Past exam of the plate tectonics course of the University of Oxford / 2024 / 2 / a / Solution

Cascadia subducts a young, warm plate and lacks the enormous plateau and crustal root of the central Andes; the central Andean Nazca Plate system also supports much deeper intraslab earthquakes. These are regional comparisons: neither western margin is uniform along its length.

First, the Juan de Fuca Plate entering Cascadia is young, typically of order 10 million years old, because its spreading ridge lies close to the margin. Its lithosphere is comparatively thin and warm. Much of the Nazca Plate entering the central Andes is older and colder. A cold plate carries low temperatures farther into the mantle, sustaining deep intraslab seismicity; the warm Cascadia slab loses its brittle seismogenic character much shallower. This concerns earthquakes within the slab, not the size of megathrust earthquakes: both margins can generate very large plate-interface events. The official warm-slab synthesis treats Cascadia as an endmember.^[ref]

Second, the central Andes have undergone major sustained crustal shortening and tectonic thickening, building a high plateau and locally roughly 60–70 km thick continental crust. Cascadia has an active volcanic arc and mountains but no equivalent continuous central-Andean plateau and crustal root. The difference reflects the accumulated history of convergence, upper-plate deformation, slab dip, trench motion and mechanical coupling, not just the instantaneous existence of subduction. The Andes include flat-slab segments with gaps in arc volcanism as well as normally dipping segments.^[ref][ref]

A young warm incoming plate and a faster, colder incoming plate can therefore generate different thermal structures, dehydration patterns and deformation histories. Plate age alone is insufficient to predict the thickness of the overriding continental crust.