Todd A. Ehlers

Apatite (U–Th)/He thermochronometry: methods and applications to problems in tectonic and surface processes

In the last decade apatite (U–Th)/He thermochronometry has emerged as an important tool for quantifying the cooling history of rocks as they pass through the upper 1–3 km of the crust. The low closure temperature of this technique (∼70°C) has gained the interest of geomorphologists and tectonocists because it is applicable to interdisciplinary studies in landform evolution, structural geology, and geodynamics. We discuss current analytical techniques, the temperature calibration of the method, and sample quality considerations. Results from 1D, 2D and 3D thermo-kinematic numerical models are used to illustrate applications of He thermochronometry to problems in tectonics and landform evolution.

Late Miocene exhumation and uplift of the Washington Cascade Range

The Washington Cascade Range is a complex, polygenetic mountain range that dominates the topographic, climatic, and cultural configurations of Washington State. Although it has been the locus of ongoing arc magmatism since the Eocene, most of the range is distinct from the southern part of the arc in Oregon and California in that bedrock uplift has produced high surface elevations and topographic relief, rather than volcanic burial or edifice construction. (U-Th)/He and fission-track ages of bedrock samples on the east flank of the range record relatively rapid cooling in the early Tertiary, but slow exhumation rates (∼0.2 km/m.y.) through most of the Oligocene. Samples on the west flank suggest rapid cooling in the late Miocene (8–12 Ma), and age variations in vertical transects are consistent with a pulse of rapid exhumation (0.5–1.0 km/m.y.) at that time. Apatite He ages as young as 1–5 Ma in several areas suggest that high cooling and possibly exhumation rates persist locally. Accelerated exhumation rates ca. 10 Ma are also observed in the Coast Mountains of British Columbia and southeast Alaska, ∼1500 km to the north, suggesting a large-scale mechanism for the exhumation pulse at that time.