Late Cenozoic exhumation of the Cascadia accretionary wedge in the Olympic Mountains, northwest Washington State

Abstract

The apatite fission-track method is used to determine the exhumation history of the Olympic subduction complex, an uplifted part of the modern Cascadia accretionary wedge. Fission-track ages are reported for 35 sandstones from the Olympic subduction complex, and 7 sandstones and 1 diabase from the Coast Range terrane, which structurally overlies the Olympic subduction complex. Most sandstone samples give discordant results, which means that the variance in grains ages is much greater than would be expected for radioactive decay alone. Discordance in an unreset sample is caused by a mix of detrital ages, and in a reset sample is caused by a mix of annealing properties among the detrital apatites and perhaps by U loss from some apatites. Discordant grain-age distributions can be successfully interpreted by using the minimum age, which is the pooled age of the youngest group of concordant fission-track grain ages in a dated sample. The inference is that this fraction of apatites has the lowest thermal stability, and will be the first to reset on heating and the last to close on cooling. Comparison of the minimum age with depositional age provides a simple distinction between reset samples (minimum age younger than deposition) and unreset samples (minimum age older than deposition). The success of the minimum-age approach is demonstrated by its ability to resolve a well-defined age-elevation trend for reset samples from the Olympic subduction complex. Microprobe data suggest that the apatites that make up the minimum-age fraction are mostly fluorapatite, which has the lowest thermal stability for fission tracks among the common apatites.

Reset minimum ages are all younger than 15 Ma, and show a concentric age pattern; the youngest ages are centered on the central massif of the Olympic Mountains and progressively older ages in the surrounding lowlands. Unreset localities are generally found in coastal areas, indicating relatively little exhumation there. Using a stratigraphically coordinated suite of apatite fission-track ages, we estimate that prior to the start of exhumation, the base of the fluorapatite partial annealing zone was located at ∼100 °C and ∼4.7 km depth. The temperature gradient at that time was 19.6 ± 4.4 °C/km, similar to the modern gradient in adjacent parts of the Cascadia forearc high.

Apatite and previously published zircon fission-track data are used to determine the exhumation history of the central massif. Sedimentary rocks exposed there were initially accreted during late Oligocene and early Miocene time at depths of 12.1–14.5 km and temperatures of ∼242–289 °C. Exhumation began at ca. 18 Ma. A rock currently at the local mean elevation of the central massif (1204 m) would have moved through the alpha-damaged zircon closure temperature at about 13.7 Ma and ∼10.0 km depth, and through the fluorapatite closure temperature at about 6.7 Ma and ∼4.4 km depth. On the basis of age-elevation trends and paired cooling ages, we find that the exhumation rate in the central massif has remained fairly constant, ∼0.75 km/m.y., since at least 14 Ma. Apatite fission-track data are used to construct a contour map of long-term exhumation rates for the Olympic Peninsula. The average rate for the entire peninsula is ∼0.28 km/m.y., which is comparable with modern erosion rates (0.18 to 0.32 km/m.y.) estimated from sediment yield data for two major rivers of the Olympic Mountains.

We show that exhumation of this part of the Cascadia forearc high has been dominated by erosion and not by extensional faulting. Topography and erosion appear to have been sustained by continued accretion and thickening within the underlying Cascadia accretionary wedge. The rivers that drain the modern Olympic Mountains indicate that most of the eroded sediment is transported into the Pacific Ocean, where it is recycled back into the accretionary wedge, either by tectonic accretion or by sedimentary accumulation in shelf and slope basins. The influx of accreted sediments is shown to be similar to the outflux of eroded sediment, indicating that the Olympic segment of the Cascadia margin is currently close to a topographic steady state. The record provided by our fission-track data, of a steady exhumation rate for the central massif area since 14 Ma, suggests that this topographic steady state developed within several million years after initial emergence of the forearc high.