (1) seismology, e.g., the large stress drops constrained from analysis of seismic radiation patterns of some earthquakes [Imanishi and Ellsworth, ; Malagnini et al., ; Viegas et al., ] or the (debated) breakdown of the scaling between radiated energy and seismic moment [Abercrombie, ; Kanamori and Heaton, href="#b37" rid="b37" class=" bibr popnode">2000], (2) geophysics, e.g., the lack of a pronounced heat flow anomaly along major fault zones [Lachenbruch and Sass, href="#b40" rid="b40" class=" bibr popnode">1992; Fulton et al., href="#b22" rid="b22" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_461764699">2013] or the large seismic slip accommodated in fault patches in the Sumatra 2004 of moment magnitude (Mw) 9.3 (15 m of max slip [Stein and Okal, href="#b74" rid="b74" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_461764712">2005]) and the Tohoku 2011 Mw 9.0 (50 m of max slip [Fujiwara et al., href="#b21" rid="b21" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_461764714">2011]) events, and (3) geology, e.g., estimates of coseismic frictional strength obtained from ancient exhumed faults [e.g., Di Toro et al., href="#b13" rid="b13" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_461764701">2006; Griffith et al., href="#b27" rid="b27" class=" bibr popnode">2009] or active deep-drilled seismic faults [Chester et al., href="#b9" rid="b9" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_461764695">2013; Hirono et al., href="#b31" rid="b31" class=" bibr popnode">2007].A number of physical mechanisms have been proposed to explain the dynamic weakening behavior observed in experiments and postulated to occur on faults (see Di Toro et al. [href="#b15" rid="b15" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_461764703">2011], Niemeijer et al. [href="#b50" rid="b50" class=" bibr popnode">2012], and Rice and Cocco [href="#b61" rid="b61" class=" bibr popnode">2007] for a summary). In particular, mechanical data and microstructural investigations of experimentally deformed bare rocks are consistent with flash heating of asperities [Goldsby and Tullis, href="#b25" rid="b25" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_461764704">2011; Violay et al., href="#b79" rid="b79" class=" bibr popnode">2014], frictional melting [Di Toro et al., href="#b13" rid="b13" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_461764706">2006; Hirose and Shimamoto, href="#b34" rid="b34" class=" bibr popnode">2005; Niemeijer et al., href="#b49" rid="b49" class=" bibr popnode">2011; Spray, href="#b73" rid="b73" class=" bibr popnode">2005], silica gel weakening [Di Toro et al., href="#b12" rid="b12" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_461764713">2004; Goldsby and Tullis, href="#b24" rid="b24" class=" bibr popnode">2002], and superplasticity (grain boundary sliding accommodated by dislocation motion or diffusion) [Green et al., href="#b26" rid="b26" class=" bibr popnode">2010; Holdsworth et al., href="#b35" rid="b35" class=" bibr popnode">2013; Schubnel et al., href="#b64" rid="b64" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_461764705">2013]. However, all faults generate a millimeter to centimeter thick layer of gouge during rupture and seismic slip [Reches and Dewers, href="#b57" rid="b57" class=" bibr popnode">2005], even within their deeper roots (6–15 km [e.g., Sibson, href="#b66" rid="b66" class=" bibr popnode">1977; Snoke et al., href="#b72" rid="b72" class=" bibr popnode">1999]). This raises the following questions: Which dynamic-weakening mechanisms occur in gouge-bearing faults? How might the presence of gouge modify the occurrence and/or efficacy of these weakening processes at seismic slip rates? Lubrication due to the presence of powders (i.e., powder lubrication) [Han et al., href="#b29" rid="b29" class=" bibr popnode">2010; Reches and Lockner, href="#b56" rid="b56" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_461764702">2010; Tisato et al., href="#b76" rid="b76" class=" bibr popnode">2012] is inconsistent with the rapid recovery of frictional strength at the end of sliding. Moreover, in exposed fault zones it is commonly observed that slip tends to be localized along very thin surfaces within gouge [e.g., Chester and Chester, href="#b8" rid="b8" class=" bibr popnode">1998; Fondriest et al., href="#b18" rid="b18" class=" bibr popnode">2013; Sibson, href="#b67" rid="b67" class=" bibr popnode">2003], leading some workers to suggest that once strain is localized within gouge the system will emulate bare surface slip behavior [e.g., Smith et al., href="#b71" rid="b71" class=" bibr popnode">2012; T. Tullis, personal communication, 2013]. But is it appropriate to extrapolate rock friction behavior obtained in rock-on-rock friction experiments to natural gouge-bearing faults? Furthermore, how does the effective normal stress affect this behavior? Interestingly, the results from Smith et al. [href="#b70" rid="b70" class=" bibr popnode">2013b] on calcite gouge and Han et al. [href="#b28" rid="b28" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_461764697">2007] on (cohesive) calcite-bearing marble suggest that the shear stress or strength of calcite gouge is a factor of 2 or greater than marble bare surfaces at seismic slip velocities despite having localized strain.Serpentinite is a common rock type in the oceanic lithosphere, and earthquakes may propagate into serpentinized mantle along mid-oceanic ridges, transform faults, and subduction zones; the latter alone release about 85–90% of the global seismic moment [Scholz, href="#b63" rid="b63" class=" bibr popnode">2002]. For this reason, the frictional behavior of serpentinite has been studied over a wide range of slip rates from plate rates to seismic slip rates [e.g., Hirose and Bystricky, href="#b32" rid="b32" class=" bibr popnode">2007; Kohli et al., href="#b39" rid="b39" class=" bibr popnode">2011; Reinen et al., href="#b59" rid="b59" class=" bibr popnode">1992]. Moreover, serpentine group minerals are expected to react to talc, olivine, and enstatite due to frictional heating during rapid slip. These minerals are thought to be stable in the geologic record and could therefore provide evidence for seismic slip [e.g., Kohli et al., href="#b39" rid="b39" class=" bibr popnode">2011]. Currently, the only widely accepted evidence for ancient seismic faulting is the presence of pseudotachylytes [Sibson, href="#b65" rid="b65" class=" bibr popnode">1975]. Other proposed geologic evidence for seismic slip includes thermally altered biomarkers in sedimentary rocks [Polissar et al., href="#b55" rid="b55" class=" bibr popnode">2011], peculiar crystal-plastic features [Bestmann et al., href="#b5" rid="b5" class=" bibr popnode">2012; Smith et al., href="#b69" rid="b69" class=" bibr popnode">2013a, href="#b70" rid="b70" class=" bibr popnode">2013b], injection of fluidized gouge [Fondriest et al., href="#b17" rid="b17" class=" bibr popnode">2012; Lin, href="#b41" rid="b41" class=" bibr popnode">2011; Rowe, href="#b62" rid="b62" class=" bibr popnode">2013], and the combination of mirror-like surfaces with truncated and exploded grains [Fondriest et al., href="#b18" rid="b18" class=" bibr popnode">2013; Siman-Tov et al., href="#b68" rid="b68" class=" bibr popnode">2013]. As a consequence, the occurrence of serpentine breakdown minerals in slipping zones could be indicative of ancient seismicity in faults exhumed from seismogenic depths, outlining the importance for further field studies of exhumed fault zones hosted in oceanic rocks.Employing a rotary-shear apparatus, we extend the study of the frictional behavior of serpentinite rocks to higher normal stresses (up to 96.6 MPa for bare surfaces and 22.4 MPa for gouges) and slip velocities (up to 4.3 m/s for bare surfaces and 6.5 m/s for gouges) than investigated previously. We also explore differences in dynamic frictional-weakening behavior observed on serpentine gouge and during tests on initially bare surfaces of serpentine by conducting relatively short-displacement, high-velocity experiments while varying the normal stress between tests. Following each experiment, the slip surfaces and wear material were analyzed with X-ray powder diffraction (XRPD) and several microstructural analysis techniques. These analyses, coupled with 1-D thermal modeling, allow us to constrain the effects of velocity, normal stress, shear heating, strain localization, and dehydration reactions on dynamic frictional weakening of serpentine and, by extension, other materials.
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