Spatially Variable Exhumation Rates in Orogenic Belts: An Andean Example

Abstract

The Cordillera Real of the Bolivian Andes is a large, tectonically active mountain range that dominates sediment influx into the Amazon Basin, but rates of exhumation in the orogen are poorly known. We present 20 new apatite fission track ages from two valleys in the Cordillera Real to constrain patterns of mountain range exhumation over 106–107 yr. We interpret these and previously published data from a third valley using a 2‐D thermal model that accounts for topographic and advective influences on measured cooling ages. Exhumation rates in the Cordillera Real are ∼0.2–0.6 mm/yr, comparable to rates in parts of Denali, the Washington Cascades, the Olympic Mountains, and the European Alps and an order of magnitude slower than rates in Taiwan, Nanga Parbat, the Greater Himalaya of Nepal, and the Southern Alps of New Zealand. Three‐ to fourfold cooling age variations in the Cordillera Real imply at least twofold exhumation rate variations within and between valleys over distances of only tens of kilometers. Topography in the cross‐valley dimension affects exhumation rate estimates by 20%–30% in the downstream portions of two sample transects. Along‐valley topographic effects are less significant in this setting, affecting exhumation rate estimates by <15%. The most significant along‐valley topographic effects are associated with long‐wavelength mountain shape, including both retreat of the closure temperature isotherm near the mountain crest and compression of low‐temperature isotherms farther down the mountain flank. Locally varying phenomena (e.g., subregional structural history or transient patterns of local channel incision) must exert important controls on long‐term erosion patterns in order to produce observed short‐wavelength exhumation rate variations. Comparison of exhumation rate estimates with modern erosion rates suggests that long‐term and short‐term average erosion rates likely vary by less than twofold.