Structural history of the Chugach metamorphic complex in the Tana River region, eastern Alaska: A record of Eocene ridge subduction

摘要

The Chugach metamorphic complex of southern Alaska is an Eocene high-temperature (T), low-pressure (P) fore-arc metamorphic belt related to subduction of the Kula-Farallon spreading center beneath western North America. The Chugach metamorphic complex has a three-phase ductile deformational history that records major changes in kinematic axes during a short interval of geologic time (8 m.y.). The earliest deformation (D₁) is a regional event recognized throughout the flysch subterrane of the Chugach terrane. D₁ is a regional layer-parallel slaty/phyllitic cleavage developed during accretion and subsequent shortening. In the Chugach metamorphic complex, D₁ predates high-temperature metamorphism. During prograde metamorphism, there were two major structural events. D₂ records orogen-parallel extensional accompanied by vertical shortening with components of pure shear and top-to-the-east simple shear. D₂ is synchronous with melt injections (Ti₂) in the gneissic core of the complex and large plutons throughout the complex. D₃ records a return to subhorizontal contraction perpendicular to the margin and is interpreted as a dextral transpressional event. D₃ contraction produced a dramatic thickening of the complex in a regional-scale D₃ anticlinorium. In the gneissic core, the presence of melt (Ti₃) strongly influenced D₃. Finite strain data and field observations indicate that both F₂ and F₃ have axes that are parallel to the stretching direction, yet these are not sheath folds because strains are too low. Instead the structures are examples of folds that developed with their axes parallel to the elongation axis. Together these observations provide further evidence for our previous interpretations that the Chugach metamorphic complex is a manifestation of an Eocene plate reorganization at ca. 56–52 Ma. Plate models predict that before 56 Ma the Kula–Farallon–North American triple junction migrated southward and is associated with a time-transgressive fore-arc plutonic belt. After plate reorganization, the triple junction either backtracked northward (Kula Plate model) or continued southward with intermittent northward motion (Pacific-Farallon model). We interpret the D₂-to-D₃ progression as either a result of highly oblique subduction of the Kula plate followed by more orthogonal—but still dextral-oblique—convergence of the Farallon plate (Kula Plate model), or a special case of Pacific–Farallon–North American interaction.