This project is a follow up to a series of previous CD funded Rocky Mountain focused projects building on the discovery of a low velocity anomaly in the mantle beneath central Colorado. The proposed project involves a wide variety of techniques assembled to address the cause of the so called "Aspen anomaly", its history, and its effect on crustal evolution. The PIs are interested in the connection between changes in the mantle causing modification of surface topography. They have observed that the Aspen anomaly is located beneath the highest portion of the Rocky Mountains. They equate low mantle seismic velocities with low mantle densities to cause uplift. They will investigate whether the very large velocity variations in the mantle beneath this area are: a) in the lithosphere or asthenosphere, b) reflect Proterozoic lithospheric sutures reactivated by Cenozoic tectonism, c) are caused by a plume, and d) are caused by the presence of melt. These are all important and relevant questions for Continental Dynamics.
Specifically, the PIs will use: - passive seismology to better define the anomaly using tomography, receiver functions, surface waves, and anisotrophy; - geological studies (magmatic history, low temperature thermochronology, tectonic geomorphology) to identify Cenozoic tectonic and magmatic history to define long term and current history; - geodynamical models of topography uplift and mantle processes to study their connection.
The goals of our project were to better understand the timing and mechanisms by which high elevations in the Colorado Rocky Mountains were attained. The research conducted by Kirby and colleagues at Penn State focused on using the history of river incision along the Colorado River and its tributaries to reconstruct uplift of the Rockies. Many workers attribute incision and the development of deep canyons along the upper Colorado River to one of several processes - 1) development of Grand Canyon during drainge integration across the western flank of the Colorado Plateau; 2) an increase in erosive potential of river systems as a consequence of climate change, or 3) local differential uplift of the Rockies. One of the primary conclusions of our research is that the onset of rapid incision in the upper Colorado drainage began prior to cutting of Grand Canyon, and thus must reflect more local changes in the river system. It does not seem likely that incision in the Rockies is the consequence of drainage integration downstream. By developing new chronology of volcanic deposits and lava flows, we are also able to show that the onset of incision appears to be similar along rivers to the north (White, Yampa and Little Snake Rivers). However, the amount of incision decreases from south to north. This pattern is not expected to result from a simple result of a change in climate. In addition, the amount of incision appears to correlate with how steep the rivers are - rivers that have incised more deeply are still steeper than their counterparts to the north. This counter-intuitive finding suggests that some local process is maintaining steep rivers, despite greater incision. These spatial patterns in both river steepness and incision appear to be driven by differential uplift along the western slope of the Rockies. The differences are interpreted to reflect uplift driven by changes in the buoyancy of the upper mantle beneath the range, and we suggest that these differences can be explained by differential uplift of a few hundred meters in the past 10 million years. Our results present new evidence that the Rockies indeed have a youthful component to their growth.
