STEEP (SainT Elias TEctonics and Erosion Project) is a five year, multi-disciplinary study that addresses the evolution of the highest coastal mountain range on Earth - the St. Elias Mountains of southern Alaska and northwestern Canada. This orogen has developed over the past few million years as the Yakutat block, a continental-oceanic terrane, has attempted subduction beneath the eastern end of the Aleutian arc-trench system. The ~500 km-long, 150-km-wide St. Elias mountain range is the product of the dynamic balance between rapid uplift induced by crustal covergence and rapid exhumation by a regional system of large, fast-moving temperate glaciers. Most sediments are deposited either on a broad shelf or in deepsea fans and provide a complete record of the tectonic, climatic, erosional, and eustatic events that have accompanied the orogeny. Such a fresh and currently active "mini-orogen" is ideal for the integrated project we propose here.
The overarching goal of our project is to develop a comprehensive model for the St. Elias orogen that accounts for the interaction of regional plate tectonic processes, structural development, and rapid erosion. Our focus is on the partitioning of deformation within the system from upper mantle flow to near-surface faulting and exhumation. Three basic questions guide us:
1) What is the nature of the upper mantle interactions that drive this orogenic system? In particular, is the orogen driven by passive subduction of a microplate or by forceful subduction driven by the Pacific plate; is continental crust being subducted; and how does upper mantle flow respond to the plate interaction?
2) How does the sedimentary cover respond to interaction of the three-plate/microplate interaction as it is stripped from basement along large-scale fault systems? That is, is the microplate behaving as an indentor or is it forcing lateral escape of the cover as the collision progresses? At what depth, and with what geometry do these separations occur?
3) How do surface processes, particularly areas of rapid glacial erosion, affect localization of deformation and slip-partitioning? Specifically, is the spatial association of large glaciers with areas of active deformation coincidental, or is the active deformation localized by rapid exhumation?
To address these questions we propose an integrated onshore-offshore study involving active source and passive source seismology, GPS-based geodetic studies, geologic studies, surface process studies, geochronology, and geodynamic modeling. Question 1 (crustal structure and upper mantle) will be addressed by a large-scale passive seismic study as well as offshore seismic profiling. These studies collectively will constrain the geometry and kinematics of the large-scale plate/microplate interactions in the system. Question 2 (sedimentary cover response) will be addressed through a combination of geologic studies onland, analysis of offshore seismic data (both existing data and the new data to be acquired in this study), GPS-based geodesy, and thermochronology. Question 3 (erosion/tectonics linkage) will be addressed by adding additional data from surface process studies and modeling. Data obtained from these studies will allow development of a realistic, quantitative geodynamic model of the St. Elias orogen. This model will be developed through integration of the diverse data sets into a comprehensive thermal/mechanical model for the Quaternary history of the system as well as kinematic models for the long-term geologic evolution of the orogen. The result will have fundamental implications for general problems of the interplay between erosion and tectonics, the geodynamics of microplate accretion at mantle to supracrustal depths, and climatic influence on long-term exhumation of mountain belts.