Super-scale failure of the southern Oregon Cascadia margin

Using SeaBeam bathymetry and multichannel seismic reflection records we hav e identified three large submarine landslides on the southern Oregon Cascad ia margin. The area enclosed by the three arcuate slide scarps is approxima tely 8000 km(2), and involves an estimated 12,000-16,000 km(3) of the accre tionary wedge. The three arcuate slump escarpments are nearly coincident wi th the continental shelf edge on their landward margins, spanning the full width of the accretionary wedge. Debris from the slides is buried or partia lly buried beneath the abyssal plain, covering a subsurface area of at leas t 8000 km2. The three major slides, called the Heceta, Coos Basin and Blanc o slides, display morphologic and structural features typical of submarine landslides. Bathymetry, sidescan sonar, and seismic reflection profiles rev eal that regions of the continental slope enclosed by the scarps are chaoti c, with poor penetration of seismic energy and numerous diffractions. These regions show little structural coherence, in strong contrast to the fold t hrust belt tectonics of the adjacent northern Oregon margin. The bathymetri c scarps correlate with listric detachment faults identified on reflection profiles that show large vertical separation and bathymetric relief. Reflec tion profiles on the adjacent abyssal plain image buried debris packages ex tending 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 se diments, rather than a thrust fault, mark the base of slope. The age of the three major slides decreases from south to north, indicated by the progres sive northward shallowing of buried debris packages, increasing sharpness o f 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 1 10 ka, 450 ka, and 1210 ka. This series of slides traveled 25-70 km onto th e abyssal plain in at least three probably catastrophic events, which may h ave been triggered by subduction earthquakes. The lack of internal structur e in the slide packages, and the considerable distance traveled suggest cat astrophic rather than incremental slip, although there could have been mult iple events. The slides would have generated large tsunami in the Pacific b asin, possibly larger than that generated by an earthquake alone. We have i dentified a potential future slide off southern Oregon that may be released in a subduction earthquake. The occurrence of the slides and subsequent su bduction of the slide debris, along with evidence for margin subsidence imp lies 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 deb ris and the corresponding scarp on the continental slope implies that the f orearc is translating northward at a substantial fraction of the margin-par allel convergence rate.