Late quaternary evolution of the Manastash Anticline and Manastash Range Front, Yakima Fold Belt, Washington: influence of tectonics and climate

Evaluating the geomorphic evolution of surface processes along the Manastash Ridge and range front provided insight into the structural growth and evolution of the Manastash anticline, Yakima Fold Belt, Washington. Based on multiple independent data sets documenting episodic baselevel lowering of the Kittitas Valley relative to the Manastash Ridge range front, I infer Quaternary growth of the Manastash anticline above a south-dipping master ramp thrust. Uplift rates within the core of the anticline, calculated using luminescence dating of strath terraces, are 0.16-0.18 m/ky over the last 90 ky. Further support for Quaternary tectonic activity includes two LiDAR-identified fault scarps at the base of the Manastash range front west of the canyon entrance. Each fault scarp correlates with thrust faults evident on a seismic reflection line across the range front. Where these fault scarps occur at the base of the range front, alluvial fans are progressively truncated and uplifted by each fault scarp. Fan growth and entrenchment along the range front is driven by episodic climatic fluctuations that foster cycles of aggradation (transport-limited conditions) and incision (supply-limited conditions). I used luminescence age determinations and tephrochronology to determine that the timing of aggradation of alluvial units (Qf3, Qf2) correlates to the late Pleistocene (MIS Stage 5) glacial-interglacial climate transition. While climate aggradation and degradation cycles control fan building events, tectonic faulting along the Manastash range front is concurrent and has elevated fans above the Kittitas Valley. Range front fault displacement of alluvial fans instigates headward retreat of stream knickpoints across multiple tributaries. Therefore, both climatic and tectonic forcing mechanisims contribute to the Quaternary landscape evolution of the southern Kittitas Valley and Manastash range front.