Collaborative Research: Evaluating the contribution of crustal deformation to the present-day tectonics of convergent margins: the southern Cascadia forearc

Subduction zones are the sites of the Earth's largest and most damaging earthquakes. Between earthquakes, the Earth's crust accumulates strain, which is observed in gradual movement of GPS markers affixed to the ground (geodetic observations). In addition, the Earth's crust deforms over longer timescales of millions of years, such as the uplift of mountain ranges. The relationship between geodetic observations and long-term deformation is not well understood, especially with respect to earthquake hazards in subduction zones. This problem is particularly challenging at the southern end of the Cascadia Subduction zone, offshore of northern California and southern Oregon, where the Earth's crust is influenced both by subduction and by tectonic plate motion transferred from the San Andreas fault system to the south. However, the geologic characteristics of southern Cascadia make this region well-suited for understanding and isolating the processes that could drive upper crustal deformation and earthquakes. These characteristics include a combination of high topography, high uplift rates, and high erosion rates; rocks and deposits suitable for dating; and three potential, and testable, processes that could generate crustal deformation and subduction zone earthquakes. The goal of this research is to better understand how subduction zones work and to anticipate the size and timing of future earthquakes. In addition to the research objectives, this project includes partnering with faculty at Hoopa Valley Elementary School to develop geoscience field and laboratory exercises for sixth grade students. Hoopa Elementary is located in the heart of the region of scientific focus and serves primarily American Indian students. Hoopa Elementary School teachers will join the research team for summer field work. The project's university faculty and students will join the teachers in developing hands-on activities and field trips that will enable sixth grade students to practice each step of scientific research using real data - the results from this research. The research project would also advance other desired societal outcomes such as full participation of women and underrepresented minorities in STEM and development of a diverse, globally competitive STEM workforce through graduate and undergraduate student training and support of an early career researcher.The southern end of the Cascadia plate boundary in North America is marked by transition from Cascadia lithospheric subduction to San Andreas transform faulting. This complex region of deformation, the Mendocino Triple Junction, is migratory in space and time. Localized rock uplift and erosion rates, terrace formation, and river channel morphology have responded to northward movement of the Mendocino Triple Junction and possibly the Blanco Fracture Zone, which is a physiographic boundary between the Juan de Fuca plate and its Gorda segment. Limited understanding of the long-term deformation in the upper-plate of the Cascadia forearc and its tectonic drivers make it difficult to isolate the earthquake-cycle signal within observed patterns of present-day deformation. In particular, overprinting from different geologic signals - migratory differences in the character of the subducting plate and the propagating wave of crustal thickening associated with the Mendocino Triple Junction - requires an evaluation of deformation and topographic change across a range of timescales. This project is an integrated study of the Late Cenozoic uplift, exhumation and erosion of Southern Cascadia. By using multiple geochronologic proxies that are sensitive to different rates and timings of processes (i.e., AHe thermochronology [rock exhumation >1Ma], cosmogenic radionuclide burial dating on buried surfaces that are presently uplifted [uplift rate constrained from ~0-5Ma], and cosmogenic radionuclide-derived basin averaged erosion rates [averaged over last ~100ka]), it will be possible to develop a record of exhumation and erosion through time and detect spatial variations in the southern forearc. The preservation of relict landscape remnants will be exploited to reconstruct long-wavelength deformation/uplift patterns and to quantify relief production in the Late Cenozoic. Finally, geodynamic models will be used to explore the mechanisms driving permanent upper plate deformation, and address how tectonic deformation of southern Cascadia may impact the signal recorded in observed geodetic data. This research will aid estimation of earthquake hazards at subduction zones by isolating and identifying the contribution of (recoverable) earthquake cycle deformation and of tectonically-driven, geologic time scale deformation at a site well suited to record ongoing tectonic deformation and associated strain.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.