STRUCTURAL ANALYSIS AND EVOLUTION OF THE ENTIAT FAULT ZONE, A MAJOR EOCENE STRUCTURE OF THE NORTH CASCADES, WASHINGTON

The poorly understood, NW-striking Entiat fault is a major structure at the southern end of the Coast Mountains-North Cascades orogen in Washington, and was active during regional Eocene dextral transtension. This ~175-km-long fault in part separates amphibolite-facies rocks of the Swakane Biotite Gneiss and Napeequa complex (quartzite, amphibolite, biotite schist, metaperidotite, marble) from non-metamorphosed clastic rocks of the Eocene Chumstick basin. The Entiat fault zone experienced at least two episodes of movement, and has been interpreted to record both dextral strike-slip and normal slip. For this study, outcrop and microstructural analysis were conducted along a 55-km-long segment of the fault to better understand the fault-zone evolution.

The main strand of the fault is sharp and marked by a ~100-m-wide brittle damage zone in the crystalline rocks and a narrower zone (<20 m wide) in the sedimentary rocks. A ≥25-m-wide zone of greenschist-facies mylonites lies ~700 m from the main fault in the metamorphic rocks. Foliation in the mylonites strikes sub-parallel to obliquely to the main fault and dips moderately to steeply to both the NE and SW. Lineations generally trend NW or SE; 75% of the measured plunges are gentle and imply dominantly strike-slip motion. Kinematic indicators (S-C, asymmetric porphyroclasts) suggest dextral movement. Bulging recrystallization in quartz, accompanied by microfracturing of plagioclase, indicates low-temperature ductile deformation (Regime 1 of Hirth and Tullis [1992]), compatible with the retrograde assemblage of chlorite-muscovite-quartz in the mylonites. The mylonites were strongly overprinted at lower temperatures by cataclastic deformation marked by micro-scale faults and intragranular and transgranular fractures.

In summary, movement in the Entiat fault zone consisted of an early ductile deformation, followed by reactivation and migration of deformation to the SW under brittle conditions. Displacement included both dip-slip and dextral strike-slip motion.