Kinematic and thermochronologic studies of Cordilleran metamorphic complexes (Pioneer, Idaho; North Cascades, Washington)

Abstract

This thesis addresses the exhumation of metamorphic rocks in two settings in the North American Cordillera: (1) exhumation of the Pioneer core complex rocks in Idaho, related to detachment activity for which the deformation kinematics are analyzed in detail in this thesis; (2) exhumation of rocks associated with incision of the Skagit Gorge and Ross Lake drainages, North Cascades, Washington state, as traced by fission-track age-elevation relations. In the first study, kinematic analysis was aimed at determining the relative contributions of pure and simple shear strain in the ~100 m thick quartzite-dominated detachment shear zone, using quartz microstructures (quartz ribbons and recrystallized grains) and crystallographic preferred orientation (measured by electron backscatter diffraction) as well as the shape preferred orientation of feldspar porphyroclasts. Results from quartz microfabric suggest a pure shear contribution of 0-70%, which likely reflects the spatial and temporal distributions of crustal thinning in this detachment system. Vorticity derived from feldspar shape fabrics indicates that an initial, pure-shear dominated fabric was overprinted by an increment of simple shear dominated strain (g ~ 1.0) that rotated feldspar clasts "in mass" out of their average orientation, likely during the formation of C' shear planes. In the second study, apatite fission-track ages were obtained from 13 samples from two subvertical profiles on the steep flanks of Skagit Gorge and Ross Lake in the North Cascades, in order to construct age-elevation relations. Results from this study are combined with previous apatite and zircon U-Th/He ages from the same samples to provide some new information about the cooling history. The Ross Lake traverse displays an enigmatic reversal of slope of age-elevation curves, possibly owing to thermal resetting from volcanic flows at ~15 Ma. The Skagit transect displays a steep age-elevation profile an provides an exhumation rate of 0.21 km/Myr, a value similar to that obtained from U-Th/He systems.