RESEARCH Revised Slip Rates for the Alpine Fault At

Home , Alpine Fault, Clarence Fault, Hokitika, Inchbonnie, Marlborough Fault System, Taramakau River

RESEARCH

Revised slip rates for the Alpine fault at Inchbonnie: Implications for plate boundary kinematics of South Island, New Zealand

R.M. Langridge1, P. Villamor1, R. Basili2, P. Almond3, J.J. Martinez-Diaz4, and C. Canora4 1GNS SCIENCE, P.O. BOX 30-368, LOWER HUTT 5010, NEW ZEALAND 2ISTITUTO NAZIONALE GEOFISICA E VULCANOLOGIA, VIA DI VIGNA MURATA 605, 00143 ROME, ITALY 3DEPARTMENT OF SOIL SCIENCE, P.O. BOX 84, LINCOLN UNIVERSITY, LINCOLN 7647, CANTERBURY, NEW ZEALAND 4DEPARTAMENTO DE GEODINÁMICA, FACULTAD DE CIENCIAS GEOLÓGICAS, UNIVERSIDAD COMPLUTENSE DE MADRID, MADRID 28040, SPAIN

ABSTRACT

The northeast-striking, dextral-reverse Alpine fault transitions into the Marlborough Fault System near Inchbonnie in the central South Island, New Zealand. New slip-rate estimates for the Alpine fault are presented following a reassessment of the geomorphology and age of displaced late Holocene alluvial surfaces of the Taramakau River at Inchbonnie. Progressive avulsion and abandonment of the Taramakau fl oodplain, aided by fault movements during the late Holocene, have preserved a left-stepping fault scarp that grows in height to the northeast. Surveyed dextral (22.5 ± 2 m) and vertical (4.8 ± 0.5 m) displacements across a left stepover in the fault across an alluvial surface are combined with a precise maximum age from a remnant tree stump (≥1590–1730 yr) to yield dextral, vertical, and reverse-slip rates of 13.6 ± 1.8, 2.9 ± 0.4, and 3.4 ± 0.6 mm/yr, respectively. These values are larger (dextral) and smaller (dip slip) than previous estimates for this site, but they refl ect advances in the local chronology of surfaces and represent improved time-averaged results over 1.7 k.y. A geological kinematic circuit constructed for the central South Island demonstrates that (1) 69%–89% of the Australian-Pacifi c plate motion is accommodated by the major faults (Alpine-Hope-Kakapo) in this transitional area, (2) the 50% drop in slip rate on the Alpine fault between Hokitika and Inchbonnie is taken up by the Hope and Kakapo faults at the southwestern edge of the Marlborough Fault System, and (3) the new slip rates are more compatible with contemporary models of strain partitioning presented from geodesy.

LITHOSPHERE; v. 2; no. 3; p. 139–152. doi: 10.1130/L88.1

INTRODUCTION tral slip rate averaging ~27 ± 5 mm/yr (Norris Last Glacial to late Holocene), and have been and Cooper, 2001) and which is also respon- used to demonstrate the variability of strike-slip Partitioning and transfer of strain at obliquely sible for the uplift of the Southern Alps. At its and dip-slip partitioning along the length of the convergent collisional plate boundaries over southern end, the Central segment of the Alpine fault (Norris and Cooper, 2001; Berryman et al., millennial time scales are poorly documented fault evolves offshore into partitioned strike-slip 1992). Dextral slip-rate measurements along the worldwide. The Alpine fault, and its transition faulting and oblique subduction in the Fiordland Central segment of the fault are high, e.g., 27 to the Marlborough Fault System in the north- region, related to the Puysegur margin (Barnes, ± 5 mm/yr (Waikukupa River); 29 ± 6 mm/yr ern South Island of New Zealand, offer such an 2009; Sutherland and Norris, 1995; Barnes et (Kakapotahi River) (Fig. 1). Geologic dip-slip opportunity using late Holocene geologic slip al., 2005), whereas at its northern end the fault rates vary along strike, ranging from >12 mm/ rates and vectors to assess an important on-land transitions into the Marlborough Fault System, yr (Gaunt Creek) to 0 mm/yr (Hokuri Creek). transitional plate boundary. a zone of distributed strike-slip deformation For such a major plate boundary structure, these The Alpine fault is a major component of (Yeats and Berryman, 1987; Van Dissen and rates show considerable variability and uncer- the collisional zone between the Australian and Yeats, 1991; Langridge and Berryman, 2005). tainty, and importantly, owing to the rugged and Paciﬁ c plates across the South Island (e.g., Cox The Alpine fault is a highly evolved fault with vegetated nature of the West Coast terrain, they and Sutherland, 2007). Through the southern total dextral displacement of bedrock geology are derived from sites spaced tens of kilometers half of the island, the northeast-striking and of ~480 km (Wellman, 1955; Cox and Suther- apart along the fault. southeast-dipping Alpine fault exhibits pri- land, 2007). Rupture of the Central segment rep- In this paper we used a geomorphic and marily dextral-reverse slip and accommodates resents a signiﬁ cant seismic hazard, capable of structural approach to calculate revised slip

50%–80% of the 37 ± 2 mm/yr of convergent generating Mw 7.8–8.0 surface-rupturing earth- rates for the Alpine fault at Inchbonnie in motion across the plate boundary (DeMets et quakes every few hundred years (Yetton, 1998, north Westland (Fig. 2) based on the offset of al., 1994; Sutherland et al., 2006; Berryman 2000; Rhoades and Van Dissen, 2003; Suther- late Holocene features. This area is important et al., 1992) (Fig. 1). This section of the fault, land et al., 2007; Wells et al., 1999). because the dextral slip rate along this portion of between Milford Sound and Toaroha River, is Late Quaternary slip rates have been esti- the fault decreases by 50%–70% compared with generally referred to as the Central segment, as mated for the Alpine fault from offset geo- those sites to the southwest (Norris and Cooper, it represents the ≥325-km-long, straight on-land morphic markers that range over more than 2001), and the site is present at a key location part of the fault with a consistently high dex- one order of magnitude in age (generally from for understanding the kinematic transition from

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168∞E 171° E 174° E North Island Tasman Sea Wellington North 47 h Island g Cook plate u ro WF T ° i Strait 40 S g AwF n ra u 42° S Hik CF 42∞S Australian MFS 39 Lake Brunner Alpine Fault Hope F 37 IB HF kF plate TR South KF PF Pacific margin Island AlpineKR fault Ocean Pacific 2 segur GC . y WR g 34 ° Fig.Fi 2 PPFZ Christchurch Pu 170 E Central segment

Southern Alps 0150Km Milford Sound HC South Alpine Fault sites HC Hokuri Creek 45° S Alpine fault Island WR Waikukupa River 45° S GC Gaunt Creek KR Kakapotahi River TR Toaroha River Dunedin IB Inchbonnie Fiordland Fault names AwF Awatere fault CF Clarence fault kF Kakapo fault HF Hope fault KF Kelly fault PF Poulter fault Stewart PPFZ Porters Pass Fault Zone Island 165° E 168° E 171° E 174° E

Figure 1. Map of major active faults of South Island, highlighting the Alpine fault and Marlborough Fault System (MFS). The Central segment of the Alpine fault is marked in bold. Fault names and localities are shown in the legend. Inset: Plate tectonic setting of New Zealand, including locations of the Puysegur and Hikurangi subduction margins. Relative motion between the Paciﬁ c and Australian plates is shown in mm/yr from De Mets et al. (1994).

the Alpine fault to the Marlborough Fault Sys- the northeast from the Central segment of the differential GPS, trench logging, soil chronol- tem. Because the previously published slip rates Alpine fault onto the Hope and Kelly faults near ogy, and AMS (accelerator mass spectrometry) at Inchbonnie of Berryman et al. (1992) (i.e., 10 Inchbonnie (Robinson, 2004; Berryman et al., radiocarbon dating. Initial studies in the Inch- ± 2 mm/yr dextral; 6 ± 2 mm/yr reverse) come 1992; Wallace et al., 2007; Stirling et al., 2002). bonnie area suggested that paleoseismic trench- from displacement of a very young surface In this paper we use our geologic slip-rate data to ing would not yield a straightforward paleo- (1 k.y.) dated using only a weathering rind tech- construct a kinematic model for the plate bound- earthquake record (see Toy, 2007; Langridge et nique, an essential part of this study has been to ary transition from the Alpine fault to the Marl- al., 2008). However, two signiﬁ cant outcomes of recognize and date alluvial surfaces using radio- borough Fault System in central South Island. trenching included a need for (1) detailed geo- metric and relative dating techniques to estimate morphic mapping of the fault scarps and Holo- slip rates averaged over a longer time, i.e., over METHODS AND RESULTS cene alluvial surfaces in the area, and (2) precise more earthquake cycles. age control on those surfaces, in order to reassess Geologic and geodetic data and their deriva- The main tools used in this study were aerial local slip rates for the Alpine fault. tive models indicate that a large proportion photograph interpretation, geomorphic mapping, In the following sections the geomor- of the tectonic plate motion is partitioned to 2-D scarp proﬁ ling, topographic surveying using phic development of the Taramakau valley is

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Tasman Sea Greymouth Taramakau River Hokitika

T O Alpine P TR KeKKelly fault Alpine KKR elll IB ffafaulta ly fault uull t FFigFig.g. 3 Southern Alps ded DDiDivideivivi MaMMaina MFS inin MFMF Clarence HHFF fault PPF F KKB Kakapo fault

0 km 50

Figure 2. Digital elevation strip map of central Westland, rotated to emphasize the Alpine fault. Large arrow indicates north. The study area at Inchbonnie (IB) is within the small box designated Figure 3. The Taramakau River is shown from the Main Divide of the Southern Alps (thin dotted line) to the coast. The maximum extent of ice from the Last Glacial in the Taramakau system—on the Australian plate side of the Alpine fault—is shown by the white area and consists of three lobes: T—Taramakau; O—Orangipuku; P—Poerua. Traces of the major regional faults of the Marlborough Fault System (MFS), including the Hope and Poulter faults (HF, PF) are highlighted by arrows. Slip-rate sites on the Alpine, Hope (MF, McKenzie fan), and Kakapo (KB, Kakapo Brook) faults are also marked. KR—Kakapotahi River; TR—Toaroha River.

described, along with a chronosequence of late into three lobes at the range front of the South- the upthrown side of the Alpine fault by risers Holocene alluvial surfaces (Figs. 3, 4). This is ern Alps near Inchbonnie. These lobes excavated that separate them. On the downthrown side of followed by a description of the location and distinct, moraine-bounded troughs, referred to the fault the differentiation of surfaces based on height of the scarp of the Alpine fault between here as the Poerua, Orangipuku, and Taram- topography is diffi cult, as older surfaces are par- the Taramakau River and Lake Poerua (Fig. 5) akau lobes (Fig. 2). On the northwest side of tially reoccupied or buried by large fl oods from along with presentation of scarp profi les used to the Alpine fault the glacial troughs are separated the active younger fl oodplain. assess the vertical displacement history (Fig. 6). by a series of ice-sculpted, dome-shaped Creta- On the West Coast of the South Island, gray- This is followed by a description of the Harris ceous to Paleozoic granitoid hills of the Hohonu wacke alluvium is characteristic of larger rivers trench site, where a microtopographic map is Group, e.g., Mount Te Kinga (Figs. 2–4) (Sug- with headwaters in the lower grade graywacke used to characterize the lateral slip at the site. gate and Waight, 1999; Nathan et al., 2002). For rocks of the Torlesse Supergroup that crop out Two of fi ve trench exposures are discussed to more detail of the bedrock geology, see Figure 3. near the Main Divide of South Island (Nathan et provide background to the stratigraphy and age al., 2002). The modern river course (referred to of surfaces and their deposits, and the tectonic Holocene Alluvial Surfaces as I-Ø) exclusively follows the Taramakau lobe structure there. In addition, to aid interpretation Figure 4 shows a series of fanning alluvial fl owing southwest to the Tasman Sea (Figs. 2, of surface and landscape ages, soil profi les from surfaces related to the Taramakau River near 3). Surface I-1 corresponds to the most recently these trenches are also described. Last, in order Inchbonnie. During the Holocene, and fol- abandoned and faulted surface within the Tara- to derive slip rates for the site, the ages of the lowing retreat of the valley glacier, the glacial makau valley. I-1 also grades geomorphically key displaced alluvial surfaces are discussed. troughs have been partly fi lled by alluvium from to the Taramakau lobe of the system but may the Taramakau River system. In this paper we have at times spilled into the Orangipuku lobe Glaciofl uvial History of the Taramakau recognize a series of inset, overlapping fanning (Fig. 4). Buried forests and remnant “snag” trees Valley alluvial surfaces in the Inchbonnie area. Based are useful in estimating the times of construction on their geomorphology, surface texture (i.e., and abandonment of surfaces I-Ø to I-2. At site The glacial history of the Taramakau valley “smoothness”), and channel fl ow directions, it TR1 a number of in situ tree stumps are exposed is well recorded for the Last Glacial Maximum has been possible to distinguish several distinct in the bed of a secondary channel of the Tara- (LGM) by extensive moraines, one of which has “Inchbonnie” alluvial surfaces locally described makau River, southwest of Inchbonnie (Figs. 3, impounded Lake Brunner (Figs. 1, 3) (Suggate, as I-Ø to I-4 (Fig. 4). Radiocarbon samples 4) and in the adjacent edge of an eroding, sandy 1965; Suggate and Waight, 1999; Nathan et al., from exposures, and from in situ and buried tree fi ll terrace that corresponds to I-1. The outer 2002). It was recognized that during the LGM stumps, are used to infer ages for these surfaces rings from stump TR1 were dated and yielded the former Taramakau valley glacier branched (Table 1). In general, I-1 to I-4 are identifi ed on an AMS radiocarbon age of 416 ± 20 yr B.P.

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Lake Brunner 0 1 2 km

Bruc

r Australian plate rocks e Stream Fault traces (generally granodiorite) u Rive Fig. 4 Pacific plate rocks uk Concealed fault traces (generally Torlesse Schist)

rangip Pacific plate rocks Roads O (semi-schist graywacke) MTK HR

Orangipuku lobe Taramakau Taramakau Poerua lobe lobe River L ake Poerua DC F Inchbonnie - Brunner Road Alpine fault Hwy 73 Alpine fault

I-Ø Inchbonnie ?

Taipo River Hope fault BC

MTK Lakes

Active, unfaulted young I-Ø floodplain of Taramakau valley

Jacksons Harris trench site Hwy 73 Channels

Figure 3. Simplifi ed geologic and geomorphic map of the Inchbonnie area (modifi ed after Nathan et al., 2002). The Alpine fault is shown from left to right. Holocene alluvial surfaces are shown in white. In this area the Pacifi c plate is composed of a sequence of regionally metamorphosed Mesozoic sediments of the Torlesse composite terrane. Torlesse rocks are uplifted along the Alpine fault, forming a dipping crustal section of metamorphic grade IV (schists) to I (graywacke) rocks that characterize the Southern Alps. Juxtaposed against this section are Paleozoic to Mesozoic metasedimentary and plutonic rocks of the Australian plate (Suggate and Waight, 1999). The box at center marks the area of Figure 4. Hwy 73—Arthur’s Pass Highway; DCF—Dry Creek fan; HR—Hohonu Range; MTK—Mount Te Kinga.

(330–498 calibrated yr B.P. at 2σ) (Table 1). beheaded by the recent switch of the Taramakau graywacke in the Bruce Stream catchment are This date suggests that the Taramakau River River (I-Ø and I-1). These channels are pinned typically overlain by ~0.5 m of sandy to silty recently shifted to its current course and also on the west side of the Orangipuku lobe and deposits that resemble overbank deposits. A probably provides a maximum age for I-1. That form surface I-2a. previous maximum age estimate for this surface is, a forest was formerly established in a place Surface I-2b follows the central axis of the of 1100–1300 yr comes from measurements of which has been both inundated by sediment and Orangipuku lobe and is characterized by a var- weathering rinds on graywacke clasts in a gravel reoccupied by the river during the past 500 yr. iegated network of northward-fl owing under- pit within the scarp of the Alpine fault (Fig. 5) The I-2 surface is a lobate-shaped alluvial fi t braided channels that merge to form Bruce (Berryman et al., 1992). surface that extends across the Alpine fault from Stream (Fig. 4). A tree stump is exposed in the The I-3 alluvial surface traverses the area that the Taramakau River northward in the direction channel of Bruce Stream at its lower end where includes the Harris trench site, and therefore an of the Orangipuku lobe and Lake Brunner. At the I-2c surface has been superseded by I-2b essential part of this study was to understand the least three subdivisions of I-2 have been made drainage. A wood sample from the outside of extent and age of this surface. On the upthrown using old aerial photographs combined with stump B3 yielded an age of 332 ± 25 yr B.P. side of the fault near the trench site, I-3 is a typi- fi eld reconnaissance. Despite its lack of topo- (302–447 cal. yr B.P. at 2σ) (Table 1). On cally smooth geomorphic surface, as evidenced graphic separation and modifi cation by farming aerial photographs, surface I-2c is character- by fi ne-grained overbank deposits exposed practice, surfaces I-2a to I-2c are mapped from ized by a smooth geomorphic texture lacking within trenches there. On the downthrown side west to east across this fl oodplain. The spring- in distinct channels. This texture is typical of of the fault the I-3 alluvial surface is character- fed and underfi t Orangipuku River is character- surfaces covered by a veneer of overbank fl ood ized by a network of north- to northeast-directed ized by a braided network of channels that were deposits. Exposures of coarse, cobbly alluvial channels that trend northeastward into the Poerua

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deposits that cover a paleotopography on alluvial gravel (Golder Associates, 2007). Northeast of Lake Poerua, I-4 is overlain by schist-bearing alluvial fans derived from the range front of the Alpine fault (e.g., Dry Creek; DCF in Figs. 3, 4). The southwest edge of I-4 is marked by a prominent riser cut at the edge of the large push-up adjacent to the Harris trench site (Figs. 4, 5). I-4 is buried on the downthrown side of the fault by I-3 and subsequent deposition. Although there is no direct age control for I-4, it is inferred from the predominance of graywacke cobbles that it corresponds to an older course of the Taramakau River through the Poerua lobe, which has been uplifted along the trace of the Alpine fault. In summary, during the late Holocene the Taramakau River has occupied three main courses that correspond to former glacial troughs. Surfaces I-Ø to I-4 correspond to a series of inset alluvial fan lobes formed as the river migrated through avulsion from the northeast (I-4) to its current position (I-Ø).

Location and Size of the Alpine Fault Scarp

Geomorphic Expression of the Fault The stretch of the Alpine fault between the Taramakau River and Lake Poerua has received much attention because of its ease of access and exposure (Figs. 4, 5) (Berryman, 1975; Figure 4. Quickbird image of the Inchbonnie area. Outwash alluvial surfaces of the Taramakau River Berryman et al., 1992). The fault has a clear, system are labeled I-Ø to I-4 and fan from southwest to northeast across the area. The trace of the fresh trace along this stretch and also forms the Alpine fault is marked by arrows from the Taramakau River to Lake Poerua. The Harris trench site is southeastern shorefront of the lake (Langridge marked by a star. Rectangle marks the strip map shown in Figure 5. Other abbreviations are listed and McSaveney, 2008). The fault displaces allu- in the Figure 3 caption. vial surfaces I-1 to I-4 along this stretch, each of which is typically recognized by a stepwise increase in scarp height to the northeast on the lobe of the valley (Fig. 4). These underfi t chan- study area. I-4 is preserved only on the upthrown upthrown side of the fault. Along this stretch the nels grade to Lake Poerua and have dissected the side of the Alpine fault to the northeast of the fault is also characterized by a series of left-step- smooth upper surface of I-3, and therefore prob- Harris site. I-4 also characterizes the surface that ping fault traces with approximately kilometer- ably represent a reoccupation of the I-3 surface bounds the southeastern shore of Lake Poerua length sections separated by 80–100-m-wide by overbank fl ow, shown as I-2r in Figure 4. (Figs. 4, 5). Engineering soil pits excavated into stepover zones between sections (Fig. 5). North- The I-4 surface refers to the texturally I-4 near the lake show a variable thickness (0.2– east of Lake Poerua the trace of the Alpine fault smoothest and highest alluvial surface in the 1.2 m) of moderately weathered, fi ne-grained is buried by young alluvial fans that emanate

McArthur Road S1 S2 S3 Lake Brunner Rd LakeLake PPoeruaoerua 0 100 Fig. 7 Silage pit 35 m 49 ± 30 (1200 ± 100 WR) I-3 2 I-2 I-3 HR 203 ± 1 WP3 WP13 11–13 m RL Alpine fault

Alpine fault PU L1 Zb Za ZX1 Riser I-4 N I-1 I-3 I-4 Riser I-2 StoStopbanktoopb ZØ surface river pbankpbank nk Inchbonnie Riser S4

Figure 5. Strip map of the Alpine fault in the Inchbonnie–Lake Poerua area. The fault traces (hachured) consist of ~1 km sections separated by ~100-m-wide left stepover zones. The 10 topographic scarp proﬁ les, labeled ZØ through L1, are shown by dashed lines. The polygon shows the location of Figure 7, constructed around the Harris trench site. PU refers to the highest part of the push-up related to the left stepover in the fault. Radiocarbon ages from within Lake Poerua and a weathering rind (WR) age locality are marked.

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c channel ZØ ac abandoned channel I-1 SW re railroad embankment fs frontal scarp 2.3 m bs back-scarp I-2 I-1 pu push-up ZX1 dms distributed 3.1 m Legend main scarp Zb I-2 ls lake shore I-2 lf lake floor Za 3.3 m S1 I-2 I-3 HR re I-4 c I-2r I-1 to I-4 refer to alluvial surfaces pu S2 re I-4 bs I-4 S3 4.8 m ac fs I-3 c I-2r S4 fs bs L1 I-3

6.9 m ls dms 3 2 Scale (m) 1 V.E. c. 10 x 0 050100 150 lf WP3 stump NE

Figure 6. Surveyed profi les across the Alpine fault from southwest to northeast between the Taramakau River and Lake Poerua. Profi le locations are shown in Figure 5. Vertical exaggeration (V.E.) is 10×. Abbreviations of geomorphic features are shown in the legend. Five examples of the scarp height projections are shown for profi les ZØ, ZX1, Zb, HR, and L1.

TABLE 1. SUMMARY OF RADIOCARBON DATES FROM THE INCHBONNIE AND LAKE POERUA AREAS Location NZA del 13C Radiocarbon Calibrated age Material and signiﬁ cance (NZMS 260 grid ref.) lab no. age (cal. B.P.) sample ID (yr B.P.) 1σ range 2σ range

Taramakau River (K33/821298) TR1(13) 30472 –25.5 416 ± 20 342–350 & 330–369 & Sliver of wood from the outside of a tree stump (rings 1–3) exposed 452–491 441–498 in the bed of the Taramakau River. Dates a former forest that has been exposed by the shifting of the river into its current course. Bruce Stream (K32/833360) B3 31163 –22.4 332 ± 25 309–439* 302–336 & Sliver of wood from the outside of a tree stump exposed in an 356–447 active channel. Dates a former forest that has been exposed by the shifting of Bruce Stream. Lake Poerua (K32/862315) WP13(2023) 29673 –21.1 1203 ± 35 978–1165* 968–1170 Sliver of wood representing tree rings 20–23 from near the outside of stump WP13. Should closely approximate the death age of WP13. WP13(4547) 29232 –22.1 1217 ± 25 1006–1167* 984–1034 & Sliver of wood that represents tree rings 45–47 of stump WP13 at 1047–1171 Lake Poerua WP13(222) 30473 –23.0 1329 ± 20 1179–1210 & 1173–1278 Sliver of wood that represents tree rings 221–223 of stump WP13. 1227–1262 WP3 29229 –22.8 249 ± 30 153–301* 147–220 & Grab sample of wood from the outside of remnant stump WP3 267–312 within Lake Poerua. WP3 stands in ~1.9 m of water. Note: Radiocarbon age: Conventional radiocarbon age before present (AD 1950) calculated as deﬁ ned in Stuiver and Polach (1977) using Libby half-life of 5568 yr, and normalized to δ13C of –25‰. Quoted error is ±1σ. Calibrated age: calendar years before present (AD 1950) and calendar years AD/BC using calibration program Winscal5.0 (© Inst. Geological & Nuclear Sciences) and Southern Hemisphere atmospheric data from McCormac et al. (2004). A laboratory error multiplier of 1 has been applied to NZA samples. Age ranges listed are minimum and maximum values of the calibrated age range, based on a radiocarbon age error of ±2σ. *There are multiple calibration peaks; the range presented is the full range of all calibration peaks.

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from the range front of the Southern Alps and is deposits, local channel deposits, colluvium, to map the undeformed stratigraphy of the I-3 not exposed (Fig. 3). and soil units. Fault zone and anthropogenic surface and this channel. deposits have also been identifi ed in the stra- The Taramakau Gravels were deposited Fault Scarp Profi ling tigraphy. The major packages show the evolu- when the active fl oodplain of that river occupied Figure 6 shows 10 topographic profi les of the tion from an aggrading gravel surface (I-3) to the I-3 surface northeast of Inchbonnie (Fig. 4). scarp of the Alpine fault that are used to dem- overbank deposition, followed by abandon- The Taramakau Gravels comprise fl uvial gravel onstrate the progressive growth in the vertical ment, local deposition (related to reoccupa- and sand beds derived from graywacke source component of deformation along strike (profi le tion of I-3), and soil formation. The surfi cial material. The coarse, permeable nature of the locations shown in Fig. 5). Many of these pro- channel form exposed in trench 4 is a shallow gravels makes the preservation of organic mate- fi les probably document minimum scarp heights (~10 cm) erosional feature cut on fi ne-grained rial within them unlikely. Consequently, no owing to reoccupation and burial of older sur- cover materials (silt-sand and soils). Trench 4 wood or charcoal was available from these units faces on the downthrown side of the fault. A was excavated outside of the fault zone in order for dating the I-3 surface. clear fault trace is fi rst observed on the I-1 surface near the Taramakau River. Based on the pro- jection of I-1 across the fault, profi le ZØ yields a scarp height of ~2.3 m. Profi les across the I-2a and I-2b (surface profi les ZX1 and Zb) yield scarp heights of ~3.1 and ~3.3 m, respectively. I-2r A small shoulder (riser) on the upthrown side of the fault scarp marks the boundary between I-2c and I-3 southwest of Inchbonnie. Across the I-3 alluvial surface, profi les Za, S1, and HR yield typically larger scarp heights of ~6.4 ± 0.2, ~6.6 ± 0.2, and ~4.8 ± 0.5 m, respectively (Figs. 5, 6). f1 Three further profi les across the I-4 surface Main trace between the trench site and Lake Poerua (S2, I-3 C h a n n e l S3, and S4) yield scarp heights of 8.1 ± 0.6 m, 6.5 ± 0.4, and 9.3 ± 0.3 m, respectively. Finally, f5

profi le L1 was measured from tree stump WP3 ditch Stepover zone (Fig. 5; Table 1) on the fl oor of Lake Poerua to f6 the lakeshore and across the raised edge of the f5 Push-up lake. This profi le confi rmed that the steep lake a edge was the scarp of the Alpine fault, which Farm has an overall scarp height of ~6.9 ± 0.2 m there f1 (Langridge and McSaveney, 2008).

Harris Trench Site f3 f2 At the Harris farm, detailed topographic sur- I-4 veying and paleoseismic trenching were under- f7 f4 b f8 taken to estimate the lateral slip and to determine the paleo-earthquake record of the Alpine fault. I-3 The site lies at the leading edge of a prominent f6 c left stepover in the fault. Directly northeast of the site the stepover is characterized by a com- Main trace pressional bulge with a range-facing back scarp Displacements (see proﬁ le S2 in Fig. 6). At the trench site the a = 10 ± 1 m stepover zone comprises two main traces (f1, f7) b = ~ 1.1 m riser c = 11.4 ± 1.3 m and a number of subparallel, transpressive fault trace

traces (f2–f6) that displace the alluvial surface Ch a n n e l and locally a surface stream channel (Figs. 5, 7).

Site Stratigraphy and Sedimentation Meters Figure 8 shows trench 4 and an abbreviated stratigraphic legend and brief description of units at the trench site. Overall, the stratigra- Figure 7. Real-time kinematic Global Positioning System micro-topographic map of the edge of left stepover zone at the Harris trench site. Contour interval is 20 cm. Fault traces related to the phy exposed in the Harris trenches is demon- stepover are shown as solid gray lines. Strike-slip faults are marked with ticks on the downthrown strably coarse scaled and can be divided into side, and faults with dominantly reverse movement are shown with teeth on the upthrown side. ﬁ ve major late Holocene packages of units: Trenches (T1–T5) are marked by dark rectangles. A shallow local channel (I-2r; dotted arrowed line) Taramakau Gravels, Taramakau Overbank that passes through the site from south to north is used as a marker of dextral displacement.

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Harris trench 4 Surficial channel thalweg North wall

3333 1a 3434 1a 1b 1a 3434 1b 3434 3333 1b 3434 3535 r 3535 50 50

Site stratigraphy Surface Soil units 1a Topsoil (unit 1a) Fault zone (mx) I-2r Local channel deposits 1b Subsoil (unit 1b)

Taramakau Overbank deposits 3434 35 Sand (unit 34); silt (units 30, 33, 35) Scale I-3 = 1 m Taramakau Gravels 50 r Gravel; gravel clasts (unit 50)

Figure 8. North wall of Harris trench 4, showing the site stratigraphy and geomorphology in cross section. The stratigraphy of the deposits is correlated with the I-3 surface, I-2r reoccupation, and the shallow channel that traverses the site.

The Taramakau Overbank deposits comprise polynomial surface that honors the data values f6). This scarp was confi rmed as a rupture trace a series of sheetlike silt to sand units that blan- and predicts some degree of over- and under- in both trenches 5 and 3. Within the stepover ket the top of the gravels and fi ll in topographic estimation above and below local high and low zone the channel is guided by (runs parallel to) lows on that surface. These overbank deposits values. This technique allowed us to obtain a the oblique structures there (e.g., f3–f5) (Fig. 7). are extensive at the trench site and are consid- 1-m-resolution digital elevation model that had The amount of internal strike-slip deformation ered to be fl ood deposits from the Taramakau minimal errors compared with the meter-scale within the stepover zone is diffi cult to charac- River, laid down subsequent to aggradation of displacements across the site. terize. Based on their strike and the relatively the I-3 surface. In trench 4 the overbank depos- Continuous, multi-meter-high scarps occur shallow dips observed for fault traces f3 and its cover and fi ll an old paleochannel formed on on either side of the stepover zone at the trench f4, exposed in trenches 2 and 3, these faults are Taramakau Gravels (Fig. 8). site (e.g., f1, f7) but diminish in vertical expres- inferred to have a dominantly reverse sense of Widespread alluvial sedimentation ceased at sion as they pass into the stepover zone (Figs. 6, movement. However, as this internal component the Harris site following deposition of the Taram- 7). The main trace to the southwest (f7) steps of dextral displacement is unknown, the total of akau Overbank deposits. Local channel deposits to the left by ~80 m, to the main trace toward 22.5 ± 2.0 m must be considered a minimum refer to the units found underlying the displaced the northeast (f1). The microtopographic map value. The individual (a–c in Fig. 7) and total channel mapped through the site (see unit 1b in also shows that fault displacement is partitioned displacements presented here are equivalent to Fig. 8). Local channel deposits typically consist among north-northeast–striking structures those shown by Berryman (1975). of reworked sand and silt derived from the Tara- within the stepover, which manifest themselves makau Overbank sequence. These are associ- as oblique-slip faults and folds on the surface Structure and Faulting ated with a reoccupation of the I-3 surface that and in exposure; e.g., fault trace f6 was exposed In this study the main purpose of the trenches formed the surfi cial channels ascribed to surface at the upper end of trench 3 (Fig. 9). was to document the presence and style of fault- I-2r (Fig. 4). Following broad abandonment of At the trench site a shallow channel crosses ing related to individual fault traces within the the trench site area, soils began to develop within the fault zone at a high angle and was used as stepover and the correspondence between sur- this sequence of units. Apart from large earth- a piercing line to estimate a cumulative dextral face fault traces and their subsurface expres- quake faulting events, which acted to expose the separation of 22.5 ± 2.0 m in the fi eld across the sion. The faults and folds logged in the Har- section to colluviation, the next major event to major faults of the stepover zone. This channel is ris trenches are consistent with strike-slip to occur at the site was clearing of native forest for clearly younger than the I-3 surface and probably reverse-slip faulting. Each linear trace (f1–f8) grazing during the mid–twentieth century. originated as part of an overfl ow episode related identifi ed in Figure 7 that was intercepted in a to I-2 surface construction. At the southwest trench exposure was confi rmed as an active fault Topographic Map and Displacements edge of the stepover zone the channel crosses or fold (see Fig. 9). A microtopographic map of the Harris site is fault trace f7 and is dextrally displaced by ~11.4 Figure 9 presents an example of the expres- shown in Figure 7. We used a Leica 500 RTK- ± 1.3 m (estimated by projecting the channel sion of faulting observed at the Harris site. Fault GPS unit to survey the site. Almost 15,000 thalweg into and across the fault). The channel f6 is exposed in trench 3 as a steep, southeast- points were collected in transects with a 1-m is also dextrally displaced 10 ± 1 m across a sec- dipping zone of oblique-slip faulting, identifi ed point spacing and a measurement accuracy of ond main trace of the fault at the northwest edge by the juxtaposition of a section of Taramakau ±5 cm. The scattered data points were gridded of the stepover zone (f1 in Fig. 7). Within the Overbank deposits against a shear zone char- through a Triangular Irregular Network inter- stepover zone the channel is further displaced by acterized by graywacke cobbles rotated toward polation method (Akima, 1978) using a quintic ~1.1 m across a small, broad scarp (fault trace vertical. The juxtaposition of sands and silts

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Soil profile Harris trench 3 Soil NE wall horizons 1a 1 A 1b / 301c/30 bA/B ‘Brown’ 31 soil bBw 34 mxmx C 3532 50

Unit 50 2C graywacke gravels mxmx

f6 1 meter Fault AB zone Figure 9. Photograph and annotated log of fault zone f6 in Harris trench 3. (A) Image shows an inceptisol (brown soil) formed in sandy to silty Taramakau Overbank deposits preserved in the lee of the fault scarp. White lines and crosses belong to the meter grid. (B) Overbank deposits and soil are drag folded against fault zone f6, identifi ed by dragged con- tacts and rotated cobbles of the Taramakau Gravels. Horizons within the soil profi le are identifi ed in the margin. Unit 1a soil (A horizon) is in part a disturbed post-European soil.

against gravel is clear evidence for faulting, At the southern shore of Lake Poerua a cut oxcal/OxCal.html), it is possible to order the whereas, in addition, the Taramakau Overbank slab was extracted from the stump of an in three dates and place time constraints on each deposits are drag folded beyond vertical adjacent situ, large (2.9 m circumference) podocarp tree dated ring section by inserting gaps in the OxCal to the fault zone (Fig. 9). An additional trench (probably Matai; Prumnopitys taxifolia) (WP13 program that correspond to counted annular log from trench 1, displaying steep, oblique-slip in Figs. 4, 5). We infer that this tree began grow- ring gaps (Fig. 11). In this way it is possible to faulting, can be viewed in Toy (2007). ing on soft sediments following the full aban- “shave” the probability density functions that donment of I-3 (i.e., post–Taramakau Overbank represent each calibrated age (e.g., Biasi and Age of the Faulted Surfaces at the deposits) so that WP13 probably provides a Weldon, 1994) and ultimately to reﬁ ne the dis- Harris Site reasonable maximum age with respect to the tribution for the death age of tree WP13. Based offsets at the trench site. Podocarps are intoler- on this analysis and an estimate of 22 rings to the Dendrochronologic Age of I-3 ant of saturated soil, such that when the local exterior of the tree, tree WP13 stopped growing To estimate slip rates for the faulted channel groundwater table rose, this tree effectively at ca. 947–1064 cal. yr B.P. (Fig. 11). at the Harris site it is necessary to determine a drowned and was preserved in place. The seedling age, i.e., the time when tree tractable age for either the channel or the surface The age analysis of stump WP13 indicated WP13 began growing, is considered to repre- that it cuts into. Dates for the I-3 surface from when this tree both began growing and died, and sent a minimum age for the abandonment of the Taramakau Gravels and Overbank deposits hence provides a minimum age for the full aban- surface I-3. The seedling age can be estimated would likely provide maximum ages for the donment of I-3 (Fig. 10). Three AMS radiocar- by counting the ring sequence back to the cen- displacement of the channel. No deposits or bon samples were submitted from counted ring ter of the tree. Figure 10 provides an attempt materials were located to directly determine the sections from the slab, i.e., rings 22, 46, and 222 to estimate when WP13 began to grow. A tree age of the channel itself; however, local chan- (Table 1; each sample was 3 rings wide). The ring count of ≥480 rings was determined from nel deposits probably provide a reasonable age sample from near the outside of stump WP13 the cut slab of stump WP13, which implies that for the formation of the channel. Nevertheless, yielded an age of 1203 ± 35 yr B.P. The second this tree was alive at least 1500 yr ago or more. owing to the paucity of datable organic material sample (ring 46) yielded an age of 1217 ± 25 yr Because it was not possible to extract a full slab at the trench site, it was necessary to constrain B.P. The third ring section (222 ± 1) was selected from this large stump in the lake, we mapped the age of I-3 by other means. Geomorphic map- for dating on the basis of its postulated position out the visible ring structure to estimate the time ping concluded that the I-3 surface is continuous on the radiocarbon calibration curve. That is, the (rings) preserved in the core of the tree. Thus, between Lake Poerua and the Harris site. For a rings were counted to intercept a portion of the it was possible to estimate that the podocarp fault as active as the Alpine fault, which causes calibration curve that was steep and unimodal WP13 was a further 80–100 yr older than could punctuated landscape change every few hundred (1329 ± 20 yr B.P.; see Table 1 for 2σ calibrated be counted from the incomplete slab section. years (Berryman et al., 2009; Wells et al., 2001), ages). Using a Bayesian statistical technique for Therefore, this tree was at least 570 ± 10 yr old dendrochronology is a useful technique for ordering radiocarbon samples and events (i.e., when it died. In addition, it has been shown that determining surface and paleo-earthquake ages. the OxCal program; https://c14.arch.ox.ac.uk/ Matai are relatively rapid colonizers of alluvial

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surfaces and have an average colonization time Ring 0 984-1171 Ring 46 ± 1 to corer height of 28 yr (5–40 yr; Wells et al., Outer curvature of stump 968-1170 1217 ± 25 1999). Based on the tree ring countback from A Ring 22 ± 1 the OxCal distribution for the death of WP13 1203 ± 35 (Figs. 10, 11), the tree probably began grow- Ring 100 ing on abandoned surface I-3 at ca. 1530–1670 cal. yr B.P. (1590–1730 yr ago).

Relative Soil Chronosequence Ages Ring 222 ± 1 200 If the rate of soil development can be ade- 1329 ± 20 Path used for 1173-1278 ring count quately calibrated, then soil morphology can be used to estimate surface exposure age. The rate of soil development is usually calibrated through Ring 300 studies of soil chronosequences, and a number of chronosequence studies exist for the Westland Slab cut out of stump region, in which the study site is located. Tonkin 350 and Basher (1990) review three of these chronosequences and provide a general pathway of soil development. Soils develop from entisols to Ring 400 inceptisols to spodosols (U.S. Natural Resources 05cm Conservation Service, 1999) but at differing rates, depending on mean annual rainfall. In Trench 3 at the Harris site, a sequence of Taramakau Overbank deposits on gravel is preserved in fault contact with Taramakau Gravels (Fig. 9). A moderately developed inceptisol (brown soil; see Hewitt, 1998) has formed in these ﬁ ne deposits, adjacent to the fault zone 450 Circumference c. 2.9 m Radius (f6). This soil proﬁ le is described in Appendix 1. Radiocarbon sample location A similar inceptisol was logged in trench 2. In c. 46 trench 1, Taramakau Gravels with a thin silt 2-sigma calibrated age (cal yr BP) 608-764 cm cover show oxidation and reduction features that Ring 22 ± 1 are indicative of a similar amount of relative soil Radiocarbon age (yr BP) Possible central 1203 ± 35 area of tree development to those soils described above (see logs in Toy, 2007). The Wanganui River chronosequence in South Westland, reviewed by Tonkin and Basher (1990), is probably the most appro- priate for comparison with the Inchbonnie area, although the former has a mean annual rainfall B in the order of 6500 mm in contrast to Inchbon- nie’s 5000 mm. At Wanganui River the transition from entisol to inceptisol would have taken more than ~400 yr but less than ~600 yr, whereas the transition to a spodsol would have taken at least ~1500 yr but no more than 3000 yr. Assuming that soil development was not as rapid at Inch- bonnie as at Wanganui River, we conclude that the inceptisol in trench 3 is at least 400–600 yr sstumptump old but younger than 1500–3000 yr old. sscarfcarf WWP13P13 DISCUSSION

Lake Poerua Resolving Age Issues for the Inchbonnie Surfaces

To develop viable slip rates for the Alpine Figure 10. (A) Scanned section of a slab cut from a large in situ podocarp stump (WP13). Up to 480 fault at Inchbonnie, a comprehensive under- annular tree rings were counted and contoured from this scarf section. Accelerator mass spectrometry radiocarbon dates of ring sections come from near rings 22, 46, and 222. From the shape of standing of surface ages was required. While the tree rings, the central part of the tree, and subsequently the total age of the podocarp, can be this has been challenging in this area, the most estimated. (B) Photograph of stump WP13, near the shore of Lake Poerua. promising techniques for dating the alluvial

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OxCal v4.0.5 Bronk Ramsey (2007); SHCal04 Southern Hemisphere atmospheric curve (McCormac et al., 2004) WP13 death age Event [Tree WP13 Drowns] Gap 22 ‘Shaved’ age of WP13 22 R_Date WP13 22 (1203 ± 35) Gap 24 R_Date WP13 46 (1217 ± 25) Gap 176

R_Date WP13 222 (1329 ± 20) WP13 death age WP13 counted ring age 947-1064 cal yr BP

1600 1500 1400 1300 1200 1100 1000 900 WP13 seedling age 1530-1670 cal yr BP Modeled date (cal. yr BP) Figure 11. OxCal plot of calibrated radiocarbon distributions from tree stump WP13 at Lake Poerua. Dates are stacked from youngest to oldest. The light-gray probability density functions (PDFs) are prior distributions, whereas the dark PDFs are posterior or “shaved” distributions using the counted tree ring gaps. The preferred death and seedling ages of WP13 are indicated by the arrowed ranges.

surfaces have been dendrochronology combined tion that are consistent with the early Holocene Vertical, Shortening, and Dip-Slip Rates with AMS radiocarbon dating, and relative soil and Last Glacial period for the West Coast of of Deformation chronosequence dating. These two techniques South Island (Suggate, 1965). Two of these OSL provide bounding age constraints of differing dates (14.1 ± 1.6 and 26.3 ± 4.2 k.y.) are consis- A new minimum estimate for the vertical slip precision for the I-3 surface and displacements tent with a time when the Inchbonnie area was rate for the Alpine fault at the Harris site is 2.9 upon it. The preferred age of I-3 and the offset under ice and not depositing alluvial gravels at ± 0.4 mm/yr. This result combines the vertical features, which comes from geomorphic and this location. Two other OSL dates from fi ne- scarp height from profi le HR (4.8 ± 0.4 m; Fig. 6) dendrochonologic data (1590–1730 yr ago), grained deposits within the Harris trenches (7.5 with the minimum age of the I-3 (maximum age lies within the middle of the broad age range ± 1.5 and 8.15 ± 0.81 k.y.) are also signifi cantly of I-2r) surface estimated from stump WP13 described from soil chronosequence dating. In older than the soil chronosequence ages and (1590–1730 yr). Two other profi les across the addition, both the younger (400–600 yr) and would imply very low rates of deformation for Alpine fault (Za, S1) near the Harris site across older (3000 yr) limits of the soil chronosequence the Alpine fault. Therefore, we recognize that surface I-3 yield larger scarp heights of 6.4 ± 0.2 range can be reasonably ruled out owing to the there is a problem with using OSL ages from and 6.6 ± 0.2 m, respectively, and subsequently expectation that 22 m of dextral displacement this site, i.e., poor or non-resetting of glacial age higher vertical slip rates using the available is unlikely to have occurred over such a short materials redeposited in late Holocene deposits, dates. We use profi le HR for the vertical slip rate time frame (i.e., some 400–600 yr), and that up and consequently we have rejected these dates because it comes from the same locality (Har- to 5–6 m of scarp height is unlikely to account from our results. ris site–stepover) as the dextral displacements for the vertical deformation over 3000 yr. There- Finally, as the former slip rates for the Alpine (Fig. 7). The reasons for a lower vertical slip rate fore, the preferred age of abandonment for the fault at Inchbonnie are derived from weathering at the stepover are probably due to the rejuvena- I-3 surface (i.e., effectively a maximum age rind ages of 1100 ± 100 yr on graywacke clasts tion (erosion) of the scarp post–I-3 abandonment of 1590–1730 yr), which grades toward Lake from the I-2 surface, it is important to test the by fl ooding across the I-3 surface that ultimately Poerua, fi ts with a general understanding of validity of these ages (Berryman et al., 1992). We formed the I-2r channels. These data indicate the the sequence of late Holocene fanning surfaces accept that this was the only dating technique, importance of understanding the geomorphic and soils, from northeast to southwest across and therefore best available data, for this area at data inputs that go into deriving slip rates. the Inchbonnie area. Similarly, young dates on the time. However, we infer that the sample loca- In order to calculate the reverse-slip move- exposed tree stumps in the valley and on the tion cannot be used in association with current ment and shortening, fault dip is required. fl oor of Lake Poerua (e.g., B3, TR1, WP3; all New Zealand weathering rind calibration curves Locally, measurements of hanging-wall bedrock less than 500 cal. yr B.P.) (Table 1) probably (McSaveney, 1992; M.J. McSaveney, 2009, attitudes imply that the dip of the Alpine fault relate to young avulsions of the Taramakau personal commun.). That is, the sample site on is 58° ± 5° SE (Nathan et al., 2002), whereas River that have occurred following the last one surface I-2 was formerly a forested alluvial sur- measurements from the Harris trenches show or two surface ruptures on the Alpine fault. face where the sandstone clasts were within the that the faulting is typically high angle near the A third dating technique, optically stim- soil-forming zone; c.f. dry alpine environments, ground surface (Fig. 9). In this study a dip of ulated luminescence (OSL) dating, was where chemical weathering processes are mini- 60° ± 5° SE is used with a vertical slip rate of attempted on fi ne-grained deposits of the Tara- mal. Therefore, we conclude that the Inchbonnie 2.9 ± 0.4 mm/yr to derive an estimate for the makau aggradation sequence within the Harris weathering rind data are invalid and cannot be minimum dip-slip rate of 3.4 ± 0.6 mm/yr. Simi- trenches (see Rieser and Wang, 2009). OSL age used to determine geologic slip rates from these larly, shortening rates of 1.2–2.3 mm/yr can be results have not been presented in detail, as they surfaces. The implications of this will be dis- calculated from profi le HR using the fault dip all yielded feldspar OSL ages from the silt frac- cussed in the following sections. and dip-slip rate above.

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Our new values of vertical and dip-slip rates displaced 11–13 m dextrally (Fig. 5). One of minimum rate of 13.7 ± 1.9 mm/yr at an orienta- represent a signifi cant drop compared with the these former channels was later abandoned, tion of 061° ± 3° for the Inchbonnie slip vector. published values, i.e., dip slip 6 ± 1–2 mm/ leaving an active channel that displays a dex- This value is in close agreement with the geo- yr (Norris and Cooper, 2001; Berryman et al., tral displacement of ~6 m. Although the slip detic slip rate estimated for this locality (14.7 1992). Earlier, we discounted the validity of rate based on this 11–13 m displacement is not ± 1.1 mm/yr at 062.4° ± 3°) (see Wallace et al., the slip rates calculated at Inchbonnie using the upheld in this paper, these observations sug- 2007) and implies that the differences between weathering rind dating technique. Therefore, gest that there have been single to multiple co- short-term strain accumulation from GPS and our lower estimates are valid and represent a seismic dextral displacements across the Alpine medium-term strain release from geology are more robust analysis of both the age and dis- fault at Inchbonnie of ~6 ± 1 m. In this way the minimal relative to their differing time frames. placement components of the slip rate. stepwise growth of the fault scarp between the Taramakau River and Lake Poerua (Figs. 5, Partitioning of Slip Rate across Central Dextral Slip Rate and Inchbonnie Slip 6) is mirrored by a similar increase in dextral South Island Vector displacement from the river to older surfaces to the northeast. This gives us further confi dence Geophysical, geodetic, and seismic haz- The dextral displacement of 22.5 ± 2 m on that our geomorphic assessments and slip rates ard models indicate that a large proportion of the shallow surfi cial channel measured across derived from dating the abandonment of the I-3 tectonic motion is partitioned from the Alpine three fault strands at the Harris site is used surface using AMS radiocarbon dating are more fault southwest of Inchbonnie onto the Hope together with the minimum age for the abandon- reliable and more time-averaged slip rates com- and Kelly faults (Fig. 2) (Berryman et al., 1992; ment of the I-3 surface to calculate a minimum pared to previously published data. Wallace et al., 2007; Stirling et al., 2002). The dextral slip rate of 13.6 ± 1.8 mm/yr for the The slip vector at Inchbonnie is estimated revision of slip rates at the Inchbonnie site, in Alpine fault at Inchbonnie. Again, we infer that from the combination of the dextral and short- combination with recently published slip rates the previously published value of 10 ± 2 mm/yr ening rates at the Harris site, which are used in on nearby structures such as the Hope and cannot be used as a valid slip rate for the fault the next section as vectors with direction (trend) Kakapo faults (Yang, 1991; Langridge and Ber- in this area, based on the use of weathering rind and magnitude (rate) in a kinematic circuit. The ryman, 2005), allow for a geologic analysis of ages. Nonetheless, this new value represents a strike-parallel component of the total vector the partitioning of strain about this major tran- signifi cant increase in the accepted slip rate for is 13.6 ± 1.8 mm/yr at 052°. There is a small sition in the Australian-Pacifi c plate boundary. the Alpine fault in this area, and such an increase change in the strike of the fault in the Inchbon- First- and second-order tectonic changes can (and decrease in the case of fault normal rates) nie area (effective to the NE of the Alpine-Hope be assessed by using geologic fault slip rates and has a signifi cant impact on hazard and tectonic fault junction) compared with the Central seg- vectors in a kinematic circuit model, following a models for the area. ment of the Alpine fault to the southwest of similar technique to that of Humphreys and Wel- Additional dextrally displaced geomorphic Inchbonnie (Fig. 2). To compare the resultant don (1994). Figure 12 documents a kinematic features have been diffi cult to identify across geologic slip rate with a geodetic rate, we have vector sum constructed using the rates, vectors, the fault trace. However, Berryman et al. (1992) used the dextral slip rate in combination with and uncertainties of the major regional faults. recognized an area on the I-2 surface near the the horizontal shortening rate (1.2–2.3 mm/yr The circuit passes from Inchbonnie, between the Taramakau River where four channels had been at 142°). These components yield a combined Clarence and Hope faults, crossing the western

A B ± 6 1 ±

Kakapotahi River Wairau f. Strike-slip: 29 Tasman South Shortening: 3 Sea Island Awatere f. ± 1.8 13.6 ± 0.6 ± 1.5 Inchbonnie Clarence f. western Hope S-slip: MF S-slip: 9.6 Short: 1.7 fault Short: <0.5 IB Hope f. KB PAC AUS- ± 0.4 Kelly f. Kakapo f. vector

KR Kakapo S-slip: 6.4 Poulter f.

Alpine Porters O Un-closed Pass FZ part of circuit Figure 12. Kinematic circuit of major faults, using local fault strike and slip parameters, plotted to north. (A) Map view of the circuit path (dotted, arrowed) through central South Island. (B) Vector map. The origin (O) marks the beginning of the circuit from the Kakapotahi River site (KR), SW of the Alpine-Kelly-Hope fault intersection. All other vectors (dashed) have a negative “incoming” vector sense. Vectors are divided into strike-slip and horizontal shortening components (in mm/yr). The residual vector equates to ~1.2 mm/yr at a trend of 348°. IB—Inchbonnie; MF—McKenzie fan; KB—Kakapo Brook; AUS- PAC—Australian-Paciﬁ c plate; f.—fault; FZ—fault zone.

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end of the Hope fault before returning southwest difference between the outgoing and incoming CONCLUSIONS to the Kakapotahi River site on the Alpine fault paths is only ~1.2 mm/yr at an azimuth of 348°. via the Kakapo fault (Fig. 12A). The slip vectors At fi rst order, this implies that these major faults 1. Geomorphic and geochronologic studies for the Kakapotahi River are shown as positive account for the greater proportion of strain in the Inchbonnie area have resulted in refi ned vectors, whereas all other “returning vectors” accumulation and release in this part of the plate local dextral and vertical slip rates for the Alpine have a negative sense. The Hope and Kakapo boundary. The resultant vector difference can fault. A series of late Holocene (0–2 k.y.) allu- faults are both primarily dextral-slip faults and be completely accounted for within the uncer- vial surfaces (I-Ø to I-4) have been mapped in have small to negligible shortening compo- tainties of the slip-rate estimates presented here this part of the Taramakau River valley. The nents. Slip vector data for the Hope fault comes (e.g., the large errors for Alpine fault rates). scarp of the Alpine fault crosses these surfaces, from the McKenzie fan site (9.6 ± 1.5 mm/yr at Third, excluding the Kakapo fault, the azi- growing in height stepwise from I-1 to I-4. 067°) and for the Kakapo fault from the Kakapo muths of each of the slip vectors and the plate 2. Detailed mapping and surveying at the Brook site (6.4 ± 0.4 mm/yr at 037°) (Yang, convergence direction are remarkably similar, leading edge of a push-up structure along the 1991; Langridge and Berryman, 2005). Dip-slip i.e., generally at 062°–071° compared with 070° fault at the Harris site have resulted in new esti- rates have been converted to horizontal shorten- (250°), respectively (Fig. 12). This implies that mates of vertical (4.8 ± 0.5 m) and dextral (22.5 ing rates in order to be in accordance with GPS the faults operate in accordance with the regional ± 2 m) displacement there. Based on precise data, which are expressed in a horizontal frame- stress fi eld at the plate boundary scale. Simi- dating of a remnant tree stump, the minimum work (Fig. 12). larly, the principal horizontal stress (PHS) direc- age for the abandonment of the I-3 surface at the To complete the kinematic circuit, valid tions for most faults throughout South Island are Harris trench site is ~1590–1730 yr. slip rates for the Alpine fault are required from subparallel to each other and the regional stress 3. The resultant dextral, vertical, and reverse- southwest of the Kelly fault. The Kakapotahi fi eld (Berryman, 1979). For the Alpine fault, this slip rates for the Harris site are 13.6 ± 1.8 mm/ River locality is the closest to the northeast end effectively means that owing to the step-down in yr, 2.9 ± 0.4 mm/yr, and 3.4 ± 0.6 mm/yr, of the Central segment, but it is situated within dextral slip rate that occurs from Kakapotahi to respectively. These values are somewhat larger 65 km of the Inchbonnie site, which is wholly Inchbonnie, there must be a local strike change (dextral) and smaller (dip slip) than previous northeast of the Alpine-Kelly-Hope fault junc- (i.e., as observed from 228° to 232°), step-down estimates for this area, which have been ren- tion. From the Kakapotahi River to Inchbonnie, in the rate of dip-slip motion (6 ± 1 to 4 ± 1 mm/ dered invalid by improvements in the dating of the dextral rate decreases from 29 ± 6 to 13.6 yr) consistent with the regional stress direction late Holocene surfaces locally. ± 1.8 mm/yr, with the shortening rate decrease across the plate boundary, and/or changes in the 4. A kinematic circuit using the new slip vec- from 3 ± 1 to 1.7 ± 0.6 mm/yr, assuming a near partitioning of strain release. From a kinematic tor in conjunction with other fault slip vectors surface dip of 60° SE (Norris and Cooper, 2001; perspective, this helps to explain why the high- in the transition area between the Alpine fault this study). There is an associated strike change est parts of the Southern Alps in South Island and the western end of the Marlborough Fault from 048° to 052° between these two sites, (Fig. 1) are limited to the length of the Central System is almost closed (i.e., net ~0). The analy- respectively. The dextral slip rates estimated segment of the Alpine fault, southwest of the sis shows that 69%–89% of the plate boundary in this study represent an ~50% decrease from transition to the Marlborough Fault System. strain in the area can be accommodated across the Kakapotahi River site, and there is a similar Finally, slip vectors used here are esti- the major faults (Alpine-Hope-Kakapo), with relative drop in the calculated reverse-slip rates. mated from dated surfaces that range from late approximately half of the strain release being The resultant kinematic circuit shows a num- (Inchbonnie–western Hope) to mid-Holocene transferred from the Alpine fault to the Kelly- ber of interesting features. First, the slip rates for (Kakapo fault) and represent medium- to long- Hope-Kakapo system. these four faults (or segments) do not account term estimates of strain release. Despite their 5. The higher estimated strike-slip rate for for the total convergence rate required across the differing time scales, the magnitude and direc- Inchbonnie is consistent with the amount and Australian-Pacifi c plate boundary in this area; tion of resultant slip vectors derived from the partitioning of geologic strain between the i.e., 37 ± 2 mm/yr at 250° (DeMets et al., 1994). results of this study and contemporary GPS Alpine fault and faults of the Marlborough The average strain release from the “outgoing” rates and vectors are remarkably similar at fi rst Fault System in this part of the Australian- (Kakapotahi River) and “incoming” paths of order (e.g., Wallace et al., 2007). The geologic Pacifi c plate boundary. the kinematic circuit is 29 ± 2 mm/yr at 236°, slip-rate and kinematic data presented in this or ~69%–89% of the plate rate. This suggests study confi rm a major change (junction and ACKNOWLEDGMENTS that there must be further deformation on other bend) in the Australian-Pacifi c plate boundary structures and/or within crustal blocks to the through New Zealand in central South Island. In The authors wish to thank Mark Hemphill- south and east of our circuit that account for the this area, approximately half of the Alpine fault Haley, William Ries, and Zion Klos for assis- remaining ~11%–31% of plate motion. Other motion (~52%) is partitioned onto the Hope, tance in the fi eld. We also thank Peter Harris fault systems that could account for this short- Kelly, and Kakapo faults at the southeastern for access and permission to dig trenches and fall in slip rate include the Main Divide Fault edge of the Marlborough Fault System. These to the Department of Conservation for access Zone (Cox and Findlay, 1995), the Poulter fault results have signifi cant implications for the to Lake Poerua. Fabian Hurter provided several (Berryman and Villamor, 2004), the Porters Pass treatment of seismic hazard at junctions along of the scarp profi les. The authors thank Laura Fault Zone (Cowan et al., 1996; Howard et al., major continental plate boundary systems. The Wallace, Mauri McSaveney, Kelvin Berryman, 2005), and structures within the Australian plate new, higher dextral slip rate presented from and Liz Schermer for discussions and insight- (Nathan et al., 2002) (Figs. 1, 12). Inchbonnie is also more consistent with the ful reviews that helped the content of this paper. Second, though the kinematic circuit overall strain budget that is partitioned farther to This research was funded by the New Zealand accounts for only 69%–89% of the plate conver- the northeast among faults of the Marlborough Foundation for Research, Science and Technol- gence rate, the circuit itself is virtually closed, Fault System (Van Dissen and Yeats, 1991; Holt ogy project PLT Alpine Fault earthquake geol- i.e., the net vector sum is close to zero. The net and Haines, 1995). ogy (PGST Contract CO5X0702).

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APPENDIX 1. SOIL PROFILE FROM TRENCH 3, HARRIS SITE, INCHBONNIE Suggate, R.P., 1965, Late Pleistocene Geology of the Northern Part of the South Island, New Zealand: New Zealand Soil horizon Depth Description Geological Survey Bulletin 77, 91 p. (cm) Suggate, R.P., and Waight, T.E., 1999, Geology of the Kumara- Moana Area: Lower Hutt, New Zealand, Institute of Geo- A 0–12 Silt loam; 2.5Y 4/1 (dark gray to dark grayish brown); moderately fi rm soil strength, logical and Nuclear Sciences Geological Map 24, scale brittle failure, weak fi ne nutty structure; clear wavy boundary. 1:50,000, 1 sheet plus 124 p. bA/B 12–27 Silt loam; 2.5Y 4/2 (dark grayish brown) and 2.5Y 4/1 (dark gray to dark grayish Sutherland, R., and Norris, R.J., 1995, Late Quaternary dis- brown); moderately fi rm soil strength, brittle failure, weak medium prismatic breaking placement rate, paleoseismicity, and geomorphic evo- to weak medium nutty structure; clear wavy boundary. lution of the Alpine fault: Evidence from Hokuri Creek, bBw 27–54 Silt loam; 2.5Y 5/4 (light olive brown); weak soil strength, brittle failure, weak coarse South Westland, New Zealand: New Zealand Journal of blocky breaking to moderate medium nutty structure; clear wavy boundary. Geology and Geophysics, v. 38, p. 419–430. C 54–81 Loamy sand; 5Y 4/2 (olive gray); weak soil strength, brittle failure, massive and weak Sutherland, R., Berryman, K.R., and Norris, R.J., 2006, Qua- very fi ne and fi ne granular structure; abrupt wavy boundary. ternary slip rate and geomorphology of the Alpine fault: Implications for the kinematics and seismic hazard in 2C 81–181 10YR 4/1 (dark gray) and 2.5Y 4/3 (dark grayish brown to olive brown); Note: 10YR southwest New Zealand: Geological Society of America 4/1 material is loose and single grains; 2.5Y 4/3 material is very weak and single Bulletin, v. 118, p. 464–474, doi: 10.1130/B25627.1. grains and weak medium granular. Sutherland, R., Eberhart-Phillips, D., Harris, R.A., Stern, T., Beavan, J., Ellis, S., Henrys, S., Cox, S., Norris, R.J., Ber- ryman, K.R., Townend, J., Bannister, S., Pettinga, J., Leit- REFERENCES CITED Hewitt, A.E., 1998, New Zealand Soil Classifi cation (2nd edi- ner, B., Wallace, L., Little, T.A., Cooper, A.F., Yetton, M., tion): Lincoln, New Zealand, Manaaki Press, Landcare and Stirling, M., 2007, Do great earthquakes occur on the Alpine fault in central South Island, New Zealand?, Akima, H., 1978, A method for bivariate interpolation and Research Science Serial 1, 133 p. in Okaya, D., et al., eds., A Continental Plate Boundary: smooth surface fi tting for irregularly distributed data Holt, W.E., and Haines, A.J., 1995, The kinematics of northern Tectonics at South Island, New Zealand: Washignton, points: ACM Transactions on Mathematical Software, South Island, New Zealand, determined from geologic D.C., American Geophysical Union, Geophysical Mono- v. 4, p. 148–159, doi: 10.1145/355780.355786. strain rates: Journal of Geophysical Research, v. 100, graph 175, p. 235–251. 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