Spatial and Temporal Variations of Sediment Input to the Cascadia Subduction Zone

The abundance of expandable clay (smectite) in subducting sediments is an important parameter to consider during studies of plate-boundary seismogenic zones because the mineral has an unusually low coefficient of internal friction. Furthermore, interlayer water is released during smectite-to-illite diagenesis, which increases the likelihood of excess pore pressure and low effective stress. In theory, the up-dip limit of a seismogenic zones might coincide with the down-dip dissipation of excess pore pressure. In the specific case of Cascadia Basin, the amount and types of clay on the abyssal floor change considerably in both time and space. Those variations probably influence the strength of the plate-boundary fault, the vergence of imbricate thrusts within the frontal accretionary prism, and the fluid budget in 3-D. New data from more than 200 gravity-piston cores show that distal parts of Cascadia Basin contain 4% to 70% smectite in the clay fraction (Biscaye weighting factors). Smectite is more abundant within a plume of discharge that emanates from the Columbia River, but spatial variations are not clear-cut, even within the upper meter of Holocene mud. This complexity is a product of interplay among autocyclic factors (e.g., channel switching, migration of surface currents), allocyclic variables (e.g., continental glaciation, volcanism, balance of chemical to mechanical weathering, earthquake trigger of turbidity currents), and eustatic forcing (e.g., attachment-detachment of river-canyon connections). In addition, new data from ODP Site 888 (Nitinat Fan) and DSDP Site 174 (Astoria Fan) show substantial changes in clay composition with depth. Smectite at Site 888 varies erratically from 13% to 55%. Smectite content is 11% to 64% at Site 174, with a marked increase from the younger submarine-fan facies (average 22%) to the underlying abyssal-plain facies (average 42%). Models of fault-zone mechanics and accretionary-prism hydrogeology should take this compositional variability into account.