Welcome to the new version of CaltechAUTHORS. Login is currently restricted to library staff. If you notice any issues, please email coda@library.caltech.edu
Published June 1992 | Published
Journal Article Open

Tectonic history of the eastern edge of the Alexander Terrane, southeast Alaska


Rocks exposed west of the Coast Plutonic Complex in southern southeast Alaska form an imbricate thrust belt that overprints the tectonic boundary between two of the largest allochthonous crustal fragments in the North American Cordillera, the Insular and Intermontane composite terranes. In the Alexander terrane (Insular composite terrane), lower Paleozoic metavolcanic and metasedimentary rocks (Descon Formation) and dioritic plutons are unconformably overlain by Lower Devonian clastic strata (Karheen Formation). These rocks are overlain locally by Upper Triassic basalt, rhyolite and marine clastic strata (Hyd Group). Upper Jurassic and Lower Cretaceous metavolcanic and metasedimentary strata of the Gravina sequence unconformably overlie the Alexander terrane. The Gravina sequence forms a structural package over 15 km thick and records intermittent arc volcanism along the eastern edge of the Alexander terrane. The Gravina sequence is structurally overlain by upper Paleozoic and lower Mesozoic metamorphosed basaltic strata, marble, and argillite (Alava sequence), and locally by lower Paleozoic supracrustal rocks and orthogneiss (Kah Shakes sequence). Together, these constitute the Taku terrane which we correlate with the Intermontane composite terrane. Local unconformity of Gravina sequence strata over the Alava sequence demonstrates that the Gravina sequence overlapped an earlier structural boundary between the Intermontane and Insular composite terranes. The rocks were deformed in the mid-Cretaceous by west-vergent thrusting that was was broadly coeval with arc magmatism. Deformation involved emplacement of west-directed thrust nappes over the structurally intact and relatively unmetamorphosed Alexander terrane basement. Mid-Cretaceous tonalite, granodiorite, and quartz diorite intrude rocks of the thrust belt and are locally affected by the deformation. Mid-Cretaceous deformation occurred during two episodes that were contemporaneous with the emplacement of large sill-like plutons. Older structures record ductile southwest-vergent folding and faulting, regional metamorphism, and development of axial-planar foliation. The second-generation structures developed during the later stages of southwest-directed thrust faulting, which juxtaposed rocks of contrasting metamorphic pressures and temperatures. Structural, stratigraphic, and geochronologic data indicate that the two phases of regional thrusting in southeast Alaska occurred between 113 Ma and 89 Ma. Rocks in the western part of the thrust belt were uplifted regionally by 70 Ma. Deformation involved the collapse of a marginal basin(s) and a magmatic arc, and overprinted the older tectonic boundary between the Insular composite terrane and the late Mesozoic western margin of North America (at that time the Intermontane composite terrane). Contractional deformation along the length of the thrust belt was broadly coeval with arc magmatism, and thus records intra-arc tectonism. Late Paleocene to early Eocene igneous activity and extensional (?) deformation subsequently affected the thrust belt.

Additional Information

© 1992 by the American Geophysical Union. (Received February 21, 1991; revised August 5, 1991; accepted August 8, 1991. Paper number 91TC02182. Parts of this research were supported by National Science Foundation grants EAR 86-05386 and EAR 88-034834 (to Saleeby). Additional support (to Rubin) was provided by a Geological Society of America Penrose grant,a Sigma-Xi grant-in-aid, by the U.S. Geological Survey, Alaska Branch, and by the U.S. Forest Service, Ketchikan Ranger District. Rick Allmendinger graciously provided the stereonet programs. We thank Fred Barker, Henry Berg, Dave Brew, Darrel Cowan, Weecha Crawford, John Garver, George Gehrels, Linc Hollister, Bill McClelland, Meghan Miller, Jim Monger, George Plafker, and Margi Rusmore for helpful discussions on northwest Cordilleran stratigraphy and tectonics. Jim Wright provided encouragement and advice on U-Pb zircon leaching techniques. Jeff Marshall assisted in mapping part of the area during the summer of 1987; Mark Fahnestock and Jon Nourse provided able field assistance during the summer of 1986. Field discussion with Bill McClelland and Meghan Miller are appreciated. A review by Weecha Crawford significantly improved the clarity of an earlier version of this paper. Hank Berg and Margi Rusmore thoroughly reviewed the manuscript.

Attached Files

Published - tect635.pdf


Files (1.8 MB)
Name Size Download all
1.8 MB Preview Download

Additional details

August 22, 2023
October 17, 2023