Also at CEREGE, BP 80, Europole Méditerranéen de l'Arbois, F-13545 Aix en Provence CEDEX 4, France [1] We present a user-friendly and versatile Monte Carlo simulator for modeling profiles of in situ terrestrial cosmogenic nuclides (TCNs). Our program (available online at http://geochronology.earthsciences.dal. ca/downloads-models.html) permits the incorporation of site-specific geologic knowledge to calculate most probable values for exposure age, erosion rate, and inherited nuclide concentration while providing a rigorous treatment of their uncertainties. The simulator is demonstrated with 10 Be data from a fluvial terrace at Lees Ferry, Arizona. Interpreted constraints on erosion, based on local soil properties and terrace morphology, yield a most probable exposure age and inheritance of 83.9 −14.1 +19.1 ka, and 9.49 −2.52 +1.21 × 10 4 atoms g −1 , respectively (2s). Without the ability to apply some constraint to either erosion rate or age, shallow depth profiles of any cosmogenic nuclide (except for nuclides produced via thermal and epithermal neutron capture, e.g., 36 Cl) cannot be optimized to resolve either parameter. Contrasting simulations of 10 Be data from both sand-and pebble-sized clasts within the same deposit indicate grain size can significantly affect the ability to model ages with TCN depth profiles and, when possible, sand-not pebbles-should be used for depth profile exposure dating.Components: 9500 words, 7 figures, 4 tables.

The forces that drove rock uplift of the low-relief, high-elevation, tectonically stable Colorado Plateau are the subject of long-standing debate. While the adjacent Basin and Range province and Rio Grande rift province underwent Cenozoic shortening followed by extension, the plateau experienced approximately 2 km of rock uplift without significant internal deformation. Here we propose that warming of the thicker, more iron-depleted Colorado Plateau lithosphere over 35-40 Myr following mid-Cenozoic removal of the Farallon plate from beneath North America is the primary mechanism driving rock uplift. In our model, conductive re-equilibration not only explains the rock uplift of the plateau, but also provides a robust geodynamic interpretation of observed contrasts between the Colorado Plateau margins and the plateau interior. In particular, the model matches the encroachment of Cenozoic magmatism from the margins towards the plateau interior at rates of 3-6 km Myr(-1) and is consistent with lower seismic velocities and more negative Bouguer gravity at the margins than in the plateau interior. We suggest that warming of heterogeneous lithosphere is a powerful mechanism for driving epeirogenic rock uplift of the Colorado Plateau and may be of general importance in plate-interior settings.

Study of the interaction between uplift and erosion is a major theme of our science, but our understanding of their interplay is often limited by a lack of quantitative data. A classic example is the Colorado Plateau, for which the starting and ending points are well known: The region was at sea level in the Late Cretaceous, and now, the deeply eroded land surface is at ~2 km. The path of the landscape between these endpoints is less clear, and there has been longstanding debate on the mechanisms, amounts, and timing of uplift and erosion. We use a geographic information system to map, interpolate, and calculate the Cenozoic rock uplift and erosional exhumation of the Colorado Plateau and gain insight into its landscape development through time. Initial results indicate uplift and erosion are highly spatially variable with mean values of 2117 m for rock uplift and 406 m for net erosional exhumation since Late Cretaceous coastal sandstones were deposited. We estimate 843 m of erosion since ca. 30 Ma (a larger value because of net deposition on the plateau over the early Cenozoic), which can account for 639 m of post-Laramide rock uplift by isostatic processes. Aside from this isostatic source of rock uplift, paleobotanical and fission-track data from the larger region suggest the early Cenozoic Laramide orogeny alone should have caused more than the remaining rock uplift, and geophysical studies suggest mantle sources for additional Cenozoic uplift. There is, in fact, less uplift on the plateau than proposed sources can supply. This suggests Laramide uplift of the plateau was significantly less than that of the Rocky Mountains, consistent with its prevalent sedimentary basins, and/or that there has been little or no post-Laramide uplift beyond erosional isostasy.

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[1] We pioneer a technique of surface-exposure dating based upon the characteristic form of an optically stimulated luminescence (OSL) bleaching profile beneath a rock surface; this evolves as a function of depth and time. As a field illustration of this new method, the maximum age of a premier example of Barrier Canyon Style (BCS) rock art in Canyonlands National Park, Utah, USA, is constrained. The natural OSL signal from quartz grains is measured from the surface to a depth of >10 mm in three different rock samples of the Jurassic Navajo Sandstone. Two samples are from talus with unknown daylight exposure histories; one of these samples was exposed at the time of sampling and one was buried and no longer light exposed. A third sample is known to have been first exposed 80 years ago and was still exposed at the time of sampling. First, the OSL-depth profile of the known-age sample is modeled to estimate material-dependent and environmental parameters. These parameters are then used to fit the model to the corresponding data for the samples of unknown exposure history. From these fits we calculate that the buried sample was light exposed for $700 years before burial and that the unburied sample has been exposed for $120 years. The shielded surface of the buried talus sample is decorated with rock art; this rock fell from the adjacent Great Gallery panel. Related research using conventional OSL dating suggests that this rockfall event occurred $900 years ago, and so we deduce that the rock art must have been created between $1600 and 900 years ago. Our results are the first credible estimates of exposure ages based on luminescence bleaching profiles. The strength of this novel OSL method is its ability to establish both ongoing and prior exposure times, at decadal to millennial timescales or perhaps longer (depending on the environmental dose rate) even for material subsequently buried. This has considerable potential in many archeological, geological and geo-hazard applications.Citation: Sohbati, R., A. S. Murray, M. S. Chapot, M. Jain, and J. Pederson (2012), Optically stimulated luminescence (OSL) as a chronometer for surface exposure dating,