EXTENT OF MYLONITE ZONES ON BEACON HILL, SPOKANE, WASHINGTON

Recent mapping along the western side of the Priest River complex (PRC) at Beacon Hill near Spokane, Washington, provides new insights into the transition from infrastructure to suprastructure in this metamorphic core complex. The PRC is dominated by igneous and metamorphic rocks, which were tectonically exhumed and mylonitized during early to middle Eocene extension. The Spokane Dome is a large anticline in the southern culmination of the PRC, exposing a broad mylonitic zone within the upper portions of the infrastructure.

Beacon Hill consists of two units: the Newman Lake Gneiss (NLG) and a biotite-muscovite granite. The NLG is a well-foliated orthogneiss characterized by the pervasive presence of kspar porphyroclasts. The biotite-muscovite granite is medium to coarse-grained and generally non-foliated, but does show zones of foliation and mylonitization toward the contact with the NLG. At Beacon Hill, NLG dominates the southern side while the biotite-muscovite granite occurs on the northwestern side. A main zone of mylonite (Beacon Hill mylonite zone) occurs at the contact between the two units. Numerous NNW-trending high angle faults cut both units as well as the main mylonite zone. Variably mylonitized pegmatites occur throughout the NLG and Beacon Hill mylonite zone.

The main mylonite zone, near the summit of Beacon Hill, is up to 100 feet thick and becomes less intense to the south. Lineations show a consistent 220^o-255^o orientation with gentle plunge directions to the west. Poles to foliations (and plotted lineations) define a broad bulls-eye. Kinematic indicators within the main mylonite zone indicate a top to the east up-dip direction of movement. Near the summit, the area is dominated by ultramylonite with exposed zones along the top one hundred feet that range from six inches to two feet in thickness and occur every fifteen to twenty feet. The number of individual mylonite zones decreases to the south. Types of mylonite at Beacon Hill range from protomylonite to ultra-mylonite. The NLG along the lower slopes of the southern side shows variable degrees of blastomylonitic fabric development with visible quartz rodding.