Crustal Deformation at the Leading Edge of the Oregon Coast Range Block, Offshore Washington (Columbia River to Hoh River)

Determination of slip rates for the seismotectonic elements that make up the Cascadia subduction boundary is complicated by crustal-block motion within the forearc. In particular, geodetic measurements indicate that the Oregon Coast Range block is translating north-northwest at 6-10 mm/yr relative to stable North America. Differential motion of the Oregon Coast Range block with respect to other crustal blocks that compose the forearc results in a complex pattern of faults and folds with orientations highly oblique to those expected from subduction-driven contraction.

The leading (northwestern) edge of the Oregon Coast Range block traverses coastal Washington, where it abuts subduction-complex rocks of the Olympic Mountains to the north. Block kinematics predicts north-northwest-directed contraction where the boundary trends east-northeast near Grays Harbor, Washington. Crustal deformation observed near Grays Harbor is consistent with north-northwestward motion of the Oregon Coast Range block. Deformation is localized within the more ductile subduction-complex rocks of the Olympic coast rather than the more rigid basaltic rocks that underlie the Oregon Coast Range block. The boundary near Grays Harbor is relatively discrete. Seismic-reflection and sidescan-sonar data image several faults and folds that trend east-northeast on the inner continental shelf between Grays Harbor and Cape Elizabeth, Washington, across a zone 40 km wide from south to north. In contrast, in Puget Sound to the east, where the Oregon Coast Range block abuts the relatively rigid pre-Tertiary rocks of Vancouver Island, relative motion is distributed across a zone more than 200 km wide from south to north.

An estimate of the contraction rate across the transverse coastal zone can be derived from the degree of folding and faulting of Quaternary strata. Constraints on the contraction rate come from two datums: (1) the older datum (Raft datum) is an unconformity at the base of a widespread, glacially derived deposit, and (2) the younger datum (Langley datum) is a widespread unconformity marking the base of upper Pleistocene sediments (<150 ka). Shortening of the older, ca. 900-600-ka Raft datum is ~2.0 km, corresponding to a shortening rate of 2.2-3.3 mm/yr.

Quaternary folds range in length from 3 to 15 km and have amplitudes as great as 160 m. Of the seven primary faults and associated folds imaged in this coastal zone, the southernmost structure appears to be the most important. This structure marks the boundary between the Oregon Coast Range block and the Olympic Mountains block, vertically offsets the late Pleistocene Langley datum by more than 40 m, and displaces the sea floor several meters. Other faults in this zone displace the late Pleistocene datum 2-10 m.

Block kinematics predicts transpressional shear where the block boundary trends north-northwest through Willapa Bay, Washington. Crustal deformation observed in Willapa Bay documents a component of contraction in an east-northeast direction. Lateral slip, if any, on mapped faults remains undetected. In contrast to the kinematic model, this portion of the boundary exhibits the geometry of an east-dipping thrust fault.

Faults mapped with both new and earlier seismic-reflection data are consistent with a thrust geometry. These data depict a 3-km-wide, 30-km-long fault zone trending north-northwest through Willapa Bay. However, the main strand shows a sense of vergence opposite to that of the inferred boundary thrust. The main fault strand vertically displaces a ca. 20-ka erosional surface beneath the bay 10-12 m, west side up. A vertical displacement rate of 0.5±0.2 mm/yr has been calculated across the entire fault zone. This “Willapa Bay” fault zone may include a backthrust above the block boundary. Alternatively, the fault geometry may reflect lateral juxtaposition of bedrock slivers. No shallow trace (<90 m) has been identified for the postulated block boundary fault. Nonetheless, the recency and magnitude of movement on the “Willapa Bay” fault zone implies activity on the postulated main thrust to the west.

These observations provide the first constraints on rates of Quaternary structural activity in coastal Washington. Furthermore, they suggest that this region may accommodate as much as half of the differential motion between the Oregon Coast Range block and stable North America. The remaining differential motion may be accommodated in the Olympic Mountains, which are uplifting as much as ~3.2 mm/yr as indicated by geodetic measurements, or as broad-scale buckling of the Oregon Coast Range basement beneath Grays Harbor and Willapa Bay.