KINEMATICS OF THE CHUGACH METAMORPHIC COMPLEX, SOUTHERN ALASKA: PLATE GEOMETRY IN THE NORTH PACIFIC MARGIN DURING THE LATE CRETACEOUS TO EOCENE

During the late Cretaceous to Eocene the plate geometry in the north Pacific margin was complicated, but the exact plate geometry at this time is problematic. The Chugach metamorphic complex (CMC), southern Alaska, is a high-temperature/low-pressure metamorphic sequence in the Mesozoic Chugach terrane accretionary complex, and the metamorphism is widely ascribed to a ridge subduction event produced by the complicated plate geometry. Two possible plate geometries are indicated for this time period in the north Pacific margin: 1) a single mid-ocean ridge separating the Kula and Farallon plates subducting beneath the North American plate, and 2) the existence of an additional Resurrection plate, producing simultaneous subduction of two mid-ocean ridges beneath the North American plate. Recent work on the kinematics of deformation and timing of the metamorphism have clarified this history, indicating a rapid two-phase deformational sequence occurring over a time period less than 2 m.y., which is difficult to reconcile with any existing hypotheses for the plate configuration that produced the CMC. Combining finite strain modeling of overprinting deformation sequences with plate velocity models suggests the presence of the Resurrection plate could explain the CMC formation through a 3-stage tectonic progression: 1) initial D₁ contraction produced by Resurrection plate subduction, 2) D₂ margin parallel extension produced by subduction of higher topography of the Kula-Resurrection ridge as the triple junction migrates southward, followed by 3) formation of D₃ dextral shear zones which accommodate margin parallel motion produced by oblique subduction of the Kula plate.