| Summary: | An imbricate thrust belt that extends along strike for over 2000 km overprints the tectonic boundary between two of the largest allochthonous crustal fragments (Intermontane and Insular superterranes) in the North American Cordillera and affects rocks west of the Coast Plutonic Complex in southeast Alaska, western British Columbia and northern Washington. Deformation was broadly coeval with mid-Cretaceous magmatism and involved the emplacement of west-directed thrust nappes over a structurally intact and relatively unmetamorphosed basement. The Paleozoic and lower Mesozoic Alexander terrane forms structural basement for much of the thrust belt, along a moderately northeast-dipping ramp. The western metamorphic belt of the Coast Plutonic Complex consists of the Alexander and Taku terranes, and the Upper Jurassic and Lower Cretaceous Gravina sequence. The Alexander terrane consists of lower Paleozoic metavolcanic and metasedimentary rocks (Descon Formation) and dioritic plutons that 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). The Taku terrane consists of polydeformed and metamorphosed strata that are divided into anupper Paleozoic and lower Mesozoic assemblage (Alava sequence) and a lower Paleozoic assembtage (Kah Shakes sequence). The lower Paleozoic Kah Shakes sequence consists of Devonian orthogneiss, quartz-rich metasedimentary rocks, metabasalt, meta-silicic tuff, marble, cate-silicate, and quartzite. The quartz-rich metasedimentary rocks may be correlative with the lower Paleozoic and mid-Paleozoic Yukon-Tanana terrane, which represents an east-Pacific fringing arc complex built on continental slope and rise deposits. The upper Paleozoic and lower Mesozoic Alava sequence consists of crinoidal and argillaceous marble, carbonaceous phyllite, argillite, mafic flows, pillow breccia, pyroclastic tuff, and minor quartz-rich metasedimentary rocks. The upper Paleozoic part of ...
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