EVALUATING TECTONIC AND VOLCANIC ORIGINS OF FAULTS IN THE HIGH CASCADES: ANALYSIS OF THE WHITE BRANCH FAULT ZONE, CENTRAL OREGON

Recent advances in the understanding of active tectonics in the Pacific Northwest have been greatly facilitated by the rapid expansion of airborne lidar topographic data, particularly in regions of dense forest where submeter resolutions permit the identification of subtle surface deformation. The addition of numerous Quaternary-active faults in the Oregon Cascades have added significantly to the number and distribution of known active faults in the region. However, distinguishing seismogenic faults from surface displacements produced by subsurface volcanic processes remains a persistent challenge. Here, we present an investigation of the potential link between volcanic processes and postglacial faulting in the White Branch fault zone (WBFZ) in west central Oregon.

The WBFZ is a diffuse zone of segmented normal faults, up to 45 km long and 10 km wide, which forms the contemporary western margin of the High Cascades Graben in the upper McKenzie River watershed: a region characterized by widespread normal faulting coupled with high rates of volcanic output. In an effort to refine the characterization of the fault as a potential source of strong ground shaking, we used high-resolution (0.5 m) airborne bare-earth lidar to supplement existing mapping of fault distribution, geometry, displacement, and relation to latest Pleistocene to Holocene deposits. To better understand whether the prominent postglacial expression of the WBFZ is representative of long-term tectonic processes, related to volcanic processes, or both, we sought to: 1) estimate long-term and short-term (post-LGM) slip rates, 2) evaluate the role of far-field and near-field stress on the contemporary development of the fault system, and 3) evaluate the volcanic contribution to strain accumulation. Our results favor a volcanic origin for the majority of post-LGM strain release along the WBFZ. However, a period of tectonic-earthquake clustering and associated high strain release cannot be precluded with current data. With the availability of lidar throughout the Cascades, this approach is likely to have regional applicability for future fault studies and seismic source characterization efforts.