USING MARINE TERRACES IN CENTRAL OREGON TO DETERMINE LONG-TERM UPLIFT RATES ALONG THE CASCADIA FOREARC

The long-term uplift of subduction zone forearcs can be recorded by uplifted marine terraces. Along the Oregon and California coastlines, the preservation of Late Pleistocene marine terraces records spatially-variable uplift above the Cascadia subduction zone at rates of ~0.1 mm/yr to ~1 mm/yr. Following the availability of high-resolution elevation data and modern dating methods, recent work has revealed that long-term uplift rates can vary through time and may be linked to temporal variations in fault slip rates along upper-plate faults. For example, at Yaquina Bay, Oregon, temporal variations in uplift over the last 125 thousand years are attributed to variable slip along the Yaquina Bay fault. The marine terraces preserved south of the Yaquina Bay fault extend laterally ~35 km south along the coastline to Yachats, Oregon. However, it is not clear whether the temporal variations in uplift rates observed at Yaquina Bay also extend laterally along this section of the coastline, or whether they are localized to the vicinity of the Yaquina Bay fault.

In this study, we used a 1-m horizontal resolution Lidar-derived digital elevation model to refine the location of marine terrace platforms and paleoshorelines from Waldport to Yachats, Oregon. Our work builds on previous mapping by Kelsey et al. (1996). We used ArcGIS Pro to digitally map marine terrace platforms and paleoshoreline locations. Using this map, we extracted multiple coastline-perpendicular elevation profiles to obtain measurements of terrace width and height. Preliminary results show that there are three well defined terraces at elevations of 10-15 m, 25-35 m, and 40-60 m; these terraces increase in elevation over a distance of one kilometer moving inland from the shoreline. Recent luminescence dating near Yaquina Bay (McKenzie et al., 2022) indicates that the three lowest elevation terraces preserved along this section of the Oregon coastline are 80 ka, 105 ka and 125 ka. Using the elevation and age of the three lowest elevation terraces and correcting for variations in sea level, we determine the tectonic uplift rates from 125 ka to Present. Our results will reveal whether temporal variations in long-term uplift rate are localized to the Yaquina Bay fault or record a broader-scale variation in tectonic uplift along the central Cascadia margin.