RIDGE SUBDUCTION AS A PROCESS FOR CONTINENTALIZATION OF AN ACCRETIONARY PRISM, CHUGACH METAMORPHIC COMPLEX, SOUTHERN ALASKA

A tectonic process that is commonly overlooked in the formation of continental crust is the effects of ridge subduction. This process can essentially “continentalize” a forearc accretionary complex through high grade metamorphism and anatexis, pluton emplacement, and underplating of mafic rock. This process is best illustrated in the Chugach metamorphic complex (CMC) of southern Alaska, where an accretionary prism was subjected to greenschist to amphibolite grade metamorphism under high-temperature/low-pressure conditions and largely stabilized this assemblage during Neogene deformation associated with collision of the Yakutat terrane. The CMC was formed when subduction of the Kula-Farallon (or Kula-Resurrection) ridge generated an intense thermal anomaly that converted trench-fill turbidites to a gneissic basement in the core of the CMC, and regionally produced a belt of near-trench plutonism. Transportation of the complex northward during metamorphism generated internal structures recording the deep seated effects of dextral transpression. Preliminary U/Pb ages for plutons in the western CMC at the edge of the core of gneissic basement exhibit three deformational phases that occurred within approximately 2 m.y. Since the ridge migrated from west to east during subduction, this short interval is interpreted as marking the entire time span of deformation at this location. The three deformational phases are associated with 1) initial compression of the accretionary prism, 2) the passing of the eastward migrating triple-junction beneath this location, higher temperature metamorphism and plutonism, and 3) a return to compression. This progressive suite of deformational phases under a rapid time span suggests that this pattern progressed eastward along with the migrating ridge, and the accretionary prism progressively became continental crust.