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


Chugach terrane (accretionary wedge and subduction zone)Ñ



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Chugach terrane (accretionary wedge and subduction zone)ÑComposite terrane in Chugach Mountains between Border Ranges and Contact faults in southern Alaska. In southern Alaska consists of: (1) a northern unit of blueschist and greenschist; (2) a central unit of McHugh Complex and correlative units; and (3) a southern unit of the Valdez Group and correlative units. These three units have counterparts in southeastern Alaska

Blueschist and greenschist (subduction zone, dominantly oceanic rocks)ÑChiefly narrow, fault-bounded units of blueschist and interlayered greenschist too small to depict on map (schists of Liberty Creek, Iceberg Lake, Seldovia, and Raspberry Strait). Blueschist and greenschist unit occurs in narrow, intensely-deformed, fault-bounded lenses mainly along northern or inboard margin of McHugh Complex and correlative units. Metamorphosed from blueschist to transitional greenschist-blueschist facies. Early Jurassic U-Pb sphene, Ar-Ar, and Rb-Sr mineral isochron metamorphic ages. Interlayered blueschist and greenschist units interpreted as discontinuous remnants of a subduction zone assemblage associated with formation of Jurassic Talkeetna arc to the north
CGM McHugh Complex and correlative units (subduction zone, dominantly oceanic rocks)ÑChiefly a Mesozoic subduction zone melange composed of mainly Late Triassic, Jurassic, and Early to mid-Cretaceous argillite, basalt, graywacke, radiolarian chert, and limestone, with sparse conglomerate, pillow basalt, mafic tuff, volcanic rocks, and plutonic rocks, and with exotic blocks of ultramafic rocks, and limestone. Includes McHugh and Uyak Complexes, Kelp Bay Group, and unnamed chert and basalt of Kachemak Bay previously interpreted as the Kachemak terrane by Jones and others (1987). Some limestone blocks contain Permian Tethyan fusulinids. Metamorphosed from prehnite-pumpellyite to lower greenschist facies. McHugh Complex and correlative units intruded by Middle Cretaceous trondhjemite and by early and middle Tertiary granitic plutonic rocks in outcrops that are too small to depict on map. McHugh and correlative units interpreted as disrupted oceanic crust, sea mount, and (or) trench fill assemblages. Subduction linked to Early and middle Jurassic Talkeetna arc, and to Late Jurassic and Early Cretaceous Chitina and Chisana arcs to north
CGV Valdez Group and correlative units (accretionary wedge - dominantly turbidites)ÑChiefly an accretionary complex composed of Upper Cretaceous (Campanian to Maastrichtian) flysch, mainly graywacke and slate of Valdez Group, Kodiak and Shumagin Formations, Sitka Graywacke, and unnamed metamorphic equivalents. Sparse megafossils. Local thick metabasalt sequence along southern margin. Generally exhibits lower greenschist metamorphism. Complexly folded and faulted. Metamorphic grade increases to upper amphibolite facies in the informally named Chugach metamorphic complex of Hudson and Plafker (1982) in eastern Chugach Mountains. In this area, granitic intrusive rocks of Sanak Baranof plutonic belt (50-65 Ma), and associated aureoles of amphibolite facies metamorphism weld Chugach terrane to the adjacent northern Prince William terrane by early Eocene. Elsewhere locally, early Tertiary plutons are truncated by Contact fault between the Chugach and Prince William terranes. In southern Alaska, bounded to north by Border Ranges fault and Peninsular and Wrangellia terranes. Bounded to south and west by Contact fault and Prince William terrane. Valdez Group and correlative units interpreted as trench deposit derived from a Cretaceous magmatic arc to the north and east (present-day coordinates), and partly as an intraoceanic accreted primitive arc. REFERENCES: Connelly, 1978; Connelly and Moore, 1979; Decker and others, 1980; Hudson and Plafker, 1982; Johnson and Karl, 1985; Brew and others, 1988; Roeske and others, 1989; Plafker and others, 1989b; Sisson and others, 1989; Karl and others, 1990; Lull and Plafker, 1990; Bradley and Kusky, 1992; Nelson, 1992
CH Chulitna terrane (ophiolite)ÑOccurs in central Alaska Range and consists of four major units: (1) Late Devonian ophiolite composed of serpentinite, gabbro, pillow basalt, and red radiolarian chert; and Mississippian chert; (2) Permian volcanic conglomerate, limestone, chert, and argillite; (3) Lower Triassic limestone; (4) Upper Triassic redbeds, including red sandstone, siltstone, argillite, and conglomerate, and pillow basalt, limestone, sandstone, and shale; and (5) Jurassic argillite, sandstone, and chert. Depositional contacts and (or) reworked clastic detritus form underlying rocks indicate a continuous stratigraphic sequence. Ammonites from Lower Triassic limestone show strong affinities with faunas from California, Nevada, and Idaho, but differ from assemblages in Canada. Terrane moderately to intensely folded and thrust-faulted and structurally overlies highly deformed upper Mesozoic argillite and graywacke of the Kahiltna overlap assemblage to west, and West Fork terrane to east. Chulitna terrane interpreted as an allochthonous fragment of an ophiolite and overlying igneous arc and sedimentary sequence. REFERENCES: Nichols and Silberling, 1979; Jones and others, 1980; Csejtey and others, 1992
CO Coldfoot terrane (metamorphosed continental margin)ÑOccurs along southern margin of Brooks Range and consists of two major units: (1) a structurally high sequence Late Proterozoic(?) to Cambrian(?) quartz-mica schist, quartzite, mica schist, and minor metamorphosed felsic and mafic volcanic rocks; and (2) a structurally low sequence of calc-schist and marble with local Silurian and Devonian conodonts, and locally the Devonian Ambler sequence forms in central and western Brooks Range composed of metamorphosed rhyolite and basalt volcanic flows, tuff, and breccia with stratiform massive sulfide deposits interlayered with chlorite schist, marble, calcschist, and graphitic schist. Terrane contains local metagabbro and metadiabase dikes and sills of middle Paleozoic(?) age. In central Brooks Range younger unit exposed in local structural windows beneath older unit. Terrane locally intruded by Devonian and Late Proterozoic granitic plutons metamorphosed to orthogneiss. Terrane informally named the schist belt.

Polymetamorphosed and deformed in Late Mesozoic and older(?) time. Terrane exhibits relict regional Mesozoic blueschist facies minerals overprinted by pervasive lower to middle greenschist facies minerals. Local areas of Late Proterozoic(?) amphibolite facies metamorphism. Local intense retrogressive metamorphism and deformation with formation of phyllonite at structural top adjacent to structurally overlying Angayucham terrane. Structurally overlies Hammond terrane to north, structurally overlain by Angayucham terrane to south. Coldfoot terrane correlated with Seward terrane to southwest. Coldfoot terrane interpreted as an intensely metamorphosed and deformed, displaced fragment of the North American craton margin. REFERENCES: Armstrong and others, 1986; Hitzman and others, 1982; Till and others, 1988; Dillon, 1989; Karl and others, 1989; Moore and Mull, 1989; Gottschalk, 1990; Karl and Aleinikoff, 1990; Moore and others, 1992; Patton and others, 1992


CW Clearwater terrane (island arc)ÑOccurs in east-central Alaska and consists of a structurally complex assemblage of argillite, greenstone (pillow basalt), marble derived from shallow-water limestone, and metarhyolite. Marble contains Late Triassic (late Norian) fossils. Contains fault-bounded pluton of Cretaceous(?) granodiorite. Weakly metamorphosed and penetratively deformed at lower greenschist facies. Terrane occurs as a narrow fault-bounded lens along Broxson Gulch thrust between Maclaren terrane to north and Wrangellia terrane to south. Because of the assemblage of basalt and rhyolite volcanic rocks, and shallow-water sedimentary rocks, Clearwater terrane interpreted as a shallow-level fragment of an island arc. REFERENCES: Nokleberg and others, 1985, 1989a, 1992; Csejtey and others, 1992
CZ Crazy Mountains terrane (passive continental margin)ÑOccurs in east-central Alaska in two displaced fragments and consists of two major sequences: (1) maroon and green slate, and grit, and black limestone with floating quartz grains with trace fossil Oldhamia of probable Cambrian age in slate; and (2) younger sequence of Early Devonian or older mafic volcanic rocks, agglomerate, chert, and clastic rocks, fossiliferous Lower to Middle Devonian limestone, and Upper Devonian chert-pebble conglomerate. Crazy Mountains terrane interpreted as a displaced fragment of the early Paleozoic continental margin of North America. REFERENCES: Churkin and others, 1982; Dover, 1990
DL Dillinger terrane (passive continental margin)ÑOccurs as a major unit in west-central Alaska, south of Denali fault, and in a smaller fragment along branches of the fault. Within branches of fault, terrane composed mainly of lower and middle Paleozoic carbonate and lesser clastic rocks that consist of: (1) Cambrian and Ordovician basinal limestone, banded mudstone, and silty limestone turbidites; and (2) Upper Silurian through Middle Devonian, unnamed, shallow-water limestone and dolomite. Structurally and (or) stratigraphically overlain by Mystic terrane. Unconformably overlain by Upper Cretaceous Kuskokwim Group. Stratigraphic column for area within branches of Denali fault. South of Denali fault, terrane consists chiefly of: (1) Cambrian(?) to Ordovician calcareous turbidite, shale, and minor greenstone; (2) Lower Ordovician to Lower Silurian graptolitic black shale and chert; (3) Lower to Middle Silurian laminated limestone and graptolitic black shale; (4) Middle to Upper Silurian sandstone turbidites and shale; and (5) Upper Silurian to Lower Devonian limestone, limestone, breccia, sandstone, and shale. Dextrally offset along Denali fault in southwestern Alaska Range; locally structurally and possibly stratigraphically overlain by Mystic terrane. Complexly folded and faulted. Interpreted as lower to middle Paleozoic, basinal facies equivalent to lower and middle Paleozoic rocks of Nixon Fork terrane. Dillinger terrane interpreted by R.B. Blodgett, T.K. Bundtzen, and J.B. Decker, Jr. (written commun., 1991), along with Minchumina, Mystic, and Nixon Fork terranes, as part of the larger, composite Farewell terrane. Dillinger terrane interpreted as a displaced fragment of the Paleozoic and early Mesozoic continental margin of North America. REFERENCES: Jones and others, 1982; Bundtzen and Gilbert, 1983; Gilbert and Bundtzen, 1984; Blodgett and Clough, 1985; R.B. Blodgett, T.K. Bundtzen, and J.B. Decker, Jr., written commun., 1991
GD Goodnews terrane (subduction zone - dominantly oceanic rocks)ÑOccurs in southwestern Alaska and consists of a disrupted assemblage of pillow basalt, diabase, gabbro, chert, argillite, minor limestone, volcanogenic sandstone, and ultramafic rocks. Ordovician, Devonian, Mississippian, Permian, and Triassic fossils in limestones. Mississippian, Triassic, and Jurassic fossils in cherts. Metamorphic grade of terrane ranges from prehnite-pumpellyite to greenschist to blueschist facies. Blueschist facies minerals occur in two geographically separated belts. Southern belt, south of Goodnews Bay, intruded by pre-186 Ma diorite to gabbro plutons, yields Late Triassic K-Ar blue amphibole age. Northern belt, near Kilbuck terrane, is structurally adjacent to greenschist facies unit that yields Late Jurassic K-Ar amphibole age. Goodnews terrane unconformably overlain by Lower Cretaceous marine sandstone and conglomerate derived from Kilbuck and Togiak terranes, and by Upper Cretaceous sedimentary rocks of Kuskokwim Group, and intruded by Late Cretaceous and early Tertiary plutonic rocks of Kuskokwim Mountains plutonic belt. Goodnews terrane interpreted as subduction zone complex for the Togiak island arc terrane. REFERENCES: Hoare and Coonrad, 1978; Murphy, 1987; Box, 1985a, b; Box and others, 1992; Dusel-Bacon and others, 1992; Patton and others, 1992
KI Kilbuck-Idono terrane (cratonal)ÑOccurs in two fault-bounded fragments about 330 km apart in southwestern and west-central Alaska. Chiefly metamorphosed diorite, tonalite, trondhjemite, and granite orthogneiss, subordinate amphibolite, and minor metasedimentary rocks, mainly quartz-mica schist, marble, and garnet amphibolite of the informally named Kanektok metamorphic complex of Hoare and Coonrad (1979). Metaplutonic rocks yield Early Proterozoic (2.06 to 2.07 Ga) U-Pb zircon ages of emplacement. Nd-Sm isotopic analyses indicate 2.5 Ga (Archean) crustal component. Terrane metamorphosed to amphibolite facies with retrograde greenschist facies metamorphism, possibly in Late Mesozoic. Southern fragment constitutes Kilbuck terrane of Box and others (1990); northern fragment constitutes Idono Complex of Miller and others (1991). Unconformably overlain by Upper Cretaceous sedimentary rocks of Kuskokwim Group. Kilbuck-Idono terrane interpreted as highly-displaced cratonal fragment; no known correlative cratonal rocks in North America. Possibly displaced from North Asian craton. REFERENCES: Hoare and Coonrad, 1978, 1979; Turner and others, 1983; Box and others, 1990; Miller and others, 1991
KY Koyukuk terrane (island arc)ÑOccurs in northern and southern sequences within Yukon-Koyukuk basin in west-central Alaska. Northern sequence, north of Kaltag fault, chiefly basalt, voluminous Lower Cretaceous (Neocomian) andesite flows, tuff, breccia, conglomerate, tuffaceous graywacke, mudstone, and bioclastic limestone. Fossil and isotopic ages of Late Jurassic through Early Cretaceous (Berriassian to Aptian). Southern sequence, south of Kaltag fault, consists of Middle and Late Jurassic tonalite and trondhjemite plutonic rocks that intrude Angayucham terrane basement of Koyukuk terrane. Upper Jurassic to Lower Cretaceous volcanic and related rocks associated with, and unconformably overlie tonalite-trondhjemite plutonic rocks and locally an older suite of units, similar to Angayucham terrane, with microfossil ages that range from Pennsylvanian to Triassic. Volcanic and plutonic rocks constitute Middle Jurassic to Early Cretaceous Koyukuk arc. Terrane is depositionally overlain by Lower Cretaceous marine sedimentary rocks of Yukon-Koyukuk basin, and by volcanic rocks of Yukon-Kanuti igneous belt. Terrane structurally underlain by Angayucham terrane. Koyukuk terrane and nearby Nyac and Togiak terranes are interpreted as a group of broadly coeval Jurassic and Early Cretaceous island arc terranes. REFERENCES: Patton, 1973; Box and Patton, 1989; Patton and others, 1989; Patton, 1991
LG Livengood terrane (oceanic crust)ÑOccurs in east-central Alaska and consists of a structurally deformed and poorly-exposed assemblage of: (1) serpentinized harzburgite, minor dunite, and Cambrian diabase, gabbro, and diorite; (2) Ordovician ribbon chert with graptolitic shale (Livengood Dome Chert); (3) undated (Silurian(?) to Late Proterozoic(?)) silicified dolomite with lesser chert, limestone, argillite, basalt, and volcaniclastic rocks (Amy Creek and Lost Creek units); and (4) Late Silurian limestone debris flows interbedded within siliciclastic strata; and (5) unconformably overlying Middle Devonian turbidites composed of conglomerate with clasts of mafic and ultramafic rocks, and sandstone and mudstone with minor limestone. (Cascaden Ridge unit). Terrane occurs as one of several, narrow, east-northeast trending terranes in region. Faulted against Crazy Mountains, Ruby, and Angayucham terranes to northwest. Thrust under Manley terrane to southeast. Livengood terrane interpreted as a fragment of a Cambrian ophiolite and oceanic crust with overlying Cambrian to Silurian pelagic and continental rise and slope deposits, and Devonian turbidite deposits. REFERENCES: Churkin and others, 1980; Weber and others, 1985; Jones and others, 1986; Loney and Himmelberg, 1988; Dover, 1990; Grantz and others, 1991; Blodgett 1992; Blodgett and others, 1988; Weber and others, 1992

MA Manley terrane (turbidite basin)ÑOccurs in east-central Alaska as three distinct flysch sequences: (1) Upper Triassic or Upper Jurassic unfossiliferous, carbonaceous, slate, argillite, phyllite with sparse chert- and quartzite-pebble conglomerate, and minor chert intruded by gabbro (Vrain unit); (2) Upper Jurassic to Lower Cretaceous quartzite and pebble conglomerate (Wolverine unit); and (3) Albian and Upper Cretaceous shale, siltstone, graywacke, and polymictic conglomerate (Wilber Creek unit) probably derived from a magmatic arc and from sedimentary and volcanic rocks of White Mountains and Livengood terranes. Intruded by mid-Cretaceous granitic rocks. Terrane structurally overlies Livengood terrane and structurally overlain by White Mountains terrane. Manley terrane interpreted as displaced fragment of a Mesozoic overlap assemblage, possibly originally deposited on Livengood terrane. Possible offset equivalent of Kandik River overlap assemblage. REFERENCES: Jones and others, 1981, 1986; Weber and others, 1985, 1988; Gergen and others, 1988; Dover, 1990; Grantz and others, 1991


MK McKinley terrane (sea mount)ÑOccurs between branches of Denali fault in central Alaska Range and consists chiefly of a strongly folded and faulted structural complex assemblage of: (1) fine-grained Permian flysch, mainly graywacke, argillite, and minor chert; (2) Triassic chert; (3) a thick sequence of Upper Triassic (Norian) pillow basalt; (4) Upper Jurassic(?) to Cretaceous flysch, mainly graywacke, argillite, minor conglomerate and chert; and (5) Late Jurassic(?) to Cretaceous gabbro and diabase. Also included are Mississippian to Upper Triassic chert (Red Paint subterrane) that is thrust over, and folded with the upper Mesozoic flysch. McKinley terrane interpreted as a fragment upper Paleozoic deep-sea sedimentary rocks and one or more Late Triassic seamounts. One of several terranes in tectonic lenses along Denali fault. REFERENCES: Jones and others, 1981, 1982, 1987; Gilbert and others, 1984
ML Maclaren terrane (continental margin arc)ÑOccurs in a narrow, east-west-striking lens between the Denali fault and the Broxson Gulch thrust in eastern Alaska Range. Consists of Maclaren Glacier metamorphic belt to the south and East Susitna batholith to the north. Maclaren Glacier metamorphic belt contains three fault-bounded sequences of: (1) Upper Jurassic or older argillite, flysch, and sparse andesite and marble; (2) phyllite and metagraywacke; and (3) pelitic schist and amphibolite. Protolith for Maclaren Glacier metamorphic belt may be older part of Kahiltna ovrlap assemblage. Progressively metamorphosed from lower greenschist facies to upper amphibolite facies. Higher metamorphic-grade units, adjacent to East Susitna batholith, structurally overlie lower-grade units to south. East Susitna batholith (unit TKpi) chiefly mid-Cretaceous to early Tertiary regionally metamorphosed and penetratively deformed granitic plutonic rocks. Maclaren terrane also includes Nenana terrane of Jones and others (1981, 1987). Maclaren terrane thrust over Wrangellia terrane to south and interpreted as a offset fragment of Cretaceous continental-margin arc formed in southeastern Alaska and southwestern Yukon Territory (Kluane Schist and Ruby Range batholith). REFERENCES: Smith, 1981; Nokleberg and others, 1985, 1989a, 1992; Csejtey and others, 1992; Davidson and others, 1992
MN Minchumina terrane (passive continental margin)ÑOccurs in central Alaska and consists of a complexly folded assemblage of: (1) Cambrian(?) limestone, dolomite, phyllite, and argillite; (2) Ordovician argillite, quartzite, grit, and chert; (3) Ordovician chert, argillite, shaley limestone, and siliceous siltstone; and (4) Middle to Upper Devonian limestone and dolomite. Minchumina terrane interpreted by R.B. Blodgett, T.K. Bundtzen, and J.B. Decker, Jr., written commun. (1991), along with Mystic, and Nixon Fork terranes, as part of the composite Farewell terrane. Minchumina terrane designated as separate terrane by Patton and others (1989) and in this study, and interpreted as a sequence of deep-water deposits that formed along a continental margin adjacent to the lower Paleozoic platform carbonate rocks of the Nixon Fork terrane to west. Both terranes interpreted as displaced fragments of the Paleozoic and early Mesozoic continental margin of North American. REFERENCES: Jones and others, 1981; Patton and others, 1989; R.B. Blodgett, T.K. Bundtzen, and J.B. Decker, Jr., written commun., 1991
MNK Minook terrane (turbidite basin)ÑOccurs in east-central Alaska and consists chiefly of flysch composed conglomerate, sandstone, siltstone, and shale. Conglomerate consists mainly of chert pebble and lesser quartz, slate, and sandstone fragments in a sandy and locally calcareous matrix. Shales contain rare thin argillaceous sandstone and siltstone layers. Early Permian(?) megafossils. Moderately to intensely folded and occurs as east-northeast-trending lens structurally adjacent to Ruby, Livengood, and Manley terranes. Origin of Minook terrane uncertain; may be a fragment of the Nixon Fork, Angayucham, or Tozitna terranes. REFERENCES: Chapman and others, 1971; Jones and others, 1981; Dover, 1990; Grantz and others, 1991
MY Mystic terrane (passive continental margin)ÑOccurs in central Alaska, mainly south of Denali fault and consists of a complexly deformed, but partly coherent, long-lived stratigraphic succession of: (1) Ordovician graptolitic shale and associated(?) pillow basalt; (2) Silurian massive limestone; (3) Upper Devonian sandstone, shale, conglomerate, and limestone (informally-named Yentna limestone of Fernette and Cleveland (1984)); (4) uppermost Devonian to Pennsylvanian radiolarian chert; (5) Pennsylvanian siltstone, sandstone, and conglomerate; (6) Pennsylvanian(?) and Permian flysch, chert, argillite, and conglomerate (locally plant bearing); and (7) Triassic(?) pillow basalt and gabbro. Terrane structurally and possibly locally stratigraphically underlain by rocks of Dillinger terrane. Pre-Devonian rocks assigned in part to Dillinger terrane (Gilbert and Bundtzen, 1984; Blodgett and Gilbert, 1992). Mystic terrane interpreted by R.B. Blodgett, T.K. Bundtzen, and J.B. Decker, Jr. (written commun., 1991), along with Dillinger, Minchumina, and Nixon Fork terranes, as part of the composite Farewell terrane. Mystic terrane interpreted as a displaced fragment of the Paleozoic and early Mesozoic continental margin of North America. REFERENCES: Reed and Nelson, 1980; Gilbert and Bundtzen, 1984; Blodgett and Gilbert, 1992; R.B. Blodgett, T.K. Bundtzen, and J.B. Decker, Jr., written commun., 1991
NX Nixon Fork terrane (passive continental margin)ÑOccurs in several fault-bounded fragments in central and west-central Alaska and consists of a long-lived stratigraphic succession of: (1) Late Precambrian basement of mainly pelitic and calcareous schists with minor marble, quartzite, and felsic metavolcanic rocks; (2) overlying unmetamorphosed sedimentary rocks consisting of dolograinstone and laminated dolomudstone; (3) overlying unmetamorphosed sedimentary rocks of probable Late Proterozoic age consisting of two units (interbedded sandstone, siltstone, and carbonate), and two dolostone units (Babcock and others, 1993); (4) Middle Cambrian limestone and chert; (5) Upper Cambrian to Lower Ordovician platy limestone (possibly part lower part of Novi Mountain Formation); (6) Lower Ordovician thick and platy limestone (Novi Mountain Formation); (7) Ordovician to Middle Devonian carbonate platform rocks (Telsitna, Paradise Fork, and Whirlwind Creek Formations, and Cheeneetnuk Limestone (R.B. Blodgett, written commun., 1993); and (7) Permian, Triassic, and Cretaceous fossiliferous sedimentary rocks, mainly calcareous sandstone, siltstone, conglomerate, and sparse chert. Proterozoic stratified rocks intruded by Middle Proterozoic metagranitic rocks with U-Pb zircon age of 1.27 Ga, and capped by Late Proterozoic metavolcanic rocks with U-Pb zircon age of 850 Ma. Middle Paleozoic strata grade laterally into deep-water strata equivalent to Dillinger terrane. Permian strata contain clasts of basement rocks.

Nixon Fork terrane is stratigraphically overlain by unnamed Triassic and Lower Cretaceous clastic rocks and by Upper Cretaceous Kuskokwim Group, and locally structurally overlain by Angayucham terrane. Nixon Fork terrane interpreted by R.B. Blodgett, T.K. Bundtzen, and J.B. Decker, Jr., written commun. (1991), along with Dillinger, Minchumina, and Mystic terranes, as part of the larger, composite Farewell terrane. Nixon Fork terrane is designated as separate terrane by Patton and others (1989) and in this study; interpreted as one of several displaced fragments of the Paleozoic and early Mesozoic continental margin of North America. REFERENCES: Patton and others, 1980, 1989, Blodgett and Gilbert, 1983; Blodgett and Clough, 1985; Clough and Blodgett, 1985; Palmer and others, 1985; R.B. Blodgett, T.K. Bundtzen, and J.B. Decker, Jr. written commun., 1991; Babcock and Blodgett, 1992


NY Nyac terrane (island arc)ÑOccurs in southwest Alaska and consists chiefly of andesite and basalt flows, breccia, tuff, and interbedded shallow-marine volcaniclastic rocks with Middle and Late Jurassic (Bajocian and Tithonian) fossils. Terrane intruded by Early Cretaceous gabbroic and granitic rocks and depositionally overlain by Late Cretaceous and early Tertiary volcanic rocks of the Kuskokwim Mountains igneous belt. Nyac and nearby Koyukuk and Togiak terranes are interpreted as a group of several broadly coeval Jurassic and Early Cretaceous island arc terranes. REFERENCES: Hoare and Coonrad, 1978; Box, 1985a, b; Box and others, 1992
PC Porcupine terrane (passive continental margin)ÑOccurs in northern east-central Alaska and consists of a long-lived stratigraphic succession of: (1) Precambrian (Proterozoic?) phyllite, slate, quartzite, marble, greenstone (metabasalt) rocks; (2) thick, structurally and stratigraphically complex assemblage of Cambrian(?) to Upper Devonian shallow-water limestone, dolomite, and minor chert minor shale (including Salmontrout Limestone); (3) upper Paleozoic sandstone, siltstone, argillite, massive quartzite, conglomerate, and minor limestone; and (4) Permian Step Conglomerate. Jurassic ammonite-bearing strata occur in two places in the northern part of the terrane. Terrane thrust over Kandik River assemblage to southeast, locally overthrust by Angayucham terrane, and depositionally overlain by now faulted Kandik River assemblage. Porcupine terrane interpreted as a displaced fragment of the Paleozoic and Mesozoic continental margin of North America. REFERENCES: BrosgŽ and Reiser, 1969; Imlay and Detterman, 1973; Howell and Wiley, 1987; Howell and others, 1992; Lane and Ormistron, 1979
PN Pingston terrane (turbidite basin)ÑOccurs in discontinuous, narrow, fault-bounded lenses along Denali fault in central Alaska Range. Chiefly a weakly metamorphosed, strongly folded, and faulted sequence of: (1) Early Pennsylvanian and Permian phyllite, minor marble, and chert; (2) Upper Triassic thin-bedded, laminated dark limestone, black sooty shale, calcareous siltstone, and minor quartzite; and (3) locally numerous bodies of post-Late Triassic gabbro, diabase, and diorite. Terrane contains a single slaty cleavage that parallels axial planes of locally abundant isoclinal folds. Terrane also occurs in a small lens of thin-bedded dark limestone, sooty shale, and minor quartzite in eastern Alaska Range (W.J. Nokleberg and D.H. Richter, unpub. data, 1987). Most widespread, Upper Triassic part of terrane interpreted as a turbidite-apron sequence deposited from deep-water turbidity currents that flowed from a cratonal source, such as the Yukon-Tanana terrane to the north, onto upper Paleozoic continental slope-rise deposits. REFERENCES: Jones and others, 1981, 1982; Gilbert and others, 1984
PW Prince William terrane (accretionary wedge - dominantly turbidites)ÑOccurs in southern Alaska and consists chiefly of a complexly folded and faulted, thick assemblage of graywacke, argillite, minor conglomerate, pillow basalt, and basaltic tuff, sills, and dikes, and ultramafic rocks (Orca Group, Sitkalidak Formation, and Ghost Rocks Formation). Includes Late Cretaceous and early Tertiary Ghost Rocks terrane of Jones and others (1987). Rare mega- and microfossils of Paleocene and Eocene age

Terrane bounded to north by Contact fault and Chugach terrane, to south by Aleutian megathrust or late Cenozoic accretionary prism, and to east by Yakutat terrane. In central and western parts of Prince William Sound, large areas occur without fossil control on either side of Contact fault that in this area may be a gradational accretionary zone. Sandstone petrologic studies indicate local gradational contact between Prince William and Chugach terranes (Dumoulin, 1987, 1988). In eastern part of Prince William Sound, east of Valdez and Cordova, singly deformed Orca Group of Prince William terrane is thrust under doubly-deformed and metamorphosed rocks of Valdez Group of southern Chugach terrane (Plafker and others, 1989b).



One early Tertiary granitic pluton of Sanak-Baranof plutonic belt (50-65 Ma) and local adjacent amphibolite facies metamorphism welds the northern Prince William terrane to the southern Chugach terrane. Early Tertiary zeolite to greenschist facies metamorphism. Local ophiolite exposed on Resurrection Peninsula and on Knight Island; interpreted as slabs of oceanic basement. Prince William terrane overlain by assemblages of marine clastic rocks and local continental coal-bearing rocks of post-middle Eocene age. Prince William terrane interpreted as early Tertiary oceanic lithosphere, seamounts, and deep-sea fan assemblage that was tectonically imbricated into an extensive accretionary wedge complex. REFERENCES: Tysdal and Case (1979), Moore and others, 1983; Nelson and others, 1985; Dumoulin, 1987, 1988; Plafker and others, 1989b; Nelson, 1992
RB Ruby terrane (metamorphosed continental margin)ÑOccurs in northern west-central Alaska and consists chiefly of a structurally complex assemblage of quartz-mica schist, quartzite, calcareous schist, mafic greenschist (metabasalt), quartz-feldspar schist and gneiss, and marble. Sparse Silurian and Devonian fossils occur in metasedimentary rocks. Local granitic gneiss with Devonian and Early Cretaceous U-Pb zircon age. Generally metamorphosed to greenschist facies and local amphibolite faces, with sparse relict blueschist facies metamorphic minerals. Ruby terrane is overthrust by Angayucham terrane, is unconformably overlain by Lower Cretaceous sedimentary rocks of Yukon-Koyukuk basin and by Upper Cretaceous Kuskokwim Group, and is extensively intruded by mid-Cretaceous granitic plutons. Ruby terrane interpreted as a highly metamorphosed and deformed, displaced fragment of the North American craton margin. REFERENCES: Patton and others, 1987, 1989
SD Seward terrane (metamorphosed continental margin)ÑOccurs on Seward Peninsula in western Alaska and consists chiefly of a structurally complex assemblage of mica schist, micaceous calc-schist, metavolcanic rocks, and carbonate rocks. Consists of informally-named Nome Group composed of pelitic schist, marble, quartz-graphite schist, and mafic schist (rift-related metabasalt, calc-schist, and chlorite-albite schist). Metasedimentary rocks contain Cambrian through Silurian conodonts and megafossils. Part of terrane may be Proterozoic in age. Associated carbonate rocks occur in poorly-exposed area in eastern part of terrane and range in age from Lower Cambrian to Middle Devonian. Regionally metamorphosed at blueschist facies, and retrograded to greenschist facies. Penetratively deformed. Terrane includes a small pluton (informally named Kiwalik Mountain orthogneiss of A.B. Till and J.A. Dumoulin, written commun., 1991) that yields U-Pb zircon age of 381 Ma. Central part of terrane consists mainly of Kigluaik Group that contains: (1) upper unit of pelitic and calcareous schist, marble, and lesser amphibolite and quartz-graphite schist metamorphosed up to upper amphibolite facies, and interpreted as a higher-grade counterpart of the Nome Group; and (2) lower unit of marble gneiss, mixed gneiss, and gneissic granitic rocks, metamorphosed up to granulite facies. Locally intruded by large Cretaceous plutons. Correlated with Coldfoot terrane to northeast. Seward terrane interpreted as a highly metamorphosed and deformed, displaced fragment of the North American craton margin. REFERENCES: Armstrong and others, 1986; Hudson, 1977; A.B. Till and J.A. Dumoulin, written commun., 1991; Patton and others, 1992
SM Seventymile terrane (subduction zone - dominantly oceanic rocks)ÑOccurs in east-central Alaska as scattered remnants of three highly-deformed and locally folded thrust sheets. Structurally overlies Yukon-Tanana and Stikinia terranes.

Lower thrust sheet composed chiefly structural melange of undated metasandstone, metagraywacke, and metaconglomerate, and metaandesite. Pervasively metamorphosed to lower greenschist facies in Mesozoic time. Lower thrust sheet interpreted as a fragment of an island arc.

Middle thrust sheet composed chiefly structural melange of pillow basalt, basalt, mafic tuff, chert, argillite, and limestone. Cherts and limestones contain Mississippian, Permian, and Late Triassic radiolarians or conodonts. Locally, limestone contains assemblage of giant Parafusulina, indicating tropical depositional environment (C.S. Stevens, written commun. to D.G. Howell, 1990) that also occurs in similar oceanic and (or) ophiolite terranes in the Canadian Cordillera, Klamath Mountains, and northwest Mexico. Pervasively metamorphosed to lower greenschist facies in Mesozoic time. Sparse relict glaucophane in pillow basalt. Middle thrust sheep interpreted as subduction zone melange of late Paleozoic and early Mesozoic oceanic crust and sea mounts.

Upper thrust sheet composed chiefly harzburgite, peridotite, and minor clinopyroxenite, and gabbro, and diabase with local amphibolite sole. Upper thrust sheet interpreted as possible roots to a Jurassic(?) island arc. Lower and middle thrust sheets pervasively metamorphosed to lower greenschist facies in Mesozoic time. REFERENCES: Keith and others, 1981; Foster and others, 1987


ST(?) Stikinia(?) terrane - east-central Alaska (island arc)Ñ-Occurs in eastern east-central Alaska and consists chiefly of metasedimentary rocks intruded by large Late Triassic to Early Jurassic granitic plutons of Taylor Mountain batholith. Metasedimentary rocks mainly biotite-hornblende gneiss, marble, amphibolite, quartzite, pelitic schist, quartz-feldspar schist, and metachert. Protoliths of metasedimentary rocks interpreted as forming in an oceanic or marginal basin. Stikinia(?) terrane intensely deformed. and metamorphosed to amphibolite and epidote-amphibolite facies at high temperatures and pressures, with local kyanite, during Early to Middle Jurassic. Structurally overlies Yukon-Tanana terrane and is structurally overlain by Seventymile terrane. Stikinia(?) terrane interpreted as part of an extensive early Mesozoic oceanic igneous arc. REFERENCES: Foster and others, 1987; Dusel-Bacon and Hansen, 1992
ST Stikinia terrane - southeastern Alaska (island arc)ÑOccurs in southern southeastern Alaska and consists of: (1) basal sequence consists of Late Proterozoic(?), Devonian(?), Carboniferous, and Permian andesitic, basaltic, and minor rhyolitic volcanic and volcaniclastic rocks with interbedded marine limestone and clastic sedimentary rocks; and (2) Upper Triassic to Upper Jurassic marine sedimentary clastic rocks, minor limestone, and various volcanic rocks (Hazelton and Stuhini Formations of Souther (1971) and Anderson (1989) that are locally intruded by Late Triassic and Early Jurassic granitic rocks, and by middle Tertiary intermediate plutonic rocks. Stikinia terrane is interpreted as occurring structurally or stratigraphically above Yukon-Tanana terrane. Stikinia terrane interpreted as part of an extensive early Mesozoic oceanic igneous arc. REFERENCES: Monger and Berg, 1987; Monger and others, 1982; Bevier and Anderson, 1991; Evenchick, 1991
SU Susitna terrane (sea mount)ÑOccurs in northern part of the Talkeetna Mountains and consists of thick piles of pillow basalt, deep-marine tuffaceous sedimentary rocks, sandstone, and tuff. Late Triassic (Norian) Monotis subcircularis and Heterastridium sp. locally abundant in argillite interbedded with volcanic rocks. Terrane forms rootless nappe in highly-deformed Mesozoic flysch of the Kahiltna overlap assemblage. The upper contact of basalt with flysch originally may have been depositional, but relations now obscured by subsequent shearing along the contact. Susitna terrane interpreted as possible fragment of an oceanic seamount and (or) fragment of the Peninsular terrane, possibly lithologically equivalent to the Cottonwood Bay or Chilikadrotna Greenstones, that was tectonically decoupled from its basement and faulted into the Kahiltna overlap assemblage of flysch during the mid- or Late Cretaceous. REFERENCES: Jones and others, 1980, 1982
SW Southern Wrangellia terrane (island arc)ÑOccurs in southern Alaska along southern margin of Wrangellia terrane and consists mainly of two sequences: (1). Older sequence composed of a fault-bounded assemblage of generally highly-deformed and metamorphosed sedimentary and volcanic rocks of late Paleozoic and older age (stratified part of metamorphic complex of Gulkana River and Strelna Metamorphics of Plafker and others (1989b)). Metasedimentary and metavolcanic rocks mainly chlorite schist derived from andesite, metabasalt, metatuff, calc-schist, pelitic schist, metachert, and marble that locally contains Early Pennsylvanian conodonts. (2) Younger sequence composed of metamorphosed and deformed late Paleozoic and Late Jurassic metaplutonic rocks (plutonic part of metamorphic complex of Gulkana River and Uranatina River metaplutonic unit and Chitina Valley batholith of Plafker and others (1989b)). Paleozoic metaplutonic rocks, with U-Pb ages of 308 to 312 Ma, consist of schistose hornblende diorite and gabbro, metagranodiorite, metagranite, and orthogneiss. Late Jurassic metaplutonic rocks, with an U-Pb zircon age of 153 Ma, consist mainly of hornblende diorite and tonalite. Southern Wrangellia terrane generally intensely deformed and regionally metamorphosed from greenschist facies to amphibolite facies near metaplutonic rocks. Local mylonite zones occur adjacent to, and within syntectonic Late Jurassic metaplutonic rocks.

Southern Wrangellia terrane locally structurally overlain or underlain by greenstone and Upper Triassic carbonate sequence inferred to be equivalent of Upper Triassic strata that characterize the Nikolai Greenstone, and Chitistone and Nizina Limestones of the Wrangellia terrane. Late Paleozoic part of terrane interpreted as part of late Paleozoic Skolai arc of Wrangellia terrane. Late Jurassic metaplutonic rocks constitute part of Chitina arc. Southern Wrangellia terrane locally occurs as moderate-size klippen thrust onto northern Chugach terrane during mid-Cretaceous faulting along Border Ranges fault. Southern Wrangellia terrane interpreted either as: (1) a deeper and more metamorphosed equivalent of the Wrangellia terrane to the north; or (2) as a distinct terrane structurally juxtaposed between the Wrangellia terrane to north and Peninsular terrane to south. Equivalent to Strelna terrane of Grantz and others (1991). REFERENCES: Plafker and others, 1989b; Nokleberg and others, 1989b; Grantz and others, 1991


TG Togiak terrane (island arc)ÑOccurs in southwestern Alaska and consists of two major sequences: (1) lower ophiolite sequence of Late Triassic mid-ocean-ridge pillow basalt, diabase, gabbro, and ultramafic rocks at southwestern end of unit; and (2) coherent upper stratigraphic sequence of Lower Jurassic to Lower Cretaceous marine volcaniclastic sandstone, conglomerate, shale, tuffaceous chert, and minor argillaceous limestone, and marine to non-marine andesite and basalt flows and flow breccia, and tuff. Depositionally overlain by Upper Cretaceous marine sedimentary rocks of Kuskokwim Group, and by volcanic and plutonic rocks of Kuskokwim Mountains igneous belt. Togiak terrane may depositionally overly Tikchik terrane at northeastern end of unit. Togiak and nearby Koyukuk and Nyac terranes interpreted as a group of broadly coeval Jurassic and Early Cretaceous island arc terranes. REFERENCES: Hoare and Coonrad, 1978; Box, 1985a, b; Box and others, 1992
TK Tikchik terrane (island arc)ÑOccurs in southwestern Alaska as structurally complex assemblage of andesite, dacite, and rhyolite volcanic rocks, volcaniclastic rocks, chert, tuffaceous chert, chert-clast sandstone, limestone, dolomite, and argillite that contain microfossils of pre-Late Devonian, Devonian, Mississippian, Permian, and Triassic age. Tikchik terrane depositionally overlain by Upper Cretaceous marine sedimentary rocks of Kuskokwim Group, and intruded by Late Cretaceous to early Tertiary plutonic rocks of Kuskokwim Mountains igneous belt. Tikchik terrane interpreted as possible depositional basement to Togiak (island arc) terrane. REFERENCES: Hoare and Coonrad, 1978; Hoare and Jones, 1981; Box, 1985a, b; Box and others, 1992
UM Ultramafic and associated rocks (oceanic crust?)ÑOccurs as narrow, fault-bounded, near-vertical prisms and sparse klippen discontinuously exposed for several hundred kilometers along the Denali fault in the eastern and central Alaska Range. Ultramafic rocks (Mesozoic?) mainly fine- to medium-grained pyroxenite and peridotite, and fine-grained dunite and are largely altered to serpentinite and highly sheared. Associated rocks are amphibolite, hornblende-plagioclase gneiss, marble derived from calcareous sedimentary rocks, and weakly metamorphosed hornblende gabbro, tonalite, and granite. Amphibolite and gneiss enclose ultramafic rocks. Intense strong schistosity that subparallels contacts and enclosing faults. Tonalite and granite intrude ultramafic and associated rocks; form small elongate plutons with moderate schistosity that subparallels contacts. Terrane of ultramafic and associated rocks interpreted as crustal-suture belt composed of fragments of either oceanic lithosphere, an island arc, or less likely deep-level continental lithosphere. Terrane may be derived in part from basement of terranes juxtaposed along the Denali fault. REFERENCES: Richter, 1976; Nokleberg and others, 1985, 1989a, 1992; Csejtey and others, 1992; Patton and others, 1992
WF West Fork terrane (turbidite basin)ÑOccurs in central Alaska Range and consists of two sequences: (1) Upper sequence of chert, argillite, and sandstone ranging in age from Early to Late Jurassic; and (2) lower sequence of crystal tuff and volcaniclastic sandstone and argillite that contains an Early Jurassic sandy conglomerate. Terrane occurs structurally below Chulitna terrane to west and is faulted against Broad Pass terrane to east. One of several mini-terranes enclosed in highly deformed flysch of the Kahiltna overlap assemblage. West Fork terrane interpreted as fragment of a turbidite basin or submarine fan formed adjacent to Jurassic volcanic arc. REFERENCES: Jones and others, 1982; Csejtey and others, 1992
WM White Mountains terrane (passive continental margin)ÑOccurs as one of several narrow east-northeast trending terranes in east-central Alaska and consists of three units: (1) Ordovician phyllite, slate, chert, and limestone overlain by pillow basalt, basaltic aquagene tuff, and mafic volcanic breccia (Fossil Creek Volcanics) with local volcaniclastic conglomerate containing granitic clasts dated at 560 Ma; (2) Silurian limestone and dolomite (Tolovana Limestone); and (3) undated gray, vitreous quartzite with argillite layers and Mississippian diorite and gabbro sills (Globe quartzite unit of Weber and others (1992)). Terrane moderately to intensely-deformed into east-northeast-trending upright folds and parallel high-angle faults and is similar to upper part of Crazy Mountains terrane to north across Tintina fault. White Mountains terrane structurally overlies Manley terrane to northwest and structurally underlies Wickersham terrane to southeast. White Mountains terrane interpreted as displaced fragment of the early Paleozoic continental margin of North America Cordillera. REFERENCES: Churkin and others, 1982; Weber and others, 1985, 1988; Dover, 1990; Aleinikoff and Plafker, 1989; Grantz and others, 1991



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