Tectonics seminars

Abyssal marine turbidites provide some of the longest and most spatially extensive records of subduction zone earthquake recurrence globally; however, correlation of these deposits over long distances and interpretation of synchronous emplacement requires both an understanding of the turbidite generating systems and precise dating. Here, we present an integrated suite of high-resolution bathymetry, subbottom profiles, and sediment cores from combined autonomous underwater vehicle, remotely operated vehicle, and ship-based studies at a key paleoseismic site in the southern Cascadia subduction zone. We demonstrate how widespread, earthquake-triggered landslides on the lower slope deposit discrete, proximal mass transport deposits (MTDs) that grade offshore into complex, interfingered abyssal turbidites, which correspond to records of megathrust earthquake history. We propose accretion and oversteepening of thrust folds on the lower slope both preconditions the slope to fail and provides a perpetual source of unstable material to fail during every earthquake cycle. Furthermore, we suggest the periodic and pervasive landsliding indicates coseismic deformation of the outer accretionary wedge during megathrust rupture.

Fluvial channel steepness and hilltop curvature are commonly used as proxies for erosion rate in equilibrium landscapes, but they become less reliable in rocky steeplands dominated by debris flows. We explore an alternative metric—Adf, the drainage area at which valleys transition from slope-invariant to fluvial power-law slope-area scaling—and show that it correlates with erosion rates in several regions. Combining field data with a landform evolution model, we demonstrate that Adf provides insight into where debris-flow processes dominate, and we constrain a physically meaningful debris-flow incision law that offers new insight into how steep landscapes evolve under varying climatic and tectonic conditions.