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Published June 1991 | public
Journal Article

Tectonic framework of the upper Paleozoic and lower Mesozoic Alava sequence: a revised view of the polygenetic Taku terrane in southern southeast Alaska


Fragments of upper Paleozoic and lower Mesozoic metavolcanic and metasedimentary sequences of the Taku terrane are exposed discontinuously along a narrow belt in southeast Alaska and form a distinct lithostratigraphic package in the Ketchikan area called the Alava sequence. Crinoidal and argillaceous marble, carbonaceous phyllite, argillite, mafic flows, pillow breccia, pyroclastic tuff, and quartzite characterize the sequence. These strata are unconformably overlain by Upper Jurassic to Lower Cretaceous fine- to coarse-grained epiclastic rocks of the Gravina sequence. The upper Paleozoic part of the Alava sequence may be correlative with the Yukon–Tanana terrane, whereas the Middle and Upper Triassic portion of the Alava sequence may represent a metamorphic vestige of the Stikine terrane. Both parts are now exposed on the western flank of the Coast Plutonic Complex, in contrast with their correlatives to the east. These relations suggest that the Stikine and Alexander terranes were juxtaposed prior to deposition of the Gravina sequence. The western boundary between rocks of North American affinity and allochthonous ensimatic crustal fragments of the Alexander and Wrangellian terranes lies west of the Coast Plutonic Complex.

Additional Information

© 1991 Canadian Science Publishing. Received June 21, 1990; Revision accepted December 5, 1990. Field and laboratory work for southeast Alaska regional studies has been supported by the United States National Science Foundation grants EAR 86-05386 and EAR 88-03834 (to JBS). Additional support (to CMR) was provided by Geological Society of America Penrose Grants, a Sigma-Xi grant-in-aid, and by the United States Geological Survey (USGS), Alaska Branch. We thank Fred Barker (USGS) and David Marren (University of California, Riverside) for geochemical analyses. Field assistance by Jeff Marshall was extremely helpful. We thank Lucia Crawford, George Gebrels, Bill McClelland, Meghan Miller and Margi Rusmore for helpful discussions on Alaskan geology. Bob Anderson, Bill McClelland, Nick Massey, and Glenn Woodsworth provided helpful and critical reviews.

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