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Earth Surface Dynamics An interactive open-access journal of the European Geosciences Union
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© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 03 Sep 2018

Research article | 03 Sep 2018

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This discussion paper is a preprint. A revision of the manuscript is under review for the journal Earth Surface Dynamics (ESurf).

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

Lorenz Michel1, Christoph Glotzbach1, Sarah Falkowski1, Byron A. Adams1,2, and Todd A. Ehlers1 Lorenz Michel et al.
  • 1Department of Geosciences, University of Tübingen, Tübingen, 72074, Germany
  • 2School of Earth Sciences, University of Bristol, Bristol, BS8 1RJ, United Kingdom

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 (<4Ma), and in some samples, AFT ages (5–8Ma) overlap with ZHe ages (7–9Ma) within uncertainties. Thermo-kinematic modelling shows that exhumation rates are temporally variable, with rates decreasing from >2km/Myr to <0.3km/Myr around 6–8Ma. With the onset of Plio-Pleistocene glaciation, exhumation rates increased again to values >1km/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 (107yr), but significant perturbations of steady-state occur on shorter timescales (105–106yr), e.g., during the Plio-Pleistocene glaciation.

Lorenz Michel et al.
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Status: final response (author comments only)
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Lorenz Michel et al.
Lorenz Michel et al.
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Publications Copernicus
Short summary
Mountain-building processes are often investigated by assuming a steady-state, meaning the balance between opposing forces, like e.g. mass influx and mass outflux. In this study we show that the assumption of steady-state for the Olympic Mountains is depending on the timescale and the spatial geometry, and particularly affected by the Quaternary glaciation. This indicates that mountain ranges can be out of steady-state during parts of their evolution.
Mountain-building processes are often investigated by assuming a steady-state, meaning the...