TECTONICS, VOL. 6, NO. 2, PAGES 151-173, APRIL 1987

GEOLOGIC FRAMEWORK, TECTONIC EVOLUTION, AND DISPLACEMENT HISTORY OF THE ALEXANDER TERRANE GeorgeE. Gehrels1 and Jason B. Saleeby

Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena

Abstract. The Alexander terrane consists of Devonian (Klakas orogeny). The second phase is upper Proterozoic(?)-Cambrian through marked by Middle Devonian through Lower Middle(?) Jurassic rocks that underlie much of Permian strata which accumulated in southeastern (SE) Alaska and parts of eastern tectonically stable marine environments. Alaska, western British Columbia, and Devonian and Lower Permian volcanic rocks and southwestern Yukon Territory. A variety of upper Pennsylvanian-Lower Permian syenitic to geologic, paleomagnetic, and paleontologic dioritic intrusive bodies occur locally but do not evidence indicates that these rocks have been appear to represent major magmatic systems. displaced considerable distances from their The third phase is marked by Triassic volcanic sites of origin and were not accreted to western and sedimentary rocks which are interpreted to North America until Late Cretaceous-early have formed in a rift environment. Previous Tertiary time. Our geologic and U-Pb syntheses of the displacement history of the geochronologic studies in southern SE Alaska terrane emphasized apparent similarities with and the work of others to the north indicate rocks in the Sierra-Klamath region and that the terrane evolved through three distinct suggested that the Alexander terrane evolved in tectonic phases. During the initial phase, from proximity to the California continental margin late Proterozoic(?)-Cambrian through Early during Paleozoic time. Our studies indicate, Devonian time, the terrane probably evolved however, that the geologic record of the along a convergent plate margin. Arc-type(?) Alexander terrane is quite different from that volcanism and plutonism occurred during late in the Sierra-Klamath region, and we conclude Proterozoic(?)-Cambrian and Ordovician-Early that the two regions were not closely associated Silurian time, with orogenic events during the during Paleozoic time. The available geologic, Middle Cambrian-Early Ordovician (Wales paleomagnetic, and paleontologic data are more orogeny) and the middle Silurian-earliest consistent with a scenario involving (1) early Paleozoic origin and evolution of the Alexander terrane along the paleo-Pacific margin of Gondwana, (2) rifting from this margin during 1 Now at Department of Geosciences, Devonian time, (3) late Paleozoic migration University of Arizona, Tucson across the paleo-Pacific basin in low southerly paleolatitudes, (4) residence in proximity to the Copyright 1987 paleo-Pacific margin of South America during by the American Geophysical Union. latest Paleozoic(?)-Triassic time, and (5) Late Permian(?)-Triassic rifting followed by Paper number 6T0694. northward displacement along the eastern 0Z78-7407/87/006T-0694510.00 margin of the Pacific basin. 152 Gehrels and Saleeby: Alexander Terrame

eastern Alaska, Yukon Territory, and British Columbia, and part of the coastal region of west-central British Columbia (Figure 1). The terrane is anomalous in the North American Cordillera in that it has an exceptionally long and complete geologic record: Rocks are known from every Paleozoic epoch except the Late Permian and perhaps the Early Cambrian. In addition, rocks of late Proterozoic(?)-Cambrian, Late Triassic, and Middle(?) Jurassic age occur locally. Upper Jurassic to mid-Cretaceous strata and Cretaceous intrusive bodies overlie ¾ BRITISH and intrude rocks belonging to the Alexander k\COLUMBIA and adjacent terranes. Rocks of the Alexander terrane were

dUNEAU initially described as a distinct geologic assemblageby Schuchert (1923), who suggested that they belong to a geosynclinal system (the "Alexandrian embayment") which was isolated ,IRF(?) from the main Cordilleran geosyncline. Based on their occurrence outboard of the Cordilleran miogeocline, Wilson (1968) hypothesized that CHATHAM STRAIT these rocks constitute a distinct tectonic F A U LT fragment which is exotic to North America. Berg et al. (1972) named this fragment the Alexander terrane, described its primary

! geologic components, and delineated its boundaries with adjacent terranes. IOO km We have conducted detailed field and U-Pb ! (zircon) geochronologicstudies in southern SE •] CRAIGSUBTERRANE Alaska and have reviewed the work of others to the north and south in an effort to reconstruct • ADMIRALTYSUBTERRANE the geologic and tectonic evolution of the •i• dURASSICPOST-MIDDLE STRATA Alexander terrane. In this report, we describe its geologic framework and tectonic evolution POST-dURASSICMIDDLE PLUTONI and discuss constraints and speculations on its Fig. 1. Location map of the Alexander terrane displacement history. showing regions and features referred to in the text. Adapted from Monger and Berg (1984), GEOLOGIC FRAMEWORK Tipper et al. (1981), Yorath and Chase (1981), MacKevett (1978), Campbell and Dodds (198Za, Overview b, c, 1985a, b), and Gehrels and Berg (1984). The Alexander terrane was subdivided by Location Map: TF, Totschunda fault; HF, Berg et al. (1978) and Monger and Berg (1984) Hubbard fault; BRF, Border Ranges fault; FF, into the Craig, Annette, and Admiralty Fairweather fault, DRF, Duke River fault. GB, subterranes based on apparent differences in Glacier Bay; C, Chichagof Island, AD, their geologic records. Rocks in most regions Admiralty Island; KU, Kuiu Island; KP, belong to the Craig subterrane, which consists Kupreanof Island; POW, Prince of Wales Island; of upper Proterozoic(?)-Cambrianto Middle(?) G, Gravina Island; A, Annette Island; Dk, Duke Jurassic rocks. Annette, Gravina, and Duke Island; Da, Dall Island; L, Long Island; Dn, islands and regions to the southeast have been Dundas Island; CF, Cape Fox area; QC, Queen assigned to the Annette subterrane, which is Charlotte Islands. Inset Map: AT, Alexander distinguished from the Craig subterrane based terrane; S-K, Sierra-Klamath region. on apparent dissimilarities in the Devonian and older rocks and on the absence of upper INTRODUCTION Paleozoic strata. Our mapping demonstrates, however, that Devonian and older rocks in the The Alexander terrane is a tectonic two subterranes are quite similar and that upper fragment that underlies much of southeastern Paleozoic strata are also absent in adjacent (SE) Alaska, the Saint Elias Mountains of parts of the Craig subterrane (Gehrels et al., Gehrels and Saleeby: Alexander Terrane 153

1987). In addition, mapping by Woodsworth and Early Ordovician age because (1) Middle-Upper Orchard (1985) in the Dundas Island region Cambrian metaplutonic rocks have experienced (Figure 1) indicates that upper Paleozoic strata the deformation and metamorphism, (2) less- do occur locally in the Annette subterrane. We deformed Middle Ordovician-Lower Silurian therefore assign the Triassic and older rocks in plutons intrude the Cambrian metaplutonic the Annette, Gravina, Duke, and Dundas islands rocks, (3) less-deformed upper Lower and region to the Craig subterrane and recommend Middle Ordovician strata belonging to the that the term "Annette subterrane" be Descon Formation occur near and probably abandoned. overlie rocks of the Wales suite, and (4) The Admiralty subterrane (Figure 1) is metamorphic minerals in the Wales suite yield a distinguished from the Craig subterrane on the K-At isochron date of approximately 483 Ma basis of apparent differences in their (Early Ordovician) (Turner et al., 1977). Carboniferous and older rocks. Similarities in Ordovician to middle Lower Silurian rocks. Permian and Triassic strata of the Admiralty Much of the southern Alexander terrane is and Craig subterranes indicate that the two underlain by middle(?) Lower Ordovician to subterranes have been closely associated since middle Lower Silurian strata belonging to or at least Early Permian time (Berg et al., correlative with the Descon Formation. These 1978). Discussion of the geology of the rocks include basaltic-andesitic pillow flows and Admiralty subterrane and comparison with the breccia, rhyolitic-dacitic tuff and breccia, geology of other regions are hampered by the mudstone and graywacke turbidites, and lack of detailed geologic studies in pre-Permian subordinate limestone, conglomerate, and rocks of the Admiralty subterrane. Because of shale. The volcanic rocks were apparently this lack of information, the following erupted from submarine volcanic centers, and discussion focuses on rocks belonging to the volcaniclastic strata were probably deposited in Craig subterrane. basins adjacent to these centers. Although the SouthernAlexander Terrane (FiguresZ and 3) basal contact of the Descon Formation has Pre-Middle Ordovician metamorphic rocks. nowhere been identified, we suspect that it was The oldest rocks recognized in the Alexander originally an unconformity based on the greater terrane consist of greenschist- and locally degree of deformation and metamorphism of amphibolite-facies metavolcanic and the Wales suite, and the occurrence of a thick metasedimentary rocks of the Wales sedimentary breccia at the base(?) of the metamorphic suite (Buddingtonand Chapin, Descon Formation on southern Prince of Wales 1929; Gehrels and Saleeby, 1987). These rocks Island (Herreid et al., 1978). were derived from basaltic to andesitic pillow A regionally extensive intrusive suite was flows, breccia, and tuff; rhyolitic breccia and eraplaced in the southern Alexander terrane tuff; and graywacke, mudstone, and thick during deposition of the Descon Formation. limestone layers. Most rocks have a foliation Major rock types include hornblende diorite; and lineation along which protolith features are biotite-hornblende granodiorite, strongly flattened, isoclinally folded, and leucogranodiorite, and quartz diorite; elongated. Late- to post-metamorphic hornblende-biotite quartz monzonite; structures include (1) shallow-plunging upright subordinate hornblende gabbro, biotite granite folds with wavelengths up to several kilometers, and quartz syenite; and minor hornblendite and (Z) outcrop-scale asymmetric folds that are clinopyroxenite. Our U-Pb geochronologic data apparentlyparasitic to the upright folds, and (3) indicate that most diorite and granodiorite steeply plunging kilometer-scale folds which bodies (unit Odg on Figure Z) are Middle deform the other fold sets. Ordovician in age and that quartz diorite, Rocks of the Wales suite are locally intruded quartz monzonite, granite, and quartz syenite by small bodies of penetratively deformed bodies (unit OSq on Figure Z) are Late metadiorite and metagranodiorite which yield Ordovician-Early Silurian in age. These U-Pb (zircon) apparent ages of 540 to 5Z0 Ma intrusive rocks are interpreted to be genetically (Middle-Late Cambrian) (Gehrels et al., 1987; related to basaltic-rhyolitic rocks of the Descon J.B. Saleeby, unpublisheddata, 1984). This Formation based on their similarity in minimum age constraint plus our interpretation composition and age and on observed gradations that the dioritic-granodioritic metaplutonic from intrusive bodies, through swarms of dikes rocks are genetically related to the basaltic(?)- and hypabyssal bodies, into the volcanic rocks rhyolitic(?) metavolcanic rocks suggest that (Gehrels and Saleeby, 1986, 1987). protoliths of the Wales suite are Cambrian and On east-central Prince of Wales Island, perhaps late Proterozoic in age. Middle Ordovician-Lower Silurian diorite and Deformation and metamorphism of rocks in quartz diorite locally intrude Cambrian the Wales suite is assigned a Middle Cambrian- metaplutonic rocks (Gehrels et al., 1987). This 154 Gehrels and Saleeby: Alexander Terrane

ß

o E •-• \"...'.,•2.•.:...•,.:o "' • /J,• •' F--•-., •.., 2: .... ' ''.' "'.;'..' ..' • • ::::) ,•...... •., :. .:..•,•• •

• .. .•' . ';' .i '.:" o •' 0 0 -: •'- ".'...•• -'...•::.-•"'' " ,,' • • -'-':"-'":•'"' ß .....•..-i. 1-•.. t'•"':••'•,> .... k- • ,,,":'. . ß •.. .//"•'•.. ...(,..,-.:f,.•..f..;•..,•.•,....,;.•. o : ...•..:" o .... ':-,.• .. •'.-,••-.:;'..ß ; ß E \-.,,'' •.,6•v'0 ' I ] ] ø ,-, •.-•, .•" •i.,..•-•," • - ß-':•.••,-'...... ,.•',,,0•. • ,• 0 o_/ o/ 0 . ß . •, o ...... ,,, •, ,/'•,•, o _-- ..'...., ß •....:. o • !. .;. .: ... . ß.... .'..'.. - ß ß :. '. , . (.f) .... :' .: ß..'." ::ß .."' ,.i,•• ."': .•...... ß ' ¾'.' (n• 'r:.o ...... • ._...: .• '.;.• o o',:, ,,, ...::•••,, . . ..,_• •-- •r• ß ,,,'\ . i.•.•,....•,•,• ...... o :: .• I .....:...... •...--'.'.•.'.:... '...... • o ...-" ...... •..."...... ß"'...... • :."} _•_•-o O-o " 03 .' O 113::,o I ,1•:..... ':i' "'...."•. " ...."/ t.' '.'.•. •t:'• •.."."' ;,•'•".';,; .,-[-z'_•,•....." ' •'• •'.::• '• •:..,•'i',,'• '""' '::•:.:L.'.'i.½ • •',•'•' " ' * • '__:!;.-• •...-•..•_:_'-z• • õ ..;:•.':: ..:,•'t '• •d, -Z'-'Ft•.•.':' \ • iß • .: • .' ß -:.' ..' 03• ....ß 03 +1 _..,,.'...:. • .:. ß . ß o! • r."• • . (n \ O.

•, ß•...•.....';•, •: ":; '""":'' OCF, AN ,I '"-•---Z.•"' ': .'- :i:..ß•c•c •,• <•o,' ...... • Gehrels amd Saleeby: Alexamder Terrame 155

critical intrusive relationship demonstrates that the pre-Middle Ordovician metamorphic complex served as basement to at least part of the Ordovician-Lower Silurian volcanic- plutonic-sedimentary assemblage. Upper Lower and Upper Silurian rocks. Silurian strata conform ably overlie rocks of the Descon Formation on much of north-central Prince of Wales Island and locally on Dall and Long islands. These strata consist of shallow- marine limestone of the Heceta Limestone, thick sections of turbidites belonging to or correlative with the Bay of Pillars Formation, and conglomerate layers and lenses (Muffler, 1967; Eberlein and Churkin, 1970; Eberlein et al., 1983; Brew et al., 1984; Ovenshine and Webster, 1970; G.E. Gehrels, unpublished mapping, 1984). The shallow-marine limestone was apparently deposited on or adjacent to Descon-age volcanic centers after the main phase of magmatic activity, and the turbidites and conglomeratic strata record uplift and erosion of the older volcanic-plutonic- sedimentary complex (Brew et al., 1966; Eberlein et al., 1983). The Silurian strata on north-central Prince of Wales Island pinch out toward the southeast and are not seen in the Annette, Gravina, Duke, and southern Prince of Wales islands region. Rather, to the southeast, large trondhjemite plutons and small leucodiorite bodies (containing aegirine-augite, arfvedsonite, and melanite(?) garnet) are the only upper Lower or Upper Silurian rocks recognized. These intrusive rocks are significantly different in composition and mineralogy from the Ordovician-Lower Silurian intrusive rocks and were eraplaced approximately 15 m.y. after the youngest known member of the earlier suite. Lower Devonian strata. Lower Devonian rocks in the southern Alexander terrame generally consist of fining-upward clastic strata which overlie Silurian and older rocks on a regional unconformity. In most areas these strata are referred to as the Karheen Formation. On southern Prince of Wales Island, Lower Devonian strata include conglomeratic red beds and subordinate sedimentary breccia at the base, siltstone, mudstone, and limestone in the middle, and laminated mudstone and black shale at the top of the section (Gehrels and Saleeby, 1986, 1987). These strata are locally overlain by and interbedded with basaltic- dacitic volcanic rocks. The unconformity at the base of the section combined with facies relations and sedimentary structures suggests that the conglomeratic strata were deposited in topographically rugged, subaerial to shallow- marine environments. Subsidence of the region 156 Gehrels and Saleeby: Alexander Terrane

CENTRALP. O.W. - ANNETTE,GRAVINA, DUKE, • SOUTHERN KUIU ISLANDS RO.W.ISLANDS 208 •_ __" :,•- .__"______• Vv vvv V • V v • , _ vv • v v• •,vv•v v- • •- -•-.•,z•. ENVIRONMENTRIFT(?)

-- , UPLIFT UPLIFT 245

.... i i i t ! . -, - i i i ' . '•- ' m J i •{ m[ i mI J I Iim i'i--•% .- P ,-•... ;.,_'...; .-. '. j ..,,--•-•...... •,,---.-..-..-..-..-•. ::,., :.: ...:.: .:.:•.'.::':'.'.•.'..:.::.': :..: •: :..:' -•'•;-.:::.::.: •?:;!:;< ...... 286 -- ....'"":'"'". •- ''••:• ,• •• - DISANCEr U RB - m ;•CHERT

• LIMESTONE 320 ' ' "'"'"' '""'"'•'•' •' '• r'-

M CLASTIC

STRATA $60 -- -_-••_.-.. :_• '=.z.:..v'r• i i i -- I"' r•rt '--.'•.•7•'•r i i I I I"rI

D -"'-•ANDESITEBASALT TO

-.._-.•-• ._-;= .=.¾..': ;.: •-•_= •--• =.= := =. •=-.-_• =; •x.•-. -".'- '.- ß .-:' '- .-.'..: .,'.: :-.:.-•..:..'-. •.. 408 -- '-• "'-, ',•.'-....•'-•==" '-'-•".'*-;-;-• UPLIFT,.... THRUSTING,(tT;/, - K L A K A S [• DACITERHYOLITE TO ,4j•_,-;t•;=•.' .'.':'.•:' -='.'-.' DEFORMATION, • OROGENY s '¾.'•-'••-'•"-T•, ,' ,' T-P_• ....._...: .•.._..-• v J4: . .•."=. ='.=.-'.-• ^•, •-%--•--•-•'. '.•.: '-=.'_.•v •• 438 •-,..'•/•'__ •METAMORPHISM, • • ROCKSINTRUSIVE i .-....-.._= .•..•.. :-v-• • ,-.'-T"==•• ' '•/•.' •_•, • - '-"-.' =:=-'•. -:_-;: '-..=_.• •,' -._...... '-'.'.-'.' •..•J• ??;y,,.. v o Lc AN

._....'--. =...._ c_..-_.... .•... :?• .=•.. :.-'_ '..'....;....• ..p •L•..• ._'.:..•----.-_._._._.•f:_'...••--• ,'•_•-=-j_ /•x'\'.;,5 .•, / _'! _.• :._:....,--..--...... •;/•...-_:.-:-:•:: _--•... . ._,_.. -.-,-..•./,• •,,s,., -.....:.::;._ ...... •.•,-,'•.-. •,••, ;,; .... :'-:"...•1'•....• UPLIft, WALES 505 DEFORMATION,-- OROGENY 8• METAMORPHISM

-- 9ß ••-.•r I i i _•.••'•••'•-•-• '•:-7.'. :-- , V0 L C A N I C

-'"-"'•"•"•...... • • ARC(?)

570 _..•.._..._....._ •_./•

ß

l•ig. 3. Schematic columns depicting the geologic and tectonic evolution of the Alexander terrane in southern St• Alaska (adapted from t•berlein et al. (1983), Brew et al. (1984), Gehrelsand Saleeb¾(1986, 1987), and Gehrelset al. (1987)). The geologictime scale is from Palmer (1983). Gehrels and Saleeby: Alexander Terrane 157 is recorded by superjacent shallow-marine Annette and Gravina islands, the base of the limestone overlain by graptolitic shale. section is marked by a regional unconformity On Annette and Hotspur islands, Lower overlain by a discontinuous but locally thick Devonian strata include shallow marine section of coarse, poorly sorted, massive limestone, fine-grained clastic strata, polymictic conglomerate and breccia which we olistostromal layers, conglomerate, and interpret to represent a submarine talus superjacent basaltic-andesitic volcanic rocks breccia. These conglomeratic strata grade (Gehrels et al., 1987). The occurrence of clasts upsection into several hundred meters of of Silurian metadiorite and trondhjemite in rhyolite and rhyolitic tuff, which are conglomeratic strata records uplift and erosion conform ably overlain by up to 100 m of thick- of pre-Devonian rocks prior to deposition of the bedded to massive limestone. Several hundred Karheen Formation. meters of calcareous siltstone and thin-bedded Strata belonging to or correlative with the limestone overlie the massive limestone and Karheen Formation on central Prince of Wales grade upsection into a thick sequence of Island consist of conglomeratic red beds, finer- basaltic flows and breccia. Upper Triassic grained clastic strata, and subordinate strata in the Dundas Island region include limestone (Eberlein and Churkin, 1970; Eberlein limestone, clastic strata, basalt-andesite, and et al., 1983; Ovenshine, 1975). As described by rhyolite which unconformably overlie upper Ovenshine et al. (1969), these strata become Paleozoic strata (Woodsworth and Orchard, thicker and coarser grained to the south and 1985). contain paleocurrent indicators which record Bokan Mountain Granite (Middle(?) northwest directed transport. These workers $urassic). The Bokan Mountain Granite is a interpret the Karheen Formation as part of a Middle(?) $urassic peralkaline ring-dike complex subaerial to shallow marine clastic wedge that on southern Prince of Wales Island that consists was derived from uplifted regions to the of aegirine- and arfvedsonite-bearing granite, southeast. The coarse clastic strata on aplite, porphyry, and pegmatite (Gehrels and southern Prince of Wales Island were apparently Saleeby, 1986, 1987). This intrusive body is deposited near the source region for this clastic apparently the youngest component of the wedge. Alexander terrane, as similar bodies have not Upper Paleozoic rocks. Upper Paleozoic been recognized in adjacent terranes. rocks occur in restricted areas of central Prince Faults of the southern Alexander terrane. of Wales Island and the Dundas Island region The oldest faults recognized are southwest (Figure 1). On central Prince of Wales Island, vetgent thrust faults which moved between regionally significant post-Karheen rocks earliest Silurian and middle Early Devonian consist of basaltic volcanic rocks (Coronados time. These faults have been mapped on Volcanics) and limestone (Wadleigh Limestone) Annette Island (where they are overprinted by of Middle and Late Devonian age, Upper younger high-angle faults) and on southern Devonian clastic strata and volcanic rocks (Port Prince of Wales Island, and may form the Refugio Formation), Mississippianshallow- contact between Ordovician-Silurian strata and marine limestone and chert (Peratrovich pre-Middle Ordovician metamorphic rocks on Formation), and Pennsylvanian shallow-marine Dall Island (Figure Z). limestone (Ladtones Limestone) and clastic A variety of faults were active after strata (Klawak Formation) (Eberlein and deposition of the Lower Devonian strata and Churkin, 1970; Eberlein et al., 1983) (Figures Z prior to mid-Cretaceous time. Crosscutting and 3). These strata are not recognized on relations on southern Prince of Wales Island southern Prince of Wales Island or on Annette, indicate that right-lateral, north to northeast Gravina, and Duke islands, but some units striking faults displace Devonian strata and are reappear to the southeast. According to cut by a complex set of curviplanar, northwest Woodsworth and Orchard (1985), Mississippian striking faults with left-lateral displacement. limestone in the Dundas Island region resembles These faults are in turn cut by the Keete Inlet the upper part of the Peratrovich Formation, fault, which is locally intruded by mid- and Pennsylvanian limestone and siltstone Cretaceous plutons. The Keete Inlet fault resemble the Klawak Formation. regionally juxtaposes Ordovician through Upper Triassic rocks. Upper Triassic Devonian strata over pre-Middle Ordovician volcanic and sedimentary rocks of the Hyd rocks of the Wales metamorphic suite. Its Group occur on Annette and Gravina islands curviplanar nature and regional younger over (Gehrels et al., 1987), in the Dundas Island older juxtaposition lead us to the tentative region (Woodsworthand Orchard, 1985), and conclusion that the fault is an extensional perhaps on east-central Prince of Wales Island structure with significant east-side-down(?) (Buddingtonand Chapin, 19Z9, p. 313). On displacement. 158 Gehrels and Saleeby: Alexander Terrane

Central and Northern Alexander Terrane Mississippian through Permian rocks. In SE Alaska, upper Paleozoic strata include Rocks north of the Prince of Wales Island Mississippian limestone, chert, and subordinate region have been mapped and described most gypsum (Iyoukeen Formation on Chichagof recently by Brew etal. (1984) on Kuiu, Island), Pennsylvanian limestone and chert (part Kupreanof, and northern Prince of Wales of the Saginaw Bay Formation on Kuiu Island), islands, Loney etal. (1975) on Chichagof Island, and Lower Permian limestone (Pybus Lathram etal. (1965) on Admiralty Island, Brew Formation), marine clastic strata, and etal. (1978) and Brew and Ford (1985) in the subordinate basaltic volcanic rocks and Glacier Bay region, and Campbell and Dodds conglomerate (Halleck Formation) (Kuiu- (198Za, b, c, 1983a, b) in the Saint Elias Kupreanof islands and in the Glacier Bay Mountains (Figure 1). The geologic map and region). Upper Paleozoic rocks in the Saint unit descriptions of Gehrels and Berg (1984) Elias Mountains include large syentitic and summarize the geologic relations in SE Alaska dioritic bodies of late Pennsylvanian-Early and, together with the maps and descriptions of Permian age in the northwestern part of the Campbell and Dodds, serve as the basis for the terrane, and a poorly known but apparently following discussion. widespread section of argillite, shale, siltstone, Cambrian through Devonian rocks. North sandstone, and limestone. of the Prince of Wales Island region in SE Upper Triassic strata. Upper Triassic Alaska, Silurian turbidires (similar to the Bay of strata occur along the northeastern margin of Pillars Formation) and shallow-marine the terrario in SE Alaska and are known only limestone (similar to the Heceta Limestone) are locally but may be widespread in the Saint Elias the oldest rocks exposed. Subjacent rocks Mountains. In SE Alaska the strata are include Silurian and older(?) trondhjemitic and everywhere separated from subjacent Lower metamorphic rocks on northeastern Chichagof Permian and older rocks by an unconformity and Island (Loney etal., 1975) and perhaps generally resemble the rocks on Annette and Ordovician argillite, chert, and limestone Gravina islands (Gehrels etal., 1986). Upper belongingto the Admiralty subterrane (Hood Triassic strata in the Saint Elias Mountains Bay Formation) on Admiralty Island (Lathram et include limestone, marine clastic strata, al., 1965). Lower Devonian strata include finer- basaltic volcanic rocks, and perhaps grained components of the Karheen Formation conglomerate and felsic volcanic rocks. on north central Prince of Wales Island (Ovenshine etal., 1969; Ovenshine, 1975; Eberlein etal., 1983); some or all of the TECTONIC EVOLUTION conglomeratic strata belonging to the Cedar Cove Formation on northeastern Chichagof Late Proterozoic(?)-Cambrian Through Island (Loney et al., 1975); and fine-grained Early Devonian Time marine clastic strata and limestone (unnamed units and part of the Rendu Formation) near The initial phase in the evolution of the Glacier Bay. These strata are overlain by Alexander terrane is characterized by late limestone, clastic strata, and volcanic rocks of Proterozoic(?)-Cambrian through Early Middle and Late Devonian age on Chichagof Devonian magmatic and and orogenic activity in Island and in the Glacier Bay region. Contacts the southern part of the terrario and by between the various Devonian units are Cambrian volcanism in the Saint Elias gradational or conformable where known, Mountains. General compositional similarities except on northeastern Chichagof Island where between the upper Proterozoic(?)-Cambrian Upper Devonian volcanic rocks (Freshwater Bay volcanic and intrusive rocks in the terrane and Formation) are separated from adjacent units igneous rocks in modern arc systems suggest by unconformities. that the Cambrian and older(?) rocks may have In the Saint Elias Mountains, lower formed in a volcanic arc environment. This Paleozoic rocks include basaltic to andesitic interpretation has been stated by Churkin volcanic rocks, volcaniclastic strata, limestone, (1974), Churkin and Eberlein (1977), and others and gabbro of Cambrian age, and a thick, but is difficult to demonstrate because of the widespread, and apparently continuous section regional metamorphism of rocks in the Wales of Ordovician through Upper Devonian-lower metamorphic suite and the lack of detailed Mississippian limestone and marine clastic studies of rocks in the Saint Elias Mountains. strata. Poorly known sedimentary, volcanic, In SE Alaska, these arc-type(?) rocks were and metamorphic rocks in some parts of the penetratively deformed, regionally Saint Elias region may also be of early metamorphosed, uplifted, and eroded during a Paleozoic age. Middle Cambrian-Early Ordovician tectonic Gehrels and Saleeby: Alexander Terrane 159

based on (1) trends of Silurian facies, (2) I00 km VOLCANIC - PLU TONIC • alignment of regions with Ordovician-Silurian COMPLEX plutonic and/or volcanic rocks (after restoring 150 km of dextral offset on the Chatham Strait [• PROBABLE fault) (Hudson et al., 1981), and (3) trends of structures that formed during Silurian-Early Devonian time. We speculate that the arc BASINALSTRATA / system faced to the southwest because •] PROBABLE (PRESENT-DAY) Ordovician-Lower Silurian shallow-marine strata extend for a large distance northeast of the volcanic-plutonic complex (Figure 4). • UNKNOWN CHATHAMSTRAIT FAULT The arc-type magmatism in the southern Alexander terrane ceased during middle Early Silurian time in response to the onset of a middle Silurian-earliest Devonian tectonic event which we refer to as the Klakas orogeny (Gehrels et al., 1983). Buddington and Chapin (1929, p. 281-289) originally recognized this tectonic disturbance as an unconformity separating Devonian strata from Silurian and older rocks. Brew et al. (1966) recognized that the disturbance is also recorded by Silurian- Devonian conglomeratic strata on Prince of Wales, Kuiu, and Chichagof islands. More detailed studies of these conglomeratic strata led Ovenshine et al. (1969) to suggest that Lower Devonian conglomerate on central Prince ORDOVICIAN - of Wales Island formed as part of a clastic EARLY SILURIAN wedge which represents a "basinward Fig. 4. Sketch map showing the interpreted manifestation of Late Silurian to pre-Middle distribution of tectonic elements in the Devonian diastrophism in southern southeastern Alexander terrane during Ordovician-Early Alaska." Loney et al. (1975, p. 92) reported Silurian time. We have restored 150 km of that similar conglomeratic strata on right-lateral displacement on the Chatham northeastern Chichagof Island were shed from a Strait fault (Hudson et al., 1981). subaerial source area to the southwest and suggested that uplift of the source area began during Silurian time. event which we refer to as the Wales orogeny Our studies indicate that the Klakas orogeny (Gehrels and Saleeby, 1984). Structures is manifest on Annette, Gravina, Duke, and associated with this event record regional southern Prince of Wales islands by (1) cessation shortening, but it is not known whether the of the Ordovician-Lower Silurian volcanism and tectonism occurred in response to interplate or plutonism, (2) emplacement of compositionally intraplate processes. distinct Upper Silurian leucodiorite and Volcanism, plutonism, and sedimentation trondhjemite bodies, (3) southwest-vergent resumed soon after the Wales orogeny in SE movement on thrust faults during middle Alaska and continued until approximately Silurian-earliest Devonian time, (4) deposition middle Early Silurian time. Rocks formed and deformation of a Lower Devonian talus during this period were interpreted by Monger breccia adjacent to some thrust faults, (5) et al. (1972), Churkin (1974), and many others to greenschist and locally amphibolite-facies have formed in a volcanic arc environment metamorphism of Ordovician-Lower Silurian based on their lithic similarity to rocks in young rocks in some areas, (6) regional structural and presently active volcanic-plutonic uplift (5-10 km in some areas) during middle systems. This interpretation is supported by our Silurian-earliest Devonian time, and (7) major, minor, and trace element geochemical deposition of Lower Devonian conglomeratic studies of Ordovician volcanic and plutonic red beds (Karheen Formation) in topographically rocks from Prince of Wales Island (conducted rugged alluvial to shallow-marine environments with Fred Barker) (Gehrels and Saleeby, 1987). (Gehrels and Saleeby, 1987; Gehrels et al., We interpret the arc system to have trended 1987). Genetic relations between the Klakas northwesterly across the present north orogeny and the Upper Silurian trondhjemite northwest elongation of the terrane (Figure 4) and leucodiorite bodies are uncertain. The 160 Gehrels and Saleeby: Alexander Terrane

DISTAL FACIES locally, perhaps, from syenitic sources on IOO km northeastern Chichagof Island (prior to [• INFERRED movement on the Chatham Strait fault). PROBABLE• The Lower Devonian strata of SE Alaska

PROXIMAL FACIES record the location of regions uplifted during

INFERRED the Klakas orogeny and also record regional (PRESENT- subsidence following the main orogenic phase. • PROBABLEKNOWNOR DAY) We suggest that the conglomeratic strata in the Prince of Wales Island region and on CHATHAM D UNKNOWN STRAIT northeastern Chichagof Island are near-source FAULT or proximal facies of the Karheen clastic wedge and the Cedar Cove clastic wedge, which may have been contiguous prior to displacement on CEDAR the Chatham Strait fault (Figure 5). More COVE "CLASTIC distal facies of these wedges may be WEDGE" represented by fine-grained marine clastic PALEOSLOPE strata of probable Early Devonian age in the INDICATORS Glacier Bay area and perhaps in the Saint Elias Mountains region. Middle Lower and Middle Devonian limestone and fine-grained marine KARHEEN clastic strata which overlie the conglomeratic "CLAS TIC strata indicate that the orogenic activity WEDGE" ceased and that the region subsided below sea level soon after earliest Devonian time. The structural and stratigraphic relations described above indicate that deformation during the Klakas orogeny was predominantly compressional, although regional extension EARLY DEVONIAN during later phases of the orogeny is not Fig. ,5. Sketch map showing the interpreted precluded by the field relations. As with the distribution of tectonic elements in the Wales orogeny, we are not yet able to Alexander terrane during Early Devonian time. demonstrate whether the Klakas orogeny occurred in responseto interplate (i.e., We have restored 150 km of right-lateral collisional) processesalong a convergent plate displacement on the Chatham Strait fault boundary or to intraplate processes following a (Hudson et al., 1981). phase of volcanic arc activity. compositional difference between these rocks Middle Devonian Through Early Permian Time and the Ordovician-Lower Silurian intrusive bodies, combined with their emplacement The tectonic evolution from Middle during or soon after the main phase of Devonian through Early Permian time is deformation, metamorphism, and thrusting, difficult to reconstruct with certainty because suggest that they may be anatectic in origin strata of this age occur in relatively restricted rather than part of the older magmatic arc areas of SE Alaska and have not been studied in suite. detail in the Saint Elias Mountains. Upper The Klakas orogeny is recognized in the Paleozoic strata which have been studied, central Prince of Wales Island region by the (1) however, generally accumulated in tectonically Lower Devonian conglomeratic strata described stable shallow-marine environments. by Ovenshineet al. (1969), (Z) thick layers and Widespread Middle Devonian and Lower lenses of polymictic conglomerate in upper Permian limestone and dolomite record phases Lower and Upper Silurian limestone and of tectonic stability throughout much of the turbidites, and (3} southward pinchout of the terrane, and areally restricted upper Lower and Silurian strata. Clasts in the Silurian and Upper Devonian, lower and upper Mississippian, Devonian conglomerates include limestone, and lower and middle Pennsylvanian carbonates chert, graywacke, basalt-andesite, gabbro, record stability in at least some regions during diorite, granite, and syenite (Eberlein and much of late Paleozoic time. Devonian Churkin, 1970; Ovenshine and Webster, 1970; volcanic rocks with local unconformities, upper Brew et al., 1984}. The plutonic clasts were Pennsylvanian-Lower Permian dioritic to probably eroded from volcanic-plutonic syenitic bodies, and Lower Permian volcanic complexes to the south and southeast, and rocks and conglomerate record tectonic activity Gehrels and Saleeby: Alexander Terrane 161 in some areas, but these phases of tectonic Alexander terrane has been displaced activity appear to have been more restricted in considerable distances from where it evolved time and space than the early Paleozoic phases. during Paleozoic time. We have attempted to We conclude that the Alexander terrane reconstruct its displacement history primarily evolved in a tectonically stable environment, through comparisons of its geologic and perhaps as part of an oceanic microcontinent in tectonic evolution with the history recorded in an intraplate environment, during much of late other regions. Such attempts are aided by the Paleozoic time based on the (1) long period long, complete, and well-preserved geologic (approximately 130 m.y.) of relative tectonic record of the Alexander terrane, by the stability, (Z) deposition of shallow-marine availablity of paleomagnetic data for much of limestone during much of late Paleozoic time its history, and, to a lesser degree, by the (Figure 3), and (3) scarcity of coarse clastic reported similarities of fossils from the terrane strata and regional unconformities. What and fossils from other regions. tectonic activity is recorded may have been Recognizing regions which were at one time extensional in origin, in contrast to the early associated with the Alexander terrane is Paleozoic compressional events. difficult because (1) there is considerable variation in the geology of the terrane along its Late Permian and Triassic Time length and across its width, (Z) large regions of the terrane have not been studied in detail, (3) The third phase in the tectonic evolution of there is little evidence that the terrane ever the Alexander terrane is recorded by a regional evolved on or in proximity to a continental unconformity separating Upper Triassic strata landmass,and (4) many Paleozoic orogenic belts from Lower Permian and older rocks and by have not been studied in detail and/or have been various characteristics of the Triassic strata. overprinted by younger tectonism. The The Upper Triassic strata and their subjacent implications of such associations are also unconformity are interpreted to have formed uncertain because potentially correlative rocks during a Late Permian(?)-Triassic rifting event in other regions may also have been displaced based on the (1) occurrence of a thick section of from their previous positions. Such correlative Triassic marine strata in the central Saint Elias regions must exist, however, if we are correct Mountains which MacIntyre (1984) interprets to in our interpretations that the Ordovician- have been deposited in a rift basin, (Z) Lower Silurian magmatic arc trended at an restriction of Upper Triassic strata to a belt oblique angle across the present elongation of along the eastern margin of the terrane in SE the Alexander terrane and that the terrane may Alaska, (3) bimodal (basalt-rhyolite) have been considerably larger prior to Late composition of the volcanic rocks in SE Alaska, Permian(?)-Triassic rifting. (4) lack of regional metamorphism and (or) deformation associated with the Late Permian(?)-Triassic uplift and erosion, and (5) Geologic Constraints presence of Upper Triassic talus breccia and thick lenses of conglomerate (Gehrels et al., 1985, 1987). Normal(?) movement on the Keete The geologic framework described in Inlet fault (south-central Prince of Wales Island) previous sections serves as the primary tool in identifying regions with which the Alexander and emplacement of steeply dipping mafic dikes terrane may have been associated. Specific in much of the southern part of the terrane may characteristics, such as upper Proterozoic(?)- also have occurred during this extensional Cambrian and Ordovician-Lower Silurian phase. The occurrence of a rift(?) assemblage along magmatic suites, Silurian trondhjemitic rocks of the eastern margin of the terrane suggests that anatectic(?) origin, widespread Silurian turbidites, shallow-marine carbonate, and Paleozoic rocks may have extended for a conglomerate, and Lower Devonian red beds considerable distance northeast of the present serve as fingerprints for comparison with other extent of the terrane prior to Late Triassic time and that Jura-Cretaceous strata of the regions. The interpreted tectonic evolution of the terrane also offers criteria for Gravina belt (Berg et al., 197Z) accumulated in this rifted basin. comparison. In particular, phases of Middle Cambrian-Early Ordovician and middle Silurian- earliest Devonian orogenic activity, late DISPLACEMENT HISTORY Paleozoic tectonic stability, and Late Permian(?)-Triassic rifting(?) place general A variety of geologic, paleomagnetic, and constraints on the history of potentially paleobiogeographic evidence indicates that the associated regions. 162 Gehrels and Saleeby: Alexander Terrame

TABLE 1. Palecmagnetic data frcm the Alexander terrane

Sense and •mount of Rotation Time Pal eo 1 at i tude (Relative to the Paleopole) Northern Southern Hemi sphere Hemi sphere

Late Triassic a 44 ø ñ 10ø 1Z5ø c-cwise 55ø cwise Early Permianb 9ø ß 8ø (?) 15Zø cwise ( ?) Z8ø c-cwise (?) Pennsylvanianc 8ø • 7ø 80ø c-cwise 100ø cwise Mississippianc 14ø • 8ø 85ø c-cwise 95ø cwise Late Devonianc Z1ø ß 6ø 8Zø c-cwise 98ø cwise Middle Devonian c 10ø ñ 5ø 67ø c-cwise 113ø cwise Late Ordovician c 7ø ß 8ø 6Zø c-cwise 115ø cwise Middle Ordovician c 1ø ß 19ø 103ø c-cwise 77ø cwise

Definitions: cwise, clockwise rotation since deposition of strata; c-cwise, counter-clockwise rotation. Data Sources:a Hillhouseand Grcrrme (1980); b Panuskaand Stone (1985); c Van der Voo et al. (1980).

Paleomagnetic Constraints Paleobiogeographic Constraints

Paleomagnetic studies have been conducted Fauna and flora from the Alexander terrane on rocks of Ordovician, Devonian, Mississippian, have been described in many reports, but the Pennsylvanian (Van der Voo et al., 1980), paleobiogeographic implications of these fossils Permian (Panuska and Stone, 1985), and Triassic have not been synthesized. The appendix is a age (Hillhouse and Gromme, 1980) from the compilation of reported similarities of fossils southern Alexander terrane. Table 1 from the Alexander terrane and from other summarizes the results of these studies, regions, which we present in an effort to showing the apparent paleolatitudes and the summarize similarities noted by others and to sense and amount of rotation (relative to the stimulate additional studies and discussion of paleopole) required for positions in the northern the diverse and well-preserved fossils in the and southern hemispheres. The most significant Alexander terrane. Although the reported and apparently most reliable conclusion derived similarities do not clearly identify correlatives from these studies is that the Alexander terrane of the Alexander terrane, they show that (1) evolved in relatively low paleolatitudes (either most fossils have circum-Pacific similarities, north or south of the paleoequator) during (Z) some upper Paleozoic fauna and flora are Ordovician-Pennsylvanian time and has been Tethyan or equatorial in affinity, and (3) Late displacedat least 18ø northwardwith respectto Triassic bivalves grew in equatorial or low North America since Pennsylvanian time (Van southerly paleolatitudes in the eastern part of der Voo et al., 1980). The significance of the the paleo-Pacific basin. Permian and Triassic data is less clear. Panuskaand Stone (1985) reported that remagnetization of many samples precluded a Previous Hypotheses rigorous fold test for the Permian rocks, and the moderate Late Triassic paleolatitude Evolution near its present position in the reported by Hillhouse and Gromme (1980) is not Cordillera. Churkin (1974) proposedthat the consistent with the low paleolatitude deduced Alexander terrane has not been transported from studies (described below) of Late Triassic northward relative to North America but has bivalves. only moved toward and away from the Gehrels and Saleeby: Alexander Terrame 163 continental margin during the closure and (1980), the Ordovician through Pennsylvanian opening of marginal basins. Churkin and paleolatitudes of the Alexander terrane are Eberlein (1977, po 784) offered a more detailed similar to a paleoposition in northeastern scenario in which the terrane evolved as a west California-western Nevada if it is assumed that facing volcanic arc near its present position in the Alexander terrane was in the northern the Cordillera during early Paleozoic time, hemisphere. switched polarity prior to or during Early Links between the Alexander terrane and Devonian time, and impinged on continental the Sierra-Klamath region have also been margin assemblages to the east during the suggested on the basis of U-Pb analyses of Antler orogeny. These scenarios are considered detrital zircon populations in pre-Upper unlikely because of the (1) occurrence of Devonian rocks of the Shoo Fly Complex (Sierra Permian fauna with Tethyan or equatorial Nevada Mountains). Girty and Wardlaw (1984) affinities inboard of the terrane, (Z) northward concluded that the zircon populations were displacementof at least 18ø indicatedby derived during a single cycle of erosion from a paleomagnetic data, and (3) low paleolatitude source terrane containing igneous rocks of affinity of Upper Triassic bivalves in the approximately 506 ñ ZZ Ma (Late Cambrian- terrane. Early Ordovician). They hypothesized that the Evolution near western North America in detritus was shed from the Alexander terrane proximity to rocks now in the Sierra-Klamath based on previous suggestions that the terrane region. Monger and Ross (1971) and Monger et originated in the Sierra-Klamath region, the al. (1972) hypothesized that the AleXander paleomagnetic evidence of Van der Voo et al. terrane may have evolved south of its present (1980), and the occurrence of lower Paleozoic position near western North America based on igneous rocks in the Alexander terrane. the similarity of Permian fusulinid fauna in the In spite of the numerous hypotheses linking Alexander terrane and in rocks inboard of the the Alexander terrane with rocks of the Sierra- Cache Creek terrane. They postulated that the Klamath region, our studies suggest that the terrane was originally the southern continuation geologic records of the two areas are of the inboard rocks and was then displaced fundamentally different. The main components northward after the Cache Creek terrane had of potentially correlative assemblages in the been accreted. Jones et al. (197Z) suggested Sierra-Klamath region are described by most more specifically that the terrane was workers as (1) ophiolitic rocks of Early contiguous with continental-margin assemblages Ordovician age, (Z) locally to regionally in the Sierra-Klamath region (central and disrupted Cambro(?)-Ordovician through Lower northern California) during Paleozoic time Devonian marine clastic strata and subordinate based primarily on similarities in Silurian facies volcanic rocks and limestone, (3) Lower-Middle relations and Permian fusulinid faunas in the Devonian and subordinate Ordovician-Silurian two areas. intrusive bodies, and (4) Upper Devonian Schweikert (1976) and Schweikert and through Permian andesitic and felsic volcanic Snyder (1981) followed this theme and rocks, marine volcaniclastic and clastic strata, hypothesized that the Alexander terrane was an and subordinate limestone (Irwin, 1977, 1985; east facing volcanic arc during Ordovician- Potter et al., 1977; Lindsley-Griffin and Griffin, Silurian time and that coeval rocks in the 1983• D'Allura et al., 1977; Harwood, 1983; Sierra-Klamath region accumulated in an Schweikert and Snyder, 1981}. Most of these accretionary prism along the leading edge of workers hypothesized that the upper Paleozoic this arc. They suggested that the arc rocks formed in a volcanic arc environment magmatism migrated eastward over the older after a pre-Late Devonian orogenic event. The accretionary prism during Devonian time and abundance of quartz in the lower Paleozoic then this composite arc collided with the strata suggests that they were deposited in western margin of North America during the proximity to a continental landmass, but there Antler orogeny. According to their scenario, is little agreement on whether the strata the Alexander terrane and rocks in the Sierra- formed as slope and rise deposits along a Klamath region evolved in a west facing continental margin or in a volcanic arc- volcanic arc during late Paleozoic time, and accretionary prism environment. then the Alexander terrane was displaced Critical differences between the two regions northward and accreted to its present position. include the (1} presence of upper The hypothesis that the Alexander terrane Proterozoic(?}-Cambrian arc-type(?} rocks in evolved in proximity to rocks in the Sierra- the Alexander terrane in contrast to the Lower Klamath region is consistent with the Ordovician ophiolitic rocks in the Sierra- Ordovician-Pennsylvanian paleomagnetic data Klamath region, (Z} lack of quartz-rich clastic (Table 1). According to Van der Voo et al. strata in the Alexander terrane compared to 164 Gehrels and Saleeby: Alexander Terrame their abundance in lower Paleozoic rocks of the the appropriate age (approximately 540-5Z0 Ma) Sierra-Klamath region, (3) intact Ordovician- (Gehrels et al., 1987), but these layers are too Lower Silurian volcanic-plutonic complex in the restricted in extent to have been a reasonable Alexander terrane, in contrast to the coeval source for the sandstone described by Girty and sedimentary assemblages with subordinate Wardlaw (1984). Larger plutons of this age may volcanic rocks to the south, (4) timing of occur beneath younger strata in other parts of Paleozoic orogenic phases: Middle Cambrian- the terrane, but the existence of such plutons Early Ordovician and middle Silurian-earliest remains speculative. More reasonable sources Devonian in the Alexander terrane; post-Late for the detritus described by Girty and Wardlaw Silurian and pre-Late Devonian in the Sierra (1984) are the plutons of Cambrian(?)- Nevada (Varga and Moores, 1981• Hanson et al., Ordovician age in east central Idaho (Evans, 1987• Saleeby et al., 1987)• probable Middle 1984), which are near the interpreted source for Devonian in the Klamath Mountains, and (5) much of the detritus in the Shoo Fly Complex abundance of upper Paleozoic arc-type volcanic (Schweikert and Snyder, 1981). rocks in the Sierra-Klamath region, in contrast Based on the significant differences in their to the predominant marine clastic strata and geologic records and the lack of other shallow-marine carbonate in the Alexander compelling arguments, we conclude that there terrane. is little evidence which links the Alexander In addition, the reported similarity of terrane with rocks in the Sierra-Klamath region Permian fusulinid fauna from the two regions during Paleozoic time. It is possible to envision now appears to have less significance, as the scenarios in which the two regions were distinctive fauna also occur in other regions of adjacent to one another but have different the circum-Pacific basin (Ross and Ross, 1953, geologic records and tectonic histories, but such 1955). These more recent analyses accordingly scenarios remain unsupported by the present do not draw associations between the Alexander geologic data base. terrame and rocks in the Sierra-Klamath region Evolutionin other regions. Wilson(1968), during Permian time. The occurrence of Monger and Ross (1971), and Monger et al. Permian fusulinid faunas with Tethyan or (197Z) acknowledged the possibility that the equatorial affinities outboard of the reportedly Alexander terrane is an exotic component in the similar rocks in the Sierra-Klamath region North American Cordillera but did not discuss (Jones et al., 197Z) also suggeststhat the where it may have been located prior to its Alexander terrane was located south of the Late Cretaceous-early Tertiary accretion. Our Sierra-Klamath region if it was originally search for correlatives of the Alexander terrane inboard of Cache Creek-affinity rocks. focuses on early Paleozoic orogenic belts The interpretation that the Alexander because lower Paleozoic rocks of the terrane terrame was the source for detrital zircon are widespread, distinctive, and record long- populations of apparent Late Cambrian-Early lived tectonic activity. We direct our search Ordovician age in the ShooFly Complex (Girty primarily to regions which were in central or and Wardlaw, 1984) is intriguingly consistent southern parts of the paleo-Pacific basin based with previous scenarios, but source rocks of the on (1) the observation that most fossils in the appropriate composition, age, and extent have Alexander terrane are circum-Pacific in not been recognized in the Alexander terrane. affinity, (Z) the low paleolatitudes indicated by Rocks which could reasonably yield zircon the paleomagnetic data of Van der Voo et al. populations with the reported isotopic (1980), and (3) a variety of evidence which characteristics include (1) silicic metavolcanic suggests that the Alexander terrane was rocks of the Wales metamorphic suite, (Z) displaced northward after Permian time. Such Middle Ordovician-Lower Silurian plutonic rocks evidence includes the Tethyan or equatorial containing zircons with a significant component affinity of Permian fusulinid fauna inboard of of inherited pre-Ordovician zircon, and (3) the Alexander terrane, the equatorial or low Middle-Upper Cambrian metaplutonic layers in southerly paleolatitude recorded by Triassic the Wales metamorphic suite. The silicic bivalves (appendix), and the evidence that metavolcanic rocks are not a likely source oceanic plates in the Pacific basin were in because they are in large part or entirely pre- general moving northerly prior to and during Late Cambrian in age and are very poor in accretion of the Alexander terrane zircon. The Ordovician-Lower Silurian rocks (Engebretsonet al., 1985). are also an unlikely source in that they are too According to the continental reconstructions young and do not contain detectable quantities of Scotese et al. (1979) and Scotese (1984), the of inherited zircon. Middle-Upper Cambrian paleo-Pacific basin was rimmed by continental metagranodiorite layers associated with the margins of North America, South America, Wales suite yield zircon populations of nearly Antarctica, Australia, and various fragments Gehrelsand Saleeby: Alexander Terrane 165

PALEO-PACIFIC BASIN 30 ø KAZAKHS TAN ALEXANDER SE CHINA TERRANE (present position) 0 o LACHLAN BELT

NEW ZEALAND

NS-ANT. MTNS .. BYRD ID

60 ø AF SA

Fig. 6. Siluriancontinental reconstruction (from Scoteseet al., 1979) showingthe distribution of early Paleozoic orogenicbelts along the western and southwesternmargin of the paleo-Pacific basin. AUS, Australia; ANT, Antarctica; SA, South America; North America; AF, Africa. which now reside in Asia during much of early are considered more likely by some. The Paleozoic time (Fisure 6). Along most of these depositional basement to these rocks locally continental margins there is evidence of early consists of Precambrian crystalline or stratified Paleozoic tectonic activity, and the circum- rocks but is in most areas unknown. Beginning Pacific deformational belts of Australia, in Late and locally Middle Cambrian time and Antarctica, and South America probably all continuing into the Early Ordovician, these belonged to a single complex orogenic system. rocks were penetratively deformed, regionally Because the lower Paleozoic rocks of eastern metamorphosed, and intruded by syntectonic Australia have been studied in more detail than •rranitic plutons. This orogenic event has been those of Antarctica or South America, our referred to as the Delamerian orogeny in comparisons with the Alexander terrane focus Australia and as the Ross orogeny in on the development of the Lachlan Fold Belt of Antarctica. eastern Australia. Rocks in this region have Following the Middle Cambrian-Early recently been summarized by Cooper and Ordovician orogenic activity, Lower Ordovician Grindley (1982), Cas (1983), Veevers (1984), and conglomerate and quartz-rich clastic strata and Scheibner (1985} and are only briefly outlined Middle and Upper Ordovician shallow-marine below. limestone and clastic strata were deposited in The Phanerozoic evolution of the Lachlan the western part of the belt. These strata Belt began with the deposition of Lower and •rrade eastward into deeper water clastic strata locally Middle and Upper Cambrian mafic to which were deposited throughout much of the intermediate volcanic rocks, marine clastic central La•hlan Belt. To the east, mafic to strata, and subordinate limestone in western intermediate volcanic rocks were deposited and parts of the orogen. Most workers suggest that shallow-marine limestone accumulated in some these rocks formed in a volcanic arc areas. Most workers interpret these volcanic environment, although extensional environments rocks as part of an east facing volcanic arc 166 Gehrels and Saleeby: Alexander Terrane

(referred to as the Molong arc), with the deeper experienced a Silurian-Early Devonian orogenic water clastic strata to the west deposited in a event (Figure 7). The similarities between the marginal basin (Wagga trough). two regions end in the Middle Devonian, when The tectonic evolution of the Lachlan Belt the Alexander terrane began to evolve in a changed dramatically with the onset of an tectonically stable environment whereas the orogenic event which began in Early Silurian Lachlan Belt remained tectonically active. time (Benambran orogeny), lasted through the Our preliminary studies of other early middle Silurian (locally the Quidonganorogeny), Paleozoic orogenic belts, such as the and continued into Late Silurian-earliest Caledonian-Appalachian orogen, various orogens Devonian time (Bowning orogeny). During early in Eurasia, and much of western South America, phases of this orogeny, strata in the Wagga suggest that their geologic and tectonic trough were regionally metamorphosed and evolution is significantly different from that deformed, many S-type plutons were eraplaced, recorded in the Alexander terrane. Many other and large regions were uplifted above sea level orogens also do not satisfy the criteria of and covered by terrigeneous red beds and silicic having evolved in low paleolatitudes and of volcanic rocks. In some regions, deeper water having paleo-Pacific faunal affinities. strata were deposited in relatively narrow Based on the similarities with rocks in troughs separated by shallow-marine or eastern Australia and previously adjacent terrestrial regions. The later phases of the regions of Gondwana, and dissimilarities with orogeny are manifest only as uplift and rocks in other regions, we raise the possibility erosional events. Although most manifestations that the Alexander terrane may have evolved in of this Silurian-Early Devonian orogeny proximity to the paleo-Pacific margin of apparently record crustal shortening, some Gondwana prior to Middle Devonian time aspects suggest that regional crustal extension (Gehrels and Saleeby, 1984). It is not possible was also significant. to identify a specific paleoposition along the Middle Lower to upper Middle Devonian margin because not enough is known about the rocks occur only locally and consist of shallow- early Paleozoic evolution of Antarctica, South marine limestone, clastic strata, and America, and various fragments now residing in subordinate volcanic rocks. This interval is Asia and because the lower Paleozoic rocks interpreted by Cas (1983) as being transitional presently in these orogenic belts have probably from a tectonically active phase during also been displaced since their formation. Silurian-Early Devonian time to another Hence our hypothesis is quite general in drawing tectonically active phase beginning in late associations between the Alexander terrane and Middle Devonian time. The onset of the next rocks along the paleo--Pacific margin of phase of tectonism is marked by the regional Gondwana. Middle to Late Devonian Tabberabberan The paleomagnetic data of Van der Voo et orogeny. This was followed during Late al. (1980) are consistent with a paleoposition Devonian-Early Carboniferous time by along northern parts of the Gondwana margin if widespread deposition of conglomeratic red it is assumed that the terrane was in the beds and silicic and subordinate mafic volcanic southern hemisphere. As shown on Figure 8, the rocks, and local eraplacement of both S- and I- apparent paleolatitudinal position of the type granitic plutons. Cas (1983) suggeststhat Alexander terrane (Van der Voo et al., 1980) is these Upper Devonian-Lower Carboniferous similar to that of eastern Australia (Scotese, sedimentary and bimodal volcanic rocks were 1984) during early Paleozoic time. The deposited in an extensional environment. The paleomagnetic data also apparently record final phase of regional deformation in the divergence between the two regions at Lachlan Belt occurred during the mid- approximately the same time that their Carboniferous Kanimblan orogeny. geologic records become dissimilar. The Evolution of the Alexander terrane along the paleolatitudinal data therefore indicate that the paleo-Pacific margin of Gondwana. As Alexander terrane could have been associated presently described, rocks in the Lachlan Belt with at least the northern parts of the record an early Paleozoic evolution which is in Gondwana margin during early Paleozoic time. many respects similar to that in the Alexander terrane. The most striking tectonic similarities SpeculativeDisplacement History of. are that rocks in at least some parts of the two The Alexander Terrane orogensformed in a magmatic arc(?) environment during Cambrian time, were Our hypothesis linking the Alexander terrane deformed during the Middle Cambrian-Early with rocks along the paleo-Pacific margin of Ordovician, evolved in a volcanic arc-marginal Gondwana, combined with the paleomagnetic basin regime during Ordovician time, and data (Table 1) and paleobiogeographic Gehrels and Saleeby: Alexander Terrane 167

LACHLAN ALEXANDER structures which formed prior to and perhaps FOLD BELT TERRANE after the Tabberabberan orogeny may record rifting of the Alexander terrane and other M 'KANIMBLAN tectonic fragments from the Gondwana margin. •.. O.RO•G •.• TECTONICALLY During Carboniferous-Permian time the //'//'/, STABLE 3,60 RIFTING / terrane apparently evolved near the , paleoequator, according to both paleomagnetic TABB!•RAB./ / / / / and paleobiogeographic data (Van der Voo et al., OROG. / / / / / 1980; Ross and Ross, 1983, 1985; Mamet and Pinard, 1985). Because the terrane was 4O8 BOWNING_ ' / / / / • apparently located near or south of the \••%•QUIDONGAN ///// KLAKAS•OROG. paleoequator in the eastern part of the paleo- kBE•N A•M•B "•AN 438 Pacific basin by the Late Triassic, it must have drifted eastward across the paleo-Pacific basin between Devonian and Late Triassic time. MAGMATIC ARC o Combined with the upper Paleozoic geologic record of the terrane, these constraints indicate that it may have moved eastward as part of a 505 microcontinental fragment in an intraplate oceanic environment. The Alexander terrane was apparently rifted from its northeastern continuation or from rocks belonging to another AiMAGMATICiiiilll • :11111111O•RO•GL• I' ARC? MAGMATIC tectonic fragment during Late Permian(?)- IIIIIIIII Triassic time, which, as described by Gehrels 57O iiiiiiiii and Saleeby (1985), initiated its northward IIiiiiiii ß pc Iiiiiiiii migration along the eastern margin of the I•11 paleo-Pacific basin. Fig. 7. Comparison of the main tectonic phases and events in the early Paleozoic Discussionof the ProposedDisplacement evolution of some regions of the Alexander History terrane and the Lachlan Fold Belt. The evolution of the Lachlan Belt is interpreted The displacement scenario presented above primarily from the descriptions and is derived primarily from our interpretations that the Alexander terrane is an exotic interpretations of Cas (1983), Veevers (1984), and Scheibner (1985). The time scale is from component in the North American Cordillera Palmer (1983). and that its early Paleozoic history most closely resembles that recorded along the paleo-Pacific margin of Gondwana. Although geologic and similarities (appendix), provide a basis for paleomagnetic similarities with eastern speculations on the displacement history of the Australia have been noted, precise terrane. We propose a scenario in which the paleopositions of the terrane cannot be terrane evolved as an offshore volcanic arc determined until more is known about other during late Proterozoic(?)-Cambrian time, was orogens that formed along the Gondwana involved in a regional orogenic event during the margin and until the degree of tectonic mobility Middle Cambrian-Early Ordovician, and evolved in these orogens has been assessed. The path, in a volcanic arc-marginal basin environment timing, and mechanism of its displacement during Ordovician-Early Silurian time. across the paleo-Pacific basin are also very Beginning in Early Silurian time and continuing poorly known. The only apparently reliable into the Early Devonian, the arc system was constraints are that the terrane was (1) in involved in an orogenic event which included proximity to the Gondwana margin through regional deformation, metamorphism, anatectic Silurian-earliest Devonian time, (Z) near the magmatism, uplift, erosion, and perhaps paleoequator in a tectonically stable extension. environment during late Paleozoic time, (3) We speculate that the Alexander terrane near or south of the paleoequator in the eastern was removed from the vicinity of the Gondwana part of the paleo-Pacific basin during the Late margin after or during the waning stages of this Triassic, and (4) near its present position in the Silurian-Early Devonian orogenic activity Cordillera by Late Cretaceous-early Tertiary because it did not experience the widespread time. Middle Devonian and younger tectonic events The proposed displacement scenario is recorded along the margin. Extensional offered as an alternative to hypotheses linking 168 Gehrels and Saleeby: Alexander Terrame

i i i i i i i 20 ø three distinct tectonic phases, including late Proterozoic(?)-Cambrian through Early ..... ,•LEX,•NDERTERR,•NEDevonian, Middle Devonian through Early Permian, and Late Permian(?)-Triassic. The first phase records the origin and evolution of the terrane in a volcanic arc(?) environment during late Proterozoic(?)-Cambrian time, and continued arc-type volcanism and plutonism in

60 ø the southern part of the terrane during Ordovician-Early Silurian time. The arc-type activity was interrupted during the Middle Cambrian-Early Ordovician Wales orogeny and Fig. 8. Comparison of the apparent was terminated during the middle Silurian- paleolatitudes of the Alexander terrane earliest Devonian Klakas orogeny. These (assumingit was in the southernhemisphere) orogenic events apparently record primarily and of eastern Australia. Paleolatitudes and crustal shortening, but it is not known whether their uncertainties for the Alexander terrane they occurred in response to interplate or (Van der Voo et al., 1980; Panuska and Stone, intraplate processes. Rocks of early Paleozoic 1985; Hillhouse and Groinroe, 1980) are shown age in the northern part of the terrane form a with solid bars, which have been connected to thick section of limestone and clastic strata yield a generalized apparent paleolatitudinal that apparently have not experienced either of path for the Alexander terrane. The the two early Paleozoic disturbances. paleolatitudesfor eastern Australia (dotted Rocks of Middle Devonian through Permian vertical lines) represent the paleolatitudinal age were deposited in tectonically stable range of the eastern margin of Australia on shallow-marine environments, and they record paleocontinental reconstructions by Scotese the second phase in the evolution of the (1984). These vertical lines have been Alexander terrane. Volcanic rocks of Devonian connected to yield an approximate and of Permian age occur locally, but they do paleolatitudinal path for eastern Australia. not appear to have accumulated in long-lived or Note that the uncertainty in the paleolatitude regionally significant magmatic systems. The of eastern Australia is not shown on this third stage is recorded by deposition of Triassic diagramand that, accordingto Scotese(1984), volcanic and sedimentary rocks in a rift(?) the Late Devonian paleolatitude of eastern environment and by a regional unconformity at Australia is not well constrained. the base of the Triassic section. Occurrence of the Triassic strata along the eastern margin of the terrane in SE Alaska suggests that this the Alexander terrane with rocks in the Sierra- interpreted rifting may record detachment of Klamath region, which we consider unlikely, and the terrane from previously contiguous rocks or as a specific outline which can be tested. A from another tectonic fragment. tested and modified displacement history for Previous discussions of the displacement the Alexander terrane should provide critical history of the Alexander terrane have focused information on the accretionary history of on correlations with Paleozoic rocks in the western North America and the evolution of the Sierra-Klamath region of California. Our paleo-Pacific margin of Gondwana and may also analyses of the Paleozoic geologic records of lead to a method for reconstructing the the Alexander terrane and of the Sierra- movement of pre-Mesozoic oceanic plates in Klamath region suggest, however, that there the paleo-Pacific basin. are few similarities in their geologic and tectonic history. Based on a review of the SUMMARY geology of other early Paleozoic orogens, we raise the alternative possibility that the The Alexander terrane is a unique tectonic Alexander terrane formed and evolved along the fragment in the North American Cordillera in paleo-Pacific margin of Gondwana during early that its geologic record is suprisingly long, Paleozoic time. The existing paleomagnetic complete, and well preserved. The Triassic and data and, to a lesser degree, the every Paleozoic epoch are represented in the paleobiogeographic affinities of fossil groups terrane except for the Late Permian and from the Alexander terrane are apparently perhaps the Early Cambrian. In most areas, the consistent with this hypothesis, although some rocks are only slightly to moderately deformed lower Paleozoic fossils in the terrane and are not highly metamorphosed. apparently have stronger affinities with other We suggest that the terrane evolved through parts of the circum-Pacific region. Gehrels and Saleeby: Alexander Terrane 169

We speculate further that the terrane North American although some degree of separated from the Gondwana margin during isolation is indicated (Savage et al., 1978) Devonian time, perhaps through rifting, and Corals- similar to fauna in the U.S.S.R. migrated eastward across the paleo-Pacific (Tchudinova et al., 1974) ocean basin south of the paleoequator during MIDDLE DEVONIAN: Conodonts- Cordilleran late Paleozoic time. We envision the terrame as NorthAmerican affinity (Savage,1984); a microcontinental fragment in an intraplate cosmopolitan (Klapper and Johnson, 1980) oceanic environment during this time. By Late Corals- similar tb fauna in the U.S.S.R. Triassic time the terrane was apparently near (Tchudinova et al., 1974) or south of the paleoequator in the eastern part EARLY DEVONIAN: Conodonts- similar to of the paleo-Pacific basin-- perhaps in fauna in western North America, eastern association with rocks now in western South Australia, and central Asia (Savage, 1984); America. Northward displacement of the one genus (Kimognathus)is known only from terrane along the eastern margin of the paleo- central Asia and eastern Australia (Savage Pacific basin apparently began during or soon and Gehrels, 1984) after Late Triassic time and continued until it Land plants (Baragwanatia)and graptolites- was accreted during Late Cretaceous-early similar to an occurrence in eastern Australia Tertiary time. (Churkin et al., 1969, p. 567); similar land plants also occur in North America and APPENDIX: REPORTED SIMILARITIES Europe (L.J. Hickey, written communication, BETWEEN FAUNA AND FLORA FROM THE 1985) ALEXANDER TERRANE AND FOSSIL GROUPS Corals - similar to fauna in Asiatic U.S.S.R. IN OTHER REGIONS (Churkin et al., 1970) Brachiopods- endemic fauna and also similar TRIASSIC: Bivalves- eastern part of the paleo- to fauna in western North America, Europe, Australia, and northeastern U.S.S.R. (Soja, Pacific basin and near the paleoequator 1985) (Tozer, 1982); similar to fauna along the Pacific margin of South America (Newton, SILURIAN: Brachiopods- similar to fauna in the eastern Urals (Kirk and Amsden, 1952) and in 1983; Silberling, 1985) Nevada and the Canadian Arctic ($.G. EARLY PERMIAN: Brachiopods- similar fauna Johnson, written communication, 1986) occur in the Urals, southern China, eastern Conodonts - similar to fauna in the Selwyn Alaska, the Canadian Arctic, Yugoslavia, and Basin(northern Canadian Cordillera) (M. in a melange in central Oregon (Grant, 1971); Orchard, written communication, 1986) Boreal fauna (Yancey, 1975) ORDOVICIAN: Conodonts- belong to the North Various fauna indicate a position within the Atlantic conodontprovince (Savage, 1984); paleo-Pacific basin and south of the similar to fauna in the Selwin Basin and the paleoequator (Ross and Ross, 1983, 1985) PENNSYLVANIAN: Fusulinids- similar to fauna Kechika trough (northern Canadian Cordillera) (M. Orchard, written in Japam and the Cache Creek terrame communication, 1986); belong to a peripheral (central British Columbia) (Douglass, 1971) biofacies (J.G. $ohnson, written Conodonts- generally cosmopolitan but have affinities with midcontinent North American communication, 1986) fauna (Savage and Barkeley, 1985) Graptolites- Pacific faunal province (C. Carter, written communication, 1986). Various fauna indicate a position within the paleo-Pacific basin and near the paleoequator Acknowledgments. Our field work in SE (Ross and Ross, 1983, 1985} Alaska has been supported by the U.S. MISSISSIPPIAN: Conodonts- western North Geological Survey and by research grants American affinity (Savage, 1984} awarded to G.E.G. from the California Institute Foraminifera- both Tethyan and North of Technology, the Geological Society of American affinities present (Mamet and America, and Sigma Xi. We thank Henry C. Pinard, 1985; Dutro et al., 1981} Berg for his guidance and assistance in Algal microflora- cosmopolitan, but together collecting the data summarized herein and for with foraminifers they constitute a link sharing his knowledge of the geology of SE between Eurasia and North America (Mamet Alaska. We also wish to express our and Pinard, 1985} appreciation to R.B. Blodgett, N.M. Savage, Various fauna indicate a position within the J.M. Berdan, A.J. Boucot, C. Carter, M. paleo-Pacific basin and south of the Churkin, J.T. Dutro, R.E. Grant, L.J. Hickey, paleoequator (Ross and Ross, 1985} J.G. Johnson, C.R. Newton, W.A. Oliver, M. LATE DEVONIAN: Brachiopods- generally Orchard, C.A. Ross, J.R.P. Ross, N.J. 170 Gehrels and Saleeby: Alexander Terrame

Silberling, and C.H. Stevens for assistance in Cas, R., A review of the paleogeographic and compiling the appendix. Comments by Henry C. tectonic development of the Lachlan Fold Berg, David A. Brew, and an anonymous Belt of southeastern Australia, Spec. Publ., reviewer improved the manuscript considerably. Geol. Soc. Aust., • 1-104, 1983. Churkin, M., Jr., Paleozoic marginal ocean REFERENCES basin-volcanic arc systems in the Cordilleran foldbelt, Spec. Publ., Soc. Econ. Paleontol. Berg, H. C., D. L. Jones, and D. H. Richter, Mineral., • 174-19Z, 1974. Gravina-Nutzotin belt-- Tectonic Churkin, M., Jr., and G. D. Eberlein, Ancient significance of an upper Mesozoic borderland terranes of the North American sedimentary and volcanic sequence in Cordillera: Correlation and microplate southern and southeastern Alaska, U.S. Geol. tectonics, Geol. Soc. Am. Bull., • 769-786, Surv. Prof. Pap., 800-D, D1-DZ4, 197Z. 1977. Berg, H. C., D. L. Jones, and P. J. Coney, Map Churkin, M., Jr., G. D. Eberlein, F. M. Heuber, showing pre-Cenozoic tectonostratigraphic and S. H. Mamay, Lower Devonian land plants terranes of southeastern Alaska and adjacent from graptolitic shale in southeastern Alaska, areas, U.S. Geol. Surv. Open File Rep., 78- Paleontology,• 559-573, 1969. 1085, 1978. Churkin, M., Jr., H. Jaeger, and G. D. Eberlein, Brew, D. A., and A. B. Ford, Preliminary Lower Devonian graptolites from reconnaissance geologic map of the Juneau, southeastern Alaska, Lethaia, 3_3183-ZOZ, Taku River, Atlin, and part of the Skagway 1970. l:Z50,000 quadrangles, southeastern Alaska, Cooper, R. A., and G. W. Grinalley (Eds.), Late U.S. Geol. Surv. Open File Rep., 85-395, Proterozoic to Devonian sequences of 1985. southeastern Australia, Antarctica, and New Brew, D. A., R. A. Loney, and L. J.P. Muffler, Zealand and their correlation, Spec. Publ., Tectonic history of southeastern Alaska, CIM Geol. Soc. Aust., 9_•1-103, 1982. Spec. Vol., • 149-170, 1966. D'Allura, J. A., E. M. Moores, and L. Robinson, Brew, D. A., B. R. Johnson, D. Grybeck, A. Paleozoic rocks of the northern Sierra Griscom, and D. F. Barnes, Mineral resources Nevada: Their structural and paleogeographic of the Glacier Bay National Monument implications, in Paleozoic Paleogeography of wilderness study area, Alaska, U.S. Geol. the Western United States, edited by J. H. Surv. Open File Rep., 78-494• 1-670, 1978. Stewart, C. H. Stevens, and A. E. Fritsche, Brew, D. A., A. T. Ovenshine, S. M. Karl, and S. pp. 395-408, Pacific Section, Society of J. Hunt, Preliminary reconnaissance geologic Economic Paleontologists and Mineralogists, map of the Petersburg and parts of the Port Los Angeles, Calif., 1977. Alexander and Sumdum quadrangles, Douglass, R. C., Pennsylvanian fusulinids from southeastern Alaska, U.S. Geol. Surv. Open southeastern Alaska, U.S. Geol. Surv. Prof. File Rep., 84-405, 1-43, 1984. Pap., 706, l-Z1, 1971. Buddington, A. F., and T. Chapin, Geology and Dutro, J. T., Jr., A. K. Armstrong, R. C. mineral deposits of southeastern Alaska, U.S. Douglass, and B. L. Mamet, Carboniferous Geol. Surv. Bull., 800, 1-398, 19Z9. biostratigraphy, southeastern Alaska, U.S. Campbell, R. B., and C. J. Dodds, Geology of Geol. Surv. Circ., 823-B, B94-B96, 1981. the southwest Kluane Lake Map-Area, Yukon Eberlein, G. D., and M. Churkin, Jr., Paleozoic Territory, Geol. Surv. Can. Open File Rep., stratigraphy in the northwest coastal area of 829, 198Za. Prince of Wales Island, southeastern Alaska, Campbell, R. B., and C. J. Dodds, Geology of U.S. Geol. Surv. Bull., 1284, 1-67, 1970. the Mount Saint Elias Map-Area, Yukon Eberlein, G. D., M. Churkin, Jr., C. Carter, H. Territory, Geol. Surv. Can. Open File Rep., C. Berg, and A. T. Ovenshine, Geology of the 830, 198Zb. Craig quadrangle, Alaska, U.S. Geol. Surv. Campbell, R. B., and C. J. Dodds, Geology of Open File Rep., 83-91, 1-28, 1983. the southwest Dezadeash Map-Area, Yukon Engebretson, D.C., A. Cox, and R. G. Gordon, Territory, Geol. Surv. Can. Open File Rep., Relative motions between oceanic and 831, 198Zc. continental plates in the Pacific basin, Spec. Campbell, R. B., and C. J. Dodds, Geology of Pap., Geol. Soc. Am., 206, 1-59, 1985. the Tatshenshini River Map-Area, British Evans, K. V., Ordovician plutonism in east- Columbia, Geol. Surv. Can. Open File Rep., central Idaho: Variation on a Canadian theme, 9Z6• 1983a. Geol. Soc.Am. Abstr. Programs,• 504, 1984. Campbell, R. B., and C. J. Dodds, Geology of Gehrels, G. E., and H. C. Berg, Geologic map of the Yakutat Map-Area, British Columbia, southeastern Alaska, U.S. Geol. Surv. Open Geol. Surv. Can. Open File Rep., 9Z7, 1983b. File Rep., 84-886, 1-28, 1984. Gehrels and Saleeby: Alexander Terrane 171

Gehrels, G. E., and J. B. Saleeby, Paleozoic Hillhouse, J. W., and C. S. Groinroe, geologic history of the Alexander terrane in Paleomagnetism of the Triassic Hound Island SE Alaska, and comparisons with other Volcanics, Alexander terrane, southeastern orogenic belts, Geol. Soc. Am. Abstr. Alaska, J. Geophys.Res., • 2594-2602, Programs,• 516, 1984. 1980. Gehrels, G. E., and J. B. Saleeby, Constraints Hudson, T., G. Plafker, and K. Dixon, and speculations on the displacement and Horizontal offset history of the Chatham accretionary history of the Alexander- Strait fault, U.S. 'Geol. Surv. Circ., 844• 128- Wrangellia- Peninsular superterrane, Geol. 132, 1981. Soc. Am. Abstr. Programs• 17, 356, 1985. Irwin, W. P., Review of Paleozoic rocks of the Gehrels, G. E., and J. B. Saleeby, Geologic map Klamath Mountains, in Paleozoic of southern Prince of Wales Island, Paleogeography of the Western United States• southeastern Alaska, U.S. Geol. Surv. Open edited by J. H. Stewart, C. H. Stevens, and A. File Rep., 86-275• 1986. E. Fritsche, pp. 441-454, Pacific Section, Gehrels, G. E., and J. B. Saleeby, Geology of Society of Economic Paleontologists and southern Prince of Wales Island, southeastern Mineralogists, Los Angeles, Calif., 1977. Alaska, Geol. Soc. Am. Bull., • in press, Irwin, W. P., Age and tectonics of plutonic belts 1987. in accreted terranes of the Klamath Gehrels, G. E., J. B. Saleeby, and H. C. Berg, Mountains, California and Oregon, in Preliminary description of the Klakas orogeny Tectonostratigraphic Terranes of the Circum- in the southern Alexander terrane, Pacific Reõion• edited by D. G. Howell, pp. southeastern Alaska, in Pre-Jurassic Rocks in 187-200, Circum-Pacific Council for Energy Western North American Suspect Terranes• and Mineral Resources, Houston, Texas, 1985. edited by C. H. Stevens, pp. 131-141, Pacific Jones, D. L., W. P. Irwin, and A. T. Ovenshine, Section, Society of Economic Paleontologists Southeastern Alaska-- A displaced and Mineralogists, Los Angeles, Calif., 1983. continental fragment?, U.S. Geol. Surv. Prof. Gehrels, G. E., C. J. Dodds, and R. B. Campbell, Pap., 800-B, BZ 11-BZ 17, 197 2. Upper Triassic rocks of the Alexander Kirk, E., and T. W. Amsden, Upper Silurian terrane, SE Alaska & the Saint Elias brachiopods from southeastern Alaska, U.S. Mountains of B.C. and Yukon, Geol. Soc. Am. Geol. Surv. Prof. Pap., 233-C•,C53-C66, 1952. Abstr. Programs•• 109, 1986. Klapper, G., and J. G. Johnson, Endemism and Gehrels, G. E., J. B. Saleeby, and H. C. Berg, dispersal of Devonian conodonts, J. Geology of Annette, Gravina, and Duke Paleontol., • 400-455, 1980. islands, southeastern Alaska, Can. J. Earth Lathram, E. H., J. S. Pomeroy, H. C. Berg, and Sci., • in press, 1987. R. A. Loney, Reconnaissance geology of Girty, G. H., and M. S. Wardlaw, Was the Admiralty Island, Alaska, U.S. Geol. Surv. Alexander terrane a source of feldspathic Bull., 1181-R• 1-48, 1965. sandstones in the Shoo Fly Complex, Sierra Lindsley-Griffin, N., and J. R. Griffin, The Nevada, California?, Geology, • 339-34Z, Trinity terrane: An early Paleozoic 1984. microplate assemblage, in Pre-Jurassic Rocks Grant, R. E., Taxonomy and autecology of two in Western North American Suspect Terranes, Arctic Permian rhynchonelloid brachiopods, edited by C. H. Stevens, pp. 63-76, Pacific Smithson. Inst. Contrib. Paleontol., 3_•313- Section, Society of Economic Paleontologists 335, 1971. and Mineralogists, Los Angeles, Calif., 1983. Hanson, R. E, J. B. Saleeby, and R. A. Loney, R. A., D. A. Brew, L. J.P. Muffler, and Schweikert, Composite Devonian island arc J. S. Pomeroy, Reconnaissance geology of batholith in the northern Sierra Nevada, Chichagof, Baranof, and Kruzof islands, California, Geol. Soc. Am. Bull., • in press, southeastern Alaska, U.S. Geol. Surv. Prof. 1987. Pap., 792• 1-105, 1975. Harwood, D. S., Stratigraphy of upper Paleozoic MacIntyre, D. G., Geology of the Alsek- volcanic rocks and regional unconformities in Tatshenshini Rivers area, B. Columbia Min. part of the northern Sierra Nevada, Energy, Mines, Pet. Resour. Pap., 1984-1• California, Geol. Soc. Am. Bull., • 413-4ZZ, 173-184, 1984. 1983. MacKevett, E. M., Jr., Geologic map of the Herreid, Gordon, T. K. Bundtzen, and D. L. MacCarthy quadrangle, Alaska, U.S. Geol. Turner, Geology and geochemistry of the Surv. Misc. Invest. Map• 1-103Z• 1978. Craig A-Z quadrangle and vicinity, Prince of Mamet, B. L., and S. Pinard, 9 Carboniferous Wales Island, southeastern Alaska, Geol. Algae from the Peratrovich Formation, Rep., Alaska Div. Geol. Geophys.Surv., • 1- southeastern Alaska, in Paleoalgology: 49, 1978. Contemporary Research and Applications• 172 Gehrels and Saleeby: Alexander Terrame

edited by D. F. Toomey and M. H. Nitecki, Stevens, and A. E. Fritsche, pp. 421-440, pp. 91-100, Springer-Verlag, New York, 1985. Pacific Section, Society of Economic Monger, J. W. H., and H. C. Berg, Lithotectonic Paleontologists and Mineralogists, Los terrane map of western Canada and Angeles, Calif., 1977. southeastern Alaska, U.S. Geol. Surv. Open Ross, C. A., and J. R. P. Ross, Late Paleozoic File Rep., 84-523• B1-B31, 1984. accreted terranes of western North America, Monger, J. W. H., and C. A. Ross, Distribution in Pre-Jurassic Rocks in Western North of Fusulinaceans in the western Canadian American Suspect Terranes• edited by C. H. Cordillera, Can. J. Earth Sci., 8_•259-278, Stevens, pp. 7-22, Pacific Section, Society of 1971. Economic Paleontologists and Mineralogists, Monger, J. W. H., J. G. Souther, and H. Los Angeles, Calif., 1983. Gabrielse, Evolution of the Canadian Ross, C. A., and J. R. P. Ross, Carboniferous Cordillera: A plate tectonic model, Am. J. and Early Permian biogeography,Geology, • Sci., 272• 577-60Z, 1972. 27-30, 1985. Muffler, L. J.P., Stratigraphy of the Keku Saleeby, J. B., J. L. Hannah, and R. J. Varga, Islets and neighboring parts of Kuiu and Isotopic age constraints on middle Paleozoic Kupreanof islands, southeastern Alaska, U.S. deformation in the northern Sierra Nevada, Geol. Surv. Bull., 1Z41-C• 1-52, 1967. California, Geology, in press, 1987. Newton, C. R., Paleozoogeographic affinities of Savage, N.M., Provincial affinities of conodont Norian bivalves from the Wrangellian, faunas from the Alexander terrane, Peninsular and Alexander terranes, western southeastern Alaska, Geol. Soc. Am. Abstr. North America, in Pre-Jurassic Rocks in Programs,• 644-645, 1984. Western North American Suspect Terranes, Savage, N.M., and S. J. Barkeley, Early to edited by C. H. Stevens, pp. 37-48, Pacific middle Pennsylvanian conodonts from the Section, Society of Economic Paleontologists Klawak Formation and the Ladtones and Mineralogists, Los Angeles, Calif., 1983. Limestone, southeastern Alaska, J. Ovenshine, A. T., Tidal origin of parts of the Paleontology, • 1451-1475, 1985. Karheen Formation (Lower Devonian), Savage, N. M., and G. E. Gehrels, Early southeastern Alaska, in Tidal Deposits• a Devonian conodonts from Prince of Wales Casebook of Recent Examples and Fossil Island, southeastern Alaska, Can. J. Earth Counterparts• edited by R. N. Ginsburg, pp. Sci., • 1415-1425, 1984. 127-133, Springer-Verlag, New York, 1975. Savage, N.M., G. D. Eberlein, and Michael Ovenshine, A. T., and G. D. Webster, Age and Churkin, Jr., Upper Devonian Brachiopods stratigraphy of the Heceta Limestone in from the Upper Devonian Port Refugio northern Sea Otter Sound, southeastern Formation, Suemez Island, southeastern Alaska, U.S. Geol. Surv. Prof. Pap., 700-C• Alaska, J. Paleontology, • 370-393, 1978. C170-C174, 1970. Scheibner, E., Suspect terranes in the Tasman Ovenshine, A. T., G. D. Eberlein, and M. Fold Belt System. eastern Australia. in Churkin, Jr., Paleotectonic significance of a Tectonostratigraphic Terranes of the Circum- Pacific Region• edited by D. G. Howell, pp. Silurian-Devonian clastic wedge, southeastern 493-514, Circum-Pacific Council for Energy Alaska, Geol. Soc. Am. Abstr. Programs• 1_• and Mineral Resources, Houston, Texas, 1985. 50, 1969. Schuchert, C., Sites and nature of the North Palmer, A. R., The Decade of North American American geosynclines, Geol. Soc. Am. Bull., Geology Time Scale, Geology, • 503-504, • 151-230, 1923. 1983. Schweikert, R. A., Early Mesozoic rifting and Panuska, B.C., and D. B. Stone, Latitudinal fragmentation of the C or di ll er an orogen in motion of the Wrangellia and Alexander terranes and the southern Alaska the westernU.S.A., Nature • z60•586-591,1976. Schweikert, R. A., and W. S. Snyder, Paleozoic superterrane, in Tectonostratigraphic plate tectonics of the Sierra Nevada and Terranes of the Circum-Pacific Region• adjacent regions, in The Geotectonic edited by D. G. Howell, pp. 109-120, Circum- Development of California, edited by W. G. Pacific Council for Energ• and Mineral Ernst, pp. 182-210, Prentice-Hall, Englewood Resources, Houston, Texas, 1985. Cliffs, N. $., 1981. Potter, A. W., P. E. Hotz, and D. M. Rohr, Scotese, C. R., An introduction to this volume: Stratigraphy and inferred tectonic framework Paleozoic paleomagnetism and the assembly of lower Paleozoic rocks in the eastern of Pangea, in Plate Reconstruction From Klamath Mountains, northern California, in Paleozoic Paleomagnetism• Geodyn., Set., Paleozoic Paleogeography of the Western vol. 12, edited by R. Van der Voo et al., pp. 1- United States• edited by J. H. Stewart, C. H. 10, AGU, Washington, D.C., 1984. Gehrels and Saleeby: Alexander Terrane 173

Scotese, C., R. K. Barnbach, C. Barton, R. Van Alexander terrane, southeastern Alaska, J. der Voo, and A. Ziegler, Paleozoic base maps, Geophy. Res., • 5Z81-SZ96, 1980. J. Geol., • 217-278, 1979. Varga, R. $., and E. M. Moores, Age, origin, and Silberling, N.J., Biogeographic significance of significance of an unconforrnity that predates the Upper Triassic bivalve Mortotis in island-arc volcanism in the northern Sierra Circurn-Pacific accreted terranes, in Nevada, Geology,9_, 512-518, 1981. Tectonostratigraphic Terranes of the Circurn- Veevers, J. J. (Ed.), PhanerozoicEarth History Pacific Region• edited by D. G. Howell, pp. of Australia• 418 pp., Clarendon, Oxford, 63-70, Circurn-Pacific Council for Energy and 1984. Mineral Resources, Houston, Texas, 1985. Wilson, J. T., Static or mobile earth, the Soja, C. M., A new look at an old fauna: Kindle's current scientific revolution, Proc. Am. Kasaan Island locality (southeastern U.S.A.) Philos. Soc., 11Z• 309-3Z0, 1908. reexamined, paper presented at 1st Woodsworth, G. J., and M. J. Orchard, Upper International Congress on Brachiopods, Univ. Paleozoic to lower Mesozoic strata and their de Bretagne Occidental, Brest, 85, 1985. conodonts, western Coast plutonic complex, Tchudinova, I. I., M. Churkin, Jr., and G. D. British Columbia, Can. J. Earth Sci., • Eberlein, Devonian Syringoporid corals from 13Z9-1344, 1985. southeastern Alaska, J. Paleontol., • 1Z4- Yancey, T. E., Permian marine biotic provinces 134, 1974. in North America, J. Paleontol., • 758-706, Tipper, H. W., G. J. Woodsworth, and H. 1975. Gabrielse, Tectonic assemblage map of the Yorath, C. J., and R. L. Chase, Tectonic history Canadian Cordillera and adjacent parts of the of the (•ueen Charlotte Islands and adjacent United States of America, Geol. Surv. Can. areas-- A model, Can. J. Earth Sci., • Map, 1505A t 1981. 1717-1739, 1981. Tozer, E. T., Marine Triassic faunas of North America: Their significance for assessing plate and terrane movements, Geol. G. E. Gehrels, Department of Geosciences, Rundsch., • 1077-1104, 1982. University of Arizona, Tucson, AZ 85721. Turner, D. L., G. Herreid, and T. K. Bundtzen, J. B. Saleeby, Division of Geological and Geochronology of southern Prince of Wales Planetary Sciences, California Institute of island, Alaska, Geol. Rep., Alaska Div. Geol. Technology, Pasadena, CA 91125. Geophys.Surv., • 11-16, 1977. Van der Voo, R., M. Jones, C. S. Gromrne, G. D. (Received June 10, 1986; Eberlein, and M. Churkin, Jr., Paleozoic revised November 25, 1986; paleornagnetisrn and northward drift of the accepted November 26, 1986.)