Dec 10, 1989 - and Clark [1930] and Stearns and Macdonald [1946] sug- gested that ..... ken into large blocks in their midsections, and terminate in an apron of ...
JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 94, NO. BI2, PAGES 17,465-17,484, DECEMBER
Prodigious Submarine Landslides on the
maWallan "
10, 1989
txluge
J. G. MOORE, 1 D. A. CLAGUE, • R. T. HOLCOMB,2 P. W. LIPMAN, 3 W. R. NORMARK, 1 AND M. E. TORRESAN1 The extensive area covered by major submarine mass wasting deposits on or near the Hawaiian Ridge has been delimited by systematicmapping of the Hawaiian exclusive economic zone using the side-looking sonar system GLORIA. These surveys show that slumps and debris avalanche deposits
areexposed overabout100,000 km2 oftheridgeandadjacent seafloor fromKauaito Hawaii,covering an area more than 5 times the land area of the islands.
Some of the individual
debris avalanches
are
morethan200kmlongandabout5000km3 in volume,rankingthemamongthelargestonEarth.The slopefailures that produce these depositsbegin early in the history of individual volcanoes when they are small submarine seamounts, culminate near the end of subaerial shield building, and apparently continue long after dormancy. Consequently, landslide debris is an important element in the internal structure of the volcanoes. The dynamic behavior of the volcanoes can be modulated by slope failure, and the structural features of the landslides are related to elements of the volcanoes including rift zones and fault systems. The landslides are of two general types, slumps and debris avalanches. The slumps are slow moving, wide (up to 110 km), and thick (about 10 km) with transverse blocky ridges and steep toes. The debris avalanches are fast moving, long (up to 230 km) compared to width, and thinner (0.05-2 km); they commonly have a well-defined amphitheater at their head and hummocky terrain in the lower part. Oceanic disturbance caused by rapid emplacement of debris avalanches may have produced high-level wave deposits (such as the 365-m elevation Hulopoe Gravel on Lanai) that are found on several islands. Most present-day submarine canyons were originally carved subaerially in the upper parts of debris avalanches. Subaerial canyon cutting was apparently promoted by the recently steepened and stripped slopes of the landslide amphitheaters.
INTRODUCTION
New perspectives on the extent and character of submarine slope failures have been obtained with the systematic mapping of the Hawaiian exclusive economic zone (EEZ) using the side-looking sonar system GLORIA (Geologic Long-Range Inclined Asdic). In the fall of 1988, GLORIA surveys extending 200 nautical miles from land were completed in a zone from the southeast end of the Hawaiian Ridge adjacent to the island of Hawaii northwest to beyond the island of Kauai. In this region which includes the principal Hawaiian islands, 17 well-defined major landslides were mapped (Figures 1 and 2 and Table 1), and remnants of several others partly covered by younger deposits also were found. This report documents the characteristics of these landslides, examines their morphologic variations, and attempts to relate them to the processesthat built and modified the volcanic systems of the Hawaiian Ridge. The study of submarinemass-wastingdepositscomposed of massive or poorly bedded volcanic rocks presentsdifficult problems not encountered in the better understood deposits composed of sedimentary materials. Slumps, block slides, debrisflows, and other suchprocessesare widely recognized in nearly all types of continentalmargin settings(seereviews by Moore [1978], Embley [1982], Kenyon [1987], Coleman and Prior [1988], Lee [1989]). These depositsare common in areas of high sedimentation rates and include thick accumulations of sediment with slope-parallel bedding. Features produced by sediment failure are well described on modern
•U.S. Geological Survey,MenloPark,California. 2U.S.Geological Survey,Universityof Washington, Seattle. 3U.S.Geological Survey,Denver,Colorado. This paper is not subject to U.S. copyright. Publishedin 1989 by the American Geophysical Union. Paper number 89JB02753.
delta slopes, where the relatively shallow water allows high-resolution surveys using side-looking sonar, reflection profiling, borehole drilling, and measurementsof geotechnical properties [Coleman and Garrison, 1977; Prior and Coleman, 1982, 1984;Prior et al., 1984a, b]. In slope regions with lower sediment accumulation rates, earthquakes are commonly the favored triggering mechanism for ground failure over areas of a few hundred square meters to as much
as 80,000km2 [e.g., Field et al., 1982;Normark, 1974; Kenyon, 1987] (see also review articles cited earlier). The larger mass wasting deposits from continental margins extend well into deep water and, including the debris flows associated with many submarine slope failures, may reach
several hundred kilometers in length [Jacobi, 1976;Bugge et al., 1988]. Because much of the deposits from these larger failures lie in deep water (>2000 m), less is known about their internal structure and composition as compared to continental shelf structures (see discussionby Normark and Gutmacher [1988]). The Storegga submarine slide on the
Norwegiancontinental marginexceeds5000km3 in volume and is one of the largest mass failures [Bugge et al., 1988]. It apparently represents three episodesof failure, the second of which may be associated with a tsunami deposit along the east Scottish coast [Dawson et al., 1988]. Slope failures on sedimented continental margins are comparatively easy to document as compared to those on volcanic slopes. The small-scale failures in bedded sediment appear on high-resolution reflection profiles because the missing sediment section is obvious, and the mass wasting depositsshow as lens- or wedge-shapedunits with irregular, hummocky surfaces and little or no coherent internal structure. Seismic reflection profiles can discern these deposits even when buried by later sediment, and both the scar and base of sediment mass-wasting deposits can be clearly recognized. In contrast, mass failure on the submarine slopes of oceanic volcanoes must be recognized primarily on the basis
17,465
17,466
MOORE ETAL.: SUBMARINE LANDSLIDES, HAWAII
Fig. 1. Bathymetric mapof thesoutheastern Hawaiian Ridgeshowing majormass wasting features (heavy dashed
lines)identifiedby numberin text and Table 1. Contourinterval200 fathoms(366m). Basefrom U.S. Naval Oceanographic Office [1973].
of surfacefeaturesasthebasalticrockandrubblearenearly Dana's faulting model was rejected by Hitchcock [1900], opaqueacoustically.Standardseismicreflectionprofiling who believed that erosion alone was sutficient to form the systemsgenerally do not provide subsurfaceinformation on cliffs without the additionalcomplexityof faulting. Wentthe internalstructureof slidecomponents or of the slopes worth [1927], comparing estimates of marine and fluvial
where they originated.With relatively slowaccumulationof
erosion rates, concluded that marine erosion alone was
primarilypelagicsediment,the HawaiianRidgeslopeadja- insutficient to producesomeof the largersea cliffs (e.g., centto the islandshasinsutficient sedimentcoverto provide thoseof northMolokai) and that they were probablypromuch assistancein defininglandslidefeatures.
PREVIOUS WORK ON HAWAIIAN SLOPE FAILURES
duced by faulting.
Theideaof flankfaultingeventuallyledto theconceptthat the faults bound the headwallsof giant landslides.Stearns
The role of major landslidingin shapingHawaiian volca- and Clark [1930]and Stearnsand Macdonald[1946]sugnoeshaslongbeena subjectof debate.An earlycontroversy gestedthat the 50-km-longHilina fault systemon the south concernedthe relative importanceof faultingversusmarine flankof Kilaueavolcanoowesits originto, andis at the head erosion in producinghigh cliffs along the coastsof some of, a majorlandslideon the steepseawardslope.Macdonald islands.The highcliffsof northeastOahu(NuuanuPali), as [1956]later discountedlandsliding,suggesting that largewell as thoseof eastNiihau, the Napali coastof Kauai, and volumeintrusionbeneaththe volcanocenterforciblyupnorthernMolokaiwereinterpretedby Dana [1890,p. 290]as lifted the summitcausingfaultingon the volcanoflank. resultingfrom "profoundfracturingof the mountaindome" MooreandKrivoy [1964]revivedthe landslideconceptfor and "downdrop" of the seaward part beneath the sea. Kilauea'ssouthflank and suggested that not only is the
MOOREET AL..' SUBMARINELANDSLIDES,HAWAII
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Fig. 2. Map of southeastern HawaiianRidgeshowingmajorslidesboundedby dashedlinesidentifiedby number in text andTable t; comparewith Figure1. Dottedarea,hummockyground(widelyspacedwheresubdued);hachured lines,scarps;thin, downslope-directed lines,submarine canyonsandtheir subaerialcounterparts; heavydashedline, axis of the Hawaiian Deep: dash-dottedline, crest of the Hawaiian Arch.
Hilina fault system related to gravitational sliding but also the pre-Hawaiian seafloorand hence that the slide was as that the east rift zone of the volcano forms the upper margin much as 10 km thick. Episodic movement of this landslide of a large seawardmoving slump. An analysisof geodetic has been incorporated into a model for the long-term erupand seismic evidence led Swanson et al. [1976] to support
tive behavior of Kilauea [Holcomb, 1987].
this notion of a mobile south flank of Kilauea and to add the
Irregular blocky topographicwelts extending downslope from giant amphitheaterson the submarinenorth flanks of Molokai and Oahu were interpreted as representinggiant landslidesby Moore [1964]. Macdonald and Abbott [ 1970, p. 365], however, questioned the prodigious size of these features, comparedwith previously known landslides,and proposedthat the blocky seamountsare in situ volcanoes. The results of three separate oceanographicinvestigations by Mathewson [1970], Langford and Brill [1972], and Andrews and Bainbridge [1972, p. 112] causedeach of them to reject the landslidehypothesis,with the latter two studies fhvoringthe in situvolcanoconcept.Later measurements of the magnetic stratigraphy of East Molokai volcano led
conceptthat the south flank moves in responseto forceful intrusion of magma into the rift zones. They assumedthat the volcano flank is mobile downward to a depth of 5-8 km and suggestthat the baseof the mobile zone is not definedby a singleplane or solebut that seawarddisplacementgradually decreasesdownward [seealso Crossenand Endo, 1982]. Lipman et al. [1985] assessedthe deformationassociated with the 1975earthquake(M = 7.2) on Kilauea's southflank and concludedthat a deep-seatedgravitational slump, partly related to forcible intrusionof magmainto the rift zone at its head, could explain the geologicand geophysicalobservations. They suggestedthat the sole of the creepingmasswas
17,468
MOORE ET AL..' SUBMARINE LANDSLIDES, HAWAII TABLE
No. 1 2
Name
1.
Hawaiian
Location
Slides
Area, km2
Length, a km
Width,b km
14,000 6,800 3,900 6,100 23,000 13,0007 4,900 6,100 3,500 4,600 2,300 1,700 850 950 5,200
140 100 80 50 235 •:
