Super-scale Failure of the Southern Oregon Cascadia Margin

Pure appl. geophys. 157 (2000) 1189–12260033–4553/00/081189–38 $ 1.50+0.20/0

Super-scale Failure of the Southern Oregon Cascadia Margin

CHRIS GOLDFINGER,1 LAVERNE D. KULM,2 LISA C. MCNEILL3 andPHILLIP WATTS4

Abstract—Using SeaBeam bathymetry and multichannel seismic reflection records we have identifiedthree large submarine landslides on the southern Oregon Cascadia margin. The area enclosed by the threearcuate slide scarps is approximately 8000 km2, and involves an estimated 12,000–16,000 km3 of theaccretionary wedge. The three arcuate slump escarpments are nearly coincident with the continental shelfedge on their landward margins, spanning the full width of the accretionary wedge. Debris from the slidesis buried or partially buried beneath the abyssal plain, covering a subsurface area of at least 8000 km2.The three major slides, called the Heceta, Coos Basin and Blanco slides, display morphologic and structuralfeatures typical of submarine landslides. Bathymetry, sidescan sonar, and seismic reflection profiles revealthat regions of the continental slope enclosed by the scarps are chaotic, with poor penetration of seismicenergy and numerous diffractions. These regions show little structural coherence, in strong contrast tothe fold thrust belt tectonics of the adjacent northern Oregon margin. The bathymetric scarps correlatewith listric detachment faults identified on reflection profiles that show large vertical separation andbathymetric relief. Reflection profiles on the adjacent abyssal plain image buried debris packages extending20–35 km seaward of the base of the continental slope. In the case of the youngest slide, an intersectionof slide debris and abyssal plain sediments, rather than a thrust fault, mark the base of slope. The ageof the three major slides decreases from south to north, indicated by the progressive northward shallowingof buried debris packages, increasing sharpness of morphologic expression, and southward increase inpost-slide reformation of the accretionary wedge. The ages of the events, derived from calculatedsedimentation rates in overlying Pleistocene sediments, are approximately 110 ka, 450 ka, and 1210 ka.This series of slides traveled 25–70 km onto the abyssal plain in at least three probably catastrophic events,which may have been triggered by subduction earthquakes. The lack of internal structure in the slidepackages, and the considerable distance traveled suggest catastrophic rather than incremental slip, althoughthere could have been multiple events. The slides would have generated large tsunami in the Pacific basin,possibly larger than that generated by an earthquake alone. We have identified a potential future slideoff southern Oregon that may be released in a subduction earthquake. The occurrence of the slides andsubsequent subduction of the slide debris, along with evidence for margin subsidence implies that basalsubduction erosion has occurred over at least the last 1 Ma. The massive failure of the southern Oregonslope may have been the result of the collision of a seamount province or aseismic ridge with the margin,suggested by the age progression of the slides and evidence for subducted basement highs. The lack oflatitudinal offset between the oldest slide debris and the corresponding scarp on the continental slope impliesthat the forearc is translating northward at a substantial fraction of the margin-parallel convergence rate.

Key words: Submarine, landslides, tsunami, subduction earthquakes, subduction erosion.

1 College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331,U.S.A. e-mail: [email protected]

2 College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331,U.S.A.

3 Department of Geosciences, Oregon State University, Corvallis, Oregon 97331, U.S.A.4 Applied Fluids Engineering, 5355 E. La Pasada, Suite 22, Long Beach, CA 90815, U.S.A.

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Introduction

Although the Oregon convergent margin is frequently cited as a type example ofa seaward-vergent accretionary wedge, this characterization is only applicable to asmall part of the northern Oregon margin. Taken as a whole, the Oregon marginis better characterized by significant along-strike variability in structural style andwedge morphology (Fig. 1). The northern Oregon and Washington accretionarywedge is a broad landward-vergent thrust system with widely spaced folds, and adecollement stepping down to the basement, with virtually all incoming sedimentbeing frontally accreted (FLUEH et al., 1996; MACKAY, 1995; GOLDFINGER, 1994;MACKAY et al., 1992; SNAVELY and MCCLELLAN, 1987; SILVER, 1972). Thislow-taper wedge is composed primarily of the Pleistocene Astoria and Nitinat Fans,which have been accreting outboard of a narrow, older Cenozoic accretionarycomplex. The older complex in turn lies outboard of an outer arc high andCenozoic forearc basin. The Eocene Siletzia terrane, an oceanic terrane accreted orperhaps rifted in place during the Eocene (WELLS et al., 1984; DUNCAN, 1982), isthe basement of the continental forearc, and terminates at its seaward end beneaththe forearc basin (TREHU et al., 1995; SNAVELY et al., 1980). In contrast, thesouthern Oregon margin is characterized by a steep narrow chaotic continentalslope outboard of the outer arc high and forearc basin, similar to the northernmargin, although without the low-taper accretionary wedge. Between these distinctprovinces is a limited transitional region characterized by the seaward vergentaccretionary wedge for which the Oregon margin has become known (Fig. 1). Thesteep, narrow, southern margin is poorly known, but as we will present here, it ischaracterized by massive slope failures that dominate the structure and morphologyof the continental slope between 42° and 44° N latitude. Recently, we identifiedarcuate escarpments on the Oregon continental slope enclosing regions of hum-mocky topography, and underlain by detachment surfaces that delineate at leastthree failure zones encompassing much of the southern Oregon continental slope(Fig. 2). Beneath the abyssal plain we have found widespread subsurface andpartially buried debris aprons. We first present evidence for buried slump debrisbeneath the abyssal plain, then discuss morphologic and structural evidence for theslides themselves. We then determine the age of the slides, and discuss possiblemechanisms for the apparent massive collapse of the margin, implications forearthquake and tsunami hazards, and forearc deformation.

E�idence for Massi�e Failure of the Southern Oregon Margin

Abyssal Plain

Extensive chaotic reflectors are evident in seismic reflection profiles across thesouthern Oregon abyssal plain adjacent to the continental slope (see Fig. 2 for

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Figure 1Onshore-offshore shaded relief image of the Cascadia subduction zone, Oregon, USA. This image wascompiled from USGS DEM’s onshore, offshore NOAA SeaBeam and BSSS swath bathymetry, andinterpolated surfaces generated from digitized contours where swath bathymetry was unavailable. Theimage resolution is 100 meters. Relief image (and those to follow) shown without contours and onlyminimal depth shading to emphasize morphology. Note morphologic contrast of the lower slopebetween the southern segment (42°17� N–44°14� N) the fold thrust belt of the northern Oregon segment(45° N–46° N), and the transitional zone between these two provinces. Landward-vergent (LV) and

seaward-vergent (SV) segments indicated at left.



locations). The seismic profiles are mostly unmigrated, and some are of poorquality, but nevertheless consistently show the presence of these chaotic reflectors.The seismic profiles image discrete reflector packages that are highly irregular andhummocky at their upper boundary (Figs. 3–8). Where overlain by youngerabyssal plain sediments, younger sediments have onlapped the margins of thehummocky sequences and filled depressions on the irregular upper surfaces (Figs.4–8). The hummocky sequences taper seaward (Figs. 3–7), abutting or gradinginto conformable abyssal plain strata. The sequences are found at three appar-ently discrete levels within the abyssal plain sedimentary section. Planar abyssalplain reflectors underlie these hummocky sequences and are undisturbed exceptfor minor disruption probably due to velocity effects and raypath bending causedby the overlying irregular blocks (Figs. 3–8). No single line displays all of thesecharacteristics, but collectively they characterize the buried reflector packagesquite well. The limited seismic coverage suggests that these reflectors cumulativelycover a minimum area of �8000 km2 between 42°15� N and 44°10� N, extending20–35 km seaward of the base of the continental slope. The reflector packageslocally make up as much as 30% of the sedimentary section (in two-way time)near the base of the slope. The seismic character of these reflector packages issimilar to known slumping elsewhere on the Oregon margin, and the hummockycharacter, chaotic internal structure, and seaward tapering geometry are all con-sistent with, and most probably are, slide debris aprons (MULDER and CO-

CHONAT, 1996; MOORE et al., 1994; LEE, 1989). The consistent seaward taperand highly irregular upper surfaces differentiate them from channel-levee com-plexes which otherwise can have similar reflection character. They have littleinternal structure, and do not display obvious internal cross-cutting relationships,or layering that would suggest multiple events within any given package, implyingthat each package represents a single event. This interpretation is based primarilyon industry and USGS seismic data of intermediate quality. More detaileddata may reveal greater complexity. Several profiles of higher quality exist offsouthernmost Oregon, collected with the R/V Ewing in 1994. These profiles alsoshow minimal internal structure or cross-cutting relationships, supporting a

Figure 2Shaded relief image of the southern Oregon continental margin. Beneath the abyssal plain between42°16.55� N and 44°13.75� N, reflection profiles reveal thick intervals of hummocky, chaotic reflectors(Figs. 3–8, 13; profile locations A–H shown here. R/V Farnella two-channel reflection profiles onabyssal plan shown with time ticks. Buried packages at three stratigraphic levels, shown by superposedpatterns. Upper Blanco Slide is the smaller polygon within the Blanco Slide polygon. The patternedpolygons represent the minimum distribution based on available reflection data, and are dashed whereinferred. Interpreted slump scarps indicated by white arrows. Burial of the frontal thrust by the HecetaSlide can be seen at the arrow point for the label ‘‘Deformation Front’’ at upper left. Estimated pre-slide

deformation front shown by dashed white line.

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Figure 3Unmigrated industry multichannel reflection profile (A-A� in Figs. 2 and 9) across the Cascadia plate boundary showing the Heceta slide debris on thecontinental slope, shallowly buried debris beneath the abyssal plain, and more recent slump debris labeled ‘‘44N Slide Debris.’’ Coherent planar abyssal plainreflectors can be traced at 8–10 km landward beneath the base of the slope. We suggest that the tectonic plate boundary is further east, presently buried

beneath Heceta slide material. Velocity pull-up accounts for the abrupt rise in abyssal plain reflectors beneath the slope.

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Figure 4Unmigrated industry multichannel reflection profile (B-B� in Fig. 2) across the Cascadia plate boundary showing shallowly buried debris from the Coos Basinslide beneath the abyssal plain. Buried debris package rises to the surface and is exposed at the extreme right, where the base of the slope corresponds to

the intersection of slump debris and abyssal plain sediment. No record was available for the lower slope due to gun failure.


Figure 5Migrated industry multichannel reflection profile (C-C� in Fig. 2) across the Cascadia plate boundaryshowing shallowly buried debris from the Upper Blanco Slide beneath the abyssal plain. Buried debris

package rises to the surface and is exposed at the extreme right.

catastrophic origin for the Blanco slide (interpretation by the authors, data shownby CLARKE et al., 1995).

The inferred slump aprons are found at three main levels within the abyssalplain section on multichannel reflection profiles, and a fourth smaller package(Upper Blanco; Fig. 8). The relative position of the packages within the sedimen-tary section shallows from south to north, implying a northward decreasing age forthe causal slump events (Figs. 2 and 8). The deepest package is found adjacent tothe subducting Blanco fracture zone and the Juan de Fuca-North America plateboundary, where the top of hummocky package is found at 0.9–0.95 s (twt) belowthe seafloor on USGS lines 77–49 and 77–50 (Figs. 6 and 7; MCCLELLAN andSNAVELY, 1987). A debris package buried by 0.37 s (twt) of sediment lies to thenorth of the deeper package, from about 42°42� to 43°34� N, and overlaps thedeeper package at its southern margin. This package extends �25 km seaward ofthe base of the continental slope. The minimum seaward extent is constrained bythe seaward limits of the industry dip profiles, two channel profiles collected byUSGS aboard R/V Farnella during the GLORIA EEZ surveys (EEZ-SCAN 84SCIENTIFIC STAFF, 1986) and the R/V Ewing 1994 profiles (CLARKE et al., 1995).All of the debris packages are imaged by the inboard Farnella strike line (JD 200),but the outboard line (JD 203) had an equipment failure in the area of interest, and

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Figure 6USGS unmigrated multichannel reflection profile 77–50 (D-D� in Fig. 2) across the Cascadia plate boundary showing buried debris from the Blanco slide,

older and deeper in the section than the Coos Basin slide debris. Note landward vergent thrusts in this section.


no data were collected seaward of the three slides. One Farnella dip profile (JD 176;see Fig. 2) and the R/V Ewing profiles (S. H. Clarke, pers. comm., 1995) constrainthe seaward limits of the Blanco and Coos Basin slides. The smaller Upper Blancoslide package lies stratigraphically between the Blanco and the Coos Basin slides,complicating the overlap between them (Fig. 8). The seaward limit of the Heceta slideis not well constrained and is dashed on Figure 2. The Heceta slide debris is buriedby 0.09–0.10 s of abyssal plain sediment on industry profile A-A� (Fig. 3). This unitalso overlaps the middle unit at its southern margin, and extends at least 25 kmseaward of the base of the slope. A smaller young debris apron from a lower slopeslide visible in Figures 2 and 3 overlaps the shallow northern unit. We note thatseismic imaging of such slide packages on other margins is not common, and mayin part be due to the lack of a topographic trench which would otherwise captureand limit the seaward travel of slide material (R. von Huene, pers. comm., 1998).

Continental Slope: Structural and Morphologic E�idence

We have compiled bathymetry data on the Oregon margin from NOAA SeaBeamand BSSS swath surveys, and NOAA hydrographic data (Figs. 1, 2, and 9). Thesecombined data were gridded at a 100 m spacing, and shaded relief images weregenerated from the gridded data in order to analyze the morphology of thecontinental slope adjacent to the abyssal plain debris packages (GOLDFINGER et al.,1997).

Figure 7USGS unmigrated multichannel reflection profile 77–49 (E-E� in Fig. 2) across the Cascadia plate

boundary showing buried debris from the Blanco slide beneath the abyssal plain.

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Figure 8R/V Farnella two-channel seismic data parallel to the southern Oregon margin. Records are from Julian Day (JD) 200 and JD 176. Location shown in Figure2. Three debris packages can be interpreted from these records, and the northward shallowing is evident. See text for discussion. A fourth package, called

the ‘‘Upper Blanco Slide’’ is also visible at intermediate depth between the Blanco and Coos Basin slides.


Figure 9Detail of the Heceta slide off central Oregon. Ayssal plain debris is the shallowest in the sedimentarysection opposite this feature, suggesting it is the most recent event. The slump scar (white triangles) hasbeen buried in several locations by progradational lobes (‘‘PL’’), presumably deposited during Pleis-tocene sea level low-stands. These lobes have themselves failed in secondary failures (‘‘SF’’), superimpos-ing sediment sheets on the surface of the larger chaotic blocks. The 44 North slide, a small recent slumpat the deformation front, has remobilized debris from the Heceta slide and redeposited blocks on thesurface of the abyssal plain. SeaCliff Dive 1031 and ATV dive 22–185 locations are shown. Estimatedposition of the frontal thrust beneath the slump debris shown by dashed line. Note possible NE and NW

trending conjugate faults enclosed by the Heceta scarp.

Much of the continental slope off southern Oregon has a distinctive morphol-ogy that can be easily differentiated from the northern Oregon, Washington andnorthern California margins, and from the adjacent uppermost slope (Figs. 1 and2). Four morphologic features distinguish the southern Oregon slope: 1) Lack of orpoor definition of accretionary wedge fold-thrust ridges and slope basins; 2)Hummocky surface morphology; 3) Pervasive close-spaced linear trends; 4) Largearcuate scarps enclosing domains characterized by the first three morphologicelements. Examination of shaded relief bathymetry (Fig. 2) reveals three largearcuate scarps, the northernmost of which is both the largest and best defined. Theupper slope scarp morphology is distinct in the northern area, becoming progres-


sively less so to the south. From north to south, these three large scarps are 76, 68and 65 km in length, and 33, 30, and 34 km in width. They enclose 2874 km2, 2304km2, and 2713 km2 respectively, totaling 7890 km2 in area. We have named thesefeatures the Heceta slide, the Coos Basin slide, and the Blanco slide respectivelyafter adjacent features on the margin and nearby coast. In addition to thenorthward increasing definition of the slump scar morphology, the three otherdistinguishing morphologic elements (poor fold-thrust definition, hummocky topog-raphy, and closely spaced linear trends) also exhibit a north-south variationdescribed below.

On the upper slope, multichannel and single channel reflection profiles illustratethat the arcuate scarps sharply truncate gently folded upper slope-outer shelf lateTertiary sedimentary units. Our recent structural analysis of the Oregon marginsuggests that these slides involve both the active accretionary wedge of the lowerslope, and an older accretionary complex, including the outer arc high (MCNEILL,1998). The continental framework, defined here as the seaward limit of the EoceneSiletzia terrane, appears not to be involved in the megaslides discussed here.Heceta Slide. The Heceta (northernmost) slide scarp encloses a low sloping

region of hummocky, chaotic topography. Slight evidence of the expected accre-tionary fold-thrust structural style is evident, either in bathymetric or seismicreflection data. A few faint lineations, sub-parallel to the margin are seen, and areon the same trend as clearly defined fold-thrust structures immediately to the north,suggesting continuation of these structures beneath the slump pile (Fig. 9). Thenorthern boundary between the hummocky region and the well-defined thrust beltis sharp, which we interpret as the northern limit of slump debris from the Hecetaslide. A secondary shallower slope failure appears to have contributed to burial ofthe fold thrust belt at this location (Fig. 9). Other secondary failures mark thenorthern scar along its steep eastern boundary where it lies adjacent to the edge ofthe continental shelf. Based on the morphology of the scarp and the compilation ofindustry reflection data, we infer that progradational lobes of sediment advancedover the main scarp at some time following the major slump event. These lobessubsequently failed, adding several thin sheet flows to the hummocky debris fromthe Heceta slide on the lower slope and abyssal plain (Fig. 9).

Industry profile A-A� (Fig. 3) and SeaMARC 1A sidescan imagery (Fig. 10)suggest that the base of the continental slope at the Heceta slide is not marked bya thrust fault, implying a young age for this slide that has apparently been barelyaltered by interplate convergence. The base of the slope is also not the toe of theslide, which is buried well to the west. Figure 3 shows that undeformed abyssalplain reflections extend beneath the lower slope at least 8 km east of the base of theslope. We examined the base of the slope with high-resolution SeaMARC 1Asidescan sonar, and at several locations during Sea Cliff dive 1031 and ATV dive21–184 in 1996. We found that the slope base is covered with at least several metersof draped unconsolidated sediment and no evidence of active faulting, although we


cannot exclude a thrust fault at this location. The SeaMARC 1A sidescan sonarimagery of the slope base shows no evidence of faulting, but rather a highlyirregular intersection between the partially buried slump blocks and abyssal plainstrata (Fig. 10). If there were a thrust fault at the base of the chaotic section inFigure 3, perhaps slightly landward of the sidescan and submersible dives, weestimate that the dip of the fault would have to be no more than 5° for a distanceof 8–10 km in order to fit between undeformed reflectors and the chaotic slopereflections (using reasonable estimated velocities for the abyssal plain section). Thisis an atypically shallow and perhaps unreasonable fault dip, thus we infer that theslump contact is a more plausible interpretation. Further evidence that the base ofthe slope is presently a sedimentary contact at the Heceta slide is the lobatemorphology, typical of slide surface morphology. The abrupt burial of the activeseaward-vergent frontal thrust by the Heceta slide and resulting strike change of theslope base can be observed on Figure 2 at the upper left (see Fig. 2 caption). Somethrust faulting may deform the chaotic lower slope section, but further interpreta-tion of internal structure is problematic due to the lack of migrated reflection data.

The hummocky morphology of the Heceta slide is cut by weak NE and strongNW trending lineations that terminate at the slump scarp (Figs. 2 and 9). Thesefeatures are clearly faults, nonetheless the limited seismic and sidescan sonar dataavailable are not sufficient to determine the slip sense of individual faults. The twosets of lineations may be a conjugate set of mostly strike-slip faults responding toinitial post-slump plate convergence. These apparent conjugate lineations becomeabruptly less pronounced at the southern margin of the Heceta slide where the scarabuts the northern part of the Coos Basin slide.

The detachment surface is poorly imaged in available profiles. On USGS line76-WO-1A (MCCLELLAN and SNAVELY, 1987), chaotic slope reflectors are ob-served to overlie shallow-dipping reflectors that are not multiples or reflectionsfrom the downgoing plate. These two reflector packages are separated by ashallowly west dipping reflector that merges with the topographic scarp which weinterpret as the uppermost Heceta slide detachment surface. This possible detach-ment surface is 2.0–2.2 s (twt) below the middle slope terrace on this line.Interpretation is made difficult by the water bottom multiple, which merges withthe detachment, however, the angular discordance between the upper and lowerunit reflectors clearly marks the separation between these units. The top of the slide

Figure 10SeaMarc 1A sidescan sonar image of part of the base of the continental slope. Area of figure indicatedby label in Figures 2 and 9. The base of the continental slope from about 43°15� to the northern limitof the slumped area at 44°13.75� is characterized by irregular, blocky material that we interpret as theonlapping of abyssal plain sediments on the top of the slumped debris pile. We see no evidence of athrust fault along this part of the margin in seismic, bathymetric, or sidescan data. The sidescan coverageis nearly complete along the deformation front between these latitudes. Nominal swath width is 5 km,

pixel resolution is 2.5 m. Dark line is nadir area (no data) beneath towfish.



package appears to be overlain by approximately 0.2 s (twt) of post-slump sedimentand debris apron material from secondary slides visible in the shaded relief imagery(Fig. 9). The thickness of the slide package overlying the interpreted detachment onthis line is about 1.8–2.2 km, using an interval velocity of 1800–2000 m/s for theslide package. The topographic scarp averages �660 m in height, with a range of490–834 m. We estimate that the vertical separation is about 700–800 m for theHeceta slide, based on the vertical distance from the base of growth strataoverlapping the tail of the slide, to the base of the oldest strata that overlap thescarp. The truncated and overlapping strata are shown in Figure 11. This line doesnot extend far enough west to image the scarp base; vertical separation measure-ments were made on adjacent line 7310 (not shown) which was of low imagequality.

A single traverse made across the Heceta slide scarp with an ROV (US NavyAdvanced Tethered Vehicle, ATV Dive 22–185) revealed no active surface faultingat that location (location shown on Fig. 9). However, new Simrad EM-300bathymetry data on Heceta Bank, immediately landward of the Heceta slide scarphas revealed en echelon normal faults that may represent headward migration ofthe headwall of the Heceta slide (R. W. Embley, NOAA, pers. comm., 1998).Coos Basin Slide. The Coos Basin slide scarp encloses a region that is less

hummocky, has less distinct NW-NE trending lineations, and more distinct NNWtrending lineations subparallel to the margin (Fig. 2). The NNW trending lineationsare closely spaced, and at least some are small seaward vergent thrust faults, whileothers are small normal faults with growth strata (Fig. 12). At the deformationfront, some of the deeply buried slump debris from both the Blanco and CoosBasin slides is presently being re-accreted by young landward and seaward-vergentthrust faulting, and some is apparently being subducted beneath the toe of the slope(Figs. 5–7).

The Coos Basin detachment is imaged on a migrated industry profile across theCoos Basin slide scarp and upper slope (Fig. 12). A possible low-angle detachmentsurface can be traced seaward from the base of the steep section of the fault, andlike USGS Line 76-WO-1A, separates chaotic bust mostly landward-dipping reflec-tors above the inferred detachment from a shallower dip domain below. Thewater-bottom multiple makes interpretation of this detachment itself problematic.A large gap prevents correlation of the detachment entirely to the base of the slopeon this otherwise excellent profile. Rollover folds and growth strata, suggestingthere has been slow movement down slope following the initial event, overlie theupper parts of all three detachments. A seafloor scarp in Holocene, covering strata(cored nearby and observed during an ATV dive) in several locations, indicates thatthis process has continued into recent time for at least the Coos Basin slide. Theslide package has a minimum thickness of �2.0 s (twt), the maximum depth towhich we could trace the detachment fault from the surface on this profile. Like theyounger Heceta slide, the Coos Basin slide package has a thickness of 1.8–2.0 km,using an interval velocity of 1800–2000 m/s.

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Figure 11Interpreted industry single channel sparker profile (F-F� in Figs. 2 and 9) across the northern proposed slump scar off central Oregon. Shallow seawarddipping reflectors are truncated at the scarp, which we interpret as the headwall scarp. Several other profiles show this same relationship. The line terminated

at the left (west end) of this figure.

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Figure 12Industry migrated multichannel seismic profile (G-G� in Fig. 2) across Coos Basin slump scar and slide package off central Oregon. Detachment indicatedby black arrows. The anticline at center is a rollover fold forming above the listric detachment. Detachment separates landward dipping domain (above)from seaward dipping domain (below). Faulting has continued at a slower pace since the initial failure, indicated by the faulted growth strata in the synclineat center. Also visible is a strong BSR indicating probable gas hydrate at about 0.3 s below sea floor. Note also smaller normal fault below BSR label.


Blanco Slide. The southernmost failure, the Blanco slide, is the least distinct ofthe three morphologically. It is the oldest, indicated by the depth of burial of thedebris apron, and thus we expect that modification of the slope and scarp followingthe failure event by sedimentation, other slumping events, and compression due tointerplate convergence, would be extensive. The morphology of the Blanco slidescar is indistinct, and hummocky topography within the scar is almost absent.Fold-thrust development of the accretionary wedge is greater than the adjacentCoos Basin slide. Were it not for the buried debris apron beneath the adjacentabyssal plain, and the association of this slide with the adjacent younger slopefailures, it would be difficult to identify the Blanco slide as a major slope failure.The Blanco slide scarp has been heavily modified by both subsequent slumping andsedimentation from the Rogue River, which has built a prodelta across the scarp onthe upper slope, and carved the Rogue Canyon through the lower slope to theabyssal plain (Fig. 2).

The anomalously close spacing of margin parallel faults within the southern twoslide scarps may relate to the kinematics of a slump apron under post-slidecompression. Fold wavelength and fault spacing is in part a function of thethickness of the coherent sedimentary section that can participate in the develop-ment of ramps, flats, and folds (RAMSAY, 1967; JOHNSON, 1970; SUPPE, 1985). Ingeneral, the more heterogeneous the stratigraphy, the shorter the fold wavelength.Thus we predict that a slump apron, comprising a weak and chaotic stratigraphy,will form closely spaced folds and faults relative to a well-stratified section underinterplate compression. The numerous small margin parallel normal faults suggestslope oversteepening and secondary failure of the unstable slump blocks as theslump debris is re-accreted to the margin.

Age Constraints

The three major slides clearly decrease in age northward, indicated by theirdistinctly shallowing depth of burial within the abyssal plain. Although we have nodirect means of determining the ages of the slides, we can estimate their ages usingseismic reflection profiles, velocities, and sedimentation rates. For the three slides,we first calculate the thickness of overlying sediments, then estimate event agesusing both a core-based sedimentation rate and correlation to a dated horizon fromDSDP Site 174 on the abyssal plain. Several east-west seismic profiles cross theburied abyssal plain debris that can be used to determine the thickness of overlyingsediment. In addition, one north-south profile, USGS R/V Farnella two-channelseismic data (EEZ-SCAN 84 SCIENTIFIC STAFF, 1986) reveals the presence of thedebris apron on day 200 between 0500 and 0800 (Fig. 8).Core-based Age Calculation. On the Farnella seismic records, approximately

0.09 s (twt) of post slump sediments overlie the Heceta slide debris. On profile A-A�

(Fig. 3), 0.1 s of sediment overlie the slump debris, although this can only be


measured at one point, as subsequent debris from a smaller slump mask relation-ships effectively on this line. Using an average velocity of 1540�20 m/s for thepost-slump sediment (velocity for similar Cascadia setting taken from ODP Site888; HYNDMAN and DAVIS, 1992; WESTBROOK et al., 1994) we estimate a depth ofburial of 73�6 meters.

The Coos Basin slide is crossed by two industry reflection profiles (Figs. 4 and5) in addition to the Farnella seismics on the abyssal plain, making depth of burialand age estimates somewhat more accurate. On the abyssal plain, the Coos Basinslide is overlapped by 0.37�0.02 s (twt) of post-slide sediment on these two profiles(Figs. 4 and 5). Using an interval velocity of 1580�20 m/s (also from ODP Site888), we estimate the sediments covering the slide to be 298�20 m in thickness.

The Blanco slide is crossed near its south end by USGS lines 77–50 and 77–49(Figs. 6 and 7). On these profiles, the debris apron is overlapped by 0.95–1.05 s(twt) of sediment. Using an interval velocity of 1620�30 m/s for this older section,we estimate that this slide is covered by approximately 810�55 m of sediment.(The average interval velocity is taken from the model velocities used in processingof Oregon multichannel data in MACKAY, 1995, referred to below as MACKAY,1995 velocities.)

To estimate the ages of the events, we first estimate sedimentation rates fromcores taken on the abyssal plain. The best available core is OSU core 6609-1(DUNCAN, 1968) on the abyssal plain at 42°26.0� N, 125°14.4� W (Fig. 2). Core6609-1 reaches a depth of 8.1 meters, has AMS (Accelerator Mass Spectrometer)radiocarbon ages to the bottom of the core, and has a well-dated marker horizon,the Mazama ash, that is identified in the core and verified with AMS radiocarbonages. The AMS ages were determined by NELSON et al. (1996) using planktonicforaminifera. Although we have insufficient data to directly derive an averagesedimentation rate for the entire Pleistocene in the Cascadia abyssal plain at thedesired location, we use the data from this core with some caveats. The age of thebase of the Mazama ash is 7465 yrs. based on the average of three AMS dates. Thisis in reasonably good agreement with the age of the Mt. Mazama explosion at7669�100 calibrated years before 1998 (using calibration of BACON, 1983 inADAMS, 1990). Some time must have passed between deposition of the ash on theupper slope, and the turbidite event that redeposited it off southern Oregon. Themean recurrence interval for Holocene turbidites was approximately 600 years(ADAMS, 1990), consistent with the approximately 200 years between the Mazamaexplosion and the age of the first turbidite that included the ash.

An AMS age at the base of this core, 810 cm, gives an age of 13,000�110 calyears. The sedimentation rate for the interval between the Mazama ash and thebase of the core is 370 cm in 5535�100 yrs. or 67�2 cm/1000 yrs. This rate iscomparable to a 77 cm/1000 yr. rate calculated for a point adjacent to the base ofthe slope farther north, where the Pleistocene Astoria fan sediments are approxi-mately 30% thicker (GOLDFINGER et al., 1996). It is also consistent with two


nearby cores (OSU cores 6511-5 and 6604-12) which bottom in 3.8 and 8.8 m ofpost-glacial sediment, respectively (DUNCAN, 1968). In using this sedimentationrate, we are sampling partly Holocene and partly Pleistocene sedimentationregimes. While not ideal, this method may crudely approximate the averagePleistocene rate that includes both low and high stand sediment transport regimes.We note that the Holocene sedimentation rate adjacent to the southern Oregonmargin is considerably higher than that found adjacent to the margin farther north(DUNCAN, 1968). Using data from the same core, we find the Mazama ash is at 440cm depth, giving a Holocene rate of 59 cm/1000 yr. The average rate for the entirecore is 62 cm/1000 yrs. This is in good agreement with rates derived fromdeep-towed 4.5 kHz seismic profiles (discussed further below). These data clearlyimage a profound reflectivity boundary, which we tentatively correlate to theHolocene/Pleistocene boundary identified lithologically in core 6609-1 at a depth of440 cm in the core. The 4.5 kHz data, located along the base of the slope from43°–44°N, indicate that the Holocene/Pleistocene boundary is at a depth of 5–8meters below sea floor (mbsf; Fig. 13). We speculate that the high sedimentationrate is due at least partly to the numerous small slumps and sheet flows observedin the bathymetric data. Many of these appear to be secondary failures from theoriginal large slump scars. Some are failures of the scarp itself, while others appearto be failures of progradational sediment lobes that have extended seaward over thesteep scarps (Fig. 9). Possible explanations for this high rate are discussed below,but regardless of the reason, if the Pleistocene and Holocene rates are roughlycomparable at this locality, the composite rate we calculate from core 6609-1 maybe a reasonable approximation of the average Pleistocene sedimentation rate offsouthern Oregon.

Using the composite sedimentation rate of 67�2 cm/1000 yrs., we estimate thatthe depth of burial of 73�6 m represents an event age for the Heceta slide of108�15 ka. Similarly, the age of the Coos Basin slide would be 445�42 ka, andthe age of the Blanco slide would be 1209�112 ka.Correlation to DSDP Site 174. The second method we employ to estimate

sedimentation rates is by correlation of a dated horizon from DSDP site 174 (Fig.1 inset) to the southern Oregon margin. Although the site is 123 km northwest ofthe northern slump, we used this method to verify dates derived from the pistoncore-based sedimentation rates, with little expectation of more than an approximatecheck. At Site 174A, a profound lithologic change between sand turbidites of theAstoria Fan and silt turbidites of the underlying abyssal plain sequence wasobserved at a depth of 284 m in the drill cores (KULM et al., 1973). Biostratigraphicanalysis of the cores from Site 174 yields an age of 760�50 ka (J. C. Ingle,Stanford University, written communication, 1995; see GOLDFINGER et al., 1996).Available reflection profiles, including the R/V Farnella two-channel seismics, showthat prior to the onset of Pleistocene sedimentation, the Cascadia plate boundarywas marked by a bathymetric trench, now filled with young sediments. There were

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Figure 13Deep towed 4.5 kHz seismic profile (H-H� in Fig. 9) adjacent to the base of slope and the Heceta slide off southern Oregon. Upper panel shows slide blocksfrom the 44 North slump, overlain by a semi-transparent layer correlated to post-glacial sediments in nearby cores. Buried tops of slide blocks are visible.Lower panel shows detail of transparent post-glacial sediments, including 7–8 probable turbidites. The turbidites may correspond to individual great

earthquakes as postulated by ADAMS (1990) in Cascadia Basin channel cores. Location of detail panel is immediately west of the upper panel.


apparently two main phases filling the pre-Pleistocene trench. The first phase ischaracterized by nearly flat-lying turbidites that are clearly time-transgressive in aneast-west direction, progressively onlapping the gently east-dipping older sectionbelow as the trench filled. Reflection data show little evidence of progressive northto south trench filling in this phase. The second phase was the construction of thebathymetrically expressed Astoria Fan. The boundary between these phases isabrupt and distinct on the Farnella and other reflection profiles (Fig. 8). Wecorrelated a prominent seismic reflector at the base of the fan from Site 174Aeastward to the base of the slope, using a seismic reflection profile connecting thedrill site to the margin (KULM et al., 1973; GOLDFINGER et al., 1996). This reflectoris within the lower, trench-filling unit. GOLDFINGER et al. (1996) argued that theage of the fan base reflector at Site 174 is approximately the same age at the marginbecause no significant onlap or offlap relationships are observed, and because thetrend of the seismic section is subperpendicular to the sediment transport direction.We adopt this assumption in our attempt to extend the dated horizon to the marginand then south to the slide sites. Although additional uncertainties exist insedimentation rates, age of the fan, and seismic velocities, we use the same estimateof error (50 ky) here because we are unable to quantify these errors independently.The fan section overlying the dated reflector is 0.70�0.05 s (twt) below sea floor.Using an upper fan average interval velocity of 1540�30 m/s (MACKAY, 1995velocities), we estimate this section to be 539�50 m in thickness. The averagesedimentation rate for this section is then 77�7 cm/1000 yr., including bothvelocity and age error ranges.

This dated reflector at Site 174 is not the base of the fan or trench fill sedimentat the base of the slope, where an additional 0.55�0.05 s (twt) of sediment liebetween the dated reflector and the angular unconformity at the base of the trenchfill sequence (total thickness of the fan/trench fill sequence is 1.25 s twt). Toestimate the age of the base of the fan/trench fill sequence, we have continued theaverage Pleistocene sedimentation rate of 77�7 cm/1000 yrs. downward to the fanbase 0.55 s below. For this deeper section we use a velocity of 1640�30 m/s.(MACKAY, 1995 velocities), yielding a thickness of 451�50 m for the lower fansection. The total fan thickness is then 990�100 m, and if the average sedimenta-tion rate was approximately constant at 77�7 cm/1000 yr., the age of the base ofthe fan at this location would be 1286�270 ka.

In order to estimate the ages of the slide events, we have correlated the reflectorat the base of the Astoria fan/trench fill sequence adjacent to the margin southwardfrom the intersection of the dated horizon with the margin. We have used Farnellatwo channel seismic reflection data on day JD 200 which runs subparallel to themargin (Figs. 2 and 8). We have projected its position onto this profile at the northend from other data (principally correlated from the Leg 146 site survey data, e.g.GOLDFINGER et al., 1992; MACKAY, 1995). We assume that the base of thefan-trench sequence is approximately the same age on this profile. Although there


is clearly evidence for southward onlapping of sediment packages within thefan/trench sequence, the onlapping reflectors are mostly either north of or higher inthe section than the base of the trench fill reflector we have correlated.

The fan base reflector, assumed to be approximately isochronous at 1286�270ka, is found to underlie the Heceta slide package at an average depth of 0.83�0.05s twt below seafloor along the JD 200 profile. Using an average velocity of1550�30 m/s (MACKAY, 1995 velocities), the fan thickness at this locality is643�50 m. These values yield a sedimentation rate of 50�7 cm/1000 yrs. If weapply this rate to the post-slide sediment, using the burial thickness of 73�6 m, wederive an age of 146�30 ka for the Heceta slide. We note that the JD 200 profileis 14 km seaward of the deformation front, thus the known east-west timetransgression results in an overestimate of the age of the fan base. Since we knowthe age and the thickness of the fan section at Site 174 and at the deformationfront, we can calculate the approximate rate of time transgression and apply acorrection to the correlation along JD 200. The fan base age is 760�50 ka at site174A, and 1286�50 ka at the margin, 53 km east. Thus the approximate rate oftime transgression is �9.9 ka/km (assuming linearity). The distance of line JD 200from deformation front at the Heceta slide is 14 km, thus the fan base here shouldbe 1286 ka-(14 km�9.9 ka/km)=1174 ka. Using this corrected age for the fan basewe recalculate the sedimentation rate to be 56�7 cm/1000 yrs. and the age of theHeceta slide to be 130�30 ka.

Applying similar methods to the Coos Basin slide, the fan base is found at0.92�0.05 s twt. Using the age of 1286�50 ka, the average thickness andsedimentation rate of the fan/trench fill section are 722�50 m and 56�7 cm/1000yrs., respectively. The Coos Basin slide is overlain by 0.37�0.02 s (twt) ofsediment, yielding a thickness and age of 298�20 m and 532�80 ka, respectively.The JD 200 seismic profile is 17 km seaward of the base of slope, thus applying thetime transgression correction as above, the age of the fan base is 1286–17�9.9ka/km=1118�50 ka. The corrected sedimentation rate is then 64�7 cm/1000.Using the corrected fan base age, the slide age is then 466�100 ka.

We find that the fan base is at 0.95–1.05 s (twt) beneath the Blanco slide.However, the distal portions of the Blanco slide debris apron converge with the fanbase reflector, indicating that the Blanco slide was approximately correlative withthe onset of Astoria fan/trench-fill sedimentation. Using the assumed age of the fanbase sequence, and correcting for the 15 km distance of the JD 200 profile from theslope base, the age of the Blanco slide is 1286�50 ka–15 km�9.9 ka/km=1137�50 ka.

The two independent age estimates for the three mega-slides are surprisinglysimilar, particularly considering the assumption of an isochronous fan base agealong the trench. Considering the poor data quality and errors involved, the goodagreement between these ages is surprising. Further examination of the stratigraphyand depositional mechanism suggests that our assumptions about the age of the fan


base might be more reasonable than at first glance. The relatively simple lowertrench-fill stratigraphy comprises abyssal plain sand and silt turbidites that werecored at Site 174A (KULM et al., 1973). Individual turbidites are depositedessentially instantaneously, thus if turbidite sedimentation is the principal trench-filling mode, and the depositional surface has no significant topography, individualcorrelative horizons may well be nearly isochronous over long distances. Theregional along-strike change in Juan de Fuca slab depth at these latitudes isminimal, and most of the topography on the subducting plate is filled and coveredby the lower pre-Pleistocene abyssal plain section. We therefore infer that thetrench fill sequence would be expected to show little onlap/offlap along the trenchaxis, and, in seismic sections, shows little variation other than distal thinning to thesouth. If this model for trench filling is correct, the good agreement between agesand sedimentation rates estimated from cores and seismic correlation is somewhatless surprising.

Given that the age of the Blanco slide is greater than 1 Ma, we can estimate thatincluding the orthogonal rate of plate convergence of about 34 km/Ma, the Blancoslide must have traveled �70 km onto the Juan de Fuca plate.

Discussion

The evidence for super-scale slumping of the southern Oregon Cascadia marginwas previously undetected, perhaps because many of the relevant reflection profilesare proprietary industry data, and because a full compilation of bathymetry datawas needed to recognize the scarps. The tectonic style of the slump province differssharply from that observed along the rest of the Cascadia margin. Off northernOregon and Washington, the continental margin is clearly accretionary, withyoung, well defined thrust ridges and faults characterizing a youthful wedge that islargely Pleistocene in age (SILVER, 1972; SEELY, 1977; WESTBROOK, 1994). Theaccretionary wedge is widest in Washington, 120 km from the shelf edge to thefrontal thrust. The wedge narrows both to the north and south, and is only 30–50km wide off southern Oregon (Fig. 2). The Vancouver and northern Californiamargins are also not typified by massive slope failure, though smaller slides areknown.

What is responsible for the age progression and triggering of the megaslides? Wesuggest and evaluate three possible mechanisms that may play a role in this process:1) lowered basal shear stress in the Pleistocene leading to wedge taper adjustment;2) basal erosion from seamount subduction; and 3) arc-parallel forearc extension.We do not discuss great subduction earthquakes as a mechanism here because wemake a distinction between a triggering event, such as an earthquake, and atectonic mechanism, such as seamount subduction or taper adjustment that controlsthe larger process. We assume that individual slide events most likely are triggered


by subduction earthquakes, regardless of the underlying mechanism. We also notethat the depth of the detachment surfaces is too great to call upon destabilizationof gas hydrate as a candidate mechanism for these slides (e.g., PAULL et al., 1996).The detachments are well below the stability field of gas hydrate, though it iswidespread on this margin. The slides would certainly have caused massive methanerelease, however, as slope structure and stratigraphy was strongly disrupted duringthe slide events.

Change in Basal Shear Stress

The principal reason for the extensive variations in width of the Cascadia wedgeis the accretion of the Astoria and Nitinat submarine fans in the center of themargin. Rapid deposition and re-accretion of these fans can explain both the widecentral margin, and the low-taper character (SEELY, 1977). The subduction decolle-ment is seaward vergent between 44°50� and 42°39�, and landward vergent from thatpoint northward to the Vancouver Island margin. Southward, the frontal thrust islandward vergent south of 42°39� N, and landward vergence dominates the north-ern California margin (Fig. 2). The decollement in the landward-vergent section ofthe margin steps down to a deep level close to the base of the sedimentary section,causing frontal accretion (FLUEH et al., 1996; MACKAY, 1995; GOLDFINGER,1994), whereas the seaward-vergent thrusts in southern Oregon override much ofthe incoming section. USGS profile 77–49 (Fig. 7) and other profiles crossing theBlanco and Coos Basin slides reveal that the debris aprons from these two slides arepresently being overridden. The deeper position in the sedimentary section occupiedby these packages places them below the decollement, while the fate of the northernpackage remains unclear.

There may be several causes for the shift from an accreting margin in Washing-ton and northern Oregon to a collapsing margin in southern Oregon. Clearly thesediment supply is considerably greater in the north. The Astoria and Nitinatsubmarine fans, fed by the Juan de Fuca Strait, the Columbia and Washingtoncoastal rivers, overlie the hemipelagic section, resulting in a 3–4 km thick sedimen-tary section on the incoming Juan de Fuca plate. Off southern Oregon, despite thepresence of the relatively high topography of the Klamath Mountains, the sedimentsupply is substantially lower, and the incoming section is less than 2 km inthickness. We infer that the rapid deposition of the large submarine fans con-tributes to their subsequent accretion in that high fluid pressures generated in thesection by rapid deposition tend to favor mixed-vergent and landward-vergentthrusting at the deformation front (MACKAY, 1995; SEELY, 1977). Landwardvergence in turn promotes frontal accretion because the decollement commonlysteps down approaching the basement, off-scraping the entire incoming sedimentarypackage. The high fluid pressures result in low basal shear stress on the decolle-ment, resulting in a wedge with a low critical taper angle (plate dip plus surface


slope; DAHLEN et al., 1984; DAVIS et al., 1983) which is less prone to gravitationalfailures. Beneath the lower slope off Washington, the dip of the Juan de Fuca plateis only 3–5° (FLUEH et al., 1996). The upper surface of the wedge has little slope,and locally slopes landward. Reflection data show that this is due to increasedthickness of the accreting section over time (as the fans grew) resulting in thethickest off-scraped sections presently being accreted at the frontal thrust, with thethinnest section at the rear of the wedge (WESTBROOK et al., 1994; data shown inFLUEH et al., 1996).

We suggest that increased fluid pressures generated during rapid sedimentdeposition during the Pleistocene probably caused a reduction in basal shear stresson the megathrust along much of the margin, resulting in a change to landwardvergence in the areas with the highest sedimentation rates. If the accretionary wedgewas at critical taper prior to this change in basal shear stress, a reduction in basalshear would have brought the wedge to a super-critical (i.e., oversteepened)condition. The oversteepened wedge may then have failed by gravity-driven detach-ment to re-establish a critical taper angle (e.g., DAVIS et al., 1983; DAHLEN et al.,1984). The Washington and northern Oregon upper slope and shelf are indeedpresently collapsing, however not by this mechanism. The Washington margincollapse is characterized by large listric normal faults (MCNEILL et al., 1997). Thiscollapse is apparently facilitated by plastic mobiliziation of the Hoh melange, whichunderlies the northern Oregon and Washington upper slope and shelf. Slip on thesefaults has occurred progressively since the middle Miocene, rather than catastrophi-cally as is the case in southern Oregon. Although collapse and taper reduction isoccurring on the Washington margin, it predates the rapid influx of Pleistocenesediments responsible for the low-taper Washington wedge. Thus, reduction inbasal shear stress apparently did not result in collapse of the Washington wedge.Instead, it may have resulted in stabilization of the wedge due to rapid accretion ofthe fans and resulting growth of a low-taper landward-vergent wedge that but-tresses the collapsing upper slope. This may be an example of a feedback mecha-nism countering the predictions of critical taper theory.

Might the reduction in basal shear stress have resulted in collapse of thesouthern margin? Although the large fans and associated high pore fluid pressuresare to the north, rapid trench filling occurred along the rest of the margin, includingsouthern Oregon, probably resulting in elevated pore pressures margin-wide. With-out the frontally accreted submarine fans that buttress the northern margin, thismechanism may have been effective for the seaward-vergent southern Oregonwedge, which had little to prevent seaward collapse. We conclude that thismechanism could have been a significant contributor to the failure of the southernOregon margin, given the coincident timing of Pleistocene slide events and trench-filling/fan accretion, however, it fails to explain the age progression of the slides.Were lowered basal shear stress the primary mechanism for the southern Oregonslides, the age progression might be expected to run in the opposite direction (i.e.,


north to south), given that the advance of the thickened trench fill/fan sequence andattendant high fluid pressures was from north to south.

Seamount Subduction

Seamount subduction has been observed to have profound effects on accretion-ary wedges in a variety of settings (e.g., VON HUENE et al., 1996; LALLEMAND andLE PICHON, 1987). Although no direct evidence of subducting seamounts is obviousin the Cascadia accretionary complex, a number of shallowly buried seamounts arepresent just seaward of the base of slope. Figure 14 displays one of theseseamounts, the locations of which are shown on Figure 15. A search of all theavailable seismic records on the abyssal plain in Cascadia basin reveals fewseamounts in most areas. However, the southern Oregon margin has a relativelyhigh population of seamounts in close proximity to the margin, suggesting thatother may have been recently subducted. Recent gravity and magnetics modeling ofanomalies in the margin slump province suggests that there may be accreted

Figure 14Farnella two-channel seismic profile (JD 203) showing buried seamount seaward of the southern Oregon

margin (location shown in Fig. 15).


Figure 15Total field magnetics illustrating trends of basement magnetic lineations and pseudofaults with bathy-metric contours. Contour interval is 50 m above 200 m, 500 m below. Seamount positions mapped fromreflection data are shown as circles. Filled circle is seamount shown in Figure 14. Oblong anomalymodeled by FLEMING (1996) outlined in black. Possible subducting seamount shown by dashed blackcircle. Possible Cenozoic slide scarps shown by heavy arcuate lines. Magnetics data from the National

Geophysical Data Center.


seamounts or an accreted basaltic ridge beneath the area of the three mega-slides.FLEMING (1996) used both forward and inverse modeling to examine anomalies inthis area employing shipboard gravity and magnetics data available from NGDC.Results from both gravity and magnetics suggest that a linear high velocity featureof relatively short wavelength with respect to the magnetic signature extends fromabout 45°N to 42°N beneath the upper slope (Fig. 15). At its southern end, thisfeature constitutes several separate anomalies. FLEMING (1996) tentatively inter-prets this feature as an accreted ridge/seamount chain, although alternatively itcould be interpreted as a thrust sliver of the subducting plate or Siletzia basement.FLEMING (1996) observes that the linear feature cuts across the extension of apseudofault interpreted from seafloor magnetic anomalies by WILSON (1993).Although this feature is presently a linear anomaly, we note that it need not havebeen one at the time of accretion. If incoming seamounts are sheared from thebasement and accreted, they may form a linear trend by virtue of their accretedpositions against a ‘‘backstop’’ rather than their initial positions on the subductingplate.

Using the age progression of the slope failures and the current plate convergencevector, we can examine the possibility that the subducted seamounts are an aseismicridge or linear chain colliding with the margin. If we assume that each slide wastriggered by a subducted seamount or ridge as it swept past the present center ofeach slide, we can use the present geometry to determine the orientation of such aridge or linear chain. The center to center distance along the margin is 78 km forthe Heceta and Coos Basin slides. The plate convergence vector is approximately 40mm/yr. oriented 062°, using the NUVEL 1 poles (DEMETS et al., 1990). With anage difference of 350 ka between the two slides, we calculate that a linear ridge orseamount chain would have to be oriented about 350° in order to account for theage progression. Similarly, basement ridges responsible for the Blanco-Coos Basinand Blanco-Heceta slide pairs would be oriented 332°, and 343° respectively. Thisorientation does not correspond to known fracture zones, ridge trends, pseudo-faults, or other known basement trends. However, the reasonably good agreementbetween these orientations permits this hypothesis, particularly considering that theslide triggering need not occur as the ridge swept under the center of each slide, aswe have assumed. Examples of seamount subduction elsewhere suggest that thetypical upper plate expression of a subducted seamount is a relatively narrowdeformation trail, unlike the large slope failures of southern Oregon. Thus ifsubduction of basement topography triggered the megaslides, we conclude that alinear ridge is a more likely candidate than a linear seamount chain.

Forearc Extension

Another potential influence on the development of slope failure in southernOregon may be the observed structural response of the submarine forearc to


oblique subduction. A set of WNW trending left-lateral strike-slip faults mapped inthe Oregon and Washington forearc (GOLDFINGER et al., 1997, 1992; APPELGATE

et al., 1992) results in clockwise block rotation and northward transport of theaccretionary prism (GOLDFINGER, 1994; MCCAFFREY and GOLDFINGER, 1995).These faults define a domain of forearc deformation and transport that appears toterminate in southern Oregon with the southernmost transverse fault, the Thomp-son Ridge fault at 43°15� N. This structure lies in the center of the slide province(Fig. 2). At the southern terminus of the rotating/translating forearc sliver, wepredict margin-parallel extension at the juncture between translating and non-trans-lating parts of the forearc (Fig. 16). Similar extension has been proposed for theSumatran forearc (MCCAFFREY, 1991), and the Kuril forearc (KIMURA, 1986),resulting in forearc stretching as a result of changes in trench orientation withrespect to the plate convergence vector. If this process occurs in Cascadia, it mayresult in westward slumping; since any detached blocks would be unconstrained tothe west. The Heceta, Blanco and Coos Basin slide scarps are steeper and betterexpressed on their southern flanks. The average topography enclosed by the scarpsalso has a component of southward tilt, consistent with an asymmetrical expressionof margin parallel extension. We presently cannot place sufficient constraints on the

Figure 16Kinematic model showing a possible margin-parallel extensional mechanism for the southern Oregonmargin. NW trending sinistral strike-slip faults (GOLDFINGER et al., 1997) driven by oblique subduction,cause clockwise block rotation of the fault bounded blocks. Some extension is expected at the southernterminus of the rotated terrane, similar to proposed margin parallel extension at the Sumatra and Kurile

arcs.


kinematics of wedge tectonics in southern Oregon to evaluate this hypothesisfurther, nor are we able to determine whether it is consistent with the observed ageprogression.

In summary, we do not find that any of the three mechanisms we have examinedcan be excluded on the basis of our investigation, nor are any of them clearlydominant. We also infer that some form of basal subduction erosion is taking placeoff southern Oregon, based on the deep-seated failures and subduction of the debrisaprons. We are unable to determine however, whether the margin is retreating,accreting, or in steady state. Further evidence for basal erosion is suggested bysubsidence and seaward tilt of submarine banks and other areas adjacent to theslide province. Immediately landward of the Heceta and Blanco slide scarps lieHeceta and Coquille submarine banks. These two banks are rimmed by a latePleistocene low-stand shoreline (GOLDFINGER, 1994). The shorelines around thesetwo banks have been deformed subsequent to their formation, the most significantcomponent being overall subsidence along with both southward and seaward tilt.KULM and FOWLER (1974) also found that Heceta Bank began subsiding in thePleistocene, following more than 1 km of uplift since the middle Miocene. Theseaward tilt and subsidence of the banks, like the slope failures, is consistent withbasal erosion of the southern Oregon margin.

Is there evidence of older slope failures that would suggest margin retreat overtime scales longer than later Pleistocene? There are suggestions that this may havebeen the case. Several arcuate features, even larger than the megaslides discussedhere, shape the plan view of the Oregon continental shelf (Figs. 2 and 15). We havemade preliminary attempts to determine if these huge features might be oldermargin failures. Available seismic data do delineate a sharp boundary betweencoherent Neogene strata and deeper chaotic reflectors of unknown nature. Thearcuate shape and stratigraphic boundaries are intriguing and suggestive of such anorigin, and are the subject of ongoing investigation.

Implications for Cascadia Great Earthquakes and Tsunamis

An important implication of the catastrophic southern Oregon slides is thatsouthern Oregon can be considered an area of increased tsunami hazard relative tonorthern Oregon and Washington. A subduction earthquake, or indeed any earth-quake in the southern Oregon forearc carries with it the possibility of triggering amajor slide that could generate a tsunami larger than that generated by theearthquake alone.

A characteristic tsunami amplitude along the Oregon coastline closest to thelandslide can be estimated from an empirical correlation of WATTS (1998). Wemake this estimate for the Heceta slide, the youngest and best imaged of the threelarge slides. The empirical correlation was fitted from accurate numerical experi-ments of a two-dimensional model problem described by GRILLI and WATTS


(1999). A semi-elliptic mound was translated along a straight incline with a centerof mass motion s=s0 ln[cosh(t/t0)] derived by WATTS (1998), where s0 and t0 aredetermined by landslide density and geometry. The initial landslide geometry isdescribed by the length along the incline b, the maximum thickness perpendicular tothe incline T, the incline angle �, and the initial depth of the middle of the landslided. Following the scaling analyses of HAMMACK (1973) and WATTS (1998), anychosen near-field wave amplitude is an unknown function of the five nondimen-sional numbers

�

s0 sin �= f

�db

,Tb

, �, Sg, Ha0�

(1)

where Sg and Ha0 are nondimensional dynamical quantities related to landslidemotion. The submergence number Sg=g0 sin �/d is the ratio of the vertical lengthscale of landslide acceleration to the initial landslide submergence. The Hammacknumber Ha0= t0�gd/b is the ratio of the time scale of landslide acceleration to thetime it takes long waves to propagate over the length of the landslide. Thecharacteristic distance traveled along the incline is approximately s0=4.71b=94.2km, while the characteristic duration of landslide acceleration is approximatelyt0�3.76�b/g sin �=1086 s, both assuming a landslide specific density of �=1.95.We estimate that the motion of the center of mass was approximately 22 km, theaverage of the distance traveled by the toe, 40 km, and distance traveled by thetrailing edge of the block, about 5 km (e.g., see Fig. 12). Friction and cohesionalong the failure plane have been neglected. The maximum free surface depressionabove the middle of the initial landslide position can then be estimated from

��0.218 sin1.38 �T�bd�1.25

(2)

to within about �25% for the two-dimensional model problem considered byWATTS (1998). Equation (2) is provided in terms of primitive landslide geometryand is only valid if Ha0�3.76�d/b sin ��2, indicating the generation of linearwaves (WATTS, 1998).

Based on seismic reflection data and bathymetry, the landslide parameters forthe Heceta slide are: b=20 km; T=2 km; �=1.4°; and d=1.6 km. We estimatethe initial width of the slide was 20 km, measured from the headwall to the initialposition of the toe. The initial position of the toe is presently buried, nonetheless weestimated its position by projecting the adjacent deformation front southward (Figs.2 and 9). The angle of the slip plane is estimated from seismic reflection dataconverted to depth. We estimate that the maximum free surface depression is ��61m immediately above the middle of the initial landslide position. The tsunamiamplitude above the landslide is expected to be reduced by three-dimensionaleffects, and by internal deformation of the slide blocks, which we have neglectedhere. Moreover, the first elevation wave to strike the coastline would be 3–5 times


Figure 17Shaded-relief bathymetry of the southernmost Oregon continental slope, showing a possible incipientslump measuring about 20×20 km. The toe of the slide has moved 3.5 km seaward of the deformationfront along WNW-trending tear faults (indicated by arrows). The detached block appears to haverotated seaward along a listric basal detachment, so that the eastern part is down and the western partis up relative to the adjacent continental slope, shown by U/D symbols. Location shown on Figure 2.

smaller than the depression amplitude estimated here. However, wave focusing dueto offshore bathymetry features, shoaling effects during shoreline interaction, andthe leading depression wave arriving before the elevation wave all increase thetsunami hazard (TADEPALLI and SYNOLAKIS, 1994).

The youngest of the megaslides appears to be Pleistocene, and the recurrenceinterval of these events is 350–850 ka, thus these events appear to be relatively rare.Nevertheless, there is the possibility of a new occurrence, or repeated failure of theexisting slides. For example, a large scarp 25 km across at 42°07� N appears to bean incipient slump (Fig. 17). This feature has an arcuate headwall, sidewall faults,and shows evidence of rotation toward the rear of the slide, supporting thisinterpretation. At present, the slip on this block, interpreted from the seawardgeomorphic position of the toe relative to the slope base, is approximately 3.5 km.Future intraplate or interplate earthquakes may release this block as a catastrophicfailure.


Finally, we note that although the Blanco slide is about 1 million years old, thelatitudinal limits of the slide debris on the abyssal plain are approximately the sameas the latitude limits of the scarp on the continental slope. Given that the platesshould have converged �40 km/Ma toward the northeast (055° using Nuvel 1Apoles), we expected that the slide debris should have translated north of the scarpdue to oblique convergence. The magnitude of this translation should be about 20km, the margin parallel component of 40 km of convergence, yet within theresolution of the data, no such translation is observed. Although the data resolu-tion is not high, it is more than adequate to observe a 20 km offset. We infer fromthis that there is minimal or no margin parallel motion between the slide debris andthe continental slope. This conclusion is relatively insensitive to errors in estimatingthe age of the slide. This relation suggests to us that the forearc is translatingnorthward at or near the full margin-parallel component of the plate convergencerate. Such forearc translation is observed at many subduction zones, and has beeninferred for Cascadia based on deformation rates in the submarine forearc(GOLDFINGER et al., 1992, 1997; MCCAFFREY and GOLDFINGER, 1995; WELLS etal., 1998).

Conclusions

Super-scale slumping of the southern Oregon Cascadia margin has been animportant tectonic process operating in late Quaternary time. At least threemegaslides have occurred, involving much of the accretionary prism. The massivenature of slump debris buried in the abyssal plain, and the considerable distance thedebris traveled, suggest that the slides were probably single catastrophic events,although we presently cannot exclude multiple events. The evidence of extensivedeep-seated slope failure, subsidence and tilting of adjacent submarine banks, andthe apparent subduction of slide debris suggest that the southern Oregon margin isundergoing basal tectonic erosion. This does not preclude frontal accretion, whichis occurring simultaneously. We cannot presently determine whether there is neterosion or accretion of material at the southern Oregon margin. In contrast, thenorthern Oregon and Washington accretionary wedge is currently accreting andoutbuilding as Pleistocene submarine fans are rafted landward on the subductingJuan de Fuca plate.

The Oregon megaslides may have multiple driving mechanisms. Lowered Pleis-tocene basal shear stress on the megathrust, seamount/ridge subduction, andarc-parallel extension may all play a role in the tectonics of this segment of themargin. Subduction of a basement topographic feature(s) offers the best explana-tion for the observed south to north age progression of the three megaslides,although the preferred NNW orientation of the inferred ridge does not correspondto known basement trends. A seamount province that is potentially related to this


process has been identified on the adjacent abyssal plain with seismic reflectiondata, and may continue beneath the wedge based on gravity and magnetics analysis.

Southern Oregon can be defined as an area of greater tsunami hazard relative toother margin segments by virtue of its failure mode in great slides, and due to thepresence of a large incipient slump that maybe released in a future earthquake. Therelative lack of displacement between the oldest slump debris and its correspondingscarp on the continental slope suggests little margin-parallel motion between them,despite oblique subduction. This lack of relative motion suggests that the southernCascadia forearc may be translating northward at or near the full margin-parallelcomponent of the plate rate.

Acknowledgements

Supported by National Science Foundation Grants OCE-8812731 and OCE-8821577, NOAA National Undersea Research Program Award UAF 96-0060, andby the National Earthquake Hazards Reduction Program, U.S. Geological Survey,Department of Interior, under award 14-08-001-G1800. We gratefully acknowledgethe use of an extensive collection of single and multichannel proprietary seismicdata. As part of a standard use agreement, company names and detailed tracklinenavigation are omitted. The manuscript was substantially improved by reviews byRoland von Huene, Gregory Moore, and an anonymous reviewer.

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(Received May 28, 1998, revised/accepted February 21, 1999)

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Super-scale Failure of the Southern Oregon Cascadia Margin

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