A REFINED STRUCTURAL MODEL OF THE OREGON CASCADES ARC-BACKARC TECTONIC PROVINCE

Extensive new digital fault mapping in the Oregon Cascades and backarc reveals previously unrecognized fault complexity at the full range of spatial and temporal scales. A combination of slope, aspect, curvature, and hillshade raster data calculated from 1-meter lidar bare earth models allowed for discovery of new faults and revisions to known faults. When combined with subsets of published fault layers from DOGAMI and the USGS, we present a significantly improved multi-layer fault database of the arc-backarc tectonic system.

In the High Cascades between Mt Jefferson and the Three Sisters (44°N-45°N), a generation of young NNW-trending en echelon fault scarps crosscut exposed volcanic stratigraphy and the youngest glacial valley surfaces, revealing an active cross-arc fault system that potentially connects the Sisters Fault Zone to the Breitenbush fault system. The southeast extension of the fault system apparently connects to young faults cutting the northwest flank of Newberry Caldera. This cross-arc fault system crosscuts the N-S trend of the Cascade Graben at Mt Jefferson but does not appear to reactivate the Green Ridge Fault. These NNW-trending faults presently dominate Cascade deformation between 44°N-45°N.

Further south (43°N-44°N), crosscutting faults of the Bald Mountain Caldera, Walker Rim, and Chemult Graben systems affirm a vigorous interplay between extensional fault scarps striking NW and NE. Furthermore, the dominant faulting at these latitudes is localized well to the east of the High Cascades, implying the major structure of the High Cascades is not a traditional graben. Assuming surficial fault scarp density is a proxy for where modern strain is localized, our new interpretations suggest that a wide zone of distributed faulting that extends well to the east of the High Cascades into the Central Oregon Basin-and-Range extensional province marks the eastern edge of the Oregon forearc block between 43°N-44°N.