Transcript
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Focused rock uplift above the subduction décollement at Montague and Hinchinbrook Islands, Prince William Sound, Alaska

Kelly M. Ferguson1, Phillip A. Armstrong1, Jeanette C. Arkle1,* and Peter J. Haeussler2

1Geological Sciences, California State University Fullerton, 800 N. State College Boulevard, Fullerton, California 92834, USA2U.S. Geological Survey, 4210 University Drive, Anchorage, Alaska 99508, USA

ABSTRACT

Megathrust splay fault systems in accre-tionary prisms have been identifi ed as con-duits for long-term plate motion and sig-nifi cant coseismic slip during subduction earthquakes. These fault systems are impor-tant because of their role in generating tsu-namis, but rarely are emergent above sea level where their long-term (million year) history can be studied. We present 32 apatite (U-Th)/He (AHe) and 27 apatite fi ssion-track (AFT) ages from rocks along an emergent megathrust splay fault system in the Prince William Sound region of Alaska above the shallowly subducting Yakutat microplate. The data show focused exhumation along the Patton Bay megathrust splay fault system since 3–2 Ma. Most AHe ages are younger than 5 Ma; some are as young as 1.1 Ma. AHe ages are youngest at the southwest end of Montague Island, where maximum fault displacement occurred on the Hanning Bay and Patton Bay faults and the highest shoreline uplift occurred during the 1964 earthquake. AFT ages range from ca. 20 to 5 Ma. Age changes across the Montague Strait fault, north of Montague Island, sug-gest that this fault may be a major structural boundary that acts as backstop to deforma-tion and may be the westward mechanical continuation of the Bagley fault system back-stop in the Saint Elias orogen. The regional pattern of ages and corresponding cooling and exhumation rates indicate that the Mon-tague and Hinchinbrook Island splay faults, though separated by only a few kilometers, accommodate kilometer-scale exhumation above a shallowly subducting plate at million year time scales. This long-term pattern of

exhumation also refl ects short-term seismo-genic uplift patterns formed during the 1964 earthquake. The increase in rock uplift and exhumation rate ca. 3–2 Ma is coincident with increased glacial erosion that, in combi-nation with the fault-bounded, narrow width of the islands, has limited topographic devel-opment. Increased exhumation starting ca. 3–2 Ma is interpreted to be due to rock uplift caused by increased underplating of sedi-ments derived from the Saint Elias orogen, which was being rapidly eroded at that time.

INTRODUCTION

Flat-slab subduction and collision of the Yakutat microplate have had a profound effect on southern Alaskan geology for the past ~24 m.y. (e.g., Haeussler, 2008). Deforma-tion from this interaction has penetrated as far as ~900 km inland, from the Brooks Range in the north (O’Sullivan et al., 1997a, 1997b) to the Saint Elias Mountains in the southeast. Flat-slab subduction of the Yakutat microplate has resulted in slip and deformation along sev-eral fault systems throughout the region (Fig. 1), including faults that splay off the subduction megathrust (e.g., Plafker, 1967; Bruhn et al., 2004; Haeussler et al., 2011; Liberty et al., 2013). Megathrust splay faults elsewhere in the world develop in accretionary prisms at outer ridges that fl ank the deformation front in sub-duction settings (e.g., Kame et al., 2003; Ikari et al., 2009). Some megathrust splay faults have been identifi ed as conduits for long-term plate motion and signifi cant coseismic slip dur-ing subduction earthquakes (Park et al., 2002; Kame et al., 2003, Moore et al., 2007; Ikari et al., 2009). These megathrust splay faults can be a source of tsunami generation during large megathrust ruptures, because they are typically located offshore in deep water.

Thermochronometers allow us to place million-year time-scale constraints on the exhu-

mation history of an area and to gain insight into structural systems, such as megathrust splay faults, as they accommodate the vertical transport of rock. Numerous studies using low-temperature thermochronology have focused on exhumational patterns across major fault systems associated with fl at-slab subduction in southern Alaska, including studies in the Alaska Range (e.g., Fitzgerald et al., 1995; Haeussler et al., 2008, 2011; Benowitz et al., 2011, 2012, 2013), Chugach Mountains (Little and Naeser , 1989; Buscher et al., 2008; Arkle et al., 2013), and Saint Elias Mountains (e.g., Berger et al., 2008a, 2008b; Berger and Spotila, 2008; Meigs et al., 2008; Enkelmann et al., 2008, 2009; Spotila and Berger, 2010). Some of these stud-ies detected loci of rapid exhumation, particu-larly in the Saint Elias and western Chugach Mountains, which may be the result of crustal-scale lithologic backstops to upper crustal rock deformation above the subducting Yakutat microplate.

This study targets the southern Prince Wil-liam Sound region (Fig. 1), located on the over-riding North American plate closest to the Aleu-tian Trench and ~20 km above the mega thrust décollement. Seismic imaging and thermal-mechanical models show that there is a large degree of coupling and/or underplating between the subducting Yakutat microplate and the over-riding North American plate (Brocher et al., 1991; Ratchkovski and Hansen, 2002; Zweck et al., 2002; Ferris et al., 2003; Eberhart-Phillips et al., 2006; Fuis et al., 2008) below Prince William Sound, making this area susceptible to large (moment magnitude, Mw > 8.0) earth-quakes like the 1964 Mw 9.2 Alaska earthquake (Plafker, 1965). Evidence that this region is actively accommodating deformation is shown by the tectonic analysis of ground breakage and surface warping during the 1964 earthquake on Montague and Hinchinbrook Islands (Plafker, 1967) in southern Prince William Sound. This study expands on Plafker’s (1967) original

For permission to copy, contact [email protected]© 2014 Geological Society of America

Geosphere; February 2015; v. 11; no. 1; p. 144–159; doi:10.1130/GES01036.1; 9 fi gures; 1 table; 1 supplemental fi le.Received 1 February 2014 ♦ Revision received 15 September 2014 ♦ Accepted 27 October 2014 ♦ Published online 22 December 2014

*Current address: Department of Geology, Uni-versity of Cincinnati, P.O. Box 0013, Cincinnati, Ohio 45221, USA.

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Geosphere, February 2015 145

geologic analyses by using apatite (U-Th)/He (AHe) and fi ssion-track (AFT) thermochronol-ogy in order to quantify long-term rock uplift and exhumation patterns across southern Prince William Sound. We target Hinchinbrook and Montague Islands (Figs. 1 and 2), which are the largest and most trenchward islands in Prince William Sound.

TECTONIC AND GEOLOGIC SETTING

Cretaceous–Cenozoic History of Southern Alaska

The rocks along the southern Alaska margin in Prince William Sound are part of a vast accre-tionary complex that has developed since the

early Mesozoic (e.g., Plafker et al., 1994; Brad-ley et al., 2003). The Yakutat terrane is the young-est in the terrane sequence and is composed mostly of a 15–30-km-thick oceanic plateau (e.g., Christeson et al., 2010; Worthington et al., 2012). Arrival of thickened Yakutat crust at the convergent boundary is inferred to have begun ca. 12–10 Ma (Plafker, 1987; Plafker et al., 1994; Zellers, 1995; Ferris et al., 2003; Eberhart-Phil-lips et al., 2006; Enkelmann et al., 2008, 2009) and as early as ca. 30–18 Ma (Plafker et al., 1994b; Enkelmann et al., 2008; Haeussler, 2008; Finzel et al., 2011; Benowitz et al., 2011, 2012; Arkle et al., 2013). As the collision of this rela-tively buoyant material progressed, a fold-thrust belt developed, leading to high topography in the eastern Chugach–Saint Elias Mountains, and a

marked transition between shallow subduction beneath the Prince William Sound and relatively steeper subduction of dense oceanic Pacifi c plate to the southwest (Fig. 1). Sediment shed from the growing orogen also became incorporated into and deformed within the fold-thrust belt (e.g., Plafker, 1987; Meigs et al., 2008; Pavlis et al., 2012).

The accretionary complex rocks in central and southern Prince William Sound consist of the late Paleocene to Eocene Orca Group. This is a fl ysch deposit consisting dominantly of slate and graywacke turbidites, but it also contains interbedded conglomerate, volcanic-lithic and/or pelagic sandstone, and mudstone (Nelson et al., 1985). The Orca Group extends laterally for more than 100 km and has a structural thickness

140°W

150°W

150°W64°N

61°N

59°N

59°N

Borde

r Ran

ges F

ault

Mt. LoganAnchorage

Mt.McKinley

MG

Cook

Inle

t

YAKUTATPLATE

PACIFICPLATE

CA

NA

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U.S.

Castle Mountain Fault

BG

51 mm/yr

50 mm/yr

Border Ranges Fault

Conta

ct Fa

ult

CSEF

Denali Fault

Denali Fault

Denali Fault

Transition Fault

KIZ

Fairweather Fault

PZ

Aleu

tian

Mega

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st

Wrangell Mountains

TalkeetnaMountains

central Alaska Range

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er R

iver

Alaska

Mt.Marcus Baker

area of Fig.2

BF

NORTH AMERICANPLATE

CM

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Figure 1. Regional 300 m digital elevation model base map of southern Alaska (modifi ed from the U.S. Geological Survey data repository, http:// ned .usgs .gov). Prince William Sound study area is outlined by yellow box and shows the area of Figure 2. Major faults are after Plafker et al. (1994) and the U.S. Geological Survey data repository (http:// ned .usgs .gov). Plate motion vectors (white arrows) are from Plattner et al. (2007) and Elliott et al. (2010). Interpreted region of the subducted Yakutat microplate (green boundary) and subaerial region of Yakutat microplate (green shaded portion of plate) are from Fletcher and Freymueller (2003), Eberhart-Phillips et al. (2006), and Fuis et al. (2008). CM—Chugach Mountains; KM—Kenai Mountains; SEM—Saint Elias Mountains; PZ—Pamplona fold-thrust zone; KIZ—Kayak Island zone; MG—Malaspina Glacier; BG—Bering Glacier; BF—Bagley fault; CSEF—Chugach–Saint Elias fault; MI—Montague Island; HI—Hinchinbrook Island; MSF—Montague Strait fault; MDI—Middleton Island. Modifi ed from Arkle et al. (2013).

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146 Geosphere, February 2015

of 18–20 km (Brocher et al., 1991; Plafker and Berg, 1994; Fuis et al., 2008). It was intruded by two episodes of small granitic plutons that are between ca. 56–53 Ma (the Sanak-Baranof belt) and ca. 40–37 Ma (the Eshamy suite) (Hudson et al., 1979; Plafker and Berg, 1994; Haeussler et al., 1995; Davidson et al., 2011; Garver et al., 2012, 2013; Carlson, 2012; Johnson, 2012).

Intrusions were likely formed from near-trench processes related to a slab window, which may be associated with the subduction of the Kula-Farallon-Resurrection spreading center and pos-sibly another smaller ridge (Bradley et al., 2003; Haeussler et al., 2003; Cowan, 2003; Cole et al., 2006; Madsen et al., 2006). Based on the timing of intrusion of these plutons and detrital zircon

fi ssion-track and U-Pb ages, the depositional age for the Orca Group is ca. 35 Ma in southeastern-most Prince William Sound, ca. 40–37 Ma near Latouche and Evans Islands, and ca. 60–57 Ma in central Prince William Sound (Garver et al., 2012; Davidson et al., 2011) (Fig. 2). Rocks of the Orca Group young to the southeast, toward the convergent margin.

Paleocene - Eocene Orca Group Sandstone or Conglomerate

Paleocene - Eocene Intrusions

Oligocene Intrusions

ice

Cretaceous Valdez Group Sandstone

Paleocene - Eocene Ophiolitic Rocks

basin deposits

20 km

B

B′

C

C′

A′

A

11-15.818.7

11-22.89.7

11-42.04.8

11-103.06.3

10-131.77.3

11-73.710.1

11-62.37.5

11-86.311.6

11-93.26.7

10-195.113.9

11-51.87.4

11-154.98.8

10-183.612.3

6-31.44.4

11-114.87.5

11-124.4---

11-136.0---

10-175.4---

11-145.212.3

10-144.49.8

10-234.1

10.3

10-214.9---

10-224.69.4

10-206.4---

11-168.48.7

10-154.311.3

10-164.511.4

10-116.9

13.0

10-105.4

17.8

10-97.2

21.5

10-811.216.6

10-124.711.2

11-31.17.7

Sample Label KeySample #

AHe Age

AFT Age

ZHe Age

ZFT Age

11-175.0

10.530.254.0

P379.732.6

K14---

20.7---

40.0

K4---

32.5

K5---

39.2

K6---

34.0

B84.8---

P39.115.4

P134.8

10.7

P454.910.8

P26.012.4

P446.6

13.8P439.715.3

P4216.037.4

P3310.121.5

P3212.823.5

P3412.136.6

K10---

28.5---

53.3

P518.116.4

P420.7---

B65.8---

B17.0---

HW-18---------

53.1

HW-15---------

52.7

Mon

tagu

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trai

t Fau

ltRude River Fault

Gravina FaultPat

ton

Bay F

ault

Hanni

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ault

Cape

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are

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Prince William Sound

Cordova

MIMI

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HKIHKI

LILI

NINI

KNIKNIPE

ZBRB

PB

JC

SISI

GIGI

Data SourcesAges from this studyArkle et al., 2013Carlson, 2012Buscher et al., 2008Kveton, 1989

146W

60N

147W

146W

60N

147W

Paleocene - Eocene Orca Group Sandstone or Conglomerate

Paleocene - Eocene Intrusions

Oligocene Intrusions

ice

Cretaceous Valdez Group Sandstone

Paleocene - Eocene Ophiolitic Rocks

basin deposits

Figure 2. Sample map with apatite (U-Th)/He (AHe), apatite fi ssion-track (AFT), and zircon fi ssion-track (ZFT) cooling ages. Base map (300 m digital elevation model) is modifi ed from Arkle et al. (2013). Faults (solid lines), inferred faults (dashed lines), and overlain major lithologic units are from Plafker et al. (1989) and the U.S. Geological Survey fault data repository (http:// ned .usgs .gov). The Patton Bay and Hanning Bay faults are highlighted in red. Transect lines are located for Figures 5 and 6. MI—Montague Island; HI—Hinchinbrook Island; HKI—Hawkins Island; LI—Latouche Island; KNI—Knight Island; NI—Naked Islands; GI—Green Island; SI—Smith Islands; PE—Port Etches; ZB—Zaikof Bay; RB—Rocky Bay; PB—Patton Bay; JC—Jeanie Cove.

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Regional Structures of Prince William Sound

Our study area in Prince William Sound is bound to the north by the Chugach Mountains and to the south by Hinchinbrook and Mon-tague Islands (Fig. 1). The arcuate Contact fault strikes approximately east-west in north-central Prince William Sound and bends southward in the Chugach Mountains to form the western Chugach syntaxis, a major structural bound-ary to this study area. In its southern exposures the Contact fault strikes southwest and gener-ally separates Prince William Sound from the Kenai Peninsula (Fig. 1). The Contact fault is dominantly a right-lateral strike-slip fault to the east (Bol and Roeske, 1993), but displays reverse-fault dip-slip displacement to the west in the Chugach Mountains and Prince William Sound (Bol and Gibbons, 1992). Farther toward the trench an array of faults extends southwest from the Cordova area (Fig. 2) to form the faults of Montague and Hinchinbrook Islands (Nelson et al., 1985), all of which strike north-east-southwest and dip northwest. These are the most outboard faults exposed in Prince William Sound (Fig. 2). Between these faults and the Contact fault is the Montague Strait fault, which is a high-angle normal fault that dips southeast, though the deep structural confi guration of the fault is uncertain (Haeussler et al., 2014; Liberty et al., 2013). Liberty et al. (2013) interpreted the Montague Strait fault as a major structure that separates metamorphosed Orca Group rocks to the northwest from unmetamorphosed Orca Group rocks to the south (Liberty and Finn, 2010; Haeussler et al., 2014; Garver et al., 2012). It was previously recognized as a structural dis-continuity (Nelson et al., 1985), but the nature of the fault was unknown. Subsequent National Oceanic and Atmospheric Administration multi-beam surveys collected from 1988 to 2003 reveal a seafl oor fault scarp (Liberty and Finn, 2010; Haeussler et al., 2014). New seismic refl ection profi les show that the Montague Strait fault locally had southeast-side-down normal slip during the Holocene (Liberty and Finn, 2010; Haeussler et al., 2014; Liberty et al., 2013).

Hinchinbrook and Montague Islands

Hinchinbrook and Montague Islands are elon-gate, narrow islands with steep coastlines and numerous reverse faults that strike parallel to the trend of the islands (Plafker, 1967) (Fig. 2). Average peak elevations on Hinchinbrook and Montague Islands are consistently ~800 m; the highest peaks are nearly 1000 m. Bedding is highly deformed and ranges from shallowly dipping to vertical and is overturned in some

places (Nelson et al., 1985; Plafker, 1967). In low-lying areas, unconsolidated Quaternary gla-cial till with variable thicknesses is present.

There are two main reverse faults on southern Montague Island, the Patton Bay and Hanning Bay faults, both of which ruptured during the 1964 earthquake (Plafker, 1967). After the earth-quake, the faults were traceable on land by large (6–9 m) fault scarps, landslides, and uplifted coastal platforms (Plafker, 1967). These faults generally strike northeast, or parallel to the long axis of the island, and dip ~60° northwest on average (Plafker, 1967) (Fig. 2). The Patton Bay fault can be traced on land for 35 km along the southeast coast and continues southwest on the seafl oor, perhaps as far south as Kodiak Island (Plafker, 1965; Liberty et al., 2013). Seis-mic refl ection profi les and bathymetry show the Patton Bay fault continuing offshore to the southwest for ~20 km (Plafker, 1967; Malloy and Merrill, 1972), where vertical displacements of ~15 m associated with the 1964 earthquake are mapped along seafl oor escarpments (Malloy and Merrill, 1972; Liberty and Finn, 2010; Lib-erty et al., 2013). To the north, the Patton Bay fault may continue offshore along the coast to near the northeast end of the island, where its trace is lost and the fault is likely broken into multiple strands (Fig. 2). The northern coastal extent of the Patton Bay fault is suggested by the straight nature of the coastline and numerous tri-angular facets that line the coast along northeast-ern Montague Island. However, there is little evi-dence of active faulting at the northern extent of the Patton Bay fault. Mountain-front sinuosity is nearly 1.0 and average slopes are steeper on the east coast (18°–22°) versus the west coast (10°–12°) (measurements from this study). Motion on the fault at its southern extent was dominantly dip slip with a maximum of 9 m of vertical off-set on land during the 1964 earthquake, although there was a small component (~0.5 m) of left-lateral motion (Plafker, 1967).

The Hanning Bay fault, which cuts across a small portion of the southwest coast of Mon-tague Island at Fault Cove (Fig. 2), extends for ~6 km on land with the same structural orienta-tion as the Patton Bay fault. The well-defi ned 1964 scarp at Fault Cove has a vertical offset of 6 m (Plafker, 1967). Across both the Hanning Bay and Patton Bay faults, the northwest blocks were upthrown relative to the southeast blocks during the 1964 earthquake. The block southeast of the Patton Bay fault was upthrown ~4.7 m relative to sea level, the block northwest of the Patton Bay fault was upthrown ~12 m, and the block northwest of the Hanning Bay fault was upthrown ~5 m (Plafker, 1967).

Liberty et al. (2013) showed the presence of an additional splay fault, the Cape Cleare fault,

offshore and southwest of Montague Island. Bathymetry shows a 40 m fault scarp, and sub-sequent marine terrace ~5 km south of the Patton Bay fault that decreases in height to the south-west and forms the hanging-wall block of the Cape Cleare fault. This fault is traced onshore south and east of the Patton Bay fault (Fig. 2).

Other geophysical evidence from Trans-Alaska Crustal Transect (TACT) studies (e.g., Fuis et al., 2008) shows that faults in the inlet between Hinchinbrook and Montague Islands near Zaikof Bay become listric and are con-nected at their base to the subduction décolle-ment to the northwest. These faults are along strike with those on Hinchinbrook and Mon-tague Islands, possibly indicating that the faults on Hinchinbrook and Montague Islands are also rooted at depth (Liberty et al., 2013; Haeussler et al., 2014). Land surfaces at Montague Island, along the footwall block of the Cape Cleare fault offshore, and at Middleton Island ~100 km southeast toward the Aleutian megathrust (Fig. 1), were all uplifted during the 1964 earth-quake, suggesting that these faults are likely rooted downward and are separately linked to the décollement at depth (Plafker, 1967; Malloy and Merrill, 1972; Haeussler et al., 2014).

PREVIOUS REGIONAL THERMOCHRONOLOGY WORK

An extensive data set of low-temperature thermochronometer ages generated over the past 25 years provides insights into the timing, rates, and regional exhumation patterns, and provides constraints on regions of localized rapid exhu-mation during the Neogene along the southern Alaska margin (e.g., O’Sullivan et al., 1997a, 1997b; Meigs et al., 2008; Berger et al., 2008a, 2008b; Armstrong et al., 2008; Buscher et al., 2008; Enkelmann et al., 2008, 2009; Spotila and Berger, 2010; Arkle et al., 2013).

In the Saint Elias region, exhumation rates in the past ~6 m.y. are between ~0.5 and 4 mm/yr and vary based on structural position relative to major fault systems (Berger et al., 2008a, 2008b; Berger and Spotila, 2008; Meigs et al., 2008; Enkelmann et al., 2008, 2009; Spotila and Berger, 2010; Falkowski et al., 2014). Low-tem-perature cooling ages are generally very young (AHe younger than 1.5 Ma) along the coast and abruptly increase north of the Bagley fault, suggesting that the Bagley fault is acting as a deformational backstop to thin-skinned folding and thrusting at the collision front (Berger et al., 2008b; Berger and Spotila, 2008; Enkelmann et al., 2008, 2009; Headley et al., 2013; Pavlis, 2013) (Fig. 1). South of the Bagley fault, young AHe ages are attributed to their position within the active fold-thrust belt coupled with exten-

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sive erosion at the coastal front due to its wind-ward position and heavy glaciation (Spotila and Berger, 2010). This zone of focused exhuma-tion is projected west along the Bagley-Con-tact backstop toward the Miles Glacier region, or Miles Corner (previously referred to as the Western or Katalla syntaxis; e.g., Chapman et al., 2011), where it curves southwest to even-tually connect with the Ragged Island thrust, Kayak Island zone, and the Alaska-Aleutian megathrust (Spotila and Berger, 2010) (Fig. 1). While the Miles Corner region may represent an immature indentor corner, there have been insuffi cient age constraints across the Copper River delta and into Prince William Sound to

show whether this zone of rapid exhumation continues west (Fig. 1).

In the western Chugach Mountains and north-ern Prince William Sound (Fig. 1), a bullseye of relatively rapid exhumation was identifi ed in a syntaxial bend between the Border Ranges and Contact faults (Arkle et al., 2013). AHe and AFT ages decrease northward across the Contact fault to minimum ages (averaging ca. 5 and 10 Ma, respectively) in the core of the Chugach Moun-tains between the Contact and Border Ranges faults. Zircon fi ssion-track (ZFT) ages follow a similar pattern, but with older and more scattered ages, ranging between ca. 50 and 26 Ma (transect D–D′; Figs. 1 and 3) (Little and Naeser, 1989;

Arkle et al., 2013). Between the Montague Strait and Contact faults, ages for all thermochrono-logic systems generally decrease by 50% relative to north of the Contact fault (Fig. 3). This pattern of relatively young ages and rapid exhumation north of the Contact fault is interpreted to be the result of underplating along the décollement that has been focused by a syntaxial geometry and modulated by glacial erosion at the southward fl ank of the core (Arkle et al., 2013).

The data set from this study helps to con-strain the nature of deformation west of the transition from the eastern Chugach–Saint Elias Mountains and across the Copper River delta into Prince William Sound. It also provides

southern Prince William Sound

central Prince William SoundChugach MountainsTalkeetna Mountains

Border Ranges Fault

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Montague Strait Fault

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Figure 3. Plot of thermochronometer ages along a southeast-northwest transect (D–D′) shown in Figure 1. Ages are projected onto the transect profi le from a 100 km swath. Age uncertainties are ±1σ. Samples are shown relative to faults (vertical dashed lines). Shaded regions mark approximate bounds of maximum and minimum ages for the apatite (U-Th)/He (AHe), apatite fi ssion-track (AFT), and zircon fi ssion-track (ZFT) systems. Direction of Yakutat convergence is from right to left. The ZFT ages in the Chugach Mountain region are the average of the two youngest ZFT age peaks from modern glacial deposits (Arkle et al., 2013). Figure modifi ed from Arkle et al. (2013).

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Geosphere, February 2015 149

constraints on deformation mechanisms above the subduction megathrust, but outboard of the exhumation bullseye and syntaxial bend in the western Chugach Mountains.

METHODS

This study utilizes AHe and AFT thermo-chronometers to track thermal histories up to effective closure temperatures of ~130 °C. For the AHe system, the partial retention zone (PRZ) is between ~40 °C and 70 °C (depending on the cooling rate, grain size, and effective uranium concentration), which corresponds with depths of ~2–3 km at typical geothermal gradients and surface temperatures (Farley, 2000; Farley and Stockli, 2002; Ehlers and Farley, 2003; Reiners et al., 2004; Flowers et al., 2009). For the AFT system, tracks anneal at temperatures between 60 °C and 130 °C (e.g., Wagner and Reimer, 1972; Naeser, 1979; Gleadow et al., 1986; Dumitru, 2000; Donelick et al., 2005), which corresponds to depths of ~3–6 km (depending on annealing kinetics and geothermal gradient).

We also utilize ZFT data from other studies (Carlson, 2012; Garver et al., 2012) that track the thermal histories of rock up to ~240 °C (e.g., Reiners and Brandon, 2006).

Sampling Strategy and Analytical Techniques

Samples of Orca Group sandstone (n = 32) were collected in southeastern Prince William Sound and used for AHe and AFT analysis (Fig. 2; Table 1). Samples were collected both along and across the structural grain of Hinchin-brook and Montague Islands and especially adjacent to known faults and along the length of topography from southern Montague Island to north of Cordova. At least 5 kg of unweathered sandstone was collected in order to ensure suffi -cient apatite yield. Most samples were medium- to coarse-grained sandstone with well-sorted angular to subangular clasts, composed of quartz, feldspar, lithic fragments, and minor biotite.

AHe ages were determined for 32 samples by laser and inductively coupled plasma–mass

spectrometry methods at the California Institute of Technology (Pasadena). We used 141 single apatite grains with 4–7 single grain replicates per sample for AHe analysis. Of these grains, 13 (9%) were removed from the average age cal-culations because they yielded outlier ages that were high (greater than a 2σ variance) relative to the other grains in the samples (Table SF1 in the Supplemental File1). Anomalously high ages are probably due to high U-bearing inclusions

1Supplemental File. PDF fi le that outlines meth-ods for determining inter-grain age variability using the effective uranium content of samples from this study and a corresponding fi gure, our fi ssion-track analysis, methods for reporting weighted mean AHe ages, and methods for estimating geothermal gradient specifi c to our study area and samples. It also contains tables with detailed AHe and AFT ana-lytical data and AHe/AFT exhumation rate calcula-tions for all thermochronometer data reported in this study. These supplemental tables and fi gure will be denoted throughout this manuscript with an “SF” in front of them. If you are viewing the PDF of this paper or reading it offl ine, please visit http:// dx .doi .org /10 .1130 /GES01036 .S1 or the full-text article on www .gsapubs .org to view the Supplemental File.

TABLE 1. SUMMARY OF THERMOCHRONOLOGY DATA

SampleElevation

(m)Latitude

(°N) Longitude

(°W)

atadegaeHAatadegakcart-noissiFAge*(Ma)

±1σ(Ma)

ε rate†

(mm/yr) n§Age**(Ma)

±1σ(Ma)

ε rate†

(mm/yr)10-8 7.6 60.730 147.483 16.6 1.8 0.3 4 11.2 1.9 0.310-9 1.8 60.623 147.481 21.5 2.1 0.2 4 7.2 1.4 0.410-10 0 60.520 147.405 17.8 1.7 0.3 4 5.4 1.1 0.610-11 3.8 60.430 147.414 13.0 1.2 0.3 4 6.9 1.4 0.410-12 1.2 60.232 147.489 11.2 1.1 0.4 4 4.7 0.5 0.610-13 6 59.890 147.794 7.3 0.8 0.6 5 1.7 0.6 1.310-14 1.7 60.371 147.152 9.8 0.9 0.4 5 4.4 0.2 1.710-15 0 60.344 147.033 11.3 1.1 0.4 4 4.3 0.1 1.110-16 0 60.307 146.906 11.4 1.1 0.4 5 4.5 0.9 1.210-17 0 60.290 146.665 — — — 4 5.4 0.8 0.510-18 0 60.391 146.726 12.3 1.1 0.4 4 3.6 0.2 0.810-19 13.3 60.482 146.613 13.9 1.0 0.3 4 5.1 0.9 0.610-20 0 60.472 146.474 — — — 4 6.4 0.4 0.510-21 4.9 60.530 146.157 — — — 4 4.9 0.2 0.610-22 0 60.612 145.857 9.4 0.9 0.5 4 4.6 0.7 0.610-23 5.1 60.603 145.741 10.3 1.1 0.4 5 4.1 0.4 0.711-1 0 59.880 147.347 18.7 2.3 0.3 4 5.8 0.9 0.911-2 0 59.902 147.441 9.7 0.9 0.4 4 2.8 0.8 0.511-3 759 59.914 147.623 7.7 0.7 0.5 5 1.1 0.4 1.211-4 771 59.981 147.530 4.8 0.6 0.8 5 2.0 0.6 0.711-5 525 59.996 147.582 7.4 0.5 0.5 7 1.8 0.6 0.411-6 382 59.988 147.646 7.5 0.6 0.5 4 2.3 0.2 0.911-7 484 59.976 147.445 10.1 0.8 0.4 4 3.7 0.2 0.711-8 0 59.961 147.344 11.6 0.9 0.4 4 6.3 1.0 0.711-9 0 60.198 147.083 6.7 0.6 0.7 4 3.2 0.8 0.711-10 738.3 60.223 147.126 6.3 0.6 0.6 4 3.0 0.7 0.611-11 887.7 60.412 146.550 7.5 0.7 0.5 4 4.8 0.7 0.511-12 494.4 60.396 146.679 — — — 4 4.4 0.6 0.611-13 443.1 60.404 146.506 — — — 4 6.0 0.1 0.411-14 555 60.376 146.511 12.3 1.2 0.3 4 5.2 0.6 0.511-15 0 60.279 146.518 8.8 1.2 0.5 4 4.9 1.3 0.611-16 0 60.352 146.194 8.7 1.4 0.5 4 8.4 1.8 0.36-3†† 0 59.962 147.679 4.4 0.5 1.0 4 1.4 0.2 2.5

Note: All samples are Orca Group sandstone. Latitude and longitude are North American Datum 1927. Dashes indicate no data.*Apatite fission-track ages are reported from analytical data (Table SF2 in the Supplemental File [text footnote 1]).†ε is exhumation rate for both AFT and AHe; data are from Tables SF3 and SF4 in the Supplemental File (text footnote 1) and are derived from age and closure

temperature data.§n is number of single grain replicates used for AHe age calculation.**Apatite (U-Th)/He (AHe) ages are averages of valid replicates after Ft correction and are reported as a weighted mean (discussed in Supplemental File [see text

footnote 1]).††Ages are from Arkle et al. (2013).

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Ferguson et al.

150 Geosphere, February 2015

undetected within the crystal. To evaluate the effects of radiation damage, effective uranium concentration was compared with each single-grain age (e.g., Flowers et al., 2009), but no relationship was apparent (Fig. SF1 in the Sup-plemental File [see footnote 1]). An Ft (alpha ejection) correction was applied to all raw ages to account for alpha ejection effects related to the grain size and shape (Farley et al., 1996; Farley, 2002). After outlier ages were culled, a weighted average AHe age was determined for the 32 samples using the 4–7 same-sample replicates remaining and a 1σ confi dence inter-val (see the Supplemental File [footnote 1] for details regarding weighted age calculations).

AFT ages were determined for 27 sam-ples using the external detector method (e.g., Gleadow and Duddy, 1981) and ages were computed using Trackkey version 4.2 (Dunkl, 2002). Analytic data are reported in Table SF2 in the Supplemental File (see footnote 1). Fis-sion tracks were counted in 35–40 grains in most samples; as few as 15 grains were counted in some samples with low apatite yield or poor polishing. The etch pit width (Dpar) varies from 0.93 to 1.75 µm and the average for all samples is 1.48 ± 0.29 µm. No systematic relationship was found between Dpar and AFT single-grain age, indicating that age variation between sam-ples is not caused by varying kinetic properties (Table SF2 in the Supplemental File [see foot-note 1]). Almost all have narrow single-grain age distributions and pass the P(χ2) (>5%) test, indicating that the grains either spent little time in the partial annealing zone and/or are kineti-cally invariable.

Horizontal confi ned track length measure-ments were made for three samples (Table 1). Horizontal confi ned tracks were generally diffi -cult to fi nd within grains due to low spontaneous track densities, despite undergoing Cf-252 irra-diation. In 2 samples, an average of 16 measur-able tracks was counted, and in 1 sample, 108 horizontal-confi ned tracks were counted. The range in average track lengths is 13.0–13.8 µm.

RESULTS

AHe Ages

New AHe ages range from 11.2 to 1.1 Ma (Table 1; Table SF1 in the Supplemental File [see footnote 1]). In samples collected across the topographic grain on Hinchinbrook and Montague Islands, young ages are found both at sea level and at high elevations, indicating no apparent age-elevation relationship. Samples north of the Montague Strait fault have AHe ages of 7.2 ± 1.4 and 11.2 ± 1.9 Ma (Figs. 2 and 4; Table 1) and are consistent with previ-

ous AHe ages in central Prince William Sound (Arkle et al., 2013) that average 12.5 Ma. Just south of the Montague Strait fault, three ages on Smith Islands and Green Island are between 6.9 and 4.7 Ma (Fig. 2). The youngest ages are on southern Montague Island and range from 6.3 to 1.1 Ma, with an average age of 2.9 Ma. On northern Montague Island, fi ve samples have ages between 4.5 and 3.0 Ma, with an average of 3.9 Ma. On Hinchinbrook Island, 10 AHe ages range from 8.4 to 3.6 Ma, with an average age of 5.4 Ma (Fig. 2). Farther northeast, three ages are between 4.9 and 4.1 Ma on Hawkins Island and just north of Cordova (Fig. 2). All AHe ages are younger than the sample depositional age of ca. 35 Ma (Garver et al., 2012; Carlson, 2012), and are reset.

AFT Ages

AFT ages range from 21.5 to 4.4 Ma across southern Prince William Sound (Table 1; Table SF2 in the Supplemental File [see footnote 1]). AFT ages north of the Montague Strait fault are 16.6 ± 1.8 and 21.5 ± 2.1 Ma (Fig. 2; Table 1), and are consistent with the 37.4–10.0 Ma AFT ages of Kveton (1989) and Arkle et al. (2013) between the Contact and Montague Strait faults (Fig. 4). Three samples just south of the Mon-tague Strait fault have AFT ages between 17.8 and 11.2 Ma. Those on southern Montague Island are, on average, 8.9 Ma and range from 18.7 to 4.4 Ma. On northern Montague Island, AFT ages range from 11.4 to 6.3 Ma and aver-age 9.1 Ma. On Hinchinbrook Island the average AFT age is 10.6 Ma, and ranges from 13.9 to 7.5 Ma (Fig. 2). Two samples north of Cordova have AFT ages of 9.4 and 10.3 Ma. Like their corresponding AHe ages, all of the AFT ages are younger than the ca. 35 Ma depositional ages and have been reset.

DISCUSSION

Local Analysis of Thermochronometer Ages and Relationships to Faults

Our new data show that AHe and AFT ages south of the Montague Strait fault are younger than those from rocks in the core of the Chugach Mountains to the north (Fig. 3). Overall, ages decrease by ~10–15 m.y. (~50% for the AHe and AFT systems) southward across the Montague Strait fault, similar to the age decrease north of the Contact fault (Fig. 3). ZFT ages (Carlson, 2012), however, are the same or increase south-ward across the Montague Strait fault (Fig. 3). The ZFT ages (averaging ca. 52 Ma; Carlson, 2012) southeast of the Montague Strait fault are older than the 35 Ma depositional age for the

Orca Group sandstone in this region (Hilbert-Wolf, 2012), indicating that these rocks were never buried deep enough to be reset.

AHe ages young southward along the strike of Hinchinbrook and Montague Islands long axes; the youngest ages are at southern Mon-tague Island (transect A–A′; Figs. 2 and 5). However, there is the exception of two older ages on southern Montague Island and one older age on northeastern Hinchinbrook Island (open symbols in Fig. 5). The older AHe ages (5.8 and 6.3 Ma) on Montague Island are located on the footwall block of the Cape Cleare fault (Figs. 4 and 5), indicating that those rocks were not exhumed as rapidly as other southwestern Montague Island rocks. Similarly, the sample on northeastern Hinchinbrook Island has a sig-nifi cantly older AHe age (8.4 Ma) than adjacent samples, and it is located along strike with the Cape Cleare fault farther south (Figs. 4 and 5). We infer these relatively older ages to be part of a separate structural block that was exhumed more slowly than rocks northwest of the Cape Cleare fault and northwest of the structure on northeastern Hinchinbrook Island.

In contrast to the AHe age trends, there is no systematic northeast to southwest AFT age trend (Fig. 5). The lack of an AFT age trend suggests that exhumation was relatively uniform along the islands while rocks were cooling through the AFT closure temperature.

Distinct changes in AHe and AFT age pat-terns also occur across known faults (Fig. 6). AHe and AFT ages decrease southward by more than half across the Montague Strait and Hanning Bay faults in the southwest Montague Island area (transect B–B′; Figs. 2 and 6A). However, this age change is across a broad ~25 km region and may be related to regional rock uplift and exhumation variations rather than offset slip across the Montague Strait fault. Farther southeast across the Patton Bay fault, an average ~1.5 m.y. increase in AHe ages and average ~4.5 m.y. increase in AFT ages from the hanging-wall block to the footwall block may indicate signifi cant changes in rock uplift along the Patton Bay fault in the past ~5 m.y. The contrast in ages across the Hanning Bay, Patton Bay, and Cape Cleare faults since the late Miocene indicates that these structures have been active on million-year time scales and have exhumed rocks from depths of >4 km along nar-row, fault-bounded blocks.

Faults and structural lineaments are more dispersed farther northeast on northern Mon-tague and Hinchinbrook Islands, where there is a more complex network of faults (Fig. 2) (Nelson et al., 1985). Both AHe and AFT ages of sam-ples from northern Montague and Hinchinbrook Islands decrease toward the southeast across the

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Geosphere, February 2015 151

Montague Strait fault region (transect C–C′; Figs. 2 and 6B). AHe ages are ca. 5 Ma on both sides of the Rude River fault on Hinchinbrook Island, suggesting little differential exhumation across the fault in the past 5 m.y. AFT ages decrease to ca. 2.5 Ma to the southeast of the Rude River fault (Fig. 6B), but there is no defi nitive break in the AFT ages across the Rude River fault.

Cooling and Exhumation Rates

Cooling and exhumation rates for single sam-ples were derived from sample-specifi c closure temperatures, an averaged geothermal gradient that accounts for thermal advection of rapidly cooled samples, assumed steady-state topogra-

phy, and a surface temperature of 0 °C (Péwé, 1975). For the AFT ages, a closure temperature of 110 °C was assumed (Reiners and Brandon, 2006). Note that variations in the AFT closure temperature can result from variable kinetic parameters, but Dpar for samples in this study varies little. Closure temperatures for AHe ages were determined using the program CLOSURE (Brandon et al., 1998) using average spherical radii and cooling rates specifi c to each sample. The closure depth (Zc) for each sample was cal-culated using a modifi ed equation of Brandon et al. (1998):

Zc = Tc − Tsgo

+ (h − hm )⎛⎝

⎞⎠ , (1)

where h is the sample elevation, hm is the aver-age elevation for a 10 km radius around the sam-ple location, Ts is the surface temperature, Tc is the effective closure temperature, and go is the geothermal gradient. The average elevation was computed to account for the effect of the long wavelength topography on the shape of shallow isotherms (Stüwe et al., 1994; Mancktelow and Grasemann, 1997).

For samples south of the Montague Strait fault, average cooling rates are ~6.5 °C/m.y. between AFT and AHe closure temperatures, and increase to ~16 °C/m.y. in the past 5 m.y. or less (line A, Fig. 7). On Montague Island, the average cooling rate is ~5 °C/m.y. between AFT and AHe closure temperatures, increasing to

6

6

81010

121214141616

1818

8

6

68

10101212

4 4

2

Mon

tagu

e S

trai

t Fau

lt

RRF

Gravina Fault

PBF

HBF

CCF

5.8

20.7

12.8

9.19.7

6.64.84.9

6.0

7.0

1.7

3.0

3.2

6.3

1.82.3

5.8

4.7

4.4

5.4

1.11.4

2.0

2.8

8.4

6.4

4.8

4.36.9

12.1

4.4

5.4

6.0

7.2

4.9

9.7

4.6

4.9

5.1

4.8

3.65.2

3.7

4.5

11.2

19.7

10.1

16.0

4.1Prince William

Sound Cordova

MIMI

HIHI

HKIHKI

LILI

NINI

KNIKNI

SISI

GIGI

20 km

146W

60N

147W

146W

60N

147W

0–2 2.1–4 4.1–6 6.1–8 8.1–10 10.1–15 15.1–2020.1–40

Age Contours (Ma)

Figure 4. Map of contoured apatite (U-Th)/He (AHe) ages. Contour interval is 2 m.y. HBF—Hanning Bay fault; PBF—Patton Bay fault; CCF—Cape Cleare fault; RRF—Rude River fault; MI—Montague Island; HI—Hinchinbrook Island; HKI—Hawkins Island; LI—Latouche Island; KNI—Knight Island; SI—Smith Islands; NI—Naked Islands; GI—Green Island.

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Ferguson et al.

152 Geosphere, February 2015

~20 °C/m.y. after AHe closure (line B, Fig. 7). On the hanging-wall block of the Patton Bay fault on southern Montague Island, the average cooling rate is 6.5 °C/m.y. after AFT closure and increases to ~35 °C/m.y. after AHe closure (line C, Fig. 7). These relationships across southern Prince William Sound demonstrate a regional increase in cooling rate in the past ~5 m.y., or since the time of AHe closure. More locally, cooling rates increased between ca. 5 and 2 Ma for rocks on the hanging wall of the Patton Bay fault on southern Montague Island.

A background geothermal gradient (gi) for southern Prince William Sound was computed based on the surface heat fl ow and thermal con-ductivity (Blackwell and Richards, 2004; Huang et al., 2008; Batir et al., 2013). Assuming a back-ground surface heat fl ow for Prince William Sound of 45 mW/m2 (Blackwell and Richards, 2004; Batir et al., 2013) and a thermal conduc-tivity of 2.5 W/mK for typical fi ne-grained sedi-mentary rocks (Huang et al., 2008), we compute a gi of 18 °C/km. This background gi was then given a ±20% variability to account for variations in heat fl ow and thermal conductivity resulting in a range in gi values of 14.4–21.6 °C/km (see Supplemental File [footnote 1] for analysis).

Age

(M

a)

Montague Island

Hinchinbrook Island

NESW

2

4

6

8

10

12

14

16

18

0 20 40 60 80 100 120 140 160 180

AFT AgesAHe Ages

Distance Along Transect A-A′ (km)

Figure 5. Apatite (U-Th)/He (AHe) and apatite fi ssion-track (AFT) ages projected onto line A–A′ parallel to Hinchinbrook and Montague Islands long axes (transect location shown in Fig. 2). Age uncertainties are ±1σ. Trend lines show a southward-younging of ages for the AHe system. Ages show no systematic variation from northeast-southwest for the AFT system. Hollow sample symbols are outlier ages on the footwall block of the Cape Cleare fault and southeastern Hinchinbrook Island (see text).

33

Han

ning

Bay

Fau

ltg

y

Pat

ton

Bay

Fau

lt

6

8

10

12

14

20

30

40

4

2

0

Mon

tagu

e S

trai

t Fau

ltg

M

Age

(M

a)

Cap

e C

lear

e F

aultlt

pp

5 20 25 30 35 4010 15 45 50 550Distance along B–B′ (km)

A

0 15 30 45 60 75 90Distance along C–C′ (km)

AFT AgeAHe Age

Age

(M

a)

SENW

tagu

e S

trai

t Fau

lttr

ait F

aul

Mon

g

Rud

e R

iveer

Fau

lt

6

9

12

15

18

21

24

27

3

0 ?

B

Figure 6. Cross sections across the Montague Strait fault and Montague and Hinchin-brook Islands (transect locations shown in Fig. 2), showing apatite (U-Th)/He (AHe) and apatite fi ssion-track (AFT) ages relative to fault locations. (A) B–B′. (B) C–C′. Age uncertainties are ±1σ. Colored bands gen-erally outline range in ages and red or blue line is average age for the region. Arrows along fault location markers (gray dashed lines) represent relative vertical displace-ment direction for that fault.

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Focused rock uplift at Montague and Hinchinbrook Islands

Geosphere, February 2015 153

Regional processes may affect the thermal structure of the crust overriding the Yakutat microplate and may infl uence low-temperature cooling ages. These processes include ridge subduction during the Paleocene–Eocene (e.g., Haeussler et al., 2003; Idleman et al., 2011; Benowitz et al., 2012) and later cooling related to subduction of the relatively cool Yakutat microplate. Thermal length arguments (e.g., Turcotte and Schubert, 2002) suggest that the transitory thermal effects of Paleocene–Eocene ridge subduction in the 25-km-thick overriding plate would have dissipated in ~15–20 m.y., prior to the Miocene and younger cooling docu-mented by our ages. The present-day regional geothermal gradient is relatively low, but it is consistent with fl at-slab subduction thermal models (e.g., Gutscher and Peacock, 2003) and models of thermal effects of subduction in general (e.g., Cloos, 1985). We use the rela-tively low present-day geothermal gradient in our exhumation analysis, but if the slab cooling effects were greater in the past, then the exhu-mation rates discussed here may be a minimum. The relatively young ages in our study and the local scale of the age variations suggest that regional slab refrigeration or ridge subduc-tion effects do not cause the age patterns we observe, thus we assume that cooling is related to exhumation.

It is well known that the advection of hot-ter rocks toward the surface during exhumation

causes geothermal gradients to increase (e.g., Kappelmeyer and Haenel, 1974; Powell et al., 1988; Ehlers, 2005). We apply a simple cor-rection to the background geothermal gradient by assuming erosion durations consistent with sample ages and typical exhumation rates of between ~0.5 and 2 mm/yr. Advection correc-tions increase gi between 10% and 80%, result-ing in sample-specifi c advection-corrected geothermal gradient (go) values between 23 and 38 °C/km (Tables SF3 and SF4 in the Sup-plemental File [see footnote 1]). The median of the two corrected end-member go values for each sample was used in Equation 1 to com-pute the closure depth and the resulting exhu-mation rates (ε = Zc/t) for each sample age (t) (Table 1).

For the AHe system, the average closure temperature for southern Prince William Sound is ~66 °C, average Zc is ~2.7 km, and overall average exhumation rates are 0.7 mm/yr. For the AFT system, a 110 °C closure temperature was used for all samples. The average depth to closure is ~4.5 km, and average exhumation rates are 0.4 mm/yr (Tables SF3 and SF4 in the Supplemental File [see footnote 1]). On the hanging wall of the Patton Bay fault of southern Montague Island, exhumation rates are as high as 2.5 mm/yr based on AHe closure, but aver-age 1.5 and 0.6 mm/yr based on AHe and AFT closure, respectively. Overall, exhumation rates increased in the past ~5 m.y. or less (Fig. 7)

across both Montague and Hinchinbrook Islands, but on southern Montague Island the exhumation rates increased in the past ~2 m.y.

Timing and Magnitude of Rock Uplift across Faults

AHe and AFT age differences or similarities across known faults on Montague and Hinchin-brook Islands allow estimates of relative rock uplift across the faults, and therefore estimates of long-term fault offset. Based on the AHe cooling age data, samples from the Patton Bay fault hang-ing wall cooled at a rate of 35 °C/m.y. since ca. 2 Ma, which is the average AHe age of the hang-ing-wall samples (Figs. 6A and 7). Samples from the footwall block of the Patton Bay fault cooled at a rate of 14 °C/m.y. since 3.3 Ma. Assuming constant cooling rates across the Patton Bay fault since 3.3 m.y. and an average geothermal gra-dient of 25 °C/km, the temperature change dif-ferential between the footwall and hanging-wall blocks is 65 °C, leading to a difference in rock uplift of ~2.6 km across the fault since 3.3 Ma. The Patton Bay fault is estimated to dip 60° based on surface ruptures from the 1964 earthquake (Plafker, 1967) and apparent dip estimates in the TACT Prince William Sound deep seismic refl ec-tion line (Haeussler et al., 2014; Liberty et al., 2013). Correcting for the 60° dip leads to a total offset of ~3 km across the Patton Bay fault since 3.3 Ma; note that total offset amounts are greater

0 5 10 15 20 25 30 35 400

20

40

60

80

100

120

140

160

180

200

220

Thermochronometer Age (Ma)

Tem

pera

ture

(°C

)

Max

imum

Dep

ositi

onal

Age

Max

imum

AF

T A

ge

AHe Ages

AFT Ages

AB

Average Cooling Rate

C

Figure 7. Age and closure temperature plot for all apa-tite (U-Th)/He (AHe), apatite fi ssion-track (AFT), and zircon fi ssion-track (ZFT) data south of the Montague Strait fault. Gray lines are simple cooling paths for each sample. Aver-age cooling paths for the AHe and AFT systems are shown by the bold black lines. Line A is the average cooling rate for all samples, line B is the average for Montague Island, and line C is the average cooling path on the hanging-wall block of the Patton Bay fault. Potential time-temperature paths are outlined by the green shaded region. Dashed line is the AFT cooling rate extrapolated back to 200 °C. Maximum ZFT age is constrained by the deposi-tional age of the Orca Group in southern Prince William Sound from Hilbert-Wolf (2012).

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Ferguson et al.

154 Geosphere, February 2015

if the fault dip is less, as suggested by Liberty et al. (2013). AFT age similarity across the Patton Bay fault indicates that cooling rates were likely the same on the footwall and hanging-wall blocks while cooling through AFT closure, suggesting minimal differential exhumation across the Pat-ton Bay fault between ca. 10 Ma (AFT closure) and 3.3 Ma (AHe closure).

On northern Montague and Hinchinbrook Islands, the clustering of AHe ages ca. 4.5 Ma allows estimates to be placed on the maximum amount of slip on these faults because vertical fault offset could not have been greater than the depth that corresponds to the minimum tempera-ture of the AHe partial retention zone. Assuming an average advection-corrected geothermal gra-dient of 25 °C/km and a minimum PRZ tempera-ture of 40 °C, the maximum amount of differen-tial rock uplift is ~1.6 km in the past ~4.5 m.y. on northern Montague and Hinchinbrook Islands.

The depositional age for Orca Group sand-stone (younger than 35 Ma; Hilbert-Wolf, 2012) in southern Prince William Sound is an impor-

tant constraint because it indicates that the sedi-ments were at surface temperatures after 35 Ma. If the AHe and AFT ages are reset, but the ZFT ages are not reset (Carlson, 2012), then the Orca Group had to have been buried to depths and corresponding temperatures of between ~110 °C and ~200 °C, then reexhumed after ca. 35 Ma (Fig. 7). The maximum AFT age is the minimum age that the rocks at Hinchinbrook and Montague Islands were at 110–200 °C. The shaded region in Figure 7 represents the range of cooling paths rocks may have taken prior to passing through AFT closure temperature. If the average AFT cooling rate (line A, Fig. 7) is constant from 200 °C, then this rate may be extrapolated to as long ago as ca. 23 Ma (dashed line, Fig. 7). We acknowledge that there is con-siderable uncertainty in this analysis, but it pro-vides loose constraints on the potential timing of maximum temperature estimates for rocks on Montague and Hinchinbrook Islands. Given that rocks were buried to temperatures as high as 200 °C, the maximum amount of rock uplift and

exhumation in southern Prince William Sound (assuming a constant average geothermal gradi-ent of 25 °C/km) is ~8 km since ca. 23–35 Ma. In addition, the reset AFT ages indicate that the minimum exhumation magnitude on Montague and Hinchinbrook Islands is ~4.8 km, which occurred in the past ~7–10 m.y.

Regional Analysis

One of our goals is to examine the relation-ship between the zone of very young AHe ages (younger than 1 Ma) and rapid exhuma-tion south of the Bagley fault in the Saint Elias region (Spotila and Berger, 2010) and the thermo chronology data across the Copper River delta into Prince William Sound. Based on our new ages, the region of young cooling ages in the Saint Elias Mountains does not appear to extend westward into Prince William Sound (Fig. 8), but rather bends southward at the Miles Corner to connect with the Kayak Island zone and Aleutian megathrust, as suggested by

Age Contours (Ma)a)

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AgAge e Contouo rssrs (((MaMaMa)

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Conto0–20 220 2ooursrsours2.1–40 2 0 22 12 442 1 44.1–64 1 664 1 66.1–844.1 64.1 6 6 1 86 1 88.1–10 8 1 1008 1 1010.1–158 1008.1 10 10 1 1150 1515.1–20 15 1 22005 1 2020.1–40 200 1 41 40020 1 4040.1–60 4040 1 66040 1 60

Age Contours (Ma)

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Figure 8. Contour map of new and published apatite (U-Th)/He (AHe) data from southern Alaska. Area outlined by red dashes shows the possible region of rapid exhumation from the Saint Elias Mountains continuing south along the Kayak Island zone and Aleutian mega-thrust, as described by Spotila and Berger (2010). Region labeled A indicates a possible broad deformation region that extends from the Kayak Island fault zone to southern Prince William Sound. Green line indicates interpreted subducted region of the Yakutat microplate from Fletcher and Freymueller (2003), Eberhart-Phillips et al. (2006), and Fuis et al. (2008). White dots are sample locations from this study and others. HI—Hinchinbrook Island; MI—Montague Island; MDI—Middleton Island; KI—Kayak Island; RI—Ragged Island thrust fault; PZ—Pamplona fold-thrust zone; CSEF—Chugach–Saint Elias fault; MSF—Montague Strait fault.

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Spotila and Berger (2010) (red dashed area, Fig. 8). In this interpretation, the Kayak Island zone and Ragged Mountain fault link the Miles Corner region with the Aleutian megathrust and are well-defi ned boundaries between transpres-sion and convergence. Even though the band of youngest ages either ends at Miles Corner or bends south, some focused rock uplift may be transferred west into Prince William Sound, because rocks with young ages (4 Ma AHe) continue west across the Copper River delta into the Hinchinbrook and Montague Islands region. The rapid exhumation in the Saint Elias region is focused along the Bagley backstop (Berger et al., 2008b). We propose that this backstop continues westward as the Montague Strait fault (Fig. 8), but with lower exhumation rates than in the Saint Elias orogen. A broader zone of deformation extending from the Kayak Island fault system to southern Prince William Sound (area A, Fig. 8) may also accommodate the overall rock uplift differences south of the backstops in southern Prince William Sound and the Saint Elias area. Uplifted fault blocks during the 1964 earthquake and the presence of reverse faults (Haeussler et al., 2014) outboard of southern Prince William Sound to Middleton Island are consistent with a distributed defor-mation zone.

Relationships between Exhumation and Topography

Along most active orogens, high exhumation rates coincide with high elevations and high

relief. Often the regions of high exhumation rate are offset from the highest elevations where high precipitation rates (Reiners et al., 2003; Willett et al., 2001) or erosive alpine glaciers (e.g., Tomkin, 2007; Berger et al., 2008a; Arkle et al., 2013) cause rapid erosion on the wind-ward fl ank of the orogen. The correspondence between elevations and glacier equilibrium-line altitude (ELA) suggests that glaciers behave as buzz saws that limit orogen elevation and width (e.g., Brozović et al., 1997; Meigs and Sauber, 2000; Spotila et al., 2004). In the Saint Elias Mountains, where glaciers cover much of the windward fl ank of the orogen, the coincidence of the ELA with zones of rapid exhumation sug-gests that glaciers partly control the rock uplift and exhumation (Meigs and Sauber, 2000; Spotila et al., 2004; Berger and Spotila, 2008; Berger et al., 2008a, 2008b).

Our data from Montague and Hinchinbrook Islands show a lack of correlation between AHe age (or exhumation rates and magnitudes derived from the ages) and elevation. Com-puted exhumation magnitudes along the trend of Montague and Hinchinbrook Islands were derived using the AHe exhumation rates and an exhumation duration of 2 m.y. (Fig. 9); the dura-tion is based on the average AHe age of sam-ples from the Patton Bay fault hanging wall. In general, exhumation magnitude increases from northeast to southwest, with highest exhuma-tion magnitudes at the south end of Montague Island (Fig. 9). Range-crest elevations along the island-parallel transect are ~800 m on both Hinchinbrook and Montague Islands, but eleva-

tion decreases to below sea level between them at Hinchinbrook Entrance (the inlet between Hinchinbrook and Montague Islands). Exhuma-tion magnitude is relatively constant along the transect, even across Hinchinbrook Entrance. At the southern end of Montague Island, eleva-tion decreases to sea level whereas the exhuma-tion magnitudes increase abruptly (Fig. 9). We expect that elevation would be greatest at the south end of Montague Island because it has the highest exhumation rate and is not a focus region of high orographic precipitation or the alpine glacier–dominated windward fl ank of an orogen. The lack of coincidence between high exhumation magnitude (or rate) and high-elevation regions has been documented in other areas, especially where Quaternary glaciers eroded the landscape. In central and north-ern Fiordland, New Zealand, AHe ages are typically 1–3 Ma, but elevations are <1500 m (House et al., 2005). House et al. (2005) inter-preted the changes in ages across Fiordland to be due to differential exhumation across faults, but extensive glaciation caused regional ero-sion across the faults after ca. 2 Ma (Sutherland et al., 2009). Along the Fairweather corridor adjacent to the Fairweather fault in southeast Alaska, AHe ages are typically younger than 1 Ma (McAleer et al., 2009) across 50 km regions of low topography that are relatively glacially denuded. McAleer et al. (2009) suggested that glacial erosion in the Fairweather corridor was high enough to limit the development of topography even where exhumation rates were highest. Whereas Hinchinbrook and Montague

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parallel topographic profile (gray dashed line, transect A–A′ from Fig. 2) and sample exhu-mation magnitudes (green trian-gles). Exhumation magnitudes represent exhumation for past 2 m.y. at sample-specifi c exhuma-tion rates. Elev.—elevation.

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Islands are not currently glacially dominated or a focus region of high orographic precipitation, Pleistocene glaciers extended ~100 km south of their present location (Kaufman and Manley, 2004). Thus, we infer that glaciers were able to erode Montague and Hinchinbrook Islands rap-idly enough to limit topographic growth even where rock uplift rates were the highest, attest-ing to the buzz saw potential of glaciers (e.g., Brozović et al., 1997; Meigs and Sauber, 2000; Spotila et al., 2004).

Although glacial erosion likely limited topo-graphic growth, the maximum elevations are probably also limited by the narrowness of rock uplift along the splay faults. Both Hinchinbrook and Montague Islands are at most 20 km wide, and <10 km in many locations. For example, AHe ages on the Yucaipa Ridge block along the San Andreas fault in southern California are 0.7–1.6 Ma across a 5–10-km-wide ridge between fault strands (Spotila et al., 1998). Elevations along the ridge are ~1000 m above base level. In this case, where glaciers have not eroded the landscape, oversteepening of the slopes between the fault strands regulated the maximum eleva-tions along the rapidly exhuming narrow ridge (Spotila et al., 1998). If rock uplift on Montague and Hinchinbrook Islands is mostly focused along megathrust fault splays, then the lack of high topography may be partly due to the nar-rowness of the uplifted region.

Causes of Rock Uplift and Exhumation

The AHe age patterns (Fig. 4) clearly indi-cate that long-term exhumation rates increase to the southwest and are highest at the south-west end of Montague Island. The southwest increase in exhumation rate and magnitude also mimics the southwest increase in the amount of uplift and fault offset in the 1964 earthquake (Plafker, 1967, 1969). There was no measured offset on the Cape Cleare fault onshore after the 1964 earthquake, but seismic data of Lib-erty et al. (2013) show a 40 m scarp offshore to the southwest. Thus the long-term pattern of exhumation refl ects the short-term seismogenic uplift patterns. Given (1) the narrow geometry of Montague and Hinchinbrook Islands; (2) the relatively young thermochronometer ages from samples across the islands that are adjacent to relatively older ages north of the Montague Strait fault; (3) a well-defi ned topographic expression of faults onshore and their seis-mic and bathymetric expression offshore; and (4) the correlation between long-term exhuma-tion and coseismic deformation, we infer that exhumation is controlled dominantly by rock uplift along faults. We also infer that Hinchin-brook and Montague Islands have remained the

focus of exhumation for at least the past 2–3 m.y., with higher rates to the southwest.

Even though we interpret the majority of exhumation to be on narrow fault-bounded blocks on Montague and Hinchinbrook Islands, we cannot rule out the possibility that rock uplift is spread over a broader region offshore to the southeast. AHe ages on the footwall of the Cape Cleare fault are ca. 6 Ma, indicating late Mio-cene and younger exhumation, but at a lower long-term rate than rocks on the hanging-wall block. In addition, the shorelines on the Cape Cleare fault footwall and on Middleton Island located ~100 km to the southeast were uplifted in the 1964 earthquake (Plafker, 1969). Thus the exhumation focused along fault splays may be part of broader region of rock uplift caused by underplating along the megathrust. Liberty et al. (2013) reprocessed the TACT deep seismic refl ection data and interpreted each of the splay faults to sole into the subduction megathrust separately; they showed a subhorizontal mega-thrust located 18–20 km beneath Montague Island and a lens-shaped zone of refl ections below the megathrust, interpreted as thickening due to underplating and duplexing and coinci-dent with where the Patton Bay, Hanning Bay, and Cape Cleare faults splay off the megathrust (Haeussler et al., 2014). Liberty et al. (2013) inferred that the thickened region causes lock-ing of the megathrust below the western part of Prince William Sound (Zweck et al., 2002), which initiates the splay faults that project to the surface parallel to one another. Underplating and duplexing along the megathrust may also cause broad rock uplift that extends southward and away from the more focused rock uplift and exhumation on Montague and Hinchinbrook Islands. Increased rock cooling starting ca. 2–3 Ma on southern Montague Island is coin-cident with increased mountain glacial erosion worldwide (Herman et al., 2013) and with the onset of glacial sedimentation in the Saint Elias region (Lagoe et al., 1993; Lagoe and Zellers, 1996; Cowan et al., 2013). Climate cooling and glacier erosion may have enhanced erosion rates on Montague and Hinchinbrook Islands during the Pleistocene, but if glacial erosion was the sole cause of increased rock uplift in southern Prince William Sound, then other parts of Prince William Sound should have had the same rapid uplift during this time. Pavlis et al. (2012) suggested that sediments originally shed from the Saint Elias orogen 2–3 Ma in response to cooling climate and glacial erosion caused duplex stacking and underplating under the Yakataga segment farther to the southeast. Mankhemthong et al. (2013) used gravity and magnetic data from the Chugach Mountains north of Prince William Sound to suggest that

sediments shed from the Saint Elias Mountains were carried along and underplated above the Yakutat microplate. Thus, the increased volume of subducting sediments starting ca. 2–3 Ma may have also enhanced underplating along the megathrust under southern Prince William Sound, which then increased rock uplift along the megathrust splay faults. We infer that the high rock uplift rate since 2–3 Ma on Montague Island is due mainly to splay faulting and related underplating. Recent glaciation at the surface has accommodated this accelerated exhumation across a narrow region, but has also masked the topographic effects of increased rock uplift.

CONCLUSIONS

The thermochronology data from this study provide insight into the style of deformation above the subduction décollement in the Prince William Sound of southern Alaska. Our ages, combined with previously published ages, show that southern Prince William Sound, between the Cape Cleare and Montague Strait faults, is a region of focused exhumation and deformation likely caused by Yakutat fl at slab subduction. AHe and AFT ages on Montague and Hinchin-brook Islands are as young as 1.1 and 4.4 Ma, respectively; the youngest ages are at the south-west end of Montague Island. These ages vary across major faults, especially on southwestern Montague Island across the Hanning Bay, Pat-ton Bay, and Cape Cleare faults. Exhumation rates across the Patton Bay fault are ~2 times higher on the hanging-wall block than on the footwall block, leading to as much as 3 km of slip across the Patton Bay fault in the past 3.3 m.y., with decreasing slip to the northeast. The northeast to southwest increase in exhuma-tion rate is coincident with the trend of increas-ing coseismic uplift from the 1964 earthquake, suggesting that fault-related rock uplift is long lived. Thermochronometer ages are 2–5 times greater north of the Montague Strait fault than to the south, suggesting that this fault is part of a major structural transition that acts as a mechanically strong backstop to deformation, and faster exhumation to the south on Montague and Hinchinbrook Islands. The Montague Strait fault backstop may be a westward mechanical continuation of the Bagley fault system back-stop in the Saint Elias orogen, but exhumation is slower in the Montague and Hinchinbrook Islands area, where deformation may be distrib-uted between the Aleutian Trench and southern Prince William Sound.

Splay faulting above the subduction décolle-ment is interpreted as the primary cause for rapid exhumation in southern Prince William Sound. More specifi cally, rock is being uplifted via

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splay faulting that is rooted to the Yakutat–North America plate interface. Underplating at the plate interface in the past ~5 m.y., with increased effects since ca. 3–2 Ma, may be enhancing or driving splay fault formation in southern Prince William Sound. The lack of correlation between exhumation rates or magnitudes and topogra-phy suggests that a cooling climate and glacial erosion played a role in limiting topographic growth. Notably, this study shows that splay faults with historic coseismic rupture separated by only a few kilometers can facilitate kilometer-scale exhumation above shallowly subducting plates at million-year time scales.

ACKNOWLEDGMENTS

We thank K. Farley and L. Hedges (California Institute of Technology, Pasadena) for (U-Th)/He analyses. We also thank Matan Salmon for his help as fi eld assistant in the Prince William Sound. Insightful reviews by J. Benowitz, C. Davidson, Associate Edi-tor T. Pavlis, and an anonymous reviewer helped us focus and improve this paper. Funding was provided by Donors of the Petroleum Research Fund adminis-tered by the American Chemical Society and the John T. Dillon Alaska Research Fund administered by the Geologic Society of America.

REFERENCES CITED

Arkle, J.C., Armstrong, P.A., Haeussler, P.J., Prior, M.G., Hartman, S., Sendziak, K.L., and Brush, J.A., 2013, Mechanisms of rock uplift above fl at slab subduction in the western Chugach Mountains and Prince William Sound of the southern Alaska syntaxis: Geological Society of America Bulletin, v. 125, p. 776–793, doi: 10 .1130 /B30738 .1 .

Armstrong, P.A., Haeussler, P.J., Sendziak, K.L., and Arkle, J.C., 2008, The western Chugach core: Locus of sub-duction-related exhumation? [abs.], in Garver, J.I., and Montario, M.J., eds., FT2008: Proceedings, 11th Inter-national Conference on Thermochronometry, Anchor-age, Alaska, 2008, Abstracts with Programs, p. 8–10.

Batir, J.F., Blackwell, D.D., and Richards, M.C., 2013, Updated surface heat fl ow map of Alaska: Geothermal Resources Council Transactions, v. 37, p. 2–23.

Benowitz, J.A., Layer, P.W., Armstrong, P.A., Perry, S.E., Haeussler, P.J., Fitzgerald, P.G., and VanLaningham, S., 2011, Spatial variations in focused exhumation along a continental-scale strike-slip fault: The Denali fault of the eastern Alaska Range: Geosphere, v. 7, p. 455–467, doi: 10 .1130 /GES00589 .1 .

Benowitz, J.A., Haeussler, P.J., Layer, P.W., O’Sullivan, P.B., Wallace, W.K., and Gillis, R.J., 2012, Cenozoic tectono-thermal history of the Tordrillo Mountains, Alaska: Paleocene–Eocene ridge subduction, decreas-ing relief, and late Neogene faulting: Geochemistry Geophysics Geosystems, v. 13, Q04009, doi: 10 .1029 /2011GC003951 .

Benowitz, J.A., Layer, P.W., and VanLaningham, S., 2013, Persistent long-term (c. 24 Ma) exhumation in the Eastern Alaska Range constrained by stacked thermo-chronology, in Jourdan, F., et al., eds., Advances in 40Ar/39Ar dating: From archaeology to planetary sci-ences: Geological Society of London Special Publica-tion 378, p. 225–243, doi: 10 .1144 /SP378 .12 .

Berger, A.L., and Spotila, J.A., 2008, Denudation and defor-mation in a glaciated orogenic wedge: The St. Elias orogen, Alaska: Geology, v. 36, p. 523–526, doi: 10 .1130 /G24883A .1 .

Berger, A.L., and 10 others, 2008a, Quaternary tectonic response to intensifi ed glacial erosion in an orogenic wedge: Nature Geoscience, v. 1, p. 793–799, doi: 10 .1038 /ngeo334 .

Berger, A.L., Spotila, J.A., Chapman, J.B., Pavlis, T.L., Enkelmann, E., Ruppert, N.A., and Buscher, J.T.,

2008b, Architecture, kinematics, and exhumation of a convergent orogenic wedge: A thermochronological investigation of tectonic-climatic interactions within the central St. Elias orogen, Alaska: Earth and Plan-etary Science Letters, v. 270, p. 13–24, doi: 10 .1016 /j .epsl .2008 .02 .034 .

Blackwell, D.D., and Richards, M., 2004, Geothermal map of North America: American Association of Petroleum Geologists, scale 1:6,500,000.

Bol, A.J., and Gibbons, H., 1992, Tectonic implications of out-of-sequence faults in an accretionary prism, Prince William Sound, Alaska: Tectonics, v. 11, p. 1288–1300, doi: 10 .1029 /92TC01327 .

Bol, A.J., and Roeske, S.M., 1993, Strike-slip faulting and block rotation along the contact fault system, east-ern Prince William Sound, Alaska: Tectonics, v. 12, p. 49–62, doi: 10 .1029 /92TC01324 .

Bradley, D., Kusky, T., Haeussler, P., Goldfarb, R., Miller, M.L., Dumoulin, J., Nelson, S.W., and Karl, S., 2003, Geologic signature of early Tertiary ridge subduction in Alaska, in Sisson, V.E., et al., eds., Geology of a transpressional orogen developed during ridge-trench interaction along the North Pacifi c margin: Geological Society of America Special Paper 371, p. 19–49, doi: 10 .1130 /0 -8137 -2371 -X .19 .

Brandon, M.T., Roden-Tice, M.K., and Garver, J.I., 1998, Late Cenozoic exhumation of the Cascadia accretion-ary wedge in the Olympic Mountains, northwest Wash-ington State: Geological Society of America Bulletin, v. 110, p. 985–1009, doi: 10 .1130 /0016 -7606 (1998)110 <0985: LCEOTC>2 .3 .CO;2 .

Brocher, T.M., Moses, M.A., Fisher, M.A., Stephens, C.D., and Geist, E.L., 1991, Images of the plate boundary beneath southern Alaska, in Meissner, R., et al., eds., Continental lithosphere: Deep seismic refl ections: American Geophysical Union Geodynamics Series 22, p. 241–246.

Brozović, N., Burbank, D.W., and Meigs, A.J., 1997, Cli-matic limits on landscape development in the north-western Himalaya: Science, v. 276, p. 571–574, doi: 10 .1126 /science .276 .5312 .571 .

Bruhn, R.L., Pavlis, T.L., Plafker, G., and Serpa, L., 2004, Deformation during terrane accretion in the Saint Elias orogen, Alaska: Geological Society of America Bulle-tin, v. 116, p. 771–787, doi: 10 .1130 /B25182 .1 .

Buscher, J.T., Berger, A.L., and Spotila, J.A., 2008, Exhu-mation in the Chugach-Kenai Mountain belt above the Aleutian subduction zone, southern Alaska, in Frey-mueller, J.T., et al., eds., Active tectonics and seismic potential of Alaska: American Geophysical Union Geophysical Monograph 179, p. 151–166, doi: 10 .1029 /179GM08 .

Carlson, B.M., 2012, Cooling and provenance revealed through detrital zircon fi ssion track dating of the Upper Cretaceous Valdez Group and Paleogene Orca Group in Western Prince William Sound, Alaska, in Varga, R., ed., Proceedings of the Twenty-Fifth Keck Research Symposium in Geology, Amherst, Massachusetts: Pomona College, Claremont, California, Keck Geol-ogy Consortium, p. 8–16.

Chapman, J.B., Worthington, L.L., Pavlis, T.L., Bruhn, R.L., and Gulick, S.P., 2011, The Suckling Hills fault, Kayak Island zone, and accretion of the Yakutat micro-plate, Alaska: Tectonics, v. 30, TC6011, doi: 10 .1029 /2011TC002945 .

Christeson, G.L., Gulick, S.P., van Avendonk, H.J., Worthington, L.L., Reece, R.S., and Pavlis, T.L., 2010, The Yakutat terrane: Dramatic change in crustal thick-ness across the Transition fault, Alaska: Geology, v. 38, p. 895–898, doi: 10 .1130 /G31170 .1 .

Cloos, M., 1985, Thermal evolution of convergent plate mar-gins: Thermal modeling and reevaluation of iso topic Ar-ages for blueschists in the Franciscan Complex of California: Tectonics, v. 4, p. 421–433, doi: 10 .1029 /TC004i005p00421 .

Cole, R.B., Nelson, S.W., Layer, P.W., and Oswald, P.J., 2006, Eocene volcanism above a depleted mantle slab window in southern Alaska: Geological Society of America Bulletin, v. 118, p. 140–158, doi: 10 .1130 /B25658 .1 .

Cowan, D.S., 2003, Revisiting the Baranof–Leech River hypothesis for early Tertiary coastwise transport of

the Chugach–Prince William terrane: Earth and Plan-etary Science Letters, v. 213, p. 463–475, doi: 10 .1016 /S0012 -821X (03)00300 -5 .

Cowan, E.A., Forwick, M., Bahlburg, H., Childress, L.B., Moy, C.M., Muller, J., Ribeiro, F., Ridgway, K.D., 2013, Southern Alaska glaciations recorded in deep-sea diamicts: Preliminary results from IODP Expedi-tion 341: American Geophysical Union, fall meeting abs. T22C-07.

Davidson, C., Garver, J.I., Hilbert-Wolf, H.L., and Carlson, B., 2011, Maximum depositional age of the Paleocene to Eocene Orca Flysch, Prince William Sound, Alaska: Geological Society of America Abstracts with Pro-grams, v. 43, no. 5, p. 439.

Donelick, R.A., O’Sullivan, P.B., and Ketcham, R.A., 2005, Apatite fission-track dating, in Reiners, P.W., and Ehlers, T.A., eds., Low-temperature thermochronol-ogy: Techniques, interpretations, and applications: Reviews in Mineralogy and Geochemistry Volume 58, p. 49–94, doi: 10 .2138 /rmg .2005 .58 .3 .

Dumitru, T.A., 2000, Fission-track geochronology in Qua-ternary geology, in Noller, J.S., et al., eds., Quaternary geochronology: Methods and applications: Washing-ton, D.C., American Geophysical Union Reference Shelf, v. 4, p. 131–155, doi: 10 .1029 /RF004p0131 .

Dunkl, I., 2002, Trackkey: A Windows program for calcu-lation and graphical presentation of fi ssion track data: Computers & Geosciences, v. 28, p. 3–12, doi: 10 .1016 /S0098 -3004 (01)00024 -3 .

Eberhart-Phillips, D., Christensen, D.J., Brocher, T.M., Han-sen, R., Ruppert, N.A., Haeussler, P.J., and Abers, G.A., 2006, Imaging the transition from Aleutian subduction to Yakutat collision in central Alaska, with local earth-quakes and active source data: Journal of Geophysical Research, v. 111, B11303, doi: 10 .1029 /2005JB004240 .

Ehlers, T.A., 2005, Crustal thermal processes and the inter-pretation of thermochronometer data, in Reiners, P.W., and Ehlers, T.A., eds., Low-temperature thermochro-nology: Techniques, interpretations, and applications: Reviews in Mineralogy and Geochemistry Volume 58, p. 315–350, doi: 10 .2138 /rmg .2005 .58 .12 .

Ehlers, T.A., and Farley, K.A., 2003, Apatite (U-Th)/He thermo chronometry: Methods and applications to prob-lems in tectonic and surface processes: Earth and Planetary Science Letters, v. 206, p. 1–14, doi: 10 .1016 /S0012 -821X (02)01069 -5 .

Elliott, J.L., Larsen, C.F., Freymueller, J.T., and Motyka, R.J., 2010, Tectonic block motion and glacial iso-static adjustment in southeast Alaska and adjacent Canada constrained by GPS measurements: Journal of Geophysical Research, v. 115, B09407, doi: 10 .1029 /2009JB007139 .

Enkelmann, E., Garver, J.I., and Pavlis, T.L., 2008, Rapid exhumation of ice-covered rocks of the Chugach–St. Elias orogen, southeast Alaska: Geology, v. 36, p. 915–918, doi: 10 .1130 /G2252A .1 .

Enkelmann, E., Zeitler, P.K., Pavlis, T.L., Garver, J.I., and Ridgway, K.D., 2009, Intense localized rock uplift and erosion in the St Elias orogen of Alaska: Nature Geo-science, v. 2, p. 360–363, doi: 10 .1038 /ngeo502 .

Falkowski, S., Enkelmann, E., and Ehlers, T.A., 2014, Con-straining the area of rapid and deep-seated exhumation at the St. Elias syntaxis, southeast Alaska, with detrital zircon fi ssion-track analysis: Tectonics, v. 33, p. 597–616, doi: 10 .1002 /2013TC003408 .

Farley, K.A., 2000, Helium diffusion from apatite: General behavior as illustrated by Durango fl uorapatite: Journal of Geophysical Research, v. 105, p. 2903–2914, doi: 10 .1029 /1999JB900348 .

Farley, K.A., 2002, (U-Th)/He dating: Techniques, cali-brations, and applications, in Porcelli, D., et al., eds., Noble gases in geochemistry and cosmochemistry: Mineralogical Society of America Reviews of Miner-alogy Volume 47, p. 819–844, doi: 10 .2138 /rmg .2002 .47 .18 .

Farley, K.A., and Stockli, D.F., 2002, (U-Th)/He dating of phosphates: Apatite, monazite, and xenotime, in Kohn, M., et al., eds., Phosphates: Mineralogical Society of America Reviews of Mineralogy Volume 48, p. 559–578, doi: 10 .2138 /rmg .2002 .48 .15 .

Farley, K.A., Wolf, R.A., and Silver, L.T., 1996, The effects of long alpha-stopping distances on (U-Th)/He ages:

on January 28, 2015geosphere.gsapubs.orgDownloaded from

Page 15: Focused rock uplift above the subduction décollement at ...earthsci.fullerton.edu/parmstrong/Geosphere-2015-Ferguson-144-59.… · 144 Focused rock uplift above the subduction décollement

Ferguson et al.

158 Geosphere, February 2015

Geochimica et Cosmochimica Acta, v. 60, p. 4223–4229, doi: 10 .1016 /S0016 -7037 (96)00193 -7 .

Ferris, A., Abers, G.A., Christensen, D.H., and Veenstra, E., 2003, High resolution image of the subducted Pacifi c (?) plate beneath central Alaska, 50–150 km depth: Earth and Planetary Science Letters, v. 214, p. 575–588, doi: 10 .1016 /S0012 -821X (03)00403 -5 .

Finzel, E.S., Trop, J.M., Ridgway, K.D., and Enkelmann, E., 2011, Upper plate proxies for fl at-slab subduction processes in southern Alaska: Earth and Planetary Sci-ence Letters, v. 303, p. 348–360, doi: 10 .1016 /j .epsl .2011 .01 .014 .

Fitzgerald, P., Sorkhabi, R., Redfi eld, T.F., and Stump, E., 1995, Uplift and denudation of the central Alaska Range: A case study in the use of apatite fi ssion track thermochronology to determine absolute uplift parameters: Journal of Geophysical Research, v. 100, p. 20,175–20,191, doi: 10 .1029 /95JB02150 .

Fletcher, H.J., and Freymueller, J.T., 2003, New constraints on the motion of the Fairweather fault, Alaska, from GPS observations: Geophysical Research Letters, v. 30, p. 1139, doi: 10 .1029 /2002GL016476 .

Flowers, R.M., Ketcham, R.A., Shuster, D.L., and Farley, K.A., 2009, Apatite (U-Th)/He thermochronometry using a radiation damage accumulation and anneal-ing model: Geochimica et Cosmochimica Acta, v. 73, p. 2347–2365, doi: 10 .1016 /j .gca .2009 .01 .015 .

Fuis, G.S., and 13 others, 2008, Trans-Alaska Crustal Tran-sect and continental evolution involving subduction underplating and synchronous foreland thrusting: Geology, v. 36, p. 267–270, doi: 10 .1130 /G24257A .1 .

Garver, J.I., Davidson, C., Hilbert-Wolf, H., and Carlson, B., 2012, Provenance and thermal evolution of fl ysch of the Chugach-Prince William terrane, Alaska: Geologi-cal Society of America Abstracts with Programs, v. 44, no. 7, p. 72.

Garver, J.I., Davidson, C.M., DeLuca, M.J., Pettiette, R.A., Roe, C.F., and Hilbert-Wolf, H., 2013, Constraints of the original setting of the fl ysch of the Chugach and Prince William terranes in Alaska using detrital zircon: Cordilleran Tectonics Workshop, Kingston, Ontario, Abstracts with Program, 27–30 October, session 84–2, p. 21–23, http:// minerva .union .edu /garverj /alaska /garver _CTW _2013 _abstracts .pdf.

Gleadow, A.J., and Duddy, I.R., 1981, A natural long-term annealing experiment for apatite: Nuclear Tracks and Radiation Measurements, v. 5, p. 169–174, doi: 10 .1016 /0191 -278X (81)90039 -1 .

Gleadow, A.J., Duddy, I.R., Green, P.F., Laslett, G.M., and Lovering, J.F., 1986, Confi ned fi ssion track lengths in apatite—A diagnostic tool for thermal history analy-sis: Contributions to Mineralogy and Petrology, v. 94, p. 405–415, doi: 10 .1007 /BF00376334 .

Gutscher, M.A., and Peacock, S.M., 2003, Thermal models of fl at subduction and the rupture zone of great sub-duction earthquakes: Journal of Geophysical Research, v. 108, 2009, doi: 10 .1029 /2001JB000787 .

Haeussler, P.J., 2008, An overview of the neotectonics of interior Alaska: Far-fi eld deformation from the Yaku-tat microplate collision, in Freymueller, J.T., et al., eds., Active tectonics and seismic potential of Alaska: American Geophysical Union Geophysical Mono-graph 179, p. 83–108, doi: 10 .1029 /179GM05 .

Haeussler, P.J., Bradley, D.C., Goldfarb, R.J., Snee, L.W., and Taylor, C., 1995, Link between ridge subduction and gold mineralization in southern Alaska: Geology, v. 23, p. 995–998, doi: 10 .1130 /0091 -7613 (1995)023 <0995: LBRSAG>2 .3 .CO;2 .

Haeussler, P.J., Bradley, D.C., Wells, R.E., and Miller, M.L., 2003, Life and death of the Resurrection plate: Evidence for its existence and subduction in the north-eastern Pacifi c in Paleocene–Eocene time: Geological Society of America Bulletin, v. 115, p. 867–880, doi: 10 .1130 /0016 -7606 (2003)115 <0867: LADOTR>2 .0 .CO;2 .

Haeussler, P.J., O’Sullivan, P., Berger, A.L., and Spotila, J.A., 2008, Neogene exhumation of the Tordrillo Mountains, Alaska, and correlations with Denali (Mount McKin-ley), in Freymueller, J.T., et al., eds., Active tectonics and seismic potential of Alaska: American Geophysi-cal Union Geophysical Monograph 179, p. 269–285, doi: 10 .1029 /179GM158 .

Haeussler, P.J., Armstrong, P.A., Liberty, L., Ferguson, K., Finn, S., Arkle, J., and Pratt, T., 2011, Focused exhu-mation along megathrust splay faults in Prince Wil-liam Sound, Alaska: American Geophysical Union fall meeting, abs. T41A-06.

Haeussler, P.J., Finn, S.P., Armstrong, P.A., Arkle, J.C., Liberty , L.M., and Pratt, T.L., 2014, Focused exhuma-tion along megathrust splay faults in Prince William Sound, Alaska: Quaternary Science Reviews, doi: 10 .1016 /j .quascirev .2014 .10 .013 (in press).

Headley, R.M., Enkelmann, E., and Hallet, B., 2013, Exami-nation of the interplay between glacial processes and exhumation in the Saint Elias Mountains, Alaska: Geo-sphere, v. 9, p. 229–241, doi: 10 .1130 /GES00810 .1 .

Herman, F., Seward, D., Valla, P.G., Carter, A., Kohn, B., Willett, S.D., and Ehlers, T.A., 2013, Worldwide accel-eration of mountain erosion under a cooling climate: Nature, v. 504, p. 423–426, doi: 10 .1038 /nature12877 .

Hilbert-Wolf, H.L., 2012, U/Pb detrital zircon provenance of the fl ysch of the Paleogene Orca Group, Chugach–Prince William terrane, Alaska, in Varga, R., ed., Pro-ceedings of the Twenty-Fifth Keck Research Sympo-sium in Geology, Amherst, Massachusetts: Pomona College, Claremont, California, Keck Geology Con-sortium, p. 23–32.

Huang, S.P., Pollack, H.N., and Shen, P.Y., 2008, A late Qua-ternary climate reconstruction based on borehole heat fl ux data, borehole temperature data, and the instru-mental record: Geophysical Research Letters, v. 35, L13703, doi: 10 .1029 /2008GL034187 .

House, M.A., Gurnis, M., Sutherland, R., and Kamp, P.J.J., 2005, Patterns of late Cenozoic exhumation deduced from apatite and zircon U-He ages from Fiordland, New Zealand: Geochemistry Geophysics Geosystems, v. 6, Q09013, doi: 10 .1029 /2005GC000968 .

Hudson, T., Plafker, G., and Peterman, Z.E., 1979, Paleo-gene anatexis along the Gulf of Alaska margin: Geol-ogy, v. 7, p. 573–577, doi: 10 .1130 /0091 -7613 (1979)7 <573: PAATGO>2 .0 .CO;2 .

Idleman, B., Trop, J.M., and Ridgway, K.D., 2011, Geo-chronological evidence for rapid forearc subsidence and sedimentation during Paleogene spreading ridge subduction along the southern Alaska convergent mar-gin: Geological Society of America Abstracts with Pro-grams, v. 43, no. 5, p. 439.

Ikari, M.J., Saffer, D.M., and Marone, C., 2009, Frictional and hydrologic properties of clay-rich fault gouge: Journal of Geophysical Research, v. 114, B05409, doi: 10 .1029 /2008JB006089 .

Johnson, E., 2012, Origin of Late Eocene granitoids in west-ern Prince William Sound, Alaska, in Varga, R., ed., Proceedings of the Twenty-Fifth Keck Research Sym-posium in Geology, Amherst, Massachusetts: Pomona College, Claremont, California, Keck Geology Con-sortium, p. 33–39.

Kame, N., Rice, J.R., and Dmowska, R., 2003, Effects of pre-stress state and rupture velocity on dynamic fault branching: Journal of Geophysical Research, v. 108, 2265, doi: 10 .1029 /2002JB002189 .

Kappelmeyer, G., and Haenel, R., 1974, Geothermics with special reference to application: Berlin, Gebruder Borntraege, 238 p.

Kaufman, D.S., and Manley, W.F., 2004, Pleistocene maxi-mum and late Wisconsin glacier extents across Alaska, U.S.A., in Ehlers, J., and Gibbard, P.L., eds., Qua-ternary glaciations—Extent and chronology, Part II: North America: Developments in Quaternary Science Volume 2: Amsterdam, Elsevier, p. 9–27.

Kveton, K.J., 1989, Structure, thermochronology, prove-nance and tectonic history of the Orca Group in south-western Prince William Sound, Alaska [Ph.D. thesis]: Seattle, University of Washington, 402 p.

Lagoe, M.B., and Zellers, S.D., 1996, Depositional and microfaunal response to Pliocene climate change and tectonics in the eastern Gulf of Alaska: Marine Micro-paleontology, v. 27, p. 121–140, doi: 10 .1016 /0377 -8398 (95)00055 -0 .

Lagoe, M.B., Eyles, C.H., Eyles, N., and Hale, C., 1993, Timing of late Cenozoic tidewater glaciation in the far North Pacifi c: Geological Society of America Bul-letin, v. 105, p. 1542–1560, doi: 10 .1130 /0016 -7606 (1993)105 <1542: TOLCTG>2 .3 .CO;2 .

Liberty, L.M., and Finn, S., 2010, High resolution imaging of megathrust splay faults in Prince William Sound, Alaska, Department of Geosciences, Boise State University, U.S. Geological Survey Project Award G09AP00049, 19 p.

Liberty, L.M., Finn, S., Haeussler, P.J., Pratt, T., and Peter-son, A., 2013, Megathrust splay faults at the focus of the Prince William Sound asperity, Alaska: Journal of Geophysical Research, v. 118, p. 5428–5441, doi: 10 .1002 /jgrb .50372 .

Little, T., and Naeser, C., 1989, Tertiary tectonics of the Bor-der Ranges fault system, Chugach Mountains, Alaska: Deformation and uplift in a forearc setting: Journal of Geophysical Research, v. 94, p. 4333–4359, doi: 10 .1029 /JB094iB04p04333 .

Madsen, J.K., Thorkelson, D.J., Friedman, R.M., and Mar-shall, D.D., 2006, Cenozoic to recent plate confi gura-tions in the Pacifi c Basin: Ridge subduction and slab window magmatism in western North America: Geo-sphere, v. 2, p. 11–34, doi: 10 .1130 /GES00020 .1 .

Malloy, R.J., and Merrill, G.F., 1972, Vertical crustal move-ment on the sea fl oor, in The great Alaska earthquake of 1964: Volume 6, Oceanography and Coastal Engi-neering: Washington, D.C., National Academy of Sci-ences, p. 252–265.

Mancktelow, N.S., and Grasemann, B., 1997, Time-depen-dent effects of heat advection and topography on cool-ing histories during erosion: Tectonophysics, v. 270, p. 167–195, doi: 10 .1016 /S0040 -1951 (96)00279 -X .

Mankhemthong, N., Doser, D.I., and Pavlis, T.L., 2013, Interpretation of gravity and magnetic data and devel-opment of two-dimensional cross-sectional models for the Border Ranges fault system, south-central Alaska: Geosphere, v. 9, p. 242–259, doi: 10 .1130 /GES00833 .1 .

McAleer, R.J., Spotila, J.A., Enkleman, E., and Berger, A.L., 2009, Exhumation along the Fairweather fault, south-eastern Alaska, based on low-temperature thermo-chronometry: Tectonics, v. 28, TC1007, doi: 10 .1029 /2007TC002240 .

Meigs, A., and Sauber, J., 2000, Southern Alaska as an example of the long-term consequences of mountain building under the infl uence of glaciers: Quaternary Science Reviews, v. 19, p. 1543–1562, doi: 10 .1016 /S0277 -3791 (00)00077 -9 .

Meigs, A., Johnston, S., Garver, J., and Spotila, J., 2008, Crustal-scale structural architecture, shortening, and exhumation of an active, eroding orogenic wedge (Chugach/St Elias Range, southern Alaska): Tectonics, v. 27, TC4003, doi: 10 .1029 /2007TC002168 .

Moore, G.F., Bangs, N.L., Taira, A., Kuramoto, S., Pang-born, E., and Tobin, H.J., 2007, Three-dimensional splay fault geometry and implications for tsunami gen-eration: Science, v. 318, p. 1128–1131, doi: 10 .1126 /science .1147195 .

Naeser, C.W., 1979, Thermal history of sedimentary basins: Fission track dating of subsurface rocks, in Scholle, P., and Schluger, R., eds., Aspects of diagenesis: Society of Economic Paleontologists and Mineralogists Spe-cial Publication 26, p. 109–112, doi: 10 .2110 /pec .79 .26 .0109 .

Nelson, S.W., Dumoulin, J.A., and Miller, M.L., 1985, Geo-logic map of the Chugach National Forest, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-1645-B, 16 p., scale 1:250,000.

O’Sullivan, P.B., Plafker, G., and Murphy, J.M., 1997a, Apa-tite fi ssion-track thermotectonic history of crystalline rocks in the northern St. Elias Mountains, Alaska, in Domoulin, J.A., and Gray, J.E., eds., Geological stud-ies in Alaska by the U.S. Geological Survey, 1995: U.S. Geological Survey Professional Paper 1574, p. 283–293.

O’Sullivan, P.B., Murphy, J.M., and Blythe, A.E., 1997b, Late Mesozoic and Cenozoic thermotectonic evolution of the central Brooks Range and adjacent North Slope foreland basin, Alaska: Including fi ssion track results from the Trans-Alaska Crustal Transect (TACT): Journal of Geophysical Research, v. 102, no. B9, p. 20,821–20,845, doi: 10 .1029 /96JB03411 .

Park, J.O., Tsuru, T., Kodaira, S., Cummins, P.R., and Kaneda, Y., 2002, Splay fault branching along the Nan-kai subduction zone: Science, v. 297, p. 1157–1160, doi: 10 .1126 /science .1074111 .

on January 28, 2015geosphere.gsapubs.orgDownloaded from

Page 16: Focused rock uplift above the subduction décollement at ...earthsci.fullerton.edu/parmstrong/Geosphere-2015-Ferguson-144-59.… · 144 Focused rock uplift above the subduction décollement

Focused rock uplift at Montague and Hinchinbrook Islands

Geosphere, February 2015 159

Pavlis, T.L., 2013, Kinematic model for out-of-sequence thrusting: Motion of two ramp-flat faults and the production of upper plate duplex systems: Journal of Structural Geology, v. 51, p. 132–143, doi: 10 .1016 /j .jsg .2013 .02 .003 .

Pavlis, T.L., Chapman, J.B., Bruhn, R.L., Ridgway, K., Worthington, L.L., Gulick, S.P.S., and Spotila, J., 2012, Structure of the actively deforming fold-thrust belt of the St. Elias orogen with implications for glacial exhu-mation and three dimensional tectonic processes: Geo-sphere, v. 8, p. 991–1019, doi: 10 .1130 /GES00753 .1 .

Péwé, T.L., 1975, Quaternary geology of Alaska: U.S. Geo-logical Survey Professional Paper 835, p. B1–B145, scale 1:500,000.

Plafker, G., 1965, Tectonic deformation associated with the 1964 Alaska earthquake: Science, v. 148, p. 1675–1687, doi: 10 .1126 /science .148 .3678 .1675 .

Plafker, G., 1967, Surface faults on Montague Island asso-ciated with the 1964 earthquake, in The Alaska earth-quake, March 27, 1964: Regional effects: U.S. Geo-logical Survey Professional Paper 543, p. G1–G42, 2 sheets, scales 1:31,680, ~1:20,000.

Plafker, G., 1969, Tectonics of the March 27, 1964 Alaska earthquake, in The Alaska earthquake, March 27, 1964: Regional effects: U.S. Geological Survey Professional Paper 543, p. I1–I74 p., 2 sheets, scales 1:2,000,000, 1:500,000.

Plafker, G., 1987, Regional geology and petroleum potential of the northern Gulf of Alaska continental margin, in Scholl, D.W., et al., eds., Geology and resource poten-tial of the continental margin of western North America and adjacent ocean basins–Beaufort Sea to Baja Cali-fornia: Circum-Pacifi c Council for Energy and Mineral Resources Earth Sciences Series, v. 6, p. 299–268.

Plafker, G., and Berg, H.C., 1994, Overview of the geol-ogy and tectonic evolution of Alaska, in Plafker, G., and Berg, H.C., eds., The geology of Alaska: Boulder, Colorado, Geological Society of America, Geology of North America, v. G-1, p. 989–1021.

Plafker, G., Nokleberg, W., and Lull, J., 1989, Bedrock geology and tectonic evolution of the Wrangellia, Peninsular, and Chugach terranes along the Trans-Alaska Crustal Transect in the Chugach Mountains and southern Copper River Basin, Alaska: Journal of Geo-physical Research, v. 94, p. 4255–4295, doi: 10 .1029 /JB094iB04p04255 .

Plafker, G., Moore, J.C., and Winkler, G., 1994, Geology of the southern Alaska margin, in Plafker, G., and Berg, H.C., eds., The geology of Alaska: Boulder, Colorado, Geological Society of America, Geology of North America, v. G-1, p. 389–449.

Plattner, C., Malservisi, R., Dixon, T.H., LaFemina, P., Sella, G.F., Fletcher, J., and Suarez-Vidal, F., 2007, New con-straints on relative motion between the Pacifi c plate and Baja California microplate (Mexico) from GPS measurements: Geophysical Journal International, v. 170, p. 1373–1380, doi: 10 .1111 /j .1365 -246X .2007 .03494 .x .

Powell, W.G., Chapman, D.S., Balling, N., and Beck, A.E., 1988, Continental heat fl ow density, in Haenel, R., et al., eds., Handbook of terrestrial heat-fl ow density determinations: Boston, Massachusetts, Kluwer Aca-demic, p. 167–222.

Ratchkovski, N.A., and Hansen, R.A., 2002, New evidence for segmentation of the Alaska subduction zone: Seis-mological Society of America Bulletin, v. 92, p. 1754–1765, doi: 10 .1785 /0120000269 .

Reiners, P.W., and Brandon, M.T., 2006, Using thermochro-nology to understand orogenic erosion: Annual Review of Earth and Planetary Sciences, v. 34, p. 419–466, doi: 10 .1146 /annurev .earth .34 .031405 .125202 .

Reiners, P.W., Ehlers, T.A., Mitchell, S.G., and Montgom-ery, D.R., 2003, Coupled spatial variations in precipita-tion and long-term erosion rates across the Washington Cascades: Nature, v. 426, p. 645–647, doi: 10 .1038 /nature02111 .

Reiners, P.W., Spell, T.L., Nicolescu, S., and Zanetti, K.A., 2004, Zircon (U-Th)/He thermochronometry: He dif-fusion and comparisons with 40Ar/39Ar dating: Geo-chimica et Cosmochimica Acta, v. 68, p. 1857–1887, doi: 10 .1016 /j .gca .2003 .10 .021 .

Spotila, J.A., and Berger, A.L., 2010, Exhumation at oro-genic indentor corners under long-term glacial con-ditions: Example of the St. Elias orogen, southern Alaska: Tectonophysics, v. 490, p. 241–256, doi: 10 .1016 /j .tecto .2010 .05 .015 .

Spotila, J.A., Farley, K.A., and Sieh, K., 1998, Uplift and erosion of the San Bernardino Mountains associ-ated with transpression along the San Andreas fault, California, as constrained by radiogenic helium ther-mochronometry: Tectonics, v. 17, p. 360–378, doi: 10 .1029 /98TC00378 .

Spotila, J.A., Buscher, J.T., Meigs, A.J., and Reiners, P.W., 2004, Long-term glacial erosion of active mountain belts: Example of the Chugach–St. Elias Range, Alaska: Geology, v. 32, p. 501–504, doi: 10 .1130 /G20343 .1 .

Stüwe, K., White, L., and Brown, R., 1994, The infl uence of eroding topography on steady-state isotherms. Appli-cation to fi ssion track analysis: Earth and Planetary Science Letters, v. 124, p. 63–74, doi: 10 .1016 /0012 -821X (94)00068 -9 .

Sutherland, R., Gurnis, M., Kamp, P.J.J., and House, M.A., 2009, Regional exhumation history of brittle crust dur-ing subduction initiation, Fiordland, southwest New Zealand, and implications for thermochronologic sam-pling and analysis strategies: Geosphere, v. 5, p. 409–425, doi: 10 .1130 /GES00225 .SA2 .

Tomkin, J.H., 2007, Coupling glacial erosion and tectonics at active orogens: A numerical modeling study: Journal of Geophysical Research, v. 112, F02015, doi: 10 .1029 /2005JF000332 .

Turcotte, D., and Schubert, G., 2002, Geodynamics (second edition): Cambridge, UK, Cambridge University Press, 472 p.

Wagner, G.A., and Reimer, G.M., 1972, Fission track tec-tonics: The tectonic interpretation of fi ssion track ages: Earth and Planetary Science Letters, v. 14, p. 263–268, doi: 10 .1016 /0012 -821X (72)90018 -0 .

Willett, S.D., Slingerland, R., and Hovius, N., 2001, Uplift, shortening, and steady state topography in active mountain belts: American Journal of Science, v. 301, p. 455–485, doi: 10 .2475 /ajs .301 .4 -5 .455 .

Worthington, L.L., van Avendonk, H.J.A., Gulick, S.P.S., Christeson, G.L., and Pavlis, T.L., 2012, Crustal struc-ture of the Yakutat terrane and the evolution of sub-duction and collision in southern Alaska: Journal of Geophysical Research, v. 117, B01102, doi: 10 .1029 /2011JB008493 .

Zellers, S.D., 1995, Foraminiferal sequence biostratigraphy and seismic stratigraphy of a tectonically active mar-gin: The Yakataga Formation, northeastern Gulf of Alaska: Marine Micropaleontology, v. 26, p. 255–271, doi: 10 .1016 /0377 -8398 (95)00031 -3 .

Zweck, C., Freymueller, J.T., and Cohen, S.C., 2002, Three-dimensional dislocation modeling of the postseismic response to the 1964 Alaska earthquake: Journal of Geophysical Research, v. 107, 2064, doi: 10 .1029 /2001JB000409 .

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doi: 10.1130/GES01036.1 2015;11;144-159Geosphere

 Kelly M. Ferguson, Phillip A. Armstrong, Jeanette C. Arkle and Peter J. Haeussler Hinchinbrook Islands, Prince William Sound, AlaskaFocused rock uplift above the subduction décollement at Montague and  

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