Paleoseismology of the Mount Narryer fault zone, Western Australia

Geological Society of America Bulletin, v. 1XX, no. XX/XX 1

Paleoseismology of the Mount Narryer fault zone, Western Australia: A multistrand intraplate fault system

Beau B. Whitney1,2,†, Dan Clark3, James V. Hengesh2, and Paul Bierman4

1Centre for Energy Geoscience, University of Western Australia M005, 35 Stirling Highway, Crawley, Perth, WA 6009, Australia2 Centre for Offshore Foundation Systems, University of Western Australia M053, 35 Stirling Highway, Crawley, Perth, WA 6009, Australia

3Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia4Department of Geology, University of Vermont, Burlington, Vermont 05405, USA

ABSTRACT

Our paleoseismological study of faults and fault-related folds comprising the Mount Narryer fault zone reveals a mid- to late Qua-ternary history of repeated morphogenic earthquakes that have influenced the plan-form and course of the Murchison, Roderick, and Sanford Rivers, Western Australia. The dominant style of deformation involves fold-ing of near-surface sediments overlying dis-crete basement faults. Carbon-14, optically stimulated luminescence, and in situ–pro-duced 10Be constrain the timing of the events and late Quaternary slip rates associated with fault propagation folds in tectonically uplifted and deformed alluvial channel deposits. A flight of five inset fluvial terraces is preserved where the Murchison River flows across the Roderick River fault. These terraces, which we infer to be coseismic, are consistent with at least four late Quaternary seismic events on the order of moment magnitude (Mw) 7.1 within the last ~240 k.y. Secondary shears ex-pressed on the folds indicate a component of dextral strike-slip displacement. Quaternary slip rates on the underlying faults range from 0.01 to 0.07 mm yr–1, with a total slip rate for the zone between 0.04 and 0.11 mm yr–1. These rates are intermediate to those in the adjacent Mesozoic basin (>0.1 mm yr–1) and Precam-brian craton (<0.005 mm yr–1) and so provide insight into how tectonic strain is partitioned and transferred across a craton margin.

INTRODUCTION

Intraplate earthquakes in continental interiors are rare compared with interplate settings, and only a few paleoseismological investigations

have been completed in Australia to document fault behavior and styles of deformation (e.g., Crone et al., 1992, 1997, 2003, 2009; Machette et al., 1993; Crone and Machette, 1994; Clark and McCue, 2003; Quigley et al., 2010; Clark et al., 2014a). The current seismotectonic mod­els describing earthquake activity within stable continental regions are based predominately on data from the stable continental regions of eastern North and South America and Aus­tralia (Talwani, 2014). Though sparse, these data broadly reveal a consistent pattern of mor­phogenic earthquake occurrence, where faults rupture episodically, with clusters of morpho­genic events separated by tens of thousands to hundreds of thousands of years of quiescence (Crone et al., 2003; Clark et al., 2012, 2014b). Moreover, there is a tendency for preexisting (e.g., Proterozoic to Mesozoic) basement struc­tures to become reactivated under neotectonic stress (Clark and Leonard, 2003; Dentith et al., 2009) rather than the formation of new struc­tures. Extended crust tends to accommodate more neotectonic strain than the non extended Precambrian shields and Phanerozoic accre­tionary terranes (Schulte and Mooney, 2005; Bezerra et al., 2011; Clark et al., 2012; Bar­tholomew and Van Arsdale, 2012; Talwani, 2014). The mechanisms by which strain is partitioned between intraplate tectonic environ­ments (i.e., extended crust and cratonic crust) are almost completely unexplored.

This study examines the Mount Narryer fault zone on the northwest margin of the Yilgarn cra­ton in Western Australia (Fig. 1) and presents data on the tectonic geomorphology and paleo­seismic history of the fault zone. Ages of tec­tonically deformed sediments were determined using carbon­14, optically stimulated lumines­cence (OSL), and measurements of in situ–produced 10Be. The Mount Narryer fault zone straddles the boundary between the Mesozoic extended crust of the Carnarvon Basin and the

Archean crust of the Yilgarn craton. As such, the Mount Narryer fault zone is an ideal candidate with which to study how strain is partitioned between continental margin structural domains and cratonic interiors.

REGIONAL GEOLOGY AND TECTONICS

The Mount Narryer fault zone is an ~120­km­long, 15­km­wide, N10°E­trending system of faults. There are five individual fault strands that range in length from 11 to 68 km. From north to south, the fault strands are named Mount Narryer West, Mount Narryer East, Roderick River, Sanford River West, and Sanford River East (Fig. 2).

The Mount Narryer fault zone is located within the Narryer terrane, at the northwest corner of the Archean Yilgarn craton (Fig. 1). The Narryer terrane consists of a hetero­geneous series of north­northeast–trending metasedimentary and metaplutonic belts that amalgamated and became part of the Yilgarn craton ca. 2.5 Ga (Williams and Myers, 1987; Myers, 1995a). The Mount Narryer fault zone forms an active subset of an anastomosing net­work of Precambrian basement faults mapped primarily on the basis of aeromagnetic data (Myers, 1995a, 1995b; Sheppard et al., 2004). The fault zone occupies a portion of the tran­sition between the Yilgarn cratonic terranes to the east and the Carnarvon sedimentary basin to the west.

Reactivation of Gondwana­era rift­related structures is well documented in the Phanero­zoic terranes to the west of the Narryer ter­rane (cf. Clark et al., 2011, 2012; Whitney and Hengesh, 2015a, 2015b). The most recent struc­tural reactivation is attributed to the change in intraplate stress that accompanied the reorgani­zation of the Australian plate boundaries that ini­tiated during the late Neogene (Coblentz et al.,

GSA Bulletin; Month/Month 2015; v. 1xx; no. X/X; p. 1–21; doi: 10.1130/B31313.1; 14 figures; 4 tables.; published online XX Month 2015.

†beau .whitney@ uwa .edu .au; bbwhitney@ gmail .com

For permission to copy, contact editing@geosociety.org © 2015 Geological Society of America

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1998; Hillis and Reynolds, 2000; Densley et al., 2000; Kaiko and Tait, 2001; Audley­Charles, 2004, 2011; Cathro and Karner, 2006; Keep et al., 2007; Hall, 2011; Hengesh et al., 2011; Clark et al., 2011; Hengesh and Whitney, 2014). While evidence for Quaternary surface rupture is reported on the Mount Narryer East and West faults within the Narryer terrane (e.g., Williams, 1979; Keep et al., 2012), little is known of the seismo­tectonic context and rupture history of the faults.

Historical seismicity in the Narryer terrane is higher than in the interior of the craton (cf. Everingham et al., 1982; Clark et al., 2012) and includes the 1959 Mw 5.9 and 1941 ML 7.2 Mee­berrie events. Focal mechanisms derived from seismicity data indicate the faults in the region are predominately accommodating an oblique­reverse (transpressional) sense of motion (Revets et al., 2009; Keep et al., 2012).

REGIONAL GEOMORPHOLOGY AND CLIMATE

The region is characterized by a low­relief landscape. Low­gradient alluvial valleys and sheetwash plains are occasionally interrupted by granitic inselbergs or remnant spines of meta­morphic ranges. Rivers are ephemeral and have narrow incised channels (tens of meters wide) inset within broad (kilometers wide) braid plains (alluvial channel belts). A prominent knickpoint at the Kalbarri Gorge, a few hundred

kilometers downstream from the study region, has effectively disconnected the upper Murchi­son catchment from effects of changes in base level related to Quaternary sea­level fluctuations (Fig. 1; English et al., 2012).

Scarps and vegetation lineaments associated with the faults in the Mount Narryer fault zone are evident in the landscape. The fault scarps are east facing, linear to slightly arcuate, and are clearly visible on aerial photographs. The scarps have influenced the fluvial processes of the three major regional rivers (Sanford, Roder­ick, and Murchison Rivers). They have locally diverted and captured surface water and formed ephemeral ponds, warped and uplifted alluvial surfaces, formed fluvial terraces, affected chan­nel planform and gradient, and impounded Lake Wooleen. The long­lived rivers in the region are characteristically graded (Mackin, 1948) along most of their reaches, except where their channels are affected by faults; we observed no terraces preserved along the rivers away from the fault crossings. Where not affected by faults, channel belts are weakly incised, and the regional stream gradient is consistently 1:1200 (Jutson, 1950). The channel responses proximal to faults are similar to, and locally more evolved than, the ephemeral stream responses to grow­ing folds in the Carnarvon Basin northwest of the Mount Narryer fault zone (Whitney and Hengesh, 2015a).

The Mount Narryer East and West lineaments were previously identified by Williams (1979)

as eroded fault scarps. Clark (2004) identified an ~1­km­long section where the eastern fault is associated with a 1.5–2.2­m­high east­facing scarp. Williams (1979) observed that the pro­nounced vegetation lineaments were defined by Acacia aneura (Mulga) trees. He dated selected trees growing along the lineaments using dendrochronology and suggested that the lineaments might have been related to an orphan earthquake in 1885 that was felt in the town of Geraldton, 300 km to the southwest. Until this study, no subsurface data have been gathered to confirm if the vegetation lineaments are related to tectonic structures, or to determine if the ages of the trees are related to a morphogenic earthquake.

The region is arid with seasonal and incon­sistent rainfall. River flow regimes are highly variable. Average annual rainfall in the region is 225 mm yr–1 (Law, 1992), but the maximum recorded daily rainfall is 160 mm (Murgoo Sta­tion, 007064: http:// www .bom .gov .au /climate /data /index .shtml). The channel beds are scoured ferricrete surfaces that were exposed as a result of these rapid, high­discharge flows and episodic flood events.

Three principal regolith units exist in the study area: bedrock and residual soils that are weathered in place (saprolite); weathered allu­vium in which ferricrete or duricrust horizons have developed; and Holocene­age unconsoli­dated mobile alluvium. Weathered and mobile alluvia are genetically similar. Their parent material is derived from granitoid and green­schist bedrock that underlie the drainage basins (Myers, 1995b). The alluvia consist dominantly of quartz sands and fine gravels transported and deposited in river channels and on adjacent alluvial plains. The weathered alluvium is indu­rated. It is traditionally mapped as Tertiary in age (Williams et al., 1983), but it is demonstra­bly still forming. In proximity to river channels, a deep weathering profile has developed within the alluvium, forming a fine­grained ferruginous conglomerate (ferricrete) known throughout the region as “coffee rock.” It is a deep red color and forms platy slabs or bricks a few centimeters to several decimeters thick.

Ferricrete development is regionally exten­sive within low­lying alluvial units beneath sheetwash surfaces. The ferricrete and duricrust units discussed in this paper are mapped as partly dissected hardpan and are found along all the main drainage courses (e.g., Williams et al., 1983). The Murchison cement, also found along old drainage channels, is included in this unit and is interpreted as a groundwater­precipitated carbonate­cemented alluvium (Williams et al., 1983). Locally, the Murchison cement contains aboriginal artifacts and fossils of extinct Holo­

Perth

Phanerozoic / Precambrian boundary

SYMBOLS

Phanerozoic / Oceanic boundary

Meeberri e-Darling fault system

25°0

′0″S

120°0′0″E

PILBARACRATON

YILGARNCRATON

PerthBasin

Carn

arvo

n Ba

sin

Study Area(Figure 2)

MNfz

Proterozoic Basins

NarryerTerrane

Indi

an O

cean

WESTERNAUSTRALIA

WES

TER

N

AUS

TRALIA

SHEAR

ZONE

Knickpoint

Km250

Figure 1. Regional geologic map of Western Australia showing major tectonic elements in the vicinity of the Mount Narryer fault zone (MNfz).

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Paleoseismology of the Mount Narryer fault zone, Western Australia

Geological Society of America Bulletin, v. 1XX, no. XX/XX 3

1

2

3

4

Lake Wooleen

Roderick River

Sanford River

Murchison River –27°S

116.25°EKilometers

0 20

N

S

W E5

116°E

–27.5°S

Mur

chiso

n Ri

ver

Fig. 6

Mt. Narryer West fault

Mt. Narryer East fault

Roderick River fault

Sanford River West fault

Sanford River East fault

1

2

3

4

5

Key

WS

MGS

M

MNS

MBS

MNS Mt. Narryer stationMBS Meeberrie stationWS Wooleen stationMGS Murgoo stationM Murchison

1941 Meeberrie epicenter with 50 km radius

Mt. Narryer

Trench location

MNw

Rn

Rm

Rs

Sn

Paleo-Roderick Riverchannels

Figure 2. Image of the Mount Narryer fault zone showing faults, trench sites, stations (pastoral houses), and the 1941 Meeberrie earthquake epicenter. The epicentral uncertainty of ~50 km is shown with the radius. The trench sites are: MNw—Mount Narryer West; Rn—Roderick north; Rm—Roderick middle; Rs—Roderick south; Sn—Sanford north.

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cene marsupials (Merrilees, 1968; Murszewski, 2013), indicating a rapid rate of cementation during latest Quaternary time.

Away from river channels, the alluvium is less weathered and less indurated, and a thin (<2 m) ferruginous duricrust capstone is com­mon. The duricrust is a weakly developed ferri­crete, which is soft when saturated but hardens when dry and exposed (Twidale, 1982; Anand and Paine, 2002). In map view, the ferri­crete and duricrust surfaces are polygonally weathered.

Unconsolidated alluvium unconformably overlies the duricrust and ferricrete units. It is predominately mobilized by river flooding and comprises channel bed sands and large sand bars adjacent to river channels. Adjacent to channel belts, sheetwash plains cover low­gra­dient slopes. The sheetwash plains are covered in megaripples with a distinct waveform pattern visible on air photos and satellite imagery. The ripples are alternating fine­ and coarse­grained sheetwash bed­form deposits. These features are referred to as “wanderrie” (Mabbutt, 1963) and have similar form and genesis to relict sheetflood bed forms observed in the arid southwestern United States (Wells and Dohrenwend, 1985).

The mobile alluvium consists predomi­nately of sands and silts with occasional fine gravels. Other mobile alluvium includes beach berms around Lake Wooleen. The bed of Lake Wooleen is an aggrading depocenter for fine­grained lacustrine sediment. Eolian dunes were not observed in the region.

METHODS

We initially conducted aerial and field recon­naissance in the Mount Narryer region to inspect lineaments and geomorphic features that had been identified using Google Earth®, and 30 m bin Shuttle Radar Topography Mis­sion elevation data (Landgate, 2010). Following this initial field verification, we used 1:50,000 scale black and white stereographic pairs of aerial photographs to map lineaments and evi­dence of neotectonic deformation. A number of the lineaments had been visited during a prior reconnaissance investigation conducted in 2003 (Clark, 2004). We used this work and the Wil­liams (1979) reconnaissance report as guides during our subsequent field investigations.

Based on the results of the aerial recon­naissance, field inspections, and desktop stud­ies, we identified a number of potential trench locations to investigate the paleoseismic his­tory of the Mount Narryer fault zone. A 22 ton track­mounted excavator was used to excavate trenches across fault scarps at sites that were least altered by erosion and/or grazing, and that were determined to have the greatest potential to preserve deposits useful in constraining recent fault movements. Five trenches were exca­vated across scarps associated with three faults, including the Mount Narryer West fault (n = 1), the Roderick River fault (n = 3), and the San­ford River East fault (n = 1; Fig. 2). Trench walls were cleaned and logged at a scale of 1:20 using manual paleoseismological techniques (McCal­

pin, 2009). Tectonic folds were reconstructed per Ragan (2009) and using StructureSolver™ to determine horizontal shortening and vertical offset across the structures.

OSL samples were collected by driving 40­mm­diameter brass tubes into cleaned sec­tions of trench walls. Bag samples were col­lected from around the OSL sample annulus to determine dose rate, mineralogy, and particle size. Thirteen samples were collected and sub­mitted for OSL analysis. Samples were col­lected from lacustrine and sheetwash silt and sand units and colluvial sand and gravel units exposed in the trench walls (Table 1). Samples were preprocessed at the University of Illi­nois at Chicago luminescence dating research labora tory for U, Th, and K measurements. The analytical procedures employed in sampling, processing, and analyzing the sedimentary materials for OSL were as detailed in Forman et al. (2014). U, Th, and K content was deter­mined by inductively coupled plasma–mass spectrometry analyses on the prepared samples by Activation Laboratory, Ltd. We collected one sample of freshwater snail shells for radio­carbon analysis from the Roderick middle trench (Fig. 2) wall. The sample was processed at Beta Analytic, Inc., by accelerator mass spec­trometry (AMS) methods (Table 2).

Three surface samples were collected from exposures of ferricrete for analysis of in situ–produced 10Be. Global positioning system coor­dinates and elevations were recorded at each sample location (Table 3). Topographic shield­

TABLE 1. OPTICALLY STIMULATED LUMINESCENCE (OSL) DATA

Sample Depth

Equivalent dose rate

(Gy)*

Over-dispersion

(%)†U

(ppm)§Th

(ppm)§K

(%)§H2O(%)

Cosmic dose(mGy/yr)#

Dose rate(mGy/yr)

OSL age(yr)

Mount Narryer West siteMNW 1 0.46 34.57 ± 1.95 23 ± 3 2.6 ± 0.1 16.8 ± 0.1 1.56 ± 0.01 5 ± 2 0.21 ± 0.02 3.38 ± 0.21 10,220 ± 785MNW 2 0.9 52.50 ± 4.13 38 ± 5 1.9 ± 0.1 20.1 ± 0.1 1.59 ± 0.01 5 ± 2 0.20 ± 0.02 3.38 ± 0.21 15,515 ± 1475

Roderick Middle SiteRM 1 0.6 17.74 ± 0.98 23 ± 3 1.8 ± 0.1 12.5 ±0.1 1.89 ± 0.02 5 ± 2 0.21 ± 0.02 3.23 ± 0.19 5490 ± 430RM 2 1 35.97 ± 2.38 25 ± 3 1.9 ± 0.1 12.6 ± 0.1 2.07 ± 0.02 5 ± 2 0.20 ± 0.02 3.36 ± 0.21 10,700 ± 925RM 3 1.5 35.97 ± 2.38 30 ± 4 3.0 ± 0.1 11.1 ± 0.1 1.80 ± 0.02 5 ± 2 0.19 ± 0.02 3.26 ± 0.19 24,630± 2120RM 4 1 131.38 ± 5.75 16 ± 2 2.4 ± 0.1 9.7 ± 0.1 1.68 ± 0.01 5 ± 2 0.20 ± 0.02 2.92 ± 0.15 44,940 ± 3120RM 5 1 156.36 ± 8.22 20 ± 3 5.2 ± 0.1 15.4 ± 0.1 1.92 ± 0.02 5 ± 2 0.20 ± 0.02 5.42 ± 0.35 28,840 ± 2120RM 6 1.3 92.41 ± 6.20 29 ± 4 7.7 ± 0.1 16.0 ± 0.1 1.26 ± 0.01 5 ± 2 0.19 ± 0.02 4.05 ± 0.26 22,785 ± 1920

Roderick South siteRSØ1 0.5 51.61 ± 3.00 26 ± 3 5.1 ± 0.1 31.8 ± 0.1 2.43 ± 0.02 5 ± 2 0.21 ± 0.02 5.80 ± 0.38 8900 ± 690

Lake Wooleen siteWLØ1 1.57 61.53 ± 2.45 12 ± 2 15.8 ± 0.1 38.8 ± 0.1 2.55 ± 0.02 25 ± 2 0.19 ± 0.02 7.21 ± 0.47 8355 ± 655

Sanford North siteSNS 1 0.42 6.74 ± 0.42 28 ± 4 2.3 ± 0.1 16.0 ± 0.1 1.53 ±0.01 5 ± 2 0.21 ± 0.02 3.23 ± 0.19 2085 ± 170SNS 2 1.15 12.58 ± 0.66 22 ± 3 2.4 ± 0.1 16.5 ± 0.1 1.53 ± 0.01 5 ± 2 0.20 ± 0.02 3.28 ± 0.20 3835 ± 285SNS 3 1.4 80.90 ± 5.95 30 ± 5 2.5 ± 0.1 16.5 ± 0.1 1.41 ± 0.01 5 ± 2 0.19 ± 0.02 3.18 ± 0.19 25,400 ± 2285

Note: Ages include random and systematic errors and were calculated from the datum year A.D. 2010. Italic ages are in disequilibrium and only provide a minimum limiting age.

*Equivalent dose calculated on quartz fraction with about 100–400 grains/aliquot and analyzed under blue-light excitation (470 ± 20 nm) by single aliquot regeneration protocols (Murray and Wintle, 2003).

†Values reflect precision beyond instrumental errors; values of ≤25% (at 2σ limits) indicate low spread in equivalent dose values and a unimodal distribution.§U, Th, and K contents were analyzed by inductively coupled plasma–mass spectrometry by Activation Laboratory, Ltd., Ontario, Canada.#From Prescott and Hutton (1994).

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Paleoseismology of the Mount Narryer fault zone, Western Australia

Geological Society of America Bulletin, v. 1XX, no. XX/XX 5

ing was <5° at each site and so is not further con­sidered. All three samples were collected from the uplifted alluvial surface above the Roderick River scarp. Samples were processed at the Uni­versity of Vermont using standard methods of quartz purification (Kohl and Nishiizumi, 1992) and beryllium extraction (Corbett et al., 2011), including weak acid ultrasonic etching, dissolu­tion in HF, anion and cation chromatography, and oxidation to BeO before mixing with Nb for AMS analysis. The 10Be9Be ratios were mea­sured at the Scottish University Environmental Research Center (Xu et al., 2010) and were normalized to NIST standard (NIST_27900) with an assumed ratio of 2.79 × 10–15 based on a half­life of 1.36 m.y. (Nishiizumi et al., 2007) equivalent to the assumed value for 07KNSTD. The 10Be concentrations from each sample were averaged in order to estimate a mean concen­tration for the exposed surface. We used the production model from Lal (1991) and Stone (2000) to infer an age for the surface using the online CRONUS calculator (Balco et al., 2008). We used a Monte Carlo simulation to develop a probabilistic estimate for the abandonment age of the terrace surface by considering the aver­age and standard deviation (1.97 ± 0.51 × 106 atoms/g) of the 10Be measured in our three sam­ples, the concentration of 10Be inherited when the alluvial material was deposited (Fink et al., 2000), and the most probable erosion rate of the surface after abandonment.

RESULTS

We present the results of our study on a site by site basis. Trench logs and data from five trenches across the Mount Narryer West, Rod­

erick River, and Sanford River East fault strands are presented in the following sections (Fig. 2). We present the Mount Narryer, Roderick north, and Sanford north trench logs on the same fig­ure because all three trenches exposed a similar style of fold­related deformation.

Mount Narryer East and West Faults

The eroded Mount Narryer East and West fault scarps previously mapped near Mount Narryer are associated with strong vegetation lineaments (Fig. 2; Williams, 1979). The east­ern fault trends N37°E, is 10­km­long, and has a slightly curvilinear trace. The western fault trends N35°E, is 32­km­long, and is relatively straight. We observed no topographic expres­sion of the western fault, and the eastern fault only has relief over an along­strike distance of a few hundred meters, where up to 2.2 m of west­side­up displacement of the sheetwash plains and bedrock terrain is preserved (Clark, 2004). Elsewhere, either the scarp has been eroded or is buried.

The Mount Narryer West trench is located across the mapped trace of the Mount Narryer West fault, where the fault is delineated by the contact on the ground surface between ferri­crete and alluvium, which coincides with the vegetation lineament identified by Williams (1979; site MNw on Figs. 2 and 3A). The trench exposed a deformed ferruginous duricrust (unit 4; Fig. 4A). The ferruginous duricrust has developed within older alluvial sands and fine gravels and has a platy structure defined by 10–60­mm­thick tabular plates. In plan view, the plates have a polygonal pedogenic struc­ture. The duricrust is unconformably overlain

by undeformed alluvial sands and gravels (unit 1) along a clear and irregular contact. Near the eastern end of the trench, a sandy gravel lens (unit 2) occurs at the base of the younger allu­vium (unit 1) and fills a 4­m­wide notch that is eroded into the ferricrete surface (Fig. 4A). The gravels in unit 2 are fine grained and sub­rounded to rounded, have lenticular beds, and are carbonate cemented.

The ferricrete is folded into a syncline­anti­cline pair. The limbs of the western anticline dip 7°W and 23°E, and the eastern limb of the syncline dips 4°W. The alluvium overlying the ferricrete is not tilted or deformed. We infer that the fold overlies a fault at depth consistent with the bedrock fault mapping of Myers (1995b).

We infer a single event from relations in the Mount Narryer West trench. Structural relations indicate that 0.4 m of horizontal shortening and 1.3 m of vertical displacement have occurred across the underlying fault perpendicular to the axis of the overlying fold pair (Whitney and Hengesh, 2013). Reconstructing the fold in relation to the ground surface indicates that a minimum of 1.5 m of material has been eroded from the fold crest. Erosion has removed the entire uplift and continued to degrade the ferri­crete surface.

The gravel unit has an OSL age of 15,515 ± 1475 yr (sample MNW 2; Table 1; Fig. 4A). The overlying sheetwash sand has an OSL age of 10,220 ± 785 yr (sample MNW 1; Table 1; Fig. 4A). Both samples confirm a latest Pleistocene to Holocene age for the undeformed alluvium and a minimum limiting age for deformation of the ferricrete.

Roderick River Fault

The Roderick River fault is expressed at the surface as a scarp that is the most prominent geomorphic lineament in the Mount Narryer fault zone. The scarp is a 68­km­long, topo­graphically west­side­up, up to 6.8­m­high flexure that trends in a N25°E direction (Fig. 2). The Roderick River fault influences the course of both the Roderick and Murchison Rivers. The Roderick River is deflected and impounded by the east­facing Roderick River fault scarp

TABLE 2. 14C DATA FROM THE RODERICK MIDDLE TRENCH SITE

Sample location MaterialLaboratory

number

Measured radiocarbon age*

(yr)

Calibrated radiocarbon age (2σ)(yr B.P.)†

Roderick Middle SiteRM #4 Shells (Bithyniidae Gabbia) Beta-330169 38,460 ± 310 42,810 ± 410

Note: Sample consists of individual intact disaggregated shells and experienced acid etch pretreatment.*Beta Analytical Laboratories, Inc.†Radiocarbon ages were calibrated by Beta Analytical Laboratories using the INTCAL09 database and the

simplified approach of Talma and Vogel (1993).

TABLE 3. MEASURED AND INTERPRETTED 10Be DATA

Sample name

Lat.(°S)

Long.(°E)

Elevation/ pressure

(m)

Elevation pressure

flag

Sample thickness

(cm)

Sample density (g/cm3)

Shielding correction

10Be concentration

(atoms/g)

Uncertainty in 10Be

concentration(atoms/g)

10Be standard

Al concent.

Al uncert.

Al stand.

CRN1 26.95 116.22 291 std 4 2.06 1 1956841.802 18734.32718 NIST_27900 0 0 KNSTDCRN2 26.96 116.22 292 std 5 2.02 1 2487218.222 31751.40726 NIST_27900 0 0 KNSTDCRN3 27.02 116.22 292 std 4 1.98 1 1474549.019 30328.21046 NIST_27900 0 0 KNSTDCRN_ave 26.98 116.22 292 std 4.33 2.02 1 1972869.681 26937.98163 NIST_27900 0 0 KNSTD

Note: CRN_ave was derived by considering the three samples as aliquots of one sample and then averaging the measured concentrations, uncertainty, elevations, and locations of the three aliquots.

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(Fig. 5). The river flows in a southwest direction until it is deflected northward along the scarp for 22 km to the confluence with the oncom­ing Murchison River. Ephemeral Lake Wooleen occurs midway along this deflected reach. West of the scarp, the Roderick River floodplain is uplifted and abandoned, except where the active Murchison River channel has downcut through the scarp and incised into the uplifted alluvial deposits, forming a sequence of fluvial terraces. (Figs. 5 and 6). The terraces are strath surfaces that have been cut into alluvial ferricrete (paleo–B horizon). Fluvial erosion removed the overly­ing soil horizons from the cemented B­horizon during strath formation.

The 6 km reach of the Roderick River follow­ing the base of the scarp between the north end of Lake Wooleen and the Murchison confluence is referred to as the Irrida channel (Figs. 5 and 6). During flood events, flow in the Murchison River backs up at the choke point presented by Murchison Canyon. The constriction forces the Murchison River to overflow to the south through Irrida channel and into Lake Wooleen. Photographs from the 2006 floods show a tur­bid red­brown Murchison River flowing into the blue­green waters of Lake Wooleen from the north (David Pollock, 2011, personal com­mun.). However, during the waning flood stage, flow in the Irrida channel reverses to the north, effectively draining the northern portion of Lake Wooleen.

The maximum height of the Roderick River scarp along Irrida channel is ~6.8 m. The scarp height decreases along trend both to the north and south. To the south, the face of the scarp is buried by alluvium. To the north, it has been eroded by the Murchison River, and the height of the scarp decreases systematically as the flight of strath terraces step down toward the active Murchison River channel (Fig. 6).

Five strath terraces are cut into the uplifted alluvial ferricrete of the Roderick and Murchi­son paleochannels. The highest terrace is T5. It occurs at an elevation of 292 m, ~11 m above the bed of the Murchison River. A lower terrace T4 occurs at an elevation of 289 m and is pre­served on both sides of the canyon (cf. Fig. 6). T3 occurs at an elevation of 287 m and is pre­served between an overflow channel and the Murchison River. T2 occurs at an elevation of 285 m and is preserved on both sides of the river. T1 is a 100­m­wide channel belt into which the Murchison River has most recently incised. T1 occurs at an elevation of 283 m. Where the T1 strath surface outcrops from beneath sand bars and vegetation, it is 1.5–2 m above the ferricrete bed of the active river channel.

The T2 terrace is preserved along 17 km of the Murchison River upstream of the canyon (Figs.

5 and 6). North of the T2 terrace, the river has a 3500­m­wide channel belt within a braid plain (T1) and an anastomosing channel form (Fig. 5, reach M­A). The channel belt (T1) straight­ens and narrows abruptly to 375­m­wide adja­cent to the uplifted T2 terrace (reach M­B). The channel belt (T1) continues to narrow, becoming 100­m­wide through the canyon (reach M­C). Approximately 2.3 km downstream of the can­yon, the channel sinuosity increases to a mean­dering form, and the channel belt (T1) widens to 1400 m (reach M­D). A channel that is graded to the T1 terrace incises T2 west of the Murchi­son River upstream of the canyon (Fig. 6). This channel and the overflow channel are incised between the T3 and T4 terraces at the same eleva­tion as the T2 terrace (285 m) and still carry flow during large discharge events.

An anticlinal fold axis is exposed in the eroded scarp face south of Murchison Canyon and trends 352° (Fig. 6). The fold limbs dip 14°E and 10°W. Series of northeast­trending shear zones are exposed in the surface of the alluvium above the scarp edge near this location (Figs. 7A and 7B). We observed one of these shear zones cutting obliquely through the eroded scarp face (Figs. 6, 7C, and 7D). Shear zones are oriented between N14°E and N38°E and dip from 20° to vertical. Every shear zone we observed dips vertically or to the west. They are 6–50­m­long and less than 1.5­m­wide.

Irrida pool lies within the Irrida channel and effectively demarcates the sill between Lake Wooleen and the Murchison River (Fig. 5). The sill extends for 3 km along the Irrida channel at 286 m elevation. Irrida channel is cut into allu­vial ferricrete, and the surface of the channel is irregular and scoured. Ferricrete blocks in the channel are imbricated, indicating flow direc­tion from Lake Wooleen (south to north).

South of Lake Wooleen, the scarp deflects drainages where it crosses alluvial surfaces and then continues into bedrock terrain for an additional 28 km (Fig. 2). Along this section, topographic expression is generally subtle, although the trace of the scarp is weakly expressed as a vegetation lineament on aerial photographs. Thirteen kilometers southwest of Lake Wooleen, the Roderick River scarp impounds two ponds on the margin of an insel­berg. The scarp is 0.6­m­high and developed in granitic saprolite. The parent rock is a highly sheared foliated granite containing a steeply dipping shear fabric (>70°W) that crops out as 10–20­mm­wide tabular fins trending par­allel to the scarp. The scarp has ponded sedi­ment on the upstream (SE) side. The ponds are grass covered, in contrast to the surface above the scarp, which is sparsely vegetated with Mulga trees.

Mt. Narryer West trench

Roderick north trench

Roderick middle trench

Roderick south trench

Sanford north trench

A

B

C

D

E

Figure 3. Photographs of the trench sites discussed in text. (A) Mount Narryer West trench. (B) Roderick north trench. (C) Rod-erick middle trench (Land Cruiser for scale, just visible behind spoils pile). (D) Roderick south trench. (E) Sanford north trench.

as doi:10.1130/B31313.1Geological Society of America Bulletin, published online on 20 August 2015

Paleoseismology of the Mount Narryer fault zone, Western Australia

Geological Society of America Bulletin, v. 1XX, no. XX/XX 7

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as doi:10.1130/B31313.1Geological Society of America Bulletin, published online on 20 August 2015

Whitney et al.

8 Geological Society of America Bulletin, v. 1XX, no. XX/XX

Roderick North TrenchThe Roderick north trench is located across

the Roderick River fault scarp near the north­western corner of Lake Wooleen and south of the Irrida channel (site Rn on Fig. 2). At this location, the scarp is 0.6­m­high, and the eleva­tion of the scarp crest is 290 m (Fig. 3B). The trench exposed a basal package of indurated alluvium with CaCO3­filled cracks and pebble stringers (unit 5) overlain by a platy ferruginous duricrust (unit 4) formed in sands and fine grav­els (Fig. 4B). Unit 4 is unconformably overlain by younger alluvium (unit 1). The top of the duricrust unit consists of 10–40­mm­thick tabu­lar plates that are less cemented and more iron rich than the material below.

The ferruginous duricrust (unit 4) is folded and forms an anticline. The overlying alluvium (unit 1) is undeformed and appears to be depos­ited on paleotopography that represents the former surface of the duricrust (Fig. 4B). The eastern limb of the fold dips 14°E. The western limb was not exposed in the trench, but it was observed on the ground surface as tilted blocks of in situ duricrust. The western limb dips 10°W. A 0.8­m­wide shear zone at the ground surface intersects the western limb of the fold. The shear zone strikes N40°E, dips between 30° and 65°W, and truncates the western fold limb. On the western side of the shear zone, the western fold limb dips 7°W and diminishes further to the west.

We infer a single event from relations in the Roderick north trench. Fold reconstructions indicate that 0.4 m of horizontal shortening and 1.4 m of vertical displacement have occurred across the basement fault perpendicular to the axis of the overlying fold (Whitney and Hengesh, 2013). Reconstructing the fold in rela­tion to the ground surface indicates that a mini­mum of 1.75 m of ferruginous duricrust have been eroded from the fold crest.

Roderick Middle TrenchThe Roderick middle trench was located

along the Roderick River fault across a subdued 110­m­long, 4­m­high, east­facing scarp at the western margin of Lake Wooleen within a subtle paleo­overflow channel (site Rm on Figs. 2 and 5; Fig. 3C). Above the scarp, the Roderick River

paleo­floodplain slopes gently to the west. The elevation of the crest of the scarp at this location is approximately coincident with the T3 terrace in Murchison Canyon.

Four primary units were observed in the trench exposures (Fig. 8). A basal colluvium (unit 4)

consists of angular gravel­size clasts of platy duricrust in a sandy matrix. Near the eastern end of the trench, the colluvium is weakly bedded with aligned gravels and coarse sand stringers. Unit 4 is overlain by a 1.3­m­thick lacustrine deposit (unit 3) that contains 0.2–0.4­m­thick

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Figure 5. Map of stream channel planforms within the Mount Narryer fault zone. Note: the active channel belt on the Murchison River dramatically narrows at Murchison Canyon. Trench sites: RN—Roderick north; RM—Roderick middle; RS—Roderick south; SN—Sanford north.

as doi:10.1130/B31313.1Geological Society of America Bulletin, published online on 20 August 2015

Paleoseismology of the Mount Narryer fault zone, Western Australia

Geological Society of America Bulletin, v. 1XX, no. XX/XX 9

beds of fine sand and silt with abundant pockets of freshwater snail shells (unit 3; family Bithyniidae, genus Gabbia; Whisson, 2012. per­sonal commun.). The basal contact of the lacus­trine deposits (unit 3) is clear and irregular to wavy. A duricrust cap (unit 2) has formed in the top of the lacustrine unit at the higher­elevation western end of the trench. The duricrust is char­acterized by 10–40­mm­thick platy clasts simi­lar to the other duricrusts observed in the Mount Narryer West and Roderick north trenches. The duricrust weakens to the east, has been partly removed by erosion, and is unconformably overlain by massive red­brown sheetwash sands (unit 1). The sand is fine to coarse and contains a weakly developed platy pedogenic structure. Where unit 2 has been removed by erosion, stage II carbonate coatings have developed along ped faces in the lacustrine deposits (unit 3). The base of the sheetwash sand is clear and irregular. The soil profile in the sheetwash sand is more devel­oped above the lake margin to the west.

The shells within unit 3 yielded a 14C age of 42,810 ± 410 cal. yr B.P. (sample RM4 [14C]; Fig. 8; Table 2). Although the shell samples may contain pedogenic or groundwater carbonate, which has inherent residency problems (Gile et al., 1981), the same shell­rich unit yielded an OSL age of 44,940 ± 3120 yr (sample RM4; Table 1) which is in agreement with the 14C age.

Two other OSL samples provide only limit­ing minimum ages because of disequilibrium in their U and Th decay series (daughter excess or deficit), likely due to reactions with ground­water (Forman, 2014, personal commun.), as has been documented elsewhere (Roberts, 1997). The minimum limiting OSL age of the colluvium (unit 4) is 22,785 ± 1920 yr (sample RM6; Fig. 8; Table 1). The minimum limiting OSL age of the overlying lacustrine unit (unit 3) is 28,840 ± 2120 yr (sample RM5; Fig. 8; Table 1). These two ages are stratigraphically reversed and are incompatible with the two independent ages of the overlying shell horizon.

Another sample from the lacustrine unit has an OSL age of 24,630 ± 2120 yr (sample RM3; Fig. 8; Table 1). This sample is at a similar stratigraphic position (20 m west) as the shell horizon that has an age ~20 k.y. older. We have a higher confidence that unit 3 is ca. 43 ka, given the corroborating stratigraphically concordant ages measured from the shell horizon using OSL and 14C. The base of the sheetwash sand (unit 1) above the duricrust (unit 2) has an OSL age of 10,700 ± 975 yr near the scarp crest and an age of 5490 ± 430 yr near the base of the scarp closer to the lake.

The stratigraphic and structural relations sug­gest that at least two surface­rupturing events occurred prior to the deposition and burial of

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as doi:10.1130/B31313.1Geological Society of America Bulletin, published online on 20 August 2015

Whitney et al.

10 Geological Society of America Bulletin, v. 1XX, no. XX/XX

the fault trace by unit 1. The unit 4 colluvium is inferred to have been derived from an initial scarp­forming event that deformed the ferrugi­nous duricrust. This was then overlaid by lacus­trine deposits (units 2 and 3) that were ponded against the scarp. The warping of unit 3 is inferred to be related to a second event.

Roderick South TrenchThe Roderick south trench was excavated

along the Roderick River fault across a scarp and dry pond ~13 km south of Lake Wooleen (site Rs on Figs. 2 and 5; Fig. 3D). The trench exposed a discrete fault plane (strike 18°, dip 32°W) that juxtaposes highly weathered bed­

rock (unit 6) on the hanging wall against col­luvial duricrust (unit 5) in the footwall (Fig. 9); the duricrust is warped into the fault zone. The bedrock is composed of porphyritic granite weathered to saprolite and contains large peds and a weak residual structural fabric that dips steeply to the west.

C D

A B

Figure 7. (A) Shear zone in ferricrete above the Roderick River scarp; white lines delineate shear zone margins; notebook is ~0.19 m long. Trend of shear zone is indicated by white arrows. (B) Detail of shear fabric in ferricrete; white arrows indicate trend of shear zone; global positioning system unit is ~0.1 m long. (C–D) Shear zone exposed in the Roderick River scarp face. Shear zone cuts across bedded ferricrete (white lines) and dips to the northwest. Shear zone is outlined in red and shaded pink. View is to the southwest. Notebook is ~0.19 m long.

as doi:10.1130/B31313.1Geological Society of America Bulletin, published online on 20 August 2015

Paleoseismology of the Mount Narryer fault zone, Western Australia

Geological Society of America Bulletin, v. 1XX, no. XX/XX 11

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as doi:10.1130/B31313.1Geological Society of America Bulletin, published online on 20 August 2015

Whitney et al.

12 Geological Society of America Bulletin, v. 1XX, no. XX/XX

Sediments exposed in the footwall consist of colluvium (unit 4) and duricrust (unit 5). The unit 4 colluvium consists of micaceous biotite­rich sandy gravel that contains fresh quartz clasts and numerous ghost (thoroughly weath­ered) clasts of saprolite peds (Fig. 9). A cumulic stage IV carbonate­cemented duricrust is devel­oped in the colluvium (unit 5). It has a platy pedogenic structure with 10–30­mm­thick hori­zontally elongated plates coated in thin discon­tinuous carbonate laminae. Near the fault trace, the unit 5 duricrust developed in the colluvium is broken and displaced and is overprinted by a more recent weathering profile. The colluvium (unit 4) is overlain by cemented lacustrine sedi­ments (unit 3) at the eastern end of the trench beneath the pond surface. The western limit of the lacustrine sediments coincides with the veg­etation lineament, and a hydrological transition from sheetflow deposits to ponding on the sur­face. A layer of unconsolidated lacustrine sand and silt (unit 2) overlies the cemented lacustrine unit. The lower contact of unit 2 is an erosional unconformity that is abrupt and wavy. The sur­ficial deposit (unit 1) overlying the duricrust (unit 5) and saprolite (unit 6) on the western side of the exposure consists of sheetwash sand that interfingers with the lacustrine deposits in the middle of the trench exposure. The OSL age for unit 2 is 8900 ± 690 yr (sample RS01; Table 1; Fig. 9); however, the material is in disequilib­rium and therefore only provides a limiting minimum age for the lacustrine unit (Forman, 2014, personal commun.).

The stratigraphic and structural relations indi­cate that at least one surface­displacement fault­ing event occurred prior to the deposition and burial of the fault trace by unit 1. The presence of lacustrine deposits (units 2 and 3) is consis­tent with ponding against the scarp.

Sanford River Fault

The Sanford River East and Sanford River West faults are associated with prominent geo­morphic scarps that cut across the 6­km­wide Sanford River floodplain and deflect and alter the course of the river (Fig. 2). The channel planform adjustments, scarp characteristics, and surface hydrology indicate that the surface of the floodplain west of the scarps has been uplifted and deformed. The river has a multi­channel anastomosing pattern within an ~6­km­wide channel belt until it encounters the Sanford River East fault (the main scarp; Fig. 5, reach S­A). The channel has aggraded upstream of the scarp. A single channel is incised through the scarp near the southern river bank and flows to the west within a 400­m­wide channel belt (Fig. 5, reach S­B). The floodplain west of the

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as doi:10.1130/B31313.1Geological Society of America Bulletin, published online on 20 August 2015

Paleoseismology of the Mount Narryer fault zone, Western Australia

Geological Society of America Bulletin, v. 1XX, no. XX/XX 13

main scarp is abandoned, except where crossed by the single channel. The scarp associated with the Sanford River West fault also deflects surface flow along its base. A broad clay pan is ponded against the scarp in the middle of the channel. Upon crossing the Sanford River West fault, the channel fans out and regains its anastomosing planform across a 4­km­wide belt (Fig. 5, reach S­C).

The Sanford River East fault is 25­km­long and oriented in a N22°E direction. North of the river, the main fault bifurcates (Figs. 2 and 5), and a N18°W­oriented segment splays from the main trace and extends for 12 km across a sheetwash surface. A shorter, 4­km­long lin­eament crosses the northern edge of the river between the two faults. South of the river, the scarp continues for 13 km across a sheetwash surface. The scarp associated with the Sanford River East fault is a degraded, 3­m­high, west­side­up flexure that deforms ferricrete devel­oped in alluvium. West­flowing drainage is col­lected along the base of the scarp and diverted toward the river. Near its southern end, the scarp bifurcates (Fig. 2). A pond is confined between the bifurcated splays. Relief across the western splay is less than 0.5 m and is coincident with a marked vegetation lineament visible on aerial photographs and in the field. The Sanford River West fault and a smaller lineament between the two faults were not visited in the field.

The Sanford north trench was located across the main Sanford River East fault scarp ~5 km south of the river (site Sn on Figs. 2 and 5; Fig. 3E). The stratigraphy consists of a basal structureless indurated alluvium (unit 5) with a ferru ginous duricrust (unit 4) capstone (Fig. 4C). The ferruginous duricrust is formed in alluvial sands and fine gravels. The duricrust is up to 0.6­m­thick and forms an eroded cap rock. Poorly sorted gravel­rich colluvium (unit 3) unconformably overlies the duricrust and consists of broken platy angular clasts of duricrust (unit 4) within a sandy matrix. While coarser gravels are concentrated near the base of the unit, the colluvium is undeformed, homo geneous, and contains no bedding. Rela­tively flat­lying sheetwash sand (unit 1) overlies the colluvium (unit 3) and the duricrust (unit 4). Unit 1 consists of well­sorted, fine to medium sands with occasional fine gravel and local pebble stringers. Similar to the underlying duri­crust, the sands are weakly cemented at the sur­face, and the degree of cementation decreases with depth. The colluvium (unit 3) has an OSL age of 25,400 ± 2285 yr (sample SNS3; Table 1). The overlying sands have stratigraphi­cally concordant OSL ages of 3835 ± 285 yr (sample SNS2; Table 1) and 2085 ± 170 yr (sample SNS1; Table 1).

The duricrust (unit 4) is deformed and folded into an anticline (Fig. 4C). The eastern limb dips up to 30°E, and the western limb dips 5°W. Duricrust exposed on the ground surface to the west of the trench dips 31°W. The crest of the anticline has been eroded and fragmented duri­crust clasts from unit 4 are redeposited within the colluvium (unit 3) to the east. The relatively flat­lying colluvium and sheetwash sands bury the eastern limb of the folded duricrust (unit 4) and associated colluvium (unit 3), indicating that the deformation predates the deposition of unit 1.

The stratigraphic and structural relations indi­cate that at least one surface­displacement fault­ing event occurred prior to the deposition and burial of the fault trace by unit 1. Fold recon­structions yield 0.9 m of horizontal shortening and suggest that 1.4 m of vertical displacement has occurred across the underlying fault perpen­dicular to the axis of the overlying fold (Whit­ney and Hengesh, 2013). Reconstructing the fold in relation to the ground surface indicates up to 2.9 m of duricrust have been eroded from the fold crest at the Sanford north trench site.

DISCUSSION

Geomorphological and trenching data indi­cate that surface deformation within the Mount Narryer fault zone is generally expressed by flexural folding above buried faults (Fig. 10). We interpret the Mount Narryer fault zone scarps as reflecting the development of fault­propagation folds in partly indurated alluvial sediments above steeply dipping basement faults. Right­lateral transpressional motion is expressed at the surface through two distinct styles of deforma­

tion. Contractional deformation is accommo­dated by folding of surficial sediments along a fold that has an axis parallel to the fault strike direction. The lateral component of motion is accommodated in the upper indurated alluvial section as a series of discrete shear zones that cut the hanging wall of the fault­propagation folds (Fig. 10). This general style of deforma­tion is documented on all three structures we investigated in the field. The left­stepping en echelon configuration of the Mount Narryer fault zone is consistent with this interpretation, and with the regional pattern of neotectonic deformation documented along the reactivated extended continental margin to the northwest in the Western Australia shear zone (Whitney and Hengesh, 2013, 2015a, 2015b; Whitney et al., 2014; Hengesh and Whitney, 2014).

Stress

The geometry of the fold scarps, where fold amplitude can be twice the vertical displace­ment, and the general linearity of scarps at the regional scale suggest that the underlying faults are steeply dipping. Furthermore, most of the shear zones are on the western limbs of the folds (on the hanging wall above the faults). All of the shear zones we observed dip vertically or steeply to the west (Fig. 10). The shear zone orientations provide information on the state of stress and enable an estimation of the maximum horizontal stress direction (SHmax).

Hillis and Reynolds (2000) predicted a maxi­mum horizontal stress direction of ~76° for the region that contains the Mount Narryer fault zone based on data from a few hundred kilome­ters northwest and southwest. The orientations

+BEDROCK

FERRICRETE interlayered with Murchison Cement

Fold axis

Shear zone

Figure 10. Schematic block diagram showing the style of deformation observed in the Mount Narryer fault zone. All of the observed shear zones dip vertically or to the west.

as doi:10.1130/B31313.1Geological Society of America Bulletin, published online on 20 August 2015

Whitney et al.

14 Geological Society of America Bulletin, v. 1XX, no. XX/XX

of the fold axes align essentially perpendicular to SHmax (s1 = 75°; Fig. 11). Shears are oriented 22° to 46° clockwise of the fold axis, consis­tent with the pattern for secondary synthetic Riedel shears. The average shear zone orienta­tion is 34°, consistent with the synthetic Riedel (R1) direction in a dextral­transpressional set­ting (Fig. 11). The SHmax azimuth of 75° we derived from geological data confirms the Hillis and Reynolds (2000) estimate of SHmax and extends this pattern of regional stress across the former extended margin and into the mar­gin of the Yilgarn craton. The pattern of oblique dextral­transpression observed in the geology is consistent with the focal mechanism solutions for small­ to moderate­magnitude earthquakes recorded by a temporary seismic network in the region (Revets et al., 2009; Keep et al., 2012).

Coseismic surface rupture of synthetic Riedel shears may indicate an early stage of strike­slip fault evolution (Lin and Nishikawa, 2011). The lack of conjugate Riedels and the sigmoidal geometry of shear zones also may suggest the recent onset of fault activity, similar to “stage a” in the Riedel experiment (e.g., Tchalenko, 1970, p. 1627). Within alluvial sediments, the fault system exhibits an early phase of shear zone evolution, with strike­slip deformation being dispersed through a belt of shear zones rather than being concentrated on a principal fault at the surface. Further, the presence of folded rather than faulted cover sediments may also suggest an early stage of fault growth (Finch et al., 2003) or the recency of fault reactivation. Alternatively, the apparent juvenile expression of the faults in the alluvial sediments (ferricrete) could relate to the young (Pleistocene) age of the sediments themselves (i.e., not old enough to have experienced more than a few earthquake events) rather than constraining the onset of

faulting. However, the regional­scale tectonic geomorphology demonstrates that the most topographically developed fault­related features are confined within alluvial sediments rather than bedrock terrains. The alluvial settings are more susceptible to erosional processes (within or adjacent to channels), and alluvial material is more mobile and deformable than granitic bed­rock. If tectonic deformation were long­lived in the Mount Narryer fault zone, then one would expect the morphogenic indicators to be better preserved and expressed in the bedrock settings compared to the alluvial settings; that is not the case. Therefore, we interpret the data to provide evidence for a recent onset of fault reactivation in the Mount Narryer fault zone.

A dominant control on stress within the Aus­tralian plate is from far­field plate­boundary forcing mechanisms (e.g., Wellman, 1981; Hillis et al., 2008). A recent onset of deformation is consistent with the onset of collision at Suva­Roti Ridge on the northern plate margin between 1.8 and 0.2 Ma (Harris et al., 2009; Roosmawati and Harris, 2009; Audley­Charles, 2011) and is interpreted as being the driving mechanism for the most recent phase of fault reactivation along the western extended continental margin (Whit­ney and Hengesh, 2013; Hengesh and Whitney, 2014; Whitney et al., 2014).

Age Control and Erosion Rates

We contend that strath terraces in Murchison Canyon were formed by an uplift event fol­lowed by river incision. Figure 12 presents a schematic chronologic model of terrace­form­ing events along the Roderick River fault scarp at Murchison Canyon. After each uplift event, the newly formed terrace remained active for a limited period of time (lag time) during subse­

quent flood events while the river incised and attempted to reestablish grade (Fig. 13). This is evident along the Sanford River where it is crossed by the scarp. The scarp is 3 m above the active river channel, yet the uplifted braid plain on the west side of the scarp is intermit­tently reoccupied during high­flow events. Similarly, the T2 terrace in Murchison Canyon can be episodically reoccupied, as occurred during the 2006 floods. With repeated tectonic events (uplift­incision cycles), the older terraces progressively become more isolated from flood events and the resulting intermittent shielding of the strath surface by mobile sediments, and ultimately are removed from the fluvial sys­tem. Once this occurs, cosmogenic nuclides can begin to accumulate continuously in the terrace surface material, in this case ferricretized allu­vium. In ephemeral river systems, a lag time may separate the earthquake uplift event and the channel re­establishing grade, as documented in the Carnarvon Basin (Whitney and Hengesh, 2015a). Therefore, the measured 10Be con­centrations from the strath surface relate most closely to the time of abandonment and isolation of the terrace from fluvial processes, rather than the uplift event itself.

Erosion Rates of Sampled SurfacesThe 10Be sample locations were selected in

areas that minimized potential erosion: They were isolated from regionally significant ero­sional processes, removed from fluvial pro­cesses (>6 m above the active ephemeral chan­nel), located near the apex of a gently sloping alluvial surface; and in places where there were no wanderrie (indicative of sheetwash flow). Rainfall impact and wind slowly lower the ferri­crete surface by disaggregating ferricrete peds into their fine and coarse constituents and then winnowing away the fines. These processes lead to the formation of a thin unconsolidated coarse sand lag that armors the ferricrete from subse­quent rain attack.

Reported landscape surface­lowering rates around Australia range from <1 to 10 m/m.y. (Wellman and McDougall, 1974; Bishop, 1984, 1985; Young and MacDougall, 1993; Bierman and Turner, 1995; Belton et al., 2004; Chappell, 2006; Vasconcelos et al., 2008), with the most climatically similar sites to the Mount Narryer region being reported as 0.3–4.0 m/m.y. (Bier­man and Caffee, 2002). However, their samples were collected almost exclusively from granite outcrops. Maximum erosion rates between 0.5 and 0.85 m/m.y. have been estimated on Pleis­tocene clastic ferricrete materials in Brazil (do Nascimento Pupim et al., 2015), which is a simi­lar material to the ferricrete found in the Mount Narryer fault zone; these are some of the lowest

R Synth

etic

R’ Antitheticσ1

σ1

σ3

σ3

75°

345° n =10 sector angle = 15°vector mean = 34°max = 40%

n =8 sector angle = 15°vector mean = 345°max = 62.5%

Figure 11. Rose diagram of fold attitudes and shear zone atti-tudes from the Roderick River fold indicating SHmax direction overlain by a dextral-transpres-sional strain ellipse. Fold atti-tudes are oblique to the main scarp face, which presumably aligns with the basement fault at depth (Fig. 10). The oblique fold axes are smaller immature features superimposed along the main fold axis.

as doi:10.1130/B31313.1Geological Society of America Bulletin, published online on 20 August 2015

Paleoseismology of the Mount Narryer fault zone, Western Australia

Geological Society of America Bulletin, v. 1XX, no. XX/XX 15

outcrop erosion rates reported worldwide. Their samples were collected from a gently sloping forested location with an annual precipitation six times higher (1350 mm yr–1) than the Mur­chison region. We conservatively use the lowest value of their range (0.5 m/m.y.) as a maximum erosion rate constraint on the uplifted alluvial ferricrete surface for our analysis.

Sample S­1, collected from the terrace riser 1 m in elevation below the T5 surface, has a 10Be concentration equivalent to the average of samples S­2 and S­3, which were collected from the T5 surface (Table 3). Similar and high con­centrations of 10Be imply stability of the sample locations. Based on its geomorphic setting and surface characteristics, we infer that erosion has been negligible (0.1 m/m.y.) since isolation of the sampled surfaces from fluvial processes; we use a value of 0.1 mm yr–1 as a minimum con­straint for our analyses.

Age Estimates for the Terrace SurfacesWe considered the measured concentrations

of 10Be from the three surface samples as indica­tors of surface stability, and we used the average concentration, along with our best estimates of inherited 10Be at the time of sediment deposi­tion (inheritance) and erosion rate, to infer the timing of terrace stabilization. We used Monte Carlo simulations to infer the most likely timing for stabilization of the alluvial surface. Because local values of 10Be concentration in contem­porary stream sediment were not available, we used the nearest reported 10Be concentrations measured from samples of surficial sediment southwest of our study area (Fink et al., 2000) as a local proxy for inherited 10Be in the area we sampled. We generated a normal distribution of inheritance values using the mean value of 1 × 106 and standard deviation of 2 × 105 and using the minimum and maximum of the three reported values (0.81–1.20 × 106 atoms g–1) as the P16.6 and P83.3 values (Fig. 14A). We also generated a log­normal distribution of erosion

rates with a mean of 0.27 m/m.y. and a standard deviation of 0.19 m/m.y. The lower tail of this distribution assumes our minimum estimate of 0.1 m/m.y. is a P10 value and ends at the physi­cal minimum of zero (Fig. 14B). The upper tail assumes our high estimate of 0.5 m/m.y. is a P90 value. The resultant Monte Carlo simula­tion yields 10,000 age estimates with a mean value of 239 ka (SD = 54 k.y.; Fig. 14C). The ca. 240 ka value provides a maximum limiting

constraint on the time of T5 abandonment by flu­vial processes, which we suggest coincides with the onset of cutting of the T3 terrace in response to the occurrence of the antepenultimate paleo­seismic event (Figs. 12 and 13).

This interpretation requires that a total of 4 m of incision has occurred through the ferricrete at Murchison Canyon since ca. 240 ka. This yields an incision rate of ~16 m/m.y., though the rate likely was punctuated and higher immediately

braidplainchannel channel

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active

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Hanging wall uplift

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T4

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intermittently active

intermittently active

intermittently active

Braidplain position pre-E4

T5 removed from �uvial erosion and Be accumulation initiates

10

Figure 12. Schematic terrace develop-ment chronology of the Roderick scarp at Murchi son Canyon. Perspective is looking at the scarp face. Time progresses from older (top of page) to younger (bottom of page). Lines with X’s designate earthquake events (E1–E4). The terrace configuration is shown immediately after the earthquake and after the fluvial system has responded and terrace evolution has progressed (R1–R4). The terrace development chronology progresses from earthquake, then response to earthquake, then response, etc.

as doi:10.1130/B31313.1Geological Society of America Bulletin, published online on 20 August 2015

Whitney et al.

16 Geological Society of America Bulletin, v. 1XX, no. XX/XX

after each uplift event and lower during inter­seismic intervals. The contrast in erosion rates between the T5 surface (<0.5 m/m.y.) and inci­sion of Murchison Canyon (16 m/m.y.) illustrate the disequilibrium in the landscape near the fault zones.

The fold reconstructions (Fig. 4) provide estimates for how much the fold crests have been eroded since the onset of folding. We used Monte Carlo simulations to generate trun­cated normal distributions of erosion rates for the three sites. We constrained the distribution between minimum and maximum erosion rates, >0 m/m.y. and <16 m/m.y., which we selected as

a maximum credible value based on incision in Murchison Canyon. The eroded fold crest mea­surements were divided by the resultant 10,000 erosion rates to provide populations of estimates of the time since focused erosion began for each site (i.e., a proxy for the time of initial fold for­mation). The probability distributions of the age estimates are shown on Figure 14D. Median age estimates of the eroded folds are 190, 220, and 360 k.y. for sites MNw, Rn, and Sn, respectively (Fig. 14D). The median age estimate coincides with a median erosion rate of 8.1 m/m.y. at each site. This erosion rate is higher than the regional average because the uplifted fold crests have

steepened the local gradient, thereby triggering more rapid erosion.

The geomorphological settings of the sites are variable. The MNw trench site is located on a subtle interfluve that is covered by sheetwash bed forms. The Rn site is along the margin of Lake Wooleen away from any channelization. The Sn site is in an area that is actively diverting surface flow and ephemeral channels. Based on the surface hydrology at each of the three sites, the erosion rate at site MNw likely is lower than the rates estimated here, which would increase the age of the feature. Conversely, the erosion rate at site Sn could be higher than estimated, which would reduce the age of the feature. Not­withstanding the uncertainty, all three features are of mid­ to late Pleistocene age (Fig. 14D).

Although OSL ages provide a minimum limit­ing age constraint on the most recent uplift event on structures in the region, the OSL ages more likely relate to late Quaternary climatic events when sediment was mobilized and deposited rather than discrete paleoseismic events.

Slip Rates

Erosion rate estimates and 10Be concentra­tions in terms of terrace abandonment age pro­vide an estimate for uplift and slip rates. We

complete �uvial abandonment

Gray = periods when the river is active at each terrace elevation

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= earthquake

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LT

LT

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Figure 13. Schematic chronol-ogy showing relations among measured 10Be concentrations, terrace formation, and terrace abandonment. Arrows designate earthquake events (E1–E4).

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Figure 14. (A) Probability dis-tribution for 10Be inheritance concentrations. (B) Probability distribution for erosion rate estimates. (C) Probability dis-tribution of exposure ages from Monte Carlo analyses on ero-sion and inheritance inputs; mean and one standard devia-tion ages shown. (D) Box plot of probability distributions for the age of the fold crests. Black line shows median value, which coincides with an erosion rate of 8.1 m/m.y.; Q1 and Q3 limits coincide with erosion rates as shown. Q1 and Q4 tails are not shown. MNw—Mount Narryer West; Rn—Roderick north; Sn—Sanford north.

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estimated slip rates from the trench data at the MNw, Rn, and Sn sites using the amount of fold deformation and the estimated fold ages based on the erosion rate estimates. Whitney and Hengesh (2013) calculated the amount of dip­slip displacement by reconstructing the folds. We present their data with the 10Be­based age inferences in Table 4.

We also estimated long­term slip rates on the Roderick River fault using the elevation dif­ference between the highest and lowest terrace surfaces near Murchison Canyon. The eleva­tion difference between the T5 and T1 terraces is 9 m. This yields a minimum of 10.0 m of dip­slip displacement on a 74°­dipping fault plane (Whitney and Hengesh, 2013). Since the 10Be­based age inferences indirectly estimate the tim­ing of the antepenultimate event and constrain the timing of the younger events, only three of the four terrace­forming events are constrained to have occurred since ca. 240 ka. The uniform 2–3 m terrace riser heights indicate characteris­tic earthquake displacements (cf. Schwartz and Coppersmith, 1984), and we infer that a total of 7.5 m of dip­slip displacement has occurred on the Roderick River fault in the past 240 k.y. This yields a dip­slip rate of 0.033 ± 0.007 mm yr–1 (Table 4).

The en echelon geometry of the Mount Narryer fault zone (Fig. 2), and the shears on the face of the fold scarps (Figs. 10 and 11) indicate that a component of the total slip is lateral. In addition, microseismicity data from the region indicate dominantly oblique­slip displacements (Revets et al., 2009; Keep et al., 2012). The 30°–45° angle between the regional stress direc­tion and the trend of the master faults suggests that up to half of the total slip may be lateral. Therefore, the dip­slip rates can reasonably be estimated as half the total slip rates (Table 4).

The estimated total fault slip rates range from 0.01 to 0.07 mm yr–1 (Table 4). The slip rates estimated from the eroded fold crests at the trench locations are lower than the rates esti­mated from the Roderick River scarp height. This may be due to punctuated erosional pro­cesses that become smoothed over the longer­term record; the erosion rate along a scarp will decrease with time from its peak immediately after a seismic event (e.g., Hanks, 2000).

Morphogenic Events

The relative youth (Holocene) of undeformed mobile alluvium overlying mid­Pleistocene folded duricrust suggests late Pleistocene to Holocene sediment stripping and mobilization of sheetwash sands and lacustrine units in rela­tion to climatic perturbations rather than tec­tonic events. Consequently, we were unable to

obtain numerical age control for individual mor­phogenic earthquakes. However, based upon the length of the scarps, and the amplitude of the folds exposed in the trenches, we infer a mini­mum of one morphogenic earthquake event on the Mount Narryer West and one on the Sanford River East faults based on the trench stratigra­phy. On the Mount Narryer West fault, the ferri­crete was folded by a scarp­forming earthquake resulting in 0.4 m of horizontal shortening and 1.3 m of vertical displacement (Fig. 4A). The fold scarp was subsequently eroded. The unde­formed 10,220 ka sheetwash sands overlying the scarp provide a minimum limiting age for the most recent event on this fault. A pocket of sediment has accumulated on the downthrown side of the fold and has provided an environ­ment conducive to vegetation growth, which has accentuated the expression of the lineament.

On the Sanford River East fault (Sanford north trench), mid­Pleistocene duricrust was folded, resulting in 0.9 m of horizontal shorten­ing and 1.4 m of vertical displacement. Subse­quently, the scarp was eroded. Undeformed ca. 26 ka colluvium and late Holocene sheetwash sands were deposited on the eroded scarp and provide a minimum limiting age for the most recent morphogenic seismic event on the fault.

A more detailed interpretation of the history of events along the Roderick River fault can be deduced from our observations of abandoned terraces at the Murchison Canyon site. There, we interpret the five terraces cut into alluvial ferricrete as indicating four morphogenic events along this portion of the fault (Fig. 12). Sub­sequent to each faulting event, the Murchison River incised through the fold in an attempt to reestablish grade. The terrace risers are between 2 and 3 m high, which provide an estimate for the vertical component of single­event dis­placements.

We interpret the following uplift and ero­sion sequence at Murchison Canyon (Fig. 12). An original braid plain surface with multiple ephemeral channels flowed across the structure. An uplift event occurred, and subsequent ero­sion and incision of the channels and braid plain surface preserved a terrace on the south side of

the river (T5). A second uplift event occurred. A terrace was formed on both sides of the river (T4) as subsequent erosion and incision confined the Murchison River within the developing canyon. At this time, the surface of T5 was 5 m higher than the Murchison River, and the 10Be sample locations were 4–5 m higher than the river. This elevation above the river precluded the surfaces from being flooded and intermittently shielded, which allowed 10Be to accumulate on the sam­pled surfaces from this time onward (Fig. 13). Then, a third uplift event occurred. Subsequent erosion and incision concentrated flow in one of the two channels that cut through the canyon and preserved a remnant terrace between the two channels (T3). The most recent uplift event and erosion sequence then occurred, preserv­ing a terrace (T2) along the southern margin of the canyon. T1 is the active floodplain terrace. In this sequence model, three of the four uplift events have occurred since ca. 240 ka. The lag time between uplift event and terrace abandon­ment is not constrained (Fig. 13). However, given the uncertainties addressed in our proba­bilistic analyses, the limiting age estimate of ca. 240 ka approximates the timing of the antepen­ultimate event.

The geomorphology of the canyon suggests that the locus of uplift is south of the Murchison River (the fold structure is doming), and each event is pushing the river slightly to the north (Figs. 5 and 6). The north side of the canyon is functionally the inside of a meander bend in terms of large­scale channel architecture, yet it is the outside of this bend (the south side) that has preserved the flight of terraces. The Murchi­son River appears to be incising through the fold limbs both upstream and downstream of Mur­chison Canyon. The channel that crosses the T2 terrace upstream of the canyon was probably abandoned due to folding during the most recent event, forcing the river to the east by steepening the eastern limb of the fold. The active channel now runs along the eastern margin of the chan­nel belt (Fig. 5, reach M­B).

South of the canyon, the trench data along Lake Wooleen from the Roderick middle trench indicate that colluvium from slopes along the

TABLE 4. ESTIMATED SLIP RATES FOR FAULTS WITHIN THE MOUNT NARRYER FAULT ZONE

Fault segmentAge estimate

(Ma) ±m.y.Dip slip*

(m)Dip-slip rate

(mm/yr)

Oblique-slip (total slip) rate

(mm/yr)†

Narryer west 0.19 1.4 0.007 0.015Roderick River 0.22 1.5 0.007 0.014Sanford east 0.36 1.7 0.005 0.009Roderick terraces 0.24 0.05 7.5 0.033 ± 0.007 0.066 ± 0.015

Min MaxMount Narryer fault zone total 0.04

*From Whitney and Hengesh (2013).†Estimated 50% of total slip is lateral.

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western margin of the lake was deposited prior to ca. 43 ka. The colluvium appears to slope toward the lake. Presumably, it was shed from the scarp face, though we did not encounter the ferricrete scarp in our trench. Lacustrine sedi­ments then were deposited on the colluvium. A duricrust capstone developed in the lacustrine unit. The duricrust capstone was eroded and then buried by sheetwash deposits.

The Roderick south trench provides the only evidence for discrete faulting to the surface. The exposed bedrock displays a distinctly different style of deformation compared to the alluvial deposits in all the other trenches. We interpret two events from the stratigraphy of the Roder­ick south trench. The penultimate event formed a scarp by faulting the highly weathered granitic saprolite. The scarp shed colluvium to the east. The colluvium includes coarse blocks of gra­nitic saprolite. A soil profile developed a duri­crust in the colluvium. Then, the most recent event faulted the colluvial duricrust, forming another scarp that was subsequently eroded. A younger weathering profile overprinted the faulted duricrust. Both of these events occurred before the deposition of the lacustrine unit prior to at least 9 ka. Based on the weathering profiles of the colluvial units and apparent scarp denuda­tion, we suggest that the two events could have occurred much longer ago (>240 ka?), and the 9 ka age of the lacustrine unit relates to a much more recent climatic event.

1941 Meeberrie Earthquake and 1885 Earthquake Events

Williams (1979) conducted dendrochrono­logical analyses of trees along the Mount Narryer lineaments. He identified a relatively uniform age of the trees sampled along the lineaments, and he tentatively suggested that the Mount Narryer faults ruptured during the 1885 M ~6–7 Geraldton­Northhampton earthquake. How­ever, based on the undeformed sheetwash sedi­ments (>10 ka) observed in our trenches, and the scarp morphology (e.g., lineaments relate to the boundary between duricrust and overlying sheetwash sediments on the east­facing limb of the eroded folds), we conclude that the last movement on the fault was much older. Likely, the uniform age of the trees he dated represents a climatic event or a flood­dispersed seed ger­mination event rather than an earthquake event.

We did not observe evidence for surface deformation resulting from the 1941 Meeberrie event in any of the three structures we trenched, nor did we observe any geomorphological evi­dence that suggested a 70­yr­old event occurred on any of the features we visited during our site investigations. The epicenter for the Meeberrie

earthquake is poorly constrained (Everingham et al., 1982). It is unlikely that the earthquake occurred on the Sanford River West trace that we did not visit, because that fault segment is too short to have independently accommodated a large­magnitude event. Eyewitnesses from Meeberrie station, located between the Roder­ick and Mount Narryer faults (Fig. 2) recalled the “shaking and rumble, like a passing loco­motive” propagating from north to south (Carol McTaggart, former resident of Meeberrie sta­tion, 2012, personal commun.). The Sanford West fault is ~50 km south of Meeberrie sta­tion. Damage and shaking at Mount Narryer, Meeber rie, and Wooleen stations were more intense than at Murgoo station, which is located proximal to the Sanford fault segments (Reg Seaman, resident of Murgoo station, 2012, per­sonal commun.; McCue, 2014).

If the Meeberrie earthquake caused surface deformation, either it has not been observed, or the deformation occurred on a fault that has not been investigated. There are a few candi­date features within a 50 km radius that could relate to the Meeberrie event (e.g., Curbur scarps, Meeberrie fault) and that also should be investigated. Alternatively, it is possible that the earthquake did not rupture to the surface. Given the blind­fault nature and subtle surface expres­sion of paleoseismic evidence we have observed in the region (Whitney and Hengesh, 2015a, 2015b), the Meeberrie earthquake may not have been morphogenic.

Regional Strain Gradient

The Western Australia shear zone, proximal to the plate boundary, accommodates slip on the order of 8 mm yr–1 across a broad zone of reactivated structures (Hengesh and Whitney, 2014). In the Barrow Island region offshore in the Carnarvon Basin, slip rates decrease to ~4 mm yr–1, with roughly 75% of slip being accommodated by dextral displacements within the Western Australia shear zone (Hengesh and Whitney, 2014). Slip rates continue to decrease to the south in the Western Australia shear zone. In the onshore southern Carnarvon Basin, indi­vidual fault structures accommodate <0.1 mm yr–1 of slip (Whitney et al., 2014; Whitney and Hengesh, 2015a, 2015b). The slip rate estimates from the Mount Narryer fault zone are lower still than the Mesozoic basins to the northwest and higher than the cratonic interior (<0.005 mm yr–1) to the southeast (McPherson et al., 2013).

The Mount Narryer results provide an addi­tional data point with which to quantify the hypothesis suggesting the existence of a strain gradient extending ~2000 km from Australia’s

plate margin into the interior of the Yilgarn cra­ton (McPherson et al., 2013; Hengesh and Whit­ney, 2014; Whitney and Hengesh, 2013, 2015a, 2015b; Whitney et al., 2014). Slip rates on faults within the Narryer terrane are found to be inter­mediate between the two orders of magnitude spread of rates between the adjacent extended continental crust and cratonic crust, suggesting that the gradient is smooth rather than stepped or abrupt.

CONCLUSIONS

We provide paleoseismological data for at least four and possibly six, mid­ to late Qua­ternary morphogenic earthquake events across three fault strands within the Mount Narryer fault zone. At least four events have occurred on the Roderick River fault. The 10Be measure­ments of a suite of samples from an uplifted alluvial terrace sequence allow us to estimate a maximum limiting age of the antepenultimate event of ca. 240 ka. An earlier event occurred prior to 240 ka, whereas the penultimate and most recent events occurred since 240 ka. Two of these four events are expressed as faulting of bedrock at the surface, whereas the other events are expressed as faulting of bedrock near the surface, resulting in fault propagation folding at the surface. There is evidence for at least one folding event on the Mount Narryer West and Sanford River East faults. Whether the fault strands ruptured independently or in multistrand events remains unclear.

The Mount Narryer fault zone is a well­expressed multistrand intraplate fault system. Faults in the system exhibit evidence for repeated morphogenic events. Slip rates for the faults in the zone range from 0.01 to 0.07 mm yr–1, with a total late Quaternary slip rate for the zone between 0.04 and 0.11 mm yr–1. These rates are intermediate to those in the adjacent Mesozoic basin (>0.1 mm yr–1) and Precambrian craton (<0.005 mm yr–1), and they suggest that strain is transferred smoothly across these geological boundaries rather than along abrupt, stepped transitions.

Surface deformation within the Mount Narryer fault zone is predominately fault propa­gation folding expressed as flexure of pedogenic horizons in surficial materials. The orientation of discontinuous shear zones oblique to the main structural trends indicates a component of dextral displacement on the underlying faults consistent with an SHmax oriented at 75°. The overall style of deformation suggests a recent (mid­late Pleistocene) onset of fault activation, as individual shear zones and fault strands are not through­going, and parent faults have not reached the surface through alluvial cover. Mid­

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Paleoseismology of the Mount Narryer fault zone, Western Australia

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to late Pleistocene reactivation of the Mount Narryer fault zone is attributable to the most recent pulse of collision at the plate boundary. Reactivation is consistent with the timing of collision between the Scott Plateau (Austra­lian continental crust) and Suva­Rote Ridge (Banda arc terranes) on the northern Austra­lian plate margin between 1.8 and 0.2 Ma (cf. Roosmawati and Harris, 2009; Hengesh and Whitney, 2014). The decreasing strain gradient from the plate boundary into the plate interior in the Western Australia shear zone along the for­merly rifted continental margin and craton edge suggests that, in this region of intraplate Austra­lia, far­field plate­boundary forces are driving intraplate tectonics, as has been suggested by others (e.g., Cloetingh and Wortel, 1986; Reyn­olds et al., 2002; Sandiford and Quigley, 2009). The timing of fault reactivation elsewhere in the Western Australia shear zone also is consistent with this interpretation. Collectively, the rates of tectonic deformation observed in the Mount Narryer fault zone, and the Western Australia shear zone as a whole, do not satisfy the criteria used to define stable continental regions. Tec­tonic processes are ongoing, and, as such, this part of western Australia should no longer be considered a stable continental region.

ACKNOWLEDGMENTS

We acknowledge the financial support provided by the Chevron Australia Business Unit (project PDEP AES 12­P1ABU­82) for this project and the interest and support for this research shown by Dan Gillam. We thank Stephen White and the Geological Survey of Western Australia, who provided access to their aerial photograph collection. We thank Martha Whitney and Christopher Slack for field assistance. We thank Dylan Rood at Scottish University Environmental Research Center for making the accelerator mass spectrometry measurements. We thank Steven Forman for complet­ing optically stimulated luminescence analyses. We thank Richard Koehler for detailed comments and constructive review of an earlier version of this manu­script. We thank Wendy Bohon for her review of this manuscript. All of the reviews helped to improve the manuscript. We thank the folks in the Shire of Murchi­son who allowed us to access and excavate their lands and provided us their local knowledge and history of the 1941 Meeberrie earthquake. They include the Mc­Taggarts at Mount Narryer station and the Seamens at Murgoo station. We are grateful to David Pollock and Frances Jones at Wooleen station for their hospitality and logistical support.

This work forms part of the activities of the Centre for Offshore Foundation Systems (COFS), currently supported as a node of the Australian Research Coun­cil Centre of Excellence for Geotechnical Engineering and as a Centre of Excellence by the Lloyd’s Register Foundation. The Lloyd’s Register Foundation invests in science, engineering, and technology for pub­lic benefit, worldwide. Whitney is the recipient of a scholarship for international research fees from The University of Western Australia. Dan Clark publishes with the permission of the chief executive officer of Geoscience Australia.

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as doi:10.1130/B31313.1Geological Society of America Bulletin, published online on 20 August 2015

Paleoseismology of the Mount Narryer fault zone, Western Australia

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 Beau B. Whitney, Dan Clark, James V. Hengesh and Paul Bierman multistrand intraplate fault systemPaleoseismology of the Mount Narryer fault zone, Western Australia: A  

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