DEFORMATION IN AND NEAR THE EOCENE LEAVENWORTH FAULT ZONE, NORTH CASCADES, WASHINGTON

The North Cascades is an excellent area to study basins and strike-slip faults that formed during regional Eocene transtension. This study focuses on the southern flank of the North Cascades crystalline core where the Eocene Leavenworth fault separates Eocene clastic rocks of the Chumstick Formation to the east, from the older Swauk Formation to the west. The Leavenworth fault zone is 80 km long, up to ~5 km wide, and has N-S- and NW-striking segments. The movement history is poorly understood and controversial. Cheney and Hayman (2009) suggest that it is a reverse fault zone, whereas Evans (1988) and Tabor et al. (1982), amongst others, propose that it is a dextral structure with a dip-slip component. Structural analysis and mapping were conducted along a 20 km-long segment of the southern part of the fault zone to evaluate its movement history. Beds in the Swauk Formation (n=285) are oblique to the NW-striking (~320-330^˚) fault segments, on average strike WNW, and most commonly dip SW (maxima of 70˚). On a stereographic projection, poles to beds define a fold axis plunging 1˚ toward 119˚. Beds in the Chumstick Formation (n= 66) strike WNW, and have a moderately steep SW-dipping maxima. The poles to beds define a girdle and a fold axis plunging 16˚ towards 306˚. Map scale (>2 km wavelength) folds in both the Swauk and Chumstick units trend 270˚-315˚, obliquely (5-50^˚) to the fault zone. Overall, the obliquity of these folds and the axes determined from stereographic projections to the Leavenworth fault are compatible with dextral strike-slip. Outcrop-scale faults (n=22) in Swauk rocks strike N-S and dip to the E. Slickensides (n=88) in the Swauk and Chumstick units have a variety of strikes, and rakes range from gentle to steep. Eocene Teanaway basalt dikes (n=20) intrude the Swauk Formation in the fault zone and strike nearly N (359˚), which contrasts with the strike of 040^˚ determined by Mendoza (2008) west of the fault zone. This change may result from rotation of the strain field in the fault zone. Finally, these observations, particularly the orientation of the fold axes relative to that of the Leavenworth fault zone, support dextral strike-slip motion.