CLIMATE, TECTONICS, AND TOPOGRAPHIC EVOLUTION OF THE WASHINGTON CASCADES: INSIGHTS FROM COUPLED PROCESS MODELS AND THERMOCHRONOMETRY

The Washington Cascade Mountains are part of a prominent 1200 km long by ~150 km wide active mountain belt that extends north into British Columbia, and south into Oregon and California. The central Washington Cascades are characterized by both high precipitation rates and topographic relief. Prevailing westerly winds impose a strong orographic effect across the range such that precipitation rates are as high as 4 m/yr on the windward side of the range and less than 0.5 m/yr on the leeward side. Topographic relief across the range is 2.7 km and is some of the most dramatic relief in the contemporaneous U.S.A. Topographic profiles across the range demonstrate a pronounced topographic asymmetry such that the drainage divide is located 7/10ths of the distance across the range. The strong orographic precipitation gradient and tectonic activity of the central Cascades lend to an investigation of the interaction between climate, tectonics, and topography. We use apatite (U-Th)/He cooling ages and a coupled atmospheric, surface process, and kinematic models to investigate the topographic evolution of the Washington Cascade Mountains, over the last 14 million years. The atmospheric model predicts orographic precipitation as a function of atmospheric moisture content, prevailing wind strength, temperature, and topographic slope. The coupled atmospheric and surface-process models predict plan-form topographic evolution as a function of tectonic uplift, and processes of hillslope and fluvial erosion. Exhumed (U-Th)/He sample ages are calculated using a cooling-rate dependent model of helium diffusion in apatite. Model predictions for transient topography that are consistent with cooling ages suggest: (1) long-term enhanced precipitation on the windward (western) side of the range resulted in erosion rates roughly an order of magnitude higher (~0.35 mm/yr) than the leeward side of the range (~0.04 mm/yr), (2) a pulse of high erosion rates that propagates across the range in the windward direction as the drainages develop, and (3) a strong orographic forcing explains the present day asymmetric topography and drainage divide position.