Deformation and structure in the Chugach metamorphic complex, southern Alaska: Crustal architecture of a transpressional system from a down plunge section

Abstract

Multiple Paleogene dextral shear zones are identified in the Chugach metamorphic complex (CMC) in southern Alaska providing an unusual down-plunge view of the deformation associated with transpressional strike-slip systems at mid- to lower-crustal levels. Along the northern flank of the CMC, the brittle Stuart Creek fault is structurally continuous with the ductile Harry9s Gulch shear zone. Field relationships suggest the brittle fault transfers slip structurally downward into a localized, ∼2-km-wide shear zone deformed under greenschist-facies conditions. At amphibolite-facies, under mid-crustal conditions, deformation is localized in anastomosing shear zones ranging from 10s of m to 2–3 km in width that are cryptic as they pass downward into lower-crustal migmatitic gneiss. Strike-slip–related deformation in these migmatitic gneisses is more dispersed, suggesting more distributed flow. Absence of a symmetric cleavage fan associated with the steep ductile shear zones and variations in finite strain indicate that an attachment zone model (e.g., Teyssier and Cruz, 2004) is not applicable to the deep crustal structure of the CMC strike-slip system. We discuss two as yet indistinguishable hypotheses to account for the deep crustal structure of the CMC strike-slip system: (1) a narrow detachment zone model where detachment occurs at the metamorphic transition from schist to gneiss, and (2) a differential folding model where crustal-penetrating shear zones are masked by variations in folding mechanisms at different crustal levels.