Geological Society of America Bulletin, Published Online on 5 April 2013 As Doi:10.1130/B30738.1

Home , Chugach Mountains, Denali, Mount Marcus Baker, Mount Muir (Alaska)

Geological Society of America Bulletin, published online on 5 April 2013 as doi:10.1130/B30738.1

Geological Society of America Bulletin

Focused exhumation in the syntaxis of the western Chugach Mountains and Prince William Sound, Alaska

Jeanette C. Arkle, Phillip A. Armstrong, Peter J. Haeussler, Michael G. Prior, Sean Hartman, Kassandra L. Sendziak and Jade A. Brush

Geological Society of America Bulletin published online 5 April 2013; doi: 10.1130/B30738.1

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Geological Society of America Bulletin, published online on 5 April 2013 as doi:10.1130/B30738.1

Focused exhumation in the syntaxis of the western Chugach Mountains and Prince William Sound, Alaska

Jeanette C. Arkle1,†, Phillip A. Armstrong1,§, Peter J. Haeussler2, Michael G. Prior1, Sean Hartman1, Kassandra L. Sendziak1, and Jade A. Brush1 1Geological Sciences, California State University–Fullerton, 800 N. State College Boulevard, Fullerton, California 92834, USA 2U.S. Geological Survey, 4210 University Drive, Anchorage, Alaska 99508, USA

ABSTRACT ~11 km of rock uplift north of the Contact fi ned by the maximum curvature of major fault fault and ~4–5 km of rock uplift in Prince systems and topographic trends (Figs. 1 and The western Chugach Mountains and William Sound to the south. These data are 2A), and thus it is a critical region for a more Prince William Sound are located in a syn- consistent with a deformation model where complete understanding of the effects of plate- taxial bend, which lies above fl at-slab subduc- the western Chugach core has approached boundary deformation in southern Alaska. tion of the Yakutat microplate and inboard of long-term exhumational steady state, though Long-term deformation in the western the Yakutat collision zone of southern Alaska. exhumation rates have probably increased Chugach Mountains is evident by high peaks The syntaxis is characterized by arcuate fault in the last ~5 m.y. We interpret that rock up- that reach elevations of ~4 km and rugged topog- systems and steep, high topography, which lift in the overriding wedge has been driven raphy, constituting the greatest topographic re- suggest focused uplift and exhumation of the dominantly by underplating, with long-term lief of any emergent accretionary prism in the accretionary prism. We examined the exhu- vertical displacement concentrated at the world. Much of the topography is infl uenced by mation history with low-temperature thermo- southern edge of the Chugach Mountains and glacial erosion, as this area retains one third of chronometry of 42 samples collected across centered on the Contact fault system. Though the present glaciated area of Alaska and is situ- the region. These new apatite (U-Th)/He, our data do not unequivocally differentiate ated along the southern Alaska coast exposed to apatite fi ssion-track, zircon (U-Th)/He, and between Pliocene tectonic- or climate-related high coastal precipitation. The western Chugach zircon fi ssion-track ages, combined with ages causes for increased exhumation in the last syntaxis as defi ned in this study is the area lo- from surrounding regions, show a bull’s- ~5 m.y., we interpret the increased rates to be cated south of the Border Ranges fault system eye pattern, with the youngest ages focused due to increased infl ux of underplated sedi- and includes northern Prince William Sound, on the western Chugach syntaxis. The ages ments that are derived from erosion in the where the topographic grain and the general have ranges of ca. 10–4 Ma, ca. 35–11 Ma, ca. Saint Elias orogen collision zone. strike of bedding, cleavage, and major fault 33–25 Ma, and ca. 44–27 Ma, respectively. The systems display the greatest curvature between youngest ages are located on the south (wind- INTRODUCTION the Kenai Mountains to the west and the eastern ward) side of the Chugach Mountains and Chugach–Saint Elias Mountains region to the just north of the Contact fault. Sequentially Many mechanisms have been proposed east (Figs. 1 and 2). The western Chugach syn- higher closure temperature systems are nested for driving rock uplift and mountain building taxis is part of the long-lived Alaskan orocline across Prince William Sound in the south, throughout Alaska, but most have been related (Carey, 1958; Glen, 2004), which is thought to the Chugach Mountains, and the Talkeetna to the collision and fl at-slab subduction of the have developed during the Late Cretaceous and Mountains to the north. Computed exhu- Yakutat microplate, which began subducting in Paleocene (Coe et al., 1985; Plafker, 1987). We mation rates typically are 0.2 mm/yr across the Cenozoic (Plafker, 1987; Bruhn et al., 2004; refer to the region roughly surrounding Mount Prince William Sound, increase abruptly to Haeussler, 2008) and possibly as early as the late Marcus Baker in the western Chugach Moun- ~0.7 mm/yr across and adjacent to the Con- Eocene–early Oligocene (Finzel et al., 2011). tains and northern Prince William Sound, where tact fault system, and decrease to ~0.4 mm/yr The subducted Yakutat microplate has been geo- rock uplift and exhumation rates are highest, as north of the core of the Chugach Mountains. physically imaged (Eberhart-Phillips et al., 2006; the Chugach core (Fig. 1). High rock uplift and The abrupt age and exhumation rate changes Fuis et al., 2008), and these images show that the exhumation within structural syntaxes is com- centered on the Contact fault system suggest subducted Yakutat microplate extends from the mon in orogenic belts throughout Alaska (e.g., that it may be a critical structural system for Saint Elias Mountains region in the southeast to Fitzgerald et al., 1995; O’Sullivan and Cur- facilitating rock uplift. Our data are most the Alaska Range in the northwest (Fig. 1). The rie, 1996; Enkelmann et al., 2009; Spotila and consistent with Yakutat fl at-slab subduc- Yakutat collision is driving deformation into Berger, 2010) and elsewhere (e.g., Zeitler et al., tion starting in the Oligocene, and since then central interior Alaska (e.g., Haeussler, 2008); 2001; Koons et al., 2002; Finnegan et al., 2008), however, the timing of mountain building in the and thus it is important to understand the causes western Chugach Mountains and Prince Wil- and mechanisms of deformation in different †Current address: Department of Geology, Univer- sity of Cincinnati, P.O. Box 0013, Cincinnati, Ohio liam Sound region and its relationship to Yakutat types of orogenic systems. 45221, USA. collision and subduction remain unclear. This The purpose of this study is to quantify ex- §E-mail: [email protected] region is composed of a major syntaxis, as de- humation in the western Chugach syntaxis and

GSA Bulletin; Month/Month 2013; v. 1xx; no. X/X; p. 1–18; doi: 10.1130/B30738.1; 9 ﬁ gures; 2 tables; Data Repository item 2013172.

Arkle et al.

64°N 150°W 140°W Much of the permanent Yakutat-related defor- Mt. Hayes eastern C A mation in southern Alaska is accommodated in Alaska Range U N . A S. D the Saint Elias orogen, located southeast of the A Chugach core, and many studies have addressed Mt.McKinley Denali Fault Denali Fault central deformation and exhumation in this orogenic Alaska Range belt (e.g., O’Sullivan and Currie, 1996; Bruhn Talkeetna Wrangell Kilometers et al., 2004; Pavlis et al., 2004; Spotila et al., Mountains Mountains 100 61°N 2004; Meigs et al., 2008; Berger et al., 2008a, 2008b; Berger and Spotila, 2008; Enkelmann Tordrillo Mt.Marcus Borde et al., 2008, 2009; Spotila and Berger, 2010). Mountains Chugach r Ranges Fault Denali F Baker Mountains stle Mountain Fault Mt. Logan Thermochronology data from the Saint Elias Ca ault Anchorage Contact Fault St. Elias Mountains Mountains and eastern Chugach Mountains, es- Kenai Prince CSEF pecially around the Bering and Malaspina Gla- Mountains William Sound BG MG ciers, document the highest exhumation rates Mt. 59°N in Alaska of ~4 mm/yr (Spotila et al., 2004; SAB Fairweather PZ 50 mm/yr Berger et al., 2008a, 2008b; Berger and Spotila, Cook Inlet pole KIZ YAKUTAT 2008; Enkelmann et al., 2008, 2009; Spotila and MICROPLATE DRZ Fairwea order Ranges Fault Berger, 2010). These authors attributed rapid B tact Fault ther F 55 mm/yr Transition Fault and recent rock uplift in this region to the out- Con a 59°N PACIFIC ult board location along the current oblique Yakutat PLATE Aleutian collision front, the position in structural syn- Megathrust taxes, and extreme glacial erosion. Figure 1. Regional 300 m digital elevation model (DEM) of Alaska showing major faults Sparse apatite helium age data in the Chugach (after Plafker et al., 1994; U.S. Geological Survey fault data repository) and relative plate and Kenai Mountains around the periphery of motion vectors (DeMets et al., 1994; Leonard et al., 2007; Elliott et al., 2010). Approxi- the western Chugach core (Fig. 2A) indicate mate region of the Yakutat microplate is shown by thick dashed line; approximated from relatively slow exhumation rates of ~0.2 mm/yr Fletcher and Freymueller (2003), Eberhart-Phillips et al. (2006), and Fuis et al. (2008). (Buscher et al., 2008) since the mid-Tertiary. SAB is the pole of southern Alaska block rotation approximately located from Haeussler These data imply that far slower exhumation et al. (2008). The dashed box outlines the western Chugach Mountains and Prince William has occurred above the subduction zone than Sound study area shown in Figure 2, and the dashed ellipse shows the approximate region expected and yield rock uplift and exhumation of the Chugach core as defi ned in this study. KIZ—Kayak Island zone, PZ—Pamplona rates that seem enigmatic relative to the rugged zone, DRZ—Dangerous River zone, CSEF—Chugach–Saint Elias fault, BG—Bering Gla- topography and high relief. Buscher et al. (2008) cier, MG—Malaspina Glacier. proposed rapid exhumation in the Chugach core, but they did not have data from that region. to investigate mechanisms of mountain build- 2008). Elastic dislocation models indicate the Rocks of the Western Chugach Mountains ing. We present new low-temperature thermo- Yakutat and North American plates are coupled and Prince William Sound chronology age constraints on the spatial and below Prince William Sound and that the area temporal patterns of exhumation that allow us is partly or completely locked at the megathrust Southern Alaska is dominated by a number to: (1) quantify the magnitude of rock uplift and interface (Zweck et al., 2002). This high degree of accreted composite terranes that become exhumation; (2) provide new estimates of the of plate coupling may be strongly related to younger progressively southward. The study long-term exhumation rates; and (3) interpret Yaku tat underplating and structural discontinui- area consists mainly of two composite terranes the evolution of the western Chugach Moun- ties that are geophysically imaged below parts (Fig. 2A): (1) the Mesozoic Chugach composite tains in regard to the styles and causes of defor- of the western Chugach Mountains and Prince terrane to the north and (2) the Paleogene Prince mation and uplift. Our fi ndings provide insight William Sound (Eberhart-Phillips et al., 2006; William composite terrane that rims the south- into the understanding of the near- and far-fi eld Fuis et al., 2008). The difference between ob- central Alaskan margin (Plafker et al., 1994; effects of fl at-slab subduction, and its timing, in served and modeled global positioning system Plafker and Berg, 1994). south-central Alaska. (GPS) velocities shows NW-directed horizontal The Chugach accretionary complex, some- velocities of ~5–10 mm/yr in the region between times referred to as a terrane, consists of the GEOLOGIC SETTING the Border Ranges fault zone and Contact fault Permian to mid-Cretaceous McHugh Complex in the western Chugach Mountains (Suito and and the Late Cretaceous Valdez Group (Bradley Tectonic Setting Freymueller, 2009). The Border Ranges–Con- et al., 2003). The study area is dominated by tact fault region, however, only accounts for a the Valdez Group, which varies from lithic-rich The western Chugach Mountains and Prince small fraction of the total relative North Ameri- metasedimentary and volcaniclastic graywacke, William Sound make up a region of recent tec- can–Yakutat northwest convergence of ~45–50 to fl ysch and turbidite sequences with sand- tonic activity, as indicated by the 1964 Mw 9.2 mm/yr (DeMets et al., 1994; Fletcher and Frey- stone compositions (dominantly quartzofeld- megathrust earthquake, which attests to the mueller, 2003; Freymueller et al., 2008; Elliott spathic), and minor basalt (Plafker et al., 1994). accumulation of elastic strain in the Alaska et al., 2010) or the 35–57 mm/yr northwest ve- These rocks were metamorphosed from zeolite forearc, at least over decadal or shorter time locity of Prince William Sound area relative to to greenschist facies in the study area (Plafker scales (Zweck et al., 2002; Freymueller et al., North America (Freymueller et al., 2008). et al., 1994).

2 Geological Society of America Bulletin, Month/Month 2012 Geological Society of America Bulletin, published online on 5 April 2013 as doi:10.1130/B30738.1

Focused exhumation in the syntaxis of the western Chugach Mountains and Prince William Sound, Alaska 60° 62°N 61°N 61°N 60°N 62°N 145°W 145°W Ma 1 5 15 25 70 10 Cordova Cordova ult a 040 Kilometers Valdez Valdez

ct Fault

Conta Contact F Fault Chugach Chugach Mountains Mountains

Sound Sound

nges nges a Prince William Prince William Prince William Prince William

ing Bay fault, WP—Wells Passage , Passage WP—Wells ing Bay fault,

order R order

Border Ranges Fault Ranges Border B Baker Baker owing studies: P, C, and W—this study; C, and owing studies: P,

Mt. Marcus Mt. Marcus

ault ault

Talkeetna Talkeetna

Mountains Mountains

ontact F ontact

Contact F Contact C

Fault ain Kenai Kenai

150° ault ault F Mountains Mountains

150°W

order Ranges Ranges order

B Border Ranges F Ranges Border

Castle Mount Castle Castle Mountain Fault Mountain Castle Anchorage Anchorage 61°N 60°N 62°N 61°N 60° 62°N C Ages AFT B AHe Ages .9 - - - B2 17 62°N 60°N 61°N Cordova - - - 11.5 B5 - - - 4.8 B8 ssion-track ages (C) with faults (solid lines), inferred faults (dashed lines), and ssion-track ages (C) with faults (solid lines), inferred Kilometers 146°W Cretaceous Valdez Group Sandstone Permian to Cretaceous McHugh Complex Composite Wrangellia & Other Rocks Terrane Valdez 5 01020304050 HI A Chugach Mountains

CG Contact Fault Sound 147°W P6 - 5.6 21.1 33.9 - - P5 18.1 16.4

P7 - - - 15.0

Prince William Prince William

- - - H2 9.7

Border Ranges Fault Ranges Border y Fault y 7.2 - - P12 10.0 - P4 - - 20.7

MI - - - P65 6.6

Paleocene-Eocene Orca Group Sandstone Paleocene-Eocene Orca Group Conglomerate Paleocene-Eocene Ophiolites - - - UI

P19 27.0

C4 - - -

36.0 H3 Patton Ba Patton P21 3.8 - - Fault Strait Montague 6.5 P35 1.4 4.4 - - - LN6 - 9.3 47.8

- - - 3.8 P52 LN3 EF 6.6 - - P44 13.8 B′ 4.0 P16 6.5 25.3 - 9.7

- - P43 15.3

Baker

LN5 HBF

LN7 LN4 - 21.2 - 170.0 SB P37 9.7 - - 32.6 P53 5.3 8.4 - - K3 P34 12.1 - - 36.6

4.8 - P13 - LN9 10.7 1 Mt. Marcus

P42 16.0 37.4 - - College Fiord College K1 K K2 Talkeetna - - - 4.2 Mountains 31.3 - P66 7.6 P24 12.6 LCI LN8 - 17.5 - 49.6 K1 - - - 37.5 WP K6 - 34.0 - - K10 - 28.5 - 53.3 EXPLANATION P54 8.5 5.2 - - Island Esther

P3 9.1 - - 15.4

- 20.7 - 40.0 - - - K14 P56 26.2 CF K13 - - - H7 - - 130.4 - P67 10.8 HF

K9 P33 21.5 10.1 - - PNJ K4 - 32.5 - - K8 - - - P69 7.3 ------P64 P31 5.9 4.0 P68 8.4 - - 12.0 K11 K12 H6 4.9 P45 10.8 - - LN2 - 24.3 - 37.3 K5 - 39.2 - - K7 - - - P63 7.9 P58 6.4 - - 13.3 H8 - - - 76.2 - - - P32 B10 23.5 - - 12.8 18.1 Mt. 6.0 P2 - - Basin Deposits Oligocene Intrusions 12.4 Ice Paleocene-Eocene Intrusions - - - Muir P62 6.7 - - - P59 4.9 K11 ------28.2 44.0 K7 - - - C1 47.7 LN1 - 21.1 - 36.8 0 - - - K13 - - - 40.2 P70 4.7 - - - B9 25.2 ------P61 T6 38.5 5.1 - - - 8.2 P6 Icefield B12 14.9 - - - Sargent K3 - - - K9 - - - 35.1 43.4 Whittier

K12 - - - 36.9 - - - T1 20.4

- - - Fault Contact T3 18.1 ERT K2 - - - 39.1 - - - K8 - - - 46.3 WH1 15.7 - - - T2 17.7 - 33.1 - - K15 149°W

T12 73.4 - - - Arm

RB1 - 31.9 - - - - - T9 16.4 - - - B11 13.5

- H1 30.2 - - A′ Arm Knik T13 60.4 - - - This study

Turnagain Other samples Kenai - - - 22.0 B13 - - - H4 59.1 Mountains

Sample AHe AFT ZHe ZFT 150°W

Border Ranges Fault Ranges Border Sample legend - - -

B14 19.9 Castle Mountain Fault Mountain Castle 61° 60°N A Icefield Harding Inlet Cook Anchorage major lithologic units (after Plafker et al., 1989; U.S. Geological Survey fault data repository). Large bold letters and markers in A indicate cross-section locations for Fig- locations for indicate cross-section A in Large bold letters and markers et al., 1989; U.S. Geological Survey fault data repository). Plafker lithologic units (after major fault, EF—Eaglek HBF—Hann thrust, CF—Culross River same as in C. ERT—Eagle colors in B are Age contour 3, 4, and 6. ures , CG—Columbia Glacier Glacie r, YG—Yale PNJ—Port Nellie Juan, UI—Unakwik Inlet, HF—Harriman Fiord, HG—Harvard Glacier, Passage, SB—Squaw Bay, CP—Culross to the foll blocks correspond The letters in the sample number Island, LCI—Latouche MI—Montague Island. HI—Hinchinbrook et al. (2011). H—Hacker et al. (2008); (2006); B—Buscher Armstrong T—Hoffman and (1989); RB—Parry et al. (2001); K—Kveton (1989); LN—Little and Naeser Figure 2. Sample location map (A), contoured apatite helium ages (B), and contoured apatite ﬁ apatite helium ages (B), and contoured 2. Sample location map (A), contoured Figure

Geological Society of America Bulletin, Month/Month 2012 3 Geological Society of America Bulletin, published online on 5 April 2013 as doi:10.1130/B30738.1

Arkle et al.

The Prince William composite terrane is the systems and rugged topography. Numerous which we combine with ages from previously youngest part of the accretionary complex and faults are untraceable under glaciers and fi ords published studies (Fig. 2A). Most of the samples is composed of the late Paleocene to middle Eo- and/or are unidentifi able because of similar rock were collected at sea level in Prince William cene Orca Group, which consists of thick fl ysch types across them. However, the arcuate nature Sound from Montague Island in the south to the complexes, basaltic fragments, hemipelagic of the fi ords and glaciers that mirror the struc- northern reaches of fi ords that are carved deep mudstone, and conglomerate metamorphosed tural grain imply that faults are commonly pres- into the Chugach Mountains. Ten non-sea-level from prehnite-pumpellyite to greenschist facies ent and concealed. samples were collected (elevation to ~1.7 km) in (Winkler, 1992; Plafker et al., 1994; Nelson Mount Marcus Baker and adjoining peaks the western Chugach Mountains. et al., 1999; Bradley et al., 2003). reach elevations of ~4000 m in the western Samples were collected from Paleocene–early After Valdez and Orca Group deposition, plu- Chugach Mountains. Tidewater glaciers, which Eocene Sanak-Baranof belt near-trench granitic tons of gabbroic to granitic compositions were dominate northern Prince William Sound, move plutons, late Eocene–Oligocene intrusions, Late emplaced along the southern margin of Alaska at modern rates of tens to hundreds of meters Cretaceous Valdez Group sandstone, and Paleo- (Hudson et al., 1979; Plafker et al., 1994; Brad- per year (Field, 1975; Molnia, 2008) and erode cene–Eocene Orca Group sandstone and con- ley et al., 1993, 2003). Igneous intrusions crop massive areas, including the ~300-m-deep fi ord glomerate. These intrusions and sedimentary out within the study area from two discon- down gradient of the largest glacier in the sound, rocks mostly predate the initiation of Yakutat tinuous belts: (1) the Paleocene–early Eocene Columbia Glacier. Drainages along the northern collision (Table 1). Samples were also collected Sanak-Baranof belt intrusions, and (2) the late fl ank of the western Chugach Mountains feed from modern outwash deposits at the ends of two Eocene–Oligocene intrusions (Hudson et al., the Matanuska and Copper Rivers to the north glaciers on the north side of the Chugach Moun- 1979; Plafker et al., 1994). U-Pb ages on the and east. tains divide and two glaciers on the south side. Sanak-Baranof belt intrusions range from 61 to Detailed locations of the glacial deposit samples 48 Ma. U-Pb and Ar-Ar ages from the Oligo- THERMOCHRONOLOGY and the spatial extent of the glaciers are given in cene intrusions range from 39 to 29 Ma (Nelson the Data Repository (Fig. DR1 [see footnote 1]). et al., 1985; Winkler, 1992; Bradley et al., 1993, Thermochronology Background Details of the sample processing and analytical 2000; Haeussler et al., 2003). Smaller felsic to techniques are included in the Data Repository intermediate dikes, sills, and stocks are common This study focuses on the transport of ma- text (see footnote 1). General methods and spe- throughout the Valdez Group and Orca Group terial in Prince William Sound and western cifi cs for our study are given next. (Winkler, 1992; Plafker et al., 1994). Chugach Mountains through closure tempera- Apatite (U-Th)/He ages (AHe) were deter- tures upwards of ~240 °C. We use comple- mined for 38 samples. For each sample, four Geomorphic and Structural Setting mentary low-temperature thermochronologic to seven single-grain ages were determined by systems with different effective closure tem- laser and inductively coupled plasma–mass The diffuse tectonic regime that extends re- peratures; these temperatures are ~65 °C and spectrometry (ICP-MS) techniques at Caltech. gionally from the Aleutian Islands to British ~180 °C for apatite and zircon (U-Th)/He ages, Raw ages were corrected for alpha ejection Columbia is characterized in southern Alaska and ~110 °C and ~240 °C for apatite and zir- effects (Ft correction) following methods de- by an arcuate architecture between the Alaska con fi ssion-track ages, respectively (Wolf et al., scribed by Farley et al. (1996) and Farley (2002) Range to the north and the Chugach–Saint Elias 1996; Farley, 2002; Reiners, 2005; Reiners and (Table DR1 [see footnote 1]). Several samples Mountains to the south (Fig. 1). The arcuate Brandon, 2006; Ketcham et al., 2007; Flowers display single-grain age outliers; these outliers Border Ranges fault system may defi ne a back- et al., 2009). The combination of multiple (n = 27 out of 166 total grains) are nearly all stop to deformation where the Chugach terrane thermochronometers may provide general spa- signifi cantly older than the mean of the other is thrust up against the arc-basement Peninsular- tial and temporal constraints on exhumation rate single-grain ages in that sample. The percent- Wrangellia terrane (Little and Naeser, 1989; changes and causes of rock uplift, and provide age of outliers from our single-grain analy- Pavlis and Roeske, 2007; Fuis et al., 2008). insight into the degree of steady state of an oro- ses (16%) is higher than the 8%–10% outliers To the south, the arcuate Contact fault system gen (Brandon et al., 1998; Willett et al., 2001; from multiple-grain aliquot replicate analyses separates the Valdez and Orca Groups and is the Willett and Brandon, 2002; Batt and Brandon, from the Kenai Peninsula and western Chugach southern boundary between the Chugach and 2002; Ehlers and Farley, 2003; Reiners and Mountains (Buscher et al., 2008) or from the Prince William terranes. The kinematics and Brandon, 2006). Saint Elias region (Berger et al., 2008b; Spotila location of the Contact fault in this area are con- and Berger, 2010). These old age outliers are in- tentious because of uncertainties in dating, lack Field Approach and Methods terpreted to be the result of excess helium from of fossils for correlation, structural complexity, undetected inclusion phases such as zircon in and the similarity of metamorphism and lithol- Samples generally were collected along a the apatite grains. Two samples with four single- ogy (Dumoulin, 1988; Bol and Gibbons, 1992). northwest-trending, ~160-km-long transect grain ages each have one single-grain age that is This study addresses the location of the Con- that is perpendicular to the structural grain and signifi cantly younger than the other three ages tact fault system and possible phases of major subparallel to Yakutat motion (Fig. 2A). New (Table DR1 [see footnote 1]). We exclude these displacement. ages from 42 samples are reported in this study young outlier ages from the average age, but The western Chugach Mountains are situ- (Tables 1 and 2; Tables DR1, DR2, and DR31), this does not signifi cantly affect our results and ated along the southern Alaska coast and are interpretations. After outlier removal, the aver- exposed to abundant coastal precipitation and 1GSA Data Repository item 2013172, Fission- age relative standard deviation for 34 of the AHe glaciation (Péwé, 1975; Post and Meier, 1980; track and (U-Th)/He tables and analytical methods, ages is ±17.2% (1σ). Four AHe ages reproduce temperature-time modeling, glacial outwash sampling σ Molnia, 2008). The western Chugach Moun- methods, and sample processing methods, is avail- poorly at >40% (1 ). The AHe ages reported in tains and Prince William Sound are etched by able at http://www.geosociety.org/pubs/ft2013.htm Table 1 are average ages of single-grain repli- glaciation, as evident from the extensive fi ord or by request to [email protected]. cates with outliers removed.

4 Geological Society of America Bulletin, Month/Month 2012 Geological Society of America Bulletin, published online on 5 April 2013 as doi:10.1130/B30738.1

Focused exhumation in the syntaxis of the western Chugach Mountains and Prince William Sound, Alaska

TABLE 1. SUMMARY OF THERMOCHRONOLOGY DATA Zircon data Apatite ﬁatadkcart-noiss tadeH/)hT-U(etitapA a

Rock Lat Long Elev. ∆h ZHe age AgeAFT ZcAFT ε rateAFT AgeAHe Tc AHe ZcAHe ε rateAHe Sample type* (°N)† (°W)† (m) (m)§ (Ma)# (Ma)# (km)** (mm/yr)†† (Ma)# (°C)§§ (km)** (mm/yr)†† 2P 41200534.84158.06gT 14.010.55.1±4.21 15.080.3365.0±0.6 3P 57−001.84197.06gT 13.027.46.1±4.51 13.048.2464.0±1.9 4P 86−043.74198.06ggT 31.086.2067.2±7.02 5P 26−063.74188.06ggT 41.006.2955.1±1.8192.047.40.3±4.61 P6 Tgg 60.95 147.41 0 −143 33.9 ± 2.4 21.1 ± 1.4 4.66 0.22 5.6 ± 0.6 68 2.93 0.52 7P 052−024.74100.16ggT 51.013.2658.3±0.51 21P 682−025.74150.16coT 54.015.40.1±0.01 83.007.2663.0±2.7 31P 681−010.84119.06gT 34.016.40.1±7.01 2.0±8.4 95.048.276 P16 Kvs 61.14 147.83 0 −360 25.3 ± 0.5 6.5 ± 0.8 4.44 0.68 4.0 ± 0.6 67 2.68 0.67 12P 754−017.74102.16svK 76.043.41.1±5.6 66.094.2567.0±8.3 P24 Kvs 61.07 148.12 0 −322 31.3 ± 1.8 12.6 ± 1.8 4.48 0.35 4.2 ± 0.3 64 2.59 0.62 13P 49−1612.84179.06gT 93.017.43.2±0.21 07.028.2466.0±0.4 23P 102−073.84115.06gT 91.064.2950.3±8.2102.006.45.3±5.32 33P 311−091.84176.06gT 82.058.2562.1±1.0122.096.45.2±5.12 43P 9.74174.06gT 47−06 22.027.2263.2±1.2131.037.42.3±6.63 53P 841−086.74169.95soT 60.156.45.0±4.4 34.282.3571.0±4.1 73P 34−019.74190.06coT 3 51.067.49.5±6.2 72.075.2859.0±7.9 24P 44−079.74137.06gT 61.015.2650.5±0.6131.067.46.4±4.73 34P 99−079.74118.06ggT 13.007.49.1±3.51 52.064.2654.1±7.9 44P 312−039.74178.06ggT 33.095.46.1±8.31 14.096.2464.0±6.6 54P 89−001.84198.06gT 44.007.40.2±8.01 06.089.2861.0±9.4 1HW 144−017.84187.06dfT 61.045.2667.1±7.51 25P 966−007.74162.16svK 46.044.2865.0±8.3 35P 426−078.74181.16svK 94.081.40.1±4.8 54.063.2665.0±3.5 45P 32−000.84160.16svK 5 45.075.43.1±5.8 35.047.2663.0±2.5 85P 114−052.84160.16svK 33.093.44.1±3.31 93.094.24640.0±4.6 95P .84160.16svK 067−093 54.022.2668.0±9.4 06P 271000133.84101.16svK 73.040.3368.0±2.8 16P 93−87723.84101.16svK 65.058.2462.1±1.5 26P 513−30513.84101.16svK 5.2262.0±7.6 83.01 36P 335−35213.84101.16svK 82.032.2166.0±9.7 46P .84180.16svK 736−072 73.061.2261.0±9.5 56P 271−86945.74143.16svK 14.056.2268.0±6.6 66P 875−257158.74105.16svK 92.091.2167.0±6.7 76P 071−004131.84176.16dfT 086.2463.1±8.01 82. 86P 672−473131.84116.16svK 23.096.2464.1±4.8 96P 302−772131.84193.16svK 63.026.2260.1±3.7 07P 665−43155.84172.16svK 15.054.2669.0±7.4 *Rock type abbreviations: Tg—Eocene–Oligocene intrusions; Tgg—Sanak-Baranof intrusions; Tfd—Tertiary felsic intrusions; Toc & Tos—Paleocene–mid-Eocene Orca Group conglomerate and sandstone; Kvs—Cretaceous Valdez Group sandstone. †Sample datum is NAD 27. §∆h—difference between the actual elevation of a sample and the average elevation around a 10 km radius of a sample (Brandon et al., 1998). #Ages are reported from analytical data (Tables DR1 and DR2 [see text footnote 1]) and errors for ﬁ ssion-track (FT) and He ages are reported as 1σ and one standard error, respectively. **Zc—elevation-corrected closure depth (Brandon et al., 1998). For AFT samples, a closure temperature of 110 °C is used. ††ε—apatite ﬁ ssion-track and apatite helium exhumation rate calculated from the elevation-corrected closure depth. §§Tc—AHe effective closure temperature calculated for individual grains with diffusion parameters from Farley (2000).

Zircon (U-Th)/He ages (ZHe) were deter- for two of the three samples. In one Valdez and lower He retentivity (Reiners, 2005). After mined for three samples. For each sample, ages Group sandstone sample, two aliquots have ages excluding the two outlier grains, the average re- were determined for four aliquots containing that are much younger than the other two. We producibility of the ZHe samples is ±9% (1σ). three to four similar-sized grains each (Table exclude these two young ages from the sample Apatite fi ssion-track ages (AFT) were deter- DR1 [see footnote 1]) by laser and ICP-MS mean age, because (1) they are younger than the mined for 22 samples. Analytic data are reported techniques at Caltech. Raw ages were corrected same sample and/or nearby apatite fi ssion-track in Table DR2 (see footnote 1), and ages are sum- for alpha ejection effects. The ZHe ages reported age, and (2) they have relatively high effective marized in Table 1. Ages were determined using in Table 1 are average ages of aliquot replicates. uranium concentrations (1000–2000 ppm), standard external detector methods (see Data The four replicates show good reproducibility which may cause signifi cant radiation damage Repository [footnote 1]). Fission tracks were

TABLE 2. GLACIAL OUTWASH ZIRCON FISSION-TRACK (ZFT) DATA Lat Long Elev. ZFT age (Ma)§ Sample Loc.* (°N)† (°W)† (m) P1 P2 P3 P4 Mean Mean-2 P19 Harvard 61.26 147.7 0 19.1 ± 2.0 (5) 27.8 ± 1.4 (54) 33.3 ± 2.6 (33) 44.1 ± 3.8 (8) 30.4 27.0 P56 Barry 61.12 148.16 0 23.0 ± 1.3 (44) 29.6 ± 1.8 (51) 45.4 ± 3.5 (6) – 27.6 26.2 C1 Knik 61.41 148.58 91 24.0 ± 4.8 (2) 44.9 ± 1.4 (46) 61.4 ± 5.1 (43) 98.3 ± 10.6 (9) 52.3 44.0 C4 Mat. 61.78 147.76 610 22.2 ± 5.9 (2) 37.6 ± 2.7 (17) 52.9 ± 3.5 (48) 65.3 ± 4.7 (34) 54.0 36.0 *Loc.—glacier name; Mat.—Matanuska. †Sample datum is NAD 27. §P numbers are peak ages. Errors are 1σ. Numbers in parentheses are percentage of grains. Mean is weighted mean of all ages. Mean-2 is weighted mean of two youngest peaks.

Geological Society of America Bulletin, Month/Month 2012 5 Geological Society of America Bulletin, published online on 5 April 2013 as doi:10.1130/B30738.1

Arkle et al. counted in 20–30 grains for granitic samples A A′ and in 40 grains for sandstone samples; many of Southeast Northwest the sandstone grains contained no spontaneous Prince William Sound Chugach Mountains Talkeetna Mountains tracks. Horizontal confi ned track lengths were 80 insuffi cient for measurement due to low sponta- Mt. Marcus AHe Baker neous track densities, but Cf-252 irradiation al- AFT Border Ranges lowed revelation of suffi cient horizontal confi ned Valdez ZHe Fault 70 tracks in two representative samples. Dpar was ZFT Contact measured on most age-dated grains, and average Fault (Ma) Thermochronometer Age sample Dpar varies from ~1.0 µm to ~2.5 µm, ZFT 60 with no systematic variation between sample M pooled ages. Single-grain age distributions for Orca χ2 SBB Int. K AFT 50 each sample are narrow and display P( ) (Gal- AHe braith, 1981) typically greater than 80% (average ZFT of all samples is 82%), indicating little variation 40 in intergrain annealing kinetics and/or relatively AFT little time spent in the apatite partial annealing H zone. The average relative standard deviation for E-O Int. 30 B the 22 AFT samples is ±13% (1σ). Zircon fi ssion-track (ZFT) age distributions 20 were determined for four samples (samples C1, Yakutat C4, P19, P56; Fig. 2A; Table 2) from modern Convergence glacial outwash sediments collected at the ends of Barry and Harvard Glaciers on the south side AHe 10 of the Chugach Mountains divide and Knik and Matanuska Glaciers on the north side (Fig. 2A). 0 Ages were determined using standard external 0 50 100 150 200 250 detector methods (see Data Repository [foot- Distance southeast to northwest A-A′ (km) note 1]) for up to 100 grains per sample. Be- cause the glaciers effectively sample bedrock Figure 3. Plot of thermochronology ages along a NW-SE trend (profi le location shown in (Valdez Group sandstone) along their entire Fig. 2) parallel to the relative Yakutat convergence (convergence from left to right). Ages length (Fig. DR1 [see footnote 1]), large age dis- from a 100-km-wide swath are projected onto the profi le along curved paths parallel to the persion is expected. The age distributions were structural grain. Age uncertainties are ±1σ. Shaded boxes approximately bound the aver- decomposed with a binomial peak-fi tting algo- age area of age-distance relations and are estimated in some areas. Curves are third-order rithm (Galbraith and Green, 1990; Brandon, polynomial fi t lines used to show the nested pattern. The zircon fi ssion-track (ZFT) ages in 1992) using the program Binomfi t of Brandon the Mount Marcus Baker area are detrital ages from modern glacial outwash deposits, and (2002). Three or four age peaks were deter- letter labels correspond to glacier names: H—Harvard; B—Barry; M—Matanuska; K— mined for each sample (see Data Repository and Knik. The width of the location error bar on the glacial samples corresponds to the distance Fig. DR2 [see footnote 1]). Detrital ZFT peak along the transect crossed by the glacier. The small dot on the age error bar is the age of ages that are fully reset (i.e., are younger than two youngest age peaks (see text). Age-location relationships show that sequentially higher- the source rocks) provide valuable information temperature thermochronometers are nested across the entire width of the forearc. Verti- on exhumation rate and magnitude for the ex- cal bars on left show approximate depositional ages for Valdez Group (Valdez) and Orca humed source region (e.g., Garver et al., 1999; Group (Orca) and intrusion ages for Sanak-Baranof belt (SBB Int.) and Eocene-Oligocene Bernet and Garver, 2005). (E-O Int.) intrusions.

THERMOCHRONOLOGY AGE RESULTS located on Montague Island (P35) from Tertiary eye pattern of younger ages that is located at the Orca Group sandstone yields the youngest AHe area of maximum structural curvature between The new thermochronology results for sam- age in the sample suite of 1.4 ± 0.1 Ma (Fig. the Contact and Border Ranges fault zones (Fig. ples from this study are discussed next, and then 2A). The core region of the Chugach Mountains 2B) (Arkle, 2011; Armstrong et al., 2011). they are synthesized with published data. north of the Contact fault includes samples collected at sea level in ﬁ ords that extend into the Apatite Fission-Track Ages Apatite (U-Th)/He Ages mountains. AHe ages across the region between the Contact and Border Ranges fault systems New AFT ages range from 4.4 ± 0.5 Ma to 37.4 New AHe ages range from 1.4 ± 0.1 Ma to generally increase from ca. 4 Ma to 8 Ma (Figs. ± 4.6 Ma (Table 1; Fig. 2). AFT ages generally 20.7 ± 2.7 Ma (Table 1; Fig. 2A). AHe ages gen- 2A and 3); this increase is at least partly related decrease from ca. 20–37 Ma in Prince William erally decrease from ca. 10–20 Ma south of the to the higher elevation of the more northern sam- Sound to 8–13 Ma in the Chugach Mountains Contact fault zone in Prince William Sound to ples. Combined with ages from the surrounding core (Figs. 2A and 3). C-axis–corrected horizon- 4–8 Ma north of the Contact fault in the Chugach region, AHe ages get younger from the south to tal conﬁ ned track lengths from two Cf-252 irra- Mountains core (Figs. 2A and 3). One sample north and from the east to west to form a bull’s- diated samples are 13.89 ± 1.08 µm for a sample

6 Geological Society of America Bulletin, Month/Month 2012 Geological Society of America Bulletin, published online on 5 April 2013 as doi:10.1130/B30738.1

Focused exhumation in the syntaxis of the western Chugach Mountains and Prince William Sound, Alaska

north of the Contact fault and 13.68 ± 1.37 µm sitional age and therefore have been reset. The for both AHe and AFT ages, the spatial age gra- for a sample to the south. The long track lengths weighted mean age of the reset peak ages is dients are steepest from south to north across indicate relatively rapid cooling through the apa- 52 Ma, and the two youngest two peaks have a the Contact fault system and generally more dif- tite partial annealing zone. Like the AHe ages, weighted mean age of 44 Ma. fuse from the Kenai Mountains and Valdez area AFT ages form a bull’s-eye pattern of younger Figure 3 shows age distributions of the gla- northeastward and northwestward, respectively, ages centered between the Contact fault and cial outwash, and the approximate width of the toward Mount Marcus Baker (Figs. 2B and 2C). Mount Marcus Baker (Fig. 2C). The sample with range traversed by the glaciers is shown in Fig- Ages for all of the thermochronologic sys- the youngest AHe age from Montague Island ure DR1 (see footnote 1). Importantly, all sam- tems get younger into the core of the Chugach in southern Prince William Sound also yielded ples have one or more Oligocene or younger age Mountains when projected onto a NW-SE–ori- the youngest AFT age (4.4 Ma) in the data set. peaks. The southern glacier samples have 90% ented transect (Fig. 3). Age gradients are es- The relatively young AHe and AFT ages on or more grain ages younger than ca. 33 Ma, pecially steep across the Contact and Border Montague Island indicate that signiﬁ cant strain whereas the northern glacier samples have <5% Ranges fault systems. With the exception of partitioning is occurring in southernmost Prince grain ages younger than 33 Ma. The age distri- the southernmost sample on Montague Island, William Sound with respect to the Chugach core butions suggest that at least parts of the range, AHe ages decrease from an average of 15 Ma region farther north. especially the southern (windward) side, were in Prince William Sound to an average of 5 Ma exhumed through the ZFT closure temperature north of the Contact fault system (Figs. 3 and 4), Zircon (U-Th)/He Ages since ca. 30 Ma. and then they increase to 20–74 Ma with distance north of the Border Ranges fault system. Two Valdez Group sandstone samples from Regional and Local Synthesis of Ages AFT ages average ca. 30 Ma in Prince William the College Fiord area north of the Contact fault Sound, decrease to ca. 12 Ma north of the Con- yielded ZHe ages of 25.3 ± 0.5 Ma and 31.3 ± The combination of our new AHe and AFT tact fault system (Figs. 3 and 4), and then in- 1.8 Ma (Table 1; Fig. 2A). One granitic sample ages with published data from the surrounding crease to 20–40 Ma north of the Border Ranges from south of the Contact fault in Prince Wil- regions form a bull’s-eye pattern of young ages fault system. ZHe ages decrease from 34 Ma liam Sound yielded an age of 33.9 ± 2.4 Ma. located in the Chugach core region (Figs. 2B, in Prince William Sound (n = 1) to 30–25 Ma 2C, and 3). The spatial distribution of the ages north of the Contact fault (n = 2), and then in- Zircon Fission-Track Ages closely follows the arcuate structural grain, and, crease to >70 Ma north of the Border Ranges

The four glacial outwash samples yielded ZFT ages with large age dispersion. We calculated weighted mean ages of all peaks and weighted peak ages of the youngest two peaks in order to contrast cooling rates and timing on either side of the range (Table 2). The weighted mean of all peaks can be interpreted to characterize average cooling rate and timing over the entire catchment, whereas the younger peaks capture the most recent phases of cooling. We group the two youngest peaks because the youngest peak (P1 in Table 2; Fig. DR2 [see footnote 1]) is less than ~5% of the total grains in three of the four samples. On the south side of the range, the Barry Glacier age distribution is decomposed into three peaks of 23 Ma (44%), 30 Ma (51%), and 45 Ma (6%), with a weighted mean age of 28 Ma (Table 2). The two youngest peaks have a weighted mean age of 27 Ma. Harvard Glacier has four peaks of 19 Ma (5%), 28 Ma (54%), 33 Ma (33%), and 44 Ma (8%), with a weighted mean age of 30 Ma. The two youngest peaks have a weighted mean age of 27 Ma. On the north side of the range, ZFT weighted mean ages from two glaciers are older than those on the south. Matanuska Glacier has four peaks of 22 Ma (2%), 38 Ma (17%), 53 Ma (48%), and 65 Ma (34%), with a weighted mean Figure 4. Age-location profi le across the Contact fault into the western Chugach Mountain age of 54 Ma. The two youngest peaks have a syntaxis (profi le location shown in Fig. 2). The shaded boxes show the 1σ range of ages about weighted mean age of 36 Ma. Knik Glacier has the mean age for each thermochronometer to the north and south of the Contact fault. Age four peaks of 24 Ma (2%), 45 Ma (46%), 61 Ma ranges and averages only include samples within a 50 km swath around the profi le line. (43%), and 98 Ma (9%). The three youngest Tos—Orca Group sandstone; Kvs—Valdez Group sandstone. Topography is exaggerated peaks are younger than the Valdez Group depo- 16×, and the subsurface is schematic.

Geological Society of America Bulletin, Month/Month 2012 7 Geological Society of America Bulletin, published online on 5 April 2013 as doi:10.1130/B30738.1

Arkle et al. fault. ZFT ages from Orca Group sandstone tion over data gaps. However, the contoured in- intrusion was emplaced in the upper crust at samples average ca. 40 Ma in Prince William crease of AHe ages (Fig. 2) away from Chugach depths of ~3–4 km, so that it rapidly cooled to Sound (Kveton, 1989); north of the Contact core southwestward into the Kenai Mountains ambient crustal temperatures. Heating associ- fault system, the weighted average of the two and eastward toward Valdez seems reasonable ated with these intrusions probably also par- youngest age peaks from glacial outwash sam- given the relatively older peripheral AHe ages tially or totally reset the detrital ZFT ages in ples is ca. 27 Ma south of the range divide and (Buscher et al., 2008) in these areas. the host Orca Group sandstones in Prince Wil- ca. 35–45 Ma north of the divide. Bedrock ZFT Minor differences in AHe ages across College liam Sound, which average ca. 40 Ma (Kveton, ages increase to 38–50 Ma north of the Border and Harriman Fiords suggest that differential 1989). Between 35 Ma and ca. 12 Ma, cooling Ranges fault. Thus, all the age systems: (1) have rock uplift may be partitioned along faults ob- was very slow at <1 °C/m.y., and since then been reset in the syntaxial core region between scured by glaciers and the fi ords (Figs. 2 and 4). cooling has been ~5 °C/m.y., though the rate is the Contact and Border Ranges fault systems The largest difference in AHe ages is observed poorly constrained over the last ~12 m.y. The and (2) collectively form a regionally nested in the northern reach of College Fiord, where cooling histories from AFT and AHe modeling system of ages with the major age changes, ages are ca. 3.8–4.0 Ma and ca. 5.2–5.3 Ma on (Figs. 5B and 5C) generally agree with the cool- especially for the lower-temperature AHe and the southeast and northwest sides, respectively ing histories based on the average ages and clo- AFT systems (Fig. 3), centered on the Contact (Fig. 4). Assuming a typical geothermal gradi- sure temperatures for the north and south sides and Border Ranges fault systems. ent for this region of 22 °C/km (Magoon, 1986) of the Contact fault (Fig. 5A). These regional The most signifi cant age contrast occurs in and monotonic exhumation since 5 Ma, this age cooling patterns refl ect signifi cant differences in southern Prince William Sound at Montague difference corresponds to a 0.2 mm/yr differ- rates and timing of exhumation in the western Island. Our one sample is from the footwall ence in exhumation rate and ~1000 m of south- Chugach Mountains syntaxial core north of the of the Hanning Bay fault and the hanging wall side-up motion across northern College Fiord. Contact fault and the Prince William Sound re- of the Patton Bay fault. These two faults are Although the AHe age difference is relatively gion to the south. splays off the megathrust and produced verti- small (~1.5 m.y.) across College Fiord, we spec- cal displacement up to ~9 m from the 1964 Mw ulate that these data combined with the arcuate EXHUMATION RATES 9.2 earthquake (Malloy, 1964; Plafker, 1967). geomorphology of fi ords and glacial valleys Thus, the relatively young AHe and AFT ages may indicate that internal subsidiary faults are Spatially averaged exhumation rates are on Montague Island indicate that rapid long- present just inboard of the Contact fault. Sub- commonly determined with multiple samples term (106 yr) rock uplift and exhumation are sidiary faults could cause focused rock uplift in from vertical transects (e.g., Fitzgerald et al., occurring in the southernmost Prince William narrow blocks between faults of the overall fault 1995; O’Sullivan and Currie, 1996; Reiners and Sound with respect to the Chugach core far- system. If present, these faults probably control Brandon, 2006). In this study, exhumation rates ther north. Buscher et al. (2008) postulated that the locations and orientations of glacier and are calculated from single AHe and AFT ages young exhumation near Cordova may bend to fi ord systems. using a modifi ed approach from Brandon et al. the southwest and continue along through-going (1998). Exhumation rates are also derived from structures to Montague Island, which may result TEMPERATURE HISTORIES AHe ages from an ~1 km elevation transect and in rapid rock uplift in southernmost Prince Wil- from a spatially and vertically varying sample liam Sound related to thin-skinned thrusting or There are clearly signifi cant differences in set from across the core of the western Chugach pop-up structures associated with megathrust cooling histories from the regions north and Mountains. splay faults (Ferguson et al., 2011; Haeussler south of the Contact fault. In Figure 5A, the et al., 2011). different thermochronometers are grouped and Single-Sample Elevation-Corrected Though most of the age variation occurs averaged for these two broad regions and plot- Exhumation Rates along a NW-SE transect (Fig. 3), signifi cant age ted as a function of closure temperature. In order reduction occurs along a SW to NE trend in the to better assess the temperature histories from To constrain exhumation rates for single sam- area of Whittier (Fig. 2). AHe ages are ca. 15 Ma, these two regions, we used the program HeFTy ples at sea level, a typical AFT closure tempera- and one AFT age is 33 Ma at Whittier and along (Ketcham, 2005) to model the AFT and AHe ture of 110 °C is used (Reiners and Brandon, Turnagain Arm, but ages decrease to 5–6 Ma data from two representative samples from north 2006). AHe closure temperatures vary with grain and 12–13 Ma, respectively, just 10 km to the (P13) and south (P34) of the Contact fault. These size, cooling rate, and radiation damage (Farley NE. Farther SW in the Kenai Mountains, AHe samples were chosen because they have suffi - et al., 1996; Farley, 2000; Flowers et al., 2009). ages are 13–20 Ma, indicating that AHe ages cient age, track length, and kinetic data for mod- See Data Repository for effective uranium con- may be relatively old across the Kenai Moun- eling and because they both are granitic samples centration (eU) evaluation (see footnote 1). AHe tains region (Buscher et al., 2008). The abruptly with known emplacement ages of 35–36 Ma closure temperatures were calculated using the younger ages NE of Whittier may indicate the (Nelson et al., 1985). The parameters used in program CLOSURE from Brandon et al. (1998) presence of an undocumented through-going the modeling and representative model runs are (Table 1). We do not account for radiation dam- NW-striking structure that may be an impor- given in the Data Repository (see footnote 1). age (Flowers et al., 2009) in our AHe closure tant structural transition dividing the Kenai and North of the Contact fault, the postemplace- temperature estimates, but we expect radiation western Chugach Mountains. It is also possible ment cooling rate was relatively constant at damage to have minimal effect because our that this may mark the western limit of the high ~5 °C/m.y. from >200 °C to 60 °C between 35 samples show little or no correlation of intra- P-wave velocity imagined by Eberhart-Phillips and ca. 4 Ma. In the last <5 m.y., the cooling sample single-grain ages with eU concentration et al. (2006) and/or underplated material imaged rate was 10 °C/m.y. (Fig. 5B). In contrast, the (Table DR1 [see footnote 1]). AFT closure tem- by Fuis et al. (2008). The diffuse pattern around sample from south of the Contact fault cooled peratures vary mainly with cooling rate and an- the periphery of the Chugach core may result rapidly from >200 °C to ~80 °C ca. 35–40 Ma nealing kinetics (e.g., Donelick et al., 2005). We from a lack of ages, which requires interpola- (Fig. 5C). This rapid cooling indicates that the do not account for variable kinetic parameters

8 Geological Society of America Bulletin, Month/Month 2012 Geological Society of America Bulletin, published online on 5 April 2013 as doi:10.1130/B30738.1

Focused exhumation in the syntaxis of the western Chugach Mountains and Prince William Sound, Alaska

Thermochronometer Age (Ma) 0 5 1015202530354045 0

50 AHe 75 C) o 100 AFT 125 P13 P34 150 Temperature ( Temperature 175 ZHe 200 A 225 ZFT 250

Sample P13 20 0.35 TL = 13.89 +/- 1.08 μm 20 Sample P34 0.30 n = 97 AFT = 10.7 Ma 40 0.25 40 AFT = 36.6 Ma AHe = 4.8 Ma 60 0.20 AHe = 12.1 Ma

Frequency 60 0.15 80 0.10 80 100 0.05 0.00 100 120 86420 201816141210 120 Length (μm) 0.35 Temperature (°C) Temperature (°C) TL = 13.68 +/- 1.37 μm 140 140 0.30 n = 51 160 160 0.25 0.20 180 180 Frequency 0.15 200 0.10 200 0.05 220 220 0.00 B C 86420 201816141210 240 240 Length (μm) 50 40 30 20 10 0 50 40 30 20 10 0 Time (Ma) Time (Ma)

Figure 5. (A) Temperature versus age plot for multiple thermochronometers. Typical closure temperatures for apatite (U-Th)/He (AHe), apatite fi ssion-track (AFT), zircon (U-Th)/He (ZHe), and zircon fi ssion-track (ZFT) ages are 64 °C, 110 °C, 180 °C, and 240 °C, respectively. The horizontal shaded regions represent the partial retention or annealing zones for the respective thermochronometers. Plot shows average age of samples (±1σ) to the north (diamonds) and south (circles) of the Contact fault, respectively. The diagonal shaded areas show the upper and lower limits of the sample ages for each region. The solid lines are the best-fi t temperature-time (T-t) histories from B and C. (B–C) Modeled temperature-time histories for samples north (P13) and south (P34) of the Contact fault. Gray shaded regions represent the range of acceptable histories for each sample. Insets show track length histograms with model fi ts (curve). TL—mean track length. Model parameters are given in the Data Repository (see text footnote 1).

Geological Society of America Bulletin, Month/Month 2012 9 Geological Society of America Bulletin, published online on 5 April 2013 as doi:10.1130/B30738.1

Arkle et al. in AFT closure temperatures for exhumation A A′ Southeast Northwest rate calculations because our measured kinetic Precipitation 4

parame ters (Dpar) show only small variation (m/yr) from sample to sample (average ~1.5 µm). How- 3 2 ever, variations in T for both AHe and AFT will c Weather Track 1 contribute error to our calculated exhumation 0 rates. Calculated exhumation rates also assume 2.4 Mt. Marcus a constant geothermal gradient (g ) of 22 °C/km 1.2 4.0 0 Baker (Magoon, 1986), an average surface tempera- AHe AFT ture (T ) of 0 °C (Péwé, 1975), and steady-state Border 3.5 s 1.0 Contact topography. Fault Ranges To account for the effect of long-wavelength Montague Fault 3.0 Strait topography on the shape of shallow isotherms 0.8 Fault Max. Elevation (km) (e.g., Stüwe et al., 1994; Mancktelow and 2.5 Grasemann, 1997), sample elevations were cor- 0.6 2.0 rected relative to the mean local elevation. The Ave. elevation-corrected closure depth (Zc) for AHe 1.5 and AFT samples is calculated using a modiﬁ ed 0.4 Min. equation described by Brandon et al. (1998): Exhumation Rates (mm/yr) 1.0

⎛ − ⎞ 0.2 = ′ +Δ = TTcs+− 0.5 ZZcc h⎜ ⎟ ()hhm , (1) ⎝ g0 ⎠ 0.0 0 where ∆h is the difference between the actual el- 0 50 100 150 200 250 Distance (km) evation of a sample h and the average elevation ′ hm around a sample, and Zc is the effective depth Figure 6. Exhumation rates computed from apatite (U-Th)/He (AHe) to the closure temperature (Tc) below the mean ages (diamonds) and apatite fi ssion-track (AFT) ages (squares) elevation. We use the average elevation hm for a across the Prince William Sound, western Chugach Mountains, and 10-km-radius area around each sample derived Talkeetna Mountains (plotted along transect A-A′ shown in Fig. 2). from (90 m) digital elevation model (DEM) Exhumation rates are single-sample elevation-corrected rates (see data. The ∆h ranges from –760 m to 214 m text). Minimum, maximum, and average elevations are shown by (Table 1). The modeled exhumation rate (ε) is gray and black curves. The upper plot shows mean annual precipi- a function of the thermochronometer age (t) and tation modifi ed from Péwé (1975). ε the elevation-corrected closure depth: = Zc/t. Average closure temperatures and depths for the AHe samples are ~64.2 °C and ~2.7 km, sample yields Miocene to recent AHe and AFT Mountains syntaxis. The ages range from ca. and average AFT closure depths are ~4.8 km exhumation rates of 2.4 and 1.0 mm/yr. 4–6 Ma at sea level to 7–8 Ma at 1.3–1.7 km (Table 1) . AHe ages yield exhumation rates that With the exception of the Montague Island elevations. A regression yields an average exhu- range from 0.1 to 2.4 mm/yr and average 0.5 sample, maximum exhumation rates occur just mation rate of 0.4 mm/yr and a projected AHe mm/yr (Table 1; Fig. 6). AFT ages yield exhu- north of the Contact fault and south (windward) closure temperature depth of 1.9 km, which is mation rates that range from 0.1 to 1.0 mm/yr of the range divide in the region where the fi ords ~800 m shallower than the calculated depth of and average 0.4 mm/yr. Estimating errors in ex- extend the farthest into the range (Fig. 6). Note ~2.7 km. We acknowledge that the scatter of sea- humation rates is diffi cult due to the complex that the highest exhumation rates were derived level samples signifi cantly affects the slopes, but uncertainties in Zc. However, using the average from sea-level samples, even though the aver- the maximum slope allowed by the data is 0.5 relative uncertainties for AHe and AFT ages of age elevation around the fi ords may be >1 km. mm/yr with a projected closure depth of 2.7 km, 17% and 12%, respectively, and assuming a con- Farther north, the exhumation rates decrease to and the minimum slope is 0.4 mm/yr with a clo- servative uncertainty of ±25% for Zc, standard ~0.2 mm/yr at the Border Ranges fault. Thus, the sure depth of 1.7 km. This indicates that either error propagation leads to ~30% error in the ex- exhumation rates display an asymmetric pattern, this time-averaged rate estimate may be lower humation rate calculations. with the highest rates concentrated along the than the average elevation-age slope, or that AHe exhumation rates north of the Contact Contact fault system and on the windward side exhumation may have increased at or after the fault in the Chugach core and fi ords typically of the range where glaciers extend to the fi ords. youngest low-elevation sample age of ca. 4 Ma. are 0.4–0.7 mm/yr and two times higher than The ranges of slopes yield rates consistent with rates south of the Contact fault in Prince Wil- Age-Elevation Relations the average single sample rates (Fig. 6) and sug- liam Sound, where they typically are 0.1–0.2 gest time-averaged exhumation of the Chugach mm/yr (Fig. 6). The AFT-derived rates show We explore age-elevation relationships for the Mountains core has been ~0.4–0.5 mm/yr since the same spatial pattern, producing an average regional data and for a single vertical transect at least 8 Ma. However, samples closest to the exhumation rate of 0.5 mm/yr north of the Con- (Fig. 7). Figure 7A shows the regional AHe age Contact fault yield single sample exhumation tact fault and average of ~0.2 mm/yr south of the versus elevation relations for samples collected rates that are higher than this average rate. The Contact fault. South of the Montague Strait fault from the northernmost fi ords to north of Mount consistent pattern and magnitude of AHe- and in southernmost Prince William Sound, one Marcus Baker, all within the western Chugach AFT-derived exhumation rates imply that long-

10 Geological Society of America Bulletin, Month/Month 2012 Geological Society of America Bulletin, published online on 5 April 2013 as doi:10.1130/B30738.1

Focused exhumation in the syntaxis of the western Chugach Mountains and Prince William Sound, Alaska

term exhumation of the Chugach core has been A. Regional Age-Elevation Profile B. Mt. Muir Age-Elevation Transect steady since at least the mid-late Miocene (AFT 2.0 P66 1.5 ages in the Chugach core), but they do not pre- P60 1.5 clude an increase in exhumation rate in the last 1.0 P61 ~4–5 m.y. This is consistent with the increase in P69 P68 cooling rates (Fig. 5B), which also suggests an 1.0 P62 0.5 increase in rock uplift and exhumation rates in P65 the last ~4–5 m.y. or less. 0.5 P63 P59 P64 Although the regional elevation profi le gives P70 0 P24 P54 P53 246810 an average exhumation rate, an additional verti- P52 0 P64 cal elevation transect spanning 1000 m elevation 246810 –0.5 collected from the northwest side of Harriman Fiord on Mount Muir (Figs. 1 and 8A) shows –0.5 Elevation (km) –1.0 the potential complications with age-elevation y = 0.42x – 1.9 Elevation (km) profi les in faulted regions. AHe ages at sea level –1.0 y = 0.47x – 2.7 are 4–6 Ma, increase to 8 Ma at 250 m, decrease –1.5 to 5 Ma at 750 m, and increase again to 8 Ma –1.5 at 1000 m to form a zigzag pattern (Fig. 7B). –2.0 The general age-elevation trend (~0.5 mm/yr) –2.0 is consistent with the regional trend in Figure 7A, and the sea-level sample ages are the same –2.5 –2.5 (4–6 Ma). However, the zigzag pattern suggests the profi le was offset by faults, so that the age –3.0 –3.0 trend reverses with higher elevation (Prior et al., AHe Age (Ma) AHe Age (Ma) 2010). Faults are diffi cult to recognize in the monotonous rocks of the Valdez Group, but at Figure 7. Apatite (U-Th)/He (AHe) age-elevation profi les from the western Chugach syn- least one reverse fault offsetting a felsic dike taxis. The regional age-elevation profi le (A) combines samples collected from northern fi ords and the sample transect was apparent in the fi eld to north of Mount Marcus Baker. The regression indicates an exhumation rate of 0.4 mm/yr (Fig. 8B). Haeussler et al. (2008) documented a and a predicted closure depth of ~1.9 km. A local transect on Mount Muir (B) shows an off- similar pattern of cooling ages in the Tordrillo set pattern of ages with similar cooling histories (small dashed lines) that is consistent with Mountains of the western Alaska Range, which reverse faulting (arrows). The exhumation rate derived from all the samples projected to the they attributed to thrust faults. Our age-elevation predicted closure depth (~2.7 km) gives a rate of 0.5 mm/yr. Numbers next to the diamonds transect is also in the region where fi ord and gla- correspond to sample numbers in Figure 2 and Table 1. cial valley trends may be controlled by faulting, as discussed earlier. Thus, reverse faulting may be ubiquitous across the windward region adja- since the mid-late Miocene, and perhaps longer. constrain the total magnitude of rock uplift cent to the Contact fault and may be the princi- Alternatively, the steep south-to-north AFT and to ~11 km since ca. 30–35 Ma in the western pal mechanism by which rocks of the southern AHe age (and exhumation rate) gradients cen- Chugach core north of the Contact fault sys- edge of the western Chugach Mountains are up- tered on the Contact fault system may be due to tem. In contrast, very slow cooling of the re- lifted and exhumed. general rock uplift of the region rather than off- gion between the Contact fault and Montague set across a fault system, perhaps above a grow- Island from ~100 °C indicates 4–5 km of exhu- INTERPRETATIONS AND DISCUSSION ing duplex or antiformal stack, as suggested by mation in this part of Prince William Sound for Pavlis et al. (2012) for the fold-and-thrust belt the same time period. Our exhumation magni- Exhumation of the Western Chugach of the Saint Elias orogen. tudes outside the Chugach core area are greater Mountains and Northern Prince The combined cooling data and modeling than those reported by Buscher et al. (2008), William Sound (Fig. 5) provide constraints on the magnitude who interpreted 2–3 km of exhumation since of rock uplift in the western Chugach syntaxis the Miocene for the Kenai and Chugach Moun- A critical result from this study is the major and can be used to estimate the magnitude tains; our estimates are higher because they are difference in rock uplift centered on the Con- of rock uplift above the subducting Yakutat based on a longer time period at about the same tact fault system, which has been the subject of slab. Assuming a relatively low, but constant exhumation rate. We note that the present-day debate as to whether it is a structure that has (in depth and time) geothermal gradient of geothermal gradient of 22 °C/km is relatively accommodated Miocene to recent deformation 22 °C/km (Magoon, 1986) and surface tem- low, presumably due to cooling related to sub- (Dumoulin, 1988; Bol and Gibbons, 1992). perature 0 °C, we can estimate exhumation duction of the relatively cool Yakutat micro- AHe and AFT ages decrease by 50% (Fig. 4), magnitudes. The highest-temperature thermo- plate; the present-day geothermal gradient is and the average exhumation rate increases by chronometer (ZFT) indicates that the maxi- consistent with the thermal models of Gutscher a factor of two from south to north across the mum amount of exhumation in the western and Peacock (2003) for fl at-slab subduction of Contact fault (Fig. 6). These fi ndings suggest Chugach Mountain core since the time of clo- the Yakutat microplate. However, if the cooling that the Contact fault system may be a major sure is ~9–10 km. Assuming the average ele- effects of subduction (e.g., Cloos, 1985) were structural boundary that has accommodated a vation was zero before rock uplift began and greater in the past, then the exhumation magni- signifi cant amount of differential rock uplift a current mean elevation of 1 km, these data tude may be greater than reported here.

Geological Society of America Bulletin, Month/Month 2012 11 Geological Society of America Bulletin, published online on 5 April 2013 as doi:10.1130/B30738.1

Arkle et al.

x related cooling than what we observe. Our data Mt. Muir (2319 m) ABx

x indicate that relatively constant cooling started x

x ca. 30–35 Ma regionally, and perhaps earlier in x

1000 m x

x areas not affected by intrusion heating, and these Reverse fault x

x results are more consistent with long-term exhu-

x mation in the western Chugach syntaxis related x

x x to a late Eocene–early Oligocene initiation of ﬂ at-slab subduction as suggested by Finzel et al. (2011). However, our thermal model results (Fig. 5) show that there are acceptable cooling curves with possible rapid cooling at 10–15 Ma; thus,

x we cannot completely rule out a middle Miocene

x cooling event with our data.

Serpentine Glacier x

x ~100 m Exhumational Steady State? x x Harriman Fiord x x We suggest that the age-location relation- x ships in the western Chugach core indicate that Figure 8. Field photographs along the Mount Muir vertical transect (Figs. 2A and 7B). the orogen has approached exhumational steady (A) Photo graph taken from Harriman fi ord near the base of Serpentine Glacier looking state as defi ned by Willett and Brandon (2002). northwest along the Mount Muir elevation transect. White dotted line shows the transect lo- In exhumational steady state, the ages will dis- cation. Black solid line shows approximate location of reverse fault. (B) Photograph showing play spatially nested reset zones that are orga- south-dipping reverse fault (solid line) that offsets a felsic dike (x-line). Field of view is ~300 m. nized with the highest closure temperature ages located in the center. Steady-state models (Batt et al., 2001; Batt and Brandon, 2002; Willett and Constraints on Timing of Flat-Slab Finzel et al. (2011) used stratigraphic, prov- Brandon, 2002) show that as erosion begins to Subduction enance, thermochronologic, and geochronologic keep pace with rock uplift at million-year time data to argue that fl at-slab–related processes be- scales, the spatial extent of reset ages broad- Our new exhumation and cooling history gan in southern Alaska in late Eocene–early ens, and the ages across the region show little constraints provide insight into the competing Oligo cene time. Their analysis suggested that variation. The ages from each system across the hypotheses of subduction initiation of the buoy- (1) regional exhumation in the upper plate above western Chugach core are all reset and show ant fl at-slab portion of the Yakutat microplate. and around the fl at-slab subduction began ca. relatively little age variation with distance in the Deformation related to Yakutat microplate sub- 43 Ma; (2) fl at-slab subduction essentially shut centers of the reset zones, particularly for the duction generally is thought to have initiated as off magmatism ca. 32 Ma; and (3) fl at-slab sub- lowest-temperature systems (Fig. 3). The spatial normal oceanic crustal subduction ca. 30 Ma duction caused enhanced basin subsidence along age distribution clearly shows that exhumation (Plafker, 1987; Plafker et al., 1994). In this model, the northern and western borders of the fl at-slab is focused in the core of the Chugach range at the buoyant, continental-like part of the Yakutat region. We suggest that prior to thick Yakutat ar- the base of the highest topography. AHe and microplate began to subduct at a shallow angle rival, emplacement of granitic intrusions at ca. AFT ages in the Chugach core cluster at ca. starting in the middle Miocene (ca. 12–15 Ma); 35–36 Ma (Nelson et al., 1985) in Prince Wil- 5–6 Ma and 8–12 Ma, respectively, over a broad this timing is based on the age of thick succes- liam Sound partially or totally reset detrital ZFT area of ~70 km (Figs. 3 and 4). Additionally, the sions of Neogene strata such as the Yakataga For- ages in the region (Kveton, 1989). In our study, higher-temperature ages are nested and are reset mation overlying the Yakutat microplate (Plafker AFT and ZHe ages south of the Contact fault are in the center. The relatively older ages and larger et al., 1994). This timing also is consistent with synchronous with Eocene–Oligocene magma- age variations north of the Border Ranges fault the geophysically imaged length of the thickened tism, and they indicate rapid cooling followed in the Talkeetna Mountains, as well as in the Yakutat microplate extending 650 km inboard by relatively slow and nearly steady cooling Kenai Mountains to the southwest and Chugach from the present-day collisional front (Ferris to the present (Figs. 5A and 5C). Additionally, Mountains to the east, indicate relatively little et al., 2003), which would take ~12 m.y. at mod- three of the four detrital samples from modern exhumation and that exhumational steady state ern plate motion rates of 55 mm/yr as suggested glaciers that cross the Chugach core yield reset does not apply there. The nested pattern of by Koons et al. (2010). Several low-tempera- ZFT age populations with largest peak ages be- ages is similar to other natural settings where ture thermochronometry studies from the Saint tween 28 and 45 Ma (Table 2). These ages are exhumational steady state has been suggested; Elias orogen (e.g., O’Sullivan and Currie, 1996; probably not related to intrusion heating because these include the Olympic Mountains (Batt Enkelmann et al., 2008; Meigs et al., 2008), there are only small isolated intrusions in the and Brandon, 2002); the Southern Alps of New the Alaska Range (e.g., Fitzgerald et al., 1995; Chugach core crossed and sampled by the gla- Zealand (Sutherland, 1996; Walcott, 1998; Batt Benowitz et al., 2011; Haeussler et al., 2008), ciers, and thus we interpret the reset detrital ZFT and Brandon, 2002); and Taiwan (Willett et al., and the Talkeetna Mountains (Parry et al., 2001; ages to be related to rock uplift and exhumation 2003). Even though we interpret the Chugach Hoffman and Armstrong, 2006) are consistent cooling. Although there may have been a slight core to be in or approaching exhumational with widespread Miocene to Holocene deforma- increase in cooling at ca. 10–15 Ma, both north steady state, increased Pliocene exhumation due tion potentially related to fl at-slab subduction, and south of the Contact fault (Fig. 5), we sug- to the dynamic feedbacks of increased rock up- but by themselves they do not directly constrain gest that arrival of a thick slab at that time would lift and climate-related glacial activity cause the the initiation of fl at-slab subduction. In contrast, have caused considerably more exhumation and range to be out of topographic steady state.

12 Geological Society of America Bulletin, Month/Month 2012 Geological Society of America Bulletin, published online on 5 April 2013 as doi:10.1130/B30738.1

Focused exhumation in the syntaxis of the western Chugach Mountains and Prince William Sound, Alaska

Mechanisms of Rock Uplift in the Western Brandon, 2002; Willett and Brandon, 2002). the subduction interface cause dynamic uplift of Chugach Mountains In the underplating case, the ages from a spe- a broad forearc ridge. Buscher et al. (2008) also cifi c thermochronometer would show little age suggested underplating as a possible mechanism One of the primary goals of this study is to variation across the wedge, indicating uniform for the relatively minor uplift in the Kenai and address mechanisms of deformation and moun- uplift of the wedge, and the zones of reset ages Chugach Mountains surrounding the Chugach tain building in the orogenic wedge (western would be nearly symmetric, with location of the core. An underplating interpretation is also con- Chugach Mountains) located more than 200 km minimum age for each system in the same re- sistent with geophysical data that show under- northwest of the trench and the Yakutat col- gion (Batt et al., 2001; Batt and Brandon, 2002; plated material below the western Chugach lision front. Buscher et al. (2008) provided a Willett and Brandon, 2002). Mountains and Prince William Sound, which summary of potential rock uplift mechanisms We suggest that the pattern of ages and exhu- is thought to be a previously underplated frag- for the Kenai and Chugach Mountain region, mation in the western Chugach Mountains more ment of the Yakutat terrane (Fuis et al., 2008). which included fl exure of the upper plate, dy- closely matches model predictions of wedge Seismic tomography also shows high P-wave namic changes caused by density contrasts in deformation driven dominantly by underplating velocities under the western Chugach Mountain the mantle, frontal accretion, and underplating. (Fig. 9). All of the thermochronometers show core (Eberhart-Phillips et al., 2006), which may Numerical and theoretical models show that for little variation in age across the wedge, indi- indicate bulk rock uplift of deeper buried rocks, convergent orogens under steady-state condi- cating nearly uniform rock uplift in the core of consistent with underplating. Refl ectors from tions, thermochronometers and topographic the Chugach (Fig. 3). In the western Chugach deep seismic-refl ection profi les from just south- distributions have the potential to discriminate Mountains, the zones of reset ages are nearly west of the Kenai Peninsula are interpreted to between frontal accretion and underplating in evenly distributed across the wedge, or they are represent Eocene underplating southeast of the convergent orogens (Willett et al., 2001; Willett asymmetric to the south in the opposite direc- Border Ranges fault system (Moore et al., 1991; and Brandon, 2002). These models show an ac- tion of convergence. Ye et al., 1997). New gravity and published cretionary wedge bounded by conjugate reverse The distribution of topography and location magnetic data and 2.5-dimensional cross-sec- faults with an accretionary front (pro-wedge) of the topographic divide also imply that the tional models across the Cook Inlet basin and and the opposing fl ank (retro-wedge) (Koons, vertical component of displacement is more Border Ranges fault system are consistent with 1990; Willett et al., 1993, 2001; Batt et al., 2001; significant than the horizontal component. low-density sediments being subducted beneath Batt and Brandon, 2002). Topographic profi les show that the topographic the accretionary complex (Mankhemthong et al., One end-member model considers that divide is located about equal distances between 2013). Bulk uplift is also consistent with the crustal deformation is dominantly driven by the Contact and Border Ranges fault systems thermal-mechanical models of Koons et al. mass fl ux into and through the orogen, such that (Fig. 6), which form the pro- and retro-wedge (2010), which show increased vertical velocities all crustal material is added by frontal accretion. boundaries, and this is consistent with model in their convergent sub-boundary area above the This model requires that the wedge shortens predictions of wedge evolution dominantly subducting Yakutat microplate and adjacent to internally (i.e., pure shear). Material at the sur- driven by underplating. Pavlis and Bruhn (1983) the Chugach core region. face moves at a constant vertical velocity, but proposed underplating for this area whereby Even though we interpret underplating as the the horizontal velocity of material decreases lin- deep-seated fl ow and ductile deformation above dominant driver of rock uplift, the steep gradient early from the pro-wedge to the retro-wedge, re- sulting in an asymmetric topographic form with Prince William Chugach Mountains Talkeetna the main range drainage divide offset toward Mountains Sound (Wedge) the retro-wedge (Koons, 1990; Willett et al., (Pro-Wedge) (Retro-Wedge) Focused uplift 2001; Whipple and Meade, 2006). The second Contraction Extension? & shortening Distributed uplift end-member model addresses underplating as Mt Marcus the dominant mode of material accretion. Sub- Contact Fault Erosion Baker stantial underplating below the orogenic wedge Erosion would result in uniform uplift of the wedge. Montague Montague Chug. Island Strait Fault This process requires little to no internal short- Terr. Border Ranges Fault ening (Willett et al., 2001), but the entire wedge Prince Will- Terr. Wrangellia may still be carried in the convergence direction. Yakutat microplate Terrane In contrast to the frontal accretion model, the Underplating topographic divide would be located at approximately equal distances from the fl anks, as long as orographic effects do not force the divide to migrate (Willett et al., 2001). Thermochronometers may refl ect material paths related to these end-member scenarios. In Figure 9. Schematic model of rock uplift for the western Chugach the frontal accretion case, ages would decrease Mountains and Prince William Sound. The Yakutat microplate sub- linearly from the pro-wedge to near the topo- ducts at shallow angle under Prince William Sound and the Chugach graphic divide where erosion rates are highest. Mountains. Yakutat material is being underplated under the region. Additionally, the nested zones of reset ages for Underplating drives uplift of the Chugach Mountains between the higher-temperature ages would be successively Contact and Border Ranges faults, but exhumation may be focused shifted in the direction of convergence toward along the southern edge of the wedge, as shown by the longer sche- the retro-wedge (Batt et al., 2001; Batt and matic arrows, adjacent to the Contact fault.

Geological Society of America Bulletin, Month/Month 2012 13 Geological Society of America Bulletin, published online on 5 April 2013 as doi:10.1130/B30738.1

Arkle et al. in AFT and AHe ages and associated exhuma- Gibbons, 1992); alternatively, we suggest that Drivers of Exhumation in the Last 5 tion rates centered on the Contact fault system initial effects of fl at-slab subduction may have Million Years: Tectonics and Climate indicate that it may be a critical structure, or resulted in late Eocene–early Oligocene uplift structural zone (Bol and Gibbons, 1992), for along and between the Border Ranges and Con- Our results show that cooling rates in the rock uplift along the pro-wedge fl ank. This age tact fault systems. Shortening related to poten- western Chugach core have increased sometime pattern indicates more rapid exhumation along tial underplating is also evident in GPS data; in the last ~4–5 m.y., which is consistent with the Contact fault structural zone, where narrow geodetic models indicate that, after subtraction nearly every low-temperature thermochronol- fault-bounded blocks have lower mean eleva- of viscoelastic locking models from observed ogy study in southern Alaska (e.g., Fitz gerald tions than the more slowly exhumed rocks far- velocities, the residual velocities in the College et al., 1995; O’Sullivan and Currie, 1996; ther north in the core of the range. Thus, material Fiord area are moving ~5–10 mm/yr NNW at Spotila et al., 2004; Berger et al., 2008a, 2008b; within the majority of the wedge interior may decadal time scales with respect to areas north of Berger and Spotila, 2008; Enkelmann et al., experience little to no shortening, but shortening the Border Ranges fault system (Suito and Frey- 2008, 2009; Meigs et al., 2008; Spotila and and related rock uplift instead are concentrated mueller, 2009). Thus, our thermochronology Berger, 2010) and with increases in Pliocene along the wedge boundary. data are consistent with other data and indicate sedimentation rates (e.g., Zellers, 1993). The With underplating along the megathrust, that underplating probably causes bulk rock up- Pliocene increase in cooling rate is interpreted the underplated Yakutat material may become lift across the western Chugach Mountains. Ad- as being related to changes in tectonic activity, highly coupled with the overriding crust of the ditionally, exhumation rates are two times higher climate change and related increases in glacial southern Alaska block (Zweck et al., 2002). The adjacent to and within the Contact fault system, erosion, or both. Many studies show that inten- locked interface may carry the crustal wedge and we therefore interpret that the bulk of per- sifi cation of glacial activity increased in the last northwestward into the Wrangellia-Peninsular manent shortening at million-year time scales ~2.5 m.y. in southern Alaska (Lagoe et al., 1993; terrane along the Border Ranges fault zone, related to underplating and coupling along the Lagoe and Zellers, 1996) and in the Northern which acts as a ramp-like backstop (Little and megathrust is focused on reverse faults in a nar- Hemisphere worldwide (Zachos et al., 2001; Naeser, 1989). The net effect of northward row region along the Contact fault system. Lisiecki and Raymo, 2005). Next, we review under thrusting of the Yakutat coupled to the Underplating above the subducting Yaku- some of these causes of increased cooling rates overriding wedge is to uplift the region between tat microplate is probably not occurring only for southern Alaska and their potential effects the Contact and Border Ranges fault systems under the Chugach core, where exhumation on the western Chugach core. (Fig. 9); abundant geological data indicate rock rates are greatest, but is widely distributed In the Alaska Range adjacent to the Denali uplift across this region. The Border Ranges along the Kenai and Chugach Mountain system fault, various tectonic-related mechanisms are fault system is a complex fault system that has (Moore et al., 1991; Fuis et al., 2008), where interpreted to have caused rapid or increased focused deformation during a long Jurassic to exhumation rates are much lower. However, the exhumation rates in the last ~5 m.y. Fitzgerald Neogene history with periods of both strike-slip Chugach core is located above the center of the et al. (1995) interpreted a break in slope of AFT and dip-slip motion (Pavlis and Roeske, 2007). subducted Yakutat microplate (Fig. 1) (Eber- age versus elevation profi le (4500 m) at 5–6 Ma Dip-slip motion on high-angle faults is thought hart-Phillips et al., 2006) and at the location of to represent an increase in exhumation rate at to be generally restricted to the Eocene (Little the maximum curvature of the Border Ranges that time, which they interpreted to be caused and Naeser, 1989) and the Neogene (Little and and Contact fault systems that border the by a change in plate motion (Engebretson Naeser, 1989; Pavlis and Bruhn, 1983) and is re- Chugach core to the north and south. We specu- et al., 1985) that resulted in a greater compo- lated to forearc basin accretionary prism devel- late that underplated material that is affi xed to nent of contraction across the Denali fault in the opment (Pavlis and Roeske, 2007). Rocks south the overriding plate is essentially jammed into Mount McKinley area. Haeussler et al. (2008) and east of the Border Ranges fault system were this core region, where processes such as three- interpreted cooling histories from AFT and uplifted along these high-angle faults. Tertiary dimensional fault duplexing and/or antiformal AHe data from the Tordrillo Mountains in the deformation of the Cook Inlet began in the Neo- stacking concentrate rock uplift and exhuma- western Alaska Range to refl ect rapid cooling gene on the northwest and downdropped side of tion, as suggested by Pavlis et al. (2012) for the starting ca. 6 Ma caused by counterclockwise the Border Ranges fault system (Haeussler et al., Saint Elias orogen. East of the Chugach core, motion of southern Alaska south of the Denali 2000; Bruhn and Haeussler, 2006; Haeussler displacement in the overriding plate has been fault due to the far-fi eld effects of Yakutat sub- and Saltus, 2011). The Contact fault system is dominantly strike slip along major fault systems duction. Benowitz et al. (2011) used multiple also a complex fault system that displays domi- (Bol and Gibbons, 1992; Pavlis and Roeske, thermochronometers to interpret rapid Neogene nantly strike-slip notion to the southeast (Bol and 2007), and underplating under this region will cooling in the eastern Alaska Range as the con- Roeske, 1992) and reverse-fault dip-slip motion not be forced up against a backstop. In the tinuing effects of long-term (~22 m.y.) uplift to the northwest in the Chugach core (Bol and Chugach core area, the underplated rocks and along the Denali fault system locally affected Gibbons, 1992). However, detailed structural overriding Chugach terrane will be forced up by transpressional variation due to changes in analysis by Bol and Gibbons (1992) showed against the backstop (Fig. 9) to focus exhuma- fault strike. They also correlated their ages with that the Contact fault system in the Chugach tion. Farther southwest and in the Kenai Moun- climatic cooling and suggested that at least part core represents an out-of-sequence reverse fault tains, the exhumation decreases toward the of the exhumation in the last ~3 m.y. may be system that formed during accretionary prism subducted edge of the shallow-dipping Yakutat related to increased glacial erosion. development. Plate-motion changes (Enge- microplate as defi ned by Eberhart-Phillips et The highest Neogene exhumation rates in bretson et al., 1985; Stock and Molnar, 1988) al. (2006) (Fig. 1). In the lateral transition from Alaska are from the Saint Elias Range and east- generally are interpreted as the cause of initia- shallow subduction to steeper subduction, the ern Chugach Mountains in southeast Alaska tion or reactivation of dip-slip deformation along uplift effects of underplating in the upper plate and are constrained by thermochronologic data the Border Ranges and Contact fault systems in may diminish so that exhumation rates decrease (O’Sullivan and Currie, 1996; Spotila et al., the Eocene (Little and Naeser, 1989; Bol and to the southwest. 2004; Berger et al., 2008a, 2008b; Berger and

14 Geological Society of America Bulletin, Month/Month 2012 Geological Society of America Bulletin, published online on 5 April 2013 as doi:10.1130/B30738.1

Focused exhumation in the syntaxis of the western Chugach Mountains and Prince William Sound, Alaska

Spotila, 2008; Enkelmann et al., 2008, 2009; only in the Chugach core area and not along the exhumational steady state, but an increase in Meigs et al., 2008; Spotila and Berger, 2010). Kenai Peninsula and east of the Chugach core the amount of subducted sediments may have This area is located along the current oblique near Valdez (Fig. 2). As suggested by Buscher caused enhanced underplating and increased Yakutat collision front and is being actively de- et al. (2008), the relatively older AHe ages in exhumation rates during the Neogene. formed in a thin-skinned fold-and-thrust belt. these other areas away from the Chugach core The rapid exhumation here (up to 4 mm/yr) is where the glacial ELA and precipitation pat- CONCLUSIONS caused by thrust stacking and shortening in the terns are comparable suggest that tectonics is orogenic wedge (Meigs et al., 2008; Berger et the main driver of exhumation because climate- We present a thermochronology data set for al., 2008a), perhaps focused along fault sys- related exhumation should affect all these areas the western Chugach Mountains and Prince tems that acted as backstops (Enkelmann et al., equally. Climate and related glacier erosion play William Sound that constrains exhumation in a 2008; Berger et al., 2008b), or above a grow- a secondary role in modulating short-term ex- major syntaxial bend. Our ages, in combination ing three-dimensional antiformal stack (Pavlis humation patterns and keep the topography in a with published ages, defi ne a regional pattern et al., 2012). Rapid exhumation is also caused dynamic state, rather than a steady state. Thus, of focused exhumation centered in the western by structural complexities adjacent to structural on the orogen scale, it seems that the fundamen- Chugach core, where ~11 km of exhumation bends along syntaxial bends that form structural tal control on rock uplift and exhumation in the has occurred since the late Eocene–early Oligo- aneurysms (Enkelmann et al., 2009; Spotila and Chugach core is tectonics, which produces posi- cene. The age distributions support a model of Berger, 2010). tive feedbacks among the windward polarity of exhumation in the western Chugach Mountains The rapid Neogene exhumation in the Saint precipitation, glacial activity, and exhumation. and Prince William Sound that is driven domi- Elias orogen is focused on the windward fl ank Our data do not uniquely constrain the tec- nantly by underplating above fl at-slab subduc- of the orogen, suggesting that climate and re- tonic causes of focused exhumation in the tion. Rock uplift is probably concentrated by the lated glacier activity caused and/or enhanced Chugach core during the last ~5 m.y. Changes arcuate structural system in the syntaxial core rapid rock uplift and erosion. Glaciers currently in plate motion in the last 5.6 m.y. (Engebret- region. Further, the highest exhumation rates cover much of the orogen, and most of the land son et al., 1985) could have caused increased are centered on the Contact fault system, which surface was covered by glaciers that extended rock uplift along the fault systems by changing may be a critical structural system for facilitat- beyond the current coast in the late Pleistocene the convergence 10°–15° counterclockwise to a ing and accommodating long-term (106 yr) rock (Péwé, 1975). Present coastal precipitation is more northwesterly direction. We do not favor uplift in the orogenic wedge. ~3 m/yr on the windward side and decreases the plate-motion change as the main cause of All the thermochronometer systems display a to <0.5 m/yr on the leeward side (Péwé, 1975). focusing exhumation in the Chugach core be- nested spatial-age pattern across the Prince Wil- The spatial coincidence of glacier equilibrium cause the counterclockwise change would liam Sound, western Chugach, and Talkeetna line altitudes (ELA) with zones of rapid exhu- place a greater component of convergence on Mountains suggesting that the Chugach core re- mation suggests that glaciers control, at least in the Kenai Peninsula area, where long-term gion has approached exhumational steady state. part, localized rock uplift and exhumation in the exhumation is not focused. Our preferred in- However, exhumation rates have increased in Saint Elias orogen (Meigs and Sauber, 2000; terpretation is that underplating, which may the last ~4–5 m.y., perhaps in response to in- Spotila et al., 2004; Berger and Spotila, 2008; have started in the Oligocene, was enhanced creased tectonic activity as documented in the Berger et al., 2008a, 2008b). However, Meigs in the Neogene. Pavlis et al. (2012) used cross- Saint Elias region to the southeast and/or Plio- et al. (2008) argued that thermochonologic data section restorations to suggest that the Yakataga cene climate change that resulted in more gla- do not allow the unique distinction between segment of the Saint Elias orogen may have cial activity. Although our data do not uniquely orographic/glacier-controlled exhumation and formed due to subduction of sediments eroded distinguish between a climate or tectonic cause tectonic-controlled exhumation. from the orogen in the last 2–3 m.y. The sub- for the Pliocene increase in exhumation rate, In the western Chugach and northern Prince duction of these sediments may have caused the lack of recent exhumation elsewhere along William Sound, modern precipitation is ~3 m/yr duplex stacking and regional antiform devel- the coastal margin adjacent to the Chugach core on the windward side of the range and Prince opment beneath the coastal mountains, which region suggests that tectonics are responsible William Sound, and it decreases to <0.4 m/yr drives the long-term rock uplift and rapid ex- for the exhumation patterns. We speculate that on the leeward side of the range. The modern humation observed there. New gravity and pub- accelerated rock uplift in the Chugach core dur- ELA is ~450 m and was below modern sea level lished magnetic data and 2.5-D cross-sectional ing the Pliocene was caused by underplating of during the Last Glacial Maximum along the models across the western Chugach Mountains sediments eroded from the rapidly exhuming windward side of the western Chugach Moun- are consistent with low-density sediments being Saint Elias orogen that were subducted with the tains and Kenai Peninsula (Péwé, 1975). Thus, subducted beneath the accretionary complex Yakutat microplate. Thus, the fundamental con- it is possible that increased precipitation and in- since the Neogene (Mankhemthong et al., 2013). trol on rock uplift in the Chugach core in the last creased erosion due to glacial activity starting ca. We speculate that for the last ~5 m.y., coincident ~4–5 m.y. is probably tectonics, which produces 2.5 Ma (Lagoe et al., 1993; Lagoe and Zellers, with deposition of the Yakataga Formation and positive feedbacks among precipitation, glacial 1996) could have caused the rapid Neogene ex- foreland basin development in the Saint Elias activity, and exhumation. humation in the Chugach core. Unfortunately, orogen (Plafker, 1987; Lagoe et al., 1993), fl at- ACKNOWLEDGMENTS our thermochronologic data do not better con- slab subduction may have carried the sediments strain this younger part of the history because northwest with the Yakutat microplate, where We thank D. Bowman and B. Rhodes for tectonic as much as 1–2 km of erosion, perhaps associ- they were focused into the region beneath the discussions and J. Garver for his help with the zir- ated with climate change and glacier activity, Chugach core and underplated. Thus, focused con fi ssion-track analyses. K. Farley and L. Hedges (Caltech) are thanked for (U-Th)/He analyses. T. Pav- could have occurred since the 4–5 Ma resetting underplating may have been occurring in the lis provided a preprint of a paper on gravity data and of our lowest-temperature thermochronometers Chugach core since the middle Cenozoic, interpretations. R. Arkle (U.S. Geological Survey) (AHe). However, the young AHe ages occur which allowed this part of the range to reach provided help with data contouring. J. Freymueller

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Arkle et al.

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