Chris Goldfinger, L.D. Kulm, R.S. Yeats, J.D. Chaytor
Modern rates of active deformation have thus far been based mainly on the tectonic geomorphology of subaerial landforms. However, the world's principal plate boundaries are mostly located offshore, making such studies difficult at many active continental margins. However, sidescan sonar, swath bathymetry, high-resolution seismic reflection profiling, and submersible observations allow the mapping of deformed stream channels, abrasion platforms, and submerged lowstand shorelines, leading to the determination of deformation rates. We have previously used sidescan sonar imagery, seismic reflection profiles, and dated piston cores to determine Holocene slip rates of several left-lateral strike-slip faults on the Juan de Fuca (JDF) plate, immediately west of the Cascadia deformation front off Oregon and Washington. These faults offset the baseof-slope channel; their slip rates are determined from offset isopachs of landward thickening Pleistocene submarine fan deposits, and from surficial offset of dated channels. Recently, we have discovered a late Pleistocene lowstand shoreline rimming three major submarine banks along the outer shelf off Oregon.
In 1993-95, we investigated this shoreline with AMS 150 kHz sidescan sonar and the DELTA submersible and found spectacular shoreline features, including barrier islands, estuaries, and headlands. The preservation of these ephemeral features attests to the very rapid rise of sealevel at the close of the Pleistocene. Submarine traverses found pholad borings, oxidized cross stratified sands, and shallow-water fossil debris at the base of beach cliffs. We also conducted sidescan and submersible traverses across Nehalem and Coquille banks and found similar features. Coquille Bank, in fact, was formerly an island. Datable intertidal fossils like the barnacles at right indicate the age of last submergence of the latest glacial shoreling at 14-18 ka.
Shoreline angles were originally horizontal and close to sea level. Now these shoreline angles are warped and faulted, locally to depths of 200-300 m, deeper than the Wisconsin lowstand of -120 m, indicating that the shoreline angles reflect vertical tectonics rather than eustatic sea level change. Two of the major submarine banks, Heceta and Nehalem, are strongly tilted to the south (margin parallel), and deformed by faults and folds, while the third appears relatively undeformed. The submerged shoreline is at about the depth of the lowstand at the north ends of these two blocks, while the south ends are deeper, indicating overall subsidence of these blocks. Both banks are known to have been uplifted approximately 1 km since the Miocene, so the recent subsidence could reflect one of many vertical fluctuations, or a significant change in margin tectonics. Subsidence of the banks may be due to gravitational collapse or tectonic erosion of the margin, despite the presence of an accreting prism of sediments to the west.
The deformation of the outer shelf is on a much longer time-scale than moderd geodetic techniques, but can be compared to uplift rates onshore determined with geodetic techniques and tide gauges, as well as deformation of marine terraces to gain insight into the processes and rates over a range of time scales. We hope these data will help us to identify along-strike variability that may indicate segmentation of the Cascadia plate boundary.
Cascadia Deformed Shorelines
Figure 2. Color shaded relief image of Heceta Bank, Oregon
looking to the southeast. EM-300 multibeam data collected by MBARI and
NOAA PMEL/Newport. Rough bank top is exposed Miocene strata. Note
smooth wave-cut platform surrounding the bank at a depth of 100-150 m.
Figure 3. AMS-150 sidescan sonar mosaic of the late Pleistocene low-stand shoreline and wave-cut platform at Heceta Bank, Oregon continental shelf. Visible in the mosaic are the smooth, low-reflectivity abrasion platform at left; the high reflectivity bedrock outcrops at right; and the paleo-seacliff separating these regions. At center, a paleo stream/river entered the sea at a small estuary. The former stream channel has evidence of unidirectional current indicators as observed directly from teh DELTA submersible. Other peripheral nearshore features such as tombolos and offshore bars (not shown) were surprisingly well preserved. Intertidal rock boring Pholad clams and mussels were recovered from the low seacliff.
Figure 4. AMS-150 sidescan sonar image of a former sea-stack on the northern Oregon continental shelf at Nehalem Bank. The top of the stack was eroded at a previous abrasion platrom level, then later was awash at this site at a water depth of 140 m. Coarser high-backscatter gravel, likely derived from the stack, is the high backscatter material surrounding the stack. Swath width of image is 1000m.
Chaytor, J. D., Chris Goldfinger, C., Meiner, M. A., Huftile, G. J., Romsos, C. G., and Legg, M. R., 2008, Measuring Vertical Tectonic Motion at the Intersection of the Santa Cruz-Catalina Ridge and Northern Channel Islands Platform, California Continental Borderland, Using Submerged Paleoshorelines, Bulletin of the Geological
Society of America, v. 120. P. 1053-1071.
Goldfinger, C., Kulm, L.D., McNeill, L., Yeats, R.S., Hummon, C., Huftile, G., Schneider, C., Neim, A.R., Tsutsumi, H., and Chen, Y.J., 1994, Cascadia subduction zone: Active deformation of the Oregon continental shelf: Oregon Academy of Sciences, Proceedings, v. XXX, p. 38.
We thank Kevin Redman and many other colleagues at Williamson and Associates for collection of the AMS 150 sidescan data. We thank DELTA pilots David and Rich Slater, and Chris Ijames, ROPOS pilots Kieth XX and Kieth XXX, and the officers and crews of the Thomas G. Thompson from 2000-2005. We thank LaVerne Kulm, Lisa McNeill, Roland Bergmann, Roland von Heune, Lisa McNeill, Kerry Sieh, Ray Wells, Kelin Wang, Alan Niem, and Yuichi Sugiyama for discussion of these issues over the years. Thanks to Alan Mix for use of his sealevel compilation.