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 degrees C in lawsonite blueschist facies rocks at the lowest structural levels, through similar to 433 degrees C in overlying epidote blueschists, 546 +/- 20 degrees C in albite-epidote amphibolite facies rocks, to 650-730 degrees 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 similar to 100 degrees 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 similar to 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 degrees C/km across the subduction channel likely existed during the high-T event, decreased during underplating, and reached zero by similar to 90 Ma. New geothermometry shows that high-pressure rocks on Catalina define an inverted metamorphic temperature zonation The zonation is disrupted, and the present structure does not represent an original inverted thermal gradient The zonation formed by progressive underplating in a cooling subduction zone following a high-T metamorphic event