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Focused rock uplift above the subduction décollement at Montague and Hinchinbrook Islands, Prince William Sound, Alaska
Kelly M. Ferguson1, Phillip A. Armstrong1, Jeanette C. Arkle1,* and Peter J. Haeussler2 1Geological Sciences, California State University Fullerton, 800 N. State College Boulevard, Fullerton, California 92834, USA 2U.S. Geological Survey, 4210 University Drive, Anchorage, Alaska 99508, USA
ABSTRACT exhumation also refl ects short-term seismo- mation history of an area and to gain insight genic uplift patterns formed during the 1964 into structural systems, such as megathrust Megathrust splay fault systems in accre- earthquake. The increase in rock uplift and splay faults, as they accommodate the vertical tionary prisms have been identifi ed as con- exhumation rate ca. 3–2 Ma is coincident transport of rock. Numerous studies using low- duits for long-term plate motion and sig- with increased glacial erosion that, in combi- temperature thermochronology have focused nifi cant coseismic slip during subduction nation with the fault-bounded, narrow width on exhumational patterns across major fault earthquakes. These fault systems are impor- of the islands, has limited topographic devel- systems associated with fl at-slab subduction in tant because of their role in generating tsu- opment. Increased exhumation starting ca. southern Alaska, including studies in the Alaska namis, but rarely are emergent above sea 3–2 Ma is interpreted to be due to rock uplift Range (e.g., Fitzgerald et al., 1995; Haeussler level where their long-term (million year) caused by increased underplating of sedi- et al., 2008, 2011; Benowitz et al., 2011, 2012, history can be studied. We present 32 apatite ments derived from the Saint Elias orogen, 2013), Chugach Mountains (Little and Naeser , (U-Th)/He (AHe) and 27 apatite fi ssion-track which was being rapidly eroded at that time. 1989; Buscher et al., 2008; Arkle et al., 2013), (AFT) ages from rocks along an emergent and Saint Elias Mountains (e.g., Berger et al., megathrust splay fault system in the Prince INTRODUCTION 2008a, 2008b; Berger and Spotila, 2008; Meigs William Sound region of Alaska above the et al., 2008; Enkelmann et al., 2008, 2009; shallowly subducting Yakutat microplate. Flat-slab subduction and collision of the Spotila and Berger, 2010). Some of these stud- The data show focused exhumation along the Yakutat microplate have had a profound effect ies detected loci of rapid exhumation, particu- Patton Bay megathrust splay fault system on southern Alaskan geology for the past larly in the Saint Elias and western Chugach since 3–2 Ma. Most AHe ages are younger ~24 m.y. (e.g., Haeussler, 2008). Deforma- Mountains, which may be the result of crustal- than 5 Ma; some are as young as 1.1 Ma. tion from this interaction has penetrated as far scale lithologic backstops to upper crustal rock AHe ages are youngest at the southwest as ~900 km inland, from the Brooks Range deformation above the subducting Yakutat end of Montague Island, where maximum in the north (O’Sullivan et al., 1997a, 1997b) microplate. fault displacement occurred on the Hanning to the Saint Elias Mountains in the southeast. This study targets the southern Prince Wil- Bay and Patton Bay faults and the highest Flat-slab subduction of the Yakutat microplate liam Sound region (Fig. 1), located on the over- shoreline uplift occurred during the 1964 has resulted in slip and deformation along sev- riding North American plate closest to the Aleu- earthquake. AFT ages range from ca. 20 eral fault systems throughout the region (Fig. 1), tian Trench and ~20 km above the mega thrust to 5 Ma. Age changes across the Montague including faults that splay off the subduction décollement. Seismic imaging and thermal- Strait fault, north of Montague Island, sug- megathrust (e.g., Plafker, 1967; Bruhn et al., mechanical models show that there is a large gest that this fault may be a major structural 2004; Haeussler et al., 2011; Liberty et al., degree of coupling and/or underplating between boundary that acts as backstop to deforma- 2013). Megathrust splay faults elsewhere in the the subducting Yakutat microplate and the over- tion and may be the westward mechanical world develop in accretionary prisms at outer riding North American plate (Brocher et al., continuation of the Bagley fault system back- ridges that fl ank the deformation front in sub- 1991; Ratchkovski and Hansen, 2002; Zweck stop in the Saint Elias orogen. The regional duction settings (e.g., Kame et al., 2003; Ikari et al., 2002; Ferris et al., 2003; Eberhart-Phillips pattern of ages and corresponding cooling et al., 2009). Some megathrust splay faults et al., 2006; Fuis et al., 2008) below Prince and exhumation rates indicate that the Mon- have been identifi ed as conduits for long-term William Sound, making this area susceptible tague and Hinchinbrook Island splay faults, plate motion and signifi cant coseismic slip dur- to large (moment magnitude, Mw > 8.0) earth- though separated by only a few kilometers, ing subduction earthquakes (Park et al., 2002; quakes like the 1964 Mw 9.2 Alaska earthquake accommodate kilometer-scale exhumation Kame et al., 2003, Moore et al., 2007; Ikari (Plafker, 1965). Evidence that this region is above a shallowly subducting plate at million et al., 2009). These megathrust splay faults can actively accommodating deformation is shown year time scales. This long-term pattern of be a source of tsunami generation during large by the tectonic analysis of ground breakage and megathrust ruptures, because they are typically surface warping during the 1964 earthquake on *Current address: Department of Geology, Uni- located offshore in deep water. Montague and Hinchinbrook Islands (Plafker, versity of Cincinnati, P.O. Box 0013, Cincinnati, Thermochronometers allow us to place 1967) in southern Prince William Sound. This Ohio 45221, USA. million-year time-scale constraints on the exhu- study expands on Plafker’s (1967) original
Geosphere; February 2015; v. 11; no. 1; p. 144–159; doi:10.1130/GES01036.1; 9 fi gures; 1 table; 1 supplemental fi le. Received 1 February 2014 ♦ Revision received 15 September 2014 ♦ Accepted 27 October 2014 ♦ Published online 22 December 2014
144 For permission to copy, contact [email protected] © 2014 Geological Society of America Downloaded from geosphere.gsapubs.org on January 28, 2015
Focused rock uplift at Montague and Hinchinbrook Islands
64°N 150°W 140°W CANADA
U.
S. Denali Fault Denali FaultMt.McKinley central Alaska Range Alaska
Talkeetna Wrangell Mountains Mountains 61°N NORTH AMERICAN PLATE D Mt.Marcus Baker Border Ran Denali Faul CM ges Fault Mt. Logan Castle Mountain Fault t Anchorage BF SEM Copper River CSEF KM MSF HI BG MG Prince Fairweather Fault 59°N William Sound Cook Inlet D MI 50 mm/yr area of Fig.2 PZ KIZ MDI YAKUTAT Border Ranges Fault PLATE Transition Fault 59°N Contact Fault PACIFIC 0 100 km PLATE 51 mm/yr Aleutian 150°W Megathrust
Figure 1. Regional 300 m digital elevation model base map of southern Alaska (modifi ed from the U.S. Geological Survey data repository, http:// ned.usgs .gov). Prince William Sound study area is outlined by yellow box and shows the area of Figure 2. Major faults are after Plafker et al. (1994) and the U.S. Geological Survey data repository (http://ned .usgs .gov). Plate motion vectors (white arrows) are from Plattner et al. (2007) and Elliott et al. (2010). Interpreted region of the subducted Yakutat microplate (green boundary) and subaerial region of Yakutat microplate (green shaded portion of plate) are from Fletcher and Freymueller (2003), Eberhart-Phillips et al. (2006), and Fuis et al. (2008). CM—Chugach Mountains; KM—Kenai Mountains; SEM—Saint Elias Mountains; PZ—Pamplona fold-thrust zone; KIZ— Kayak Island zone; MG—Malaspina Glacier; BG—Bering Glacier; BF—Bagley fault; CSEF—Chugach–Saint Elias fault; MI—Montague Island; HI—Hinchinbrook Island; MSF—Montague Strait fault; MDI—Middleton Island. Modifi ed from Arkle et al. (2013). geologic analyses by using apatite (U-Th)/He early Mesozoic (e.g., Plafker et al., 1994; Brad- marked transition between shallow subduction (AHe) and fi ssion-track (AFT) thermochronol- ley et al., 2003). The Yakutat terrane is the young- beneath the Prince William Sound and relatively ogy in order to quantify long-term rock uplift est in the terrane sequence and is composed steeper subduction of dense oceanic Pacifi c plate and exhumation patterns across southern Prince mostly of a 15–30-km-thick oceanic plateau to the southwest (Fig. 1). Sediment shed from William Sound. We target Hinchinbrook and (e.g., Christeson et al., 2010; Worthington et al., the growing orogen also became incorporated Montague Islands (Figs. 1 and 2), which are the 2012). Arrival of thickened Yakutat crust at the into and deformed within the fold-thrust belt largest and most trenchward islands in Prince convergent boundary is inferred to have begun (e.g., Plafker, 1987; Meigs et al., 2008; Pavlis William Sound. ca. 12–10 Ma (Plafker, 1987; Plafker et al., 1994; et al., 2012). Zellers, 1995; Ferris et al., 2003; Eberhart-Phil- The accretionary complex rocks in central TECTONIC AND GEOLOGIC SETTING lips et al., 2006; Enkelmann et al., 2008, 2009) and southern Prince William Sound consist of and as early as ca. 30–18 Ma (Plafker et al., the late Paleocene to Eocene Orca Group. This Cretaceous–Cenozoic History of 1994b; Enkelmann et al., 2008; Haeussler, 2008; is a fl ysch deposit consisting dominantly of slate Southern Alaska Finzel et al., 2011; Benowitz et al., 2011, 2012; and graywacke turbidites, but it also contains Arkle et al., 2013). As the collision of this rela- interbedded conglomerate, volcanic-lithic and/or The rocks along the southern Alaska margin tively buoyant material progressed, a fold-thrust pelagic sandstone, and mudstone (Nelson et al., in Prince William Sound are part of a vast accre- belt developed, leading to high topography in the 1985). The Orca Group extends laterally for tionary complex that has developed since the eastern Chugach–Saint Elias Mountains, and a more than 100 km and has a structural thickness
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Ferguson et al.
P45 P13 4.9 4.8 Gravina Fault 10.8 10.7 147W P4 P5 146W P44 20.7 P2 P3 18.1 P43 6.6 --- 6.0 9.1 16.4 9.7 13.8 12.4 15.4 15.3 10-8 11.2 C 16.6 B8 10-22 P42 4.8 4.6 P33 NI 16.0 --- 9.4 10.1 10-9 37.4 Prince William A′ 21.5 7.2 21.5 Sound 10-21 4.9 Rude River Fault P32 SI 11-11 --- Cordova 10-20 12.8 P34 10-19 4.8 5.1 6.4 23.5 12.1 10-10 7.5 13.9 --- 10-23 36.6 5.4 HKI 4.1 17.8 11-12 11-13 10.3 10-14 4.4 6.0 B1 B6 10-11 4.4 --- 5.8 10-15 --- 7.0 6.9 9.8 --- 4.3 --- 13.0 RB 11.3 10-18 11-16 KNI 3.6 K10 ZB 8.4 --- 12.3 ′ 28.5 11-10 PE C 8.7 --- 3.0 11-14 53.3 6.3 10-16 HI 5.2 Montague Strait GIFault 4.5 11-15 10-17 12.3 11.4 4.9 10-12 5.4 8.8 4.7 --- K14 P37 11.2 11-9 B --- 9.7 11-5 3.2 20.7 32.6 1.8 6.7 --- 7.4 40.0 MI 11-6 11-4 2.3 2.0 11-7 K6 7.5 4.8 3.7 --- 10.1 60N 34.0 LI 11-8 6-3 20 km 60N 6.3 1.4 K5 11-3 PB 11.6 K4 4.4 --- 1.1 Sample Label Key --- 39.2 7.7 11-1 Data Sources 32.5 5.8 Sample # 11-17 Ages from this study Hanning Bay Fault B′ 18.7 HW-15 10-13 JC 11-2 AHe Age 5.0 Arkle et al., 2013 --- 1.7 2.8 --- 7.3 9.7 AFT Age 10.5 Carlson, 2012 --- HW-18 52.7 --- A --- ZHe Age 30.2 Buscher et al., 2008 --- Kveton, 1989 53.1 ZFT Age 54.0 146W
Cape Cleare Fault 147W Patton Bay Fault ice Paleocene - Eocene Intrusions Paleocene - Eocene Ophiolitic Rocks basin deposits Oligocene Intrusions Cretaceous Valdez Group Sandstone Paleocene - Eocene Orca Group Sandstone or Conglomerate
Figure 2. Sample map with apatite (U-Th)/He (AHe), apatite fi ssion-track (AFT), and zircon fi ssion-track (ZFT) cooling ages. Base map (300 m digital elevation model) is modifi ed from Arkle et al. (2013). Faults (solid lines), inferred faults (dashed lines), and overlain major lithologic units are from Plafker et al. (1989) and the U.S. Geological Survey fault data repository (http:// ned .usgs .gov). The Patton Bay and Hanning Bay faults are highlighted in red. Transect lines are located for Figures 5 and 6. MI—Montague Island; HI—Hinchinbrook Island; HKI—Hawkins Island; LI—Latouche Island; KNI—Knight Island; NI—Naked Islands; GI—Green Island; SI—Smith Islands; PE—Port Etches; ZB—Zaikof Bay; RB—Rocky Bay; PB—Patton Bay; JC—Jeanie Cove. of 18–20 km (Brocher et al., 1991; Plafker and Intrusions were likely formed from near-trench fi ssion-track and U-Pb ages, the depositional age Berg, 1994; Fuis et al., 2008). It was intruded by processes related to a slab window, which may for the Orca Group is ca. 35 Ma in southeastern- two episodes of small granitic plutons that are be associated with the subduction of the Kula- most Prince William Sound, ca. 40–37 Ma near between ca. 56–53 Ma (the Sanak-Baranof belt) Farallon-Resurrection spreading center and pos- Latouche and Evans Islands, and ca. 60–57 Ma and ca. 40–37 Ma (the Eshamy suite) (Hudson sibly another smaller ridge (Bradley et al., 2003; in central Prince William Sound (Garver et al., et al., 1979; Plafker and Berg, 1994; Haeussler Haeussler et al., 2003; Cowan, 2003; Cole et al., 2012; Davidson et al., 2011) (Fig. 2). Rocks of et al., 1995; Davidson et al., 2011; Garver et al., 2006; Madsen et al., 2006). Based on the timing the Orca Group young to the southeast, toward 2012, 2013; Carlson, 2012; Johnson, 2012). of intrusion of these plutons and detrital zircon the convergent margin.
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Focused rock uplift at Montague and Hinchinbrook Islands
Regional Structures of places (Nelson et al., 1985; Plafker, 1967). In offshore and southwest of Montague Island. Prince William Sound low-lying areas, unconsolidated Quaternary gla- Bathymetry shows a 40 m fault scarp, and sub- cial till with variable thicknesses is present. sequent marine terrace ~5 km south of the Patton Our study area in Prince William Sound is There are two main reverse faults on southern Bay fault that decreases in height to the south- bound to the north by the Chugach Mountains Montague Island, the Patton Bay and Hanning west and forms the hanging-wall block of the and to the south by Hinchinbrook and Mon- Bay faults, both of which ruptured during the Cape Cleare fault. This fault is traced onshore tague Islands (Fig. 1). The arcuate Contact fault 1964 earthquake (Plafker, 1967). After the earth- south and east of the Patton Bay fault (Fig. 2). strikes approximately east-west in north-central quake, the faults were traceable on land by large Other geophysical evidence from Trans- Prince William Sound and bends southward in (6–9 m) fault scarps, landslides, and uplifted Alaska Crustal Transect (TACT) studies (e.g., the Chugach Mountains to form the western coastal platforms (Plafker, 1967). These faults Fuis et al., 2008) shows that faults in the inlet Chugach syntaxis, a major structural bound- generally strike northeast, or parallel to the between Hinchinbrook and Montague Islands ary to this study area. In its southern exposures long axis of the island, and dip ~60° northwest near Zaikof Bay become listric and are con- the Contact fault strikes southwest and gener- on average (Plafker, 1967) (Fig. 2). The Patton nected at their base to the subduction décolle- ally separates Prince William Sound from the Bay fault can be traced on land for 35 km along ment to the northwest. These faults are along Kenai Peninsula (Fig. 1). The Contact fault is the southeast coast and continues southwest strike with those on Hinchinbrook and Mon- dominantly a right-lateral strike-slip fault to on the seafl oor, perhaps as far south as Kodiak tague Islands, possibly indicating that the faults the east (Bol and Roeske, 1993), but displays Island (Plafker, 1965; Liberty et al., 2013). Seis- on Hinchinbrook and Montague Islands are also reverse-fault dip-slip displacement to the west mic refl ection profi les and bathymetry show rooted at depth (Liberty et al., 2013; Haeussler in the Chugach Mountains and Prince William the Patton Bay fault continuing offshore to et al., 2014). Land surfaces at Montague Island, Sound (Bol and Gibbons, 1992). Farther toward the southwest for ~20 km (Plafker, 1967; Malloy along the footwall block of the Cape Cleare the trench an array of faults extends southwest and Merrill, 1972), where vertical displacements fault offshore, and at Middleton Island ~100 km from the Cordova area (Fig. 2) to form the of ~15 m associated with the 1964 earthquake southeast toward the Aleutian megathrust faults of Montague and Hinchinbrook Islands are mapped along seafl oor escarpments (Malloy (Fig. 1), were all uplifted during the 1964 earth- (Nelson et al., 1985), all of which strike north- and Merrill, 1972; Liberty and Finn, 2010; Lib- quake, suggesting that these faults are likely east-southwest and dip northwest. These are the erty et al., 2013). To the north, the Patton Bay rooted downward and are separately linked to most outboard faults exposed in Prince William fault may continue offshore along the coast to the décollement at depth (Plafker, 1967; Malloy Sound (Fig. 2). Between these faults and the near the northeast end of the island, where its and Merrill, 1972; Haeussler et al., 2014). Contact fault is the Montague Strait fault, which trace is lost and the fault is likely broken into is a high-angle normal fault that dips southeast, multiple strands (Fig. 2). The northern coastal PREVIOUS REGIONAL though the deep structural confi guration of the extent of the Patton Bay fault is suggested by the THERMOCHRONOLOGY WORK fault is uncertain (Haeussler et al., 2014; Liberty straight nature of the coastline and numerous tri- et al., 2013). Liberty et al. (2013) interpreted the angular facets that line the coast along northeast- An extensive data set of low-temperature Montague Strait fault as a major structure that ern Montague Island. However, there is little evi- thermochronometer ages generated over the past separates metamorphosed Orca Group rocks dence of active faulting at the northern extent of 25 years provides insights into the timing, rates, to the northwest from unmetamorphosed Orca the Patton Bay fault. Mountain-front sinuosity is and regional exhumation patterns, and provides Group rocks to the south (Liberty and Finn, nearly 1.0 and average slopes are steeper on the constraints on regions of localized rapid exhu- 2010; Haeussler et al., 2014; Garver et al., 2012). east coast (18°–22°) versus the west coast (10°– mation during the Neogene along the southern It was previously recognized as a structural dis- 12°) (measurements from this study). Motion on Alaska margin (e.g., O’Sullivan et al., 1997a, continuity (Nelson et al., 1985), but the nature the fault at its southern extent was dominantly 1997b; Meigs et al., 2008; Berger et al., 2008a, of the fault was unknown. Subsequent National dip slip with a maximum of 9 m of vertical off- 2008b; Armstrong et al., 2008; Buscher et al., Oceanic and Atmospheric Administration multi- set on land during the 1964 earthquake, although 2008; Enkelmann et al., 2008, 2009; Spotila and beam surveys collected from 1988 to 2003 reveal there was a small component (~0.5 m) of left- Berger, 2010; Arkle et al., 2013). a seafl oor fault scarp (Liberty and Finn, 2010; lateral motion (Plafker, 1967). In the Saint Elias region, exhumation rates in Haeussler et al., 2014). New seismic refl ection The Hanning Bay fault, which cuts across a the past ~6 m.y. are between ~0.5 and 4 mm/yr profi les show that the Montague Strait fault small portion of the southwest coast of Mon- and vary based on structural position relative locally had southeast-side-down normal slip tague Island at Fault Cove (Fig. 2), extends for to major fault systems (Berger et al., 2008a, during the Holocene (Liberty and Finn, 2010; ~6 km on land with the same structural orienta- 2008b; Berger and Spotila, 2008; Meigs et al., Haeussler et al., 2014; Liberty et al., 2013). tion as the Patton Bay fault. The well-defi ned 2008; Enkelmann et al., 2008, 2009; Spotila and 1964 scarp at Fault Cove has a vertical offset of Berger, 2010; Falkowski et al., 2014). Low-tem- Hinchinbrook and Montague Islands 6 m (Plafker, 1967). Across both the Hanning perature cooling ages are generally very young Bay and Patton Bay faults, the northwest (AHe younger than 1.5 Ma) along the coast Hinchinbrook and Montague Islands are elon- blocks were upthrown relative to the southeast and abruptly increase north of the Bagley fault, gate, narrow islands with steep coastlines and blocks during the 1964 earthquake. The block suggesting that the Bagley fault is acting as a numerous reverse faults that strike parallel to southeast of the Patton Bay fault was upthrown deformational backstop to thin-skinned folding the trend of the islands (Plafker, 1967) (Fig. 2). ~4.7 m relative to sea level, the block northwest and thrusting at the collision front (Berger et al., Average peak elevations on Hinchinbrook and of the Patton Bay fault was upthrown ~12 m, 2008b; Berger and Spotila, 2008; Enkelmann Montague Islands are consistently ~800 m; and the block northwest of the Hanning Bay et al., 2008, 2009; Headley et al., 2013; Pavlis, the highest peaks are nearly 1000 m. Bedding fault was upthrown ~5 m (Plafker, 1967). 2013) (Fig. 1). South of the Bagley fault, young is highly deformed and ranges from shallowly Liberty et al. (2013) showed the presence of AHe ages are attributed to their position within dipping to vertical and is overturned in some an additional splay fault, the Cape Cleare fault, the active fold-thrust belt coupled with exten-
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Ferguson et al. sive erosion at the coastal front due to its wind- show whether this zone of rapid exhumation Arkle et al., 2013). Between the Montague Strait ward position and heavy glaciation (Spotila and continues west (Fig. 1). and Contact faults, ages for all thermochrono- Berger, 2010). This zone of focused exhuma- In the western Chugach Mountains and north- logic systems generally decrease by 50% relative tion is projected west along the Bagley-Con- ern Prince William Sound (Fig. 1), a bullseye of to north of the Contact fault (Fig. 3). This pattern tact backstop toward the Miles Glacier region, relatively rapid exhumation was identifi ed in a of relatively young ages and rapid exhumation or Miles Corner (previously referred to as the syntaxial bend between the Border Ranges and north of the Contact fault is interpreted to be the Western or Katalla syntaxis; e.g., Chapman Contact faults (Arkle et al., 2013). AHe and AFT result of underplating along the décollement that et al., 2011), where it curves southwest to even- ages decrease northward across the Contact fault has been focused by a syntaxial geometry and tually connect with the Ragged Island thrust, to minimum ages (averaging ca. 5 and 10 Ma, modulated by glacial erosion at the southward Kayak Island zone, and the Alaska-Aleutian respectively) in the core of the Chugach Moun- fl ank of the core (Arkle et al., 2013). megathrust (Spotila and Berger, 2010) (Fig. 1). tains between the Contact and Border Ranges The data set from this study helps to con- While the Miles Corner region may represent faults. Zircon fi ssion-track (ZFT) ages follow a strain the nature of deformation west of the an immature indentor corner, there have been similar pattern, but with older and more scattered transition from the eastern Chugach–Saint Elias insuffi cient age constraints across the Copper ages, ranging between ca. 50 and 26 Ma (transect Mountains and across the Copper River delta River delta and into Prince William Sound to D–D′; Figs. 1 and 3) (Little and Naeser, 1989; into Prince William Sound. It also provides
D′ D Northwest Southeast central Prince southern Prince Talkeetna Mountains Chugach Mountains William Sound William Sound 80 Montague Border Ranges AHe Strait Fault Fault AFT 70 ZHe Contact Fault ZFT
60
50
40
30 Thermochronometer Age (Ma) 20
10
0 300 250 200 150 100 50 0 Distance southeast to northwest D-D′ (km)
Figure 3. Plot of thermochronometer ages along a southeast-northwest transect (D–D′) shown in Figure 1. Ages are projected onto the transect profi le from a 100 km swath. Age uncertainties are ±1σ. Samples are shown relative to faults (vertical dashed lines). Shaded regions mark approximate bounds of maximum and minimum ages for the apatite (U-Th)/He (AHe), apatite fi ssion-track (AFT), and zircon fi ssion-track (ZFT) systems. Direction of Yakutat convergence is from right to left. The ZFT ages in the Chugach Mountain region are the average of the two youngest ZFT age peaks from modern glacial deposits (Arkle et al., 2013). Figure modifi ed from Arkle et al. (2013).
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Focused rock uplift at Montague and Hinchinbrook Islands
TABLE 1. SUMMARY OF THERMOCHRONOLOGY DATA atadegakcart-noissiF atadegaeHA Elevation Latitude Longitude Age* ±1σ ε rate† Age** ±1σ ε rate† Sample (m) (°N) (°W) (Ma) (Ma) (mm/yr) n§ (Ma) (Ma) (mm/yr) 10-8 7.6 60.730 147.483 16.6 1.8 0.3 4 11.2 1.9 0.3 10-9 1.8 60.623 147.481 21.5 2.1 0.2 4 7.2 1.4 0.4 10-10 0 60.520 147.405 17.8 1.7 0.3 4 5.4 1.1 0.6 10-11 3.8 60.430 147.414 13.0 1.2 0.3 4 6.9 1.4 0.4 10-12 1.2 60.232 147.489 11.2 1.1 0.4 4 4.7 0.5 0.6 10-13 6 59.890 147.794 7.3 0.8 0.6 5 1.7 0.6 1.3 10-14 1.7 60.371 147.152 9.8 0.9 0.4 5 4.4 0.2 1.7 10-15 0 60.344 147.033 11.3 1.1 0.4 4 4.3 0.1 1.1 10-16 0 60.307 146.906 11.4 1.1 0.4 5 4.5 0.9 1.2 10-17 0 60.290 146.665 — — — 4 5.4 0.8 0.5 10-18 0 60.391 146.726 12.3 1.1 0.4 4 3.6 0.2 0.8 10-19 13.3 60.482 146.613 13.9 1.0 0.3 4 5.1 0.9 0.6 10-20 0 60.472 146.474 — — — 4 6.4 0.4 0.5 10-21 4.9 60.530 146.157 — — — 4 4.9 0.2 0.6 10-22 0 60.612 145.857 9.4 0.9 0.5 4 4.6 0.7 0.6 10-23 5.1 60.603 145.741 10.3 1.1 0.4 5 4.1 0.4 0.7 11-1 0 59.880 147.347 18.7 2.3 0.3 4 5.8 0.9 0.9 11-2 0 59.902 147.441 9.7 0.9 0.4 4 2.8 0.8 0.5 11-3 759 59.914 147.623 7.7 0.7 0.5 5 1.1 0.4 1.2 11-4 771 59.981 147.530 4.8 0.6 0.8 5 2.0 0.6 0.7 11-5 525 59.996 147.582 7.4 0.5 0.5 7 1.8 0.6 0.4 11-6 382 59.988 147.646 7.5 0.6 0.5 4 2.3 0.2 0.9 11-7 484 59.976 147.445 10.1 0.8 0.4 4 3.7 0.2 0.7 11-8 0 59.961 147.344 11.6 0.9 0.4 4 6.3 1.0 0.7 11-9 0 60.198 147.083 6.7 0.6 0.7 4 3.2 0.8 0.7 11-10 738.3 60.223 147.126 6.3 0.6 0.6 4 3.0 0.7 0.6 11-11 887.7 60.412 146.550 7.5 0.7 0.5 4 4.8 0.7 0.5 11-12 494.4 60.396 146.679 — — — 4 4.4 0.6 0.6 11-13 443.1 60.404 146.506 — — — 4 6.0 0.1 0.4 11-14 555 60.376 146.511 12.3 1.2 0.3 4 5.2 0.6 0.5 11-15 0 60.279 146.518 8.8 1.2 0.5 4 4.9 1.3 0.6 11-16 0 60.352 146.194 8.7 1.4 0.5 4 8.4 1.8 0.3 6-3†† 0 59.962 147.679 4.4 0.5 1.0 4 1.4 0.2 2.5 Note: All samples are Orca Group sandstone. Latitude and longitude are North American Datum 1927. Dashes indicate no data. *Apatite fission-track ages are reported from analytical data (Table SF2 in the Supplemental File [text footnote 1]). †ε is exhumation rate for both AFT and AHe; data are from Tables SF3 and SF4 in the Supplemental File (text footnote 1) and are derived from age and closure temperature data. §n is number of single grain replicates used for AHe age calculation. **Apatite (U-Th)/He (AHe) ages are averages of valid replicates after Ft correction and are reported as a weighted mean (discussed in Supplemental File [see text footnote 1]). ††Ages are from Arkle et al. (2013).
constraints on deformation mechanisms above We also utilize ZFT data from other studies spectrometry methods at the California Institute the subduction megathrust, but outboard of the (Carlson, 2012; Garver et al., 2012) that track of Technology (Pasadena). We used 141 single exhumation bullseye and syntaxial bend in the the thermal histories of rock up to ~240 °C (e.g., apatite grains with 4–7 single grain replicates western Chugach Mountains. Reiners and Brandon, 2006). per sample for AHe analysis. Of these grains, 13 (9%) were removed from the average age cal- METHODS Sampling Strategy and culations because they yielded outlier ages that Analytical Techniques were high (greater than a 2σ variance) relative This study utilizes AHe and AFT thermo- to the other grains in the samples (Table SF1 in chronometers to track thermal histories up to Samples of Orca Group sandstone (n = 32) the Supplemental File1). Anomalously high ages effective closure temperatures of ~130 °C. For were collected in southeastern Prince William are probably due to high U-bearing inclusions the AHe system, the partial retention zone (PRZ) Sound and used for AHe and AFT analysis is between ~40 °C and 70 °C (depending on the (Fig. 2; Table 1). Samples were collected both 1Supplemental File. PDF fi le that outlines meth- ods for determining inter-grain age variability using cooling rate, grain size, and effective uranium along and across the structural grain of Hinchin- the effective uranium content of samples from this concentration), which corresponds with depths brook and Montague Islands and especially study and a corresponding fi gure, our fi ssion-track of ~2–3 km at typical geothermal gradients and adjacent to known faults and along the length of analysis, methods for reporting weighted mean surface temperatures (Farley, 2000; Farley and topography from southern Montague Island to AHe ages, and methods for estimating geothermal Stockli, 2002; Ehlers and Farley, 2003; Reiners north of Cordova. At least 5 kg of unweathered gradient specifi c to our study area and samples. It also contains tables with detailed AHe and AFT ana- et al., 2004; Flowers et al., 2009). For the AFT sandstone was collected in order to ensure suffi - lytical data and AHe/AFT exhumation rate calcula- system, tracks anneal at temperatures between cient apatite yield. Most samples were medium- tions for all thermochronometer data reported in this 60 °C and 130 °C (e.g., Wagner and Reimer, to coarse-grained sandstone with well-sorted study. These supplemental tables and fi gure will be 1972; Naeser, 1979; Gleadow et al., 1986; angular to subangular clasts, composed of quartz, denoted throughout this manuscript with an “SF” in front of them. If you are viewing the PDF of this Dumitru, 2000; Donelick et al., 2005), which feldspar, lithic fragments, and minor biotite. paper or reading it offl ine, please visit http://dx .doi corresponds to depths of ~3–6 km (depending AHe ages were determined for 32 samples .org /10 .1130 /GES01036 .S1 or the full-text article on on annealing kinetics and geothermal gradient). by laser and inductively coupled plasma–mass www .gsapubs .org to view the Supplemental File.
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Ferguson et al. undetected within the crystal. To evaluate the ous AHe ages in central Prince William Sound Orca Group sandstone in this region (Hilbert- effects of radiation damage, effective uranium (Arkle et al., 2013) that average 12.5 Ma. Just Wolf, 2012), indicating that these rocks were concentration was compared with each single- south of the Montague Strait fault, three ages on never buried deep enough to be reset. grain age (e.g., Flowers et al., 2009), but no Smith Islands and Green Island are between 6.9 AHe ages young southward along the strike relationship was apparent (Fig. SF1 in the Sup- and 4.7 Ma (Fig. 2). The youngest ages are on of Hinchinbrook and Montague Islands long plemental File [see footnote 1]). An Ft (alpha southern Montague Island and range from 6.3 axes; the youngest ages are at southern Mon- ejection) correction was applied to all raw ages to 1.1 Ma, with an average age of 2.9 Ma. On tague Island (transect A–A′; Figs. 2 and 5). to account for alpha ejection effects related to northern Montague Island, fi ve samples have However, there is the exception of two older the grain size and shape (Farley et al., 1996; ages between 4.5 and 3.0 Ma, with an average of ages on southern Montague Island and one older Farley, 2002). After outlier ages were culled, 3.9 Ma. On Hinchinbrook Island, 10 AHe ages age on northeastern Hinchinbrook Island (open a weighted average AHe age was determined range from 8.4 to 3.6 Ma, with an average age symbols in Fig. 5). The older AHe ages (5.8 for the 32 samples using the 4–7 same-sample of 5.4 Ma (Fig. 2). Farther northeast, three ages and 6.3 Ma) on Montague Island are located replicates remaining and a 1σ confi dence inter- are between 4.9 and 4.1 Ma on Hawkins Island on the footwall block of the Cape Cleare fault val (see the Supplemental File [footnote 1] for and just north of Cordova (Fig. 2). All AHe ages (Figs. 4 and 5), indicating that those rocks were details regarding weighted age calculations). are younger than the sample depositional age of not exhumed as rapidly as other southwestern AFT ages were determined for 27 sam- ca. 35 Ma (Garver et al., 2012; Carlson, 2012), Montague Island rocks. Similarly, the sample ples using the external detector method (e.g., and are reset. on northeastern Hinchinbrook Island has a sig- Gleadow and Duddy, 1981) and ages were nifi cantly older AHe age (8.4 Ma) than adjacent computed using Trackkey version 4.2 (Dunkl, AFT Ages samples, and it is located along strike with the 2002). Analytic data are reported in Table SF2 Cape Cleare fault farther south (Figs. 4 and 5). in the Supplemental File (see footnote 1). Fis- AFT ages range from 21.5 to 4.4 Ma across We infer these relatively older ages to be part sion tracks were counted in 35–40 grains in southern Prince William Sound (Table 1; Table of a separate structural block that was exhumed most samples; as few as 15 grains were counted SF2 in the Supplemental File [see footnote 1]). more slowly than rocks northwest of the Cape in some samples with low apatite yield or poor AFT ages north of the Montague Strait fault are Cleare fault and northwest of the structure on polishing. The etch pit width (Dpar) varies from 16.6 ± 1.8 and 21.5 ± 2.1 Ma (Fig. 2; Table 1), northeastern Hinchinbrook Island. 0.93 to 1.75 µm and the average for all samples and are consistent with the 37.4–10.0 Ma AFT In contrast to the AHe age trends, there is no is 1.48 ± 0.29 µm. No systematic relationship ages of Kveton (1989) and Arkle et al. (2013) systematic northeast to southwest AFT age trend was found between Dpar and AFT single-grain between the Contact and Montague Strait faults (Fig. 5). The lack of an AFT age trend suggests age, indicating that age variation between sam- (Fig. 4). Three samples just south of the Mon- that exhumation was relatively uniform along ples is not caused by varying kinetic properties tague Strait fault have AFT ages between 17.8 the islands while rocks were cooling through (Table SF2 in the Supplemental File [see foot- and 11.2 Ma. Those on southern Montague the AFT closure temperature. note 1]). Almost all have narrow single-grain Island are, on average, 8.9 Ma and range from Distinct changes in AHe and AFT age pat- age distributions and pass the P(χ2) (>5%) test, 18.7 to 4.4 Ma. On northern Montague Island, terns also occur across known faults (Fig. 6). indicating that the grains either spent little time AFT ages range from 11.4 to 6.3 Ma and aver- AHe and AFT ages decrease southward by in the partial annealing zone and/or are kineti- age 9.1 Ma. On Hinchinbrook Island the average more than half across the Montague Strait and cally invariable. AFT age is 10.6 Ma, and ranges from 13.9 to Hanning Bay faults in the southwest Montague Horizontal confi ned track length measure- 7.5 Ma (Fig. 2). Two samples north of Cordova Island area (transect B–B′; Figs. 2 and 6A). ments were made for three samples (Table 1). have AFT ages of 9.4 and 10.3 Ma. Like their However, this age change is across a broad Horizontal confi ned tracks were generally diffi - corresponding AHe ages, all of the AFT ages are ~25 km region and may be related to regional cult to fi nd within grains due to low spontaneous younger than the ca. 35 Ma depositional ages rock uplift and exhumation variations rather track densities, despite undergoing Cf-252 irra- and have been reset. than offset slip across the Montague Strait fault. diation. In 2 samples, an average of 16 measur- Farther southeast across the Patton Bay fault, able tracks was counted, and in 1 sample, 108 DISCUSSION an average ~1.5 m.y. increase in AHe ages and horizontal-confi ned tracks were counted. The average ~4.5 m.y. increase in AFT ages from range in average track lengths is 13.0–13.8 µm. Local Analysis of Thermochronometer the hanging-wall block to the footwall block Ages and Relationships to Faults may indicate signifi cant changes in rock uplift RESULTS along the Patton Bay fault in the past ~5 m.y. Our new data show that AHe and AFT ages The contrast in ages across the Hanning Bay, AHe Ages south of the Montague Strait fault are younger Patton Bay, and Cape Cleare faults since the than those from rocks in the core of the Chugach late Miocene indicates that these structures have New AHe ages range from 11.2 to 1.1 Ma Mountains to the north (Fig. 3). Overall, ages been active on million-year time scales and have (Table 1; Table SF1 in the Supplemental File decrease by ~10–15 m.y. (~50% for the AHe and exhumed rocks from depths of >4 km along nar- [see footnote 1]). In samples collected across AFT systems) southward across the Montague row, fault-bounded blocks. the topographic grain on Hinchinbrook and Strait fault, similar to the age decrease north of Faults and structural lineaments are more Montague Islands, young ages are found both the Contact fault (Fig. 3). ZFT ages (Carlson, dispersed farther northeast on northern Mon- at sea level and at high elevations, indicating no 2012), however, are the same or increase south- tague and Hinchinbrook Islands, where there is a apparent age-elevation relationship. Samples ward across the Montague Strait fault (Fig. 3). more complex network of faults (Fig. 2) (Nelson north of the Montague Strait fault have AHe The ZFT ages (averaging ca. 52 Ma; Carlson, et al., 1985). Both AHe and AFT ages of sam- ages of 7.2 ± 1.4 and 11.2 ± 1.9 Ma (Figs. 2 2012) southeast of the Montague Strait fault are ples from northern Montague and Hinchinbrook and 4; Table 1) and are consistent with previ- older than the 35 Ma depositional age for the Islands decrease toward the southeast across the
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Focused rock uplift at Montague and Hinchinbrook Islands
4.9 4.8 6.6 20.7 18 Gravina Fault 16
6 19.7 147W 6.0 9.7 14 146W 9.1 12 8 16.0 10 8 11.2 4.8 6 NI 10.1 4.6 4.1 7.2 Prince William RRF SI Sound 4.9 Cordova 12.8 5.1 5.8 12.1 5.4 6.4 HKI 4.8 7.0 4.4 6.0 6.9 4.4 3.6 6 KNI trait Fault 4.3 5.2 HI 4.5 8.4 5.4 GI 3.0 4.9 1212 Montague S 10 4.7 3.2 8 MI 6 20 km 4 9.7 4 Age Contours (Ma) 0–2 LI 1.8 2.1–4 60N 2.3 2.0 4.1–6 60N 6.3 6.1–8 1.4 3.7 8.1–10 HBF 1.1 2.8 10.1–15 2 15.1–20 5.8 20.1–40 1.7 PBF
CCF 146W 147W
Figure 4. Map of contoured apatite (U-Th)/He (AHe) ages. Contour interval is 2 m.y. HBF—Hanning Bay fault; PBF—Patton Bay fault; CCF—Cape Cleare fault; RRF—Rude River fault; MI—Montague Island; HI—Hinchinbrook Island; HKI—Hawkins Island; LI— Latouche Island; KNI—Knight Island; SI—Smith Islands; NI—Naked Islands; GI—Green Island.
′ Montague Strait fault region (transect C–C ; Figs. phy, and a surface temperature of 0 °C (Péwé, where h is the sample elevation, hm is the aver- 2 and 6B). AHe ages are ca. 5 Ma on both sides 1975). For the AFT ages, a closure temperature age elevation for a 10 km radius around the sam- of the Rude River fault on Hinchinbrook Island, of 110 °C was assumed (Reiners and Brandon, ple location, Ts is the surface temperature, Tc is suggesting little differential exhumation across 2006). Note that variations in the AFT closure the effective closure temperature, and go is the the fault in the past 5 m.y. AFT ages decrease to temperature can result from variable kinetic geothermal gradient. The average elevation was ca. 2.5 Ma to the southeast of the Rude River fault parameters, but Dpar for samples in this study computed to account for the effect of the long (Fig. 6B), but there is no defi nitive break in the varies little. Closure temperatures for AHe ages wavelength topography on the shape of shallow AFT ages across the Rude River fault. were determined using the program CLOSURE isotherms (Stüwe et al., 1994; Mancktelow and (Brandon et al., 1998) using average spherical Grasemann, 1997). Cooling and Exhumation Rates radii and cooling rates specifi c to each sample. For samples south of the Montague Strait
The closure depth (Zc) for each sample was cal- fault, average cooling rates are ~6.5 °C/m.y. Cooling and exhumation rates for single sam- culated using a modifi ed equation of Brandon between AFT and AHe closure temperatures, ples were derived from sample-specifi c closure et al. (1998): and increase to ~16 °C/m.y. in the past 5 m.y. temperatures, an averaged geothermal gradient or less (line A, Fig. 7). On Montague Island, the that accounts for thermal advection of rapidly ⎛ − ⎞ average cooling rate is ~5 °C/m.y. between AFT = Tc Ts + − Zc ⎝ ⎠ (h hm ), (1) cooled samples, assumed steady-state topogra- go and AHe closure temperatures, increasing to
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Ferguson et al.
~20 °C/m.y. after AHe closure (line B, Fig. 7). SW NE Montague Hinchinbrook On the hanging-wall block of the Patton Bay 18 Island Island fault on southern Montague Island, the average cooling rate is 6.5 °C/m.y. after AFT closure and 16 increases to ~35 °C/m.y. after AHe closure (line AFT Ages 14 AHe Ages C, Fig. 7). These relationships across southern Prince William Sound demonstrate a regional 12 increase in cooling rate in the past ~5 m.y., or since the time of AHe closure. More locally, 10 cooling rates increased between ca. 5 and 2 Ma for rocks on the hanging wall of the Patton Bay Age (Ma) 8 fault on southern Montague Island.
6 A background geothermal gradient (gi) for southern Prince William Sound was computed 4 based on the surface heat fl ow and thermal con- 2 ductivity (Blackwell and Richards, 2004; Huang et al., 2008; Batir et al., 2013). Assuming a back- ground surface heat fl ow for Prince William 0 20 40 60 80 100 120 140 160 180 Sound of 45 mW/m2 (Blackwell and Richards, Distance Along Transect A-A′ (km) 2004; Batir et al., 2013) and a thermal conduc- tivity of 2.5 W/mK for typical fi ne-grained sedi- Figure 5. Apatite (U-Th)/He (AHe) and apatite fi ssion-track (AFT) ages projected onto mentary rocks (Huang et al., 2008), we compute line A–A′ parallel to Hinchinbrook and Montague Islands long axes (transect location a gi of 18 °C/km. This background gi was then shown in Fig. 2). Age uncertainties are ±1σ. Trend lines show a southward-younging of given a ±20% variability to account for variations ages for the AHe system. Ages show no systematic variation from northeast-southwest for in heat fl ow and thermal conductivity resulting the AFT system. Hollow sample symbols are outlier ages on the footwall block of the in a range in gi values of 14.4–21.6 °C/km (see Cape Cleare fault and southeastern Hinchinbrook Island (see text). Supplemental File [footnote 1] for analysis).
0 ? 3
6
9
12 trait Faul t
15 g tague Strait Fault
Age (Ma) 18 r Faul Mon e AFT Age 21 Figure 6. Cross sections across the Montague AHe Age Strait fault and Montague and Hinchin-
24 ude Riv R brook Islands (transect locations shown in 27 Fig. 2), showing apatite (U-Th)/He (AHe) B 0 15 30 45 60 75 90 Distance along C–C′ (km) and apatite fi ssion-track (AFT) ages relative NW SE to fault locations. (A) B–B′. (B) C–C′. Age 0 uncertainties are ±1σ. Colored bands gen- erally outline range in ages and red or blue 2 line is average age for the region. Arrows 334 along fault location markers (gray dashed 6 lines) represent relative vertical displace- g 8 ment direction for that fault.
10 Montague Strait Fault M t l Age (Ma) 12
14 gy
20 Patton Bay Fault Hanning Bay Fault p
30 Cape Cleare Fault
40 15 20 25 30 35 40 45 50 55 A 0 5 10 Distance along B–B′ (km)
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Focused rock uplift at Montague and Hinchinbrook Islands
Figure 7. Age and closure temperature plot for all apa- tite (U-Th)/He (AHe), apatite Thermochronometer Age (Ma) 0 5 10 15 20 25 30 35 40 fi ssion-track (AFT), and zircon 0 fi ssion-track (ZFT) data south of the Montague Strait fault. 20 Gray lines are simple cooling paths for each sample. Aver- 40 age cooling paths for the AHe A AHe Ages and AFT systems are shown by 60 C B the bold black lines. Line A is 80 the average cooling rate for all samples, line B is the average 100 AFT Ages for Montague Island, and line C is the average cooling path 120 Average Cooling R on the hanging-wall block of the Patton Bay fault. Potential 140 Temperature (°C) Temperature time-temperature paths are a outlined by the green shaded 160 te region. Dashed line is the AFT
180 Age Maximum AFT cooling rate extrapolated back Age Maximum Depositional to 200 °C. Maximum ZFT age 200 is constrained by the deposi- tional age of the Orca Group in 220 southern Prince William Sound from Hilbert-Wolf (2012).
Regional processes may affect the thermal causes geothermal gradients to increase (e.g., across both Montague and Hinchinbrook structure of the crust overriding the Yakutat Kappelmeyer and Haenel, 1974; Powell et al., Islands, but on southern Montague Island the microplate and may infl uence low-temperature 1988; Ehlers, 2005). We apply a simple cor- exhumation rates increased in the past ~2 m.y. cooling ages. These processes include ridge rection to the background geothermal gradient subduction during the Paleocene–Eocene (e.g., by assuming erosion durations consistent with Timing and Magnitude of Haeussler et al., 2003; Idleman et al., 2011; sample ages and typical exhumation rates of Rock Uplift across Faults Benowitz et al., 2012) and later cooling related between ~0.5 and 2 mm/yr. Advection correc- to subduction of the relatively cool Yakutat tions increase gi between 10% and 80%, result- AHe and AFT age differences or similarities microplate. Thermal length arguments (e.g., ing in sample-specifi c advection-corrected across known faults on Montague and Hinchin-
Turcotte and Schubert, 2002) suggest that the geothermal gradient (go) values between 23 brook Islands allow estimates of relative rock transitory thermal effects of Paleocene–Eocene and 38 °C/km (Tables SF3 and SF4 in the Sup- uplift across the faults, and therefore estimates of ridge subduction in the 25-km-thick overriding plemental File [see footnote 1]). The median long-term fault offset. Based on the AHe cooling plate would have dissipated in ~15–20 m.y., of the two corrected end-member go values for age data, samples from the Patton Bay fault hang- prior to the Miocene and younger cooling docu- each sample was used in Equation 1 to com- ing wall cooled at a rate of 35 °C/m.y. since ca. mented by our ages. The present-day regional pute the closure depth and the resulting exhu- 2 Ma, which is the average AHe age of the hang- ε geothermal gradient is relatively low, but it is mation rates ( = Zc/t) for each sample age (t) ing-wall samples (Figs. 6A and 7). Samples from consistent with fl at-slab subduction thermal (Table 1). the footwall block of the Patton Bay fault cooled models (e.g., Gutscher and Peacock, 2003) For the AHe system, the average closure at a rate of 14 °C/m.y. since 3.3 Ma. Assuming and models of thermal effects of subduction temperature for southern Prince William Sound constant cooling rates across the Patton Bay fault in general (e.g., Cloos, 1985). We use the rela- is ~66 °C, average Zc is ~2.7 km, and overall since 3.3 m.y. and an average geothermal gra- tively low present-day geothermal gradient in average exhumation rates are 0.7 mm/yr. For dient of 25 °C/km, the temperature change dif- our exhumation analysis, but if the slab cooling the AFT system, a 110 °C closure temperature ferential between the footwall and hanging-wall effects were greater in the past, then the exhu- was used for all samples. The average depth to blocks is 65 °C, leading to a difference in rock mation rates discussed here may be a minimum. closure is ~4.5 km, and average exhumation uplift of ~2.6 km across the fault since 3.3 Ma. The relatively young ages in our study and the rates are 0.4 mm/yr (Tables SF3 and SF4 in The Patton Bay fault is estimated to dip 60° based local scale of the age variations suggest that the Supplemental File [see footnote 1]). On the on surface ruptures from the 1964 earthquake regional slab refrigeration or ridge subduc- hanging wall of the Patton Bay fault of southern (Plafker, 1967) and apparent dip estimates in the tion effects do not cause the age patterns we Montague Island, exhumation rates are as high TACT Prince William Sound deep seismic refl ec- observe, thus we assume that cooling is related as 2.5 mm/yr based on AHe closure, but aver- tion line (Haeussler et al., 2014; Liberty et al., to exhumation. age 1.5 and 0.6 mm/yr based on AHe and AFT 2013). Correcting for the 60° dip leads to a total It is well known that the advection of hot- closure, respectively. Overall, exhumation rates offset of ~3 km across the Patton Bay fault since ter rocks toward the surface during exhumation increased in the past ~5 m.y. or less (Fig. 7) 3.3 Ma; note that total offset amounts are greater
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Ferguson et al. if the fault dip is less, as suggested by Liberty tant constraint because it indicates that the sedi- exhumation in southern Prince William Sound et al. (2013). AFT age similarity across the Patton ments were at surface temperatures after 35 Ma. (assuming a constant average geothermal gradi- Bay fault indicates that cooling rates were likely If the AHe and AFT ages are reset, but the ZFT ent of 25 °C/km) is ~8 km since ca. 23–35 Ma. the same on the footwall and hanging-wall blocks ages are not reset (Carlson, 2012), then the In addition, the reset AFT ages indicate that the while cooling through AFT closure, suggesting Orca Group had to have been buried to depths minimum exhumation magnitude on Montague minimal differential exhumation across the Pat- and corresponding temperatures of between and Hinchinbrook Islands is ~4.8 km, which ton Bay fault between ca. 10 Ma (AFT closure) ~110 °C and ~200 °C, then reexhumed after ca. occurred in the past ~7–10 m.y. and 3.3 Ma (AHe closure). 35 Ma (Fig. 7). The maximum AFT age is the On northern Montague and Hinchinbrook minimum age that the rocks at Hinchinbrook Regional Analysis Islands, the clustering of AHe ages ca. 4.5 Ma and Montague Islands were at 110–200 °C. The allows estimates to be placed on the maximum shaded region in Figure 7 represents the range One of our goals is to examine the relation- amount of slip on these faults because vertical of cooling paths rocks may have taken prior to ship between the zone of very young AHe fault offset could not have been greater than the passing through AFT closure temperature. If ages (younger than 1 Ma) and rapid exhuma- depth that corresponds to the minimum tempera- the average AFT cooling rate (line A, Fig. 7) tion south of the Bagley fault in the Saint Elias ture of the AHe partial retention zone. Assuming is constant from 200 °C, then this rate may be region (Spotila and Berger, 2010) and the an average advection-corrected geothermal gra- extrapolated to as long ago as ca. 23 Ma (dashed thermochronology data across the Copper River dient of 25 °C/km and a minimum PRZ tempera- line, Fig. 7). We acknowledge that there is con- delta into Prince William Sound. Based on our ture of 40 °C, the maximum amount of differen- siderable uncertainty in this analysis, but it pro- new ages, the region of young cooling ages in tial rock uplift is ~1.6 km in the past ~4.5 m.y. on vides loose constraints on the potential timing the Saint Elias Mountains does not appear to northern Montague and Hinchinbrook Islands. of maximum temperature estimates for rocks on extend westward into Prince William Sound The depositional age for Orca Group sand- Montague and Hinchinbrook Islands. Given that (Fig. 8), but rather bends southward at the stone (younger than 35 Ma; Hilbert-Wolf, 2012) rocks were buried to temperatures as high as Miles Corner to connect with the Kayak Island in southern Prince William Sound is an impor- 200 °C, the maximum amount of rock uplift and zone and Aleutian megathrust, as suggested by
140°W a)a) AgeAge ContContoContoursouo rss ((Ma)Ma) Denali Fault 0-2020–2-22 2.12122.1–4-44 4.14414.1–6-66 6.1616.1-8–88 8.18188.1–10-1010 0 10.1010.1–10 1-111-1155 15.1515.5 1-2121–2200 20.2020.0 1-4141–400 44040.40.1-6161601–6600
Mt. Marcus Border Ranges Fault Baker BorderCHUGACH Ranges Fault Anchorage CORE Bagley Fault CSEF MSF Copper River “Miles “Seward PRINCE Corner” Corner” WILLIAM RI SOUND HI MI 59°N A Bering Malaspina Contact Fault Glacier PZ Fairweather Fault KI Glacier COASTAL MDI ST. ELIAS
150°W150°W150°W 100100 km Aleutian Megathrust
Figure 8. Contour map of new and published apatite (U-Th)/He (AHe) data from southern Alaska. Area outlined by red dashes shows the possible region of rapid exhumation from the Saint Elias Mountains continuing south along the Kayak Island zone and Aleutian mega- thrust, as described by Spotila and Berger (2010). Region labeled A indicates a possible broad deformation region that extends from the Kayak Island fault zone to southern Prince William Sound. Green line indicates interpreted subducted region of the Yakutat microplate from Fletcher and Freymueller (2003), Eberhart-Phillips et al. (2006), and Fuis et al. (2008). White dots are sample locations from this study and others. HI—Hinchinbrook Island; MI—Montague Island; MDI—Middleton Island; KI—Kayak Island; RI—Ragged Island thrust fault; PZ—Pamplona fold-thrust zone; CSEF—Chugach–Saint Elias fault; MSF—Montague Strait fault.
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Focused rock uplift at Montague and Hinchinbrook Islands
Spotila and Berger (2010) (red dashed area, relief. Often the regions of high exhumation tion decreases to below sea level between them Fig. 8). In this interpretation, the Kayak Island rate are offset from the highest elevations where at Hinchinbrook Entrance (the inlet between zone and Ragged Mountain fault link the Miles high precipitation rates (Reiners et al., 2003; Hinchinbrook and Montague Islands). Exhuma- Corner region with the Aleutian megathrust and Willett et al., 2001) or erosive alpine glaciers tion magnitude is relatively constant along the are well-defi ned boundaries between transpres- (e.g., Tomkin, 2007; Berger et al., 2008a; Arkle transect, even across Hinchinbrook Entrance. sion and convergence. Even though the band et al., 2013) cause rapid erosion on the wind- At the southern end of Montague Island, eleva- of youngest ages either ends at Miles Corner ward fl ank of the orogen. The correspondence tion decreases to sea level whereas the exhuma- or bends south, some focused rock uplift may between elevations and glacier equilibrium-line tion magnitudes increase abruptly (Fig. 9). We be transferred west into Prince William Sound, altitude (ELA) suggests that glaciers behave as expect that elevation would be greatest at the because rocks with young ages (4 Ma AHe) buzz saws that limit orogen elevation and width south end of Montague Island because it has continue west across the Copper River delta (e.g., Brozović et al., 1997; Meigs and Sauber, the highest exhumation rate and is not a focus into the Hinchinbrook and Montague Islands 2000; Spotila et al., 2004). In the Saint Elias region of high orographic precipitation or the region. The rapid exhumation in the Saint Elias Mountains, where glaciers cover much of the alpine glacier–dominated windward fl ank of region is focused along the Bagley backstop windward fl ank of the orogen, the coincidence an orogen. The lack of coincidence between (Berger et al., 2008b). We propose that this of the ELA with zones of rapid exhumation sug- high exhumation magnitude (or rate) and high- backstop continues westward as the Montague gests that glaciers partly control the rock uplift elevation regions has been documented in other Strait fault (Fig. 8), but with lower exhumation and exhumation (Meigs and Sauber, 2000; areas, especially where Quaternary glaciers rates than in the Saint Elias orogen. A broader Spotila et al., 2004; Berger and Spotila, 2008; eroded the landscape. In central and north- zone of deformation extending from the Kayak Berger et al., 2008a, 2008b). ern Fiordland, New Zealand, AHe ages are Island fault system to southern Prince William Our data from Montague and Hinchinbrook typically 1–3 Ma, but elevations are <1500 m Sound (area A, Fig. 8) may also accommodate Islands show a lack of correlation between (House et al., 2005). House et al. (2005) inter- the overall rock uplift differences south of the AHe age (or exhumation rates and magnitudes preted the changes in ages across Fiordland to backstops in southern Prince William Sound derived from the ages) and elevation. Com- be due to differential exhumation across faults, and the Saint Elias area. Uplifted fault blocks puted exhumation magnitudes along the trend but extensive glaciation caused regional ero- during the 1964 earthquake and the presence of of Montague and Hinchinbrook Islands were sion across the faults after ca. 2 Ma (Sutherland reverse faults (Haeussler et al., 2014) outboard derived using the AHe exhumation rates and an et al., 2009). Along the Fairweather corridor of southern Prince William Sound to Middleton exhumation duration of 2 m.y. (Fig. 9); the dura- adjacent to the Fairweather fault in southeast Island are consistent with a distributed defor- tion is based on the average AHe age of sam- Alaska, AHe ages are typically younger than 1 mation zone. ples from the Patton Bay fault hanging wall. In Ma (McAleer et al., 2009) across 50 km regions general, exhumation magnitude increases from of low topography that are relatively glacially Relationships between Exhumation northeast to southwest, with highest exhuma- denuded. McAleer et al. (2009) suggested that and Topography tion magnitudes at the south end of Montague glacial erosion in the Fairweather corridor Island (Fig. 9). Range-crest elevations along was high enough to limit the development of Along most active orogens, high exhumation the island-parallel transect are ~800 m on both topography even where exhumation rates were rates coincide with high elevations and high Hinchinbrook and Montague Islands, but eleva- highest. Whereas Hinchinbrook and Montague
SW Montague Hinchinbrook Hawkins NE Island Island Island 1000 8
7
800 Exhumation Magnitude (km) Island Elev. Profile Sea Level Sample 6 Figure 9. Plot comparing island- > 300m Sample parallel topographic profile 600 (gray dashed line, transect A–A′ 5 from Fig. 2) and sample exhu- mation magnitudes (green trian- 400 4 gles). Exhumation magnitudes
represent exhumation for past 2 Elevation (m) 3 m.y. at sample-specifi c exhuma- 200 tion rates. Elev.—elevation. 2 0 1
-200 0 0 20 40 60 80 100 120140 160 Distance Along A-A′ (km)
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Islands are not currently glacially dominated or focus of exhumation for at least the past 2–3 sediments shed from the Saint Elias Mountains a focus region of high orographic precipitation, m.y., with higher rates to the southwest. were carried along and underplated above the Pleistocene glaciers extended ~100 km south of Even though we interpret the majority of Yakutat microplate. Thus, the increased volume their present location (Kaufman and Manley, exhumation to be on narrow fault-bounded of subducting sediments starting ca. 2–3 Ma 2004). Thus, we infer that glaciers were able to blocks on Montague and Hinchinbrook Islands, may have also enhanced underplating along erode Montague and Hinchinbrook Islands rap- we cannot rule out the possibility that rock uplift the megathrust under southern Prince William idly enough to limit topographic growth even is spread over a broader region offshore to the Sound, which then increased rock uplift along where rock uplift rates were the highest, attest- southeast. AHe ages on the footwall of the Cape the megathrust splay faults. We infer that the ing to the buzz saw potential of glaciers (e.g., Cleare fault are ca. 6 Ma, indicating late Mio- high rock uplift rate since 2–3 Ma on Montague Brozović et al., 1997; Meigs and Sauber, 2000; cene and younger exhumation, but at a lower Island is due mainly to splay faulting and related Spotila et al., 2004). long-term rate than rocks on the hanging-wall underplating. Recent glaciation at the surface Although glacial erosion likely limited topo- block. In addition, the shorelines on the Cape has accommodated this accelerated exhumation graphic growth, the maximum elevations are Cleare fault footwall and on Middleton Island across a narrow region, but has also masked the probably also limited by the narrowness of rock located ~100 km to the southeast were uplifted topographic effects of increased rock uplift. uplift along the splay faults. Both Hinchinbrook in the 1964 earthquake (Plafker, 1969). Thus the and Montague Islands are at most 20 km wide, exhumation focused along fault splays may be CONCLUSIONS and <10 km in many locations. For example, part of broader region of rock uplift caused by AHe ages on the Yucaipa Ridge block along the underplating along the megathrust. Liberty et al. The thermochronology data from this study San Andreas fault in southern California are 0.7– (2013) reprocessed the TACT deep seismic provide insight into the style of deformation 1.6 Ma across a 5–10-km-wide ridge between refl ection data and interpreted each of the splay above the subduction décollement in the Prince fault strands (Spotila et al., 1998). Elevations faults to sole into the subduction megathrust William Sound of southern Alaska. Our ages, along the ridge are ~1000 m above base level. separately; they showed a subhorizontal mega- combined with previously published ages, show In this case, where glaciers have not eroded the thrust located 18–20 km beneath Montague that southern Prince William Sound, between landscape, oversteepening of the slopes between Island and a lens-shaped zone of refl ections the Cape Cleare and Montague Strait faults, is a the fault strands regulated the maximum eleva- below the megathrust, interpreted as thickening region of focused exhumation and deformation tions along the rapidly exhuming narrow ridge due to underplating and duplexing and coinci- likely caused by Yakutat fl at slab subduction. (Spotila et al., 1998). If rock uplift on Montague dent with where the Patton Bay, Hanning Bay, AHe and AFT ages on Montague and Hinchin- and Hinchinbrook Islands is mostly focused and Cape Cleare faults splay off the megathrust brook Islands are as young as 1.1 and 4.4 Ma, along megathrust fault splays, then the lack of (Haeussler et al., 2014). Liberty et al. (2013) respectively; the youngest ages are at the south- high topography may be partly due to the nar- inferred that the thickened region causes lock- west end of Montague Island. These ages vary rowness of the uplifted region. ing of the megathrust below the western part across major faults, especially on southwestern of Prince William Sound (Zweck et al., 2002), Montague Island across the Hanning Bay, Pat- Causes of Rock Uplift and Exhumation which initiates the splay faults that project to ton Bay, and Cape Cleare faults. Exhumation the surface parallel to one another. Underplating rates across the Patton Bay fault are ~2 times The AHe age patterns (Fig. 4) clearly indi- and duplexing along the megathrust may also higher on the hanging-wall block than on the cate that long-term exhumation rates increase cause broad rock uplift that extends southward footwall block, leading to as much as 3 km to the southwest and are highest at the south- and away from the more focused rock uplift and of slip across the Patton Bay fault in the past west end of Montague Island. The southwest exhumation on Montague and Hinchinbrook 3.3 m.y., with decreasing slip to the northeast. increase in exhumation rate and magnitude also Islands. Increased rock cooling starting ca. The northeast to southwest increase in exhuma- mimics the southwest increase in the amount 2–3 Ma on southern Montague Island is coin- tion rate is coincident with the trend of increas- of uplift and fault offset in the 1964 earthquake cident with increased mountain glacial erosion ing coseismic uplift from the 1964 earthquake, (Plafker, 1967, 1969). There was no measured worldwide (Herman et al., 2013) and with the suggesting that fault-related rock uplift is long offset on the Cape Cleare fault onshore after onset of glacial sedimentation in the Saint Elias lived. Thermochronometer ages are 2–5 times the 1964 earthquake, but seismic data of Lib- region (Lagoe et al., 1993; Lagoe and Zellers, greater north of the Montague Strait fault than erty et al. (2013) show a 40 m scarp offshore 1996; Cowan et al., 2013). Climate cooling to the south, suggesting that this fault is part to the southwest. Thus the long-term pattern of and glacier erosion may have enhanced erosion of a major structural transition that acts as a exhumation refl ects the short-term seismogenic rates on Montague and Hinchinbrook Islands mechanically strong backstop to deformation, uplift patterns. Given (1) the narrow geometry during the Pleistocene, but if glacial erosion and faster exhumation to the south on Montague of Montague and Hinchinbrook Islands; (2) the was the sole cause of increased rock uplift in and Hinchinbrook Islands. The Montague Strait relatively young thermochronometer ages from southern Prince William Sound, then other parts fault backstop may be a westward mechanical samples across the islands that are adjacent to of Prince William Sound should have had the continuation of the Bagley fault system back- relatively older ages north of the Montague same rapid uplift during this time. Pavlis et al. stop in the Saint Elias orogen, but exhumation Strait fault; (3) a well-defi ned topographic (2012) suggested that sediments originally is slower in the Montague and Hinchinbrook expression of faults onshore and their seis- shed from the Saint Elias orogen 2–3 Ma in Islands area, where deformation may be distrib- mic and bathymetric expression offshore; and response to cooling climate and glacial erosion uted between the Aleutian Trench and southern (4) the correlation between long-term exhuma- caused duplex stacking and underplating under Prince William Sound. tion and coseismic deformation, we infer that the Yakataga segment farther to the southeast. Splay faulting above the subduction décolle- exhumation is controlled dominantly by rock Mankhemthong et al. (2013) used gravity and ment is interpreted as the primary cause for rapid uplift along faults. We also infer that Hinchin- magnetic data from the Chugach Mountains exhumation in southern Prince William Sound. brook and Montague Islands have remained the north of Prince William Sound to suggest that More specifi cally, rock is being uplifted via
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Focused rock uplift at Montague and Hinchinbrook Islands splay faulting that is rooted to the Yakutat–North 2008b, Architecture, kinematics, and exhumation of a the Chugach–Prince William terrane: Earth and Plan- convergent orogenic wedge: A thermochronological etary Science Letters, v. 213, p. 463–475, doi: 10 .1016 America plate interface. Underplating at the investigation of tectonic-climatic interactions within /S0012 -821X (03)00300 -5 . plate interface in the past ~5 m.y., with increased the central St. Elias orogen, Alaska: Earth and Plan- Cowan, E.A., Forwick, M., Bahlburg, H., Childress, L.B., effects since ca. 3–2 Ma, may be enhancing or etary Science Letters, v. 270, p. 13–24, doi: 10 .1016 /j Moy, C.M., Muller, J., Ribeiro, F., Ridgway, K.D., .epsl .2008 .02 .034 . 2013, Southern Alaska glaciations recorded in deep- driving splay fault formation in southern Prince Blackwell, D.D., and Richards, M., 2004, Geothermal map sea diamicts: Preliminary results from IODP Expedi- William Sound. The lack of correlation between of North America: American Association of Petroleum tion 341: American Geophysical Union, fall meeting exhumation rates or magnitudes and topogra- Geologists, scale 1:6,500,000. abs. T22C-07. Bol, A.J., and Gibbons, H., 1992, Tectonic implications of Davidson, C., Garver, J.I., Hilbert-Wolf, H.L., and Carlson, phy suggests that a cooling climate and glacial out-of-sequence faults in an accretionary prism, Prince B., 2011, Maximum depositional age of the Paleocene erosion played a role in limiting topographic William Sound, Alaska: Tectonics, v. 11, p. 1288– to Eocene Orca Flysch, Prince William Sound, Alaska: 1300, doi: 10 .1029 /92TC01327 . Geological Society of America Abstracts with Pro- growth. Notably, this study shows that splay Bol, A.J., and Roeske, S.M., 1993, Strike-slip faulting and grams, v. 43, no. 5, p. 439. faults with historic coseismic rupture separated block rotation along the contact fault system, east- Donelick, R.A., O’Sullivan, P.B., and Ketcham, R.A., 2005, by only a few kilometers can facilitate kilometer- ern Prince William Sound, Alaska: Tectonics, v. 12, Apatite fission-track dating, in Reiners, P.W., and p. 49–62, doi: 10 .1029 /92TC01324 . Ehlers, T.A., eds., Low-temperature thermochronol- scale exhumation above shallowly subducting Bradley, D., Kusky, T., Haeussler, P., Goldfarb, R., Miller, ogy: Techniques, interpretations, and applications: plates at million-year time scales. M.L., Dumoulin, J., Nelson, S.W., and Karl, S., 2003, Reviews in Mineralogy and Geochemistry Volume 58, Geologic signature of early Tertiary ridge subduction p. 49–94, doi: 10 .2138 /rmg .2005 .58 .3 . ACKNOWLEDGMENTS in Alaska, in Sisson, V.E., et al., eds., Geology of a Dumitru, T.A., 2000, Fission-track geochronology in Qua- We thank K. Farley and L. Hedges (California transpressional orogen developed during ridge-trench ternary geology, in Noller, J.S., et al., eds., Quaternary interaction along the North Pacifi c margin: Geological geochronology: Methods and applications: Washing- Institute of Technology, Pasadena) for (U-Th)/He Society of America Special Paper 371, p. 19–49, doi: 10 ton, D.C., American Geophysical Union Reference analyses. We also thank Matan Salmon for his help as .1130 /0 -8137 -2371 -X .19 . Shelf, v. 4, p. 131–155, doi: 10 .1029 /RF004p0131 . fi eld assistant in the Prince William Sound. Insightful Brandon, M.T., Roden-Tice, M.K., and Garver, J.I., 1998, Dunkl, I., 2002, Trackkey: A Windows program for calcu- reviews by J. Benowitz, C. Davidson, Associate Edi- Late Cenozoic exhumation of the Cascadia accretion- lation and graphical presentation of fi ssion track data: tor T. Pavlis, and an anonymous reviewer helped us ary wedge in the Olympic Mountains, northwest Wash- Computers & Geosciences, v. 28, p. 3–12, doi:10 .1016 focus and improve this paper. Funding was provided ington State: Geological Society of America Bulletin, /S0098 -3004 (01)00024 -3 . by Donors of the Petroleum Research Fund adminis- v. 110, p. 985–1009, doi: 10 .1130 /0016 -7606 (1998)110 Eberhart-Phillips, D., Christensen, D.J., Brocher, T.M., Han- tered by the American Chemical Society and the John <0985: LCEOTC>2 .3 .CO;2 . sen, R., Ruppert, N.A., Haeussler, P.J., and Abers, G.A., Brocher, T.M., Moses, M.A., Fisher, M.A., Stephens, C.D., 2006, Imaging the transition from Aleutian subduction T. Dillon Alaska Research Fund administered by the and Geist, E.L., 1991, Images of the plate boundary to Yakutat collision in central Alaska, with local earth- Geologic Society of America. beneath southern Alaska, in Meissner, R., et al., eds., quakes and active source data: Journal of Geophysical REFERENCES CITED Continental lithosphere: Deep seismic refl ections: Research, v. 111, B11303, doi: 10 .1029 /2005JB004240 . 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Geosphere
Focused rock uplift above the subduction décollement at Montague and Hinchinbrook Islands, Prince William Sound, Alaska
Kelly M. Ferguson, Phillip A. Armstrong, Jeanette C. Arkle and Peter J. Haeussler
Geosphere 2015;11;144-159 doi: 10.1130/GES01036.1
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