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

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REFERENCES: Sainsbury, 1969; Hudson, 1977; A.B. Till and J.A. Dumoulin, written commun., 1991
YP Yukon-Prong terrane (metamorphosed continental margin) (southeastern Alaska)Ñ Occurs in eastern southeastern Alaska and consists chiefly of Late Proterozoic(?) and (or) early and middle Paleozoic mica schist, granitic orthogneiss, amphibolite, metavolcanic rocks, quartzite, marble, and calc-silicate rocks. Protoliths are quartz-rich and pelitic sedimentary rocks, carbonate rocks, intermediate to mafic volcanic rocks, ultramafic rocks, and sparse metamorphosed middle Paleozoic granitic plutons. Terrane includes lower Paleozoic metasedimentary rocks of Nisling assemblage (marble, metamorphosed quartz sandstone, pelitic schist, and metabasalt), middle Paleozoic metasedimentary and metavolcanic rocks of Ruth assemblage and Kah Shakes sequence (pelitic schist, marble, mafic and felsic metavolcanic rocks, metagranodiorite and orthogneiss) (former Tracy Arm terrane), and the informally named Paleozoic East Behm gneiss complex of metaplutonic rocks of Saleeby and Rubin (1990). Metamorphosed stratified and plutonic rocks correlated with Yukon-Tanana terrane in east-central Alaska. Metamorphosed before Late Triassic to amphibolite-facies

Yukon Prong terrane intruded by Late Cretaceous and Early Tertiary intermediate plutonic rocks, and middle Tertiary intermediate plutonic rocks. Bounded to west by structural complex of Behm Canal terrane. Possibly stratigraphically(?) or structurally overlain by Stikinia terrane. Interpreted as a highly metamorphosed and deformed, displaced fragment of the North American craton margin. REFERENCES: Gehrels and others, 1990; Saleeby and Rubin, 1990; Rubin and Saleeby, 1991; McClelland and others, 1991, 1992; Samson and others, 1991; Gehrels and McClelland, 1992

YT Yukon-Tanana terrane (metamorphosed continental margin) (east-central Alaska)ÑOccurs in eastern Alaska and adjoining Yukon Territory of northwestern Canada and consists of an enormous tract of poly-deformed and poly-metamorphosed, middle Paleozoic and older sedimentary, volcanic, and plutonic rocks. Contains three major sequences of metasedimentary, metavolcanic, and metaplutonic rocks that occur in various subhorizontal to gently folded thrust slices. Major sequences are: (1) Mississippian, Devonian, and locally older metasedimentary rocks, mainly pelitic schist, quartz schist, quartz-feldspar schist, and sparse marble (Fairbanks, Birch Hill, and Chena River subterranes, and metasedimentary parts of Jarvis Creek Glacier, Hayes Glacier, Lake George, and Macomb subterranes; Totatlanika Schist, and Keevey Peak Formation); (2) Devonian and Mississippian orthogneiss and augen gneiss (metaplutonic part of Lake George and Macomb subterranes); and (3) Devonian and Mississippian intermediate and siliceous metavolcanic rocks (Butte subterrane, metavolcanic rock parts of Jarvis Creek, Hayes Glacier, and Lake George subterranes; Cleary sequence within Fairbanks schist unit; and Spruce Creek sequence in the Kantishna Hills area). The extrusive metavolcanic rocks and coeval metamorphosed hypabyssal igneous rocks, mainly metamorphosed andesite, dacite, and keratophyre flows and tuffs, are interlayered with the Mississippian, Devonian, and locally older metasedimentary rocks.

U-Pbigneous isotopic analyses indicate Devonian and Mississippian extrusion of intermediate and siliceous metavolcanic rocks (Cleary sequence of Fairbanks subterrane, and Spruce Creek sequence) (Aleinikoff and Nokleberg, 1989; Nokleberg and others, 1989a; J.N. Aleinikoff, T.K. Bundtzen, and W.J. Nokleberg, unpub. data, 1993). Conodont and megafossil ages indicate Mississippian and Devonian ages for metasedimentary rock part of Jarvis Creek Glacier subterrane, and Totatlanika Schist. One known Late Proterozoic metagranitic pluton intrudes metasedimentary rocks in northern part of Chena River subterrane (Nokleberg and others, 1989a), indicating at least part of metasedimentary rocks are pre-Late Proterozoic in age. Locally near Denali fault are abundant Cretaceous(?) metadiabase and metagabbro dikes, sills, and small plutons.

Yukon-Tanana terrane contains an intense subhorizontal to gently dipping structural fabric defined by parallel schistosity and composition layering that are subparallel to major subhorizontal faults that separate major thrust slices. Faults between major thrust slices exhibit both older, early Mesozoic thrust, and younger, mid-Cretaceous extensional displacements. Higher-grade units, with abundant Devonian and Mississippian orthogneiss and augen gneiss, and upper amphibolite to eclogite facies metamorphism, generally occur in deeper structural levels. Lower-grade units, with abundant Devonian and Mississippian volcanic rocks and upper greenschist facies metamorphism, generally occur in higher structural levels. Intense zone of retrogressive metamorphism and mylonitic schists along southern margin of terrane. Major periods of amphibolite to eclogite facies metamorphism in Early Jurassic and mid-Cretaceous time.

Southern margin of Yukon-Tanana terrane faulted against mainly Windy and Wrangellia terranes. Northwestern margin thrust under Wickersham terrane. Southern part of terrane interpreted as structurally underlain in part by Mesozoic flysch of Kahiltna overlap assemblage. Northeastern margin faulted North America craton margin, Kandik River, Angayucham, and Crazy Mountains terranes along Tintina fault. Yukon-Tanana terrane locally structurally overlain by formerly extensive klippen of Seventymile and Stikinia(?) terranes in east-central Alaska. In same area, locally unconformably overlain by Upper Cretaceous and lower Tertiary conglomerate, sandstone, coal, and rhyolite to basalt tuff and flows, and intruded by extensive mid-Cretaceous, Late Cretaceous, and early Tertiary granitic plutons. Yukon-Tanana terrane interpreted as a highly metamorphosed and deformed, displaced fragment of the North American craton margin. Former Yukon Crystalline terrane of Churkin and others, 1982. REFERENCES: Foster, 1976; Foster and others, 1987; Nokleberg and others, 1989a, 1992; Stanley and others, 1990; Dusel-Bacon, 1991; Pavlis and others, 1993; Dusel-Bacon and Hansen, 1992

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