Upper-Crustal Cooling of the Wrangellia Composite Terrane in the Northern St. Elias Mountains, Western Canada

Home , Mount Wood (Yukon), Wrangellia Terrane

RESEARCH

Upper-crustal cooling of the Wrangellia composite terrane in the northern St. Elias Mountains, western Canada

Sarah Falkowski1 and Eva Enkelmann2 1UNIVERSITY OF TÜBINGEN, DEPARTMENT OF GEOSCIENCES, 72074 TÜBINGEN, GERMANY 2UNIVERSITY OF CINCINNATI, DEPARTMENT OF GEOLOGY, CINCINNATI, OHIO 45221-0037, USA

ABSTRACT

This study presents the long-term exhumation history of the Wrangellia composite terrane of the remote and ice-covered northern St. Elias Mountains in southwest Yukon, northwest British Columbia, and adjacent Alaska. Detrital zircon and apatite fission-track age distributions are presented from 21 glacial catchments. The detrital sampling approach allows for a large spatial coverage (~30,000 km2) and access to material eroded beneath the ice. An additional five bedrock samples were dated by zircon fission-track analysis for a comparison with detrital results. Our new thermochronology data record the Late Jurassic–mid-Cretaceous accretion of the Wrangellia composite terrane to the former North American margin and magmatism, which reset the older thermal record. The good preservation of the Jurassic–Cretaceous record suggests that Cenozoic erosion must have been limited overall. Nonetheless, Eocene spreading-ridge subduction and Oligocene–Neogene cooling in response to the ongoing Yakutat flat-slab subduction are evident in the study area despite its inboard position from the active plate boundary. The results further indicate an area of rapid exhumation at the northern end of the Fairweather fault ca. 10–5 Ma; this area is bounded by discrete, unmapped structures. The area of rapid exhumation shifted southwest toward the plate boundary and the center of the St. Elias syntaxis after 5 Ma. Integrating the new data with published detrital thermochronology from the southern St. Elias Mountains reveals an evolving concentration of deformation and exhumation, possibly within a large-scale, transpressional structure providing important constraints for geodynamic models of syntaxes.

LITHOSPHERE; v. 8; no. 4; p. 359–378; GSA Data Repository Item 2016150 | Published online 20 May 2016 doi:10.1130/L508.1

INTRODUCTION 1 TiF 10 2 DF Yakutat slab YCT The Wrangellia composite terrane is the classic example of an accreted 3 WT G and displaced terrane within the terrane mosaic of the western North 5 PT 7 Fig. 2 American margin (Jones et al., 1972; Coney et al., 1980). The composite KT 4 BRF A 6 terrane has been studied to understand continental growth by terrane sub- CMC 8 AT duction and accretion processes and its spatial and temporal transitions 6 9 between compressional, transpressional, and extensional deformation 60ºN (e.g., Coney et al., 1980; Rusmore and Woodsworth, 1991; Andronicos YM FF et al., 1999, 2003; Gehrels et al., 2009; Israel et al., 2013). Due to the CPWT Alaska Fig. 4 TF WT long history of accretion, terranes experience continued deformation dur- ko n NWT Yu ing their accretion, possible postaccretional displacement, and continued 60ºN PAC B.C. QCF subduction and accretion at their outboard margins. Arc AMT 140ºW This study investigates the Wrangellia composite terrane of the north- Aleutian 140ºW ern St. Elias Mountains in southwest Yukon, northwest British Columbia, Figure 1. Terrane map of southern Alaska and western Canada after Silber- and adjacent Alaska (Figs. 1 and 2). This area has been influenced by two ling et al. (1994). The thick, dashed line indicates the surface projection major accretion events—the Late Jurassic–mid Cretaceous Wrangellia of the Yakutat slab after Eberhart-Phillips et al. (2006). Plate vectors after composite terrane accretion to the North American margin and the Late Plattner et al. (2007) and Elliott et al. (2010). TiF—Tintina fault; DF—Denali fault; BRF—Border Ranges fault; FF—Fairweather fault; QCF—Queen Eocene–Present oblique accretion and flat-slab subduction of the Yaku- Charlotte fault; TF—Transition fault; AMT—Aleutian megathrust; YCT— tat microplate (Fig. 1) (e.g., Nokleberg et al., 1994; Plafker et al., 1994). Yukon composite terrane; KT—Kahiltna terrane; WT—Wrangellia terrane; Evidence of the Yakutat flat-slab subduction is found in areas above the PT—Peninsular terrane; AT—Alexander terrane; CPWT—Chugach–Prince subducted slab located in south-central Alaska and the Alaska Range (Fig. William terrane; YM—Yakutat microplate; PAC—Pacific plate; G—Gravina- 1) (e.g., Enkelmann et al., 2008; Benowitz et al., 2011, 2014; Finzel et al., Nutzotin belt; CMC—Chugach metamorphic complex; A—Anchorage; 2011; Brennan and Ridgway, 2015). Our study area is located east of the NWT—Northwest Territories; B.C.—British Columbia (inset map). Numbers indicate locations of 1—eastern Alaska Range; 2—central Alaska Range; 3—Talkeetna Mountains; 4—Matanuska Basin; 5—western Alaska Range; *Emails: Falkowski: [email protected]; Enkelmann: eva.enkelmann@ 6—eastern and western Chugach Mountains; 7—Wrangell Mountains; 8— uc.edu St. Elias Mountains; 9—St. Elias syntaxis; 10—Mackenzie Mountains.

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/8/4/359/3040051/359.pdf by guest on 30 September 2021 FALKOWSKI AND ENKELMANN km Denali Fault 50 KLB9 1 ik tshenshini 33 Ta KL D Lake r Aishih 25 River 18 KL D 29 r KL D 0 KLD2 0 KLD2 3 KLD2 7 17 Alsek Rive Downstream KL D KLB4 1 KLB5 9 39

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Malaspina Alaska Yukon

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Valley

Klutlan Gl 25

St. Elias Mountains

Bagley Ice 61° N 60° N 0

A Wrangell volcanic belt Figure 2. (A) Satellite image (Google Earth) and (B) digital elevation model (ASTER GDEM, 30 m; NASA, USA, and METI, Japan) of the St. Elias Mountains and Fairweather, Kluane, Kluane, Elias Mountains and Fairweather, of the St. Japan) and METI, USA, NASA, 30 m; GDEM, model (ASTER and (B) digital elevation (Google Earth) image (A) Satellite 2. Figure Alsek that the large Note (Gl.—Glacier). given Elias Mountains are St. the northern Names of main glaciers draining Alaska. Canada and southeastern in western ranges and Ruby CSEF— fault; CcF—Connector fault; River DRF—Duke and 25). 26, 18, 17, 13, 9, 40, 39, 85, (KLD67, area study of the eastern most of the glacial catchments comprises catchment River fault. Creek ECF—Esker CHF—Chaix Hills fault; Elias fault; Chugach–St.

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Yakutat slab and north and east of the indenting Yakutat plate corner where transcurrent faults since the Late Cretaceous, even though plate motion deformation transitions from transpressional to compressional (Fig. 2B). directions changed through time (e.g., Doubrovine and Tarduno, 2008). This area is known as the St. Elias syntaxis and gained recent attention due Today, the geodynamics of southern Alaska are dominated by the col- to its complex structural setting and locally concentrated, high deforma- lision of the Yakutat microplate, which is a 15–30-km-thick, westward tion rates (e.g., O’Sullivan et al., 1997; Pavlis et al., 2004; Enkelmann et tapering oceanic plateau capped by a remnant Campanian–Maastrichtian al., 2009, 2015a, 2015b; Koons et al., 2010). The Wrangellia composite accretionary prism (Yakutat Group) in the east and Cenozoic sediments terrane has generally been ascribed the role of the backstop to Cenozoic in the west (Fig. 3) (Plafker, 1987; Worthington et al., 2012). The Fair- accretion with limited amounts of exhumation (e.g., Pavlis and Roeske, weather–Queen Charlotte transform plate boundary (Fig. 1) formed in the 2007; Enkelmann et al., 2008, 2010; Spotila and Berger, 2010). However, mid-Eocene and resulted in the northwestward translation of the Yakutat geophysical data and large-scale geodynamic models suggest that deforma- microplate (e.g., Plafker et al., 1994; Haeussler et al., 2003). A prominent, tion at the current plate boundary is partly transferred inland to the northwestward bend in the plate boundary and associated topography marks the ern margin of the Wrangellia composite terrane and farther to the eastern St. Elias syntaxis, which is the transition zone between transform motion deformational front of the North American Cordillera (e.g., Mackenzie and subduction (Fig. 3). During the past decade, various data have been Mountains in the Northwest Territories; Fig. 1) (e.g., Lahr and Plafker, published that illuminate the structural situation and geologic evolution 1980; Mazzotti and Hyndman, 2002; Koons et al., 2010; Doser, 2014). of the St. Elias orogen with a strong emphasis on the southern flanks We use detrital zircon and apatite fission-track (ZFT and AFT, respec- located in Alaska. Particularly geophysical data, glacial geomorphology, tively) analyses on modern sand deposits of glacial and fluvial catch- and detrital thermochronology provided insight into the geologic pro- ments to investigate the long-term cooling and exhumation history of cesses underneath the ice cover (e.g., Bruhn et al., 2004, 2012; Berger et the remote, rugged, and extensively glaciated area located north and east al., 2008; Enkelmann et al., 2008, 2009, 2010, 2015a, 2015b; Elliott et of the St. Elias syntaxis (Figs. 1 and 2). The detrital thermochronology al., 2010, 2013; Chapman et al., 2012; Pavlis et al., 2012; Worthington et approach proved to be very powerful to reveal the spatial pattern of rock al., 2012; Van Avendonk et al., 2013), but those data are lacking from the exhumation occurring above and below the ice fields and glaciers of the northern side, located in Canada (Fig. 2A). In the following, we summa- southern St. Elias Mountains (Enkelmann et al., 2008, 2009, 2010, 2015a; rize what is known about the St. Elias orogeny and highlight some of the Grabowski et al., 2013; Falkowski et al., 2014, 2016). The combination of major open questions, some of which we are able to address in this study. two thermochronometric methods recording cooling through temperatures The northern Fairweather fault is characterized by transpression with of 250 ± 40 °C and 110 ± 10 °C, respectively (Gleadow and Duddy, 1981; strike-slip motion along the Fairweather fault and thrusting along north- Brandon et al., 1998), allows identifying changes of cooling through time. east-dipping faults in the Yakutat foothills (Fig. 3) (Plafker and Thatcher, We present 3537 new single-grain ages (detrital ZFT and AFT) recording 2008; Elliott et al., 2010). West of the syntaxis, the Cenozoic cover is being the late Mesozoic–Cenozoic cooling and exhumation in the northern St. deformed and imbricated by the shallow, northwest-dipping thrusts of the Elias Mountains and investigate the temporal and spatial far-field effects fold-and-thrust belt. It was suggested that the Fairweather fault ends in of the Yakutat microplate collision. oblique-extensional splay faulting beneath the Seward Glacier and dextral motion is accommodated along the Bagley fault beneath the Bagley BACKGROUND Ice Valley (Figs. 2A and 4; Ford et al., 2003; Bruhn et al., 2004, 2012). The geometry and slip history of the ice-covered Bagley fault is not well Key to understanding the cooling record of an orogen is information known, but it may have accommodated significant vertical motion in the about the geology in general and particularly the formation age of the past ~5 m.y. as a backthrust and dextral motion since ca. 20 Ma consis- rocks, possible sources of thermal overprint, and the tectonic setting. tent with the model of counterclockwise rotation and escape tectonics of This information allows differentiating between magmatic and/or meta- southern Alaska (Berger et al., 2008; Bruhn et al., 2012). Our study area morphic cooling, tectonic and/or erosional denudation, and evaluating is located northeast of the syntaxis and the inferred Connector fault that climate-tectonic interactions. In the following, we summarize what is possibly transfers strain from the Fairweather to the Denali fault (Figs. known about our 30,142 km2 study area, which includes the northern St. 2A and 3). Large-scale geodetic studies indicate that strain is transferred Elias Mountains and the Kluane and Ruby ranges (Fig. 2A). Because from the plate boundary inland to the Denali fault and to the eastern Cor- most of the area is heavily glaciated, the geology must be inferred from dilleran deformation front (e.g., Mazzotti and Hyndman, 2002; Brennan the ice-free ridges and surroundings. Only the two catchments in the and Ridgway, 2015; Marechal et al., 2015). However, in the southwest northwestern study area and the large Alsek River catchment contain corner of Yukon, seismic and GPS station cover is poor (e.g., Doser, 2014; considerable ice-free areas (Fig. 2A). Marechal et al., 2015) and leaves geophysical models ambiguous about specific structures in the study area. It is uncertain whether the Connector Tectonic Setting fault exists and, if so, when it became active. The role of the Duke River fault and whether it is responsible (rather than the Connector fault) for The North American Cordillera is the classic study area for terrane the bypassing of the eastern Denali fault is also uncertain (see slip rates accretion processes, which started during Mesozoic time and are char- in Fig. 3) (e.g., Lahr and Plafker, 1980; Doser, 2014; Marechal et al., acterized by major strike-slip faults such as the Tintina and Denali faults 2015; Brennan and Ridgway, 2015). The Duke River fault occupies the (Fig. 1; Jones et al., 1972; Coney et al., 1980). Pacific plate motions have reactivated suture and thrusts Alexander terrane rocks over Wrangellia been highly oblique relative to the North American plate since at least terrane rocks (Cobbett et al., 2010). Evidence for its activity and ~10–13 84 Ma, indicating a long-lived transpressional tectonic setting, consistent km of vertical motion comes from muscovite growth (ca. 104–84 Ma) in with suggested large (hundreds to thousands of kilometers) displacements the Cretaceous, folding of Miocene Wrangell lava in the Pliocene, and of terranes (e.g., Lowey, 1998; Doubrovine and Tarduno, 2008; Garver current seismicity (Cobbett et al., 2010; Cobbett, 2011; Doser, 2014). and Davidson, 2015). Despite the fact that the paleogeography of some Seismic studies have imaged the Yakutat slab underneath the fold-and- terranes is still under debate (e.g., Garver and Davidson, 2015), it is thrust belt, south-central Alaska, and the Alaska Range (Figs. 1 and 3; important for our study that terranes have been situated between major e.g., Eberhart-Phillips et al., 2006; Bauer et al., 2014), but seismic data

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~12 mm/yr ~8 mm/yr ~6 mm/yr ~10 mm/yr W ~2 mm/yr YCT Fig. 2 E EAR DF Surface TotF DRF projection ? Yakutat slab ~5 mm/yr CcF ~42 mm/yr Rotation of ? Southern k Alaska Bloc Mt. Logan ? WCT CPWT ? ~15 km thic BgF BRF

Fold-thrustbelt k Yakutat FF Group YF Cenozoic sediments YM/NAM ~30 mm/yr ~50 mm/yr Slip rate Shortening rate Yakutat Microplate St. Elias syntaxis ~30 km thick area

Figure 3. Schematic block model of the Yakutat–North American collision and subduction zone (not to scale). The Yakutat microplate subducts at a flat angle (9°–11° beneath the fold-and-thrust belt; Bauer et al., 2014). Strike-slip and shortening rates are given for main active structures (slip rates from Matmon et al., 2006; Kalbas et al., 2008; Elliott et al., 2010; shortening rates after Marechal et al., 2015). YCT—Yukon composite terrane; WCT—Wrangellia composite terrane; CPWT—Chugach– Prince William terrane; YM—Yakutat microplate; NAM—North American plate; EAR—Eastern Alaska Range; DF—Denali fault; DRF—Duke River fault; TotF—Totschunda fault; CcF—Connector fault; BgF—Bagley fault; BRF—Border Ranges fault; FF—Fairweather fault; YF—Yakutat fault.

have not been able to image the northeastern edge of the slab beneath the accretion of the Wrangellia composite terrane (e.g., Rubin et al., 1990; St. Elias Mountains better than locating it somewhere east of Mount St. Dusel-Bacon et al., 1993; Rusmore et al., 2000; Gehrels, 2001). Elias (Bauer et al., 2014). It is certain that the slab is not currently passing The Wrangellia composite terrane comprises in the study area the beneath the study area (Fig. 3), but it might have in the past. Paleozoic–early Mesozoic island arc assemblages of the Alexander and Wrangellia terranes (Figs. 1 and 5A). The two terranes were stitched Geology of the Study Area together by Upper Pennsylvanian plutons but share a common tectonic setting since at least the Late Devonian (Gardner et al., 1988; Israel et al., The catchments investigated in this study comprise mainly rocks of 2014). The history of accretion of the Wrangellia composite terrane to the Wrangellia composite terrane and, to a smaller extent, rocks of the the former North American margin is not entirely resolved, but the two Yukon composite terrane north of the Denali fault (upper Alsek River units were likely close in the Late Jurassic, when backarc basin sedimen- catchment) and the Chugach–Prince William terrane south of the Border tation occurred along the inboard margin of the Wrangellia composite Ranges fault (catchment KLD29; Figs. 2B and 4). The Yukon composite terrane (e.g., Trop and Ridgway, 2007). In the study area, these backarc terrane in the study area comprises mostly early Cenozoic plutonic rocks basin turbidites were fed by the adjacent Chitina arc (ca. 160–130 Ma) of the Kluane arc (ca. 85–45 Ma), which are part of the widespread Coast and are represented by the Upper Jurassic–Lower Cretaceous Dezadeash plutonic complex (ca. 175–45 Ma) of the North American Cordillera (e.g., Formation (Figs. 4 and 5; Berg et al., 1972; Eisbacher, 1976; Lowey, Armstrong, 1988; Erdmer and Mortensen, 1993; Gehrels et al., 2009). 1992). Remnants of the Chitina arc are represented by the widespread St. Associated with the intrusions, Cretaceous sedimentary rocks have been Elias plutonic suite (Figs. 4 and 5). Approximately 30–50 km north of metamorphosed (Kluane metamorphic assemblage; Fig. 4). Rocks of the the Chitina arc, the Chisana arc formed ca. 120–105 Ma (Fig. 5B) due to Aishihik metamorphic assemblages (late Proterozoic–Mississippian) and continued subduction at the outboard margin of the Wrangellia composite Aishihik plutonic complex (ca. 186 Ma) are exposed within the study terrane. Remnants are the scattered Kluane Ranges plutonic suite in the area as well (Erdmer and Mortensen, 1993; Johnston and Erdmer, 1995; wider Denali fault zone (Figs. 4 and 5). The Eocene–Oligocene Amphi- Johnston et al., 1996; Johnston and Canil, 2007; Israel et al., 2011). Along theatre Formation overlaps the Wrangellia and Alexander terrane rocks the entire North American Cordillera, the southern margin of the Yukon in the Denali and Duke River fault zones, where local strike-slip basins composite terrane and the northern margin of the Wrangellia composite formed (Figs. 4 and 5A) (e.g., Ridgway and DeCelles, 1993). During the terrane are characterized by regional, mid-Cretaceous folding, thrust- Early to mid-Miocene, Wrangell lava leaked along transtensional faults ing, and metamorphic overprint, which is interpreted to reflect the final and overlies older units in the Duke River fault area (Figs. 4 and 5A)

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TotF 62º N DF

BRF ? DR F 61ºN CcF

? BgF L ? CSEF ? E ? CHF

ECF Previous bedrock MF 60ºN thermochronology ? Ages (Ma) 0–5 ZFT PZ 5–10 YF 10–20 ZHe 20–40 AFT 40–60 FF 60–100 AHe 050 100 100–150 km

59ºN 144ºW 142ºW 140ºW 138ºW136ºW Regional Wrangellia Composite Terrane WCT Intrusives Alluvium Quaternary Wrangell Lava Oligocene–Holocene Kluane Ranges Suite Mid-Cretaceous Intrusives Paleocene–Eocene Amphitheatre Formation Eocene–Oligocene St. Elias Suite Yakutat Microplate Upper Jurassic– Dezadeash Formation (flysch) Lower Cretaceous Poul Creek, Yakataga, Kulthieth Fm. Upper Jurassic–Lower Cretaceous (marine, fluvial, glacio-marine) Icefield Ranges Suite Eocene–Present Chitistone Limestone Lower Permian Upper Triassic Yakutat Group (flysch & mélange) Faults Upper Cretaceous Nikolai Greenstone Certain 230 Ma Concealed Chugach-Prince William Terrane Undifferentiated WT and AT basement ? Uncertain McHugh Complex, Orca & Valdez Groups (arc-backarc basin assemblages) (flysch & mélange, subordinate basalts) Proterozoic–Mesozoic WaterIce Cretaceous–Eocene L: Mount Logan Yukon Composite Terrane E: Mount St. Elias Undifferentiated YCT Sampled catchments Kluane Metamorphic Assemblage (continental margin sequences) Mesozoic Paleozoic–Mesozoic New bedrock samples

Figure 4. Geologic map of the larger study area with previous bedrock thermochronometric data: ZFT—zircon fission-track; ZHe—zircon (U-Th)/He; AFT—apatite fission-track; and AHe—apatite (U-Th)/He) from the sampled catchments (dark blue outlined) and their immediate surroundings. Thermo- chronometer data from O’Sullivan and Currie (1996); O’Sullivan et al. (1997); McAleer et al. (2009); Spotila and Berger (2010); and Enkelmann et al. (2010, 2015b). Higher-temperature chronometer data can be found in Figure S1 (see footnote 1). DF—Denali fault; DRF—Duke River fault; TotF—Totschunda fault; CcF—Connector fault; BRF—Border Ranges fault; BgF—Bagley fault; CSEF—Chugach–St. Elias fault; CHF—Chaix Hills fault; MF—Malaspina fault; ECF—Esker Creek fault; PZ—Pamplona zone; FF—Fairweather fault; YF—Yakutat fault; WCT—Wrangellia composite terrane; WT—Wrangellia terrane; AT—Alexander terrane; YCT—Yukon composite terrane. Digital geologic maps from (i) Alaska: Wilson et al. (2015), Geologic map of Alaska SIM3340, http://dx.doi.org/10.3133/sim3340; U.S. Geological Survey (March 2016); (ii) Yukon: Bedrock Geology 250K, http://data.geology.gov.yk.ca/Compilation/3 (February 2015); and (iii) British Columbia: Ministry of Energy, Mines, and Petroleum Resources, British Columbia, BC_digital_geology_II83, http://www .empr.gov.bc.ca/Mining/Geoscience/BedrockMapping/Pages/BCGeoMap.aspx (March 2015).

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(Skulski et al., 1991, 1992; Israel et al., 2006). Typical intraplate calc-alka- Alexander Wrangellia Arc-relates line volcanics and plutonic rocks occur more extensively in the Wrangell A Terrane Overlap Terrane plutonic suites (Ma) assemblages (WCT, YCT) Mountains located west of the study area (26–0 Ma; Richter et al., 1990; Trop et al., 2012) and reflect the progressive oblique subduction of the Wrangell Wrangell Wrangell Yakutat microplate and slab edge volcanism (Fig. 1) (Richter et al., 1990). c Volcanics 23 suite Arc The Chugach–Prince William terrane represents a vast accretionary complex derived from a long-lived volcanic arc, most likely the Coast 33 Amphitheatre Fm. plutonic complex (ca. 175–45 Ma) of today’s British Columbia (Dumoulin, 1988; Farmer et al., 1993; Garver and Davidson, 2015). A narrow band enozoi 5 Coast of Upper Cretaceous flysch of the Valdez Group is exposed in the south- 56 Kluane Plutonic Arc ern study area (Fig. 4). This area is part of the Eocene greenschist- to 66 Complex amphibolite-facies Chugach metamorphic complex (Fig. 1) that resulted from near-trench intrusions of the Sanak-Baranof belt associated with a Kluane spreading-ridge subduction ca. 62–47 Ma (Hudson et al., 1977a, 1979; Ranges Chisana Bradley et al., 1993; Pavlis and Sisson, 1995; Gasser et al., 2011, 2012). suite Arc 4 2 Chitina 145 St. Elias Previous Geochronologic and Thermochronologic Data Dezadeash Fm. suite 3 Arc Higher-temperature chronometric data have been published from the 40 39 Mesozoic study area, particularly K-Ar and Ar/ Ar ages that are largely interpreted 201 McCarthy Fm. as emplacement ages of plutons or lava and some metamorphic overprint Chitistone Lmst. (e.g., Hudson et al., 1977a, 1977b; Dodds and Campbell, 1988; Farrar et Nikolai Fm. al., 1988; Richter et al., 1990; Erdmer and Mortensen, 1993; Johnston Icefield Fm. . et al., 1996; Trop et al., 2012). In contrast, only few lower-temperature Icefield 252 Hasen Creek Fm. chronometer data have been reported from the study region including cC Ranges Station Creek Fm suite apatite and zircon (U-Th)/He (AHe and ZHe, respectively) and FT ages

Skolai Gr 1 (Fig. 4) (O’Sullivan et al., 1997; McAleer et al., 2009; Spotila and Berger, Steele Creek- Skolai/Sicker Bullion Fm. Mount Constantine Arc 2010). For reasons of clarity, the higher-temperature chronometers are Goatherd Fm. Gabbro Complex omitted from Figure 4 but are shown in Figure S11. For compilations of thermochronometric ages from the entire St. Elias Mountains, the reader Paleozoi Donjek Fm. 541 is referred to Enkelmann et al. (2010, 2015a), Spotila and Berger (2010), Bedrock samples: Gasser et al. (2011), and Falkowski et al. (2016). Overall, the low-tem- 1 KLB44 2 KLB41 3 KLB42 4 KLB91 5 KLB5 perature chronometric data reflect the ongoing subduction and collision of the Yakutat microplate and efficient glacial erosion starting 6–5 Ma (e.g., Lagoe et al., 1993; Berger et al., 2008; McAleer et al., 2009; Enkelmann et al., 2010, 2015b; Spotila and Berger, 2010). Exhumation is most rapid and deepest in the St. Elias syntaxis area, interpreted from young (≤5 B Wrangel 50 km Ma) detrital ZFT and AFT ages (Enkelmann et al., 2009, 2010, 2015a; l Arc Falkowski et al., 2014) and bedrock AFT and AHe ages (McAleer et al., DF YCT BRF WCT Kluane 2009; Spotila and Berger, 2010; Enkelmann et al., 2015b). AHe bedrock Chisana ages generally increase northward from the plate boundary and within the CPWT Arc Wrangellia composite terrane but form a bulge of younger ages northeast- Chitina Arc ward from the indenting Yakutat plate corner suggesting deformation and Arc exhumation occurred also inboard of the plate boundary (Fig. 4; Spotila and Berger, 2010). While the AHe ages record cooling of rocks within 60ºN the upper ~2–3 km of the crust (closure temperature of 60 ± 15 °C; Farley, YM 2000), our new ZFT and AFT data set allows us to study the longer-term FF cooling history of a broad area and analyze material eroded below the ice. TF 140ºW METHODS Figure 5. (A) Stratigraphic summary of arc-backarc system assemblages of the Alexander and Wrangellia terranes as well as overlap assemblages and We sampled 21 glacial and glacio-fluvial catchments in the northern plutonic suites exposed within the sampled catchments in southwest Yukon St. Elias Mountains and Kluane and Ruby ranges (Table 1 and Fig. 2). and northwest British Columbia. Sampled units for bedrock zircon fission- Sampling locations were chosen to include all large glaciers that drain the track analysis are indicated (cf. Table 2). Fm.—Formation; Lmst.—Limestone; Gr.—Group. After Israel (2004) and Beranek et al. (2012). (B) Approximate 1 GSA Data Repository Item 2016150, Table S1: Comparison of previous and new locations of magmatic arcs after Dodds and Campbell (1988), Plafker et detrital ZFT data from catchments KLD40, KLD65, and KLD110; Text S1: Analytical pro- al. (1994), and Trop and Ridgway (2007). YCT—Yukon composite terrane; cedures; Figure S1: Geologic map of the St. Elias Mountains with higher-temperature WCT—Wrangellia composite terrane; CPWT—Chugach–Prince William ter- chronometer data from the sampled catchments and their immediate surroundings; rane; YM—Yakutat microplate; DF—Denali fault; BRF—Border Ranges fault; Datasets S1 and S2: ZFT and AFT single-grain ages, respectively, is available at FF—Fairweather fault. www.geosociety.org/pubs/ft2016.htm, or on request from [email protected].

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TABLE 1. DETRITAL SAMPLE LOCATIONS WITH CORRESPONDING CATCHMENTS Sample Latitude Longitude Glacier or stream name Catchment area Terrane (°N) (°W) (km2) KLD9 60.1755 138.0282 Between Dusty and Lowell glaciers 142AT KLD13 60.0746 137.9578 Fisher Glacier 540AT KLD17 60.0355 137.9487 Plug, Tough, and Super Cub creeks (south of Fisher Glacier)262 AT KLD18 59.9917 137.7755 East side Alsek River, east of Tweedsmuir Glacier341 AT KLD20 59.9427 137.8286 Alsek River, downstream of Fisher Glacier16,997 YCT, AT, WT KLD23 59.7288 137.9442 Alsek River, downstream of Tweedsmuir Glacier17,687 YCT, AT, WT KLD25 59.6033 138.0264 Battle Glacier 410AT KLD26 59.6302 138.0249 Vern Ritchie Glacier 270AT KLD27 59.4961 137.7543 Alsek River, downstream of Battle Glacier19,255 YCT, AT, WT KLD29 59.4498 137.7180 Konamoxt, Staircase, Melbern, Tikke, Grand Paciﬁc, Moxt, 888CT, AT Jarl, and Hay glaciers KLD33 59.5738 137.3309 Henshi Creek236 AT KLD39 60.5689 138.0397 Felsite Glacier, Felsite Creek236 AT KLD40 60.4803 138.1062 Dusty Glacier 843AT KLD65 60.8760 138.6269 Kaskawulsh, Stairway, and Atrypa glaciers, Canada Creek 1752 AT KLD66 60.8469 138.6771 Canada Creek 193AT KLD67 60.8254 138.4923 Maxwell Glacier 164AT KLD78 61.5497 140.4056 St. Clare and Bull Creeks (east of Klutlan Glacier) 460AT (WVB) KLD85 60.7683 138.2723 Disappointment Glacier 480AT KLD105 61.0251 139.3327 Kluane Glacier 707AT KLD106 61.2564 139.6159 Donjek and Kluane glaciers1944AT KLD110 61.9872 140.5581 White River (Klutlan, Nesham, Brabazon, Mount Wood, 6067 WT, AT Russell, Gifﬁn, Guerin, Natazhat, Brooke, Lime, and (WVB) Middle Fork glaciers, St. Clare and Bull creeks) Note: Catchment areas were calculated using ArcGIS and a 30 m digital elevation model (ASTER GDEM; NASA, USA, and METI, Japan). Abbre- viations: AT—Alexander terrane; CT—Chugach terrane; WT—Wrangellia terrane; WVB—Wrangell volcanic belt; YCT—Yukon composite terrane.

high Icefield region to the north and east (Fig. 2A). Smaller catchments ZFT ages to the detrital cooling age record (Fig. 2 and Table 2). Per sam- located farther inland in the Kluane Ranges were sampled to better confine ple, ~100 single-grain ages were analyzed using the z-calibration method exhumation signals obtained from regions closer to the St. Elias syntaxis (Hurford, 1990). Analytical details on the fission-track dating procedure (Fig. 2). Three samples are from the Alsek River, which is the only river can be found in Text S1 (see footnote 1). For extraction of detrital age crossing the St. Elias Mountains and transporting material into the Gulf components, we used the software BINOMFIT (Brandon, 1992, 1996), of Alaska (from north to south KLD20, KLD23, and KLD27; Fig. 2). which employs binomial peak fitting to the single-grain age distributions. Note that the Alsek River catchment encompasses most of the catchments in the eastern study area (Fig. 2). Also, catchment KLD110 comprises RESULTS catchment KLD78, catchment KLD105 is contained within KLD106, and catchment KLD66 lies within the reaches of catchment KLD65 (Fig. 2). Bedrock Zircon Fission-Track Analysis At each sample location, 3–5 kg of medium- to coarse-grained sand were collected for detrital ZFT and AFT analysis. Additionally, five bed- Five bedrock ZFT ages from samples located within the analyzed rock samples (1–3 kg each) have been collected that allow comparing their catchments are presented in Table 2. Two bedrock samples from the upper

TABLE 2. ZIRCON FISSION-TRACK AGES OF BEDROCK SAMPLES Sample Latitude Longitude Elevation Location descriptionLithology ZFT age ±1σ (Ma) (°N) (°W) (m asl) N KLB5 60.8142 137.4896 681 Catchment of KLD20, 23, 27 Granitoid 43.2 ± 3.5 (W side Pine Lake) (CPC) 7

KLB91 60.4086 137.0494 725 Catchment of KLD20, 23, 27 Mudstone 109.9 ± 8.9 (SW side Dezadeash Lake) (Dezadeash Group) 10

KLB41 60.4277 138.2428 1383 Catchment of KLD40 Granite 154.2 ± 10.7 (east, Dusty Glacier terminus) (St. Elias suite) 13

KLB42 60.4027 138.6824 1750 Catchment of KLD40 (central) Granite 101.4 ± 6.1 (St. Elias suite) 18

KLB44 60.4282 139.0935 2637 Catchment of KLD40 (west) Granodiorite 9.4 ± 0.6 (Iceﬁeld Ranges suite) 37

Note: The zircon ﬁssion-track (ZFT) ages represent pooled ages, as all samples demonstrated age homogeneity by passing the χ2-test (e.g., Galbraith, 2005). ζ = 119.6 ± 5.4 cm2 yr; dosimeter glass: IRMM541. asl—above sea level; N—number of single grains dated per sample; CPC—Coast plutonic complex.

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Alsek River catchment yielded ZFT ages of ca. 43 Ma (KLB5) and ca. Detrital Fission-Track Analysis 110 Ma (KLB91). Sample KLB5 was taken from a granitoid of the Coast plutonic complex (Table 2 and Fig. 4). U-Pb and K-Ar ages from the In the following, the results of 21 detrital, glacio-fluvial samples, all area (Fig. S1 [see footnote 1]) suggest that KLB5 has Early Eocene of which were dated by ZFT analysis (2187 new ZFT single-grain ages) crystallization and K-Ar cooling ages. Mudstone sample KLB91 was and 15 of them also by AFT analysis (1350 new AFT single-grain ages), collected from the Upper Jurassic–Lower Cretaceous Dezadeash Forma- are reported. Results of age component extraction are shown in Figure 6 tion and exhibits a mid-Cretaceous cooling age that is probably related and summarized in Table 3, which divides the ZFT and AFT age popu- to regional deformation during the final stage of Wrangellia composite lations into time intervals that combine similar age populations but also terrane accretion. This deformational event resulted in low- to medium- follow different regional tectonic settings. grade metamorphism within the flysch deposits in the collision zone (e.g., The 15 AFT samples each yielded 2–3 age populations that range Dusel-Bacon et al., 1993) and reset the ZFT system of sample KLB91. between 331.0 ± 70.1 Ma (2.5%, KLD13) and 8.7 ± 2.4 Ma (34%, KLD33), Three granitoid samples from the east-west–elongated Dusty Glacier but are mostly Late Cretaceous–Miocene (Table 3). Age clusters can be catchment (KLD40, Fig. 2) exhibit decreasing ZFT cooling ages of ca. observed at ca. 90–80 Ma and ca. 30 Ma (Table 3). With the exception of 154 Ma, ca. 101 Ma, and ca. 9.4 Ma from east to west and with increas- KLD105, samples with a ca. 30 Ma age population (12%–63%; Table 3) ing elevations (1383–2637 m above sea level [asl]; Table 2). According occur in the southern part of the study area. Furthermore, some samples to the geologic map of southwest Yukon (Israel, 2004) the westernmost contain Late Miocene 13–8 Ma AFT age populations (KLD23, KLD26, sample KLB44 was collected from the Early Permian (290–270 Ma) Ice- KLD27, KLD29, KLD33, KLD40, and KLD85; Table 3), the majority field Ranges plutonic suite, and KLB41 and KLB42 represent the Upper of which are located in the southern part as well. Jurassic–Lower Cretaceous St. Elias plutonic suite (160–130 Ma; Figs. 4 From the study area, three detrital ZFT samples have been published and 5). This indicates that sample KLB41 cooled below ~250 °C shortly previously (Enkelmann et al., 2015a). These have been collected from the following its emplacement, while KLB42 either cooled much slower or same glacial catchments as samples KLD40, KLD65, and KLD105. The experienced a heating event and was reset, which left KLB41 unaffected previous and new data generally agree. For a comparison of previous and even though the samples were taken only 20 km apart from another (Fig. new ZFT data for these catchments, see Table S1 (see footnote 1). Each 2B). The youngest cooling age of ca. 9.4 Ma of KLB44 reflects exhu- of the new detrital ZFT samples yielded 2–4 different age populations mational cooling and occurs in roughly the same area as a young AHe with age peaks that range between 253.8 ± 28.9 Ma (14%, KLD110) and age of ca. 4.3 Ma reported by Spotila and Berger (2010) and relatively 2.5 ± 1.5 Ma (3%, KLD78). Most age peaks fall into the Cretaceous; close to ca. 3.6 Ma AHe (McAleer et al., 2009) and 4.5 Ma AFT ages some cluster around 130 Ma and 120–115 Ma, while Late Cretaceous (O’Sullivan et al., 1997) (Fig. 4). age peaks show a wide range (Table 3). Early Cenozoic detrital ZFT

KLD9 67.1 -4.8/+5.1 Ma (12.3%) KLD13 6.5 -0.7/+0.8 Ma (10.8%) KLD17 24.5 -5.8/+7.5 Ma (1.8%) N=102 119.7 -6.1/+6.4 Ma (87.7%) N=106 115.5 -5.8/+6.1 Ma (89.2%) N=106 71.1 -9.1/+10.4 Ma (10.7%) 122.9 -11.8/+13.1 Ma (66.5%) 191.6 -35.9/+44.0 Ma (21.1%) Probabilit y

0100 200300 0100 200300 0100 200300

KLD18 129.9 -18.0/+20.9 Ma KLD20 90.7 -7.8/+8.5 Ma (22.2%) KLD23 49.1 -9.5/+11.8 Ma (2.8%) N=104 (14.7%) N=103 133.8 -7.3/+7.7 Ma (77.8%) N=105 88.6 -6.9/+7.5 Ma (32.4%) 155.7 -9.1/+9.6 Ma 125.2 -7.3/+7.7 Ma (64.8%) (85.3%) Probabilit y

0100 200300 0100 200300 0100 200300 Zircon fission-track age (Ma) Zircon fission-track age (Ma) Zircon fission-track age (Ma) ((continuecontinuedd))

Figure 6. Results of binomial peak fitting of zircon fission-track (ZFT) (A) and apatite fission-track (AFT) (B) ages using BINOMFIT (Brandon, 1992, 1996). Black curves—fitted peaks; gray curves—measured ZFT and AFT age distributions (±1s). Given are the peak ages (average age of the age populations) with asymmetric 1s standard deviation and the fraction of the grains that make up the age population in that sample. N = number of dated grains per sample. (Continued on following two pages.)

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A (continued )

KLD25 21.7 -2.4/+2.7 Ma (4%) KLD26 15.8 -2.5/+3.0 Ma (5%) KLD27 9.7 -1.1/+1.2 Ma (3.8%) N=105 45.6 -2.7/+2.9 Ma (50.7%) N=105 40.0 -3.9/+4.3 Ma (9.4%) N=105 99.7 -8.9/+9.7 Ma (21%) 102.8 -6.4/+6.8 Ma (45.2%) 84.7 -6.4/+6.9 Ma (52.6%) 158.0 -8.5/+9.0 Ma (75.2%) 121.8 -11.0/+12.1 Ma (33%) Probabilit y

0100 200300 0100 200300 0100 200300

KLD29 14.3 -1.4/+1.5 Ma (9%) KLD33 107.4 -11.0/+12.3 Ma KLD39 60.4 -7.5/+8.5 Ma (7.7%) N=105 23.9 -1.3/+1.3 Ma (52.7%) N=104 (14.4%) N=102 78.5 -7.0/+7.7 Ma (29.6%) 44.8 -3.0/+3.2 Ma (19.8%) 134.1 -7.3/+7.7 Ma 104.0 -6.9/+7.4 Ma (50.4%) 91.1 -6.1/+6.6 Ma (18.5%) (85.6%) 150.7 -14.6/+16.2 Ma (12.3%) Probabilit y

0100 200300 0100 200300 0100 200300

KLD40 7.6 -0.4/+0.4 Ma (63.6%) KLD65 55.7 -4.6/+5.0 Ma (11.9%) KLD66 77.3 -5.2/+5.6 Ma (34.8%) N=105 92.7 -4.9/+5.2 Ma (36.4%) N=104 84.4 -5.5/+5.9 Ma (38.5%) N=103 116.5 -9.2/+9.9 Ma (58.3%) 113.5 -8.9/+9.7 Ma (35.2%) 166.1 -55.7/+83.2 Ma (6.9%) 156.3 -18.0/+20.3 Ma (14.3%) Probability

0100 200300 0100 200300 0100 200300

KLD67 59.1 -4.6/+5.0 Ma (13.9%) KLD78 2.5 -1.1/+1.9 Ma (3%) KLD85 44.4 -4.6/+5.2 Ma (8.7%) N=103 87.5 -5.5/+5.9 Ma (46.8%) N=104 9.3 -0.7/+0.8 Ma (48.7%) N=105 68.8 -10.7/+12.7 Ma (24.6%) 119.8 -7.6/+8.1 Ma (39.3%) 12.8 -1.0/+1.1 Ma (41.3%) 87.2 -6.9/+7.5 Ma (51.5%) 46.1 -3.8/+4.1 Ma (7.1%) 130.5 -11.1/+12.2 Ma (15.2%) Probability

0100 200300 0100 200300 0100 200300

KLD105 48.9 -4.1/+4.5 Ma (14%) KLD106 71.3 -6.2/+6.8 Ma (29.2%) KLD110 6.2 -0.6/+0.7 Ma (16.1%) N=104 100.1 -5.2/+5.5 Ma (86%) N=103 100.1 -8.5/+9.3 Ma (47.9%) N=104 10.9 -0.6/+0.6 Ma (58%) 143.5 -12.9/+14.1 Ma (22.9%) 51.5 -4.0/+4.3 Ma (11.5%) 253.8 -26.0/+28.9 Ma (14.4%) Probability

0100 200300 0100 200300 0100 200300 Zircon fission-track age (Ma) Zircon fission-track age (Ma) Zircon fission-track age (Ma)

Figure 6 (continued ).

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KLD9 86.3 -5.8/+6.2 Ma (70.8%) KLD13 49.1 -9.7/+12.0 Ma (25%) KLD18 34.7 -3.2/+3.5 Ma (63.4%) N=72 153.2 -17.8/+20.1 Ma (29.2%) N=104 83.5 -5.7/+6.1 Ma (72.5%) N=33 129.5 -12.7/+14.1 Ma (36.6%) 331.0 -58.1/+70.1 Ma (2.5%) Probabilit y

0 100 200 300 0 100 200 300 0 100 200 300

KLD20 17.9 -2.2/+2.5 Ma (15.1%) KLD23 10.6 -0.7/+0.8 Ma (60.6%) KLD26 9.4 -0.9/+1.0 Ma (70%) N=95 66.6 -5.3/+5.7 Ma (48.5%) N=105 33.4 -2.7/+3.0 Ma (33.7%) N=73 47.2 -5.0/+5.6 Ma (30%) 105.5 -9.7/+10.7 Ma (36.4%) 81.0 -10.5/+12.0 Ma (5.7%) y Probabilit

0 100 200 300 0 100 200 300 0 100 200 300

KLD27 11.3 -1.0/+1.1 Ma (37.3%) KLD29 13.1 -1.2/+1.3 Ma (54.3%) KLD33 8.7 -1.9/+2.4 Ma (34.2%) N=107 29.5 -2.5/+2.7 Ma (40.7%) N=103 21.1 -3.4/+4.1 Ma (34.1%) N=95 18.6 -3.1/+3.7 Ma (45.8%) 74.9 -6.1/+6.7 Ma (22%) 31.5 -6.6/+8.4 Ma (11.6%) 32.3 -4.1/+4.7 Ma (20%) y Probabilit

0 100 200 300 0 100 200 300 0 100 200 300

KLD39 18.7 -1.7/+1.9 Ma (15.3%) KLD40 8.8 -0.8/+0.9 Ma (54.5%) KLD65 58.5 -5.2/+5.7 Ma (40.9%) N=106 62.5 -11.6/+14.3 Ma (20.8%) N=105 85.0 -4.8/+5.1 Ma (45.5%) N=52 102.8 -8.7/+9.5 Ma (59.1%) 91.3 -6.2/+6.7 Ma (63.8%) y Probabilit

0 100 200 300 0 100 200 300 0 100 200 300

KLD67 50.4 -6.3/+7.2 Ma (33.5%) KLD85 12.2 -0.9/+0.9 Ma (63.6%) KLD105 31.4 -9.3/+13.2 Ma (4.4%) N=97 74.2 -8.1/+9.1 Ma (56%) N=105 100.3 -5.7/+6.1 Ma (36.4%) N=98 88.4 -4.6/+4.9 Ma (95.6%) 164.2 -29.2/+35.4 Ma (10.5%) y Probabilit

0 100 200 300 0 100 200 300 0 100 200 300 Apatite fission-track age (Ma) Apatite fission-track age (Ma) Apatite fission-track age (Ma)

Figure 6 (continued ).

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y. – – – – – – – – – – – – – – – – – – – – >145 Ma 158.0 ± 9.0 [75.2] 164.2 ± 35.4 [10.5] 253.8 ± 28.9 [14.4] 166.1 ± 83.2 [6.9] 156.3 ± 20.3 [14.3] 150.7 ± 16.2 [12.3 ] 153.2 ± 20.1 [29.2] 155.7 ± 9.6 [85.3] 191.6 ± 44.0 [21.1] 331.0 ± 70.1 [2.5] . Alsek River are given separatel – –– – –– –– – – –– – –– – –– 145–100 Ma 9.7 ± 6.4 [87.7] 19.8 ± 8.1 [39.3] 16.5 ± 9.9 [58.3] 13.5 ± 9.7 [35.2] 15.5 ± 6.1 [89.2 ] 1 1 1 11 1 133.8 ± 7.7 [77.8] 105.5 ± 10.7 [36.4] 125.2 ± 7.7 [64.8] 143.5 ± 14.1 [22.9] 130.5 ± 12.2 [15.2] 102.8 ± 9.5 [59.1] 104.0 ± 7.4 [50.4] 129.9 ± 20.9 [14.7] 122.9 ± 13.1 [66.5] 129.5 ± 14.1 [36.6] 107.4 ± 12.3 [14.4] 134.1 ± 7.7 [85.6] 121.8 ± 12.1 [33.0] 102.8 ± 6.8 [45.2] –– –– – –– –– – 100–60 Ma TIONS 66.6 ± 5.7 [48.5] 81.0 ± 12.0 [5.7 ]– 90.7 ± 8.5 [22.2] 99.7 ± 9.7 [21.0] 88.6 ± 7.5 [32.4] 74.9 ± 6.7 [22.0] 74.2 ± 9.1 [56.0] 87.5 ± 5.9 [46.8] 88.4 ± 4.9 [95.6] 77.3 ± 5.6 [34.8] 71.3 ± 6.8 [29.2] 84.4 ± 5.9 [38.5] 68.8 ± 12.7 [24.6] 87.2 ± 7.5 [51.5] 78.5 ± 7.7 [29.6] 92.7 ± 5.2 [36.4] 85.0 ± 5.1 [45.5] 67.1 ± 5.1 [12.3] 71.1 ± 10.4 [10.7] 62.5 ± 14.3 [20.8 ] 91.3 ± 6.7 [63.8] 83.5 ± 6.1 [72.5] 91.1 ± 6.6 [18.5] 84.7 ± 6.9 [52.6] 86.3 ± 6.2 [70.8] 100.1 ± 5.5 [86] 100.1 ± 9.3 [47.9] 100.3 ± 6.1 [36.4] yr (S. Falkowski), dosimeter glass: IRMM541; apatite fission-track (AFT): ζ = 2 1.5] 1.9] AGE POPULA .8 [2.8] Ap—apatit e; N—Number of single grains dated per sample – – – – – – – –– –– – ––– location from north to south; s amples taken along the 60–35 Ma ’s 49.1 ± 11 50.4 ± 7.2 [33.5] 59.1 ± 5.0 [13.9] 51.5 ± 4.3 [1 46.1 ± 4.1 [7.1] 48.9 ± 4.5 [14] 55.7 ± 5.0 [1 44.4 ± 5.2 [8.7] 58.5 ± 5.7 [40.9] 60.4 ± 8.5 [7.7] 49.1 ± 12.0 [25.0] 44.8 ± 3.2 [19.8] 47.2 ± 5.6 [30.0] 40.0 ± 4.3 [9.4] 45.6 ± 2.9 [50.7] 19.6 ± 5.4 cm 1.6] A PA TITE FISSION-TRACK – –– – – – – – –– –– – – – – – – –– –– –– –– – – – – Peak ages of distribution components ±1 σ (Ma) [Grain fraction (%)] 35–15 Ma AND 24.5±7.5 [1.8 ] 17.9 ± 2.5 [15.1] 33.4 ± 3.0 [33.7] 29.5 ± 2.7 [40.7] 31.4 ± 13.2 [4.4] 18.7 ± 1.9 [15.3 ]– 34.7 ± 3.5 [63.4] 23.9 ± 1.3 [52.7] 21.1 ± 4.1 [34.1] 31.5 ± 8.4 [1 18.6 ± 3.7 [45.8] 32.3 ± 4.7 [20] 21.7 ± 2.7 [4.0] L ZIRCON ––– – –––– 15–5 Ma .5 ± 0.8 [10.8] 9.7 ± 1.2 [3.8 ] 1.3 ± 1.1 [37.3] 6.2 ± 0.7 [16.1] 9.3 ± 0.8 [48.7] 8.8 ± 0.9 [54.5] 7.6 ± 0.4 [63.6] 8.7 ± 2.4 [34.2] 9.4 ± 1.0 [70.0] 10.6 ± 0.8 [60.6] 10.9 ± 0.6 [58.0] 12.8 ± 1.1 [41.3] 12.2 ± 0.9 [63.6] 14.3 ± 1.5 [9.0] 13.1 ± 1.3 [54.3] 15.8 ± 3.0 [5.0 ]– ABLE 3. DETRI TA T (Brandon, 1992, 1996). Zircon fission-track (ZFT): ζ = 1 –– – – –– – –1 –– –– – –– –– –– –– –– – –– –– –– – – – –– –– –– –– –6 –– –– – – – – – – –– ≤ 5 Ma 2.5 ± 1.9 [3.0] 1 (Ma) 69–1 80–2 245–7 132–3 430–8 132–3 547–3 300–1 224–1 192–4 266–8 220–4 316–9 173–3 108–3 263–6 161–1 238–58 236–36 332–21 195–43 187–35 294–49 281–21 251–52 206–39 206–34 335–38 197–47 375–36 214–45 360–22 433–22 279–76 218–13 256–105 Age range N 95 97 98 52 72 33 95 73 105 103 105 105 107 103 104 104 104 103 103 104 105 105 102 105 105 102 106 104 106 106 104 105 103 104 105 105 yr (E. Enkelmann), dosimeter glass: IRMM540-R. Samples are ordered according to the catchment 2 r r r r r r Zr Zr Zr Zr Zr Zr Zr Zr Zr Zr Zr Zr Zr Zr Zr Ap Ap Ap Ap Ap Ap Ap Ap Ap Ap Ap Ap Ap Ap Ap Min r Results of binomial peak-fitting using BINOMFIT 0Z 3Z 7Z 7Z 8Z 3Z 9 3 6 5 10 KLD2 KLD2 KLD2 KLD1 KLD106 KLD105 KLD66 KLD67 KLD65 KLD78 Note: 237.0 ± 5.0 cm Single-grain ages are given in the Data Repository (Datasets S1 and S2; see text footnote 1). Min—mineral; Zr—zircon; KLD85 KLD39 KLD1 KLD1 KLD40 KLD9 KLD1 KLD2 KLD3 KLD2 KLD2 Sample Alsek Rive

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age populations show some common age peaks in the range 55–40 Ma; almost the entire previous thermal record of the Cambrian–Upper Triassic these peaks can be seen in eight of the samples throughout the study area basement rocks of the Wrangellia composite terrane. The Late Jurassic (Table 3 and Fig. 2B). In contrast, three age populations with 25–22 Ma cooling age populations (around 155 Ma) of the Alexander terrane are age peaks (KLD17, KLD25, and KLD27; Table 3) occur in the southern assignable to magmatic activity and subsequent cooling of the Chitina study area close to or in the Chugach terrane (cf. Table 3 and Fig. 2B). arc–St. Elias plutonic suite (Fig. 5) (e.g., Dodds and Campbell, 1988; The largest young age populations are derived from the two catchments Plafker et al., 1989, 1994). After cessation of Chitina arc activity (ca. located in the Wrangell volcanic belt (KLD78 and KLD110; Table 3 and 130 Ma), rocks cooled probably due to thermal relaxation, as recorded Figs. 2B and 4) and are therefore considered to be of volcanic origin and by the large group of ≤130 Ma ZFT and AFT age peaks (Table 3). Mag- reflect instantaneous cooling at the time of eruption. Looking at the distri- matic intrusions and heating of the crust might have uplifted the crust and bution of Wrangell lava in the study area, ZFT and AFT ages from sample resulted in erosion and exhumation. Renewed magmatic activity of the KLD85 and the Alsek River samples could reflect volcanic cooling as well Chisana arc began ca. 120 Ma (Fig. 5), and remnants can be found only (cf. Figs. 2B and 4). Reported ages for the Alsek volcanic field within in the Wrangellia terrane part of the study area (KLD110 and KLD78, these catchments range from 13.5 to 10.8 Ma (Stevens et al., 1982; Dodds Alsek River catchment), but a characteristic cooling signal like that of the and Campbell, 1988) to 16.4–15.4 Ma (Trop et al., 2012). If the younger St. Elias plutonic suite is not detected from these catchments (Table 3). range is valid, the ca. 12.2 Ma AFT age population of KLD85 could be of However, some catchments in the Alexander terrane indicate late Early volcanic origin. Samples KLD20 and KLD23 from the Alsek River contain Cretaceous cooling. The area has been in a forearc position at that time, ca. 10.6 Ma (61%) and ca. 11.3 Ma (37%) AFT age populations, respec- but no sedimentary strata are preserved. Therefore, cooling age popula- tively (Table 3), but considering that the mid-Miocene Alsek volcanics tions of this time might reflect an uplifted area experiencing erosion. make <5% of the catchment area, it is likely that those age populations This kind of setting must have continued throughout the mid- and Late represent an exhumational rather than a volcanic signal. Mid-Miocene and Cretaceous but with enhanced exhumation indicated by the large cooling younger ZFT and AFT age populations in other catchments (other than age populations dating this time interval that are found throughout the KLD110, KLD78, and KLD85) are considered as exhumational signals study area (Table 3). Final accretion of the Wrangellia composite terrane to because Wrangell lava is either absent or occurs in only very small areas, North America caused deformation that is recorded along the entire length and most importantly, are younger than mid-Miocene. No younger than of the composite terrane during the mid-Cretaceous (e.g., McClelland and mid-Miocene Wrangell volcanic ages have been reported for the study Gehrels, 1990; Dodds and Campbell, 1992; Dusel-Bacon et al., 1993; area (e.g., Skulski et al., 1991; Trop et al., 2012). The youngest (<10 Ma) Gehrels, 2001; Ridgway et al., 2002), while the southern margin deformed non-volcanic ZFT cooling age peaks occur in catchments KLD13 (6.5 ± due to continued subduction and accretion of sediments (e.g., Plafker et al., 0.8 Ma, 11%), KLD27 (9.7 ± 1.2 Ma, 4%), and KLD40 (7.6 ± 0.4 Ma, 1989). Furthermore, the Denali and Border Ranges faults became active 64%). Catchments KLD13 and KLD40 lie within the ca. 19,200 km2 Alsek as major dextral strike-slip faults in the Late Cretaceous due to very rapid River catchment of sample KLD27 (Fig. 2B). Considering the fact that oblique subduction, placing the study area in a transpressional setting the other two samples collected from the Alsek River upstream of KLD27 (Engebretson et al., 1985; Little and Naeser, 1989; Smart et al., 1996; do not contain such young ZFT ages (Table 3), the young ages of KLD27 Doubrovine and Tarduno, 2008). At the northern margin, the Duke River must be derived from the southern part of the Alsek River catchment. thrust fault also became active or reactivated, placing greenschist-facies The ZFT age distributions also contain few Jurassic and older age Alexander terrane rocks over prehnite-pumpellyite–facies Wrangellia peaks (>145 Ma, mostly >10%; Table 3), mainly 165–150 Ma. It is pos- terrane rocks (Cobbett et al., 2010). Thus, our cooling age populations sible that no older cooling record is preserved, but it should also be noted represent a mixture of tectonic and erosional denudation and possibly that with increasing cooling age or uranium content, it becomes increas- metamorphism associated with this mid- to Late Cretaceous orogeny. ingly difficult to date zircons by the fission-track method due to overlap- Similarly, the Coast plutonic complex in British Columbia records very ping tracks. In many samples, a few grains (<7) could not be dated due high exhumation and erosion rates during the Late Cretaceous–Paleocene to track densities that were too high. resulting in the vast Chugach–Prince William accretionary complex (e.g., Hollister, 1982; Farmer et al., 1993; Garver and Davidson, 2015). DISCUSSION Even though the detrital thermochronologic data also record Cenozoic cooling, the fact that the Late Jurassic–Cretaceous cooling record is so The record of rock cooling in the study area can be divided into phases dominant and well preserved suggests a limited amount of erosion in the of terrane deformation that encompass the Mesozoic arc magmatism study area since that time. In contrast, the detrital FT record from the and accretion of the Wrangellia composite terrane, as well as Cenozoic Chugach–Prince William terrane and Yakutat microplate contains barely deformation due to continued accretion of terranes at the new continen- any older than Cenozoic cooling (Enkelmann et al., 2008, 2009, 2010, tal margin. While the Mesozoic deformational history of the study area 2015a; Grabowski et al., 2013; Falkowski et al., 2014, 2016). Figure 7 resembles the history of the entire North American Cordillera, the Ceno- illustrates the detrital ZFT and AFT cooling signals of the study area zoic history is more unique due to the inboard location from the St. Elias in comparison to the northern Fairweather fault area and the southern syntaxis. These aspects of older and younger terrane deformation will be and western St. Elias Mountains in form of single-grain pie charts. It is outlined in the following. Older, Paleozoic–early Mesozoic deformation important to note that single-grain ages have large uncertainties and are is preserved in the structural and metamorphic record (e.g., Dodds and not used for the interpretation of cooling phases. Nevertheless, the visu- Campbell, 1992; Dusel-Bacon et al., 1993; Cobbett et al., 2010; Beranek alization with pie charts gives a good overview of the spatial occurrence et al., 2014) but not in the detrital FT data. of cooling ages. The abrupt younging of cooling age populations south of the Border Ranges fault (except the St. Elias syntaxis area, which is Mesozoic Cooling discussed below) is well observed in KLD29 with mainly Oligocene and younger cooling ages of both ZFT and AFT systems (Table 3 and Carboniferous–Early Cretaceous ZFT and AFT age populations are Fig. 7), and in samples CH46, CH21, and WR23 (Fig. 7; Enkelmann et dominated by magmatic phases and associated metamorphism that reset al., 2008). AHe bedrock samples from the western St. Elias Mountains

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r r

10 Ma

tshenshini Rive tshenshini Downstream Alsek Rive Ta

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Alsek Composit e r KLD6 5 Rive Alsek

r KLD2 9 Alsek Rive Alsek KLD6 6 KLD2 0 KLD2 3 KLD2 7 154 Ma 101 Ma KLD105 9.4 Ma ier

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Chitina River r 75

T age 5

– AF T

Chugach-Prince William

5 < < 35

–35 Alsek Rive

60 s

0 –6

0 0 0 10 > 20

–100 N W ° Copper River sample ZFT fission track ages (Ma) Detrital sample Detrital samples Berdock samples with ZF Previous detrital sample

200 60 45° Detrital single zircon/apatite 1 Figure 7. Detrital zircon fission-track (ZFT) and apatite fission-track (AFT) data from Enkelmann et al. (2008, 2009, 2010, 2015a), Falkowski et al. (2014) (black dots for sample locations), and for sample locations), dots (black (2014) et al. Falkowski 2015a), 2010, (2008, 2009, et al. from Enkelmann (AFT) data fission-track (ZFT) and apatite fission-track zircon Detrital 7. Figure Small pie events. tectonic with certain that correlate time ranges to binned with regard as pie charts presented are ages Single-grain sample locations). dots for (blue and red this study robust a more combined for were distributions with similar age catchments several sample where a composite represent pie charts larger whereas sample, one detrital represent charts com - Alexander except charts, pie of composite color the label to and correspond combined areas indicate of catchments outlines colored The Yakutat ages). number of single-grain (higher signal the outlines line blue Thick shown. are ages ZFT new bedrock The five combined. samples were which indicate arrows the three case, In that corner. in upper right charts posite belt is outlined volcanic Wrangell The in the south. fault Ranges and the Border in the north with the Denali fault terrane composite Wrangellia lines outline the brown thick microplate; fault. FF—Fairweather fault; Ranges BRF—Border fault; CcF—Connector DF—Denali fault; line. orange with a dashed,

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show the same trend (Spotila et al., 2004; Enkelmann et al., 2010; Spotila motions resulted in a more orthogonal convergence, westward directed and Berger, 2010). subduction in south-central Alaska, the establishment of the Aleutian arc, The upper Alsek River catchment is located in the Yukon composite and initiation of the Fairweather–Queen Charlotte transform plate bound- terrane and mostly comprises rocks of the Coast plutonic complex. Based ary (Fig. 1; Engebretson et al., 1985; Atwater and Stock, 1998; Haeussler et on the 43 Ma ZFT age of bedrock sample KLB5 (Table 2) and other K-Ar al., 2003; Doubrovine and Tarduno, 2008). Even after plate reorganization, cooling ages (Fig. S1 [see footnote 1]) we expected Eocene ZFT cool- relative plate motion was primarily oblique rather than orthogonal in our ing ages from the plutons and mid-Cretaceous ages from the Wrangellia study area, and ZFT and AFT cooling age populations are closer in time suture zone, e.g., from the Dezadeash Formation (ca. 110 Ma ZFT age to the suggested spreading-ridge subduction at that portion of the margin of KLB91; Table 2 and Fig. 7). However, those cooling ages are barely (ca. 53–50 Ma; Bradley et al., 1993; Gasser et al., 2011, 2012). However, present in the Alsek River samples (KLD20, KLD23, and KLD27; Table it is not possible to determine the dominant driver of cooling, and it might 3). This is most likely because of the morphology of the large Alsek River have been a combination of different influences during the Eocene. Prior catchment and long transport distance (up to 270 km). The catchment to spreading-ridge subduction, the downgoing oceanic crust becomes is characterized by low-relief topography north of the Denali fault, and increasingly younger, thicker, and buoyant, which results in a stronger sediment is stored in the wide valleys before the river enters the more coupling with the upper plate and possibly uplift. Then, the subduction rugged topography of the St. Elias Mountains (Figs. 2 and 7). The sand- of the spreading-ridge and slab-window passage allows for an upwelling sized material we collected along the Alsek River thus originates likely of hot mantle material, which together can result in the uplift and erosion from the large glaciers that drain the high Icefield region but not from of the upper plate as well as a significant increase in the geothermal gra- the upper Alsek River catchment. dient (e.g., Thorkelson and Taylor, 1989; Cloos, 1993; Sakaguchi, 1996; Bradley et al., 2003). The change in relative plate motions might have had Cenozoic Cooling an effect on rock exhumation as well, suggested by the 43 Ma increase in exhumation rates found at Mount Logan (O’Sullivan and Currie, 1996). We discuss the Cenozoic cooling recorded in our study area in terms However, our Eocene age populations do not cluster around the time of of three different phases: (1) Eocene spreading-ridge subduction and plate motion change (47–43 Ma) and are generally older, suggesting that slab-window passage followed by plate reorganization; (2) Oligocene– spreading-ridge subduction and slab-window development had a strong Present Yakutat flat-slab subduction; and (3) rapid, deep St. Elias syntaxis impact on the rock heating and subsequent cooling in our study area. exhumation. Benowitz et al. (2012) also described a scenario of spreading-ridge subduction, flat-slab subduction, and slab-window evolution to be responsible Eocene Spreading-Ridge Subduction and Plate Reorganization for uplift and Eocene cooling ages in the western Alaska Range and entire Paleocene–mid Eocene ZFT and AFT age populations (ca. 60–45 Ma; southern Alaska rather than plate reorganization. Table 3) occur throughout the entire study area and are similar to the ZFT age populations found in the fold-and-thrust belt and in the Yakutat Oligocene–Present Yakutat Flat-Slab Subduction foothills, south of the active plate boundary (Fig. 7; Enkelmann et al., Oligocene–Early Miocene cooling age populations are dominant in the 2009; Falkowski et al., 2014). However, the provenance of the Eocene– detrital AFT samples from the southern catchments of the study area, but Oligocene fold-and-thrust belt sediments and the Yakutat Group is much they are generally sparse in the detrital ZFT record (Table 3; orange bins farther south along the western North American margin (Plafker et al., of the pie charts in Fig. 7). The fact that only apatite records this cooling 1994; Perry et al., 2009; Garver and Davidson, 2015). Eocene cooling phase indicates that the total amount of exhumation since the Oligocene has also been reported from farther west, based on ca. 45 Ma detrital ZFT was limited to ~3.0–4.5 km (assuming a paleogeothermal gradient of 25–35 and AFT age populations found in the eastern and central Alaska Range °C/km). In contrast, Oligocene and younger cooling ages are far more (Lease et al., 2016) and thermal history modeling of K-feldspar 40Ar/39Ar abundant in the detrital ZFT and AFT data farther south and west in the St. data from the western Alaska Range (Fig. 1; Benowitz et al., 2012). This Elias Mountains (Fig. 7; Enkelmann et al., 2009, 2010, 2015a; Grabowski widespread Eocene cooling phase is well recognized in the geologic record et al., 2013; Falkowski et al., 2014). Oligocene and younger detrital and of southern Alaska and documents oblique subduction of a spreading bedrock cooling ages have also been found farther west in the Chugach ridge that resulted in diachronous (ca. 62–47 Ma), east-to-west–migrating, and Talkeetna Mountains and in the Alaska Range (Fig. 1), and they have near-trench plutonism within the Chugach–Prince William accretionary been associated with plate coupling due to the flat-slab subduction of the prism (Hudson et al., 1977a, 1979; Bradley et al., 1993; Pavlis and Sis- Yakutat microplate (Benowitz et al., 2011, 2012, 2014; Arkle et al., 2013; son, 1995; Gasser et al., 2011, 2012). The inboard effects are not well Lease et al., 2016). The effect of the flat-slab subduction on the upper understood, but the sedimentary record shows Eocene episodes of uplift plate has also been shown by sedimentologic and provenance studies (e.g., and subsidence inferred from the change from marine to coarse-grained, Trop and Ridgway, 2007; Brennan and Ridgway, 2015; Finzel et al., 2015) non-marine sedimentation and magmatism farther west on the Wrangellia and numerical modeling studies (Jadamec et al., 2013). During the same composite terrane (Matanuska Valley and Talkeetna Mountains; Fig. 1). time, deformation farther southeast is dominated by transpression along This was interpreted as an effect of the migrating slab window beneath the Fairweather fault (Chapman et al., 2012; Falkowski et al., 2014, 2016). southern Alaska (Trop et al., 2003; Trop and Ridgway, 2007). We suggest that the Oligocene cooling signal in our study area is The thermochronologic record of the spreading-ridge subduction has related to exhumation due to subduction of the Yakutat microplate. Even been more ambiguous because it is difficult to distinguish between cool- though the Yakutat slab is not presently beneath the study area (Fig. 3), ing due to thermal relaxation and rock exhumation (e.g., Benowitz et al., it probably was in the past. The Late Eocene–Oligocene Amphitheatre 2012; Lease et al., 2016). Because the slab window passed beneath the Formation was deposited in a strike-slip basin that developed in the Denali central and eastern Alaska Range ca. 60–56 Ma, Lease et al. (2016) sug- and Duke River fault zones in transpressional and transtensional settings, gested that the Eocene detrital ZFT and AFT age populations (ca. 45 Ma) and sediment was sourced from local, high-relief topography (Ridgway record cooling due to the 47–43 Ma reorganization of the Pacific plates and DeCelles, 1993). This indicates oblique convergence and subduc- that followed the spreading-ridge subduction. This change in relative plate tion of the thick Yakutat microplate during that time and is consistent

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with erosion and exhumation in the study area. Furthermore, leaking of derived sediments deposited 5–4 Ma and exposed in the fold-and-thrust Wrangell lava at transtensional faults in the Miocene and Wrangell arc belt (Enkelmann et al., 2008, 2009). This short lag time between cooling volcanism beginning 26 Ma are consistent with the passage of the Yakutat below ZFT closure temperature and deposition requires very rapid exhu- microplate beneath the study area and the location of the slab edge closer mation (Enkelmann et al., 2010). The occurrence of rapid exhumation is to the Duke River fault zone at that time (Richter et al., 1990; Skulski et also supported by the finding that detrital AFT cooling age peaks from al., 1991, 1992; Trop et al., 2012). Compared to regions farther west, the the syntaxis region are essentially the same as the detrital ZFT age peaks amount of exhumation is smaller in the study area possibly due to the (Enkelmann et al., 2015a), suggesting rapid cooling below both the ZFT shorter time that the slab was present beneath the study area. and AFT closure temperatures (i.e., ca. 250 °C and ca. 110 °C, respectively) Coupling between the Yakutat microplate and the upper plate prob- in 1–2 m.y. The amount of exhumation was, however, limited (ca. 10 km), ably increased over time as the crustal thickness of the subducting plate which is implied by the facts that higher-temperature 40Ar/39Ar systems of increased (Worthington et al., 2012). A mid-Miocene cooling signal (ca. detrital samples from the Seward and Hubbard glaciers (Falkowski et al., 15–12 Ma) has been observed in bedrock and detrital samples from the St. 2016) and bedrock ages at the surrounding ridges are much older (e.g., at Elias syntaxis area and the northern Fairweather fault and was interpreted Mount Logan; O’Sullivan and Currie, 1996). Enkelmann et al. (2015a) to reflect increasing resistance to subduction of the Yakutat microplate showed that the focus of deformation and the area of most rapid exhuma- (O’Sullivan and Currie, 1996; Meigs et al., 2008; Grabowski et al., 2013; tion shifted south of the plate boundary (Fig. 8B) after ca. 2 Ma based on Falkowski et al., 2014). A few ca. 18–10 Ma AFT and ZFT age peaks thermochronology data integrated with geophysical, structural, and sur- are also found in the southern study area, especially in the southernmost face processes data. This southward shift was explained by the rheologic KLD29 catchment, where a large (54%) ca. 13 Ma AFT age peak and a modification of the incoming Yakutat microplate, which resulted from smaller ca. 14 Ma ZFT age peak are found (Table 3). The large ca. 12 increasing erosion rates due to the onset of glaciation at 5.6 Ma and glacial Ma (64%) AFT age peak of catchment KLD85 (Table 3) farther north intensification with the global climate shift at 2.6 Ma and onset of 100 k.y. in the study area is left out here because it might be a volcanic signal glacial cycles in the mid-Pleistocene (Enkelmann et al., 2015a). The result- as discussed earlier. Likewise, KLD110 and KLD78 are excluded here. ing high sedimentation rates on the Yakutat microplate (>2 mm/yr up to All other mid-Miocene cooling age populations, however, likely reflect 14 mm/yr; Hallet et al., 1996; Lagoe and Zellers, 1996; Jaeger et al., 1998; distal response to Yakutat–North American plate coupling and collision. Gulick et al., 2015) allowed the development of a décollement and a shift of deformation to shallow thrusts at the coast. With the emergence of the St. Elias Syntaxis Exhumation southern St. Elias Mountains that intercept with atmospheric circulation, Detrital ZFT data revealed that exhumation with extremely high rates the modern precipitation pattern was established and caused a focusing of and from greater depth occurred below the ice in a ca. 4800 km2 area erosion in the southern syntaxis region (Enkelmann et al., 2015a). encompassing the Seward and Hubbard-Valerie glacial catchments and Our new data from the northern region of the syntaxis suggest that the northern Fairweather fault (Fig. 7; Enkelmann et al., 2008, 2009, 2010, rapid exhumation has already been focused in the wider syntaxial region 2015a; Grabowski et al., 2013; Falkowski et al., 2014, 2016). Detrital ZFT prior to 5 Ma. The youngest ZFT age peaks of our study area (ca. 7.6–6.5 cooling age populations of <5 Ma are much younger than expected from Ma; Table 3) in KLD40 and KLD13 occur adjacent to the eastern and surrounding bedrock data, even when considering the lower elevation southeastern Hubbard Glacier catchment, which is also reflected in a sources of detrital samples (below the ice compared to bedrock samples small ca. 10 Ma ZFT age peak in the southernmost Alsek River sample from ice-free ridges) and heat advection in response to rapid exhuma- (KLD27) (Figs. 7 and 8A). Furthermore, the southern parts of catchments tion (Spotila and Berger, 2010). The regional extent of rapid and deep KLD27 and KLD23 and catchment KLD26 record major ca. 10 Ma AFT exhumation was defined based on the occurrence of young detrital ZFT age populations suggesting Late Miocene exhumation (Table 3). Bedrock age populations in glacial catchments (cf. composite pie charts in Fig. 7 data from the Dusty Glacier catchment (KLD40) allow to further confine and reddish-colored catchments in Fig. 8A), whereby the northern and the ca. 7.6 Ma age peak (64%; Table 3) to the upper (western) part of eastern boundaries were uncertain due to the lack of data from catchments the catchment. There is a stark contrast between young bedrock ages at draining to the north. Our new data show that no <5 Ma detrital ZFT age higher elevations (ca. 9.4 Ma ZFT age; this study; and ca. 4.3 Ma AHe populations occur on the northeast side of the speculated Connector fault age; Spotila and Berger, 2010) occurring in the western catchment and (Table 3; excluding volcanic cooling signals). Whether the fault trace is much older ZFT (>100 Ma; this study) and AHe (ca. 16.6 Ma; Spotila the limit of the rapidly exhuming area is unknown due to the large catch- and Berger, 2010) bedrock ages in the eastern catchment (Fig. 8A). This ments, but the location of the inferred Connector fault is an obvious choice. contrast suggests a vertical offset across an unrecognized structure with Seismic activity of the Art Lewis Glacier fault, the southern part of the significant exhumation on its western side after ca. 10 Ma. This is con- Connector fault, indicates dextral shearing (Bruhn et al., 2012), but it is sistent with the KLD40 detrital ZFT and AFT age populations with peak possible that the fault accommodated significant vertical motion in the ages at ca. 7.6 Ma and ca. 8.8 Ma, respectively, which are the same within past as it is suggested based on geodynamic models that imply a south- 1s errors (Table 3 and Fig. 8A). Considering the different elevations of dipping reverse fault or shear zone north of the Fairweather fault in the bedrock and detrital samples, as well as heat advection and the dependence syntaxis area (Koons et al., 2010). While geologic and seismic studies on fault geometry and slip history, we do not attempt to estimate rates suggest that inland strain transfer along the Connector fault might be a or exact timing of exhumation here, other than a window of 10–5 Ma. young feature (<1 m.y.; Lahr and Plafker, 1980; Doser, 2014), the Art Taken together, the spatial exhumation pattern confirms the suggested Lewis Glacier fault has probably been active for several million years “bull’s-eye” pattern for the St. Elias syntaxis, similar to other colliding (written communication between G. Plafker and Kalbas et al., 2008). plate corners where efficient erosion occurs, e.g., in the Himalaya (e.g., However, the ice cover hampers geologic mapping, and glacial erosion Enkelmann et al., 2011; Koons et al., 2013; Bendick and Ehlers, 2014) potentially removes marker features. but adds a temporal dimension to it. We suggest a progressive focusing Regardless of the structural accommodation of exhumation, further and increasing rates of post–ca. 10 Ma exhumation in the syntaxial area, evidence for rapid and deep exhumation starting ca. 5 Ma in the syntaxial accommodated by discrete structures such as the proposed fault in the region comes from 6 to 5 Ma detrital ZFT age populations found in locally Dusty Glacier catchment and the inferred Connector fault (Figs. 8B and

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A 140ºW 10–5 Ma ZFT age populations B Focus of most rapid exhumation ≤ 5 Ma ZFT age populations < 2 Ma ~5–2 Ma ~10–5 Ma DF Catchments BRF Detrital sample ? Detrital sample (Alsek River) BgF Bedrock ZFT CcF sample CcF CHF Bedrock AHe Fig. 8C sample (Spotila & Berger, 2010) ECF FF 60ºN BRF 9.4 Ma 4.3 Ma ? BF 154 Ma ? KLD40 16.6 Ma DRF 101 Ma YF KLD13 0 50 60ºN km 140ºW Y57 C SW NE FF CcF ? part of KLD27 BF YF FF

0 75 150 km

Figure 8. (A) Locations of catchments with >5% of 10–5 Ma (purple) and ≤5 Ma (red) detrital zircon fission-track (ZFT) age populations (this study; Enkelmann et al., 2009; Falkowski et al., 2014). Age populations representing young volcanics are excluded. (B) Map view of the migrating focus of most rapid exhumation at ca. 10–5 Ma, ca. 5–2 Ma, and <2 Ma based on detrital and bedrock thermochronometric ages. Note that the outline of the ca. 10–5 Ma area of rapid exhumation is uncertain (dashed line), where <5 Ma exhumation overprinted possible earlier signals. (C) Schematic block model of possible structural accommodation of <10 Ma exhumation in the St. Elias syntaxis area. The colored bars at the top indicate the migrating focus of most rapid exhumation through time; color code as in B. DF—Denali fault; CcF—Connector fault; BRF—Border Ranges fault; BgF—Bagley fault; FF—Fairweather fault; CHF—Chaix Hills fault; ECF—Esker Creek fault; BF—Boundary fault; YF—Yakutat fault.

8C). The southwestern limit of suggested ca. 10–5 Ma rapid exhumation systems that develop to accommodate transpressional deformation, have is not clear because the area overlaps with the area where <5 Ma exhu- been suggested previously to occur in the area, e.g., the Yakutat foothills mation signals prevail and potentially conceal 10–5 Ma exhumation. The thrusts and Fairweather fault or the Art Lewis Glacier fault and thrust focus of exhumation shifted south over time and into the area between faults at Mount Logan (Bruhn et al., 2004, 2012). However, our finding Connector and Fairweather faults where we find clear evidence in the of a much larger region of rapid exhumation since ca. 10 Ma requires detrital ZFT age population record (Syntaxis/Nunatak Fjord composite geodynamic models to take into account the larger amount of deforma- chart in Fig. 7 and reddish polygon in Figs. 8A and 8B; Falkowski et al., tion accommodated within the syntaxis area in the past. 2014). Few, small ≤5 Ma detrital ZFT age populations were also found in the hanging wall of the Yakutat fault south of the Fairweather fault, IMPLICATIONS AND CONCLUSION suggesting that exhumation increased there as well, along shallower paths, however (Falkowski et al., 2014). The exhumation pattern in combination The new thermochronology data from the northern St. Elias Mountains with inferred structures north of the Fairweather fault and known struc- reveal the long-term cooling history of the Wrangellia composite terrane tures (thrust faults) south of the Fairweather fault in the Yakutat foothills that comprises two major phases of terrane collision and accretion to the indicate that exhumation might have been accommodated within a large- western margin of North America. The first phase was the accretion of scale, two-sided flower structure with a migrating focus of deformation the Wrangellia composite terrane during Late Jurassic–mid-Cretaceous (Fig. 8C). Partial flower structures, i.e., linked strike-slip and thrust fault time. This phase was characterized by widespread magmatism that almost

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fully reset the thermal record of the study area. The fact that the Juras- (ca. 10–5 Ma) with deformation and exhumation probably accommodated sic–Cretaceous record is so well preserved in the study area indicates the along the Fairweather fault and southwest-dipping faults north of it. The strong thermal effect of magmatism to the entire upper crust and hinders driver or drivers for the focusing of exhumation in the syntaxial region are the record of coeval cooling due to erosion or tectonic rock exhumation uncertain. However, a 20° clockwise rotation of Pacific–North American by means of thermochronology. The subsequent, Late Cretaceous phase relative plate motion has been attributed to increased transpression and of exhumation is, however, recorded. It was related to a transpressional changes in the structural setting facilitating vertical motion and impeding tectonic setting with rapid, oblique plate convergence that resulted in lateral slip. Even though 5 Ma (Engebretson et al., 1985) is often cited as crustal shortening and thickening, and translation of terranes, similar to timing of plate motion change (e.g., Fitzgerald et al., 1995; O’Sullivan and observation from the entire North American Cordillera (e.g., Farmer et Currie, 1996; Bruhn et al., 2012; Falkowski et al., 2016), other estimates al., 1993; Hollister and Andronicos, 2006). that do not rely on fixed hot spots might be more appropriate at ca. 8 Ma The second phase of terrane accretion is related to the ongoing oblique (Atwater and Stock, 1998), which is more consistent with a ca. 10–5 Ma flat-slab subduction and collision of the Yakutat microplate. After passage onset of exhumation processes in the syntaxis. With glaciation of the area, of the spreading ridge during the Eocene, the North American margin was and increasing thickness of the subducting slab, the focus of deformation affected by the subduction of the Yakutat microplate that initiated in the shifted southward ca. 5 Ma and caused rapid and deep-seated exhumation Late Eocene. Both events deformed particularly the outboard accretionary in the center of the structure between Connector fault and thrust faults in wedge (e.g., Hudson et al., 1977a, 1979; Bradley et al., 1993; Pavlis and the Yakutat foothills. The focusing of deformation in the North American Sisson, 1995) but are also evident in the thermochronometric record of plate might have been due to its weakness relative to the indenting thick the regions that are located farther inboard. The collision of the Yakutat oceanic basement rocks of the Yakutat microplate. The southward shift of microplate is inherently different from the Wrangellia composite terrane deformation continued and since <2 Ma deformation and the most rapid accretion due to the smaller size and its nature as an oceanic plateau with exhumation is concentrated along northeast-dipping faults that parallel increasing crustal thickness (Christeson et al., 2010; Worthington et al., the northern Fairweather fault and imbricate the metasedimentary rocks 2012). While accretion of the Wrangellia composite terrane was associated of the Yakutat Group, and along the northwest-dipping, shallow thrusts with arc magmatism, flat-slab subduction suppresses magmatism above of the fold-and-thrust belt comprising thick Cenozoic cover strata (Enkel- the slab and confines it along the slab edges. However, the stronger cou- mann et al., 2015a). This migration of deformation and exhumation over pling between downgoing and upper plate results in surface uplift above time presents an important observational data set for future geodynamic the slab (e.g., Chugach and Talkeetna Mountains), focused deformation models that aim to understand mechanisms at plate corners, which must in weak crustal sections and reactivation of preexisting structures (Alaska be treated as four-dimensional systems. Range and Denali fault; e.g., Fitzgerald et al., 2014; Lease et al., 2016). The lack of magmatism and the inboard transfer of strain results in rock ACKNOWLEDGMENTS cooling due to exhumation that is recorded in the thermochronology data This study was funded by the Deutsche Forschungsgemeinschaft (DFG grants EN-941/1 and EN-941/1-2). We are grateful to our very skilled helicopter pilot, Doug Makkonen, for his efforts in the areas above the flat slab. Similar to southern Alaska, flat-slab sub- during our sampling campaign and Philipp Widmann for his assistance in the field. We thank duction of the Juan Fernandez Ridge of the Nazca plate transfers strain editor Kurt Stüwe, Sarah M. Roeske, and an anonymous reviewer, whose comments and much farther inboard and east of the Andes than in the regions north suggestions helped us to improve this manuscript. and south of the subducted ridge in central Argentina (e.g., Alvarado et REFERENCES CITED al., 2009; Richardson et al., 2013). This process reactivated old base- Alvarado, P., Pardo, M., Gilbert, H., Miranda, S., Anderson, M., Saez, M., and Beck, S., 2009, ment structures and formed the Sierras Pampeanas. 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