Examination of the Interplay Between Glacial Processes and Exhumation in the Saint Elias Mountains, Alaska

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

Examination of the interplay between glacial processes and exhumation in the Saint Elias Mountains, Alaska

Rachel M. Headley1,2, Eva Enkelmann2,3, and Bernard Hallet1 1Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195-1310, USA 2Institut für Geowissenschaften, Universität Tübingen, Tübingen, D-72074, Germany 3Department of Geology, University of Cincinnati, Cincinnati, Ohio 45221, USA

ABSTRACT concepts of glacial erosion should be used ogy of deglaciated landscapes (Brocklehurst with caution, as oversimplifi cation can fail and Whipple, 2002, 2007; Champagnac et al., The combination of large, temperate to account for important site-specifi c differ- 2009), and on conceptual (Whipple et al., 1999), glaciers and rapid crustal convergence in ences in geologic and glacial conditions. analytical (Tomkin and Roe, 2007), and numeri- the Saint Elias Mountains (southeastern cal models (Tomkin and Braun, 2002; Tomkin, Alaska, USA, and Yukon Territory and Brit- INTRODUCTION 2007; Herman and Braun, 2008; Yanites and ish Colombia, Canada) provides an excep- Ehlers, 2012). Direct field investigations of tional opportunity to study the interactions The interactions among climate, tectonics, regions currently being eroded by massive gla- between the tectonic and surface processes and surface processes in active orogens have ciers have only recently gained attention (Ehlers that have shaped most active orogens on received much attention through a variety of et al., 2006; Herman et al., 2010). Earth during much of the Quaternary. This modeling efforts and fi eld investigations (e.g., The heavily glaciated Saint Elias Mountains research fi rst provides a review of thermo- Molnar and England, 1990; Beaumont et al., in southeastern Alaska provide a glimpse at how chronometric data sets recording exhuma- 1992, 2001; Koons, 1995; Zeitler et al., 2001; many orogens likely appeared and functioned tion under two major glacier systems of the Wobus et al., 2003; Bookhagen et al., 2005; during much of the Quaternary; the area is an Saint Elias Mountains, the Bagley-Bering Hooks et al., 2009). Erosion can signifi cantly excellent location to study links between glacial and the Seward-Malaspina systems. These infl uence the geodynamics in active mountain erosion and exhumation in a tectonically active data sets are integrated over the single gla- belts (e.g., Brozović et al., 1997; Zeitler et al., mountain belt. We fi rst review existing data of cier systems and used in conjunction with 2001). Many studies have focused on fl uvial the geology, glaciology, and thermochrono- glaciological data to investigate the inter- erosion (e.g., Zeitler et al., 2001; Whipple and metric record from the Saint Elias Mountains. actions of glacial erosion and tectonics. Meade, 2004), but the importance of glacial Then, we reexamine the detrital data reported in Despite their proximity, the glaciological erosion has recently gained attention in view Enkelmann et al. (2008, 2009) to obtain robust processes and geological settings of these of (1) the exceptionally high modern erosion cooling age information over the individual two glacial systems differ signifi cantly. On rates documented for many glaciers (e.g., Hallet glaciers. This information is then integrated the east side of the orogen, sediments eroded et al., 1996; Delmas et al., 2009; Koppes and with published glaciological data and new cal- from bedrock underneath the Malaspina Montgomery, 2009), and (2) the role of glacial culations of the hydrological potential for the Glacier refl ect regions of rapid erosion under erosion in curtailing the height of mountain Bagley-Bering Glacier. The purpose is to gain the slowly moving Seward Ice Field. Because ranges, a concept known as the “glacial buzz- new insights on the interaction between glacial the Seward Ice Field overlies a localized zone saw” that highlights the widespread associa- erosion and rock exhumation for individual gla- of major faulting and rapid exhumation, the tion between the heights of mountains and the cial systems. We emphasize a number of results strained and fractured bedrock is primed for snowline (Brozović et al., 1997; Mitchell and that merit further attention. (1) The two major erosion. On the west side, the Bering Glacier Montgomery, 2006; Egholm et al., 2009). Many glacier systems covering much of the mountain is the primary outlet for the Bagley Ice Field, active orogens were heavily glaciated during range share many similarities in terms of their which covers half of the crest of the orogen; much of the Pliocene–Pleistocene (e.g., Hima- size, ice dynamics, and underlying rock units, however, few if any of the sediments at its laya, Andes, European Alps, Southern Alps of but produce signifi cantly different detrital sig- terminus originate from under the Bagley New Zealand, northwestern Cordillera of North nals of exhumation rates, suggesting a strong Ice Field. Sediment transport is likely hin- America), but most of these regions currently infl uence of the different structural settings dered by subglacial freeze-on processes contain only small alpine glaciers and remnants within the orogeny on the glacial erosion. that reduce the sediment-carrying capacity of larger ice fi elds (e.g., Porter, 1989; Meigs and (2) From the thermochronometry, the spatial of subglacial rivers, though glacial surges Sauber, 2000; Tomkin and Braun, 2002; Tomkin pattern of exhumation rates shows little correla- are likely exceptions that deposit sediment and Roe, 2007). Consequently, the studies of the tion with the rate of ice motion, and the most far beyond the active margin of the glacier. coupling between glacial erosion and tectonic intense exhumation in the orogen occurs in a Our study concludes that the widely invoked processes are largely based on the geomorphol- region of relatively slow ice fl ow.

Geosphere; April 2013; v. 9; no. 2; p. 229–241; doi:10.1130/GES00810.1; 4 fi gures; 2 tables. Received 16 April 2012 ♦ Revision received 13 September 2012 ♦ Accepted 1 December 2012 ♦ Published online 5 February 2013

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BACKGROUND continent by the Aleutian megathrust in the west, forming the Chugach metamorphic complex the Chugach–Saint Elias thrust fault in the north, (Plafker et al., 1977, 1994; Barker et al., 1992; Geology and the Fairweather transform fault in the east Cowan, 2003; Pavlis et al., 2003; Sisson et al., (Figs. 1 and 2). 2003). Metamorphic grades decrease generally The tectonic setting of southern Alaska is Southeastern Alaska comprises a complex westward, from amphibolite facies gneiss and dominated by the ongoing convergence of the mosaic of terranes that have been accreted schist around the eastern Bagley Ice Field and Yakutat terrane with North America (Fig. 1). to North America since the Mesozoic (e.g., Seward ice fi eld, to greenschist metamorphic Recent geophysical studies characterize the Yaku- Plafker et al., 1994). From north to south these rocks in the west (Fig. 2). tat terrane as buoyant overthickened (15–30 km) terranes are the Wrangellia, Chugach, Prince The Chugach terrane is separated from the oceanic crust (Ferris et al., 2003; Eberhart-Phil- William, and Yakutat (Fig. 2). The Border Prince William terrane by the Contact fault, lips et al., 2006; Worthington et al., 2008, 2012; Ranges fault is the suture between the late which is thought to have initiated as a subduc- Christeson et al., 2010) that was transported Paleozoic–Jurassic basement of the Wrangellia tion thrust during the Paleogene (Plafker et al., northward along the dextral Fairweather fault sys- terrane in the north and the Chugach terrane in 1994). Although the Contact fault cannot be tem (Plafker et al., 1977; Plafker, 1987), causing the south (Fig. 2) (Pavlis and Roeske, 2007). observed directly because it is buried under the uplift and exhumation in southern Alaska since The Chugach and Prince William terranes thick ice of the Bagley and Seward Ice Fields, the Late Eocene–Early Oligocene (Finzel et al., represent a Mesozoic–Paleogene subduction it can be inferred with confi dence from geo- 2011; Benowitz et al., 2011). Today the Yakutat accretionary complex, which was metamor- detic measurements, structural analysis of the terrane is separated from the Pacifi c plate by the phosed and intruded by the Sanak-Baranof surrounding region, and geological observa- Transition fault, and from the North American plutonic belt during Eocene ridge subduction, tions on isolated nunataks (e.g., Bruhn et al.,

North American Km 100 Alaska 200 Canada 64 Plate Alaska Range Denali Fault 63 Mt. McKinley

62 Wrangell Range Chugach - Saint Elias Anchorage 61 .

CSEF Mt. Saint Elias 60 Yakutat Cook Inlet BRF Border Range F terrane 59 Transition Fault 47 mm/yr

58

Fairweather F.

51 mm/yr 57 Aleutian Trench Pacific Plate

56N 154W 152 150 148 146 144 142 140 138 136 134 132 130

Figure 1. Tectonic setting of southern Alaska with major tectonic structures and geographic features, including the Bering and Seward glacier systems (light blue). The Last Glacial Maximum extent is shown by the red line, and the Pleistocene maximum is shown by the blue line; both are from the work of Kaufman and Manley (2004). The Yakutat–North America plate motion vector is from global positioning system measurements by Fletcher and Freymueller (2003), and Pacifi c plate–North American plate motion is from Kreemer et al. (2003). CSEF—Chugach–Saint Elias fault.

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A 143W 142W Chitina River 140W Valley Yukon Alaska Wrangellia terrane CH46,47 Border Range F. CH44 Tana Gl. Chugach terrane Contact F. Mt.Logan 43zFT 5959m CSEF Bagley Ice Field 4-40aFT Yakutat Chugach terrane terrane Prince William terrane Seward Ice Mt.Vancouver Y7 Field 4812m Bering Glacier Y11 0.6aHe 10zH Y10 Yakutat Saint Elias 5489m 3.5zHe 2aHe Y12 4aFT Y3 3aFT 55zHe Throat Y9 terrane 1.5aHe Y8 4aHe 7aHe Fairweather F.

MF Seward Yakutat 60N Vitus lake terrane 60N Malaspina Glacier Icy Bay IB3

IB1 Yakutat Kayak Island zone IB4 Gulf of Alaska Bay

0 20 40km Pamplona zone 143W 142W 141W 140W

B Tana Glacier Bering Glacier Malaspina Glacier (CH46,47) n= 208 (Y8,9,10) n=309 (IB1,3,4) n= 312

50 200 Yakutat, Kultieth & 0 100 150 Poul Creek Fms. Age (Ma) Chugach terrane (Y3,7,11,12) n=411 (CH44) n= 104

probability density Detrital zircon FT age distribution peak fitted composite age populations

measured age distribution 0 50 100 150 200 0 50 100 150 200 Age (Ma) Age (Ma)

Figure 2. (A) Map of the Saint Elias Mountains with main structures and geological units. The Bagley-Bering glacier system is outlined in yellow, the Seward-Malaspina system is in red. Sample locations of detrital thermochronometry data are shown as boxes: red for samples from the Bering, Tana, and Malaspina Glaciers, purple for other drainages used for comparison (see Table 2). Faults (F.) are labeled in italics: MF—Malaspina fault, CSEF—Chugach–Saint Elias fault. Bedrock ages (in Ma) from around the Seward Ice Field are shown: aHe and zHe are apatite and zircon U-Th/He ages and aFT and zFT are apatite and zircon fi ssion-track (FT) ages. Ages are from Berger and Spotila (2008), Enkelmann et al. (2010), O’Sullivan et al. (1995), and O’Sullivan and Currie (1996). (B) New calculated probability density plot and peak fi tting results of the combined zircon FT age distribution data from Enkelmann et al. (2008, 2009).

2004; Ruppert et al., 2008; Elliott et al., 2010). south. During the Yakutat collision, the Paleo- tion (younger than 5.6 Ma) are located farther Moreover, it was possibly reactivated during gene to Pleistocene sedimentary cover of the south and offshore (Plafker, 1987; Plafker et al., the Yakutat collision, most likely as a dextral Yakutat terrane was accreted to North America 1994). In contrast to the Kultieth and Poul Creek compressional transform fault and the western in the evolving fold-and-thrust belt (Fig. 2; Formations that derived from rocks located sev- continuation of the Fairweather fault (Fig. 2) Plafker et al., 1994). Mainly Eocene to Miocene eral hundreds of kilometers farther south, the (Savage and Lisowski, 1986; Sauber et al., sediments of the Kultieth and Poul Creek For- Yakataga Formation is derived from Chugach 1997; Bruhn et al., 2004). The Chugach–Saint mations are exposed in the northern and western terrane rocks and reworked Kultieth and Poul Elias thrust fault is the suture between the Prince part of the fold-and-thrust belt, whereas the sedi- Creek Formations (e.g., Enkelmann et al., 2008, William terrane and the Yakutat terrane in the ments of the syncollisional Yakataga Forma- 2010; Perry et al., 2009).

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Glacial Setting and Sedimentation The syncollisional Yakataga Formation is William Sound and submarine features that composed of marine and glaciomarine sedi- show glacial erosion or deposition, including a The Seward and Bagley Ice Fields occupy ments and records a long history of glaciation series of prominent shelf-crossing sea valleys the orogenic spine of the Saint Elias Mountains starting ca. 5.6 Ma (Plafker, 1987; Lagoe et al., (e.g., Mann and Hamilton, 1995; Kaufman and at ~2000 m elevation (Fig. 2). The Seward Ice 1989, 1993; Plafker et al., 1994). Glaciers large Manley, 2004; Berger et al., 2008a). Field funnels ice southward through the Seward enough to reach the Gulf of Alaska have gener- It is possible to quantify the amount of gla- Throat (~40 km long, 4–6 km wide), a nar- ally been sustained since ca. 5 Ma (Péwé, 1975). cial erosion by measurements of sediment strata row passage through the prominent range that During the last major glaciation, and likely previ- to estimate approximate orogen-wide erosion bounds most of the ice fi eld in the south. The ice ous glaciations, the ice was substantially thicker rates over shorter time scales (101–104 yr). then spreads out toward the coast into the mas- than today and extended over much of the conti- Based on sedimentation rates of 7.9 mm/yr, sive Malaspina piedmont glacier (Figs. 2 and 3). nental shelf (Péwé, 1975; Mann and Hamilton, Sheaf et al. (2003) estimated minimum ero- In the area of highest topography near Mount 1995; Kaufman and Manley, 2004). Glaciers sion rates of 5.1 mm/yr over the past 10,000 yr Logan and Mount Saint Elias, the Seward Ice were suffi ciently massive to reach the ocean for the full Saint Elias Mountains. Jaeger et al. Field connects with the Bagley Ice Field. The and shed debris-laden icebergs since the latest (1998) determined that the deposition rates Bagley ice fl ows westward for >100 km before Miocene–Pliocene, possibly during a period that from the previous 100 years vary signifi cantly diverging into the Bering and Tana Glaciers. The was warmer than today (Péwé, 1975; Lagoe and from those rates averaged over the Holocene; Bering Glacier (~75 km long, 8–10 km wide) Zellers, 1996). Evidence for Last Glacial Maxi- these rates were measured in shallow water off fl ows south toward the Gulf of Alaska, and the mum (LGM) ice extent to the edge of the conti- the coast of the Saint Elias Mountains, particu- Tana Glacier (~30 km long, 3–4 km wide) drains nental shelf, 150 km off the present coastline, is larly in fjords and bays into which glaciers had to the north into the Chitina Valley (Fig. 2). given by erosional features on islands in Prince extended during the LGM. However, in deep troughs that had never been overrun by glaciers, the rates on the two times scales were found to be very similar over both the past ~100 yr and 6.71 the Holocene (Jaeger et al., 1998). Even consid- ering corrections for overestimates of erosion rates due to the enhanced sediment fl ux carried Seward Ice Field by increased water discharge as modern gla- 6.70 Contact F. ciers retreat, the coastal Alaskan glaciers have maintained generally high erosion rates (1–10

6.69 mm/yr) for more than 10,000 yr (compiled by Koppes and Hallet, 2006; Koppes and Mont- m) 6 CSEF gomery, 2009). Considerably less is known about erosion rates over longer time scales, 6.68 105–106 yr (Sheaf et al., 2003). These rates may have been slightly lower (2–5 mm/yr), based on thermochronometry data from the region south 6.67 Velocity of the Seward and Bagley ice fi elds (Berger and Agassiz Glacier (m/yr) Spotila, 2008). 2000 F. On the scale of specifi c glaciers, sediment 6.66 yield and water discharge rates have also been alaspina 1500 M used to determine erosion rates that compare

UTM Zone 7N Northing (10 60N favorably to the regional rates. The erosion rate 6.65 1000 of the Variegated Glacier near Yakutat Bay was found to average ~3 mm/yr (Humphrey and 500 Malaspina Glacier Raymond, 1994) and Icy Bay glaciers averaged 6.64 0 ~9 mm/yr (Koppes and Hallet, 2006). For the Seward-Malaspina system, a sediment yield of 108 m3/yr, if sustained, corresponds to an effec- 6.63 10 km tive erosion rate of ~11 mm/yr averaged over a basin area of ~5000 km2 (Molnia et al., 1978; 140W Hallet et al., 1996). For the Bagley-Bering sys- 5.1 5.2 5.3 5.4 5.5 5.6 UTM Zone 7N Easting (105 m) tem, contemporary sediment yields correspond to basin-averaged erosion rates of ~2 mm/yr Figure 3. The Seward-Malaspina glacier system and glacier veloc- based on rough estimates (Merrand and Hallet, ity distribution. Through the Seward Throat, the velocities range 1996) for non-surge periods. During outburst from >100 m/yr to ~2000 m/yr (adapted from Headley et al., 2012). fl oods associated with surges, the much higher The Seward Ice Field shows velocities >25 m/yr (adapted from Ford sediment yields refl ect enhanced sediment et al., 2003), reaching a maximum of ~100 m/yr. Mapped faults are transport and more vigorous erosion, possibly shown as black lines, assumed faults (F.) are dashed lines. CSEF— corresponding to minimum erosion rates of ~10 Chugach–Saint Elias fault; UTM—Universal Transverse Mercator. mm/yr (Merrand and Hallet, 1996).

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Both the Seward-Malaspina and the Bagley- to the Malaspina lobe but are not considered the Bering Glacier (Merrand and Hallet, 1996; Bering-Tana systems surge on a decadal to multi- here. According to work by Washburn (1935), Fleisher et al., 2003). The discharge in the Seal decadal time scale. The surges likely result from Sharp (1958), and Post (1972), the folded River, which drains proglacial Vitus Lake to the partial or complete blockage of the subglacial moraines of the Malaspina piedmont refl ect a Gulf of Alaska, increased by an average of 1100 (and possibly englacial) fl uvial network, and series of surges. Different regions of the lobe m3/s above the base fl ow of 1550 m3/s over a 6 the buildup of basal water that leads to fast slid- are expected to surge at different times, includ- day fl ood period in 1994 (Merrand and Hallet, ing (Raymond, 1987). Surges can end abruptly ing regions infl uenced by the Agassiz Glacier, 1996). Offshore in the Bering Trough, sediment when the subglacial hydrological network distorting existing moraines (Washburn, 1935; deposits record many discrete packages of rap- opens and connects with the glacier margin, Sharp, 1958). The effects of surges between idly settled sediment; these packages have been characteristically releasing large volumes of 1976 and 2006 include surface lowering, heavy interpreted as historical surges that disrupt the sediment-laden water in outburst fl oods (Ray- crevassing, distortion of moraines on the pied- normal sedimentation packages (Jaeger et al., mond, 1987; Paterson, 1994). This profound mont lobe, and terminus encroachment into bor- 1998; Jaeger and Nittrouer, 1999). reorganization of the subglacial hydrological dering lakes (Muskett et al., 2008). While not nearly as extensive as on the network during surges has a strong impact on There are excellent surface velocity mea- Seward-Malaspina, ice motion has been stud- the subglacial transport of sediment (Raymond, surements available for the Seward-Malaspina ied through sparse global positioning system 1987). Surging is characteristic of, and has been system. Synthetic aperture radar interferom- (GPS) measurements (Larsen et al., 2007; extensively studied for, several other major etry (InSAR) derived surface velocities through LeBlanc et al., 2008; Bruhn et al., 2010) and glaciers in southeastern Alaska, including the the Seward Throat were presented in Head- InSAR (Fatland and Lingle, 2002). Summer nearby Varie gated Glacier in the eastern part ley et al. (2012). Additional surface veloci- velocities range from <300 m/yr to >1000 m/yr of the Saint Elias Mountains (Raymond, 1987; ties were measured for the Seward Ice Field on both the Bering Glacier and the Bagley Ice Lingle and Fatland, 1998). (Ford et al., 2003). Surface velocities increase Field. However, the longer-term velocity dis- There are other processes that contribute to by more than an order of magnitude as the ice tribution of this glacier system is diffi cult to the erosion and transport of sediment out of from the Seward Ice Field funnels through the determine because of seasonal variations and mountainous terrain. Periglacial processes, i.e., Seward Throat, accelerating from ~100 m/yr to the surge cycles. headwall erosion due to frost weathering, rock >1800 m/yr in the central and lower portion of In contrast to the Seward-Malaspina system, fall, and landslides, can also deliver consider- the throat (Fig. 3). While the surface velocity is the basal topography of the Bering Glacier is able debris to the glacier surface, where it can well defi ned for the Seward-Malaspina Glacier, well defi ned, although is lacking for beneath be carried along with glacier fl ow and mixed the ice thickness has proven diffi cult to measure most of the Bagley Ice Field. Figure 4 shows with subglacially eroded sediment at the glacial except for through the Malaspina lobe (Conway a surface elevation profi le measured using air- terminus (e.g., Hallet and Roche, 2010). Previ- et al., 2009). These diffi culties are due to the borne laser altimetry along an approximate ous work on other glaciers has shown that this extreme constriction, existence of ample water, fl owline (Echelmeyer et al., 2002) and the supraglacially-sourced sediment can account and heavy crevassing of the Seward Throat. glacier thickness measured using an airborne for a large range of the total sediment fl ux, ice-penetrating radar over most of the Bering from <10% to as much as 25%–60% for small Bagley-Bering-Tana Glacier System Glacier (Conway et al., 2009). glaciers (Syvitski, 1989; Hunter et al., 1996; Much research has focused on the Bering Arsenault and Meigs, 2005). However, both Glacier surge cycle (discussed in detail in Bruhn Thermochronometry in the the Seward-Malaspina and Bagley-Bering-Tana et al., 2010). Evidence of the surges includes Saint Elias Mountains systems cover the vast majority of their drain- drastic changes to the velocity, historical records age areas (~103 km2), with only small portions of the glacier advancing several kilometers into The main geological events that have been of the surrounding slopes exposed to supra- proglacial Vitus Lake, and folded moraine loops recorded in the Saint Elias orogen using geo- glacial processes, so supraglacial material likely on its surface (e.g., Post, 1972; Lingle and Fat- chronometry and thermochronometry are sum- accounts for only a small percentage of the total land, 1998, 2003). In contrast, the Tana Glacier marized in Table 1. The tectonic and erosional sediment load. At the bed of the glacier, once does not seem to surge or be heavily infl uenced evolution of the Saint Elias Mountains derived loose material is produced through erosion, by the Bering surges (Fatland and Lingle, 2002). from thermochronometry was reviewed in detail subglacial rivers are largely responsible for the The Bering Glacier has surged at least six times (see Enkelmann et al., 2010). In general, there is sediment evacuation and transport through to in the twentieth century; a well-documented a wealth of bedrock apatite U-Th/He and fi ssion- the rivers that emanate from the termini of many surge occurred in 1993–1995 (Sauber and Mol- track (FT) ages that refer to cooling through temperate glaciers (e.g., Hunter et al., 1996; nia, 2004). These surges affect the surface pro- 55–65 °C and 100–110 °C, respectively (e.g., Alley et al., 1997; Riihimaki et al., 2005). For fi le and velocity upglacier to within 20 km of Farley, 2000; Carlson et al., 1999) and reveal glaciers in Alaska and Canada, 80%–98% of the the divide from the Seward Ice Field (Lingle and rapid exhumation along the southern fl anks sediment load is transported by glacial rivers Fatland, 2003). Other effects are evidenced well of the Saint Elias Mountains and along the (Stravers and Syvitski, 1991; Hunter, 1994), beyond the glacier’s margin. Sediment fl ux dur- Fairweather fault (Spotila et al., 2004; Berger making glacial-fl uvial sediments natural targets ing the fl oods that characteristically punctuate et al., 2008b; Berger and Spotila, 2008; Meigs for probing the erosional effi ciency of glaciers. the end of surges can exceed normal fl uxes by et al., 2008; McAleer et al., 2009; Spotila and orders of magnitude (Humphrey and Raymond, Berger, 2010; O’Sullivan et al., 1995). The rapid Seward-Malaspina Glacier System 1994; Fleisher et al., 2003), and can transport exhumation coincides with the highest rates of The Seward Ice Field provides the bulk of sediment hundreds of kilometers beyond the annual precipitation (>5000 mm/yr), whereas the ice forming the piedmont Malaspina Gla- glacier front (Jaeger et al., 1998). Large outburst older cooling ages on the drier (<1000 mm/yr) cier (Fig. 3). Other smaller glaciers, including fl oods, with massive sediment delivery, were north side of the mountain range indicate less the Agassiz to the west (Fig. 3), also contribute documented during the 1993–1995 surge of rapid exhumation (O’Sullivan and Currie, 1996;

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Ice A 6.73 TanaT G. Thickness a (km) 6.72 1.2 Contact F. m) 6 Bagley Ice Field 6.71 1.0 60.5N A 6.70 CSEF 0.8

6.69 0.6 Bering G. 6.68

0.4 A’A’

UTM Zone 7N Northing (10 6.67 10 km 0.2 6.66 143W 142W 141W 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 UTM Zone 7N Easting (105 m)

2000 B A A′ 1500 Glacier Fl ow Direction 1000

500 Unknown bed topography

Elevation (m) 0 Prince William Ya k u t a t CSEF 500 Terrane Terrane 0 20 40 60 80 100 120 Distance Along Glacier (km)

Figure 4. (A) Ice thickness and surface elevation measurements and thickness extrapolations for the Bagley-Bering-Tana glacier system. The colored lines show the thickness of the ice from radar (Conway et al., 2009). Black squares show the laser altimetry surface profi le by Echelmeyer et al. (2002) with the thickness (colored circles) extrapolated along this profi le using a nearest neighbor algorithm. CSEF— Chugach–Saint Elias fault; G—glacier; F.—fault; UTM—Universal Transverse Mercator. (B) Profi le of the Bagley-Bering ice surface and bed topog raphy shown along profi le A-A′. The lightest shaded region represents the glacier ice; other shades represent different bed geologies. The purple shows regions where the bed topography ascends downglacier more steeply than 20% of the surface slope, and the green region shows where the bed topography ascends more than 70% greater.

Spotila et al., 2004; Berger et al., 2008b; Berger mation and erosion, with the most rapid and Seward-Malaspina Glacier System and Spotila, 2008; Meigs et al., 2008; Spotila deep-seated exhumation occurring at the Saint Proglacial sand samples collected around and Berger, 2010; Enkelmann et al., 2010). At Elias syntaxis region, which is covered by the the Malaspina Glacier revealed the youngest the southern fl anks the bedrock cooling ages Seward Ice Field (Enkelmann et al., 2008, 2009, zircon FT cooling ages found in this orogen, do not show variations in exhumation along the 2010). For these detrital zircon FT studies, sand with single grain ages as young as 0.5 Ma and orogenic strike and were suggested to show the was collected from the main river channel that promi nent component age populations that highest rates of erosion (>3 mm/yr) within an emerges from the glacier; 3–4 kg of sand were make up 24%–41% of the entire grains and orogen-parallel band that coincides with the collected at various spots to obtain a well-mixed peak at 2–3 Ma (Table 2; Enkelmann et al., region between the modern and LGM equilib- sample. Sample locations were usually within 2009). These young ages suggest a region of rium line altitude (ELA) (Berger and Spotila, 100 m and 1 km from the ice front (Enkelmann localized intense exhumation with cooling 2008; Berger et al., 2008a). Zircon U-Th/He et al., 2008, 2009). The measured zircon FT age rates to 300 °C/Ma. The U/Pb crystallization and FT cooling ages have also been reported distributions of individual samples were decon- ages of these young detrital zircons FT grains for bedrock and detrital material from the Saint volved into age populations using the binomial revealed that cooling is clearly due to exhu- Elias Mountains that refer to cooling through peak-fi tting method (Galbraith and Green, 1990; mation (FT age << U/Pb age) and, more sur- 180 ± 20 °C and 250 ± 40 °C, respectively Brandon, 1992, 1996), and the peak age of the prisingly, that the source region is composed (Reiners et al., 2004; Brandon et al., 1998). Zir- age populations and their size with respect to the of Chugach terrane rocks and Yakutat Group con FT cooling ages from modern proglacial entire sample have been reported (Enkelmann rocks (Enkelmann et al., 2009). While the sediments revealed a different pattern of exhu- et al., 2008, 2009, 2010). Yakutat Group crops out at the southeastern

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TABLE 1. SUMMARY OF THE MAIN GEOLOGIC EVENTS IN SOUTHEAST ALASKA AND THE CHRONOMETRIC RECORD Time (Ma) Event Chronometric record 0–10 Beginning of subduction collision of thick Reset and partially reset bedrock apatite fi ssion-track (FT) and U-Th/He ages from the fold-and-thrust belt, and Yakutat crust and formation of the reset bedrock apatite and zircon FT and U-Th/He cooling ages from the Prince William and Chugach terranes fold-and-thrust belt (Spotila et al., 2004; Berger et al., 2008b; Berger and Spotila, 2008; Meigs et al., 2008; Perry et al., 2009; Enkelmann et al., 2010) Reset zircon FT ages from Chugach terrane and Yakutat basement (Enkelmann et al., 2009) 10–40 Several phases of exhumation in southern Bedrock apatite and zircon FT and U-Th/He cooling ages of the Chugach terrane (O’Sullivan and Currie, 1996; and southeastern Alaska due to Yakutat Spotila et al., 2004; Berger et al., 2008b; Berger and Spotila, 2008; Meigs et al., 2008) subduction Bedrock FT, U-Th/He, and Ar/Ar ages in southern Alaska and basin inversion due to fl at slab subduction (Benowitz et al., 2011; Finzel et al., 2011) Several Eocene to Miocene zircon FT age populations in glacial sediment originating from the Chugach terrane (Enkelmann et al., 2008, 2010) Similar zircon FT populations found in the syncollisional Yakataga Formation that is sourced from the Chugach terrane (Perry et al., 2009; Enkelmann et al., 2010) 50–60 Metamorphism and magmatism related Zircon U/Pb crystallization ages of glacial sediment derived from the Chugach terrane (Enkelmann et al., 2008, to the ridge subduction, reactivation of 2009) the Border Range fault Zircon U-Pb, K-feldspar, amphibole, and biotite 40Ar/39Ar ages of various intrusive and metamorphic rocks of the Chugach terrane and Border Range fault zone (Sisson et al., 2003) 90–125 Last major exhumation and/or cooling Bedrock zircon FT and U-Th/He ages from the Wrangellia terrane north of the Chitina Valley (O’Sullivan and phase in the Wrangellia composite Currie, 1996; Enkelmann et al., 2010) terrane K-feldspar 40Ar/39Ar multidomain modeling and hornblende, biotite ages of rocks north of the Border Range fault (Sisson et al., 2003; Enkelmann et al., 2010)

margin of the Seward Ice Field and the Seward Bagley-Bering-Tana Glacier System Meigs et al. (2008) suggested that only the Throat, the Chugach terrane rocks only occur Three detrital zircon FT samples from the upper 5 km of the sediments are involved in north of the Contact fault under the Seward Bering Glacier were collected from river out- the cycle of lateral material input and erosional Ice Field (Enkelmann et al., 2009). Field lets on the eastern side of the glacier at the low- exhumation; this is supported by thermal-kine- observations provide additional evidence for a est elevations possible: 6 m, 60 m, and 70 m matic modeling of the Yakutat–North America Chugach terrane source of the sediments; most (Fig. 2) (Enkelmann et al., 2009). As the Bering collision (Enkelmann et al., 2010). North of the pebbles and cobbles in the Malaspina detritus Glacier terminates in Vitus Lake, contemporary Bagley Ice Field bedrock apatite U-Th/He and are gneisses, migmatites, and granites, all typi- fl uvial sediment samples were not collected FT ages are older and range from 8 to 30 Ma cal of the Chugach terrane and Chugach meta- further west because the subglacial rivers exit (e.g., Spotila et al., 2004; Berger et al., 2008b; morphic complex. This rapid and deep-seated the glacier well below lake level (Fig. 2). The Berger and Spotila, 2008; Meigs et al., 2008). rock exhumation is not evident in any cool- age populations of the three individual sam- Two sand samples collected from the Tana ing ages of the surrounding bedrock (Fig. 2) ples are generally similar in their peak ages Glacier yielded zircon FT age populations that (O’Sullivan et al., 1995; O’Sullivan and Currie, and population size (Table 2), suggesting that were similar to one another with small age 1996; Berger et al., 2008b; Berger and Spotila, (1) all the material is representative of sediment populations (4%–12%) that peak between 11 2008), suggesting that erosion, and presum- currently carried by the main rivers under the and 17 Ma and the main age components that ably rock uplift, are highly localized under the Bering Glacier; and (2) the samples are repre- peak at 21–35 Ma (Table 2; Fig. 2) (Enkelmann Seward ice fi eld while uplift and erosion are sentative of the basin, well mixed, and largely et al., 2008). The zircon FT age distributions much slower on the ice-free ridges. The young- unaffected by local sources within the basin, from the Tana Glacier are similar to the result est cooling age known from the surround- such as landslide material. The main zircon obtained from Granite Creek (CH44; Fig. 2), ing bedrock is a 0.6 Ma apatite U-Th/He age FT age populations of these three Bering Gla- a catchment located just east of the Tana Gla- from the Prince William terrane located north- cier samples peak between 23 and 70 Ma and cier and composed entirely of Chugach terrane west of the Seward Throat (Fig. 2) (Berger are similar to the age population peaks found rocks (see Table 2) (Enkelmann et al., 2008). and Spotila, 2008). Due to the much lower in the detritus of smaller glacial catchments closure temperature of the apatite U-Th/He located east of the Bering Glacier (see Table 2; NEW ANALYSIS AND RESULTS system, this sample indicates cooling rates Enkelmann et al., 2008, 2009); they are also of ~100 °C/Ma and permits much shallower similar to the age population peaks reported Thermochronometry exhumation. However, bedrock zircon cooling from bedrock samples of the Kultieth and Paul ages from the area surrounding the Seward Ice Creek Formations collected east of the Bering The individual detrital zircon FT results pre- Field are sparse (Fig. 2). Two zircon FT ages Glacier (Meigs et al., 2008; Perry et al., 2009). sented in Table 2 (see also Enkelmann et al., from the Mount Logan massif were both 43 Ma However, bedrock apatite cooling ages from the 2008, 2009) show that samples derived from (O’Sullivan and Currie, 1996), and a sample same formations are young (0.5–5 Ma; Spotila the same glacier system are similar in their age from a nunatak in the Seward Ice Field yielded et al., 2004; Berger and Spotila, 2008; Berger distributions, suggesting that they represent the a 10 Ma zircon U-Th/He age (Enkelmann et al., 2008b), but the non-reset ages of the same source material. We combine the single- et al., 2010). Two samples from the Prince Wil- higher-temperature systems indicate that these grain FT ages of the samples from the same liam terrane and the Yakutat Group south of the rocks were exhumed from shallow depths (e.g., glacier into a single, aggregate grain age distri- Seward Ice Field yielded 3.5 Ma and 55 Ma Spotila et al., 2004; Meigs et al., 2008; Perry et al., bution and repeated the peak-fi tting procedure zircon U-Th/He ages, respectively (Fig. 2) 2009; Enkelmann et al., 2010). Based on apatite (shaded in Table 2; Fig. 2). In this way the age (Enkelmann et al., 2010). and zircon FT studies in the fold-and-thrust belt, population result of a specifi c glacier is more

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) robust and allows for a more instructive com- ) ) ) ) ) ) 1 ) 7 1 ) 8 3 7 1 7 6 ( ( ( ( ( ( ( ). 6 1 parison between glaciers and also other areas. ( ( σ 0 3 0 0 1 1 1 1 1 8 1 8 1 1 3 1 The probability density plots of the combined ± ± ± ± ± ± ± ± ± 2 6 5 4 7 6 7 1 6 sample analyses with the peak fi tted age popula- 1 2 1 7 9 8 1 0 1 1 1 1 1 1 1 tions are shown in Figure 2B. For the Seward-Malaspina glacier system, the ) ) combined detrital zircon ages show that 30% NA, 7 ) ) ) ) ) ) ) ) ) ) ) 3 1 6 8 5 3 0 4 8 7 7 1 1 2 ( ( 4 4 1 2 2 3 2 4 3 3 2 of all grains make up an age population that ( ( ( ( ( ( ( ( ( ( ( ) 2 1 ) 1 1 6 3 2 5 4 1 3 4 4 5 3 6 8 1 ( ( peaks at 2.1 ± 0.2 Ma and another population ± ± ± ± ± ± ± ± ± ± ± ± ± 8 3 8 2 5 6 4 6 7 1 8 5 1 2 7 7 6 6 6 6 5 56 ± 2 (59) 5 5 5 7 5 6 6 6 6 that peaks at 9.7 ± 1 Ma (12%) (Table 2). The remaining grains are distributed over three older age populations that peak at 28 ± 2.5 Ma, 55 ± ) ) ) ) ) ) ) 5 Ma, and 96 ± 10 Ma (Table 2). The combined 9 5 5 4 4 0 5 4 5 4 4 4 5 3 ) ) ( ( ( ( ( ( ( Bering Glacier sample yielded three main age 5 2 5 2 2 8 2 3 4 5 5 ( ( ± ± ± ± ± ± ± populations that peak at 28 ± 3.5 Ma, 44 ± 8 Ma, 9 1 2 1 9 4 5 5 6 3 4 4 41 ± 5 (71) ± 15 (29) 67 4 4 3 4 4 4 and 67 ± 11 Ma, and only 2% of all grains make up a population that peaks at 11.8 ± 3.5 Ma ) ) (Table 2). We also combined the single zircon 7 4 ) ) ) ) ) ) 3 1 ) 5 3 7 2 2 4 ( ( FT ages of four samples from smaller glacial 6 3 4 2 3 6 2 ( ( ( ( ( 5 ( 3 ( . . 6 5 3 3 3 2 1 3 4 catchments located east of the Bering Glacier ± ± ± ± ± ± ± ± ± utions were deconvolved into component age populations using binomial 6 8 2 5 4 2 4 3 3 and found similar peak fi tting results with main ...... 5 5 4 4 4 4 0 2 4 3 3 3 3 3 3 3 3 3 age populations that peak at 24 ± 3 Ma, 34 ± 4 Ma, 46 ± 4 Ma, and 67 ± 6 Ma, and only 3% ) ) ) ) that make up a population that peak at 11 ± 1 0 8 0 ) ) ) ) ) ) 2 2 2 1 4 9 8 3 5 2 ( ( ( ( 3 4 4 3 1 1 3 Ma (3%; Table 2). The combined Tana Glacier ) ) ( ( ( 5 5 ( 5 7 ( ( . . . . 7 7 c glacier systems or bedrock areas. P—peak of modeled age populations (in Ma ± 2 4 1 2 4 2 3 1 2 2 3 2 3 sample yielded three main populations that peak ( ( ± ± ± ± ± ± ± ± ± ± 8 9 4 1 1 4 3 8 1 7 3 1 ...... 2 2 at 18 ± 3 Ma, 25 ± 4 Ma, and 35 ± 5 Ma, and a 2 6 5 8 5 7 4 4 3 4 2 2 2 2 2 2 2 2 2 5% peak at 12 ± 3 Ma (Table 2). Overall, this analysis shows that although the ) ) ) 2 4 ) 5 ) ) ) Seward and Bagley Ice Fields are connected 1 2 4 8 8 4 2 ( ( ( ( 4 2 2 ( ( 8 ( 4 3 0 . . . and cover the same main rock units and struc- . 1 2 1 3 1 2 1 ± ± ± ± ± ± ± ture (Contact fault), there is a strong difference 7 . 4 5 3 2 7 9 ...... 1 8 1 4 7 0 8 in the exhumation signal between the Bagley- 2 1 2 1 1 2 1 Bering-Tana system and the Seward-Malaspina system. The age populations found in Bering ) ) ) ) ) ) ) tting of grouped samples from specifi 9 5 2 2 4 3 ) 2 ) ) ( ( ( ( ( ( Glacier sediment are remarkably similar to the 1 5 4 3 2 3 6 ( ( 5 2 ( 5 ( ...... old Yakutat cover samples (Fig. 2B) and are also 2 2 1 3 2 3 2 3 1 3 locations are shown in text Figure 2. Zircon grain age distrib ± ± ± ± ± ± ± ± ± ±

7 similar to the zircon FT age distribution reported 5 2 2 7 8 . 9 2 7 7 ...... 9 1 3 0 8 9 0 1 4 8 1 1 1 1 1 for consolidated Kultieth Formation and Poul Creek Formation rocks (Table 2) (Meigs et al., 2008; Perry et al., 2009). This result suggests ) ) ) 0 8 4 —number of single grain ages. 3 2 3 that the sampled rivers emanating from the Ber- ( ( ( N 2 2 3 . . . ing Glacier do not transport material from the 0 0 0 ± ± ± Chugach and Prince William terranes that are 1 1 1 . . . 2 2 3 underneath the Bagley Ice Field and north of the Chugach–Saint Elias fault. Only material eroded by the Bering Glacier is transported, in spite of 3 3 1 8 9 4 2 3 5 4 0 2 4 0 7 5 3 0 1 1 3 0 1 0 0 5 0 0 0 0 0 0 0 0 0 0 0 0 NP0P1P2P3P4P5P6P7 4 1 3 1 1 1 1 103 2.6 ± 0.1 (41) 2 1 3 2 1 1 1 1 2 1 1 the ice from the Bagley Ice Field dominantly fl owing into the Bering Glacier. A similar differ- )

2 ence in the thermo chrono logic results between 1 Y

, the Tana Glacier and sample CH44 might be ) ) 1 0 7 1 1 4 expected, as the Tana Glacier also drains the Y Y , H BERING, AND MALASPINA GLACIER SEDIMENTS COMPARED WITH THOSE OF THE CHUGACH TERRANE AND A COMBINED YAKUTAT TERRANE SAMPLE YAKUTAT COMBINED A AND TERRANE THE CHUGACH THOSE OF WITH GLACIER SEDIMENTS COMPARED AND MALASPINA BERING, , 7 * C )

9 Bagley Ice Field. However, the Tana Glacier Y , 4 Y , § 6 TABLE 2. ZIRCON FISSION-TRACK AGE POPULATIONS FROM INDIVIDUAL SAMPLES AND NEW PEAK FITTING RESULTS OF THE COMBINED AGES FROM TA AGES FROM THE COMBINED OF AND NEW PEAK FITTING RESULTS SAMPLES FROM INDIVIDUAL AGE POPULATIONS 2. ZIRCON FISSION-TRACK TABLE 4 , 3 4 shows a similar pattern with zircon FT age pop- H 8 Y * ( H Y * § C ( ( n C d ulations similar to that of CH44, which samples ( o d e e i ssion-track ages are from Enkelmann et al. (2008, 2009). Sample t e n n d i a n a only Chugach terrane rocks north of the Bagley e i b r r n m b i r m e b t m o Ice Field (Table 2; Fig. 2). The U/Pb ages (zir- o o F c m h c t Zircon fi c o h * *

t con crystallization) from Tana Glacier zircons a c a g tting (Brandon, 1992, 1996). Gray shaded bands indicate the results of peak fi t 6 7 e g * † † n i † u a i † † † 4 4 t u 3 l r † † † k n

Ages from Enkelmann et al. (2009). Ages from consolidated samples of the Kultieth and Poul Creek Formations Perry et al. (2009; no error bars given). are also similar to the Chugach terrane sam- h H H u e a A *Ages from Enkelmann et al. (2008). Note: **Ages from consolidated samples of the Kultieth and Poul Creek Formations Meigs et al. (2008). † § a K Poul Creek Formation** 20 Y Kultieth Formation C Glacier Tana Consolidated Yakutat cover samples Yakutat Consolidated C Poul Creek Formation Y8 IB4 IB3 Seward-Malaspina Glacier Yakutat (Kultieth and Poul Creek Formations) Yakutat T Combined Seward-Malaspina (IB1, IB3, IB4) B IB1 Y11 Y Bering Glacier C Y9 Y12 Y7 Y10 Parentheses indicate the size of age population (in percent). peak fi ple (CH44) and do not show any contribution

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from the Prince William terrane (Enkelmann which can be determined simply from the sur- transferred through the subglacial hydrological et al., 2008). Rather, the Tana sediments show face and bed topography (e.g., Gudmundsson, system. Subglacial freezing would tend to choke the typical Eocene–Miocene cooling signal of 2003). With the current glacier surface elevation or close subglacial conduits, thereby constrict- the Chugach terrane that is also found in the and slope, ice under the Bagley Ice Field would ing the fl ow and decreasing the bedload sedi- Kultieth Formation and Paul Creek Formation have to be much thicker than the thickest ice that ment–carrying capacity of the conduits, as this rocks that were sourced by the Chugach rocks has currently been measured. Whereas radar capacity increases nonlinearly with discharge and are now located in the fold-and thrust belt measurements have only revealed maximum (e.g., Alley et al., 1997). This decrease, together (Perry et al., 2009) (samples Y3,7,11,12; Fig. 2). ice thicknesses of just slightly over 1 km (Gim with the relatively low hydraulic potential gradi- et al., 2008), ice at 40 km down the glacier’s ent on the upglacier side of transverse bedrock Glaciology length (Fig. 4B) would need to be 3000 m thick ridges, would be conducive to sediment deposi- for a reverse bed gradient between 40 and 60 km tion (Alley et al., 1998). In the case that con- To integrate the glaciological and detrital to prevent subglacial water fl ow toward the Ber- duits are completely blocked, only sediment that thermochronometric data, we investigate the ing terminus. This analysis suggests that subgla- is incorporated in the ice accreted at the base transport capacity of the subglacial hydrologi- cial water should fl ow from the Bagley Ice Field or entrained higher in the glacier would move cal system of the Bagley-Bering-Tana system. to the Bering Glacier despite overdeepenings, beyond the region of ice freeze-on. Over time, Based on estimates of precipitation and on mea- consistent with estimates of the size of the basin material that is stored under the glacier because surements of proglacial discharge compared to that is required, based on estimates of regional of loss of sediment-carrying capacity would be the size of the basin, meltwater from the entire ice melt rates, to account for the measured pro- poorly represented in the detrital samples col- Bagley-Bering system drains primarily to the glacial discharge (Merrand and Hallet, 1996; lected from rivers at the glacier margin. Gulf of Alaska (Merrand and Hallet, 1996; Jos- Josberger et al., 2006). During surges the subglacial hydrological berger et al., 2006). Discharge from the Tana A downglacier decrease in subglacial hydraulic system transforms considerably. Surges are Glacier is considered negligible, though it is potential is a necessary but not suffi cient con- likely initiated when the system is not able to admittedly not well studied. The lack of sedi- dition for effective sediment transport; other convey the meltwater at the glacier bed, caus- ment derived from under the Bagley Ice Field in requirements arise from thermodynamic con- ing the basal water pressure to generally rise the discharge from the Bering Glacier leads us to straints on the carrying capacity of the hydro- and accelerate sliding. Ultimately, the system examine the sediment transport capacity of the logical network. Because the ice is at the fails, marking the end of the surge, and releases subglacial hydrological system during quiescent pressure melting point, water driven from higher major sediment-laden fl oods (Humphrey and (i.e., non-surge) phases of the glacier. We use the to lower pressure areas can be supercooled due Raymond, 1994; Jaeger and Nittrouer, 1999; known thickness profi le for the glacier to deter- to the melting point dependence on pressure. Fleisher et al., 2003). In front of the Bering Gla- mine whether water can fl ow along the entire With further depressurization, this supercooled cier, the massive discharge of the Seal River length of the glacier, and whether supercooling water can freeze at the base of the glacier. This increased by ~70% during the 1994 fl ood and and refreezing of water might impede water fl ow process and its consequences, with respect to ero- the suspended sediment fl ux increased 8 or 9 and sediment transport under the glacier. sion and sediment transport, have been studied fold (Merrand and Hallet, 1996); this highlights The direction of subglacial water fl ow is extensively (Röthlisberger, 1972; Hooke, 1991; the sensitive dependence of sediment fl ux on dis- determined by the direction of the hydraulic Alley et al., 2003). In general, basal freezing is charge (e.g., Alley et al., 1997). Further support potential gradient (∇Φ) at the glacier bed expected if the bed ascends downglacier more for sediment evacuation from the Bagley-Bering (Röthlis berger, 1972; Shreve, 1972): ∇Φ = steeply than 20%–70% of the glacier surface system largely during glacier surges is evident ρ ∇ ∇ ρ ig zs – zb, where i is the density of ice, g is slope; this percentage depends on the air content in the Bering Trough, where fi ne-grained, high-

gravitational acceleration, zs is the measured of the water (Alley et al., 2003). Such steeply porosity mud deposits, representative of fast

elevation of the glacier surface, and zb is the bed ascending bed slopes occur on the downstream sedimentation, abruptly punctuate coarser and elevation (Shreve, 1972). The expression shows end of overdeepening in the Bagley-Bering sys- bioturbated sand and mud layers (Jaeger et al., that the glacier surface gradient tends to dictate tem: the bed slopes commonly reach >20% and 1998; Jaeger and Nittrouer, 1999). We hypoth- the direction of subglacial water fl ow. At the occasionally exceed 70% of the glacier surface esize that much of the sediment eroded below glacier bed, an impermeable bed with reverse slope (Fig. 4B). Direct evidence for sub glacial the Bagley Ice Field is evacuated during surges. gradient (dipping upglacier) does not necessar- supercooling under the Bering Glacier has The youngest zircon FT age population found in ily preclude water fl ow, as it does subaerially. been reported; it includes frazil ice growth on the Bering Glacier and Tana Glacier sample that However, in cases of relatively steep, opposing sediment traps suspended in Vitus Lake, due peaks ca. 12 Ma, but comprises only <5% of the bed surfaces, at least 10 times steeper than the presumably to freezing of supercooled water entire sample, may highlight the cooling signal ice surface, subglacial water fl ow and sediment emanating from the glacier terminus in the lake from underneath the Bagley Ice Field, which transport would be blocked. (Fleisher et al., 1998). is not transported effi ciently during quiescent Using the defi nition of hydraulic potential periods. This hypothesis could be tested through and the known ice thickness profi le (Fig. 4B), DISCUSSION a study of sediments deposited by such fl oods we calculate that the subglacial hydraulic poten- proglacially and offshore. tial is suffi cient to drive subglacial water fl ow Transport Capacity and Surges of the downglacier, despite overdeepenings under the Bagley-Bering Glacier Rapid Erosion Underneath the Bagley Ice Field (Fig. 4B). As the glacier sur- Seward-Malaspina Glacier System face refl ects the dynamically induced stresses The effects of subglacial supercooling are within the moving glacier over an uneven bed, likely to infl uence sediment transport under the The large age population peak ca. 2 Ma found we can reasonably approximate the basal pres- Bering Glacier. Supercooling and subsequent in the Malaspina sediment is exceptional within sure as the weight of overlying ice per unit area, freezing could limit the amount of sediment the orogen and indicates very fast erosion in

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the catchment, particularly north of the Seward of rapid exhumation and long-term, focused 2010). The effect of crustal strain on making Throat under the sluggish Seward Ice Field, erosion, inferred from bedrock apatite U-Th/He the bedrock more erodible can focus erosion, where ice surface velocities do not exceed 100 ages, corresponds in a rough sense to the region crustal deformation, and rock uplift into narrow m/yr (Ford et al., 2003) (Fig. 3). Material from between the modern and LGM ELA (Berger regions, contributing to the interplay between around the accumulation area of the glacier and Spotila, 2008; Berger et al., 2008b). This tectonics, erosion, and topography (e.g., Zeitler found at the terminus demonstrates that, unlike was consistent with the common assumption et al., 2001; Koons et al., 2011). for the Bering Glacier, material is successfully that the glacial erosion rates tend to be high- evacuated by the subglacial drainage system est near the regional ELA. The ELA is where Integration of Thermochronometry despite the existence of a number of overdeep- the long-term accumulation and ablation of ice and Glaciology enings under the Seward Throat (Sharp, 1958; are balanced, and therefore where the ice fl ux Muskett, 2007; Headley et al., 2012). is maximized (e.g., Andrews, 1972; Brozović On an orogen scale, the Saint Elias Moun- The Seward Throat (Fig. 3) seems an ideal et al., 1997; Egholm et al., 2009). However, we tains have been showcased as an archetype of location to consider the impact of sliding velocity suggest that a direct connection between the climatic infl uence on tectonics (Spotila et al., on erosion rate, as the surface velocity increases region of maximum erosion rate and the ELA is 2004; Berger et al., 2008b; Whipple, 2009). by more than an order of magnitude as ice funnels unlikely due to a large tectonic infl uence. There is no doubt that climate affects glaciers, from the Seward Ice Field through the narrow Defi ning the location of the ELA in the Saint and that glaciers are effi cient at eroding and gap in the range (Fig. 3). The erosion rate is gen- Elias Mountains is not a trivial task. First, the evacuating sediments from the range. However erally expected to scale with basal sliding speed current ELA is located high up in the mountain compelling, using simplifi ed stand-ins such as (Hallet, 1979, 1996; Humphrey and Raymond, range, with estimated altitudes ranging from the ELA or the sliding velocity for the many 1994). Numerical analysis of glacier dynamics ~1000 m to 1100 m for the Seward-Malaspina complicated processes related to glacial ero- has shown that sliding accounts for the majority (Meier et al., 1971; Péwé, 1975) and the Bagley- sion appears to not always capture the full rock of the ice motion through this fast-moving reach Bering systems (Péwé, 1975; Tangborn, 2002; exhumation hisstory of this region, especially (Headley et al., 2012). Erosion in the Seward Berger and Spotila, 2008). Throughout much of when considering the changing tectonic and Throat might be expected for many reasons. The the Quaternary, however, the ELA was ~300– climatic histories. This study helps build a basis exceptional convergence of ice funneling through 600 m lower (Péwé, 1975), when glaciers were for further and more detailed analysis of the this region accounts for extremely rapid fl ow, also much more extensive. The ELA likely spatial and temporal patterns of glacial erosion fast sliding (exceeding 800 m/yr in places), and intersected the ice surface tens of kilometers in conjunction with sedimentation and erosion by inference, fast erosion (Headley et al., 2012). south of the current shoreline due to both the on the continental shelf over the Quaternary. The Seward Throat also traverses the Malaspina shallow surface gradients and the substantial By combining existing thermochronologic data fault, which is the northeastern continuation thickness of these massive glaciers, although into bulk samples representing single glaciers, of the Pamplona fault zone; the active defor- the ice thickness history is not established. In we have a snapshot of from where and how gla- mational front of the Yakutat–North American this region, the location of the ELA likely swept cially eroded bedrock has been transported. By subduction boundary (Figs. 1 and 2) (Plafker through a broad region during the Quaternary as integrating these results with glaciological evi- et al., 1994; Bruhn et al., 2004; Worthington et al., the glaciers expanded over 100 km onto the con- dence for patterns of erosion rate and sediment 2008), where many active faults converge (Chap- tinental shelf during recurring periods of cooler transport, we can make interpretations of how man et al., 2008). GPS measurements show the climate and lower sea level (Mann and Hamil- larger scale patterns of exhumation and glaciers highest rates of crustal convergence across this ton, 1995) (Fig. 1). interact. fault zone (Elliott et al., 2008). However, we A reinterpretation of the apatite U-Th/He lack supporting evidence for rapid exhumation ages in the fold-and-thrust belt suggests that a CONCLUSIONS from the detrital thermo chronom etry. High rates simple correspondence between young cooling of exhumation are not apparent from the detrital ages and the region of modern and past ELAs Our integration of glaciological and thermo- thermochronometry because the zircons of the is perhaps too simple: the zone of rapid exhu- chronometric data over the Saint Elias Moun- underlying rocks (belonging to the Yakutat ter- mation was found to roughly parallel the active tains is a powerful tool to reveal new insights rane) are not reset. Sediment strata have not been northeast-striking thrust fault systems (Enkel- on how glacial erosion shapes orogens over geo- heated high enough to erase entirely the cooling mann et al., 2010) (Fig. 2A). In light of this logical time scales. This study emphasizes that, signal from their source area, before they were recent work, we stress that the highest erosion even for glaciers that occupy the same climatic exhumed again. rates, inferred from the highest cooling rates, and orogenic setting, the patterns of erosion In contrast, the young zircon grains originating and deepest exhumation in the Saint Elias oro- rates and sediment evacuation are infl uenced from Chugach terrane rocks under the Seward gen occur at the tectonic corner of the Yakutat both by glacier-specifi c factors (basin architec- Ice Field indicate an unexpected region of rapid indenter instead of a focused band related to ture, glacial dynamics, subglacial hydrology) exhumation in an area with low surface veloci- the ELA position (Fig. 2). Generally, a zone of and local tectonics. Therefore, a detailed con- ties and where rapid exhumation is not expressed rapid erosion would tend to persist and account sideration of both glaciology and tectonics, on by the few bedrock ages that have been reported for the deep exhumation only if it is offset by scales ranging from the thickness of glaciers to from this area (O’Sullivan and Currie, 1996; rock uplift, otherwise the erosion by the glacier the width of the orogen, is necessary as a basis Enkelmann et al., 2010). Particularly, the 10 Ma would carve deep basins that would eventually for understanding the spatial patterns of glacial zircon U-Th/He age from the Seward Ice Field cease to be eroded (e.g., Alley et al., 2003). erosion and rock exhumation. nunatak suggests large gradients in exhumation Moreover, a region where faulting continu- The Seward Ice Field is a locus of rapid rock above and beneath the glacier. ously strains, weakens, and fractures the rock exhumation even though ice velocities are mod- A major fi nding reported in recent studies in may be an optimal setting for effi cient glacial est. This suggests that the underlying bedrock is this region is that a distinct orogen-parallel band erosion (Laitakari et al., 1985; Dühnforth et al., particularly erodible not because the lithologies

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