Timing of early Tertiary ridge subduction in southern Alaska: A section in <i>Geologic studies in Alaska by the U.S. Geological Survey, 1992</i>

We present a new compilation of 158 isotopic ages from Tertiary plutons that intrude the accretionary prism (Chugach-Prince William composite terrane) of southern Alaska. Two broad plutonic age groups are present: Paleocene to Eocene (the Sanak-Baranof plutonic belt), and Oligocene to Miocene. Plutons of the Sanak-Baranof belt are broadly coeval with magmatism along the axis of an Andean-type arc that lay farther inboard. A plot of age versus position along strike shows that near-trench magmatism progressed from about 66-63 Ma in the west to about 50 Ma in the east. We interpret the near-trench magmatism as the product of subduction of the Kula Farallon ridge (or another unidentified spreading center) and suggest that the age progression tracks a migrating trench-ridge-trench triple junction along the continental margin.