Crustal stabilization through the processes of ridge subduction: Examples from the Chugach metamorphic complex, southern Alaska

A tectonic process commonly overlooked in the growth and formation of continental crust is ridge subduction. This process can stabilize a forearc accretionary complex through high grade metamorphism, anatexis, pluton emplacement, and underplating of mafic rock. An excellent example of the process is in the Chugach metamorphic complex (CMC), southern Alaska, formed as a result of late Cretaceous to Paleogene subduction of the Kula–Farallon (or Kula–Resurrection) ridge. Ridge subduction subjected the trench-fill turbidites of the accretionary prism to greenschist to amphibolite grade metamorphism under high-temperature/low-pressure conditions, converting them to a gneissic basement in the core of the CMC, and regional near-trench plutonism. In addition to high grade metamorphism and near-trench plutonism, the CMC experienced three distinct phases of deformation associated with triple-junction position in the process of becoming stabilized continental crust: 1) accretionary prism-related compression and metamorphism, 2) eastward migration of the triple-junction and associated ridge subduction producing orogen-parallel extension, peak metamorphism, and mafic underplating, and 3) dextral transpression following the migrating triple junction. This combination of progressive deformation and tectonic phases produces continental crust, effectively becoming a stable tectonic backstop by the time of Neogene Yakutat terrane collision, the present-day locus of deformation.