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 grainage 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 α-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...

Orogeny and orography: The effects of erosion on the structure of mountain belts

“…They found that apatite ages were reset over a much broader area (outer bold line in Figure 20) consistent with exhumation of the entire core but to a lesser degree. Cooling rates derived from these apatite data were the same as those based on the zircon ages, suggesting that exhumation rates have remained constant over at least the last 12 Ma [Brandon et al, 1998]. This supports the premise that the exhumation rates are in steady state, and insofar as erosion rates depend on the topography, the topography must also be in steady state.…”

Dynamics of sediment subduction‐accretion at convergent margins: Short‐term modes, long‐term deformation, and tectonic implications

“…A simple explanation is an earlier P-C phase, followed by the transition to P-U-(C?). This combination accounts for the tectonic underplate, the uplift and denudational exhumation of the core of the Olympic Mountains, and the rotation of the Coast Range Terrane [Tabor and Cady, 1978;Brandon et al, 1998]. The rotation of the backstop clearly indicates a change of mode and demonstrates that the backstop ceased to act in this role when the mode changed.…”

“…In contrast, the Miocene to recent evolution of the Olympic Mountains segment of the subduction zone has much in common with the P-U mode [Brandon et al, 1998; Willett, submitted manuscript, 1999]. This segment of the subducting plate has an uncharacteristically shallow dip of 11 o or less beneath the accretionary wedge.…”

“…This segment of the subducting plate has an uncharacteristically shallow dip of 11 o or less beneath the accretionary wedge. Such a shallow dip, coupled with an enhanced sediment influx, would cause the wedge to grow and become subaerially exposed sooner than elsewhere [Brandon et at., 1998]. Once exposed to tectonically coupled denudation, the system may have become P-U (Willett, submitted manuscript, 1999).…”

Seismic and non-seismic influences on coastal change in Alaska--Fieldtrip guide and conference abstracts, 5th International Conference of IGCP 588

“…4.3.3). This pattern of exhumation has been found by the study of the sand-size glacial outwash material using zircon fission-track dating (Enkelmann et al, , 2010, which records the time of cooling below 250 ± 40°C (Brandon et al, 1998). Exhumation of 8-10 km since 2-3 Ma have been estimated for the eastern syntaxis region and explained through the development of a positive feedback between localized strain concentration and efficient glacial erosion .…”