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Bruns, T.R., 1984

Tectonics of an allochthonous terrane, the Yakutat block, northern Gulf of Alaska

Bibliographic Reference

Bruns, T.R., 1984, Tectonics of an allochthonous terrane, the Yakutat block, northern Gulf of Alaska: University of California, Santa Cruz, Ph.D. dissertation, 240 p., illust., maps.


Marine geophysical and geological data delineate the late Cenozoic structure and tectonic history of the northern Gulf of Alaska continental margin, and indicate that part of the margin, the Yakutat block, is an allochthonous terrane that has moved with the Pacific plate for at least the last 5 m.y. The block is currently colliding with southern Alaska and subducting along the Wrangell Benioff zone. The Yakutat block is bounded onshore by the Fairweather Fault and the Chugach-Saint Elias fault system, and offshore by the Queen Charlotte fault system, by Kayak Island and its offshore structural extension, and by the Transition fault at the base of the continental slope from Cross Sound to Kayak Island. Magnetic and structural data indicate that the block is subducting at Kayak Island, and continues west of Kayak Island to at least the Kenai Peninsula in the lower, subducted plate. The major area of deformation within the Yakutat block occurs in a fold and thrust belt on the northwest margin of the block, the region of maximum convergence between the Yakutat block and southern Alaska. This deformation reflects collision and the seaward propagation of thrust faults during Pliocene and Quaternary time within the sedimentary strata covering the block. The Transition fault is a major tectonic boundary that has been inactive during Pliocene and Quaternary time. Prior to the Pliocene, tectonism along the fault juxtaposed Oligocene oceanic basement against Mesozoic and Paleocene rocks of the Yakutat block and truncated the Palogene basin of the block. This tectonism caused no major deformation of the Yakutat block or accretion along the Transition fault. The Transition fault was most likely a transform fault; subduction along the fault prior to the Pliocene is unlikely. Faunal assemblages from the Paleogene strata of the Yakutat block require about 30o of northward motion of the block in the last 50 m.y. A speculative model for the block is that it originated as a composite oceanic-continental terrane during subduction of the Kula-Farallon spreading center beneath North America about 45 m.y. ago, and has since moved north with the Kula and Pacific plates.

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