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  • 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, and the northwestward migration of the Pacific plate. 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 geologic mapping near the northern edge of this transitional zone suggests that extensional deformation persists into northeastern Oregon. The study area on the Oregon-Idaho border, is a complex zone characterized by NNWtrending primarily sinistral-oblique 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 - 730 m dip-slip separation, marks the southern boundary of a half-graben; whereas the Sturgill Peak fault, - 10 to 15 km long and not associated with a half-graben, has a minimum dip-slip separation of - 660 m. In contrast, the transfer zone informally named the Powder River Peninsula Fault Zone, or PRPFZ, houses numerous -NNW-striking, 2 to 3 km-long normal and sinistral-oblique normal faults with dip-slip separations of < 130 m. Evidence of late Pleistocene-Holocene deformation within the transfer zone is found in at least four places, and at two of those involves sinistral-oblique normal faults (-10-15 m vertical separation on a bedrock scarp) cutting late Pleistocene Bonneville flood gravels, -14 ka, with flood gravels both entrained in the fault plane and interbedded with Holocene colluvium implying significant activity in the late Pleistocene and possibly the early Holocene. Additionally, one fault in the PRPFZ cuts Holocene (?) soils/clays and deforms Pleistocene ash beds. Slip rates calculated from slickenlines and separations in the PRPFZ range from 0.01 mm/yr to possibly as high as 2.8 mm/yr to. This detailed slip data for the PRPFZ can be used as a proxy for the more regional deformational styles. Although the Halfway and Sturgill Peak faults have been interpreted to represent the southeastern extension of the Olympic-Wallowa Lineament (OWL), the tectonic and structural significance of the OWL remains poorly understood. It is suggested here that these structures reflect the continuation of Basinand-Range style deformation into northeastern Oregon, and western Idaho, and any connection to the OWL to be circumstantial. Furthermore, the presence of these structures poses a potential seismic hazard to local hydroelectric facilities.
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