UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY

A revley o f the regional geoiopy and tectonics o f southeastern Alaska

George E. ~ e h r e l r l and Henry C. 0erg2

Open-Fil e Report 88-659

Thfs r e p o r t i s pre l iminary and has not been reviewed f o r conformity wi th U.S. Geological Survey e d i t o r i a l standards and s t r a t i graphic nomenclature.

l0epartment o f Geosciences Univers i ty o f Arizona Tucson, Arizona 85721

*115 Ma1 vern Ave. F u l l e r ton , Cal i f o r n i a 92632

INTRODUCTION

Southeastern (SE) Alaska, a l so known as the "panhandle" o f Alaska, i s an

approximately 52,000 square m i l e area of i n t e n s e l y g l ac ia ted and h e a v i l y

forested mountains t h a t r i s e ab rup t l y from a complex system o f deep f i o r d s and

i n 1 and marine waterways. Primary physiographic fea tures i n SE A1 aska inc lude

the Coast Mountains, which u n d e r l i e the easternmost p a r t o f the panhandle, and

a myriad of small and l a r g e i s l a n d s t o the west which comprise the Alexander

a rch ipe l ago.

The panhandle i s under la in by an exceedingly complex and heterogeneous

assemblage of rocks, and i s c u t by an i n t r i c a t e network o f t h rus t , normal, and

s t r i ke-sl i p f a u l t s (Buddington and Chapin, 1929; Gehrel s and Berg, 1984).

These rocks record a l ong and n e a r l y complete geologic h i s t o r y beginning i n

the Proterozoic, represent ing every Phanerozoic per iod, and con t i nu ing through

the Holocene. We here in d j v i d e these rocks i n t o f i v e terranes t h a t apparent ly

conta in d i s t i n c t geologic records, and a v a r i e t y o f s t r a t i f i e d , p l u t o n i c , and

metamorphic assemblages t h a t are i n depos i t fona l , i n t r u s i v e , o r unknown

contac t w i t h the terranes. The d i s t r i b u t i o n o f these terranes and assemblages

i s discussed below i n reference t o the Coast Range b a t h o l i t h , which under l i es

the western Coast blountains and serves as a prominent geologic marker i n the

panhandle.

This r e p o r t begins w i t h a summary o f the reg iona l geology of SE Alaska

which i s der fved p r i m a r i l y from the synthesis o f Gehrels and Berg (1984) and

from more recent s tudies. Next, we discuss the components and c h a r a c t e r i s t i c s

o f each o f the terranes and 1 i t h i c assemblages t h a t comprise the panhandle and

then o u t l i n e s i m i l a r i t i e s and d i f fe rences among the terranes. We then present

a general overview o f the tec ton i c evo lu t i on o f SE Alaska.

SUmARY OF REGIONAL GEOLOGY

S t r a t i f i e d rocks

Pre-Jurass ic s t r a t i f i e d rocks i n SE Alaska ( f i g . 1) c o n s t i t u t e a s e r i e s

o f nor thwest -e longate b e l t s t h a t va ry i n depos i t i ona l age and I n degree of

deformat ion and metamorphism. These b e l t s occur a long t h e eas t and west

f l anks o f t h e Coast Range b a t h o l i t h ( f i g . 2 ) and on t he i s l a n d s t o t he west.

Rocks e a s t of t he b a t h o l i t h i n c l u d e a r e l a t i v e l y narrow b e l t of P ro te rozo i c (? )

sch i s t , gneiss, and marb le (Werner, 1977, 1978; Monger and Berg, 1987) and

Devonian, Carboniferous, Permian, and T r i a s s i c sedimentary and vo l can i c rocks

( S t i k i n e t e r r a n e ) t h a t extend eastward i n t o t he i n t e r i o r o f B r i t i s h Columbia.

Rocks ad jacen t t o t h e bath01 i t h on t he west i n c l u d e a narrow and p o o r l y known

assemblage o f moderate ly t o s t r o n g l y deformed Permian and T r i a s s i c

metasedimentary and metavolcanic rocks (Taku t e r r a n e ) . The most ex tens i ve

assemblage o f p r e J u r a s s i c s t r a t a occurs on i s l a n d s t o t he west, where a

v i r t u a l l y complete s e c t i o n o f sedimentary and vo l can i c rocks ranges from the

1 a t e s t P ro te rozo i c (? ) -Cambri an through the La te T r i a s s i c (A1 exander t e r rane ) . Upper Mesozoic s t r a t a i n c l ude Jura-Cretaceous graywacke and ma f i c -

i n t e rmed ia te vo l can i c rocks bo th eas t and west o f t he b a t h o l i t h . S t r a t a on

t he west (Gravina b e l t ) ove r l ap bo th b e l t s o f p re -Jurass ic rocks and grade

f rom r e l a t i v e l y nondeformed on t he west t o high-grade s c h i s t and gneiss toward

the east . A t h i r d upper Mesozoic assemblage (Chugach t e r rane ) occurs a long

the west coas t o f no r t he rn SE Alaska and c o n s i s t s o f s t r o n g l y deformed and

d i s r u p t e d Cretaceous graywacke and vo l can i c rocks. Cenozoic s t r a t a a re

widespread and occur on bo th s ides of t he b a t h o l i t h . They range from

Paleocene t o Holocene i n age, w i t h some vo l can i c rocks e r u p t i n g as r e c e n t l y as

360 +/- 60 years ago ( E l l i o t t and o thers , 1981).

With in the Coast Range b a t h o l i t h , m e t a s t r a t i f i e d rocks occur as pendants

of amphibol i te- t o g ranu l i t e - fac ies sch i s t , gneiss, and marble der ived from

p re -Te r t i a ry p r o t o l i t h s . Some o f these rocks may be c o r r e l a t i v e w i t h

Pro terozo ic (?) metamorphic rocks along the eastern f l a n k o f the b a t h o l i t h ,

some were probably der ived from Cretaceous and o l d e r s t r a t i f i e d rocks t o the

eas t and west, and some p r o t o l i t h s may n o t occur elsewhere i n SE Alaska.

I n t r u s i v e rocks

I n t r u s i v e rocks ( f i g . 1) range i n age from Cambrian t o Miocene b u t most

are Cretaceous-early T e r t i a r y i n age. Paleozoic p l utons occur on the i s l ands

west of the Coast Range bathol i t h and inc lude: (1) small bodies o f Cambrian

metagranodior i t e and metad ior i t e , ( 2 ) Ordovician-Early S i 1 u r i a n calc-a1 ka l i n e

gran i t o ids , ( 3 ) mid-Si l u r i a n t o e a r l i e s t Devonian trondhjemi t e and

l e u c o d i o r i t e , and ( 4 ) Pennsylvanian-Permian syen i te and d i o r i t e . T r i a s s i c

p lu tons inc lude a pyroxene gabbro on Duke I s l a n d and a l a r g e g ranod lo r i t e body

eas t o f the Coast Range b a t h o l i t h . Jurass ic p lu tons occur i n a b e l t from

Baranof I s l a n d t o the west sfde o f G lac ie r Bay, on southern Pr ince o f Wales

I s1 and, and perhaps i n c lose assoc ia t ion w i t h Jura-Cretaceous s t r a t a a1 ong the

west f l ank of the Coast Range b a t h o l i t h . Most o f the pre-Cretaceous i n t r u s i v e

su i tes i n the panhandle appear t o be cogenetic w i t h vo lcan ic rocks which occur

nearby . Cretaceous-earl y T e r t i a r y p l utons i ncl ude: (1) grani t o i ds o f E a r l y

Cretaceous and e a r l y T e r t i a r y age on i s l ands west o f the Coast Range b a t h o l i t h

and along i t s eastern f lank , ( 2 ) b e l t s o f mid-Cretaceous u l t r a m a f i c bodies and

Late Cretaceous granod ior i t i c p l utons west o f the bathol S th , ( 3 ) Cretaceous-

e a r l y T e r t i a r y p lu tons t h a t are the pr imary cons t i t uen ts o f the Coast Range

b a t h o l i t h , and (4 ) Oligocene-Miocene a l k a l i c g r a n i t o i d s which t rend west-

northwest across the batholith and belts t o the west.

Regional metamorphism and deformation

Several phases of deformation and metamorphism have punctuated the

. evolution of SE Alaska ( f ig . 1 ) . The most significant and widespread event

occurred during Late Cretaceous-early Tertiary time, when strat if ied

protol i ths w i t h i n the Coast Range bathol t h (now pendants) and a1 ong i t s

western f l a n k were regionally metamorphosed t o amphi bol i te-granul i t e facies,

strongly deformed, and intruded by a variety of plutonic suites. Rocks east

of the bathol i t h (Sti kine terrane) were metamorphosed original ly prior to mid-

Paleozoic time and uplifted, eroded, and perhaps deformed during the Middle

Triassic Tahl t a n orogeny (Souther, 1971; Monger, 1977). Metamorphic rocks

along the west flank of the batholith progressively decrease in metamorphic

grade and degree of deformation toward the west. In the lower grade

assemblages, pre-Cretaceous tectonic events are recorded. An unconformity a t

the base of the Jura-Cretaceous section (Gravi na be1 t) indicates t h a t Triassic

and older strata b o t h along the west flank of the bathol i th ( T a k u terrane) and

on the islands t o the west (Alexander terrane) were up1 ifted and eroded, and

a t least locally deformed and metamorphosed, between Late Triassic and Late

Jurassic time (Gehrel s and Berg, 1984; McClel land and Gehrel s, 1987b). West

of the belt of Jura-Cretaceous strata, Paleozoic rocks (Alexander terrane)

record deformational and metamorphic events during Middle Cambrian-Early

Ordovician and middle Sil urian-earl i e s t Devonian time, and an up1 i ft and

erosional event during the L a t e Permian(?)-Triassic.

Faults

The most conspicuous structural features in SE Alaska are regional

strike-slip fault zones t h a t c u t the bedrock into a great jigsaw pattern ( f ig .

2; Twenhofel and Sainsbury, 1958). Three major s t r i k e - s l i p systems dominate.

On the west, t he panhandle i s t runcated a t the North American cont inenta l

margin by the Fairweather f a u l t system. Fau l t s of thSs system are known from

geologic mapping and from marine and onshore geophysical s tud ies t o be a c t i v e

r i g h t - 1 a t e r a l s t ruc tu res wf t h considerable displacement. To the south they

connect w i t h the Queen Char lo t te f a u l t system; t o the no r th they splay i n t o a

se t o f compt ex t h r u s t f a u l t s (P la fker , 1967). The second major s t r i k e - s l i p

system i s the Chatham S t r a i t f a u l t , which o f f s e t s rocks as young as mid-

T e r t i a r y by as much as 150 km (Lathram, 1964; Hudson and others, 1982a). This

f a u l t i s apparent ly t runcated t o the southwest by the Fa i rweather-Queen

Char lo t te f a u l t system and connects northward w i t h the Denal i f a u l t system.

The d i f f e r e n c e i n displacement between the Chatham St ra f t f a u l t (150 km) and

the Denal i f a u l t (350 km?: Lanphere, 1978; Nokleberg and others, 1985) i s an

unresolved problem. The t h i r d major s t r i k e - s l i p system i n SE Alaska i s the

Clarence S t r a i t f a u l t , which coinc ides w i t h a major topographic l ineament b u t

has on ly approximately 15 km o f dex t ra l displacement (Gehrels and others,

1987).

I n a d d i t i o n t o these s t r i k e - s l i p f a u l t s , t h rus t , low-angle normal, and

steep d i p - s l i p f a u l t s are s i g n i f i c a n t . The o l d e s t known o f these are

southwest-vergent t h r u s t s i n the southern panhandle whjch moved du r ing middle

S i l u r f a n - e a r l i e s t Devonian time. They were fo l lowed by normal movement on

gen t l y d ipp ing f a u l t s on southern Pr ince o f Wales I s l a n d (Keete I n l e t f a u l t o f

Redman, 1981) probably dur ing L a t e Permian(?)-Tr iass ic t ime (Gehrels and

Sal eeby, 1987a) . Southwest-vergent t h r u s t f au l t s have been mapped on l y

l o c a l l y along the west f l ank o f the Coast Range b a t h o l i t h (Berg and others,

1988; Rubin and Saleeby, 1987a; Gehrel s and McClelland, 1988), b u t probably a re

much more widespread. Such f a u l t s along the west coast o f B r i t i s h Columbia,

j u s t south of the panhandle, reg iona l 1 y juxtaposed high-grade metamorphic

rocks southwestward over lower-grade rocks dur ing Late Cretaceous-early

T e r t i a r y t ime (Crawford and others, 1987). Fau l t s of s i m i l a r age and s t y l e

b u t steeper- o r i e n t a t i o n have a1 so been recognized a1 ong the eas t f l ank o f the

Coast Range b a t h o l i t h (Berg and others, 1978; Crawford and others, 1987).

Soon a f t e r movement on these t h r u s t f au l t s , the Coast Range b a t h o l i t h and

adjacent rocks t o the west were up1 i f t e d along high-angle f a u l t s , one o f which

i s r e f e r r e d t o as the Coast Range megalineament (Brew and Ford, 1978).

TERRANES AND LITHIC ASSEMBLAGES

The rocks o f SE Alaska were i n i t i a l l y d i v i d e d i n t o reg iona l geologic

be1 t s o r assemblages by Buddington and Chapin (1929). Schuchert ( 1923) noted

t h a t rocks i n one o f these b e l t s record a geologic h i s t o r y which i s

s i g n i f i c a n t l y d i f f e r e n t from o ther regions o f the nor thern C o r d i l l e r a -- t h i s

assemblage he r e f e r r e d t o as the "Alexandrian embayment" w i t h i n the

Cordi 11 eran Geosyncl i ne. Geosyncl i nal theory dominated syntheses o f

southeastern Alaska geology u n t i l : (1) J.T. Wilson (1968) recogn i ted t h a t

Paleozoic rocks o f the panhandle occur outboard o f coeval miogeocl i nal s t r a t a

and must there fore have been accreted t o North America; (2) Monger and Ross

(1971) documented the Tethyan o r equator ia l a f f i n i t y o f Permian fusul i n i d

faunas inboard of southeastern Alaska; and ( 3 ) Berg and others (1972)

recognized t h a t the pre-Jurassic rocks o f southeastern Alaska belong t o

several d i s t i n c t t e c t o n i c fragments, ' terranes ,I1 t h a t have d ispara te geologic

records. The te r rane concept was app l i ed i n a more comprehensive fashion by

Berg and o thers (19781, wherein they d i v ided a l l rocks of SE Alaska i n t o (1 )

fundamental 1 y d i s t i n c t , f au l t-bounded tec ton i c fragments ( t ec tonos t ra t i g raph ic

ter ranes) , and ( 2 ) assemblages t h a t were emplaced i n t o o r deposited on more

than one te r rane and are accord ing ly i n t e r p r e t e d t o have formed a f t e r adjacent

fragments were juxtaposed.

I n t h i s r e p o r t we subdivide the rocks o f SE Alaska i n t o a v a r i e t y o f

t ec ton i c assemblages ( f i g u r e s 1 and 2). F i ve of these are i d e n t i f i e d as

terranes because they apparent ly have d i s t i n c t geologic records. These

inc lude the A1 exander, Chugach, S t i k i n e ( S t i k i n i a ) , Taku, and Wrangell i a

terranes. The o ther assemblages of SE Alaska c o n s i s t of metamorphic rocks o f

unknown t e c t o n i c a f f i n i t y , and of rocks t h a t a re known o r reasonably

i n t e r p r e t e d t o be i n depos i t iona l o r i n t r u s i v e con tac t w i t h the terranes.

Such l i t h i c assemblages inc lude: (1) Jura-Cretaceous s t r a t a o f the GravSna # be1 t ( p a r t o f the Gravina-Nutzot in be1 t o f Berg and others, 1972); ( 2 )

metamorphic pendants and screens o f unknown t e c t o n i c a f f i n i t y i n the Coast

Range bath01 i th ; ( 3 ) Cretaceous-early T e r t i a r y p l u t o n i c s u i t e s i n the

b a t h o l i t h ; ( 4 ) p l u t o n i c s u i t e s west o f the b a t h o l i t h o f Early, mid-, and Late

Cretaceous, Eocene-01 i gocene, and 01 jgocene-Miocene age; and ( 5 ) Quaternary-

T e r t i a r y s t r a t a t h a t are d i s t r i b u t e d throughout SE A1 aska.

Alexander te r rane (Ac and Aa on f i g . 2 )

The Alexander te r rane comprises a v a r i e t y o f s t r a t 4 f f ed, metamorphic,

and p l u ton i c rocks o f 1 a t e s t Precambrian(?)-Cambrian through Midd le(?)

Jurass ic age t h a t u n d e r l i e much o f the Alaskan panhandle and cont inue

northward i n t o the Sa in t E l i a s Mountains reg ion o f B r i t i s h Columbia and Yukon

(Berg and others, 1972; Churkin and Eber le in, 1977; Gehrels and Saleeby,

1987a). Vol c a n i c l a s t i c t u r b i d i tes, s h a l l ow-marine carbonate, and subordinate

conglomerate o f S i l u r i a n age a re the most widespread u n i t s i n the terrane.

Pre-S i lu r ian rocks occur p r i m a r i l y i n the southern p a r t o f SE Alaska, upper

Paleozoic rocks occur i n r e l a t i v e l y r e s t r i c t e d areas, and Upper T r i a s s i c

s t r a t a crop o u t i n a f a i r l y narrow b e l t along the eastern margin of the

terrane,

The o l d e s t rocks recognized are arc- type rnetasedimentary and metavolcanic

rocks (Wales Group) t h a t were metamorphosed and deformed dur ing the Middle

Cambrian-Early Ordovic ian Wales orogeny (Gehrels and Saleeby, 1984, 1987a).

These rocks form t h e depos i t iona l and i n t r u s i v e basement t o an arc-type

volcanic-p lutonic-sedimentary complex o f E a r l y Ordovician-Early S i l u r i a n age

which under l i es much o f the southern p a r t o f the terrane. Ordovician c h e r t

and a r g i l l i t e on Admira l ty I s l a n d and marine c l a s t i c s t r a t a and carbonate i n

northernmost SE Alaska and the Sa in t E l i a s Mountains reg ion are i n t e r p r e t e d t o

have formed i n a deep- t o shallow-marine bas in behind t h i s arc system (Gehrels

and Saleeby, 1987a). This phase of arc-type a c t i v i t y ceased w i t h onset o f t he

middle S i l u r i a n - e a r l i e s t Devonian Klakas orogeny, whtch i s mani fest i n the

southern p a r t o f the terrane as: (1) southwest-vergent t h rus t i ng ; ( 2 ) reg iona l

metamorphism and deformation i n some areas; ( 3 ) u p l i f t ( l o c a l l y >5 km) and

eros ion of the arc complex; and ( 4 ) generat ion o f a n a t e c t i c ( ? ) t rondhjemite

and 1 eucodi o r i t e bodies . Upper Paleozoic s t r a t a i n much o f the Alexander terrane cons i s t p r i m a r i l y

of shal low-marine carbonate, c l a s t i c s t r a t a , and subordinate mafic-

in te rmed ia te vo lcan ic rocks. These s t r a t a now occur i n r e s t r i c t e d eros ional

remnants b u t probably were much more widespread o r i g i n a l l y . The predominance

of shallow-marine l imestone, the lack of r e g i o n a l l y s i g n i f i c a n t unconformi t ies

o r t h i c k conglomerate i n t h e sect ion, and r e s t r i c t i o n of vo lcanic rocks t o the

Middle and Upper Devonian and the Lower Permian suggest t h a t the te r rane

evolved i n a t e c t o n i c a l l y s tab le environment compared t o the e a r l y Paleozoic

orogenic and magmatic activity . Upper Triassic strata over1 i e the Permian and 01 der rocks on a regional

unconformity. In the southern part of the terrane, the section generally

consists, from bottom t o t o p , of basal conglomerate and sedimentary breccia,

rhyolite and rhyol i t i c tuff , massive 1 imestone, calcareous argill i te , and

basal tic-andesi t i c pi 11 ow flows and breccia. Toward the northwest, the amount

of rhyolite and conglomerate decreases and the proportion of mafic-

intermediate volcanic rock increases. These strata and their subjacent

unconformity are interpreted t o have formed in a r i f t environment based on:

(1) the bimodal (basal t-rhyol i t e ) composition of the volcanic rocks; ( 2 )

occurrence of the section in a relatively narrow belt along the eastern margin

of the terrane; (3) stratigraphic evidence for syndeposi tional faul ting; and

( 4 ) evidence for Late Permian(?)-Triassic up1 i f t and erosion without

accompanying deformation and metamorphism (Gehrels and others, 1986).

The youngest component of the terrane i s the Bokan Mountain Granite,

which i s a Middle(?) Jurassic peralkaline ring-dike complex on southern Prince

of Wales Island (Thompson and others, 1982; Saint-Andre and others, 1983;

Armstrong, 1985). The tectonic significance of this body i s as yet unknown.

The Alexander terrane was subdivided by Berg and others (1978) and Monger

and Berg (1987) into the Craig, Annette, and Admiralty subterranes based

primarily on regional variations in stratigraphy and in degree and age of

metamorphism and deformation. Gehrels and others (1987) suggest t h a t the

Annette and Craig subterranes should no t be differentiated, however, because

they: (1) share similar pre-Middle Devonian and Triassic l i th ic assemblages;

( 2 ) record the same early Paleozoic tectonic histories; and (3) b o t h lack

upper Paleozoic strata. We accordingly i ncl ude rocks of the Annette

subterrane w i t h rocks o f t he C ra ig subterrane.

Re la t i ons among t h e Admi ra l t y (Aa on f i g . 2 ) and C r a i g (Ac on f i g . 2 )

subter ranes a r e more d i f f i c u l t t o assess because rocks o f the Admi ra l t y

subterrane: ( 1 ) have o n l y l o c a l l y been s tud ied i n d e t a i l ; ( 2 ) have y i e l d e d few

f o s s i l s ; and ( 3 ) a r e r e g i o n a l l y metamorphosed and deformed i n most areas.

However, s t ud ies t o date on Kupreanof I s l a n d (Mu f f l e r , 1967; Brew and others ,

1984; McCle l land and Gehrels, 1987a, b ) and Admi ra l t y I s l a n d (Lathram and

o thers , 1965) and i n t h e C h i l k a t Range (Lathram and o thers , 1959; Brew and

others , 1985; Forbes and o thers , 1987) suppor t t h e i n t e r p r e t a t i o n t h a t

Paleozoic s t r a t a o f t h e Admi ra l t y subter rane reco rd a d i f f e r e n t h i s t o r y f rom

coeval r ocks i n t h e r e s t o f the te r rane . I n general , t he Admi ra l t y subter rane

c o n s i s t s o f Ordov ic ian and Devonian t o T r i a s s i c bas ina l c l a s t i c s t r a t a , maf i c -

i n t e rmed ia te vo l can i c rocks, and subord ina te 1 imestone ( f i g . 1 ) . An

assemblage o f metasedimentary and metavolcanic rocks i n t he no r the rn C h i l k a t

Range (Four Winds complex o f Forbes and o thers , 1987) i s h e r e i n i n t e r p r e t e d t o

be p a r t o f t h e Admi ra l t y subter rane based on apparent s i m i l a r i t i e s w i t h rocks

on Admi ra l t y and Kupreanof I s l ands . Forbes and o the rs (1987) argue t h a t

metamorphism o f a t l e a s t some rocks i n t he Four Winds complex occur red d u r i n g

La te Devonian-Early M i s s i s s i p p i a n t ime. The t i m i n g o f t h i s event and t he

o v e r a l l bas ina l cha rac te r o f Adm i ra l t y s t r a t a c o n t r a s t w i t h t h e h i s t o r y o f t he

C ra ig subter rane and suppor t t h e i r d i s t i n c t i o n as two subterranes.

The e a r l i e s t re1 i a b l e 1 i n k between the two subter ranes occurs d u r i n g

Permian t ime, when c l a s t s o f c h e r t from t h e Admi ra l t y assemblage were

depos i ted on C ra ig s t r a t a and then t h e Pybus Dolomi te was deposi ted over bo th

subter ranes (Muf f le r , 1967; Berg and o thers , 1978; Jones and o thers , 1981).

T r i a s s i c s t r a t a of t he two subter ranes a re apparen t l y c o r r e l a t i v e ( M u f f l e r ,

1967; Gehrels and others, 1986). Thus, p r i o r t o Permian time, the Admira l ty

subterrane may have been a basinal , vo l can i c - r i ch fac ies adjacent t o a

t e c t o n i c a l l y stab1 e, dominantly s h a l l ow-marine fac ies (Cra ig subterrane) , o r

i t may have been a d i s t i n c t t ec ton i c fragment. \

The displacement h i s t o r y of the Alexander te r rane p r i o r t o i t s Mesozoic

accretion i s poo r l y constra ined and i s a sub jec t o f considerable speculat ion.

Apparent ly re1 f a b l e cons t ra in t s on i t s pal eoposi t i o n i n c l ude: (1) Nd-Sr

i s o t o p i c data which i n d i c a t e t h a t the te r rane comprises j uveni 1 e c r u s t a l

ma te r i a l s and was n o t i n p rox im i t y t o a l a r g e cont inenta l landmass p r i o r t o

Jurass ic t ime (Samson and others, 1987, 1988); ( 2 ) paleomagnetic data which

suggest t h a t the te r rane evolved near the paleoequator from Ordovician through

Pennsylvanian t ime (Van der Voo and others, 1980); ( 3 ) biogeographic

i n d i c a t i o n s o f a l a t e Paleozoic p o s i t i o n between eastern and western P a c i f i c

faunal realms (Mamet and Pinard, 1985); (4 ) the low p a l e o l a t i t u d e and eastern

P a c i f i c a f f i n i t y o f T r i a s s i c b i va l ves o f the te r rane (Tozer, 1982; Newton,

1983; S i l b e r l f n g , 1985); and ( 5 ) occurrence o f the te r rane outboard o f the

Cache Creek terrane, which conta ins Permian f u s u l i n i d faunas of Tethyan

( e q u a t o r i a l ) a f f i n i t y (Monger and Ross, 1971) and apparent ly remained i n an

oceanic s e t t i n g i n t o Jurassic t ime (Cordey and others, 1987). Accret ion

aga ins t the western margin of inboard terranes probably began dur ing Ear ly -

Middle Jurassic t ime (McClelland and Gehrel s, 1987b) and was completed by the

e a r l y T e r t i a r y (Monger and others, 1982).

Chugach te r rane ( C on f i g . 2)

The Chugach te r rane has two s t r u c t u r a l components ( P l a f k e r and others,

1977; Johnson and K a r l , 1985; Gehrel s and Berg, 1984): (1) a s t rong ly deformed

b u t coherent assemblage o f fl yschoidal graywacke, a r g i l l i t e , and s l a t e o f

probable E a r l y Cretaceous age; and ( 2 ) a deformed and d i s r u p t e d assemblage

(me1 ange) of b l ocks o f bas ic vo l can i c rocks, r ad io1 a r i a n che r t , u l t r a m a f i c

rocks, l imestone, and p l u t o n i c rocks i n a m a t r f x of cher ty , tu f faceous

a r g i l l i t e . The composi t ion and age o f some b locks suggest t h a t they p robab ly

were de r i ved f rom Permo-Carboniferous and T r i a s s i c s t r a t a o f Wrangel l ia .

Greenschis t - t o amph ibo l i t e - f ac i es reg iona l metamorphism o v e r p r i n t s l o c a l

remnants of b l u e s c h i s t - f a c i e s metamorphism. Rad io l a r i ans i n c h e r t i n t h e

m a t r i x o f t he melange a re g e n e r a l l y E a r l y Cretaceous i n age. The two

assemblages a r e s t r u c t u r a l l y i n t e r l e a v e d i n many places, b u t i n general t he

d i s r u p t e d assemblage tends t o l i e s t r u c t u r a l l y above and eas t o f t h e coherent

f l y s c h assernbl age.

J u x t a p o s i t i o n o f t he t e r r a n e aga ins t inboard assemblages i s thought t o

have occur red d u r i n g Late Cretaceous-ear ly T e r t i a r y t ime (Decker and o thers ,

1980), a1 though Cowan ( 1982) hypothes izes t h a t t he t e r rane was d i sp l aced

nor thward f rom an o r i g i n a l p o s i t i o n near Vancouver I s l a n d a f t e r 40 Ma.

S t i k i n e t e r r a n e ( S t i k i n i a : S on f i g . 2)

The most s i g n i f i c a n t components o f S t i k i n e t e r r a n e i n and ad jacen t t o SE

Alaska I n c l u d e Devonian carbonate, Carboni ferous a rc - type vo l can i c and

sedimentary rocks, Lower Permian bas ina l s t r a t a , Upper Permian p la t f o rma l

l imestone, and Upper T r f a s s i c t o M idd le Ju rass i c a rc - type vo lcan ic , p l u t o n i c ,

and c l a s t i c sedimentary rocks ( f i g . 1) (Monger, 1977; Gehrels and Berg, 1984;

Robert G. Anderson, o r a l commun., 1987). Most of t he t e r r a n e i n B r i t i s h

Col umbia i s covered by Jura-Cretaceous sedimentary and vo l can i c rocks;

T r i a s s i c and o l d e r s t r a t a c rop o u t p r i m a r i l y a long t h e eas te rn f l a n k of t h e

Coast Range b a t h o l i t h . The boundary between these s t r a t a and high-grade

metamorphic rocks i n t h e b a t h o l i t h i s i n most areas o b l i t e r a t e d by p l u tons o f

the batholith. In some regions, however: (1) a steep west-side-up fault

juxtaposes high-grade rocks against s trata of the Stikine terrane; ( 2 ) strata

be1 onging to the S t i kine terrane grade w i t h increasing metanorphi sm and

deformation into the high-grade rocks; and ( 3 ) Upper Triassic strata

apparently overlie high-grade metamorphic rocks (Souther, 1971; Berg and

others, 1978; Werner, 1977, 1978; Bultman, 1979; Monger and Berg, 1987; Brew

and others, 1985; Hill and others, 1985; Crawford and others, 1987)

Rocks of the Stikine terrane are known t o have been displaced because

they occur outboard of Cache Creek rocks containing Permian fusulinid faunas

of Tethyan or equatorial affinity (Monger and Ross, 1971), and their primitive

Nd-Sr i sotopic signature precludes primary relations with North America

(Samson and others, 1987). Their accretionary hi story remains en4 gmatic,

however, because: (1) paleomagnetic d a t a from Permian and Triassic rocks of

Stikine do not record significant latitudinal transport relative t o North

America (May and Butler, 1986; Irving and Monger, 1987); ( 2 ) the Cache Creek

terrane apparently remained in an oceanic setting through Early to Middle

Jurassic time (Cordey and others, 1987); and ( 3 ) paleomagnetic data from

Cretaceous rocks and from older rocks with interpreted Cretaceous magnetic

signatures record large-scale northward transport in Cretaceous-early Tertiary

time (Irving and others, 1985; Marquis and Globerman, 1987).

Taku terrane ( T on fig. 2)

The Taku terrane i s a poorly known assemblage of deformed and

metamorphosed strata of Early Permian, Middle and Late Triassic, and perhaps

pre-Permian age (Sil berl i ng a n d others, 1982; Brew and Grybeck, 1984). A s

mapped on figure 2, the terrane also contains a significant proportion of

Upper Jurassic t o mid-Cretaceous strata of the Gravina belt (Gehrels and Berg,

1984; Rubin and Saleeby, 1987b) and lower Paleozoic rocks o f the Alexander

t e r r a n e (Sal eeby, 1987). Regional l y s i g n i f i c a n t p re -Jurass ic components

inc lude : Permian c r i no ida l marble i n t e r c a l a t e d w i t h pe l i t i c p h y l l i t e and

f e l s i c m e t a t u f f ; Permian(?) b a s a l t i c me ta tu f f , agglomerate, and p i l l o w f lows;

Midd le and Upper T r i a s s i c carbonaceous 1 imestone, s l a t e , and p h y l l i te ; undated

q u a r t z i t e and q u a r t z o f e l dspa th ic gneiss presumably de r i ved f rom f e l SIC

volcan i c rocks; and metaconglomerate c o n t a i n i n g c l a s t s of g r a n i t i c rocks,

q u a r t z i t e , and f i ne -g ra ined c l a s t i c s t r a t a (Berg and o thers , 1978, 1988; Brew

and Grybeck, 1984; Brew and o thers , 1984, 1985; Rubjn and Saleeby, 1987a, b;

Gehrel s and McClel land, 1988; FlcClel l a n d and Gehrel s, 1988). Rocks o f

probable Jura-Cretaceous age i n c l u d e g reensch is t - and amph ibo l i t e - f ac i es

metagraywacke, meta-argi 11 i t e , metabasal t i c p i 11 ow f lows and b recc ia , and

metacongl omerate.

Rocks o f the Taku t e r r a n e can be subd iv ided i n t o t h r e e assemblages, t h e

r e l a t i o n s among which a re as y e t unknown (Monger and Berg, 1987). To t h e

northwest, Permian(?) -Tr iass ic metabasal t i c p i 11 ow f l ows predominate and a re

o v e r l a i n unconformably by less-deformed Jura-Cretaceous f l y s c h o f t he Gravina

b e l t (Redman, 1984; Gehrels and Berg, 1984). Near Haines ( f i g . 2 ) , Upper

T r i a s s i c b a s a l t and o v e r l y i n g T r i a s s i c - J u r a s s i c ( ? ) sediments a re r e p o r t e d t o

be geochemical l y and b i o s t r a t i g r a p h i c a l l y s i m i l a r t o Wrangell i a n s t r a t a i n

western B r i t i s h Col umbia and southern A1 aska ( P l a f k e r and Hudson, 1980; Davis

and Plafker , 1985; P la f ke r and o thers , 1988). I n southern SE Alaska,

T r i a s s i c ( ? ) metasedimentary rocks predominate b u t con tac t r e l a t i o n s w i t h Jura-

Cretaceous s t r a t a a r e obscured by Late Cretaceous-ear ly T e r t i a r y deformat ion

and metamorphism. Rubin and Saleeby (1987a, b ) and Saleeby (1987) r e p o r t t h a t

much of what has been mapped as t he southern Taku t e r r a n e c o n s j s t s o f lower

Paleozoic metasedimentary and metavolcanic rocks o f t he Alexander te r rane. In

c e n t r a l SE A1 aska, the te r rane inc ludes both Permian-Triassic metasedimentary

and metavolcanfc rocks and a t h i c k sequence of metarhyo l i te , metabasalt, and

qua r t z - r i ch metagraywacke o f unknown age and a f f i n i t y (Gehrel s and McCl e l 1 and,

1988; McCl e l 1 and and Gehrel s, 1988).

Contact r e l a t i o n s w i t h adjacent terranes are poo r l y known. To the

southwest, the te r rane i s general 1 y t h r u s t southwestward over Jura-Cretaceous

s t r a t a o f the Gravina b e l t . To the nor theast , rocks o f the te r rane increase

i n metamorphic grade t o amphiboli t e and l o c a l l y granul i t e f ac ies and are

i n t ruded by p lu tons o f t he Coast Range bathol i t h . We t e n t a t j v e l y draw the

eastern boundary along the western edge o f t h e bathol i th, a1 though metamorphic

rocks w i t h i n the b a t h o l i t h a re 1 i t h f c a l l y i nd i s t f ngu ishab le from high-grade

members of the Taku terrane.

Wrangel l ia te r rane ( W on f i g . 2)

A coherent sequence o f u n f o s s i l i f e r o u s s t r a t a on Chichagof and Baranof

Is lands i s i n t e r p r e t e d t o be a fragment o f Wrangel l ia on the bas is of

s i m i l a r i t i e s i n l i t h i c types and age ( P l a f k e r and others, 1976; Jones and

others, 1977; Berg and others, 1978). The sequence i s d i s t i ngu i shed by: (1 )

t h i c k , ma in ly subaeri a1 b a s a l t f lows (Goon Dip Greenstone) s imi 1 a r t o those o f

the Middle o r Upper T r i a s s i c N i k o l a i Greenstone i n southern Alaska (Jones and

others, 1977); ( 2 ) shallow- t o deep-marine carbonate (Whi t e s t r i p e Marble) t h a t

i s s i m i l a r t o the Upper T r i a s s i c Ch i t i s tone Limestone; and ( 3 ) p e l i t i c

sedimentary rocks s i m i l a r t o the Upper T r i a s s i c and Jurassic McCarthy

Formation. Jurass ic tonal i t i c p l utons are t he youngest components o f the

terrane, The sequence apparent ly o v e r l i e s a heterogeneous assemblage o f

upper(?) Paleozoic mafic vo lcanic rocks, pyroc l a s t i c rocks, c l a s t i c

sedimentary rocks, and minor c h e r t and marble rang ing i n metamorphic grade

f rom g reensch i s t t o amph ibo l i t e f ac i es .

The con tac t s between these rocks and ad jacen t t e r ranes a re p o o r l y known.

To t he e a s t they a r e apparen t l y jux taposed a g a i n s t rocks o f t he Alexander

t e r rane a long t h e Perf 1 S t r a i t f a u l t , a r i g h t - l a t e r a l f a u l t w i t h T e r t i a r y

o f f s e t , b u t t h e o r i g i n a l boundary between t h e two i s d i f f i c u l t t o i d e n t i f y due

t o abundant Jurass ic(?) -Cretaceous p lu tons and widespread metamorphism and

deformat ion a1 ong t h e boundary. E l sewhere Wrangell i a i s jux taposed a g a i n s t

rocks o f t h e Chugach t e r r a n e a long t he Border Ranges f a u l t , which i s

i n t e r p r e t e d as a west-vergent t h r u s t (P l a f k e r and o thers , 1976).

I f c o r r e l a t i o n s w i t h rocks i n o t h e r p a r t s o f Wrange l l i a a re c o r r e c t , then

the rocks i n SE Alaska must a1 so have been t r anspo r ted cons iderab le d is tances

nor thward s i nce La te T r i a s s i c t ime (Jones and o thers , 1977; H i l l h o u s e and

Gromme, 1984; Tozer, 1982).

Gravina b e l t (G on f i g . 2 )

The Gravina b e l t comprises Upper Ju rass i c t o mid-Cretaceous marine

a r g i l l i t e and graywacke, in te rbedded a n d e s i t i c t o b a s a l t i c vo l can i c and

vo lcan ic1 a s t i c rocks, subord inate p o l y m i c t i c conglomerate, and perhaps p l u tons

rang ing f rom qua r t z d i o r i t e t o d u n i t e and p e r i d o t i t e (Berg and others , 1972,

1978; Gehrel s and Berg, 1984). These s t r a t a occur i n a narrow be1 t sepa ra t i ng

t he Alexander and Taku te r ranes and reco rd the t r a n s i t i o n from lower-grade

rocks on t h e wes t t o h igher-grade rocks a long t h e f l a n k o f t he Coast Range

b a t h o l i t h . I n genera l , t h e metamorphic grade increases from greensch is t o r

sub-greenschis t fac ies on t he west t o amph ibo l i t e and l o c a l l y g r a n u l i t e f a c i e s

toward t h e east . Contact r e l a t i o n s w i t h ad jacen t t e r ranes a re uncer ta in .

Al though s t r a t a o f t h e Gravina b e l t a re i n t e r p r e t e d t o d e p o s i t i o n a l l y o v e r l i e

rocks o f t h e Alexander t e r rane (Berg and o thers , 1972), depos i t i ona l con tac t s

between t h e two assemblages a re appa ren t l y preserved o n l y on Gravina I s l a n d

(Berg, 1973) and Kupreanof I s 1 and (McClel l a n d and Gehrel s, 1987a). I n

- con t ras t , Gravina rocks a re known t o d e p o s i t i o n a l l y o v e r l i e metamorphosed and

deformed rocks of t h e Taku t e r r a n e i n no r t he rn SE Alaska (Redman, 1984).

S i m i l a r r e l a t i o n s may a l s o occur i n southern SE Alaska (Gehre ls and Berg,

1984; McClel l a n d and Gehrels, 1987b; Char les Rubin, o r a l commun., 1987).

The eas te rn marg in o f t he Gravina b e l t i s i n most areas d i f f i c u l t t o

identify. I n southern SE Alaska, high-grade metamorphic rocks assigned t o t h e

Taku t e r r a n e were p robab ly de r i ved i n p a r t f rom Gravina be1 t p r o t o l i t h s . To

the nor th , Gravina s t r a t a a re bound t o t he e a s t by eas t -d i pp ing t h r u s t

(Gehrel s and McClel land, 1988) and perhaps s t r i k e - s l i p f a u l t s , and may l o c a l l y

be i n t r u d e d by p l u tons be long ing t o t h e western p a r t o f t h e Coast Range

ba tho l i th,

Regional r e l a t i o n s suggest t h a t t h e Gravina b e l t i s p a r t o f a bas ina l

assemblage t h a t accumulated a long t he eas te rn marg in o f a p r e v i o u s l y

jux taposed Alexander t e r r a n e and Wrange l l i a and con t inued eastward across Taku

t e r r a n e (Berg and o thers , 1972, 1978). The c o n t i n u a t i o n o r c o r r e l a t i v e o f

Gravina s t r a t a eas t o f t he Taku t e r r a n e has n o t y e t been i d e n t i f i e d ,

Metamorphic rocks o f t he Coast Range ba tho l i t h (M on f i g . 2)

Metasedimentary and metavolcanic rocks comprise n e a r l y 50% o f t h e Coast

Range ba tho l i t h and c o n s i s t p r i m a r i l y o f pe l i t i c , semi-pel i t i c , and

quar tzo fe l dspa th ic s c h i s t and gnei ss and subord ina te amphi b o l i te , q u a r t z i t e ,

marble, and c a l c - s i l i c a t e . P r o t o l i t h s a re i n t e r p r e t e d by most workers t o be

a r g i l l aceous marine s t r a t a , 1 imestone and ( o r ) do lomi te , che r t , and

subord inate ma f i c t o f e l s i c vo l can i c rocks. Some rocks may a l s o have been

de r i ved from p l u t o n i c p r o t o l i ths. P r o t o l i t h ages o f Cretaceous(?) ,

J u r a s s i c ( ? ) , T r i a s s i c , Permian(?), Carboni ferous(?) , and P ro te rozo i c (? ) a re

i n d i c a t e d by: ( 1 ) r e l a t i o n s n o r t h and e a s t o f Juneau which suggest t h a t t he

metamorphic rocks grade eastward i n t o T r i a s s i c and o l d e r s t r a t a o f S t i k i n i a ,

and a re a t l e a s t l o c a l l y o v e r l a i n by t he T r i a s s i c s t r a t a (Souther, 1971;

Werner, 1977, 1978; Bul tman, 1979; Brew and o thers , 1985); ( 2 ) a p r e l i m i n a r y

Rb-Sr i sochron o f P ro te rozo i c apparent age determined on high-grade

metamorphic rocks a1 ong t h e A1 aska-Br i t i sh Col umbia border n o r t h o f Juneau

(L.J. Werner and R.L. Armstrong, i n Monger and Berg, 1987; Werner, 1977, -

1978); and ( 3 ) r e l a t i o n s i n t h e Coast Range b a t h o l i t h southeast o f SE Alaska

whfch suggest t h a t s i m i l a r metamorphic rocks were de r i ved f rom Cretaceous and

Ju rass i c s t r a t a (Douglass, 1986), from t h e Ju rass i c Bowser Lake Group

(Woodsworth and others , 1983), and f rom s t r a t a o f probable Permian (H i 11 ,

1985) and pre-Permian (Hutchison, 1982) age (Hi11 and o thers , 1985).

Regional amph ibo l i te - and l o c a l l y g r a n u l i t e - f a c i e s metamorphism occur red

p r i m a r i l y d u r i n g La te Cretaceous-ear ly T e r t i a r y t ime, a1 though metamorphism

p r i o r t o d e p o s i t i o n o f Jura-Cretaceous s t r a t a o f t h e Gravina b e l t , p r i o r t o

T r i a s s i c t ime, and between Carboniferous and P ro te rozo i c t ime may have

p r e v i o u s l y a f fec ted va r i ous p a r t s o f t h e Coast Range b a t h o l i t h .

I n t r u s i v e s u i t e s of t he Coast Range b a t h o l i t h

P lu tons o f t he Coast Range b a t h o l i t h comprise t h ree d i s t i n c t s u i t e s t h a t

become p r o g r e s s i v e l y younger toward t he east , and a l a r g e u n i t of und iv ided

g ranod io r i t i c rocks rang ing i n age from E a r l y ( ? ) Cretaceous t o Paleocene

( f i g u r e s 1 and 2). The o l d e s t s u i t e c o n s i s t s o f narrow b u t ve ry l o n g sheet-

l i k e masses of hornblende-dominant t o n a l i t e and qua r t z d i o r i t e t h a t extend i n

a l i n e a r f ash ion a long t he western margin o f t he b a t h o l i t h . These t a b u l a r

bodies form a complex of tonal i t i c s i l l s (Brew and Ford, 1981) and are

interpreted as marking a fundamental tectonic boundary within the batholith

(Berg and others, 1978; Brew and Ford, 1981; Gehrel s and Berg, 1984). Strong

foliation and lineation within the bodies and the i r contact relations with

country rocks suggest t h a t they were intruded during the l a t e r stages of

regional metamorphism and deformation within the Coast Range batholith. U-Pb

(zircon) dates on individual bodies appear t o young southward from near 70 Ma

i n northern southeastern Alaska (Barker and others, 1986), through 67-64 Ma in

central SE Alaska (Gehrels and others, 19841, t o approximately 60 Ma jus t

south of the panhandle (Armstrong and Runkle, 19791,

East of the tona l i t ic s i l l s are discrete plutons to large batholithic

complexes of Paleocene tonal i te-granodiori t e . These bodies are commonly

elongate b u t appear to have been emplaced a f t e r most of the deformation in the

batholith. In southern SE Alaska these bodies engulf the tonali t i c s i l l s and

constf tute most of the bathol i t h . The youngest and vol umetrical 1 y most

significant su i te in the bathol i t h consists of huge bodies of bioti te-dominant

granodiorite of Eocene age (Gehrels and others, 1984; Gehrels and Berg, 1984).

These rocks were apparently empl aced a t shallow crustal levels, as the i r

volcanic cover (Sloko Volcanics) i s locally preserved adjacent t o the pl utons.

Barker and Arth (1984), Barker and others (19861, and Arth and others

(1988) conclude t h a t plutons in the batholith are the roots of an Andean-type

arc formed in response to subduction of oceanic crust . In contrast , Monger

and others (1982), Kenah and Holl i s t e r (19831, and Crawford and others (1987)

indicate tha t some components may be anatectic me1 ts generated during the main

phase of deformation and metamorphism in the Coast Range batholith.

Intrusive sui tes west of the Coast Range batholith

Intrusive bodies west of the Coast Range batholith belong t o several

suites, including, from 01 dest t o youngest:

(1) Large, generally is01 ated granodiori te pl utons t h a t intrude A1 exander

and Wrangell i a . K-Ar dates on these bodies are generally Early Cretaceous,

b u t some bodies may be coeval with similar plutons i n the Saint Elias

Mountains region t h a t yield Jurassic K-Ar dates. Berg and others (1972)

suggest t h a t these intrusions formed in response t o plate convergence along

the western margin o f A1 exander and Wrangel 1 ia terranes.

( 2 ) Zoned ul tramaflc complexes ranging in composition from duni te,

commonly in the centers of the complexes, t o cl inopyroxeni te (Taylor, 1967;

Irvine, 1967, 1974). These bodies yield K-Ar dates o f Early to mid-Cretaceous

(Lanphere and Eberlein, 1966). The ul tramafic bodies intrude Triassic and

older rocks of the Alexander terrane, Permo-Triassic(?) rocks of Taku terrane,

and probably Jura-Cretaceous strata of the Gravina be1 t. I t has been argued

that the pl utons are subvolcanic t o m a f i c , cl inopyroxene-bearing flows in the

Gravina be1 t ( Irvine, 1973, 1974). They may have been formed in response to

plate convergence along the Pacific margin (Berg and others, 19721, o r in a

transtensional environment within which the Gravina basin was local ized

(Gehrel s and Sal eeby, 1985 1.

(3) Granodiori t i c , tonal i t i c , and subordinate quartz monzoni te to quartz

diorite bodies immediately west of the Coast Range batholith t h a t intrude

strata of the Taku terrane and Gravina bel t . Most intrusions contain bioti te

and(or) hornblende, many contain garnet, muscovite, and primary epidote, and

some are pyroxene-beari ng. K - A r , 4 0 ~ r / 3 9 ~ r , and U-Pb dates indicate

emplacement primarily during mid-Cretaceous time (Saleeby, 1987; Rubin and

Saleeby, 1987a, b; Sutter and Crawford, 1985). Geobarometric studies of epidote

and garne t i n these bodies suggest t h a t they c r y s t a l l i z e d a t mid- t o l owe r

c r u s t a l l e v e l s (Zen and Hammarstrom, 1984a, b) . A r t h and o the rs (1988)

conclude t h a t these p lu tons formed i n a subduc t fon- re la ted i s l a n d a r c based on

geochemical and i s o t o p i c analyses i n t h e Ketch ikan area, b u t Crawford and

o the rs (1987) argue t h a t they formed i n response t o c r u s t a l ana tex is d u r i n g

mid-Cretaceous compressional deformat ion.

( 4 ) Large g ranod io r i t i c bodies o f probable Eocene-01 igocene age t h a t

f n t r u d e Chugach, Wrangell i a , and A1 exander t e r ranes i n nor thwestern SE A1 aska.

The rocks range f rom muscovi te and 1 ocal l y garne t -bear ing g r a n o d i o r i te ,

g r a n i t e , and t ona l i t e i n t he Baranof I s l and -G lac ie r Bay area, t o b i o t i t e - and

hornbl ende-bearSng qua r t z d i o r i t e and g r a n o d i o r i t e i n t h e C h j 1 k a t Range.

(5) Stocks o f b i o t i t e - , hornblende-, and pyroxene-bearing g ran i t e , a1 k a l i

gran i te , qua r t z monzoni t e , g r a n o d i o r i t e , and d i o r i t e , and o f 1 ayered and

l o c a l l y toned bodies of gabbro, qua r t z gabbro, and o t h e r ma f i c -u l t r ama f i c

i n t r u s i v e s . These s tocks occur i n two d i s t i n c t reg ions, one ex tend ing f rom

the Coast Range b a t h o l i t h eas t o f Ketch lkan northwestward t o t he no r the rn t i p

o f Ku iu I s l and , and t h e o t h e r i n no r t he rn SE Alaska on Chichagof I s l a n d and i n

the G l a c i e r Bay area. K - A r dates on these bodies a r e g e n e r a l l y Ol igocene and

Miocene. The bodies i n southern and c e n t r a l SE Alaska i n t r u d e t he Coast Range

b a t h o l i t h , Gravina b e l t , and t h e Taku te r ranes , whereas t h e no r the rn

assembl age i n t r u d e s A1 exander , Wrangell , and Chugach te r ranes .

Qua te rna ry -Te r t i a r y s t r a t a ( Q T on f i g . 2)

Qua te rna ry -Te r t i a r y s t r a t a u n d e r l i e l a r g e reg ions o f Kupreanof, southern

Admira l ty , and western Baranof I s l ands and occur i n many o the r more r e s t r i c t e d

areas throughout SE Alaska. F i g u r e 2 shows the d i s t r i b u t i o n o f these strata

o n l y where they cover l a r g e reg ions. I n c e n t r a l SE Alaska these s t r a t a

i n c l u d e t h e e a r l y t o m idd le T e r t i a r y Kootznahoo Format ion (non-marine

sandstone, shale, and congl omerate) and Admiral ty I s 1 and Vol can ics ( basal t and

andesi t e ) , and younger basal t i c t o r hyo l i t i c v o l c a n i c rocks and assoc ia ted

sediments. West o f G l a c i e r Bay, 01 igocene t o Miocene s t r a t a ( n o t shown on

f i g . 2 ) be long t o t he Cenotaph Volcanics ( b a s a l t ) and t h e Topsy Format ion

(mar ine ca lcareous sandstone and s i l t s t o n e ) . Quaternary and T e r t i a r y basal t i c

t o r h y o l i t i c vo l can i c rocks and subord ina te sediments a l s o occur a t M t ,

Edgecumbe (west o f S i t k a ) , i n t he Coast Range bath01 i t h eas t o f Ketch ikan and

Petersburg, i n t h e western Pr ince o f Wales I s l a n d reg ion , on i s l a n d s i n Cross

Sound, and i n many o t h e r areas.

RELATIONS AMONG TERWES

The p r ima ry and present-day r e l a t i o n s among t h e t e r ranes o f southeastern

Alaska a r e c o n t r o v e r s i a l . U n c e r t a i n t i e s about t h e i r s i m i l a r i t i e s and

d i f ferences and about the ex is tence and na tu re of boundaries between them

a r i s e f rom t h e f a c t t h a t t he geology o f much of t h e panhandle has been s tud ied

o n l y i n reconnaissance fash ion, and because many c r i t i c a l r e l a t i o n s w i t h i n and

between t e r r a n e s a r e obscured by Cretaceous-ear ly T e r t i a r y metamorphism,

deformation, and ( o r ) p l utonism. To date, two fundamenta l ly d i f f e r e n t

i n t e r p r e t a t i o n s o f t h e t e c t o n i c framework o f SE Alaska have been proposed.

Berg and o the rs (1978) and Monger and Berg (1987) be1 $eve t h a t the Alexander,

Chugach, S t i k i n e , Taku, and Wrangell i a te r ranes , and t h e metamorphic r ocks o f

the Coast Range b a t h o l i t h , each have d i s t i n c t l i t h i c components and t e c t o n i c

h i s t o r i e s , and t h a t each i s ( o r was) f a u l t bounded. I n con t ras t , o t h e r

workers ( f o r example, Brew and Ford, 1983, 1984) b e l i e v e t h a t t he d i f f e r e n c e s

between most o f these te r ranes r e s u l t f rom f a c i e s changes w i t h i n a s i n g l e

c r u s t a l fragment. For example, Brew and Ford (1984) suggest t h a t Permo-

T r i a s s i c rocks o f t h e Taku and S t i k i n e t e r ranes a re f ac i es -equ i va len t s of t h e

upper p a r t s o f t he Alexander t e r rane , and t h a t t h e metamorphic rocks of t h e

Coast Range b a t h o l i t h and t he o l d e r rocks o f S t i k i n i a a re f a c i e s equ i va len t s

o f t h e lower p a r t s o f t he Alexander te r rane .

I n t he f o l l o w i n g sec t i ons we assess what i s known about t h e s i m i l a r i t i e s

and d i f f e r e n c e s among Alexander, Taku, Wrange l l i a , and S t i k i n e t e r ranes and

the metamorphic rocks i n t he Coast Range b a t h o l i t h , and o f f e r some t e n t a t i v e

i n t e r p r e t a t i o n s about p r imary r e l a t i o n s among the te r ranes .

A1 exander-Wrangel 1 i a re1 a t i o n s

The Alexander and Wrange l l i a t e r ranes were o r i g i n a l l y i n t e r p r e t e d as

separate t e c t o n i c e n t i t i e s p r i o r t o t h e i r j u x t a p o s i t f o n d u r i n g Ju rass i c t ime

(Berg and o thers , 1978; Coney and o thers , 1980). Th i s i n t e r p r e t a t i o n needs t o

be m o d i f i e d because Pennsyl vani an-Permian d i o r i t i c and syeni t i c i n t r u s i v e

bodies a r e "ow known t o i n t r u d e b o t h t e r ranes and t h e i r boundary i n southern

Alaska and southwestern Yukon (MacKevett and o thers , 1986). These r e l a t i o n s

i n d i c a t e t h a t Alexander and Wrange l l i a have been i n p r o x i m i t y s ince a t l e a s t

Pennsylvanian-Permian t ime. I n a d d i t i o n , i t i s 1 i k e l y t h a t Upper T r i a s s i c

rocks o f t he Alexander t e r rane a r e f a c i e s equ i va len t s o f t h e Upper T r i a s s i c

r i f t assemblage i n Wrangell i a (Gehrel s and o thers , 1986).

Pre-Pennsyl vanian re1 a t i o n s between t he two ter ranes, however, a r e as y e t

uncer ta in . The Devonian through Permian v o l c a n i c and bas ina l sedimentary

assemblages t h a t c h a r a c t e r i z e Wrange l l i a (Jones and others , 1977; Brandon and

others , 1986) a r e d i f f e r e n t f rom the carbonate-dominated upper Paleozoic rocks

o f much o f t h e Alexander t e r rane ( C r a i g subter rane) . It i s poss ib l e t h a t they

c o r r e l a t e w i t h d i s t a l sedimentary and vo l canogenic components o f t h e Admi ra1 t y

subterrane, b u t a r i g o r o u s comparison of these two assemblages must a w a i t more

d e t a i l e d s t u d i e s o f t he Admi ra l t y subterrane.

A1 exander-Taku re1 a t i o n s

Rigorous comparisons between t he Taku t e r r a n e and ad jacen t assemblages

a re h indered by a l a c k o f age c o n s t r a i n t s on most p r o t o l j t h s o f Taku. Recent

s tud ies have a l s o shown t h a t , i n many areas, rocks p r e v i o u s l y assigned t o t he

Taku t e r r a n e a re p a r t s o f o t h e r assemblages. I n t he Ketch ikan area, f o r

example, Saleeby (1987) and Rubin and Saleeby (1987a, b ) conclude t h a t t h e Taku

t e r rane o f Berg and o the rs (1978) c o n s i s t s o f lower Paleozoic rocks o f t he

Alexander terrane, Jura-Cretaceous s t r a t a of the Gravina b e l t , and Permo-

T r i a s s i c metasedimentary and metavolcanic rocks o f t r u e Taku a f f i n i t y . I n

cont ras t , i n nor thern SE Alaska, P l a f k e r and Davis (1985) and Plafker and

others (1988) argue on the bas is o f geochemical and b i o s t r a t i g r a p h i c

s i m i l a r i t i e s t h a t T r i a s s i c s t r a t a o f the Taku te r rane are c o r r e l a t i v e w - i i

Wrangell i a basal t s and ove r l y i ng sediments. These re1 at ions, the prox imi ty o f

Taku rocks t o o l d e r Alexander te r rane s t r a t a near Ketchikan, and the pre-

T r i a s s i c l i nkage of Alexander te r rane and Wrangel l ia are cons j s ten t w i t h a

scenario i n which the T r i a s s i c rocks i n Alexander, Wrangel l ia, and Taku

terranes are p a r t s o f a once-contiguous r i f t assemblage. The d i f f e rences i n

Upper T r i a s s i c rock types among the th ree terranes may r e f l e c t vary ing

pos i t i ons i n the extensional envjronment: bimodal vo lcan ics and coarse

conglomerate o f the Alexander te r rane may have formed on th i cke r , more evolved

c r u s t near the basin margin; f lood basal t s of Wrangell i a may be the r e s u l t o f

extension w i t h i n ensimat ic o r less-evol ved c rus t ; and b a s a l t and f ine-gra ined

c l a s t i c s t r a t a o f Taku may have formed w i t h i n an e n t i r e l y basinal regime.

The p r i n c i p l e arguments aga ins t pr imary 1 i n k s between Alexander and Taku

are tha t : (1 ) Permian rocks are known i n o n l y two r e s t r i c t e d regions of the

Alexander te r rane i n the panhandle but apparent ly c o n s t i t u t e much of the Taku

terrane; ( 2 ) i n s p i t e o f t h e i r present-day c lose prox imi ty , T r i a s s i c l i t h i c

types and s t r a t i g r a p h i c r e l a t i o n s i n the two terranes are q u i t e d i f f e ren t ; and

( 3 ) z i r con populat ions i n U-Pb samples from the Alexander te r rane do n o t show

evf dence o f i n h e r i tence (Gehrel s and Saleeby, 1987b; Gehrel s and others,

19871, whereas Cretaceous i n t r u s i v e s i n the Taku terr8ane have i n h e r i t e d

s i g n i f i c a n t Precambrian z i rcon components (Rubin and Saleeby, 1987a, b ) .

Current data are thus general 1 y cons i s ten t w i t h a scenario i n which

A1 exander, Wrangell i a, and Taku nay have been c l o s e l y associated du r ing

T r i a s s i c and perhaps Permian time. The data do not, however, r u l e ou t the

p o s s i b i l i t y t h a t Taku remained fundamentally separate from Alexander and

Wrangel l ia i n t o Jurassic t ime.

C o r r e l a t i o n o f metamorphic rocks o f the Coast Range b a t h o l i t h

The h igh metamorphic grade, pene t ra t i ve deformation, and l ack o f

p r o t o l i t h age c o n t r o l 1 i m i t arguments concerning the reg f onal t ec ton i c

a f f i n i t y o f metamorphic rocks o f the Coast Range b a t h o l i t h . Most r e l a t i o n s

i nd i ca te , however, t h a t rocks along the eastern margin o f t he b a t h o l i t h e i t h e r

are metamorphic equivalents o f S t i k i ne s t r a t a o r be1 ong t o a metamorphic

complex t h a t i s o v e r l a i n and i n t ruded by T r i a s s i c rocks o f S t i k i ne . These

metamorphic rocks are apparent ly i n d i s t i n g u i s h a b l e from rocks i n t he western

p a r t of the b a t h o l i t h and i n the eastern, h igh grade, p a r t o f Taku terrane.

I n add i t i on , i n h e r i t e d Precambrian z i r con components a l so occur i n i n t r u s i v e

bodies i n bo th the Taku te r rane (Rubin and Saleeby, 1987a, b ) and the Coast Range

bathol i t h (Gehrel s and others, 1984). Thus, a1 though tonal i t i c s i l l s and ( o r )

the Coast Range mega1 ineament may mark the pr imary boundary between the

bathol i t h and the Taku te r rane (Berg and others, 1978; Brew and Ford, 1978,

1984; Gehrel s and Berg, 1984), i t i s n o t y e t poss ib le t o document a

s i g n i f i c a n t change i n p r o t o l i t h content across e i t h e r one. As i n most

prev ious syntheses, we draw the boundary w i t h i n the t o n a l i t i c s i l l s because

they mark the on l y recognized major d i s c o n t i n u i t y between metamorphic rocks t o

the eas t and the west.

Brew and Ford (1984) have suggested t h a t metamorphic rocks i n the Coast

Range b a t h o l i t h a re the metamorphic equ iva len ts of s t r a t a i n the lower p a r t o f

the Alexander terrane. Beyond a crude comparison of p ropor t ions o f general

rock types, t h i s p o s s i b i l i t y i s d i f f i c u l t t o t e s t g e o l o g i c a l l y because so

l i t t l e i s known about the p r o t o l i t h age of the metamorphic rocks.

I s o t o p i c a l l y , however, the two can be d i s t i ngu i shed on the bas is of the: (1)

presence o f i n h e r i t e d t i rcon components i n the bath01 i t h (Gehrel s and others,

1984) b u t n o t i n the Alexander te r rane (Gehrel s and Saleeby, 1987b, Gehrels

and others, 19871, and (2) s i g n i f i c a n t l y more p r i m i t l v e 87~ r /86~r and

143~d /144~d i n 1 t i a l r a t i o s i n the A1 exander te r rane (Samson and others, 1987,

1988) than i n i n t r u s i v e bodies o f the Coast Range b a t h o l i t h (Barker and

others, 1986; A r th and others, 1988).

TECTONIC H I STORY OF SOUTHEASTERN ALASKA

The c u r r e n t l y decipherable tec ton i c h i s t o r y o f SE A1 aska begins dur f ng

l a t e s t Pro terozo ic (?) -Ear ly Cambrian t ime w i t h the format ion o f an arc- type

basement f o r t he Alexander terrane. Arc-type a c t i v i t y , punctuated by Late

Cambrian-Early Ordovician and middle S i l u r i a n - e a r l i e s t Devonian orogenic

events, cont inued through S i l u r i a n and perhaps i n t o Devonian time. The e a r l y

Paleozoic paleoposi t i o n o f the te r rane wS thSn t h f s t e c t o n i c a l l y ac t i ve , i n t r a -

oceanic regime i s problematic. Gehrels and Saleeby (1984, 1987a) argue t h a t

the te r rane bears tec ton i c s i m i l a r i t i e s w i t h orogenic systems t h a t formed

along the pa leo-Pac i f i c margins o f Aus t ra l i a , Antarc t ica , and c r u s t a l

fragments now r e s i d i n g i n Asia, and hypothesize t h a t the te r rane may have

formed w i t h i n the western p a r t of the pa leo-Pac i f i c Ocean basin. As noted by

Savage (1987), however, e a r l y Paleozoic faunas o f the A1 exander te r rane are

d i f f e r e n t from faunas found i n eastern Aus t ra l i a , and more c l o s e l y resemble

North American forms. I n apparent c o n t r a s t t o bo th comparisons, Nd-Sr

i s o t o p i c data i n d i c a t e t h a t t he te r rane i s cons t ruc ted of j u v e n i l e c r u s t a l

m a t e r i a l s and t h a t i t was n o t near any con t i nen ta l landmasses dur ing e a r l y

Paleozoic t ime (Samson and others, 1987, 1988).

P ro te rozo i c (? ) rocks a long the eastern margin o f t he Coast 4ange

b a t h o l i t h d i f f e r f rom those i n Alexander te r rane because they are dominated by

qua r t z - r i ch c l a s t i c s t r a t a and y i e l d Rb-Sr i s o t o p i c data cons i s ten t w i t h an

age o f approx imate ly 900 Ma (L.J. Werner and R.L. Armstrong, - i n Monger and

Berg, 1987). These rocks may have formed i n a con t i nen ta l margin environment

and are repo r ted by Werner (1977, 1978) and Bul tman (1979) t o form the

depos i t iona l basement t o T r i a s s i c rocks o f t he S t i k i n e te r rane. The p r i m i t i v e

Nd-Sr i s o t o p i c s ignature o f o t h e r rocks o f the S t i k i n e te r rane i nd i ca tes ,

however, e i t h e r t h a t t h i s o l d e r metamorphic basement does no t extend eastward

beneath t h e main p a r t o f S t i k i n i a , o r t h a t t he T r i a s s i c s t r a t a o v e r l y i n g the

metamorphic rocks are n o t p a r t o f S t i k i n i a .

Beginning i n Devonian t ime, Wrangell i a , S t i k i n i a , and the Admira l ty

subterrane o f t he Alexander t e r rane a1 1 evolved i n an environment

charac ter ized by arc- type vo lcan ic rocks, bas ina l marine c l a s t i c sediments,

and subord inate carbonate. I n con t ras t , upper Paleozoic rocks o f t he Cra ig

subterrane record t e c t o n i c s t a b i li t y through a t l e a s t 1 a t e Pennsylvanian t ime

and perhaps through the mid-Permian. Wrangel l ia and the Cra ig and Admi ra l ty

subterranes were probably i n c l o s e p rox im i t y du r i ng most o f t h i s t ime

(MacKevett and others, 1986; M u f f l e r , 1967; Berg and others, 1978; Jones and

others, 1981). These r e l a t i o n s , combined w i t h Nd-Sr i s o t o p i c data from upper

Paleozoic rocks o f the Cra ig subterrane and S t i k i n e ter rane, r e q u i r e the l a r g e

tectonic fragments in and adjacent t o the panhandle to have evolved i n an

intraoceanic realm th rough 1 ate Paleozoic ti me. Similarities between

Alexander terrane faunas and b o t h North American and Tethyan forms (Mamet and

Pinard, 1985; Ross and Ross, 1983, 1985) are consistent w i t h an intraoceanic

post tion within the paleo-Pacific basin.

Triassic rocks of the Alexander terrane and Wrangellia are interpreted to

have formed in a r i f t environment (Jones and others, 1977; Gehrels and others,

1986). In Wrangellia, huge volumes of tholei i t ic flood basalt covered the

terrane, whereas in Alexander terrane a bimodal volcanic suite was erupted

along the eastern margin of the terrane. Triassic basalt and andesite of the

Stikine terrane are interpreted t o have erupted within a volcanic arc

environment, presumably related t o subduction a1 ong the eastern (inboard)

margin of the terrane (Monger and ROSS, 1971). The tectonic environment of

Triassic rocks of the Taku terrane i s unknown.

The tectonic history o f SE Alaska after Triassic time i s dominated by

accretion of Alexander and Wrangell ia against Stikinia or other inboard

terranes. An unconformi t y separating Upper Jurassic to mi d-Cretaceous strata

of the Gravina be1 t from underlying Triassic and older rocks i s the f f r s t

evidence of this accretionary activity, Prior t o Late Jurassic time, Permo-

Triassic rocks of the Taku terrane were deformed and regionally metamorphosed

( ~ e h r e l s and Berg, 19841, and rocks of the eastern Alexander terrane were

deformed and disrupted a long the Duncan Canal shear tone on Kupreanof Island

(McClel land and Gehrel s , 1987a). This deformation i s interpreted to record

either the northward movement o f the Alexander-Wrangellia terrane along the

Cal ifornia-Washington continental margin (Gehrel s and Saleeby, 19851, or the

i n i t i a l j u x t a p o s i t i o n aga ins t t h e western marg in o f S t i k i n i a (McCl e l 1 and and

Gehrel s, 1987b) . Rocks o f t h e Gravina b e l t accumulated a long t h e western margin o f a

marine b a s i n o f unknown w id th . As proposed by Berg and o t h e r s ( 1972),

vo lcanism w i t h j n t h i s bas in was p robab ly d i s t a l l y r e l a t e d t o g r a n i t i c

p l u ton i sm i n t h e Alexander t e r rane , t o f o rma t i on o f the Chugach acc re t i ona ry

complex, and t o p l a t e convergence a long t h e outboard marg in o f Alexander and

Wrangell i a , The Gravina be1 t as a depos i t i ona l b a s i n may have formed as: (1)

a back-arc bas in o r r i f t behind a wes t - fac ing a r c (Berg and others , 1972; Brew

and Ford, 1983), ( 2 ) a c o l l a p s i n g sedimentary b a s i n t h a t records c l o s u r e o f

t he su tu re between t h e Alexander and S t i k i n e t e r ranes ( P a v l i s , 1982); ( 3 ) a

western c o n t i n u a t i o n o f the Jura-Cretaceous mar ine bas in t h a t formed on t h e

S t l k i n e t e r r a n e (Mu l l e r , 1977); o r ( 4 ) as a p u l l - a p a r t s t r u c t u r e i n a r i g h t -

l a t e r a l t r ans fo rm system a long which t h e outboard t e r ranes were t r anspo r ted

nor thward (Gehrel s and Saleeby, 1985). Wi thout a d d i t i o n a l i n f o rma t i on , a1 1 of

these scenar ios apparen t l y r e n a i n v i ab le .

S t r u c t u r a l a c c r e t i o n o f t he Alexander t e r r a n e a g a i n s t t h e western marg in

of S t i k i n i a began soon a f t e r d e p o s i t i o n o f mid-Cretaceous s t r a t a of t h e

Gravina b e l t (Berg and o thers , 1972, 1978; Coney and o thers , 1980; Monger and

others , 1982). Th i s acc re t i ona ry event i s recognized as movement on west-

vergent t h r u s t f a u l t s , widespread deformat ion and reg iona l metamorphism o f

rocks w i t h i n t h e Coast Range bath01 i t h and a long i t s western f l a n k , and

a n a t e c t i c ( ? ) p l u ton ism w i t h i n t h e su tu re zone sepa ra t i ng Alexander and

S t i k i n i a (Monger and o thers , 1982). These events apparen t l y cu lm ina ted

between approx imate ly 95 Ma and 65 Ma, and were f o l l o w e d soon a f t e r by r a p i d

up1 i ft of the Coast Range bath01 1 th . Thermobarometric s tud ies w i t h i n the

b a t h o l i t h i n d i ca te t h a t up1 i f t ra tes o f 2 m/yr were achieved a t about 55 Ma,

b r i n g i n g rocks t h a t formed a t over 20 km depth t o the surface ( H o l l i s t e r ,

1982; Crawford and others, 1987). Although the p e t r o l o g i c evidence f o r r a p i d

and la rge-sca le u p l i f t f s compell ing, the d e t r i t u s shed from the u p l i f t e d

orogen has n o t been i d e n t i f i e d , and the major s t ruc tu res along which the

b a t h o l i t h was u p l i f t e d have been recognized i n on l y a few areas.

I n t r u s i o n o f Oligocene-Miocene gabbro and g r a n i t e having low i n i t i a l

87~r /86~r , and of swarms o f lanprophyre and quar tz porphyry d ikes suggest

s t i l l younger post-accret ionary tectonism, poss ib l y tapping mantle sources.

These i n t r u s i o n s t rend west-northwester ly, across the reg iona l northwest

t rends of the Coast Range b a t h o l i t h , Taku terrane, Gravina b e l t , and Alexander

te r rane . Southeastern A1 aska cont inues t o be t e c t o n i c a l l y ac t i ve , w i t h Holocene

f a u l t i n g and up1 i f t (Hudson and others, 1982b), and e rup t i on o f l ava f lows as

r e c e n t l y as about 360 +/- 60 years ago ( E l 1 i o t t and others, 1981).

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Jones, D.L., Berg, H.C., Coney, Peter , and Ha r r i s , An i ta , 1981, S t r u c t u r a l and s t r a t i g r a p h i c s i g n i f i c a n c e o f Upper Devonian and M i s s i s s i p p i a n f o s s i l s f rom the Cannery Formation, Kupreanof I s l and , southeastern Alaska, i n The U.S.

/ ' ~ e o l o g i c a l Survey i n Alaska: Accompl ishrnents d u r i n g 1979, e d i t 3 by N.R.D. ; A l b e r t , and T r a v i s Hudson, pp. 109-112: U.S. Geolog ica l Survey C i r c u l a r 823-B.

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