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.
The St. Elias mountains of southern Alaska is the fastest deforming active mountain belt in North America. Mountains are uplifting rapidly because of the rapid shortening across the range, at a rate of about 1.5 inches per year (4 cm/year) caused by the collision of thickened crust with southern Alaska. At the same time, the mountains are being eroded rapidly by vast areas of temperate, fast flowing ice. This project was initially formulated to study interactions between erosion and tectonics, and very specifically the role of the "glacial buzzsaw" in the destruction of topography. The primary role of scientists at the University of Alaska Fairbanks was to work on the identification of the active structures that take up the present-day deformation, and evaluate how these related to the longer-term crustal-scale geological structures. We also worked on trying to separate present-day vertical motions from tectonics from vertical motions from glacial isostatic adjustment, which is the response to the net loss of ice across the region. We have identified the present-day active structures within the St. Elias orogen and placed them within their structural context. We found that the Alaska-Aleutian megathrust at depth extends farther to the east than expected based on surface information. Not only does it exist, but it is locked and building up for future earthquakes. This indicates that very large earthquakes could occur east of the 1964 rupture zone, perhaps larger than the events that occurred at the beginning of the 20th century. We used seismic tomography to determine the crustal-scale structure of the region. This seismic velocity model reveals slower velocities along the northern edge of the Yakutat microplate in the St. Elias coastal range corresponding to tectonically off-scraped and deformed rocks of the Yakutat sedimentary cover. These slower velocities can be traced as deep as 30 km and contrast to the offshore area in the south and region north of the Bagley fault, which is the northern limit of the main active tectonic deformation. The upper part of the velocity anomaly corresponds to the up to 15 km of young sediments that have been scraped off by the collision, which have slower velocities than the average crust and the reference model. The lower part of the velocity anomaly corresponds to the rocks of the Yakutat oceanic plateau, which have a slower velocity than the average middle and lower crust. The region between the Bagley fault and the Wrangell volcanic field is characterized by higher crustal velocities, while the Wrangell volcanic field is characterized by slower velocities both near the surface and at depth. A strong slow velocity anomaly corresponding to the Copper River sedimentary basin can be traced down to 20 km depth. The seismic tomography shows that the active tectonic structures correspond at the large scale with large contrasts in the rock properties of the crust.
