Is the Inverted Field Gradient in the Catalina Schist Terrane Primary or Constructional?

New geothermometry using laser-Raman data on carbonaceous material from low and intermediate grade rocks on Santa Catalina Island, California, together with existing thermobarometric data, show that there is a quasi-continuous increase in peak metamorphic temperature from 327 ± 8°C in lawsonite blueschist facies rocks at the lowest structural levels, through ∼433°C in overlying epidote blueschists, 546 ± 20°C in albite-epidote amphibolite facies rocks, to 650–730°C in amphibolite facies rocks at the top of the sequence. Rocks of different metamorphic grade are separated from one another by tectonic contacts across which temperature increases by ∼100°C in each case. Previously published geochronological data indicate that peak metamorphism in the highest grade rocks at 115 Ma preceded deposition of blueschist facies metasediments by ∼15 million years, so that the present inverted grade sequence does not represent an original inverted temperature gradient. The present structure results from progressive underplating of oceanic rocks in a cooling subduction zone following a high-T metamorphic event at 115 Ma. An inverted temperature gradient of ≥100°C/km across the subduction channel likely existed during the high-T event, decreased during underplating, and reached zero by ∼90 Ma.