Structural, stratigraphic, and geochronologic analysis of the Alexander-Taku terrane boundary and the overlapping upper Jurassic to lower Cretaceous gravina sequence, southeast Alaska

• Main Author: Rubin, Charles Martin
• Format: Thesis
• Language: English
• Published: California Institute of Technology 1991
• Subjects: tectonics; Alaska; geochronology; structural geology; Geology; FOS Earth and related environmental sciences; Yukon; Pacific; Taku; Stikine; Ramp The
• Online Access: https://dx.doi.org/10.7907/zq82-2077; https://resolver.caltech.edu/CaltechTHESIS:04132011-115331837

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 the Alava sequence is probably correlative with the mid- to late Paleozoic portion of the Yukon-Tanana terrane. The Middle and Upper Triassic portion of the Alava sequence may represent a metamorphiC vestige of the Stuhini Group, now exposed on the western flank of the Coast Batholitic belt. Upper Jurassic and Lower Cretaceous metavolcanic and metasedimentary strata of the Gravina sequence unconformably overlie both the Alexander and Taku terranes. These rocks form two distinct lithotectonic units in southern southeast Alaska. The lower unit consists of coarse marine pyroclastic and volcaniclastic strata, mafic flows, breccia, and fine-grained tuff which are locally intruded by hypabyssal bodies of diorite and quartz diorite. Fine- to coarse-grained turbidites and related channel-fill deposits comprise the epiclastic part of the Gravina sequence. Conglomerate units contain mostly volcanic and plutonic lithic clasts that suggest they were derived from a composite igneous source. Clasts from the channel-fill deposits yield Pb-U zircon ages of 154 to 158 Ma. The pyroclastic and volcaniclastic rocks represent remnants of a Late Jurassic oceanic arc system that was constructed on a composite basement consisting of the Alexander and Taku terranes; the Taku terrane is inferred to represent the westernmost extent of the Stikine and Yukon-Tanana terranes. These data suggest that the Intermontane (Stikine and Yukon-Tanana terranes) and Insular (Alexander terrane) superterranes were juxtaposed prior deposition of the Upper Jurassic and Lower Cretaceous Gravina sequence. The lower Paleozoic to Early Cretaceous rocks were deformed in the mid-Cretaceous and tectonism was broadly coeval with arc magmatism. Deformation involved the emplacement of west-directed thrust nappes over a structurally intact and relatively unmetamorphosed basement. Mid-Cretaceous tonalite, granodiorite, and quartz diorite intrude rocks of the thrust belt and were 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, 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. The presence of syntectonic kyanite-staurolite-garnet-biotite assemblages in the more eastern high-strain zones indicates that at least some of the reverse faults were generated at depths in excess of 20 km during the later stages of thrust faulting and associated uplift. Paleocene and younger (?) deformation has also affected rocks on the western margin of the Coast Plutonic Complex. Younger fabrics are dominated by low to moderate west-dipping foliation surfaces that are axial planar to asymmetric east-vergent folds. The east-verging fabrics have transposed earlier mid-Cretaceous fabrics . Late Paleocene pegmatite dikes are highly deformed and are affected by the west-dipping structures. Exposure of mid-crustal level rocks might be related to a reversal in vergence during Paleocene time, in which deep levels of the mid-Cretaceous thrust system were transported upward along east-vergent structures. A swarm of hornblende-bearing diabase dikes cross-cut all structures and fabrics. These dikes trend northeast 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 minimum rate of ≈ 0.9 mm/yr. Mid-Cretaceous 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 (i.e., 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.