Deformation Rates Based on Submerged 18ka Shorelines

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.

    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 can be tied directly to uplift rates onshore determined with geodetic techniques and tide gauges, as well as deformation of marine terraces. We hope these data will help us to identify along-strike variability that may indicate segmentation of the Cascadia plate boundary.

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