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TECTONICS, VOL. 11, NO. 3, PAGES 586-602, JUNE 1992
TECTONIC
HISTORY
OF THE ALEXANDER
OF THE EASTERN
EDGE
TERRANE, SOUTHEAST
ALASKA
Charles
M. Rubin! andJason
B. Saleeby
Intermontanecompositeterrane). Contractionaldeformation
alongthe lengthof the thrustbelt wasbroadlycoevalwith arc
magmatism, and thus records intra-arc tectonism. Late
Paleoceneto earlyEoceneigneousactivityandextensional
(?)
deformationsubsequently
affectedthethrustbelt.
Divisionof Geologicaland PlanetarySciences,California
Instituteof Technology,Pasadena
INTRODUCTION
Abstract.Rocksexposed
westof theCoastPlutonicComplex
in southern southeastAlaska form an imbricate thrust belt that
overprintsthe tectonicboundarybetweentwo of the largest
allochthonouscrustal fragments in the North American
Cordillera,the InsularandIntermontane
compositeterranes.In
the Alexander terrane (Insular compositeterrane), lower
Paleozoicmetavolcanicand metasedimentary
rocks(Descon
Formation)anddioriticplutonsareunconformably
overlainby
Lower Devonian clastic strata (Karheen Formation). These
rocksare overlainlocally by Upper Triassicbasalt,rhyolite
and marine clasticstrata(Hyd Group). Upper Jurassicand
Lower Cretaceous
metavolcanicandmetasedimentary
strataof
the Gravinasequenceunconformably
overlie the Alexander
terrane. The Gravinasequenceformsa structuralpackageover
15 km thick and recordsintermittentarc volcanismalong the
eastern
edgeof theAlexander
terrane.TheGravinasequence
is
structurally
overlainby upperPaleozoicandlowerMesozoic
metamorphosed
basalticstrata,marble,and argillite (Alava
sequence),
and locallyby lowerPaleozoicsupracrustal
rocks
and orthogneiss(Kah Shakessequence). Together,these
Stmcturallyimbricatedmetamorphic
rocksareintermittently
exposed along the western flank of the Coast Plutonic
Complexandextendfor approximately2000 km alongstrike,
from northernWashingtonto southeastAlaska (Figure 1).
Detailedstructural,stratigraphic,
andgeochronologic
datafrom
the metamorphicrocks are critical for understandingthe
accretionaryhistory of the northwesternCordillera. The
metamorphicrocks containmantle-derived,juvenile crustal
materialrepresented
by the Alexanderterraneon the westand
fringing continental volcanic arc rocks and continentally
derived slope-and-risedeposits of the North American
continent on the east. One of the outstanding tectonic
problemsin the northwesternCordillera is the natureof the
boundarybetweenallochthonous
ensimaticcrustalfragments,
consistingof the Alexanderand Wrangellia terranes(Insular
compositeterraneof Wheeler and McFeely [1987]), and the
westernmarginof late MesozoicNorth America(Intermontane
compositeterrane of Wheeler and McFeely [1987]). The
Ketchikanarea providesa strategiclink acrossthis boundary
because it is relatively unaffected by Tertiary plutonism.
Distinctive stratigraphicsequences
recordthe late Mesozoic
continent-marginhistoryof westernNorth America. Recent
with the
work [Crawford et al., 1987; Rubin et al., 1990a] shows that
Intermontanecompositeterrane. Local unconformityof
regionallymetamorphosed
rocksform a lateMesozoicfold and
thrust belt and involve both crystalline basement and its
constitute
the Taku
terrane which we correlate
Gravinasequencestrataover the Alava sequencedemonstrates
that the Gravina sequenceoverlappedan earlier structural
boundarybetweenthe Intermontaneand Insular composite
terranes. The rocks were deformedin the mid-Cretaceousby
west-vergentthrustingthat was wasbroadlycoevalwith arc
magmatism. Deformationinvolved emplacementof westdirectedthrustnappesoverthestructurally
intactandrelatively
unmetamorphosedAlexander terrane basement. MidCretaceoustonalitc,granodiorite,and quartzdiorite intrude
rocks of the thrust belt and are locally affected by the
deformation.Mid-Cretaceousdeformationoccurredduringtwo
episodes
thatwerecontemporaneous
with theemplacement
of
large sill-like plutons. Older structuresrecord ductile
southwest-vergent folding and faulting, regional
metamorphism,and developmentof axial-planarfoliation.
The second-generation
structuresdevelopedduringthe later
stagesof southwest-directed
thrustfaulting,whichjuxtaposed
rocksof contrastingmetamorphicpressuresand temperatures.
Structural,stratigraphic,
andgeochronologic
dataindicatethat
the two phasesof regional thrustingin southeastAlaska
occurred between 113 Ma and 89 Ma.
Rocks in the western
part of the thrustbelt were uplifted regionallyby 70 Ma.
Deformationinvolvedthecollapseof a marginalbasin(s)anda
magmaticarc, and overprintedthe older tectonicboundary
betweenthe Insularcompositeterraneandthe late Mesozoic
western margin of North America (at that time the
1Nowat Departmont
of Goology,
Contral
Washington
University,Ellensburg.
Copyright1992 by the AmericanGeophysical
Union.
Paler number91TC02182.
0278 -7407/92/91TC-02182 $10.00
volcanic
and basinal
cover.
Mid-Cretaceous
thrust belt
deformationis well documentedand wasbroadlycoevalwith
arc magmatism,involvingthe emplacementof west-directed
thrust nappes over a structurally intact and relatively
unmetamorphosedbasement. The presence of inverted
metamorphicisogradsbeneaththrustfaultsindicatesthat hot
metamorphic and plutonic rocks were translated over a
relativelycoldbasement.
In thispaper,we describethe generalstratigraphyandbroad
structuralgeometryof metamorphicrocksfrom the Ketchikan
area that include the Alexander
and Taku terranes and Gravina
sequence. U-Pb zircon geochronologicdata and a relative
structural chronology allow us to establish the timing of
deformation in relation to isotopically dated plutons.
Furthermore, by establishing the structural setting of
syntectonicandposttectonic
plutonsandwith the useof zircon
geochronology,
we are able to establishthe absolutetiming
and tectonic settingof thrust belt evolution. Stratigraphic
correlations, regional structure, and U-Pb zircon
geochronology,discussedin detail by Rubin and Saleeby
[1991a,b, c], are summarizedbelow. The datapresentedin
this paper are based upon derailed geologic field mapping
along the shorelinesand ridges of Cleveland Peninsula,
Revillagigedo,Annette,Gravina,and smalleradjacentislands
(Figure2).
GEOLOGIC
SETYING
The Alexanderterraneformsstructuralbasementfor mostof
the rocks that lie west of the CoastPlutonicComplex. The
Alexander terrane consistsof a structurallyintact lower
Paleozoic
ensimatic
arcsequence
overlain
by middlePaleozoic
clasticand carbonatestratathat are unconformablycappedby
RubinandSaleeby:
Tectonic
Historyof Eastern
Edgeof Alexander
Terrane
587
•.•.•o.o
[•qlntermontaneSuperterrone
Stikine
Insular Superterrane
Cache
Creek •
Quesnel
Alexander
terrane
•l Wrangellia
Coast
Plutonic
Complex
:• Fra•7ments
inthe
SanJuan
NW Cascades
System
Insular Superterrane
Study Area
T'['l
Cretaceous
accretionaryoceanic
complexes
• arc
Upper
Pz
- lower
Mzprimitive
& rift
assemblages
I• island
Lower
Pz,
lower
Mzprimitive
arc
assemblages
!:• CoastPlutonicComt•lex
Intermontane Superterrane
,• arc
Upper
Pz-lower
Mz
assemblages
,• accretionary
Upper
Pz- lower
Pzoceanic
complexes
• Upper
Pz
lower
Mz
basinal assemblagesmarginal
).'•
ß Lower
Pzbasinal
assemblages
':...•.•:,i.(x,
/
Pacific
Ocean
'
Cc
Vancouver
I
I
100 km
Fig.1.Generalized
geologic
mapof thenorthwestern
Cordillera
withstudyareaindicated.
Areasof midCretaceous
andyounger
deformation
arestippled.Terranemapof theCanadian
Cordillerais shownin
inset. Pz is Paleozoic; Mz is Mesozoic.
anUpperTriassic
rift assemblage
[Gehrels
andSaleeby,
1987].
In mostareas,rocksof the Alexanderterraneare only slightly
deformed
andarenothighlymetamorphosed,
exceptnearthe
eastern
boundary
wheretheyareoverprinted
by lateMesozoic
deformational
structures.UpperJurassicto LowerCretaceous
marinepyroclastic
andbasinalstrataof theGravinasequence
depositionally
overliethe Alexanderterrane. The middle
Paleozoicand lower MesozoicAlava sequenceand lower to
mid-PaleozoicKah Shakessequence(part of the Taku terrane
of Berget al. [1988])structurally
overlietheAlexander
terrane
andGravinasequence.Locally,channel-filldepositsof the
Gravinasequence
overlietheAlavasequence
andthusforman
overlapbetweentheAlexander
terraneandtheAlavasequence
[Rubin and Saleeby, 1991a). The Kah Shakessequence
locally occupieshigher structurallevels on northeast
Cleveland Peninsula, western Revillagigedo Island and
northwestern
PortlandPeninsula(Figure 2) and consistsof
lower Paleozoic meta-silicic tuff, quartzite, marble,
metabasalt,
calc-silicate,andorthogneiss.
A regionallyextensivemid-Cretaceous
west-vergent
thrust
belt containingimbricatedGravina,Alava, and Kah Shakes
sequences
occursalongpartsof the easternboundaryof the
Alexanderterrane(Figure 3). The thrustbelt consistsof an
imbricateseriesof west-vergentthrustsheets;on Cleveland
Peninsula the imbricated package has a total structural
thickness of over 15 km.
On the basis of field relations and
geochronology
discussed
here,thethrustbeltwasactiveovera
relativelyshortperiodof timeandwasbroadlycoevalwitharc
magmatism.
GEOLOGIC
FRAMEWORK
Alexander Terrane
Lower Paleozoic metavolcanicrocks. The southernpart of
Cleveland Peninsula and part of the western side of
Revillagigedo
Islandareunderlain
by metavolcanic
rocks,with
subordinatephyllite and marble of Ordovician to Early
Silurian(?) age (Figure2). The lower contactof this mafic
588
RubinandSaleeby:
Tectonic
History
ofEastern
Edgeof Alexander
Terrane
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Ketchikan
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PALEOCENEAND YOUNGERINTRUSIVEROCKS
(Coast Plutonic
+
+
+
+
+
+
+
+
+
+
+
+
+
+
U. PALEOZOIC& L. MESOZOICALAVASEOUENCF
+l•'-•
Tonalite,
quartz
diorite,
&granodiorite
Complex)
<•l'•Metamorphosed
mafic
pillow
flows,
tuff
&breccia,
•
Pegmatite
dikeswarm
MIDDLE CRETACEOUS INTRUSIVE ROCKS
?• Tonalite,
granodiorite,
diorite,
&gabbro
:?•-•'•
Zoned
ultramafic
complexes
/ gabbro
U.JURASSIC
& L. CRETACEOUS
GRAVINASEQ{IENCE
Metamorphosed
tuff,greywacke,argillite,
!• conglomerate,
basalt-andesite
tuff,
breccia
&
argillite,marble,& quartzite
PALEOZOIC& L. MESOZOICALEXANDERTERRANF
.• Triassic
siltstone,
limestone,
basalt,&conglomerate,
rhyolite
"• Devonian
sandstone,
siltstone,
& marble conglomerate,
.•"",'•
Ordovician-Silurian
basaltic
andesite
tuff,
breccia,
pillowed
flows,
& hypabyssal
rocks
x}[•Silurian
trondhjemite
&local
diorite
•]
Ordovician-Silurian
tonalite,
diorite,
&gabbro
'"'•Strike
&dipofbedding'"•Geologic
contact
"x (dashed
where
inferredI•l Cambrian
&older
(?)
meta-igneous
rocks
Strike
&dipoffoliation '"&dotted
where
covered)
pillowflows,& hypabyssalintrusiverocks
Strike
&dip
cross-cutting'•-•.
Thrust
Fault
cleavage
..
(dashed where inferred
PALEOZOIC
KAHSHAKESSEQUENGE
Devonian
orthogneiss,
lowerPaleozoic
quartz::..:..:.•
bearing
psammitic
rocks,
silicic
metavolcanic
•,, Trend
&plunge
oflineation
&dotted
where
covered)
U-Pb
zircon •
sample
location
rocks,amphibolite,
metapelite,
quartzite
& marble
EASTBEHMCANALGNEISSCOMPLEX
•
...
High
Angle
Fault
(dashed
where
inferred j•{;• Lower
Paleozoic,
tonalite
gneiss,
gneiss,
amphibolite,
& psammitic
gneissdiorite
& dottedwhere covered)
Rubin
andSaleeby:
Tectonic
History
ofEastern
Edge
ofAlexander
Terrane
Northern
Cleveland
589
Peninsula
Union Bay
SW
• O,o•
' '•O%e&•
• Ultramafic
Complex
•
%
o
o
+.•
•
SW
Garnet - Out
•
•
•
•
•
/ •
to'In
or-/
I
SW
intrusive
rocks
'
•
o•.
%,.
'½
Southorn
Clovoland
Poninsula
•
øø•
2km I
•
•
Vixen
Inlet
Eaton
Point
NE
Pluton
A'
I
2 km I
•
Bell
Island
NRFZ
Moth
Bay
•.a•t
.....
Pluton
•
contact
aureole
SRFZ •
B'
Flat.lying
aplito
andquartz
voins
Carroll
Inlet
NE
C'
Gravina
sequence
:•Mesozoic
Alava sequence
Alexander
terrane
+F:'•'l
Ultramafic
&gabbroic
intrusive
rocks
Kah Shakes sequence
I 4kin
Fig. 3. Geologiccrosssections:A-A', northernClevelandPeninsula;B-B', southernClevelandPeninsula;
C-C', CarrollInlet. Locationsof crosssections
shownin Figure2. No verticalexaggeration.
sequence
is not exposed.The uppercontactis faultedagainst
theUpperJurassic
to LowerCretaceous
Gravinasequence
and
is depositionally
overlainby theDevonianKarheenFormation
on western Cleveland Peninsula. The minimum age of
metavolcanicrocks on the southwesternpart of Cleveland
Peninsulais constrained
by a crosscuttingtrondhjemite
dike,
with single-fraction
U-Pb zirconageof 443 + 4 Ma (sample
1, Tables1 and 2). Althoughtherewasinsufficientmaterial
to obtainmultiple analyses,the internallyconcordantLate
Ordovicianageis similarto agesof metaigneous
rocksof the
DesconFormation on Prince of Wales Island [Saleebyet al.,
1984]. Similar metavolcanicand metasedimentary
rocks on
centralandsouthernPrinceof WalesIslandrangein agefrom
Early Ordovicianto Early Silurianand are assignedto the
Descon Formation [Eberlein et al., 1983; Herreid et al., 1978;
Gehrelsand Saleeby, 1987; Gehrelset al., 1987]. On the
basisof similarrocktypes,stratigraphic
position,andage,we
assignthe mafic metavolcanicand metasedimentary
strataon
southwestern Cleveland
Peninsula to the Descon Formation.
Thereis a possibilitythatthe metavolcanic
rockson southern
ClevelandPeninsularepresentslightlydeformedWalesGroup,
inasmuchas the Wales Group lies regionally beneath the
DesconFormation;however,suchan interpretationdoesnot
fundamentally
affectour analysis.
On Cleveland Peninsula, the Descon Formation consistsof
lower greenschistfaciesmetabasaltflows, breccia,and tuff.
The mafic metavolcanicstratacontain euhedralaugitc and
feldsparphenocrysts
in a paleto dark greentuffaceousmatrix
comprisedof albite, chlorite,epidote,and white mica. The
mafic metavolcanicstrataare locallyinterlayeredwith marble
and argillite;marblebedsrangein thicknessbetween1 and 3
m. The DesconFormationin this region containsa greater
proportionof mafic metavolcanicrocks than are presentin
correlative strata on Prince of Wales Island. No fossils have
been recovered from these lower Paleozoic rocks on Cleveland
Peninsula.
Metaplutonic Complex. Metamorphosedgabbro, diorite,
and plagioclaseand quartz porphyritic granodioriteunderlie
much of southwesternCleveland Peninsula (Figure 2).
Plagioclaseand quartz porphyritic granodioriteoccurs as
homogeneous, massive to foliated bodies containing
oligoclase, quartz, interstitial microperthite, and minor
hornblende. Hererogenousbodies of foliated and layered
diorite,quartzdiorite,andgabbroconsistof texturallyvaried,
fine- to medium-grained
plagioclase,hornblende,
biotite,and
minorquartz.The widespread
foliationis definedby micasand
mafic phases. Minor hornblenditeand clinopyroxenitesills
and dikesdisplaycomplexintrusiverelationswith the diorite
andgranodiorite.Interlayeredfoliateddikesandsillsof diorite
and gabbroare crosscut by porphyriticintrusivebodies. The
quartzporphyriticanddioriteplutonicrocksintrudescreens
and
septaof foliatedaugite-phyricmetabasaltand marblethat are
part of the DesconFormation. All intrusiveunitsare, in turn,
crosscutby leucogabbroand diorite pods and sills and by
quartzand feldsparveins. Zircon from a foliatedmetadiorite
dike yieldsa U-Pb ageof 445 + 8 Ma (sample2, Tables1 and
2). The metaplutonicrocksexposedon ClevelandPeninsula
are similar in composition,texture,lithology,and intrusive
relationsto metaplutonicrocks exposedto the west across
Clarence Strait on Kaasan Peninsula [Eberlein et al., 1983].
On the basisof thesegeologicrelationsand the continuityof
exposurealong the southwesternside of ClevelandPeninsula
andthe eastsideof KaasanPeninsula,the metaplutonic
rocks
on ClevelandPeninsulaare interpretedto be correlativewith
similar rocks on Kaasan Peninsula.
U-Pb zircon geochronology. All zircon fractionsfrom the
Fig.2. Geologic
mapshowing
distribution
of geologic
units,majorstructures,
andzirconsample
locations
onCleveland
Peninsula
andRevillagigedo
andadjacent
islands.
Abbreviations
areAB,Alava
Bay;B, BackIsland;
Bn,Barton
Island;
BI, BellIshmd
Pluton;
BP,Bushy
PointPluton;
C, CarolInlet;
EP,EatonPointPluton;
Gc,GnatCove,G, George
Inlet;MB,MothBayPluton;
POW,Princeof Wales
Island;
SB,Spacious
Bay;SI,SpireIsland;
T, Thorne
Arm;UB,UnionBayUltramafic
Complex.
NRFZ
isnorthern
Revillagigedo
Island
faultzone;
SRFZissouthern
Revillagigedo
Island
faultzone.Adapted
fromBerg[1972,1973](parts
of Annette
andGravina
islands),
Gehrels
andSaleeby
[1987](Prince
of
WalesIsland,partsof Annette,Duke,andGravinaislands),
andthiswork(Cleveland
Peninsula
and
Revillagigedo
andadjacent
islands).
A-A',B-B',andC-C'indicate
locations
ofcross
sections
inFigure
3.
590
RubinandSaleeby:
Tectonic
HistoryofEastern
Edgeof Alexander
Terrane
TABLE 1. U-Pb Geochronologic
SampleLocationsFrom the KetchikanArea
Sample
1
Latitude
N55ø30'00"
Longitude
131ø59 ' 6"
FieldSeahag
Nonfoliated,medium-grained
trondhjemitedike - intrudes
greenschist
faciesmetavolcanic
Zircon Properties
2:1; Sub=An>Eu;irregularshapes;colorless,
grey-tint;inclusionscommon
rocks and marble
2
N55ø31'24"
13101 ' 61"
Slightly alteredand foliated
medium-grainedquartzdiorite, in
a heterogeneoussequenceof
diorite,amphibolite,homblendite,
and pyroxenite
2:1; Sub>Eu>An;irregularshapes;colorless,
grey-to pink -int; inclusionscommon
3
N55ø46'43"
131ø48
' 3"
Highly foliatedmedium-grained
1:1; An>Sub>Eu;rounded,irregularshapes;
grey-to red-tint;inclusionscommonin all
grains:Detrital zircon
micaceousquartzite
4
N55ø54'15"
131ø11'6"
Massive,very fine-grained
gabbroicpodsanddikesthat are
interstitial
3:1;Eu>Sub>An;
colorless,
pink-tintinclusion
free
to homblendite
associatedwith the Union Bay
UltramaficComplex
5
N55ø55'00"
131033'20"
Massive,slightlyfoliated,mediumgrainedbiotite tonalite,part of the
2:1; Eu>Sub>An;pale pink-tintinclusionfree
Bell Island Pluton
6
N55ø59'57"
13203'5"
Nonfoliated,
medium-grained
biotite granodiorite,part of the
3:1;Eu>Sub>An;
colorless,
pink-tintinclusion
free
Eaton Point Pluton
7
N55ø7'58"
132ø51'13"
Nonfoliated,medium-grained
porphyriticgranodiorite,exposed
on the northernshoreof Spacious
Bay, par t of the EatonPoint
3:1;Eu>Sub>An;
colorless,
pink-tintinclusion
free
Pluton
8
N55ø48'42"
131047'4"
Massiveto slightlyfoliated,
medium-to coarse-grained
epidote-bearing
granodiorite,part
of the BushyPointPluton
3:1;Eu>Sub>An;
colorless,
verypalepink-tint,
inclusionfree
Abbreviations
areEu, euhedral;
Sub,subhedral;
An, anhedral.2:1, length:width
ratiosof zircongrains.Colordetermined
under
reflectedlight.
Paleozoicsamplesyielddiscordant
U-Pb andPb-Pbdata. Both
samples(samples1 and 2) havePb-Pb agesfrom 443 to 445
Ma andU-Pb agesin the 330-430 Ma range(Table2). Zircon
in these samples lacks optically distinguishedcores or
compositionalzoning. We interpretthe discordanceas the
result of episodiclead loss during Mesozoic and Cenozoic
time, due to a younger thermal disturbance. This
interpretationis supportedby hornblendeand biotite K-Ar
ages,as youngas ~79 Ma, from PaleozoicAlexanderterrane
rockson AnnetteandDuke islands[SmithandDiggles,1981].
Both polyphasedeformationand mid-Cretaceous
greenschist
faciesmetamorphism
haveaffectedtheAlexanderterranerocks
and perhapscontributedto the isotopicdisturbance. This
interpretationis in accordwith that of Gehrelsand Saleeby
[1987], who report similar disturbanceof U-Pb isotopic
systemsin metaplutonic rocks of the Alexander terrane.
Alternatively, the U-Pb isotopicdisturbancemay be due to
later mid-Teritary hyrdothermalactivity and associatedfluid
flow [MagaritzandTaylor, 1986].
Karheen Formation. Interbedded siliceous argillite and
micaceouslimestoneoverlainby distinctivepebbleto cobble
conglomerate
layers,30-50 cm thick, unconformably
overlie
metaplutonicrockson the shorelinesouthof Niblack Hollow
on westernCleveland Peninsula(Figure 2). Clasts in the
conglomerate
are matrixsupported
andare usuallylessthan5
cm in diameter in an argillaceouslimestonematrix. Clasts
consistof roundedto subangular
plutonicandvolcanicclasts,
and vein quartz. Basalticsills anddikesintrudethesequence.
No fossilshave beenrecoveredfrom the clasticsequence.On
the basis of similarities of stratigraphic position and
lithology,the clasticstrataare interpretedas part of theLower
Devonian conglomeratic part of the Karheen Formation,
exposedon central Prince of Wales Island [Eberlein et al.,
1983]. On Princeof Wales Island,similarstrataconsisting
of
pebble to cobble conglomerate, limestone, shale, and
greywacke unconformably overlie metaplutonic diorite
[Eberleinet al., 1983]. The conglomeraticstratarecorduplift
anderosionof Ordovicianand SilurianrocksduringtheKlakas
orogeny[Gehrelset al., 1987].
Hyd Group. Upper Triassic strataof the Hyd Group on
Annette and Gravina islands consist of limestone, fine- to
coarse-grained
clasticsedimentaryrocks,andbasaltto rhyolite
volcanicrocks (Figure 2) [Berg, 1972, 1973]. Theserocks
unconformablyoverlie pre-Devonianstrataof the Alexander
terraneand are unconformablyoverlainby the UpperJurassic
to Lower CretaceousGravinasequence(seediscussion
below).
TheseUpperTriassicstratarecentlydescribed
by Gehrelset al.
[1987] are not present on Cleveland Peninsula and
RevillagigedoIsland.
Taku Terrane
Alava sequence.The Alava sequence(part of the the Taku
terraneof Berg et al. [1988]) is a distinctiveupperPaleozoic
RubinandSaleeby:
Tectonic
History
ofEastern
EdgeofAlexander
Terrane
and lower Mesozoic stratifiedsequenceand, along with the
Gravinasequence,
is exposedalongtheeasternboundaryedge
of the Alexander terrane along its 100-km extent in the
Ketchikanarea. The sequenceis fault boundedon the eastby
lower to middlePaleozoicorthogneiss,
and locally on eastern
RevillagigedoIslandthe sequence
is intrudedby Cretaceous
andyoungerfoliatedtonalireandgranodiorite.
The Alava sequence,
in southernsoutheast
Alaska,consists
of two members:a lower memberof UpperPennsylvanian
and
Lower Permian metavolcanic and metasedimentary
assemblage,
anda uppermemberof Middleto UpperTriassic
mixedmetasedimentary
andmetavolcanic
assemblage
[Rubin
andSaleeby,1991a]. The lowermemberconsists
of massive
crinoidalmarbleinterlayeredwith blackargilliteandphyllite,
and mafic metavolcanicflows, breccia,tuff, pillowed flows,
and minor quartzite. The upper member comprises
carbonaceous
and siliceousnodularlimestone,argillite,mafic
metavolcanictuff, breccia,andpillowedflows. The baseof
the Alava sequenceis not exposed, and the sequence
structurally overlies either the Gravina or Kah Shakes
sequences
or the Alexanderterrane. Locally, strataof the
Gravinasequence
unconformably
overliethe Alava sequence
[Rubinand Saleeby,1991b,c].
On thebasisof lithology,broadageconstraints,
andtectonic
setting,the Alava sequencemight be correlativewith the
upperPaleozoicpartsof the Yukon-Tananaterrane,whereas
thelowerMesozoicportionof theAlavasequence
is correlated
591
with the Triassicportionof the Stikine terrane(e.g., Stuhini
Group[Rubinand Saleeby,1991a]).
Kah Shakessequence.The Kah Shakessequence
(partof the
Taku and Tracy Arm terranesof Berg et al. [ 1988] comprises
metamorphicrocks exposedalong the westernflank of the
CoastPlutonicComplexand forms screensand septawithin
part of the Coast Plutonic Complex (Figures 1 and 2)
[SaleebyandRubin, 1990a]. The sequenceis boundedon the
westby a thrustfault in whichtheAlexanderterraneformsthe
lower plate, and is intrudedon the east by the voluminous
CretaceousandTertiaryplutonicrocksof theCoastbatholithic
belt. On southernmostRevillagigedoIsland, the Kah Shakes
sequencestructurallyoverliesthe Alexanderterranealongan
east-dippingthrustfault. The sequenceconsistsof a thick
succession
of quartziteandcarbonate-richturbidires,siliceous
and quartzo-feldspathic
schist,subordinatemetabasalt,pelitic
schist, and orthogneiss (Figure 4). Highly deformed
micaceous
quartzitefromeasternClevelandPeninsula
yieldeda
very small,heterogeneous
zirconpopulation(sample3, Table
1). Becauseof the very smallsamplesize,onefraction(62-80
gm) was analyzedfor its "average"isotopicproperties. A
Precambrian
207pb-206pb
average
ageof 2.03Gafor the
quartziteis a minimum age for the oldestcomponentin the
fractionandindicatesthatPrecambrianrocksformedpartof the
ultimate sourcefor the quartz-richdetritus. To the southon
Portland
Peninsula,
similarPrecambrian
207pb-206pb
ages
on Kah Shakes quartzite have been reported [Saleeby and
TABLE 2. U-Pb ZirconIsotopicAge Datafor theAlexanderTerrane,Kah ShakesSequence
andMid-Cretaceous
Plutons
Atomic Ratios
Concentrations,
ppm
Amount
206p
b
206pb, 207pb* 207pb
*
238U
Sample
Fractiøn,1
Analyzed,
238
U206pb*
204pb
gm
mg
IsotopicAges,Ma
235U
206Pb*
206pb* 207pb
* 207pb*
238U
235U
206pb*
Alexander terrane
1
45-62
0.6
130
7.9
7110
TrondhjemiteDike
0.07289(44) 0.05403 0.05578(14) 438.0
438.7
443+4
2
2
2
45-62
62-80
>80
2.7
2.3
1.0
1167
991
1201
53.1
49.2
67.1
42995
50280
5742
Foliated QuartzDiorite
0.052550(36) 0.40425 0.05581(72) 330.2
0.053064(34) 0.44088 0.05582(19) 359.3
0.064319(54) 0.49498 0.05583(04) 401.9
344.7
370.9
408.3
444+26
445+8
445+5
0.2
93
4.6
32
MicaceousQuartzite
0.032484(07) 0.56088 0.12529(17)
452.1
2.8
3.1
3.7
462
508
587
5.5
5.8
6.9
7007
14901
6889
Kah Shakessequence
3
45-62
206.1
2032
Mid-Cretaceous
plutons
4
4
4
45-62
62-80
80-100
replicate 80-100
2.7
434
5.1
12982
4
100-120
3.2
493
5.8
10950
42
>120
3.3
449
602
11894
UnionBay UltramaficComplex
0.013690(07) 0.09061 0.048026(11) 87.7
0.013219(07) 0.08053 0.048053(09) 84.7
0.103470(08) 0.08926 0.048077(15) 86.3
0.013476(07) 0.08917 0.048019(09) 86.3
0.013676(07) 0.09074 0.048144(20) 87.6
0.015931(09) 0.10541 0.048011(18) 101.9
88.1
85.2
86.8
86.7
88.2
101.7
101+6
101+4
102+7
99+10
105+11
99+ 18
91.3
90.6
105+7
102+11
95.4
92.6
95.8
115+15
103+13
95+26
Bell Island Tonalite
5
5
45-80
80-100
4.3
6.3
1190
449
14.3
5.7
8037
412
0.014910(09) 0.09406
0.014082(11) 0.09331
6
6
6
<25
25-45
45-62
3.6
5.2
9.0
692
898
725
8.9
11.2
9.5
6955
7421
5532
0.014784(10) 0.09850
0.014368(09) 0.09524
0.014976(10) 0.09897
0.048132(07) 90.7
0.048083(19) 90.2
Eaton Point Granodiorite
0.04834(08)
0.04809(11)
0.04795(39)
94.6
92.8
95.8
592
RubinandSaleeby:TectonicHistoryof EasternEdgeof AlexanderTerrane
TABLE 2. (continued)
Concentrations,
ppm
Amount
AtomicRatios
IsotopicAges, Ma
206p
b 206p
b, 207p
b, 207p
b, 206p
b, 207p
b, 207pb,
Sample
Fractiøn,1
Analyzed,
238
U206pb*
204pb
238
U 235
U 206Pb
* 238
U 235
U 206pb*
gm
mg
Mid-Cretaceous
plutons(continued)
Spacious
Bay PorphyriticGranodiorite
7
7
7
62-80
80-100
>100
7.5
5.9
2.9
752
870
536
9.7
11.3
6.9
7598
6468
9036
0.014910(10) 0.09852
0.01492(10) 0.09847
0.014935(10) 0.99129
0.047941(22) 95.4
0.047819(18) 95.6
0.048157(37) 95.6
8
8
62-80
80-120
2.4
5.8
2248
1368
28.7
17.5
BushyPointEpidote-Bearing
Granodiorite
1713
0.01479(10) 0.09772 0.04793(09)
1333
0.01476(10) 0.09774 0.04793(09)
94.6
94.4
95.4
95.4
95.6
95+26
90+11
106+35
94.6
94.4
95+5
93+5
i Fractions
separated
bygrain
size
and
magnetic
properties.
Magnetic
properties
are
nonmagnetic
using
20øside
and
2øfront
slopes
at
1.7 A on FranzIsodynamicSeparator.Sampleshand-picked
to 99.9% purityprior to dissolution.Dissolutionandchemicalextraction
technique
modifiedfromKrogh[1973].
2 Zircon
fraction
leached
inaPFAteflon
beaker
ona hotplatewithHFfor7 days
at~100øC
andfluxed
twice
for2 days
in6NHC1
priorto sampledigestion.
3 Radiogenic:
nonradiogenic
correction
based
upon40 picogram
blankPb(1:18.78;
15.61;38,50)andinitialPbapproximations:
206/204 = 48.6; 207/204 = 15.6; 208/204 = 38.2.
4 Decay
constants
usedin
agecalculation:
3,238U = 1.55125
x 10©, 3,235U= 9.98465
x 10© [Jaffey
etal.,1971];
238U/235U
atom
= 137.88.
Uncertainties
(+)inradiogenic
milos
calculated
byquadratic
sum
oftotal
derivatives
of238Uand206pb*
concentration
and207pb*/206pb*
equations
witherrordifferentials
defined
asfollows:
(1)isotope
ratiodeterminations
fromstandard
errors
(B/N)of
massspectrometer
runsplusuncertainties
in fractionation
corrections
basedon multiplerunsof NBS 981,983, andU500 standards;
(2)
spikeconcentrations
from rangeof deviationsin multiplecalibrationswith normalsolutions;(3) Spikecompositions
from external
precisions
ofmultiple
isotope
ratiodeterminations;
(4)uncertainty
innatural
238U/235U
fromChenandWasserburg
[1981];
and(5)
nonradiogenic
Pb isotopiccompositions
from uncertainties
in isotoperatio determinations
of blankPb and uncertainties
in composition
of initialPb fromestimates
of regionalvariations
basedon references
givenaboveandconsideration
of rocktype.
Rubin, 1989, 1990a]. On the basisof lithologicsimilarities,
the presenceof Precambriandetritalzircon,spatialproximity
andtectonicsetting,theKah Shakessequence
is interpretedas
correlative with the Yukon-Tanana terrane, which is now
recognizedwithin roof pendantsand metamorphic
screensas
far southaslatitude59ø,eastof Juneau[WheelerandMcFeely,
1987; Gehrelset al., 1990; Gehrelset al., 1991; Rubin et al.,
1990b;Rubinand Saleeby,1991a].
GravinaSequence
Upper Jurassicand Lower Cretaceousstrata structurally
overlie
lower
Paleozoic
rocks on southeastern Cleveland
Peninsulaand locally lie unconformablyover Triassicrocks
on Annetteand GravinaIslands(Figure2) [Berg,1972, 1973;
Brew and Karl, 1987; Rubin and Saleeby, 1991c]. These
stratabelongto the Gravina-Nutzotinbelt [Berget al., 1972]
which we informallycall the Gravinasequencein southern
southeasternAlaska. The sequenceis exposedalong the
easternmarginof the Alexanderterranein muchof southeast
Alaska. Near Ketchikan,the Gravinasequence
consists
of two
distinctive members; however, stratigraphic thickness is
uncertaindueto complexdeformation.
On Cleveland Peninsula,the lower contact of the Gravina
sequence
is a thrustfault with thelowerPaleozoicrocksof the
Alexanderterrane(Figure5). There,theGravinasequence
has
an approximate structural thickness of 15 km. The lower
memberconsistsof argillite, calcareousargillite, waterlaid
coarsepyroclasticdeposits,tuff, lavas, and minor intrusive
rocks. Pyroclastic volcanic depositsdominate the lower
member on Cleveland
Peninsula
and contain massive to
pillowed flows and flow breccia,commonlywith claststhat
are angular to subangularin shape. The age of the lower
memberis poorly constrainedand, on the basisof lithologic
and stratigraphic similarities with Upper Jurassic strata
exposedon GravinaIsland[Berg,1973],thelowermemberon
ClevelandPeninsulais interpretedasLate Jurassicin age.
The upper member consistsof argillite, tuff, slate, and
conglomerateand is well exposedon Cleveland Peninsula,
Revillagigedo, Gravina, and adjacent islands. The lower
contactof the member is not exposed;however, locally the
uppermemberoverliesboth the lower memberof the Gravina
sequence
and unconformably
overliestheupperPaleozoicand
lower MesozoicAlava sequence.The uppercontactis a thrust
fault with adjacent terranes. Lithologic units consist of
argillaceousand tuffaceousturbidiresand pebbleto cobble
conglomerate,with a total structuralthicknessof 900 m.
Conglomeratebedsoccurin a distinctivemappablehorizon,
exposed between southeastern Cleveland Peninsula and
southeastern
RevillagigedoIsland. Clastsin the conglomerate
yield U-Pb zircon ages of 154 Ma to 158 Ma [Rubin and
Saleeby, 1991c]. These Late Jurassicagesfor the granitic
clasts provide a maximum age for deposition. Based on
geologicrelationswith similarstrataexposedto the north and
sparsefossildatain theKetchikanarea[BergandCruz, 1982],
an Early Cretaceousage is inferred for the upper member
clastic and tuffaceous strata.
Mid-Cretaceous Intrusive Rocks
Mafic and ultramafic rocks. Mid-Cretaceousmafic and
ultramafic bodies underlie part of Cleveland Peninsula,
Revillagigedo,and Duke islands(Figure 2), The ultramafic
complexesform a linear belt that spansapproximately560
RubinandSaleeby:
Tectonic
Historyof Eastern
Edgeof Alexander
Terrane
593
Fig. 4. Amphibolite-facies
rocksof quartzite
andmarblein theKah Shakessequence
exposed
northof
RudyerdInleton theeast-sideof BehmCanal.
120
110
I
100
i
i
90
i
80 Ma
i
Sample #
Cleveland ,/""/
Peninsula
/ /
breccia
{-• ,
I Grav,
.... q.....
6•n
Point
k ¾/
/;-.m
Argillite
1•%_
½ / •01•';;J•;• I• Maf,
ctu,
andvo,
can,cf,ows
• 7•
Spacious
Bay
• _ •y
• gg2•nO
/ • 100
...... ,
Pom,
•• ,oliation
=•,ng
Fig. 5. Geologic sketch map of the contact between the
Alexander terraneand the Gravina sequence,exposedin an
!sland
4 [.•._..•__
__TertiaryPb loss...•-r'
HFLeach
unnamed cove northeast of Caamano Point, southern
Cleveland Peninsula. Regional location is shown in inset
(KetchikanC-6 quadrangle).
Bay
I
i
i
I
120
110
100
90
!
80 Ma
238
U/2o6
Pbage
km, from northwestern British Columbia to southeastAlaska
[Taylor, 1967]. The complexesintruderocksof the Alexander
terrane,andGravinaandAlava sequences.
The bodiesoccuras
zoned ultramaficcomplexesrangingin compositionfrom
dunite,in thecenter,to pyroxenite
andhornblende
pyroxenite
on the outer border [Taylor, 1967; Irvine, 1967, 1974].
Interstitial gabbroicpods and lensesare presentin the
hornblendite.Plagioclasefrom the gabbrois extensively
saussuritized.Zircon from gabbropods and veins within
hornblendite
in theUnionBay UltramaficComplex(Table1)
Fig. 6. Concordiaplotted separatelyfor each sample,with
scale
for207pb/206pb
ratioshown.Eachsegment
contains
data pointsand error bars for indicatedsamplesof igneous
rocks from Cleveland Peninsula and Bell Island. Bars at ends
of concordia
segments
showuncertainty
in 207/206pb
values
of concordia
based
onuncertainties
in 238Uand235Udecay
constantsfrom Mattinson's[1987] treatmentof Jaffey et al.
[1971] data. Concordiadiagramafter Tera and Wasserburg
[1972]. Linear regressionand errorsin lower and upper
intercepts
areadaptedfromYork [1969].
yielddiscordant
238U-206Pb
ages
ranging
from84.7to88.1
Ma (sample4, Table 2). A leachedcoarsezircon fraction
yields
a concordant
238U_206Pb
ageof-101.5Ma(sample
4,
Table2). Fivefractions
aredispersed
off concordia
(Figure6),
whichmay reflectlater disturbance
of the U-Pb isotopic
systems
inzircon.
The238U-206pb
and235U-207pb
ages
areslightly
discordant
at---86Ma;however,
the207pb-206pb
agesare olderandrangebetween99 and 105 Ma (Table2).
Discordance
probablyreflectslater isotopicdisturbance
of
Cretaceouszircon populationsrather than inheritancefrom
olderxenocrystic
zircon. This interpretation
is consistent
with
the lack of compositional
zoningor opticallydistinguished
coresin zircon. Widespread
Miocenevolcanism,
hydrothermal
activity and associatedfluid flow have beenrecognizedin
southeast
Alaska [Magaritzand Taylor, 1986] and may have
beenresponsible
for theisotopicdisturbance
in theUnionBay
zircon. On the basisof the foregoing,the gabbroicpods
594
Rubin
andSaleeby:
Tectonic
History
ofEastern
Edge
ofAlexander
Terrane
withinthe hornblendite
are assigned
an approximate
ageof
102Ma,which
represents
theoldest
207pb-206pb
ageforthe
sample
andthe238U-206Pbagefor theleached
zircon
fraction. K-Ar analyseson hornblendeand biotite of the
ultramaficbodiesyield 100-110Ma coolingages[Lanphere
and Eberlein,1966]. K-Ar analyseson the Union Bay
ultramaficbody are consistentwith the 108 Ma U-Pb zircon
ageobtainedfrom a pegmatiteassociated
with theDuke Island
ultramaficcomplex[J.B.Saleeby,unpublished
data,1990].
Quartzdioriteandgranodiorite.NorthernRevillagigedo
IslandandCleveland
Peninsula
areunderlain
by a composite
batholithcomposedof massiveto foliated,medium-to coarse-
grainedhornblende
quartzdiorite,tonalite,andgranodiorite
(Figure2). TheBellIslandPlutononnorthern
Revillagigedo
andBell IslandsandnorthernClevelandPeninsula
is partof
thecomposite
body(Figure2). Subhorizontal
dikesandveins
of quartz-feldspar
pegmatiteand subordinate
agmatiteare
presentthroughout
thebatholith.The rockscontainvarying
proportions
of plagioclase,
microcline,quartz,hornblende,
and
brown biotite. Apatite, zircon, sphene, ilmenite, and
magnetite
occurasaccessory
minerals.A leucocratic
quartz
diorite(Bell IslandPluton,sample5, Table 1) yieldsU-Pb
zircon age of 90.5 + 7 Ma (Table 2). These data are
consistentwith a preliminaryU-Pb zircon age of 89 Ma
reportedby Arthet al. [1988]. K-Ar analyses
yieldhornblende
cooling ages that range from 86 to 55.5 Ma, and biotite
coolingagesthatrangefrom74 to49 Ma [SmithandDiggles,
1981]. The batholithintrudesthe Gravinasequence
on
northernClevelandPeninsula,
theKah Shakessequence
on
by Zen [1985, 1988],Zen andHammarstrom[1984],Hollister
et al. [1987], and Cook et al. [1991]. The rocks are
characterized
by apple-green,
euhedralepidoteandcontain
quartz,
plagioclase,
microcline,
hornblende,
biotite,
andlocally
garnet. Magnetite,ilmenite,apatite,zircon,sphene,and
allaniteoccurasaccessory
minerals.
Theplutons
areusually
massive
in theinteriorandfoliatedalongtheirmargins
and
intrudetheGravinaandAlavasequences
and,locally,the
Alexanderterrane.The bodiesdisplayintrusivecontacts
and
arelocallyin faultcontact
withadjacent
metasedimentary
and
metaplutonic
rocks.U-Pbzirconagesfor theplutons
range
between94 Ma and 101 Ma, andlocallyzirconsystematics
revealProterozoic
inheritance(e.g., the Moth Bay Pluton
[Rubin
andSaleeby,
1987]).A hornblende
Ar40/Ar
39plateau
ageof 97 Ma isreported
by SutterandCrawford[1985]forthe
Moth Bay Pluton.
U-Pb zircongeochronology.
Mid-Cretaceous
ageswere
determinedfor all four plutons on northern Cleveland
Peninsula
andRevillagigedo
andBellislands.Theporphyritic
granodioriteof SpaciousBay (sample7) shows95 Ma
concordance
of all thefractions,
withslightdispersion
of its
coarsefractionoff concordia
whichprobablyreflectsminor
contamination
by olderzirconfromitscountry
rocks(Figure
6). The EatonPointgranodiorite
(sample6) is discordant,
with its coarsefractionlying on concordiaat - 95.8 Ma
(Figure6). The BushyPointepidote-bearing
granodiorite
(sample8) yieldsa U-Pbageof 94.5Ma, withPb-Pbagesof
95 + 5 and93 +5 Ma, respectively
(Table2). The two finer
fractions
aredispersed
off concordia,
whichmayreflectlater
east Behm Canal, and the Alava sequenceon western
Revillagigedo
Island.
disturbance
of theU-Pbisotopic
systems
in zircon,perhaps
dueto the emplacement
of the youngerBell Islandpluton
Plagioclaseporphyriticgranodiorite,quartzdiorite,and
tonalite. Elongate stocks, plutons, sills, and dikes of
plagioclaseporphyryand quartz diorite intrude much of
Revillagigedoand adjacentislandsand ClevelandPeninsula
(Figure 2). The Eaton Point Pluton, exposedon northern
ClevelandPeninsula,is assignedto this unit. Locally,the
rockscontainabundant
plagioclase
phenocrysts,
whichmake
up to 50% of the rock and range between 1 and 4 cm in
diameter. The groundmass
containsfine-grainedquartz,
microcline,biotite,minorhornblende,
epidote,and garnet.
Euhedralgarnetoccurswithincoresof plagioclase,
butit also
occurswithinthegroundmass.
Anhedraltoeuhedral
epidoteis
interstitial to plagioclasecrystals. The plutonsintrude
metasedimentary
and metavolcanicrocks assignedto the
GravinaandAlavasequences,
andlocallytheAlexander
terrane
(?), and have narrow contactaureolesthat overprintthe
metamorphic
foliation. The sills anddikesare parallelwith
foliationand locally crosscutthe metamorphic
framework
(sample5) or Miocenehydrothermal
activitydiscussed
above.
A youngerisotopicdisturbance
is recordedby K-Ar ages
[SmithandDiggles,1981]. QuartzdioriticrocksyieldK/Ar
rocks, increasingin abundancetoward the east. The Eaton
PointPluton,a medium-grained
granodiorite,
yieldsa U-Pb
zircon age of 95+3 Ma (sample6, Tables 1 and 2). A
plagioclase
porphyriticgranodiorite
phaseof theEatonPoint
Plutonyieldsa U-Pb ageof 95+1Ma (sample7, Tables1 and
2). A granodiorite
phaseof the BushyPointPluton(Bushy
PointPluton,sample8, Table1) yieldsa U-Pb ageof 94.5
Ma (Table2). Thesedataare consistent
with a preliminary
discordant
238U-206pb
zircon
ageof92.7Mareported
by
Arth et al. [1988]. K-Ar analysesyield 82 + 2.5 Ma
(hornblende)
and86 _+2.7 Ma (biotite)coolingages[Smith
andDiggles, 1981].
Epidote-hornblende
tonalite,quartzdiorite,andgranodiorite.
Epidote-bearing
granodiorite,
quartzdiorite,andtonaliteoccur
as elongateplutonson southwestern
andpart of northeastern
Revillagigedo
Island(Figure2). Thesedistinctive
plutons
continue
asa beltfarthernorthintothePetersburg
andJuneau
area(Figure1) [BrewandMorrell,1983].Thepetrology
and
mineralchemistry
of theepidote-bearing
plutonsarediscussed
agesof---74 Ma (biotite)and81.8 to 79 Ma (hornblende)
on
northernClevelandPeninsulaand on adjacentislands
southwestof Bell Island (uncertainties
in the K-Ar datesare
reportedat the lc• level). The dispersion
of the finer fractions
may also be due in part to contaminationwith older zircon.
Disturbance
of the U-Pb isotopicsystemsmay havebeen
morecomplexthana singleepisode
of leadloss,perhaps
due
to a combinationof Pb loss and contaminationof older zircon.
Thequartzdioriteof BellIsland(sample
5) yieldsa U-Pbage
of 90.5 Ma, with olderPb-Pbagesof 102+ 11 and 105+ 7
Ma, respectively(Table2). A 90 Ma ageis consistent
with
nearly
concordant
206pb/238U
zircon
ages
of•-89 and91Ma
[Arth et al., 1988];Pb-Pbageswerenot reportedin their
study.
STRUCTURAL
GEOLOGY
Only recentlyhavemid-Cretaceous
basement-involved
thrust
faultsand associated
nappestructures
beenrecognized
in
southernsoutheast
Alaska [Berget al., 1972, 1988;Crawford
et al., 1987;Rubinet al., 1990a;RubinandSaleeby,1991b].
Near Ketchikan, deformation of lower Paleozoic to mid-
Cretaceous
supracrustal
andplutonicrocksis localizedalong
the eastern boundary of the Alexander terrane and is
characterizedby an imbricateseriesof west- to southwest-
vergent thrust sheets. Based on regional geologicand
geochronologic
relations,the west-vergentfold and thrust
systemwasactivebetweenabout113 and89 Ma. Younger
intrusiverocksdo notdisplaythemid-Cretaceous
thrust-related
fabrics. Both the younger intrusiverocks and the rocks
affectedby mid-Cretaceous
deformation
weresubsequently
affectedby late Paleoceneto earlyEocenedeformation
and
associateduplift [Hollister, 1982; Crawford et al., 1987;
Gehrels and McClelland, 1988; Gehrels et al., 1992;
RubinandSaleeby:
Tectonic
History
ofEastern
EdgeofAlexander
Terrane
McClelland
et
al.,
1992,a,
b].
Deformation
and
595
plutonicbodies.Olderfabricsrecordductilesouthwest-vergent
folding and faulting and regional metamorphism,whereas
youngerfabricsare characterized
by crenulationcleavageand
thrustfaultingand associatedfolding. Structuralstyle varies
along the strike of the orogeniczone and betweenstructural
levelsdue to changesin rock type,overallmetamorphic
grade,
and proximity to plutonic rocks. Cross sectionscannotbe
rigorouslybalanceddueto (1) changesin bulk-rockvolume,
(2) the presenceof ductilestrainandassociated
plasticflow,
and (3) the absenceof exposedstep-upanglesalong thrust
metamorphismwhich precededmid-Cretaceoustectonismare
discussed
by McClellandandGehrels[1990] andSaleebyand
Rubin[1990b]andappearto represent
pre-LateJurassic
dextral
strike-slipdismemberment
of the Alexanderterrane,possibly
during its accretion, and subsequentLate Jurassic-earliest
Cretaceous
extensionacrosstheaccretionary
suture.
Mid-Cretaceousdeformation occurredduring three main
episodesthat correspondwith the emplacementof sill-like
faults.
First-Generation
Mid-Cretaceous
Thrust Faults
The lowest thrust sheets consist of lower Paleozoic schist,
marble, and metaigneousrocks of the Alexander terrane
(Figures 3a and 3b). These rocks are characterizedby
mesoscopicfolding, nonpenetrativefoliation, and thrust
faulting and display lower-greenschist facies mineral
assemblages.Earlydeformationalfabrics (D1) are
characterized by mesoscopicnortheast dipping foliation
surfaces
(S1)anda bedding-foliation
intersection
lineation
(L0xl). Thrustfaultstrendnorthwest
anddipmoderately
to
ß •=58
'• I,,I=$4
the northeast. The Gravina sequencestructurallyoverliesthe
Alexanderterrane,and on ClevelandPeninsulathe sequence
consistsof six separatewest-vergentthrustsheets(Figure3b).
The basalthrustfault separating
the Alexanderterranefrom the
Gravina sequence crosscutsthe metamorphic foliation,
displaysa cataclasticfabric,andjuxtaposesshallowlydipping
Gravinasequenceargilliteover the moderatelydippinglower
PaleozoicDesconFormationof the Alexanderterrane(Figure
5). Lower Paleozoicand upperProterozoiccrystallinerocksof
ß I,,1=$$ 0•=•"1
the Alexander
to foliation surfaces on northwest Cleveland Peninsula, (b)
polesto foliation surfaceson southernClevelandPeninsula,
(c) trendand plungeof small-scalefoldsandpolesto foliation
surfacesin higher level thrustnappeson northernCleveland
Peninsula,and (d) poles to foliation surfaces,and trend and
plungeof elongationlineationin higherlevel thrustnappes.
:•;?.i
:'•'""'"'•'•:
.........
":•?•!•!?;%.......;:;:•%.•..•
...........
•'•'•" ½:"'•
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..:.:.;:.:.:-:•
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'•?:::•-•.•.
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"'":'}::?;'•:•[::;•g}-:•5:::;;.:-•-'.-:
:•:.::-'--.-:-::..:';:.
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-.-.,.,.•
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:•:•:.;..:•:½:.,'
.......½i•
-•:'"'"';•'•'--'*•'
%":"}•:;•.
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'• :' •
''.'.'½'"'.;:4'
• '"•;:;*'".*•
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•':'%..i':-;•u..E•:.
• .........:.:-'::;•.r,':•:•½...:
½,'-.•,•
-...--:-:..----.----•
... •:•.... '-!.•}
..;½:. :..--•'.
4-•".
%•'::•.......?..:::.:•:::-:•?•.::.:;:..•;;%•:::::•:;.:...•:•,•;...::•:,•½.•.•:.%..,::
'•,. ....;:•.•:.•;:.;•:....:
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•.• ½ß •-.-..
......
:.;..
. ½';•:':"•5
. .:-..,:•..:....
•...
.......
::. ...,
terrane form structural basement to the thrust
belt. In general, thrust faults trend northwest, and their
orientation generally follows the regional metamorphic
foliation (Figure2).
There is a predominanceof moderate,northeast-dipping
foliation surfaces (Figure 7a)which are axial planar to
asymmetric, west- to southwest vergent folds. These
asymmetricfolds (Figure 8) are truncatedby thrustfaultsand
Fig. 7. Lower hemisphere,
equal-areaplot showing, (a) poles
.
""'
......
. ,--:.•;• ....•
.........
4:•.
......
...:-
.%... .
.:.:..
....
s:•:..•
•,
.
.•-:½&' .•....
ß.:,•..•
.......
,
½'-.•
•. -•:.
...•: _
.•
•.
.
-.;•.,. •....
:.
..
'".
...:•...
:•.
•-..;..;;•:;;.•...:;.
;..:.:
.:•..?';.•:•."....•,;;
- .;
:
"-
. :.:
•b
ß
.......
.....: .:.
•%• .... '......................
•.::;::•;:•:,.
" ,;' ß•.
; ;...'
.... ....:½:,:':•}•:•,.."':'";::•i':'i.•
..............
. ,.•...... }.&.
%.... .•..
..';;:•':;?•;•:::;•-;
.....
: ....•;:•:•...:• ß
•.--,%}::.•.-•.,;•:
...........
:•::•-•'•½•½:•.::;,
..
,.• •,.
'•:'•.:;{:8
•:•,. '.,'...Q•-.•:?
.-.:•
...... ::..,½
L::'.;::,.
...
_•';"
...
•
....,.-,:.:....
.•.
.-..•..
-,-•.'
............... ,• .............
..................
.•:
Fig. 8. Asymmetricfoldstruncatedby moderatelydippingthrustfaultsin the Permianpart of the Alava
sequence,exposedon RevillagigedoIsland. This fault zoneplacesPermianmarbleand metabasaltover
Triassiccarbonaceous
pelitic schist.
596
Rubin•mdSaleeby'
Tectonic
Historyof Eastern
Edgeof Alexander
Terrane
appearkinematically
linkedto thethrustfaults.In well-bedded
strata,compositionallayering and foliation are commonly
subparallel,
whereasmassivevolcanicflowsandbrecciasdo
not displaya foliation and were deformedas rigid bodies.
Foliationparallelsflatteningfabricsthat are well developed
within conglomeratic
horizons.The S1 foliationis definedby
micagrainsandelongate,slightlyflattenedplagioclase
grains
that are parallel or subparallelto compositionallayering.
Rocksof the Gravinasequenceare structurallyoverlainto the
northeast,alongthe PointFrancisfault system(Figure2), by
upper greenschist facies rocks belonging to the upper
Paleozoicand lower MesozoicAlava sequence.Moderately
dipping,west- to southwest-vergent
ductile fault zonesare
characteristic
of the structurallyhigherthrustsheets.Thereis
a predominanceof moderate northeast-dippingfoliation
surfaces(S1) which are axial planarto asymmetric,west- to
southwest-vergent folds (Figures 7b and 7c). These
asymmetricfoldsare truncatedby thrustfaults(Figure8) and
appearto be kinematicallylinked,as above,to thrustfaults.
The Alava sequenceis locally structurally overlain by
amphibolitefaciesrocksassignedto the lower PaleozoicKah
Shakessequence. Kah Shakesrocks are characterizedby
attenuatedfold limbs and a moderatelydevelopeddown-dip
elongationlineationthat is parallelto fold axes. On northern
Cleveland Peninsula and northeastRevillagigedo Island
(Figure2), Kah Shakesfoliationtrendsnorthwest.
Metamorphicgradesincreasefromlowerto higherstructural
levels. In the lowest structurallevel, phyllite in the Gravina
sequencecontainswhite mica, chlorite,epidote,plagioclase,
and calcite which are typical of lower greenschistfacies
metamorphism. Fossils are preserved in carbonaceous
limestoneand argillite. Contact metamorphicaureolesare
presentnear plutonsand are best observedon Spire Island
locatednortheastof Annette Island (Figure 2). There relict
andalusite,replacedby white mica, occursin phyllite in a
narrow zone adjacent to the Spire Island diorite, which
indicates low pressureand low to moderate temperature
conditions. Garnet porphyroblastsoccur in phyllite of the
Gravina sequenceadjacent to the Union Bay ultramafic
complex. There, the regionalfabric, definedby the planar
orientationof biotite and white mica, overprintsthe contact
metamorphicaureole,showingthat the emplacementof the
ultramaficcomplexpr•eded regionalmetamorphism.
Mafic volcanicrocksare widespreadin the lower structural
levels and contain relict euhedral phenocrysts of
clinopyroxene,hornblende,and plagioclasein a fine-grained
microgranular matrix of relict pyroxene, amphibole,
metamorphicactinolite,chlorite,epidote,albite,quartz,white
mica,andcalcite,typicalof low-temperature
greenschist-facies
metamorphism. Relict clinopyroxene and hornblende
phenocrystsin the volcanic strata are unstrainedand are
partially replacedby chloriteand white mica. Preliminary
oxygen isotopic analyses on mafic phenocrystsindicate
primary igneous •5180 values which imply that the
phenocrystsdid not interactwith hydrothermalfluids (C.M.
Rubin and H. P. Taylor, Jr., unpublished data, 1987).
Andalusite in contact aureoles,lower greenschistfacies
metamorphic mineral assemblages,and abundant relict
phenocrysts
in volcanicstrataimply pressures
no greaterthan
3.5 kbar [e.g., Holdaway, 1971] and generally low
temperatures(<400øC, [e.g., Turner, 1981]) in the lower
structural levels.
Toward the east, in successivelyhigher structurallevels,
biotite and garnetappear,indicatinguppergreenschist
facies
metamorphism.Locally, stauroliteis presentin pelitic strata,
and hornblende-plagioclase-garnet+ epidote mineral
assemblages
are presentin mafic metavolcanicrocks. These
lower amphibolite mineral assemblages,combined with
pressureestimatesobtainedfor the Moth Bay Pluton, and
based on aluminium content in hornblende [Zen and
Hammarstrom, 1984], and the experimentalcalibration of
Johnsonand Rutherford[1989] imply pressureof 5 to 6 kbar
and temperatures
of--- 550øC. With the samegeobarometer,
pressureestimatesfor the BushyPoint Pluton,locatedto the
northeast(Figure 2) [Zen and Hammarstrom,1984], yield
pressuresin excessof 6 kbar. In higherstructurallevels,on
northernClevelandPeninsulaandRevillagigedo
Island(Figure
2), kyanite+ sillimaniteschistoccuras eitherbladedcrystals
up to 1 cm long or bundles of rods which crosscutthe
dominant northeast trending foliation. The presenceof
kyanite-staurolitemineral assemblages,midcrustal level
plutons(e.g., BushyPoint Pluton), and the occurrenceof
tonaliresillsanddikesthatlack contactaureolessuggestupper
amphibolitetemperatures
of at least550ø C andpressures
of6-7 kbar [e.g., Turner, 1981] in the higher structuralthrust
sheets.
Second-Generation
Mid-Cretaceous
Thrust Faults
The SouthernRevillagigedo Island fault zone (Southern
Revillagigedo Island shear zone of Cook et al. [1991]),
exposedon southwestern
RevillagigedoIslandbetweenThorne
Arm and Carroll Inlet (Figures2 and 9), is a major structural
boundary in the Ketchikan area. The fault zone dips
moderately northeastwardand juxtaposesmid-Cretaceous
tonaliticsills, dikes,and a largepluton,alongwith their host
rocks, over nondeformed mid-Cretaceous sills and dikes and
Gravina sequencemetasedimentaryand metavolcanicrocks
(Figure 3b). To the northwest,the fault zone cuts tonalitic
sills and segmentsof the Moth Bay Pluton. Regional
metamorphicand structuralfabricsare truncatedby the fault
zone. Fabricsproducedby simpleshearare well preservedin
the fault zone. Igneousrocks in the fault zone commonly
displayblastomylonitictextures,and, locally, relict igneous
textures are completely overprinted by ultramylonite
consistingof very fine grainedquartz-bioite-epidiote
mineral
assemblages.
Rotatedplagioclase
porphyroclasts,
asymmetric
recrystallizedtails, and S-C fabrics in the igneousrocks
suggesttop to the northwestor an oblique,senseof motion.
Br•ciated, oblateclastsof weakly deformedplutonicrocks
(Figure 10) are surroundedby a foliated micaceous,locally
mylonitic matrix and commonlydisplay S-C fabricswhich
indicatea similarsenseof shear. The timingof motionon the
Black Mountain fault is constrainedby two lines of evidence.
The fault zoneaff•ts plutonicrockswith a U-Pb zirconageof
101 Ma [Rubin and Saleeby, 1987], so much of the
deformation must have postdatedthe emplacementof the
pluton. Althoughmuchof the deformationpostdatesthe 101
SWAndalusite
contact aureole
Southern
Revillagigedo
P>6kbarNE
Island Fault Zone
Moth Bay Pluton
,
lkm
•
:::/•O
Gravina
sequence
W Alava
sequence
schist
phyllite
&
schist
•
Alexander
terrane
metaplutonic
rocks
•"•Qquartz
Mid-Cretaceous
diorite, & tonalite,
granodiorite
Fig. 9. Geologic cross section across the Southern
RevillagigedoIsland fault zone, exposedon southwestern
Revillagigedo
Island.
RubinandSaleeby:
Tectonic
History
ofEastern
EdgeofAlexander
Terrane
597
..
.
:i:.......-•.:.::•
. -•,'.
•.:•:
:•:•,..::..:•.•u•.•
7}•'
:, .?
..•..
"'
:
Fig. 10. Weaklydeformedtonaliticclastsin a myloniticmatrix. Thesefabricsdevelopedwithin the
Southern
Revillagigedo
Islandfaultzoneon southem
Revillagigedo
Island.
Ma Moth Bay pluton, contactmineralassemblages
locally
overprintblastomyloniticfabricsin the fault zone,implying
that intrusion and contact metamorphismwere partly
contemporaneous
with faulting.
The fault zone probablyextendsto the north along the
southwestside of RevillagigedoIsland (Figure 2), where a
thrust fault strikesapproximatelyN20ø-30øW. Associated
southwest-vergentasymmetric folds trend S60ø-70øEand
plungemoderatelyto the southeast
(Figure1la). Numerous
quartz veins are present in the fault zone. In the footwall,
Gravinasequence
rocksarewellbedded
andarecharacterized
by
lowermost greenschistfacies mineral assemblagesand
unstrained
relict phenocrysts.On the basisof theserelations,
the footwallblock appearsto haveundergone
minor finite
strain and metamorphism. In contrast, the upper plate
containsrocksthatlocallyhavebiotite-grade
greenschist
facies
estimate.By usingdifferencesin metamorphic
and igneous
INtel
ON•22
-7.9
+ 0.6 kbar to -8.5_+1 kbar.
Thesepressuresare in agreementwith the inferredestimated
crystallization
pressurefor the Moth Bay plutonbasedon the
presenceof magmaticepidote[Zen and Hammarstrom,1984].
Pressures
of 3.0-3.5 kbar in the lowerplatecanbe inferredby
the andalusitecontactaureoleand associatedregionallower
greenschistfacies mineral assemblages. Assuming that
pressureestimatesare correctand were locked in prior to
faulting,the hangingwall was uplifted-14 km with respectto
the footwall.
A minimum dip-slip displacement of
N
N
*N.7
IN.•4
On the basis of aluminum
contents in hornblende from the Moth Bay Pluton
[Hammarstromand Zen, 1986; Cook et al., 1991] and using
the experimentalcalibrationof Johnsonand Rutherford[1989],
a pressurerangefrom 6.0 to 7.9 kbaris recordedfor thepluton
exposed in the upper plate of the thrust. The ranges in
pressure
maybe dueto variabilityof AIT in hornblende.
mineralassemblages
andwell-developedfoliationsurfacesand
that are cut by numerousdeformedsills and dikes of diorite.
As thefaultzoneis approached,
abundantquartzveinsappear
in themetamorphosed
hangingwall lithologies.
The amountof displacement
on the fault zoneis difficult to
N
pressures on rocks exposed on either side of the fault,
however,a qualitativedisplacement
estimatecanbe obtained.
The presenceof kyanite-stauroliteschistgives a minimum
pressureof 4.5 kbar for the hangingwall metasedimentary
rocks. In addition,thermobarometric
estimatesfor the upper
plate were determinedby Cook et al. [1991] and rangefrom
N•51
CJ..
40'
Fig. 11. Lowerhemisphere
equal-area
plot showing(a) polesto foliationsurfaces,
small-scale
folds,and
lineationsalongthesouthern
Revillagigedo
faultzone,(b) polesto foliationsurfaces
andsmall-scale
folds
of D1 and D2 fabrics,and (c) Contourof polesto dike orientation.C.I. is confidenceinterval[after
Kamb, 1959]. Solidcirclesare S1 foliation;Opensquaresare S2 foliation;Asterisksare trendand
plungeof smallscalefolds;Opencirclesaretrendandplungeof linearion.
598
Rubin
andSaleeby:
Tectonic
History
ofEastern
Edge
ofAlexander
Terrane
approximately 24 km is inferred across the Southern
fault zone is difficult to estimate, due to the absenceof
Revillagigedo
Islandfaultzone,assuming
thefaultwasnot
subsequently
rotated
duringpost-Cretaceous
time. Although
elongation
lineationstrendobliqueto the dip of the fault,
stratigraphic
cutoffsalongthe fault zone. A minimumof
displacement
of 7 kmbetween
thehanging
wallandfootwall
is inferred,
onthebasisof metamorphic
pressure
estimates
on
concurrentstrike-slipdisplacementacrossthe fault cannotbe
quantified.
peliticsupracrustal
rocksexposedon eithersideof thefault
zone. Strike-slip displacementacrossthe fault cannotbe
quantified;
however,
on thebasisof thesoutheast
plunging
stretchinglineation associatedwith asymmetricfolds,
substantial
displacement
maybelikely.
Third-Generation Ductile Fault Zones
Steeplydipping,north-to northwest-striking
ductilefault
zones crosscutthe earlier (S 1) fabrics in the easternand
northernpartof thethrustbelt(Figure2). The faultzonesare
characterized
by a stronglydeveloped
foliationwhichreorients
earlierS1 fabrics. Structural
relationssuggest
thatreverse
faulting occurredafter F1 folding and associated
thrust
faulting, and possiblyduring the late stagesof the
emplacement
of the mid-Cretaceous
tonalireplutons.The
Zonationof the ThrustBelt
High-angle
reversefaults,represented
by thenorthern
and
southernRevillagigedofault zones,dividethe thrustbelt into
three of the four mid-Cretaceousstructuraldomainsshownin
Figure13. The subsurface
geometry
of thethrustfaultsand
ductile
reverse
faultsisnotaccurately
known.Bycombining
constraints,
a reasonable
and
relationships
between
thrust
faulting,
regional
metamorphism, availablegeologicandpetrologic
internally
consistent
subsurface
geometry
canbeconstructed.
andlaterreverse
faultingdescribed
belowsuggest
thatthese
are
likelytheresultof a singleprogressive
deformational
event.
Thenorthern
Revillagigedo
Islandfaultzone,exposed
onthe
northwestern
portionof Revillagigedo
Island(Figure2), dips
steeplynortheastward.In the fault zone, earlier northwest-
trending
S1 fabrics(regional
foliation)arepreserved
withina
compositionally
bandedbiotiteschistandare completely
transposed
and tightlyrefoldedwithin a seriesof ductileshear
zones(Figure12). The metamorphic
rocksare cut by
numerous
quartzveinsin whichnorthwest-trending
S1 quartz
veins
arefolded
andattenuated;
laterquartz
veins
formed
along
theaxialsurfaces
of S2 folds.Mostaxialplanesof S2folds
trendbetween
N40øto 60øWanddipfrom70øto thenortheast
to 90ø. Folds in bandedbiotite schistare disharmonic,
generallytightto isoclinalandcommonly
asymmetric.
The
foldaxestrendS40ø to 80øEandplungemoderately
to the
southeast.
Thelimbsof thefoldsareextremely
attenuated
and
locallydisplayisolatedasymmetric
floatinghinges.A welldeveloped
elongation
lineation
is associated
withS2foliation
andtrends
S40øto60øE,plunging
moderately
tosteeply
tothe
southeast.
The northernRevillagigedo
Islandfault zoneseparates
supracrustal
metamorphicsequences
of widely differing
metamorphicgrade.In the footwall block, southof the fault
zone, supracrustalrocks display garnetgreenschist-facies
mineralassemblages,
whereas
to thenorthin thehanging
wall, kyanite-bearing
amphibolite
gradeschistandthe midCretaceous
Bell Islandplutonare present.On thebasisof
thesegeologicrelations,topto thenorthor a reverse-sense
of
motionis recorded.The magnitude
of displacement
on the
Domainal
shear
zones
with
isolated fold hinges
Youngershear zones (S2)
andquartzveins
Domain1, locatedon thewestern
portionof thethrustbelt,
consists
of rocksthathaveundergone
little finite strainand
thatcontain
low-temperature
andlow-pressure
metamorphic
mineralassemblages.
Typically,rocksin thisdomainarewell
beddedand consistentirelyof the Alexanderterraneand
Gravinasequence.The centralregion,domain2, consists
of
imbricatethrustnappesof rocksthatare assigned
to the
Alexander
terrane
andGravina
andAlavasequences.
Therocks
displaywell-developedmetamorphicfabricsand contain
mediumpressure
and temperature-sensitive
metamorphic
minerals. The appearanceof mid-Cretaceous
tonaliteand
granodiorite
is characteristic
of this domain. Highertemperature
andhigher-pressure
(?) mineralassemblages
are
recorded by rocks in domain 3, which consists of the
Alexanderterrane,and Kah Shakes,Alava, and Gravina
sequences. There, amphibolitefacies rocks are dominant.
Well-developed
elongation
lineations,
highlyattenuated
fold
limbs, and polyphasefabrics characterizethis domain.
Metamorphic
rocksare intrudedby tonaliticsillsanddikes,
and,locally,lit-par-litstructures
are well developed
in the
high-grade
gneisses
of domain3. Typically,rocksin Domain
3 are intrudedby late Paleoceneintrusive rocks and are
overprinted
by earlyTertiaryextensional
deformation.
Mid-Cretaceous
shortening
wasaccommodated
through
the
development
of crenulation
cleavage
andis bestexposed
near
NahaBayalongthenorthwestern
shore
of Revillagigedo
Island
(Figure2). This fabricis definedby mesoscopic,
northeast
trending
spaced
cleavage
(S2),S1- S2 intersection
lineation
(Llx2), andassociated
west-to northwest-vergent
asymmetric
kinkfolds(Figure1lb). Thecleavage
formssmall-scale
folds
in phyllite,whereas
in massive
metavolcanic
rocksa planar
widelyspaced
cleavage
ispresent
(Figure1lb). Thecleavage
is definedby theconcentration
of phyllosilicates
in foldlimbs.
New mineralgrowthparallelto the kink fold axial planes
consistof fine-grainedbiotiteand garnet.Quartzgrainsare
recrystallized
andslightlyflattened
totheaxialplanes.
LatePaleocene
Deformation
view towards
the southeast
Folded S1 foliation and quartz veins
Paleoceneand younger(?) deformationhasaffectedrockson
0
30 cm
I
I
Fig. 12. Geologicsketchof transposed
foliation(S1) and
associated
isoclinal
foldsin banded
kyaniteschist
alongthe
Northern
Revillagigedo
faultzone.Elongation
lineation
dips
moderatelyto the southeast.
thewestern
marginof theCoastPlutonic
Complex
(Figure2).
Deeperstructural
levelsareexposed
alongtheeastern
edgeof
theAlexander
terrane
andtheAlavasequence
andaredominated
by low to moderate
east-dipping
sheetsof schist,gneiss,and
pht/llonitetermedthe east Behm Canal gneisscomplex
[Saleeby, 1987; Saleeby and Rubin, 1990a]. Shallow
northeast-plunging elongation lineations are common;
RubinandSalecby:
Tectonic
History
ofEastern
Edgeof Alexander
Terrane
Domain I
Domain II
Domain III
599
Domain IV
I
Southern
Revillagigedo
FaultZone
Northern
Revillagigedo bend
insection
Fault
Zone
•,• Med.
Pt.
Francis
Fault
\•
temperature Late
Low
pressure
Med.
pressure
Teritary
•,•
Low strain
•1 Moderatestrain
maficdikes
\
•
Med.
pressure Low
Low
-strain
med.
pressure
High
temperature
(?) (?)
High strain
Lowtemperature\
6km
mgals
nT
Southern
Revillagigedo
Imbricate
thrust
sheet•
\ Kyanite
' }]'- '•' ' "Coast
Plutonic
mid-Cretaceous
Complex
Fault Zone
50,000 0
54,000 -60
56,000 -100
Bøugtte•vity
Profi/e
0 0
0
0
0
0
0
-'"-'"'•O'""•'•O
'•-Oøroo,,
Aeromagnetic
Profile
(afterU.S.G.S.,1977)
DOMAIN I: AT & GS
DOMAIN III: AT, KSS, AS, & GS (?)
DOMAIN II: AT, GS, & AS
DOMAIN IV: KSS & AT
,[• intrusive
Mid-Cretaceous
• Tertiary
intrusive
rocks
rocks
CMR '89
Fig. 13. Aeromagneticandgravityprofilesandgeologiccrosssectionacrossthe westernwall rocksof the
CoastPlutonicComplex showingthe major mid-Cretaceousstructuraldomains. Abbreviationsare AS,
Alava sequence;AT, Alexanderterrane;GS, Gravinasequence;and KSS, Kah Shakessequence.Med. is
medium. Domain IV is not discussed
in thispaper.
however, penecontemporaneous
recrystallizationoverprints
thisfabric. There is a predominance
of moderatewest-dipping
foliation surfaceswhich are axial planar to asymmetriceastverging folds. These east-vergingfabrics have reoriented
earliermid-Cretaceous
fabrics.Late Paleocenepegmatitedikes
are highlydeformedand are affectedby theseeast-and westdipping structures(Figure 2) [Saleeby and Rubin, 1989].
Exposureof the gneisscomplexmay haveresultedfrom eastvergentPaleocenedeformation,whendeeplevelsof the midCretaceousthrust system were transportedupward along
younger east-vergent structures. A swarm of Tertiary
hornblende-bearing
diabasedikescrosscutsall structuresand
fabrics. Thesedikes trendnortheast(Figure 1lc) and mark a
regionalchangein the overalltectonicsettingduringMiocene
time.
DISCUSSION
AND
TECTONIC
HISTORY
Three major lithetectonicassemblages,
the Alexanderand
Taku terranesand the Gravina sequence,are presentalong a
northwest-trendingbelt on the western side of the Coast
PlutonicComplex. The upperPaleozoicportionof the Alava
sequencemay representthe westernmostextentof the YukonTananaterrane,an assemblage
of lower Paleozoiccontinental
slope-and-rise
depositsanddismembered
fragmentsof a mid- to
late Paleozoic arc system. The lower Mesozoic part of the
Alava sequencehas lithelogic similarities to parts of the
Stikine terrane. Structuralbasementis compositeand consists
of the Alexanderand Taku (Alava andKah Shakessequences)
terranos. The Alexanderterraneformsdepositionalbasement
to the Gravina sequence;locally the epiclasticstrata of the
Gravinasequencelie unconformablyover the upperPaleozoic
and lower Mesozoic Alava sequence[Rubin and Saleeby,
1991c], thus forming an overlapbetweenthe two basement
constituents(Alexanderterraneand Alava sequence).These
datasuggestthe Alexanderterranewas adjacentto the western
margin of North America prior to depositionof the Upper
Jurassicto Lower Cretaceous
Gravinasequence.
The recordof sedimentation,magmatism,and deformation
within the Gravinabelt spansan intervalof almost50 m.y.,
from the Late Jurassicto the end of the Early Cretaceous.
Beginningin the Late Jurassic,primitive arc-type(?) basaltic
to basaltic andesitevolcanic rocks of the Gravina sequence
weredepositedon basinalepiclasticstratathatunconformably
overlie Triassicand older portionsof the Alexanderterrane
(Insularcompositeterrane)and the upperPaleozoicand lower
Mesozoic Alava sequence(e.g., Yukon-Tananaand Stikine
terranes and Intermontane composite terrane [Rubin and
Saleeby, 1991a]). Thus the Insular and Intermontane
compositeterraneswere adjacentduring constructionof the
overlying Late Jurassic to Early CretaceousGravina arc.
Coarse-grainedepiclasticand volcanicbasinal strataof the
Gravinasequence
overliethe oldervolcanicrocks. Provenance
of the clastic rocks indicates uplift erosion of the older
plutonic arc edifice, perhapswithin an extensionaltectonic
setting[Rubinand Saleeby,1991c]. Followingdepositionof
the Gravinasequence,zonedmafic and ultramaficcomplexes
were emplacedinto the Alexanderterraneon Duke andAnnette
Islands (e.g., the Duke Island ultramafic complex) and the
Gravina sequence(e.g., the Union Bay ultramaficcomplex)
and are present as numerous smaller bodies that intrude
adjacentterranes. These enigmaticbodiesform a linear belt
parallelto the Gravinasequence
alongthe easternedgeof the
Alexander terraneand may have been emplacedduring the
onset of mid-Cretaceous deformation, possibly in an
extensionalsetting. After depositionof the Gravinasequence
and after the emplacementof the zonedultramaficcomplexes,
significantmid-Cretaceousdeformationand metamorphism
affectedtheserocksandtheirbasementcomponents.
Mid-Cretaceous
deformation
is recorded
on Cleveland
Peninsula,RevillagigedoIsland, and the easternportionsof
Annette,Gravina, and adjacentislands. Asymmetricwest to
southwest-vergent
folds formedduringthrustfaulting. Thrust
600
RubinandSaleeby:
Tectonic
HistoryofEastern
Edgeof Alexander
Terrane
sheetsof thelower PaleozoicKah Shakessequence
overliethe
Alexander terrane and Gravina and Alava sequencenappes;
however, locally on Portland Peninsula, the Kah Shakes
sequencelies structurallybelow deformedAlexanderterrane
[Saleebyand Rubin, 1990b]. Complex thrustgeometriesand
out of sequencethrust faults may have resultedfrom the
reactivation of different primary basement components.
Deformation
is recorded
on
Cleveland
Peninsula
and
Revillagigedoand adjacentislandsby (1) west- to southwestvergentthrustfaulting, (2) pervasivemetamorphism,
ranging
from lower greenschist to amphibolite facies mineral
assemblages,
(3) emplacement
of calc-alkalinesills,dikes,and
plutons,(4) high-anglereversefaulting, and (5) uplift that
recordsat least 14 km of verticaltransportby Late Cretaceous
time. Available age constraintsfrom Revillagigedo and
adjacentislandsindicatethatthisdeformafional
eventbeganin
the Albian (~113 Ma), the ageof the youngeststratainvolved
in thrustfaulting,and musthave ceasedby ~74 Ma, basedon
biotite K-Ar cooling ages [Smith and Diggles, 1981] from
mid-Cretaceousplutons. Asymmetric west- to southwestvergentfolds formedduringthis faulting. The presenceof
thrustsheetscomposedof crystallinebasementmay imply a
zone of recouplingwithin middle levels of the continental
crustduring the mid-Cretaceous.The extentof southwestdirected thrusting of the Yukon-Tanana terrane over the
Gravina sequenceand its compositebasementis difficult to
assess; however, it is possible that a subhorizontal
d6collementseparatingthe Yukon-Tanana and Alexander
terrane
extends
beneath
northern
British
Columbia
and
southernYukon Territory [RubinandSaleeby,1991b].
The emplacementof mid-Cretaceoussills, dikes, and
elongateplutonswasbroadlycoevalwith deformation.The
details of the relation between deformation and pluton
emplacement,however, are complicated[Crawford and
Crawford, 1991]. Thesecalc-alkalineintrusiverocksformed
in response
to plateconvergence
alongthewesternmarginof
the Alexander terrane and at middle crustal levels of a mid-
Cretaceouscontinent-marginarc [Barker and Arth, 1984].
Mid-Cretaceousagesreportedhere,togetherwith olderU-Pb
datesfrom the Ketchikanarea [C. M. Rubin, unpublisheddata,
1988]indicatean overallprogression
of olderto youngermidCretaceousigneousages from the southwestto northeast
(Figure 14). This trendmay reflecta northeastmigrationof
the mid-Cretaceousarc between 100 and 90 Ma. Steeply
dippingreversefaultsbecameactiveduringthelate stagesof
thrustfaulting. Vertical motionon thesefaultsresultedin the
juxtapositionof differing crustallevelsacrossthe thrustbelt
andwasperhapsresponsible
for the initial uplift of the deeper
levels of the arc by the latest Cretaceous,reflectedby an
average uplift rate of ~ 0.9 mm/yr [Rubin and Saleeby,
199lb]. K-Ar coolingagesindicatethat the westernsideof
the thrustbelt cooledbelow the biotiteclosuretemperatureby
~ 74 Ma (Figure 14) [Smith and Diggles, 1981]. Highly
deformed late Paleocene to early Eocene tonalitic and
granodioriticintrusivesare presenton easternRevillagigedo
Island and western Portland Peninsula, and belong to a
discontinuousbelt of sills and elongateplutons that were
intrudedinto overthickened
crustalongthewesternflank of the
Coast Plutonic Complex. Rapid uplift accompaniedthe
emplacementof theseplutons[Hollister, 1982; Crawfordet
al., 1987] andmay recordthegravitationalcollapseof themidCretaceousthrust belt, although a contractionalsetting has
alsobeenproposed.An abruptstepin the K-Ar coolingages
indicatesthat the easternpart of the thrustbelt passedthrough
the biotite closure temperatureby ~58 Ma. Uplift in the
region was probablyaccommodated
by east-side-upfaults.
Alternatively,it is possiblethat the older K-At coolingages
may reflect cooling due to underthrustingof cold material
NE
SW
120
120
[] •a,,--ExceeeArgon ?
lOO
100
+
[]
80
4-4.
[]
ß
80
Isotopic Age
Determinations
[] K-Ar biotite
60
60
I K-Ar hornblende
4' U-Pb zircon
4O
•
o
I
20
•
I
40
i
I
60
,
•
80
,
40
100
Distance across thrust belt (km)
Projected onto a SW- NE section
Fig. 14. U-Pb zircon agesand K-Ar coolingagesfor 100-90
Ma plutonsin the Ketchikanarea. U-Pb agesare after Arth et
al. [1988] andRubin and Saleeby(thispaperandunpublished
data).K-Ar dataarefrom SmithandDiggles[1981].
beneaththe arc coeval during the eastwardmigration of
magmatism.In thisview, theeastwardchangeof K-At biotite
ages may reflect not regional uplift but flattening of the
subducting
slabandtheresultantunderflowof a relativelycold
subductingplate (for example,seeDumitru [1990]). Presentday thermalgradientsin muchof the regionare low [Yorathet
al., 1985], consistentwith a forearcsettingfor the western
part of the CoastPlutonicComplex.
Limitedgravityandaeromagnetic
dataalongthetrendof the
thrustbelt help definethe present-daydeepercrustalstructure
[Barnes, 1972a, b; 1977; United StatesGeological Survey
(USGS), 1977]. The dramaticdecreasein Bouguergravity
anomaly values, from about +10 mGal in the nondeformed
portionsof the Alexanderterraneto about-115 mGal in the
interiorof the thrustbelt, alongthe westernedgeof the Coast
PlutonicComplex(Figure13), may reflecta thickenedcrust
and associatedlarge volumesof graniticmaterial. A strong
Bouguergravitygradientcoincides
withthethickerpartsof the
thrustwedgeand thepresenceof mid-Cretaceous
andyounger
intrusiverocks. A steepmagneticfield gradientfrom low
magneticstrataof nondeformed
Alexanderterraneon thewest
to highly magneticeasternportionsof the thrustbelt and the
westernedge of the CoastPlutonicComplex[USGS, 1977]
mirrors the Bouguergravity gradient. Bouguergravity data
[Barnes, 1977] suggesta crustal thicknessof ~ 25-30 km
beneaththe southeastAlaska archipelago(i.e., Alexander
terraneand Gravinasequence).Crustalthicknessbeneaththe
CoastPlutonicComplexmay be as thick as 40 km; however,
gravitydata alonecannotresolveMoho depths. The presence
of thinnedlithospherein which midcrustallevelsare exposed
on the Earth's surfaceposesfundamentalquestionson the
characterand natureof the lithosphericstructurebeneaththe
Coast Plutonic Complex. Tertiary and younger (?) crustal
thinningmay havebeenaccomplished
by extension;however,
the role of extensionis presently difficult to assess. A
northeast-trending
swarmof late Tertiarymafic dikescutsall
Cretaceousand early Tertiary structuralelementsand records
northwest-southeast
extension. This dike swarmmay reflect
regionalextensionassociated
with the openingof HecateStrait
during the late Tertiary. The modern structuralsetting is
dominated by dextral strike-slip motion on the Queen
Charlottefault system.
The mid-Cretaceous southeast Alaska orogen was
characterized by thrust faulting, metamorphism,and the
emplacement
of arc-relatedtonaliticandgranodiorificplutons.
RubinandSalecby:
Tcctonic
History
of Eastern
Edgeof Alexander
Terrane
Igneous activity and deformation were broadly coeval,
suggesting
an intra-arcsetting.Mid-Cretaceous
rocksexposed
at the surfacetodaywere formedin middle crustallevelsof a
continent-margin
arcandwereinvolvedin tectonicreworking
of an oldercrustalboundarybetweentheAlexanderterraneand
the westernmargin of North America. Most penetrative
deformation
occurred
wherestructural
reactivation
of differing
basementcomponents,suchas the Alexander terraneand the
Kah Shakessequence,
resultedin complexfault geometries.
Emplacement
of the early Tertiarytonaliticto graniticplutons
followedmid-Cretaceous
crustalshortening
anduplift. These
data clearly indicatethat deformationand magmatismwere
integral parts of the ongoing tectonic evolution of a
convergent
continentalmarginanddid notresultfroma shortlived collisionalevent. Becausecrustalshorteningwas
broadlycontemporaneous
with arc magmatism,thesubduction
of oceaniclithosphere
playeda majorrolein crustalshortening
in a noncollisionalsetting. During the evolution of the
orogen, mid-Cretaceousmagmatic processesthermally
weakened
andallowedcompressional
failureof theoverriding
continentalplate.
601
Acknowledgments.
Partsof thisresearchwere supportedby
National ScienceFoundationgrantsEAR 86-05386 and EAR
88-034834 (to Saleeby). Additionalsupport(to Rubin) was
providedby a GeologicalSocietyof AmericaPenrosegrant,a
Sigma-Xi grant-in-aid, by the U.S. Geological Survey,
Alaska Branch, and by the U.S. Forest Service, Ketchikan
Ranger District. Rick Allmendingergraciouslyprovidedthe
stereonetprograms.We thankFred Barker,Henry Berg, Dave
Brew, Darrel Cowan,WeechaCrawford,JohnGarver,George
Gehrels,Linc Hollister, Bill McClelland, Meghan Miller, Jim
Monger, George Plafker, and Margi Rusmore for helpful
discussions on northwest Cordilleran stratigraphy and
tectonics.Jim Wright providedencouragement
andadviceon
U-Pb zircon leachingtechniques. Jeff Marshall assistedin
mappingpart of the area during the summerof 1987; Mark
Fahnestockand Jon Nourse provided able field assistance
during the summer of 1986. Field discussionwith Bill
McClellandand MeghanMiller are appreciated.A reviewby
Weecha Crawford significantlyimprovedthe clarity of an
earlierversionof this paper. Hank Berg and Margi Rusmore
thoroughlyreviewedthemanuscript.
REFERENCES
Arth, J. G., F. Barker, and T. W. Stem, Coast
Batholith and Taku plutons near Ketchikan,
Alaska:
Petrography,
geochronology,
geochemistry,and isotopic character,Am. J.
Sci., 288-A, 461-489, 1988.
Barker, F., and J. G. Arth, Preliminary results,
CentralGneissComplexof the CoastPlutonic
batholith, southeastern Alaska: the roots of a
high-K, calc-alkalinearc?,Phys. Earth Planet.
Inter., 35, 191-189, 1984.
Barnes,D. F., Simple Bouguergravity anomaly
map of Ketchikan 1:250,000 quadrangle,
southeastern Alaska, showing station
locations,anomalyvalues,and generalized10milligal contours,U.S. Geol. Surv. Open File
Barnes,D. F., Simple Bouguergravity anomaly
map of Craig 1:250,000 quadrangle,
southeastern Alaska, showing station
locations,anomalyvalues,and generalized10milligal contours,U.S. Geol. Surv. Open File
Map, 1972b.
Barnes, D. F., Interpretation of the available
gravity, in Mineral Resourcesof the Tracy
Arm-Fords Terror wildemess study area and
vicinity, editedby D. A. Brew, A. L. Kimball,
D. Grybeck,and J. C. Still,. Geol. Surv. Open
File Rep., 77-649, 300 pp, 1977.
Berg, H. C., Geologic map of Annette Island,
Alaska, scale 1:63,000, U.S. Geol. Surv. Misc.
Geol. Invest.Map, 1-684, 8p., 1972.
Berg, H. C., Geology of Gravina Island, Alaska,
U.S. Geol.Surv.Bull. 1373, 41 pp., 1973.
Berg, H. C., and E. L., Cruz, Map showing
of
geochronology of detrital zircons from the
Yukon crystalline terrane in the northem Coast
Mountains batholith, southeastern Alaska,
Can. J. Earth Sci., 28, 899-911, 1991.
Gehrels, G. E., W. C. McClelland, S. D. Samson,
U.S.A., Circum-Pacific Plutonic terranes,
editedby J.A. Roddick, Geol. Soc. Am. Mere.,
the
159, 171-194, 1983.
between Cape Fanshaw and Taku Inlet,
Chen, J. H., and G. J. Wasserburg, Isotopic
determinationof uranium in picomole and
subpicomolequantities, Anal. Chem., 53,
2060-2067, 1981.
Cook, R. D., M. L. Crawford, G. I. Omar, and W.
A. Crawford, Magmatism and deformation,
southern Revillagigedo Island, SE Alaska,
P. J. Patcherr,and M. J. Orchard, Geology of
western
flank
of the
Coast
Mountains
southeasternAlaska, Tectonics,in press1992.
Hammarstrom,J. M., and E-A. Zen, Aluminum in
hornblende:
An
empirical
igneous
geobarometer,Am. Mineral., 71, 1297-1313,
1986.
Herreid, G., T. K. Bundtzen, and D. L. Tumer,
Geology and geochemistryof the Craig A-2
quadrangleand vicinity, Prince of Wales
Crawford, M. L, and W. A. Crawford, Magma
Island, southeastern
Alaska,Geol.Rep. Alaska
Div. Geol. Geophys.Surv.,48, 49p., 1978.
emplacementin a convergenttectonic orogen,
southern Revillagigedo Island, southeastern Holdaway, M. J., Stability of andalusiteand the
aluminumsilicatephasediagram,Am. J. Sci.,
Alaska,Can. J. Earth Sci., in press,1991.
Crawford, M. L., L. S. Hollister, and G.J.
271, 97-131, 1971.
Geol. Soc. Am. Bull., 103, 829-841, 1991.
Map, 1972a.
locations
during 1987, editedby J.P. Galloway, and T.
D. Hamilton, U.S. Geol. Surv. Circ., 1016,
143-146, 1987.
Brew, D. A., and R. P. Morrell, Intrusive rocks
and plutonic belts in southeasternAlaska,
fossil
collections
and
related
Woodsworth,
Cmstaldeformation
andregional Hollister,L. S., Metamorphicevidencefor rapid
metamorphism across a terrane boundary,
(2 mm/yr) uplift of a portion of the Central
Coast Plutonic Complex, British Columbia,
Gneiss Complex, Coast Mountains, British
Tectonics, 6, 343-361, 1987.
Columbia, Can. Mineral., 20, 319-332, 1982.
Dumitru, T. A., SubnormalCenozoicgeothermal Hollister, L. S., G. C. Grisson,E. K. Peters,H H.
gradients in the extinct Sierra Nevada arc:
Consequences
of laramide and post-laramide
shallow-angle subduction,J. Geophys.Res.,
95, 4925-4941, 1990.
Eberlein, G. D., M. Churkin, Jr., C. Carter, H. C.
Berg, and A.T. Ovenshine, Geology of the
Craig quadrangle, Alaska, scale 1:250,000,
U.S. Geol.Surv.OpenFile Rep., 83-91, 53p,
1983.
Stowell, and V. B. Sisson, Confirmation of the
emprical correlation of A1 in homblende with
pressure solidification
of calc-alkaline
plutons, Am. Min., 72, 231-239, 1987.
Irvine, T. N.,
The Duke Island ultramafic
complex, southeasternAlaska, in Ultramafic
and RelatedRocks,editedby P. J. Wyllie, pp.
84-97, John Wiley, New York, 1967.
Irvine, T. N., Petrology of the Duke Island
ultramafic complex, southeastern Alaska,
Gehrels,G. E., and W. C. McClelland, Outline of
samples in the Ketchikan area and Prince
Mere. Geol. Soc. Am. 138, 1974.
the TakuterraneandGravinabelt in the Cape
Rupert quadrangles,southeastern
Alaska, scale
Fanshaw-WindhamBay region of central Jaffey,A. H., K. F. Flynn, L. E. Glendenin,W. C.
1:250,000, U.S. Geol. Surv. Open File Rep.,
southeastern Alaska, Geol. Soc. Am. Abstr.
Bentley, and A. M. Essling, Precision
82-1088, 27p., 1982.
Programs,20, 163, 1988.
measurement of half-lives and specific
Berg, H.C., D. L. Jones, and D. H. Richter, Gehrels,G. E., and J. B. Saleeby,Geology of
activities
of 235Uand238U,Phys.Rev.C,4,
Gravina-Nutzotinbelt: Tectonic significance
1889-1906, 1971.
Prince of Wales Island, southeasternAlaska,
of an upperMesozoicsedimentary
andvolcanic
Geol. Soc. Am. Bull., 98, 123-137, 1987.
Johnson, M. C., and M. J. Rutherford,
sequencein southernand southeasternAlaska, Gehrels,G. E., J.B. Saleeby,and H. C. Berg,
Experimental calibration of aluminium-inU.S. Geol. Surv. Prof. Pap. 800-D, D1- D24,
Geology of Annette, Gravina, and Duke
homblendegeobarometer
with applicationto
1972.
Islands, southeasternAlaska, Can. J. Earth
Long Valley caldera (California) volcanic
Berg,H. C., R. L. Elliott,andR. D. Koch,
Sci.,24,866-881,1987.
rocks, Geology, 17, 837-841, 1989.
Geologic
mapof theKetchikan
andPrince Gehrels,
G.E.,W. C.McClelland,
S.D. Samson,Kamb, W. B., Theory of preferredorientation
Rupertquadrangles,
Alaska,scale1:250,000,
P.J. Patchett,
andJ. L. Jackson,
Ancient
U.S. Geol. Surv.Misc.Invest.Set.,Map, 1-
continental
marginassemblage
in thenorthem
1807, scale 1:250,000, 1988.
Brew,D. A., andS. M. Karl, Reexamination
of
the contacts and other features of the Gravina
Coast Mountains, southeast Alaska and
northwestCanada,Geology,18, 208-211,
1990.
developed by crystallization under stress, J.
Geol., 67, 153-170, 1959.
Krogh, T. E., A low-contamination method for
hydrothermal decompositionof zircon and
belt, southeastern
Alaska,in Geologicstudies Gehrels,G. E., W. C. McClelland,S. D. Samson,
extraction of U and Pb for isotopic age
determinations,Geochim.Cosomochim.
Acta,
in Alaska by the U.S. Geological. Survey
37, 485-494, 1973.
P.J.
Patchett, and J. L. Jackson, U-Pb
602
Rubin
andSaleeby:
Tectonic
History
ofEastem
Edge
ofAlexander
Terrane
Lanphere,M. A., and G. D. Eberlein,Potassiumargon ages of magnetite-bearing ultramafic
complexesin southeastem
Alaska, Spec.Pap.
Geol. Soc. Am. 87, 94, 1966.
Magaritz, M., and H. P. Taylor, Jr., Oxygen
18/Oxygen16 and D/I-I studiesof plutonic
granitic and metamorphic rocks acrossthe
Cordilleran
batholiths
of
southern
British
Columbia, J. Geophys Res., 91, 2193-2217,
1986.
Martinson, J. M., U-Pb ages of zircon: a basic
examination of error propagation, Chem.
Geol., 66, 151-162, 1987.
McClelland,W. C., andG. E. Gehrels,Geologyof
the Duncan Canal shear zone: Evidence for
Early to Middle Jurassic deformation of the
Alexander terrane, southeastern
Alaska, Geol.
Soc. Am. Bull., 102, 1378-1392, 1990.
McClelland,W. C., G. E. Gehrels,S. D. Samson,
and P. J. Patcherr, Structural and
geochronologicrelations along the westem
Implications
for continent-margin
tectonism,
Geology,18, 276-280, 1990a.
Rubin,C. M., M. M. Miller, andG. E. Smith,
Tectonic development of Cordilleran mid-
Evidencefor convergentmargintectonism,in
terrane in
central southeastern
Alaska, J. Geol., 1992b.
Rubin, C. M., and J. B. Saleeby, The inner
boundary zone of the Alexander terrane in
southernSE Alaska, A newly discoveredthrust
belt, Geol. Soc. Am. Abstr. Programs, 19,
455, 1987.
Rubin, C. M., and J. B. Saleeby, Tectonic
frameworkof the Upper Paleozoicand Lower
Mesozoic Alava sequence:A revisedview of
the polygenitic Taku terrane in southern
southeastAlaska, Can. J. Earth Sci., 28, 881893, 1991a.
Rubin, C. M. and J. B. Saleeby,Thrusttectonics
and intracontinentalshorteningin southeast
Alaska and westernBritish Columbia,in Thrust
Tectonics, editedby by K. McCLay, Unwin
Hyman, London,in press.1991b.
Rubin, C. M., and J. B. Saleeby, The Gravina
Sequence - Remnants of a mid-Mesozoic
oceanic arc in southem southeast Alaska, J.
Geophys. Res., 96, 14551-14568, 1991c.
Rubin,C. M., J. B. Saleeby,D. S. Cowan,M. T.
Brandon,and M. F. McGroder,Regionally
extensivemid-Cretaceouswest-vergentthrust
system
in
the
northwestern Cordillera:
systematicsin three Apollo 14 basaltsand the
Paleozoic
andearlyMesozoic
palcogeographic problemsof the initial Pb in lunar rocks,Earth
relations;Sierra Nevada, Klamath Mountains,
Planet. Sci. Lett., 14, 281-304, 1972.
andrelatedrocks,editedby D. S. Harwoodand Turner, F. J., MetamorphicPetrology,2nd ed.,
M. M. Miller, Spec.Pap. Geol. Soc.Am.,225,
524 pp., publisher,New York, 1981.
1-16, 1990b.
United States Geological Survey (USGS),
Saleeby,J. B., The inner boundaryzone of the
Aeromagneticmap of the Ketchikan, Prince
Alexander terrane in southern SE Alaska: Part 11
Rupert, and northeasternCraig quadrangles,
southern
Revillagigedo
Island(RI) to CapeFox
(CF), Geol. Soc. Am. Abstr. Programs,19,
828, 1987.
Saleeby,J. B., and C. M., Rubin, The western
marginof the CoastPlutonicComplex(CPC)
in southernmostSE Alaska, Geol. Soc. Am.
Abstr. Programs,21, 139, 1989.
Saleeby,J. B., and C. M., Rubin, The East Behm
of the Coast Mountains batholith:
Canal Gneiss Complex (southernsoutheast
Stikine River to Cape Fanshaw, central
Alaska) and some insights into basement
southeastern
Alaska,J. Struct.Geol.,in press1992a. tectonicsalong the Insular suturebelt, Geol.
McClelland,W. C., G. E. Gehrels,S. D. Samson,
Assoc. Can. Abst. Programs, 15, All6,
and P. J. Patchett, Protolith relations of the
1990a.
belt and Yukon-Tanana
1967.
Paleozoic volcanoplutonic complexes: Tera, F., and G. S. Wasserburg,
U-Th-Pb
flank
Gravina
complexes of southeasternAlaska, in
Ultrarnaficand RelatedRocks,editedby P. J.
Wyllie,pp. 209-230,JohnWiley,New York,
Saleeby, J. B., and C. M. Rubin, Tectonic
Alaska, scale 1:250,000, U.S. Geol. Surv.
OpenFile Rep. Map 77-359, 1977.
Wheeler, J. O., and P. McFeely, Tectonic
assemblagemap of the CanadianCordillera,
Geol. Surv. Can. OpenFile Rep. 1565, 1987.
Yorath,
C.J.,
G.
J.
Woodsworth,
R.
P.
Riddihough,R.G. Curde,R. D. Hyndman,G. C
Rogers, D. A. Seemann, and A.D. Collins,
Centennial Continent/Ocean Transect # 8,
Intermontane Belt (Skeena Mountains) to
Insular Belt (Queen Charlotte Islands), Geol.
Soc. America. Centennial Continent/Ocean
TransectSer., 8, 7 pp. GeologicalSociety of
America,Boulder,Colo., 1985.
intercalation
of crystalline
nappesandbasinal York,D., Least-squares
fittingof a straightline
overlap relations in southern SE Alaska:
Implicationsfor the initial stagesof Insular
superterraneaccretion, Eos Trans. AGU, 71,
1591, 1990b.
Saleeby,J. B., G. E. Gehrels,G. D. Ebefiein,and
H. C. Berg, Progressin lead/uraniumzircon
studies of lower
Paleozoic
rocks
of
the
southernAlexander terrane, U.S. Geol. Surv.
Circ., 868, 110-113, 1984.
Silbefiing,
N.J., Wardlaw,B. R., andH. C. Berg,
New paleontologicage determinationsform
the Taku terrane,Ketchikanarea, southeastem
Alaska, U.S. Geol. Surv.Circ., 844, 117-119,
with correlatederrors,Earth Planet. Sci. Lett.,
5, 320-324, 1969.
Zen, E-A., Implicationsof magmaticepidotebearing plutons on crustal evolution in the
accreted
terranes
of
northwestern
North
America, Geology,12, 266-269, 1985.
Zen, E-A., Tectonicsignificance
of high-pressure
plutonit rocks in the westerns(sic) cordilleran
of North America, in Metamorphism and
CrustalEvolutionof the WesternUnitedStates,
RubeyVolume7, editedby W. G. Ernst,pp.
41-68, Prentice-HaH,EnglewoodCliffs, N.J.,
1988.
1982.
Zen E-A., and J. M. Hammarstrom,
Magmatic
Smith,J. G., andM. F. Diggles,Potassium-argon epidote and its petrologle significance,
determinations in the Ketchikan and Prince
Rupertquadrangles,
southeastem
Alaska, U.S.
Geol. Surv. Open File Rep., 78-73N, 1-6,
1981.
Geology, 12, 515-528, 1984.
C. M. Rubin, l•partrnent of Geology,Central
WashingtonUniversity,Ellensburg,WA 98926.
J. B. $aleeby,
170-25
Geology,
California
Sutter,
J.F.,andM. L. Crawford,
Timing
of Institute
ofTechnology,
Pasadena,
CA91125.
metamorphism
and uplift in the vicinity of
PrinceRupert,BritishColumbia,
Geol.Soc. (Received
February
21, 1991;
Am. Abstr.Programs,17, 411, 1985.
revisedAugust5, 1991;
Taylor, H. P., Jr., The zoned ultramarie accepted
August8, 1991.)
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