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 objective of this project is to understand how the mountains in the central and eastern Alaska form along the Denali fault system and to understand the geological processes responsible for their formation. This was a joint project between scientists and students from the University of Alaska in Fairbanks, the University of California at Davis and Syracuse University in New York. The Denali fault is the major geologic structure in south-central Alaska and is the locus for the formation of the Alaska Range, including North America’s highest mountain Mt. McKinley. We confirmed that movement along the fault is partitioned into strike-slip and thrust fault components and that thrusting uplifts the mountains. The curve of the Denali fault means that the rate of slip decreases to the west while the amount of thrusting increasing. Thrust faults occur north and south of the Denali fault system. We determined the timing of the uplift events in the Alaska Range by constraining episodes of when rocks cooled quickly, using a variety of dating techniques on a number of minerals. A number of different uplift and exhumation (erosion) episodes occurred since about 43 million years ago, with episodes of greater uplift and erosion beginning at about 25 million years ago, 15-10 million years ago and about 6 million years ago. The event that started about 6 million years ago is what dominantly formed the central Alaska Range and North America’s highest mountain, Mt McKinley. Glaciation started in the Alaska Range at about 2.5 million years ago and cooling due to that enhanced erosion is observed indirectly in the cooling record. So what is driving uplift of these mountains along the Denali fault? The answer to that question is active plate boundary processes along Alaska’s southern coast where the Pacific plate is subducting beneath the North American plate, which is also what is driving collision of the Yakutat microplate into southeastern Alaska, providing greater coupling between the leading edge of this microplate and interior Alaska. Strain from this collision is transferred inland along the Denali fault and when that strain is transferred onto thrust faults, as the Denali fault curves, mountains form above these faults. We also answered the long standing question as to why the high mountains in the central Alaska Range (including the very high peaks of Mts Hunter, Foraker and McKinley) form south of the Denali fault whereas those in the eastern Alaska Range form north of the Denali fault. This is because the Alaska Range has formed within much weaker rocks of an ancient collision (or suture) zone. A much stronger zone of rocks, the Yukon composite terrane defines the northern edge of this suture zone. Where the Yukon terrane lies well north of the Denali fault, the weaker rocks deform to the north of this fault, as in the eastern Alaska range. On the other hand, when the Yukon terrane lies right next to the Denali fault weaker rocks deform preferentially to the south. This is a very nice example of a simple pattern of preferential deformation in weaker rocks being deformed up against an irregular-shaped strong boundary, with deformation controlled by the Denali fault running through the middle. In 2002 a magnitude 7.9 earthquake originated on the Sustina Glacier thrust fault in the eastern Alaska Range, before transferring to the Denali fault, travelling east and then southeast down the Totschunda Fault. We investigated the cooling history of rocks on the side of the Sustina Glacier thrust fault that is being pushed up (the hanging wall) and the side that is being thrust under the hanging wall (the footwall of the fault). We did this to better understand the role of thrust faults in the Alaska Range and also to see if this fault had been extremely active over geologic time. We found that while deformation has happened in the past this fault was not active or behaved in a similar way to what it is today, until about 15 million years ago and then has only moved every now and again. In contrast, the Denali fault has remained very active for about the last 50 million years, and it is this fault that is primarily driving deformation of rocks and earthquake activity in the region.
