U. S. Geological survey pamphlet introduction, tectonic definitions, acknowledgments

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Upper (Kanuti) thrust panelÑChiefly serpentinized pyroxenite, harzburgite, dunite, wehrlite, and gabbro with cumulate and tectonic structures, and intrusive gabbro and diabase. Basal contact of ophiolite locally marked by amphibolite metamorphosed at the time of structural emplacement in the Middle Jurassic (Boak and others, 1987). Includes Misheguk Mountain terrane of Grantz and others (1991) and other isolated klippen of ultramafic and mafic rocks structurally overlying Narvak thrust panel. Kanuti thrust panel interpreted as part of an ophiolitic assemblage emplaced during Middle or Late Jurassic onto an apparently unrelated middle (Narvak) thrust panel of basalt and oceanic sedimentary rocks that constitute the root of an island arc, possibly the tectonically suprajacent Koyukuk terrane. REFERENCES: Patton and others, 1977, 1989; 1992; Wirth and others, 1986; Boak and others, 1987; Loney and Himmelberg, 1985, 1989; Mayfield and others, 1988; Karl, 1992; Patton, 1992a, b
AP Aurora Peak terrane (continental margin arc)ÑOccurs in eastern and central Alaska Range and consists of Silurian through Triassic metasedimentary rocks, and Late Cretaceous and early Tertiary metagranitic rocks. Older metasedimentary rocks consist mainly of fine- to medium-grained and polydeformed calc-schist, marble, quartzite, and pelitic schist. One conodont fragment from marble indicates a Silurian to Triassic age. Protoliths for metasedimentary rocks include marl, quartzite, and shale. Correlated with Kluane Schist in southwest Yukon Territory. Younger metaplutonic sequence (unit Kpf) consists of regionally metamorphosed and penetratively deformed schistose quartz diorite with granodiorite, granite, and sparse amphibolite derived from gabbro and diorite. U-Pb zircon and Rb-Sr whole-rock isochron isotopic analyses of the metaplutonic rocks indicate emplacement ages of 71 and 74 Ma. Isotopic analysis of lead from samples of metagranitic rocks show moderate radiogenic values, and derivation from a source of about 1.2-Ga age.

Minimum structural thickness of terrane several thousand meters. Terrane is twice ductily metamorphosed and deformed. Core of terrane exhibits older, upper amphibolite facies metamorphism and associated mylonitic schist. Upper amphibolite facies metamorphism probably occurred during syntectonic intrusion of the Late Cretaceous metagranitic rocks. Margin of terrane exhibits younger middle greenschist facies metamorphism and formation of blastomylonite along an intense, younger schistosity. Younger greenschist facies minerals and younger intense schistosity occur mainly along the margins of the terrane, adjacent to bounding Denali and Nenana Glacier faults. Aurora Peak terrane interpreted as a displaced fragment of a North American continental-margin arc that was tectonically separated from the Kluane Schist and the Ruby Range batholith in southwestern Yukon Territory. REFERENCES: Brewer, 1982; Nokleberg and others, 1985, 1989a, 1992; Aleinikoff and others, 1987

BC Behm Canal (metamorphic)ÑOccurs in eastern southeastern Alaska and consists of structurally interleaved thrust slices of Gravina-Nutzotin overlap assemblage, Alexander, Wrangellia, Stikinia, and Yukon-Tanana terranes, Alava sequence of Rubin and Saleeby (1991), Kah Shakes sequence of Saleeby and Rubin (1990) and Rubin and Saleeby (1991), and Ruth assemblage of McClelland and Gehrels (1990). Chiefly mica schist, granitic orthogneiss, calc-silicate rocks, siliceous paragneiss, minor marble and amphibolite. Alava sequence consists of crinoidal and argillaceous marble, carbonaceous phyllite, argillite, mafic flows, pillow breccia, tuff, and quartzite of late Paleozoic and Middle and Late Triassic age that are correlated by Rubin and Saleeby (1991) with rocks of the Stikinia terrane. Behm Canal terrane metamorphosed to greenschist- to amphibolite-facies in the Late Cretaceous and Early Tertiary, after accretion. Intruded by middle and Late Cretaceous granitic, gabbroic, and ultramafic rocks, and by Late Cretaceous and Early Tertiary intermediate plutonic rocks. Includes part of the former Taku terrane of Monger and Berg (1987). Locally structurally overlain by the lower Paleozoic Kah Shakes sequence and an early Paleozoic Behm Canal gneiss complex that are both interpreted as part of the Yukon Prong terrane in southeastern Alaska. Behm Canal terrane interpreted as a series of interleaved fault slices of various adjacent terranes. REFERENCES: Saleeby and Rubin, 1990; Rubin and Saleeby, 1991
BP Broad Pass terrane (metamorphic)ÑOccurs in central Alaska Range as a structural melange composed chiefly of chert, argillite, phyllite, bedded tuff, limestone, and flysch including graywacke, argillite, and chert-pebble and polymictic conglomerate. Chert contains Late Devonian(?) to Mississippian radiolarians, and limestone blocks contain Silurian and Devonian fossils (Jones and others, 1980). Poorly exposed and structurally complex. One of several mini-terranes enclosed in highly-deformed flysch of the Kahiltna overlap assemblage. Broad Pass terrane interpreted as a structural melange of diverse metasedimentary rocks that formed during Cretaceous deformation of enclosing Kahiltna overlap assemblage. REFERENCES: Jones and others, 1982; Csejtey and others, 1992

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