HOLOCENE DISPLACEMENT ON THE BOULDER CREEK FAULT NEAR BELLINGHAM, WA AND IMPLICATIONS FOR KINEMATICS OF DEFORMATION OF THE CASCADIA FOREARC

A recent lidar survey of the North Fork Nooksack valley discloses an east-west fault scarp 35 km NE of Bellingham. The 4 km-long south-side-up Kendall scarp has a maximum preserved height of 3 m and cuts latest Pleistocene outwash surfaces. The scarp is on the mapped trace of the Boulder Creek fault, a south-dipping normal fault that juxtaposes Eocene Chuckanut Formation with older rocks to the north. Evidently the fault was reactivated as a thrust under regional N-S compression, an idea proposed by Dragovich and co-authors to explain the 1990 Deming earthquake swarm. Recognition of the Kendall scarp reinforces the utility of lidar surveys for identifying young fault scarps in the forested terrain of western Washington: in the last 8 years we have found over a dozen scarps on nearly as many faults.

Geologic and far-field geodetic observations suggest long-term shortening between Portland, OR and Vancouver, BC of ~6-8 mm/yr in response to clockwise rotation of the forearc. Preliminary interpretation of GPS data suggests significantly slower NNE-SSW shortening of the Puget Sound region. Aggregate Holocene N-S shortening calculated from the height of recognized fault scarps and observed local uplift is ~40 m, or ~3 mm/yr. However, large areas of the forearc have not yet been surveyed with lidar, and additional shortening on not-yet-recognized scarps is likely. If fault slip largely reflects thrusting in the direction of regional contraction, there may be a surplus of fault slip. Forearc deformation may be more complicated than simple contraction by thrusting, and may involve lateral escape of forearc blocks westward over the trench. Strike slip observed in trenches on the WNW-trending Rocky Point and Little River scarps supports this inference.

Active shortening on the Boulder Creek fault at the latitude of Bellingham demonstrates that forearc deformation is not confined to the Olympic-Wallowa lineament and suggests that the southern end of the Coast Mountains buttress may be deforming.