Oblique subduction of the Juan de Fuca plate beneath the North American plate characterizes the tectonic setting of the Pacific Northwest. North American plate deformation at the latitude of central Oregon consists of the clockwise-rotation of the Siletzia block in the forearc and the extensional Basin and Range province in the Cascadia backarc. Siletzia rotates westward and northward with respect to the backarc. Geodetic velocity vectors change in direction and decrease in magnitude from west to east across the Cascade Arc, which indicates that one or more structural boundaries exist within the forearc-backarc transition in Central Oregon to accommodate the separation. A combination of normal and right-lateral shear ought to characterize the differential motion across the structural boundary.
The Sisters fault zone, an active, diffuse fault system, may account for a significant fraction of the differential motion of Siletzia with respect to the Basin and Range province in central Oregon. Both heat flow and volcanic productivity decrease to the northwest across the Sisters fault zone, which suggests the Sisters fault zone is a fundamental structure. The northwest-striking Sisters fault zone extends from Newberry Volcano to just south of Mt. Jefferson. Prominent fault scarps up to 10 m in relief characterize individual fault traces. Whereas normal separation characterizes fault surface expression, lateral separation has not been recognized to date. Sisters fault zone faults located on the NW flank of Newberry are associated with Quaternary to Holocene lava flows. A new, Lidar-based evaluation of individual fault segments comprising the Sisters fault zone is used to examine the fault kinematics, slip rate, and their relationship to volcanic processes. New geochronologic analyses of several Quaternary lava flows provide constraint on the slip rates across the Sisters fault zone. Isostatic gravity analysis reveals the geometry of crustal-scale structures and the influence of the Sisters fault zone upon the subsurface. Detailed study of the Sisters fault zone has direct implications for Pacific Northwest tectonics and, importantly, for volcanic and earthquake hazards.