Oceanic plateau accretion for young oceanic plates: Geodynamics models of Siletzia

The Siletzia oceanic plateau was accreted to the western margin of North America in the Eocene, marking the most recent accretion event in this area and the formation of the modern Cascadia subduction zone. Siletzia formed on or near the spreading ridge of two oceanic plates, and its chemical composition shows that its origin is a mixture of mid-ocean ridge basalts and hotspot volcanism likely associated with the Yellowstone plume. The plateau formed between 56 and 49 Ma, and accretion to the continent occurred at around 50 Ma. Plate reconstructions date the age of the oceanic plate during accretion at about 10 Ma. Therefore, Siletzia accretion occurred in a unique environment where the oceanic plate was young and likely hot and weak. Observation of modern equivalent plateaus show a conduit of hot, weak, partially melted mantle below the islands, which creates an especially weaken zone below the plateau.   

Using 2D thermo-mechanical numerical models, we explore the dynamics as an oceanic plateau is carried into a subduction zone to determine the conditions under which the plateau is accreted to the overlying continent. Our models examine the effects of variations in age of the oceanic plate, weakening of the plate due to the plateau creation and the structure of the continent. We also test the effect two different boundary conditions: (1) forced plate convergence at 4 cm/yr and (2) free subduction, where plate convergence is driven dynamically by the negative buoyancy of the oceanic plate.

Results show that in models with an old oceanic plate (>50 Ma), the plateau is readily subducted into the deeper mantle with little disruption to the subduction system for both boundary conditions. In contrast, for a young oceanic plate (~10 Ma), subduction stalls as the plateau enters the subduction zone, leading to accretion of the plateau and parts of the oceanic lithosphere to the continental margin. With no imposed convergence, all plate motions cease, whereas forced convergence is accommodated by formation of a new subduction zone outboard of the terrane when the plate is weakened by the formation of the plateau. Otherwise, deformation occurs within the interior of the oceanic plate, causing the oceanic plate to break 900 km seaward of the subduction zone. These models demonstrate that Siletzia accretion to North America may have occurred due to the young plate age, but in some models, accretion only lasts for around 10 Myr as continued plate convergence causes entrainment and subducting of the plateau. If the modern Cascadia subduction zone formed as a new plate boundary west of Siletzia, continued plate convergence may have been driven by the older subducting plate to the south. Ongoing work is using 3D models to assess this in more detail.