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