Most sandstone samples give discordant results, which means that the variance in grain 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.
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 wa 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.
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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 at depths of 12.1-14.5 km and tempeartures 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 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/myr, since at least 14 Ma.
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.
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