How steady are steady-state mountain belts? – a re-examination of the Olympic Mountains (Washington State, USA)

Abstract. The Olympic Mountains of Washington State (USA) represent the aerially exposed accretionary wedge of the Cascadia subduction zone and are thought to be in flux steady-state, whereby the mass outflux (denudation) and influx (tectonic accretion) into the mountain range are balanced. We use a multi-method approach to investigate how temporal variations in the influx and outflux could affect previous interpretations of flux steady-state. This includes published and new thermochronometric ages for (U-Th)/He dating of apatite and zircon (AHe and ZHe, respectively) and fission track dating of apatite and zircon (AFT and ZFT, respectively), 1D thermo-kinematic modelling of thermochronometric data and independent estimates of outflux and influx. We present 61 new thermochronometric ages. AHe ages are generally young ( 2 km/Myr to  1 km/Myr. This demonstrates that the material outflux is varying through time, requiring a commensurate variation in influx to maintain flux steady-state. Evaluation of the offshore and onshore sediment record shows, that the material influx is also variable through time and that the amount of sediment residing in the wedge is spatially variable. Our quantitative assessment of influx and outflux indicates that the Olympic Mountains could be in flux steady-state on long timescales (107 yr), but significant perturbations of steady-state occur on shorter timescales (105–106 yr), e.g., during the Plio-Pleistocene glaciation.