STRIKE-SLIP FAULTING IN THE CASCADIA BACKARC: NEOTECTONIC MAPPING AND IRSL GEOCHRONOLOGY REVEAL NORMAL-OBLIQUE DEXTRAL SLIP RATES ALONG THE ACTIVE TUMALO FAULT, SISTERS FAULT ZONE, CENTRAL OREGON, USA

The active northwest-trending transtensional Sisters fault zone in Deschutes County, Oregon, is nestled in the central Cascadia backarc amid extensional, contractional, and strike-slip tectonic domains of the Pacific Northwest. The relationship and potential kinematic linkage between the faulting within the central Cascadia backarc and the various deformation styles throughout Western North America are unclear. The >45-km-long, northwest-striking Tumalo fault is the longest fault strand of the Sisters fault zone. Previous studies note that the sense of slip on the Tumalo fault is mainly unknown. However, recent studies identify normal and strike-slip separations on the fault, although our understanding of dextral motion is poorly constrained. Here we present new neotectonic mapping, topographic analysis, and IRSL geochronology that reveals active normal-oblique slip on the Tumalo fault. The Tumalo fault dextrally offsets Middle Pleistocene volcanic deposits by 105.5 ± 35.0 m and Late Pleistocene/Early Holocene fluvial terrace risers by 29.2 ± 9.0 m. Previous ages from ³⁹Ar/⁴⁰Ar of the volcanic deposits yield a minimum slip rate of 0.4 ± 0.1 mm/yr, and our IRSL ages of the younger geomorphic landforms yield a maximum slip rate of 1.3 ± 0.4 mm/yr for the Tumalo fault. Active normal-oblique slip within the Sisters fault zone suggests kinematic linkages between crustal deformation in the central Cascadia backarc and transtension in the Klamath Graben, dextral slip in the Walker Lane Belt, Basin & Range extension, and clockwise rotation of the Cascadia forearc. Additionally, new geologic slip rates within the Sisters fault zone will improve current geodetic models, inform future seismic hazard assessments, and may stimulate further neotectonic, paleoseismic, and geodetic investigation of crustal deformation and strain distribution on faults throughout the Cascadia backarc.