ABSTRACT

The remarkable match between the glacial equilibrium line altitude (ELA) and summit elevations in many active mountain ranges of the world has led to the supposition that glaciers act as an erosional buzzsaw. Since most observed rates of glacial erosion are very high they are thought to be able to effectively remove most topography tectonically uplifted through the ELA. However, the glacial buzzsaw idea is based largely on circumstantial evidence. Only preliminary attempts have been made to theoretically simulate quantitative predictions of this process. This proposed research aims to test the glacial buzzsaw hypothesis using modern technical and computational methods. Understanding the importance of glacial erosion is complicated by the fact that elevated topographies are often the result of plate convergence, and so an appreciation of the feedbacks involved between orogenic wedge mechanics and surface processes is required. Development of a coupled geodynamic and surface process model that incorporates glacial erosion in an active critical orogenic wedge is proposed to examine these feedbacks. Preliminary model predictions indicate that the temporal and spatial patterns of uplift and erosion for both a weak and strong glacial buzzsaw in an active orogen contrast significantly with erosion patterns of an orogen that responds to the buzzsaw by passive isostasy. Testing the model predictions to judge whether glacial erosion acts as a 'strong' or 'weak' buzzsaw will be achieved by applying combined apatite (U-Th)/He and fission-track low temperature thermochronology to evaluate changes in erosion rates and spatial erosion patterns following the onset of late Cenozoic glaciation in the Patagonian Andes - the orogen where the match between the height of the ELA and summit elevation was first recognized. The Patagonian Andes are an exceptional natural laboratory. As well as having a suitable and well-documented tectonic and climatic history, their north-south range provides an opportunity to study a spatially varying glacial history. Two different transects will be targeted: one at 39S to 41S (Valdivia, Pucn, Bariloche), and another between 46S and 51S (Canal Baker, Lago General Carrera, South Patagonian Icefield). This will include the collection of both near-vertical and trans-orogen sample profiles. The proposed work will address the following research questions: 1) Does the glacial buzzsaw hypothesis explain the behavior of surface erosion in the Patagonian Andes? 2) Was the buzzsaw diachronous or synchronous along the length of the Patagonian Andes? 3) Is the orogen close to steady state, or still responding to the drop in ELA? and 4) How accurate are current models of glacial erosion over regional scales and long time periods?
Broader Impact
The long-term behavior of the earth system is dependent on dynamic interactions between climate, albedo, tectonics, orogenic topography, weathering, and greenhouse gases, such as CO2. In particular, there has been much speculation about the role that erosional fluxes from orogenic landscapes might play in moderating the greenhouse effect, given the important role of silicate weathering in moderating CO2 concentrations in the atmosphere. Orogenic topography as influences the distribution of precipitation, and can also change global atmospheric circulation, if the topography gets high enough, as is presently the case in the Himalaya and central Andes. Thus, there remains broad interest in the factors that might influence the evolution of orogenic topography, or limit the maximum size of the topography. This research will make a significant contribution towards understanding the role that alpine glaciation plays in controlling the maximum height of mountain ranges. The multidisciplinary approach of this proposal will foster a number of national and international educational and research links. Students and researchers from Louisiana State, Yale, GFZ Potsdam, the Universidad de Chile will be involved in this project. Students from two US institutions will have an opportunity to combine field research and theory. The proposed study has the potential to provide research support for an alumnus of the NSF funded Geoscience Alliance to Enhance Minority Participation in the Earth Sciences (GAEMP) project, through co-PI Jonathan
Tomkin's participation in the program at LSU. The results of the research will be integrated into a GAEMP summer school program, which emphasizes the value of ongoing earth science research to minority undergraduates.
This award is being supported with funds from the Geomorphology and Land Use Dynamics Program, the Tectonics Program, and the Americas Program in the Office of International Science and Engineering.

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