Kveton, K.J., 1989, Structure, thermochronology, provenance, and tectonic history of the Orca Group in southwestern Prince William Sound, Alaska: University of Washington, Seattle, Ph.D. dissertation, 202 p., illust., map (folded in pocket).
The Paleogene Orca Group is part of an extensive accretionary prism or complex that includes the Upper Cretaceous Valdez Group. The Orca consists of several northeast-trending belts of turbidites and local basaltic sections bounded by thrust faults. Available evidence suggests that the components of this accretionary complex accumulated farther south then were transferred from the down-going plate to the accretionary prism at a subduction zone. Two major phases of deformation can be discerned in the Orca Group. The earlier phase is only present in one of the belts and is characterized by intense stratal disruption interpreted to have developed during underthrusting beneath the accretionary wedge. The later phase of deformation is present in all three belts studied, is characterized by east-vergent thrust faulting and folding, and is interpreted to have developed during underplating to the base of the prism along the belt-bounding thrust faults. Features indicative of deep burial of the belts in the Orca Group include: Maximum temperatures of over 200°C based on vitrinite reflectance, prehnite-pumpellyite facies metamorphism, and deformation dominated by pressure-solution, consistent with underthrusting followed by underplating as the process by which these belts were accreted. Fission-track dates on detrital zircon and apatite from sandstones in these belts of the Orca record differential uplift as the means by which these belts passed through the 200°C and 110°C isotherms. The focus of the present study in the vicinity of the Bainbridge fault, which has been proposed as a terrane-bounding structure between the Valdez and Orca Groups, revealed similar sandstones on both sides. Previous regional studies of sandstone petrography found no evidence of other terrane-bounding faults between the Valdez and Orca Groups in western Prince William Sound or on the adjacent Kenai Peninsula. The Valdez and Orca Groups must have been derived from the same, possibly evolving, source area through time and followed similar paths of northward translation. A comparison of the age spectra of fission-track dates on detrital zircon from the Orca Group with histograms of dates from potential source areas in southwestern Alaska, the Canadian Cordillera, and Washington state indicates that the most likely source of sediments to the Orca Group was the Coast Plutonic Complex in the Canadian Cordillera. The Valdez and Orca Groups were successively derived from the Coast Mountains of western Canada, sequentially translated northward along dextral strike-slip faults, and were incorporated into growing accretionary complexes. The Valdez Group was likely incorporated into a subduction zone associated with the latest Cretaceous to Paleocene Alaska Range-Talkeetna Mountains arc, whereas the fission-track data indicate that the Orca Group must have been incorporated into the subduction complex associated with the late Eocene to Oligocene phase of the Aleutian arc.
Theses and Dissertations