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Mass wasting along the Hawaüan Ridge: giant debris avalanches C.L.Jacobs Institute of Oceanographic Sciences, Deacon Lahoratory. Brook Road. Wormley. Godalming, Surrey GlJ8 SL:B, UK

lntroduction GLORIA Iang-range sidescan sonar surveys along the Hawaiian Ridge were undertaken between 1986 and 1991 as part of the United States Geological Survey (USGS) programme to map the seafloor within the entire US 200-mile Exclusive Economic Zone (EEZ). The surveys revealed many irregularly speckled Iobes of high acoustic backscatter on the submarine flanks and adjacent seafloor of virtually all the islands and shoals. Coincident with these speckled areas, 3.5 kHz profiles showed the topography to be irregular, with local highs frequently reaching hundreds of metres above the surrounding seafloor.

Deep towed high resolution sidescan sonar and camera studies off the coast of the island of Hawaii Cover territory imaged by GLORIA and containing these irregular Iobes) indicate that the high backscattering is due to acoustic reflection from angular blocks (Moore et al. 1992). The size, shape and content of the deposits allow them to be defined as debris avalanches (Varnes 1978). Upslope from the avalanche deposits, the submarine flanks of the volcanoes typically show broad amphitheatre-like cusps down which the deposits flowed. Subaerial mapping suggests that the slope failures are related to the location of laterally extensive volcanic rift zones.

Methods GLORIA Iang-range sidescan sonar data forms the basis of this study. The sonar is described in detail by Somers et al. 0978) and data processing methods by

Chavez (1986). Abrief resume of interpretation methods is given by EEZ-Scan 84 Scientific Staff 0988) and references therein. The swath width for this survey was 22.5 km to either side with a nominal track separation of 30 km. Along-track dimensions of the GLORIA footprint (the area of the seafloor insonified by the cone of sound from one acoustic pulse) increases from about 100m in the near-field, to about 1 km at far range (because of the 2.7° beam-width). The across-track dimension remains approximately constant at about 50 m.

Debris avalanche deposits The commonest features observed on the sonar data off the Hawaiian Islands are lobate or fan-shaped areas characterized by high acoustic backscattering. They extend seaward for many kilometres across the deep seafloor and have been mapped along the entire island chain (e.g. Normark et al. 1987; Moore et al. 1989; Holcomb and Searle 1991). High resolution acoustic profiles across these regions consistently reveal a rough hummocky topography dominated by acoustically opaque steep-sided blocks. The height of individual blocks varies from a few to several hundred metres above the surrounding seafloor, in some cases giving the bathymetric contours a strange shape or orientation (Fig. 5.1). Photographie studies of younger slides off the island of Hawaii reveal individual blocks down to >1m in size (Moore et al. 1992). These characteristics, and the amphitheatre-like cusps that mark the slip-planes, allow the features to be interpreted as debris avalanche deposits (Fig. 5.2).

Debris avalanche deposits are the most common type of mass wasting deposit araund the islands and shoals, though other types, notably rotational slumps, are also seen (Torresan et al. 1990, 1991). At present no models are available to explain why some areas are eroded by debris avalanches and some by rotational slumps; structurally and petrologically the source areas are very similar. One possibility is that the debris avalanches are the end result of movement that initiated as rotational slumps. As the slumped blocks of Iava move downslope across the uneven slopes of the volcano, they are likely to disintegrate into the myriad

Atlas of Deep Water Environments: Architectural style in turbidite systems. Edited by K.T. Pickering, RN. Hiscott, N.H. Kenyon, F. Ricci Lucchi and R.D.A. Smith. Published in 1995 by Chapman & Hall, London. ISBN 0 412 56110 7.

of avalanche blocks that are now mapped scattered across the seabed.

In some cases the avalanche deposits have travelled upslope indicating considerable momentum and therefore velocity. As a comparison, velocities of subaerial debris avalanches have been observed to exceed 35m s 1 (Rogers et al. 1992).

The debris avalanche deposits are covered with varying amounts of overburden Sedimentation. Cover thickness varies from <1m off the island of Hawaii to >30m off Midway Atoll. Generally, the overburden sediments thicken toward the west, suggesting that the avalanche deposits were formed in an age-progressive sequence in the same way as the islands themselves. Therefore, it is probable that each avalanche event is linked to a particular stage in the volcanic evolution of each island (or shoal).

Nuuanu debris avalanche Lying to the north of the island of Oahu, is the largest and most spectacular of the debris avalanche deposits mapped along the Hawaiian Ridge (Fig. 5.2). It was formed by the collapse of the northern flank of the Koolau volcano along a volcanic rift zone. Onshore the line of failure is marked by a line of steep cliffs, the Nuuanu Pali, that are parallel to the coastline.

The Nuuanu debris avalanche deposit forms an amalgam with the adjacent Wailau debris avalanche deposit, a similar slope failure deposit originating from the northern flank of the neighbouring island of Molokai. Tagether they cover araund 25 000 km2 of the seafloor. The debris avalanche deposit extends seaward for about 230 km from its head-wall at the Nuuanu Pali to its toe (as detected by GLORIA). It crosses and partially in-fills the Hawaiian Deep (Fig. 5.2), then runs upslope for c. 120 km, with a vertical rise of araund 300m. A longitudinal topographic profile across the avalanche deposit and its cartoon interpretation (Fig. 5.3) show a number of significant features including the bathymetric spikes of the !arge debris blocks and the partially disguised first-order features such as the Hawaiian Deep and Arch. The steep slope may be the back-wall of the slip-face, whilst the break of slope at about 3500 m at the

southern end of Line 1 may represent the base of the slip-plane from where the avalanche originated. The interpretation cartoon indicates possible initial rotational slip-planes both onshore and offshore and the probable shape of the pre-infilled Hawaiian Deep. The outermost scarp face may represent either an older slip-plane from a now-buried avalanche deposit or the natural base of the Koolau volcano.

The precise Iimits of the avalanche deposit are difficult to define because the finer-grained component of the deposit shows no acoustic cantrast with the in situ pelagic sediments (Fig. 5.4). The chaotic nature of the deposit and the acoustic properlies of the entrained blocks also makes accurate thickness measurements difficult. However, despite these difficulties, Moore et al. (1989) estimate that up to 5000 km3 of material may have been transported, ranking this one of the largest mass-movement deposits of any type so far discovered.

The Nuuanu debris avalanche deposit contains some of the largest blocks ever described as individual components of a debris deposit. The largest block was originally interpreted as a tablemount, the Tuscaloosa Seamount (Fig. 5.5). This tabular block has basal dimensions of c. 30 x 17km and a flat, slightly back­tilted summit that lies 1800 m above the surrounding seafloor, giving it a volume in excess of 900 km1.

Summary GLORIA sidescan sonar surveys along the Hawaiian Ridge have allowed definitive identification and accurate mapping of giant debris avalanche deposits. These mass-movement deposits rank amongst the largest so far discovered and must make them an extreme end-member of any turbidite-debris flow classification scheme. Movement of individual debris blocks of over 900 km3 was hitherto unprecedented. These findings may require re-examination of some olistostrome deposits in collision zones and may allow well-constrained tsunami deposits on the Hawaiian Islands and araund the Pacific rim to be linked to specific debris avalanches.

Acknowledgements I should like to thank Dr D.G. Masson for comments on this manuscript and the science and marine teams who collected the data during the joint United States Geological Survey - Institute of Oceanographic Seiences survey of the Hawaiian EEZ.

Heferences Chavez, Jr, P.S. 1986. Processing techniques for digital sonar

images from GLORIA. Photograrnrnetric Engineering and Remale Sensing, 52, 1133-1145.

EEZ-Scan 84 Scientific Staff 1988. Physiography of the western United States Exclusive Economic Zone. Geology, 16, 131-134.

Holcombe, R.T. and Searle, R.C. 1991. Large Iandslides from oceanic volcanoes. Marine Geotechnology, 10, 19-32.

Moore, J.G. , Clague, D.A. , Holcombe, R.T., Lipman, P.W., Normark, W.R. and Torresan, M.E. 1989. Prodigious submarine Iandslides on the Hawaiian Ridge. Journal of Geophysical Research, 94, 17 465-17 484.

Moore, J.G ., Normark, W.R. and Gutmacher, C.E. 1992. Major Iandslides on the submarine flanks of Mauna Loa volcano, Hawaii. Landslide News, 6 , 13-16.

Norma1'k, W.R., Lipman, P.W. , Wilson, J.B., Jacobs, C.L. , Johnson, D.P. and Gutmacher, C.E. 1987. Hawaiian GLORIA leg II. United States Geo!ogical Suroey, Open File Report, 87-298.

Rogers, W.P., English, D. , Schuster, R.L. and Kirkham, R.M. 1992. Large rock slide/ debris avalanche in the San Juan Mountains, Southwestern Colorado, USA, July 1991. Landslide News, 6, 22-24.

Somers, M.L., Carson, R.M. , Revie, J.A., Edge, R.H. , Barrow, B.J. and Andrews, A.G. 1978. GLORIA II- An improved long-range side-scan sonar. In: Oceanology International 1978, Technical sessionj, B.P.S. Pubbeations Ltd, London, pp. 16-24.

Torresan, M.E. , Clague, D.A., Jacobs, C.L. and Moore, J.G. 1990. Maturation of large-scale mass-wasting along the Hawaiian Ridge. American Association of Petroleum Geologists Bulletin, 74, 1005-1006.

Torresan, M.E., Clague, D.A. and Jacobs, C.L. 1991. Cruise Report, Hawaiian GLORIA Cruise F12-89-HW. United States Geological Suroey, Open File Report, 91-127.

Varnes, D.]. 1978. Slope movement types and processes. In: Schuster, R.L. and Krizek, R.J. (eds) Landslides Analysis and Contra!. Special Report 176, National Academy of Sciences, Washington DC.

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Fig. 5.1. Sehemarie bathymetry of the study area illustrating the huge size and odd shapes of some of the individual debris avalanche blocks. It also shows how the deposit has in-filled the Hawaiian Deep (shaded) and travelled upslope. The contour interval is lOOOm on the slope, otherwise 500m. The location of the bathymetric profiles in Fig. 5.3 are also shown.

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Fig. 5.2. Line-drawn interpretation of the sidescan sonar mosaic. Also shown is the location of Koolau volcano and the rift zone along with the top and bottarn of the submarine slope. The location of the study area relative to the Hawaiian islands is shown below.

Bathymetric Profiles

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Fig. 5.3. Composite longitudinal bathymetric profiles across the Nuuanu debris avalanche deposit (see Fig. 5.1 for location) and cartoon interpretation. The seaward Iimit of blocks N

imaged by GLORIA is over 100 km onto the 1\ Hawaiian Arch and about 300 km above the axis U of the now buried Hawaiian Deep. The steep slope may be the back-wall of the slip-face, whilst the break of slope at about 3500 m at the southern end of Line 1 may represent the base of the slip-plane from where the avalanche originated. The outermost scarp face may represent either an older slip-plane from a now­buried avalanche deposit or the natural base of the Koolau volcano.

Fig. 5.4. (Right) Digitally processed GLORIA sidescan sonar mosaic of the Nuuanu and Wailau debris avalanche deposits to the north of the islands of Oahu and Molokai. Light tones are areas of high acoustic backscatter. The area covered by the avalanche deposits Cover 25 000 km2) is characterized by a blotchy-to-speckled texture. For interpretation see Fig. 5.2.

Fig. 5.5. (Above) Close-up of a GLORIA sonograph over the Tuscaloosa Seamount. The dark marginal areas are pelagic sediments on the 'normal' seabed, the lighter tones represent the irregular slopes of the seamount, and the summit plateau, c. 8 km by 20 km in size, is the medium-grey area in the centre of the figure.

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