The Story of a Yakima Fold and How It Informs Late Neogene and Quaternary Backarc Deformation in the Cascadia Subduction Zone, Manastash Anticline, Washington, USA

Abstract

The Yakima folds of central Washington, USA, are prominent anticlines that are the primary tectonic features of the backarc of the northern Cascadia subduction zone. What accounts for their topographic expression and how much strain do they accommodate and over what time period? We investigate Manastash anticline, a north vergent fault propagation fold typical of structures in the fold province. From retrodeformation of line‐ and area‐balanced cross sections, the crust has horizontally shortened by 11% (0.8–0.9 km). The fold, and by inference all other folds in the fold province, formed no earlier than 15.6 Ma as they developed on a landscape that was reset to negligible relief following voluminous outpouring of Grande Ronde Basalt. Deformation is accommodated on two fault sets including west‐northwest striking frontal thrust faults and shorter north to northeast striking faults. The frontal thrust fault system is active with late Quaternary scarps at the base of the range front. The fault‐cored Manastash anticline terminates to the east at the Naneum anticline and fault; activity on the north trending Naneum structures predates emplacement of the Grande Ronde Basalt. The west trending Yakima folds and west striking thrust faults, the shorter north to northeast striking faults, and the Naneum fault together constitute the tectonic structures that accommodate deformation in the low strain rate environment in the backarc of the Cascadia Subduction Zone.

Exhumation Timing in the Oregon Cascade Range Decoupled From Deformation, Magmatic, and Climate Patterns

“…Compressional structures east of the High Cascades have been tracked westward into the arc, suggesting that this segment of the Cascade arc was constructed on crust that was being shortened after circa 17 Ma (Beeson & Tolan, 1990; Blakely et al, 2011; Brocher et al, 2017; Hammond, 1979). The southernmost cooling ages from Reiners et al (2002, 2003) are situated at the same latitude as the northern Yakima Fold Belt and are younger than circa 17 Ma (Figure 10) (Blakely et al, 2011; Kelsey et al, 2017; Staisch et al, 2017, 2018). Because the Yakima Fold Belt structures and shortening are restricted to this segment of the Cascades, the Yakima Fold Belt may contribute to rock uplift in only southern Washington or northern Oregon, but not along the entire Cascade Range.…”

Crustal Structure and Quaternary Acceleration of Deformation Rates in Central Washington Revealed by Stream Profile Inversion, Potential Field Geophysics, and Structural Geology of the Yakima Folds

“…We also investigated the possibility that the post‐16 Ma growth of the north‐trending Hog Ranch‐Naneum anticline (Kelsey et al, ), the fold hinge of which trends through the east end of the profiled streams (Figure a), affected the profile inversion results. In a comparison of streams east of the Yakima River and west of the Yakima River, we did not see a notable difference in relative rock uplift rate history derived from inversion results.…”

“…Here the Yakima River incised orthogonally through the Manastash Ridge, Umtanum Ridge, and Selah Butte anticlines, from north to south (Figure ). The Yakima Canyon has incised over 600 m through these folds and provides excellent exposure of the typically north‐vergent anticlines and occasionally overturned basaltic strata (Bentley, ; Kelsey et al, ; Figure ).…”

“…Geologic mapping is modified based on field and LiDAR observations from the 1:100,000 scale mapping (Schuster, ; Walsh, ) available at http://www.dnr.wa.gov/geologyportal. Mapped fault traces for the Manastash Ridge are adopted from Kelsey et al (). Bedding orientations are sourced from Walsh (), Schuster (), and Miller ().…”

“…Cross‐section B‐B′ was designed to capture the structural relationships between Umtanum Ridge and Selah Butte anticlines, is 20 km in length, and is parallel to A‐A′ (Figure ). Our structural analyses and interpretations are guided by existing geologic mapping at 1:100k scale (Schuster, ; Walsh, ), with bedding orientations supplemented from (Miller, ), existing structural cross sections of Manastash Ridge anticline (Kelsey et al, ), stratigraphy from exploration borehole logs (Czajkowski et al, ), and forward modeling of aeromagnetic and gravity data (Figures and ).…”