Evidence of Crustal Faulting and Deformation in the Muckleshoot Basin, Washington

Abstract:

The Muckleshoot basin of western Washington, sandwiched between the Seattle Uplift on the west and the Cascade Range on the east, is deforming under north-south shortening and clockwise rotation of the north Cascadia forearc. Accommodating the regional strain are crustal faults in the Puget Lowland that cluster around three azimuths: east-west, northwest-southeast, and north-northwest-south-southeast. Evidence for all three groups appears on the periphery of the Muckleshoot basin. In this study, we add gravity measurements to an existing database to better define the geometry of the Muckleshoot basin and its relation to previously mapped faults appearing on the basin margins. A northwest-trending gravity high bisects the basin into two sub-basins, a larger one to the south and a smaller one to the north. We suggest that the gravity high is associated with a deep basement structure and its orientation is consistent with northward-directed crustal shortening. Regional-residual separation methods and derivative maps show pronounced magnetic lineations that extend faults expressed along the basin margins to east-west trending faults that cross the Puget Sound. Three intersecting cross-sectional models produced for this study are consistent with the following hypotheses: (1) the northwest-trending White River and Green River faults mapped on the eastern basin margin appear as south-verging, steeply dipping reverse faults in the central basin; (2) the north-northwest trending Franklin fault, mapped previously as a strike-slip fault, projects into the basin and shows little vertical offset in the single profile it crosses, and (3) the northwest trajectory of both the White River and Green River faults appears to curve southward as the faults traverse the Muckleshoot basin, following east-west oriented gravity and magnetic anomalies that cross the Puget Sound. Results from the study suggest that the faults and folds in Muckleshoot basin are actively interacting with other faults in the Puget Lowland to accommodate regional strain in the overriding plate.