The upper crust in the forearc of the Cascadia subduction zone hosts a complex network of faults that accommodate trench-parallel and trench-normal shortening due to oblique subduction and northward migration of the Oregon/Washington forearc block. Outside of the Seattle area, the seismic potential of major faults, as well as how they connect in a 3-D network, is poorly known. The trench-normal Doty fault, a major, north-dipping forearc fault crosses the I-5 corridor south of the Centralia-Chehalis urban area. Its length, orientation, and hypothesized total offset are comparable to the active Seattle fault, but it is unclear if the Doty fault poses a similar modern seismic hazard. We present preliminary results from the Chehalis Basin project (fieldwork summer 2018). We seek to define the Doty fault’s length, structure, dip, and linkages with smaller, likely transpressive faults to accommodate 3-D crustal deformation. We investigate possible blind faults south of the mapped Doty fault, near the site of a proposed flood-control water retention facility, and present evidence for recent fault activity. A multi-disciplinary approach is critical for regional investigations given dense foliage and glacial cover. Thus, we apply aeromagnetic and ground magnetic data, a regional gravity grid, high-resolution gravity lines, seismicity from a local broadband network, targeted geologic mapping, provenance characterization of Quaternary to Neogene sediments, dating, Lidar interpretation and field reconnaissance of geomorphic features to our research questions. The aeromagnetic data and prior geologic mapping suggest the Doty fault connects to unnamed NNW-striking faults to the west, and our new data will confirm or refute this hypothesis. Initial mapping and aeromagnetic interpretation suggest transpressive faults exist NNE of the Doty fault, which together bound a discrete region of uplift (Lincoln Creek uplift). There is little seismicity in the region recorded by the PNSN regional seismic network, and our PASSCAL array will help confirm the existence or absence of small, local earthquakes that could indicate neotectonic activity.

Miocene-Pliocene strata in the Pacific Northwest preserve a rich record of landscape evolution and coincident faunal shifts. Considerable research efforts in the past century have been aimed at understanding major drainage reorganization and its relation to tectonics, volcanism, climate change, and aquatic biota. Many studies have focused on fish fossils, which show that Miocene fish diversity, particularly salmonoids, displays great adaptive plasticity. However, the details and mechanisms for river reorganization are still debated. Here we present new and recent detrital zircon provenance results from modern and ancestral river sands collected throughout Oregon, Washington, and Idaho. We synthesize our new results and interpretations with existing paleontological evidence for basin isolation and drainage capture. Detrital zircons from the Columbia Basin (CB) consistently show populations derived from the Snake River Plain (SRP) throughout late Miocene-Pliocene time. However, comparisons of Miocene-Pliocene detrital zircons from the CB to modern major rivers and tributaries in the CB and SPR show that the upstream eastern SRP is a major contributor. CB strata do not require zircons sourced from the western SRP, where Pliocene Lake Idaho existed in a large, deep, and occasionally internally drained basin. Based on the age and provenance results, we suggest that the transiting Yellowstone Hotspot divided the modern SRP into two basins: the western basin was isolated and possibly closed, while the eastern basin drained northward into the modern Clark Fork and Columbia Rivers. This scenario is consistent with fish, mollusk, and rodent fossil evidence from the SRP and CB. In addition, the detrital zircon data indicate a Miocene confluence of the Columbia and Clearwater rivers south of the Saddle Mountains anticline, but north of the current Columbia-Snake River confluence. We also find that the Salmon River may have been captured by the Clearwater River sometime between 4.6 and 8.5 Ma. Prior to this time, the Salmon River likely drained into the SRP. Lastly, we find that faunal localities in southern Oregon suggested to contain evidence for fluvial connection between the western SRP and California are ~2.5 Ma, younger than the incision of Hells Canyon.

The Rochester and Adna 7.5 minute quadrangles in the Washington forearc of the Cascadia subduction zone encompass the Doty fault, a large forearc fault crossing the I-5 corridor south of Centralia. We have begun a cooperative geological and geophysical study of the area to assess the seismic hazard to a water retention facility that has been proposed to mitigate flooding along the Chehalis River and the I-5 corridor. This region between Olympia and Portland is undergoing north-south compression, clockwise rotation, and regional uplift associated with both subduction processes and the northward migration of the forearc block. Past studies identified multiple faults that strike NW-SE and E-W in the northern and southern parts of the study area, respectively. The Kopiah, Scammon Creek, Salzer Creek and Doty faults all interact within our study area, in ways that are poorly understood. An integrated geophysical investigation will assist the State-Federal cooperative mapping program called STATEMAP efforts to produce detailed 1:24,000 scale geologic maps of the area. Geophysical field work in the summer of 2018 includes a roughly 15 x 32 km gravity grid with ~2 km station spacing. Station spacing along known geologic structures is ~1 km to provide greater resolution. Results from our coarse gravity grid will provide targets for additional high resolution profiles. A high resolution ground magnetic grid also extends across both quadrangles, and preliminary results demonstrate its efficacy at elucidating structure. Seismic profiles acquired by the USGS across the Doty fault will constrain our geophysical modeling, which will combine the high resolution gravity and magnetic profiles in a geologic model of the subsurface to support the mapping efforts of the STATEMAP program. The data and models will provide insight about total offset across these faults, precisely identify locations of faults that are not exposed at the surface, and allow us to better understand the structure of these faults. These interpretations will allow us to more accurately understand the potential seismic risk these faults pose to nearby population centers and infrastructure.