= Solution
\b[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>: \b[both margins can generate very large plate-interface events]. The official warm-slab synthesis treats <Cascadia> as an endmember.https://www.usgs.gov/publications/cascadia-subduction-zone-and-related-subduction-systems-seismic-structure-intraslab[USGS, 2002, warm-slab comparison and the 55 km deep Nisqually earthquake]{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.https://pubs.usgs.gov/of/2015/1031/e/pdf/of2015-1031-E.pdf[USGS, Seismotectonics of South America, tectonic summary and depth sections]{ref}https://escweb.wr.usgs.gov/share/mooney/54seismicvelocitystruct.pdf[USGS-hosted crustal seismic synthesis, central-Andean crustal thickness]{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>.
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