Structural relationships in the eastern syntaxis of the St. Elias orogen, Alaska

ABSTRACT

The eastern syntaxis in the St. Elias oro-

gen (Alaska, USA) is one of the most com-

plex and least understood regions within

the southern Alaska coastal mountain belt.

The syntaxis contains many features unique

to the orogen that are essential to under-

standing the structural architecture and tec-

tonic history of the collision between North

America and the allochthonous Yakutat

microplate. The eastern syntaxis contains

the transition from transpressional struc-

tures associated with the Queen Charlotte–

Fairweather fault system in the east to the

Yakataga fold-and-thrust belt (YFTB) to

the west. Throughout the eastern syntaxis,

a prominent uncon formity at the base of the

synorogenic Yakataga Formation records an

erosional event related to the development of

the YFTB. Strain accumulations in the east-

ern YFTB predate the deposition of the Yaka-

taga Formation, extending estimates for the

early development of the St. Elias orogen.

Structural and stratigraphic relationships in

the eastern syntaxis suggest that forethrusts

associated with the transpressional system

shut down and were overprinted by fold-

and-thrust structures in the Early to latest

Miocene. Basement in the eastern syntaxis

consists of the Yakutat Group, part of the

Chugach accretionary complex, which is car-

ried by numerous low-angle thrust faults in

the eastern syntaxis. Exposures of basement

and fault patterns within the syntaxis have

implications for tectonic reconstructions of

the Yakutat microplate and the geodynamics

of the orogen.

INTRODUCTION

Throughout most of the Neogene, the Yaku-

tat microplate traveled northward relative to

North America, impinging upon the southern

margin of Alaska and initiating both proximal

and far-fi eld orogenesis (Plafker, 1987; Pavlis

et al., 2004; Leonard et al., 2007). The Yaku-

tat microplate is located between the transition

from dextral strike-slip motion along the Queen

Charlotte–Fairweather fault system to oblique

convergence in the core of the St. Elias orogen

and ultimately to subduction at the Aleutian

Trench. One of the most dynamic areas within

the St. Elias orogen is the eastern syntaxis, an

area of high topography and structural diversity

where northwest-striking, basement-involved

trans pressional systems to the southeast are

juxta posed against the east- to northeast-strik-

ing, thin-skinned Yakataga fold-and-thrust belt

(YFTB) to the west (Fig. 1). The intersection

of these two structural systems is poorly under-

stood, but critical to unraveling the tectonic his-

tory of the Yakutat microplate.

We present an updated description of the east-

ern syntaxis, characterize the structural architec-

ture of the YFTB, and utilize stratigraphic and

structural relationships to help constrain the tim-

ing and evolution of deformation in the eastern

syntaxis, including the initiation of the YTFB.

In addition to an overview of the eastern YFTB,

we present a focused study of the Samovar

Hills, a key exposure located in the core of the

eastern syntaxis at the intersection of structural

styles (Fig. 1). The Samovar Hills offer singular

insight into the evolution of the YFTB and pro-

vide a template for interpreting geologic obser-

vations throughout the Yakutat microplate. Our

study of the eastern syntaxis helps to resolve the

geodynamics of the St. Elias orogen in a criti-

cal, and poorly understood, location within the

Yakutat microplate.

TECTONIC SETTING

In the eastern Yakutat microplate, onshore

basement exposures consist of fl ysch and accre-

tionary mélange of the Yakutat Group, part of

the late Mesozoic Chugach accretionary com-

plex (Fig. 1) (Plafker et al., 1977). The entire

Mesozoic accretionary complex from southern

Alaska to Vancouver Island was intruded by

distinctive near-trench plutons and locally meta-

morphosed to high-grade low-pressure–high-

temperature metamorphic assemblages during

an Early Eocene oceanic spreading ridge sub-

duction event (Pavlis and Sisson, 1995; Bradley

et al., 2003; Sisson et al., 2003). A portion of

this accretionary complex was excised from the

Cordilleran margin to form part of the Yakutat

microplate, and transported northward. As a

result, in the present collision, variably meta-

morphosed assemblages of the Chugach accre-

tionary complex form both the autochthonous

backstop and part of the allochthonous base-

ment (Plafker, 1987).

The autochthonous rocks are equivalent to

the Late Cretaceous to Paleocene Valdez and

Orca Groups, which were variably metamor-

phosed from greenschist to upper amphibo-

lite facies (Richter et al., 2005). The Valdez

Group is part of the Chugach terrane, which

was accreted to the southern Alaska margin in

the Late Cretaceous (Amato and Pavlis, 2010).

For permission to copy, contact [email protected]

© 2012 Geological Society of America 105

Geosphere; February 2012; v. 8; no. 1; p. 105–126; doi:10.1130/GES00677.1; 17 fi gures; 1 table.

Structural relationships in the eastern syntaxis

of the St. Elias orogen, Alaska

James B. Chapman1,*,†, Terry L. Pavlis1,†, Ronald L. Bruhn2,†, Lindsay L. Worthington3,†, Sean P.S. Gulick4,†,

and Aaron L. Berger5,†

1University of Texas at El Paso, Department of Geological Sciences, 500 West University Boulevard, El Paso, Texas 79968, USA2University of Utah, Department of Geology and Geophysics, 115 S. 1460 E, Salt Lake City, Utah 84112, USA3Texas A&M University, Department of Geology and Geophysics, MS 3115, College Station, Texas, 77843, USA4University of Texas Institute for Geophysics, J.J. Pickle Research Campus, Building 196, 10100 Burnet Road (R2200), Austin,

Texas 78758, USA5ConocoPhillips Company, Houston, Texas 77252, USA

*Present address: SandRidge Energy, 123 Robert S. Kerr Avenue, Oklahoma City, Oklahoma 73102, USA.

†Emails: Chapman: [email protected]; Pavlis: [email protected]; Bruhn: [email protected]; Worthington: [email protected]; Gulick: [email protected]; Berger: [email protected].

Neogene Tectonics and Climate-tectonic Interactions in the Southern Alaskan Orogen themed issue as doi:10.1130/GES00677.1Geosphere, published online on 23 January 2012

Chapman et al.

106 Geosphere, February 2012

The Orca Group is part of the Prince William

terrane, which was accreted in the Early Eocene

(Plafker et al., 1994). The Contact fault forms

the suture between the Prince William ter-

rane and Chugach terrane (Plafker, 1987). The

Chugach St. Elias fault (CSEF) forms the suture

between the Prince William terrane and the

Yakutat microplate (Fig. 1).

The Yakutat Group is regionally metamor-

phosed to zeolite to prehnite-pumpellyite

facies and locally metamorphosed to green-

schist facies (Dusel-Bacon, 1994; this study).

Building upon mapping by Plakfer and Miller

(1957), Richter et al. (2005) subdivided the

Yakutat Group into fl ysch and mélange assem-

blages. Paleontological data from outcrop and

cores (Jones and Clark, 1973; Rau et al., 1983)

suggest that the Yakutat Group is Campanian to

Maastrichtian in age (65.5–83.5 Ma; Gradstein

et al., 2005). Haeussler et al. (2005) estimated

a maximum deposition age of 72–74 Ma from

detrital zircon data for the Yakutat Group, an

age they interpret as within ~5 Ma of the true

depositional age.

200 m

È

È

1

2

Yakutat sedimentary rocks

Yakutat Group

Insular terrane

Bering

Glacier

Yaku

tat B

ay

Icy Bay

Malaspina

Glacier

Transition Fault

Pam

plon

a fo

ld

and

thru

st b

elt

Denali Fault

Fairweather Fault

Malaspina Fault

Insular terrane

Chugach

Terrane

Chugach Terrane

CSEF

Yakutat

Lituya Bay Fault

Mt. St. Elias

Mt. LoganPWT

Pacific Plate

20 mm/yr

Pacific

Yakutat

Ala

ska

Can

ada

Yakataga

Yakutat

Microplate

Alsek R

iverYakutat Sea Valley

Samovar Hills

Fig. 3

1

È

international border

bathymetric contour

fault

wellsuture

“Dangerous River Zone”

Area of Eastern

Syntaxis

Border Ranges Fault

Yakutat Foothills

Esker Creek Fault

Chugach terrane

Yakutat microplate Prince William terrane

“Dan

gero

us R

iver Z

one”

Bagley/Contact Fault

??

138° W140° W142° W

60° N

59° N

58° N

50 km

Figure 1. Overview map of the eastern Yakutat microplate and surrounding area. Outline of Alaska is inset in white. The interpretation

of the Dangerous River zone is from Plafker et al. (1994). Plate motion velocity vectors are from Elliott et al. (2010). Wells: 1—Chaix Hills

#1A, 2—Riou Bay #1. PWT is Prince William terrane, CSEF is Chugach St. Elias fault.

as doi:10.1130/GES00677.1Geosphere, published online on 23 January 2012

Eastern syntaxis, St. Elias orogen

Geosphere, February 2012 107

In contrast to the eastern Yakutat microplate,

geophysical studies indicate that basement in the

central and western Yakutat microplate consists

of a 15–30-km-thick oceanic plateau that is sub-

ducting beneath and accreting to North America

(Ferris et al., 2003; Pavlis et al., 2004; Eberhart-

Phillips et al., 2006; Gulick et al., 2007; Christe-

son et al., 2010). The relationships and transi-

tion between basement lithologies across the

Yakutat microplate are unclear, although Plafker

(1987) and Plafker et al. (1994) suggested that

the transition occurs along the Dangerous River

zone (DRZ) (Fig. 1), which they interpreted as

a steeply dipping crustal boundary. The DRZ

was originally defi ned from offshore seismic

refl ection data as the western edge of a base-

ment uplift across which the Cenozoic sedimen-

tary cover sequence for the Yakutat microplate

thins dramatically eastward (Bruns 1982, 1983).

Plafker (1987) further delineated the location of

the DRZ onshore and assigned a series of faults

in the Samovar Hills to the DRZ.

A thick sedimentary sequence that spans

most of the Cenozoic overlies Yakutat micro-

plate basement (Fig. 2). The sedimentary cover

reaches a total stratigraphic thickness of >10 km

(Worthington et al., 2010) and thins to near zero

across the offshore DRZ (Bruns, 1982; 1983).

In the eastern syntaxis area, three main strati-

graphic units compose the cover sequence: the

Kulthieth Formation, the Poul Creek Formation,

and the Yakataga Formation (Plafker, 1987). Sev-

eral minor stratigraphic units are present locally,

including the siltstone of Oily Lake (Plafker,

1987), herein referred to as the Oily Lake mem-

ber of the Kulthieth Formation (Fig. 2). Forami-

nifera analysis for the Oily Lake member in

the Samovar Hills indicates an Early to Middle

Eocene age (Ulatisian Stage, ca. 42.5–49.5 Ma;

McDougall, 2007) (Plafker et al., 1994).

The Kulthieth Formation is typically 2–3 km

thick and consists of fl uvial to shallow-marine

deltaic deposits including coarse arkosic sand-

stone, shale, and coal beds (Plafker, 1987;

Landis , 2007). West of the study area, the

Tokun and Stillwater Formations are the marine

equivalents to the Kulthieth Formation (Plafker,

1987). In the Samovar Hills, the basal Kul-

thieth Formation consists of a white tuffaceous

conglomerate, 1–3 m thick, that grades upward

into ~500 m of coarse sandstone with coal beds

as thick as several meters (Landis, 2007; this

study). The Kulthieth Formation in the Samo-

var Hills is late Early Eocene to early Middle

Eocene in age (Plafker, 1987).

The Poul Creek Formation conformably

overlies the Kulthieth Formation and includes

organic-rich shale, siltstone, glauconitic sand-

stone, and interbedded tuff (Plafker, 1987;

Brennan et al., 2009). The Poul Creek Forma-

tion is ~1 km thick and is characterized by low

sedimentation rates and was deposited in marine

waters along a narrow shelf fringing low-lying

coastal mountains (Eyles et al., 1991). The Poul

Creek Formation is latest Eocene to Early Mio-

cene in age (Plafker, 1987). The Yakataga For-

mation rocks are thick interbedded glaciomarine

sandstone, mudstone, and diamictite deposited

on the continental shelf (Eyles and Lagoe,

1990), and are the youngest within the orogen

(Middle Miocene to present). The 0–6-km-thick

Yakataga Formation contains numerous internal

angular unconformities associated with fold and

fault growth (Miller, 1957; Worthington et al.,

2008, 2010; Broadwell, 2001).

Yakataga Fold-and-Thrust Belt

In the western and central Yakutat micro-

plate, the sedimentary cover is decoupled from

the Yakutat oceanic plateau basement to form the

thin-skinned YFTB. Offshore, the deforma-

tion front is defi ned by the Pamplona zone, an

active fold-and-thrust system that accommodates

~6 mm/yr of shortening (Worthington et al.,

2010; Chapman et al., 2008). The Pamplona

zone trends onshore into the Malaspina fault,

the active deformation front in the eastern syn-

taxis area (Fig. 3). Global positioning system

(GPS) data suggest that the Malaspina fault may

accommodate ~20 mm/yr convergence (Elliott

et al., 2010). Because the Malaspina fault and

structures within the Pamplona zone are oriented

oblique to the plate boundary (Figs. 1 and 3), the

width of the active orogenic wedge varies across

the region from a minimum of ~35 km in the

vicinity of the Samovar Hills to >100 km in

the central YFTB (Plafker et al., 1994).

The depth to the basal detachment in the

eastern YFTB is constrained by earthquake

relocations, including a series of aftershocks

associated with the 1979 St. Elias earthquake

(Bauer et al., 2008; Estabrook et al., 1992) and

offshore seismic data (Worthington et al., 2010),

together suggesting that the basal detachment in

the eastern YFTB is located at ~12–16 km depth

(Fig. 4). This depth is comparable to the basal

detachment in the central YFTB where detach-

ment depths are estimated as 10–17 km (Berger

et al., 2008a; Wallace, 2008; Doser et al., 2007;

Worthington et al., 2010). Apatite U-Th/He ages

(Berger et al., 2008a, 2008b; Spotila and Berger,

2010), zircon fi ssion-track ages (Enkelmann

et al., 2010), and structural restorations (Meigs

et al., 2008; Wallace, 2008), however, suggest

that much of the sedimentary cover exposed

at the surface today was never buried >5 km,

indicating that none of the rocks now exposed

in the YFTB were deeply buried beneath thrusts

prior to exhumation. Previous interpretations

of the YFTB structure at depth include a series

of duplexes and imbricate thrusts in Cenozoic

sedimentary rocks alone (Berger et al., 2008a;

Chapman et al., 2008) or also including structur-

ally thickened basement rocks (Wallace, 2008).

The interpretation of structure at depth has con-

siderable implications for structural reconstruc-

tions and estimates of total shortening.

West East

Cen

ozo

ic

Ple

ist.

Pli

o.

Mio

ceneNeo

gen

eP

aleo

gen

e

Mes

ozo

icC

reta

ceous

Lat

eO

ligoce

ne

Eoce

ne

Pal

eoc.

~Ma

2

5

23

34

56

65

Tol

unconformity

Tpc

Yakataga Formation

Kulthieth

Formation

Yakutat Group

Kym

?

Ti

QTy

Tk

Ky

unconformity

schem

atic

thic

knes

s (k

m)

1

2

3

4

5

6

7

8

9

Figure 2. Simplified strati-

graphic column for the eastern

syntaxis area in the Yakutat

microplate. Stratigraphic thick-

nesses are schematic and show

that the Paleogene sedimen-

tary section thins toward the

east. Kym—Cretaceous Yakutat

Group mélange; Ky—Yakutat

Group flysch; T i—Tertiary

intrusive rocks; Tk—Kulthieth

Formation; Tol—Oily Lake

mem ber of the Kulthieth Forma-

tion; Tpc—Poul Creek For-

mation; Paleoc—Paleocene;

Plio—Pliocene; Pleist—Pleisto-

cene; QTy—Quaternary–Ter-

tiary Yakataga Formation.


Chapm

an et a

l.

108

Geo

sphere, F

ebru

ary 2

012

?

?

?

?

?

?

4.8 cm/yr

?

?

?

?

?

?

20 km20 km

N

A

A′

?

?

?

?

?

?

?

?

Fig. 5

Fig. 7

Fig. 8

Fig. 6

Fig. 10

Fig. 9

Fig. 12

Chugach Terrane

(not shown)

Mt. St. Elias

Icy Bay

Mal

aspin

a Fau

lt

CSEF

Chaix Hills Fault

Malaspina

Glacier

Contact Fault / Bagley Fault

Seward Glacier

Sewar

d Thr

oat

CSEF ?

Dome Pass Fault

Bagley Icefield

Mt. Augusta

Samova

r Hill

s

Ala

skaC

anada

Agass

iz G

laci

er

Libbe

y G

laci

er F

ault

Karr HillsGuyot

Hills

Chaix

Hills

Esker Creek Fault

Haydon

Peak

Mt. Logan

Mt. Huxley

Mt. Owens

FairweatherFault

140° W140.5° W141° W

60.5° N

60.25° N

141.5° W

60° N

Ti

QTy

Tk K

y

Kvm

Kvm

Kvm

Kym

Kym

Kym

Ky

Ky

Tk

Tk

QTy

QTy

Ti

Tk

Kvm

Tyn

dall

Gla

cier T

pc

Tann F

jord

YahtseGlacier

Coal Glacier

Fault

Cretaceous Yakutat Group (Ky)

Cretaceous Yakutat Group melange (Kym

)

Cretaceous Valdez Group/Chugach terrane (Kvm

)

Tertiary instrusive rocks (Ti)

thermochronology sample

fold axis fault

unconsolidated Quaternary sediments

international boundary

Quaternary-Tertiary Yakataga formation (QTy)

Tertiary Kulthieth formation (Tk)

Tertiary Poul Creek formation (Tpc

)

ÈChaix Hills #1A

Figure 3. Geologic map of the eastern syntaxis area. Location is in Figure 1. CSEF is Chugach St. Elias fault.

as doi:10.1130/GE

S00677.1

Geosphere, published online on 23 January 2012



Total shortening estimates for the St. Elias

orogen, based on plate reconstructions and

exhumed material, are ≥300 km (Pavlis et al.,

2004; Berger et al., 2008a; Chapman et al., 2008).

Shortening estimates from published structural

reconstructions are lower, generally <100 km

(Meigs et al., 2008; Chapman et al., 2008;

Wallace, 2008). The YFTB is currently accom-

modating >3.5 cm/yr of the total 4–5 cm/yr

convergence between the Yakutat microplate

and North America (Elliott et al., 2009; 2010;

Sauber et al., 1997; Fletcher and Freymueller,

2003). These rates are thought to have remained

similar since the earliest Pliocene (DeMets et al.,

1994). In contrast, estimates of shortening since

deposition of the Yakataga Formation using

published cross-section restorations range from

5 to 14 mm/yr (Chapman et al., 2008; Meigs

et al.; 2008; Wallace, 2008). Explanations for

the differences in the rate of shortening include

underestimates of fault displacement (Meigs

et al., 2008; Chapman et al., 2008) and struc-

tural thickening of Yakutat microplate basement

at depth (Wallace, 2008).

In addition to questions surrounding the

structural architecture at depth in the YFTB,

there is considerable uncertainty concerning

the potential fault geometry beneath the Bagley

Icefi eld and upper Seward glacier, north of the

YFTB (Fig. 1). West of the Yakutat microplate,

the Contact fault is a north-dipping reverse fault

(Plafker et al., 1994). Although covered by ice,

the Contact fault connects with the Fairweather

fault in the eastern syntaxis area (Plafker and

Thatcher, 2008) (Figs. 1 and 3). Geodetic data

(Savage and Lisowski, 1986; Sauber et al.,

1997) and fault studies adjacent to the Bagley

Icefi eld (Bruhn et al., 2004) suggest that the

Contact fault accommodates some component

of dextral shear and may be reactivated as a

strike-slip fault in this location (Pavlis et al.,

2004; Bruhn et al., 2004). Bruhn et al. (2004)

proposed that the oblique convergence of the

Yakutat microplate was partitioned between dip

slip on north-dipping thrust faults in the YFTB

and dextral strike-slip motion on a steeply dip-

ping to vertical Contact fault.

Geomorphic and bedrock apatite [U-Th]/He

(AHe) data were presented that indicated south-

side-up motion across the Bagley Icefi eld

(Berger et al., 2008a). Earthquake relocations

reveal a loosely defi ned, south-dipping limit to

seismicity beneath the central YFTB (Berger

et al., 2008a; Doser et al., 2007) interpreted

(Berger et al., 2008a) as a new fault, the south-

dipping Bagley fault, which would represent an

orogen-scale backthrust and suggest that the St.

Elias orogen is doubly vergent. Furthermore,

in Berger et al. (2008a, 2008b) AHe data were

combined with data from mineral systems with

higher closing temperatures including bedrock

apatite fi ssion track (AFT), zircon [U-Th]/He

(ZHe), and zircon fi ssion track (ZFT) to sug-

gest that motion on the Bagley fault initiated

or greatly increased in the Quaternary, coinci-

dent with the end to motion on the CSEF in the

central YFTB. In this scenario, two faults could

exist beneath the Bagley Icefi eld: the younger,

south-dipping Bagley fault and the older, north-

dipping to vertical Contact fault. If the Contact

fault dips shallowly to the north, as proposed

by Plafker et al. (1994), then the Bagley fault

as proposed in Berger et al. (2008a) may cut the

older Contact fault.

The model of Berger et al. (2008a, 2008b)

is complicated, however, by young (<3 Ma)

detrital ZFT ages originating from the Valdez

Group beneath the Malaspina–Seward glacier

system (Enkelmann et al., 2009, 2010). These

data suggest rapid exhumation along the trace

of the Contact fault beneath the upper Seward

glacier. Enkelmann et al. (2008, 2010) inter-

preted these ages as evidence for exhumation of

transpressional fault slivers along a reactivated

Contact fault system, and suggested that the

Bagley backthrust is unnecessary. In this model,

the Contact fault is either north dipping (Enkel-

mann et al., 2008) or nearly vertical (Enkelmann

et al., 2010).

Spotila and Berger (2010) also suggested

that exhumation of transpressional fault slivers

beneath the upper Seward glacier is responsible

for the young ZFT ages; however, they suggested

that these slivers originate along the Fairweather

fault in the eastern syntaxis and maintained that

west of the eastern syntaxis area, the Bagley fault

is a viable model for the apparent south-side-

up motion across the Bagley Icefi eld. Further

analysis is needed to determine the geometry

and nature of this important structure.

EASTERN SYNTAXIS

We defi ne the eastern syntaxis as an ~30°

bend in trend of the St. Elias orogen (Fig. 1)

that encompasses the transition from basement-

involved transpressional structures to thin-

skinned fold-and-thrust structures. Figure 3

5

0

–5

–10

–15

Ele

va

tio

n (

km

)

no vertical exaggeration5 km

Yakutat oceanic plateau basement

?

?

~5 degrees

~ 10 degrees

Bag

ely

Fau

lt ?

Riou Bay #1

projected ~15 km

Malaspina

Fault

Malaspina Glacier

CSEFMt. St. Elias

Chaix Hills

Fault

Chaix Hills #1A

Haydon

PeakLibbey Glacier

FaultSeward Glacier

Contact

Fault

A′A

?

QTy

KvmK

ym

Ky ?

Tk

QTy

Tk

Tk

Tpc

Tk

Kvm

Kym

Tpc

Conta

ct F

ault ?

Conta

ct Fault ?

(South) (North)

areas of concentrated

aftershocks from the

1979 St. Elias Earthquake

Figure 4. Cross-section A–A′ through the Chaix Hills. Location is in Figure 3. CSEF is Chugach St. Elias fault. The fault geometry beneath

the Seward glacier is unknown and the faults shown represent a range of possibilities (Berger et al., 2008a; Enkelmann et al., 2010). The

internal folding and faulting is not shown for the Cretaceous Yakutat Group fl ysch (Ky), Yakutat Group mélange (Kym), or the Tertiary

Kulthieth Formation (Tk) at depth (Q—Quaternary). There is a general increase in fold shortening and asymmetry up structural section.


Chapman et al.


is a geologic map of the eastern syntaxis area

compiled from previously published geologic

maps (Plafker and Miller, 1957; Campbell and

Dodds, 1982; Richter et al., 2005), new map-

ping in the 2005–2007 fi eld seasons, aerial

and satellite photograph interpretation, and

spectral lithologic mapping using Landsat The-

matic Mapper (TM) imagery. We also present

a schematic cross section through the orogen

in Figure 4 illustrating several main structural

features. Our mapping suggests that the east-

ern syntaxis contains at least four major thrust

sheets including, from south to north, the Mala-

spina–Esker Creek fault, Chaix Hills fault,

Libbey Glacier fault, and CSEF. In the follow-

ing we report on each of the major thrust faults

and associated thrust sheets.

Malaspina Fault

The Malaspina fault is not exposed due to ice

cover of the Malaspina Glacier (Fig. 3), but best

estimates of its trace suggest that it is oriented

oblique (~070) to the major thrust faults struc-

turally higher within the thrust belt (080–115),

but subperpendicular to the current Yakutat

microplate motion (~337) (Elliott et al., 2010).

This orientation suggests the Malaspina fault

accommodates predominantly dip-slip motion.

We have constrained the location of the Mala-

spina fault with the structural geometry of the

Yakataga Formation in the southern Chaix Hills,

earthquake relocations (Estabrook et al., 1992),

and data from the Chaix Hills #1A well (drilled

by Standard Oil in 1961), which penetrated the

Poul Creek Formation (Plafker et al., 1975)

(Figs. 3 and 4). The Riou Bay #1 well (drilled by

Standard Oil in 1962) reached a total measured

depth of 4300 m in the Yakataga Formation

(Plafker et al., 1975). Offset of the Yakataga–

Poul Creek Formation contact between the

Chaix Hills #1A well and the projected depth

of the top of the Poul Creek Formation in the

Riou Bay #1 well suggests >1200 m of throw

across the fault, assuming limited deformation

southeast of the Malaspina fault (Fig. 4).

West of Icy Bay the trace of the Malaspina

fault is unknown, but it ostensibly connects

with the easternmost fault of the Pamplona zone

offshore, an active fault system (Worthington

et al., 2010) (Fig. 1). To the east, the Mala-

spina fault bounds the southern Samovar Hills,

where it intersects the Esker Creek fault. The

Esker Creek fault also forms the leading edge of

deformation to the eastern YFTB and is a north-

dipping reverse fault that places Yakutat Group

basement rocks on the Yakataga Formation

(Bruhn et al., 2004; Plafker and Thatcher, 2008).

Bedding in the Malaspina thrust sheet dips

10°–30° north to northwest in the Chaix Hills

and southern Karr Hills. The Yakataga anticline

is present in the Guyot Hills and northern Karr

Hills and contains distinctive angular uncon-

formities and growth strata within the Yakataga

Formation (Miller, 1957). Broadwell (2001)

suggested the Yakataga anticline in the Guyot

Hills originated as a detachment fold with a

detachment at ~3 km depth. Plafker and Addi-

cott (1976) reported an angular discordance of

as much as 60° across an angular unconformity

associated with the Yakataga anticline in the

northern Karr Hills, leading us to suggest that

the Yakataga anticline may not immediately die

out along strike to the east. However, we did not

observe the Yakataga anticline in exposures on

the west side of Taan Fjord (Fig. 3), indicating

that the Yakataga anticline may trend into the

Chaix Hills fault or be eroded.

East of the Chaix Hills, the Yakataga For-

mation dips to the west on the crest of a large

plunging anticlinorium in the Samovar Hills.

We suggest that the west-plunging structure in

the Samovar Hills may provide a down-plunge

view of the subsurface structure to the west in

the Chaix Hills (Fig. 3), and use this relationship

as a template for the construction of the cross

section in Figure 4.

We interpret a small fault south of the main

Malaspina fault based on relocations of after-

shocks from the 1979 St. Elias earthquake

(Fig. 4) (Estabrook et al., 1992). This fault is

entirely covered by the Malaspina Glacier. The

magnitude of slip on this fault is unknown and

the offset shown in Figure 4 is schematic.

Chaix Hills Fault

The Chaix Hills fault bounds the Malaspina

thrust sheet to the north. The Chaix Hills fault

is covered by the Agassiz Glacier to the east and

the Yahtse Glacier to the west, but it is exposed

on either side of the Taan Fjord (Fig. 3). In this

location, the Chaix Hills fault consists of two

fault strands: the main fault placing Kulthieth

Formation to the northwest over Yakataga For-

mation to the southeast, and a splay fault to the

south placing Poul Creek and basal Yakataga

Formation on Yakataga Formation (Figs. 3

and 5). Updip reverse slip on the main north-

dipping Chaix Hills fault was reported in Bruhn

et al. (2004). The fault surface of the southern

splay fault along the Taan Fjord also dips to

the north and displays reverse slip (Table 1).

Yakataga strata in the immediate vicinity of

the hanging wall of the Chaix Hills splay fault

also display a well-developed, spaced cleavage

with bedding-cleavage intersections plunging

58° toward 340. This cleavage was absent in the

footwall. Although total offset of the southern

splay fault is uncertain, the presence of cleavage

suggests that the block of Yakataga strata in the

hanging wall of the Chaix Hills splay fault was

carried to its present position from beneath a

thick Yakataga section. The results suggest that

locally the Chaix Hills fault is predominantly

dip slip and that the orogen-parallel component

of motion is partitioned elsewhere. There is

no clear surface evidence for or against recent

motion on the Chaix Hills fault.

Taan Fjord

Chaix Hills FaultChaix Hills Splay Fault SN

~250 m

QTyT

k

QTy

Figure 5. Photograph looking

east toward the Chaix Hills

fault and Chaix Hills splay

fault. Location is in Figure 3.

QTy—Quaternary–Tertiary

Yakataga Formation; Tk—Ter-

tiary Kulthieth Formation.

TABLE 1. FAULT GEOMETRY AND SLIP VECTORS

FaultOrientation(strike/dip)

Slip vector(plunge/trend) Shear sense

esrever450/64°05/492tluafyalpsslliHxiahClamronlartxed941/45°07/811tluafkeerCztivraMesreverlartxed503/54°88/703tluafkeerCsbbuHrewoLesreverlartxed503/81°48/303tluafkeerCsbbuHreppU

lartxed553/01°09/5531tluaFesrever643/01°01/7422tluaF

esreverlartxed470/44°45/0033tluaFlartxed513/50°09/5134tluaF

Chugach St. Elias fault (eastern syntaxis area)

350–360/10–25°




In the northern Chaix Hills, the Kulthieth

Formation displays a general structural stratifi -

cation from a moderately homoclinal section in

the lower Kulthieth Formation to a more com-

plexly folded section in the upper Kulthieth For-

mation. The upper Kulthieth Formation appears

to be detached from the lower Kulthieth Forma-

tion along extensive coal beds near the middle

of the stratigraphic section. West of the Tyndall

Glacier, a contact between the Kulthieth and

Yakataga Formations is preserved in a gentle

(~145° interlimb angle), west-plunging syn-

cline. This exposure reveals a critical fi eld rela-

tionship: the Yakataga Formation overlying an

angular unconformity, on complexly deformed

rocks of the upper Kulthieth Formation (Fig. 6).

This angular unconformity is herein referred to

as the basal Yakataga unconformity.

We observed folds within the Kulthieth For-

mation during transects on either side of Tyndall

Glacier. Folds within the Kulthieth Formation

are upright, symmetrical to asymmetric, verging

to the south. Overturned limbs of south-verging

folds are increasingly common to the north, up

structural section (cf. Figs. 6, 7, and 8). Fold

wavelengths are a few hundred meters and

folds commonly occur within stratigraphic

intervals of several tens of meters, bounded by

bedding-parallel detachments. Interlimb angles

vary from open to tight with a majority of close

folds. Hinge thickening is common within coal

and shale beds.

The base of the Yakataga Formation consists

of ~5 m of red-green rounded pebble conglom-

erate in a shale matrix overlain by ~3 m of green

volcaniclastic sedimentary rock. A bed-parallel

detachment is present along the contact between

the pebble conglomerate and green volcaniclas-

tic unit, but is above the angular unconformity.

It is unclear whether this detachment surface is

tectonic in origin or the base of a gravity slide in

the Yakataga Formation. Other exposures of the

basal Yakataga unconformity in the eastern syn-

taxis area do not contain a detachment surface.

The Poul Creek Formation is notably absent at

this locality, indicating erosion to the level of at

least the upper Kulthieth Formation; however,

a small exposure of the Poul Creek Formation

reappears on the east side of the Tyndall Glacier

(Fig. 3) (Richter et al., 2005), indicating relief

on the unconformity surface or structural relief on

the Poul Creek–Kulthieth Formation contact.

The base of the Poul Creek Formation east of

the Tyndall Glacier provides a structural marker

across the Chaix Hills fault. Based on the depth

of the Poul Creek Formation in the Chaix Hills

#1A well and surface structural data from the

Chaix Hills, we estimate that the main Chaix

Hills fault has >4.5 km throw (Fig. 4). This

estimate does not take into account structural

thickening and imbrication within the Kulthieth

Formation and may not be easily related to total

displacement.

The Coal Glacier fault (Richter et al., 2005),

within the Chaix Hills thrust sheet, is a reverse

fault that places the upper Kulthieth Formation

?

unconformity

Mike

bedding tracefault

QTy

Tk

Tk

Tk

S N

~500 m

approximate scale for ridgeline

bed-parallel detachment?

~500 m

Tk

W E


west at the QTy—Quaternary–

Tertiary Yakataga Formation–

Kulthieth Formation (Tk) con-

tact in the northern Karr Hills.

Location is in Figure 3.


northwest across the Agassiz

Glacier at folds within the Ter-

tiary Kulthieth Formation (Tk).

Location is in Figure 3.


Chapman et al.


over the lower Yakataga Formation west of the

Tyndall Glacier and over the Poul Creek Forma-

tion east of the Tyndall Glacier (Fig. 3). Total off-

set along this fault is poorly constrained, but it is

at least ~350 m based on the stratigraphic thick-

ness of the Yakataga Formation in the footwall.

The fault is covered by the Yahtse Glacier to the

west and obscured within the Kulthieth Forma-

tion to the east and may die out along strike.

East of the Tyndall Glacier area, the Kulthieth

Formation dips to the north-northwest and older

stratigraphic levels are exposed to the east toward

the basement-cover contact between the Kul-

thieth Formation and the Yakutat Group north of

the Samovar Hills (Fig. 3). The exposure of the

basement-cover contact is related to both ero-

sion and depositional thinning of the Kulthieth

Formation. Our observations were taken from

helicopter, but near the contact, the Kulthieth

Formation exhibits pervasive intraformational

folding with numerous bed-parallel detach-

ments along shale and coal layers (Fig. 7). The

largest folds are south-verging asymmetric folds

with interlimb angles <60° and wavelengths of

several hundred meters. The folds are enclosed

within stratigraphic packages of a few to several

hundred meters. Within the core of the larger

folds, additional bed-parallel detachments and

smaller folds are commonly observed (Fig. 7).

Despite the intraformational folds, a fairly intact

stratigraphic section of the Kulthieth Formation

appears to be exposed along strike from its base

at the Yakutat Group north of the Samovar Hills

to the base of the Poul Creek Formation east of

the Tyndall Glacier. Similar to the Malaspina

thrust sheet, the map pattern in the Chaix Hills

thrust sheet north of the Samovar Hills may pro-

vide an along-strike view of the deeper struc-

tural confi guration beneath the Tyndall Glacier

area to the west; however, signifi cant ice cover

handicaps clear conclusions.

Northeast of the Samovar Hills, the Chaix

Hills fault appears to connect with the Dome

Pass fault of Plafker and Miller (1957) that

thrusts Yakutat Group onto the Yakataga Forma-

tion. Across the Seward glacier, the Dome Pass–

Chaix Hills fault system apparently connects

with the Boundary fault north of Yakutat Bay

that emplaces Yakutat Group on Yakutat Group

(Plafker and Thatcher, 2008). Isolated lenses of

the Kulthieth Formation are present east of the

Seward glacier throat in the immediate footwall

of the Dome Pass–Chaix Hills fault system,

expanding the depositional extent of the Kul-

thieth Formation to the east (Plafker and Miller;

1957; R. Bruhn, T. Pavlis, and G. Plafker, 2000,

personal observations).

Libbey Glacier Fault

We observed the Libbey Glacier fault (Bruhn

et al., 2004) in the northern Chaix Hills at the

base of Haydon Peak and in a nunatak in upper

Libbey Glacier (Figs. 8 and 9). The Libbey Gla-

cier fault in this location was originally mapped

as the Coal Glacier fault (Plafker and Miller,

1957), although that name was subsequently

applied to the fault to the south (Richter et al.,

2005). Rocks in the footwall of the Libbey Gla-

cier fault consist of the Kulthieth Formation,

although the classifi cation of the rocks in the

immediate hanging wall is less certain (Plafker

and Miller, 1957; Richter et al., 2005). The base

of Haydon Peak, in the hanging wall of the Lib-

bey Glacier fault, contains tightly folded dark

brown to dark gray siltstone with subordinate

fi ne-grained sandstone, pebble conglomerate,

basaltic tuff, light gray tuffaceous sandstone,

and light green to gray intrusive rocks (Plafker

and Miller, 1957; this study) (Fig. 9). Plafker

and Miller (1957) assigned these rocks to an

unnamed siltstone sequence, and correlated the

siltstone sequence to rocks west of the Tyndall

Glacier in the hanging wall of the Libbey Glacier

fault and to rocks exposed north of the Samo-

var Hills in the Chaix Hills thrust sheet. Strati-

graphic relationships suggest that the Plafker

and Miller (1957) siltstone sequence underlies

and is conformable with the Kulthieth Forma-

tion. Sparse paleontological data from the expo-

sure north of the Samovar Hills suggest that the

lowermost Kulthieth Formation is Paleocene in

age in that location, providing an upper bracket

for the age of the siltstone sequence (Addicott

and Plafker, 1971).

Subsequent mapping (by Bruhn et al., 2004,

and G. Plakfer, compiled by Richter et al., 2005),

reassigned the rocks of the siltstone sequence at

the base of Haydon Peak and north of the Samo-

var Hills to the Yakutat Group fl ysch assemblage.

The siltstone sequence west of the Tyndall Gla-

cier, in the hanging wall of the Libbey Glacier

light-colored slope wash

Libbey Glacier

Fault

Tk

Tk

Tyndall Glacier

Kym

?

Tpc

?

S

~250 m

N

?

?K

ym?

Tpc

?

Figure 8. Photograph look-

ing west at the Libbey Glacier

fault across the Tyndall Gla-

cier. Location is in Figure 3.

This outcrop was only observed

from across the Tyndall Gla-

cier, and the rocks in the hang-

ing wall could be Cretaceous

Yakutat Group mélange (Kym)

or sedimentary rocks of the

Yakutat microplate such as

Tertiary Poul Creek Forma-

tion (Tpc). Tk—Kulthieth For-

mation. Light-colored slope

wash obscures darker brown

to reddish-brown rocks in the

footwall of the fault that cuts

the fold in the upper indetermi-

nate unit.




fault, was reassigned to the Kulthieth Formation

with associated mafi c intrusive rocks (Richter

et al., 2005). Our observations immediately west

of the Tyndall Glacier in the hanging wall of the

Libbey Glacier fault, however, suggest greater

complexity. In that area, the rocks in the hang-

ing wall of the Libbey Glacier fault consist of

dark brown to reddish-brown mudstone and

siltstone with abundant felsic dikes and sills

that are tightly folded into recumbent folds with

wavelengths of a couple hundred meters (Fig. 8).

Localized faults are present in the core of the

folds and lose displacement upsection (Fig. 8).

Farther west, coarser-grained, light-colored sand-

stone of the Kulthieth Formation is clearly pres-

ent in the Libbey Glacier thrust sheet; however,

additional study is needed to positively identify

the assemblages immediately west of the Tyn-

dall Glacier in the hanging wall of the Libbey

Glacier fault. Based on lithologic and structural

similarities, we suggest that these rocks may be

part of the Yakutat Group mélange, the Kulthieth

Formation, the Poul Creek Formation, or some

combination of these.

Half-way up the southern fl ank of Haydon

Peak is a prominent color change that marks

a lithologic contact separating Yakutat Group

mélange below from lighter colored, brownish-

red sedimentary rocks lacking intrusive rocks

above (Fig. 9). Although only observed from

a distance, these rocks appear to be folded into

series of recumbent anticline-syncline pairs

with wavelengths of hundreds of meters to sev-

eral hundred meters (Fig. 9). Plafker and Miller

(1957) suggested that the contact was a steeply

dipping angular unconformity with local offset

along numerous minor faults. Our observations

demonstrate that the contact is subhorizontal and

suggest that the contact may be a fault contact,

although faulting may have locally exploited

an existing angular unconformity. Richter et al.

(2005) assigned the rocks on top of Haydon

Peak to the Yakutat Group, although they did

not recognize a lithologic change along the

southern fl ank other than a mapped fault. The

lithology and style of deformation (cf. Fig. 7)

suggest that the rocks at the top of Haydon Peak

may be equivalent to the Kulthieth Formation

present at moderate elevation on the west side

of the Tyndall Glacier. Furthermore, G. Plafker

(2011, personal commun. to T. Pavlis) observed

coal at some locations on the top of Haydon

Peak during reconnaissance work. The younger

over older relation interpreted here between

the Kulthieth Formation and the Yakutat Group

mélange is consistent with the possibility of a

faulted unconformity. We traced the contact on

the southern fl ank of Haydon Peak to the east

where it climbs in elevation and ultimately is

truncated by the CSEF along the southeast fl ank

of Mount St. Elias (Fig. 3).

South of Mount Augusta, Richter et al. (2005)

mapped rocks in the hanging wall of the Libbey

Glacier fault as greenschist facies Valdez Group.

Like the Yakutat Group, the Valdez Group is

part of the Chugach accretionary complex and

is litho logically similar to the Yakutat Group,

but was accreted to the southern Alaskan margin

prior to the collision of the Yakutat microplate

(Plafker, 1987). Campbell and Dodds (1982),

however, assigned these rocks to the Yakutat

Group. Where observed in this location, the

hanging wall of the Libbey Glacier fault contains

dark brown to gray phyllite and low-grade chlo-

ritic schist (Fig. 10) that we correlate along strike

to the lower metamorphic grade Yakutat Group

mélange exposed south of Mount St. Elias and

west of the Seward glacier throat. The footwall

contains intensely folded, light-colored, fi ne-

grained siltstone and argillite with little apparent

metamorphism (Fig. 10), similar to the Yakutat

Group fl ysch in the Samovar Hills and else-

where. This interpretation of the Libbey Glacier

fault shifts the position of the CSEF higher up

the fl ank of Mount Augusta, consistent with the

mapping of Campbell and Dodds (1982).

Spotila and Berger (2010) analyzed samples

from either side of the Libbey Glacier fault,

south of Mount Augusta, for apatite and zir-

con (U-Th)/He (AHe and ZHe) cooling age

determinations (Fig. 3, yellow circles). In the

footwall of the fault, to the south, Spotila and

Berger (2010) reported an AHe age of 0.59 Ma

and Enkelmann et al. (2010) reported a ZHe age

of 55.29 Ma. In the hanging wall to the north,

Berger and Spotila (2008) reported an AHe

age of 0.56 Ma and Enkelmann et al. (2010)

reported a ZHe age of 3.48 Ma. This unusual

pattern of AHe and ZHe age pairs is similar to

ages measured across the CSEF in the central

YFTB (Berger et al., 2008a); in that study, it was

suggested that the CSEF was active ca. 5 Ma

during the closure of the younger ZHe age, but

inactive since ca. 1 Ma based on the similar AHe

ages. A similar history of deformation may exist

for the Libbey Glacier fault at this location, sug-

gesting that signifi cant differential exhumation

across the Libbey Glacier fault has not occurred

over the past ~0.5 Ma and that current rapid

exhumation is the result of uplift associated

with other structures. The ZHe cooling age in

the footwall predates the development of the St.

Elias orogen and limits the total exhumation in

the footwall block to 6–7 km.

Chugach–St. Elias Fault

The CSEF separates sedimentary rocks of the

Yakutat microplate from the Orca Group of

the Prince William terrane in the western and

Mt. St. EliasWestern shoulder of

Haydon Peak

Tyndall Glacier

contact

Tk

Kym

folded

intrusives

SENW

~ 1.5 km

approximate scale

along flank of Haydon Peak

fold axes

Figure 9. Photograph look-

ing northeast at a prominent

lithologic contact on Haydon

Peak. Location is in Figure 3.

Tk—Tertiary Kulthieth Forma-

tion; Kym—Cretaceous Yakutat

Group mélange.


Chapman et al.


central Yakutat microplate (Fig. 1). In the east-

ern syntaxis, however, the Orca Group appears

to be absent and rocks of the Yakutat microplate

are in fault contact with metamorphic rocks

correlated to the Valdez Group of the Chugach

terrane (Plafker, 1987; Plafker et al., 1994).

We label this fault contact the CSEF, follow-

ing previous studies (e.g., Plafker et al., 1994),

although it is unclear if the CSEF of the western

and central Yakutat microplate is the same as the

CSEF in the eastern syntaxis region.

The CSEF in the eastern syntaxis places

complexly deformed amphibolite metamorphic

facies rocks onto the Yakutat Group mélange.

The CSEF marks one of the most distinc-

tive lithologic changes within the orogen, but

because of the extreme topography in the east-

ern syntaxis region, we relied heavily on aerial

photography and satellite imagery to determine

the trace of the CSEF. In addition to moving the

trace of the CSEF higher on Mount Augusta

as discussed here, we assign much of the area

south of Mount Huxley to the Yakutat Group

mélange (Fig. 3) and place the CSEF closer to

the base of Mount Huxley along a prominent

topographic escarpment and spectral boundary

identifi ed in Landsat TM imagery.

The rugged topography of the eastern syn-

taxis allows for a robust estimation of the three-

dimensional geometry of the CSEF. We draped

a geologic map over a 60 m digital elevation

model for the eastern syntaxis area and used a

script written by one of us (R. Bruhn, at the Uni-

versity of Utah) to match intersections between

fault planes and topography to the observed

fault trace (Fig. 11). To estimate the robustness

of the best-fi t fault planes, we varied fault dip

(±~10°) and fault strike (±~20°) and plotted the

resulting intersection lines against the observed

fault trace. We then constructed vertical sec-

tions spaced 1 km apart along the CSEF and

oriented perpendicular to the trace of the CSEF

at that point. We determined point intersections

between the plotted lines and the vertical sec-

tion plane. Next, we calculated the distance in

meters between the point intersection of the

estimated fault trace and the point intersection

of the observed CSEF trace within each vertical

section plane. We summed these distances for

each fault intersection trace, which we used as

an approximate measure of misfi t. We repeated

this procedure for each fault segment of the

CSEF shown in Figure 11 (A, B, and C). We

report misfi t as a relative percent by dividing the

misfi t for each fault trace against the summation

of the minimum misfi t and two maximum misfi t

traces fi t to that segment.

The results suggest that in the eastern syn-

taxis area, the CSEF dips 10°–30° toward 350–

360 (Fig. 11). We have a high degree of confi -

dence in the estimated fault geometry around

Mount St. Elias (segment A, Fig. 11) and Mount

Augusta (segment C, Fig. 11); however, where

the CSEF passes beneath the Newton Glacier

(segment B, Fig. 11), our estimation of fault dip

is not robust. In general, the dip of the CSEF

in the eastern syntaxis is signifi cantly shallower

than in the central YFTB, where it was reported

(Bruhn et al., 2004) that the CSEF dips to the

north at 40° with almost pure dip-slip motion.

The best-fit fault plane intersection to the

CSEF trace west of Mount St. Elias suggests

that the fault dip is as shallow as 10°. Because

of the steep topography, the fault plane for

segment A (green line, Fig. 11) intersects the

surface on the north side of the ridge between

Mount Huxley and the Bagley Icefi eld. On the

north side of Mount Huxley, fault segment A

dips in the same direction, but less steeply than

topography. Based on this geometric projec-

tion and the juxtaposition of amphibolite grade

rocks of the Valdez Group to the east against

lower grade rocks of the Orca Group to the

west, we tentatively map the CSEF at this loca-

tion (Figs. 3 and 11). The position of the CSEF

at this location west of Mount Huxley is roughly

coincident with an unnamed fault, separating

the Valdez Group and Orca Group, included in

regional geologic maps (Plafker, 1987; Plafker

et al., 1994; Richter et al., 2005). Our interpre-

tation suggests that the CSEF in the eastern

syntaxis area may be a partial klippe (Figs. 3,

4, and 11). This relationship suggests that the

CSEF may no longer root into its original fault

system at depth and may have been offset by

faults beneath the Bagley Icefi eld and upper

Seward glacier (Berger et al., 2008a; Enkel-

mann et al., 2010; Spotila and Berger, 2010).

Faults East of the Seward Glacier Throat

A few linear glacial valleys trend east-north-

east and may contain unrecognized structures

within the Yakutat Group mélange east of the

Seward glacier throat (Fig. 3). During limited

reconnaissance mapping, we identifi ed scattered

small nunataks of Yakutat Group fl ysch south of

Mount Owens, but were unable to determine if

these were in fault contact beneath the Yakutat

Group mélange, similar to the structural rela-

tionship along the Libbey Glacier fault, or large

blocks within the mélange. We also observed

a lithologic and metamorphic juxtaposition

between greenschist facies phyllites on a ridge

just north of Mount Owens and zeolite facies

graywacke to the south within the Yakutat Group

on the northern slope of Mount Owens. Finally,

the upper Seward glacier throat itself is a good

candidate for an unrecognized structure based

on the elevation difference and offset between

the Valdez Group at Mount Augusta and expo-

sures of the Valdez Group east of Mount Owens

along the Fairweather fault system (Fig. 3)

(Campbell and Dodds, 1982), although the

Mt. Augusta

Mt. St. Elias

B

photograph B

Kym

Ky

KymK

y

SW

~400 m

NE

Figure 10. Photographs looking north at the Libbey Glacier fault

south of Mount Augusta. Location is in Figure 3. Kym—Cretaceous

Yakutat Group mélange; Ky—Yakutat Group fl ysch.




fault relationships between these two areas are

covered by the Seward glacier and are unclear.

We tentatively map faults at these locations and

emphasize the need for further study (Fig. 3).

SAMOVAR HILLS

The Samovar Hills make up the core of the

eastern syntaxis region and were mapped by

Plafker and Miller (1957) and Plafker (1987).

We spent fi ve weeks in the 2006 and 2007

fi eld seasons in the Samovar Hills working on

detailed mapping and fault kinematic studies.

We focused our efforts on the two major fault

systems in the central Samovar Hills, the Hubbs

Creek and Marvitz Creek faults, and on the large

anticlinorium in the western Samovar Hills. The

results of our mapping are presented as a geo-

logic map in Figure 12 and as two cross sections

across the Samovar Hills (Figs. 13 and 14).

These cross sections were created by load-

ing the geologic map into structural modeling

software (2DMove created by Midland Valley,

www.mve.com) and projecting structural fea-

tures onto the plane of section. Contacts, faults,

and bedding traces were projected in both the

dip direction and along strike using a calculated

trend and plunge of various fold axes for the

along-strike projection following the methods

outlined in Pavlis and Bruhn (2011). For our

fault studies, we measured fault surfaces and

shear sense indicators directly wherever pos-

sible. For fault zones with large populations of

240

270

300

10

20

30

40

50

0 10 20 30 40 50

240

270

300

0

5

10

15

20

25

10 15 20 25 30

240

250

260

270

280

10

20

30

40

5 10 15 20 25 30

fault d

ip (

°)

percent relative misfit

fault d

ip (

°)

percent relative misfit percent relative misfit

fault d

ip (

°)

fault s

trik

e (

°)

fault s

trik

e (

°)

fault s

trik

e (

°)

slope

Seward Glacier

Seward

thro

at

Bagley Icefield

Strike Dip

270 10

270 30

260 25

CSEF

Mt. St. Elias

Mt. Augusta

Tynd

all G

laci

er

5 km

Fault Trace

Mt. Huxley

Mt. Owens

Haydon

Peak

80o0o

A

AB C

B C

Newton Glacier

A

B

C

Figure 11. Slope map of the eastern syntaxis area with the mapped position of the Chugach St. Elias fault (CSEF) shown in red. Green (A),

yellow (B), and orange (C) lines represent fault plane intersections with topography. Variable fault plane orientations were chosen to best

match the observed trace of the CSEF. The goodness of fi t for each segment is evaluated using a relative percent misfi t. Methodology and

description of misfi t calculations are discussed in the text.


Chapman et al.


1000

1400

1200

800

600

1000

10001000

800

600

1200

?

B

hcv

modern river alluvium

stream terraces or ice-dammed

lake deposits

moraines or glacial till

landslide deposits

Yakataga formation

Kulthieth formation

Oily Lake member

Yakutat Group Hubbs Creek volcanics

B′

C′

C

54

140°45′W 140°40′W 140°36′W

60°10′N

60°8′N

60°7′N

?

2 km

Agassiz G

lacier

Samovar Hills

Malaspina Glacier

Oily

Lake

Mussell Creek

Marvitz C

reek

Hubbs C

reek

Malasp

ina Fault

Hubbs Creek Fault

hcv

Contour Interval = 200 m

Esker Creek Fault

hcv

Upper HubbsCreek Fault

Marvitz

Cre

ek F

ault

?

?

Lower HubbsCreek Fault

1

1 fault number

2

3

4

Qal

Ky

Tk

Qal

Qal2

Qm

Qls

QTy

Tol

QTy

QTy

QTy

Ky

Ky

Ky

Tol

Tol

Qal

Qal2

Qal2

Qm

Qm

Tk

Qls

fold axial trace

strike and dip of bedding

Fig. 16

Figure 12. Geologic map of the Samovar Hills. Location is in Figure 3. Q—Quaternary, T—Tertiary, K—Cretaceous.




B′B

?

0

1

2

3

–11 km

Hubbs Creek

Fault

Malaspina-Esker

Creek Fault

no vertical exaggeration

hcvMalaspina Glacier

Ele

vation (

km

)

bend insection

?

Agassiz Glacier

overturnedbedding

erosionalunconformity

erosionalunconformity

Marvitz C

reek Fault

?

hcv

Ky

Tk

QTy

QTy

QTy

Tk

Tk

T ol

Ky

?

South North

strike and dip data

fold axial trace

bedding traces fault

presenttopography

Figure 13. Cross-section B–B′ through the Samovar Hills, oriented approximately in the dip direction. Location is in Figure 12. QTy—Qua-

ternary–Tertiary Yakataga Formation; Tk—Kulthieth Formation; Ky—Cretaceous Yakutat Group fl ysch; hcv—Hubbs Creek volcanics.

hcvoverturnedfolds

Marvitz CreekFault

no vertical exaggerationMalaspina Fault

Esker

Creek F

ault

HubbsCreek Fault

?

C C′

hcv

0

1 km

2

Ele

vation (

km

)1

3

–1

strike and dip data

fold axial trace

bedding traces fault

presenttopography

?

Ky

Tk

QTy

Tol

QTy

Ky

Ky

QTy

West East

Figure 14. Cross-section C–C′ through the Samovar Hills, oriented approximately in the strike direction. Location is in Figure 12. QTy—

Quaternary Yakataga Formation; Tk—Kulthieth Formation; Tol—Oily Lake member; Ky—Cretaceous Yakutat Group fl ysch; hcv—Hubbs

Creek volcanics.


Chapman et al.


fault planes and shear sense indicators with little

variation, we used a simple average for overall

fault geometry. Where fault zones were buried

or complex, populations of subsidiary faults

were measured and paleostrain calculated using

a variation on the technique of Krantz (1988)

(following Bruhn and Pavlis, 1981; Bruhn et al.,

2004). These methods assume that fault slip

occurs in the direction of maximum resolved

shear stress and that the null-motion direction

(m-pole) is perpendicular to the slip vector

within the fault plane. For each subsidiary fault,

we measured the fault plane orientation, the slip

direction (slickenline orientation), and estimated

shear sense. We plotted each slip direction and

calculated each m-pole based on the orientation

of the individual fault planes. We then contoured

the m-poles and calculated the best-fi t linea-

tion (β-axis). Where exact fault geometry was

unknown and kinematic indicators varied, we fi t

a plane between fault strike, as determined from

map trace, and the β-axis to estimate the overall

fault zone orientation. The overall slip vector

for the fault zone was assumed to occur along

the intersection of the estimated fault plane

and the great circle orthogonal to the β-axis.

As expected, the calculated slip vectors gener-

ally were within a few degrees of the best-fi t

great circle for directional (slickenline) data. We

attempted to ensure that our fault plane and slip

vector calculations were also consistent with

fi eld observations of displacement based on

other geologic constraints. Fault zones that had

subsidiary fault populations with large variation

in slip direction or with multiple slickenlines on

the fault surfaces were discarded, and data for

those fault zones are not presented here. In these

cases, we suspect that multiple phases of defor-

mation may have occurred. Best estimates of

fault geometry and average slip vectors are dis-

cussed in the following and presented in Table 1

and Figure 15.

fault 1

Upper Hubbs Creek Fault

slickensides

Lower Hubbs Creek Fault

m-p

ole

s

slic

ken

lines

fold

axes

Fold Axes

Yakataga Fm.

Kulthieth Fm.

Oily Lake memberTrend of the Hubbs Creek fault

fold-and-thrust belt

β-axis

slip vector

n = 19 n = 13

main fault

conjugate

fault 2

n = 49n = 15

BA

fault 4

n = 11

slip vector

Marvitz Creek Fault

n = 3

slip vector

fault 3

β-axis

slip vector

plane to β-axis

n = 18

fault plane

fault plane

slip vectorslip vector

Yakataga Fm. Kulthieth Fm. Oily Lake mem.

n = 9n = 103n = 75

C

Figure 15. (A) Equal area, lower-hemisphere, stereonet plots of geometry and kinematic data for selected faults in the Samovar Hills area

(faults labeled in Fig. 12). For the Lower Hubbs Creek Fault and fault 3, the fault plane and slip vector were determined using a variant

on the paleostrain method of Krantz (1988) (see text); all other faults and kinematic data were measured directly. The best fi t slip vector

and fault geometry for each fault are presented in Table 1. (B) Fold axes within the Samovar Hills area. The single plot of the fold axis in

the Yakataga Formation (Fm.) comes from the large plunging box anticline. Two distinct populations of fold axes are present and related to

deformation associated with an older transpressional system (green circle) and the Yakataga fold-and-thrust belt (blue circle). (C) Poles

to bedding in the Samovar Hills, separated by rock unit; mem.—member).




Hubbs Creek Fault

The Hubbs Creek fault system separates three

distinct fault blocks with different stratigraphic

sequences: (1) a western fault block that contains

several hundred meters of Kulthieth Formation

unconformably overlying Yakutat Group base-

ment; (2) a fault splay between the two strands of

the Hubbs Creek fault that exposes only vol canic

rocks, the Hubbs Creek vol canics, but does not

expose the base of the volcanic sequence; and

(3) an eastern fault block that exposes only

the Yakutat Group basement (Fig. 12). The

Yakataga Formation overlaps all three of these

fault blocks along the angular basal Yakataga

unconformity. Because of these stratigraphic

relationships, we divide the Hubbs Creek fault

into the lower Hubbs Creek fault, where the

Hubbs Creek volcanics abut the Kulthieth For-

mation, and the upper Hubbs Creek fault, where

the fault places the Yakataga Formation against

itself (Fig. 12).

The lower Hubbs Creek fault is buried by

young sediments in a stream valley, but sub-

sidiary fault populations suggest that the fault is

a dextral oblique thrust fault dipping northeast

(Table 1; Fig. 15). This is in good agreement

with a measurement by Plafker (1987) east of

Hubbs Creek suggesting that the fault dips 75°

toward ~035. The Hubbs Creek fault is diffi cult

to trace farther to the southeast, however, where

it projects toward a prominent, undated land-

slide and may ultimately merge with the Esker

Creek fault system (Fig. 12).

The lower Hubbs Creek fault also appears to

truncate steeply dipping faults that bound the

Hubbs Creek volcanics (Figs. 12 and 13). We

identifi ed a small outcrop south of the trace of

the lower Hubbs Creek fault where the Hubbs

Creek volcanics were juxtaposed against the

Yakutat Group along a subvertical fault with

almost pure dextral strike-slip motion (fault 1,

Table 1; Fig. 15). We did not directly observe

the fault bounding the eastern side of the Hubbs

Creek volcanics, but traced the lithologic con-

tact to the boundary with the overlying Yakataga

Formation where it intersects a younger fault

zone (discussed herein). Along the projected

trace of the eastern bounding fault to the Hubbs

Creek volcanics, bedding within the Yakataga

Formation contained minor dextral offset

(<15 m), possibly as a result of limited reactiva-

tion of this fault since deposition of the Yakataga

Formation.

A syncline that plunges 38° toward 323 in the

Kulthieth Formation and 37° toward 341 in

the Yakataga Formation is located in the foot-

wall of the Hubbs Creek fault, suggesting that

folding postdates the basal Yakataga uncon-

formity. Adjacent to the fault, a series of upright

to overturned folds in the Kulthieth Formation

with axial planes subparallel to the main fault

plane are superimposed on the eastern limb of

this syncline. These folds have wavelengths of

~10–30 m and interlimb angles <30°. These

folds are not present in the Yakataga Forma-

tion along the Hubbs Creek fault and may be

older than the Yakataga Formation or related to

mechanical differences in the stratigraphy.

The upper Hubbs Creek fault is exposed in

a small topographic notch along the ridgeline

and dips steeply to the northeast with dextral

motion (Table 1; Figs. 12 and 15). Despite the

similar geometry to the lower Hubbs Creek

fault, the slip vector for the upper Hubbs Creek

fault plunges 25°–30° less steeply. A low-angle,

north-dipping thrust fault intersects the upper

Hubbs Creek fault and places the Yakataga

Formation over the Hubbs Creek volcanics and

Yakutat Group along strike (fault 2, Table 1;

Figs. 12 and 15). The fault cuts bedding in the

Yakataga Formation at a low angle; this, with

the younger-on-older thrust relationship, implies

that this fault postdates deposition and folding

of the Yakataga Formation. Displacement on

this low-angle fault is unknown, but the fault

appears to tip out within ~2 km to the east (Fig.

12). This fault may have locally exploited the

basal Yakataga unconformity as a detachment.

The lower plunge of the slip vector for the upper

Hubbs Creek fault may be recording motion on

this younger, shallowly dipping fault, in which

case the slip vector for the lower Hubbs Creek

fault may be more representative of the primary

older movement history.

In map pattern, the basal Yakataga uncon-

formity shows little to no offset across the

Hubbs Creek fault, suggesting limited dis-

placement since deposition of the Yakataga

Formation (Fig. 12). The orientation of the slip

vector for the lower Hubbs Creek fault is com-

parable to the regional dip and dip direction of

bedding in the Yakataga Formation (25°–40°

toward 310–360). Structural restoration to a

time equivalent with the basal Yakataga uncon-

formity removes the regional, northward dip of

the Yakataga Formation in the hanging wall of

the Malaspina–Esker Creek fault system and

reduces the plunge of the slip vector on the

lower Hubbs Creek fault, increasing the strike-

slip component of motion.

Oily Lake Member

Between the Hubbs Creek fault and the Mar-

vitz Creek fault the Yakutat Group basement

is overlain by 100–150 m of green to maroon,

mottled, tuffaceous mudstone and siltstone (Fig.

12) that we correlate to the siltstone of Oily Lake

mapped by Plafker (1987). We refer to this unit

as the Oily Lake member of the Kul thieth For-

mation following preliminary sedimentological

work (Landis, 2007; Witmer et al., 2009). Local

gravel conglomerates are present throughout

the Oily Lake member and have angular clasts

suspended in a mudstone matrix. A thin (1–2 m)

rounded pebble conglomerate is deposited

unconformably on the Yakutat Group at the base

of the Oily Lake member (Fig. 16).

In the central Samovar Hills, the Oily Lake

member thins to the northeast, where it is ulti-

mately cut out by an angular unconformity at

the base of the Kulthieth Formation (Fig. 16).

We traced this contact to the southwest where

the Oily Lake member is truncated against the

Marvitz Creek fault. Bedding within the Oily

Lake member generally dips 40°–60° toward

020–070. Several folds are present within the

Oily Lake member with axes plunging an aver-

age of 14° toward 298 (Fig. 15). The strike of

bedding and trend of fold axes is subparallel to

the strike of the Hubbs Creek fault and Mar-

vitz Creek fault, but at a high angle to bedding

within the overlying Kulthieth Formation that

dips 45°–50° toward 300–340.

The Oily Lake member is also exposed in

the southern Samovar Hills where it crops out

along the north side of Marvitz Creek (Fig. 12).

The map pattern suggests that bedding may dip

moderately southwest. However, the limited

exposures consist of highly fractured mudstones

with poor indicators of bedding orientation.

Marvitz Creek Fault

The Esker Creek fault bounds the southeast-

ern Samovar Hills and intersects or connects

with the Malaspina fault near an ~45° bend in

the topographic front of the Samovar Hills. The

Marvitz Creek fault splays off of the Esker Creek

fault system near this intersection, striking sub-

parallel to the trace of the Hubbs Creek fault

(Fig. 12). To the south, the Marvitz Creek fault

juxtaposes Yakutat Group basement and the Oily

Lake member against the Kulthieth Formation;

however, displacement decreases toward the

north where the Marvitz Creek fault splits into

multiple fault strands (Figs. 12 and 14).

Although the Marvitz Creek fault is obscured

by young river gravel along much of its trace, we

observed a small segment of fault rocks re lated

to the structure on the west side of Marvitz

Creek. The fault dips steeply to the southwest

and displays apparent normal displacement of

Kulthieth Formation over the Oily Lake mem-

ber (Table 1). The steeply dipping fault may

have originated as a reverse fault that was tilted

by younger folding. The fault strikes to the

north where it dies out along the Kulthieth For-

mation–Oily Lake member contact. We did not


Chapman et al.


observe any offset of bedding within the Kul-

thieth Formation to the northwest.

We also measured subsidiary fault popu-

lations at two fault splays from the Marvitz

Creek fault that bound a small exposure of the

Hubbs Creek volcanics. The western splay fault

emplaces the Hubbs Creek volcanics against the

Oily Lake member (fault 3, Table 1; Fig. 15).

We could not follow this fault to the north within

the Oily Lake member due to poor exposure,

and we did not observe any signifi cant offset

of the angular unconformity at the base of the

Kulthieth Formation. The eastern splay fault is

also poorly exposed, but juxtaposes the Yaku-

tat Group and the Hubbs Creek volcanics and

appears to be a nearly vertical, dextral strike-slip

fault (fault 4, Table 1; Fig. 15). This fault did not

offset the base of the Oily Lake member (Fig.

12) and may predate the main Marvitz Creek

fault. Plafker (1987) and Plafker et al. (1994)

suggested that the contact between the Hubbs

Creek volcanics and Yakutat Group at the Mar-

vitz Creek locality is depositional, a relationship

supported by well data to the southeast (Plafker,

1971; 1987; Rau et al., 1983). The fault rocks

we observed along this contact may be related

to minor reactivations of an originally deposi-

tional contact. Between the two splay faults, the

Hubbs Creek volcanics are in depositional con-

tact with the Oily Lake member.

Samovar Hills Anticlinorium

Perhaps the most prominent structural feature

of the Samovar Hills is a large (~10 km wide)

box anticline in the Yakataga Formation that

plunges 30° toward 270 (Plafker, 1987; Bruhn

et al., 2004; this study). The fold is consistent

with a hanging-wall anticline above the Mala-

spina thrust fault with a steeply dipping fore-

limb (~80°) and a more moderately dipping

backlimb (30°–40°) (Figs. 13 and 14). Res-

toration of the cross section in Figure 13 sug-

gests >3 km of slip across the Malaspina fault

to construct the anticlinorium. The thickness of

the Yakataga Formation in the footwall of the

Malaspina fault shown in Figure 13 is unknown,

but may be an underestimate based on the Riou

Bay #1 well, drilled in the footwall of the Mala-

spina fault in the Chaix Hills, that encountered

>4 km of Yakataga Formation (Fig. 4). Within

the Yakataga Formation, dip domains in the box

anticline are sharply divided by kink folds; how-

ever, the expression of the anticline in the deeper

structural and stratigraphic levels has remained

unclear. As part of an effort to resolve the gen-

eral structure of the Samovar Hills, we mapped

extensively the Kulthieth Formation and the

center of the anticline.

Just below the basal Yakataga unconformity

in the box anticline, the strike of bedding in

the Kulthieth Formation is subparallel to the

Yakataga Formation, but dips 10°–30° more

steeply than the overlying Yakataga Forma-

tion in a similar downdip direction, excluding

overturned bedding in the Kulthieth Formation

on the southern limb of the anticline (Fig. 14).

This observation not only suggests that defor-

mation began prior to the deposition of the

Yakataga Formation in the Samovar Hills, but

also suggests that deformation related to this

specifi c anticline predates the Yakataga For-

mation. Removal of the dip in the Yakataga

Formation to a time equivalent with the basal

Yakataga unconformity yields an approximate

restoration to a gentle anticline in the Kulthieth

Formation plunging 10°–20° toward the west

with a forelimb dipping ~20° to the south and

a backlimb dipping 10°–15° to the north (inter-

limb angle of 145°–150°).

Toward the core of the anticlinorium, bedding

in the Kulthieth Formation steepens and a series

of parasitic (wavelengths of ~1 km) overturned

folds plunge 30°–65° toward the north and west

(Plafker, 1987) (Fig. 13). We did not observe

any angular unconformities at this stratigraphic

level of the Kulthieth Formation or any sig-

nifi cant faults that cut bedding. We observed

pervasive bedding-parallel detachments along

coal and shale layers and evidence for fl ow of

these strata into overthickened hinge zones.

We suspect that a roof detachment or series of

detachment surfaces separates the overturned

parasitic folds and the more modestly folded

bedding higher in the Kulthieth Formation, near

the basal Yakataga unconformity. Similar struc-

tures are common in the coal-bearing Kulthieth

strata throughout the YFTB. At the base of the

overturned folds, exposed near the ridgeline

on the west side of Marvitz Creek, there is a

major detachment, indicating that the parasitic

folds are intraformational detachment folds. We

tentatively correlate this detachment zone to

another bedding-parallel detachment at a simi-

lar stratigraphic interval between Marvitz and

Hubbs Creek (Figs. 12–14). The location of the

overturned folds within the core of the Samo-

var Hills anticlinorium and near the projected

intersection of the axial planes to the box fold in

the Yakataga Formation suggests that much of

the deformation at this stratigraphic and struc-

tural level may be related to tightening of the

anticlinorium after deposition of the Yakataga

Formation.

Numerous upright folds in the lower Kul-

thieth Formation that plunge 15°–25° toward

the west are in a complex area structurally

and stratigraphically beneath the detachment

zone at the base of the overturned folds. The

decrease in the plunge of the folds compared

to the overturned folds and the box anticline

erosional unconformity

erosional unconformity

~ 10 m

Ky

Tk

Tol

WestEast

Figure 16. Photograph looking north at the basal Yakataga unconformity in the Samovar

Hills. Location is in Figure 12. Tk—Tertiary Kulthieth Formation; Tol—Oily Lake member;

Ky—Cretaceous Yakutat Group fl ysch.




in the Yakataga Formation is indicative of an

overall decrease in plunge of the Samovar

Hills anticlinorium along trend to the east. The

largest folds have wavelengths to 150 m, with

many superimposed smaller folds (wavelengths

<20 m) (Fig. 13). Locally, bedding-parallel

de tach ments may exist structurally beneath

these smaller folds, including along the Marvitz

Creek fault, but we did not observe detachments

at the base of the Kulthieth Formation west of

Marvitz Creek, where the Kulthieth Formation

is separated from the Oily Lake member by an

angular unconformity.

Beneath the unconformity, near the center of

the Samovar Hills anticlinorium, the Oily Lake

member contains a series of tight, overturned

folds verging southwest with fold axes plunging

10°–15° toward the northwest (Fig. 12). These

folds are subparallel to the northern Marvitz

Creek fault and may be related to strain accu-

mulated across the Marvitz Creek fault zone.

We suggest that the Marvitz Creek fault, the

tight folds in the Oily Lake member adjacent to

the Marvitz Creek fault, and shortening within

the core of the Samovar Hills anticlinorium are

interrelated.

DISCUSSION

History of Deformation

The eastern syntaxis region and the Samo-

var Hills in particular record the clearest record

of the three-dimensionality in the deformation

history for the Yakutat microplate. This history

includes an early strike-slip to oblique transpres-

sional environment as the Yakutat microplate

traveled northward along the North American

margin, complex structural overprints as these

transpressional structures were translated into

the Alaskan eastern syntaxis region, and waning

transpressive deformation and the initiation of

fold-and-thrust style deformation.

In the Samovar Hills, the Hubbs Creek vol-

canics are in fault contact with the Yakutat

Group along a small fault segment intersecting

the Marvitz Creek fault and on the east side of

Hubbs Creek (Fig. 12). At Hubbs Creek, dex-

tral strike-slip movement along the fault bound-

ing the east side of the Hubbs Creek volcanics

occurred before deposition of the Yakataga For-

mation. The fault segment intersecting the Mar-

vitz Creek fault is truncated to the north by an

unconformity at the base of the Early to Middle

Eocene (ca. 42.5–49.5 Ma; McDougall, 2007)

Oily Lake member, placing relatively precise

timing constraints on the slip observed along

this segment. Although scant, the available evi-

dence suggests a very early history of dextral

strike-slip deformation at this point within the

Yakutat microplate, consistent with models for

the tectonic evolution of the Yakutat microplate

as it was translated northward along the North

American margin (Plafker, 1987; Pavlis et al.,

2004) and with the observation of a long-lived

strike-slip southwestern boundary to the Yaku-

tat microplate, the offshore Transition fault

(Christeson et al., 2010).

Series of open folds with axial planes subpar-

allel to the Hubbs Creek fault and Esker Creek

fault are present within the Oily Lake member

(Figs. 12 and 15). Southeast of the Samovar

Hills, the Hubbs Creek fault connects with the

Esker Creek fault, which is a forethrust asso-

ciated with the transpressional margin in the

eastern Yakutat microplate (Bruhn et al., 2004;

Plafker and Thatcher, 2008) (Fig. 3). The folds in

the Oily Lake member record shortening subper-

pendicular to these forethrusts and may indicate

the development or continuation of transpres-

sional to northeast-convergent deformation.

We suggest that this deformation may partially

record the northwest translation of the Yakutat

microplate along the southeast Alaska margin

by the right-lateral Contact, Fairweather, and

Queen Charlotte fault systems. The folds in the

Oily Lake member are truncated by the angular

unconformity at the base of the Kulthieth Forma-

tion (Figs. 12 and 16). The Kulthieth Formation

in the Samovar Hills is late Early Eocene to early

Middle Eocene in age (Plafker, 1987), constrain-

ing the deformation in the Oily Lake member to

the Early to Middle Eocene. The unconformity

at the base of the Kulthieth Formation is likely

related to this deformational event.

Following deposition of the Kulthieth Forma-

tion, the record of deformation in the Yakutat

microplate expands dramatically. In the Samo-

var Hills, the Kulthieth Formation is folded

within the Samovar Hills anticlinorium and jux-

taposed with the Hubbs Creek volcanics along

the Hubbs Creek fault. Because the Yakataga

Formation is incorporated into the Samovar

Hills anticlinorium, but the basal Yakataga

unconformity is minimally offset by the Hubbs

Creek fault, we propose that displacement along

the Hubbs Creek fault preceded development of

the anticlinorium (Fig. 12). At the least, devel-

opment of the anticlinorium continued into

Yakataga Formation time, whereas motion on

the Hubbs Creek fault had essentially ended.

This observation is an important structural

relationship that locally demonstrates the end

of deformation associated with a transpres-

sional system in the Hubbs Creek–Esker Creek

fault system and the initiation and growth of

the YFTB associated with the Samovar Hills

anticlinorium and Malaspina fault. The timing

of this transition is constrained by the basal

Yakataga unconformity.

Basal Yakataga Unconformity

The precise age of the base of the Yakataga

Formation in the eastern syntaxis is uncertain

because the age of the Yakataga Formation

varies across the orogen, generally decreasing in

age from west to east (Plafker et al., 1994). The

oldest recorded age from outcrops at Icy Bay is

Early Pliocene (ca. 3.0–3.5 Ma), although the

base of the section is not exposed (Lagoe and

Zellers, 1996). At Yakataga Reef in the central

YFTB and in offshore industry wells south-

west of Icy Bay, the base of the Yakataga For-

mation is latest Miocene to earliest Pliocene

in age (ca. 5–6 Ma) (Arnaud, 2010; Lagoe and

Zellers, 1996; Zellers, 1995), probably close to

the age of the base of the Yakataga Formation

in the eastern syntaxis area. The basal Yakataga

unconformity is an angular unconformity above

folded Middle Eocene rocks of the Kulthieth

Formation in the Samovar Hills. Thus, in the

eastern syntaxis area, the unconformity cuts

out Middle Eocene to latest Miocene–earli-

est Pliocene strata. In the central and western

YFTB a more complete stratigraphic section is

present and the Yakataga Formation commonly

conformably overlies older strata, including the

Late Eocene to Early Miocene Poul Creek For-

mation (Plafker, 1987; Brennan et al., 2009),

suggesting that the erosional and deformational

event related to the basal Yakataga uncon formity

in the eastern syntaxis occurred between the

Early Miocene and the latest Miocene to earli-

est Pliocene. Because the Yakutat microplate

converges obliquely with North America, this

deformation event may be time transgressive.

In this scenario, we speculate that deformation

may have progressed west to east, as the Yakutat

microplate was translated northwestward into

the southern Alaskan margin.

In addition to constraining the timing of fold-

and-thrust belt development in the Samovar

Hills area, the basal Yakataga unconformity

records strain accumulation in the YFTB prior

to the deposition of the Yakataga Formation.

West of the Tyndall Glacier in the northern Karr

Hills (Fig. 6) and in the Samovar Hills anticlino-

rium, the development of fold-and-thrust struc-

tures predates the Yakataga Formation. Despite

an angular unconformity reported at the base of

the Yakataga Formation in parts of the central

YFTB (Miller, 1971; Plafker, 1987; Wallace,

2008), previous studies have generally assumed

the initiation of fold-and-thrust belt–style defor-

mation and deposition of the Yakataga Forma-

tion were roughly coincident (Eyles et al., 1991;

Plafker et al., 1994; Pavlis et al., 2004; Meigs

et al., 2008; Berger et al., 2008b; Chapman

et al., 2008; Wallace, 2008). This assumption

may be good in the western to central YFTB,


Chapman et al.


but the assumption is less appropriate toward

the eastern YFTB because deposition of the

Yakataga Formation was diachronous.

The presence of deformation prior to deposi-

tion of the basal Yakataga Formation has con-

siderable implications for reconstructing the

structural history of the YFTB and reconciling

the present rate of deformation with the total

observed shortening. Plafker et al. (1994) noted

45% shortening in the Paleogene compared

to 25% shortening in the Neogene in the cen-

tral YFTB; in Chapman et al. (2008), >75 km

shorten ing was reported in the Paleogene strata

compared to ~25 km shortening in the Yakataga

Formation alone. In Chapman et al. (2008), it

was assumed that the disparity between defor-

mation recorded in the Paleogene and Neogene

sections was related to either duplexing of the

Kulthieth Formation and erosion of the Yakataga

Formation above the structurally higher thrust

sheets (e.g., Meigs et al., 2008), or nondepo-

sition of the Yakataga Formation on what are

now the structurally higher thrust sheets. How-

ever, the discrepancies in shortening between

the Paleogene section and Yakataga Formation

could also be related to deformation in the Early

to Late Miocene, as indicated by eroded struc-

tures beneath the basal Yakataga unconformity.

The level of erosion recorded by the basal

Yakataga unconformity, the amount of shorten-

ing within the Kulthieth Formation, and the

degree of structural discordance across the basal

Yakataga unconformity provide indirect evi-

dence that deformation may have signifi cantly

preceded deposition of the Yakataga Formation

in the eastern syntaxis area. However, if the cur-

rent high rates of convergence across the Yakutat

microplate extended into Miocene, much of this

deformation could have occurred within only a

few million years prior to the deposition of the

Yakataga Formation. Additional work is needed

to more precisely constrain the timing and

amount of shortening in the eastern syntaxis area.

Reducing the total shortening recorded in

the YFTB since the onset of deposition of the

Yakataga Formation in the latest Miocene

decreases estimates of the rate of shortening over

that time period. This exacerbates an already

unresolved mismatch between current rates of

convergence and total observed shortening from

cross-section restorations (Worthington et al.,

2010; Chapman et al., 2008; Meigs et al., 2008;

Wallace, 2008). Structural restoration of the

Yakataga Formation in Figure 4 yields >17 km

of shortening and, when combined with the

range of ages for the Yakataga Formation, sug-

gests a minimum shortening rate of ~2–6 mm/yr.

This rate is an order of magnitude less than GPS

derived shortening rates of >3.5 cm/yr across the

eastern YFTB (Elliott et al., 2009, 2010).

St. Elias Orogenic System

There is evidence for active slip only on the

Malaspina fault (Elliott et al., 2010; Estabrook

et al., 1992) among the major north-dipping

thrust faults within the eastern YFTB. AHe ages

decrease slightly to the south across the Chaix

Hills fault, although this decrease is not statis-

tically signifi cant as an indicator of fault slip

(Berger et al., 2008b). AHe age pairs suggest

that the Libbey Glacier fault may not be cur-

rently active (Berger et al., 2008b). We suggest

that the CSEF may be a partial klippe that no

longer roots into a deeper fault system (Figs. 4

and 11), indicating that the CSEF is probably

not active. The position of the CSEF at high

elevation on the steep southern fl anks of Mount

St. Elias and Mount Augusta also suggests that

the CSEF is not active (Figs. 3 and 4). Although

the exact mechanisms remain unclear, active

shortening within the eastern St. Elias oro-

genic wedge appears to be concentrated at toe

of the wedge, at the Malaspina fault, and

be neath the Bagley Icefi eld and upper Seward

glacier along the Bagley-Contact fault system.

This spatial relationship forms a narrow oro-

genic wedge with a cross-sectional width of as

little as 35 km and a basal décollement dipping

5°–10° toward the north at 12–16 km depth. The

surface slope of the wedge changes across the

Libbey Glacier fault, with an average slope of

15° north of the fault and an average slope of 2°

in the thrusted sedimentary cover south of the

Libbey Glacier fault (Fig. 4).

Many researchers suggest that the Malaspina

fault may be the youngest part of the YFTB

because of the narrow width of the eastern

YFTB, active slip on the Malaspina fault, the

position of the Malaspina fault at the eastern end

of the YFTB, and because the Malaspina fault

represents the deformational limit to the eastern

YFTB (Plafker et al., 1994; Bruhn et al., 2004;

Chapman et al., 2008). Therefore, the evidence

for signifi cant contraction on the Malaspina

fault in pre-Yakataga time and perhaps even

pre-Kulthieth time as recorded by deformation

in the Samovar Hills is surprising. The long-

lived record of contraction on the Malaspina

fault and structural overprinting in the Samovar

Hills may indicate that the eastern syntaxis has

remained fi xed over time, consistent with geo-

dynamic models for the St. Elias orogen (Koons

et al., 2010).

To the southeast of the eastern syntaxis area,

the orogenic margin is slightly transpressive

with dextral strike slip partitioned on the Fair-

weather fault and dip slip partitioned onto fore-

thrusts such as the Esker Creek fault (McAleer

et al., 2009; Plafker and Thatcher, 2008; Elliott

et al., 2010; Bruhn et al., 2004). The orientations

of the Esker Creek fault and Fairweather fault

become increasingly perpendicular to the direc-

tion of current Yakutat microplate motion as they

connect with the Malaspina fault and Bagley-

Contact fault system, respectively, in the eastern

syntaxis area (Fig. 3). This change in orientation

corresponds to an increase in the width of the

YFTB toward the west and creates a restrain-

ing bend along the Fairweather-Contact fault

system in the northern eastern syntaxis (Fig. 1).

We suggest that both the Bagley-Contact fault

system and the Malaspina fault accommodate

an increased amount of convergence across the

eastern syntaxis as their orientations change.

Structural style changes across the orogenic

wedge, with folding increasing up structural sec-

tion. Folds at the toe of the wedge in the Mala-

spina thrust sheet are generally open, upright,

and have relatively long wavelengths of as much

as a few kilometers. Folds in the Kulthieth For-

mation within the Chaix Hills and Libbey Gla-

cier thrust sheets have shorter wavelengths of

hundreds of meters and are generally asymmet-

ric, verging to the south with steep to overturned

southern limbs. In the structurally highest thrust

sheets, the Yakutat Group mélange is intensely

folded, often with chaotic bedding, although

much of this structural complexity is probably

inherited from its deposition and deformation

within an accretionary complex.

CSEF

In the eastern syntaxis, the CSEF places

amphibolite facies rocks of the Valdez Group on

the low-grade to unmetamorphosed Cretaceous

and Tertiary rocks of the Yakutat microplate

(Fig. 3) (Plafker et al., 1994). The presence of

these high-grade metamorphic rocks has several

implications for the structure of the St. Elias

orogen in the eastern syntaxis area. The Con-

tact fault forms the southern boundary to the

Chugach terrane and Valdez Group (Plafker,

1987); however, the Contact fault is located

north of the high-grade rocks of Valdez Group

where it connects with the Fairweather fault

beneath the upper Seward glacier (Fig. 1). We

propose that the low-angle CSEF in the eastern

syntaxis is an erosional remnant of the Contact

fault–Fairweather fault system preserved at high

elevation with Valdez Group strata in its hang-

ing wall. Plafker et al. (1994) suggested that

these rocks were translated into their present

position along the Fairweather fault.

If the CSEF in the eastern syntaxis is an ero-

sional remnant of the Contact fault–Fairweather

fault system, as we propose, it raises questions

concerning the CSEF in the central and west-

ern Yakutat microplate. Specifi cally, how does

the CSEF of the western and central YFTB




connect into the fault systems of the eastern

syntaxis? Also, how do Prince William ter-

rane rocks between the CSEF and the Bagley-

Contact faults to the west relate structurally to

the Yakutat mélange in the eastern syntaxis?

These are essential questions that will need to

be addressed in further research.

The high metamorphic grade at high eleva-

tion and young AHe and ZHe ages for the Val-

dez Group in the St. Elias area (Berger et al.,

2008b; Enkelmann et al., 2010) indicate sig-

nifi cant tectonic exhumation. The geometry of

the dextral Fairweather fault to Contact fault

transition in the eastern syntaxis area forms a

restraining bend that is also consistent with tec-

tonic exhumation (Figs. 1 and 3). It is unclear if

this exhumation may be related to slip along the

south-dipping Bagley backthrust, as proposed

in Berger et al. (2008a), structural thickening

of the Yakutat Group at depth south of the Con-

tact fault, or some combination of both. The

Chugach terrane exposed at the surface north

of the Bagley Icefi eld and upper Seward glacier

displays a lower metamorphic grade and has

produced older AHe ages (Berger et al., 2008b),

suggesting signifi cantly less tectonic exhuma-

tion north of the Bagley-Contact fault.

Implications of the Yakutat Group in the

Eastern Syntaxis

It is generally accepted that the Yakutat

microplate was excised from the Cordilleran

margin and subsequently carried northward;

however, there is considerable debate concern-

ing where it originated. The most specifi c res-

toration of the Yakutat microplate proposes that

the DRZ is a displaced segment of the Chatham

Strait fault (Plafker, 1987; Plafker et al., 1994)

(Fig. 17). The Chatham Strait fault is a dextral

strike-slip fault linking the Denali fault and

Queen Charlotte fault system and is located

~600 km southeast of the Samovar Hills. The

northern suture of the Chugach accretionary

complex, the Border Ranges fault, is truncated

by the Chatham Strait fault, suggesting a total

post-Cretaceous displacement of ~150–200 km

(Hudson et al., 1982; Plafker, 1987; Gehrels,

2000). Plafker et al. (1994) suggested that in

Paleocene to Eocene time, the Chatham Strait

fault cut through previously accreted terranes,

the accretionary complex, and into the Kula or

Pacifi c plate to juxtapose the Yakutat Group and

oceanic crust across the fault. Plafker (1987)

proposed that motion on the Fairweather fault

initiated in the Oligocene, trapping this segment

of oceanic crust and continental crust (Plafker,

1987; Plafker et al., 1994), and cited the Hubbs

Creek fault and Marvitz Creek fault in the Samo-

var Hills as a possible exposure of the DRZ.

The results of our research in the Samovar

Hills suggest that the Hubbs Creek and Mar-

vitz Creek faults may not be part of the DRZ

as defi ned by Plafker (1987). The Yakutat

Group underlies the Kulthieth and Yakataga

Formations west of the Hubbs Creek fault and

is present within the core of the Samovar Hills

anticlinorium in the hanging wall of the Mala-

spina fault (Fig. 13). The Marvitz Creek fault

abruptly loses displacement and tips out near

the center of the Samovar Hills anticlinorium;

we propose that this is related to the formation

of the anticlinorium. This provides evidence

that the Marvitz Creek fault did not have sig-

nifi cant displacement after deposition of the

Kulthieth Formation, but it does not rule out the

possibility that this fault represents a reactivated

older basement fault. The Samovar Hills anti-

clinorium plunges to the west within the Mala-

spina thrust sheet and shows no discontinuity

with the observed structure in the Chaix Hills.

No evidence exists to indicate that the Hubbs

Creek and Marvitz Creek faults mark the DRZ

or to determine how much farther westward

the Yakutat Group is present in the subsurface

between the Chaix Hills and Malaspina–Esker

Creek faults.

The Yakutat Group–sedimentary cover con-

tact dips to the northwest within the Chaix Hills

thrust sheet, broadly similar to the westward

plunge direction of the Samovar Hills anti-

clinorium in the Malaspina thrust sheet. This

orientation preserves a stratigraphically com-

plete, albeit deformed, section of the Kulthieth

Formation with younger stratigraphic levels

exposed along strike to the west (Fig. 3). Addi-

tional subsurface information is required to

determine a western limit to the Yakutat Group

at depth within the Chaix Hills thrust sheet.

The Yakutat Group is also exposed within

the Libbey Glacier thrust sheet and structurally

higher, unnamed thrust sheets in the Mount St.

Elias region. Our analysis revises the position

of the CSEF, placing it higher along the south-

ern fl ank of Mount St. Elias and Mount Huxley.

Here, a low-angle thrust separates high-grade

metamorphic rocks of the St. Elias massif to the

north from rocks of dramatically different meta-

morphic grade and composition to the south,

including the Yakutat Group. This relationship

suggests that basement exposures south of

Mount St. Elias are not part of the North Ameri-

can backstop. This interpretation of these rock

assemblages in the Libbey Glacier thrust sheet

places Yakutat Group strata >40 km west of the

Samovar Hills (Fig. 3). Our observations did not

allow us to determine how much further west

the Yakutat Group is present in the Libbey Gla-

cier thrust sheet.

Our observations of the Yakutat Group in

the eastern syntaxis raise questions about the

relationship between the Chatham Strait fault

and the DRZ. If the DRZ is a lithologic bound-

ary between Yakutat Group to the east and

basaltic basement to the west, as proposed by

Plafker (1987) and Plafker et al. (1994), then the

55° N

140° W

Pacific Plate

Ale

utia

n Tre

nch

PWTFairw

eather Fault

Transition Fault

North American

Plate

CG

250 km

MF ECFD

enali Fault

Chath

am S

trait Fau

lt

Contact Fault

BRF

BRF

CSEF

Yakutatmicroplate

60° N

145° W 135° W

IT

?

Figure 17. Regional fault and terrane map. CG—Chugach ter-

rane; PWT—Prince William terrane; IT—Insular terrane; CSEF—

Chugach St. Elias fault; MF—Malaspina fault; ECF—Esker Creek

fault; BRF—Border Ranges fault.


Chapman et al.


predicted length of the DRZ is ~275 km from

the Transition fault offshore to the Samovar

Hills (Fig. 1). The predicted length of the DRZ

increases to >325 km by including the exposures

of the Yakutat Group within the Libbey Glacier

thrust sheet. These lengths do not include a res-

toration of the fold-and-thrust belt or consider

shortening within the Yakutat Group. If oceanic

basalts were emplaced next to the Yakutat

Group along the DRZ by dextral movement on

the Chatham Strait fault, then we would expect

at least 275 km of slip on the Chatham Strait

fault, signifi cantly more than the documented

displacement. The problem is exacerbated by

the presence of the Admiralty Island volcanics

(25–27 Ma), which are offset ~100 km along

the Chatham Strait fault (Hudson et al., 1982;

Ford et al., 1996; Loney et al., 1967). The offset

of these volcanics suggests that at least half of

the total post-Cretaceous displacement on the

Chatham Strait fault may have occurred after

the Yakutat microplate was excised from the

margin (Plafker et al., 1994).

CONCLUSIONS

The eastern syntaxis of the St. Elias oro-

gen contains at least four major thrust faults,

including the Malaspina fault, Chaix Hills fault,

Libbey Glacier fault, and the CSEF. We have

revised previous interpretations of the trace

of the CSEF, which is the Yakutat microplate

suture, to higher along the south and east fl ank

of Mount Augusta and higher along the south

fl ank of Mount St. Elias and Mount Huxley.

The CSEF dips 10°–30° toward 350–360 in

the eastern syntaxis area, shallower than the

CSEF in the central YFTB, and may be over-

lain by a partial klippe. The St. Elias orogenic

wedge in the eastern syntaxis area is narrow;

convergence is currently accommodated at the

toe on the Malaspina fault–Esker Creek fault

system and beneath the Bagley Icefi eld and

upper Seward glacier on the poorly understood

Bagley-Contact fault system that connects with

the Fairweather fault to the southeast. The

geometry and intersection of the major fault

systems suggest that convergence increases

toward the eastern syntaxis area as the Esker

Creek fault intersects with the Malaspina fault

and as the Fairweather fault transitions into

the Bagley-Contact fault system. Evidence

from the Samovar Hills for contraction on the

Malaspina fault in pre-Yakataga time suggests

that the Malaspina fault may not be among the

youngest structures within the YFTB; however,

it does appear to be one of the most active.

The Yakutat Group is carried in thrust sheets

to the north and west of the previously mapped

position of the DRZ in the Samovar Hills and

may be structurally thickened in the subsur-

face. The contact between the Kulthieth For-

mation and Yakutat Group is exposed in the

Chaix Hills thrust sheet north of the Samovar

Hills, and the Libbey Glacier thrust sheet car-

ries Yakutat Group strata where exposed at Hay-

don Peak and Mount Augusta. These locations

extend the estimates for the length of the DRZ

to >325 km. In the Samovar Hills, the Marvitz

Creek fault forms a splay off the Malaspina–

Esker Creek fault system that abruptly loses dis-

placement within the core of the Samovar Hills

anti clinorium. The Hubbs Creek fault records

older strike-slip motion, but does not bound the

Yakutat Group to the west. The presence of

the Yakutat Group at high structural levels in the

eastern syntaxis area casts doubt on the genetic

link between the DRZ and the Chatham Strait

fault, and the Marvitz Creek fault and Hubbs

Creek fault in the Samovar Hills do not appear

to be related to the Chatham Strait fault. The

Chatham Strait fault is a key constraint in some

tectonic reconstructions of the Yakutat micro-

plate (Plafker, 1987; Plafker et al., 1994). Relax-

ing this constraint allows for a greater transport

distance of the Yakutat microplate; however, it

does not preclude the possibility that the Yaku-

tat microplate originated along the southeastern

Alaska margin.

We conclude that fold-and-thrust–style defor-

mation began in the Early to latest Miocene in

the eastern syntaxis area. The basal Yakataga

unconformity, observed throughout the eastern

YFTB, cuts out Middle Eocene to latest Mio-

cene strata. The erosional event responsible

for the unconformity occurred between the

Early Miocene and latest Miocene. Structural

relationships in the Samovar Hills suggest that

this erosional event brackets the transition from

basement-involved transpressional deforma-

tion to fold-and-thrust–style deformation within

the Malaspina–Esker Creek thrust sheet as the

Yakutat microplate was translated into the syn-

taxis area. Angular relationships across the basal

Yakataga unconformity also demonstrate that

deformation related to the eastern YFTB initi-

ated prior to the deposition of the Yakataga For-

mation. The initiation of the fold-and-thrust belt

was coincident with an end to oblique transpres-

sional deformation. The Kulthieth Formation

and older strata in the eastern syntaxis record

the earliest stages of fold-and-thrust–style defor-

mation. It is unclear whether the timing of the

transition to fold-and-thrust deformation is only

applicable to the eastern syntaxis or may apply

to the YFTB to the west in a synchronous or

time-transgressive manner. Extending the tim-

ing for the initiation of the YFTB reduces geo-

logic estimates of rates of convergence over the

life of the orogen.

ACKNOWLEDGMENTS

This research was supported by the St. Elias Ero-sion and Tectonics Project (STEEP) through National Science Foundation grants EAR-0409009 to Pavlis, EAR-0408959 to Bruhn, and EAR-0408584 to Gulick. Adam Barker provided valuable assistance in the fi eld. Reviews by Wes Wallace, Jean-Daniel Champagnac, and Eva Enkelmann signifi cantly improved the manu-script. This is UTIG (University of Texas Institute for Geophysics) contribution 2431.

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Structural relationships in the eastern syntaxis of the St. Elias orogen, Alaska

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