RAPID, TEMPORALLY VARIABLE SLIP RATES ON AN UPPER-PLATE FAULT ABOVE THE CASCADIA SUBDUCTION ZONE

Determining whether fault slip rates change over geologic time requires multiple displaced markers of known age. Along many subduction margins, most structures in the overriding plate are not exposed subaerially. Displaced marine terraces at Yaquina Bay in Newport, Oregon, provide an opportunity to constrain both vertical and horizontal displacement rates since ca. 125 ka on an active upper-plate fault. We combine optically stimulated luminescence (OSL) ages of marine terrace sands and platform elevations derived from high resolution topographic data to estimate and compare uplift rates of marine terraces north and south of the Yaquina Bay fault. We find that terrace uplift rates south of Yaquina Bay have been relatively constant at ~0.2-0.5 mm/yr since ca. 125 ka, and are, on average, lower than uplift rates north of Yaquina Bay. These results require north-side-up displacement along the Yaquina Bay fault. North of the fault, however, uplift rates are variable and increased from ~0.1-0.2 mm/yr (between 125-106 ka) to ~1.8 mm/yr ± 0.3 mm/yr (between 106-84 ka). Post-84 ka, uplift rates decreased to ~0.7 mm/yr. These results suggest little to no vertical fault motion along the Yaquina Bay fault between 125-106 ka. Throw rates then increased to ~1.3 mm/yr between 106-84 ka, and subsequently decreased to ~0.4 mm/yr from 84 ka to present. By comparing the width (location of the paleo-shoreline angle) of marine terraces north and south of Yaquina Bay, with the assumption that the coastline has remained approximately co-linear since ca. 125 ka, we estimate horizontal displacement rates along the Yaquina Bay fault. Horizontal displacement rates are an order of magnitude higher than vertical displacement rates and indicate a period of relatively high fault slip at horizontal rates of ~14-30 mm/yr in the ~20 kyrs between 106-84 ka. Horizontal displacement rates then decrease to ~4-6 mm/yr post-84 ka.

Our results suggest that (1) slip on upper-plate faults can be variable over ~20 kyr periods and, (2) horizontal displacement rates on the Yaquina Bay fault, at times, approach plate convergence rates of 35 mm/yr. Plate-convergence-like slip rates may be explained by concurrent faulting of upper-plate faults during subduction megathrust events, as was observed in the 2016 Kaikoura earthquake, South Island, New Zealand.