Kaikoura Canyon, New Zealand: active conduit from near-shore sediment zones to trench-axis channel

Marine Geology 162 Ž1999. 39–69 www.elsevier.nlrlocatermargeo

Kaikoura Canyon, New Zealand: active conduit from near-shore sediment zones to trench-axis channel Keith B. Lewis ) , Philip M. Barnes National Institute of Water and Atmospheric Research (NIWA), New Zealand Oceanographic Institute, EÕans Bay Parade, P.O. Box 14901, Kilbirnie, Wellington, New Zealand Received 30 September 1998; accepted 4 June 1999

Abstract Kaikoura Canyon is one of a few major, active conduits between a near-shore sediment transport system and a deep-ocean channel. It is the sink for mobile zones of gravel, sand and mud that migrate northwards off northeastern South Island, New Zealand. It is presently the primary source for the 1500 km long Hikurangi Channel, which supplies overbank turbidites to a filled trench, an ocean-plateau basin and a distal fan-drift. Swath data, seismic profiles, side-scan sonographs, aerial photographs, cores, grab samples and current meter data are used to define the shape and texture of the canyon and adjacent shelf in order to better understand how coastal processes and canyon interact to supply sediment to the deep-ocean. Kaikoura Canyon is 60 km long, up to 1200 m deep and generally U-shaped in profile. Its head is within 500 m of the shore, and within 200 m of rocky projections from the shore-platform, in a mountain-backed bay without large rivers. The canyon head incises the 18 m depth contour and boulders, pebble gravel and megarippled coarse sand reach the canyon rim. Fine sand migrating northward along the shelf under the influence of waves and currents is trapped in a southward-projecting, canyon-head gully, which incises the thickest part of the Holocene sediment prism. It is estimated that about 1.5 = 10 6 m3 of sediment falls into the canyon head each year. Tensional fractures around the canyon rim suggest that sediments in the canyon-head gully are unstable. Gravel turbidites, with post last glacial age shells, are at or near the seabed in the lower canyon but are blanketed by many thin, silt and sand, possibly storm-generated, turbidites in the upper canyon. The top gravel contains a twig that is about 170 years old, suggesting that the last major collapse in the canyon head coincides with many onshore rockfalls triggered by rupture of a major, strike-slip, plate-boundary fault in about 1833. An underlying gravel is about 300 years old and may again coincide with fault rupture. Most of the large, earthquake-triggered, failures may ‘‘ignite’’ to form self-perpetuating, autosuspension flows, that feed a 1500 km long, deep-sea, turbidite channel. q 1999 Elsevier Science B.V. All rights reserved. Keywords: submarine canyon; turbidite; shelf environment; submarine landslides; New Zealand

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Corresponding author. Fax: q64-4-386-2153

0025-3227r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 5 - 3 2 2 7 Ž 9 9 . 0 0 0 7 5 - 4

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K.B. Lewis, P.M. Barnesr Marine Geology 162 (1999) 39–69

1. Introduction 1.1. Significance of Kaikoura Canyon Submarine canyons have long been recognised as a common, almost ubiquitous feature of the world’s continental slopes, being regarded as generally the result of erosion by high velocity, sediment-charged flows ŽDaly, 1936; Shepard and Dill, 1966; Stanley and Kelling, 1978; Shepard, 1981; Pickering et al., 1989.. At most places, heads of canyons incise only the outer edges of continental shelves at water depths of 100–150 m ŽVon der Borch, 1968; May et al., 1983; O’Leary, 1996.. These canyons, now generally remote from abundant near-shore supplies of sediment, are either inactive or only very infrequently active. They are relics of glacial periods of lowered sea-level, when voluminous quantities of sediment were carried right across what are now continental shelves by rivers ŽPickering et al., 1989.. At a few places, generally either off major rivers ŽDroz et al., 1996., or on narrow, tectonically active margins ŽSavoye et al., 1993; Gardner et al., 1996; Schwalbach et al., 1996., canyons have cut headward to retain their abundant coastal sediment supply. Only these canyons remain frequently active today. Kaikoura Canyon, off northeastern South Island, New Zealand ŽFig. 1., is one these active canyons ŽCarter et al., 1982.. It cuts deeply into a narrow, tectonically active, continental margin, but is remote from the mouth of a sediment-charged river. Instead, it has been inferred to be the sink for a coastal sediment transport system that carries large amounts of debris northwards from the rivers that drain South Island’s rapidly rising mountains ŽCarter and Herzer, 1979.. Most canyon systems end at slope-toe, delta-like, depositional fans, with radiating, aggradational, distributary channels less than a few hundred kilometers long. A very few merge into deep ocean, channel systems that meander for many hundreds or even several thousand kilometers across a deep ocean floor ŽCarter, 1988; Hesse, 1995; Clark and Pickering, 1996.. Kaikoura Canyon is one of these. It is the main source of the 1500 km long Hikurangi Channel ŽFig. 1., which supplies turbidites to the Hikurangi Trough, a sediment-filled structural trench, as well as to low parts of the oceanic Hikurangi Plateau, and to

a distal ‘‘fan-drift’’ at the edge of the Southwest Pacific Basin, where settling turbidity currents are swept by the northward Deep Western Boundary Flow into the Pacific ŽCarter and McCave, 1994; Lewis, 1994.. Because it is the major active conduit between South Island’s eroding Southern Alps and the deep ocean basins to the northeast, the Kaikoura Canyon is inferred to play a crucial role in the sediment dynamics off eastern New Zealand. The objective of the present study is to better characterise its roll in trapping shelf sediments and funnelling them offshore by more clearly defining the morphological and sedimentological characteristics of both the canyon and the continental shelf at its head, and relating these to processes of sediment transport. 1.2. Structural and oceanographic setting of Kaikoura Canyon Kaikoura Canyon is located close to, and perhaps within, a broad zone where the boundary between the Australian and Pacific plates comes ashore in northeastern South Island ŽFig. 1.. It is at the southern apex of Hikurangi Trough, which marks subduction of the Pacific Plate beneath eastern North Island. Relative plate motion becomes increasing oblique towards the southern end of the trough and merges into a broad, transform zone of strike-slip and collision, which extends through the continental margin and mountains of northeastern South Island to the Alpine Fault ŽWalcott, 1978; Bibby, 1981; Lamb and Bibby, 1989; Kneupfer, 1992; Barnes et al., 1998.. Although Kaikoura Canyon is near the southern edge of the highly oblique collision zone ŽVan Dissen and Yeats, 1991., there is evidence that the plate boundary zone is in the processes of propagating south of Kaikoura towards Pegasus Bay ŽCarter and Carter, 1982; Cowan et al., 1996.. Because of the proximity of plate boundary deformation, the canyon is subject to periodic severe seismic shaking ŽVan Dissen, 1991. and to rapid sediment influx from rapidly rising mountain ranges ŽBell, 1976; Gibb and Adams, 1982.. As well as being close to a structural boundary, Kaikoura Canyon is close to a significant oceanographic boundary. It is in the convergence zone separating cool water that has flowed around the

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Fig. 1. Location of Kaikoura Canyon Žwithin box. in relationship to regional bathymetry Žcontours at 250 m intervals., the Pacific–Australian plate boundary Žthick broken line with flags indicating subduction., an active deep-ocean channel system Žline of square dots., and geostrophic currents Žlarge open arrows..

southern end of New Zealand as the Southland Current ŽFig. 1., and warm water that has flowed around the northern end of New Zealand as the East Cape Current. The main boundary between the two extends eastwards along the southern slope of the Chatham Rise ŽCarter and Herzer, 1979., but the Southland Current, which flows northeastwards along

the continental shelf and upper slope off southeastern South Island, bifurcates off Banks Peninsula and part of it continues through the gap between Banks Peninsula and Chatham Rise, reaching northwards at least as far as Kaikoura ŽFig. 1. ŽGreig and Gilmour, 1992.. There, it interacts with eddies shed by the East Cape Current ŽChiswell, 1996..

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K.B. Lewis, P.M. Barnesr Marine Geology 162 (1999) 39–69

K.B. Lewis, P.M. Barnesr Marine Geology 162 (1999) 39–69

2. Methods The head of Kaikoura Canyon and the adjacent shelf between Kaikoura Peninsula and Haumuri Bluffs was surveyed in April 1996 using a Klein 595 Sidescan Sonar system and an Echotrak DF3200 sounder ŽFig. 2, inset.. The side-scan was generally operated on a 300 m swath-width with tracks 300 m apart or less, except on the broad shelf north of the canyon where tracks were 600–700 m apart. Returning echoes were logged digitally but records were generally interpreted without slant-range correction. Samples to ‘‘groundtruth’’ the side-scan data were obtained with a small Shipek grab, with sampling concentrated on weak backscatter substrate types that are difficult to differentiate using sonar techniques. High resolution, single-channel, seismic profiles of the same area were obtained in December 1996 using an EG & G Uniboom profiling system, which records layers up to a few tens of meters below the seabed with a resolution of several meters. Tracks using the Uniboom were in a grid with onshore–offshore tracks ranged from 600 to 2000 m apart. On both occasions, surveys were undertaken with position fixing to "2 m accuracy using Differential GPS. Some additional side-scan, sounding and sampling data were available from a small area on the outer shelf off Kaikoura Peninsula, having been collected as part of a study of biomedicinal sponges in the area ŽPage and Battershill, in press.. Sounding data were also available from archive and hydrographic databases and aerial photographs showing the shore platform were made available by Terralink, NZ. The deeper parts of the canyon were surveyed in November 1993, using an EM12D, dual multibeam, swath-mapping system hull-mounted on the French Research Vessel L’Atalante ŽFig. 2, inset., as part of the French–New Zealand GeodyNZ Programme to investigate the geodynamics of the plate boundary through New Zealand ŽCollot et al., 1996.. The

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EM12D measures depth and seabed reflectivity along a swath that is up to seven times wider than the water depth. Its 12 kHz sound may penetrate several meters into soft sediment to backscatter from buried layers, as does the sound of other long-range, swath-mapping systems ŽKenyon, 1992.. Depth measurements were corrected for variations of sound velocity within the water column using data from expendable bathythermographs. Seismic reflection profiles were obtained using L’Atalante’s six-channel, twin 75 cubic in. airgun system and selected records were processed at the Institute of Geological and Nuclear Sciences ŽIGNS.. Piston cores and high resolution seismic data were available from archive, having been collected during exploratory surveys or while in transit on cruises between 1979 and 1993. Cores were split, logged, photographed and, in some cases, X-rayed before being subsampled for analysis. Grab and core samples were analysed using standard settling tube and pipette methods. An Aanderaa current meter was moored 5 m above the seabed in the axis of the upper canyon from 26 March 1996 to 29 April 1996. Records were processed with a 72 h filter to show tidal components and net drift.

3. Sediment zones on a narrowing shelf 3.1. Shelf morphology The continental shelf off northeastern South Island typically ranges from 15 to 30 km wide but Kaikoura Canyon and the associated Conway Trough are incised deeply into it ŽFig. 2.. South of the Kaikoura Canyon, the continental shelf landward of the Conway Trough narrows from 9 km wide at the southern end of the Conway Trough to about 4 km immediately south of the Kaikoura Canyon head and

Fig. 2. Kaikoura Canyon — morphology, cores and nearby geology. ŽA. Bathymetry of Kaikoura Canyon area with contours at 50 m intervals, showing positions of cores Žq.. Shelf break at broken hatchured line. Thrust faults at broken, flagged lines and simplified geology onshore. Inset: location of main datasets: Ž1. nearhore data collected for this study; Ž2. deep-sea swath and seismic data from GeodyNZ programme ŽCollot et al., 1996. Ž3. data collected for biomedicinal sponge study ŽPage and Battershill, in press.; Ž4. archived and hydrographic data. ŽB. Digital terrain model of same area as A, viewed from southeast, with vertical exaggeration of 5 = , and illumination from the east.

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K.B. Lewis, P.M. Barnesr Marine Geology 162 (1999) 39–69

the shelf break shoals from about 100 m to less than 40 m deep ŽFigs. 2A and 3; Carter et al., 1982;

Herzer and Bradshaw, 1985.. Smaller canyons incise the edge of the narrowing shelf, including one,

Fig. 3. Bathymetry of upper Kaikoura Canyon, with contours at 50 m interval, and of the adjacent continental shelf, with broken contours at 10 m intervals. Shelf break is broken hatchured line. Shelf sediment distributions from side-scan backscatter patterns Žrefer to Fig. 5. and samples. Thickness of modern, shelf sediment prism in meters Ždotted lines. from boomer profiles Žsee Fig. 6.. Location of Figs. 5A–E, 6A–E and 7A–D shown. Heavy broken and dotted line shows edge of unstable or slumped seabed in canyon head and mid-canyon.

K.B. Lewis, P.M. Barnesr Marine Geology 162 (1999) 39–69

northeast of Haumuri Bluffs, that reaches to within 1.5 km of the headland and cuts the 25 m contour ŽFig. 3.. South of the canyon, shelf contours are generally smooth, indicative of sediment cover, but locally irregular contours indicate rocky ‘‘reefs’’ projecting seaward from a rocky shore. Landward of the head of Kaikoura Canyon, between Oaro and Pinnacle Rock ŽFig. 3., the shelf is less than 1.5 km wide for over 7 km and narrows to only 500 m wide just north of Goose Bay ŽFig. 4.. The depth of the shelf break at the canyon rim ranges from about 35 m off Pinnacle Rock and Oaro to only 18 m at Goose Bay ŽFigs. 3 and 4.. Irregular contours indicate rocky ‘‘reefs’’ projecting seaward from wide, rocky shore platforms. Off Goose Bay, only 200 m separates the 10 m deep seaward end of a ‘‘reef’’ and the canyon rim. The narrowing and shoaling shelf between Oaro and Goose Bay is above the left wall of a steep-sided, canyon-head gully ŽFigs. 3 and 4.. North of Kaikoura Canyon, the shelf is much wider ŽFig. 3.. Seaward of a 7 km long pebbly beach between Kaikoura Peninsula and Pinnacle Rock, smooth contours, a gently sloping Ž0.68. inner shelf and more steeply sloping Ž1.58–2.08. mid-shelf indicate a near-shore sediment prism ŽLewis, 1973a; Lewis, 1974.. A subsidiary canyon head cuts the 25 m contour off the centre of the bay. Otherwise, the shelf break along the northern wall of the canyon, ranges from 40 m deep off Pinnacle Rock to 125 m deep southeast of Kaikoura Peninsula. There, irregular contours indicate a rocky topography characteristic of a relict, outer shelf, wave-cut platform. 3.2. Shelf backscatter patterns Side-scan sonographs of the continental shelf between Haumuri Bluffs and Kaikoura Peninsula are characterised by seven different intensities and patterns of backscatter that can be related to distinct rock and sediment types ŽFigs. 4 and 5.. The patterns are interpreted from samples, from aerial photos, and by comparison with pattern in southern North Island ŽCarter and Lewis, 1995. and elsewhere ŽBelderson et al., 1972.. They are the following: Ž1. Sharply defined areas of no backscatter Žwhite. close inshore that correspond to protruding rocks and

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rocky shore platform mapped from aerial photographs ŽFig. 5A.. Ž2. Large Ž) 3 m., strongly backscattering patches Žblack. with large shadows Žwhite. that correlate with irregular sounding traces characteristic of rock outcrops ŽFig. 5A–C.. Ž3. Speckle-pattern, with small Ž- 3 m. strongly backscattering patches and small shadow areas, identified as boulder gravel, which typically borders rocky outcrops or overlies rock platforms ŽFig. 5A, C.. Ž4. Dark, strongly backscattering seabed representing pebble gravel, with some speckles from larger clasts ŽFig. 5A.. Ž5. Regular stripes of strong and weak backscatter representing megaripples of coarse sand with fine gravel ŽFig. 5B, D.. The megaripples are aligned sub-parallel to the regional contours and have wavelengths of 0.8–1.2 m. Shadows at the edge of megarippled areas indicate that they are incised below the level of adjacent fine sand ŽFig. 5D.. Ž6. Featureless, weakly backscattering Žlight. seabed that coincides with fine sand on the shelf and muddy fine sand or sandy mud on the upper slope ŽFig. 5.. Ž7. Moderately backscattering seabed with mottled lighter and darker patches, generally on the outer shelf and upper slope that sampling indicates is mud with an abundance of upright growing, horse mussels Atrina zealandica ŽFig. 5E.. 3.3. Groundtruthing shelf backscatter patterns Although samples from the area are available from archive, only samples from the present survey, which were located with reference to specific sonograph backscatter patterns at the time of collection, are used to characterise these patterns ŽTable 1.. Backscatter patterns identified as shore platform Ž1., rocky reef Ž2. and boulders Ž3. are generally unambiguous and attempts to sample them generally yielded only encrusting organisms. Two samples obtained from flat, strongly backscattering seabed Ž4. are gravel with median diameters in the pebble gravel size range ŽTable 1A.. Clasts are rounded Mesozoic greywacke with rare, smoothly rounded pebbles of white Palaeogene limestone, similar to those out-

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K.B. Lewis, P.M. Barnesr Marine Geology 162 (1999) 39–69

cropping near Haumuri Bluffs, and modern shell debris. Megarippled areas Ž5. are lithic coarse sand with median diameters generally ranging from 0.3 to 0.7 w Ž1.8–1.2 mm. ŽTable 1B.. One sample outside this range is bimodal and may include underlying pebble gravel. Most of the samples from weakly backscattering areas Ž6. on the continental shelf are well sorted fine sand with a median diameter of 2.1–2.8 w Ž0.24–0.14 mm.. Several samples from protected spots in the lee of rocky reefs or in canyon heads have a significant proportion of silt Ž13%–35%. and a median diameter of 3.1–3.3 w Ž0.12–0.10 mm.. Samples from weakly backscattering areas on the upper slope are typically sandy mud with 18%–55% sand and a median diameter of 4.9–5.8 w Ž33–18 mm.. Samples from areas of patchy backscatter on the outermost shelf and upper slope Ž7. have abundant living and dead shells of the horse mussel Ž A. zealandica.. The shells are commonly 300–400 mm long and grow upwards out of the sandy mud presenting a good reflective target for sound waves. The sediment between the shells generally has 18%–38% sand and a median diameter ranging from 4.9 to 6.3 w Ž33–13 mm.. Thus, side-scan backscatter images show the distribution of coarser sediment types on the continental shelf, but fine sand and mud must generally be differentiated using sample data. 3.4. Shelf sediment distributions On the continental shelf, the distribution of the main rock and sediment types correlates partly with

depth and partly with local geomorphology ŽFig. 3.. Rock, gravel and megarippled coarse sand generally occur close to the shore and cover most of the shelf where it is less than 1.5 km wide. Featureless, well-sorted, fine sand occurs at most places where the shelf is more than 1.5 km wide and it extends out to depths of about 35–40 m. Mud occurs generally at depths below about 35–40 m. The exception is seaward of Kaikoura Peninsula, where gravel extends out the 125 m deep shelf break. Because sediment zones correlate partly with depth, sediments at the canyon rim change from boulders, pebble gravel and megarippled coarse sand at the canyon head, to sand and mud where the canyon incises the mid-shelf, and to Žprobably relict. gravel where it incises the outer shelf. Some of the gravel occurs in onshore–offshore aligned bands, apparently related to the trends of rock outcrops. Megarippled coarse sand also occurs in onshore–offshore aligned bands, even in areas away from rock outcrops ŽFigs. 4 and 5B, D.. The trend of the megaripples is generally near perpendicular to the trend of the band and sub-parallel with both the regional contours and with the approach of refracted, large swells from the south ŽCarter and Herzer, 1979.. Contact between the megarippled coarse sand and fine sand is always a sharp line. They are two distinct populations and shadows on side-scan sonographs ŽFig. 5D. indicate that the megaripples are recessed below the level of the fine sand and are presumably inliers in the blanketing fine sand. A smooth blanket of well-sorted, fine sand covers most of the continental shelf south of the canyon and

Fig. 4. Detail of canyon head showing the canyon-rim shelf break intersecting the 20 m contour and the pebble and boulder strewn shelf within 500 m of shore platform east of Goose Bay. Sand moving along the shelf from the south is inferred to enter the canyon-head gully mainly along its western wall. Small linear scarps along the upper part of the western wall Žhatchured lines. suggest tension associated with incipient slope failure of sediment Žand perhaps rock. in the canyon-head gully. Fig. 5. Side-scan backscatter patterns; Ž1. no backscatter from exposed rocky shore platform and contiguous projecting ‘‘reefs’’; Ž2. strong backscatter and shadows Žwhite. from submarine rock outcrops; Ž3. speckled pattern from boulder gravel; Ž4. moderate backscatter with fine speckling from pebble gravel; Ž5. zebra stripes of megarippled coarse sand; Ž6. weak backscatter from fine sand or mud Ždistinguished by sampling.; Ž7. blotchy backscatter from muddy sediments with abundant horse mussels. Horizontal lines are at 10 m intervals. Positions of sonographs shown on Fig. 3. ŽA. Shows a variety of backscatter pattern at the outer edge of a shore platform reef near Goose Bay; ŽB. shows rock outcrops up to several meters high projecting from a megarippled coarse sand; ŽC. shows mainly boulder gravel, with individual boulders up to several meters in diameter, beside a low rocky reef; ŽD. shows a white shadow in front of patches of megarippled coarse sand, indicating that the megaripples are recessed below the level of the surrounding fine sand; ŽE. shows the boundary between sand and mud with patches of horse mussels on the outer shelf off Haumuri Bluffs.

K.B. Lewis, P.M. Barnesr Marine Geology 162 (1999) 39–69

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K.B. Lewis, P.M. Barnesr Marine Geology 162 (1999) 39–69

K.B. Lewis, P.M. Barnesr Marine Geology 162 (1999) 39–69

most of the bay north of the canyon ŽFig. 3.. A change to sandy silt below about 35 m deep was defined partly by sampling and partly by a change in backscatter associated with a change to muddy sediment containing living and dead valves of the large horse mussels A. zealandica. More strongly backscattering mud with Atrina extends from about 30 to 80 m deep. 3.5. The shelf sediment prism High resolution seismic profiles from the nearshore area ŽFig. 2A, inset. show that the continental shelf is blanketed by a prism of sediments that overlies a terraced erosion surface ŽFig. 6.. The prism is generally thickest on the mid-shelf where the water depth is 25–40 m ŽFig. 3. The canyon-head gully incises the mid-shelf at about this depth but the near-shore sand prism is much thinner near to the canyon rim. The prism’s maximum thickness is over 40 m in the southern shelf near Haumuri Bluffs, and about 25 m on the wide shelf north of the canyon ŽFig. 3.. Like shelf sediment prisms elsewhere ŽLewis, 1973b; Herzer, 1981; Carter et al., 1986; Barnes, 1996., the shelf sediment prism is here considered to have been deposited on a transgressive wave-planed erosion surface during and since the last post-glacial rise of sea-level. The inner edge of terraces on the erosion surface represent palaeoshorelines cut during still-stands. There are palaeoshorelines at about 56, 45–47 and 30–33 m below present sea-level ŽFig. 6A–C.; these occur a few meters deeper than the same sequence on wider shelves to the south ŽCarter and Carter, 1986.. On the outer shelf north of the canyon, there are buried channels, recessed up to 10 m below the level of the adjacent erosion surface ŽFig. 6D., that are probably the drowned extensions of the Kowhai and Kohutara Rivers. Where the shelf is wide, the sediment prism thins towards the outer shelf, thinning to nothing on the wide shelf seaward of Kaikoura Peninsula ŽFig. 6E.. Sediment thicknesses are difficult to determine on the outer shelf because of an opaque reflector within the seaward part of the sediment prism ŽFig. 6C.. Such reflectors are characteristic of small amounts of gas in the sediment ŽSchubel, 1974; Hampton and Kvenvolden, 1981.. This mid-prism reflector is about 30–40 m below the sea surface at its inner edge so

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Table 1 Stna

Depth %Grav %Sand %Silt %Clay Median Ž w .

(A) Substrate 4 — flat graÕel S1005 21 m 83 16 S1010 30 m 68 10 (B) Substrate 5 — megaripples S1021 11 m – 100 S1037 16 m – 100 S1024 20 m – 100 S0996 22 m – 100 S1003 22 m 3 97 S1035 24 m 58 41 S0998 26 m – 100 S1001 29 m 1 99

– 14

– 8

y3.3 Ž9 mm. y2.2 Ž5 mm.

– – – – – – – –

– – – – – – – –

0.7 0.4 0.4 0.6 0.7 y1.7 0.6 0.3

(C) Substrate 6 — weakly reflectiÕe fine sand and mud S1021 11 m – 98 – – 2.7 S0992 11 m – 99 – – 2.7 S1018 12 m – 70 29 1 3.1 S1017 13 m – 99 – – 2.5 S1036 14 m – 100 – – 2.5 S1022 15 m – 99 – – 2.7 S0993 16 m – 99 – – 2.6 S1023 18 m – 85 13 1 3.2 S1004 20 m – 100 – – 2.2 S1016 22 m – 99 – – 2.7 S0999 22 m – 99 – – 2.1 S0997 23 m – 100 – – 2.7 S1034 24 m – 98 1 – 2.5 S1007 24 m – 99 – – 2.1 S0994 25 m – 99 – – 2.7 S1000 27 m – 99 – – 2.6 S0995 30 m – 98 – – 2.8 S1033 32 my – 57 35 7 3.3 S1002 33 m – 99 – – 2.6 S1015 34 m – 99 – – 2.6 S1028 35 m – 96 4 – 3.1 S1025 36 m – 55 35 10 3.5 S1008 40 m – 38 43 18 4.9 S1027 41 m – 26 56 18 5.5 S1009 44 m – 30 45 25 5.8 S1029 63 m – 25 64 – 5.0 S1011 100 m – 25 54 21 5.7 S1012 185 m – 18 63 18 5.8 (D) Substrate 7 — patchy, moderately reflectiÕe mud (with horse mussels) S1031 32 m – 26 58 15 5.2 S1030 37 m – 20 61 17 5.2 S1014 40 m – 23 54 22 6.2 S1013 62 m – 24 48 28 6.3 S1032 62 m – 18 66 16 5.6 S1026 63 m – 34 52 14 4.9

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K.B. Lewis, P.M. Barnesr Marine Geology 162 (1999) 39–69

that it must be indicative of processes that occurred during or since the later stages of the post-glacial rise of sea-level. Where the shelf is narrow, the sediment prism is truncated at the shelf break by collapse or by rapid thinning onto the upper slope ŽFig. 6A.. Around the canyon head, parts of the outermost shelf and canyon rim are stepped, with a series of apparently back-tilted blocks ŽFig. 7B, C and D.. Off Oaro, the blocks, and the troughs between them, trend NW–SE oblique to shelf and to the canyon head ŽFig. 4.. An interpretation of these scarps in terms of slope stability in the canyon head is discussed later.

4. Kaikoura Canyon 4.1. Canyon morphology and structure The 60 km long Kaikoura Canyon is incised 600–1200 m below the adjacent shelf and slope. It follows a broadly curved route from the narrow, mountain-backed shelf between Oaro and Goose Bay to the 2000 m deep apex of the Hikurangi Trough ŽFig. 2.. At the canyon-head between Oaro and Goose Bay, the seabed falls to 600 m deep in about 1 km, an average slope of 258–308. This slope is the left wall Žlooking down-slope. of a narrow, steep-sided, V-profiled, canyon-head gully that extends sub-parallel with the coast, for about 3 km. The axis of the gully slopes at about 128 from the 32 m isobath at the shelf edge off Oaro down to the 600 m isobath in the main canyon off Goose Bay ŽFigs. 3, 8 and 9.. The top of the left wall defines the narrowing and shallowing shelf between Oaro and Goose Bay. Seismic profiles proved difficult to obtain in the steepsided canyon-head gully but 3.5 kHz records appear to show chaotic sediment infill that is at least 70 m

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thick in the upper gully decreasing to about 20 m thick in the lower gully, and contrasting with parallel-bedded turbidites in the main canyon ŽFig. 8.. The upper canyon, between the 600 and 1300 m isobaths in the canyon axis, obliquely incises Tertiary sediments of the continental shelf for 14 km ŽCarter et al., 1982.. It has an asymmetric, U-shaped profile and its steep northern Žleft. wall is 1000–1200 m high with average slopes of 208–308 and cliffs of more than 458 ŽFigs. 2 and 3.. The southern wall slopes more gently at only 108–208. From the bottom of the canyon-head gully at 600 m deep to the confluence with Conway Trough ŽFig. 3., the canyon-floor is 300–700 m wide, with a few tens of meters of parallel-bedded, turbidite fill ŽFig. 8., and an axial slope of about 48 ŽFig. 9.. Conway Trough trends almost orthogonal to the canyon and its trend is mirrored in the canyon’s northern wall by a small tributary canyon. Below the confluence with the Conway Trough, the slope is only 1.58 ŽFig. 9. and the broadly U-shaped floor reaches 1.5 km wide near the 1250 m isobath. Near the 1300 m isobath, the eastward-trending, high-walled upper canyon changes to a southwardtrending, slump-modified central section. Just beyond the change in trend, the bathymetry ŽFigs. 2 and 3. and seismic profiles ŽFig. 10. suggest that a large slope failure deposit from the right Žwestern. bank almost blocks the canyon axis. The deposit covers an area of about 8 km2 with a head scarp of several more square kilometers ŽFig. 3.. The migrated profile, which crosses its northern edge, shows a reflection-free, possible debris avalanche deposit over 200 m thick overlain by parallel-bedded turbidites about 150 m thick ŽFig. 10.. In front of the toe of the slump, the floor of the canyon is restricted to a passage only a few hundred meters wide between the slump and eastern Žleft. wall. Axial slope remains at 1.58–28.

Fig. 6. High resolution ŽUniboom. profiles of the continental shelf showing seabed Žstippled. and last post-glacial transgressive surface Žline of dots. and outer shelf opaque reflector Žline of vertical bars.. Between the transgressive surface and the seabed is the modern shelf sediment prism. Horizontal time lines are at 25 ms Žapproximately 19 m. intervals. Vertical exaggeration approximately 10:1. Location of profiles shown in Fig. 3. ŽA. Shows the full width of a narrow shelf sediment prism above the canyon head, with palaeoshorelines ŽS. at 30 and 47 m below present sea-level ŽM is the seabed multiple.; ŽB. shows a 46 m palaeoshoreline ŽS. on the mid-shelf west of Kaikoura Peninsula; ŽC. shows a thick sediment prism with a mid-prism opaque layer and only faint indications of a 47 m palaeoshoreline ŽS. beneath the mid-shelf off Oaro; ŽD. shows a drowned and buried river channel from the outer shelf east of Pinnacle Rock; ŽE. shows the seaward limit of the shelf sediment prism on the outer shelf southwest of Kaikoura Peninsula.

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Below the slump-modified central section, the lower canyon continues in a broad curve into the upper end of the Hikurangi Trough ŽFig. 2.. The slope of the canyon axis reduces gradually to about 18 in the southern end of the Hikurangi Trough. The canyon walls are lower and more gently sloping and incised into deforming slope and trough sediments ŽLewis and Bennett, 1985; Barnes, 1994.. The canyon floor is almost flat-floored, ranging from gently concave to slightly convex in the middle, and reaches a maximum of 5 km wide. Thickness of fill in the lower canyon has not been determined. 4.2. Canyon backscatter and cores Sonar backscatter images show that much of the continental slope on either side of Kaikoura Canyon has moderate backscatter intensity, even in areas of high relief ŽFigs. 2 and 11.. Kaikoura Canyon and some the small Kowhai Canyons to the north are conspicuous because they produce strong backscattering from at least parts of their axes. Upper Kaikoura Canyon produces moderate backscatter except along the base of the steep northern wall and at the wall itself where backscattering is strong. In the central canyon, an area of low backscatter corresponds to the inferred slump or avalanche deposit ŽFigs. 2, 3 and 11.. The lower canyon has strong sonar backscatter across almost the full width of its near-flat floor and this continues into the southern end of the Hikurangi Trough ŽFigs. 2 and 11.. In contrast, to the Kaikoura and Kowhai Canyons, the axis of the large Pegasus Canyon, which begins at the shelf edge to the south, is indistinguishable in its backscatter intensity from adjacent slopes. All grab samples from the continental slope, including the strongly reflective parts of the canyon axes, are muddy ŽCarter and Herzer, 1986.. There is no evidence at the seabed of textural difference that might produce contrasting backscatter. However, cores ŽFigs. 12 and 13. do show major differences between areas of contrasting backscatter ŽFigs. 2 and

Fig. 8. Tracings of 3.5 kHz records of the canyon-head valley and upper end of the main canyon near Goose Bay, with shape of canyon-walls modified from narrow-beam echo soundings. Positions shown in Fig. 3. ŽA. Upper canyon-head valley showing irregular, reflection-free and possibly re-eroded sediment fill. ŽB. Lower canyon-head valley with thin fill. ŽC. Upper part of main canyon with flat, parallel-bedded turbidite fill.

11.. Cores S861, Q313, S862 from the Conway Trough and S863, S864 from upper and middle Kaikoura Canyon ŽFig. 2. have layered silts and thin sands over 3 m thick ŽFigs. 12 and 13.. However, in core S863, from the foot of the northern wall in the upper canyon, 3 m of layered sands and silts rest on a graded pebble gravel ŽFig. 12.. In the central

Fig. 7. Side-scan sonographs of the canyon rim showing topography and backscatter of back-tilted blocks and shelf edge channels. No slant-range correction is applied so that first return is a seabed sounding. Horizontal lines are equivalent to 10 m. Location of sonographs shown in Fig. 3. ŽA. Shows several small canyon-head channels east of Pinnacle Rock, the centre one is apparently clear of sediment, the one on the right is sediment-filled; ŽB. shows a small depression trending sub-parallel with the canyon rim off Oaro and inferred to be formed by a tension at the head of incipient slope failure; low backscatter suggests infilling of the trough with fine sediment; ŽC. shows back-tilted slump blocks on the upper slope off Goose Bay; ŽD. shows small back-tilted slump blocks on the upper slope off Pinnacle Rock.

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Fig. 9. Slope along the canyon axis from canyon-rim shelf edge off Oaro and off Goose Bay to the Hikurangi Trough. The dotted line is a smoothed ‘‘equilibrium profile’’ that may indicate tectonically elevated lower canyon and possible ‘‘ponding’’ of low velocity flows in the upper canyon.

canyon, core S864, with only layered silts and sands, was also obtained away from the axis of the canyon. In contrast, all three cores from the strongly backscattering lower canyon ŽFig. 2. contain a gravel layer only 0.1–0.2 m below the seabed. Core Q312, described by Carter et al. Ž1982. contains two pebble gravel layers each over 0.6 m thick. The others, S875 and S874, penetrated only a top layer of gravel beneath surface mud ŽFigs. 12 and 13.. Radiocarbon dates of shells and a wood fragment from the gravel layers provide a maximum age for emplacement of the gravels ŽTable 2.. The gravel that was buried by over 3 m of layered sands and silts in the upper canyon contained a shell dated at 3300 yr BP ŽFig. 12.. Shell fragments from a top gravel layer at other stations in the lower canyon ranged from about 16,300 yr BP ŽS874. to about 2200 yr BP ŽS875.. However, a sample of twig from the short core with the 2200 yr BP shell gave an age of only 122 " 85 yr BP, which equates to AD 1828 "85 yr. The lower of the two gravel layers cored at Q312 has a fauna of inner shelf mollusca, a Chione shell being dated at 4670 " 150 yr BP ŽCarter et al., 1982.. A twig from the same layer has a radiocarbon age of 251 " 64 yr BP, which equates to AD 1699 " 64 yr. 4.3. Canyon currents Current measurements, which were undertaken for one month, at 5 m above the seabed in the 925 m deep axis of the upper canyon in 1996, show that

semi-diurnal tidal flows are aligned along the canyon and reach up to 0.25 msy1 ŽFig. 14.. The flood tide is diagonally up the slope to the northwest and the ebb tide is down the axis of the canyon to the east. A possible correlation between tides and temperature fluctuations suggests a slight upwelling of cold water in flood tides and a slight downwelling of warmer water during ebb tides. Because ebb tides are slightly stronger than flood tides, there is a net drift down the canyon. Because of the slight northerly component of the flood time, the net drift over a 33 day period averaged 1.5 cmrs towards the northeast.

5. Interpretations and discussion 5.1. Quantities of sediment input The rivers of eastern South Island supply nearly 40 million tonnes of sediment to the oceans each year from the rising Southern Alps. About 2%–5% is of the input is coarse sand and gravel Žfluviatile bedload., about 30% is medium and fine sand, about 60% is mud Žsand and mud together constitute fluviatile suspension load., and about 4% is dissolved load ŽGriffiths and Glasby, 1985; M. Hicks, 1997, personal communication.. Much of the river input drifts northeastwards along the inner continental shelf under the influence of a particularly active hydraulic regime ŽCarter and Herzer, 1979.. Although a detailed sediment budget for eastern South Island is beyond the scope of this paper, we can nevertheless

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Fig. 10. Migrated seismic profile of the central canyon with interpretation below, position in Fig. 3. Shows reflection-free slide or debris avalanche deposit Žscreened in interpretative diagram., with overlying canyon axis turbidites disturbed by dewatering structures.

make some gross estimates of sediment supply to the Kaikoura Canyon. Since the post-glacial rise of sea-level, the largest headlands have at least partly compartmentalised the inshore sediment transport system. To the south of Kaikoura, Banks Peninsula ŽFig. 1., acts as an effective barrier to the transport of gravel and much of the sand from South Island’s large southern rivers. Fine fractions that do move around the peninsula are

mainly trapped in the sheltered, muddy waters of Pegasus Bay, where an eddying circulation also traps much of the sediment from the rivers that flow into the bay ŽHerzer, 1979; Herzer, 1981; Gibb and Adams, 1982; Barnes, 1996.. North of Pegasus Bay, the Hurunui, Waiau and Conway rivers supply sediment directly to the northeastward-moving coastal sediment transport systems that ends at Conway Trough and Kaikoura Canyon ŽCarter et al., 1982..

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Fig. 11. Backscatter imagery of Kaikoura Canyon and adjacent areas. Compare with bathymetry of same area in Figs. 2 and 15. Shows strong backscatter in the lower Kaikoura Canyon ŽKC., in several small Kowhai Canyons ŽKO., and in the proximal Hikurangi Trough ŽHT., but not in the Conway Trough ŽCT. or Pegasus Canyon ŽPC.. ŽData from L’Atalante EM12D survey..

These rivers supply an estimated 6–7 = 10 6 tonnes of suspended sediment annually to the near-shore sediment system ŽGriffiths and Glasby, 1985.. Assuming a density of 1.3 tonnesrm3 for sand and mud ŽGibb and Adams, 1982., they contribute roughly 5 = 10 6 m3ra, equivalent to 5 km3rka. Of this, less than 2 = 10 6 m3ra Ž2 km3rka. is sand. Three small rivers close to Kaikoura are estimated to supply over 1 = 10 6 tonnesra ŽGriffiths and Glasby, 1985.; the two that reach the sea southwest of the peninsula and close to the canyon head probably contribute about 0.7 = 10 6 tonnes, equivalent to 0.5 = 10 6 m3ra Ž0.5 km3rka., of which about 30% or

0.15 = 10 6rka Ž0.15 km3rka. is likely to be sand. Rivers north of the Kaikoura Peninsula probably supply a separate northeastward moving sediment system that contributes little to the canyon, the groyne-like Kaikoura Peninsula limiting feedback to the south. On the continental shelf between the Hurunui, Wairau and Conway rivers and the Kaikoura Canyon, there is zone of modern, mobile sediment only a few kilometers wide ŽCarter and Carter, 1982; Carter et al., 1982; Carter and Herzer, 1986.. Off Haumuri Bluffs, the zone of mobile, fine sand is about 3 km wide and the modern Žpost last glacial. sediment

Fig. 12. Core logs from the axes of Conway Trough and Kaikoura Canyon. Depths down core in meters, with water depth shown below each core log. Scale at top is each core indicates approximate percentage of mud down the core. From proximal Conway Trough to upper Kaikoura Canyon, cores show an increasing proportion of silty turbidites Žthin lines of dots. and sandy turbidites Žthicker stippled zones., compared with hemipelagic mud Žclear.. In the upper canyon, a gravel turbidite is buried by more than 3 m of silty and sandy turbidites. In the lower canyon, gravel turbidites are buried by only a thin veneer of mud. Positions of dated samples are shown with ages in years BP Žbefore 1950.. Young dates are from twigs, older ones are from shells.

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prism is a maximum of 40 m thick. Assuming a modern sediment prism with a similar profile between the southern rivers and Kaikoura Canyon, it is

57

estimated that the volume of mainly sandy modern sediment stored on the 50 km of shelf supplied by the southern rivers is about 3 km3. This is equivalent

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to the total river input for about 600 years or the sand input for about 1500 years. Since deposition on the inner transgressive shelf erosion surface began between 12,000 and 6000 years ago, over 90% of total local river input and over 75% of all river sand Žwith no allowance being made for coastal erosion or

higher inputs in cooler conditions. has disappeared beyond the shelf break. The sediment prism is not simply a product of modern processes. Its lower part is an onlapping record of the later stages of the post-glacial rise of sea-level, with the submerged shorelines at about 56

Fig. 13. Photographs of silt, sand and gravel turbidites. ŽA. X-radiograph of core S863 from upper Kaikoura Canyon, 1.05–1.3 m below the seabed, showing closely spaced silt and sandy turbidites. ŽB. Photograph of core S874 ŽLower Kaikoura Canyon. showing graded pebble gravel to coarse sand turbidite with mud above and below.

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Table 2 Radiocarbon ages of shell and wood samples from gravel layers in cores Core

Position

Water depth Žm.

Core Depth Žm.

Age

Dated sample

S863 S875 S875 Q312 Q312 S874

Upper canyon Lower canyon Lower canyon Lower canyon Lower canyon Lower canyon

1253 1590 1590 1625 1625 1843

3.25 0.2 0.2 1.5 1.05 0.35

3310 " 73 yr BP 2189 " 69 yr BP 122 " 85 yr BP Žc. AD 1828. 4670 " 150 yr BP 251 " 64 yr BP Žc. AD 1699. 16,295 " 83 yr BP

shell piece; top gravel shell piece; top gravel twig; top gravel shallow water shell ŽChione.; lower gravel twig; lower gravel shell piece; top gravel

Dating by Institute of Geological and Nuclear Sciences, Lower Hutt, New Zealand.

m, 46 and 34 m below present sea-level recording temporary halts in both shoreline transgression and onlap of the coastal sediment prism between 12,000 and 9000 years ago ŽLewis, 1973a; Carter et al., 1986.. After sea-level more-or-less stabilised at its present level about 6000 years ago, the inner shelf sediment-prism began to prograde over the transgressive parts of the prism to seaward. The opaque reflector beneath the outer shelf is interpreted to be from the surface of gas-rich sediment that may result from slow, organic-rich deposition in the interval between rapid coastal onlap and subsequent progradation. A similar opaque layer of early to mid-Holocene age occurs beneath the mid and outer shelf elsewhere around New Zealand ŽLewis, 1973a; Carter and Carter, 1986.. If the opaque reflector does represent an early to mid-Holocene hiatus, then the 2 km3 or so of sediment above the opaque layer still represents less than 10% of the total input from rivers and about 25% of sand input since the end of the rise of sea-level. Within the accuracy of the estimates, the proportion of sediment carried beyond the shelf is still about 90% of the total. Thus, it is estimated that, of the 5 = 10 6 m3 supplied to the sediment transport compartment southwest of the Kaikoura Canyon each year, over 4.5 = 10 6 is carried beyond the shelf break. Most of this is mud deposited as ‘‘hemipelagic’’ drape on slopes and in deep-sea troughs. Of the approximately 2 = 10 6rka of sand that is input, maybe about 0.5 = 10 6rka builds out the shelf sediment prism and about 1.5 = 10 6rka is carried along the shelf. Much of this falls into the Kaikoura Canyon. From north of the canyon head to Kaikoura Peninsula, we estimate that there is only about 0.3

km3 of sediment on the continental shelf. Since about 0.5 = 10 6rka is input from the two small rivers there, this volume could have accumulated in about 600 years so that, during the Holocene, over 90% of the input is again carried away. In the dominantly northeasterly sediment transport system that prevails on this coast, much of the lost, mobile, fine sediment is likely to be swept around Kaikoura Peninsula leaving only a minor proportion being

Fig. 14. Current–vector scatter-plots, superimposed on bathymetry, using data from current meter moored 5 m above the seabed, at 925 m deep, in the axis of the upper Kaikoura Canyon from 26 March 1996 to 29 April 1996 showing flood flow shorewards towards the WNW and ebb flow eastwards down the canyon axis.

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input to Kaikoura Canyon. We suggest that less than 0.1 = 10 6rka is likely to be input from this source. Thus, the total input into the upper Kaikoura Canyon is estimated to be of the order of 1.5 km3rka. 5.2. How graÕel, sand and mud reach the canyon Sediment input to the shelf is separated, by shelf hydrodynamic processes, into several distinct size populations that have sharp boundaries with one another. Boulder gravel, pebble gravel, megarippled coarse sand, well-sorted fine sand and sandy mud all form distinct populations that are inferred to move separately to the canyon rim. An indication of the minimum Žthreshold. velocities in which grains of various sizes and densities will move in unidirectional Žtidal and mean. flows may be obtained from empirical formulae developed by Yalin Ž1972., and modified in terms to bed roughness Ža function of grain-size and bedform. according to formulae developed by Lettau Ž1969. in Dyer Ž1980.. These formulae, which have been evaluated in the field by Heathershaw Ž1981., have been applied to New Zealand sediment types similar to those at Kaikoura by Carter and Lewis Ž1995.. Applying these formulae to sediments from Kaikoura ŽTable 3., it is evident that greywacke cobbles and boulders would require a highly unlikely shelf flow of 3.35 mrs to move them. Pebble gravel is stable until velocities reach 0.83 mrs, although it will move at 0.51 mrs if it were formed into low gravel waves Žnot evident in sonographs from this area.. Coarse lithic sand is moved at flow speeds as low as 0.20 mrs in its typical megarippled state, but would require 0.29 mrs if it forms a smooth seabed. Fine quartz sand is moved by flows of 0.19 mrs in its typical rippled bedform but is swept from areas of megarippled coarse sand at speeds of only 0.16 mrs. Oscillatory wave motions initiate sediment movement at somewhat different speeds and at very different maximum depths depending on the size of the waves. Although we have no accurate measurements of wave and swell conditions on the Kaikoura shelf, measurements on the open shelves south of Banks Peninsula and south of Wellington ŽPickrill and Mitchell, 1979. are likely to be broadly similar; each of these places is exposed to the prevailing southerly swell from southern ocean storm centres. In such

situations, a small summer swell has a period of the order of 6 s and a height of about 1.25 m, a ‘‘typical’’ or most frequent swell has a period of about 10 s and is about 2.25 m high, and the largest annual swell has about a 16 s period and a height of about 4.75 m ŽCarter and Lewis, 1995.. The threshold velocities of typical Kaikoura sediments in each of these swell conditions and the depths to which each sediment type might be set in motion are calculated using the methods of Komar and Miller Ž1973. that are summarised in Pickrill Ž1983.. Although the results ŽTable 4. are only a gross approximation, they indicate that cobbles and boulders are rarely moved much beyond the surf zone, small pebbles may be stirred by typical or average swells as far as the canyon rim off Goose Bay, megarippled coarse sand may be set in motion to the outer limit of its occurrence around the canyon head by typical 10 s swells, and well-sorted fine sand is also stirred by 10 s swells to the outer limit of its occurrence. Thus, most sediments are stirred by commonly occurring swells on the shelf around the canyon. The exception are the pebbles and boulders on the outer shelf off Kaikoura Peninsula, which are beyond the reach of modern wave processes and are believed to be relict from lowered sea-level. Gravel input into the Kaikoura Canyon is probably mainly derived locally ŽFig. 15.. A dearth of gravel around rocky reefs that extend into 12–20 m deep sandy seabed near and north of Haumuri Bluffs, indicates that these headlands and reefs provide an effective barrier to gravel migration. Gravel is now compartmentalised within the larger embayments. Between Haumuri Bluffs and Kaikoura Peninsula, gravel is inferred to be derived from several small local rivers, from limited coastal erosion, and from reworking of the post-glacial transgressive surface. In general, boulder gravel is likely to be mainly in situ close to or on the rock pavement from which it was derived. Much of the gravel supply progrades beaches but, where the shelf is narrow and the shelf break shallow at the canyon head, gravel may be stirred by waves and moved as far as the canyon rim by rip currents and underflows that develop in response to a strong onshore component of wind-drift at the sea surface. There, large waves from southeasterly directions will have lost little of their energy on a shoaling shelf before they reach the gravel zone.

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Table 3 Unidirectional threshold velocities in meters per second, measured 1 m above the seabed ŽU100 . for four grain types in three bed roughness conditions known to occur on the Kaikoura shelf. Common conditions in bold and special conditions are in parentheses Grain size

No bedform Ž z 0 s 1 mm.

Ripples Ž z 0 s 3 mm.

Megaripples Ž z 0 s 8 mm.

Cobbles Ž64 mm. Pebble gravel Ž4 mm. Coarse lithic sand Ž0.7 mm. Fine quartz sand Ž0.18 mm.

3.34 m r s 0.83 m r s 0.29 mrs 0.22 mrs

NrA NrA NrA 0.19 m r s

NrA 0.51 mrs 0.20 m r s Ž0.16 mrs.

Relict gravels on the outer continental shelf off Kaikoura Peninsula have not been buried by younger sands and muds, partly because input from the south is limited by the sediment-trapping canyon and partly because fine sediment is unable to settle permanently on a rough seabed where tidal and oceanic currents are constricted and strengthened as they pass a peninsula ŽChiswell, 1996.. Current meter measurements taken 3 m above 90 m deep gravels off Kaikoura Peninsula, as part of a study of biomedicinal sponges, indicate maximum velocities of up to 0.53 mrs, with a mean flow ŽSouthland Current. component of up to 0.25 mrs towards the north ŽPage and Battershill, in press.; this is almost twice the normal velocity along this coast ŽCarter and Herzer, 1979. A combined tidal maximum and mean flow at 90 m may be sufficient to mobilise coarse sand in an area of high bed roughness ŽPage and Battershill, in press., and even pebbles may move when, in addition, they are stirred by large swells ŽTable 4.. The tongues of megarippled coarse sand that extend for up to 1 km seawards from the shore platform to the 30–40 m deep canyon rim ŽFig. 4. have been inferred to be stirred by typical swells in this area ŽTable 4.. The megaripples’ symmetry, and their orientation sub-parallel with bathymetric contours, is compatible with formation by the refracted

wavefronts of large southerly swells. The onshore– offshore trend of megaripple zones implies some additional effect, perhaps concentration of offshore bottom currents that remove covering fine sand, during wind-induced onshore surface flow. Elsewhere around the head of the canyon, fine sand and mud are the dominant size fractions. Although fine sand is easily moved, the narrowness of the shelf at the head of Kaikoura Canyon is an effective barrier to transport so that sand bodies north and south of the canyon head are essentially isolated, although they are derived from hinterlands with similar suites of Mesozoic greywackes, Tertiary marls and Quaternary gravels ŽGregg, 1963; Lensen, 1963.. The sand from the south, which migrates around Haumuri Bluffs on a 3 km wide front between the shore platform and a sharp boundary with Atrina-rich mud at 40 m deep ŽFig. 3., is stirred almost constantly by southerly swells ŽTable 4. enhanced by local wind waves ŽCarter and Herzer, 1979.. Thus, it easily migrates under the influence of mean flow, tides, and storm-generated currents ŽKomar and Miller, 1973.. The mean flow ŽSouthland Current., although relatively weak with surface speeds ranging from 0.07– 0.08 mrs ŽHeath, 1972, 1973. to 0.13–0.14 mrs, and up to 0.25 mrs off a major headlands ŽCarter and Herzer, 1979; Chiswell, 1996., nevertheless pro-

Table 4 Maximum depth of stirring of four grain diameters in three swell conditions, with threshold speeds in parentheses, calculated using formulae of Komar and Miller Ž1973. Grain size

Small swell Ž6 sr1.25 m.

‘‘Typical’’ swell Ž10 sr2.25 m.

Large swell Ž16 sr4.75 m.

Cobbles Ž64 mm. Pebble gravel Ž4 mm. Coarse lithic sand Ž0.7 mm. Fine quartz sand Ž0.18 mm.

- 1 m Ž2.00 mrs. 6 m Ž0. 61 mrs. 15 m Ž0.25 mrs. 19 m Ž0.15 mrs.

2 m Ž2.15 mrs. 17 m Ž0.65 mrs. 39 m Ž0.27 mrs. 52 m Ž0.18 mrs.

7 m Ž2.30 mrs. 34 m Ž0.70 mrs. 122 m Ž0.29 mrs. 175 m Ž0.21 mrs.

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Fig. 15. Diagrammatic representation of modern and low sea-level sediment supply to Kaikoura Canyon and proximal Hikurangi Trough showing localised input of gravel, inner shelf transport of sand and offshore deposition of mud. The major gravel–sand–silt turbidity currents originate in the head of the Kaikoura Canyon, with smaller sand and silt laden flows originating on the shelf near the distal end of Conway Trough During periods of glacially lowered sea-level, the main sediment input is via the Conway Trough and the Pegasus Canyon, rather than at the head of Kaikoura Canyon.

vides a net drift of swell-stirred sediments towards the northeast. Similarly, the tides, which flood strongly to the northeast and ebb more weakly to the southwest, can move shelf sands to the northeast. The combined effect of northeast-flowing, flood tides, northeastward mean flow ŽSouthland Current., southerly storm-induced wind drift and barotropic flow, is more than sufficient to move sand northeastwards along the inner shelf during normal conditions, and on the mid to outer shelf during southerly storm conditions ŽCarter and Herzer, 1979.. In addition, strong southerly winds can induce an offshore component of flow, due to a downwelling of an

onshore sea-surface flow. Strong northeasterly winds during summer combined with spring ebb tides can limit or even reverse the northward coastal drift ŽGreig and Gilmour, 1992.. In a two-steps-forward-and-one-step-back motion, sand from the southern rivers moves northeastwards along the shelf towards the Kaikoura Canyon ŽFig. 15.. Sand in the more slowly moving, outer part of the mobile sand zone, which coincides with the thickest part of the shelf sediment prism, is intercepted by a small, un-named canyon that feeds into the distal end of Conway Trough ŽFigs. 3 and 15.. The inner, more regularly mobile sand continues

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northwards, slowed by the groyne effect of rock and boulder ‘‘reefs’’; a drop in height of a few meters from south to north across the ‘‘reef’’ north of Haumuri Bluffs attests to this effect. Approaching the southern rim of the Kaikoura Canyon, the shelf sediment prism thins to less than 10 m, indicating edge effects associated with the canyon. Fine sand reaches the rim of Kaikoura Canyon at the southern end of the V-shaped canyon-head gully off Oaro, where the canyon rim is 32 m deep, and also along the shoaling rim that trends obliquely inshore to the 18 m isobath off Goose Bay. North of this, rocky reefs and boulder gravel extending to the canyon rim increase turbulence and help to funnel sand offshore into the canyon, providing an effective barrier to transport of sand further to the north. Goose Bay is the end of the main eastern South Island near-shore sand transport system. The mud fraction moves offshore mainly as suspension load ŽFig. 15.. Surface plumes of low salinity, muddy water from rivers move northwards within 5 km of the coast during southerly winds and in diffuse eddies further from shore during northerly winds ŽCarter and Herzer, 1979; Carter et al., 1982.. Some of the suspended mud is carried beyond the shelf break into deep water where it is rarely remobilised. In calm conditions, some settles first on the shelf, where it is remobilised by the action of moderate to large swells, perhaps many times, and moved northwards by currents like the fine sand bedload ŽCarter and Carter, 1985.. Resuspended mud that is swept beyond the shelf break in dilute mid-water or denser bottom-hugging sediment gravity flows is deposited widely on the slope. Some reaches the canyons, where it may settle to be remobilised as part of more concentrated turbidity currents and debris flows ŽFig. 15.. 5.3. Frequent collapse in the canyon head The canyon-head gully of Kaikoura Canyon is positioned to trap a major part of mobile sand that has rounded the groyne-like rocky ‘‘reef’’ to the south and to tap the thickest part of the near-shore sediment prism. Its location and trend suggests that it may be eroded into an offshore extension of the Hundalee Fault’s shatter zone ŽGregg, 1963. ŽFig. 2..

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Seismic reflection data indicates that soft sediment has accumulated in the canyon-head gully ŽFig. 8. on a steep, earthquake-prone slope. Side-scan sonar evidence of tensional head-faults suggests that the rim of the gully is subject to incipient slope failure ŽFigs. 4 and 7.. The extent of potentially instability is inferred to include sediment, mainly sand, that is accumulating rapidly in the canyon-head gully. Assuming a failure surface that outcrops near the bottom of the gully-fill, potentially unstable material in the canyon head is estimated to cover an area of about 3 km2 and to average about 80 m thick. Thus, it has a volume of the order of about 240 = 10 6 m3 Ž0.24 km3 .. If most of the metastable material is soft sediment, and if indeed about 1.5 = 10 6 m3 of sediment pours into this part of the canyon head each year, then the metastable deposit accumulated in the order of 160 years. If incipient failure includes some Tertiary mudstone and limestone that outcrops around the canyon rim ŽCarter et al., 1982. then the accumulation time may be less. The rough estimate of 160 years required for accumulation of the canyon-head deposit is realistic given the presence of a 170 ŽAD 1828. "85 year old twig in the top gravel layer. It is perhaps significant that there has been no large seismic event centred in the Kaikoura area since historic records began there in about 1840, but lichen-dating of rock-falls indicated a major earthquake caused extensive slope failure onshore in the vicinity of the Hope Fault ŽFig. 2. in about 1833 ŽBull and Brandon, 1998.. We infer that the same severe ground-shaking that caused wide-scale rockfalls onshore may also have cause failure of metastable sediments and flushing out of the canyon-head gully offshore. The modern deposit has accumulated since then. In a plate-boundary environment, seismic ground motions would seem a more plausible trigger than the severe storm events those that have triggered catastrophic failure elsewhere ŽTsutsui et al., 1987.. Rupture of neighbouring plate-boundary faults is capable of producing MM VIII intensity earthquakes at Kaikoura and the recurrence interval for such events is of the order of a century or two ŽVan Dissen, 1991.. Extensive rockfalls occur onshore with about the same frequency ŽBull and Brandon, 1998.. The lower of two gravel layers cored in the canyon contained a twig dated at AD 1699 " 64 years and it

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is possible that this indicates collapse in the canyon head at the same time as an earlier period of extensive hillside collapse in the vicinity of the Hope Fault in about AD 1728 ŽBull and Brandon, 1998.. Since the inferred head-scarps cuts obliquely across the shelf contours and includes areas of gravel as well as sand and mud, they suggest how both modern and basal transgressive gravel of late Pleistocene and early Holocene age might be included in large-scale sediment-gravity flows down the canyon. Gravel might also be input along the northern wall. There are small scarps, interpreted as head scarps of incipient slope failures, near the canyon rim off Pinnacle Rock ŽFig. 3.. In addition, the northern shelf is exposed to stirring by the full force of southerly storms and to offshore bottom currents that result from trapped onshore surface setup. These may be carrying sediments, including perhaps the basal transgressive gravel, over the canyon rim. Strong backscatter and buried pebbles along the toe of the northern wall may be partly from this source and irregular topography, notably at the bend adjacent to the outer shelf, suggests failure of the Tertiary sediments into which the canyon is cut ŽCarter et al., 1982.. 5.4. Deposition in the canyon axis The mud that drapes most of the continental slope, including the main canyon axis, is inferred to be mainly a ‘‘hemipelagic’’ rain, derived directly from offshore dispersing river plumes ŽCarter et al., 1982., and also from shelf mud resuspended during storms. In some cases, resuspended mud may form clouds that are dense enough to hug the bottom or spread out at some mid-water level. Although the canyon is now also floored by mud, cores show that silt and sand-charged turbidity currents have repeatedly flooded both Kaikoura Canyon and the lower Conway Trough, and flows heavily charged with gravel have flowed down Kaikoura Canyon ŽFigs. 12 and 15.. Silty and sandy turbidites in cores from the Conway Trough ŽFig. 12. indicate some input from the shelf south of Kaikoura Canyon. A very few, thin, wispy silt layers in the southern, proximal Conway Trough indicate minimal input where the shelf is relatively wide. However, an increasing proportion

of graded sand layers with 30%–67% sand near their base in the middle and distal Conway Trough suggests an increasing frequency of input of dilute flows via canyons that intersect the shelf mobile sediment prism ŽFigs. 2 and 15.. A layer with angular mudstone pebbles in the distal Conway Trough suggests some failure of canyon walls. Core S862 from the upper Kaikoura Canyon has an average of more than three graded sand turbidites per meter, and silt layers that, in places, are only 20 mm apart ŽFig. 13A.. A reduced level of bioturbation compared to the Conway Trough might also indicate a higher frequency of fauna-destroying turbidites in the canyon. Although some of the sand and silt turbidites may be derived from the Conway Trough, many silts, sands and the muddy pebble gravel at 3 m below the seabed are inferred to have come from the walls of Kaikoura Canyon. The gravel could have come from the canyon-head, but, because the core is from the toe of the precipitous northern wall, it could also have come from above. The gravel could be of any age less than 3310 years old, the age of shell fragment extracted from it; it could correlate with the inferred 1833 gravel in the lower canyon. If the muddy gravel indicates large-scale failure during earthquake stress and perhaps transported by grainflows or debris flow processes ŽLowe, 1982., then the eight sandy turbidites and at least 33 silty turbidites may record relatively dilute turbidity currents, perhaps those formed by mobilisation of shelf sediments during major storm events. Late Quaternary deposition in the upper canyon may be at least partly the result of ponding above slump or debris avalanche deposits from the walls; these deposits have not yet been removed by the erosive forces that carved the canyon and presumably removed many other such deposits. However, larger flows have re-established a relatively even axial profile around the toe of the large mid-canyon avalanche deposit ŽFig. 10. by deposition above and apparently below it. Core S864, from the western flank of the central canyon above the axial strong backscatter, containing only sandy and silty turbidites. It presumably records the upper parts of flows rather than their coarsest fractions The lower canyon, below about 1500 m deep, has gravel beneath a thin Ž0.1 m. surface mud layer over a wide area. Strong seabed backscatter ŽFig. 14.

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suggests that the near seabed gravels overlie the whole of the 1–5 km wide, flat or slightly domed, canyon floor as far as the head of the Hikurangi Trough. The ages of shells from the top gravel layer range from 2189 yr BP to 16,300 yr BP but gravel and shells may be redeposited from a variety of layers around the canyon rim, including the Late Pleistocene post-glacial transgressive surface; the age of the shells is only a maximum possible age for the deposit, not the age of the deposit. This is clearly demonstrated by the two samples that contained both shell and twigs, the shell being considerably older than the twig. However, can it be assumed that the top gravel layer at all places in the canyon is the same deposit? The flanks of the upper and middle canyon may record only the upper parts of some gravel-charged flows, and gravelly flows may spread across the wide lower canyon, perhaps in a braided pattern to be deposited preferentially in different places on different occasions. It has been inferred that canyon-channel systems tend towards a smooth, equilibrium, axial profile, much like river systems, and they do this by erosion and deposition that compensate for tectonic and slope-failure changes to the equilibrium profile or to base level ŽCarter, 1988.. Since Kaikoura Canyon is in an active tectonic zone, with inferred active thrust faulting, anticlinal growth and slope failure on the adjacent continental slope ŽLewis and Pettinga, 1992; Barnes, 1994., it may be assumed that these processes are continuing to modify the canyon profile, which responds to restore the profile by deposition in some segments and erosion in others. In the Kaikoura Canyon, it may be that tectonic uplift has effectively lifted the floor of the central section causing erosion with slope failure there and deposition above and below. 5.5. Long distance flows from Kaikoura? It is well documented that earthquakes produce large-scale slope failure and turbidity currents capable of travelling long distances ŽHoutz and Wellman, 1962; Krause et al., 1970; Hughes Clarke et al., 1990; Garcia and Hull, 1994; Garfield et al., 1994.. Major earthquakes, related to dextral offset on neighbouring plate boundary faults, are expected once every few centuries ŽVan Dissen and Yeats, 1991.

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and each might be expected to increase pore pressure, induce high sensitivity and initiate failure in soft, rapidly deposited sediments ŽMorgenstern, 1967.. In addition, storms on narrow, exposed shelves can generate breaking internal waves and edge-waves with an antinode in the canyon head capable of resuspending large quantities of sediment into a riplike flow ŽInman et al., 1976.. Once started by whatever mechanism, failure in the head of the Kaikoura Canyon may grow by headward erosion at the shelf edge and by entraining down-slope sediments deposited since the last major event ŽHughes Clarke et al., 1990.. It may ‘‘ignite’’ into a major, self-perpetuating, turbidity current ŽParker, 1983.. Margin failures that deposited a significant gravel layer must have been large. If the 0.6 m thick gravel layers in core Q312 covered much of the lower canyon, then the gravel fraction alone may have been almost 10 = 10 6 m3. Since most of the sediment pouring into the canyon head is sand, this implies that the total volume of sediment that failed and transformed into a turbidity current had a volume of at least an order of magnitude greater, that is, of the magnitude of the present metastable block in the canyon-head gully. The mechanism by which a soft sediment slide or debris flow metamorphoses into a turbidity current has been inferred mainly from physical and theoretical modelling ŽAllen, 1971.. A collapsing pile of sediment that moves rapidly down slope rapidly increase its volume by two to six times after entraining water via regularly spaced tunnels beneath its overhanging head. On a steep slope, the resulting flow can become supercritical, resulting in additional mixing at the upper face. If there is an erodable substrate, particularly one that includes mud deposited since the last major flow, then the leading edge of the flow scours fine sediment while the tail deposits coarse sediment ŽSimpson, 1987. and the flow itself may rapidly attain a stable state of self perpetuating autosuspension ŽBagnold, 1962; Kenyon et al., 1995.. In some cases, deposition of course, gravelly sediment in the lower parts of canyons is attributed to hydraulic jump, where a flow suddenly reduces its velocity and increases its thickness as it passes from a supercritical, high velocity phase to a subcritical, low velocity, high turbulence phase, with its consequent change from kinetic to potential en-

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ergy ŽRothwell et al., 1992.. The hydraulic jump increases turbulence, reduces density and mean grain size and increases fluid entrainment to a level that is consistent with a long distance, self perpetuating flow ŽKomar, 1971; Pantin, 1979.. If the slope is steep, a turbidity current may even ‘‘explode’’ or ‘‘ignite’’ into a self-accelerating, ignitive autosuspension, which erodes more than it deposits, thereby increasing its power and its ability to erode yet more until a state of ‘‘catastrophic equilibrium’’ is reached ŽPantin, 1979, 1983; Parker et al., 1986.. Whether a flow attains this state or not may depend more on the availability of erodable sediment, and hence the time and rate of deposition since the last such flow, than on its initial characteristics. In the central Hikurangi Trough, tephra chronology suggests that turbidity currents originate at Kaikoura once every few centuries ŽLewis, 1994.. A general formula for autosuspension currents, one that balances gravity against friction ŽPantin, 1979., estimates that an intermediate-sized flow may reach a speed of about 30 mrs in the lower Kaikoura Canyon ŽH.M. Pantin, pers. commun., 1998. and, provided that there is an excess of sediment available for entrainment, such flows may travel for great distances and deposit sediment over enormous areas ŽGarcia and Hull, 1994; Hesse, 1995; Klaucke et al., 1997; Klaucke et al., 1998.. In this area, it has been suggested that flows originating at Kaikoura can travel for over 1500 km and deposit sediment over all of the Hikurangi Trough, the central Hikurangi Plateau and the distal Hikurangi fandrift ŽFig. 1; Lewis, 1994; Carter and McCave, 1994.. Theory also predicts that, in any particular morphological setting, catastrophic equilibrium flows will tend towards similar velocities and concentrations, producing the remarkable uniformity of turbidites that occur in modern and fossil basins ŽPantin, 1983..

6. Conclusions Ø Kaikoura Canyon is presently a major conduit for sediment between the inner continental shelf and the deep-sea. It is a major sink for the northwardmigrating near-shore sediment drift off eastern South Island and a major source to the turbidite basins off eastern North Island.

Ø The canyon is 60 km long, U-shaped in profile, and incised 1200 m below the adjacent outer shelf and upper slope. It follows a sigmoidal Ž; shaped. course from close to the shore to the 2000 m deep apex of the Hikurangi Trough where it merges into the 1500 km long Hikurangi Channel. Ø The canyon rim is only 18 m deep and 500 m from shore Ž200 m from the end of shore-platformconnected, rocky ‘‘reefs’’. at its shallowest point. It is remote from any major rivers. Ø A V-profiled canyon-head gully, which is aligned sub-parallel with the coast, traps sandy and muddy sediment migrating along the shelf from the south. The gully’s left wall trends obliquely landward from 32 m deep to 18 m deep incising zones of megarippled coarse sand, pebble gravel and even boulder gravel along a narrowing shelf. Coarse sand and pebbles are inferred to be carried over the canyon rim in storm conditions. Ø Rough estimates suggest that about 1.5 = 10 6 3 m ra of mainly sandy sediment falls into the canyon head to form a metastable deposit in the canyon-head gully that now has a volume of about 240 = 10 6 m3, representing about 160 years of input. Ø The upper canyon contains sandy and silty turbidites that may be ponded behind major slope failures or tectonic elevations. There is a gravel layer 3 m beneath sandrsilt turbidites. Ø Gravel, with only a thin hemipelagic cover, occurs widely in the wide lower canyon. Ø Gravel turbidites contain shells dating from the post-glacial rise of sea-level to late Holocene age but these record the age of the shelf sediments that fell into the canyon head, rather than the age of the turbidity current that carried them to the lower canyon. A wood fragment from the top gravel layer dates from AD 1828 Ž"85 years. and suggests that the last gravel turbidite may have been triggered by the same plate boundary fault rupture that produced extensive slope failure on land in about 1833. Large earthquakes at Kaikoura and turbidites in the southern Hikurangi Trough have been inferred to occur every few centuries. Acknowledgements Our gratitude to Jean-Yves Collot of ORSTOM, Villefranche, Jean Delteil, University of Nice, and

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IFREMER, France for permission to use the deepwater swath bathymetric and seismic data collected during the joint France–New Zealand GeodyNZ Project. We are also grateful to Greg Foster, NIWA, for collection and processing of most of the shallowwater datasets, to Bryan Davy, IGNS, Lower Hutt, for processing the seismic profile across the lower canyon, and to Richard Garlick, NIWA, for preparation of maps and figures. Lionel Carter, NIWA, made considerable improvements to the draft manuscript, and journal referees Douglas Masson, Southampton Oceanographic Centre, UK and Cecilia McHugh, Queens College, University of New York both provided constructive critiques that enhanced the readability and content of our paper. References Allen, J.R.L., 1971. Mixing at turbidity current heads, and its geological implications. J. Sedim. Petrol. 41, 97–113. Bagnold, R.A., 1962. Auto-suspension of transported sediment; turbidity currents. Proc. R. Soc. Ser. A 265, 315–319. Barnes, P.M., 1994. Continental extension of the Pacific Plate at the southern termination of the Hikurangi subduction zone: the North Mernoo Fault Zone, offshore New Zealand. Tectonics 13, 735–754. Barnes, P.M., 1996. Active folding of Pleistocene unconformities on the edge of the Australian–Pacific plate boundary zone, offshore North Canterbury, New Zealand. Tectonics 15, 623– 640. Barnes, P.M., Mercier de Lepinay, B., Collot, J.-Y., Delteil, J., ´ Audru, J.-C., 1998. Strain partitioning in a transition zone between oblique subduction and oblique continental collision: Hikurangi margin, New Zealand. Tectonics 17, 534–557. Belderson, R.H., Kenyon, N.H., Stride, A.H., Stubbs, A.R., 1972. Sonographs of the Sea Floor — A Picture Atlas. Elsevier, Amsterdam, 185 pp. Bell, D.H., 1976. High intensity rainstorms and geological hazards: cyclone Alison, March 1975, Kaikoura, New Zealand. Bull. Int. Assoc. Eng. Geol. 14, 189–200. Bibby, H.M., 1981. Geodetically determined strain across the southern end of the Tonga Kermadec Hikurangi subduction zone. Geophys. J. R. Astr. Soc. 66, 513–533. Bull, W.B., Brandon, M.T., 1998. Lichen dating of earthquakegenerated regional rockfall events, Southern Alps, New Zealand. Geol. Soc. Am. Bull. 110, 60–84. Carter, R.M., 1988. The nature and evolution of deep-sea channel systems. Basin Res. 1, 41–54. Carter, R.M., Carter, L., 1982. The Motunau fault and other structures at the southern edge of the Australian–Pacific. Tectonophysics 88, 133–159. Carter, R.M., Carter, L., 1985. The Motunau Fault revisited. Tectonophysics 5, 164–166.

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