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

A fully coupled two-dimensional kinematic and thermal model of a steady sta te accretionary wedge, constrained by an extensive data set of fission trac k and (U-Th)/He ages for apatite and zircon, is here used to investigate th e development of the Olympic Mountains segment of the Cascadia accretionary wedge. The model has two main free parameters: (epsilon) over dot(max), th e maximum rate of erosion for a generic erosion function operating at the t op of the wedge, and alpha, the distribution of sedimentary accretion into the wedge. The best fit values for (epsilon) over dot(max), and alpha and t heir confidence limits are determined through an iterative search of parame ter space. This study represents the first time that such inversion methods have been used to quantify the thermal-kinematic evolution of an accretion ary wedge. Our results suggest that horizontal transport plays an important role in the exhumation trajectories experienced by material passing throug h the Cascadia wedge. At a 95% confidence interval, 80 to 100% of the sedim entary 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 wed ge 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 se ts from the Olympic Mountains are all best fit by (epsilon) over dot(max) o f 0.9-1.0 mm yr(-1), despite variation in the timescales relevant to Mounta ins are all best fit by the three chronometers. This result supports the hy pothesis that the Olympic Mountains segment of the Cascadia accretionary we dge has been in a flux steady-state since similar to 14 Ma. The demonstrati on of a flux balance across the Cascadia margin also suggests that margin-p arallel transport has not had a significant role in driving uplift of the O lympic Mountains.