Physical properties of surface outcrop cataclastic fault rocks...

Article

Volume 13, Number 1

28 January 2012

Q01018, doi:10.1029/2011GC003872

ISSN: 1525-2027

Physical properties of surface outcrop cataclastic fault rocks,Alpine Fault, New Zealand

C. BoultonDepartment of Geological Sciences, University of Canterbury, PB 4800, Christchurch 8042,New Zealand ([email protected])

B. M. CarpenterDepartment of Geosciences, Pennsylvania State University, 522 Deike Building, University Park,Pennsylvania 16802, USA

V. ToyDepartment of Geology, University of Otago, P.O. Box 56, Dunedin 9054, New Zealand

C. MaroneDepartment of Geosciences, Pennsylvania State University, 522 Deike Building, University Park,Pennsylvania 16802, USA

[1] We present a unified analysis of physical properties of cataclastic fault rocks collected from surfaceexposures of the central Alpine Fault at Gaunt Creek and Waikukupa River, New Zealand. Friction experi-ments on fault gouge and intact samples of cataclasite were conducted at 30–33 MPa effective normal stress(sn′) using a double-direct shear configuration and controlled pore fluid pressure in a true triaxial pressurevessel. Samples from a scarp outcrop on the southwest bank of Gaunt Creek display (1) an increase in faultnormal permeability (k = 7.45 � 10�20 m2 to k = 1.15 � 10�16 m2), (2) a transition from frictionally weak(m = 0.44) fault gouge to frictionally strong (m = 0.50–0.55) cataclasite, (3) a change in friction rate depen-dence (a-b) from solely velocity strengthening, to velocity strengthening and weakening, and (4) anincrease in the rate of frictional healing with increasing distance from the footwall fluvioglacial gravelscontact. At Gaunt Creek, alteration of the primary clay minerals chlorite and illite/muscovite to smectite,kaolinite, and goethite accompanies an increase in friction coefficient (m = 0.31 to m = 0.44) and fault-perpendicular permeability (k = 3.10 � 10�20 m2 to k = 7.45 � 10�20 m2). Comminution of frictionallystrong (m = 0.51–0.57) cataclasites forms weaker (m = 0.31–0.50) foliated cataclasites and fault gougeswith behaviors associated with aseismic creep at low strain rates. Combined with published evidence oflarge magnitude (Mw � 8) surface ruptures on the Alpine Fault, petrological observations indicate thatshear failure involved frictional sliding within previously formed, velocity-strengthening fault gouge.

Components: 7900 words, 5 figures, 2 tables.

Keywords: Alpine Fault; fault rocks; friction; permeability; surface rupture.

Index Terms: 8034 Structural Geology: Rheology and friction of fault zones (8163).

Received 13 September 2011; Revised 19 December 2011; Accepted 19 December 2011; Published 28 January 2012.

Boulton, C., B. M. Carpenter, V. Toy, and C. Marone (2012), Physical properties of surface outcrop cataclastic fault rocks,Alpine Fault, New Zealand, Geochem. Geophys. Geosyst., 13, Q01018, doi:10.1029/2011GC003872.

Copyright 2012 by the American Geophysical Union 1 of 13

http://dx.doi.org/10.1029/2011GC003872

1. Introduction

[2] The Alpine Fault accommodates Pacific-Australian plate boundary convergence on a singlenortheast-southwest striking structure with a sur-face trace at least 800 km long and a cumulativeoffset of �470 km [Wellman, 1953]. The centralsegment of the Alpine Fault, between Haast Riverand Toaroha River, accommodates �70% of the37 � 2 mm/yr relative motion between the plateboundaries (Figure 1) [Sutherland et al., 2007].Detailed field mapping within this segment showsthat in the near surface, the Alpine Fault is seriallypartitioned into alternating oblique thrust andstrike-slip segments with average strikes rangingfrom 010° to 050° and 070° to 090°, respectively[Norris and Cooper, 2007]. Along-strike dimen-sions of the segments (typically ≤3 km) correspondto the spacing of major rivers [Norris and Cooper,1997], and the segmented structures likely mergeinto a single oblique structure at a depth compa-rable to the topographic relief (�1000–1500 m)[Townend et al., 2009].

[3] Multiple lines of geological, seismological, andpaleoseismological evidence indicate that accumu-lated elastic strain on the central segment is releasedevery few hundred years in Mw 7.5–8 earthquakeswith horizontal displacements up to 8 m and verticaldisplacements up to 1.5 m [e.g., Sutherland et al.,2007, and references therein]. Geodetic studies fur-ther indicate that strain release is entirely coseismic,with interseismic creep at or near zero from thesurface to 13–18 km depth [Beavan et al., 2010].Extremely rapid (�5–10 mm/yr) exhumation alongthe central segment has exposed a sequence ofmylonites, ultramylonites and cataclastic (brittlefrictional) fault rocks in the uplifted hanging wall[Little et al., 2005]. Displacement on the shallowestpart of this section of the Alpine Fault (<1 kmdepth) is localized in a narrow zone of fault gougeand cataclasite. Outcrops of shattered, hydrother-mally altered mylonites that have been thrust overQuaternary gravels yield estimates of strike-slipdisplacement rates of �25 mm/yr and dip-slip dis-placement rates of <10 mm/yr [Norris and Cooper,2007].

[4] Our study combines new petrological obser-vations with the first laboratory measurements ofhydrological and frictional properties of cataclas-tic fault rocks exposed on oblique thrust segmentsof the Alpine Fault at Gaunt Creek and WaikukupaRiver, South Island, New Zealand (Figure 1). X-raydiffraction (XRD) results quantify in situ and along-strike changes in the mineralogy of fault gouges.

Numerical constraints on the frictional and hydro-logical properties of the fault gouges and cataclasitesalso inform a discussion of the mechanisms govern-ing earthquake rupture nucleation and propagationon this seismically active crustal-scale structure.

2. Surface Exposure Fault Rocks

[5] Over the last 5–8 Ma, oblique slip on theAlpine Fault has exhumed deformed rocks fromdepths up to 35 km (Figure 1) [Little et al., 2005].Along-strike, brittle-ductile fault rocks form in anear-homogeneous quartzofeldspathic Alpine Schistprotolith containing small amounts of metabasiteand metachert. The 1 km thick mylonite sequencebecome progressively deformed with increasingproximity to a 10 to 50 m thick damage zone con-taining fault rocks in sharp, striated, faulted contactwith fluvioglacial gravels [Norris and Cooper, 2007].In the hanging wall, pseudotachylytes (solidifiedfriction melts) are preserved in several settingswithin the mylonite-cataclasite sequence [Toy et al.,2011]. The widespread occurrence of pseudotachy-lyte indicates that earthquakes nucleate and propa-gate dynamically within these rocks in the uppercrustal seismogenic zone [Sibson and Toy, 2006].Exposures in outcrop, trench, and borehole sectionsprovide further evidence that earthquake rupturespropagate to the Earth’s surface within a thin(<60 cm), moderately dipping (38–45°) plane offine-grained fault rocks (Figure 2).

[6] In this study, we systematically sampled rocksforming the fault core and hanging wall of aprominent, well-documented surface outcrop of theAlpine Fault on the south bank of Gaunt Creek(hereafter referred to as the GC scarp exposure)(Figures 2a–2c) [Cooper and Norris, 1994; Warrand Cox, 2001]. We also collected fault gougefrom a new exposure of the Alpine Fault located inan excavation adjacent to a terrace riser 360 mnortheast of the GC scarp exposure (hereafterreferred to as the GC terrace exposure) (Figure 2d).

[7] At Gaunt Creek, quartzofeldspathic mylonitesare thrust over fluvioglacial gravels 14C datedat circa 12,650 years B.P. [Cooper and Norris,1994]. Four general types of fault rock are foundadjacent to and within the fault core; each rock typesampled contains microstructural evidence for strainlocalization and cataclasis sensu stricto (Figure 2b)[Sibson, 1977]. These cataclastic fault rocks aredescribed below, following the fault rock nomen-clature of Sibson [1977]. Unit 1 is a gray-browncataclasite derived from Quaternary fluvioglacial

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gravels deposited on the Australian plate. Unit 1varies in thickness from 0 to 40 mm, forms agradational contact with the underlying clast-supported gravels, and contains ≥60% matrix grains(matrix grains are by definition <0.1 mm in diam-eter). Unit 1 is absent where cobbles or bouldersform the contact with Units 2 or 3.

[8] Unit 2 is a discontinuous gray-brown, fine-grained fault gouge that overlies and forms a sharpcontact with Unit 1. In the GC scarp exposure, thecontact is oriented 059/42°SE with a prominent setof 0/059 striations overprinted by a faint 36/114 set.Unit 2 varies in thickness and color; where present,it is always less than 2 cm thick and contains ≥90%matrix grains. Visible grains include quartz, feldspar,carbonate, opaques and phyllosilicates set in a fine-grained matrix cemented with cryptocrystallinecarbonate and amorphous iron oxide. Rare clastsinclude subrounded reworked fault gouge, meta-morphic quartz and vein quartz. Unweathered Unit2 gouge has a foliation defined by phyllosilicates,with uniform extinction and color viewed throughthe sensitive tint plate, indicating that this mineralaggregate has a crystallographic preferred orienta-tion (Figure 2e). Unit 2 weathers rapidly in outcrop,becoming indurated, stained with iron oxides, andfractured by numerous empty tension cracks. Thisalteration makes sampling intact wafers impractical.

[9] Unit 3 is a gray incohesive fault gouge thatweathers to a brown indurated material; it formsa sharp undulating contact with either Unit 1 orUnit 2 (Figures 2a–2d). Hand specimens and thinsections of Unit 3 display variations in grain size,with some samples containing >30% visible clasts

>1 mm. Clast long axes are tilted toward the SEwith respect to the fault plane, indicating a top-to-the-northwest shear sense (Figure 2b). In the GCscarp exposure, Unit 3 varies in thickness between6 and 8 cm and occasionally forms irregular con-tacts with Unit 4 (Figure 2c). In the GC terraceexposure, Unit 3 reaches a maximum thickness of60 cm (Figure 2d). Because it appears continuousalong strike, Unit 3 is the fault core lithologysampled and tested for permeability and frictionalproperties in this study.

[10] Unit 4 is a pale green cataclasite that is foliatedin places. Unit 4 cataclasite contains fractured,sometimes sericitized, feldspars and recrystallizedquartz aggregates with undulose extinction andfinely sutured grain boundaries, separated by afaint anastamosing foliation defined by fine-grainedchlorite, illite-muscovite and calcite (Figure 3a).The cataclasite commonly contains altered pseu-dotachylyte and/or ultracataclasite [e.g., Toy et al.,2011].

[11] Near the contact with Unit 3, the lower 4–8 cmof Unit 4 contains fault plane-parallel layers offinely comminuted incohesive cataclasite up to1 cm thick oriented 059/50°SE with striationsplunging 44/117; this lithology is referred to as theUnit 4 foliated cataclasite (Figure 3b). Micro-structures in Unit 4 provide evidence of multipledeformation mechanisms; each deformation mech-anism would have dominated at different condi-tions of strain rate, temperature and pressure. Unit 4is the hanging wall lithology sampled for perme-ability and frictional properties in this study.

Figure 1. Location map for places mentioned and outcrops sampled in this study of the central segment of theAlpine Fault.


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Figure 2. Field photographs of Alpine Fault sampling localities. (a) At the Gaunt Creek scarp exposure, the AlpineFault emplaces Pacific Plate mylonites over Australian Plate fluvioglacial gravels as indicated by the white arrow.Scree obscures much of the detailed structure of the outcrop. (b) Line drawing on a photograph of a freshly scouredcataclastic fault rocks at the GC scarp exposure. The continuation of Unit 2 is obscured by an aspect change inUnit 3. Detailed descriptions of Units 1–4 are given in the text. (c) Line drawing on a photograph of an irregularcontact between Unit 3 fault gouge and into Unit 4 cataclasite, GC scarp exposure. Here, Unit 2 is truncated byUnit 3. (d) Incohesive fault gouge with rounded black clasts of ultramylonite (Umyl) and angular, crushed clasts ofpale green cataclasite (Ccl) exposed in an excavated terrace 360 m NE of the GC scarp exposure illustrated inFigures 2a–2c. (e) Line drawing on a photograph of the fault rock sequence exposed on the Waikukupa Thrust.Scree obscures the Unit 4 cataclasite. Y and P shears in Unit 3 are labeled. The contact between Unit 1 and the flu-vioglacial gravels is gradational.


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[12] For comparison, we sampled Unit 3 fault gougeand Unit 4 cataclasite from a surface exposure ofthe Waikukupa Thrust located 25 km to the south-east of Gaunt Creek (Figure 1). The WaikukupaThrust emplaces Pacific Plate mylonites and cata-clasites over Australian Plate fluvioglacial gravelsolder than 40,000 years B.P. The WaikukupaThrust was abandoned approximately 20,000 B.P.when slip was transferred to the Hare Mare

imbricate thrust [Norris and Cooper, 1997]. Wheresampled, Waikukupa Thrust Unit 3 is an 8 cm thick,reverse-graded fault gouge oriented 055/56°SE; thefault gouge forms a sharp contact with underlyingfootwall gravels with striations plunging 15/066(Figure 2e). Clasts within the Unit 3 fault gougeinclude subangular to subrounded unaltered to ret-rogressed quartzofeldspathic and metabasic mylo-nite set in a fine-grained matrix of quartz, feldspar,

Figure 3. Photomicrographs of cataclastic fault rocks collected from Alpine Fault thrust segments. (a) Unit 4 cata-clasite comprising boudinaged, microcracked feldspar (Fsp)-quartz (Qtz) porphyroclasts in a matrix of comminutedquartz-feldspar plagioclase-calcite (Cal) with concentrations of opaques defining a spaced but discontinuous foliation.GC scarp exposure; plane polarized light = PPL. (b) Unit 4 foliated cataclasite with boudinaged, microcracked, seri-citized feldspar porphyroclast pods (Fsp + Ser) in a matrix comprising anastomosing layers of phyllosilicates (Phyl)and fine grained gouge cemented with goethite (Gt) and calcite (Cal). Alignment of layered phyllosilicates indicatedby bold white lines. GC scarp exposure; cross polarized light = XPL. (c) Unit 3 random fabric fault gouge showingangular to rounded clasts of reworked fault gouge (dark brown material, labeled “fg”). Other clasts are variably calcite-cemented mylonite fragments. GC scarp exposure; PPL. (d) Unit 3 fault gouge with phyllosilicates mantling a clastof quartzose mylonite cut by calcite veins. Reworked fault gouge clasts are also apparent. GC terrace exposure;XPL. (e) Incipiently altered Unit 2 fault gouge containing empty tension cracks (labeled “tc”) that crosscut alignedphyllosilicate-rich layers oriented subparallel to the fault plane. GC scarp exposure; XPL. (f ) Unit 3 fault gouge con-taining clasts of weakly foliated quartz-biotite (Bt)-opaque (Op) mylonite (Myl) and metabasic mylonite with a con-tinuous foliation defined by elongate chlorite (Chl) and epidote (Ep). Waikukupa River; PPL.


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albite, chloritized biotite and amphibole, sericitizedfeldspar, epidote, titanite, and calcite (Figure 3f).Unit 4 is a cataclasite with mylonite clasts that havebeen subject to in situ fragmentation, translationand rotation in a matrix of comminuted quartz,feldspar, illite-muscovite, chlorite, calcite and pyrite.

3. Analytical Methods

[13] For quantitative X-ray diffraction, approxi-mately 1.5 g of each oven-dried sample was groundfor 10 min in a McCrone micronizing mill underethanol. The resulting slurries were oven-dried at60°C, thoroughly mixed with an agate mortar andpestle, and lightly back pressed into stainless steelsample holders. XRD patterns were recorded insteps of 0.017°2q on a PANalytical X’Pert Pro

Multipurpose Diffractometer using Fe-filtered CoKa radiation, variable divergence slit, 1° antiscatterslit and fast X’Celerator Si strip detector. Quanti-tative analysis was performed using the commercialpackage SIROQUANT. Smectite identification wasdone using XRD patterns obtained from orientedslides of Mg-saturated, glycerolated <2 mm clayseparates. Results are listed in Table 1.

4. Experimental Methods

[14] Large blocks of fault core gouges (Unit 3) andadjacent hanging wall cataclasites (Unit 4) werecollected from the outcrop using a portable rocksaw. These blocks were cut into wafers 6–8 mmthick, 54 mm wide, and 61 mm long; the waferlong axis was cut parallel to the fault shear direction

Table 2. Summary of Experiment Details and Resultsa

Experiment Lithology sn′ (MPa) Pc (MPa) Pp (MPa) k (m2) ms

p2798 Gaunt Creek Scarp U3 Gouge 31 25 10 7.45 E-20 0.44p2862b Gaunt Creek Scarp U3 Gouge 33 25 10 n/a 0.57p3151 Gaunt Creek Scarp U4 Fol. Cataclasite 31 25 10 1.41 E-18 0.50p2799 Gaunt Creek Scarp U4 Cataclasite 30 25 10 1.15 E-16 0.51p2863 Gaunt Creek Scarp U4 Cataclasite 30 25 10 n/a 0.55p3370 Gaunt Creek Terrace U3 Gouge 31 25 10 3.10 E-20 0.31p2800 Waikukupa River U3 Gouge 31 25 10 1.71 E-20 0.39p2830 Waikukupa River U3 Gouge 31 25 10 1.16 E-20 0.41p2801 Waikukupa River U4 Cataclasite 30 25 10 1.45 E-17 0.54p2828 Waikukupa River U4 Cataclasite 30 25 10 1.23 E-16 0.57aEffective normal stress (sn′ ) represents the combined effects of applied normal load, confining pressure (Pc) and pore pressure (Pa).bExperiment p2862 was done on a powdered sample.

Table 1. Summary of Surface Outcrop Alpine Fault Rock Mineralogy

Experiment Lithology Mineralogya

n/ab Gaunt Creek Scarp U2 Gouge Quartz 25%, Orthoclase 5%, Albite 20%, Calcite 5%,Illite-Muscovite 16%, Smectite 14%, Kaolinite 7%, Lizardite 9%

p2798, p2862 Gaunt Creek Scarp U3 Gouge Quartz 30%, Orthoclase 6%, Albite 26%, Calcite 7%,Mg-Calcite 1%, Illite-Muscovite 17%, Smectite 7%,Kaolinite 6%

p3151 Gaunt Creek ScarpU4 Fol. Cataclasitec

Quartz 9%, Orthoclase 3%, Albite 27%, Calcite 2%,Illite-Muscovite 16%, Smectite 36%, Chlorite 2%,Kaolinite 4%

p2799, p2863 Gaunt Creek Scarp U4 Cataclasite Quartz 49%, Orthoclase 6%, Albite 19%, Calcite 2%,Illite-Muscovite 12%, Chlorite 8%

p3370 Gaunt Creek Terrace U3 Gouge Quartz 35%, Orthoclase <1%, Albite 17%, Calcite 6%,Illite-Muscovite 32%, Chlorite 9%

n/a Waikukupa River U2 Gouge Quartz 27%, Orthoclase 1%, Albite 19%, Calcite 17%,Illite-Muscovite 17%, Smectite 16%, Chlorite 2%, Pyrite <1%

p2830 Waikukupa River U3 Gouge Quartz 28%, Albite 26%, Calcite 25%, Illite-Muscovite 14%,Chlorite 5%, Actinolite 2%, Pyrite <1%

p2828 Waikukupa River U4 Cataclasite Quartz 35%, Albite 33%, Calcite 6%, Mg-Calcite 1%,Illite-Muscovite 6%, Chlorite 19%, Pyrite <1%,Laumontite <1%

aResults are normalized to 100% and do not include estimates of unidentified or amorphous materials.bn/a, not applicable.cData are from the <2 mm fraction only. Fol., foliated.


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as indicated by striation measurements. A total of10 double-direct shear friction experiments wereconducted on Gaunt Creek and Waikukupa Riverfault rocks (Table 2).

[15] We deformed fault rock samples in a servo-hydraulic controlled biaxial testing apparatus fittedwith a pressure vessel [Samuelson et al., 2009;Ikari et al., 2011b]. We sheared all samples undersaturated, drained conditions at room temperature.We held effective normal stress (sn′), confiningpressure (Pc), and pore pressure (Pp) constant at�31 MPa, 25 MPa, and 10 MPa, respectively(Table 2). These variables are geologically reason-able if we assume hydrostatic pore fluid pressure,an effective normal stress appropriate for 1 kmdepth on an oblique thrust fault striking NW-SE, and a stress tensor with s1 = 45 MPa,sv = s2 = s3 = 25 MPa. Along the Alpine Fault, themaximum compressive stress (s1) is subhorizontaltrending 120° and the least principal stress (s3) issubhorizontal trending 030°, with some local vari-ance between sv = s2 and sv = s3 expected for acombination of strike-slip and reverse faulting[Leitner et al., 2001].

[16] Flow-through permeability tests were con-ducted prior to shearing intact wafers of fault rock.In each experiment, two wafers were placedbetween a three-piece steel block assembly fittedwith porous, stainless steel frits and then jacketed(see Samuelson et al. [2009] for a complete descrip-tion of the experimental apparatus and methods).In the pressure vessel, normal (sn), confining (Pc),and pore fluid (Pp) pressures were applied to the

sample, resulting in a true triaxial stress state.Pore fluid (University Park, Pennsylvania, USAtap water with pH = 7 and total cation concen-tration [Ca2+ + Mg2+] = 2–4 mmol/L) was plum-bed directly into the fault rock samples andallowed to equilibrate until steady state boundaryconditions were reached. We imposed a differentialfluid pressure to induce flow normal to the sheardirection of the intact wafers. When flow ratereached steady state, permeability was calculatedusing Darcy’s law.

[17] In the double-direct shear configuration, faultrock samples are sheared by driving the centerblock with a servo controlled vertical piston at aconstant displacement rate to attain steady statefrictional behavior (Figure 4a). At steady state, thecoefficient of sliding friction m was determinedfrom the ratio of shear stress t to effective normalstress sn′ assuming cohesion of zero. We thenperformed velocity step tests (Figure 4b) to deter-mine the rate and state dependence of friction. Loadpoint velocity was varied between 1 and 100 mm/s,and in some cases 300 mm/s, to measure the frictionrate parameter (a-b) = Dm/D ln V, where a is thedirect effect, b is the evolution effect, Dm is thechange in steady state sliding friction, and V isthe sliding velocity [e.g.,Marone, 1998] (Figure 4b).We also carried out a series of slide-hold-slide teststo measure the frictional healing rate. In these tests,the load point was driven at 10 mm/s, heldmotionless for a prescribed time, t, between 1 and300s and then driven at 10 mm/s again. Frictionalhealing Dm is the difference in peak friction,

Figure 4. (a) Plot of coefficient of friction (m = t/s′n) versus strain for a double-direct shear experiments on GC scarpexposure Unit 4 hanging wall cataclasite (Ccl) and GC terrace exposure Unit 3 fault gouge. After measuring the fric-tion coefficient at steady state sliding, (b) velocity step tests were performed and (c) slide-hold-slide tests were per-formed. Measurement accuracy is �0.002 MPa for load and �0.1 mm for displacement.


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following a hold, relative to the steady state slidingfriction prior to the hold. The healing rate is thechange in Dm per factor of 10 change in hold timemeasured in seconds (Figure 4c).

5. Results

5.1. Permeability

[18] All permeability measurements were madeon intact wafers of cataclastic fault rocks at effec-tive normal stresses between 30 and 31 MPa. Thefault-perpendicular permeability of Unit 3 faultgouges varies between k = 1.16 � 10�20 m2 andk = 7.45 � 10�20 m2 (Table 2); these values aretypical for natural and experimental fault gouges[e.g., Faulkner and Rutter, 2000, 2001]. The fault-perpendicular permeability of Unit 4 is almost3 orders of magnitude greater than Unit 3, increas-ing from k = 1.41 � 10�18 m2 in the Gaunt CreekUnit 4 foliated cataclasite to k = 1.15 � 10�17 m2 inGaunt Creek Unit 4 cataclasite and k = 1.23� 10�16

m2 and k = 1.45 � 10�17 m2 in Waikukupa ThrustUnit 4 cataclasites. This increase in permeabilitycorresponds with a decreasing abundance of phyl-losilicate layers and an overall increase in meangrain size (Figures 2 and 3). These results areconsistent with measurements on similar fault rockscollected from the Nojima Fault Zone [Lockner et al.,2000] and Median Tectonic Line in Japan [Wibberleyand Shimamoto, 2003].

5.2. Friction

[19] Values of steady state sliding friction coeffi-cients for fault core (Unit 3) and hanging wall cat-aclasites (Unit 4) range from m = 0.31 to 0.57(Table 2 and Figure 5a). The Unit 3 fault gouges arethe weakest, with m = 0.31 and m = 0.44 for intactwafers of fault gouge collected from the GC terraceand GC scarp exposures, respectively. The frictioncoefficients of Waikukupa Thrust fault gouge,m = 0.39 and m = 0.41, lie within this range.Hanging wall cataclasites are stronger, with intactwafers of Unit 4 foliated cataclasite having am = 0.50. Intact wafers of Gaunt Creek andWaikukupa Thrust cataclasites have friction coef-ficients of m = 0.51–0.55 and m = 0.54–0.57,respectively. The powdered sample of Gaunt CreekUnit 3 has a higher friction coefficient, m = 0.57,than its wafer equivalent, which is consistent withrecent experiments showing that phyllosilicaterich rocks are generally stronger in powder form[Collettini et al., 2009; Ikari et al., 2011b].

Figure 5. Summary plots of the frictional properties ofAlpine Fault cataclastic fault rocks. (a) A plot of coef-ficient of friction (m = t/sn′ ) against effective normalstress sn′ . (b) Friction rate parameter (a-b) is plottedagainst the upstep load point velocity V (mm/s). (c) Fric-tional healing rates are plotted as the change in the coef-ficient of friction (Dm) as a function of hold time (t).Symbols given in the legend of Figure 5a are valid forall plots. Abbreviations are foliated cataclasite (FCcl),cataclasite (Ccl), powder (pwdr), GC terrace exposure(Tce), and GC scarp exposure (Scarp).


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5.3. Friction Rate Dependence

[20] We performed velocity step tests in sixexperiments to determine rate/state friction consti-tutive parameters (Figure 4b). This parameteriza-tion is useful for inferring whether shear will bestable or unstable under conditions of tectonicfaulting [e.g., Marone, 1998]. The friction rateparameter, a-b, describes the variation in steadysliding friction with slip velocity. Materials aresaid to be velocity strengthening if a-b > 0; thesematerials tend to slide stably, accommodatingstrain aseismically. Materials exhibiting negativea-b values are termed velocity weakening; thesematerials may undergo a frictional instability thatresults in earthquake nucleation and seismic rup-ture propagation [Scholz, 2002].

[21] Results from velocity step tests are summa-rized in Figure 5b. Jacket failure precluded mea-suring the friction rate parameters and frictionalhealing rates of intact wafers of Unit 3 from theGC scarp exposure; a powdered sample of Unit 3from that locality exhibited velocity-strengtheningbehavior (a-b = 0.00012–0.0016). Unit 3 faultgouges collected from the GC terrace exposureand Waikukupa Thrust displayed clear velocity-strengthening behavior (a-b = 0.0052–0.018)(Figures 4b and 5b). Gaunt Creek Unit 4 foliatedcataclasite also exhibited velocity-strengtheningbehavior (a-b = 0.0019–0.016). These units alsoshow strong positive a-b rate dependence, a behav-ior observed in other clay gouges [Ikari et al.,2011a].

[22] Intact wafers of Unit 4 cataclasites a range ofa-b values, depending on the magnitude of thevelocity perturbation. Values of a-b for the cata-clasites range between –0.0016 and 0.00082. Weobserve no systematic relationship between thesize of the velocity perturbation and the value ofa-b in the cataclasites (Figure 5b). Other fric-tionally strong gouges (m ≥ 0.5) commonlyexhibit both velocity-strengthening and velocity-weakening behavior in velocity step experiments[Ikari et al., 2011a].

5.4. Frictional Healing

[23] Frictional strength varies with the real area ofcontact. Under stationary conditions, asperity con-tacts can heal with time, increasing the likelihoodof unstable frictional sliding in some materials[Marone, 1998; Scholz, 2002]. Slide-hold-slide testswere performed to measure the rate of frictionalhealing (Figure 4c). As in the velocity stepping

experiments, data on GC scarp exposure Unit 3 arefrom gouge powder, which healed at a faster ratethan intact wafers of all lithologies (Figure 5c).

[24] Intact wafers of GC terrace exposure Unit 3and Unit 4 foliated cataclasite weakened with holdtime for all hold times between t = 3s and t = 100s(Figures 4c and 5c). The gouge layers have verylow permeabilities, and consolidation during holdtime may have resulted in elevated pore fluidpressures and thus lower friction coefficients [e.g.,Byerlee, 1993; Faulkner and Rutter, 2001]. How-ever, we observe no systematic correlation betweenhold time and frictional healing across the rangeof hold times imposed. The results of the GC ter-race exposure Unit 3 experiment are discussedfurther below.

[25] GC scarp exposure Unit 4 nonfoliated catacla-site, Waikukupa Thrust Unit 3 fault gouge, andWaikukupa Thrust Unit 4 cataclasite all showed apositive correlation between hold time and Dm(Figure 5c). This behavior is consistent withmechanistic interpretations of healing in rocks com-posed of framework silicates, where the real areaof surface contact increases with time because ofcontact junction growth and/or strengthening[Dieterich, 1978; Marone, 1998].

6. Discussion

[26] Along thrust segments at Gaunt Creek andWaikukupa River, the east dipping Alpine Faultplane is overlain by 10–50 m of cataclasite com-prising clasts of quartzofeldspathic and metabasicmylonites in a finely comminuted chloritizedmatrix. Near the striated basal contact with footwallfluvioglacial gravels, strain localization in thehanging wall has resulted in further grain sizereduction and fault gouge formation. In places,mechanically aligned phyllosilicates form a planarfabric aligned subparallel to the fault plane (e.g.,Figure 3e).

[27] Quantitative X-ray diffraction analyses indi-cate that strain localization and grain size reductionwas accompanied by changes in the nature andabundance of phyllosilicate phases at the GauntCreek localities (Table 1). At the GC scarp expo-sure, fault gouges comprising Units 2 and 3 containa high percentage of clay minerals (57–60%) rela-tive to Unit 4 cataclasite (20%). Unit 4 cataclasitecontains higher temperature clay minerals chlorite(clinochlore-1MIIb) and muscovite (2M1). Unit 3,however, contains the lower temperature clayminerals smectite (montmorillonite) and kaolinite


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as well as chlorite (clinochlore-1MIIb) [e.g., Mooreand Reynolds, 1997]. Based on mineralogy,microstructures and grain size, the Unit 4 foliatedcataclasite contains components of both the cata-clasite and fault gouge (Table 1) (Figure 3b).Basal Unit 2 gouges contain predominantly low-temperature phyllosilicates, with the exception oflizardite (1M) at the GC scarp exposure (Table 1).The origin of lizardite (1M) is unknown; it ispossible this mineral was derived from footwallgravels or a protolith present downdip or alongstrike.

[28] Hand specimen and thin section identificationof relict clasts in the fault gouges and cataclasites,along with X-ray diffraction analysis, indicate thatcomminution of hanging wall mylonites and pro-tocataclasites formed the granulated cataclasitesand gouges analyzed in this study. By an analysisof garnet fragments in cataclasites, Cooper andNorris [1994] reached the same conclusion, butthey could not discount a contribution from foot-wall granitoids at depth. Most mylonite hangingwall rocks are quartzofeldspathic, composed of theprimary minerals: quartz, plagioclase, muscovite,and biotite with minor amounts of garnet, ilmenite,and calcite. Metabasic mylonites contain amphi-bole, as reflected in the mineralogy of the Waiku-kupa Thrust samples [Toy et al., 2010].

[29] In addition to primary minerals, GC exposurefault gouges and foliated cataclasite fault gougescontain kaolinite and the weak dioctahedral claymineral smectite (Table 1) (m = 0.15–0.32) [Safferand Marone, 2003]. Warr and Cox [2001] foundthat smectite forms from low-temperature alterationin Alpine Fault mylonite-derived cataclasites andgouges. Smectite can form from low-temperaturealteration (<120°C) of the primary minerals illite/muscovite and chlorite. In our samples, smectiteoccurrence coincides with a depletion in chloriterelative to adjacent hanging wall cataclasites. Oxi-dized alteration of chlorite forms smectite, whichfurther alters into kaolinite and amorphous ironoxide (goethite) [e.g., Craw, 1984; Chamberlainet al., 1999]; these minerals are all present inUnits 2, 3 and 4 on the GC scarp exposure. Warrand Cox [2001] suggested that the growth ofsmectite leads to mechanical weakening in AlpineFault gouges, but its association with a secondarycement strengthens the GC scarp exposure faultgouge (m = 0.44) relative to the GC terrace expo-sure fault gouge (m = 0.31).

[30] GC terrace exposure Unit 3, GC scarp expo-sure Unit 4 cataclasite, Waikukupa Thrust Unit 3

fault gouge and Waikukupa Thrust Unit 4 catacla-site all contain the primary mineral phases quartz,albite � orthoclase, calcite, illite-muscovite (2M1),and chlorite (clinochlore-1MIIB). These mineralsare likely to remain stable to subgreenschist con-ditions (<320°C) near the base of the brittle seis-mogenic zone [Warr and Cox, 2001]. Using ageothermal gradient of 40°C, this equates to a depthof 8 km on the Alpine Fault [Toy et al., 2010]. Ateffective normal stresses >60 MPa, however, clay-rich gouge behavior becomes increasingly nonlin-ear, and pressure solution creep may compete withfriction to accommodate grain sliding in clay-richgouges containing a soluble mineral phase such asquartz or calcite [e.g., Rutter, 1983; Bos and Spiers,2002; Niemeijer and Spiers, 2007; Gratier et al.,2011]. Whether deformation is accommodated inthe gouges via frictional sliding and cataclasis orpressure solution creep depends on many factors,foremost temperature and magnitude of differentialstress resolved on the Alpine Fault [Rutter, 1976;Cox and Etheridge, 1989; Gratier et al., 2009].

[31] Our experiments were conducted at roomtemperature on saturated fault gouges loaded ateffective normal stresses �30 MPa and loadingrates between 1 and 300 mm/s. At these conditions,clay-rich Unit 3 fault gouges are frictionally weak,exhibit velocity-strengthening frictional behavior,and do not undergo frictional healing. The weak(m = 0.31), velocity-strengthening (a-b = 0.0075–0.016), nonhealing (Dm = –0.003 after t = 100 s)properties of the GC terrace exposure Unit 3 gouge(illite-muscovite 32%; chlorite 9%) best repre-sents the frictional behavior of clay-rich faultgouges at depth.

[32] Figure 4a illustrates that the GC terrace expo-sure Unit 3 fault gouge did not display peak fric-tion. Experimental observations indicate that alignedlayers of chlorite (clinochlore-1MIIb) and illite-muscovite (2M1) facilitated strain accommodationduring the early stages of shearing, suppressingpeak strength [Saffer and Marone, 2003]. At higherstrains during the slide-hold-slid tests, little or nocontact strengthening occurred in these aligned claygrains, and we observed no frictional healing withhold time [e.g., Bos and Spiers, 2000].

[33] An additional factor influencing peak strengthand frictional healing is dilation. Dilation is aphysical response to shear-induced strain localiza-tion observed in many granular materials. Byincreasing the thickness of the shearing granularlayer, dilation increases effective normal stress,decreases pore fluid pressures, and suppresses


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frictional instabilities. The lack of frictional healingin the GC terrace exposure fault gouge suggeststhat aligned clay layers do not exhibit classicalhealing behavior governed by a combination offrictional strengthening during the hold and dilan-tancy strengthening upon reshear. These units havevery low permeabilities, and time-dependent com-paction may result in high pore fluid pressures andundrained loading [Lockner and Byerlee, 1994;Garagash and Rudnicki, 2003; Faulkner et al.,2011; Schmitt et al., 2011].

[34] Unit 3 fault gouges also exhibit velocity-strengthening behavior, a frictional property thatpromotes stable (aseismic) sliding, inhibits earth-quake rupture nucleation, and may contribute toearthquake rupture arrest at shallow depths (≲3 km)[Marone, 1998]. However, paleoseismological,geomorphological and geodetic evidence suggeststhat the Alpine Fault fails coseismically in largeearthquakes, forming surface ruptures with anaverage slip per event of �8 m dextrally and �1 mvertically [Sutherland et al., 2007].

[35] Along the Alpine Fault, the upper stabilitytransition, which defines the updip limit of the seis-mogenic zone, is either too shallow to arrest seis-mic rupture propagating from below, or the upper,stable region is not sufficiently velocity strength-ening to arrest rupture [Marone, 1998]. Alternatively,theoretical considerations, numerical modeling andhigh-velocity rock friction experiments show thatrapid undrained loading in low-permeability faultgouge results in thermal pressurization of porefluids at slip rates greater than 0.1 m/s and dis-placements on the order of a few microns [e.g.,Sibson, 1973; Lachenbruch, 1980; Rice, 2006].

[36] Sufficient shear heating can induce thermalpressurization, a weakening mechanism that oper-ates before seismic slip speeds are obtained[Schmitt et al., 2011]. Faulkner et al. [2011] pre-sented experimental data on saturated clay-richgouges sheared in a high-velocity rotary apparatusat low normal stress (1.63 MPa) and high slipvelocity (1.3 m/s). They attributed the immediatefrictional weakening to rapid thermal pressurizationof the pore fluid, which resulted in a negligiblecritical slip weakening distance and fracture energy.A rapid loss of strength at the tip of an earthquakerupture may make propagation through clay richgouges energetically favorable [e.g., Tinti et al.,2005; Bizzarri and Cocco, 2006a, 2006b]. As slipspeeds grow to within a few orders of magnitude ofelastic wave speeds, however, additional factorscontribute to the physics of earthquake propagation

through velocity-strengthening material, includingelastically radiated energy, fracture energy, andporomechanical effects [Rice, 1993; Lapusta et al.,2000; Perfettini and Ampuero, 2008].

[37] The inclusion of reworked clasts of fault gougeand hanging wall cataclasites in Unit 3 fault gougesindicates that large magnitude surface ruptures onthe Alpine Fault have occurred repeatedly withinthe same fault core. Local pore fluid overpressuresare indicated by the irregular contacts betweenUnit 3 fault gouge and Unit 4 cataclasite at the GCscarp exposure (Figure 2c). Questions remain,however, about whether these irregular contactsformed interseismically by creep, compaction, andpore fluid pressurization in the impermeable, clay-rich gouge or coseismically as gouge injectionstructures formed by shear-induced thermal pres-surization [e.g., Cowan, 1999; Meneghini et al.,2010; Lin, 2011].

7. Conclusions

[38] Oblique thrust fault segments of the AlpineFault form surface outcrops at Gaunt Creek andWaikukupa River. Samples of cataclastic fault rockswere collected and investigated using a double-direct shear configuration under controlled porepressure. At effective normal stresses between 30and 31 MPa, fault core gouges have fault normalpermeabilities up to 3 orders of magnitude lowerthan hanging wall cataclasites (Table 2). Faultcore gouges are velocity strengthening and havelower friction coefficients (m = 0.31–0.44) thanhanging wall cataclasites. Cataclasites are friction-ally strong (m = 0.50–0.57), exhibit normal to highrates of frictional healing, and display both velocity-strengthening and velocity-weakening behaviors.These results support general models of maturefault cores as weak with respect to the surroundingcrust [e.g., Rice and Cocco, 2007; Carpenter et al.,2011].

[39] Current geodetic estimates indicate that theAlpine Fault is locked to a depth of 13–18 km, andtotal elastic strain accumulation is released in largemagnitude (Mw 7.5–8) earthquakes that propagatealong-strike distances between 300 and 600 km[Beavan et al., 2010; Sutherland et al., 2007]. Pet-rological observations of clasts of reworked faultgouge in fault core rocks sampled in this studyreveal that earthquake ruptures preferentially prop-agated through velocity-strengthening fault gougesexposed in surface outcrops at Gaunt Creek andWaikukupa River. Understanding the mechanisms


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facilitating earthquake rupture propagation throughclay-rich gouges remains an important goal in rockmechanics and earthquake physics.

Acknowledgments

[40] The first author received support for this research fromthe Claude McCarthy Foundation and the University of Canter-bury Roper Scholarship. We would like to thank to Sam Hainesfor doing preliminary X-ray diffraction analyses andMark Ravenfor doingmore detailed, quantitative analyses. Greg De Pascale iscredited with discovering the Alpine Fault terrace exposure atGaunt Creek. The laboratory work was supported by NSF grantsOCE-0648331, EAR-0746192, and EAR-0950517 to C. Maroneand Marsden grant UOO0919 to V. Toy. Reviews by DanFaulkner, an anonymous reviewer, and Editor Thorsten Beckergreatly improved the quality of this manuscript.

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