GSA Bulletin: Late Quaternary Strike Slip on the Eastern Part of The

Late Quaternary strike slip on the eastern part of the Awatere fault, South Island, New Zealand

T. A. Little Victoria University of Wellington, Research School of Earth Sciences, P.O. Box 600,Wellington, R. Grapes } New Zealand

G. W. Berger Desert Research Institute, Quaternary Sciences Center, Reno, Nevada 89506-0220

ABSTRACT INTRODUCTION AND REGIONAL nary strike-slip rates at several sites along the CONTEXT fault; (3) to compare the degree of “partitioning” New Zealand straddles the obliquely con- of oblique slip at different locations along the vergent boundary between the Pacific and In the northeastern part of South Island, New fault; and (4) to identify and discuss the last ma- Australian plates. In central South Island, Zealand, obliquely convergent motion between jor surface rupture along the fault. We also pre- plate motion is accommodated by oblique col- Pacific and Australian plates is accommodated sent new data on the age of late Quaternary ter- lision of continental crust along the Alpine across the ~150-km-wide Marlborough fault sys- races in the Awatere Valley. fault in the Southern Alps, and in North Island tem (Fig. 1). The Marlborough fault system is a Near Blenheim, the NuvelÐ1a plate model by subduction of oceanic crust beneath the transition zone linking the Hikurangi subduction predicts 39 mm/yr of motion of the Pacific plate continental Hikurangi margin. Between these margin offshore of the North Island (Lewis and relative to the Australian plate (DeMets et al., two zones, oblique convergence is accommo- Pettinga, 1993) to the continental collision zone 1990, 1994) (Fig. 1). At 258¡, this vector re- dated across the ~150-km-wide Marlborough of the Southern Alps in the South Island (Norris solves into 36 mm/yr of dextral slip parallel to a fault system, which is transitional between the et al., 1990). The Awatere fault is one of the four mean strike of ~055¡, and 15 mm/yr of shorten- two different styles of margins. Dextral slip in principal dextral-slip faults in the Marlborough ing orthogonal to that direction. Dextral-slip the Marlborough fault system is partitioned fault system, but its neotectonic features have faults of the Marlborough fault system are among four principal faults, of which the Awa- been little studied relative to other active strike- spaced 30Ð40 km apart and are ~20Ð40 km tere fault is one. Our data on the eastern part slip faults in New Zealand (e.g., Lensen, 1968; above a subduction interface that dips northwest of the Awatere fault provide insight into styles Kieckhefer, 1979; Grapes and Wellman, 1988; (Anderson et al., 1993). Strongly coupled in the of surface faulting and active deformation in Berryman, 1990; Cowan, 1990; Knuepfer, 1992; southern North Island, this interface becomes continental transpression zones. We (1) docu- Wood et al., 1994; Van Dissen and Berryman, locked beneath the Marlborough faults in the ment the segmentation and kinematics of 1996). The Awatere fault extends offshore into northeast South Island, as indicated by changing oblique slip on the fault, including a dis- Cook Strait (Uruski, 1992; Henrys et al., 1995), patterns of geodetic strain, seismicity, and trench crepency between long-term and short-term and may link with the Wellington fault (Carter et fill (Bibby, 1981; Reyners et al., 1998; Collot et accumulation of vertical motion; (2) describe al., 1988); thus data on late Quaternary slip on the al., 1996). Subduction terminates ~50Ð90 km an along-strike gradient in the degree of slip Awatere fault are important for seismic hazard southwest of Kaikoura (Reyners and Cowan, partitioning of oblique plate motion; (3) mea- evaluation. Although the stratigraphy of Quater- 1993; Anderson et al., 1993). Offshore of the sure late Quaternary strike-slip rates of 6Ð8 nary loess deposits covering extensive alluvial Marlborough fault system, oblique thrusts ac- mm/yr at several sites along the fault, rates terraces in the Awatere Valley is well known complish minor accretion; these become land- that decrease eastward to <1.5 mm/yr into a (Eden 1989), slip rates on the Awatere fault are ward vergent on the Canterbury shelf (Lewis and clockwise-rotating domain near the coast; (4) poorly constrained, due largely to lack of precise Pettinga, 1993; Barnes, 1996). present data on the timing and magnitude of age data for these terraces. The South Island is remarkable for the oblique- the fault’s last major surface rupture, in 1848, After discussing the slip kinematics of the slip character of its active faults. In the Marlbor- which resulted in a probable ~100+-km-long Awatere fault, we present neotectonic data from ough fault system, “partitioning” of slip into dis- rupture and strike slip of 6Ð8 m; and (5) pre- three detailed study regions along the eastern part crete belts of inland strike-slip faulting and sent new data on the age of late Quaternary of its trace (Fig. 2). As a case study, the paper pro- seaward reverse-slip faulting is, at best, imper- terraces in the Awatere Valley. vides insight into styles of surface faulting and fectly developed (Bibby, 1981; Anderson et al., active deformation in zones of continental trans- 1993; Braun and Beaumont, 1995). In the central pression. Our objectives are (1) to document the Southern Alps, it is apparently absent (Norris et al., *e-mail: [email protected] kinematics of slip on this active oblique-slip fault 1990). In northeast Marlborough the average fault system; (2) to measure and compare late Quater- strike is, at ~055¡, about 23¡ from the relative plate Data Repository item 9803 contains additional material related to this article.

GSA Bulletin; February 1998; v. 110; no. 2; p. 127Ð148; 14 figures; 3 tables.

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Blenheim NN 0 25 Cook Strait

km Wairau fault ° 39 mm/yr (3.5-6) ±2 Australliian Area of 065 Pllate Figure 2 e g Saxton s as n ra ur a River u o 42°S l eastern sectionko ik Molesworth e ° ai a Late Miocene- M K K Kekerengu section 072. nd Early Pliocene d Inla fault H f. k Barefell's Awatere basin s re Pass wate E Seaward Jordan Stream A fault (3-9) ° f. Kaikoura 080 lt (3-8) pe u Ho fa ne pi Clarence f. (20-25) Al Hikurangi TroughPaciifiic 173°E Pllate

Figure 1. Simplified tectonic map of northeastern South Island, New Zealand, showing principal elements of the Marlborough fault system and location of the late MioceneÐearly Pliocene Awatere basin. Bold arrows are NuvelÐ1a plate motion vectors for Australian-Pacific relative plate motion calculated from rotation pole of DeMets et al. (1990, 1994). Labeled half-arrows are local azimuths of fault displacement on the eastern part of Awatere fault (this study), the central (Molesworth) section of the Awatere fault (McCalpin, 1996), and on the central Alpine fault (Norris et al., 1990; Norris and Cooper, 1995). Numbers in parentheses indicate ranges of available late Quaternary dextral-slip rates (in mm/yr) obtained for strands of the Marlborough fault system. Compiled from Van Dissen and Yeats (1991), Knuepfer (1992), and Lensen (1963).

motion vector. Long-term oblique-reverse slip on blocks has been an important mode of plate presented stratigraphic, geomorphic, and paleo- the Awatere, Clarence, and Hope-Kekerengu faults boundary deformation since the Miocene (e.g., seismological data relating to the Molesworth is well expressed by mountainous topography on Roberts, 1992; Little and Roberts, 1997). section of the Awatere fault. the northwest side of each fault, whereas shorter Offset of metamorphic isograds near the term displacement vectors based on offset Quater- PREVIOUS WORK western end of the fault led Suggate et al. (1961) nary features are often variable in sense and more to infer 7Ð8 km of dextral slip, a figure in- nearly pure strike-slip (e.g., horizontal/vertical slip Early investigations of the Awatere fault in- creased to ~13 km by McClean (1986). Using a ratios of >10) (Lensen, 1968; Berryman, 1979; clude those by McKay (1886), King (1934), and Late Miocene facies boundary in the Awatere Kieckhefer, 1979; Kneupfer, 1992). Net oblique- Lensen (1963). Terrace offsets across the fault at basin (Fig. 1), Little and Jones (1998) infer a reverse slip on the Clarence fault is driving uplift the Grey and Saxton Rivers were reported by minimum finite dextral slip of 34 ± 10 km of the Seaward Kaikoura Ranges at ~10 mm/yr Lensen (1964a, 1973). Following Cotton (1947a, across the eastern Awatere fault system. They (Wellman, 1979; Van Dissen and Yeats, 1991). 1947b), Lensen (1964a) inferred a reversal in also discuss the kinematics, timing, and magni- Cumulative fault-slip rates across the Marlbor- sense of throw on the fault during the Holocene. tude of Neogene slip on the several active and ough fault system suggest that its four principal Knuepfer (1988, 1992) used pebble weathering inactive strands that compose this system. Little strands accommodate most of the relative plate rind and soil morphology dating to calculate late (1995, 1996) examined populations of faults motion during periodic large earthquakes Quaternary dextral-slip rates at the above two and extension fractures within ~3 km of the (Knuepfer, 1992; Holt and Haines, 1995). Focal sites. Eden (1983, 1989) determined a late Qua- Awatere fault, measured bulk displacement gra- mechanisms of large events (Ms >5.8) are dextral ternary stratigraphy of the Awatere Valley based dients related to this brittle deformation, and reverse or oblique reverse on northeast-striking on loess deposits and tephras, mapped the allu- concluded that regional deformation has ac- nodal planes (Anderson et al., 1993). Paleomag- vial terraces, and discussed patterns of their inci- crued chiefly as the result of coseismic slip netic data show that clockwise rotation of crustal sion and uplift. McCalpin (1992a, 1992b, 1996) along the main Awatere fault.

128 Geological Society of America Bulletin, February 1998

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STUDY METHODS Repository, Part C (see footnote 1). Fault atti- fault. There, the majority of H/V ratios are tudes (Fig. 3A) define an apparently bimodal ~3Ð5, and normal in sense (McCalpin, 1996). Active traces of the Awatere fault were mapped mixture of 060¡Ð065¡-striking strands that dip McCalpin (1996) used a rigid block model simi- between the coast and Grey River by plotting field northwest at >80¡ (e.g., coastal and Nina Brook lar to that used in Figure 3B to model his throw data on 1:20 000 to 1:16 000 scale aerial photo- sections); and (2) 050¡Ð055¡-striking strands that vs. strike data, assuming a northwest fault dip of graphs, and compiling onto 1:25 000 scale topo- dip northwest at 40¡Ð70¡ (e.g., Dumgree and 45¡. That this model often underestimates throw graphic maps. East of Black Birch Stream (Fig. Grey River sections). The complex Blairich River (Fig. 9 in McCalpin, 1996) suggests that actual 2), >90% of the fault was examined on the section includes both types of sections alternating dips may be steeper than 45¡. The Molesworth ground. Farther west, field observations in the ma- at length scales between 100 m and 5 km. The section has variable scarp facing directions indi- jor stream gorges were supplemented by aerial two orientations of strands intersect in a gently cating essentially horizontal slip at the surface photograph interpretation between streams. Surfi- southwest-plunging line, perhaps an indication of along an azimuth of ~072¡ (about 8¡ clockwise cial and bedrock geologic maps were prepared for the oblique-reverse mean direction of slip on the from that documented by us on the eastern sec- each of the three detailed study regions, and the corrugated surface of this mature fault at depth tion of the fault, and closer to the azimuth of rel- Quaternary stratigraphic record examined. At (Fig. 3A). Slip associated with late Quaternary ative Pacific-Australian plate motion). Grey River and Lake Jasper, low-level vertical earthquake rupturing and scarp development, aerial photographs (scale of ~1:4700 or ~1:2600) however, appears more nearly horizontal than NEOTECTONICS OF THE AWATERE were used as a base for plotting and interpreta- this plunging intersection lineation would imply. FAULT NEAR THE COAST tion. Outside the detailed study areas, displace- Fault sections that strike ~064¡ are fissure-like, ments were measured with tape and/or by level- causing little or no scarp on Late Quaternary al- Coastal Fault Traces and Offset Features ing. In the detailed study regions, terrace offsets luvial terrace surfaces, and are inferred to be pure were surveyed to centimeter precision using strike slip; those at 065¡Ð067¡, such as the Nina The coastal trace of the Awatere fault strikes a Sokkisha, model Set 4, total-station EDM Brook and western coastal strands, are slightly 060¡Ð065¡, dips 80¡Ð85¡ northwest. Near Bound- theodolite. Topographic data for terrace risers oblique-normal (downthrown to the northwest), ary Stream, the fault splits into two strands that were gridded and contoured by computer to al- and those striking at <064¡, such as the Dumgree bound a narrow fault wedge that down-drops hill- low unbiased measurement of finite slip-vectors section, are slightly oblique-reverse. slopes by 75Ð150 cm (Fig. 4). The northern of in three dimensions. See Data Repository, Part A Just west of the Blairich River, the strike of these splays uplifts late Miocene-early Pliocene for details of surveying techniques1. Terraces the Awatere fault swings from 055¡ to 064¡ (Fig. marine conglomerates and mudstones (Mu and were dated using tephrochronology, 14C dating, 2, location X), providing a small-scale example Pmc units) on its northwest side against Quater- thermoluminescence (TL) dating of the fine of the slip-kinematic pattern. Along the ~600-m- nary alluvial gravels (Qpm) to the southeast. At feldspar fraction from loess, stratigraphic corre- long, 064¡-striking trace, a terrace is displaced the coast, it is covered by a landslide. Although lation with dated terraces, or weathering-rind dat- 0Ð0.9 m vertically, and variably upthrown to the poorly defined at the surface, the southern splay is ing of graywacke sandstone pebbles. Details of southeast and northwest, indicating essentially inferred to extend eastward to the coast beneath a our TL dating methods are included in Data pure strike slip along this near-vertical part of the series of dextrally offset stream channels, and to Repository, Part B (see footnote 1). fault, which is well exposed in the Blairich bound an elongate fault wedge of loess (Fig. 4). River. Farther west, the fault bends sharply to a West of Boundary Stream, the Awatere fault trace KINEMATICS OF THE EASTERN PART strike of 045¡. Here, the terrace is upthrown by 8 consists chiefly of a single, poorly-drained furrow OF THE AWATERE FAULT m. Continuing west, this ~2-km-long contrac- or trench, 60Ð150 cm wide, but is locally compli- tional jog swings to an 052¡ strike as vertical slip cated by bifurcations, surface bulges, and jogs and The Awatere fault traverses >200 km of rugged on the terrace progressively decreases to ~4.5 m sag ponds as wide as tens of meters. Ridge spurs terrain between the Marlborough coast and the (Fig. 3B). The gradient in throw as a function of are locally dextrally offset by 25Ð70 m across the Alpine fault. The ~55-km-long Molesworth sec- fault strike can be modeled as a translation be- coastal section of the Awatere fault (see Data tion of the fault (Fig. 1) strikes 075¡Ð080¡, and tween curved, rigid blocks, but only if the fault is Repository, Part C (see footnote 1). bifurcates southwestward across the headwaters assigned a southeast dip of 75¡ ± 3¡ and a strike The coastal section of the Awatere fault dex- of the Wairau and Clarence Rivers (Lensen, slip of 7 ± 1 m (Fig. 3B), an unreasonably small trally offsets the Miocene-Pliocene unconformity 1963). The eastern ~100 km of the fault trace has value given the ca. 15 ka age of the Starborough by ~4 km (Fig. 4). This offset provides an upper a mean strike of ~057¡, but is characterized by 1 terrace (see below). We infer that the terrace limit on the fault’s finite dextral slip, because re- smooth to abrupt changes in strike at length gravels have detached from basement and de- verse slip and erosion have probably contributed scales of 10Ð104 m (Fig. 2). The eastern part of formed nonrigidly along a surface trace that is to this strike separation (Little and Jones, 1998). the fault can be divided into sections or strands, offset laterally from the main fault at depth. Farther south, an older, inactive fault with strong <4 to 30 km in length, on the basis of discontinu- Elsewhere along the Awatere fault, ratios of hor- topographic and gravity expression juxtaposes ity, overstepping relationships, attitude, or sense izontal/vertical slip (H/V) for displaced late differing facies of early Pliocene marine rocks of late Quaternary throw (Fig. 2). Summaries of Quaternary landforms typically range between (Pm unit), but is now largely concealed beneath neotectonic features and offsets observed along 10 and 40, but vary in relation to strike to as low Quaternary river terraces (Eden, 1989). The each of these strands are presented in Data as ~2 (offset spur on Dumgree contractional jog) western end of the inactive fault, the Fuchia fault, or as high as ~115 (on pure strike-slip sections diverges eastward from the main Awatere fault, that strike ~064¡). juxtaposing Mesozoic basement graywacke to 1GSA Data Repository item 9803, methods and supplemental data, is available on request from Docu- McCalpin (1996) also reported a dependence the north (JKp unit, Fig. 2) against late Miocene ments Secretary, GSA, P.O. Box 9140, Boulder, CO of fault throw (or H/V ratio) to local fault strike strata to the south (MPal unit). On the basis of 80301. E-mail: [email protected]. along the Molesworth section of the Awatere offset of a distinctive facies boundary in late

130 Geological Society of America Bulletin, February 1998

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A Normal Separations B A N at Grey River rigid block scarp model (not predicted by rigid translation model) slip = 7m; dip = 75° n throw = slip tan(dip) sin(φ) tio * * ec s e 10 re g Inset: m 9 φ u D 8 no vertical slip n D tio s 7 U ec tion s ec . r. s 6 R B y ina re l, N 5 G ta predicted gradient in throw as bend in fault Co 4 3

throw (m) 2 ° Intersection ~245 1 local fault strike of fault planes 0 045 050 055 060 065 N = 18 30 20 10 0 φ High H/V ratio of offset terraces near local fault obliquity, eastern end of Blairich River section (not predicted by rigid translation model)

Figure 3. (A) Lower-hemisphere, equal-area stereogram showing attitudes of the Awatere fault (great circles) determined: (i) where fault trace has been mapped across deeply incised canyons at 1:25 000 scale; and (ii) at EDM theodolite survey sites. Solid diamonds are slip vectors calculated from horizontal/vertical slip ratio of displaced terrace risers, ridge spurs, or channels. Data are from this paper, from G. J. Lensen (unpub. data for offset terrace riser, ridge spur, and channels on Blairich River section), and from P. R. Wood (unpub. data for offset spur on the Dumgree section). Bold arrow is mean slip azimuth, inferred from trend of strike-slip fault strands and from intersection of fault sets (bold ellipse). (B) Plot

of throw of Stb1 terrace vs. local fault strike for contractional fault jog west of Blairich River (Fig. 2, location X). Box dimensions are uncertainties in displacement and strike. Fault obliquity (φ) is angular deviation of scarp from azimuth of 064¡ (see inset). Curve labeled “predicted gradient in throw” corresponds to rigid-translation model of fault block motion for hypothetical case of a curved fault that dips 75¡ and that has a strike slip of 7 m. The three data points with throws of 4.5Ð5.2 m are from Lensen (unpub. data). A gap in data for strikes of 052¡Ð065¡ occurs because of the angular shape of the fault jog.

Miocene deep-marine rocks (Mu unit, Fig. 2), loess and laminated, waterlain silt resting con- The dissected upper surface of the Qpm grav- Little and Jones (1998) infer that the inactive formably on gravel. Gravel beds beneath the els dips ~1.5¡ to the northeast, whereas the lower southern strand has accumulated at least 34 ± 10 loess were tilted southeast to dips of 10¡Ð15¡ 40 km of the modern Awatere River is inclined at km of finite dextral slip. near the fault (Fig. 5A). Eden (1983, 1989) iden- a nearly constant gradient of ~0.29¡ (Eden, tified the ca. 350 ka Rangitawa tephra (Kohn et 1989). Given the ca. 350 ka age of the Qpm ter- Quaternary Stratigraphy Near the Coast al., 1992; Alloway et al., 1993) near the base of race, this dip suggests northeast tilting of the this loess. Berger and Pillans (1994) obtained a gravels at ~3¡/m.y. if the lower part of the river At the coast, >30 m of gravel are on Pliocene TL age of 346 ± 70 ka from loess just below this has maintained an equilibrium gradient during bedrock along a scoured unconformity near sea tephra at White Bluffs. We interpret the elongate, the late Quaternary Period. The inferred tilting level (Qpm unit, Fig. 4). Imbrication and round- fault-bounded wedge of loess to have been de- rate agrees with progressively smaller inclina- ing of the clasts, some of which are mafic plu- posited in the lee of the Awatere fault scarp, tions of late Quaternary terrace surfaces of de- tonic rocks characteristic of the Inland Kaikoura which formed a distinct topographic feature at creasing age in the Awatere Valley (Eden, 1989), Range (Fig. 1), indicate that they were trans- the time of deposition. The present coastal strand and, in a general way, with the ~10¡Ð20¡ seaward ported along the paleo-Awatere River. Beds of of the Awatere fault has thus been active for >350 dip of early Pliocene beds near the coast (Little, siltstone, 0.5Ð2 m thick, and minor sandstone oc- ka. To the south, the ancestral strand does not ap- 1995). Seaward tilting at the coast probably re- cur interbedded with the gravel, and increase in pear to cut the Qpm gravels, nor does it offset the flects an uplift gradient between the rising Inland

number and thickness northward toward the Downs 1 (Do1) terrace of Eden (1989) at the Kaikoura Range (Wellman, 1979; Van Dissen Awatere fault (Fig. 5, A and B). North of the in- coast, which has a TL age of 57 ± 5 ka (calendar and Yeats, 1991), and the subsiding Wairau basin ferred southern fault splay, they thicken abruptly years B.P.) (Berger and Pillans, 1994) (Fig. 4, lo- in Cook Strait (Fig. 1). into a 13-m-thick, 50Ð200-m-wide wedge of cations W and X).

Geological Society of America Bulletin, February 1998 131

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At the coast, a river terrace (Do2) is incised ~16 m below remnants of the Qpm surface, and cov- A ered by a 3.7-m-thick loessic cover bed containing the Kawakawa tephra near its base (Fig. 5B NW SE and Table 1). Because Eden (1989) defined the Loess Coastal Scarp Apron: Late Pleistocene alluvial gravel Stratigraphic Downs 2 (Do ) terrace to be the youngest terrace landslide loess & pebbly (unit Qpm) intercalated with 0.5-3 m-thickcolumns 2 laminated silt beds of silt, loess that increase tread that is overlain by the ca. 22.6 ka Kawakawa in Fig. 5b 50 m in abundance NW tephra (14C years, Pillans et al., 1993), we corre- Pmc Qpm Downs Terr. (Do2) late Do2 with his Downs 2 terrace. ? Qpm 0 Rangitawa N tephra Canyon Offset at Boundary Stream distance not to scale (~350 ka) Poles to 5 Pmc bedding planes ? Awatere fault Pmc The Do terrace can be followed westward lt 2 au f Pliocene Bedrock re from the coast into the Boundary Stream, where te a e w n A la it is cut by the Awatere fault and vertically dis- p g in dd placed by 3.8 m in a northwest-up sense (Fig. Mean be 6A). There, Do2 gravels fill a canyon incised into indurated Neogene conglomerate, and terminate B 0 topsoil laterally against this buttress unconformity (Fig. loess 6B). Southeast of the Awatere fault, a veneer of silt Do alluvium overlies ~10 m of Qpm gravels coarse 2 pebble Downs2 terrace

above an erosional tread, and is capped by ~0.5 m gravel Downs surface (Do2) of loess. Northwest of the fault, 1Ð3 m of Do 0 A horizon gray topsoil (20 cm) 2 silt lens Btg horizon mottled gray- gravel and loess overlie Miocene bedrock (Mu). Qpm Unit 2 orange, structured loess, Cover Sequence compact @ base (80 cm) The Boundary Stream has cut down along the boulder- 1 bearing, C horizon, massive sloping interface between unconsolidated terrace 10 cobbly gray loess (60 cm) coarse Pebble gravel lens, gravels and the hard conglomerate ridge to the pebble 2 thickens to S. (40 cm) east. Incision of the bedrock canyon must have gravel massive gray-brown loess, incl. some root occurred prior to its infilling by Do2 gravels. The channels (1.85 m) canyon wall is cut and laterally displaced by the 3 Kawakawa tephra (10 cm) silt lens v vv vv vv sample TL92-90 Awatere fault. On the basis of a detailed survey of loess (5 cm) finely laminated silts, fine its trace, the fault dips 80¡ northwest at this site. 4 sands (40 cm) foreset-bedded Qpm unit alluvial gravels Contours that are level with the Do2 terrace were fine pebble gravel 20 metres surveyed on the canyon wall on either side of the boulder- fault. Because the fault vertically displaces the bearing, cobbly Do2 terrace by 3.8 m, two contour segments are c. pebble separated by this vertical distance. The segments gravel Figure 5. (A) Detailed profile across Awatere fault near coast showing relationship of fault to late Quater- are interpreted as an offset line of original terrace boulder- truncation against the bedrock canyon wall. Dex- bearing, nary gravel-loess unit (Qpm). See Figure 4 for approxi- cobble mate location of profile. Lower-hemisphere stereogram tral slip of the line is 32 ± 10 m (Fig. 6A), within gravel error of the 35 ± 5 m dextral offset of the western pebbly silt lens shows gently southeast-tilted attitude of late Pleistocene 30 coarse gravel beds adjacent to Awatere fault plane (bold great bank of the present-day Boundary Stream (mea- pebble sured with tape). Two nearby spurs are also offset elev. gravel circle). (B) Stratigraphic columns of late Quaternary ~5 m Pliocene sequence near coast. See Figures 4 and 5A for location of 35Ð40 m dextrally (Fig. 4). The canyon wall and basement modern stream bank have similar dextral offsets meters (Pmc unit) measured sections. No vertical exaggeration. despite their differences in age. This correspon- dence indicates that the stream has remained laterally confined against the erosion-resistant

bedrock wall since 23 ka. graben between the two sections. North of the The oldest terrace (Stb1) is a useful datum for lake, the Blairich River strand dips steeply measuring vertical displacements. At the Ross-

NEOTECTONICS OF THE AWATERE northwest, and has a reverse sense of dip slip, more stream (unofficial name), Stb1 is displaced FAULT NEAR LAKE JASPER placing basement graywacke (JKp) against late 3.3 m vertically by the Blairich River section of Quaternary terraces and lake deposits (Fig. 7B, the fault. Farther east, the Blairich strand bifur- Fault Traces Near the Lake Jasper Pull-Apart sections AÐA′ and B-B′). South of the lake, the cates into secondary splays bounding a narrow

Nina Brook section dips 80Ð85¡ northwest, and horst (Fig. 7A). Vertical slip of Stb1 dies out east The junction between the Nina Brook and has a normal sense of dip slip, uplifting late of Taylor Pass Road. The Stb1 terrace emerges Blairich River sections of the Awatere fault in the Miocene strata (Mu) in its footwall (Fig. 7B, sec- beneath marsh deposits on the eastern side of lower Awatere Valley defines a right-stepping tion CÐC′). Both fault strands have steep, fresh- Lake Jasper, where its synclinal warping defines jog that has an offset of ~250 m (Fig. 7A). Lake looking scarps that displace a sequence of late a depression between the two fault strands. On Jasper is fault bounded on two sides and is on a Quaternary river terraces (Fig. 7A). the north, this broad depression is flanked by an

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There are degradational terraces between Stb TABLE 1. REPRESENTATIVE ELECTRON MICROPROBE ANALYSES 1 OF QUATERNARY TEPHRAS and the modern Awatere River. Eden (1983) Oxides* Nina Brook White Bluffs Kawakawa† mapped and profiled these to within 1 km of the (TL403B) (TL92-90) Tephra Lake Jasper study area. Of these, the Starborough (wt%) (±1σ) (±1σ) (±1σ) 2 terrace (Stb2), located 7.5Ð11 m below Stb1, is SiO2 77.94 ± 0.37 78.10 ± 0.45 78.43 ± 0.31 the most extensive. At Rossmore stream, Stb TiO 0.18 ± 0.04 0.17 ± 0.04 0.14 ± 0.04 2 2 consists of 1.5Ð3 m of locally derived, bouldery Al2O3 12.54 ± 0.23 12.52 ± 0.19 12.15 ± 0.11 FeO¤ 1.22 ± 0.11 1.31 ± 0.09 1.17 ± 0.11 gravel overlying Torlesse bedrock, and its tread is MgO 0.25 ± 0.11 0.14 ± 0.03 0.12 ± 0.03 overlain by ~0.5 m of loess (Fig. 8). At a site 2 CaO 1.06 ± 0.08 1.13 ± 0.09 1.08 ± 0.08 km to the southeast, across the Awatere River Na O 3.81 ± 0.24 3.97 ± 0.11 3.62 ± 0.18 2 from Lake Jasper, the downwind correlative of K2O 2.99 ± 0.18 2.97 ± 0.18 3.09 ± 0.16 # H2O 4.49 ± 0.82 4.62 ± 0.93 5.27 ± 1.07 this loess is ~1.8 m thick (Fig. 9B). At this site n = 15 shards 11 shards 14 sites (sample MRMA95Ð1 in Fig. 2), we obtained a Note: Location of TL403B is shown in Figure 7a; its stratigraphic position is shown TL age of 58.2 ± 6.9 ka on unaltered (C horizon) in Figure 8. Location of TL92-90 is shown in Figure 4; its stratigraphic position is shown in Figure 5b. loamy silt, collected 8.5 cm above the gravel *Glass chemistry determined using JEOL JXA-733 electron microprobe. Analytical tread of Stb (Table 2). This age contradicts the µ 2 conditions: 8 nAmp beam current; 20 m beam diameter. Analyses are recalculated internally consistent set of age constraints for the to 100% (volatile-free). † Average analysis of Kawakawa Tephra from 14 New Zealand localities in Carter older and higher Stb1 terrace. We conclude that et al. (1995). the dated MRMA95Ð1 sample contains fluvial ¤All iron as FeO. # silt derived by recycling of older terrace deposits H2O by difference from 100%. so that its TL was not reset (zeroed) by sunlight exposure (Data Repository, Part B [see footnote 1]). This TL dating result, therefore, does not pro-

alluvial fan containing beds that dip into the basin ognized by its position as the lowest major (>6 m vide a useful age for the Stb2 terrace. ′ at 3¡Ð5¡ (Fig. 7B, section CÐC ). On the south, thick) aggradational terrace above the modern To estimate the age of the Stb2 terrace we con- throw on the Nina Brook section of the fault in- Awatere River (Eden, 1983, 1989), and is now sider its elevation below Stb1 in relation to latest creases westward from ~4 m at Nina Brook, to well dated by several geochronologic methods. A Quaternary river-incision rates. If the Awatere ~10 m near Taylor Pass Road, where the fault reworked, waterlain tephra, 6Ð14 cm thick, is River has incised at a uniform rate during the late bends and bifurcates, to >16.5 m beneath Lake sandwiched between seams of peaty claystone. Quaternary, then the 18% ± 2% lower elevation

Jasper (8 m drop from Stb1 to lake floor, plus Microprobe analyses of glass shards (Table 1) of the Stb2 terrace above the modern river relative >8.5 m thickness of lake sediment in auger sam- show a rhyolitic composition indistinguishable to the ~15.2 ka Stb1 terrace (Eden, 1989) indi- ple, Fig. 7B, B-B′). This gradient suggests that from that of the Kawakawa tephra. Correlation cates a 12.5 ± 0.3 ka age for the lower surface. 14 the down-faulted Stb1 surface is tilted in trap- with the tephra is further confirmed by C dating The actual rate of river incision, however, was door-like fashion toward the southwest end of the of organic claystones immediately below and probably considerably slower during the early Lake Jasper basin. There, intersecting scarps above the tephra, yielding ages of 20.6 ± 0.3 ka Holocene due to sparse vegetative cover and high bound a steep-walled depression that is ~15 m and 20.5 ± 0.2 ka, respectively (14C yr B.P., Table bedloads derived from glacial outwash (e.g.,

deep. An ~300-m-long restraining bend borders 2). Near Lake Jasper, the Stb1 tread is covered by Bull, 1991). Thus 12.5 ka is considered to be a the southeast shore of the lake. 0.6Ð1 m of loess; however, at Seddon, which is maximum estimate for the age of the Stb2 terrace. downwind relative to the prevailing northwester- In accord with our interpretation, Eden (1989)

Late Quaternary to Holocene Stratigraphy lies, it is covered by >2 m of loess. We collected concluded that Stb2 was ~5 k.y. younger than and Geochronology Near Lake Jasper a loess sample near Seddon (SEDN95Ð1) at 10 Stb1, on the basis of the ~1 m mean difference in cm above the Stb1 gravel tread (Figs. 2 and 9A). the thickness of their loessic covers. Stb2 in the The oldest exposed Quaternary sediments fill- This sample yielded a TL age estimate of 15.2 ± lower Awatere valley may correlate with mo- ing the Lake Jasper graben are the Starborough 1 1.3 (1σ) (Table 2 and Data Repository Fig. i, see raines and regionally extensive outwash terraces

(Stb1) terrace gravels of Eden (1989) (Fig. 8). footnote 1). We interpret this result as closely near the headwaters of the Awatere River, dated 14 The well-rounded shape of most clasts, some of postdating the time of abandonment of the Stb1 by C as between 9460 ± 150 and 12600 ± 160 which are mafic plutonic rocks, suggest transport terrace by the Awatere River (Data Repository, yr B.P. (McCalpin, 1992a, 1992b, 1996).

along the paleo-Awatere River. Secondary input Part B [see footnote 1]). Another minimum age Below Stb2, younger terrace remnants (here by local streams is indicated by grading of the constraint for the terrace is provided by the called t3) are ~5Ð6 m above the modern stream Stb1 surface into tributary vallies, and by inter- ~6090 B.P. age of wood fragments in a pebbly beds of the Nina Brook and Rossmore stream beds of subangular gravel containing abundant mudstone (14C yr B.P., Table 2) that was augered (Figs. 7A and 10A). These incised meanders are

clasts of red chert and argillite. In the graben, Stb1 from a debris flow deposit, 10.5 m beneath the easily recognized by their sinuosity, position be- ′ gravels thicken from ~12Ð15 m to >25 m by ad- surface of Lake Jasper (Figs. 7B, section B-B , low Stb2, and lack of loess soil or cover. Mea- dition of a fanglomerate unit, the base of which is and 8). These data suggest a protracted time surement of 235 weathering-rind thicknesses on

not exposed. The fanglomerate consists of orga- (>6000 yr) of Stb1 gravel aggradation, consistent 47 sandstone pebbles exposed on the t3 surface at nized gravel beds, 0.3Ð3 m thick, and minor with occurrence of at least one paleosol in the Rossmore stream (Fig. 10A) yielded a symmetri- 2Ð10-cm-thick layers of carbonaceous mudstone gravels (Fig. 8). The muds interbedded with the cal probability density distribution with a mean and/or laminated siltstone. gravels probably accumulated in a structurally thickness of 0.59 ± 0.17 mm (1σ). Using the

The Stb1 terrace is the most areally extensive controlled lacustrine basin not unlike present-day method of McSaveney (1992), this result yields terrace in the lower Awatere Valley, is easily rec- Lake Jasper. an exposure age of 400 ± 70 yr for the t3 surface.

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luvial fan downlapping onto (i.e., younger than)

the Stb1 terrace is dextrally offset ~112 ± 10 m away from its source gully by the Dumgree section of the Awatere fault (Table 3). Together these data indicate ~110Ð125 m of dextral slip on the

Awatere fault since deposition of Stb1. West of Lake Jasper at Rossmore stream, an alluvial fan (Qf) is truncated by the Awatere fault, which has a ~5.5-m-high scarp and is footed by a marshy, ~0.5Ð1-m-deep trench (Fig. 7A). Incised

into the older fan, the Stb1 terrace is displaced vertically 3.3 ± 0.2 m by the fault. The Awatere fault offsets terrace risers at the margins of the

Stb1, Stb2, and Stb3 terraces. Dextral-reverse offset of the 9Ð10-m-high riser between the Stb1 and Stb2 terraces on the western side of the stream is especially clean and well preserved. We estimated the three-dimensional displacement vector for this offset by constructing a detailed topographic map of the terraces, their risers, and fault trace using an EDM-theodolite and 210 survey points (Data Repository, Part A (see footnote 1). The strike (052¡) and dip (65¡) of the Awatere fault were measured using repeated three-point solutions on widely spaced survey points along the fault trace. Dashed lines in Fig- ure 10B are two structural contours on the fault plane. An arbitrary reference contour was selected near the midline of the terrace riser on the south side of the fault. Near its point of fault con- tact, the riser contour is assumed originally to have had the same trend on both sides of the fault. This trend is well preserved on the northwest side of the fault, where the riser is straight. Extreme possibilities of riser curvature for the eroded southeast side are represented by the shaded region bounded by arrows: either the northwest trend was achieved distant from the fault (right arrow), or in close proximity to the fault (left arrow). The difference in elevation between riser contours that are correlated across the fault is simply the vertical slip of the lower Figure 6. (A) Simplified EDM total-station theodolite map of Boundary Stream trace of the σ terrace (Stb2), which is 2.7 ± 0.3 m (1 ). Thus, Awatere fault, showing relationship of Downs 2 terrace to bedrock paleocanyon wall (gray on the northwest side of the fault, a midline riser shading). See Figure 4 for location. North (measured with a Brunton compass) is accurate to contour that is 2.5 m higher than the southeast ±1¡; distances have a precision of ±2 mm. For simplicity, degradational terraces of unknown reference contour is projected to the fault sur- age that are incised into Downs 2 surface are not shown. (B) Northeast-southwest cross section face. The shaded rectangle on the northwest side across the buttress unconformity (no vertical exaggeration). See A for location of cross section. is a graphical representation of uncertainty in the throw measurement. Horizontal slip of the riser is measured parallel to fault strike, yielding a Displaced Relict Channels and Risers on the streams have been uplifted away from their head- value of 63 ± 3 m. The error reflects both uncer- Terraces Near Lake Jasper waters and are well preserved as relict channels tainties in projection across the scarp and in (wind gaps). On the northern side, the headwaters throw measurement.

East of Lake Jasper on the southeast side of the have been dammed and deflected by the rising The riser between the Stb2 and t3 terraces is Awatere fault, Stb1 terrace gravels onlap against scarp to flow into Lake Jasper or tributaries of 2.5Ð4 m high. On the west bank of Rossmore a ridge of gentle paleohills (probably older grav- Nina Brook, and are partly concealed by marshy stream, t3 on the south side of the fault occurs as els) protruding ~5 m above level of the terrace deposits. Two channels can be identified on the a small remnant of an isolated meander removed (Fig. 7A). Near the margins of the ridge, several north side of the fault, yielding estimates of dex- from the fault trace, so that a reliable estimate of

south-flowing relict stream channels occur on the tral slip of 123 ± 10 m and 124 ± 10 m (Table 3). dextral slip for the t3 riser cannot be measured Stb1 surface. On the southern side of the fault, the Similarly, ~5 km to the east of Nina Brook, an al- there (Fig. 10A).

Geological Society of America Bulletin, February 1998 135

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/110/2/127/3382870/i0016-7606-110-2-127.pdf by guest on 29 September 2021 s to site er ef tion Z r . Loca te xima o ppr and the scale bar is a xt. phs (scale ~1:2600), a r mentioned in te g - y (see alle ial photo V e ter a w tical aer A er ion, el v g v e w-le asper r e J e compiled on lo er y of the Lak g ta w eolo y g p). Da nar ter tion of ma e 7. (A) Qua or loca igur . 2 f F ig F

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meters Grey River Terraces A A' 260 3.3 m At least six alluvial terraces occur above the throw on Stb1 present bed of Grey River. These are best developed on the northeast side of the river, where 200 Stb1 fan gravels Stb Blairich River section 1 Stb1 some are offset by the Awatere fault (Lensen, 1964a). The gravels were deposited during two 140 ? ? Torlesse greywacke ? ? major periods of aggradation (Fig. 12). The older JKp period culminated with the TH terrace surface, 80 Late Miocene strata and the younger culminated with the SG terrace Mu (nomenclature of Lensen, 1964a). Knuepfer 20 (1988) reported a pebble-weathering-rind age estimate for the SG surface of 4190 ± 1390 yr B.P., but considered it to be an underestimate of the real age, which he suggested was 5Ð10 ka, on the B B' basis of soil morphology. We suggest that the SG 260 Lake terrace is correlative to the well-dated, regionally sediments extensive Stb1 terrace, on the basis of their equiv- 200 auger > 16.5 m alent positions as the lowest aggradational terrace sample Nina Brook section Stb above modern stream level. That glass shards are Blairich River section throw on Stb1 1 140 not present in the <20 cm thick loess cover of the Torlesse greywacke Stb1 SG terrace is consistent with this correlation, but ? ? ? ? Late Miocene strata not conclusive. 80 JKp Mu? Mu All the other terraces at Grey River formed chiefly by incision into the two gravel units, lack 20 loess, and are degradational. Knuepfer (1988, 1992) reported composite soil morphology and pebble weathering rind ages of 4520 ± 1780, and

2380 ± 820 for two of these: the SG1 and SG2 terraces, respectively. In Table 2 his modal thickness C C' data for pebble-weathering rinds on these two 260 terraces have been recalculated using the calibra- fan gravels tion curve of McSaveney (1992), yielding ages of 200 8-9 m 2454 ± 980 and 2122 ± 700, respectively. These Stb throw on Stb Stb1 1 1 Nina Brook section are younger than, but within the 1σ overlap of er- ? 140 rors of those reported by Knuepfer (1988). Torlesse greywacke Late Miocene 80 Terrace Displacements JKp ? Mu? Mu 20 Three terrace risers at Grey River are measur- horizontal scale = vertical scale ably displaced by the Awatere fault (Lensen, 1964a). Our strike-slip estimates are less than those of Lensen (1964a), who did not provide an estimate of measurement errors, and are less than Figure 7. (B) Vertical cross sections across Lake Jasper pull-apart structure (see A for loca- (but within error of) those of Knuepfer (1992). tions). Scale is slightly enlarged relative to the map. The three-dimnesional nature of the fault plane and offset risers may not have been fully ac- counted for in previous tape measureÐbased esti- NEOTECTONICS OF THE AWATERE eastern valley wall, reinforcing a scarp that is mates. The older data may also differ from ours FAULT NEAR THE GREY RIVER also upthrown on the southeast. The same mo- because of difficulties involved with defining and tion on the western side of the valley, however, matching riser lineations objectively while in Fault Traces indents the downstream wall, thus negating the the field. Because our measurements are three- scarp. A short, right-stepping bend in the fault dimensional, take into account the local strike Structure contours drawn on the Awatere fault has caused localized subsidence of a small sag and dip of the fault plane, and use computer con- trace near the Grey River indicate that it strikes on the SG terrace (Fig. 11A, location W). A pos- touring to define riser lineations, we believe that ~053¡ and dips northwest at 40¡Ð60¡ (Fig. 11A). sible southern splay to the Awatere fault crosses they are the most reliable estimates available East of the river, the fault has an uphill-facing the Grey River at a projecting bluff of crushed (Table 3). We surveyed 157 points on these ter- scarp, 3Ð6 m high; but to the west the fault trace graywacke, along which the stream makes a dex- races using an EDM total-station theodolite is indistinct. This difference in scarp relief is be- tral jog, and below which its channel abruptly (Data Repository, Part A (see footnote 1), and cause dextral slip causes a step to form on the narrows (Fig. 11A, location Z). from those data constructed a detailed topo-

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Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/110/2/127/3382870/i0016-7606-110-2-127.pdf by guest on 29 September 2021 Figure 8. Late Quaternary stratigraphy near Lake Jasper pull-apart. Locations of measured sections are specified by New Zealand coordinate grid references. Triangular symbol refers to subangular-angular gravel clasts that were derived locally, rather than by axial transport along the Awatere River.

TABLE 2. NEW GEOCHRONOLOGIC DATA FOR LATE QUATERNARY TERRACES, AWATERE VALLEY, NEW ZEALAND Sample number NZ grid Location Stratigraphic position Terrace tread Material Lab number Field number reference Wk-3414 TL-403a P29/922496 Nina Brook 4 cm below Kawakawa Tephra Starborough 1 Peaty mud (3 cm) Wk-3415 TL-403d P29/922496 Nina Brook 14 cm above Kawakawa Tephra Starborough 1 Peaty mud (5 cm) Wk-3671 TL-17/2a P29/591487 Lake Jasper Auger sample 8.6 m below floor Lake sediment above Wood in pebbly mud of Lake Jasper Starborough 1 debris flow MRMA95-1 TL-M-c P29/922469 South bank Awatere 10.7 cm above alluvial gravel Starborough 2 Loess River SEDN95-1 SEDN95-1 P29/996487 South bank Awatere 10 cm above alluvial gravel Starborough 1 Loess River

Sample number 14C data* Thermoluminescence (TL) data† ¤ ¤ ¤ # Lab number Field number Libby age New age K2O U Th Plateau DE DR** TL age (ka ± 1σ) (ka ± 1σ) (%) (ppm) (ppm) (¡C) (Gy) (Gy/ka) (ka ±1σ) Wk-3414 TL-403a 20.60 ± 0.30 21.21 ± 0.30 N.A. N.A. N.A. N.A. N.A. N.A. N.A.

Wk-3415 TL-403d 20.50 ± 0.15 21.09 ± 0.15 N.A. N.A. N.A. N.A. N.A. N.A. N.A.

Wk-3671 TL-17/2a 6.09 ± 0.20 6.27 ± 0.20 N.A. N.A. N.A. N.A. N.A. N.A. N.A.

MRMA95-1 TL-M-c N.A. N.A. 2.287 ± 0.050 2.52 ± 0.18 6.43 ± 0.58 210Ð300 (TB) 182 ± 20 3.13 ± 0.15 58.2 ± 6.9 N.A. N.A. 2.428 ± 0.050 2.43 ± 0.31 8.3 ± 1.0 N.A. N.A. 2.502 ± 0.050 2.05 ± 0.25 7.11 ± 0.83

SEDN95-1 SEDN95-1 N.A. N.A. 2.400 ± 0.050 1.92 ± 0.33 9.4 ± 1.1 230Ð300 (PB) 54.2 ± 4.3 3.59 ± 0.14 15.1 ± 1.3 N.A. N.A. 2.066 ± 0.050 1.99 ± 0.28 6.71 ± 0.91 *14C age determinations are from the University of Waikato Radiocarbon Dating Laboratory. †TL dating was done by G. W. Berger at the Desert Research Institute, Reno, Nevada. ¤ In K2O, U, and Th columns, the second and third rows of data represent surrounding sediments where different in composition from the first row (representing TL sample). For SEDN95-1, second row depicts underlying gravel. For MRMA95-1, second row depicts loess 23.5 cm above TL in Btg horizon, and third row depicts underlying gravel. # Readout (“glow-curve”) temperature interval over which weighted (by inverse variance) DE value (next column) has been calculated. PB = “partial bleach (R-beta)” (Berger, 1988), and TB = “total-bleach” method. PB used ~117 J/cm2 of 435Ð750 nm light for SEDN95-1 and 184 J/cm2 for MRMA95-1 (Fig. 10c). TB used ~1470 J/cm2 of full-spectrum Hg-lamp light. Optical fluences measured with calibrated Model 1400Aradiometer from International Light using a 400Ð1000 nm (340Ð1100 nm at 10% cut) flat-response filter over Si photodiode detector Model SEL033. **Gamma or “environmental” dose-rate component for SEDN95-1 and MRMA95-1 includes estimated (Aitken, 1985, p. 72–73) geometric contribution from gravel.

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seismic modification of fault scarps in the verti- Starborough 1 Starborough 2 A B cal dimension (on the basis of the equivalence of terrace surface terrace surface riser heights), we conclude that lateral trimming (P29/996487) (P29/922469) of offset risers also did not occur. For this reason meters 0 meters 0 we interpret riser offsets as full estimates of strike A horizon A horizon gray silt loam, fine sandy slip since the time of terrace abandonment, rather friable, crumbly loam than as minimum estimates of strike slip, as is sometimes assumed. AB horizon gray-brown silt loam, THE 1848 EARTHQUAKE weakly blocky, more 0.5 compact than A 0.5 AB horizon Recently discovered historical documents, in- fine sandy gravel lens silt loam, cluding an 1852 survey map of part of the coastal massive section, establish that the Marlborough earthquake of 1848 (Eiby, 1980) occurred on the Awa- B horizon tere fault (Grapes and Little, unpublished data), gray-brown-brown silt loam, causing surface rupturing for a distance of >100 1 weakly blocky- 1 km inland from the coast; the rupture extended prismatic; some southwestward beyond the junction with the bioturbation; firm Btg horizon Molesworth section of the Awatere fault to as far clayey silt loam west as Barefell’s Pass (Fig. 1), where the eastern w/ colloidal mottles; friable, some rootlets; section of the Awatere fault appears to terminate. Btg horizon firm Smallest observed offsets of streams channels, ris- gray-brown/gray/orange- ers, and spurs suggest that dextral strike slip dur- brown mottles; Cu horizon 1.5 silty clay loam, 1.5 massive, well-sorted ing this event was ~5Ð8 m (see Table 3). At Nina prismatic to blocky; loamy silt-sand; distinctly Brook, the Awatere fault cleanly transects an in- some root channels; softer and lighter cised meander loop cut into Pliocene bedrock, hard in color than above TL sample displacing its vertical walls dextrally by 7 ± 2 m ledge MRMA95-1 and uplifting the t surface flooring this meander alluvial gravel 3 BC horizon Starborough 2 by ~20 cm on the southeast side (Fig. 7A, location brown silty clay loam, terrace sequence Z). About ~1 km to the east, a small ridge spur is 2 root channels, 2 similarly offset by 7 ± 1.5 m (Fig. 2, location W). weak nut structure At a site 6 km to the east, a stream gully is dex- TL sample trally offset by 5 ± 2 m (Fig. 2, location, Z). This SEDN95-1 site lies on the part of the 1848 rupture that is plot- ted and annotated on the 1852 survey map. The alluvial gravel Starborough 1 smallest strike slip of a terrace riser that we mea- terrace sequence sured at the Grey River is ~6 m. Using the empirical regressions for strike-slip faults of Wells and Figure 9. Stratigraphy of loess cover beds of terraces at thermoluminescence (TL) dating Coppersmith (1994), ~6 m of maximum coseis- sites. Sample sites are shown in map of Figure 2 and also specified by New Zealand grid refer- mic slip and ~100 km of surface rupture length ences (in parentheses). Section A was logged by B. J. Pillans (1995, personal commun.); section suggest that the 1848 earthquake had a moment B is modified from data for the same site in Table 3 of Eden (1989). magnitude, M, of 7.4 ± 0.1. This estimate is probably a minimum, because the complete length of rupture was not observed and because coseismic graphic map of each offset riser. The lowest ter- tere fault, which he interpreted as the result of slip also included a vertical component.

race riser, between the SG2 and SG1 terraces, is vertical fault movements occurring while the Vertical slip inferred for the 1848 event varies offset dextrally by ~6 m and upthrown to the stream continued to occupy the lower terrace sur- laterally in both sense and magnitude (see Table southeast by ~3.5 m (Fig. 11B). The riser be- face. Postseismic smoothing of scarps by erosion 3). Throw estimates are typically 0.5Ð2 m in a

tween SG1 and SG is offset dextrally by ~15 m of the upthrown side and/or aggradation of down- northwest-up sense. Near the coast, terraces ~1 m and upthrown to the southeast by 1.5 m (Fig. thrown side could result in riser-height differ- above modern stream level are upthrown by ~1.0

11C). The riser between the SG and Th1 terraces ences across the fault. Riser heights on either side and 1.8 m (locations Y and Z, Fig. 4A). At the has been modified by gullying along the scarp of the fault at the Grey River, however, are indis- Black Birch Stream (Fig. 2), the youngest faulted and deposition of fan and marsh sediments (Fig. tinguishable at the 1σ level (Fig. 12). For this rea- alluvial terrace is vertically offset by ~1.4 m, but 11A). Thus, projection of this riser to the fault is son we conclude that stream occupation of the in an up-to-the southeast sense. Up-to-the south- subject to a large uncertainty (Fig. 11D). For this extensive terrace surfaces was relatively short east throw during 1848 may have reached a max-

oldest riser we obtain a dextral slip of 99 ± 34 m lived. Renewed stream incision soon stranded ris- imum near the Grey River, where the SG2 terrace and a throw of ~1.5 m (up to the southeast). ers above stream level, protecting them from later is upthrown by ~3.5 m. Lensen (1964a) reported differences in the modification or erosion (except perhaps during Recent 14C and paleoseismological data indi- height of terrace risers on either side of the Awa- floods). Because there is no evidence for post- cate that the 1848 earthquake did not cause rup-

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and Cooper (1995) inferred that stress gradients caused by steep topography play an important role in controlling the position of these segments along the Alpine fault. Scarp elements of different strike on the Awatere fault, however, vary widely in scale, and their location and spacing cannot be related in a simple way to topography. We interpret the bimodal segmentation pattern to be related to selective reactivation of older basement faults. In a transpression zone with boundaries converging at an oblique angle of 10¡Ð30¡, the longest and intermediate incremental strain

axes (e1 and e2) will be nearly equal in magnitude (e.g., Tikoff and Teyssieur, 1994). Thus, in an elas- tically isotropic solid, the principal compressive σ σ σ σ stress difference ratio R = ( 1Ð 2)/( 1Ð 3) will be σ σ near unity, and either 2 or 3 can bevertical. Such a state of stress probably exists today in the northeast South Island, as shallow earthquakes include both thrust and strike-slip focal mechanisms recording horizontal shortening to the east-southeast, but a variable elongation direction flipping between the horizontal and vertical (Reyners et al., 1998). This stress state leads to a condition where vertical strike-slip faults and dipping oblique- reverse faults, with different strikes, will have equally high slip tendencies, measured as the ratio of shear stress to normal stress on those planes (e.g., Morris et al., 1996). This relationship probably contributes to the segmented fault pattern characteristic of major oblique-slip faults. The Awatere fault appears to undergo a progressive lateral change in the degree to which oblique plate motion is partitioned along the fault (Fig. 1). Near the coast pure strike-slip segments strike ~064 ± 2¡ (Fig. 3, A and B). This slip azimuth is ~5¡Ð10¡ discordant to the local mean fault strike, and ~13Ð15¡ from the plate motion vector implied by the NuvelÐ1a model (DeMets et al., 1990, 1994), thus reflecting a partly slip-partitioned style of oblique plate motion. Farther southwest, the strike of pure strike- Figure 10. (A) EDM total-station theodolite map of Awatere fault and terraces in stream west slip segments, at ~072¡ (McCalpin, 1996), is of Rossmore Station, near Lake Jasper. See Figure 7A for location of map. Local coordinate sys- slightly anticlockwise from the ~30-km-long tem (in meters) is relative to origin fixed at instrument station A, which has an NZ (New trace of the Molesworth section of the fault, and Zealand) grid location of P29/905484 (±100 m) and an elevation of 195 ± 15 m. only ~3¡Ð5¡ from the plate-motion vector. Still farther southwest, vertical strike-slip parts of the strongly segmented Alpine fault trend ~080¡, turing on the Molesworth section of the Awatere fers that the intersection between these two about ~25¡ from the overall strike of the fault, fault (McCalpin, 1996), and therefore propagated strands may be an important boundary between and are essentially parallel to plate motion (case past it (Fig. 1). At the Saxton River on the earthquake rupture segments. of little or no partitioning) (Norris et al., 1990; Molesworth section, a relict channel is offset dex- Norris and Cooper, 1985). The lateral change in trally by 7.2 ± 0.5 m, probably the result of the last DISCUSSION AND INTERPRETATION degree of partitioning along the Awatere fault surface rupturing event (Lensen, 1973) (Table 3). OF DATA corresponds spatially with an important transi- McCalpin (1996) inferred from trenching data tion in orogenic boundary conditions at depth: that the last two ruptures on the Molesworth sec- Kinematics of Oblique-Slip Faulting subduction of oceanic lithosphere to the north- tion occurred 522Ð597 and 2400Ð4800 (calendar east, in the area of the Hikurangi margin, is re- years) B.P., each resulting in 6Ð8 m of dextral slip. Like the Alpine fault, the trace of the oblique- placed to the southwest by continent-continent Because the Molesworth section did not rupture slip Awatere fault is broken into strike-slip and collision in the Southern Alps. The subduction in 1848, whereas the coastal section did, one in- oblique-reverse sections of differing strike. Norris interface beneath the Marlborough fault system

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Seismic Significance of Awatere Fault B Segmentation 0 Along the ~65 km of strike length examined here, the eastern Awatere fault is broken by 3 obvious discontinuities: a contractional bend along Stb 2 the Dumgree Station oblique-thrust section (1 km

-2 m 8 m 0 m 12 m 4 m wide, 20¡ change in strike); an extensional bend at the Lake Jasper pull-apart (~250 m, ~10¡); and

14 m one complex zone of horse-tail splaying in the

-50 -4 m Black Birch Range (Fig. 2). Fault-trace rough-

+2 m ness on major strike-slip faults decreases with finite slip, and the density of large steps on the Awatere fault is comparable to that observed on

+2.0 m other strike-slip faults of similar finite offset ± "B" H = 63 3 m (Wesnousky, 1988). V = 2.7 ± 0.3 m -0.5 m structural Jogs and bends on strike-slip faults will cause contours on fault local variations in stress distribution and Meters North -100 Qf 65 strength that can influence the propagation of scarp apron dynamic slip on faults, as well the nucleation U and recurrence of earthquakes (e.g., King, 1986; Stb2 D Sibson, 1986a; Barka and Kadinsky-Cade, 1988; Sanders, 1989; Schwartz, 1989; Aki, 1995). Geo- logical and geophysical data for active strike-slip gully fault zones show that stepovers of small offset (<1Ð2 km wide, <5¡ bend) do not generally cause

-150 -0.5m arrest of dynamic earthquake ruptures, although they may retard passage of the rupture front (e.g., Sibson, 1986b; Barka and Kadinsky-Cade, Stb 1 1988; Wesnousky, 1988; Knuepfer, 1989; Scholz, 1990; Wald and Heaton, 1994; Zachariansen and Sieh, 1995). By analogy, the small section -200 -150 boundaries on the eastern Awatere fault would Meters East probably not prevent lateral propagation of major earthquake ruptures. The last seismic rupture on Figure 10. (B) Detailed contour map of offset riser between the Stb1 and Stb2 terraces. Sym- the Awatere fault in 1848 apparently propagated bols are the same as in A. Topographic contours are drawn at a 50 cm interval. Stippled areas rep- through all three of these stepovers. By contrast, resent well-preserved parts of the terrace riser that are little affected by erosion and deposition the junction between eastern and Molesworth near the fault scarp. Dashed lines are structure contours on the Awatere fault. V is vertical throw sections of the Awatere fault may be an important of the Stb2 terrace across the fault (= 2.7 m); H is the horizontal slip of the reference riser contour mechanical boundary controlling the size and lo- (in meters). cation of earthquake ruptures, because surface rupturing in 1848 appears to have been confined to the eastern section. is strongly coupled in northeast South Island, be- length of that structure. This conclusion is also The stratigraphic record indicates that sections coming completely locked to the southwest of implied by the southwest-plunge of the (bi- of the Awatere fault have a longevity measured Kaikoura. The southwestward increase in cou- modal) fault-segment lineation intersection; on a scale of 104Ð107 yr. Fault-scarpÐderived al- pling across the underlying subduction interface however, it is commonly contradicted by slip- luvial fans on the Grey River section imply that may prevent a slip-partitioned style of oblique vector data for late Quaternary offsets, which are the fault has had topographic expression since ca. motion (e.g., McCaffrey, 1992). Offshore, a generally more nearly horizontal (Fig. 3A). At 6 Ma (Little and Jones, 1998). The Lake Jasper weak low-angle thrust is no longer available to the Grey River, on the Molesworth section of the pull-apart graben has persisted for >22 k.y., as in- take up the margin-orthogonal component of fault, and at Black Birch Stream (Fig. 2), late dicated by the age of syntectonic fanglomerates motion; whereas onshore, dipping oblique-slip Quaternary terrace offsets across the fault are thickening into the graben. The inactive coastal faults may have a greater slip tendency up-to-the-southeast (e.g., Cotton, 1947a), imply- strand was abandoned between ca. 3 Ma and 350 (shear/normal stress ratio) than potential strike- ing a slightly north-plunging slip vector. Geo- ka (age of faulted and unfaulted rocks along its slip faults due to the increasing magnitude of logically recent rupturing events along the fault trace). After this, slip was transferred onto the horizontal compressive stress transmitted onto are thus less reverse slip than the conspicuous currently active coastal strand sometime after ca. vertical planes in the upper plate. topographic relief and segmentation patterns 3 Ma (implied by its small finite offset; Little and Elevated topography occurs along the north- along the fault would seem to record. These data Jones, 1998) and prior to 350 ka (age of syntec- west side of the Awatere fault, indicating a net imply a scale or time dependency of slip parti- tonic loess wedge along scarp). oblique-reverse sense of slip along the entire tioning behavior in transpression zones.

Geological Society of America Bulletin, February 1998 141

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/110/2/127/3382870/i0016-7606-110-2-127.pdf by guest on 29 September 2021 TABLE 3. SELECTED LATE QUATERNARY DISPLACEMENTS ON THE EASTERN AWATERE FAULT Site Location Offset Terrace Fault Fault Dextral slip¤ Vertical-slip Up H/V (NZ grid ref.) feature* name† strike dip (m) (m) side ratio Boundary Stream P28/022566 psv, ag Do2 49 80 NW 32 ± 10 3.8 ± 0.2 NW 8 Tributary to Stafford Ck. P28/972529 ch N.A. 65 86 NW 5 ± 2 N.D. SE N.D. East of Kennel Brook P28/948519 ch N.A. 55 N.D. 8 ± 2 N.D. NW N.D. West of Kennel Brook (1) P28/956517 sp N.A. 45 N.D 13 ± 3 N.D. NW N.D. West of Kennel Brook (2) P28/946516 ch N.A. 33 40-60 NW 6 ± 2 N.D. NW N.D. West of Kennel Brook (3) P28/946516 ch N.A. 33 40-60 NW 14 ± 3 N.D. NW N.D. West of Kennel Brook (4) P28/945515 fn Stb 1 33 40-60 NW 112 ± 10 N.D. NW N.D. East of Nina Brook (1) P28/933501 sp N.A. 55 60-82 NW 7 ± 1.5 0.7 ± 0.2 SE 10 East of Nina Brook (2) P29/925497 ch, ag Stb 1 65 82 NW 123 ± 10 N.D. SE N.D. Nina Brook P29/922495 ch, rs t3 64 82 NW 7 ± 2 0.2 ± 0.1 SE 35 West of Nina Brook P29/918494 ch, ag Stb 1 64 82 NW 124 ± 10 8 ± 0.5 SE 16 Rossmore Stream (1) P29/904484 rs, dg Stb 2 52 65 NW 63 ± 3 2.7 ± 0.3 NW 23 Rossmore Stream (2) P29/904484 rs, ag Stb 1 52 65 NW N.A. 3.3 ± 0.2 NW N.D. West of Blairich River P29/855445 rs, ag Stb 1 64 N.D. 92 ± 10b 0.8 ± 0.4 NW 115 Near Black Birch Strm. (1) P29/819418 sp N.D. 45 N.D. 6 ± 2b N.D. SE N.D. Near Black Birch Strm. (2) P29/818416 sp, os N.D. 45 N.D. 8 ± 2b N.D. SE N.D. Near Black Birch Strm. (3) P29/813411 sp N.D. 40 N.D. 4 ± 2b N.D. SE N.D. Gosling Stream P29/799396 os N.D. 55 N.D. 5 ± 2c N.D. SE N.D. Grey River (1) 029/583238 rs, dg SG2 53 50 NW 5.6 ± 2.1 3.5 ± 0.2 SE 2 Grey River (2) 029/583238 rs, dg SG1 53 50 NW 14.5 ± 2 1.5 ± 0.3 SE 10 Grey River (3) 029/583238 rs, ag SG 53 50 NW 99 ± 34 1.3 ± 0.4 SE 76 Saxton River (1) N30/225020 ch T5 65 N.D. 7.2 ± 0.5 N.A. SE N.A. Saxton River (2) N30/225020 rs T5 065 N.D. 8 ± 2 (7.2Ð7.6)a 0.45 ± 0.15 (0.35) SE 18 Saxton River (3) N30/225020 rs T4 065 N.D. 15 ± 4 (11.5Ð12)a 2.7 ± 0.3 (0.6) SE 6 Saxton River (4) N30/225020 rs T3 065 N.D. 35 ± 5 (35.5Ð42)a 3.5 ± 0.5 (3.4) SE 10 Saxton River (5) N30/225020 rs T2 065 N.D. 52 ± 5 (62Ð70)a approx. none N.D. N.D. Saxton River (6) N30/225020 rs, ch T1 065 N.D. 66 ± 5 (60Ð64)a variable NW N.D.

Site Terrace age Source of Dextral-slip rate** Comments, supplemental age data# (yr B.P.) age data# (mm/yr) Boundary Stream 23,000 ± 1000 this study, 1 1.4 +0.5/Ð0.5 Downs 2 terrace of Eden (1989), filling bedrock paleocanyon (surveyed) Tributary to Stafford Ck. N.A. N.A. N.D. Offset of deeply incised stream gully (taped distance) East of Kennel Brook N.A. N.A. N.D. Northern strand; offset stream gully (taped distance) West of Kennel Brook (1) N.A. N.A. N.D. Northern strand; offset spur (taped distance) West of Kennel Brook (2) N.A. N.A. N.D. Southern strand; offset of small stream channel (taped distance) West of Kennel Brook (3) N.A. N.A. N.D. Southern strand; offset of small stream channel fed by same upstream channel as above West of Kennel Brook (4) <15,100 ± 1300 this study, 2 >7.4 +1.4/Ð1.2 Offset of fan apex from source canyon; fan downlaps onto Starborough 1 terrace (taped distance) East of Nina Brook (1) N.A. N.A. N.A. Offset spur on northern of two strands on hillside northeast of View Hill (taped distance) East of Nina Brook (2) 15,100 ± 1300 this study, 2 8.1 +1.5/Ð1.2 Offset relict stream channel on Starborough 1 surface (taped distance) Nina Brook 400 ± 70 this study, 3 17.5 +9.8/Ð6.9 Incised meander loops below Starborough 2 surface (taped distance) West of Nina Brook 15,100 ± 1300 this study, 2 8.2 +1.6/Ð1.1 Offset relict stream channel on Starborough 1 surface (taped distance) Rossmore Stream (1) 10,500 ± 2000 this study, 4 6.0 +1.8/Ð1.0 Starborough 2 terrace of Eden (1989) (surveyed) Rossmore Stream (2) 15,100 ± 1300 this study, 2 N.A. Starborough 1 terrace of Eden (1989) (surveyed) West of Blairich River 15,100 ± 1300 this study, 5 6.1 +1.3/Ð1.1 Starborough 1 terrace of Eden (1989) Near Black Birch Strm. (1) N.D. N.D. N.D. N.A. Near Black Birch Strm. (2) N.D. N.D. N.D. N.A. Near Black Birch Strm. (3) N.D. N.D. N.D. N.A. Gosling Stream N.D. N.D. N.D. N.A. Grey River (1) 2122 ± 700 6 2.6 +2.8/Ð1.4 83170 ± 360 (mode), 82860 ± 1090 (mean); 9soil morph., 2380 ± 820 Grey River (2) 2454 ± 980 6 5.9 +5.3/Ð2.3 83530 ± 420 (mode), 84220 ± 1530(mean);9soil morph., 4520 ± 1780 Grey River (3) 15,100 ±1300 this study, 5 6.6 +3.1/Ð2.6 Correlated with Starborough 1 terrace of Eden (1989); 85390 ± 480 (mode), 4190 ± 1390 (mean) Saxton River (1) 1285 ± 45Ð1665 ± 651 6,7 4.9 +1.3/Ð2.0 7For channel-fill, 14C age = 1285 ± 45 or 1172 ± 111 cal. yr BP Saxton River (2) 1665 ± 651 6 4.8 +5.1/Ð2.2 82480 ± 250 (mode), 102780 (mode); 82450 ± 1110 (mean); 9soil morph., 2000 ± 500 Saxton River (3) 3251 ± 1167 6 4.6 +4.5/Ð2.1 84660 ± 620 (mode), 104828 (mode); 84420 ± 1510 (mean); 9soil morph., 4000 ± 1000 Saxton River (4) 3959 ± 1510 6 8.8 +7.5/Ð3.4 85460 ± 770 (mode), 105624 (mode); 85710 ± 1760 (mean) Saxton River (5) 5809 ± 2582 6 9.0 +8.7/Ð3.4 87150 ± 1100 (mode), 107420 (mode); 87100 ± 2220 (mean) Saxton River (6) 9312 ± 3298 6 7.1 +4.2/Ð2.1 89410 ± 1570 (mode), 1010,207 (mode); 87940 ± 3800 (mean) Note: N.D.—not determined; N.A.—not applicable. *Abbreviations of offset features: ag, aggradation terrace; dg, degradation terrace; rs, terrace riser; ch, relict stream channel on terrace; os, offset stream; psv, paleo- stream valley incised into bedrock; sp, ridge spur; fn, alluvial fan. †Same terrace abbreviations as used in Figs. 4, 7, and 12. ¤For offset risers, calculated by dividing horizontal offset of riser by age of lower terrace surface. Sources of displacement data: no superscript, from this study; in parentheses, from McCalpin (1996); afrom G. J. Lensen (1973) and G. Blick (unpub. data), as quoted in Kneupfer (1992); bfrom G. J. Lensen (unpub. field notes, lodged at Gracefield office of New Zealand Institute of Geological and Nuclear Sciences, to which data errors have been arbitrarily assigned); cfrom P. R. Wood (unpub. field notes, lodged at Gracefield office of New Zealand Institute of Geological and Nuclear Sciences, to which data errors have been arbitrarily assigned). #Key to age data sources: 1Age of Kawakawa Tephra in terrace cover-bed located <50 cm stratigraphically above terrace tread; 2TL age of loess located stratigraphically 12 cm above Starborough 1 terrace tread at Seddon, a result that is consistent with tephrochronology and 14C dating of Starborough 1 gravels in Nina Brook; 3Torlesse pebble weathering-rind age calculated using technique and calibration equation of McSaveney (1992); 4age based on TL age of higher Starborough 1 terrace (2) and relative position of degradational Starborough 2 terrace below that older surface (see text); 5age assigned by correlation with Starborough 1 terrace (see 2, above); 6Torlesse pebble weatherng- rind age recalculated using method of McSaveney (1992) from modal rind thickness data in Kneupfer (1988); 7the 14C age reported by Kneupfer (1992); 8Torlesse pebble weathering-rind age from Kneupfer (1988); 9soil morphology age from Kneupfer (1988); 10pebble weathering-rind age calculated from modal rind thickness data of Kneupfer (1988) using age equation of Whitehouse et al., 1986 (in McCalpin, 1996). **Uncertainties in this quotient are asymmetrical, so maximum and minimum uncertainty values are cited separately (+/Ð)

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Figure 11. (A) Quaternary geologic map of the Grey River area, Awatere Valley (see Fig. 2 for location). Data were compiled on low- level vertical aerial photographs (scale ~1:4700), and scale bar on the figure is approximate. Circled Z and W are locations cited in text. Late Quaternary terraces TH

(oldest), TH1, SG, SG1, and SG2 (youngest) are equivalent to those

of Lensen (1964a); whereas SG0 and SG3 are newly recognized.

Lake Jasper is a kilometer-scale pull-apart of subsidence/strike slip in this graben has been heights on both sides of the fault, and small dif- graben developed along a dilational jog that is ad- >0.15 since ca. 15 ka, a first-order relationship ferences in terrace widths that are inconsistent jacent to a contractional jog of similar width. The that is probably at least in part related to the with a model of riser trimming by streams that stepovers at Lake Jasper and Dumgree Station basin’s depth (e.g., ten Brink et al., 1996). continue to occupy the terraces during a series of join sections of the Awatere fault that are in-plane surface-rupturing earthquakes (for an opposite with one another outside of the bends. Because Late Quaternary Slip Rate on Central Awatere case, see Berryman, 1990; Berryman and Van the curved Awatere fault trace resembles theoret- Fault Dissen, 1993). For these reasons and because ical and experimental propagation paths of mode geochronologic methods typically date abandon- 2 dislocations (Segall and Pollard, 1980; Du and New dating of alluvial terraces and surveying ment of a terrace, not terrace-modifying events, Aydin, 1993; Moore and Lockner (1995), these of terrace offsets allows better estimates of the such as aggradation, occupation, or lateral trim- two sets of adjacent double-bends may have late Quaternary dextral-slip rate to be made for ming by the stream, we believe that riser offsets formed as a curved fault splay detoured around a the Awatere fault. We assume that horizontal off- provide reliable estimates of strike slip since the strong patch on a planar Awatere fault, although set of a riser accrues after the stream’s abandon- dated events of terrace abandonment. the depth of locking is unknown, and may be less ment of its lower terrace surface (e.g., Knuepfer, Late Quaternary dextral offsets observed along than that likely for the nucleation of large earth- 1992). At Lake Jasper and the Grey River, this the Awatere fault range from ~4 m to ~285 m quakes (Fig. 13). Our data indicate that the ratio conclusion is supported by equivalent riser (Fig. 14A). The majority of spurs and streams cut

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B C D -89

-93 120 SG 300 Active fan

155 structural contours on fault plane -112 m SG1 -103 SG1 D -97

-101 U D SG2 -108

throw U 100 throw 1-2 m Active fan 3.5 m

-107 SG 1-2 m throw Meters North

U -101

Meters North 130 D ± 200 H = 99 ± 34 m H = 14.5 2 m ± V = 1.5 ± 0.3 m V = 1.3 0.4 m

structural contours Meters North TH1 on fault plane -106 structural contours on fault plane SG 80 -109 SG1 SG SG2 -95

SG1 H = 5.6 ± 2.1 m V = 3.5 ± 0.2 m 105 500 600 Meters East

340 350 360 Meters East 400 420 440 Meters East

Figure 11. Insets (B), (C), and (D) are detailed maps of terrace risers offset by the Awatere fault. Elevations and coordinates (in meters) are relative to origin fixed at instrument station 11a, which has an elevation of 616 m, and an approximate NZ (New Zealand) grid reference of O29 580235.

Figure 12. Schematic profile through late Quaternary terrace sequence at Grey River compiled from field observations of outcrops along Grey River and from surveyed terrace elevations. Mean heights (±1σ) of terrace risers are specified for areas immediately north (N) and south (S) of the Awatere fault. Terrace nomenclature after Lensen (1964).

by the fault are offset by 25Ð60 m, implying (mean ~7.5Ð8 mm/yr) since late Quaternary time, of the T1 (ca. 9 ka), T2 (ca. 6 ka) T3 (ca. 4 ka), and postglacial ages for these landforms of ~4Ð10 ka. whereas offset of the ca. 10 ka Stb2 riser at Ross- T4 (ca. 3 ka) terraces (Knuepfer, 1992) constrain Offsets of 4Ð8 m and 10Ð15 m are interpreted as more stream by ~63 m indicates an early Holo- slip rates on the Molesworth section of the fault the product of the 1848 and penultimate earth- cene slip-rate of 6.0 +1.8/Ð1.0 mm/yr. Farther to between 3 and 18 mm/yr (means are ~7, ~9, ~9, quakes, respectively. The Molesworth section of west on the Molesworth section of the Awatere and ~5 mm/yr, respectively). Because pebble- the fault also has numerous late Quaternary off- fault, early Holocene slip rates of 6Ð7 mm/yr are weathering-rind dating has yielded age under- sets of <50 m, and modes at 6Ð8 m and 12Ð14 m, also inferred since deposition of the youngest estimations at sites in the Awatere Valley, we in- which McCalpin (1996) interpreted as the prod- glacial aggradation terrace at 9460 ± 150 to fer that at least some Holocene slip rates inferred ucts of the last two earthquakes on this part of the 12,600 ± 160 (calendar years) B.P (McCalpin, from such data are likely to be too large (Data fault. On the basis of this comparison, we infer no 1996), on the basis of 66Ð72 m dextral offsets Repository, Part D, see footnote 1). obvious difference in slip rate between the east- of risers and alluvial fans near Isolated Flat. Given the uncertainties in dating, especially in ern and central sections of the fault. Postglacial terraces have been dated by pebble- pebble-weathering-rind ages, the data in Figure Excluding the coastal section, dextral-slip, and weathering-rind dating, yielding slip-rates that 14B can be interpreted as evidence that the Awa- riser age data for all available sites along the are considerably less precise than those obtained tere fault has slipped at an essentially steady rate of Awatere fault indicate a mean slip rate of 6Ð8 from the older terraces. For example, at the Grey 7 ± 1 mm/yr during the past 15 k.y. Knuepfer mm/yr during late Quaternary time, and suggest River, pebble-weathering-rind ages of ca. 4 to ~2 (1992) proposed a late Holocene lessening in slip

a possible deceleration in slip rates during the ka for the SG1 and SG2 terraces imply late Holo- rate at the Grey and Saxton River sites, and in- Holocene Epoch (Fig. 14B and Table 3). Risers cene slip rates of ~4 to ~11 mm/yr (mean ~6), ferred that slip rates have been episodic on time

related to the ~15 ka Stb1 terrace are offset and ~1 to ~5 mm/yr (mean ~3), respectively. At scales of 5 k.y. This conclusion is not well sup- 110Ð124 m, indicating slip-rates of ~6Ð9 mm/yr the Saxton River, pebble-weathering-rind dating ported at the 95% level of confidence, except in the

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Dumgree Station

strong patch? (asperity) Lake Jasper oblique ag S Figure 13. Schematic three-dimensional oad thrust Br interpretation of Lake Jasper pull-apart Rossmore stream structure and Dumgree Station contractional scarp bend. The adjacent extensional and contrac- U tional bends link otherwise in-plane sections of the Awatere fault, and may have formed by D an oblique splay bypassing a strong patch or ~500 m “asperity” at depth before propagating back in-plane with the main fault. Scale is approximate, especially the vertical dimension. depth unspecified

contours on fault plane

case of the youngest recorded Holocene offsets at coast, dextral stream offsets of ~150Ð300 m are duction in dextral-slip rate along the Awatere both the Grey and Saxton Rivers. At the Saxton observed only on streams cut into the ca. 350 ka fault will be converted into a coastal clockwise River, 12Ð14 m of slip have accrued since the last Qpm unit, implying that the channels may have rotation rate of ~7.5¡/m.y. Paleomagnetically ob- two earthquake events since 2400Ð4800 (calendar been established at about that time (Fig. 4). If so, served clockwise rotations of ~20¡Ð35¡ since 4 years) B.P. (preferred pebble-weathering-rind ages these offsets imply a mean slip rate of <1 mm/yr. Ma (Roberts, 1992, 1995) are well within error of in McCalpin, 1996), implying a slip rate of only Near the coast, <4 km of dextral slip has offset a this prediction. 2.5Ð5.8 mm/yr. At the Grey River only 3.5Ð8 m of ca. 5 Ma unconformity, implying an average slip

strike slip has affected the SG2 terrace (1848 earth- rate of <0.8 mm/yr; or, alternatively, an inception CONCLUSIONS quake), which has a pebble weathering-rind age of age for the coastal strand of <2.7 m.y. (assuming 1.8Ð2.9 ka, implying a slip rate of only 1.2Ð5.4 a Quaternary slip rate of ~1.5 mm/yr). Several conclusions derived from the data pre- mm/yr (Table 3). Thus there is sound evidence for The coastward decrease in dextral-slip rates sented in this paper provide important insight a lessening in slip rate on the Awatere fault during requires that the plate-margin parallel component into the kinematics of oblique-slip faulting and the past 3 k.y. One possibility is that this reflects an of plate motion be accommodated in some other the partitioning of slip on the obliquely conver- increasing share of strike slip being accommo- way than by strike slip on the Awatere fault. Each gent New Zealand margin, on Quaternary strike- dated on the Hope and Porters’s Pass fault systems of the Marlborough faults splays and/or termi- slip rates on the Awatere fault, and on the nature to the south (Fig. 1). nates in or near Cook Strait, where strike-slip and magnitude of the last major earthquake on traces on the modern sea floor are nonlinear and the Awatere fault in 1848. Seaward Decrease in Late Quaternary Slip discontinuous, and the major onshore strike-slip Across a wide range in scales, the oblique-slip Rate faults cannot be traced offshore very far, if at all Awatere fault is segmented into strike-slip and (Carter et al., 1988; Browne, 1992; Barnes et al., oblique-reverse sections of differing strikes. We The rate of late Quaternary slip on the active 1995). We interpret the seaward reduction in late attribute this pattern to selective reactivation of coastal strand of the Awatere fault is distinctly Quaternary slip rates on the Awatere fault to be preexisting faults in a crustal transpression zone slower than that at Lake Jasper, indicating an accommodated by (1) transfer of slip onto mi- that contains intermediate and least compressive along-strike reduction in dextral-slip rate. Offset nor strike-slip splays near the coast, such as the principal stresses of nearly equal magnitude. The of the precisely dated Downs 2 terrace and paleo- Vernon and Hog Swamp faults (Fig. 2), and long-term accumulation of reverse slip across canyon wall at the Boundary Stream imply a slip (2) by clockwise vertical-axis rotation of coastal the Awatere fault expressed by topographic relief rate of only 1.4 ± 0.5 mm/yr since ca. 23 ka (Fig. northeast Marlborough and Cook Strait (Little is commonly contradicted by late Quaternary 14B). This rate is actually a maximum, because and Roberts, 1997). This process can also ex- offsets that are more nearly strike slip (or even (1) incision of the bedrock canyon must have pre- plain why the ancestral strand of the coastal slightly normal slip), implying a scale depen- dated its filling by Downs 2 alluvium; thus, some Awatere fault was supplanted by the currently dence or recent change in the degree of slip par- of that landform’s dextral slip may predate the active coastal strand, which is slightly less clock- titioning in this part of the Marlborough trans- dated terrace tread; and (2) the age of the terrace wise striking. For a mean fault spacing of 35 km, pression zone. may be somewhat older than the ca. 22.6 ka the simplest block-rotation model (Little and Near the coast, the local azimuth of slip on the Kawakawa Tephra deposited on it. Near the Roberts, 1997) predicts that an ~4.5 mm/yr re- Awatere fault makes a 13¡Ð15¡ angle relative to

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the NuvelÐ1a plate velocity; farther southwest, there is little or no discordance between these vectors. We infer that increasing coupling on the A 10 1848 earthquake Hikurangi subduction interface to the southwest 9 frictionally inhibits a slip-partitioned style of n = 67 last two surface ruptures oblique motion in the overlying plate. Offshore, a 8 Last glacial advance (incl. Starborough 2) weak megathrust becomes unavailable to take up a margin-orthogonal component of motion. On- 7 spurs, terrace risers (~10-12 ka) shore, dipping oblique-reverse faults acquire a 6 greater slip tendency (shear/normal stress ratio) than steep strike-slip faults, because locking of 5 Glacial maximum (incl. Starborough 1) terrace risers (~15 ka) the subduction interface increases transmission 4 of horizontal compressive stress onto vertical planes in the upper plate. 3 Coastal streams incised into Qpm unit (<350 ka) Offsets of well-dated terrace risers on both the Frequency 2 eastern and Molesworth sections of the Awatere fault indicate a mean strike-slip rate of 7 ± 1 1 mm/yr along inland parts of the fault during late Quaternary time (since ca. 15 ka). There may 5

25 45 65 85 have been a decrease in this rate between 15 and 105 145 165 185 205 225 245 265 285 125 10 ka, and probably one since ca. 3 ka (last two Dextral Displacement (m) surface rupturing events). Deceleration in slip rate may reflect an increasing share of strike slip being accommodated on the Hope and Porter’s Pass fault systems farther to the south. A seaward reduction in slip rate to <1.4 ± 0.5 B mm/yr (since ca. 23 ka) and termination of the 140 Boundary Strm Marlborough faults is accommodated by splay- Grey River Starb. 1 ing and vertical-axis rotations. Rotation may Saxton River 120 Starb. 1 have caused misorientation and abandonment of Lake Jasper inland segments an older fault strand near the coast. Blairich River The 1848 earthquake probably caused a sur- 100 6 mm/yr Isolated Flat face rupture on the eastern Awatere fault that ex- 8 mm/yr tended inland from the coast for ~100 km, and 80 SG = Starb. 1(?) propagated past an intersection with the more in- T1 land Molesworth section of the fault. This earth- 60 quake probably had a coseismic strike slip of T2 Star- borough 2 5Ð8 m, a locally variable throw of 0Ð3.5 m, and Do2 40 a moment magnitude, M, of >7.4. Dextral-Slip (m) T3

SG1 ACKNOWLEDGMENTS 20 T4 T5 coastal segment Downs 2 t3 1.4 mm/yr This study was funded by the New Zealand 0 SG2 Foundation for Research, Science, and Technol- 0 10 20 ogy grant 94-VIC-30Ð907. B. J. Pillans logged soil profiles included as parts of Figures 4B and Terrace Age (ka) 8A, and recognized the Kawakawa tephra on the Downs 2 terrace at the coast. A. Benson measured offset stream channels near Lake Jasper as part of an Master of Science degree study. We thank P. R. Figure 14. (A) Histogram of 67 late Quaternary dextral displacements along the Awatere fault Wood and the Institute of Geological and Nuclear (east of Grey River) compiled from this study, P. R. Wood and G. J. Lensen (unpub. data, Insti- Sciences for providing detailed aerial photo- tute of Geological and Nuclear Sciences), and Knuepfer (1992). Data are grouped into 5 m in- graphs of the Grey River, and access to unpub- tervals. Offset features include ridge spurs, terrace risers, streams, and relict channels. (B) Plot lished field data of P. R. Wood and G. J. Lensen; of dextral slip of terrace risers vs. terrace age for sites along Awatere fault. Horizontal offset of D. Winchester and P. R. Wood for help with sur- terrace risers is assumed to accrue after abandonment of lower terrace surface. Dashed refer- vey planning; B. J. Pillans and M. J. Hannah for ence lines correspond to dextral-slip rates of 1.4, 6.0, and 8.0 mm/yr. Terrace risers at each site help in sampling of loess for thermoluminescence are specified by name of lower terrace. See Tables 2 and 3 for description and sources of data. dating; M. Kaufman, M. Lowry, and W. McClea Data labeled Isolated Flat refer to last glacial aggradation surface and related alluvial fans that for assistance in surveying; M. J. McSaveney for have been radiocarbon dated as between ca. 9.4 and 12.6 ka (from McCalpin, 1992b, 1996). computation of pebble-weathering-rind ages;

146 Geological Society of America Bulletin, February 1998

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D. Townsend for measuring rind thicknesses; v. 20, p. 403Ð406. Eiby, G. A., 1980, The Marlborough earthquakes of 1848: New Berger, G. W., Pillans, B. J., and Palmer,A. S., 1994, Test of ther- Zealand Department of Scientific and Industrial Research J. A. Carter for separation of glass shards from moluminescence dating of loess from New Zealand and Bulletin, no. DB225, 82 p. loess and preparation of probe mounts; and D. W. Alaska: Quaternary Science Reviews, v. 13, p. 309Ð333. Grapes, R., 1993, Terrace correlation, dextral displacements, Berger, G. W., Pewe, T. L., Westgate, J. A., and Preece, S., and slip rate along the Wellington Fault, North Island, Burbank, B. J. Pillans, S. Beanland, P. R. Wood, 1996, Age of Sheep Creek tephra (Pleistocene) in central New Zealand: Comment: New Zealand Journal of Geol- K. Berryman, R. J. Van Dissen, R. I. Walcott, Alaska from thermoluminescence dating of bracketing ogy and Geophysics, v. 36, p. 131Ð133. W. McClea, and J. Goff for discussions. 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