The 1,200 km long Alaskan Denali Fault system is a major intra-continental right-lateral strike-slip fault system. Since the 2002 Denali earthquake (magnitude 7.9), which initiated along a previously undescribed thrust fault in the eastern Alaska Range, scientists have increasingly focused on how much slip and convergence occurs along this fault system, and where deformation is being accommodated. Mountainous terrain and basins have formed along the Denali fault and the locations and age of formation of these features can be used to constrain the distribution of crustal deformation through time. The eastern Alaska Range, the topographic signature of the eastern Denali Fault, rises dramatically from the tundra (about 200 meters) to sharp glaciated peaks (about 4,000 meters), forming a narrow but high-relief region immediately north of the fault. As the fault continues west, the topography drops significantly, then rises again to form the central Alaska Range, home to Mt. McKinley and Denali National Park. Uplift of the Alaska Range is related to plate tectonic processes associated with subduction and accretion along Alaska's southern margin, but why the uplift is focused along the fault in the eastern Alaska Range is not clear. This research project seeks to understand the thermal history of rocks in the eastern Alaska Range and hence place constraints on regional patterns of exhumation and uplift through time. This will allow evaluation of the relative importance of near-field boundary conditions versus far-field driving forces. The research team is conducting a high-resolution multi-technique thermochronological approach combined with macro-and microstructural work along the eastern Denali Fault. This approach will document variations in exhumation rates during the last 30 million years to understand exhumation patterns in the mountains. Structural studies are focused on the regions with the most extreme exhumation, both in terms of rate and total amount, to understand what controls these patterns. Linking the structural history to exhumation rates will permit the temporal and spatial influence of geodynamic drivers such as changes in Pacific versus North America plate motion, dip of the subducting slab, and the collision of buoyant Yakutat block at the eastern edge of the subduction zone to be evaluated.
This study has relevance to fundamental problems of major strike-slip fault systems, including what causes localized exhumation and how strike-slip deformation is transferred into the lower crust. Results will contribute to earthquake hazard prediction in this region by constraining locations of high neotectonic deformation, variations in Denali fault geometry and identification of reverse faults. A better understanding of contributing factors for seismic behavior along the Denali Fault will benefit seismic hazard maps. The Trans-Alaska oil pipeline and future 26 billion dollar Alaska gas pipeline cross the eastern Alaska Range and Denali fault. The pipeline is designed to withstand strike-slip motion, but if the there is a significant dip-slip component the effects could be disastrous.
The Alaska Range in central Alaska includes a number of high peaks, including Mt. McKinley, the highest peak in North America. The high peaks of the Alaska Range form a curved mountain chain, following the surface trace of the Denali fault, a major, active right-lateral strike-slip fault. Strike-slip faults are associated primarily with horizontal movement of rocks, but along one part of the Denali fault the rocks have moved up from more than 10 miles deep in the crust and continue to be uplifted. This research project aimed to determine where the greatest amount of deformation in the crust occurs, expressed as shortening and uplift of the rocks, and why it occurs where it does. Our results show that the most significant shortening in the eastern Alaska Range, between the Parks and Richardson Highways, is very localized at deeper levels of the crust, below ~ 6-8 miles, where rocks deform plastically by recrystallizing. This zone of crust that has experienced extreme uplift occurred where the fault bends slightly in a counter-clockwise manner, and rocks on the north side of the bend in the fault have moved up at an average rate of 1 mm/yr for at least 16 million years. As the rocks rise to shallower levels of the crust, where they deform brittley and earthquakes occur, the zone of contraction widens significantly and occurs along thrust faults both north and south of the Denali fault proper. The location of where the shortening in the upper, brittle crust is less predictable than in the lower, plastic crust, and in addition to faults consistent with shortening, we found faults that show evidence that the crust on the north side of the Denali fault is being stretched parallel to the fault. The implications of the research for seismic hazard in the region are significant. The Denali fault crosses the Alaska oil pipeline, and a magnitude 7.9 earthquake in 2002 offset the pipeline by more than 16 feet. The offset was associated with purely strike-slip motion, but our research shows that earthquakes with vertical motion also might be expected in this region. A gas pipeline has been proposed immediately west of the research area, and a major dam has been proposed immediately south of it. Future large engineering construction will need to consider the possibility of large earthquakes that could cause significant vertical motion as well as the horizontal motion.
