LATE PLEISTOCENE FAULTING IN A TRANSTENSIONAL TRANSFER ZONE, HELLS CANYON, OREGON-IDAHO BORDER

A zone of diffuse deformation ~600 km-wide extending from northern California to Washington has developed resulting from the oblique subduction of the Juan de Fuca plate beneath North America. This zone is marked by a change in structural style from transtension in the southern Oregon and western Nevada to transpression in southern and western Washington. The transition occurs across a relatively inactive zone in northeastern Oregon. New mapping near the northern edge of this transitional zone suggests that transtensional deformation persists into northeastern Oregon. The study area on the Oregon-Idaho border, is a complex zone characterized by NNW-trending oblique-reverse and normal faults linking the NW-trending Halfway and Sturgill Peak (both down to the NE) normal faults to the NW and SE, respectively. The Halfway fault, ~20 km-long ~ 250 m normal separation, marks the southern boundary of a half-graben; whereas the Sturgill Peak fault, ~ 10-15 km long and not associated with a half-graben, has a minimum separation of 640 m. In contrast, the transfer zone houses numerous ~NNW-striking, 2 - 3 km-long normal and dextral-reverse faults with dip-slip separations of < 90 m. Evidence of Late Pleistocene-Holocene deformation within the transfer zone includes two dextral reverse faults (~1.75 m vertical, ~3 m horizontal separation) cutting late Pleistocene Bonneville flood gravels (~14 ka; implying a ~ 0.25 mm/yr slip rate). A syncline in the hangingwall of the Robinette normal fault folds >20 m of Quaternary sediment, including two ash beds (~65-72 ka and ~ 46 ka). Although the Halfway and Sturgill Peak faults have been interpreted to represent the southeastern extension of the Olympic-Wallowa Lineament (O.W.L.), the tectonic and structural significance of the O.W.L. remains poorly understood. We therefore consider these structures to reflect the continuation of Basin-and-Range style deformation into northeastern Oregon, and western Idaho, and any connection to the O.W.L. to be circumstantial. Furthermore, the presence of these structures poses a potential seismic hazard to local hydroelectric facilities.