Fabric induced weakness of tectonic faults

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Fabric induced weakness of tectonic faults
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  Click Here for Full Article Fabric induced weakness of tectonic faults André Niemeijer, 1,2,3,4 Chris Marone, 2,3 and Derek Elsworth 1,3 Received 10 November 2009; revised 30 December 2009; accepted 8 January 2010; published 4 February 2010. [ 1 ] Mature fault zones appear to be weaker than predicted by both theory and experiment. One explanation involvesthe presence of weak minerals, such as talc. However, talcis only a minor constituent of most fault zones and thusthe question arises: what proportion of a weak mineral isneeded to satisfy weak fault models? Existing studies of fault gouges indicate that >30% of the weak phase isnecessary to weaken faults  ‐  a proportion not supported byobservations. Here we demonstrate that weakening of fault gouges can be accomplished by as little as 4 wt% talc, provided the talc forms a critically ‐ aligned, through ‐ goinglayer. Observations of foliated fault rocks in mature, large ‐ offset faults suggest they are produced as a consequence of ongoing fault displacement and thus our observationsmay provide a common explanation for weakness of maturefaults.  Citation:  Niemeijer, A., C. Marone, and D. Elsworth(2010), Fabric induced weakness of tectonic faults,  Geophys. Res. Lett. ,  37  , L03304, doi:10.1029/2009GL041689. 1. Introduction [ 2 ] The apparent weakness of mature faults with largeoffsets (>5 km) and the state of stress in the Earth ’ s crust have long been a matter of debate. The lack of a heat flowanomaly [  Lachenbruch and Sass , 1980, 1992] and the ori-entation of principal stresses around major fault zones[  Zoback et al. , 1987;  Zoback  , 2000] have been used tosuggest that some tectonic faults slip under much lower resolved shear stress than inferred from  “ classical ”  rock mechanics experiments [  Engelder et al. , 1975;  Dieterich ,1978;  Marone et al. , 1990]. Moreover, the existence of lowangle normal faults (i.e., faults that dip at an angle <30 °)can only be explained by extreme fault zone weakness[ Collettini and Sibson , 2001]. Possible explanations for weak faults include low effective stress, via elevated fluid pressures [  Rice , 1992;  Faulkner and Rutter  , 2001], the presence of weak phyllosilicates or clays [  Imber et al. , 2001; Collettini and Holdsworth , 2004;  Moore and Rymer  , 2007; Collettini et al. , 2009a], dynamic weakening [  Melosh , 1996;  Di Toro et al. , 2006;  Ampuero and Ben ‐   Zion , 2008], theoperation of stress ‐ enhanced dissolution ‐  precipitation creep[e.g.,  Rutter and Mainprice , 1979] or a combination of these[  Bos and Spiers , 2002;  Niemeijer and Spiers , 2005, 2006,2007]. However, the origin of fault weakness remains poorly understood and a matter of much interest  [ Scholz  ,2000].[ 3 ] Recent field observations of talc in both the SanAndreas fault [  Moore and Rymer  , 2007] and an exhumedlow angle normal fault in Italy [ Collettini et al. , 2009b] haveled to speculations that the weakness of these faults can beexplained by the presence of talc, which has very lowfriction. However, considering that the San Andreas fault isover a 1000 km long and that only minor amounts (2  –  3 wt%)of talc were discovered, the question arises: how much talcis needed to satisfy weak fault models? Recent experimentson synthetic mixtures of talc and a second, strong phase(quartz sand) suggests that 30 to 50 wt% talc is needed[ Carpenter et al. , 2009]. However, because of the layeredstructure of talc (high aspect ratio), this mineral is likelyto form a through ‐ going connected layer, which wouldsignificantly weaken the composite gouge as long as shear can be localized on the talc ‐  bearing foliation. Indeed,recent experiments on foliated fault rocks [ Collettini et al. ,2009a] show that fabric can induce significant weaknesseven under cataclastic conditions. Moreover, experimentson analogue fault gouges, using synthetic mixtures of salt and muscovite/kaolinite [  Bos and Spiers , 2002;  Niemeijer and Spiers , 2005, 2006] show that weakening can occur with as little as 10 wt% of the weak mineral, provided that the  “ strong ”  mineral deforms by pressure ‐ solution and theweak mineral forms a through ‐ going foliation. These experi-ments show that frictional slip can occur under low shear stress, provided that a weak and continuous foliation is present, which extends the slip importance of phyllosilicate ‐  bearing foliated rocks into the brittle regime. Previouswork under ductile deformation conditions has establishedthe role of fabric in weakening due to the plasticity andinter  ‐ connectivity of ductile phyllosilicate phases [ Sheaand Kronenberg  , 1993;  Wintsch et al. , 1995]. However,the details of foliation development and the amount of material required to induce weakening under brittle, cata-clastic conditions remain unclear.[ 4 ] We investigated the affect of fabric on fault strength inlaboratory experiments conducted under tectonic stresses.Layers of synthetic fault gouge were composed of quartzand talc, and the role of fabric was examined by comparinglayers containing thin zones of pure talc with layers formedfrom homogeneous mixtures of the two components. Wereport on the effects of talc interlayer thickness, normalstress, sliding velocity, and net strain on the strength of thecomposite gouges. 2. Experimental Methods [ 5 ] Our friction experiments were conducted under roomtemperature and humidity ( ∼ 20%) with the double ‐ direct  1 Department of Energy and Mineral Engineering,  Pennsylvania State University, University Park, Pennsylvania, USA. 2 Department of Geosciences,  Pennsylvania State University,University Park, Pennsylvania, USA. 3 G3 Center and Energy Institute,  Pennsylvania State University,University Park, Pennsylvania, USA. 4 Now at Istituto Nazionale di Geofisica e Vulcanologia, Rome,Italy. Copyright 2010 by the American Geophysical Union.0094 ‐ 8276/10/2009GL041689$05.00 GEOPHYSICAL RESEARCH LETTERS, VOL. 37, L03304, doi:10.1029/2009GL041689, 2010 L03304  1 of   5  shear configuration at constant normal stress inside a servo ‐ controlled, biaxial loading frame (for details, see auxiliarymaterial) [ Scott et al. , 1994;  Mair and Marone , 1999;  Niemeijer et al. , 2008]. 1 We used Ottawa quartz sand (F110,U.S. Silica Company, average grain size of 127 m m) and, in a subset of experiments, blue sand (Kelly ’ s Crafts, Ross, Ohio,U.S.A., average grain size 200  m m) which has similar fric-tional properties and provides contrast between the sand andtalc to facilitate visual observations. The talc was from theBalmat mine, New York, USA provided by Ward ’ s NaturalScience and was crushed using a shatter box and sieved to<125  m m. After each experiment, the sample assembly wascarefully taken apart in order to recover the sample. The talclayers were typically recovered intact, but the surroundingsand was not cohesive and could not be recovered. 3. Results [ 6 ] Figure 1a shows the evolution of friction with shear displacement for the two end ‐ member gouges in this study 1 Auxiliary materials are available in the HTML. doi:10.1029/ 2009GL041689. Figure 1.  (a) Evolution of friction with shear displacement for the end ‐ member cases (100% talc, p2284 and 100% quartz, p2280). Normal stress as indicated; velocity ‐ stepping sequence is 1 ‐ 3 ‐ 10 ‐ 30 ‐ 100 ‐ 300  m m/s for a displacement of 0.5 mm at each velocity. (b) Evolution of friction with shear displacement for four talc ‐ quartz mixtures: three with a talc interlayer (0.75, 1 and 2 mm ‐ thick) and one (p2279) in which 25 wt% of talc was mixed homogeneously with granular quartz. Normalstress and velocity history same as Figure 1a.  NIEMEIJER ET AL.: FABRIC INDUCED WEAKNESS  L03304L03304 2 of 5  (i.e., 100% talc and 100% quartz), where the friction coef-ficient is defined as the measured shear stress divided by themeasured normal stress. The pure talc sample (experiment  p2284) shows an initial peak stress followed by a gradualweakening. In contrast, the quartz layer (p2280) shows a minor initial peak stress and continues to strengthen duringshearing.Strainhardeningislikelyduetograinsizereduction by cataclasis resulting in gouge densification and strength-ening [  Marone et al. , 1990]. The frictional strength for purequartz (0.6) and pure talc (0.25) are consistent with resultsfrom other studies [  Marone et al. , 1990;  Mair and Marone ,1999;  Moore and Lockner  , 2008].[ 7 ] The behavior of homogeneous mineral mixtures dif-fers from layers that included a thin zone of talc (Figure 1b). Note that the relative amount of talc in the homogeneousmixture is larger than for experiments with talc interlayers(25 wt%  vs.  <16 wt% talc, see also Table 1 of Text S1 of the auxiliary material). The evolution of friction with shear displacement is a strong function of layer fabric. Homoge-neous mixtures (p2279) exhibited a gradual roll ‐ over instress, without a peak stress value, followed by strainhardening until a steady state frictional strength wasreached. Note that frictional strength of the mixture is verysimilar to that of pure quartz (Figure 1). In contrast, allsamples with a talc interlayer were weaker than the homo-geneous mixtures (Figure 1). Layers with a talc interlayer exhibited a peak in shear stress during initial loading, at thelowest normal stress, followed by gradual strain weakening(Figure 1). At higher normal stresses, these layers exhibitedcontinuous strain hardening. We varied the location of thetalc interlayer, from the centre to the edges of the compositelayer, and found that it had a negligible effect (see auxiliarymaterial).[ 8 ] We investigated the effects of sliding velocity, normalstress, shear strain, grain size, and fabric orientation. For samples with talc interlayers, friction varied systematicallywith sliding velocity, in the range 1 to 1000  m m/s. Frictionincreased with increasing sliding velocity and the evolutionof friction varied with velocity (see auxiliary material).Fabric orientation also had an important effect on frictionalstrengthofcompositelayers.Whenthetalcfabricwasrotatedtoward the R1 Riedel shear orientation ( ∼ 3° relative to theshearing direction), the strength of the composite layer wasconsistently lower (up to  ∼ 0.05 in friction) than when thetalc zone was parallel to the shear direction (see auxiliarymaterial).[ 9 ] Post experiment inspection of the samples showed that the quartz was incohesive, whereas the talc interlayers weresufficiently cohesive to be recovered intact (Figure 2). Sam- ples with a talc interlayer showed a tendency to develop a larger  ‐ scale fabric including a wavy, anastasmosing appear-ance of the talc interlayer ( “ flow perturbation folds ” ). Thewavelength and amplitude was roughly 0.5  –  1 mm and200 m m,respectively(Figure2b).TalcinterlayersalsoshowedevidenceofstronginternalshearasevidencedbyRiedelshear  partings and as shear along the talc/gouge interface. Thesurfaces of the talc layer are characterized by a series of striations (similar to slickensides) oriented in the direction of shear (Figure 2c). The width and spacing of the striations is ∼ 0.1 mm and 0.5 mm, respectively, which is consistent withsmearing (scoring) by quartz grains from the surroundingmaterial. The striations are more pronounced in experiments Figure 2.  (a) Side ‐ view of the sample assembly after shearing (experiment p2218, see Table 1 of Text S1 of theauxiliary material). Quartz was incohesive whereas thethin talc layer was cohesive (protuding layers) and strongenough to be preserved intact. (b) Zoomed side ‐ view of thetalc interlayer in an experiment with blue sand (p2221). Note undulations and evidence of flow pertubation foldingwithin the talc interlayer. (c) Top view of talc interlayer surface (p2221). Striations are parallel to the shear direction(from top to bottom).  NIEMEIJER ET AL.: FABRIC INDUCED WEAKNESS  L03304L03304 3 of 5  with blue sand which has a slightly larger grain size than our standard granular quartz. 4. Discussion and Implications [ 10 ] Consideration of our complete data set shows that the bulk frictional strength of composite layers decreases sys-tematically with talc interlayer thickness (Figure 3, note that the talc interlayer thickness was corrected for initial com- paction). The coefficient of friction ranges from 0.6 to 0.2,which is consistent with existing results for pure quartz and pure talc [  Marone et al. , 1990;  Mair and Marone , 1999; Carpenter et al. , 2009]. With increasing thickness of the talcinterlayer, we find two critical values. The first correspondsto a talc interlayer thickness of   ∼ 200  m m and indicates theonset of weakening relative to pure quartz (Figure 3). Thesecond critical thickness is at   ∼ 800  m m and corresponds to a fully ‐ weakened layer, with the frictional strength of puretalc. Layers with a talc interlayer thickness between 200  m mand 800  m m exhibit a range of frictional strengths betweenthat for pure quartz and pure talc.[ 11 ] One expects that weakening may vary with layer normal stress, for example due to flow folding of the talcinterlayer under shear and enhanced compaction (thinning)of the talc interlayer, and our data show some tendency for this. Increasing the grain size of the sand surrounding thetalc fabric resulted in a higher friction for the bulk layer.This is consistent with flow folding of the talc interlayer anddeeper penetration, into the talc layer, by larger quartz grains(see auxiliary material). The effect of grain size diminisheswith increasing net strain and normal stress, which is con-sistent with thinning of the talc interlayer, additional grain penetration, and fabric disruption. Additional work is nec-essary to understand how these critical thicknesses vary withgrain size, boundary roughness for the composite layer, andother factors.[ 12 ] We investigated the effect of fabric on the rate/statefrictional properties of the composite layers. Homogeneousmixtures exhibited very similar behavior to that of purequartz (see auxiliary material). In contrast, the frictional behavior for layers with talc fabric were very similar to that for pure talc. Homogeneous mixtures exhibited  “ classical ” rate and state behavior, with a peak in friction upon animposed jump in loading velocity followed by a gradualdecay to a new steady state value. Layers with talc fabric,however, simply jumped to a new steady state level without any evolution in friction (see auxiliary material for details).All layers exhibited velocity strengthening frictional behav-ior, but the values of the friction rate parameter,  a ‐ b , and thevariation with velocity vary with gouge fabric. Homoge-neous mixtures exhibited  a ‐ b  values very similar to those of  pure quartz, whereas layers with talc fabric exhibited valuessimilar to pure talc (see auxiliary material for details).[ 13 ] In summary, our mechanical data and observationssuggest that the frictional strength of tectonic faults can beextremely low if talc is present in an interconnected net-work. The degree of weakening depends on the thickness of the weak layer. In nature, talc and most other weak minerals(clays and phyllosilicates) are formed by chemical and hy-dration reactions [  Imber et al. , 1997;  Jefferies et al. , 2006; Schleicher et al. , 2006;  Collettini et al. , 2009b]. Therefore,mature fault zones without inherited weak minerals, should be frictionally strong unless fluids or local chemical reac-tions produce the weak phases  in ‐  situ . The formation of such authogenic phyllosilicates and clays has been docu-mented in several settings [ Wintsch et al. , 1995;  Imber et al. ,1997;  Holdsworth , 2004;  Schleicher et al. , 2006;  Collettiniet al. , 2009b] and grain size reduction and associated reac-tion rate enhancement is a possible agent to localize thereaction products (e.g., talc) as coatings on the surfaces of strong minerals, ultimately forming a weak through ‐ goingfoliation.[ 14 ] These observations have important implications for methods of evaluating fault strength and for the strength andevolution of mature, long ‐ lived faults. The importance of accounting for the role of fabric in measurements of fric-tional strength is clear. Moreover, observations of weak minerals in fault zones coupled with our conclusion that even trace quantities of weak phyllosilicates can induce fault weakness if the minerals form a connected, through ‐ goingarrangement indicates that we must understand coupling between fault processes and fabic development [  Evans and Chester  , 1995;  Wintsch et al. , 1995;  Imber et al. , 1997].This is consistent with other observations that zones of activefault slip may be only microns to millimeters in thickness onfault mélanges that are maybe decimeters to 10s of meterswide and have accommodated kilometers of slip during their lifetime [e.g.,  Chester and Chester  , 1998;  Schulz and Evans ,1998]. However, the findings of the present work suggest that mechanical deformation coupled with chemical trans-formations are necessary ingredients to develop the requisitenetwork of weak minerals that may provide reduced strengthat only trace concentrations of the weakening phase. Theseingredients are key if such networks are to form, reorganize Figure 3.  Plot of steady state friction (at v = 10  m m/s) vs.talc interlayer thickness. The value of talc interlayer thick-ness at each steady state was calculated assuming that theinterlayer of talc thins and compacts as measured for puretalc. The friction values were taken at steady state at 10  m m/sfrom experiments p2280, p2219/p2220, p2222, p2218, and p2284. The critical talc interlayer thickness for the onset of weakening and for   “ maximum ”  weakening are indicated.These thresholds are presumably a function of normal stressand the roughness of the talc interlayer surface (e.g., via grain size of the granular quartz in our experiments).  NIEMEIJER ET AL.: FABRIC INDUCED WEAKNESS  L03304L03304 4 of 5  and endure during the large ‐ scale offset of faults sustainedover long periods.[ 15 ]  Acknowledgments.  We wish to thank Bob Holdsworth and ananonymous reviewer for their helpful suggestions which helped improvethis paper. We would also like to thank Steve Swavely for his technicalsupport and Brett Carpenter and Cristiano Collettini for early discussionson this topic. This work was supported by NSF grants EAR  ‐ 0510182and ANT ‐ 0538195, the Netherlands Organisation for Scientific Research(N.W.O.) grant 825.06.003. AN was in part supported by the EuropeanResearch Council Starting grant 205175, USEMS project (P.I. Giulio DiToro). 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