EOCENE TO QUATERNARY SLIP ALONG THE ENTIAT FAULT, NORTH CASCADES, WA

The NW-striking Entiat fault extends ~175 km through the North Cascades range from Wenatchee in the SE to the Skagit River in the NW. Published work suggests Eocene dextral-normal slip during Chumstick basin formation and exhumation of the deepest part of the Cascades metamorphic core. Total estimated dextral slip varies from ~30 km in the south to ~5 km in the north, with a debated normal or reverse dip-slip component. In the SE, a wide brittle fault zone consisting of 2-3 main fault strands overprints a narrower mylonite zone; previous work shows the fault was active during basin deposition SW of the fault from 49-44 Ma. In the NW, the bedrock fault separates terranes with different cooling histories and is inferred to have been active during Eocene dextral transtension and exhumation of metamorphic core rocks NE of the fault; there is no direct dating of fault rocks. Our work further constrains the age and kinematics of ductile shearing along the Entiat fault and suggests Quaternary reactivation of parts of the fault zone.

New mapping and structural analysis along the NW portion of the fault indicates the presence of a mylonite zone ~500 m wide, subparallel to and cut by the brittle fault trace. Dextral shear on the steeply dipping NW-striking foliation occurred during greenschist-facies metamorphism. White mica in the mylonitic fabric yields a 45.4 +/- 0.3 Ma ⁴⁰Ar/³⁹Ar age, interpreted to reflect dextral shearing at depth. The fault trace in the NW is not well imaged by lidar, so its more recent history is uncertain.

Recent lidar and paleoseismic data along the SE portion of the Entiat fault suggests Holocene reactivation. This ~60 km segment is dominated by landslides along a dramatic fault-line scarp between sedimentary and metamorphic rocks, confounding identification of Quaternary fault scarps. We map ~12 km of Quaternary scarps cutting hillslope colluvium, glacial moraines, and glacial outwash fans. Kinematics are variable with some traces exhibiting 1-2 m of SW-side up dip slip, and others exhibiting NE-side up motion or possible dextral slip. Published trenching studies suggest at least one earthquake occurred in the last 5000 years, consistent with our mapping of scarps cutting probable last glacial maximum moraines. Further research is needed to determine the slip rate, kinematics, and length of the active section of the Entiat fault.