LONG-TERM SLIP ACROSS THE YAQUINA BAY FAULT REVEALED BY IRSL DATING OF UPLIFTED MARINE TERRACES

Variable long-term uplift rates along the Cascadia upper plate have previously been determined from uplifted marine terrace platforms along the coastline. In southern Cascadia, long-term uplift rates are generally higher (~0.25 mm/yr) than in central Cascadia (less than ~0.1 mm/yr). However, within these broader regions, local regions of high uplift (up to 1 mm/yr) have been attributed to Quaternary active faulting within the Cascadia upper plate.

For example, north of Yaquina Bay (YB), Newport, Oregon, the emergence of younger marine terraces and higher elevations of older marine terraces, compared to south of YB, is explained by an east-northeast-striking active upper-plate fault - the Yaquina Bay fault (Kelsey et al. 1996). Previous age assignments for marine terraces, from relative soil development and associations of terraces to marine oxygen isotope stages, were used to infer differential uplift rates across the YB fault: ~0.6-0.8 mm/yr north of YB versus ~0.04 - 0.4 mm/yr south of YB. In this work, we quantify the depositional ages of sand on the mapped marine terraces on either side of YB using infrared-stimulated luminescence (IRSL) of K-feldspar, to (1) test the use of K-feldspar IRSL to date the depositional age of marine terrace sands, and (2) evaluate the previous MIS 5 marine terrace age assignments across YB.

We present fifteen new IRSL ages for marine terraces at either side of YB. Our preliminary IRSL ages (91±20 ka – 161±39 ka) confirm the previous age assignments of the MIS 5c and 5e marine terraces. Our IRSL ages for the lowermost terrace (corresponding to MIS 5a, north of the bay) are anomalously low, and may reflect post-depositional processes (such as accumulation of aeolian sands) that have contaminated the depositional age signal of the MIS 5a terrace. However, the emergence of this younger marine terrace north of YB, and not south of YB, suggests differential uplift across the bay. These preliminary IRSL ages and the difference in elevation (~50 m) of MIS 5c and 5e terraces across the bay confirm earlier estimates of differential uplift of ~0.4-0.6 mm/yr across the Yaquina Bay fault. We use these new results to investigate the possible slip history of the Yaquina Bay fault and its role in accommodating permanent upper-plate uplift and strain over multiple subduction earthquake cycles.