HOLOCENE REACTIVATION OF AN EOCENE BEDROCK FAULT IN THE EASTERN CASCADE MOUNTAINS, WASHINGTON

Geodetic data show that ~1-2 mm/yr of N to NE-directed shortening occurs across Washington State east of the Cascades. Folding in the Yakima region accommodates some of the shortening but the distribution on individual structures is unknown. Active faulting in the east Cascades poses a seismic hazard to major infrastructure along the Columbia River. We use surficial geologic mapping based on airborne lidar and paleoseismic trenching to investigate active deformation along the southern portion of the Entiat fault NW of Wenatchee, WA.

The Entiat fault is an Eocene-age NW-trending structure that dips steeply to the SW and juxtaposes Cretaceous metamorphic rock against Eocene sedimentary rocks. Mapping reveals a 5 km long, NW-trending, SW-side-up lineament in the Swakane Creek drainage basin that forms a bench on steep hillslopes and is clearly expressed on ridge crests but is typically concealed beneath valley deposits. Measurements on lidar imagery indicate at least 3 m of vertical separation of the land surface across a structure that dips steeply (~60-90°) NE and SW. Stream channels and ridge crests show no resolvable lateral offset. The bench coincides with a previously mapped strand of the Entiat fault but not the primary basin-bounding fault.

Trenching exposed deeply weathered bedrock covered by two wedge-shaped gravel deposits coincident with the bench. Sharp contacts dividing each unit and a distinct shingled depositional fabric suggest these sediments did not form in situ and are consistent with infilling of event-generated accommodation space. We interpret these deposits as colluvial wedges formed during at least one SW-side-up earthquake that folded the bedrock, generating the accommodation space necessary to deposit these units. Preliminary radiocarbon ages of charcoal from within the colluvial wedges suggest a maximum depositional age of ~5000 cal yr BP, making any earthquake-related deformation Holocene in age. Reactivation of the Entiat fault in the modern stress regime could result in an earthquake of M~6 based on a rupture length the size of this lineament (Wells and Coppersmith, 1994), and imply renewed activity on other similarly oriented bedrock faults in WA.