Neotectonic Investigation of the Chehalis Basin, Southwestern Washington, USA

Date: 4/24/2019

Time: 06:00 PM

Room: Grand Ballroom

The Chehalis Basin in southwest Washington was an active depocenter from the Oligocene through the Miocene, and has since been uplifted and incised to create the Doty Hills. Oblique subduction of the Juan de Fuca plate and rotation of the Oregon forearc are recognized as driving forces for uplift of the Doty Hills and the Cascadia margin, but what structures accommodate strain in the upper crust remain unclear. We undertake a multidisciplinary effort to understand uplift within this region and potential hazard posed by upper crustal faults like the Doty fault. Recent gravity anomaly maps show steep gradients co-located with mapped faults, suggesting kilometer scale total displacement. New detailed mapping confirms this interpretation and shows the Doty fault separating steeply dipping Miocene rocks in the footwall from the overlying Eocene Crescent Formation in the hangingwall. New kinematic data found on fracture surfaces near the fault suggest a lateral component to offset rather than pure dip-slip. Field and lidar observation along an approximately 40 km by 3 km swath which includes the Doty fault and surrounding structures did not reveal tectonic scarps. Shallow seismicity (<20 km) in the area of the Doty fault is sparse suggesting a locked fault or aseismic deformation since recording began by the Pacific Northwest Seismic Network. Without tectonic scarps and shallow seismicity, we intend to use reconnaissance scale mapping of marine terrace surfaces to the west and inland fluvial and glacial outwash terraces to the east in conjunction with forthcoming optically stimulated luminescence (OSL) and ¹⁰Be cosmogenic surficial ages to test hypotheses for the timing of deformation.

Presenting Author: Tabor J. Reedy

Authors