Paleogene tectonic evolution of the Gulf of Alaska region
University of California, Santa Cruz
196 p., Illus. (some color), Maps
Alaska Resources Library & Information Services: QE604 B65; Rasmuson Library: ALASKA QE627.5 A4 B65 1990a (microfiche)
The Chugach and Prince William terranes compose a Mesozoic through Paleogene accretionary complex rimming the Gulf of Alaska. This thesis consists of two studies that help constrain the Paleogene tectonic history of the accretionary complex: (1) a paleomagnetic study of the Resurrection Peninsula ophiolite and (2) a comparative structural and kinematic study of the Contact fault system in E. and W. Prince William Sound. The rocks of the Resurrection Peninsula comprise an ophiolite (57 $\pm$ 1 Ma) within the Chugach/Prince William accretionary complex. The paleomagnetism of pillow basalt and sheeted dikes yields a mean paleolatitude of 54 $\pm$ 7$\sp\circ$, which implies 13 $\pm$ 9$\sp\circ$ of poleward displacement. The mean paleostrike of the sheeted dikes is N26$\sp\circ$E $\pm$ 12$\sp\circ$. I interpret the paleomagnetism to indicate that the Kula-Farallon ridge intersected North America near northern Washington and trended NNE in Early Eocene time. The Contact fault system juxtaposes flysch of the latest Cretaceous Valdez Group (Chugach terrane) with flysch of the late Paleocene through Early Eocene Orca Group (Prince William terrane). In the eastern Prince William Sound study area, faults composing Contact Fault system moved with dominantly dextral slip. Anomalous structural trends between faults appear to result from clockwise block rotation in response to dextral shear during oblique subduction. In western Prince William Sound we found that we cannot distinguish the Chugach from the Prince William terrane on the basis of fossil ages, lithology, deformation, or metamorphism. Furthermore, the major faults in the study are relatively late-stage reverse faults related to internal deformation of the prism. The Chugach and Prince William terranes appear to represent older and younger parts of a continuously evolved accretionary prism, best considered a single terrane. West-northwest shortening characterizes the deformation in western Prince William Sound, whereas dextral-slip faulting is dominant in eastern Prince William Sound. We ascribe this difference to development of the Alaska Orocline coeval with changes in plate motions, which resulted in convergence in the western Gulf remaining relatively orthogonal, while convergence in the eastern Gulf became increasingly oblique.
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