Collaborative Research: A four-dimensional view of deformation in the Eastern Alaska Range - where did the slip on the Denali fault go?

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The Denali fault in south-central Alaska ruptured in a 7.9 magnitude earthquake in 2002, one of the largest continental strike-slip (horizontal motion of blocks of rocks past each other) earthquakes ever recorded. The event brought attention to this little-studied but major fault, which crosses the trans-Alaska oil pipeline as well as the two main highways in Alaska. Scientists predicted Denali fault earthquakes to have only strike-slip motion, but instead, the 2002 quake started as a thrust (putting one block of rock over another) earthquake and uplifted rocks in the Alaska Range along a previously unknown thrust fault. The unexpected earthquake uplift pattern provided the scientific community additional evidence that thrust faults can siphon lateral motion from the strike-slip Denali fault. If these types of fault interactions persist for millions of years, then determining the amount of thrust faulting next to the Denali fault could help solve a long-lived controversy of how much total displacement has taken place across the Denali fault and explain why the Denali fault is surrounded by large mountains on all sides (forming the Alaska Range). Unraveling the history of thrust faulting and uplift next to the Denali fault will not only help chip away at these intriguing scientific questions but will also inform how significant these faults have been in the past and where the greatest seismic hazards in the Alaska Range are today.To further understand how faults in the region may be transforming lateral slip from the Denali fault into uplift of the Alaska Range, project researchers will install 7 temporary seismometers in this area to record earthquakes and ground motion. By interpreting these seismic signals, the data can produce 3-dimensional CAT-scan type image of where the faults are in the subsurface and the root of the mountains. Project investigators will also conduct geologic mapping studies, which provide a view of where the faults are at the surface. The geologic maps, combined with 3D images of the rocks at depth, will show where major faults exist at depth and if they cut the earth's entire crust. The project will involve several types of isotopic dating methods to determine the timing and rate of motion on these faults. The age information will provide the fourth dimension to the study, time, and will show which faults have been most active in both the distant and recent past. The ultimate goal of the study is to document how faults that have thrust and strike-slip motion connect into the main Denali fault, and what role they play in uplifting the mountain range. The results are important for updating the seismic hazard potential for the oil pipeline, which survived the strike-slip earthquake without breaking but is not engineered to withstand a large thrust earthquake. This project will provide training for students in field methods and laboratory applications and the overall results will be distributed to the public through outreach talks, spoken word performances, and the creation and installation of a highway wayside interpretative sign.The modern and deep time constraints on the horizontal offset history of the Denali Fault have been shown to vary east to west over a distance of ~ 200 km, with as much as 360 km of slip "missing" in the Cenozoic. The potential that the missing slip has been taken up through crustal shortening and/or strike-slip faulting off the main strand of the fault has not been thoroughly investigated. The eastern Alaska Range has significantly thicker crust in an ancient suture zone region compared to neighboring crustal blocks. Preliminary results indicate that a significant component of this crustal thickening occurred since the Oligocene through a combination of high-angle and low-angle thrust faults. This study will test the hypothesis that crustal shortening and thickening could play a major role in reducing the strike-slip component of the Denali fault. An alternate hypothesis is that strike-slip faulting within the suture zone is the important mechanism for bleeding off the slip. Neither of these hypotheses have been fully investigated by previous researchers in this region. To address this research question, project researchers will collect and combine geologic map data, thermochronology, and seismic imaging to determine the mechanisms, timing, and extent of crustal shortening and thickening, and compare that to previously unrecognized strike-slip faulting in the southern Alaska Range suture zone. Seven broadband seismometers, which will be installed over a region of 80 by 30 km (average spacing 15-20 km), will provide a more detailed view of the crustal thickness and intra-crustal discontinuities than the EarthScope Transportable Array can provide. Low-T thermochronometry will include isotopic systems that record cooling from 400 to 65 degrees C, and when combined with sediment tracking (detrital zircons) and structural studies will provide a 4-d analysis of the region's crustal structure. These new results, integrated with existing thermochronology, geochronology, and seismic data, will yield important geologic constraints on how slip dissipates along long-lived transpressive strike-slip faults.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.