THE MONROE FAULT, ANTICLINE, AND SYNCLINAL BASIN—A POTENTIALLY ACTIVE FAULT AND FOLD SYSTEM IN THE SKYKOMISH RIVER VALLEY, SNOHOMISH COUNTY, WASHINGTON

New geologic mapping of the Sultan, Monroe and Lake Chaplain 7.5ʹ quadrangles reveals a major reverse fault in the lower Skykomish River Valley (SRV) that juxtaposes Eocene Mount Persis volcanic rocks with the J-K western mélange belt. We term this east-trending fault the Monroe fault (MF). The MF is bounded by the Monroe anticline and syncline and was likely an Eocene extensional fault that was rejuvenated as a north-vergent reverse fault driven by regional north-south compression.

Relations indicate that the SRV has been a structurally-controlled fluvial feature since at least mid-Pleistocene (>300 ka). Both Pleistocene nonglacial fluvial deposits and Holocene Skykomish River alluvium have a Skykomish River provenance (SP) reflective of the compositionally distinct Cascade granitic source area. Pleistocene SP alluvium in the SRV is correlated with the ‘Olympia beds’, Whidbey Fm, Hamm Creek fm, and older nonglacial sequences on the basis of their distribution, geochemical and modal composition, stratigraphy, geophysics and geochronology. New detrital zircon U-Pb ages from Olympia beds confirm the Cascade provenance of this ancient alluvium. The Monroe anticline overlies the MF as an upper plate flexural slip fold. This anticline folds SP Olympia beds as young as ~22 ka according to multiple radiocarbon and OSL/IRSL ages. North of the MF, the Monroe synclinal basin is cored by SP Olympia beds over a long axial area and locally preserves a ~300-m thick composite sequence of folded SP alluvium spanning several nonglacial episodes. Buried Tertiary strata in the syncline suggest growth folding since the Oligo-Eocene. The MF and Seattle fault systems are similar; they are both east-west trending, north-verging structures responding to regional north-south compression with southerly hanging wall uplifts bounded by major basins to the north.

The NW-trending southern Whidbey Island fault zone (SWIF) extends into the NNW-trending Rattlesnake Mountain fault zone (RMFZ) along the westernmost edge of the map area. The MF intersects the SWIF-RMFZ at an azimuthal inflection in this composite structure. We suspect the MF, RMFZ and SWIF are primarily reverse, strike-slip, and oblique slip faults, respectively, with the MF an eastward Cascade foothills partition of the SWIF accommodating only north-south contraction.