Evidence for Deformation and Faulting in the Muckleshoot basin, Washington

Abstract

The Muckleshoot basin of Western Washington state extends from the western base of the Cascades to approximately 10 km east of the Puget Sound. Mapped traces of the Tacoma and White River fault systems, to the west and southeast, respectively, stop abruptly at the basin margins, where sediments cover the underlying structures recorded in deeper strata. This research presents new crustal models based on analyses of gravity and magnetic data and explores possible connectivity of mapped faults to the east and west of the basin. As discussed in previous studies, the Muckleshoot basin appears to be segmented into two smaller basins by a northwest-southeast-trending gravity high that runs roughly parallel to the trend of the White River fault. Gravity and magnetic anomaly maps created in this study support the existence of the two sub-basins and explore any structural control over the location of the two. Two-dimensional cross-sectional models indicate the sub-basins are approximately 20 km wide and have Eocene-aged sediments buried up to roughly eight to nine km below the surface. Deformation of Quaternary sediments suggests that the basin is actively deforming today as a consequence of plate subduction to the west. Pseudo-three-dimensional models constructed from three intersecting profiles extracted from the gravity and magnetic data indicate that the White River fault bends slightly to the west, aligning with a splay of the Tacoma fault. This geometry is consistent with north-directed compression and clockwise rotation postulated from geodetic models. The geometry revealed in the models raises the potential for an interacting fault system in which rupture along one segment could trigger or advance failure on another.