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.
Indiana University was one of several institutions involved in the large collaborative project given the acronym STEEP (St Elias Erosion and tectonic Project). The big problem STEEP addressed was an idea related to glacial erosion. It is well established that glaciers are extremely aggressive agents of erosion. Although more controversial, it its also widely accepted that erosion and mountain building can be linked through a positive feedback mechanism (removing material by erosion promotes motion on faults that would restore the topography removed by erosion). STEEP aimed to address this issue through a multidisciplinary study of the most heavily glaciated real estate in North America - the St Elias range in southeastern Alaska. The project assembled specialists in structural geology, sedimentary geology, glaciology, space geodesy, geodynamic modeling, seismology, marine geology, marine geophysics, and geochronology. STEEP has made major contributions to understanding of the problem of links between glacial erosion and tectonics. The Indiana University group mainly focused on seismic imaging methods to understand how deformation of the St. Elias range was related to crustal and upper mantle structure. We used state-of-the-art seismic imaging methods to illuminate the large scale structure of this region. Major results from this work were: It was previously suggested that there was a large scale discontinuity in the upper mantle defining a tear in the subducting plate. The imaging shows this is absolutely not true and the plate interface is continous. The Wrangell Volcanoes were previously recognized as anomalous in several ways. Seismic imaging allowed us to construct a 3D model of plate motion that demonstrates the Wrangells are easily explained by conventional plate tectonics models. That is, the volcanoes are directly linked to subduction of crust of the geologic feature known as the Yakutat Block. We developed a thermal model for the subduction of the Yakutat block. It is widely accepted that collision of the Yakutat block with southeastern Alaska is the main driving force for mountain building in all of Alaska. Using our 3D model of the plate geometry and sediment thickness estimates from STEEP offshore seismic data, our model predicts strongly elevated temperatures in the mantle downstream from the Yakutat block. This explains the anomalously shallow magma generation in the Wrangells and a falloff in the mantle seismicity rate by a factor of nearly 100 in lithosphere of Yakutat origin compared to Pacific plate (standard oceanic crust) origin. This grant supported a PhD student (Mark Bauer) who completed his degree during the grant period. Our large scale, 3D model of the geometry of the plate system in southeastern Alaska should provide the foundations for future work in this area. This is particularly time critical due to the planned deployment of the Earthscope Transportable Array in Alaska in the next year.
