BRITTLE STRUCTURES IN THE ROSS LAKE NATIONAL RECREATION AREA, NORTHERN CASCADES, WASHINGTON

Considerable research has been conducted on the Cretaceous and Paleogene crystalline core of the North Cascades, but little is known about the brittle deformation of the rocks. We measured outcrop-scale faults and post-metamorphic dikes along the Highway 20 corridor in the Skagit Gneiss Complex, which constitutes the highest grade part of the core and is characterized by Eocene cooling ages (Ar/Ar and K-Ar). The adjacent lower-grade metamorphic rocks of the Little Jack unit and ca. 49 Ma Golden Horn batholith were examined in less detail. NE-striking faults that dip to the NW are dominant throughout the region, although there is much scatter in orientations and multple fault sets cut some outcrops. The mean orientation is N30E, 73NW. Many of the faults are listric. Rakes of slickenside striae are highly variable and striae are commonly oblique. Many faults cut veins and sheets, and some truncate the late dikes. Separations typically range from 3.5 cm to 21 cm. The sense of motion on faults was determined from “drag” and locally by Riedel shears. These faults generally display a component of normal (NW-side down) motion. Some faults are associated with extensively altered gouge zones that average 25 cm across, and range from 5 cm to 90 cm. Post-metamorphic dikes have orientations similar to those of the faults. Measured dikes have a mean thickness of 3.5 m. Many dikes are fine-grained, and others have plagioclase phenocrysts set in a medium-gray groundmass. The latter dikes are spatially associated with the Golden Horn batholith.

The ages of most of the faults and dikes can only be constrained as Eocene or younger. Some of the dikes are presumably ca. 49 Ma, the age of the Golden Horn batholith. The faults and dikes are commonly oriented at high angles to the subhorizontal, NW-trending stretching lineations in the Skagit Gneiss Complex. The extension directions are thus broadly compatible and suggest a similar strain field of orogen-parallel to –oblique stretch during early ductile and subsequent brittle deformation.