INSIGHTS INTO THE SPATIAL, TEMPORAL, AND STRUCTURAL EVOLUTION OF INTERSECTING FAULT ZONES IN THE SOUTH-CENTRAL WALLA WALLA BASIN, NE OREGON

Recent geologic mapping by the Oregon Department of Geology and Mineral Industries (DOGAMI) has focused on the Walla Walla River basin (WWB) of NE Oregon, with the chief objective to establish its geologic and structural framework. The oldest rocks in the WWB are tholeiitic basaltic lava flows of the Miocene Columbia River Basalt Group (CRBG). CRBG lavas are overlain by upper Miocene-lower Pleistocene sedimentary rocks and upper Pleistocene glacial outburst megaflood slackwater deposits in the Walla Walla Valley. The CRBG is mantled in upland areas by m’s of Pleistocene-Holocene loess. Loess locally hosts air-fall tephras chemically correlated with the ~7.7 ka cal yr B.P. Mount Mazama and 13.7-13.4 ka cal yr B.P. Glacier Peak eruptions, and a 171-130 ka tephra from an unknown source. Three major fault zones intersect in the WWB: Wallula, Milton-Freewater, Hite. The Wallula fault zone is a set of locally active, WNW-striking right-lateral strike-slip faults and N-S-striking normal faults mapped for ~120 km between Kennewick, WA and Milton-Freewater, OR. West and northwest of the Wallula fault zone strain in eastern Washington is accommodated along ~E-W-trending Yakima Folds. East of Wallula Gap, WA the Wallula fault zone transfers strain to the Hite fault zone across the Milton-Freewater fault zone, a set of NNW-striking linear, vertical to subvertical, normal and right-lateral oblique-slip faults. The Hite fault zone is defined by a 20- to 25-km-wide set of NNE-striking, dominantly down-on-the-west, left-lateral oblique-slip faults, mapped for ~140 km between McKay Creek, OR and Pomeroy, WA. Mapped WWB faults cut the CRBG, with large-scale offset postdating the ~14 Ma Umatilla Member. Distinctive lineaments in loess observed from 1 m lidar DEMs run parallel to or along strike with mapped bedrock faults, suggesting late Pleistocene-Holocene offsets. Global Navigation Satellite System data, although limited in this area, suggests low strain rates, as compared to areas closer to the Cascadia subduction zone. The WWB is, however, a region capable of generating substantial earthquakes; the epicenter of the M 6.0 1936 Milton-Freewater earthquake was relocated to an area south and east of Umapine, OR. This event with intensities reaching MMI VII+, was the most damaging earthquake to occur within Oregon until the 1991 Klamath Falls and 1993 Scotts Mills events.