The correspondence between seismic velocity anomalies in the crust and mantle and the differential incision of the continental-scale Colorado River system suggests that signifi cant mantle-to-surface interactions can take place deep within continental interiors. The Colorado Rocky Mountain region exhibits low-seismic-velocity crust and mantle associated with atypically high (and rough) topography, steep normalized river segments, and areas of greatest differential river incision. Thermochronologic and geologic data show that regional exhumation accelerated starting ca. 6-10 Ma, especially in regions underlain by low-velocity mantle. Integration and synthesis of diverse geologic and geophysical data sets support the provocative hypothesis that Neogene mantle convection has driven long-wavelength surface deformation and tilting over the past 10 Ma. Attendant surface uplift on the order of 500-1000 m may account for ~25%-50% of the current elevation of the region, with the rest achieved during Laramide and mid-Tertiary uplift episodes. This hypothesis highlights the importance of continued multidisciplinary tests of the nature and magnitude of surface responses to mantle dynamics in intraplate settings.

The Appalachian Mountains (eastern United States) are the archetypal old, longdecaying orogen from which major theories for long-term landscape evolution have been derived. However, given the variability of relief and topographic correlation with geologic and tectonic history, it is diffi cult to describe the orogen as old and uniformly decaying. Long-term and short-term estimates suggest slow and steady erosion at ~20 m/m.y.; however, intermediate-time-scale data like sediment accumulation rates and river incision suggest unsteadiness, which we assess using apatite (U-Th)/He thermochronology. All

The relationship between the rise of the Andean mountain chain and the onset of aridity on its western margin is poorly understood. Canyon incision on both the eastern and western flanks of the chain is interpreted generally as a direct response to increased rock uplift, but may equally have been the result of climate change. Here we record some of the oldest canyon incision on the western Andean margin by tracking the downward migration of the local water table using (U-Th)/He hematite geochronology on vertical drill-core transects. Our data constrain the incision history of the Quebrada de Parca canyon in northern Chile from ca. 16 Ma to the present day. The erosional and topographic response of the Quebrada de Parca river suggests that incision was induced by a switch to a more arid climate in the middle Miocene, which reduced regional precipitation and river discharge. Geomorphic analysis of the modern river suggests that the Central Andes have gained only ∼700 m of elevation since incision began, and had therefore reached at least 50% of their current elevation by middle Miocene time. We thus conclude that the onset of aridity at ca. 16 Ma occurred subsequent to the main Andean uplift.