OBLIQUE SLIP HISTORY OF ACTIVE FAULTS ALONG THE WESTERN MARGIN OF THE HOOD RIVER GRABEN, NORTH-CENTRAL OREGON

The Cascade volcanic arc in northern Oregon is host to the Hood River Valley, a zone of intra-arc crustal extension bounded on the east by the impressive west-facing scarp of the Hood River fault zone. Twenty km to the west, the Gate Creek-Wyeth fault (GCWF) is a 10–20 km-long, NNW-striking, ENE-dipping zone of faults recently discovered on lidar between Mt. Hood and the Columbia River. Field and lidar observations of the primary scarp on the west flank of Mt. Defiance indicate 1–2 m vertical separation of a post-glacial colluvial surface with little scarp diffusion and a 1–2-m-high scarp preserved within a larger compound scarp, suggesting a recent surface-rupturing earthquake. Nearby, the scarp exhibits ~50 m of right separation and ~15 m of vertical separation of a late Quaternary glacial moraine crest. A 100–200-m-thick sequence of plateau-forming Pliocene(?) basalt flows of the High Cascades are cut by the GCWF, with several hundred meters of right separation of the eroded plateau edge and 100–200 m vertical separation of the slightly degraded plateau surface, further supporting a protracted history of right-lateral normal-oblique fault slip. Pending geochemistry and Ar/Ar geochronology will help correlate lava flows across the GCWF and improve slip rate estimates. Further bedrock mapping and geochemistry will attempt to quantify fault offset in underlying flows of the Columbia River Basalt Group, which exhibit west-side-up bedrock facets along the GCWF in the steep slopes of the Columbia River Gorge. Comparison of fault offset in Miocene and Pliocene units will constrain the onset of fault activity. The GCWF and related faults in this Mt. Hood fault zone are antithetic to the Hood River fault zone and may represent the western margin of a ~20 km-wide Hood River graben. Mapping on the Washington side of the Columbia River has identified several youthful NNW-oriented escarpments that could be related to the northward propagation of rifting in the High Cascades graben and/or incipient, mountain-scale landsliding that is pervasive on the northern side of the Columbia River Gorge. Dextral-oblique motion across the Mt. Hood fault zone suggests that the Hood River graben may accommodate both intra-arc rifting as well as the northward translation of rotating crustal blocks in the upper plate of the Cascadia subduction zone.