Structural development of the Western Metamorphic Belt adjacent to the Coast Plutonic Complex, southeastern Alaska: Evidence from Holkham Bay

Abstract

The Coast plutonic complex in southeastern Alaska and British Columbia is bordered on the west by low‐ to medium‐grade metamorphic rocks of the western metamorphic belt. This belt includes intensely deformed rocks underlying a prominent topographic low, the Coast Range megalineament. Structural data from the northern part of the belt near Holkham Bay, southeastern Alaska, constrain the shear sense during polyphase deformation. Tectonite fabrics within the western metamorphic belt record at least three deformational events. Variation in foliation development, fold geometry, and metamorphism indicate that deformation was heterogeneously partitioned. Four tectonic zones can be defined; the peninsular, Coast Range megalineament, coherent, and straight zones. The earliest recognized deformation resulted in folding that transposed bedding into a tectonite layering (S 1 ) throughout the western metamorphic belt. These structures are inferred to have developed during late Cretaceous crustal thickening that resulted in moderate‐ to high‐pressure metamorphism. Later deformation (probably early Tertiary) resulted in abundant mesoscopic and macroscopic F 2 folds of S 1 and development of shear bands in the Coast Range megalineament zone. The vergence and variation in plunge of F 2 folds are compatible with subvertical east‐side‐up displacement across the Coast Range megalineament zone (uplift of the Coast plutonic complex). Shear bands indicate right‐lateral strike‐slip motion occurred during the waning stages of metamorphism, probably following development of F 2 folds. Brittle faults parallel to the Coast Range megalineament contain subhorizontal slickensides suggesting additional strike‐slip movement.