Physical Properties of Surface Outcrop Cataclastic Fault Rocks, Alpine Fault, New Zealand

Home , Cataclasite, Fault gouge, Mylonite, Waikukupa River

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. Boulton Department of Geological Sciences, University of Canterbury, PB 4800, Christchurch 8042, New Zealand ([email protected]) B. M. Carpenter Department of Geosciences, Pennsylvania State University, 522 Deike Building, University Park, Pennsylvania 16802, USA V. Toy Department of Geology, University of Otago, P.O. Box 56, Dunedin 9054, New Zealand C. Marone Department 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 surface exposures of the central Alpine Fault at Gaunt Creek and Waikukupa River, New Zealand. Friction experiments 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 pressure vessel. Samples from a scarp outcrop on the southwest bank of Gaunt Creek display (1) an increase in fault normal permeability (k=7.45 Â 10À20 m2 to k = 1.15 Â 10À16 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 dependence (a-b) from solely velocity strengthening, to velocity strengthening and weakening, and (4) an increase in the rate of frictional healing with increasing distance from the footwall fluvioglacial gravels contact. 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 Â 10À20 m2 to k = 7.45 Â 10À20 m2). Comminution of frictionally strong (m = 0.51–0.57) cataclasites forms weaker (m = 0.31–0.50) foliated cataclasites and fault gouges with behaviors associated with aseismic creep at low strain rates. Combined with published evidence of large magnitude (Mw 8) surface ruptures on the Alpine Fault, petrological observations indicate that shear 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.

1. Introduction Numerical constraints on the frictional and hydrological properties of the fault gouges and cataclasites [2] The Alpine Fault accommodates Pacific- also inform a discussion of the mechanisms govern- Australian plate boundary convergence on a single ing earthquake rupture nucleation and propagation northeast-southwest striking structure with a sur- on this seismically active crustal-scale structure. face trace at least 800 km long and a cumulative offset of 470 km [Wellman, 1953]. The central 2. Surface Exposure Fault Rocks segment of the Alpine Fault, between Haast River and Toaroha River, accommodates 70% of the [5] Over the last 5–8 Ma, oblique slip on the 37 Æ 2 mm/yr relative motion between the plate Alpine Fault has exhumed deformed rocks from boundaries (Figure 1) [Sutherland et al., 2007]. depths up to 35 km (Figure 1) [Little et al., 2005]. Detailed field mapping within this segment shows Along-strike, brittle-ductile fault rocks form in a that in the near surface, the Alpine Fault is serially near-homogeneous quartzofeldspathic Alpine Schist partitioned into alternating oblique thrust and protolith containing small amounts of metabasite strike-slip segments with average strikes ranging and metachert. The 1 km thick mylonite sequence from 010° to 050° and 070° to 090°, respectively become progressively deformed with increasing [Norris and Cooper, 2007]. Along-strike dimen- proximity to a 10 to 50 m thick damage zone con- sions of the segments (typically ≤3 km) correspond taining fault rocks in sharp, striated, faulted contact to the spacing of major rivers [Norris and Cooper, with fluvioglacial gravels [Norris and Cooper, 2007]. 1997], and the segmented structures likely merge In the hanging wall, pseudotachylytes (solidified into a single oblique structure at a depth compa- friction melts) are preserved in several settings rable to the topographic relief (1000–1500 m) within the mylonite-cataclasite sequence [Toy et al., [Townend et al., 2009]. 2011]. The widespread occurrence of pseudotachy- [3] Multiple lines of geological, seismological, and lyte indicates that earthquakes nucleate and propa- paleoseismological evidence indicate that accumu- gate dynamically within these rocks in the upper lated elastic strain on the central segment is released crustal seismogenic zone [Sibson and Toy, 2006]. Exposures in outcrop, trench, and borehole sections every few hundred years in Mw 7.5–8 earthquakes with horizontal displacements up to 8 m and vertical provide further evidence that earthquake ruptures displacements up to 1.5 m [e.g., Sutherland et al., propagate to the Earth’s surface within a thin 2007, and references therein]. Geodetic studies fur- (<60 cm), moderately dipping (38–45°) plane of ther indicate that strain release is entirely coseismic, fine-grained fault rocks (Figure 2). with interseismic creep at or near zero from the [6] In this study, we systematically sampled rocks surface to 13–18 km depth [Beavan et al., 2010]. forming the fault core and hanging wall of a Extremely rapid (5–10 mm/yr) exhumation along prominent, well-documented surface outcrop of the the central segment has exposed a sequence of Alpine Fault on the south bank of Gaunt Creek mylonites, ultramylonites and cataclastic (brittle (hereafter referred to as the GC scarp exposure) frictional) fault rocks in the uplifted hanging wall (Figures 2a–2c) [Cooper and Norris, 1994; Warr [Little et al., 2005]. Displacement on the shallowest and Cox, 2001]. We also collected fault gouge part of this section of the Alpine Fault (<1 km from a new exposure of the Alpine Fault located in depth) is localized in a narrow zone of fault gouge an excavation adjacent to a terrace riser 360 m and cataclasite. Outcrops of shattered, hydrother- northeast of the GC scarp exposure (hereafter mally altered mylonites that have been thrust over referred to as the GC terrace exposure) (Figure 2d). Quaternary gravels yield estimates of strike-slip [7] At Gaunt Creek, quartzofeldspathic mylonites displacement rates of 25 mm/yr and dip-slip dis- 14 placement rates of <10 mm/yr [Norris and Cooper, are thrust over fluvioglacial gravels C dated 2007]. at circa 12,650 years B.P. [Cooper and Norris, 1994]. Four general types of fault rock are found [4] Our study combines new petrological obser- adjacent to and within the fault core; each rock type vations with the first laboratory measurements of sampled contains microstructural evidence for strain hydrological and frictional properties of cataclas- localization and cataclasis sensu stricto (Figure 2b) tic fault rocks exposed on oblique thrust segments [Sibson, 1977]. These cataclastic fault rocks are of the Alpine Fault at Gaunt Creek and Waikukupa described below, following the fault rock nomen- River, South Island, New Zealand (Figure 1). X-ray clature of Sibson [1977]. Unit 1 is a gray-brown diffraction (XRD) results quantify in situ and along- cataclasite derived from Quaternary fluvioglacial strike changes in the mineralogy of fault gouges.

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Figure 1. Location map for places mentioned and outcrops sampled in this study of the central segment of the Alpine Fault. gravels deposited on the Australian plate. Unit 1 >1 mm. Clast long axes are tilted toward the SE varies in thickness from 0 to 40 mm, forms a with respect to the fault plane, indicating a top-to- gradational contact with the underlying clast- the-northwest shear sense (Figure 2b). In the GC supported gravels, and contains ≥60% matrix grains scarp exposure, Unit 3 varies in thickness between (matrix grains are by definition <0.1 mm in diam- 6 and 8 cm and occasionally forms irregular con- eter). Unit 1 is absent where cobbles or boulders tacts with Unit 4 (Figure 2c). In the GC terrace form the contact with Units 2 or 3. exposure, Unit 3 reaches a maximum thickness of 60 cm (Figure 2d). Because it appears continuous [8] Unit 2 is a discontinuous gray-brown, fine- along strike, Unit 3 is the fault core lithology grained fault gouge that overlies and forms a sharp sampled and tested for permeability and frictional contact with Unit 1. In the GC scarp exposure, the properties in this study. contact is oriented 059/42°SE with a prominent set of 0/059 striations overprinted by a faint 36/114 set. [10] Unit 4 is a pale green cataclasite that is foliated Unit 2 varies in thickness and color; where present, in places. Unit 4 cataclasite contains fractured, it is always less than 2 cm thick and contains ≥90% sometimes sericitized, feldspars and recrystallized matrix grains. Visible grains include quartz, feldspar, quartz aggregates with undulose extinction and carbonate, opaques and phyllosilicates set in a fine- finely sutured grain boundaries, separated by a grained matrix cemented with cryptocrystalline faint anastamosing foliation defined by fine-grained carbonate and amorphous iron oxide. Rare clasts chlorite, illite-muscovite and calcite (Figure 3a). include subrounded reworked fault gouge, meta- The cataclasite commonly contains altered pseu- morphic quartz and vein quartz. Unweathered Unit dotachylyte and/or ultracataclasite [e.g., Toy et al., 2 gouge has a foliation defined by phyllosilicates, 2011]. with uniform extinction and color viewed through [11] Near the contact with Unit 3, the lower 4–8cm the sensitive tint plate, indicating that this mineral of Unit 4 contains fault plane-parallel layers of aggregate has a crystallographic preferred orienta- finely comminuted incohesive cataclasite up to tion (Figure 2e). Unit 2 weathers rapidly in outcrop, 1 cm thick oriented 059/50°SE with striations becoming indurated, stained with iron oxides, and plunging 44/117; this lithology is referred to as the fractured by numerous empty tension cracks. This Unit 4 foliated cataclasite (Figure 3b). Micro- alteration makes sampling intact wafers impractical. structures in Unit 4 provide evidence of multiple [9] Unit 3 is a gray incohesive fault gouge that deformation mechanisms; each deformation mech- weathers to a brown indurated material; it forms anism would have dominated at different condi- a sharp undulating contact with either Unit 1 or tions of strain rate, temperature and pressure. Unit 4 Unit 2 (Figures 2a–2d). Hand specimens and thin is the hanging wall lithology sampled for perme- sections of Unit 3 display variations in grain size, ability and frictional properties in this study. with some samples containing >30% visible clasts

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Figure 2. Field photographs of Alpine Fault sampling localities. (a) At the Gaunt Creek scarp exposure, the Alpine Fault 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 scoured cataclastic fault rocks at the GC scarp exposure. The continuation of Unit 2 is obscured by an aspect change in Unit 3. Detailed descriptions of Units 1–4 are given in the text. (c) Line drawing on a photograph of an irregular contact between Unit 3 fault gouge and into Unit 4 cataclasite, GC scarp exposure. Here, Unit 2 is truncated by Unit 3. (d) Incohesive fault gouge with rounded black clasts of ultramylonite (Umyl) and angular, crushed clasts of pale green cataclasite (Ccl) exposed in an excavated terrace 360 m NE of the GC scarp exposure illustrated in Figures 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 fluvioglacial gravels is gradational.

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Figure 3. Photomicrographs of cataclastic fault rocks collected from Alpine Fault thrust segments. (a) Unit 4 cataclasite comprising boudinaged, microcracked feldspar (Fsp)-quartz (Qtz) porphyroclasts in a matrix of comminuted quartz-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, sericitized 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 indicated by bold white lines. GC scarp exposure; cross polarized light = XPL. (c) Unit 3 random fabric fault gouge showing angular 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 clast of 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 aligned phyllosilicate-rich layers oriented subparallel to the fault plane. GC scarp exposure; XPL. (f) Unit 3 fault gouge containing clasts of weakly foliated quartz-biotite (Bt)-opaque (Op) mylonite (Myl) and metabasic mylonite with a continuous foliation defined by elongate chlorite (Chl) and epidote (Ep). Waikukupa River; PPL.

[12] For comparison, we sampled Unit 3 fault gouge imbricate thrust [Norris and Cooper, 1997]. Where and Unit 4 cataclasite from a surface exposure of sampled, Waikukupa Thrust Unit 3 is an 8 cm thick, the Waikukupa Thrust located 25 km to the south- reverse-graded fault gouge oriented 055/56°SE; the east of Gaunt Creek (Figure 1). The Waikukupa fault gouge forms a sharp contact with underlying Thrust emplaces Pacific Plate mylonites and cata- footwall gravels with striations plunging 15/066 clasites over Australian Plate fluvioglacial gravels (Figure 2e). Clasts within the Unit 3 fault gouge older than 40,000 years B.P. The Waikukupa include subangular to subrounded unaltered to ret- Thrust was abandoned approximately 20,000 B.P. rogressed quartzofeldspathic and metabasic mylo- when slip was transferred to the Hare Mare nite set in a fine-grained matrix of quartz, feldspar,

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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 Scarp Quartz 9%, Orthoclase 3%, Albite 27%, Calcite 2%, U4 Fol. Cataclasitec 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. albite, chloritized biotite and amphibole, sericitized Multipurpose Diffractometer using Fe-filtered Co feldspar, epidote, titanite, and calcite (Figure 3f). Ka radiation, variable divergence slit, 1° antiscatter Unit 4 is a cataclasite with mylonite clasts that have slit and fast X’Celerator Si strip detector. Quanti- been subject to in situ fragmentation, translation tative analysis was performed using the commercial and rotation in a matrix of comminuted quartz, package SIROQUANT. Smectite identification was feldspar, illite-muscovite, chlorite, calcite and pyrite. done using XRD patterns obtained from oriented slides of Mg-saturated, glycerolated <2 mm clay 3. Analytical Methods separates. Results are listed in Table 1.

[13] For quantitative X-ray diffraction, approximately 1.5 g of each oven-dried sample was ground 4. Experimental Methods for 10 min in a McCrone micronizing mill under ethanol. The resulting slurries were oven-dried at [14] Large blocks of fault core gouges (Unit 3) and 60°C, thoroughly mixed with an agate mortar and adjacent hanging wall cataclasites (Unit 4) were pestle, and lightly back pressed into stainless steel collected from the outcrop using a portable rock – sample holders. XRD patterns were recorded in saw. These blocks were cut into wafers 6 8mm steps of 0.017°2q on a PANalytical X’Pert Pro thick, 54 mm wide, and 61 mm long; the wafer long axis was cut parallel to the fault shear direction

Table 2. Summary of Experiment Details and Resultsa 2 Experiment Lithology sn′ (MPa) Pc (MPa) Pp (MPa) k (m ) ms p2798 Gaunt Creek Scarp U3 Gouge 31 25 10 7.45 E-20 0.44 p2862b Gaunt Creek Scarp U3 Gouge 33 25 10 n/a 0.57 p3151 Gaunt Creek Scarp U4 Fol. Cataclasite 31 25 10 1.41 E-18 0.50 p2799 Gaunt Creek Scarp U4 Cataclasite 30 25 10 1.15 E-16 0.51 p2863 Gaunt Creek Scarp U4 Cataclasite 30 25 10 n/a 0.55 p3370 Gaunt Creek Terrace U3 Gouge 31 25 10 3.10 E-20 0.31 p2800 Waikukupa River U3 Gouge 31 25 10 1.71 E-20 0.39 p2830 Waikukupa River U3 Gouge 31 25 10 1.16 E-20 0.41 p2801 Waikukupa River U4 Cataclasite 30 25 10 1.45 E-17 0.54 p2828 Waikukupa River U4 Cataclasite 30 25 10 1.23 E-16 0.57

a Effective 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.

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Figure 4. (a) Plot of coefficient of friction (m = t/s′n) versus strain for a double-direct shear experiments on GC scarp exposure Unit 4 hanging wall cataclasite (Ccl) and GC terrace exposure Unit 3 fault gouge. After measuring the friction coefficient at steady state sliding, (b) velocity step tests were performed and (c) slide-hold-slide tests were performed. Measurement accuracy is Æ0.002 MPa for load and Æ0.1 mm for displacement. as indicated by striation measurements. A total of sample, resulting in a true triaxial stress state. 10 double-direct shear friction experiments were Pore fluid (University Park, Pennsylvania, USA conducted on Gaunt Creek and Waikukupa River tap water with pH = 7 and total cation concen- fault rocks (Table 2). tration [Ca2+ +Mg2+]=2–4 mmol/L) was plum- bed directly into the fault rock samples and [15] We deformed fault rock samples in a servo- allowed to equilibrate until steady state boundary hydraulic controlled biaxial testing apparatus fitted conditions were reached. We imposed a differential with a pressure vessel [Samuelson et al., 2009; fluid pressure to induce flow normal to the shear Ikari et al., 2011b]. We sheared all samples under direction of the intact wafers. When flow rate saturated, drained conditions at room temperature. reached steady state, permeability was calculated We held effective normal stress (s ′), confining n using Darcy’s law. pressure (Pc), and pore pressure (Pp) constant at 31 MPa, 25 MPa, and 10 MPa, respectively [17] In the double-direct shear configuration, fault (Table 2). These variables are geologically reason- rock samples are sheared by driving the center able if we assume hydrostatic pore fluid pressure, block with a servo controlled vertical piston at a an effective normal stress appropriate for 1 km constant displacement rate to attain steady state depth on an oblique thrust fault striking NW- frictional behavior (Figure 4a). At steady state, the SE, and a stress tensor with s1 = 45 MPa, coefficient of sliding friction m was determined sv = s2 = s3 = 25 MPa. Along the Alpine Fault, the from the ratio of shear stress t to effective normal maximum compressive stress (s1) is subhorizontal stress sn′ assuming cohesion of zero. We then trending 120° and the least principal stress (s3)is performed velocity step tests (Figure 4b) to deter- subhorizontal trending 030°, with some local vari- mine the rate and state dependence of friction. Load ance between sv = s2 and sv = s3 expected for a point velocity was varied between 1 and 100 mm/s, combination of strike-slip and reverse faulting and in some cases 300 mm/s, to measure the friction [Leitner et al., 2001]. rate parameter (a-b)=Dm/D ln V, where a is the direct effect, b is the evolution effect, Dm is the [16] Flow-through permeability tests were con- change in steady state sliding friction, and V is ducted prior to shearing intact wafers of fault rock. the sliding velocity [e.g., Marone, 1998] (Figure 4b). In each experiment, two wafers were placed We also carried out a series of slide-hold-slide tests between a three-piece steel block assembly fitted to measure the frictional healing rate. In these tests, with porous, stainless steel frits and then jacketed the load point was driven at 10 mm/s, held (see Samuelson et al. [2009] for a complete descrip- motionless for a prescribed time, t, between 1 and tion of the experimental apparatus and methods). 300s and then driven at 10 mm/s again. Frictional In the pressure vessel, normal (s ), confining (P ), n c healing Dm is the difference in peak friction, and pore fluid (Pp) pressures were applied to the

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

5. Results

5.1. Permeability

[18] All permeability measurements were made on intact wafers of cataclastic fault rocks at effective normal stresses between 30 and 31 MPa. The fault-perpendicular permeability of Unit 3 fault gouges varies between k = 1.16 Â 10À20 m2 and k = 7.45 Â 10À20 m2 (Table 2); these values are typical for natural and experimental fault gouges [e.g., Faulkner and Rutter, 2000, 2001]. The fault- perpendicular permeability of Unit 4 is almost 3 orders of magnitude greater than Unit 3, increasing from k = 1.41 Â 10À18 m2 in the Gaunt Creek Unit 4 foliated cataclasite to k = 1.15 Â 10À17 m2 in Gaunt Creek Unit 4 cataclasite and k = 1.23 Â 10À16 m2 and k = 1.45 Â 10À17 m2 in Waikukupa Thrust Unit 4 cataclasites. This increase in permeability corresponds with a decreasing abundance of phyllosilicate layers and an overall increase in mean grain size (Figures 2 and 3). These results are consistent with measurements on similar fault rocks collected from the Nojima Fault Zone [Lockner et al., 2000] and Median Tectonic Line in Japan [Wibberley and Shimamoto, 2003].

5.2. Friction

[19] Values of steady state sliding friction coefficients for fault core (Unit 3) and hanging wall cataclasites (Unit 4) range from m = 0.31 to 0.57 (Table 2 and Figure 5a). The Unit 3 fault gouges are the weakest, with m = 0.31 and m = 0.44 for intact wafers of fault gouge collected from the GC terrace and GC scarp exposures, respectively. The friction coefficients of Waikukupa Thrust fault gouge, m = 0.39 and m = 0.41, lie within this range. Hanging wall cataclasites are stronger, with intact wafers of Unit 4 foliated cataclasite having a Figure 5. Summary plots of the frictional properties of m = 0.50. Intact wafers of Gaunt Creek and Alpine Fault cataclastic fault rocks. (a) A plot of coef- Waikukupa Thrust cataclasites have friction coef- m t s ′ ficient of friction ( = / n) against effective normal ficients of m = 0.51–0.55 and m = 0.54–0.57, stress sn′. (b) Friction rate parameter (a-b) is plotted m respectively. The powdered sample of Gaunt Creek against the upstep load point velocity V ( m/s). (c) Fric- m tional healing rates are plotted as the change in the coef- Unit 3 has a higher friction coefficient, = 0.57, ficient of friction (Dm) as a function of hold time (t). than its wafer equivalent, which is consistent with Symbols given in the legend of Figure 5a are valid for recent experiments showing that phyllosilicate all plots. Abbreviations are foliated cataclasite (FCcl), rich rocks are generally stronger in powder form cataclasite (Ccl), powder (pwdr), GC terrace exposure [Collettini et al., 2009; Ikari et al., 2011b]. (Tce), and GC scarp exposure (Scarp).

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5.3. Friction Rate Dependence experiments, data on GC scarp exposure Unit 3 are from gouge powder, which healed at a faster rate [20] We performed velocity step tests in six than intact wafers of all lithologies (Figure 5c). experiments to determine rate/state friction constitutive parameters (Figure 4b). This parameteriza- [24] Intact wafers of GC terrace exposure Unit 3 tion is useful for inferring whether shear will be and Unit 4 foliated cataclasite weakened with hold stable or unstable under conditions of tectonic time for all hold times between t = 3s and t = 100s faulting [e.g., Marone, 1998]. The friction rate (Figures 4c and 5c). The gouge layers have very parameter, a-b, describes the variation in steady low permeabilities, and consolidation during hold sliding friction with slip velocity. Materials are time may have resulted in elevated pore fluid said to be velocity strengthening if a-b > 0; these pressures and thus lower friction coefficients [e.g., materials tend to slide stably, accommodating Byerlee, 1993; Faulkner and Rutter, 2001]. How- strain aseismically. Materials exhibiting negative ever, we observe no systematic correlation between a-b values are termed velocity weakening; these hold time and frictional healing across the range materials may undergo a frictional instability that of hold times imposed. The results of the GC ter- results in earthquake nucleation and seismic rup- race exposure Unit 3 experiment are discussed ture propagation [Scholz, 2002]. further below.

[21] Results from velocity step tests are summa- [25] GC scarp exposure Unit 4 nonfoliated catacla- rized in Figure 5b. Jacket failure precluded mea- site, Waikukupa Thrust Unit 3 fault gouge, and suring the friction rate parameters and frictional Waikukupa Thrust Unit 4 cataclasite all showed a healing rates of intact wafers of Unit 3 from the positive correlation between hold time and Dm GC scarp exposure; a powdered sample of Unit 3 (Figure 5c). This behavior is consistent with from that locality exhibited velocity-strengthening mechanistic interpretations of healing in rocks com- behavior (a-b = 0.00012–0.0016). Unit 3 fault posed of framework silicates, where the real area gouges collected from the GC terrace exposure of surface contact increases with time because of and Waikukupa Thrust displayed clear velocity- contact junction growth and/or strengthening strengthening behavior (a-b =0.0052–0.018) [Dieterich, 1978; Marone, 1998]. (Figures 4b and 5b). Gaunt Creek Unit 4 foliated cataclasite also exhibited velocity-strengthening 6. Discussion behavior (a-b = 0.0019–0.016). These units also show strong positive a-b rate dependence, a behav- [26] Along thrust segments at Gaunt Creek and ior observed in other clay gouges [Ikari et al., Waikukupa River, the east dipping Alpine Fault 2011a]. plane is overlain by 10–50 m of cataclasite comprising clasts of quartzofeldspathic and metabasic [22] Intact wafers of Unit 4 cataclasites a range of a-b values, depending on the magnitude of the mylonites in a finely comminuted chloritized velocity perturbation. Values of a-b for the cata- matrix. Near the striated basal contact with footwall clasites range between –0.0016 and 0.00082. We fluvioglacial gravels, strain localization in the observe no systematic relationship between the hanging wall has resulted in further grain size size of the velocity perturbation and the value of reduction and fault gouge formation. In places, a-b in the cataclasites (Figure 5b). Other fric- mechanically aligned phyllosilicates form a planar tionally strong gouges (m ≥ 0.5) commonly fabric aligned subparallel to the fault plane (e.g., exhibit both velocity-strengthening and velocity- Figure 3e). weakening behavior in velocity step experiments [27] Quantitative X-ray diffraction analyses indi- [Ikari et al., 2011a]. cate that strain localization and grain size reduction was accompanied by changes in the nature and 5.4. Frictional Healing abundance of phyllosilicate phases at the Gaunt Creek localities (Table 1). At the GC scarp expo- [23] Frictional strength varies with the real area of sure, fault gouges comprising Units 2 and 3 contain contact. Under stationary conditions, asperity con- – tacts can heal with time, increasing the likelihood a high percentage of clay minerals (57 60%) rela- of unstable frictional sliding in some materials tive to Unit 4 cataclasite (20%). Unit 4 cataclasite [Marone, 1998; Scholz, 2002]. Slide-hold-slide tests contains higher temperature clay minerals chlorite were performed to measure the rate of frictional (clinochlore-1MIIb) and muscovite (2M1). Unit 3, healing (Figure 4c). As in the velocity stepping however, contains the lower temperature clay minerals smectite (montmorillonite) and kaolinite

9of13 Geochemistry Geophysics 3 BOULTON ET AL.: ALPINE FAULT PROPERTIES 10.1029/2011GC003872 Geosystems G as well as chlorite (clinochlore-1MIIb) [e.g., Moore fault gouge and Waikukupa Thrust Unit 4 catacla- and Reynolds, 1997]. Based on mineralogy, site all contain the primary mineral phases quartz, microstructures and grain size, the Unit 4 foliated albite Æ orthoclase, calcite, illite-muscovite (2M1), cataclasite contains components of both the cata- and chlorite (clinochlore-1MIIB). These minerals clasite and fault gouge (Table 1) (Figure 3b). are likely to remain stable to subgreenschist con- Basal Unit 2 gouges contain predominantly low- ditions (<320°C) near the base of the brittle seis- temperature phyllosilicates, with the exception of mogenic zone [Warr and Cox, 2001]. Using a lizardite (1M) at the GC scarp exposure (Table 1). geothermal gradient of 40°C, this equates to a depth The origin of lizardite (1M) is unknown; it is of 8 km on the Alpine Fault [Toy et al., 2010]. At possible this mineral was derived from footwall effective normal stresses >60 MPa, however, clay- gravels or a protolith present downdip or along rich gouge behavior becomes increasingly nonlin- strike. ear, and pressure solution creep may compete with friction to accommodate grain sliding in clay-rich [28] Hand specimen and thin section identification gouges containing a soluble mineral phase such as of relict clasts in the fault gouges and cataclasites, quartz or calcite [e.g., Rutter, 1983; Bos and Spiers, along with X-ray diffraction analysis, indicate that 2002; Niemeijer and Spiers, 2007; Gratier et al., comminution of hanging wall mylonites and pro- 2011]. Whether deformation is accommodated in tocataclasites formed the granulated cataclasites the gouges via frictional sliding and cataclasis or and gouges analyzed in this study. By an analysis pressure solution creep depends on many factors, of garnet fragments in cataclasites, Cooper and foremost temperature and magnitude of differential Norris [1994] reached the same conclusion, but stress resolved on the Alpine Fault [Rutter, 1976; they could not discount a contribution from foot- Cox and Etheridge, 1989; Gratier et al., 2009]. wall granitoids at depth. Most mylonite hanging wall rocks are quartzofeldspathic, composed of the [31] Our experiments were conducted at room primary minerals: quartz, plagioclase, muscovite, temperature on saturated fault gouges loaded at and biotite with minor amounts of garnet, ilmenite, effective normal stresses 30 MPa and loading and calcite. Metabasic mylonites contain amphi- rates between 1 and 300 mm/s. At these conditions, bole, as reflected in the mineralogy of the Waiku- clay-rich Unit 3 fault gouges are frictionally weak, kupa Thrust samples [Toy et al., 2010]. exhibit velocity-strengthening frictional behavior, and do not undergo frictional healing. The weak [29] In addition to primary minerals, GC exposure (m = 0.31), velocity-strengthening (a-b = 0.0075– fault gouges and foliated cataclasite fault gouges 0.016), nonhealing (Dm = –0.003 after t = 100 s) contain kaolinite and the weak dioctahedral clay properties of the GC terrace exposure Unit 3 gouge mineral smectite (Table 1) (m = 0.15–0.32) [Saffer (illite-muscovite 32%; chlorite 9%) best repre- and Marone, 2003]. Warr and Cox [2001] found sents the frictional behavior of clay-rich fault that smectite forms from low-temperature alteration gouges at depth. in Alpine Fault mylonite-derived cataclasites and gouges. Smectite can form from low-temperature [32] Figure 4a illustrates that the GC terrace expo- alteration (<120°C) of the primary minerals illite/ sure Unit 3 fault gouge did not display peak fric- muscovite and chlorite. In our samples, smectite tion. Experimental observations indicate that aligned occurrence coincides with a depletion in chlorite layers of chlorite (clinochlore-1MIIb) and illite- relative to adjacent hanging wall cataclasites. Oxi- muscovite (2M1) facilitated strain accommodation dized alteration of chlorite forms smectite, which during the early stages of shearing, suppressing further alters into kaolinite and amorphous iron peak strength [Saffer and Marone, 2003]. At higher oxide (goethite) [e.g., Craw, 1984; Chamberlain strains during the slide-hold-slid tests, little or no et al., 1999]; these minerals are all present in contact strengthening occurred in these aligned clay Units 2, 3 and 4 on the GC scarp exposure. Warr grains, and we observed no frictional healing with and Cox [2001] suggested that the growth of hold time [e.g., Bos and Spiers, 2000]. smectite leads to mechanical weakening in Alpine [33] An additional factor influencing peak strength Fault gouges, but its association with a secondary and frictional healing is dilation. Dilation is a cement strengthens the GC scarp exposure fault physical response to shear-induced strain localiza- gouge (m = 0.44) relative to the GC terrace expo- tion observed in many granular materials. By sure fault gouge (m = 0.31). increasing the thickness of the shearing granular [30] GC terrace exposure Unit 3, GC scarp expo- layer, dilation increases effective normal stress, sure Unit 4 cataclasite, Waikukupa Thrust Unit 3 decreases pore fluid pressures, and suppresses

10 of 13 Geochemistry Geophysics 3 BOULTON ET AL.: ALPINE FAULT PROPERTIES 10.1029/2011GC003872 Geosystems G frictional instabilities. The lack of frictional healing through velocity-strengthening material, including in the GC terrace exposure fault gouge suggests elastically radiated energy, fracture energy, and that aligned clay layers do not exhibit classical poromechanical effects [Rice, 1993; Lapusta et al., healing behavior governed by a combination of 2000; Perfettini and Ampuero, 2008]. frictional strengthening during the hold and dilan- [37] The inclusion of reworked clasts of fault gouge tancy strengthening upon reshear. These units have and hanging wall cataclasites in Unit 3 fault gouges very low permeabilities, and time-dependent com- indicates that large magnitude surface ruptures on paction may result in high pore fluid pressures and the Alpine Fault have occurred repeatedly within undrained loading [Lockner and Byerlee, 1994; the same fault core. Local pore fluid overpressures Garagash and Rudnicki, 2003; Faulkner et al., are indicated by the irregular contacts between 2011; Schmitt et al., 2011]. Unit 3 fault gouge and Unit 4 cataclasite at the GC [34] Unit 3 fault gouges also exhibit velocity- scarp exposure (Figure 2c). Questions remain, strengthening behavior, a frictional property that however, about whether these irregular contacts promotes stable (aseismic) sliding, inhibits earth- formed interseismically by creep, compaction, and quake rupture nucleation, and may contribute to pore fluid pressurization in the impermeable, clay- earthquake rupture arrest at shallow depths (≲3 km) rich gouge or coseismically as gouge injection [Marone, 1998]. However, paleoseismological, structures formed by shear-induced thermal pres- geomorphological and geodetic evidence suggests surization [e.g., Cowan, 1999; Meneghini et al., that the Alpine Fault fails coseismically in large 2010; Lin, 2011]. earthquakes, forming surface ruptures with an average slip per event of 8 m dextrally and 1m 7. Conclusions vertically [Sutherland et al., 2007]. [38] Oblique thrust fault segments of the Alpine [35] Along the Alpine Fault, the upper stability transition, which defines the updip limit of the seis- Fault form surface outcrops at Gaunt Creek and mogenic zone, is either too shallow to arrest seis- Waikukupa River. Samples of cataclastic fault rocks mic rupture propagating from below, or the upper, were collected and investigated using a double- stable region is not sufficiently velocity strength- direct shear configuration under controlled pore eningtoarrestrupture[Marone, 1998]. Alternatively, pressure. At effective normal stresses between 30 theoretical considerations, numerical modeling and and 31 MPa, fault core gouges have fault normal high-velocity rock friction experiments show that permeabilities up to 3 orders of magnitude lower rapid undrained loading in low-permeability fault than hanging wall cataclasites (Table 2). Fault gouge results in thermal pressurization of pore core gouges are velocity strengthening and have m – fluids at slip rates greater than 0.1 m/s and dis- lower friction coefficients ( =0.310.44) than placements on the order of a few microns [e.g., hanging wall cataclasites. Cataclasites are friction- m – Sibson, 1973; Lachenbruch, 1980; Rice, 2006]. ally strong ( =0.50 0.57), exhibit normal to high rates of frictional healing, and display both velocity- [36] Sufficient shear heating can induce thermal strengthening and velocity-weakening behaviors. pressurization, a weakening mechanism that oper- These results support general models of mature ates before seismic slip speeds are obtained fault cores as weak with respect to the surrounding [Schmitt et al., 2011]. Faulkner et al. [2011] pre- crust [e.g., Rice and Cocco, 2007; Carpenter et al., sented experimental data on saturated clay-rich 2011]. gouges sheared in a high-velocity rotary apparatus at low normal stress (1.63 MPa) and high slip [39] Current geodetic estimates indicate that the – velocity (1.3 m/s). They attributed the immediate Alpine Fault is locked to a depth of 13 18 km, and frictional weakening to rapid thermal pressurization total elastic strain accumulation is released in large – of the pore fluid, which resulted in a negligible magnitude (Mw 7.5 8) earthquakes that propagate critical slip weakening distance and fracture energy. along-strike distances between 300 and 600 km A rapid loss of strength at the tip of an earthquake [Beavan et al., 2010; Sutherland et al., 2007]. Pet- rupture may make propagation through clay rich rological observations of clasts of reworked fault gouges energetically favorable [e.g., Tinti et al., gouge in fault core rocks sampled in this study 2005; Bizzarri and Cocco, 2006a, 2006b]. As slip reveal that earthquake ruptures preferentially prop- speeds grow to within a few orders of magnitude of agated through velocity-strengthening fault gouges elastic wave speeds, however, additional factors exposed in surface outcrops at Gaunt Creek and contribute to the physics of earthquake propagation Waikukupa River. Understanding the mechanisms

11 of 13 Geochemistry Geophysics 3 BOULTON ET AL.: ALPINE FAULT PROPERTIES 10.1029/2011GC003872 Geosystems G facilitating earthquake rupture propagation through Cowan, D. S. (1999), Do faults preserve a record of seismic clay-rich gouges remains an important goal in rock slip? A field geologist’s opinion, J. Struct. Geol., 21, 995–1001, mechanics and earthquake physics. doi:10.1016/S0191-8141(99)00046-2. Cox, S., and M. Etheridge (1989), Coupled grain-scale dilatancy and mass transfer during deformation at high fluid pres- Acknowledgments sures: Examples from Mount Lyell, Tasmania, J. Struct. Geol., 11, 147–162, doi:10.1016/0191-8141(89)90040-0. Craw, D. (1984), Ferrous-iron-bearing vermiculite-smectite [40] The first author received support for this research from series formed during alteration of chlorite to kaolinite, Otago the Claude McCarthy Foundation and the University of Canter- Schist, New Zealand, Clay Miner., 19, 509–520, doi:10.1180/ bury Roper Scholarship. We would like to thank to Sam Haines claymin.1984.019.4.01. for doing preliminary X-ray diffraction analyses and Mark Raven Dieterich, J. H. (1978), Time-dependent friction and the for doing more detailed, quantitative analyses. Greg De Pascale is mechanics of stick-slip, Pure Appl. Geophys., 116,790–806, credited with discovering the Alpine Fault terrace exposure at doi:10.1007/BF00876539. Gaunt Creek. The laboratory work was supported by NSF grants Faulkner, D. R., and E. H. Rutter (2000), Comparisons of water OCE-0648331, EAR-0746192, and EAR-0950517 to C. Marone and argon permeability in natural clay-bearing fault gouge under high pressure at 20°C, J. Geophys. Res., 105, 16415– and Marsden grant UOO0919 to V. Toy. Reviews by Dan 16426, doi:10.1029/2000JB900134. Faulkner, an anonymous reviewer, and Editor Thorsten Becker Faulkner, D. R., and E. H. Rutter (2001), Can the mainte- greatly improved the quality of this manuscript. nance of overpressured fluids in large strike-slip fault zones explain their apparent weakness?, Geology, 29(6), 503–506, doi:10.1130/0091-7613(2001)029<0503:CTMOOF>2.0.CO;2. References Faulkner, D. R., T. M. Mitchell, J. Behnsen, T. Hirose, and T. Shimamoto (2011), Stuck in the mud? Earthquake nucle- Beavan, J., P. Denys, M. Denham, B. Hager, T. Herring, and ation and propagation through accretionary forearcs, Geophys. P. Molnar (2010), Distribution of present-day vertical defor- Res. Lett., 38, L18303, doi:10.1029/2011GL048552. mation across the Southern Alps, New Zealand, from 10 years Garagash, D. I., and J. W. Rudnicki (2003), Shear heating of a of GPS data, Geophys. Res. Lett., 37, L16305, doi:10.1029/ fluid-saturated slip-weakening dilatant fault zone: 1. Limiting 2010GL044165. regimes, J. Geophys. Res., 108(B2), 2121, doi:10.1029/ Bizzarri, A., and M. 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