Formation of a metamorphic complex along an obliquely convergent margin: Structural and thermochronological evolution of the Chugach Metamorphic Complex, southern Alaska

Abstract

This study documents the structural and metamorphic evolution of middle to lower crust along an oblique convergent curved continental margin during a time span of ∼60 Myr. Our study documents the importance of variable obliquity during convergence which led to the development of overprinting fabrics and distinct exhumation histories along strike of the margin. We present structural analyses,40Ar/39Ar, Rb/Sr, and zircon fission track ages from middle to lower crust exposed along the southern Alaskan margin in the Chugach Metamorphic Complex. Together with the metamorphic history and additional geochronology from the literature we derive the following tectonic evolution for this area: accretion of sediments during dextrally oblique convergence led to strain‐partitioned D1structures and greenschist‐facies metamorphism prior to circa 55 Ma. At ∼55–51 Ma, a margin‐parallel stretching phase with vertical thinning (D2) affected the margin and led to andalusite‐sillimanite grade metamorphism and the onset of partial melting. A switch back to dextral transpression (D3) shortly after D2led to rapid cooling of the western and central parts of the complex associated with exhumation of parts of the core of the complex until circa 46 Ma. The southeastern part of the complex cooled and exhumed regularly and slowly until circa 5 Ma due to its highly oblique orientation relative to the convergence direction. An increase in cooling and exhumation occurred after circa 5 Ma in the entire southeastern part of the complex, associated with the Neogene collision of the Yakutat terrane.