GEOLOGIC CONSEQUENCES OF PLATE REORGANIZATION - AN EXAMPLE FROM THE EOCENE SOUTHERN ALASKA FORE-ARC

Observation of relative timing of deformation, metamorphism, and pluto nism in a high-temperature-low-pressure metamorphic belt in the easter n Chugach Mountains of Alaska leads to a model of ridge subduction fol lowed by plate reorganization to account for the abnormally high geoth ermal gradients in the fore arc. Between 56 and 53 Ma, a change in the direction of the Kula-Farallon spreading halted the previous southwar d migration of the Kula-Farallon-North American triple junction. This forced the triple junction to migrate back north along the plate margi n, enlarging the slab window beneath the accretionary margin. The expa nded slab window produced a large-scale thermal manifestation now reco gnized as the Chugach metamorphic complex. The accretionary complex re sponded to plate reorganization by orogen-parallel extension associate d with oblique subduction of the Kula plate. Contraction began again f ollowing passage of the triple junction and subduction of the Farallon plate.