VERTICAL DEFORMATION NEAR WILLAPA BAY, WASHINGTON

The Cascadia margin deforms in response to convergence between the Juan de Fuca and North America plates and the northward migration of the bounding Mendocino and Explorer triple junctions. Northward fore-arc migration, north of the Mendocino triple junction, effectively terminates against the relatively stable Pre-Cenozoic rocks of British Columbia. Cessation of this northward translation creates a deformation gradient across the Washington fore-arc. Willapa Bay, Washington lies within this deformation gradient and the sea cliffs within it afford an excellent opportunity to measure such deformation. We generated a profile of a 5 km segment of the eastern sea cliff of Willapa Bay, using a surveying total station, which identified a long-wavelength syncline. The syncline may be an eastward continuation of the previously identified, offshore Willapa syncline and may represent surface deformation of the relatively soft Pleistocene sediment to deformation occurring at depth.

Three uplifted marine terraces, which represent the paleo-sea level high stands of 80 ka, 120 ka, and 320 ka, have been identified east of Willapa Bay. Assuming these terraces were cut rapidly (within a few thousand years) during sea-level high stands and experienced relatively constant uplift since their formation, an uplift rate can be inferred for this terrace sequence based upon the modern elevation of these surfaces.

North of Willapa Bay only the lowest and highest terrace surfaces (80 ka and 320 ka, respectively) are preserved and occur at higher elevations than their counterparts east of the bay. A submerged north-dipping reverse fault, which separates this region into two distinct structural blocks, controls the different elevations of the terraces.

The position of the Willapa syncline suggests a dominant northward-directed compressive stress through the late Pleistocene. Vertically deformed marine terraces provide an uplift rate for the southern block of 0.34 ± 0.02 mm/yr, and 0.49 ± 0.04 mm/yr for the northern block. Taken together these imply a shortening rate of 0.10 ± 0.03 mm/yr across an inferred reverse fault that dips approximately 55°. These results are consistent with similar studies elsewhere along the coast of the Pacific Northwest.