This project provides funds for a two-year renewal of the St. Elias Erosion-tectonics Project (STEEP). STEEP is a 9 institution, multidisciplinary study of the St. Elias orogen in southern Alaska that involves researchers examining the system from the outcrop to lithosphere scale. To date, STEEP has produced 17 papers with another 9 submitted or nearing submission, sponsored 71 abstracts, will have matriculated 5 masters and 4 Doctoral students by Spring 2010, and fundamentally changed our understanding of Alaskan tectonics and the interaction of tectonics and climate in mountain building. The renewal funds will be used for: 1) final processing and interpretation of some key datasets that were not acquired until year 5 of the project including the marine seismic survey (ship delays) and reoccupation of key GPS sites (weather problems in 2008); and 2) a complete integration of results which was not possible until now due to these delays. A complete integration and synthesis of these superb datasets has the potential to be transformative in our understanding of how crustal structure and tectonic forces interact with Earth surface processes of glacial erosion and sedimentary transport to grow a mountain range and a massive continental shelf.
During plate convergence the contraction between the two plates produces a wedge shaped mass of deformed material analogous to pileup of snow in advance of a plow. In a snow plow a constant surface slope is maintained by pileup of material in front of the plow, but in the earth system erosion and deposition of sediments modify the surface slope over geologic time. Thus, as uplift raises mountains above sea level the erosion of the highlands and deposition of sediments to the lowlands flatten the surface slope which forces the system to adjust through internal faulting to rebuild the surface slope to what is called critical taper. If rates of erosion increase these adjustments accelerate but tectonic adjustments are limited by plate motion rates. The fastest erosional agents on the planet are glaciers but the extent of glaciers is highly dependent on climate and surface elevation. Thus, actively-deforming, glaciated mountain systems represent key sites where we can study what happens when climate change over the last ~1 million years dramatically changed erosion rates during the rise and fall of glacial ice masses. This was the principal rationale for the St Elias Erosion and tectonics Project (STEEP) and the project focused on the development of the highest mountains in North America, the actively deforming mountain system of southern Alaska where these problems could be analyzed. STEEP produced a substantial scientific literature that is too large to summarize here, but some highlights include: STEEP established that the tectonic driver for southern Alaskan mountain building is the collision of a large oceanic plateau. This oceanic plateau is a continuous, gently-dipping slab beneath most of eastern Alaska that drives deformation as far away as the Arctic; a process analogous to ancient mountain building events that produced the Rocky Mountains of the continental United States. Prior studies had recognized the system was collisional, but the nature of the colliding block was solved by STEEP geophysical studies. STEEP represents the first whole mountain belt scale investigation to examine the interplay of erosion, deposition, and deformation across the system from source (erosional highlands) to sink (depositional basins). Although the details are debated, even within the STEEP research group, there is no doubt that climate change in the last ~1 m.y. dramatically changed the coastal St. Elias Mountain range in a complex interaction between glacial erosion and tectonics. Rapid onshore erosion shed vast piles of sediments offshore and the tectonic system reacted to this mass redistribution through shifting deformation through time; results broadly consistent with wedge taper theory, but complicated by important details of sedimentation tied directly to erosion. Within this problem STEEP researchers also recognized three-dimensional processes played important roles where the Pacific-North American transform plate boundary meets the collision zone, indicating that focused erosion and offshore sediment dispersal affected structures that created the highest coastal mountain range on earth. In spite of rapid erosion that is removing crustal mass at rates of many kilometers per million years (Figure 1), the St. Elias Mountains have been high enough for millions of years to generate sufficient snowfall to nourish glaciers that reached the Pacific. Thus the mass of crustal material removed from the mountains by erosion must be closely offset by tectonic addition of mass in the STEEP region. STEEP studies confirmed this concept and showed that zones of rapid erosion in the long term coincide closely with zones rapid deformation, high precipitation, and rapid glacial erosion. The project provided new insights into earthquake hazards in southern Alaska through paleoseismology studies conducted during the project. The project spawned new techniques including use of glacial outwash sediments for thermochronology to deduce erosion beneath glacial ice; use of LiDAR data to map bedrock geology in areas of poor outcrop; and methods for recognition of structures that form when multiple faults move simultaneously. The project supported research by students ranging from undergraduate to graduate levels to post-doctoral researchers. In all more than 20 undergraduates, 11 M. S. students, 9 Ph.D. students and 2 Post-doctoral researchers worked on the project. All of the MS and PhD students are employed in either private industry, as research scientists, or in academic positions. The clearest general impact of STEEP has been to showcase just how interwoven tectonic and surface processes are. Primarily because of STEEP, the St Elias region is emerging as the world’s premier example of how surface processes of erosion can change the landscape so fast that the tectonic system has to adjust to the changes. Additionally, the project supports emerging theories that intensification of the cycle of glaciations had a fundamental role in enhancing mountain building, but the same signal needs to be found in other glaciated mountain ranges to validate this inference. Further information can be found in an International Innovation interview (www.researchmedia.edu) or at the STEEP website (www.ig.utexas.edu/steep)
