Structural Relationships in the Eastern Syntaxis of the St. Elias Orogen, Alaska

Neogene Tectonics and Climate-tectonic Interactions in the Southern Alaskan Orogen themed issue

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, USA 2University of Utah, Department of Geology and Geophysics, 115 S. 1460 E, Salt Lake City, Utah 84112, USA 3Texas A&M University, Department of Geology and Geophysics, MS 3115, College Station, Texas, 77843, USA 4University of Texas Institute for Geophysics, J.J. Pickle Research Campus, Building 196, 10100 Burnet Road (R2200), Austin, Texas 78758, USA 5ConocoPhillips Company, Houston, Texas 77252, USA

ABSTRACT the eastern syntaxis. Exposures of basement styles (Fig. 1). The Samovar Hills offer singular and fault patterns within the syntaxis have insight into the evolution of the YFTB and pro- The eastern syntaxis in the St. Elias oro- implications for tectonic reconstructions of vide a template for interpreting geologic obser- gen (Alaska, USA) is one of the most com- the Yakutat microplate and the geodynamics vations throughout the Yakutat microplate. Our plex and least understood regions within of the orogen. study of the eastern syntaxis helps to resolve the the southern Alaska coastal mountain belt. geodynamics of the St. Elias orogen in a criti- The syntaxis contains many features unique INTRODUCTION cal, and poorly understood, location within the to the orogen that are essential to under- Yakutat microplate. standing the structural architecture and tec- Throughout most of the Neogene, the Yaku- tonic history of the collision between North tat microplate traveled northward relative to TECTONIC SETTING America and the allochthonous Yakutat North America, impinging upon the southern microplate. The eastern syntaxis contains margin of Alaska and initiating both proximal In the eastern Yakutat microplate, onshore the transition from transpressional struc- and far-ﬁ eld orogenesis (Plafker, 1987; Pavlis basement exposures consist of ﬂ ysch and accre- tures associated with the Queen Charlotte– et al., 2004; Leonard et al., 2007). The Yaku- tionary mélange of the Yakutat Group, part of Fairweather fault system in the east to the tat microplate is located between the transition the late Mesozoic Chugach accretionary com- Yakataga fold-and-thrust belt (YFTB) to from dextral strike-slip motion along the Queen plex (Fig. 1) (Plafker et al., 1977). The entire the west. Throughout the eastern syntaxis, Charlotte–Fairweather fault system to oblique Mesozoic accretionary complex from southern a prominent uncon formity at the base of the convergence in the core of the St. Elias orogen Alaska to Vancouver Island was intruded by synorogenic Yakataga Formation records an and ultimately to subduction at the Aleutian distinctive near-trench plutons and locally meta- erosional event related to the development of Trench. One of the most dynamic areas within morphosed to high-grade low-pressure–high- the YFTB. Strain accumulations in the east- the St. Elias orogen is the eastern syntaxis, an temperature metamorphic assemblages during ern YFTB predate the deposition of the Yaka- area of high topography and structural diversity an Early Eocene oceanic spreading ridge sub- taga Formation, extending estimates for the where northwest-striking, basement-involved duction event (Pavlis and Sisson, 1995; Bradley early development of the St. Elias orogen. transpressional systems to the southeast are et al., 2003; Sisson et al., 2003). A portion of Structural and stratigraphic relationships in juxtaposed against the east- to northeast-strik- this accretionary complex was excised from the the eastern syntaxis suggest that forethrusts ing, thin-skinned Yakataga fold-and-thrust belt Cordilleran margin to form part of the Yakutat associated with the transpressional system (YFTB) to the west (Fig. 1). The intersection microplate, and transported northward. As a shut down and were overprinted by fold- of these two structural systems is poorly under- result, in the present collision, variably meta- and-thrust structures in the Early to latest stood, but critical to unraveling the tectonic his- morphosed assemblages of the Chugach accre- Miocene. Basement in the eastern syntaxis tory of the Yakutat microplate. tionary complex form both the autochthonous consists of the Yakutat Group, part of the We present an updated description of the east- backstop and part of the allochthonous base- Chugach accretionary complex, which is car- ern syntaxis, characterize the structural architec- ment (Plafker, 1987). ried by numerous low-angle thrust faults in ture of the YFTB, and utilize stratigraphic and The autochthonous rocks are equivalent to structural relationships to help constrain the tim- the Late Cretaceous to Paleocene Valdez and *Present address: SandRidge Energy, 123 Robert S. ing and evolution of deformation in the eastern Orca Groups, which were variably metamor- Kerr Avenue, Oklahoma City, Oklahoma 73102, USA. syntaxis, including the initiation of the YTFB. phosed from greenschist to upper amphibo- †Emails: Chapman: jchapman@sandridgeenergy In addition to an overview of the eastern YFTB, lite facies (Richter et al., 2005). The Valdez .com; Pavlis: [email protected]; Bruhn: ron.bruhn @utah.edu; Worthington: [email protected]; we present a focused study of the Samovar Group is part of the Chugach terrane, which Gulick: [email protected]; Berger: Aaron.L Hills, a key exposure located in the core of the was accreted to the southern Alaska margin in [email protected]. eastern syntaxis at the intersection of structural the Late Cretaceous (Amato and Pavlis, 2010).

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

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142° W 140° W 138° W

Chugach Terrane Denali Fault Bagley/Contact Fault PWT Mt. Logan

Alaska Area of Eastern CSEF Syntaxis Bering ? Glacier ? Mt. St. Elias

Yakataga ovar Hil Canada am ls 1 S Esker Creek Fault 60° N È Malaspina Fault Insular terrane Icy BayÈ2 Malaspina Glacier 50 km Fig. 3 Bor der R Yakutat Bay ang a Valley Yakutat Foothills es utat Se F Pamplona fold ak au and thrust 200belt m Y Yakutat lt

Alsek River “ Yakutat D

a Chugach n

Microplate g FairweatherTerrane Fault e

Pacific r o

59° N s

Yakutat R

i v e r

Z o Lituya Bay Fault n 1 e È well ” suture fault international border bathymetric contour Pacific Plate “Dangerous River Zone”

Yakutat microplate Prince William terrane Transition Fault 58° N Yakutat sedimentary rocks Insular terrane 20 mm/yr Yakutat Group Chugach terrane

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.

The Orca Group is part of the Prince William The Yakutat Group is regionally metamor- cores (Jones and Clark, 1973; Rau et al., 1983) terrane, which was accreted in the Early Eocene phosed to zeolite to prehnite-pumpellyite suggest that the Yakutat Group is Campanian to (Plafker et al., 1994). The Contact fault forms facies and locally metamorphosed to green- Maastrichtian in age (65.5–83.5 Ma; Gradstein the suture between the Prince William ter- schist facies (Dusel-Bacon, 1994; this study). et al., 2005). Haeussler et al. (2005) estimated rane and Chugach terrane (Plafker, 1987). The Building upon mapping by Plakfer and Miller a maximum deposition age of 72–74 Ma from Chugach St. Elias fault (CSEF) forms the suture (1957), Richter et al. (2005) subdivided the detrital zircon data for the Yakutat Group, an between the Prince William terrane and the Yakutat Group into ﬂ ysch and mélange assem- age they interpret as within ~5 Ma of the true Yakutat microplate (Fig. 1). blages. Paleontological data from outcrop and depositional age.

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In contrast to the eastern Yakutat microplate, 1987), herein referred to as the Oily Lake mem- on the continental shelf (Eyles and Lagoe, geophysical studies indicate that basement in the ber of the Kulthieth Formation (Fig. 2). Forami- 1990), and are the youngest within the orogen central and western Yakutat microplate consists nifera analysis for the Oily Lake member in (Middle Miocene to present). The 0–6-km-thick of a 15–30-km-thick oceanic plateau that is sub- the Samovar Hills indicates an Early to Middle Yakataga Formation contains numerous internal ducting beneath and accreting to North America Eocene age (Ulatisian Stage, ca. 42.5–49.5 Ma; angular unconformities associated with fold and (Ferris et al., 2003; Pavlis et al., 2004; Eberhart- McDougall, 2007) (Plafker et al., 1994). fault growth (Miller, 1957; Worthington et al., Phillips et al., 2006; Gulick et al., 2007; Christe- The Kulthieth Formation is typically 2–3 km 2008, 2010; Broadwell, 2001). son et al., 2010). The relationships and transi- thick and consists of fl uvial to shallow-marine tion between basement lithologies across the deltaic deposits including coarse arkosic sand- Yakataga Fold-and-Thrust Belt Yakutat microplate are unclear, although Plafker stone, shale, and coal beds (Plafker, 1987; (1987) and Plafker et al. (1994) suggested that Landis, 2007). West of the study area, the In the western and central Yakutat micro- the transition occurs along the Dangerous River Tokun and Stillwater Formations are the marine plate, the sedimentary cover is decoupled from zone (DRZ) (Fig. 1), which they interpreted as equivalents to the Kulthieth Formation (Plafker, the Yakutat oceanic plateau basement to form the a steeply dipping crustal boundary. The DRZ 1987). In the Samovar Hills, the basal Kul- thin-skinned YFTB. Offshore, the deforma- was originally defi ned from offshore seismic thieth Formation consists of a white tuffaceous tion front is defi ned by the Pamplona zone, an refl ection data as the western edge of a base- conglomerate, 1–3 m thick, that grades upward active fold-and-thrust system that accommodates ment uplift across which the Cenozoic sedimen- into ~500 m of coarse sandstone with coal beds ~6 mm/yr of shortening (Worthington et al., tary cover sequence for the Yakutat microplate as thick as several meters (Landis, 2007; this 2010; Chapman et al., 2008). The Pamplona thins dramatically eastward (Bruns 1982, 1983). study). The Kulthieth Formation in the Samo- zone trends onshore into the Malaspina fault, Plafker (1987) further delineated the location of var Hills is late Early Eocene to early Middle the active deformation front in the eastern syn- the DRZ onshore and assigned a series of faults Eocene in age (Plafker, 1987). taxis area (Fig. 3). Global positioning system in the Samovar Hills to the DRZ. The Poul Creek Formation conformably (GPS) data suggest that the Malaspina fault may A thick sedimentary sequence that spans overlies the Kulthieth Formation and includes accommodate ~20 mm/yr convergence (Elliott most of the Cenozoic overlies Yakutat micro- organic-rich shale, siltstone, glauconitic sand- et al., 2010). Because the Malaspina fault and plate basement (Fig. 2). The sedimentary cover stone, and interbedded tuff (Plafker, 1987; structures within the Pamplona zone are oriented reaches a total stratigraphic thickness of >10 km Brennan et al., 2009). The Poul Creek Forma- oblique to the plate boundary (Figs. 1 and 3), the (Worthington et al., 2010) and thins to near zero tion is ~1 km thick and is characterized by low width of the active orogenic wedge varies across across the offshore DRZ (Bruns, 1982; 1983). sedimentation rates and was deposited in marine the region from a minimum of ~35 km in the In the eastern syntaxis area, three main strati- waters along a narrow shelf fringing low-lying vicinity of the Samovar Hills to >100 km in graphic units compose the cover sequence: the coastal mountains (Eyles et al., 1991). The Poul the central YFTB (Plafker et al., 1994). Kulthieth Formation, the Poul Creek Formation, Creek Formation is latest Eocene to Early Mio- The depth to the basal detachment in the and the Yakataga Formation (Plafker, 1987). Sev- cene in age (Plafker, 1987). The Yakataga For- eastern YFTB is constrained by earthquake eral minor stratigraphic units are present locally, mation rocks are thick interbedded glaciomarine relocations, including a series of aftershocks including the siltstone of Oily Lake (Plafker, sandstone, mudstone, and diamictite deposited associated with the 1979 St. Elias earthquake (Bauer et al., 2008; Estabrook et al., 1992) and offshore seismic data (Worthington et al., 2010), ~Ma West East together suggesting that the basal detachment in

Pleist. 2 the eastern YFTB is located at ~12–16 km depth 9 (Fig. 4). This depth is comparable to the basal Figure 2. Simplified strati- Plio. 5 Yakataga Formation detachment in the central YFTB where detach-

graphic column for the eastern Neogene ment depths are estimated as 10–17 km (Berger QTy 8 syntaxis area in the Yakutat et al., 2008a; Wallace, 2008; Doser et al., 2007; Miocene microplate. Stratigraphic thick- 23 unconformity Worthington et al., 2010). Apatite U-Th/He ages nesses are schematic and show T 7 (Berger et al., 2008a, 2008b; Spotila and Berger, that the Paleogene sedimen- pc 2010), zircon ﬁ ssion-track ages (Enkelmann tary section thins toward the 34 et al., 2010), and structural restorations (Meigs 6 east. Kym—Cretaceous Yakutat Kulthieth et al., 2008; Wallace, 2008), however, suggest Group mélange; Ky—Yakutat Cenozoic Formation T that much of the sedimentary cover exposed Group flysch; T —Tertiary ol i 5 at the surface today was never buried >5 km, Tk intrusive rocks; Tk—Kulthieth indicating that none of the rocks now exposed Eocene

Formation; T —Oily Lake Paleogene ol Yakutat Group in the YFTB were deeply buried beneath thrusts member of the Kulthieth Forma- 4 prior to exhumation. Previous interpretations

K schematic thickness (km) tion; Tpc—Poul Creek For- unconformity y of the YFTB structure at depth include a series mation; Paleoc—Paleocene; 56 of duplexes and imbricate thrusts in Cenozoic 3 Plio—Pliocene; Pleist—Pleisto- sedimentary rocks alone (Berger et al., 2008a;

Paleoc. ? cene; QTy—Quaternary–Ter- 65 Chapman et al., 2008) or also including structur- tiary Yakataga Formation. Ti 2 ally thickened basement rocks (Wallace, 2008). The interpretation of structure at depth has con- Late Oligocene Kym siderable implications for structural reconstruc- Mesozoic Cretaceous 1 tions and estimates of total shortening.

Geosphere, February 2012 107

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Chapman et al.

60.25° N 60.25° 60° N 60° ) vm ) y vm K ) ym

ym K 140° W 140° i y T fault K ) Fault pc )

Fairweather k ) y ) i ? ? Mt. Owens ?

Esker Creek Fault

t l

s international boundary thermochronology sample fold axis

Seward Glacier Seward Throat Seward

ym ? e Fig. 10

m ? o

y D Quaternary-Tertiary Yakataga formation Yakataga Quaternary-Tertiary (QT Tertiary Poul Creek formation (T Tertiary rocks (T instrusive Tertiary Group Cretaceous Yakutat (K Group melange (K Yakutat Cretaceous Group/Chugach terrane (K Valdez Cretaceous unconsolidated Quaternary sediments Tertiary Kulthieth formation (T Kulthieth Tertiary K y

K ? Fig. 12 vm K

Mt. Augusta ? ? k Glacier T

Malaspina

140.5° W 140.5° ?

y .8 cm/yr

4 Samovar Hills Samovar QT m

Mt. Logan

k c

l 0

CSEF G

Fig. 7 z i

T s s a g A Libbey Glacier Fault Glacier Libbey 20 km 2 ym

K Malaspina Fault Malaspina pc vm T K

A 60.5° N 60.5° ′ Bagley Icefield Bagley k

Mt. St. Elias Chaix Hills Fault Hills Chaix T ? Fig. 5 k Hills Chaix T È Fig. 8

Peak

Haydon

r d o j F

n n a T ?

? Tyndall Glacier Tyndall Chaix Hills #1A Fig. 9 Canada ? Alaska y

Mt. Huxley

141° W 141° ? Figure 3. Geologic map of the eastern syntaxis area. Location is in Figure 1. CSEF is Chugach St. Elias fault. 1. CSEF Location is in Figure 3. Geologic map of the eastern syntaxis area. Figure

QT cier

i lt Karr Hills

? (not shown) (not

Chugach Terrane Chugach Fau ? y

Icy Bay Coal Gla Coal

vm QT Contact Fault / Bagley Fault Bagley / Fault Contact

Fig. 6 K

N CSEF ? CSEF Yahtse Hills

Glacier

? Guyot 141.5° W 141.5°

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(North) A (South) Haydon Mt. St. Elias A′ Peak CSEF Libbey Glacier Contact 5 Malaspina Chaix Hills K Fault vm Fault Riou Bay #1 Fault Fault Seward Glacier Tk projected ~15 km Chaix Hills #1A Malaspina Glacier K Kvm ym 0 Kym T pc Contact F QTy QT Tk y Tpc

a ul –5 T t ? k K ? T y k Bagely Fault ? ntact Fault ? ~5 degrees

? Elevation (km) Co –10 ~ 10 degrees ? Yakutat ocea nic plateau basement

? areas of concentrated –15 5 km aftershocks from the no vertical exaggeration 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.

Total shortening estimates for the St. Elias 1997) and fault studies adjacent to the Bagley as proposed in Berger et al. (2008a) may cut the orogen, based on plate reconstructions and Icefi eld (Bruhn et al., 2004) suggest that the older Contact fault. exhumed material, are ≥300 km (Pavlis et al., Contact fault accommodates some component The model of Berger et al. (2008a, 2008b) 2004; Berger et al., 2008a; Chapman et al., 2008). of dextral shear and may be reactivated as a is complicated, however, by young (<3 Ma) Shortening estimates from published structural strike-slip fault in this location (Pavlis et al., detrital ZFT ages originating from the Valdez reconstructions are lower, generally <100 km 2004; Bruhn et al., 2004). Bruhn et al. (2004) Group beneath the Malaspina–Seward glacier (Meigs et al., 2008; Chapman et al., 2008; proposed that the oblique convergence of the system (Enkelmann et al., 2009, 2010). These Wallace, 2008). The YFTB is currently accom- Yakutat microplate was partitioned between dip data suggest rapid exhumation along the trace modating >3.5 cm/yr of the total 4–5 cm/yr slip on north-dipping thrust faults in the YFTB of the Contact fault beneath the upper Seward convergence between the Yakutat microplate and dextral strike-slip motion on a steeply dip- glacier. Enkelmann et al. (2008, 2010) inter- and North America (Elliott et al., 2009; 2010; ping to vertical Contact fault. preted these ages as evidence for exhumation of Sauber et al., 1997; Fletcher and Freymueller, Geomorphic and bedrock apatite [U-Th]/He transpressional fault slivers along a reactivated 2003). These rates are thought to have remained (AHe) data were presented that indicated south- Contact fault system, and suggested that the similar since the earliest Pliocene (DeMets et al., side-up motion across the Bagley Icefi eld Bagley backthrust is unnecessary. In this model, 1994). In contrast, estimates of shortening since (Berger et al., 2008a). Earthquake relocations the Contact fault is either north dipping (Enkel- deposition of the Yakataga Formation using reveal a loosely defi ned, south-dipping limit to mann et al., 2008) or nearly vertical (Enkelmann published cross-section restorations range from seismicity beneath the central YFTB (Berger et al., 2010). 5 to 14 mm/yr (Chapman et al., 2008; Meigs et al., 2008a; Doser et al., 2007) interpreted Spotila and Berger (2010) also suggested et al.; 2008; Wallace, 2008). Explanations for (Berger et al., 2008a) as a new fault, the south- that exhumation of transpressional fault slivers the differences in the rate of shortening include dipping Bagley fault, which would represent an beneath the upper Seward glacier is responsible underestimates of fault displacement (Meigs orogen-scale backthrust and suggest that the St. for the young ZFT ages; however, they suggested et al., 2008; Chapman et al., 2008) and struc- Elias orogen is doubly vergent. Furthermore, that these slivers originate along the Fairweather tural thickening of Yakutat microplate basement in Berger et al. (2008a, 2008b) AHe data were fault in the eastern syntaxis and maintained that at depth (Wallace, 2008). combined with data from mineral systems with west of the eastern syntaxis area, the Bagley fault In addition to questions surrounding the higher closing temperatures including bedrock is a viable model for the apparent south-side- structural architecture at depth in the YFTB, apatite fi ssion track (AFT), zircon [U-Th]/He up motion across the Bagley Icefi eld. Further there is considerable uncertainty concerning (ZHe), and zircon fi ssion track (ZFT) to sug- analysis is needed to determine the geometry the potential fault geometry beneath the Bagley gest that motion on the Bagley fault initiated and nature of this important structure. Icefi eld and upper Seward glacier, north of the or greatly increased in the Quaternary, coinci- YFTB (Fig. 1). West of the Yakutat microplate, dent with the end to motion on the CSEF in the EASTERN SYNTAXIS the Contact fault is a north-dipping reverse fault central YFTB. In this scenario, two faults could (Plafker et al., 1994). Although covered by ice, exist beneath the Bagley Icefi eld: the younger, We defi ne the eastern syntaxis as an ~30° the Contact fault connects with the Fairweather south-dipping Bagley fault and the older, north- bend in trend of the St. Elias orogen (Fig. 1) fault in the eastern syntaxis area (Plafker and dipping to vertical Contact fault. If the Contact that encompasses the transition from basement- Thatcher, 2008) (Figs. 1 and 3). Geodetic data fault dips shallowly to the north, as proposed involved transpressional structures to thin- (Savage and Lisowski, 1986; Sauber et al., by Plafker et al. (1994), then the Bagley fault skinned fold-and-thrust structures. Figure 3

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is a geologic map of the eastern syntaxis area and southern Karr Hills. The Yakataga anticline Chaix Hills Fault compiled from previously published geologic is present in the Guyot Hills and northern Karr maps (Plafker and Miller, 1957; Campbell and Hills and contains distinctive angular uncon- The Chaix Hills fault bounds the Malaspina Dodds, 1982; Richter et al., 2005), new map- formities and growth strata within the Yakataga thrust sheet to the north. The Chaix Hills fault ping in the 2005–2007 ﬁ eld seasons, aerial Formation (Miller, 1957). Broadwell (2001) is covered by the Agassiz Glacier to the east and and satellite photograph interpretation, and suggested the Yakataga anticline in the Guyot the Yahtse Glacier to the west, but it is exposed spectral lithologic mapping using Landsat The- Hills originated as a detachment fold with a on either side of the Taan Fjord (Fig. 3). In this matic Mapper (TM) imagery. We also present detachment at ~3 km depth. Plafker and Addi- location, the Chaix Hills fault consists of two a schematic cross section through the orogen cott (1976) reported an angular discordance of fault strands: the main fault placing Kulthieth in Figure 4 illustrating several main structural as much as 60° across an angular unconformity Formation to the northwest over Yakataga For- features. Our mapping suggests that the east- associated with the Yakataga anticline in the mation to the southeast, and a splay fault to the ern syntaxis contains at least four major thrust northern Karr Hills, leading us to suggest that south placing Poul Creek and basal Yakataga sheets including, from south to north, the Mala- the Yakataga anticline may not immediately die Formation on Yakataga Formation (Figs. 3 spina–Esker Creek fault, Chaix Hills fault, out along strike to the east. However, we did not and 5). Updip reverse slip on the main north- Libbey Glacier fault, and CSEF. In the follow- observe the Yakataga anticline in exposures on dipping Chaix Hills fault was reported in Bruhn ing we report on each of the major thrust faults the west side of Taan Fjord (Fig. 3), indicating et al. (2004). The fault surface of the southern and associated thrust sheets. that the Yakataga anticline may trend into the splay fault along the Taan Fjord also dips to Chaix Hills fault or be eroded. the north and displays reverse slip (Table 1). Malaspina Fault East of the Chaix Hills, the Yakataga For- Yakataga strata in the immediate vicinity of mation dips to the west on the crest of a large the hanging wall of the Chaix Hills splay fault The Malaspina fault is not exposed due to ice plunging anticlinorium in the Samovar Hills. also display a well-developed, spaced cleavage cover of the Malaspina Glacier (Fig. 3), but best We suggest that the west-plunging structure in with bedding-cleavage intersections plunging estimates of its trace suggest that it is oriented the Samovar Hills may provide a down-plunge 58° toward 340. This cleavage was absent in the oblique (~070) to the major thrust faults struc- view of the subsurface structure to the west in footwall. Although total offset of the southern turally higher within the thrust belt (080–115), the Chaix Hills (Fig. 3), and use this relationship splay fault is uncertain, the presence of cleavage but subperpendicular to the current Yakutat as a template for the construction of the cross suggests that the block of Yakataga strata in the microplate motion (~337) (Elliott et al., 2010). section in Figure 4. hanging wall of the Chaix Hills splay fault was This orientation suggests the Malaspina fault We interpret a small fault south of the main carried to its present position from beneath a accommodates predominantly dip-slip motion. Malaspina fault based on relocations of after- thick Yakataga section. The results suggest that We have constrained the location of the Mala- shocks from the 1979 St. Elias earthquake locally the Chaix Hills fault is predominantly spina fault with the structural geometry of the (Fig. 4) (Estabrook et al., 1992). This fault is dip slip and that the orogen-parallel component Yakataga Formation in the southern Chaix Hills, entirely covered by the Malaspina Glacier. The of motion is partitioned elsewhere. There is earthquake relocations (Estabrook et al., 1992), magnitude of slip on this fault is unknown and no clear surface evidence for or against recent and data from the Chaix Hills #1A well (drilled the offset shown in Figure 4 is schematic. motion on the Chaix Hills fault. by Standard Oil in 1961), which penetrated the Poul Creek Formation (Plafker et al., 1975) N Chaix Hills Splay Fault S (Figs. 3 and 4). The Riou Bay #1 well (drilled by Chaix Hills Fault Standard Oil in 1962) reached a total measured depth of 4300 m in the Yakataga Formation Figure 5. Photograph looking (Plafker et al., 1975). Offset of the Yakataga– east toward the Chaix Hills Poul Creek Formation contact between the QT fault and Chaix Hills splay y Tk Chaix Hills #1A well and the projected depth fault. Location is in Figure 3. of the top of the Poul Creek Formation in the QTy —Quaternary–Tertiary QT Riou Bay #1 well suggests >1200 m of throw y Yakataga Formation; Tk—Ter- across the fault, assuming limited deformation tiary Kulthieth Formation. southeast of the Malaspina fault (Fig. 4). West of Icy Bay the trace of the Malaspina ~250 m fault is unknown, but it ostensibly connects Taan Fjord with the easternmost fault of the Pamplona zone offshore, an active fault system (Worthington et al., 2010) (Fig. 1). To the east, the Mala- TABLE 1. FAULT GEOMETRY AND SLIP VECTORS Orientation Slip vector spina fault bounds the southern Samovar Hills, Fault (strike/dip) (plunge/trend) Shear sense where it intersects the Esker Creek fault. The Chaix Hills splay fault 294/50° 46/054 reverse Esker Creek fault also forms the leading edge of Marvitz Creek fault 118/70° 54/149 dextral normal Lower Hubbs Creek fault 307/88° 45/305 dextral reverse deformation to the eastern YFTB and is a north- Upper Hubbs Creek fault 303/84° 18/305 dextral reverse dipping reverse fault that places Yakutat Group Fault 1 355/90° 10/355 dextral basement rocks on the Yakataga Formation Fault 2 247/10° 10/346 reverse Fault 3 300/54° 44/074 dextral reverse (Bruhn et al., 2004; Plafker and Thatcher, 2008). Fault 4 315/90° 05/315 dextral Bedding in the Malaspina thrust sheet dips Chugach St. Elias fault 350–360/10–25° 10°–30° north to northwest in the Chaix Hills (eastern syntaxis area)

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In the northern Chaix Hills, the Kulthieth to the south. Overturned limbs of south-verging The Poul Creek Formation is notably absent at Formation displays a general structural stratiﬁ - folds are increasingly common to the north, up this locality, indicating erosion to the level of at cation from a moderately homoclinal section in structural section (cf. Figs. 6, 7, and 8). Fold least the upper Kulthieth Formation; however, the lower Kulthieth Formation to a more com- wavelengths are a few hundred meters and a small exposure of the Poul Creek Formation plexly folded section in the upper Kulthieth For- folds commonly occur within stratigraphic reappears on the east side of the Tyndall Glacier mation. The upper Kulthieth Formation appears intervals of several tens of meters, bounded by (Fig. 3) (Richter et al., 2005), indicating relief to be detached from the lower Kulthieth Forma- bedding-parallel detachments. Interlimb angles on the unconformity surface or structural relief on tion along extensive coal beds near the middle vary from open to tight with a majority of close the Poul Creek–Kulthieth Formation contact. of the stratigraphic section. West of the Tyndall folds. Hinge thickening is common within coal The base of the Poul Creek Formation east of Glacier, a contact between the Kulthieth and and shale beds. the Tyndall Glacier provides a structural marker Yakataga Formations is preserved in a gentle The base of the Yakataga Formation consists across the Chaix Hills fault. Based on the depth (~145° interlimb angle), west-plunging syn- of ~5 m of red-green rounded pebble conglom- of the Poul Creek Formation in the Chaix Hills cline. This exposure reveals a critical ﬁ eld rela- erate in a shale matrix overlain by ~3 m of green #1A well and surface structural data from the tionship: the Yakataga Formation overlying an volcaniclastic sedimentary rock. A bed-parallel Chaix Hills, we estimate that the main Chaix angular unconformity, on complexly deformed detachment is present along the contact between Hills fault has >4.5 km throw (Fig. 4). This rocks of the upper Kulthieth Formation (Fig. 6). the pebble conglomerate and green volcaniclas- estimate does not take into account structural This angular unconformity is herein referred to tic unit, but is above the angular unconformity. thickening and imbrication within the Kulthieth as the basal Yakataga unconformity. It is unclear whether this detachment surface is Formation and may not be easily related to total We observed folds within the Kulthieth For- tectonic in origin or the base of a gravity slide in displacement. mation during transects on either side of Tyndall the Yakataga Formation. Other exposures of the The Coal Glacier fault (Richter et al., 2005), Glacier. Folds within the Kulthieth Formation basal Yakataga unconformity in the eastern syn- within the Chaix Hills thrust sheet, is a reverse are upright, symmetrical to asymmetric, verging taxis area do not contain a detachment surface. fault that places the upper Kulthieth Formation

S N

~500 m

approximate scale for ridgeline

bedding trace Figure 6. Photograph looking fault unconformity west at the QTy—Quaternary– QT Tertiary Yakataga Formation– y Tk Kulthieth Formation (Tk) con-

tact in the northern Karr Hills. Tk Location is in Figure 3. Mike ?

bed-parallel detachment?

W E

Figure 7. Photograph looking Tk northwest across the Agassiz Glacier at folds within the Ter-

tiary Kulthieth Formation (Tk). Location is in Figure 3.

~500 m

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S N

Figure 8. Photograph look- Libbey Glacier Fault ing west at the Libbey Glacier fault across the Tyndall Gla- K ? ? ym cier. Location is in Figure 3. Kym? Tpc?

This outcrop was only observed Tpc? from across the Tyndall Gla- ? T cier, and the rocks in the hang- k ing wall could be Cretaceous

Yakutat Group mélange (Kym) or sedimentary rocks of the Yakutat microplate such as light-colored slope wash

Tertiary Poul Creek Forma- Tk

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- ~250 m nate unit. Tyndall Glacier

over the lower Yakataga Formation west of the the Tyndall Glacier. Similar to the Malaspina 2005). Rocks in the footwall of the Libbey Gla- Tyndall Glacier and over the Poul Creek Forma- thrust sheet, the map pattern in the Chaix Hills cier fault consist of the Kulthieth Formation, tion east of the Tyndall Glacier (Fig. 3). Total off- thrust sheet north of the Samovar Hills may pro- although the classifi cation of the rocks in the set along this fault is poorly constrained, but it is vide an along-strike view of the deeper struc- immediate hanging wall is less certain (Plafker at least ~350 m based on the stratigraphic thick- tural confi guration beneath the Tyndall Glacier and Miller, 1957; Richter et al., 2005). The base ness of the Yakataga Formation in the footwall. area to the west; however, signifi cant ice cover of Haydon Peak, in the hanging wall of the Lib- The fault is covered by the Yahtse Glacier to the handicaps clear conclusions. bey Glacier fault, contains tightly folded dark west and obscured within the Kulthieth Forma- Northeast of the Samovar Hills, the Chaix brown to dark gray siltstone with subordinate tion to the east and may die out along strike. Hills fault appears to connect with the Dome fi ne-grained sandstone, pebble conglomerate, East of the Tyndall Glacier area, the Kulthieth Pass fault of Plafker and Miller (1957) that basaltic tuff, light gray tuffaceous sandstone, Formation dips to the north-northwest and older thrusts Yakutat Group onto the Yakataga Forma- and light green to gray intrusive rocks (Plafker stratigraphic levels are exposed to the east toward tion. Across the Seward glacier, the Dome Pass– and Miller, 1957; this study) (Fig. 9). Plafker the basement-cover contact between the Kul- Chaix Hills fault system apparently connects and Miller (1957) assigned these rocks to an thieth Formation and the Yakutat Group north of with the Boundary fault north of Yakutat Bay unnamed siltstone sequence, and correlated the the Samovar Hills (Fig. 3). The exposure of the that emplaces Yakutat Group on Yakutat Group siltstone sequence to rocks west of the Tyndall basement-cover contact is related to both ero- (Plafker and Thatcher, 2008). Isolated lenses of Glacier in the hanging wall of the Libbey Glacier sion and depositional thinning of the Kulthieth the Kulthieth Formation are present east of the fault and to rocks exposed north of the Samo- Formation. Our observations were taken from Seward glacier throat in the immediate footwall var Hills in the Chaix Hills thrust sheet. Strati- helicopter, but near the contact, the Kulthieth of the Dome Pass–Chaix Hills fault system, graphic relationships suggest that the Plafker Formation exhibits pervasive intraformational expanding the depositional extent of the Kul- and Miller (1957) siltstone sequence underlies folding with numerous bed-parallel detach- thieth Formation to the east (Plafker and Miller; and is conformable with the Kulthieth Forma- ments along shale and coal layers (Fig. 7). The 1957; R. Bruhn, T. Pavlis, and G. Plafker, 2000, tion. Sparse paleontological data from the expo- largest folds are south-verging asymmetric folds personal observations). sure north of the Samovar Hills suggest that the with interlimb angles <60° and wavelengths of lowermost Kulthieth Formation is Paleocene in several hundred meters. The folds are enclosed Libbey Glacier Fault age in that location, providing an upper bracket within stratigraphic packages of a few to several for the age of the siltstone sequence (Addicott hundred meters. Within the core of the larger We observed the Libbey Glacier fault (Bruhn and Plafker, 1971). folds, additional bed-parallel detachments and et al., 2004) in the northern Chaix Hills at the Subsequent mapping (by Bruhn et al., 2004, smaller folds are commonly observed (Fig. 7). base of Haydon Peak and in a nunatak in upper and G. Plakfer, compiled by Richter et al., 2005), Despite the intraformational folds, a fairly intact Libbey Glacier (Figs. 8 and 9). The Libbey Gla- reassigned the rocks of the siltstone sequence at stratigraphic section of the Kulthieth Formation cier fault in this location was originally mapped the base of Haydon Peak and north of the Samo- appears to be exposed along strike from its base as the Coal Glacier fault (Plafker and Miller, var Hills to the Yakutat Group fl ysch assemblage. at the Yakutat Group north of the Samovar Hills 1957), although that name was subsequently The siltstone sequence west of the Tyndall Gla- to the base of the Poul Creek Formation east of applied to the fault to the south (Richter et al., cier, in the hanging wall of the Libbey Glacier

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NW SE Western shoulder of Mt. St. Elias Haydon Peak fold axes ~ 1.5 km contact folded approximate scale Figure 9. Photograph look- intrusives along flank of Haydon Peak ing northeast at a prominent lithologic contact on Haydon Tk Peak. Location is in Figure 3.

Tk—Tertiary Kulthieth Forma- tion; K —Cretaceous Yakutat ym K Group mélange. ym

Tyndall Glacier

fault, was reassigned to the Kulthieth Formation an existing angular unconformity. Richter et al. metamorphism (Fig. 10), similar to the Yakutat with associated mafi c intrusive rocks (Richter (2005) assigned the rocks on top of Haydon Group fl ysch in the Samovar Hills and else- et al., 2005). Our observations immediately west Peak to the Yakutat Group, although they did where. This interpretation of the Libbey Glacier of the Tyndall Glacier in the hanging wall of the not recognize a lithologic change along the fault shifts the position of the CSEF higher up Libbey Glacier fault, however, suggest greater southern fl ank other than a mapped fault. The the fl ank of Mount Augusta, consistent with the complexity. In that area, the rocks in the hang- lithology and style of deformation (cf. Fig. 7) mapping of Campbell and Dodds (1982). ing wall of the Libbey Glacier fault consist of suggest that the rocks at the top of Haydon Peak Spotila and Berger (2010) analyzed samples dark brown to reddish-brown mudstone and may be equivalent to the Kulthieth Formation from either side of the Libbey Glacier fault, siltstone with abundant felsic dikes and sills present at moderate elevation on the west side south of Mount Augusta, for apatite and zir- that are tightly folded into recumbent folds with of the Tyndall Glacier. Furthermore, G. Plafker con (U-Th)/He (AHe and ZHe) cooling age wavelengths of a couple hundred meters (Fig. 8). (2011, personal commun. to T. Pavlis) observed determinations (Fig. 3, yellow circles). In the Localized faults are present in the core of the coal at some locations on the top of Haydon footwall of the fault, to the south, Spotila and folds and lose displacement upsection (Fig. 8). Peak during reconnaissance work. The younger Berger (2010) reported an AHe age of 0.59 Ma Farther west, coarser-grained, light-colored sand- over older relation interpreted here between and Enkelmann et al. (2010) reported a ZHe age stone of the Kulthieth Formation is clearly pres- the Kulthieth Formation and the Yakutat Group of 55.29 Ma. In the hanging wall to the north, ent in the Libbey Glacier thrust sheet; however, mélange is consistent with the possibility of a Berger and Spotila (2008) reported an AHe additional study is needed to positively identify faulted unconformity. We traced the contact on age of 0.56 Ma and Enkelmann et al. (2010) the assemblages immediately west of the Tyn- the southern fl ank of Haydon Peak to the east reported a ZHe age of 3.48 Ma. This unusual dall Glacier in the hanging wall of the Libbey where it climbs in elevation and ultimately is pattern of AHe and ZHe age pairs is similar to Glacier fault. Based on lithologic and structural truncated by the CSEF along the southeast fl ank ages measured across the CSEF in the central similarities, we suggest that these rocks may be of Mount St. Elias (Fig. 3). YFTB (Berger et al., 2008a); in that study, it was part of the Yakutat Group mélange, the Kulthieth South of Mount Augusta, Richter et al. (2005) suggested that the CSEF was active ca. 5 Ma Formation, the Poul Creek Formation, or some mapped rocks in the hanging wall of the Libbey during the closure of the younger ZHe age, but combination of these. Glacier fault as greenschist facies Valdez Group. inactive since ca. 1 Ma based on the similar AHe Half-way up the southern fl ank of Haydon Like the Yakutat Group, the Valdez Group is ages. A similar history of deformation may exist Peak is a prominent color change that marks part of the Chugach accretionary complex and for the Libbey Glacier fault at this location, sug- a lithologic contact separating Yakutat Group is litho logically similar to the Yakutat Group, gesting that signifi cant differential exhumation mélange below from lighter colored, brownish- but was accreted to the southern Alaskan margin across the Libbey Glacier fault has not occurred red sedimentary rocks lacking intrusive rocks prior to the collision of the Yakutat microplate over the past ~0.5 Ma and that current rapid above (Fig. 9). Although only observed from (Plafker, 1987). Campbell and Dodds (1982), exhumation is the result of uplift associated a distance, these rocks appear to be folded into however, assigned these rocks to the Yakutat with other structures. The ZHe cooling age in series of recumbent anticline-syncline pairs Group. Where observed in this location, the the footwall predates the development of the St. with wavelengths of hundreds of meters to sev- hanging wall of the Libbey Glacier fault contains Elias orogen and limits the total exhumation in eral hundred meters (Fig. 9). Plafker and Miller dark brown to gray phyllite and low-grade chlo- the footwall block to 6–7 km. (1957) suggested that the contact was a steeply ritic schist (Fig. 10) that we correlate along strike dipping angular unconformity with local offset to the lower metamorphic grade Yakutat Group Chugach–St. Elias Fault along numerous minor faults. Our observations mélange exposed south of Mount St. Elias and demonstrate that the contact is subhorizontal and west of the Seward glacier throat. The footwall The CSEF separates sedimentary rocks of the suggest that the contact may be a fault contact, contains intensely folded, light-colored, fi ne- Yakutat microplate from the Orca Group of although faulting may have locally exploited grained siltstone and argillite with little apparent the Prince William terrane in the western and

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(segment B, Fig. 11), our estimation of fault dip Mt. Augusta is not robust. In general, the dip of the CSEF in the eastern syntaxis is signiﬁ cantly shallower Mt. St. Elias 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. Kym The best-fit fault plane intersection to the CSEF trace west of Mount St. Elias suggests Ky that the fault dip is as shallow as 10°. Because of the steep topography, the fault plane for photograph B segment A (green line, Fig. 11) intersects the surface on the north side of the ridge between B SW NE Mount Huxley and the Bagley Iceﬁ eld. On the north side of Mount Huxley, fault segment A K Ky ym dips in the same direction, but less steeply than topography. Based on this geometric projection 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- ~400 m 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 Figure 10. Photographs looking north at the Libbey Glacier fault the Valdez Group and Orca Group, included in

south of Mount Augusta. Location is in Figure 3. Kym—Cretaceous regional geologic maps (Plafker, 1987; Plafker

Yakutat Group mélange; Ky—Yakutat Group fl ysch. et al., 1994; Richter et al., 2005). Our interpretation suggests that the CSEF in the eastern syntaxis area may be a partial klippe (Figs. 3, central Yakutat microplate (Fig. 1). In the east- script written by one of us (R. Bruhn, at the Uni- 4, and 11). This relationship suggests that the ern syntaxis, however, the Orca Group appears versity of Utah) to match intersections between CSEF may no longer root into its original fault to be absent and rocks of the Yakutat microplate fault planes and topography to the observed system at depth and may have been offset by are in fault contact with metamorphic rocks fault trace (Fig. 11). To estimate the robustness faults beneath the Bagley Icefi eld and upper correlated to the Valdez Group of the Chugach of the best-fi t fault planes, we varied fault dip Seward glacier (Berger et al., 2008a; Enkel- terrane (Plafker, 1987; Plafker et al., 1994). (±~10°) and fault strike (±~20°) and plotted the mann et al., 2010; Spotila and Berger, 2010). We label this fault contact the CSEF, follow- resulting intersection lines against the observed ing previous studies (e.g., Plafker et al., 1994), fault trace. We then constructed vertical sec- Faults East of the Seward Glacier Throat although it is unclear if the CSEF of the western tions spaced 1 km apart along the CSEF and and central Yakutat microplate is the same as the oriented perpendicular to the trace of the CSEF A few linear glacial valleys trend east-north- CSEF in the eastern syntaxis region. at that point. We determined point intersections east and may contain unrecognized structures The CSEF in the eastern syntaxis places between the plotted lines and the vertical sec- within the Yakutat Group mélange east of the complexly deformed amphibolite metamorphic tion plane. Next, we calculated the distance in Seward glacier throat (Fig. 3). During limited facies rocks onto the Yakutat Group mélange. meters between the point intersection of the reconnaissance mapping, we identifi ed scattered The CSEF marks one of the most distinc- estimated fault trace and the point intersection small nunataks of Yakutat Group fl ysch south of tive lithologic changes within the orogen, but of the observed CSEF trace within each vertical Mount Owens, but were unable to determine if because of the extreme topography in the east- section plane. We summed these distances for these were in fault contact beneath the Yakutat ern syntaxis region, we relied heavily on aerial each fault intersection trace, which we used as Group mélange, similar to the structural rela- photography and satellite imagery to determine an approximate measure of misfi t. We repeated tionship along the Libbey Glacier fault, or large the trace of the CSEF. In addition to moving the this procedure for each fault segment of the blocks within the mélange. We also observed trace of the CSEF higher on Mount Augusta CSEF shown in Figure 11 (A, B, and C). We a lithologic and metamorphic juxtaposition as discussed here, we assign much of the area report misfi t as a relative percent by dividing the between greenschist facies phyllites on a ridge south of Mount Huxley to the Yakutat Group misfi t for each fault trace against the summation just north of Mount Owens and zeolite facies mélange (Fig. 3) and place the CSEF closer to of the minimum misfi t and two maximum misfi t graywacke to the south within the Yakutat Group the base of Mount Huxley along a prominent traces fi t to that segment. on the northern slope of Mount Owens. Finally, topographic escarpment and spectral boundary The results suggest that in the eastern syn- the upper Seward glacier throat itself is a good identifi ed in Landsat TM imagery. taxis area, the CSEF dips 10°–30° toward 350– candidate for an unrecognized structure based The rugged topography of the eastern syn- 360 (Fig. 11). We have a high degree of confi - on the elevation difference and offset between taxis allows for a robust estimation of the three- dence in the estimated fault geometry around the Valdez Group at Mount Augusta and expo- dimensional geometry of the CSEF. We draped Mount St. Elias (segment A, Fig. 11) and Mount sures of the Valdez Group east of Mount Owens a geologic map over a 60 m digital elevation Augusta (segment C, Fig. 11); however, where along the Fairweather fault system (Fig. 3) model for the eastern syntaxis area and used a the CSEF passes beneath the Newton Glacier (Campbell and Dodds, 1982), although the

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Fault Trace Strike Dip A 270 10 slope B 270 30 0o 80o C 260 25 Bagley Icefield CSEF

Seward Glacier

Mt. Huxley

B Mt. Augusta C A Newton Glacier Mt. St. Elias Haydon Peak Seward throat

Mt. Owens

5 km

Tyndall Glacier

25 50 300 40 A B C 280

20 300 40 270 30 15 30 270 260

10 270 fault dip (°) fault dip (°) fault dip (°) 20

fault strike (°) 250 fault strike (°) fault strike (°) 20 5

240

0 240 10 240 10 10 15 20 25 30 01020304050 51015202530 percent relative misfit percent relative misfit percent relative misfit

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.

fault relationships between these two areas are ﬁ eld seasons in the Samovar Hills working on software (2DMove created by Midland Valley, covered by the Seward glacier and are unclear. detailed mapping and fault kinematic studies. www.mve.com) and projecting structural fea- We tentatively map faults at these locations and We focused our efforts on the two major fault tures onto the plane of section. Contacts, faults, emphasize the need for further study (Fig. 3). systems in the central Samovar Hills, the Hubbs and bedding traces were projected in both the Creek and Marvitz Creek faults, and on the large dip direction and along strike using a calculated SAMOVAR HILLS anticlinorium in the western Samovar Hills. The trend and plunge of various fold axes for the results of our mapping are presented as a geo- along-strike projection following the methods The Samovar Hills make up the core of the logic map in Figure 12 and as two cross sections outlined in Pavlis and Bruhn (2011). For our eastern syntaxis region and were mapped by across the Samovar Hills (Figs. 13 and 14). fault studies, we measured fault surfaces and Plafker and Miller (1957) and Plafker (1987). These cross sections were created by load- shear sense indicators directly wherever pos- We spent ﬁ ve weeks in the 2006 and 2007 ing the geologic map into structural modeling sible. For fault zones with large populations of

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140°45′W 140°40′W 140°36′W ′ 2 km B 1200 QTy Contour Interval = 200 m Upper Hubbs Creek Fault 1000

1000 2 Lower Hubbs K hcv Creek Fault y Agassiz Glacier

60°10′N

1 Qm ? 800

Hubbs Creek Fault ′ 1400 C Fig. 16 1000 Q Samovar Hills m

Hubbs Creek 4 Tol 3 K QT hcv y y 54 M 1 a 200 rv Q it ls z Marvitz C re e ? ? k F Creek C a T T u ol k lt 600 Qal

0 Mussell Creek 60 60°8′N 800 ? Ky Qal2

ake Q QTy al2 Oily L Esker Creek Fault Q al B Malaspina Fault Malaspina Glacier 60°7′N

modern river alluvium Yakataga formation 1 fault number Qal QTy fold axial trace Q stream terraces or ice-dammed T Kulthieth formation al2 lake deposits k strike and dip of bedding Q m moraines or glacial till Tol Oily Lake member K Qls landslide deposits y Yakutat Group hcv Hubbs Creek volcanics

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

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B South bend in North B′ section strike and dip data present 3 fold axial trace topography

QTy bedding traces fault

erosional Malaspina-Esker unconformity 2 Creek Fault Hubbs Creek overturned Tk Fault bedding

Agassiz Glacier ? 1 Elevation (km) T ol QTy Malaspina Glacier Marvitz Creek Fault hcv ? erosional hcv unconformity Tk 0 QT ? y Ky K ? y

T k 1 km –1 no vertical exaggeration

′ 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.

C West East C′ Hubbs strike and dip data Creek Fault present QT fold axial trace topography y 3

bedding traces fault

overturned hcv folds 2

? Ky 1

Tol

QTy Marvitz Creek Elevation (km) Fault 0

Ky Tk hcv

K Creek Fault y Esker –1 1 km ? QT Malaspina Fault y no vertical exaggeration

′ 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.

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fault planes and shear sense indicators with little shear sense. We plotted each slip direction and great circle for directional (slickenline) data. We variation, we used a simple average for overall calculated each m-pole based on the orientation attempted to ensure that our fault plane and slip fault geometry. Where fault zones were buried of the individual fault planes. We then contoured vector calculations were also consistent with or complex, populations of subsidiary faults the m-poles and calculated the best-fi t linea- fi eld observations of displacement based on were measured and paleostrain calculated using tion (β-axis). Where exact fault geometry was other geologic constraints. Fault zones that had a variation on the technique of Krantz (1988) unknown and kinematic indicators varied, we fi t subsidiary fault populations with large variation (following Bruhn and Pavlis, 1981; Bruhn et al., a plane between fault strike, as determined from in slip direction or with multiple slickenlines on 2004). These methods assume that fault slip map trace, and the β-axis to estimate the overall the fault surfaces were discarded, and data for occurs in the direction of maximum resolved fault zone orientation. The overall slip vector those fault zones are not presented here. In these shear stress and that the null-motion direction for the fault zone was assumed to occur along cases, we suspect that multiple phases of defor- (m-pole) is perpendicular to the slip vector the intersection of the estimated fault plane mation may have occurred. Best estimates of within the fault plane. For each subsidiary fault, and the great circle orthogonal to the β-axis. fault geometry and average slip vectors are dis- we measured the fault plane orientation, the slip As expected, the calculated slip vectors gener- cussed in the following and presented in Table 1 direction (slickenline orientation), and estimated ally were within a few degrees of the best-fi t and Figure 15.

A Marvitz Creek Fault Lower Hubbs Creek Fault Upper Hubbs Creek Fault Fold Axes B fold axes Yakataga Fm. slip vector Trend of theKulthieth Hubbs Creek Fm. fault conjugate Oily Lake member

β-axis slip vector main fault n = 3 fault plane n = 19 n = 13 fold-and-thrust belt

fault 1 fault 2 fault 3 fault 4 slip vector fa slickensides ult slip vector slip vector pl slip vector an e β-axis m-poles slickenlines

plane to β-axis n = 15 n = 49 n = 11 n = 18

C Yakataga Fm. Kulthieth Fm. Oily Lake mem.

n = 75 n = 103 n = 9

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).

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Hubbs Creek Fault to overturned folds in the Kulthieth Formation as the Oily Lake member of the Kulthieth For- with axial planes subparallel to the main fault mation following preliminary sedimentological The Hubbs Creek fault system separates three plane are superimposed on the eastern limb of work (Landis, 2007; Witmer et al., 2009). Local distinct fault blocks with different stratigraphic this syncline. These folds have wavelengths of gravel conglomerates are present throughout sequences: (1) a western fault block that contains ~10–30 m and interlimb angles <30°. These the Oily Lake member and have angular clasts several hundred meters of Kulthieth Formation folds are not present in the Yakataga Forma- suspended in a mudstone matrix. A thin (1–2 m) unconformably overlying Yakutat Group base- tion along the Hubbs Creek fault and may be rounded pebble conglomerate is deposited ment; (2) a fault splay between the two strands of older than the Yakataga Formation or related to unconformably on the Yakutat Group at the base the Hubbs Creek fault that exposes only vol canic mechanical differences in the stratigraphy. of the Oily Lake member (Fig. 16). rocks, the Hubbs Creek vol canics, but does not The upper Hubbs Creek fault is exposed in In the central Samovar Hills, the Oily Lake expose the base of the volcanic sequence; and a small topographic notch along the ridgeline member thins to the northeast, where it is ulti- (3) an eastern fault block that exposes only and dips steeply to the northeast with dextral mately cut out by an angular unconformity at the Yakutat Group basement (Fig. 12). The motion (Table 1; Figs. 12 and 15). Despite the the base of the Kulthieth Formation (Fig. 16). Yakataga Formation overlaps all three of these similar geometry to the lower Hubbs Creek We traced this contact to the southwest where fault blocks along the angular basal Yakataga fault, the slip vector for the upper Hubbs Creek the Oily Lake member is truncated against the unconformity. Because of these stratigraphic fault plunges 25°–30° less steeply. A low-angle, Marvitz Creek fault. Bedding within the Oily relationships, we divide the Hubbs Creek fault north-dipping thrust fault intersects the upper Lake member generally dips 40°–60° toward into the lower Hubbs Creek fault, where the Hubbs Creek fault and places the Yakataga 020–070. Several folds are present within the Hubbs Creek volcanics abut the Kulthieth For- Formation over the Hubbs Creek volcanics and Oily Lake member with axes plunging an aver- mation, and the upper Hubbs Creek fault, where Yakutat Group along strike (fault 2, Table 1; age of 14° toward 298 (Fig. 15). The strike of the fault places the Yakataga Formation against Figs. 12 and 15). The fault cuts bedding in the bedding and trend of fold axes is subparallel to itself (Fig. 12). Yakataga Formation at a low angle; this, with the strike of the Hubbs Creek fault and Mar- The lower Hubbs Creek fault is buried by the younger-on-older thrust relationship, implies vitz Creek fault, but at a high angle to bedding young sediments in a stream valley, but sub- that this fault postdates deposition and folding within the overlying Kulthieth Formation that sidiary fault populations suggest that the fault is of the Yakataga Formation. Displacement on dips 45°–50° toward 300–340. a dextral oblique thrust fault dipping northeast this low-angle fault is unknown, but the fault The Oily Lake member is also exposed in (Table 1; Fig. 15). This is in good agreement appears to tip out within ~2 km to the east (Fig. the southern Samovar Hills where it crops out with a measurement by Plafker (1987) east of 12). This fault may have locally exploited the along the north side of Marvitz Creek (Fig. 12). Hubbs Creek suggesting that the fault dips 75° basal Yakataga unconformity as a detachment. The map pattern suggests that bedding may dip toward ~035. The Hubbs Creek fault is difﬁ cult The lower plunge of the slip vector for the upper moderately southwest. However, the limited to trace farther to the southeast, however, where Hubbs Creek fault may be recording motion on exposures consist of highly fractured mudstones it projects toward a prominent, undated land- this younger, shallowly dipping fault, in which with poor indicators of bedding orientation. slide and may ultimately merge with the Esker case the slip vector for the lower Hubbs Creek Creek fault system (Fig. 12). fault may be more representative of the primary Marvitz Creek Fault The lower Hubbs Creek fault also appears to older movement history. truncate steeply dipping faults that bound the In map pattern, the basal Yakataga uncon- The Esker Creek fault bounds the southeast- Hubbs Creek volcanics (Figs. 12 and 13). We formity shows little to no offset across the ern Samovar Hills and intersects or connects identiﬁ ed a small outcrop south of the trace of Hubbs Creek fault, suggesting limited dis- with the Malaspina fault near an ~45° bend in the lower Hubbs Creek fault where the Hubbs placement since deposition of the Yakataga the topographic front of the Samovar Hills. The Creek volcanics were juxtaposed against the Formation (Fig. 12). The orientation of the slip Marvitz Creek fault splays off of the Esker Creek Yakutat Group along a subvertical fault with vector for the lower Hubbs Creek fault is com- fault system near this intersection, striking sub- almost pure dextral strike-slip motion (fault 1, parable to the regional dip and dip direction of parallel to the trace of the Hubbs Creek fault Table 1; Fig. 15). We did not directly observe bedding in the Yakataga Formation (25°–40° (Fig. 12). To the south, the Marvitz Creek fault the fault bounding the eastern side of the Hubbs toward 310–360). Structural restoration to a juxtaposes Yakutat Group basement and the Oily Creek volcanics, but traced the lithologic con- time equivalent with the basal Yakataga uncon- Lake member against the Kulthieth Formation; tact to the boundary with the overlying Yakataga formity removes the regional, northward dip of however, displacement decreases toward the Formation where it intersects a younger fault the Yakataga Formation in the hanging wall of north where the Marvitz Creek fault splits into zone (discussed herein). Along the projected the Malaspina–Esker Creek fault system and multiple fault strands (Figs. 12 and 14). trace of the eastern bounding fault to the Hubbs reduces the plunge of the slip vector on the Although the Marvitz Creek fault is obscured Creek volcanics, bedding within the Yakataga lower Hubbs Creek fault, increasing the strike- by young river gravel along much of its trace, we Formation contained minor dextral offset slip component of motion. observed a small segment of fault rocks re lated (<15 m), possibly as a result of limited reactiva- to the structure on the west side of Marvitz tion of this fault since deposition of the Yakataga Oily Lake Member Creek. The fault dips steeply to the southwest Formation. and displays apparent normal displacement of A syncline that plunges 38° toward 323 in the Between the Hubbs Creek fault and the Mar- Kulthieth Formation over the Oily Lake mem- Kulthieth Formation and 37° toward 341 in vitz Creek fault the Yakutat Group basement ber (Table 1). The steeply dipping fault may the Yakataga Formation is located in the foot- is overlain by 100–150 m of green to maroon, have originated as a reverse fault that was tilted wall of the Hubbs Creek fault, suggesting that mottled, tuffaceous mudstone and siltstone (Fig. by younger folding. The fault strikes to the folding postdates the basal Yakataga uncon- 12) that we correlate to the siltstone of Oily Lake north where it dies out along the Kulthieth For- formity. Adjacent to the fault, a series of upright mapped by Plafker (1987). We refer to this unit mation–Oily Lake member contact. We did not

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the Kulthieth Formation is subparallel to the West East 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 erosional unconformity on the southern limb of the anticline (Fig. 14). This observation not only suggests that defor- Tk 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 Tol Formation to a time equivalent with the basal Yakataga unconformity yields an approximate restoration to a gentle anticline in the Kulthieth erosional unconformity Formation plunging 10°–20° toward the west with a forelimb dipping ~20° to the south and a backlimb dipping 10°–15° to the north (interlimb angle of 145°–150°). Toward the core of the anticlinorium, bedding Ky in the Kulthieth Formation steepens and a series of parasitic (wavelengths of ~1 km) overturned ~ 10 m folds plunge 30°–65° toward the north and west (Plafker, 1987) (Fig. 13). We did not observe any angular unconformities at this stratigraphic Figure 16. Photograph looking north at the basal Yakataga unconformity in the Samovar level of the Kulthieth Formation or any sig- Hills. Location is in Figure 12. T —Tertiary Kulthieth Formation; T —Oily Lake member; k ol nifi cant faults that cut bedding. We observed K —Cretaceous Yakutat Group fl ysch. y pervasive bedding-parallel detachments along coal and shale layers and evidence for fl ow of these strata into overthickened hinge zones. observe any offset of bedding within the Kul- Samovar Hills Anticlinorium We suspect that a roof detachment or series of thieth Formation to the northwest. detachment surfaces separates the overturned We also measured subsidiary fault popu- Perhaps the most prominent structural feature parasitic folds and the more modestly folded lations at two fault splays from the Marvitz of the Samovar Hills is a large (~10 km wide) bedding higher in the Kulthieth Formation, near Creek fault that bound a small exposure of the box anticline in the Yakataga Formation that the basal Yakataga unconformity. Similar struc- Hubbs Creek volcanics. The western splay fault plunges 30° toward 270 (Plafker, 1987; Bruhn tures are common in the coal-bearing Kulthieth emplaces the Hubbs Creek volcanics against the et al., 2004; this study). The fold is consistent strata throughout the YFTB. At the base of the Oily Lake member (fault 3, Table 1; Fig. 15). with a hanging-wall anticline above the Mala- overturned folds, exposed near the ridgeline We could not follow this fault to the north within spina thrust fault with a steeply dipping fore- on the west side of Marvitz Creek, there is a the Oily Lake member due to poor exposure, limb (~80°) and a more moderately dipping major detachment, indicating that the parasitic and we did not observe any signifi cant offset backlimb (30°–40°) (Figs. 13 and 14). Res- folds are intraformational detachment folds. We of the angular unconformity at the base of the toration of the cross section in Figure 13 sug- tentatively correlate this detachment zone to Kulthieth Formation. The eastern splay fault is gests >3 km of slip across the Malaspina fault another bedding-parallel detachment at a simi- also poorly exposed, but juxtaposes the Yaku- to construct the anticlinorium. The thickness of lar stratigraphic interval between Marvitz and tat Group and the Hubbs Creek volcanics and the Yakataga Formation in the footwall of the Hubbs Creek (Figs. 12–14). The location of the appears to be a nearly vertical, dextral strike-slip Malaspina fault shown in Figure 13 is unknown, overturned folds within the core of the Samo- fault (fault 4, Table 1; Fig. 15). This fault did not but may be an underestimate based on the Riou var Hills anticlinorium and near the projected offset the base of the Oily Lake member (Fig. Bay #1 well, drilled in the footwall of the Mala- intersection of the axial planes to the box fold in 12) and may predate the main Marvitz Creek spina fault in the Chaix Hills, that encountered the Yakataga Formation suggests that much of fault. Plafker (1987) and Plafker et al. (1994) >4 km of Yakataga Formation (Fig. 4). Within the deformation at this stratigraphic and struc- suggested that the contact between the Hubbs the Yakataga Formation, dip domains in the box tural level may be related to tightening of the Creek volcanics and Yakutat Group at the Mar- anticline are sharply divided by kink folds; how- anticlinorium after deposition of the Yakataga vitz Creek locality is depositional, a relationship ever, the expression of the anticline in the deeper Formation. supported by well data to the southeast (Plafker, structural and stratigraphic levels has remained Numerous upright folds in the lower Kul- 1971; 1987; Rau et al., 1983). The fault rocks unclear. As part of an effort to resolve the gen- thieth Formation that plunge 15°–25° toward we observed along this contact may be related eral structure of the Samovar Hills, we mapped the west are in a complex area structurally to minor reactivations of an originally deposi- extensively the Kulthieth Formation and the and stratigraphically beneath the detachment tional contact. Between the two splay faults, the center of the anticline. zone at the base of the overturned folds. The Hubbs Creek volcanics are in depositional con- Just below the basal Yakataga unconformity decrease in the plunge of the folds compared tact with the Oily Lake member. in the box anticline, the strike of bedding in to the overturned folds and the box anticline

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in the Yakataga Formation is indicative of an Yakutat microplate, consistent with models for Basal Yakataga Unconformity overall decrease in plunge of the Samovar the tectonic evolution of the Yakutat microplate Hills anticlinorium along trend to the east. The as it was translated northward along the North The precise age of the base of the Yakataga largest folds have wavelengths to 150 m, with American margin (Plafker, 1987; Pavlis et al., Formation in the eastern syntaxis is uncertain many superimposed smaller folds (wavelengths 2004) and with the observation of a long-lived because the age of the Yakataga Formation <20 m) (Fig. 13). Locally, bedding-parallel strike-slip southwestern boundary to the Yaku- varies across the orogen, generally decreasing in detach ments may exist structurally beneath tat microplate, the offshore Transition fault age from west to east (Plafker et al., 1994). The these smaller folds, including along the Marvitz (Christeson et al., 2010). oldest recorded age from outcrops at Icy Bay is Creek fault, but we did not observe detachments Series of open folds with axial planes subpar- Early Pliocene (ca. 3.0–3.5 Ma), although the at the base of the Kulthieth Formation west of allel to the Hubbs Creek fault and Esker Creek base of the section is not exposed (Lagoe and Marvitz Creek, where the Kulthieth Formation fault are present within the Oily Lake member Zellers, 1996). At Yakataga Reef in the central is separated from the Oily Lake member by an (Figs. 12 and 15). Southeast of the Samovar YFTB and in offshore industry wells south- angular unconformity. Hills, the Hubbs Creek fault connects with the west of Icy Bay, the base of the Yakataga For- Beneath the unconformity, near the center of Esker Creek fault, which is a forethrust asso- mation is latest Miocene to earliest Pliocene the Samovar Hills anticlinorium, the Oily Lake ciated with the transpressional margin in the in age (ca. 5–6 Ma) (Arnaud, 2010; Lagoe and member contains a series of tight, overturned eastern Yakutat microplate (Bruhn et al., 2004; Zellers, 1996; Zellers, 1995), probably close to folds verging southwest with fold axes plunging Plafker and Thatcher, 2008) (Fig. 3). The folds in the age of the base of the Yakataga Formation 10°–15° toward the northwest (Fig. 12). These the Oily Lake member record shortening subper- in the eastern syntaxis area. The basal Yakataga folds are subparallel to the northern Marvitz pendicular to these forethrusts and may indicate unconformity is an angular unconformity above Creek fault and may be related to strain accu- the development or continuation of transpres- folded Middle Eocene rocks of the Kulthieth mulated across the Marvitz Creek fault zone. sional to northeast-convergent deformation. Formation in the Samovar Hills. Thus, in the We suggest that the Marvitz Creek fault, the We suggest that this deformation may partially eastern syntaxis area, the unconformity cuts tight folds in the Oily Lake member adjacent to record the northwest translation of the Yakutat out Middle Eocene to latest Miocene–earli- the Marvitz Creek fault, and shortening within microplate along the southeast Alaska margin est Pliocene strata. In the central and western the core of the Samovar Hills anticlinorium are by the right-lateral Contact, Fairweather, and YFTB a more complete stratigraphic section is interrelated. Queen Charlotte fault systems. The folds in the present and the Yakataga Formation commonly Oily Lake member are truncated by the angular conformably overlies older strata, including the DISCUSSION unconformity at the base of the Kulthieth Forma- Late Eocene to Early Miocene Poul Creek For- tion (Figs. 12 and 16). The Kulthieth Formation mation (Plafker, 1987; Brennan et al., 2009), History of Deformation in the Samovar Hills is late Early Eocene to early suggesting that the erosional and deformational Middle Eocene in age (Plafker, 1987), constrain- event related to the basal Yakataga uncon formity The eastern syntaxis region and the Samo- ing the deformation in the Oily Lake member to in the eastern syntaxis occurred between the var Hills in particular record the clearest record the Early to Middle Eocene. The unconformity Early Miocene and the latest Miocene to earli- of the three-dimensionality in the deformation at the base of the Kulthieth Formation is likely est Pliocene. Because the Yakutat microplate history for the Yakutat microplate. This history related to this deformational event. converges obliquely with North America, this includes an early strike-slip to oblique transpres- Following deposition of the Kulthieth Forma- deformation event may be time transgressive. sional environment as the Yakutat microplate tion, the record of deformation in the Yakutat In this scenario, we speculate that deformation traveled northward along the North American microplate expands dramatically. In the Samo- may have progressed west to east, as the Yakutat margin, complex structural overprints as these var Hills, the Kulthieth Formation is folded microplate was translated northwestward into transpressional structures were translated into within the Samovar Hills anticlinorium and jux- the southern Alaskan margin. the Alaskan eastern syntaxis region, and waning taposed with the Hubbs Creek volcanics along In addition to constraining the timing of fold- transpressive deformation and the initiation of the Hubbs Creek fault. Because the Yakataga and-thrust belt development in the Samovar fold-and-thrust style deformation. Formation is incorporated into the Samovar Hills area, the basal Yakataga unconformity In the Samovar Hills, the Hubbs Creek vol- Hills anticlinorium, but the basal Yakataga records strain accumulation in the YFTB prior canics are in fault contact with the Yakutat unconformity is minimally offset by the Hubbs to the deposition of the Yakataga Formation. Group along a small fault segment intersecting Creek fault, we propose that displacement along West of the Tyndall Glacier in the northern Karr the Marvitz Creek fault and on the east side of the Hubbs Creek fault preceded development of Hills (Fig. 6) and in the Samovar Hills anticlino- Hubbs Creek (Fig. 12). At Hubbs Creek, dex- the anticlinorium (Fig. 12). At the least, devel- rium, the development of fold-and-thrust struc- tral strike-slip movement along the fault bound- opment of the anticlinorium continued into tures predates the Yakataga Formation. Despite ing the east side of the Hubbs Creek volcanics Yakataga Formation time, whereas motion on an angular unconformity reported at the base of occurred before deposition of the Yakataga For- the Hubbs Creek fault had essentially ended. the Yakataga Formation in parts of the central mation. The fault segment intersecting the Mar- This observation is an important structural YFTB (Miller, 1971; Plafker, 1987; Wallace, vitz Creek fault is truncated to the north by an relationship that locally demonstrates the end 2008), previous studies have generally assumed unconformity at the base of the Early to Middle of deformation associated with a transpres- the initiation of fold-and-thrust belt–style defor- Eocene (ca. 42.5–49.5 Ma; McDougall, 2007) sional system in the Hubbs Creek–Esker Creek mation and deposition of the Yakataga Forma- Oily Lake member, placing relatively precise fault system and the initiation and growth of tion were roughly coincident (Eyles et al., 1991; timing constraints on the slip observed along the YFTB associated with the Samovar Hills Plafker et al., 1994; Pavlis et al., 2004; Meigs this segment. Although scant, the available evi- anticlinorium and Malaspina fault. The timing et al., 2008; Berger et al., 2008b; Chapman dence suggests a very early history of dextral of this transition is constrained by the basal et al., 2008; Wallace, 2008). This assumption strike-slip deformation at this point within the Yakataga unconformity. may be good in the western to central YFTB,

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but the assumption is less appropriate toward St. Elias Orogenic System of the Esker Creek fault and Fairweather fault the eastern YFTB because deposition of the become increasingly perpendicular to the direc- Yakataga Formation was diachronous. There is evidence for active slip only on the tion of current Yakutat microplate motion as they The presence of deformation prior to deposi- Malaspina fault (Elliott et al., 2010; Estabrook connect with the Malaspina fault and Bagley- tion of the basal Yakataga Formation has con- et al., 1992) among the major north-dipping Contact fault system, respectively, in the eastern siderable implications for reconstructing the thrust faults within the eastern YFTB. AHe ages syntaxis area (Fig. 3). This change in orientation structural history of the YFTB and reconciling decrease slightly to the south across the Chaix corresponds to an increase in the width of the the present rate of deformation with the total Hills fault, although this decrease is not statis- YFTB toward the west and creates a restrain- observed shortening. Plafker et al. (1994) noted tically signifi cant as an indicator of fault slip ing bend along the Fairweather-Contact fault 45% shortening in the Paleogene compared (Berger et al., 2008b). AHe age pairs suggest system in the northern eastern syntaxis (Fig. 1). to 25% shortening in the Neogene in the cen- that the Libbey Glacier fault may not be cur- We suggest that both the Bagley-Contact fault tral YFTB; in Chapman et al. (2008), >75 km rently active (Berger et al., 2008b). We suggest system and the Malaspina fault accommodate shortening was reported in the Paleogene strata that the CSEF may be a partial klippe that no an increased amount of convergence across the compared to ~25 km shortening in the Yakataga longer roots into a deeper fault system (Figs. 4 eastern syntaxis as their orientations change. Formation alone. In Chapman et al. (2008), it and 11), indicating that the CSEF is probably Structural style changes across the orogenic was assumed that the disparity between defor- not active. The position of the CSEF at high wedge, with folding increasing up structural sec- mation recorded in the Paleogene and Neogene elevation on the steep southern fl anks of Mount tion. Folds at the toe of the wedge in the Mala- sections was related to either duplexing of the St. Elias and Mount Augusta also suggests that spina thrust sheet are generally open, upright, Kulthieth Formation and erosion of the Yakataga the CSEF is not active (Figs. 3 and 4). Although and have relatively long wavelengths of as much Formation above the structurally higher thrust the exact mechanisms remain unclear, active as a few kilometers. Folds in the Kulthieth For- sheets (e.g., Meigs et al., 2008), or nondepo- shortening within the eastern St. Elias oro- mation within the Chaix Hills and Libbey Gla- sition of the Yakataga Formation on what are genic wedge appears to be concentrated at toe cier thrust sheets have shorter wavelengths of now the structurally higher thrust sheets. How- of the wedge, at the Malaspina fault, and hundreds of meters and are generally asymmet- ever, the discrepancies in shortening between be neath the Bagley Icefi eld and upper Seward ric, verging to the south with steep to overturned the Paleogene section and Yakataga Formation glacier along the Bagley-Contact fault system. southern limbs. In the structurally highest thrust could also be related to deformation in the Early This spatial relationship forms a narrow oro- sheets, the Yakutat Group mélange is intensely to Late Miocene, as indicated by eroded struc- genic wedge with a cross-sectional width of as folded, often with chaotic bedding, although tures beneath the basal Yakataga unconformity. little as 35 km and a basal décollement dipping much of this structural complexity is probably The level of erosion recorded by the basal 5°–10° toward the north at 12–16 km depth. The inherited from its deposition and deformation Yakataga unconformity, the amount of shorten- surface slope of the wedge changes across the within an accretionary complex. ing within the Kulthieth Formation, and the Libbey Glacier fault, with an average slope of degree of structural discordance across the basal 15° north of the fault and an average slope of 2° CSEF Yakataga unconformity provide indirect evi- in the thrusted sedimentary cover south of the dence that deformation may have signifi cantly Libbey Glacier fault (Fig. 4). In the eastern syntaxis, the CSEF places preceded deposition of the Yakataga Formation Many researchers suggest that the Malaspina amphibolite facies rocks of the Valdez Group on in the eastern syntaxis area. However, if the cur- fault may be the youngest part of the YFTB the low-grade to unmetamorphosed Cretaceous rent high rates of convergence across the Yakutat because of the narrow width of the eastern and Tertiary rocks of the Yakutat microplate microplate extended into Miocene, much of this YFTB, active slip on the Malaspina fault, the (Fig. 3) (Plafker et al., 1994). The presence of deformation could have occurred within only a position of the Malaspina fault at the eastern end these high-grade metamorphic rocks has several few million years prior to the deposition of the of the YFTB, and because the Malaspina fault implications for the structure of the St. Elias Yakataga Formation. Additional work is needed represents the deformational limit to the eastern orogen in the eastern syntaxis area. The Con- to more precisely constrain the timing and YFTB (Plafker et al., 1994; Bruhn et al., 2004; tact fault forms the southern boundary to the amount of shortening in the eastern syntaxis area. Chapman et al., 2008). Therefore, the evidence Chugach terrane and Valdez Group (Plafker, Reducing the total shortening recorded in for signifi cant contraction on the Malaspina 1987); however, the Contact fault is located the YFTB since the onset of deposition of the fault in pre-Yakataga time and perhaps even north of the high-grade rocks of Valdez Group Yakataga Formation in the latest Miocene pre-Kulthieth time as recorded by deformation where it connects with the Fairweather fault decreases estimates of the rate of shortening over in the Samovar Hills is surprising. The long- beneath the upper Seward glacier (Fig. 1). We that time period. This exacerbates an already lived record of contraction on the Malaspina propose that the low-angle CSEF in the eastern unresolved mismatch between current rates of fault and structural overprinting in the Samovar syntaxis is an erosional remnant of the Contact convergence and total observed shortening from Hills may indicate that the eastern syntaxis has fault–Fairweather fault system preserved at high cross-section restorations (Worthington et al., remained fi xed over time, consistent with geo- elevation with Valdez Group strata in its hang- 2010; Chapman et al., 2008; Meigs et al., 2008; dynamic models for the St. Elias orogen (Koons ing wall. Plafker et al. (1994) suggested that Wallace, 2008). Structural restoration of the et al., 2010). these rocks were translated into their present Yakataga Formation in Figure 4 yields >17 km To the southeast of the eastern syntaxis area, position along the Fairweather fault. of shortening and, when combined with the the orogenic margin is slightly transpressive If the CSEF in the eastern syntaxis is an ero- range of ages for the Yakataga Formation, sug- with dextral strike slip partitioned on the Fair- sional remnant of the Contact fault–Fairweather gests a minimum shortening rate of ~2–6 mm/yr. weather fault and dip slip partitioned onto fore- fault system, as we propose, it raises questions This rate is an order of magnitude less than GPS thrusts such as the Esker Creek fault (McAleer concerning the CSEF in the central and west- derived shortening rates of >3.5 cm/yr across the et al., 2009; Plafker and Thatcher, 2008; Elliott ern Yakutat microplate. Specifi cally, how does eastern YFTB (Elliott et al., 2009, 2010). et al., 2010; Bruhn et al., 2004). The orientations the CSEF of the western and central YFTB

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connect into the fault systems of the eastern CG syntaxis? Also, how do Prince William ter- BRF North American rane rocks between the CSEF and the Bagley- IT Contact Fault Contact faults to the west relate structurally to Plate 60° N Denali Fault CSEF ? the Yakutat mélange in the eastern syntaxis? PWT These are essential questions that will need to MF ECF BRF be addressed in further research. Yakutat The high metamorphic grade at high eleva- microplate tion and young AHe and ZHe ages for the Val- dez Group in the St. Elias area (Berger et al., Transition Fault Chatham S 2008b; Enkelmann et al., 2010) indicate signifi cant tectonic exhumation. The geometry of Aleutian Trench Fairweather Fault the dextral Fairweather fault to Contact fault trait Fault transition in the eastern syntaxis area forms a Pacific Plate restraining bend that is also consistent with tectonic exhumation (Figs. 1 and 3). It is unclear if 250 km this exhumation may be related to slip along the south-dipping Bagley backthrust, as proposed 55° N in Berger et al. (2008a), structural thickening of the Yakutat Group at depth south of the Con- 145° W 140° W 135° W tact fault, or some combination of both. The Chugach terrane exposed at the surface north Figure 17. Regional fault and terrane map. CG—Chugach ter- of the Bagley Icefi eld and upper Seward glacier rane; PWT—Prince William terrane; IT—Insular terrane; CSEF— displays a lower metamorphic grade and has Chugach St. Elias fault; MF—Malaspina fault; ECF—Esker Creek produced older AHe ages (Berger et al., 2008b), fault; BRF—Border Ranges fault. suggesting signifi cantly less tectonic exhumation north of the Bagley-Contact fault. The results of our research in the Samovar plete, albeit deformed, section of the Kulthieth Implications of the Yakutat Group in the Hills suggest that the Hubbs Creek and Mar- Formation with younger stratigraphic levels Eastern Syntaxis vitz Creek faults may not be part of the DRZ exposed along strike to the west (Fig. 3). Addi- as defi ned by Plafker (1987). The Yakutat tional subsurface information is required to It is generally accepted that the Yakutat Group underlies the Kulthieth and Yakataga determine a western limit to the Yakutat Group microplate was excised from the Cordilleran Formations west of the Hubbs Creek fault and at depth within the Chaix Hills thrust sheet. margin and subsequently carried northward; is present within the core of the Samovar Hills The Yakutat Group is also exposed within however, there is considerable debate concern- anticlinorium in the hanging wall of the Mala- the Libbey Glacier thrust sheet and structurally ing where it originated. The most specifi c res- spina fault (Fig. 13). The Marvitz Creek fault higher, unnamed thrust sheets in the Mount St. toration of the Yakutat microplate proposes that abruptly loses displacement and tips out near Elias region. Our analysis revises the position the DRZ is a displaced segment of the Chatham the center of the Samovar Hills anticlinorium; of the CSEF, placing it higher along the south- Strait fault (Plafker, 1987; Plafker et al., 1994) we propose that this is related to the formation ern fl ank of Mount St. Elias and Mount Huxley. (Fig. 17). The Chatham Strait fault is a dextral of the anticlinorium. This provides evidence Here, a low-angle thrust separates high-grade strike-slip fault linking the Denali fault and that the Marvitz Creek fault did not have sig- metamorphic rocks of the St. Elias massif to the Queen Charlotte fault system and is located nifi cant displacement after deposition of the north from rocks of dramatically different meta- ~600 km southeast of the Samovar Hills. The Kulthieth Formation, but it does not rule out the morphic grade and composition to the south, northern suture of the Chugach accretionary possibility that this fault represents a reactivated including the Yakutat Group. This relationship complex, the Border Ranges fault, is truncated older basement fault. The Samovar Hills anti- suggests that basement exposures south of by the Chatham Strait fault, suggesting a total clinorium plunges to the west within the Mala- Mount St. Elias are not part of the North Ameri- post-Cretaceous displacement of ~150–200 km spina thrust sheet and shows no discontinuity can backstop. This interpretation of these rock (Hudson et al., 1982; Plafker, 1987; Gehrels, with the observed structure in the Chaix Hills. assemblages in the Libbey Glacier thrust sheet 2000). Plafker et al. (1994) suggested that in No evidence exists to indicate that the Hubbs places Yakutat Group strata >40 km west of the Paleocene to Eocene time, the Chatham Strait Creek and Marvitz Creek faults mark the DRZ Samovar Hills (Fig. 3). Our observations did not fault cut through previously accreted terranes, or to determine how much farther westward allow us to determine how much further west the accretionary complex, and into the Kula or the Yakutat Group is present in the subsurface the Yakutat Group is present in the Libbey Gla- Pacifi c plate to juxtapose the Yakutat Group and between the Chaix Hills and Malaspina–Esker cier thrust sheet. oceanic crust across the fault. Plafker (1987) Creek faults. Our observations of the Yakutat Group in proposed that motion on the Fairweather fault The Yakutat Group–sedimentary cover con- the eastern syntaxis raise questions about the initiated in the Oligocene, trapping this segment tact dips to the northwest within the Chaix Hills relationship between the Chatham Strait fault of oceanic crust and continental crust (Plafker, thrust sheet, broadly similar to the westward and the DRZ. If the DRZ is a lithologic bound- 1987; Plafker et al., 1994), and cited the Hubbs plunge direction of the Samovar Hills anti- ary between Yakutat Group to the east and Creek fault and Marvitz Creek fault in the Samo- clinorium in the Malaspina thrust sheet. This basaltic basement to the west, as proposed by var Hills as a possible exposure of the DRZ. orientation preserves a stratigraphically com- Plafker (1987) and Plafker et al. (1994), then the

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predicted length of the DRZ is ~275 km from may be structurally thickened in the subsur- ACKNOWLEDGMENTS the Transition fault offshore to the Samovar face. The contact between the Kulthieth For- Hills (Fig. 1). The predicted length of the DRZ mation and Yakutat Group is exposed in the This research was supported by the St. Elias Ero- sion and Tectonics Project (STEEP) through National increases to >325 km by including the exposures Chaix Hills thrust sheet north of the Samovar Science Foundation grants EAR-0409009 to Pavlis, of the Yakutat Group within the Libbey Glacier Hills, and the Libbey Glacier thrust sheet car- EAR-0408959 to Bruhn, and EAR-0408584 to Gulick. thrust sheet. These lengths do not include a res- ries Yakutat Group strata where exposed at Hay- Adam Barker provided valuable assistance in the fi eld. toration of the fold-and-thrust belt or consider don Peak and Mount Augusta. These locations Reviews by Wes Wallace, Jean-Daniel Champagnac, and Eva Enkelmann signifi cantly improved the manu- shortening within the Yakutat Group. If oceanic extend the estimates for the length of the DRZ script. This is UTIG (University of Texas Institute for basalts were emplaced next to the Yakutat to >325 km. In the Samovar Hills, the Marvitz Geophysics) contribution 2431. Group along the DRZ by dextral movement on Creek fault forms a splay off the Malaspina– the Chatham Strait fault, then we would expect Esker Creek fault system that abruptly loses dis- REFERENCES CITED at least 275 km of slip on the Chatham Strait placement within the core of the Samovar Hills fault, signifi cantly more than the documented anticlinorium. The Hubbs Creek fault records Addicott, W.O., and Plafker, G., 1971, Paleocene mollusks from the Gulf of Alaska Tertiary Province—A signifi cant displacement. The problem is exacerbated by older strike-slip motion, but does not bound the new occurrence on the North Pacifi c Rim: U.S. Geologi- the presence of the Admiralty Island volcanics Yakutat Group to the west. The presence of cal Survey Professional Paper 750-B, p. B48–B52. (25–27 Ma), which are offset ~100 km along the Yakutat Group at high structural levels in the Amato, J.M., and Pavlis, T.L., 2010, Detrital zircon ages from the Chugach terrane, southern Alaska, reveal multi- the Chatham Strait fault (Hudson et al., 1982; eastern syntaxis area casts doubt on the genetic ple episodes of accretion and erosion in a subduction Ford et al., 1996; Loney et al., 1967). The offset link between the DRZ and the Chatham Strait complex: Geology, v. 38, p. 459–462, doi: 10.1130 /G30719.1. of these volcanics suggests that at least half of fault, and the Marvitz Creek fault and Hubbs Arnaud, E., 2010, Onset of late Cenozoic glaciation in the total post-Cretaceous displacement on the Creek fault in the Samovar Hills do not appear the Gulf of Alaska: Geological Society of America Chatham Strait fault may have occurred after to be related to the Chatham Strait fault. The Abstracts with Programs, v. 42, no. 5, p. 361. Bauer, M.A., Pavlis, G.L., Landes, M., and Hansen, R., the Yakutat microplate was excised from the Chatham Strait fault is a key constraint in some 2008, Crustal and lithospheric thickness variations for margin (Plafker et al., 1994). tectonic reconstructions of the Yakutat micro- central Alaska and the Chugach–St. Elias region from plate (Plafker, 1987; Plafker et al., 1994). Relax- P and S receiver functions [abs.]: Eos (Transactions, American Geophysical Union), v. 89, abs. T53B–1945. CONCLUSIONS ing this constraint allows for a greater transport Berger, A.L., and Spotila, J.A., 2008, Denudation and defor- distance of the Yakutat microplate; however, it mation in a glaciated orogenic wedge: St. Elias orogen, Alaska: Geology, v. 36, p. 523–526, doi: 10.1130 The eastern syntaxis of the St. Elias oro- does not preclude the possibility that the Yaku- /G24883A.1. gen contains at least four major thrust faults, tat microplate originated along the southeastern Berger, A.L., Spotila, J.A., Chapman, J.B., Pavlis, T.L., including the Malaspina fault, Chaix Hills fault, Alaska margin. Enkelmann, E., and Buscher, J., 2008a, Architecture, kinematics, and exhumation of a convergent wedge: A Libbey Glacier fault, and the CSEF. We have We conclude that fold-and-thrust–style defor- thermochronological investigation of tectonic-climatic revised previous interpretations of the trace mation began in the Early to latest Miocene in interactions within the central St. Elias orogen, Alaska: of the CSEF, which is the Yakutat microplate the eastern syntaxis area. The basal Yakataga Earth and Planetary Science Letters, v. 270, p. 13–24, doi: 10.1016/j.epsl.2008.02.034. suture, to higher along the south and east fl ank unconformity, observed throughout the eastern Berger, A.L., Gulick, S.P., Spotila, J.A., Jaeger, J.M., Chap- of Mount Augusta and higher along the south YFTB, cuts out Middle Eocene to latest Mio- man, J.B., Lowe, L.A., Pavlis, T.L., Ridgway, K.R., Willems, B., and McAleer, A., 2008b, Quaternary fl ank of Mount St. Elias and Mount Huxley. cene strata. 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