Active deformation of the Pacific Northwest Continental Margin

Abstract

Tilted and uplifted marine terraces in southern Oregon show progressive landward tilting of the coastal ranges at about 5–16 × 10−8 rad. yr−1 for the last 0.25 m.y. Tide gauges in Washington and British Columbia, and ten resurveyed leveling lines running inland from the coast, indicate contemporary landward (down‐to‐the‐east) tilt rates of about 1–12 × 10−8 rad. yr−1 averaged over periods of from 10 to 50 years. The leveling lines traverse, and the terraces cut across, dipping Cenozoic strata: Pleistocene (dips to 3°), Mio‐Pliocene (dips to 30°) and Eocene (dips to 60°). Southern Oregon from Cape Blanco to the Siletz River shows geodetic or terrace tilting in the same direction as the underlying stratal dips. Hence present‐day deformation continues past deformation of the coastal ranges and is most likely related to active subduction of the Juan de Fuca plate. The steep stratal dips, lack of major active faults and historic earthquakes, and presence of very young bedding‐plane faults suggest that much of the onshore deformation and shortening within the overlying North American plate is taken up by folding rather than thrust faulting. Present shortening rates across north‐trending folds near the coast are between 0.02 and 1.9 × 10−7 yr−1. The rate of shortening decreases rapidly eastwards from the Juan de Fuca ‐ North American plate boundary. A total of about 25 mm yr−1 of permanent shortening could be occurring within the North American plate; most of it in the westernmost 40 km. The landward tilt and shortening rates are similar to those above many other subduction zones that have experienced great thrust earthquakes. While a high strain rate measured near Seattle, Washington, has been interpreted as elastic strain accumulation before a thrust earthquake, the low level of historic seismicity and the similarity of short‐ and long‐term deformation rates suggest alternatively that the subduction beneath Washington is aseismic. The issue has considerable implications for seismic hazard evaluation in the Pacific Northwest and could be resolved by a search for the effects (or lack of effects) of prehistoric great earthquakes.