Super-Scale Slumping of the Southern Oregon Cascadia Margin: 
Tsunamis, Tectonic Erosion, and Extension of the Forearc

2000 Pure and Applied Geophysics Paper  PDF

Rigging a tsunami simulation of the Central Oregon Heceta Slide at the O.H. Hinsdale Wave Facility

Using SeaBeam bathymetry and multichannel seismic reflection records on the southern Oregon continental margin, we have identified several large submarine landslides that encompass much of the accretionary wedge of the southern Oregon Cascadia margin.  The area affected by these slides is approximately 8000 km²,  and  involve an estimated 12,000 km³ of the accretionary wedge.  Debris from these slides is buried or partially buried beneath the abyssal plain, covering a subsurface area of at least 7500 km².  Three arcuate slump scars are nearly coincident with the shelf edge on their eastern margin, thus spanning the full width of the active wedge in southern Oregon.  In shaded relief SeaBeam bathymetry the accretionary wedge within the slump area is chaotic, with poorly defined thrust ridges and basins.  In reflection profiles on the slope, reflectors are commonly chaotic, with poor penetration of seismic energy and numerous diffractions.  The bathymetric scarps correlate with listric detachment faults on reflection profiles that cut deeply into the continental slope with as much as 800 m vertical separation at the surface.  The ages of the three major slides decrease from south to north.  This series of slumps traveled at least 30 km out onto the abyssal plain in at least 3 catastrophic slides, which may have been triggered by subduction earthquakes.  The structure and morphology of the slides indicates catastrophic rather than incremental slip.  The slides would have generated large tsunami in the Pacific basin, probably larger than that generated by an earthquake alone.  Mass wasting features and buried slump debris appear to terminate at the subducting Blanco Fracture Zone.  These slides and subduction of the slide debris imply that subduction erosion and narrowing of the southern Oregon margin has occurred over approximately the last 1 Ma, and may be related to tectonic transport and extension of the southern Oregon forearc.

Figure 1.  Onshore-offshore shaded relief image of the Cascadia subduction zone, Oregon, USA.  This image was compiled from onshore DEM's (USGS) onshore, offshore EEZ SeaBeam and BSSS swath bathymetry, and interpolated surfaces generated from digitized contours where swath bathymetry was unavailable.  The image resolution is 100 meters, and is the first high-resolution onshore/offshore topographic image of a major portion of the Cascadia convergent margin.  Box shows area of massive slope failures off southern Oregon shown in Figure 2.  Note the morphologic contrast between the chaotic southern Oregon continental slope and "Normal" accretionary wedge fold-thrust belt in northern Oregon.
 
 
Figure 2.  Shaded relief image of the southern Oregon continental margin, from 100 meter gridded bathymetry.  Beneath the abyssal plain between 42 16.55' N and 44 13.75' N, seismic reflection profiles show that the sedimentary section includes several thick intervals of hummocky chaotic reflectors (Figures 3, 4, 5, 6, and 8).  These buried and partially buried chaotic packages cover a subsurface area of at least 7500 km2.  We tentatively identify these chaotic packages as the debris from a series of massive slope failures.  The areal distribution of the buried packages occupy at least four stratigraphic levels, shown by colored superposed patterns.  The chaotic reflectors are buried progressively deeper southward along the margin, suggesting an age progression of slope failure from south to north.  The shaded polygons represent the minimum distribution based on available seismic reflection profiles.  The arrows on the continental slope point to proposed sump scars identified on the basis of morphology, and reflection profiles.  The morphologic evidence for the scars is stronger from south to north, also suggesting an age progression of major slope failures.  The evidence for these scars is discussed further in Figures 6, 9, 10, 11, and 12.  Colored lines show locations of seismic sections A-A' through F-F', keyed to boxes of the same color surrounding the figures.

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Figure 3.  Industry multichannel reflection profile (A-A' in Figures 2 and 6)across the Cascadia plate boundary showing the northernmost slump debris on the continental slope, shallowly buried debris beneath the abyssal plain, and recent slump debris on the abyssal plain.  Coherent planar abyssal plain reflectors can be traced at least 11 km landward beneath the base of the slope.  We suggest that no the tectonic plate boundary is further east, and no subduction thrust is present on this profile.  Velocity pull-up accounts for the abrupt rise in abyssal plain reflectors beneath the slope.  Corresponds to YELLOW polygon in Figure 2.
 

Figure 4.    Industry multichannel reflection profile (B-B' in Figure 2) across the Cascadia plate boundary showing shallowly buried debris beneath the abyssal plain.  Buried debris package rises to the surface and is exposed at extreme right, where the base of slope might better be termed a "debris front" than a deformation front (See Figures 6 and 11).  Corresponds to GREEN polygon in Figure 2.

Figure 5.  Industry multichannel reflection profile (C-C' in Figure 2) across the Cascadia plate boundary showing buried debris beneath the abyssal plain, somewhat deeper in the section that A-A' and B-B'.  The morphology of the continental slope adjacent to this older slope failure is consistent with the restoration of a fold and thrust belt on the lower slope, also shown by the basal thrust offsetting the seafloor at the base of slope.  Corresponds to BLUE polygon in Figure 2.

Figure 6.  Detail of the northernmost slump area off central Oregon.  This is the best expressed and largest of the proposed slope failures.  Abyssal plain debris is the shallowest in the sedimentary section opposite this feature, suggesting it is the most recent event.  The tectonic deformation front is buried by the debris slide.  The top of the debris 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 cm/1000 yrs (based on data in Nelson, 1968) suggests an age of approximately 50-130 ka for this event.  The slump scar has been buried in several locations by progradational lobes ("PL"), presumably deposited during Pleistocene sea level low-stands.  These lobes have themselves failed in secondary slumps ( "SF"), superimposing smaller slump piles on the surface of the larger detached block.  A small recent slump at the deformation front ( "RS") has deposited debris blocks at the surface of the abyssal plain.  Surficial morphological indicators of massive slope failure include:   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 across the scar;  4)  Blocky, convoluted base of slope;  5)  Lack of an identifiable thrust fault at base of slope.  Dashed white line shows projected tectonic deformation front beneath the debris pile.  Colored lines show locations of seismic sections A-A' and F-F', keyed to boxes of the same color surrounding the figures.
 

Figure 7.  USGS unmigrated multichannel reflection profile 77-50 (D-D' in Figure 2) across the Cascadia plate boundary showing buried debris beneath the abyssal plain, somewhat older and deeper still in the section that C-C'.   Note landward vergent thrusts in this section.  Corresponds to BLUE polygon in Figure 2.
 

Figure 8.  USGS unmigrated multichannel reflection profile 77-49 (E-E' in Figure 2) across the Cascadia plate boundary showing buried debris beneath the abyssal plain, slightly older and deeper in the section that D-DÕ.  Corresponds to BLUE polygon in Figure 2.

Figure 9.  Interpreted industry single channel sparker profile (F-F' in Figures 2 and 6) across the northern proposed slump scar off central Oregon.  Shallow seaward dipping reflectors are truncated at the scarp, which we interpret as the headwall scarp.  Several other similar profiles show this same relationship.  The line terminated at the left (west end) of this figure.

Figure 10.  Shaded-relief bathymetry of the southernmost Oregon continental slope, showing a possible incipient slump measuring about 20 x 20 km. Location shown on Figure 2.   The toe of the slide has moved 1-2 km seaward of the deformation front along WNW-trending tear faults (indicated by arrows).  The detached block appears to have rotated seaward along a listric basal detachment, so that the eastern part is down (D), and the western part is up (U), relative to the adjacent continental slope.
Figure 10.  Shaded-relief bathymetry of the southernmost Oregon continental slope, showing a possible incipient slump measuring about 20 x 20 km. Location shown on Figure 2.   The toe of the slide has moved 1-2 km seaward of the deformation front along WNW-trending tear faults (indicated by arrows).  The detached block appears to have rotated seaward along a listric basal detachment, so that the eastern part is down (D), and the western part is up (U), relative to the adjacent continental slope.

Figure 11.  SeaMarc 1A sidescan sonar image of part of the base of the continental slope.  Area of figure indicated by label SS in Figure 6.  The base of the continental slope from about 43 15' to the northern limit of the slumped area at 44deg 13.75' is characterized by irregular, blocky 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 thrust fault along this part of the margin in seismic, bathymetric, or sidescan data.  The sidescan coverage is nearly complete along the deformation front between these latitudes.
 
 
 
 
 
 
 
 
 
 

 Figure 12.  Interpreted industry multichannel seismic profile (G-G' in Figure 2) across the northern slump scar off central Oregon.  We interpret the anticline at left as a gravitationally driven fold forming above the listric detachment surface.  Faulting has continued at a slower pace since the initial failure, indicated by the faulted growth strata in the syncline at right center.
 

Discussion
 This tectonic style of the southern Oregon margin differs sharply from northern Oregon, Washington and northern California.  In northern Oregon and Washington, the continental margin is clearly accretionary, with young, well defined thrust ridges and faults characterizing a youthful 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 decollement  is seaward vergent south of 44  50', and landward vergent from that point northward to Vancouver Island.  The decollement in the landward vergent section of the margin offscrapes much of not all of the sedimentary section (Mackay at al., 1992; 1995), whereas the seaward vergent thrusts in southern Oregon override much of the sedimentary section and embedded slump debris.  The extreme narrowness of the margin, seaward vergence of the subduction decollement, and mass wasting of the southern Oregon margin suggest that the southern Oregon margin is undergoing basal subduction erosion and simultaneous frontal accretion.
 There may be several causes for the shift from an accreting margin in Washington and northern Oregon to an eroding margin in southern Oregon.  In the north the sediment supply is much greater, with the large Pleistocene Astoria and Nitinat submarine fans dominating the sedimentary section.  Off southern Oregon, despite the presence of the relatively high topography of the Klamath mountains, the sediment supply is relatively low.  The rapid deposition of the large submarine fans contributes to their subsequent accretion in that high fluid pressures generated in the section by rapid deposition tend to favor landward vergent thrusting at the deformation front (Seely, 1977;  Mackay et al., 1992; 1995).   Landward 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 destabilizing the southern Oregon accretionary wedge may have been increased fluid pressures generated during rapid sediment deposition during the Pleistocene.  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 wedge was at a critical taper angle, this would have brought the wedge to a super-critical (i.e. oversteepened) condition.  The oversteepened wedge may then have failed by gravity-driven detachment to re-establish a critical taper angle.   This hypothesis does not explain the obvious age progression, younger in the north, that is observed, in fact a reverse age progression 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 change with 2 channel seismic reflection data collected during the GLORIA/Farnella cruises of the US EEZ.

Conclusions
 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 that involve 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.  The narrowness of the accretionary wedge in southern Oregon, the extensive downslope movement, and the apparent subduction of slide debris suggests that the southern Oregon margin is undergoing tectonic erosion.  The northern Oregon and Washington accretionary wedges, in contrast, are accreting and outbuilding as Pleistocene submarine fans are rafted landward on the subducting Juan de Fuca plate.
 The deep level of detachments beneath the mega-slides in southern Oregon suggests that much of seaward part of that margin is unlikely to be involved in elastic strain accumulation leading to future subduction earthquakes.  This means that thermal and elastic "locked zone" models such as proposed by Hyndman and Wang (1995) should be modified to take into acount the rheology and geology of the margin, since the seaward portion of the seismogenic zone is unconstrained by either thermal or elastic models.   Finally, southern Oregon can be defined as an area of greater tsunami hazard by virtue of its proximity to several major slides, and due to the presence of a large incipient slump that maybe released in a future earthquake.
 

Acknowledgments
Supported by National Science Foundation Grants OCE-8812731 and OCE-8821577 and by the National Earthquake Hazards Reduction Program, U.S. Geological Survey, Department of Interior, under award 14-08-001-G1800.
 
References
MacKay, M.E., 1995, Structural variation and landward vergence at the toe of the Oregon accretionary prism: Tectonics, in press.
MacKay, M.E., Moore, G.F., Cochrane, G.R., Moore, J.C., and Kulm, L.D., 1992, Landward vergence and oblique structural trends in the Oregon margin accretionary prism: Implications and effect on fluid flow:  Earth and Planetary Science Letters, v. 109, p. 477-491.
Nelson, C.H., 1968, Marine geology of the Astoria deep-sea fan [Ph.D. Thesis]:  Oregon State University, Corvallis, Or., 287 p.
Seely, D.R., 1977, The significance of landward vergence and oblique structural trends on trench inner slopes, in M. Talwani and W.C. Pitman, eds., Island    Arcs, Deep Sea Trenches and Back-Arc Basins:  Washington, D.C.,  American. Geophysical Union,  Maurice Ewing Series I,  p. 187-198.

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