Structural and tectonic investigation of a transpressional system, Chugach metamorphic complex, southern Alaska

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\u27s 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\u27s stabile behavior is supported by present day evidence of relatively small amount of seismic and deformation activity, abruptly older cooling ages across the boundary from the Yakutat terrane, and behavior similar to that of a strengthened cratonic crust due to the presence of a mafic root beneath the complex. Plate geometry of the north Pacific margin during the late Cretaceous to Eocene responsible for the formation of the CMC was complicated, but is problematic since the exact plate geometry is unknown. Two possible plate geometries are indicated for the north Pacific during this time period: 1) a Kula-Farallon-North America plate geometry with a subducting Kula-Farallon ridge and 2) a Kula-Resurrection-Farallon-North America plate geometry with dual Kula-Resurrection and Resurrection-Farallon subducting ridges. A rapid 2 m.y. progression of deformation, metamorphism, and plutonism is indicated by deformation kinematics and geochronology, and is difficult to reconcile with pre-exisitng plate configurations for CMC formation. Finite strain modeling of the overprinting deformation sequence combined with plate velocity models suggests that the presence of the extra Resurrection plate in the north Pacific during the late Cretaceous to Eocene could explain the formation of the CMC