| Summary: | Late Cretaceous to Eocene ridge subduction beneath the Mesozic Chugach terrane accretionary complex resulted in the formation of the Chugach metamorphic complex, and anomolus near trench plutonism of the Sanak-Baranof plutonic belt. The Chugach metamorphic complex (CMC), southern Alaska, is a high-temperature/low-pressure metamorphic belt, that offers a down plunge view of the mid- to lower-crust of a dextral transpression system. A distinct progression of deformation is recognized in the complex: 1) D1 contraction throughout the Chugach accretionary complex, 2) D2 margin parallel extension and horizontal simple shear, associated with subduction of higher ridge topography, followed by 3) D3 dextral transpression associated with oblique plate subduction. Patterns of D3 folations do not mimic the symmetric pattern adjacent to the shear zones, as predicted by previous attachment zone models (e.g. Teyssier and Cruz, 2004). An asymmetric foliation pattern is observed, indicating that an attachment zone model is not applicable to deformation in the CMC. The steeply oriented dextral shear zones, which are 10's of m to 2–3 km in width in the overlying schist, however, become cryptic in the gneiss core of the complex. Two as yet indistinguishable hypotheses are discussed to explain the deep crustal structure of the CMC: 1) presence of a narrow detachment model where a detachement zone at the schist-gneiss transition and 2) a differential folding model where shear zones penetrate across the schist-gneiss transition, but are masked by variations in folding mechanisms. This distinct progression of deformation, prograde metamorphism and intrusive activity recrystallizing the curst, and formation of a thick mafic root as a result of ridge subduction have also acted to strengthen and ultimately stabilize the CMC crust relative to the Neogene Yakutat terrane currently colliding with southern Alaska. The CMC's stabile behavior is supported by present day evidence of relatively small amount of seismic and deformation activity, ...
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