Upcoming Talks and Other Things
November 10-11, 2016, National Academy of Sciences Building
2101 Constitution Ave NW Washington DC 20418
City Club Earthquake Forum, Kells Pub Portland, November 1, 5:30 pm
New Yorker Festival, Manhattan, October 3, School of Visual Arts, Theatre 1, 10 am.
NWEA Workshop, Hood River Inn, October 2.
Oregon Coast Economic Summit, August 27, Grand Ronde.
The Really Big One: A Public Forum On Earthquake Hazards
and Preparedness in the PNW, University of Oregon, Eugene, August 6, 7 PM. 156 Straub Hall.
Preliminary study of existing lake sedimentary records suggests a record of great earthquakes.
New core and high resolution reflection data illuminate thesouthern Cascadia paleoseismic record.
Seismically generated turbidites in Effingham Inlet, western Vancouver Island.
Using SeaBeam bathymetry and multichannel seismic reflection records on the southern Oregon continental margin, we have identified three large submarine landslides on the southern Oregon Cascadia margin. The area enclosed by the three arcuate slide scarps is approximately 8,000 km2, and involves an estimated 12,000-16,000 km3 of the accretionary wedge. The three arcuate slump escarpments are nearly coincident with the continental shelf edge on their landward margins, spanning the full width of the accretionary wedge. Debris from the slides is buried or partially buried beneath the abyssal plain, covering a subsurface area of at least 8,000 km2. The three major slides, called the Heceta, Coos Basin and Blanco slides, display morphologic and structural features typical of submarine landslides. Bathymetry, sidescan sonar, and seismic reflection profiles reveal that regions of the continental slope enclosed by the scarps are chaotic, with poor penetration of seismic energy and numerous diffractions. These regions show little structural coherence, in strong contrast to the fold thrust belt tectonics of the adjacent northern Oregon margin. The bathymetric scarps correlate with listric detachment faults identified on reflection profiles that show large vertical separation and bathymetric relief. Reflection profiles on the adjacent abyssal plain image buried debris packages extending 20-35 km seaward of the base of the continental slope. In the case of the youngest slide, an intersection of slide debris and abyssal plain sediments, rather than a thrust fault, mark the base of slope. The ages of the three major slides decrease from south to north, indicated by the progressive northward shallowing of buried debris packages, increasing sharpness of morphologic expression, and southward increase in post-slide reformation of the accretionary wedge. The ages of the events, derived from calculated sedimentation rates in overlying Pleistocene sediments, are approximately 110 ka, 450 ka, and 1,210 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 slide packages, and the considerable distance traveled suggests catastrophic rather than incremental slip, although there 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 slide off southern Oregon that may be released in a subduction earthquake. The occurrence of the slides and subsequent subduction of the slide debris, along with evidence for margin subsidence implies that basal subduction erosion has occurred over at least the last 1 Ma. The massive failure of the southern Oregon slope 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 of latitudinal offset between the oldest slide debris and the corresponding scarp on the continental slope implies that the forearc is translating northward at a substantial fraction of the margin-parallel convergence rate.
Figure 1. Onshore-offshore shaded relief image of the Cascadia subduction zone, Oregon, USA. This image was compiled from onshore USGS DEM's onshore, offshore NOAA SeaBeam and BSSS swath bathymetry, and interpolated surfaces generated from digitized contours where swath bathymetry was unavailable. The image resolution is 100 meters. Relief image (and those to follow) shown without contours and only minimal depth shading to emphasize morphology. Note morphologic contrast of the lower slope between 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.
(Click image for larger image)
Although the Oregon convergent margin is frequently cited as a type example of a seaward-vergent accretionary wedge, this characterization is only applicable to a small part of the northern Oregon margin. Taken as a whole, the Oregon margin is better characterized by significant along-strike variability in structural style and wedge morphology (Figure 1). The northern Oregon and Washington accretionary wedge is a broad landward-vergent thrust system with widely spaced folds, and a decollement stepping down to the basement, with virtually all incoming sediment being frontally accreted (Flueh et al., 1996; MacKay, 1995; Goldfinger, 1994; MacKay et al., 1992; Snavely and McClellan, 1987; Silver, 1972). This low-taper wedge is composed primarily of the Pleistocene Astoria and Nitinat Fans, which have been accreting outboard of a narrow, older Cenozoic accretionary complex. The older complex in turn lies outboard of an outer arc high and Cenozoic forearc basin.
The steep, narrow, southern margin is poorly known, but as we will present here, it is characterized by massive slope failures that dominate the structure and morphology of the continental slope between 42° and 44° N latitude. We have identified arcuate escarpments on the Oregon continental slope enclosing regions of hummocky topography, and underlain by detachment surfaces that delineate at least three failure zones encompassing much of the southern Oregon continental slope (Figure 2). Beneath the abyssal plain, we have found widespread subsurface and partially buried debris aprons. We first present evidence for buried slump debris beneath the abyssal plain, then discuss morphologic and structural evidence for the slides themselves. We then determine the age of the slides, and discuss possible mechanisms for the apparent massive collapse of the margin, implications for earthquake and tsunami hazards, and forearc deformation.
Much of the continental slope off southern Oregon has a distinctive morphology that can be easily differentiated from the northern Oregon, Washington and northern California margins, and from the adjacent uppermost slope (Figures 1 and 2). Four morphologic features distinguish the southern Oregon slope: 1) Lack of or poor definition of accretionary wedge fold-thrust ridges and slope basins; 2) Hummocky surface morphology; 3) Pervasive closely-spaced linear trends; 4) Large arcuate scarps enclosing domains characterized by the first three morphologic elements. Examination of shaded relief bathymetry (Figure 2) reveals three large arcuate scarps, the northernmost of which is both the largest and best defined. The upper slope scarp morphology is distinct in the northern area, becoming progressively less so to the south. From north to south, these three large scarps are 76, 68 and 65 km in length, and 33, 30, and 34 km in width. They enclose 2874 km2, 2304 km2, and 2713 km2 respectively, totaling 7890 km2 in area. We have named these features the Heceta slide, the Coos Basin slide, and the Blanco slide respectively after adjacent features on the margin and nearby coast.
In addition to the northward increasing definition of the slump scar morphology, the three other distinguishing morphologic elements (poor fold-thrust definition, hummocky topography, and closely spaced linear trends) also exhibit a north-south variation.
Figure 2. Shaded relief image of the southern Oregon continental margin. Beneath the abyssal plain between 42° 16.55' N and 44° 13.75' N, reflection reveal several thick intervals of hummocky, chaotic reflectors (Figures 3-8,13; profile locations A-H shown here. R/V Farnella two-channel reflection profiles on abyssal plan shown with time ticks. Buried packages at three stratigraphic levels, shown by superposed patterns. Upper Blanco slide is the smaller polygon within the Blanco Slide polygon. The patterned polygons represent the minimum distribution based on available reflection data, and are dashed where inferred. Interpreted slump scarps indicated by white arrows. Burial of the frontal thrust by the Heceta slide 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.
Figure 3. Industry multichannel reflection profile (A-A' in
2 and 6)across the Cascadia plate boundary showing the northernmost
debris on the continental slope, shallowly buried debris beneath the
plain, and recent slump debris on the abyssal plain. Coherent
abyssal plain reflectors can be traced at least 11 km landward beneath
the base of the slope. We suggest that no the tectonic plate
is further east, and no subduction thrust is present on this
Velocity pull-up accounts for the abrupt rise in abyssal plain
beneath the slope. Corresponds to YELLOW polygon in Figure 2.
Figure 4. Detail of the northernmost slump area off central
This is the best expressed and largest of the proposed slope
Abyssal plain debris is the shallowest in the sedimentary section
this feature, suggesting it is the most recent event. The
deformation front is buried by the debris slide. The top of the
pile is covered by 30-80 m of sediment (Figure 3), based on a sediment
velocity of 1650 m/s. An estimated sedimentation rate of 600
yrs (based on data in Nelson, 1968) suggests an age of approximately
ka for this event. The slump scar has been buried in several
by progradational lobes ("PL"), presumably deposited during Pleistocene
sea level low-stands. These lobes have themselves failed in
slumps ( "SF"), superimposing smaller slump piles on the surface of the
larger detached block. A small recent slump at the deformation
( "RS") has deposited debris blocks at the surface of the abyssal
Surficial morphological indicators of massive slope failure
1) Lack of coherent fold and thrust belt typical of accretionary
wedges; 2) Chaotic surficial morphology of area enclosed by
scar; 3) Sharply contrasting surface morphology
the scar; 4) Blocky, convoluted base of slope;
Lack of an identifiable thrust fault at base of slope. Dashed
line shows projected tectonic deformation front beneath the debris
Colored lines show locations of seismic sections A-A' and F-F', keyed
boxes of the same color surrounding the figures.
5. SeaMarc 1A sidescan sonar image of part of the base of the
slope. Area of figure indicated by label SS in Figure 6.
base of the continental slope from about 43 15' to the northern limit
the slumped area at 44deg 13.75' is characterized by irregular,
material that we interpret as the onlapping of abyssal plain sediments
on the top of the slumped debris pile. We see no evidence of a
fault along this part of the margin in seismic, bathymetric, or
data. The sidescan coverage is nearly complete along the
front between these latitudes.
This tectonic style of the southern Oregon margin differs sharply
from northern Oregon, Washington and northern California. In
Oregon and Washington, the continental margin is clearly accretionary,
with young, well defined thrust ridges and faults characterizing a
wedge that is largely Pleistocene in age. The accretionary wedge
is widest in Washington, ~100 km, and narrows southward to 30-50 km off
southern Oregon. Reflection profiles show that much of the slump
debris is presently being subducted. The subduction
is seaward vergent south of 44 50', and landward vergent from
point northward to Vancouver Island. The decollement in the
vergent section of the margin offscrapes much of not all of the
section (Mackay at al., 1992; 1995), whereas the seaward vergent
in southern Oregon override much of the sedimentary section and
slump debris. The extreme narrowness of the margin, seaward
of the subduction decollement, and mass wasting of the southern Oregon
margin suggest that the southern Oregon margin is undergoing basal
erosion and simultaneous frontal accretion.
There may be several causes for the shift from an accreting
in Washington and northern Oregon to an eroding margin in southern
In the north the sediment supply is much greater, with the large
Astoria and Nitinat submarine fans dominating the sedimentary
Off southern Oregon, despite the presence of the relatively high
of the Klamath mountains, the sediment supply is relatively low.
The rapid deposition of the large submarine fans contributes to their
accretion in that high fluid pressures generated in the section by
deposition tend to favor landward vergent thrusting at the deformation
front (Seely, 1977; Mackay et al., 1992; 1995).
vergence in turn promotes accretion because the decollement
frequently steps down to near the basement, offscraping the entire
incoming sedimentary package, where seaward vergence permits subduction
of more of the incoming section. A possible mechanism for
the southern Oregon accretionary wedge may have been increased fluid
generated during rapid sediment deposition during the
A consequent reduction in basal shear stress on the megathrust may have
led to both landward vergent accretion of the fans in the north,
and destabilization of the southern margin. If the accretionary
was at a critical taper angle, this would have brought the wedge to a
(i.e. oversteepened) condition. The oversteepened wedge may then
have failed by gravity-driven detachment to re-establish a critical
angle. This hypothesis does not explain the obvious age
younger in the north, that is observed, in fact a reverse age
might be expected based on sediment progradation from northern sources
during the Pleistocene. A better explanation may be simple basal
erosion of the wedge by seamounts on the subducting plate. There
are presently a number of seamounts near the deformation front, buried
by abyssal plain sediment. These features were imaged by chance
2 channel seismic reflection data collected during the GLORIA/Farnella
cruises of the US EEZ.
Super-scale slumping of the southern Oregon Cascadia margin has been an important tectonic process operating in late Quaternary time. At least three mega-slides have occurred, involving much of the accretionary prism. The massive nature of slump debris buried in the abyssal plain, and the considerable distance the debris traveled, suggest that the slides were probably single catastrophic events, although we presently cannot exclude multiple events. The evidence of extensive deep-seated slope failure, subsidence and tilting of adjacent submarine banks, and the apparent subduction of slide debris suggest that the southern Oregon margin is undergoing basal tectonic erosion. This does not preclude frontal accretion, which is occurring simultaneously. We cannot presently determine whether there is net erosion or accretion of material at the southern Oregon margin. In contrast, the northern Oregon and Washington accretionary wedge is currently accreting and outbuilding as Pleistocene submarine fans are rafted landward on the subducting Juan de Fuca plate.
The Oregon mega-slides may have multiple driving mechanisms. Lowered Pleistocene basal shear stress on the megathrust, seamount/ridge subduction, and arc-parallel extension may all play a role in the tectonics of this segment of the margin. Subduction of a basement topographic feature(s) offers the best explanation for the observed south to north age progression of the three megaslides, although the preferred NNW orientation of the inferred ridge does not correspond to known basement trends. A seamount province that is potentially related to this process has been identified on the adjacent abyssal plain with seismic reflection data, 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 to other margin segments by virtue of its failure mode in great slides, and due to the presence of a large incipient slump that maybe released in a future earthquake. The relative lack of displacement between the oldest slump debris and its corresponding scarp on the continental slope suggests little margin-parallel motion between them, despite oblique subduction. This lack of relative motion suggests that the southern Cascadia forearc may be translating northward at or near the full margin-parallel component of the plate rate.
Goldfinger, C., Kulm, L.D., McNeill, L.C., and Watts, P., 2000, Super-scale failure of the southern Oregon Cascadia margin: Pure and Applied Geophysics, v. 157,. p. 1189-1226.
Supported by National Science Foundation Grants OCE-8812731 and OCE-8821577, NOAA National Undersea Research Program Award UAF 96-0060, and by the National Earthquake Hazards Reduction Program, U.S. Geological Survey, Department of Interior, under award 14-08-001-G1800. We gratefully acknowledge the use of an extensive collection of single and multichannel proprietary seismic data. As part of a standard use agreement, company names and detailed trackline navigation are omitted. The manuscript was substantially improved by reviews by Roland von Huene, Gregory Moore, and an anonymous reviewer.