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╠ ├ х Other Crustal Faults in Western and Offshore Oregon
Yakima fold belt
R.S. Yeats
Willamette Valley
The Portland Basin and Tualatin Valley were discussed by Ian Madin at the meeting. There are other faults described by Yeats et al. (1996, USGS Professional Paper 1560) and shown on 1:100,000 maps. Except for the Mount Angel fault, no new information has been obtained since the last time the fault map was updated. None of these faults can be shown to cut Missoula flood deposits (Willamette Silt) of 13-16 ka age.
The largest of these faults is the NE-striking Corvallis fault, which had an earlier history of a NW over SE thrust based on gravity measurements in Chris GoldfingerТs MS thesis, but more recently the motion has been strike slip based on horizontal slickenlines in Siletz River volcanics in a quarry west of Corvallis. Parke Snavely thought the fault continued SW of Philomath into and perhaps across the Coast Range, but I know of no Quaternary evidence for this. When T. Nakata was at OSU on sabbatical, he claimed to see geomorphic evidence of activity, but he was never able to convince me of this. Because of the length of this fault and its occurrence across a population center of some 125,000 people, it very much needs LiDAR coverage, which should resolve the question of geomorphic evidence of activity. The fault can be traced NE to the Willamette River, where trenching could resolve the question of post-Missoula-flood activity. It is on the Benton County and City of Corvallis GIS maps, and is taken into account in development decisions, but the grounds for rejecting a development on or near the fault are weak based on our current knowledge.
Oregon Coast and offshore
I think that most work was incorporated into the last round of fault maps of Oregon, but here is a summary of where we are. The references for coastal and near-shore faults include Kelsey et al. (1996 GSA Bull. 108:843-860) for central Oregon, McNeill et al. (1998, in Stewart and Vita Finzi, eds., Coastal Tectonics, Geol. Soc. London Special Publ. 146:319-340), and Yeats et al. (1998 GSA Bull. 110:572-587) on the Stonewall anticline offshore. The Stonewall anticline paper has a Holocene slip rate on the blind thrust underlying the anticline of 0.9-1.3 mm/yr with the potential for an earthquake of Mw 6.8 ( 0.25 based on back-tilting of a stream crossing the anticline and the age of post-glacial eustatic sea level rise. The coastal and near-shore faults are based on deformed terraces and near-shore geophysics; see also McCrory for SW Washington, also included in the McNeill et al. paper, and other papers by Kelsey farther south in Oregon.
There is controversy about the Yaquina Bay fault, important because if it is there, it goes beneath the vintage-1930s Yaquina Bay bridge, the icon of Newport. The fault was mapped by Snavely et al. (1976) in their 15-minute map of Yaquina and Toledo quads (USGS I-867) and extended west across Hwy 101 by Kelsey et al. based on an apparent offset of late Pleistocene terraces between the N and S sides of Yaquina Bay. The N side would be up, which fits the restriction of submerged Holocene marshes to Yaquina Bay south of the fault. This terrace correlation (and thus the evidence for the fault at Hwy 101) is disputed by George Priest and Curt Peterson, and McNeill was unable to see a fault on a seismic line just off the Newport jetty or on a detailed Corps of Engineers multibeam survey of the same area. I will send a copy of this note to Harvey Kelsey who may want to weigh in.
Farther offshore are the high-slip-rate left-lateral faults published on by Goldfinger et al., with slip rates of several mm/yr. There is a terrane boundary between the accretionary prism of scraped-off sediments of the Juan de Fuca plate and North America (discussed by McNeill et al., 2000, GSA Bull. 112:1209-1224), underlain by Siletz River volcanics and Paleogene and Neogene strata. The left-lateral faults are clearly mapped in the accretionary prism but not into North America. One exception might be the Nehalem Bank fault west of Tillamook Bay, which might connect onshore with the Happy Camp fault of Wells et al., 1992, 1994; cf. McNeill et al. paper), but this fault has no counterpart in the accretionary prism. The Daisy Bank fault of Goldfinger does not have active surface expression on the shelf, but it appears to offset folds in Plio-Pleistocene marine strata in a left-lateral sense (shown in Fig. 6 of McNeill et al., 2000, GSA Bull. 112:1209-1224). If these faults are part of clockwise-rotating blocks in the accretionary prism, then there should be a north-trending fault bounding them on the east, but this has not been found. (Chris Goldfinger has been thinking about this, and may want to comment here.)
The big problem with these obviously-active faults is their ultra-low seismicity. One explanation for this is that the faults move only during subduction-zone earthquakes, as the Patton Bay fault did during the 1964 Alaska earthquake.
Chris Goldfinger, Bob Embley (of NOAA-PMEL Newport), and I are finishing papers on Heceta Bank and other offshore structures, using seismic reflection, and high-resolution multibeam imagery. ChrisТs paper focuses on the low-stand shorelines formed at Heceta Bank and elsewhere in the Oregon offshore at the last glacial maximum and subsequently deformed. My paper with Embley is a tectonic history of Heceta Bank, a follow up to my work on Stonewall Bank. Both papers should be finished in time for update of the maps.
Yakima folds
I repeat here my comments at the meeting. The last version of USGS maps, as expressed in your figure showing crustal faults in E. Washington, shows only two faults, beneath Saddle Mountains and Toppenish Ridge. This is because in Lidke et al., these two structures were the only ones with suggestions of Holocene activity, strongest at Toppenish, somewhat controversial at Saddle Mountains. The Lidke et al. report listed the others in a less-active category. I argue, following Reidel et al. (1994, Washington DGER Bull. 80:159-180), that there is no difference among the Yakima folds in their degree of activity. As Reidel et al. state, У[t]here appears to be no pattern of faulting in the YFB that would suggest that Pleistocene-Holocene faulting is any more concentrated in one part of the basin than in another. Rather, the distribution seems to suggest that the entire fold belt has continued to develop in a pattern similar to that of the Pliocene and Miocene.Ф
The convergence rate across the fold belt, as most recently summarized by McCaffrey, is less than 2 mm/yr and possibly less than 0.5 mm/yr. This is consistent with trilateration work using Geodolite summarized by Prescott and Savage in the 1980s. The best earthquake history comes from Toppenish Ridge, with 3-5 events dated as 0.5-1 ka, poor documentation; 6-7 ka, very good documentation, 10-11 ka, fair documentation; 40-60 ka, good documentation; 145-180 ka, fair-good documentation. Eliminating the youngest poorly documented event, one could explain the next two as different segments on the same structure, and the older events as recurrence intervals of tens of thousands of years.
Some structures have not moved since the deposition of Missoula flood deposits at 13-16 ka. However, if the recurrence interval is tens of thousands of years, the absence of evidence for Holocene displacement means simply that the structure has not yet had a Holocene earthquake. The geomorphic expression of a pre-Missoula flood rupture would be drastically different than for a Holocene rupture, as at Toppenish. One could even argue that Toppenish, having already had its Holocene earthquake, is less likely to have another one soon than Rattlesnake Mountain, where the fault is overlain by sediments containing Mazama ash, 7600 ybp. I donТt propose that here, but I do propose that all structures be treated the same for seismic hazard.
Doing so has little effect on the overall hazard, since faulting is distributed across a larger number of structures than what you have now. It would affect only near-field hazard for any given structure, say a structure close to the Hanford waste treatment plant or one of the dams.
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