CHARACTERIZATION AND NEOTECTONIC INVESTIGATION OF THE TRENCH-NORMAL DOTY FAULT, CHEHALIS BASIN, SW WASHINGTON, USA

We refine the tectonic evolution of the Chehalis Basin in southwest Washington with particular focus on structures that could pose a modern seismic hazard. A number of Holocene-active faults have been recognized along the Oregon and Washington forearc that accommodate shortening related to subduction-zone deformation. Here, we present the preliminary results (fieldwork 2018) of a multi-disciplinary approach integrating field-based geologic mapping and geophysical techniques. The combination of these techniques is advantageous to working in densely vegetated terrain.

The Chehalis Basin has been an active depocenter periodically throughout the Cenozoic. Evolution of the basin has been influenced both by the accretion of Siletzia and subsequent oblique convergence along the Cascadia subduction zone. Within the Chehalis Basin, detailed mapping took place along the Doty Fault, a regionally extensive, moderately to steeply dipping, multi-stranded structure that crosses Interstate-5, as demonstrated by existing maps. We seek to understand the timing of deformation, kinematics, and connectivity of the Doty Fault with other local structures. This fault juxtaposes Eocene Crescent Formation, the regional basement unit, against Miocene-aged marine sediments.

Field observations confirm a 45–70° north dip for the Doty Fault and show that it is ~43 km long. Field observations and review of available lidar did not identify any tectonic scarps along the trace of the fault. Slickenlines on subsidiary faults suggest a component of lateral motion in addition to dominantly dip-slip offset. Stratigraphic omission, steep observed gravity gradients, and topographic relief across the fault suggest kilometer-scale offset.

Forthcoming ⁴⁰Ar/³⁹Ar geochronology and paleomagnetic data from steeply dipping and folded Columbia River Basalt flows within the Miocene section will provide additional timing and kinematic information related to the Doty Fault and associated structures. Pending geophysical models will support and refine kinematic and structural interpretations. Optically stimulated luminescence geochronology and detrital zircon provenance data from uplifted coastal and inland terraces may provide additional constraints on basin-wide tectonic deformation.