EVIDENCE FOR DEFORMATION AND FAULTING IN THE MUCKLESHOOT BASIN, WASHINGTON

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. 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. Whether this high connects with a splay of the Tacoma or the Seattle faults systems is unclear. Two-dimensional cross-sectional models indicate the sub-basins are approximately 20km wide and have Eocene aged sediments buried up to roughly 8-9 km below the surface. Deformation of quaternary aged 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 by recent 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.