Tectonic synthesis of the Olympic Mountains segment of the Cascadia wedge, using two-dimensional thermal and kinematic modeling of thermochronological ages

Abstract

A fully coupled two-dimensional kinematic and thermal model of a steady state accretionary wedge, constrained by an extensive data set of fission track and (U-Th)/He ages for apatite and zircon, is here used to investigate the development of the Olympic Mountains segment of the Cascadia accretionary wedge. The model has two main free parameters: ε_(max), the maximum rate of erosion for a generic erosion function operating at the top of the wedge, and α, the distribution of sedimentary accretion into the wedge. The best fit values for ε_(max) and α and their confidence limits are determined through an iterative search of parameter space. This study represents the first time that such inversion methods have been used to quantify the thermal-kinematic evolution of an accretionary wedge. Our results suggest that horizontal transport plays an important role in the exhumation trajectories experienced by material passing through the Cascadia wedge. At a 95% confidence interval, 80 to 100% of the sedimentary sequence from the subducting Juan de Fuca Plate has been accreted at the front of the wedge offshore of the Olympics over the past 14 m.y. This frontally accreted material must then traverse the entire width of the wedge prior to its eventual exposure in the Olympic forearc high. Assessed in this two-dimensional framework, the fission track and (U-Th)/He age data sets from the Olympic Mountains are all best fit by ε_(max) of 0.9–1.0 mm yr^(−1), despite variation in the timescales relevant to the three chronometers. This result supports the hypothesis that the Olympic Mountains segment of the Cascadia accretionary wedge has been in a flux steady-state since ∼14 Ma. The demonstration of a flux balance across the Cascadia margin also suggests that margin-parallel transport has not had a significant role in driving uplift of the Olympic Mountains.

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