SPATIAL VARIABILITY IN LATE HOLOCENE AND 20TH CENTURY RSL CHANGE ALONG THE CENTRAL PACIFIC COAST OF NORTH AMERICA

20^th century rates of sea-level rise from long-term tide-gauge records must be corrected for vertical land movements that are associated with the glacial isostatic adjustment (GIA) of the solid Earth. Regional databases of relative sea level (RSL) provide a framework to assess the spatial variability of the GIA contribution. In this study we used >400 sea level data points to reconstruct late Holocene RSL changes from 12 regions from British Columbia to central California, along the central Pacific coast of North America.

During the late Holocene (last 4 ka) we document RSL fall (i.e. land-level uplift) on both the western (-0.5 ± 0.3 mm a^-1) and the eastern (-0.2 ± 0.3 mm a^-1) coasts of Vancouver Island and Queen Charlotte strait (-0.6 ± 0.2 mm a^-1). Stability or low rates of RSL rise (i.e., land-level subsidence) are documented along the northern Washington coast (0.0 ± 0.2 mm^{- 1}), Puget Sound (0.7 ± 0.2 mm a^-1) and the southern Washington coast (0.7 ± 0.1 mm a^-1). In regions beyond the maximum extent of the Cordilleran ice sheet, late Holocene RSL rise increases with distance from the former ice sheet. Rates range from 1.0 ± 0.1 mm a^-1 in Northern Oregon and up to 1.5 ± 0.2 mm a^-1 and 1.6 ± 0.2 mm a^-1 from northern and central California coasts, respectively.

This suggests that there is net Late Holocene rise along the coasts of Oregon, and California, presumably due to GIA from the Cordilleran and Laurentide Ice Sheets. The magnitudes are similar to those observed along the tectonically stable US Atlantic coast that are due to the collapse of the Laurentide proglacial forebulge. Removing the background rates of late Holocene RSL rise from tide gauges reveals a trend of increasing 20^th century rate of sea-level rise with distance from the trench. We suggest this pattern is due to ongoing interseismic deformation from the subduction of the Juan de Fuca plate beneath North America.