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Late Holocene surface ruptures on the southern Wairarapa fault, New Zealand: Link between earthquakes and the uplifting of beach ridges on a rocky coast

T.A. Little^1,*, R. Van Dissen^2,, E. Schermer^3, and R. Carne^1,#

¹SCHOOL OF GEOGRAPHY, ENVIRONMENT AND EARTH SCIENCES, VICTORIA UNIVERSITY OF WELLINGTON, P.O. BOX 600, WELLINGTON 6140, NEW ZEALAND
²GNS SCIENCE, P.O. BOX 30368, LOWER HUTT 5010, NEW ZEALAND
³GEOLOGY DEPARTMENT, WESTERN WASHINGTON UNIVERSITY, MS9080, BELLINGHAM, WASHINGTON 98225, USA

Correspondence: *Corresponding author e-mail: timothy.little{at}vuw.ac.nz.

	ABSTRACT

TOP ABSTRACT INTRODUCTION TECTONIC SETTING RESULTS DISCUSSION CONCLUSIONS REFERENCES CITED

 
The Holocene beach ridges at Turakirae Head, New Zealand, are remarkable because the fault that caused their uplift is accessible to paleoseismic trenching. Based on 40 ¹⁴C samples from eight trenches, we identify five surface-rupturing earthquakes since ca. 5.2 ka (mean earthquake recurrence of 1230 ± 190 yr). The paleoearthquake record includes two more events than were recorded by the uplift and stranding of beach ridges at Turakirae Head. We conclude that beach ridges may provide an incomplete record of paleoearthquakes on oblique-reverse faults. The southern end of the Wairarapa fault includes several splays in the near surface at variable distances from Turakirae Head. Variable partitioning of slip between these splays (and perhaps the subduction interface down-dip of them) is inferred to have caused variable magnitudes of coseismic uplift at the coast, where at least one <3 m throw is not recorded by preservation of a ridge. Variations in wave climate or sediment supply (or interseismic subsidence) may also influence the number of beach ridges preserved by governing the morphology of the storm berm and controlling its extent of landward retreat. Such retreat may cause a berm to overwhelm, or amalgamate with, the next-highest beach ridge, resulting in the omission of one ridge, as probably happened at Turakirae Head at least once. Our ¹⁴C data support the view that a widespread post–Last Glacial Maximum aggradational terrace in southern North Island, New Zealand, was abandoned soon after 12.1 cal yr B.P. From this, we infer that the Wairarapa fault has a late Quaternary slip rate of 11 ± 3 mm/yr.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION TECTONIC SETTING RESULTS DISCUSSION CONCLUSIONS REFERENCES CITED

 
Slip on reverse (or oblique-reverse) faults during large earthquakes may be accompanied by a signal of coseismic uplift (or subsidence) near the coast that may be preserved in the geological record (Atwater, 1987; Berryman, 1993; Wilson et al., 2007a). Repeated surface-rupturing earthquakes have the potential to generate a suite of uplifted coastal beach ridges that faithfully record the sequence of earthquakes on that fault. Such a paleoseismically advantageous situation might be most likely where these earthquakes have all been similarly large, involving ruptures of similar dimensions, slip, and—especially—uplift; where coastal conditions have been continuously favorable for the formation of beach ridges; and where the preservation potential of the abandoned land-forms has remained steadfastly high.

New Zealand's largest historic earthquake, the Ms ~8.2 Wairarapa fault event in 1855, resulted in uplift of the hanging wall of that dextral-reverse fault near the southern coast of the North Island (Fig. 1A) and in the generation of a set of tsunami waves up to ~9 m high (Grapes and Downes, 1997). The coseismic uplift reached a maximum near Turakirae Head, where the pre-1855 storm beach ridge was raised by as much as 6.4 m (Begg and Mazengarb, 1996; Hull and McSaveney, 1996; McSaveney et al., 2006). This fossil beach ridge is today preserved as the youngest of at least four tectonically uplifted beaches on the headland (Fig. 1B) (Aston, 1912; Wellman, 1969; McSaveney et al., 2006). In addition to its large magnitude, the historically well-documented 1855 earthquake was remarkable for influencing Charles Lyell (1868) to argue that earthquakes were associated with vertical earth movements and slip on fault planes (Grapes and Downes, 1997; Sibson, 2006), and for its extremely large coseismic strike slip (locally as high as ~18.5 m; Rodgers and Little, 2006). Since 1855, it has often been assumed that the 1855 earthquake provided an analogue for the style of deformation accompanying previous earthquakes on this fault (e.g., Grapes, 1999). A corollary is that the uplifted gravel beach ridges at Turakirae Head provide a complete record of a series of similarly expressed earthquakes that have ruptured the nearby Wairarapa fault during the past ~7000 yr (e.g., Wellman, 1969; Moore, 1987; Hull and McSaveney, 1996).

View larger version (194K): [in this window] [in a new window]   Figure 1. (A) Tectonic index map showing major active faults and other structures of the southern North Island, New Zealand (largely after Barnes, 2005; Begg and Johnston, 2000; Lee and Begg, 2002), and location of sites along the Wairarapa fault where paleoseismic data have been collected previously and during this study (large open circles). Smaller rectangles show location of detailed Wairarapa fault maps of Figure A and Figure 2. Cross section X–X' is presented in Figure 11B. Inset on upper left shows plate-tectonic setting of New Zealand (plate motions taken from DeMets et al., 1990, 1994). NIDFB—North Island Dextral Fault Belt. (B) Oblique aerial photograph of uplifted beach ridges and Holocene wave-cut platform at Turakirae Head, view looking NW (photo by Lloyd Homer, GNS Science as annotated in McSaveney et al., 2006). The two uplifted Pleistocene marine terraces in the background were studied and assigned provisional ages by Ota et al. (1981). Distance between the two headlands is about 4 kilometers.

	TECTONIC SETTING

TOP ABSTRACT INTRODUCTION TECTONIC SETTING RESULTS DISCUSSION CONCLUSIONS REFERENCES CITED

 
In central New Zealand, motion of the Pacific plate relative to the Australia plate occurs at ~39 mm/yr in a direction of ~261° (DeMets et al., 1990, 1994). The plate boundary in New Zealand is characterized by subduction of oceanic crust along the Hikurangi margin of the North Island and oblique continental collision along the Southern Alps of the South Island (Fig. 1A). Faults in the southern part of the North Island occur in a transition zone between these two plate boundary zones. There, obliquely convergent motion is partitioned between contraction-dominated folding and thrust faulting in the submerged accretionary wedge (e.g., Barnes and Mercier de Lepinay, 1997; Barnes et al., 1998) and in emergent parts of the forearc (e.g., Nicol et al., 2002, 2007), and strike-slip faulting (and vertical-axis fault-block rotations) in the onland region farther to the west (e.g., Beanland, 1995; Beanland and Haines, 1998; Mouslopoulou et al., 2007; Van Dissen and Berryman, 1996; Wallace et al., 2004). In the onshore region near Wellington, the margin-parallel component of plate motion (up to ~18 mm/yr or ~70% of the total) is mostly accommodated by dextral-slip on the Wairarapa, Wellington, Ohariu, and Shepard's Gully faults. These are the southernmost elements of the North Island Dextral Fault Belt, a belt of upper-plate strike-slip faults in the central part of the North Island (Beanland, 1995). The faults probably initiated ca. 2 Ma or more recently by reactivation of preexisting reverse faults, and they have since accommodated at most ~20 km of cumulative dextral slip (Beanland, 1995; Begg and Mazengarb, 1996; Kelsey and Cashman, 1995; Nicol et al., 2007). At the surface, the faults dip steeply (typically to the NW). Their dip slip is mostly up-to-the-west, but this component is typically a small fraction of the strike slip (<20%; e.g., Berryman, 1990; Heron et al., 1998; Rodgers and Little, 2006). In the offshore to the east and west of Wellington, upper-plate deformation is inferred to be dominantly contractional (e.g., Barnes et al., 1998, 2002; Lamarche, 2005).

Elastic dislocation modeling of global positioning system (GPS) data and seismicity data near Wellington suggest that the Hikurangi subduction zone occurs 20–25 km beneath the surface trace of these faults, and that this gently (~8°) west-dipping part of the plate interface is currently locked and accumulating elastic strain (Reyners, 1998; Darby and Beavan, 2001; Wallace et al., 2004). It seems likely that North Island dextral faults merge downdip with the underlying interface. The nature of interaction between the subducting plate interface and the intersecting North Island dextral faults is poorly understood.

The Wairarapa Fault
This NE-striking and steeply NW-dipping dextral-reverse fault bounds the western flank of the Wairarapa basin, a trough of late Cenozoic (mostly Pliocene-Pleistocene) marine and terrestrial sedimentary strata in the forearc of the Hikurangi subduction zone (Fig. 1A). The southern end of the basin is 2.5–3 km thick where it is truncated against the fault (Hicks and Woodward, 1978; Cape et al., 1990). To the west, the adjacent Rimutaka Range is >900 m high, consisting of deeply eroded Mesozoic basement rocks. Based on the gross geometry of its trace, the Wairarapa fault can be divided into central, southern, and northern sections (Fig. 1A). The central section consists of an en echelon array of mostly left-stepping fault segments that are typically 1–2 km long (Fig. A in Appendix).¹ These discontinuous, dextral-oblique faults are separated by contractional bulges or folds in the area of their overlap that cause warping of alluvial terrace surfaces (Grapes and Wellman, 1988; Rodgers and Little, 2006).

The structurally complex southern section of the fault (Fig. 2) includes the west-dipping Wharekauhau thrust and other active and inactive fault splays. The Wharekauhau thrust is locally overlapped by undeformed late Quaternary–Holocene gravels; however, some strands of the Wharekauhau fault system show recent motion (Little et al., 2008). To the west, an inferred strike-slip fault is an apparent southward continuation of the main, central section of the Wairarapa fault (Begg and Johnston, 2000). Evidence for an along-strike continuation of this western strike-slip strand has not been found at the coast to the west of Turakirae Head (Begg and Johnston, 2000). Instead, that fault appears to step southward onto a thrust segment (Muka Muka fault) entering Palliser Bay just east of Turakirae Head (Begg and Johnston, 2000; Little et al., 2008). The Rimutaka anticline to the west is an active SW-plunging fold expressed by the steep topography of the coastal ranges and by differential uplift of the Holocene wave-cut coastal platform near Turakirae Head (Ghani, 1978; Wellman, 1969; McSaveney et al., 2006). Farther east, other structures at the southern end of the Wairarapa fault zone include Wharepapa and the Battery Hill faults (Begg and Johnston, 2000), and an inferred blind thrust along the western side of Lake Onoke (Little et al., 2008). Seismic and bathymetric data suggest that at least two strands of the Wairarapa fault zone continue offshore into Palliser Bay (Barnes and Audru, 1999; Barnes, 2005).

View larger version (68K): [in this window] [in a new window]   Figure 2. Active fault traces and folds along the Wharekauhau thrust along the southern section of the Wairarapa fault zone (after Begg and Johnston, 2000; revisions by Little et al., 2008). Grid marks (in meters) refer to the New Zealand Map Grid Coordinate System.

1855 Earthquake and Raised Beach Ridges at Turakirae Head
The Wairarapa fault northeast of Wellington, New Zealand, ruptured on 23 January 1855, resulting in ground shaking landslides, regional uplift, tsunamis, and a surface rupture that broke all three of the aforementioned sections of the fault. The rupture was up to 120 km long on land and at most 40 km long on the seafloor to the SW (Grapes and Downes, 1999; Rodgers and Little, 2006). At Ms ~8.2, it was the largest earthquake in modern New Zealand history. South of Featherston, on a 16-km-long part of the central section of the rupture trace (Fig. A), Rodgers and Little (2006) mapped displaced landforms, mostly small beheaded or offset stream channels, and inferred an average single-event strike slip (in 1855) of 15.5 ± 1.4 m. There were no contemporary observations of coseismic strike slip in 1855. Three of the 16 sites yielded strike-slip estimates of 13–14.5 m for the penultimate earthquake, and several yielded estimates of the throw in 1855 (up-to-the-NW). At ~2–3 m, these throws agree with contemporary observations of the height of the scarp in the 1855 earthquake (Grapes and Downes, 1999). Based on the unusually large ratio of displacement to rupture length (D/L) for this earthquake, Rodgers and Little (2006) argued that the rupture extended several tens of kilometers downdip (W) to merge with, and co-rupture, a down-dip part of the subduction interface. This conclusion is consistent with elastic dislocation modeling of the vertical component of the coseismic motion, although it is not a uniquely determined aspect of the model (Darby and Beanland, 1992; Beaven and Darby, 2005).

A flight of uplifted gravel beach ridges occurs at Turakirae Head on the exposed and rocky southern coast of North Island, New Zealand. There, the crest of the modern storm berm (BR-1) is variably located 2–7 m above mean sea level, where it presumably marks the runup limit of present-day storm waves. The next highest beach ridge (BR-2) was abandoned as a result of uplift in 1855 (Begg and McSaveney, 2005; McSaveney et al., 2006). Contemporary observations (mostly by a surveyor; Roberts, 1855) and other geological data indicate that the 1855 uplift was approximately zero on the shore of Palliser Bay, to the east of the Muka Muka rocks (near the trace of the fault of the same name). Farther west, based on the elevation difference between BR-1 and BR-2, the 1855 uplift reached a maximum of ~6.4 m at the crest of the Rimutaka anticline, ~3 km NE of Turakirae Head (McSaveney et al., 2006). The 1855 uplift decayed westward to ~1.5 m near Wellington (Grapes and Downes, 1999). In addition to BR-1 and BR-2, Turakirae Head hosts at least three other higher and older, raised beach ridges: BR-3, BR-4, and BR-5 (Wellman, 1969). McSaveney et al. (2006) radiocarbon dated BR-3 (2380–2060 cal. yr B.P.) and BR-5 (6920–6610 cal. yr B.P.) and used the elevation difference between adjacent ridges as a measure of the coseismic uplift that caused the stranding of the upper ridge. In this way, they inferred an average incremental (coseismic) uplift of 7.3 ± 0.9 m at the crest of the anticline and a mean Holocene uplift rate of 3.5 ± 0.02 mm/yr at that location. Based on the premise that every Wairarapa fault earthquake since ca. 7 ka has caused preservation of a corresponding beach ridge at Turakirae Head, they calculated a mean recurrence interval for Wairarapa fault earthquakes of ~2200 yr.

For this study, eight paleoseismic trenches were excavated across three different sites, and 40 samples were submitted for ¹⁴C dating. See Appendix A for further information about our surveying and trenching methods. Next, we will discuss our paleoseismic results at the Pigeon Bush locality, then those at the Riverslea, and finally those at Cross Creek (Fig. 1A).

	RESULTS

TOP ABSTRACT INTRODUCTION TECTONIC SETTING RESULTS DISCUSSION CONCLUSIONS REFERENCES CITED

 
Pigeon Bush Site
Site Geology and Trench Stratigraphy
At the Pigeon Bush site (Figs. 1A and A), Grapes and Wellman (1988) interpreted two beheaded stream channels as evidence of the repeated dextral offset of a small stream gully crossing the Wairarapa fault (the last in 1855). The two channels are abruptly and orthogonally truncated at the fault on its SE side. On the NW side of the fault, a small, entrenched, and still-active source gully to these channels is similarly linear and fault-transverse (Fig. 3). This geomorphology implies that the two paleochannels were beheaded as a result of two consecutive earthquakes, and there is no evidence for a temporary phase of stream diversion parallel to the scarp (i.e., for a third or fourth slip increment; Rodgers and Little, 2006). These authors measured 18.7 ± 1.0 m of dextral slip and 1.25 ± 0.5 m of vertical slip for the younger, abandoned channel relative to the upstream gully and 32.7 ± 1.0 m and 2.25 ± 0.5 m of slip (respectively) for the older abandoned channel. This larger offset suggests that the older beheaded channel had previously been displaced by 14.0 ± 1.0 m of dextral slip and ~1.0 m of vertical slip prior to incision of the younger one.

View larger version (45K): [in this window] [in a new window]   Figure 3. Microtopographic map of the Pigeon Bush stream offset site showing location of beheaded stream channels and the Pigeon Bush 1 and 2 trenches (PB-1, PB-2, respectively). Base map is from Rodgers and Little (2006) and was surveyed using a combination of real-time kinematic differential global positioning system (GPS) methods employing a differential correction that was tied to a base station at benchmark LUCENA (NZMS270-S27A) and 3-D laser-ranging techniques. Trench corners were located by real-time kinematic GPS survey tied to the same base station. Grid marks (in meters) refer to the New Zealand Map Grid Coordinate System. M.S.L.—mean sea level; O.S.L.—optically stimulated luminescence.

View larger version (42K): [in this window] [in a new window]   Figure 4. Logs of the Pigeon Bush trenches. (A) NE wall of Pigeon Bush 1 (PB-1) trench across the younger of the two abandoned stream channels; (B) SW wall of the same trench; (C) SE wall of the Pigeon Bush 2 (PB-2) trench across the older of the two abandoned stream channels. See Figure 3 for location of the trenches. See Table 1 for 14C analytical results of the numbered samples. The unit labels and brief unit descriptions for each trench pertain only to that particular trench wall and are not meant to apply necessarily to any other trench log. See Appendix B for detailed stratigraphic descriptions of the logged units.

The fluvial part of the channel-infill sequence contains abundant detrital charcoal fragments up to 3 cm in diameter, and these are locally inter-bedded with an organic-rich paleosol (unit Ps). Four charcoal samples from the channel infill were radiocarbon dated. The first three (PB-1, PB-2, PB-3) yielded ages of 649–497 cal yr B.P. (Table 1). These overlap (at 95% confidence) with each other and with two charcoal samples (DR0425G and DR0425J) collected and dated from a nearby pit by Rodgers and Little (2006). All six samples are apparently derived from a single population of charcoals delivered into the channel by a stream that aggraded rapidly(?) after a bush fire. Based on the age of the youngest of these samples (PB-1), the fire probably took place at, or soon after, ca. 546–497 cal yr B.P. (1404–1453 cal. yr A.D.), perhaps in response to Maori burning. Another sample, PB-5, was collected in the modern soil, 32 cm below the ground, and it yielded an age of 490–315 cal yr B.P.

Interpretation of Surface-Rupturing Events
Each of the last two earthquakes on the Wairarapa fault at this site resulted in abandonment of a stream channel immediately downstream of the narrow headwater gully and incision of a new channel in downstream continuity with that gully. Hoping to date the last two earthquakes, we excavated stratigraphic trenches at right angles to each of the two channels to date their incision and abandonment. The only organic material that we found (charcoal) occurs as detrital particles within the fluvial deposits that infill the incisional scour that defines the youngest beheaded channel. This channel was cut immediately after the penultimate earthquake, resulting in the unconformity between the "Tgr" and "Si" units and the younger units above them that infill the scour (these are shown in gray patterns on Figs 4A and 4B). This channel was later displaced and abandoned as a result of slip during the most recent earthquake on the fault. Thus, our preferred age of the burn event (ca. 546–497 cal yr B.P.) provides a minimum age constraint for the penultimate earth-quake at this site (event Pb₂, Table 2). Historical data indicate that the youngest earthquake here (event Pb₁) took place in 1855.

View larger version (23K): [in this window] [in a new window]   Figure 5. Microtopographical map of the Riverslea fault trenching locality. Map is based on 1020 points that were surveyed using real-time kinematic differential global positioning system (GPS) methods. GPS survey was tied to an arbitrary local base station and was not corrected against a gazetted geodetic benchmark. Grid marks (in meters) refer to the New Zealand Map Grid Coordinate System. M.S.L.—mean sea level.

View larger version (53K): [in this window] [in a new window]   Figure 6. (A) Log of SW wall of Riverslea 1 trench (RVL-1) across folded scarp of Wharekauhau thrust near Manganui Stream. (B) Photograph of inclined (probably tectonically tilted) laminae in fine- to coarse-grained sand layer (unit ss-5 in RVL-1 trench). (C) Log of part of the NE wall of the nearby Riverslea 2 trench (RVL-2). (D) Photograph of infilled fault fissure in RVL-2 trench. See Figures 2 and 5 for trench locations. See Table 1 for 14C analytical results of the numbered samples. See Appendix C for detailed stratigraphic description of the logged units. M.S.L.—mean sea level.

Abrupt truncations and thickness changes across these and other faults in the trench suggest strike-slip motion. Units gvl-5b, and gvl-17 and si-5 (which includes a distinctive sandy interval) are truncated against the NW side of fault strand 1 without any apparent correlatives being found on the SE side of the fault. Fault strand 1 diverges upward into a wedge-shaped, soil- and colluvium-infilled fissure just beneath the topographic surface (Figs. 6C and 6D). Charcoal from unit si-15a in the faulted walls of the fissure yielded a ¹⁴C age of 547–497 cal. yr B.P. (sample RVL-2, Table 1). Charcoal from in the fissure infill (unit cgvl-16, sample RVL-3, Table 1) yielded a ¹⁴C age of 0–264 cal yr B.P.

Interpretation of Surface-Rupturing Events
The two Riverslea trenches contain evidence for two earthquakes (events Rv₁ and Rv₂ in Table 2). In trench RV-2, the most recent earthquake (Rv₁) ruptured to the surface and cut sediments younger than ca. 500 cal. yr B.P. to create the fissure. The fissure was infilled by soil debris containing "modern" charcoal and is thus consistent with this earthquake being the 1855 rupture. Based on the stratigraphic mismatches across fault strand 1, slip during the fissure event is inferred to have been primarily strike slip. In the same trench, evidence for an older (penultimate) earthquake (Rv₂) includes the depositional overlap of fault 3 (and some splays of fault 2) by undeformed channel gravel (gvl-18). A sample of fibrous wood from the faulted silt (unit si-9b) below this unconformity to the SE of fault 2 yielded an age of 897–722 cal yr B.P. (Fig. 6C). We are uncertain whether this sample was detrital or a root fragment. If it is detrital, then its age (897–722 cal yr B.P.) must predate the penultimate earthquake at this site (Rv₂). If it is a root fragment, then it does not necessarily predate that earthquake. Sample RVL-2 must postdate the penultimate earthquake.

In trench RV-1, near-surface bulging of the terrace sediments is inferred to have raised the main scarp intersected by that trench, at least in part during the 1855 earthquake. Six measurements of coseismic throw on the southern part of the Wairarapa fault's central section from the 1855 event range up to a maximum of 2.5 m in a NW-up sense (Rodgers and Little, 2006). The height of the main scarp at Riverslea, ~3 m (Fig. 5), suggests its growth in two stages. If so, these must postdate sample RV-1; that is, the penultimate earthquake must be younger than ca. 1300 cal yr B.P.

The lack of a fault in trench RV-1 and the small offset in RV-2 suggest that the primary mode of deformation to create the northwest-up scarp on the late Holocene terrace was folding. The cause of folding is interpreted to be dip slip on a northwest-dipping fault (concealed beneath the trenched gravels) that forms a part of the (here complex) Wairarapa fault zone. Based on the stratigraphic mismatches across fault strand 2, we infer that a small (several-meter?) strike-slip surface displacement accumulated in 1855 on a steep splay fault in the hanging wall of the blind structure in response to dextral-reverse slip on the main structure at depth.

The Cross Creek Pull-Apart Graben
Four trenches were excavated across opposite sides of a pull-apart graben along the Wairarapa fault at Cross Creek (Figs. 1A and 7A; Fig. A). Trenches CC-1 and CC-4 were excavated across the graben's southern bounding fault (Fig. B-i), whereas trenches CC-2 and CC-3 were excavated across its northern margin (Fig. B-ii). The swampy pull-apart graben is today watered by a small southward-flowing stream that traverses a series of diffuse scarp-lets on the north side of the main depression before it exits from the western end of the graben (Fig. 7A). The drier eastern end of the graben is abutted by a tilted terrace surface overlain by a small inactive alluvial fan (Fig. 7B; Fig. B-ii). The graben is down-faulted into the regionally extensive, post–Last Glacial Maximum (LGM) Waiohine terrace gravels (Begg and Johnston, 2000).

View larger version (56K): [in this window] [in a new window]   Figure 7. (A) Vertical aerial photograph of Cross Creek pull-apart graben showing trench sites and fault-trace locations. The photograph by Lloyd Homer (GNS Science) predates track construction that has mostly destroyed the scarplets on the NW side of the graben. (B) Microtopographical map of the NE end of the pull-apart graben showing trench locations. Map is based on 1505 points that were surveyed using real-time kinematic differential global positioning system (GPS) methods. GPS survey was tied to a base station at benchmark LUCENA (NZMS270-S27A). Grid marks (in meters) refer to the New Zealand Map Grid Coordinate System. (C) Auger transect and topographic profile across Cross Creek pull-apart graben showing depth to gravel and location of the 14C sample Auger-3. See part B for transect location. Note that scale of the profile is slightly enlarged relative to B, and that it is vertically exaggerated by 2:1. M.S.L.—mean sea level.

View larger version (103K): [in this window] [in a new window]   Figure 8. Logs of trenches CC-1 and CC-4 across the southern bounding fault of the Cross Creek pull-apart graben. See Figures A, 2, 5, and 7B for location of trenches, Figure B-i for a photograph of the trench sites, Appendix D for detailed stratigraphic descriptions, and Table 1 for 14C analytical results of the numbered samples: (A) Log of Cross Creek 1 trench (CC-1). (B) Log of the nearby Cross Creek 4 trench (CC-4). View has been inverted for ease of comparison with Figure 8A. (C) Photograph of unlogged NE wall of trench CC-4 just prior to its collapse, showing exposure of fault-draping intrapeat unconformity and location of 14C samples CC-4-2 and CC-4-3. An enlarged version of this figure is presented as Figure C.

View larger version (74K): [in this window] [in a new window]   Figure 9. Logs of trenches CC-2 and CC-3 across the northern bounding fault of the Cross Creek pull-apart graben. See Figures A, 1, 5, and 7B for location of trenches, Figure B-ii for a photograph of the trench sites, Appendix F for detailed stratigraphic descriptions, and Table 1 for 14C analytical results of the numbered samples. (A) Log of NE wall Cross Creek 2 trench (CC-2). An enlarged version of the log is presented as Figure D. (B) Log of NE wall of Cross Creek 3 (CC-3). (C) Log of SW wall of CC-3. View is inverted for ease of comparison with parts A and B. An enlarged version of the CC-3 logs are presented in Figure E.

On the down-thrown side of the fault, we were unable to excavate deeper than ~2 m because of wet, unstable ground, so the base of the peat was not exposed in either trench. In the NW parts of both CC-1 and CC-4, fault strands cutting peat terminate abruptly upward beneath younger, unfaulted sediments along a depositional contact that we refer to as the "intra-peat unconformity." On the NE wall of trench CC-1, fault strands 4 and 5 are overlapped by the undeformed peat unit, pt4 (Fig. 8A). The SW wall of CC-1 collapsed before it could be logged, as did the NE wall of trench CC-4. On the latter, we photographed and sampled another exposure of the intrapeat unconformity just before the wall collapsed (Fig. 8C). There, a steep fault strand (probably equivalent to fault strand 4 or 5 in CC-1) juxtaposes peat against silt. This fault is depositionally overlapped by an unfaulted layer of silt. The intra-peat unconformity was not exposed on the SW wall of CC-4 (Fig. 8B).

Discontinuous bodies of clastic sediment abut the fault zone to the SW and are interlayered with peaty units to the north. These clastic units include cw1, cw2, and cw3 in trench CC-1, and units cwA, cwB, and cwC in trench CC-4. All but two of these units are thickest at or near the fault, and pinch out northwestward into peat to define a wedge-shaped body that is >1.5 m long. All are truncated on their SE side against a fault, except unit dp, which occurs as small isolated blob in peat (in CC-4), and the large faulted wedge cw2 (in CC-1), some of which extends across the uplifted side of the fault zone. At the NW end of cw2, the basal contact of this wedge truncates an underlying tree in growth position. The clastic units are interpreted by us to be colluvial bodies derived by redeposition of terrace gravels eroded from the uplifted side of the fault. We distinguish two of them (unit cw1 in trench CC-1 and the aforementioned blob, dp, in trench CC-4) as being smaller (<40 cm long), more organic-rich, and more lenticular in shape than the other, wedge-shaped bodies, leading us to interpret the small gravel-bearing bodies differently (see following). The youngest colluvial wedge in trench CC-1 (cw3) is unfaulted by strand 3, but it is not in contact with fault strand 2. This wedge is overlain by an organic silt layer containing fragments of steel wire (unit wl). The youngest clastic wedge in CC-4 (unit cwC) is truncated by fault strand 2 in that trench and appears to be depositionally overlain by a unit of organic silt (unit mo) containing a line of cobbles (queried contact in Fig. 8B).

Eleven samples were ¹⁴C dated from the NE wall of trench CC-1 (Fig. 8A), six from the SW wall of trench CC-4 (Fig. 8B), and two from the unlogged (collapsed) NE wall of CC-4 (Fig. 8C). Of these, three are wood, and 16 are peat or organic clay-silt (Table 1). Two samples (CC-1-1a-i and CC-1-1a-ii) were collected from a peat that occurs interbedded with alluvial gravels on the uplifted fault block. These yielded ages between 13,160 and 12,100 cal yr B.P. (at 95% confidence). The rest of the samples were collected from the infill of the graben on the downthrown side of the fault, yielding ages of <5500 cal yr B.P. One wood sample (CC-4-11) yielded an age that was greater than that of underlying peat samples, and it is inferred to have been recycled from a near-basal part of the peat sequence. All the other samples in these trenches yielded ages that are in the correct stratigraphic order (at 95% confidence), with the exception of two peat samples (CC-1-11 and CC-4-3) that were collected in proximity (above and below) to the intrapeat unconformity. These yielded ages discordant to one another and to the remaining set of nearby samples (from both above and below the unconformity). After considering the full distribution of ¹⁴C ages, we interpret CC-1-11 and CC-4-3 as recording non-depositional events, though the reasons for this are unknown (see Appendix E). Our interpretation honors the full complement of remaining ¹⁴C ages, whereas any other explanation for the age reversal would require additional ages to be rejected. It is important to note that other interpretations of the ¹⁴C data would not affect the total number of surface rupturing events interpreted from the Cross Creek trenches.

Interpretation of Earthquakes Rupturing the Southern Bounding Fault of the Graben
On the basis of the combined data from trenches CC-1 and CC-4, we interpret five earthquakes to have ruptured the southern bounding fault of the Cross Creek graben during the past ~5.2 k.y. An older (sixth) event occurring after 12–13 ka is expressed in trench CC-1 by overlap of fault strand 3 by terrace gravel (unit tgr-3), but the minimum age of this earthquake is poorly constrained, and this event will not be discussed further. In Table 2, the five youngest events are labeled CC_S1 (youngest earthquake on the southern boundary fault) to CC_S5 (fifth-oldest earthquake on the southern boundary fault). Table 2 also identifies the specific ¹⁴C samples that constrain the maximum and minimum age limits for each of these earthquakes. All event ages are quoted in calibrated years B.P. at the 95% confidence interval. Figure 10 plots these age ranges and also labels the key ¹⁴C samples used to bracket these age ranges.

View larger version (43K): [in this window] [in a new window]   Figure 10. Time-space plot of surface-rupturing Wairarapa fault earthquake events inferred from all available types of paleoseismological data (Tables 1, 2, and 3). Data for Tea Creek trench (events labeled, T) are from Van Dissen and Berryman (1996). Diatom-based paleoenvironmental data for coastal uplift events at Lake Kohangapiripiri (events, K) are from Cochran et al. (2007). Uplift events near Turakirae Head on Palliser Bay (events, Tk) inferred from raised beach ridges (BR-1 to BR-5) are taken from McSaveney et al. (2006). Small numbers quoted in meters denote inferred single-event uplift magnitudes for a given event. For Palliser Bay data, these refer to the maximum uplifts near Turakirae Head as inferred by McSaveney et al. (2006). Name of key 14C samples that bracket the timing of rupture events in trenches (this study) are identified at maximum and minimum limits of error bars (95% confidence) and refer to the field sample names listed in Table 1. Colored horizontal bands depict the maximum and minimum age range (at 95% confidence) for each earthquake event as derived from an analysis of the composite set of trench data and 14C results.

We interpret the next-youngest earthquake, CC_S4, to have caused refreshment and collapse of the scarp, leading to emplacement of the colluvial wedge, cwB. The age of this wedge is bracketed by samples CC-4-16 and CC-4-13 to the interval 4870–3070 cal. yr B.P. In addition, we infer that the same earthquake was recorded by the intrapeat unconformity—the draping-over of fault strands 4 and 5 by peat in trench CC-1 (Fig. 8A) and the unconformable overlap of the unnamed fault by silt in trench CC-4 (Fig. 8C). We bracket the unconformity (and thus CC_S4) to the interval 3690–2970 cal. yr B.P. using samples CC-1-12 and CC-1-10. A combination of the age constraints of the colluvial wedge with those of the unconformity yields a composite age range for event CC_S4 of 3690–3070 cal. yr B.P.

The third-youngest earthquake (event CC_S3) rejuvenated the scarp to cause formation of colluvial wedge cw2, as exposed in trench CC-1. The age of this wedge is bracketed by samples CC-1-6 and CC-1-13 to the interval 2340–740 cal. yr B.P. Our preferred age for this earthquake is based on sample CC-1-6 alone, as this age (2340–2110 cal yr B.P.) is interpreted to record death of a tree by earthquake-induced toppling immediately prior to emplacement of the wedge. In trench CC-4, the same earthquake (CC_S3) is interpreted to have formed the wedge, cwC. A maximum age constraint for wedge cwC of 3340 cal. yr B.P. is provided by sample CC-4-13, which underlies it (there are no dated samples from above the wedge).

In trench CC-1, we infer that the penultimate earthquake (event CC_S2) caused emplacement of a colluvial wedge, cw3. This wedge draped across the preexisting (and gouge-laden) trace of fault 1. The age of the wedge is bracketed by samples CC-1-13 and CC-1-14 to the interval 920–800 cal. yr B.P. We infer that the uppermost units in CC-4 are condensed or have been in part eroded, perhaps as a result of deforestation and agriculture. In CC-4, a diffuse line of cobbles in the mo unit near the ground surface may be a human-disturbed equivalent to wedge cw3, but this is uncertain.

Inferred on historical grounds to be the 1855 earthquake, the most recent earthquake (CC_S1) is expressed in trench CC-1 by the rupturing of fault strand 2 upward from wedge cw3 to extend into the modern soil profile. A maximum age constraint of 970 cal. yr B.P. is provided by sample CC-1-14 from the faulted om layer, which is overlain by the wire-bearing layer, wl (undated, but assumed modern). In trench CC-4, the 1855 earthquake is inferred to have caused slip on fault strand 2, but it did not generate any colluvial wedge that is preserved near the ground surface today. If such a wedge once did exist, human-induced disturbance caused by deforestation and cultivation must have removed any evidence for it from the uppermost sediment layers.

Inferring Earthquakes from Colluvial Wedges
These interpretations rely in part on our interpretation that the large (>1.5-m-long) gravel wedges are scarp-derived colluvial units that formed as a result of fault-scarp rejuvenation during earthquakes. We infer that in the densely forested lowland settings of precolonial New Zealand, gravitational collapse of earthquake-induced fault scarps was the chief process by which large bodies of terrace gravel could be eroded from a fault scarp and redeposited in an adjacent peat basin. This interpretation was not applied, however, to the two smallest (tens of centimeters long) gravel-bearing bodies near the scarp of the southern boundary fault: specifically, units cw1 in CC-1 and dp in CC-4. This difference in interpretation was based on our consideration of the following typical attributes of earthquake-induced colluvial wedges: (1) large size (consistent with generation of a meter-high fault scarp); (2) wedge shape (thickest at the fault, where they may be truncated, and thinning away from the fault); (3) composition of clasts equivalent to exposures in the adjacent fault scarp; (4) texture consistent with transport of these clasts down the scarp; and (5) synchroneity to other wedges along the fault (or at least to other types of evidence for earthquakes along that fault).

The larger gravel bodies that we have interpreted as earthquake-induced colluvial wedges fit all of these criteria, whereas the two decimeter-sized bodies (units dp and cw-1) fit few of them. The size of the former is most plausibly attributed to the scale of coseismic shaking, scarp rejuvenation, and subsequent scarp erosion that would accompany inferred fault throws of ~1–2 m on the Wairarapa fault (Rodgers and Little, 2006). By contrast, the other two bodies are small, isolated (dp is a disconnected blob), and have a lenticular rather than wedge shape. Although both types of gravel body have apparently been derived from redeposition of the terrace gravels derived from the uplifted side of the fault, clasts in the decimeter-sized gravel bodies are supported by an organic-rich or peaty matrix, whereas the matrix of the large wedges is organic-poor. Perhaps the small bodies formed by adhesion of gravel clasts onto the roots of toppled trees. Excluding the 1855 earthquake (evidence for this youngest event appears not to be well-preserved today), all the "large" wedges at Cross Creek can be temporally correlated to other wedges (or at least earthquakes) in one or more other trenches, implying their lateral continuity along the fault. By contrast, none of the two cited "small" redeposited gravel bodies, although they are well dated (Fig. 10), could be recognized beyond a single trench wall. Next, we will show that most of the colluvial wedges at Cross Creek can be correlated across both sides of the graben.

Trenches across the Northern Bounding Fault of the Cross Creek Graben
Stratigraphy and Structure of Trenches CC-2 and CC-3
Trenches CC-2 and CC-3 were excavated across the northwestern margin of the pull-apart graben. We logged the NE wall of CC-2 (Fig. 9A) and both walls of CC-3 (Figs. 9B and 9C). Many of the stratigraphic units can easily be correlated between these three walls, so we have adopted a set of (in part) common unit names that reflects our correlation (Figs. 9A, 9B, and 9C; Appendix F). This part of the Wairarapa fault zone consists of several SE-dipping fault strands. The fault numbering in Figure 9 reflects our interpretation of how these strands correlate between the trenches. Fault slivers of strongly sheared, clay-matrix gravel (sg and stg) and peat (sp1 and sp2) contain gravel clasts that are rotated to a steep dip against the fault. In addition to their content of colluvial wedges, sediments along the northern margin of the graben reveal clear evidence of progressive deformation, such as differential tilting, angular unconformities, and fissuring. These relationships reduce the potential ambiguity of earthquake identification. Enlarged versions of the trench logs are provided in Figures D and E, and detailed unit descriptions are given in Appendix F.

None of the fault strands on the northern side of the graben cuts to the surface, and the topographic scarp is offset ~5–6 m southward relative to the subsurface fault zone. These relationships reflect lateral accretion of the uppermost gravelly layers across the fault scarp by anthropogenic processes. Wire fragments found in the s-col unit (Fig. 9C) indicate that both this unit and the overlying u-col consist of fill material pushed southward from the site of the nearby road during its excavation (Fig. 7B).

Fluvial terrace gravels (tg) in the uplifted foot-wall of the fault zone are juxtaposed across the northern boundary fault zone against a basin to the SE that is dominated by peat and organic silt. On the SE side of the fault, the down-dropped terrace gravels form an exposed depositional substrate to the peat-rich basin fill. The terrace gravels (tg) are locally capped by layers of gravelly sand and silt (units slt, gs, and ssg). These fluvial deposits are overlain by a much finer-grained sequence of organic-rich silts and peats (units pt, osi-1, osi-2, op, and pt1, pt2, pt3, and pt4). About 7 m to the SE of the fault, a deformational bulge is expressed by folding and erosion of part of the organic-rich basinal infill (Fig. 9A).

Near the fault, the peaty basinal units are interfingered with three southward-tapering gravel wedges. These wedges are progressively faulted and tilted. They are labeled, from oldest to youngest, co-1, co-2, and co-3 (Figs. 9A, 9B, and 9C). Truncated against the fault zone, and consisting of poorly sorted pebble and cobble gravel in a silt matrix, they are interpreted to be scarp-derived colluvial wedges. The oldest of these, co-1, is best exposed on the SW wall of CC-3 (Fig. 9C). On the NE wall of the same trench (Fig. 9B), it is complexly deformed (disturbed) adjacent to a large fossil tree (this was removed by the digger). There the gravel wedge has apparently been entrained into the gravel-bearing mixed units os, ogs, and cs, and also dismembered into isolated clasts. In trench CC-2, co-1 is expressed as a stone line of footwall-derived terrace cobbles (well rounded) that extends southeastward above a basal peat layer (unit pt) for >2 m away from fault strand 3 (Fig. 9A). The younger and variably tilted colluvial wedge co-2 is recognized on all three of the logged walls.

On its NW side, the co-1 wedge is faulted against a distinctive sequence of sands and pebble gravels. Elements of this sequence are found on all walls of both northern trenches as combinations of the units pg (basal pebble gravel), ss (a thin marker layer of sand), and cpg (upper layer of pebble gravel). These loosely consolidated and iron-stained deposits are distinctively well rounded and well-sorted, and we interpret them to be fluvial. On all three logged walls, the fluvial deposits depositionally overlie the terrace gravels (unit tg) on the NW side of fault strand 1 (Figs. 9A, 9B, and 9C). Between fault strands 1 and 2, an intact (unfaulted) part of the fluvial sequence depositionally overlies a substrate of strongly sheared gravel mixed with clay pug (unit sg in Fig. 9A; unit stg in Figs. 9B and 9C). In trench CC-3 (Fig. 9B), the pebble-rich basal part to fluvial package (pg unit) thickens abruptly downward to occupy the space between fault strands 1 and 2. We interpret this to be an infilled faultfissure. In CC-3 (NE wall, Fig. 9B), the upper part of the fluvial infill (cpg unit) drapes south-eastward across fault strand 2 to lie on the peat of unit pt5. In trench CC-2 (Fig. 9A), the colluvial wedge (co-3) depositionally overlies the sandy ss layer (part of the fluvial infill package). The wedge co-3 overlaps fault strands 1, 2, and 3 and is displaced by fault strand 4.

Seven samples from trench CC-2 and five from CC-3 were ¹⁴C dated (Table 1). Of these, four are wood, and the rest are peat, organic clay, or charcoal. All from trench CC-2, the wood samples (CC-2-33, CC-2-35, CC-2-37, CC-2-38) yield ages that are older than stratigraphically underlying nonwood samples (CC-2-30 and CC-2-34) but are indistinguishable from one another (ca. 5.6–5.0 ka). As explained already, the wood samples are interpreted to have been recycled from a near-basal forest layer that was disrupted soon after inception of the present graben. All the nonwood samples in trenches CC-2 and CC-3 yield ¹⁴C ages that are in the correct stratigraphic order (see Appendix G).

Interpretation of Earthquakes Rupturing the Northern Bounding Fault of the Graben
On the basis of the combined data from CC-2 and CC-3, we recognize at least four earthquakes to have ruptured the northern part of the Cross Creek graben since ca. 5.2 ka. These are labeled CC_N4 (oldest) to CC_N1 (youngest) in Table 2, and their age constraints are plotted on Figure 10.

The oldest earthquake, CC_N4, resulted in emplacement of the colluvial wedge co-1 at the site of both CC-2 and CC-3. Temporal constraints for this earthquake are provided by ¹⁴C samples CC-3-L (from unit pt, below the wedge, Fig. 9C) and from CC-3-E, CC-2-30, and CC-2-34 (from the same unit above the wedge). These ages bracket the earthquake to the period 5280–4640 cal. yr B.P. This interval overlaps with CC_S5 on the opposite side of the graben, an event that is similarly recorded by a colluvial wedge (unit cwA in Fig. 8B). We therefore infer that CC_N4 and CC_S5 represent the same earthquake. Our preferred age for event CC_N4, 5209–4842 cal. yr B.P., is based on the interpretation that the six similar-aged wood samples were derived from a forest that was toppled or damaged by this earthquake. For this preferred age, we use the age of sample CC-2-38, the youngest element of the death assemblage that we infer to have been earthquake-triggered.

We interpret the next-youngest earthquake to rupture the northern side of the graben (CC_N3) to have caused emplacement of the colluvial wedge co-2 at the site of both CC-2 and CC-3. Age constraints for this wedge are provided by samples CC-3-F, CC-2-34, and CC-2-30 from the pt unit below the wedge (Figs. 9B and 9C) and by samples CC-3-2 and CC-2-31 from above the wedge (the pt unit in Fig. 9B; the osi-2 unit in Fig. 9A). These data yield an age range for the earthquake of 3080–1991 cal. yr B.P., an interval that overlaps with CC_S3 on the opposite side of the graben, as expressed by colluvial wedges cw2 (in CC-1, Fig. 8A) and cwC (in CC-4, Fig. 8B).

The penultimate earthquake (CC_N2) to rupture the northern boundary fault of the graben caused opening of an ~1-m-deep fault fissure. This cavity was infilled initially by the pebble gravel unit pg and later by the sand of the ss unit. These well-sorted fluvial units were deposited across the pug-lined faults bounding the fissure (fault strands 1 and 2 in Fig. 9B), with the youngest part of the infill sequence (unit cpg) downlapping onto the peat unit pt4 (Fig. 9B). Presumably a small stream draining across the scarp transported the clasts. We infer that CC_N2 caused, moreover, some combination of the following: SE-ward tilting of the co-2 wedge, anticlinal bulging of the peat basin to the SE, and deposition of the colluvial wedge co-3 on the NE wall of trench CC-2 (Fig. 9A). A maximum age constraint for CC_N2 of 2150–1940 cal. yr B.P. is provided by sample CC-3-2 in unit pt5, which stratigraphically underlies the cpg member of the fissure-infilling sequence (Fig. 9B).

The final rupture at the site, CC_N1, caused renewed slip on fault strands 1 and 2 and initiation of fault strands 5, 6, and 7 (Figs. 9B and 9C). This faulting deformed the fissure-infilling sequence of stream sediments (units ss, pg, and cpg) that were deposited after earthquake CC_N2. In addition, faulting on strand 4 deformed the colluvial wedge co-3 that we attribute to CC_N2 (Fig. 9A). Inferred to be the 1855 earthquake, event CC_N1 may also have resulted in deposition of the colluvial layer grs at the site of trench CC-3; however, it is uncertain whether this is a natural deposit.

Note that the described progressive deformation sequence (opening of fissure, filling of fissure, renewed faulting) requires charcoal sample CC-3-2 (2150–1940 cal. yr B.P., from pt5 in Fig. 9B) to predate two earthquakes. Conceivably, it might predate three earthquakes if the fissure-infilling significantly predated deposition of the colluvial wedge co-3 in trench CC-2. This is because this wedge stratigraphically overlies the fissure-infilling ss unit (Fig. 9A). We view such a three-event scenario as less likely than a two-event scenario, since no other trench provides evidence for three earthquakes since ca. 2 ka. In our preferred interpretation, the fissuring and emplacement of the colluvial wedge co-3 are viewed as twin manifestations of the penultimate earthquake, CC_N2.

	DISCUSSION

TOP ABSTRACT INTRODUCTION TECTONIC SETTING RESULTS DISCUSSION CONCLUSIONS REFERENCES CITED

 
Late Holocene Rupturing History of the Southern Wairarapa Fault
By integrating the key stratigraphic and structural events observed in the eight paleoseismic trenches, and by dating and correlating these using the 40 new ¹⁴C samples, we interpret a composite surface-rupturing history that has included at least five earthquakes on the southern part of the Wairarapa fault since ca. 5.2 ka (Table 3; Fig. 10). This history and our intertrench correlations are summarized in Appendix H. Although any correlation involves interpretation, our chronology invokes the minimum number of earthquakes allowed by the data. While the dating precision for each event at each site differs, and the preferred age of specific earthquakes might vary according to interpretation, we believe that the overall number of surface-rupturing events (five since ca. 5.2 ka) is a robust outcome of the data. Not unexpectedly, preservation of the individual surface-rupturing events in this chronology is unequal between different sites and trenches because of differences in exposed stratigraphic age and degree of disruption or burial of near-surface layers as a result of human activity. Only the Cross Creek graben trenches sampled the oldest three events, whereas the Pigeon Bush and Riverslea trenches provided timing information relating to the last two events only (1855 and penultimate). As a result of such preservational differences, no single trench preserves evidence for all five events (though combinations of four are recorded in each of the four Cross Creek trenches). By correlating events between the various trenches and trench walls, and using the entire data set of ¹⁴C samples to bracket the composite event timing, we were able to significantly narrow the 95% confidence time intervals for each event (red bars in Fig. 10) relative to that which would have been derived by limiting our analysis to ¹⁴C data found in each individual trench or site (compare Table 2 to Table 3).

The apparent internal consistency in timing of these five rupturing events between the disparate trench sites suggests that these broke the entirety of the southern section of the Wairarapa fault (Fig. 1A). By contrast, only the youngest (1855) and oldest of the five southern events can be correlated to the earthquake chronology at Tea Creek trench, ~40 km to the north of Pigeon Bush (Van Dissen and Berryman, 1996) (Figs. 1A and 10). Although the Tea Creek record is based on only one trench, this apparently imperfect correlation suggests that some of the southern ruptures (not including 1855) may not corupture across the northern zone of splay fault junctions to break the entire length of the Wairarapa fault.

Comparison of Earthquake Chronology with that of Turakirae Head Beach Ridges
For a flight of uplifted strandlines to provide a complete record of paleoearthquakes, each earthquake must cause enough uplift to preserve a distinct coastal landform. Moreover, these land-forms must be distinct from others that might form by nontectonic relative sea-level changes or longer-term tectonic processes that are aseismic. Finally, any interseismic subsidence due to pre-earthquake strain accumulation or postseismic crustal relaxation must be small or predictable fractions of the overall strandline displacement (e.g., Berryman, 1987; Nelson and Manley, 1992; Wilson et al., 2007b). Turakirae Head is one of the world's best examples of a coseismically uplifted flight of Holocene beach ridges (Burbank and Anderson, 2001). Others include Mocha and Santa Maria Islands in Chile (Nelson and Manley, 1992; Bookhagen et al., 2006), the Boso Peninsula and other sites in Japan (Ota and Yamaguchi, 2004), Peninsula de Nicoya, Costa Rica (Marshall and Anderson, 1995), the Mahia Peninsula and Pakarae River mouth in New Zealand (Berryman, 1993; Ota and Yamaguchi, 2004; Wilson et al., 2007a), Cape Mendocino, California (Merritts, 1996), Taiwan (Yamaguchi and Ota, 2004), and the Gulf of Alaska (Plafker, 1969; Plafker et al., 1992; Plafker and Rubin, 1978). Although other studies have compared beach ridge uplifts to the timing of historic earthquakes as much as ~500 yr ago (e.g., Bookhagen et al., 2006; Ferranti et al., 2007), these studies typically suffer from the short time span of the historic data and from ambiguities regarding the location of the surface rupture accompanying those felt earthquakes. To our knowledge, ours is the first study attempting a one-to-one comparison between uplifted strandlines and paleoseismically documented fault ruptures over a time span of ~5 k.y. This comparison is possible because the Wairarapa fault (unlike subduction megathrusts) is exposed above sea level where it can be investigated by trenching at sites proximal to the uplifted beach ridges on its hanging wall.

McSaveney et al. (2006) identified and dated the uplift and stranding of four late Holocene beach ridges at Turakirae Head. Three of these are younger than ca. 5.2 ka. Each of these corresponds to one of our independently determined Wairarapa fault rupturing events (Fig. 10). They are the most recent earthquake (corresponds to uplift of beach ridge BR-2 in 1855), the third event (corresponds to uplift of beach ridge BR-3 at 2380–2060 cal. yr B.P.), and the fifth event (corresponds to uplift of BR-4, inferred by McSaveney et al. [2006] to have taken place at 5420 –4110 cal. yr B.P.).

Two of our trench-determined fault-rupturing events cannot be matched to a beach ridge at Turakirae Head. These are the penultimate event and fourth event (Table 3; Fig. 10). At the crest of the Rimutaka anticline, the elevation difference measured by McSaveney et al. (2006) between the stranded 1855 beach ridge (BR-2) and the next-highest stranded ridge (BR-3) is ~9.1 m (Fig. 11A). We note that this difference is >2 higher than the mean vertical spacing of the remaining three higher ridges at that site (4.8 ± 1.6 m, 1; McSaveney et al., 2006), implying that this large vertical interval may record the cumulative uplift of two earthquakes. Temporally, this beach ridge interval spans from 1855 to include both our penultimate and third events. Accordingly, we infer that a distinct beach ridge may not have been introduced (or preserved) into the landscape after the penultimate earthquake. The vertical interval between beach ridges BR-3 and BR-4 records not only the fifth event (in time) but also our "missing" event four. At 5.5 m, the height difference between these two ridges implies a mean tectonic throw of ~2.6 m for those two earthquakes (McSaveney et al., 2006). Perhaps, one of these two incremental uplifts was too small to preserve a distinct beach ridge at Turakirae Head.

View larger version (30K): [in this window] [in a new window]   Figure 11. (A) Beach-ridge topographic profiles drawn orthogonal to the Cape Palliser–Turakirae Head coastline on either side of the crest of the Rimutaka anticline (from McSaveney et al., 2006). (B) Schematic cross-section X–X' across southern North Island, New Zealand, showing major faults in upper plate of the Hikurangi subduction margin, including strike-slip and contractional faults at the southern end of the Wairarapa fault zone. Currently inactive faults are dashed. Position of the currently locked part of the subduction interface is taken from Wallace et al.'s (2004) modeling of global positioning system (GPS) data. For location of cross section, see Figure 1. M.S.L.—mean sea level.

Our comparison between the trench-based earthquake rupturing history of the Wairarapa fault and the sequence of raised beaches at Turakirae Head leads us to conclude that flights of uplifted gravel beach ridges may provide an incomplete record of paleoearthquakes on adjacent reverse-oblique faults (Fig. 11A). We note that a similar discrepancy between (less frequent) beach-ridge uplifts and (more frequent) earthquake events has been observed in Chile and Italy on the basis of historical records of earthquakes (e.g., Bookhagen et al., 2006; Cisternas et al., 2005; Lomnitz, 2004; Ferranti et al., 2007). The next section asks the question, "What processes might have led to a shortfall in the number of beach ridge uplifts relative to earthquake ruptures on the hanging wall of the Wairarapa fault?"

Nontectonic Causes for Under-Representation of Earthquakes by Beach Ridges
Consisting of coarse gravel, the raised beach ridges at Turakirae Head mantle a low-angle Holocene wave-cut platform incised into graywacke bedrock. This is a wave-battered coast, open to the south and the largest storm waves in Cook Strait. Since 1855, storm waves have built a berm crest on the modern beach that is variably 2–7 m above mean sea level (McSaveney et al., 2006), a relationship that underscores the sensitivity of berm height to local wave conditions. Due to Turakirae Head's preeminent position on the coast, and its remoteness from large rivers, its rocky coastline is only thinly covered with sediment. With little apparent input from longshore drift, a process which tends to sweep material southward away from the headland, the main source of the gravel for this sediment-starved headland has been in situ erosion of the bedrock platform (Wellman, 1967) and/or mass wasting of the adjacent hills (Hinton and McSaveney, 2007). Because the eustatic position of sea level is inferred to have remained stable throughout New Zealand since ca. 6.5 ka (Gibb, 1986), we infer that the only significant source of relative sea-level fall near Turakirae Head since ca. 5.2 ka has been tectonic uplift. In a recent review paper, Kennedy (2008) argues that Gibb's (1986) sea-level curve is supported by subsequent studies, and that there is no evidence for eustatic sea-level variations in New Zealand of more than ±1 m since ca. 6 ka. For this reason, we assume that sea-level variations have not been a key factor influencing beach-ridge formation at Turakirae Head since ca. 5.2 ka. For example, we discount the possibility that a small rise in sea level during the Holocene (by itself) could have caused erosion of a preexisting beach ridge.

Given the complexity of hydrodynamic variables involved in the formation and retreat of gravel beaches (Carter and Orford, 1993; Orford et al., 1995; Neal et al., 2003; Engels and Roberts, 2005), it is possible that variations in sediment supply or wave climate (especially as the result of large winter storms) may have contributed to formation or preservation of discrete beach ridges during some interseismic periods of the Wairarapa fault and not others.

The most plausible nontectonic scenarios for earthquake under-representation by the beach ridges probably involve some combination of the following two processes: (1) the crest of the active berm may, at times, retreat landward far enough to weld with, or overwash, the next-highest beach ridge; or (2) a beach ridge may have been so small or indistinct at the time of its (tectonic) stranding that it was later easily overwashed, or otherwise not preserved as a discrete landform on the gravelly wave-cut platform. Secular wave climate and sediment supply are the chief variables that control the size and height of a storm berm and determine whether it remains stable in one place (perhaps building up an especially high berm) or is over-washed to retreat landward (Carter and Orford, 1993; Orford et al., 1995). If these quantities varied between interseismic periods (i.e., at a time scale of ~1000–2000 yr), then preservation of a discrete beach ridge may have occurred after some earthquakes but not others. Given the extreme (nonlinear) impact of the lowest frequency–highest magnitude storm events on the extent to which gravel beach ridges can be driven landward, a period of increased storminess could cause an active storm berm to overtop (or weld with) a relict beach ridge that was originally several meters (perhaps even 5–8 m) higher than it; in this case, the key variable would be the return period of storms large enough to generate the runup capable of overwashing the higher ridge (Orford et al., 1995; Neal et al., 2002, 2003). Scenario 2 might occur because of either a reduced sediment supply (beach ridges cannot form without gravel) or because of too large a sediment supply. The latter situation might suppress formation of discrete storm berms or lead to their burial underneath younger deposits (Engels and Roberts, 2005). Clearly, the internal structure and sedimentological characteristics of the raised beach berms at Turakirae Head might provide a record of such depositional or erosional events; this would seem to be a fruitful avenue of future research that could build upon (and test) the results of our study.

Tectonic Causes for Under-Representation of Earthquakes by Beach Ridges
Alternatively (and perhaps more likely), the incomplete representation of earthquakes by beach ridges could reflect variable magnitudes of coseismic uplift at the coast during Wairarapa fault earthquakes. Figure 11B illustrates the complex structure of the Wairarapa fault zone at the southern end of the North Island near Turakirae Head, where there are multiple (alternate) fault strands in the near-surface. If the formation of a beach ridge on the hanging wall of the Wairarapa fault zone during the late Holocene was mostly controlled by the magnitude of the coseismic vertical displacement, then this would have reflected not only the direction and magnitude of slip on any rupture at depth, but also the geometry of the rupturing near-surface fault (especially its proximity to Turakirae Head and its dip). Given the observed structural complexity of the Wairarapa fault zone near the coast, one cannot assume that there was only one rupture scenario during the Holocene.

Some earthquakes may have caused a large enough coastal uplift at Turakirae Head to elevate the former storm beach beyond the reach of subsequent waves, thus preserving a discrete beach ridge, whereas others may have not. If earthquake ruptures near the coast typically reoccupy the same spatial fault plane (or near-surface splay) during each earthquake, the magnitude of the coseismic uplift may vary between earthquakes, and some may not uplift the coast enough to strand a distinct beach ridge. Dislocation modeling of vertical deformation during 1855 (Darby and Beanland, 1992; Beaven and Darby, 2005) and the extremely high slip/length ratio of that earthquake's coseismic rupture (Rodgers and Little, 2006) suggest that it may have ruptured not only the Wairarapa fault but also a contiguous segment of the subduction interface downdip of it. Perhaps some Wairarapa fault earthquakes involve deep co-rupturing of the subduction interface (as in 1855) to cause 3–9 m of coastal uplift locally near Turakirae Head, whereas others break only the upper plate, or nucleate as strike-slip ruptures in the upper plate, yielding less throw at the coast.

Alternatively, earthquake ruptures at the southern end of the Wairarapa fault may have a nearly constant slip and focal mechanism at depth, but they may cause variable coseismic uplift at the surface because of variable partitioning of slip between multiple fault splays in the upper crust. Dislocation models by Beavan and Darby (2005) require a local thrust structure to have been responsible for the very high magnitude (up to 6.4 m) but short wavelength of uplift at Turakirae Head in 1855. Coseismic slip on some oblique-slip splays (especially the Muka Muka fault; Fig. 2B) may cause significant uplift of Turakirae Head on their hanging wall, whereas slip on other splays may not. Consistent with this view, Schermer and Little (2006) and Little et al. (2008) document a temporally and spatially complex pattern of fault activation-deactivation and surface folding during the past 80 k.y. at the southern end of the Wairarapa fault zone and infer that a blind thrust is currently active near the western edge of Lake Onoke, ~10 km to the east of the Muka Muka fault (Figs. 2 and 11B). These relationships suggest a variable linkage between slip on the deeper Wairarapa fault and its splays in the near-surface. If the 1855 earthquake involved uplift of Turakirae Head along the Muka Muka fault, previous earthquakes may have ruptured to the surface along other splays to the east or west, causing little or no coastal uplift there.

Until now, we have ignored the possible contribution of aseismic regional uplift caused by sediment underplating or subduction of anomalously thick oceanic lithosphere at the Hikurangi subduction zone (e.g., Litchfield et al., 2007). Any such uplift is assumed to have accumulated slowly and steadily enough during the past ~5 k.y. to have had no significant effect on the punctuated uplift and stranding of beach ridges at Turakirae Head as a result of large Wairarapa fault earthquakes. A point less easily dismissed is the possible effect of interseismic subsidence due to elastic strain accumulation above the underlying, locked subduction interface. Permanent GPS observations along the SE coast of the North Island (and forward dislocation modeling of GPS data) suggest that it could be on the order of 1–2 mm/yr near Turakirae Head (L. Wallace, March 2008, personal commun.). If this rate of subsidence persisted for an entire Wairarapa fault interseismic period of 1–2 k.y., then up to ~2 m of a previous coseismic uplift signal might be removed. However, because the subduction interface's earthquake recurrence interval is probably much shorter (~300–625 yr; Wallace et al., 2004) than the Wairarapa fault's earthquake recurrence interval (1–2 k.y.), less subsidence is likely to accumulate before a Wairarapa fault earthquake (this amount would depend on the phase shift in the seismic cycles between these two faults). Either way, interseismic strain accumulation is predicted to reduce the height of any previously stranded beach ridge and thus increase the possibility that it would be subsequently overwashed during an especially stormy period (thus omitting one beach ridge from the preserved sequence; e.g., Marshall and Anderson, 1995). This would be most likely if the Wairarapa fault earthquake happened to occur near the end of the subduction zone fault's seismic cycle, and if that cycle was especially long. McSaveney et al. (2006) argued, however, on the basis of a comparison between survey data and historic observations of coseismic uplift in 1855 that interseismic subsidence on Palliser Bay has been negligible during the past the 150 yr.

Our preferred interpretation is that coseismic uplift has been variable at the coast near the Wairarapa fault, and that this tectonic variability, perhaps combined with contributing variations in secular wave climate or sediment supply (or interseismic subsidence), has resulted in an incomplete representation of earthquakes by beach ridges near Turakirae Head.

Rates of Slip and Earthquake Recurrence on the Wairarapa Fault
The dating undertaken as a part of this study establishes the timing of deposition and abandonment of the gravels comprising the youngest post–Last Glacial aggradation (fill) terrace in the southern Wairarapa Valley, the so-called Waiohine terrace. Our ¹⁴C age of samples CC-1-1a-I and Auger-3 bracket formation of the terrace tread to postdate 12,400 ± 300 and to predate 5440 ± 140 cal. yr B.P. (Table 1). Since the ca. 12.4 ka peat sample was deposited during the final phases of terrace aggradation, we infer that abandonment of that terrace took place at ca. 12 ka, making the Waiohine fluvial terrace age-equivalent to either the Ohakea 2 or Ohakea 3 terrace in the regional correlation scheme applied to North Island, New Zealand (e.g., Litchfield and Berryman, 2005). We note that our ¹⁴C-based age data are accordant with several other ages obtained previously from samples collected elsewhere from above or below the Waiohine terrace tread (Tompkins, 1987; Marden and Neal, 1990). They are also considerably more precise than eight late Last Glacial OSL ages of silts that overlie the gravel of Waiohine terrace along the central part of the Wairarapa fault farther north (Wang and Grapes, 2007). Those samples gave ages of 16–10 ka, with four of eight ages being younger than 13 ka. Because these cover bed silts may have accumulated after incision of the terrace by the Waiohine River, their OSL ages may underestimate the timing of terrace abandonment.

Applying a mean OSL age of 10–11 ka to displaced channels on the Waiohine terrace at Waiohine River, to which they attributed a dextral slip of 125 ± 5 m, Wang and Grapes (2007) proposed a late Quaternary dextral-slip rate for the Wairarapa fault of 11.5 ± 0.5 mm/yr. We note, however, that these authors did not present any surveyed map of these offsets, and that Lensen and Vella (1971) surveyed and mapped the same terrace in detail without recognizing these channels (their largest offset was for the terrace riser incised below the Waiohine surface, a younger landform for which they measured a dextral slip of ~99 m). Our ¹⁴C-based dating suggests that the Waiohine terrace was abandoned near Cross Creek at no later than 12 ka. Assuming that this surface was subsequently displaced laterally by at least 99 m (at least near Waiohine River), our data imply a minimum late Quaternary slip rate on the Wairarapa fault of ~8.3 mm/yr. On a shorter time scale, if we divide estimates of mean single-event strike slip on the southern Wairarapa fault during the last two earthquakes (13–16 m; from Rodgers and Little, 2006) by our revised mean earthquake recurrence interval for that fault (1230 ± 190 yr), we get a dextral-slip rate of 9–15 mm/yr. This result is within error of the above slip-rate estimate based on our ¹⁴C-derived, 10–12 ka abandonment age of the Waiohine surface (~11 ± 3 mm/yr).

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION TECTONIC SETTING RESULTS DISCUSSION CONCLUSIONS REFERENCES CITED

 
The Wairarapa fault is remarkable because a series of uplifted beach ridges on its hanging wall are preserved on the nearby coast, and the ages of these Holocene ridges can be directly compared with the ~5 k.y. chronology of surface-rupturing earthquakes derived from paleoseismological methods. We dated five surface-rupturing earthquakes on the southern part of Wairarapa fault since ca. 5.2 ka (an inter-event recurrence of 1230 ± 190 yr). Along the margins of the Cross Creek pull-apart graben, these earthquakes caused laterally and temporally variable amounts of faulting, tilting, and fissuring of scarp-proximal strata, emplacement of scarp-derived colluvial wedges, and the toppling and destruction of trees. Our late Holocene paleoearthquake record recognizes two more events than are recorded by beach ridges that were stranded on the uplifted hanging wall of the fault near Turakirae Head during the same time interval (McSaveney et al., 2006).

Our work indicates that coseismically uplifted gravel beach ridges on an open, rocky coast may provide an incomplete record of paleoearthquakes on adjacent reverse-oblique faults. In the case of the southernmost part of the Wairarapa fault zone, variable increments of coseismic uplift at Turakirae Head may have resulted from the partitioning of slip between several different fault splays in the near-surface. During event four, coseismic throw at the coast is inferred to have been so small (<3 m) that a discrete new beach ridge did not form. Variations in wave climate, sediment supply, and/or interseismic subsidence probably exerted further controls on the number of beach ridges preserved in the uplifted sequence, in particular, governing the morphology (e.g., height, elevation) of the original storm berm, and whether or not it would retreat far enough landward to overwhelm the next-highest beach ridge. A combination of such processes may have caused event four to be under-represented by a discrete, long-lived beach ridge, despite significant coseismic uplift during that earthquake.

Our ¹⁴C data indicate that a widespread Last Glacial Maximum aggradational terrace found in this part of North Island, New Zealand (Waiohine terrace), was abandoned soon after 12.4 ka. This result, combined with previous estimates of dextral slip relative to this terrace, our revised recurrence interval for the Wairarapa fault, and estimates of mean single-event slip on the fault, suggests that the southern part of the Wairarapa fault has a late Quaternary dextral-slip rate of ~11 ± 3 mm/yr.

	ACKNOWLEDGMENTS

 
We thank Kate Wilson and Vasso Mouslopoulou for their global positioning system (GPS) surveying efforts; Julia Bull, Dave Murphy, Susanne Grigull, Vasso Mouslopoulou, and Kate Wilson for assistance with trench logging; and H. Saywell, H. Brandon, P. Smith, and D. Cleal for permission to excavate on their land. This study was funded by the "It's Our Fault" Project, with additional support provided by the New Zealand Foundation for Research Science and Technology, contract CO5X0402 (Geo-Hazards and Society, GNS Science). Two anonymous reviewers and Jon Pelletier are thanked for their constructive comments. Helpful feedback on an early version of the manuscript was provided by David Kennedy and Mauri McSaveney.

	Footnotes

 
r.vandissen{at}gns.cri.nz.

schermer{at}geol.wwu.edu.

^#rachel.carne{at}vuw.ac.nz.

¹GSA Data Repository Item 2009053, Appendices A–H and Figures A–E, is available at www.geosociety.org/pubs/ft2009.htm, or on request from editing{at}geosociety.org, Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA.

Received for publication July 23, 2008. Revision received October 14, 2008. Accepted for publication October 15, 2008.

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