SEPARATION OF SILETZIA FROM THE BACKARC ALONG THE SISTERS FAULT ZONE, CENTRAL OREGON, USA?

Oblique subduction of the Juan de Fuca plate beneath the North American Plate (NAP) characterizes the tectonic setting of the Pacific Northwest (PNW). NAP 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 backarc. Siletzia rotates westward and northward with respect to the backarc. Geodetic velocity vectors change in direction and decrease magnitude from west to east across the Cascade Arc, which indicates that a structural boundary must exist within the forearc-backarc transition in Central Oregon to accommodate the differential motion. A combination of normal and right-lateral shear ought to characterize the differential motion across the structural boundary.

The Sisters Fault Zone (SFZ), an active diffuse fault zone, 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 arc volcanic productivity decrease to the northwest across the SFZ, which suggests the SFZ is a fundamental structure. The northwest-striking SFZ extends from Newberry Volcano to just south of Mt. Jefferson. Although faults of the SFZ reach the High Cascades, it is unclear whether the SFZ crosses into the forearc to the west. Prominent fault scarps up to 10 meters in relief characterize individual fault traces. Whereas normal separation characterizes fault surface expression, lateral separation has not been recognized to date. SFZ faults located on the NW flank of Newberry are associated with Holocene lava flows. A new, lidar-based evaluation of individual fault segments comprising the SFZ is used to examine the fault kinematics, slip rate, and their relationship to volcanic processes. Detailed study of the SFZ has direct implications for PNW tectonics and, importantly, for volcanic and earthquake hazards.