Structural and Kinematic Controls on Paraglacial Rock Slope Deformation at Portage Glacier, Alaska

As glaciers thin and retreat worldwide, the stability of surrounding rock slopes is increasingly at risk. This study investigates the long-term deformation of two major on-going instabilities, Portage A and Portage B, situated above Portage Glacier in Alaska. By analyzing decades of historical imagery and remote sensing data, we reconstructed the spatial evolution of these slopes, revealing progressive deformation up-glacier over the past sixty years. To further assess the links between glacier change and slope deformation, we combine structural mapping with remote sensing observations and kinematic analyses. Our results identify three distinct kinematic domains and show that progressive deformation is initiated once the glacier surface lowered below a critical elevation. This creates kinematic freedom for the rock mass to move along structural discontinuities. At Portage Glacier, the onset and progression of the instabilities are not governed solely by glacier thinning but reflect a complex, site-specific interaction between structural discontinuities and cumulative weakening from external processes. Glacier retreat and thinning act as one component within a broader “cascade system”, where multiple factors interact. Additionally, preliminary results from our three-dimensional model provide additional insights into the mechanical response of the slopes under changing boundary conditions. These findings highlight the importance of integrating structural, kinematic, and remote sensing data to better understand paraglacial slope dynamics and anticipate future instabilities in rapidly deglaciating mountain regions.