Strike-Slip Faulting in the Cascadia Backarc: Documentation of Quaternary Dextral Slip on the Tumalo Fault, Central Oregon, and Implications for Regional Kinematics

Mongovin, Daniel David Thomas

Publication

Strike-Slip Faulting in the Cascadia Backarc: Documentation of Quaternary Dextral Slip on the Tumalo Fault, Central Oregon, and Implications for Regional Kinematics

Mongovin, Daniel David Thomas

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Abstract

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 OSL/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 39Ar/40Ar ages and stratigraphic relationships of overlying and underlying volcanic deposits and out IRSL ages of the fluvial geomorphic landforms yield slip rates of 0.51 mm/yr (95% CI [0.42, 0.60]) and 1.1 mm/yr (95% CI [0.8, 1.5]) 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 may improve current geodetic models, inform future seismic hazard assessments, and stimulate further neotectonic, paleoseismic, and geodetic investigation of crustal deformation and strain distribution on faults throughout the Cascadia backarc.