JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106,NO. B9, PAGES 19,271-19,297,SEPTEMBER 10, 2001 The giant Ruatoria debris avalancheon the northern Hikurangi margin, New Zealand: Result of oblique seamountsubduction Jean-Yves Collot UMR G6osciencesAzur, Institut de Recherchepour le D6veloppement,Villefranche sur rner, France Keith Lewis and Geoffroy Lamarche National Instituteof Water and AtmosphericResearch, Wellington, New-Zealand Serge Lallemand Laboratoirede G6ophysique,Tectonique et S6dimentologie,Universit6 de MontpellierII, Montpellier,France Abstract. Despiteconvergent margins being unstablesystems, most reports of huge submarineslope failure havecome from oceanicvolcanoes and passivemargins. Swath bathymetryand seismicprofiles of the northernHikurangi subduction system, New Zealand,show a tapering65-30 km wideby 65 km deepmargin indentation, with a giant,3150+_630 km 3, blocky, debrisavalanche deposit projecting 40 km out acrosshorizontal trench fill, and a debris flow depositprojecting over 100 km. Slide blocksare well-bedded,up to 18 km acrossand 1.2 km high,the largestbeing at the avalanchedeposit's leading edge. Samples dredged from themare mainly Miocene shelfcalc-mudstones similar to thoseoutcropping around the indentation.Cores from coverbeds suggest that failure occurred-170 +_40ka, possiblysynchronously with a major extensioncollapse in the upperindentation. However, the northernpart of the indentationis much older.The steep,straight northern wall is closeto the directionof plate convergenceand probably formedaround 2.0-0.16 Ma as a largeseamount subducted, leaving in its wake a deepgroove obliquelyacross the marginand an unstabletriangle of fracturedrock in the 60ø anglebetween grooveand oversteepenedmargin front. The trianglecollapsed as a blockyavalanche, leaving a scallopedsouthern wall andprobably causing a largetsunami. Tentative calculations of compacted volumessuggest that the indentation is over 600 km 3 largerthan the avalanche, supporting a two- stageorigin that includes subduction erosion. Since failure, convergence has carried the deposits -9 km back towardthe margin,causing internal compression. The eventualsubduction/accretion of the Ruatoriaavalanche explains the scarcityof suchfeatures on activemargins and perhapsthe nature of olistostromes in fold belts. 1. Introduction Large submarineslope failure occursin a variety of forms that can be categorized by what can be regarded as end- Submarine avalanches and debris flows can be enormous. members of a continuum of gravitational processes. Perhaps Thosethat occuron slopesbetween land and deepocean basins the largest submarine landslides are rotational slumps that can be several orders of magnitude larger than the largest involve the slow or intermittent, downslope movement of landslidesonshore [Hampton et al., 1996]. They can involve largely intact, back-tilting blocks on glide planes as much as the catastrophicmovement of hundredsor even thousandsof 10 km below the seabed [Moore and Normark, 1994]. On the cubic kilometers of broken rock and sediment. They are a other hand, large catastrophic slope failure occurs as threat to offshorestructures, such as cables and platforms, and disaggregateddebris avalanches, with blocks up to many they can devastatecoastal areas both by onshoreretrogression kilometers across and run-out distancesof many tens to more at their head [Coulter and Migliaccio, 1966; Mulder and than a hundredkilometers [Bugge et al., 1987; Moore et al., Cochonat, 1996] and by generation of large tsunamis 1989; Moore and Normark, 1994]. Similar, but generally [Bondevik et al., 1997; Moore and Moore, 1984]. Ancient smaller, thinner, and more disaggregatedsediment slurries, masses of broken blocks have been described as "chaos with fewer and raftedblocks, are generallyreferred to as debris deposits",melanges or olistostromesin fold belts aroundthe flows [Enos, 1977; Masson et al., 1998]. They travel further world lAbbate et al., 1970; Ballance and Sporli, 1979; Hsu, than debris avalanches,perhaps because they travel faster in 1974; Naylor, 1981; Orangeand Underwood,1995], with the same environment [Jacobs, 1995; Weaver, 1995], and part debate often centering on whether particular deposits are of them may incorporatewater and mudto metamorphoseinto gravitationalor tectonicin origin. turbidity currentscapable of travelling a thousandkilometers or more [Garcia and Hull 1994]. Copyright2001 by the AmericanGeophysical Union. Massive margin failure can occurin a variety of geologic Papernumber 2001JB900004. settings. Perhaps the best documentedare on the flanks of 0148-0227/01/2001J B 900004509.00 oceanic "hot spot "volcanoes, wherequenching of lava has 19,271 19,272 COLLOT ET AL.: GIANT RUATORIA DEBRIS AVALANCHE, NEW ZEALAND critically oversteepenedslopes [tlolcomb and Searle, 1991]. [Mooreet al., 1976],the Aleutian Trench [Lewis et al., 1988], At Hawaiian Islands [Jacobs,1995; Moore et al., 1989; Moore Peru[Bourgois et al., 1993; Duperretet al., 1995; von Huene andNormark, 1994], Canary Islands [Masson, 1996; Masson et al., 1989],Costa Rica [Hinz,1996], and Japan [Cadet et al., et al., 1998; Urgeleset al., 1997] and Fournaisevolcano near 1987]. Subductionof oceanic asperities, commonly Reunion Island [Lenat et al., 1989], it has been shown that seamounts,have produced indentations in convergentmargins enormous rotational slumps, debris avalanches, and debris aroundthe world, with only small landslidesin their wake flows, some thousand of cubic kilometers in volume and [Lallemand et al., 1990]. ' extendingabove sea level, have collapsedcatastrophically In this paper, we documentthe massive Ruatoria debris into the surroundingdeep oceanbasin. avalancheand debris flow associatedwith a large-scale, Passivemargins are also the location of large submarine morphologicindentation of the Hikurangisubduction margin slope failure. In some cases, failure is associatedwith excess east of North Island, New Zealand. We interpret the pore pressurein sedimentaryrocks being maintainedby gas, indentationand slopefailure association from geophysical often from unstableclathrates [Bugge et al., 1987; Carpenter, datato suggestthat they result primarily from the Quaternary 1981; Lerche and Bagirov, 1998]. In other cases, failure subductionof a large seamount. We then focus on the results from rapid sedimentoverloading or tectonic stresses dynamicsof avalanchingand massbalance calculations, infer resultingfrom, among other things, isostatic rebound[Bugge that oblique seamountimpact encourageslarger margin et al., 1987]. Notable examplesoccur off Norway [Buggeet collapsecompared with orthogonalconvergence, and finally al., 1987; Jansen, 1987], South Africa [Dingle, 1980], and discussthe apparent scarcity of such features on active Northwest Africa [Massonet al., 1998; Weaver, 1995]. margins. Many of the samecauses of instability occurat convergent margins, where active forearc slopes are maintained at a 2. GeologicalSetting of Ruatoria Indentation critical angle, suggestingthat they should be a privileged and Avalanche location for catastrophicslope failure. Continental collision zones such as the Gibraltar Arc are the location of giant, The Ruatoria indentation and avalanche are located at the submarine,chaotic bodies [Torelli et al., 1997]. Moderate- to northern extremity of the Hikurangi margin, offshore from large-sized landslideshave been reportedfrom the SundaArc EastCape (Figure 1). The Hikurangi margin is at the southern Figure 1. Location of the Ruatoriaavalanche and margin indentation. Flaggedline is the convergentplate boundarybetween subductingPacific Plate and the edge of the Australian Plate east of the back arc Havre Trough and Taupo Volcanic Zone (TVZ), referred to as the KermadecForearc. The Hikurangi Plateau is thickened,seamount-studded oceanic crust being subductedat the sediment-starvedsouthern Kermadec Trench and sediment-filledHikurangi Trough. EC is EastCape. HC is the HikurangiChannel. AF is Awanui Fault. COLLOT ET AL.: GIANT RUATORIA DEBRIS AVALANCHE, NEW ZEALAND 19,273 PAC-AUS RuatoriaKnoll .;-' --.• /, ß , --• ?•... / -k x764 Core Site Gisborne Seamount , e- X765 DredgeSite -- Single-channelseismic reflection profiles ? EM12DMultichannel and MR1seismic multibeamreflection profiles 7,9ø30' I 18iø W177ø30 ' Figure2. Geophysicallines, rock samples and cores used in thisstudy. Bathymetry isat 100-m intervals. Insetshows convergence vectors between Pacific Plate (PAC), Australian Plate (AUS), and Kermadec Forearc (KER)relative to thedeformation front (flagged line). KER-AUS vector was estimated from back arc kinematics.Back arc extension rates decrease from 15-20 mm yr-• in the southernHavre Trough [Wright, 1993]to 8-12 mm yr '• inthe Taupo Volcanic Zone, on the basis of onshore geodetic triangulations [Walcott, 1987]and integration ofGPS measurements [Darby and Meertens, 1995]. The direction of extensionranges fromN124øE +_ 13 to N135øE,on thebasis of GPSdata [Darby and Meertens, 1995] and earthquake T axis azimuths[Anderson etaI., 1990: Pelletier and Louat, 1989]. Averaging these values, we estimate a rate of back arcopening (KER-AUS) of 12.5 mm yr -• in a directionN135øE tbr the latitude of theRuatoria indentation. Usingthese values, the PAC-KER convergence is54 mm yr -• in a directionN277øE. end of the Tonga-Kermadec-Hikurangisubduction system, volcanic seamounts(over 1 km high) and smaller knolls are whereconvergence between the PacificPlate (PAC) and the elongatedor alignedin ridgestrending N150øE+ 20 ø [Collotet overriding AustralianPlate