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Rubin, C.M., 1991

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Bibliographic Reference

Rubin, C.M., 1991, Structural, stratigraphic, and geochronologic analysis of the Alexander-Taku terrane boundary and the overlapping Upper Jurassic to Lower Cretaceous Gravina sequence, southeast Alaska: Pasadena, California, California Institute of Technology, Ph.D. dissertation, 358 p., illust., maps.

Abstract

An imbricate thrust belt that extends along strike for more than 2,000 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. Deformation was broadly coeval with mid-Cretaceous magmatism and involved the emplacement of west-directed supracrustal and crystalline 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. 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 sill-like plutonic bodies. Older structures record ductile southwest-vergent folding and faulting, and regional metamorphism, and contain a well-developed axial-planar foliation. The second-generation structures developed during the later stages of southwest-directed reverse faulting that juxtaposes rocks of contrasting metamorphic pressures and temperatures. Younger Paleocene deformation has also affected these rocks, where moderate to steeply E-dipping foliation surfaces have reoriented earlier mid-Cretaceous fabrics. Late Paleocene pegmatite dikes are highly deformed and are affected by these E-dipping structures. A swarm of NE-trending, hornblende-bearing diabase dikes crosscut all structures and fabrics and mark a regional change in the overall regional strain patterns during Miocene time. Structural, stratigraphic and geochronologic data suggest that regional-scale deformation in Southeast Alaska occurred between 113 Ma and 89 Ma. Rocks in the thrust belt were regionally uplifted by 70 Ma, at an average rate of ~0.9 mm/yr. Deformation involved the collapse of marginal basin(s) and a magmatic arc, overprinting the older tectonic boundary between the Insular superterrane and the late Mesozoic western margin of North America (the Intermontane superterrane). 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 deformation and uplift may mark the transition from contractional to extensional tectonism and perhaps records the collapse of tectonically thickened crust.

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