GEOLOGY OF THE PRIEST RIVER METAMORPHIC COMPLEX NEAR SPOKANE, WA: CONSTRAINTS ON THE GEOLOGIC STRUCTURE OF THE SPOKANE VALLEY AND THE SPOKANE EARTHQUAKE SWARM OF 2001

Recent bedrock mapping at the southern termination of the Priest River metamorphic complex, south and east of Spokane WA, is providing new constraints on the basement structure in the region and the potential for reactivation of buried faults in the Spokane Valley. The sequence of igneous and metamorphic rocks south of the Spokane Valley, in the Chester Creek drainage and near the Dishman Hills Natural Area, dips gently to the southwest and consists of rusty weathering kyanite- and sillimanite-bearing paragneiss overlain by megacrystic orthogneiss, leucocratic two-mica granite and banded two-mica paragneiss. We correlate this sequence of rocks with the Hauser Lake Gneiss, Newman Lake Gneiss, Mount Spokane granite, and Precambrian Belt Supergroup rocks that are present north of the Spokane Valley. East-directed mylonitization is intense within the lower paragneiss and overlying orthogneiss but fades within the two-mica granite and two-mica paragneiss. We infer that the mylonitization is a continuation of the Spokane dome mylonite zone, which occupies a similar structural position north of the Spokane Valley. A plug of foliated granite east of Dishman Hills matches the character of the Middle Eocene Mt Rathdrum granite suite north of the Valley and suggests that deformation was coeval north and south of the valley. To the east of Chester Creek, extension along the Spokane dome mylonite zone has thinned the section and juxtaposed the Hauser Lake Gneiss directly against the gneiss of Chester Creek, which we provisionally correlate with the Ravalli Group of the Precambrian Belt Supergroup. The Spokane dome mylonite zone can be traced southeastward to south of Mica Peak before it is buried beneath younger basalt and sediments, presumably rejoining the Purcell Trench fault in the vicinity of Worley, Idaho.

Based on the continuity of rock types and structural fabrics across the Spokane Valley, we conclude that east-west trending faults of the Lewis and Clark Line do not extend beneath the Spokane Valley. Consequently, recent earthquake activity in Spokane did not occur along faults of the Lewis and Clark Line.