Tectonic and glacial contributions to focused exhumation in the Olympic Mountains, Washington, USA

Tectonics and climate are major contributors to the topographic evolution of mountain ranges. Here, we investigate temporal variations in exhumation due to the onset of Pleistocene glaciation in the Olympic Mountains (Washington State, USA). We present 29 new apatite and zircon (U-Th)/He ages (AHe and ZHe), showing a decrease in ages toward the interior of the mountain range for both thermochronometric systems. Young AHe ages (<2 Ma) can be found on the western side and the interior of the mountain range. Thermokinematic modeling of sample cooling ages suggests, that ZHe ages can be explained by an ellipse-shaped exhumation pattern with lowest/ highest rates of ~0.25 and 0.9 km/m.y. These rates are interpreted as tectonically driven rock uplift, where the pattern of rates is governed by the shape of the subducted plate. However, the youngest AHe ages require a 50–150% increase in exhumation rates in the past 2–3 m.y. This increase in rates is contemporaneous with Pliocene-Pleistocene alpine glaciation of the orogen, indicating that tectonic rock uplift is perturbed by glacial erosion.