The North American Cordillera includes several blocks of metamorphic rock that apparently experienced rapid Cretaceous burial and loading, followed by rapid exhumation. These rocks are interpreted to been deposited as clastic sediments in basins within the convergent Cordilleran magmatic arc. Identification of these crustal blocks along much of the North American Cordillera leads to the hypothesis that rapid vertical motion of fault bound blocks was an integral feature during the Cretaceous. The blocks may serve as analogs for more recent processes in the Cordillera and for processes in other orogens. Therefore, it is important to fully understand the motion and quantify rates of motion in known examples. The Swakane Gneiss in the Cascades Crystalline Core of Washington was recently identified as a fault block with a Cretaceous history of rapid vertical motion. However, this idea hinges on interpretation that the Swakane Gneiss has a metasediment origin, which is based on complexly zoned zircon grains that are inferred to have a detrital origin. This interpretation requires that loading occurred at an average rate of 7 mm/year. Furthermore, ages for metamorphism and cooling require rapid exhumation and cooling. The age of metamorphism and loading must be less than that for surrounding fault blocks; however, the age is poorly constrained. This project is testing current interpretations for the Swakane Gneiss by applying zircon U-Pb and garnet Sm-Nd techniques to select samples from within the main body of the Swakane Gneiss and along its sheared margins. Laser-Ablation Inductively-Coupled Plasma Mass Spectrometry and Sensitive High Resolution Ion MicroProbe techniques are being used to fully characterize the spatial variation in age within select zircon crystals in order to test the detrital interpretation and yield new estimates for the age of igneous and possibly metamorphic activity. The timing and conditions of metamorphism are being determined by garnet Sm-Nd and phase equilibria, respectively.
Rates of processes in arcs are important for understanding arc evolution, earthquake and volcano hazards associated with arcs, and the growth of continents. Results of this study will increase the understanding of arc dynamics, deep crustal processes, and coupling between the upper and lower crust. It will furthermore provide the necessary information that will permit correlation with other metamorphic terranes in California, which are believed to be correlatives, thus enabling Cretaceous subduction over the entire west coast of North America to be better understood. Finally, the rates of burial and exhumation carry significance for subduction processes globally, and results of this study will permit comparison with other subduction terranes.
