Plate Margin Deformation and Active Tectonics Along the Northern Edge of the Yakutat Terrane in the Saint Elias Orogen, Alaska, and Yukon, Canada

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

Plate margin deformation and active tectonics along the northern edge of the Yakutat Terrane in the Saint Elias Orogen, Alaska, and Yukon, Canada

Ronald L. Bruhn1,*, Jeanne Sauber2, Michelle M. Cotton1, Terry L. Pavlis3, Evan Burgess4, Natalia Ruppert5, and Richard R. Forster4 1Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112, USA 2National Aeronautics and Space Administration, Goddard Space Flight Center, Greenbelt, Maryland 20771, USA 3Department of Geological Sciences, University of Texas, El Paso, Texas 79968, USA 4Department of Geography, University of Utah, Salt Lake City, Utah 84112, USA 5Alaska Earthquake Information Center, Geophysical Institute, University of Alaska, Fairbanks, Alaska 99775, USA

ABSTRACT generated large historic earthquakes, and is Canada provide a classic locality to study locally marked by seismicity. relationships between glaciation, tectonics, Structural syntaxes, tectonic aneurysms, and landscape evolution (Fig. 1; Worthington and fault-bounded fore-arc slivers are impor- INTRODUCTION et al., 2010; Enkelmann et al., 2010; Berger tant tectonic elements of orogenic belts world- and Spotila, 2008; Meigs et al., 2008; Jaeger wide. In this study we used high-resolution The Saint Elias and eastern Chugach et al., 2001; Meigs and Sauber, 2000). Gla- topography, geodetic imaging, seismic, and Mountains of Alaska, USA, and the Yukon, ciers mask the structural geology where they geologic data to advance understanding of how these features evolved during accretion of the Yakutat Terrane to North America. Because glaciers extend over much of the oro- U.S. Canada gen, the topography and dynamics of the gla- North American ciers were analyzed to infer the location and Dena Plate 61 N nature of faults and shear zones that lie bur- li ied beneath the ice. The Fairweather trans- Southern Alaska fault system and Elias blocks form fault system terminates by oblique- Castle Mtn & extensional splay faulting within a structural Bruin Bay faults Kenai CCSEFSEF N F syntaxis, where thrust faulting and contrac- a Peninsula iirw rw tional strain drive rapid tectonic uplift and e aath DFYakutat th rock exhumation beneath the upper Seward ere microplate r Transition fault - Glacier. West of the syntaxis, oblique plate Q u N convergence created a dextral shear zone e e n beneath the Bagley Ice Valley that may have ~55 mm/yr Is. C h been reactivated by reverse faulting when a Kodiak rrl l the subduction megathrust stepped eastward o ttte N t during the last 5–6 Ma. The Bagley fault e N fa Aleutian megathrust fault uul zone dips steeply through the upper plate to Pacific Plate lt intersect the subduction megathrust at depth, forming a fault-bounded crustal sliver capa- W ble of partitioning oblique convergence into W W strike-slip and thrust motion. Since ca. 20 Ma W the Bagley fault accommodated more than 50 Figure 1. Plate tectonic setting of southern Alaska and offshore Gulf of Alaska. The km of dextral shearing and several kilometers Yakutat microplate is labeled and shaded in tan. CSEF—Chugach Saint Elias fault: the of reverse motion along its southern ﬂ ank dur- suture between rocks of the Yakutat microplate or “terrane” and southern Alaska. DF— ing terrane accretion. The fault is considered Deformation front: the hypothesized easternmost limit of subduction of the basement or capable of generating earthquakes because crystalline rocks of the Yakutat microplate. Major plate boundaries in addition to CSEF it is suitably oriented for reactivation in the and DF are the Fairweather–Queen Charlotte fault, the Transition fault, and the Aleu- contemporary stress ﬁ eld, links to faults that tian megathrust. The region of Figure 2 is outlined in the red dashed rectangle.

*E-mail: [email protected]

Geosphere; December 2012; v. 8; no. 6; p. 1384–1407; doi:10.1130/GES00807.1; 16 ﬁ gures; 2 tables; 4 supplemental ﬁ gures. Received 27 March 2012 ♦ Revision received 5 September 2012 ♦ Accepted 13 September 2012 ♦ Published online 16 November 2012

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fl ow over folds and faults that form the tec- TECTONIC SETTING OF ically accreted rocks of the Yakutat terrane and tonic framework of the Saint Elias orogen THE YAKUTAT BLOCK the overlying metamorphic and igneous rocks (Bruhn et al., 2004, 2010), erode and transfer of the Early Tertiary plate margin of southern large volumes of rock detritus between the The Yakutat microplate is colliding into Alaska (Fig. 2; Plafker, 1987). The Bagley fault mountains and offshore realm (Hallet et al., southern Alaska at a rate of ~43–50 mm/yr (Fig. cuts through the upper plate creating a narrow 1996; Jaeger et al., 2001), and modulate the 1; Plafker, 1987; Plafker et al., 1994; Sauber et sliver of crust that is bounded to the south by tectonic stress fi eld by creating transient loads al., 1997; Elliott et al., 2010). The microplate the Chugach–Saint Elias fault and to the north on the lithosphere (Sauber et al., 2000; Sauber is a fragment of an oceanic plateau with thick by the Bagley fault. Tectonic accretion of the and Molnia, 2004; Doser et al., 2007; Sauber basaltic crust that is structurally overlain partly Yakutat terrane together with southward and and Ruppert, 2008). Regional faults that are by Cretaceous fl ysch and mélange, and blan- eastward propagation of the subduction décol- mostly buried by glaciers include the Fair- keted by Tertiary and Quaternary strata (Christe- lement created the wide foreland fold and thrust weather fault, the Malaspina and Bering gla- son et al., 2010; Worthington et al., 2012). The belt within this central segment of the orogen, cier faults, and the Bagley fault. The Bagley tectonically off-scraped and deformed rocks as well as the offshore folds and thrusts of the fault cuts through the spine of the main range of the microplate form the “Yakutat Terrane” Pamplona zone (Plafker, 1987; Bruhn et al., of the Saint Elias Mountains, where it lies within the Saint Elias orogen, while the basaltic 2004; Chapman et al., 2008; Wallace, 2008; beneath the Bagley Ice Valley, which is one crust and mantle of the microplate is subducted Worthington et al., 2010; Pavlis et al., 2012). of the most spectacular geomorphic features beneath the North American Plate margin. Structures curve southwestward toward the of the orogen (Fig. 2). The Bagley Ice Valley The rise of the Saint Elias orogen overlapped Aleutian Trench in the westernmost part of the was previously mapped as part of the Bagley in time with the onset of glaciation, resulting in Saint Elias orogen (Figs. 1 and 2) resulting in Ice Field. However, the U.S. Board of Geo- deposition of the coarse-grained glacial till and complex refolding and faulting that affects both graphic Names offi cially changed the name to glacial marine deposits of the late Miocene to the tectonic sliver of the upper plate and the Bagley Ice Valley in 1997. Quaternary Yakataga Formation (Eyles et al., Although the Saint Elias orogen is cited as a 1991), much of which is uplifted and deformed classic example of where climatic conditions by faulting and folding (Plafker, 1987). Offshore have strongly affected tectonics (e.g., Spotila in the Gulf of Alaska, the Yakutat microplate Figure 2. Fault map of the Saint Elias oro- et al., 2004; Berger et al., 2008a, 2008b), it abuts the Pacifi c Plate along the Transition fault, gen superimposed on a MODIS (Moderate is also an important example for the study a prominent submarine escarpment created by Resolution Imaging Spectroradiometer) of complex deformation and tectonic exhu- transform motion between the tectonic plates image background. See dashed red rect- mation within a plate boundary syntaxis and (Bruns, 1983; Gulick et al., 2007). Subduction angle in Figure 1 for location. Faults with oblique collision zone (Plafker, 1987; Bruhn of the Yakutat lithosphere beneath southern incontrovertible evidence for Late Pleisto- et al., 2004; Berger et al., 2008a, 2008b; Alaska occurs along a gently dipping megath- cene and younger displacement are shown Berger and Spotila, 2008; Koons et al., 2010) rust with profound and far-reaching effects on in red. Those faults that are suspected to where transform motion of the Yakutat micro- the tectonics and landscape of interior Alaska have been active, or at least partially reac- plate relative to the North American Plate (Ferris et al., 2003; Eberhart-Phillips et al., tivated, during the same time period are transitions from the Fairweather fault–Queen 2006; Bruhn and Haeussler, 2006; Haeussler, shown in purple. The orogen is divided into Charlotte transform system to tectonic accre- 2008; Abers, 2008; Benowitz et al., 2011). three segments, an eastern segment marked tion and subduction to the west. That is, the The arcuate geometry of the plate margin by the Fairweather transform fault and Yakutat microplate provides a contemporary in southern Alaska together with the NNW- coastal mountains thrust and fold belt, a example of terrane accretion that is a common directed relative motion causes a marked change central segment containing the Chugach– process during mountain building, while also in the obliquity of convergence within the Saint Saint Elias fault suture and broad foreland providing insight into the manner in which Elias orogen. Deformation in the eastern part is fold and thrust belt, and a western seg- structural syntaxes, tectonic aneurysms, and dominated by dextral strike-slip faulting along ment where the Yakutat Terrane is molded plate boundary strain partitioning evolve both the Fairweather fault and by crustal contrac- into the syntaxis of southern Alaska at the in space and time. In this study, we focus tion in a narrow coastal thrust belt that is devel- northeastern end of the Aleutian megath- on the system of faults that accommodate oped within the edge of the Yakutat microplate rust. Onshore faults are located primarily deformation within the orogen, refi ning and (Plafker, 1987; Bruhn et al., 2004). Peaks of from mapping by Plafker (1987), Bruhn et extending work on the structural geology the Fairweather Range tower above the eastern al. (2004), Chapman et al. (2008), and Chap- (Plafker, 1987; Bruhn et al., 2004; Chapman side of the Fairweather fault refl ecting slow and man et al. (2011). Offshore structures are et al., 2008, 2011, 2012; Wallace, 2008; Pav- diffuse deformation that extends far into the located from marine geophysical surveying lis et al., 2012), while integrating recently continental interior to the east and north of the reported by Worthington et al. (2008, 2012). published work on the thermochronology transform fault boundary (Mazzotti et al., 2003; The Chugach–Saint Elias fault (brown line) (Berger et al., 2008a, 2008b; Enkelmann et Elliott et al., 2010). The plate boundary bends is the original suture between the Yakutat al., 2008; Enkelmann et al., 2009; Enkelmann abruptly westward at the northern end of the Terrane and North America. The upper et al., 2010) and geodynamics (Koons et al., Fairweather fault creating a structural syntaxis part of the fault may now be abandoned 2010). In doing so, we also demonstrate how that is a locus of rapid tectonic uplift and exhu- because thrust faulting migrated southward modern remote sensing and geodetic data may mation (Plafker, 1987; Bruhn et al., 2004; Spo- over time into the foreland fold and thrust be used to document the surface morphology tila and Berger, 2010; Enkelmann et al., 2008, belt. Fault numbers are as given in Tables 1 and dynamics of large glaciers, which provide 2009; Koons et al., 2010; Chapman et al., 2012). and 2. The light yellow areas marked Bark- insight into the topography and structural West of this syntaxis, the Chugach–Saint Elias ley Ridge and Steller Ridge are prominent geology at the base of the ice. fault is the north-dipping suture between tecton- geomorphic features noted in the text.

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TABLE 1. FAULTS WITH KNOWN QUATERNARY OR HISTORICAL DISPLACEMENT Fault Rationale for displacement interpretations Reference 1. Fairweather transform fault M 7.9 earthquake, 1958 Doser (2010) 2. Fairweather boundary fault M 8.1 on September 10, 1899 (coast uplift) Plafker and Thatcher (2008) 3a: Yakutat thrust fault M 8.1 on September 10, 1899 (coast uplift) Plafker and Thatcher (2008) 3b: Olomoi thrust fault M 8.1 on September 10, 1899 (coastal Plafker and Thatcher (2008) deformation) 4: Esker and Bancas thrust fault(s) M 8.1 on September 10, 1899 (coastal uplift), Plafker and Thatcher (2008); Cotton (2011) dextral offset subglacier drainage valley 5: Malaspina foreland fault Geodetic modeling, uplift of a beach berm ca. 1899 Elliott (2011); R.L Bruhn and I. Shennan (personal (tentative correlation) commun.)

6: Malaspina fault Geodetic measurements and aftershocks to Mw 7.4 Savage and Lisowski (1986); Estabrook et al. Saint Elias earthquake (1992); Elliott (2011) 7: Pamplona zone thrust faults Earthquakes up to M 6.1 8: Cape Suckling–Bering Glacier fault Coastal uplift M 9.2 earthquake, 1964 Plafker (1969); Chapman et al. (2012) 9: Kayak Island (Structural link to Bering Glacier fault) Chapman et al. (2012) 10: Ragged Mountain Normal (a) and thrust faulting (b) (a) Tysdal et al. (1976) (b) Bruhn et al. (2004)

previously deformed rocks of the Yakutat ter- at the Martin River Glacier to the west. The micity is sparsely distributed along the length rane (Bruhn et al., 2004). The accreted terrain Bagley fault may continue farther westward of the fault with events clustering beneath the is partly indented into the plate margin, but it beneath the Miles Glacier as far as the Copper western Bagley Ice Valley (Doser et al., 2007) is also escaping toward the southwest over the River. Campbell et al. (1986) proposed that the and surrounding the Quintino Sella and Jeffer- northeastern part of the Aleutian megathrust fault dips steeply to depths of 5–6 km based ies glaciers. Seismicity is scattered beneath the (Bruhn et al., 2004; Pavlis et al., 2004; Elliott, on modeling of the regional gravity fi eld. This Seward Glacier at the eastern end of the Bagley 2011). Geological evidence for active deforma- structural confi guration creates a narrow faultfault, but is clustered to the south beneath the tion includes sinistral faulting on oblique-slip bounded sliver in the Alaskan plate margin that Malaspina Glacier and foreland fold and thrust faults (Pavlis and Bruhn, 2011; McCalpin et al., may partition displacements between thrust and belt (Fig. 4; Ruppert, 2008; Sauber and Ruppert, 2011), thrust faulting (Bruhn et al., 2010; Chap- strike-slip motion (Bruhn et al., 2004; Haq and 2008; Doser et al., 1997). man et al., 2012), and structural reactivation of Davis, 2010). Information concerning the structural geol- a deformed remnant of the Chugach–Saint Elias Rugged mountains and linear glacier-fi lled ogy of the Bagley fault is sparse and mostly suture (Fig. 2; Bruhn et al., 2004). Antislope valleys dominate the landscape surrounding circumstantial. The fault is buried beneath gla- fault scarps and landslides in mountain blocks the Bagley fault (Figs. 2 and 3). Five-thousand ciers, the surrounding bedrock is older than the attest to the instability of the steep and recently meter peaks including Mount Logan (5959 m) collision of the Yakutat microplate, and there are deglaciated slopes (e.g., Li et al., 2010; McCal- and Mount Saint Elias (5489 m) tower above few Quaternary deposits to study for evidence pin et al., 2011). the Seward Glacier basin and the eastern end of recent rupturing (Campbell and Dodds, 1982; of the Bagley Ice Valley. The Barkley and Wax- Bruhn et al., 2004; Richter et al., 2005). The ROLE OF THE BAGLEY FAULT— ell Ridges form steep north-facing mountain Bagley fault was previously considered part of HISTORY AND PREVIOUS WORK walls along the southern side of the ice valley, the Early Tertiary Contact fault plate boundary and Mount Tom White (3013 m) dominates because it juxtaposes rocks of different tectonic The Bagley fault is buried beneath the ice- the skyline of the eastern Chugach Mountains terranes that accreted to the plate margin before fi lled troughs and basins of the Seward Glacier, where the Bagley fault enters a contractional arrival of the Yakutat Terrane. The cessation the Bagley Ice Valley, and Martin River Glacier fault bend (Fig. 2). A topographic saddle at the of movement on the Bagley fault portion of (Figs. 2 and 3). The fault is bounded below by confl uence of the eastern and western arms of the Contact fault was cited as ca. 50 Ma based the easternmost Yakutat portion of the subduc- the Bagley Ice Valley marks the intersection of upon the age of a little deformed granitic pluton tion megathrust, links to the Fairweather fault the Bagley fault with one or more faults that lie within the fault zone near the head of the Miles beneath the upper Seward Glacier in the east, beneath the Bering and Tana glaciers (Bruhn et Glacier (Fig. 2; Plafker and Lamphere, 1974; and intersects the WSW-trending fault system al., 2004, 2010; Spotila and Berger, 2010). Seis- Plafker, 1987). However, recently acquired

TABLE 2. FAULTS THAT MAY HAVE QUATERNARY DISPLACEMENT (PURPLE COLOR) Fault Rationale for displacement interpretations Reference 1: Art Lewis Glacier fault Link to Fairweather fault G. Plafker (2009, personal commun.) 2: Chaix Hills fault (western part) May link to Fairweather boundary fault Geological mapping (Richter et al., 2005) 3: Cascade Glacier fault Located in contractional lobe of the terminus of Plafker and Thatcher (2008) Fairweather fault 4: Sullivan fault (thrust?) Marked by uphill facing scarp on mountainside of R.L. Bruhn (2011, personal observ) Sullivan anticline 5: Miller Creek thrust fault Mountain front and stream channel Chapman et al. (2012) geomorphology 6: Leeper fault (thrust) Geodetic measurements Elliott (2011) 7: Bagley fault South ﬂ ank geomorphology, themochronology This study; Berger and Spotila (2008); Enkelmann et al. (2009)

Geosphere, December 2012 1387

Downloaded from https://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/8/6/1384/3346159/1384.pdf by guest on 15 May 2019 Bruhn et al. THERMOCHRONOLGY 1.6 - 1.8 Ma* Exhumation age or age range for apatite He method.

** Exhumation age for apatite fission track method A Mt. Logan North Ridge Profile Early Event: 20 Ma to 10 Ma** Mt .Tom White Latest Event: 5 Ma to 2 Ma**

1.66 Ma* Tana Glacier 5 - 10 Ma * 3 - 4 Ma* 10 - 20** 6 - 10 Ma* outlet Elevation (m) Elevation Miles Glacier Western Seward Glacier Valerie Glacier Bagley Ice Valley Eastern Bagley Ice Valley Basin trough 300 Km

B South Ridge Profile Mt. Saint Elias Steller Glacier ≤1 Ma* Seward Glacier outlet Bering Glacier Mt. Miller Outlet ≤ 1.6 - 1.8 Ma* outlet ≤1 Ma* 1-2 Ma 1 Ma* ≈ ≈2 Ma* 2 Ma* Elevation (m) Elevation Miles Glacier Martin Western Seward Glacier Valerie Glacier Eastern Bagley Ice Valley River Glacier Bagley Ice Valley Basin trough Figure 3. Topographic proﬁ les of the northern (A) and southern (B) mountain crests bordering the Bagley fault along the length of the Saint Elias and Eastern Chugach Mountains. Rock exhumation ages are marked in red and based on published results by Berger and Spotila (2008), Spotila and Berger (2010), Enkelmann et al. (2009, 2010), and O’Sullivan and Currie (1996). The blue line marks the surface elevation along the centerline of the glaciers. Notice that the rock exhumation ages are older along the northern side of the Bagley Ice Valley than along the southern side, and that this pattern also applies to the region of the Miles Glacier west of Mount Tom White. See Figure 2 for locations of major geographic features. Elevation data are from the ASTER GDEM global elevation model.

145 W 144 W 143 W 142 W 141 W 140 W 139 W

r e iv R r MilesMiles GGlacierlacier e r p ive p n R o rti 60 30 N MartinMa River BagleyBag Ice Valley 60 30 N CopperC River ier ley Ice UpperUpper SewardSeward lac Valley GlacierG GlacierGlacier er ArtA Lewis Glacier ci r la t G ForelandForeland FoldFold L ing e er w BeringB Glacier andand ThrustThrust BeltBelt is G la c 60 00 N ie 60 00 N r Gulf of Alaska MalaspinaMalaspina GlacierGlacier 145 W 144 W 143 W 142 W 141 W 140 W 139 W

M=5 M=4 M=3 M=2

Figure 4. Earthquake epicenters (M > 1.5) and ﬁ rst motion focal mechanisms (M > 2.5) from Alaska Earthquake Informa- tion Center earthquake catalog (July 2005–June 2011, depth < 30 km). In the STEEP seismograph station network core area, best-located events (M ≥ 2) have errors on the order of 2–3 km or less. There are two major regions with high background seismicity; one in the Icy Bay/Malaspina region and the other west of Bering Glacier that have been observed over the last several decades, even prior to the 1979 Saint Elias earthquake. Double couple focal mechanisms contain two possible fault planes oriented normal to one another, of which one is the actual fault. Keeping this ambiguity in mind, there are mechanisms consistent with dextral shearing along or near the Art Lewis Glacier and Fairweather faults, at the eastern end of the Bagley fault in the upper Seward Glacier basin, along the western part of the Bagley fault, including one event near the intersection of the Bagley and Martin faults, and at the southwestern end of the Martin River fault. Focal mechanisms in the foreland of the orogen indicate a mixture of thrust and strike-slip fault motion, with only a few normal faulting events.

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thermochronology data reveal that the pluton 2002), Shuttle Radar Topographic Mission our goal of evaluating the role of the Bagley was uplifted and exhumed by several kilome- (SRTM; Farr and Kobrick, 2000; Rodriguez et fault in regional tectonics. The eastern end of the ters within the last 5 Ma to 10 Ma, during col- al., 2005; Muskett et al., 2003, 2008, 2009), and fault must interact with the Fairweather strike- lision and accretion of the Yakutat terrane (Fig. the Advanced Spaceborne Thermal Emission slip fault that extends into the spine of the Saint 3; Berger et al., 2008b). Farther east the moun- and Reﬂ ectance Spectrometer (ASTER) Global Elias Mountains, and also with the underlying tains along the southern side of the Bagley fault Digital Elevation Model (GDEM; ASTER thrust faults that drive uplift of the range. were also uplifted and exhumed during the last GDEM Validation Team, 2009). Elevation post- The syntaxis begins at a 10° counterclock- several million years, possibly by reverse faulting is 30 m for the ASTER GDEM and SRTM wise bend in structural and topographic grain ing (Chapman et al., 2008; Berger and Spotila, data. A 10 m posted digital elevation model at Disenchantment Bay and culminates at the 2008; Spotila and Berger, 2010; Enkelmann et over the eastern Bagley Ice Valley and western terminus of the Fairweather fault, where the al., 2010). part of the upper Seward Glacier by Intermap mountains rotate an additional 30° toward Geodetic data in the region of Figure 2 Technologies was the highest resolution DEM the west into alignment with the Bagley and include trilateration (Savage and Lisowski, (Muskett et al., 2009), and it is used to inves- Chugach–Saint Elias faults (Fig. 5). The low- 1986, 1988), VLBI (very long baseline inter- tigate the surface topography of the eastern lying foreland beneath the Malaspina Glacier ferometry), and GPS campaign-style measure- Bagley Ice Valley. is thrust obliquely beneath the mountains along ments (Sauber and Molnia, 2004) obtained Ice surface velocity was measured with offset the Bancas–Esker Creek and Malaspina faults between 1979 and 2000, and a regional net- tracking methods using both optical and syn- in this region (Bruhn et al., 2004; Plafker and work of GPS campaign-style measurements thetic aperture radar (SAR) data. Optical fea- Thatcher, 2008; Chapman et al., 2012) and the completed during the Saint Elias Erosion and ture tracking was performed using normalized Fairweather fault terminates by splaying beneath Tectonics project in the period 2005–2009 image cross correlation (COSI-CORR) soft- the upper Seward Glacier (Fig. 5; Ford et al., (Elliott et al., 2007; Elliott, 2011). The results ware (Leprince et al., 2007) on pairs of Landsat 2003). Most, if not all, of these faults ruptured of the most recent study differ from earlier images acquired over intervals of 1 month to during large to great magnitude earthquakes in ones that postulated dextral shearing along or several years (Cotton, 2011). SAR offset feature the past 110 yr but the evidence is limited in surrounding the Bagley fault in the Bagley Ice tracking was performed using normalized cross some cases by the lack of instrumental records. Valley (Savage and Lisowski, 1986; Sauber and correlation (GAMMA Software [Strozzi et al., Thrust faults formed parts of the seismic source

Molnia, 2004). Elliott’s (2011) tectonic block 2002]) on RADARSAT-1 Fine Beam, ALOS zones of two Mw 8.1 earthquakes in 1899 model of the orogen requires no contemporary PALSAR Fine Beam, and ERS1/2 data over (Plafker and Thatcher, 2008). Rupturing along motion along the Bagley fault within the lim- intervals of 24–45 days (Burgess et al., 2012). the length of the Fairweather fault in 1958 ter- its of resolution of several mm/yr, but it does Optical imagery from the KH-9 satellite, minated to the north beneath the upper Seward

place the northeastern boundary of a small Ber- Landsat 7 and 5, and ASTER data were used to Glacier, creating an Mw 7.9 earthquake (Tocher, ing Glacier tectonic block near the intersection map rock type and structure in selected areas, as 1960; McCann et al., 1980; Doser, 2010). The

between the Bering Glacier and Bagley fault. well as to map spatial patterns of crevasses and Mw 7.4 Saint Elias earthquake in 1979 initiated The Bering Glacier block is moving southwest folds on glaciers. Ice and rock structures were beneath the mountains north of the Bagley fault relative to other parts of the orogen and relative visualized by draping optical imagery over digi- and ruptured up to the south and laterally toward to the newly deﬁ ned Elias tectonic block that is tal elevation models. Orientations of tectonic the east before arresting along the northern side located to the north and west of the Saint Elias faults and folds that project beneath glaciers of the Bagley fault and near the terminus of the Mountains. Although the conclusions of the were determined by three-point and linear least- Fairweather fault (Estabrook et al., 1992). various geodetic studies differ with respect to squared calculations to determine strike and dip Faults in Disenchantment Bay and Russell strike-slip motion on the Bagley fault, all iden- where ﬁ eld measurements were not available. Fiord form a complex structural system (Fig. tify active deformation near the intersection of 5). The Fairweather fault is the primary plate the Bering Glacier and Bagley faults. GLACIOLOGY AND STRUCTURE OF boundary fault that accommodates much of the BAGLEY FAULT dextral relative plate motion. The Fairweather RESEARCH DATA AND PROCEDURES boundary fault is a subsidiary structure that par- The Eastern Syntaxis–Fairweather and allels the Fairweather fault. The southeastern Our research goal is to infer the role of the Bagley Fault Interaction part of the Fairweather boundary fault outcrops Bagley fault in accommodating deformation in a stream cut where the fault dips steeply during collision and accretion of the Yakutat The topography and structure of the orogen and slicken-lines indicate dextral shearing. terrane. Given that glaciers blanket much of rises abruptly where the plate boundary bends The presence of the fault beneath the northern the landscape, we rely primarily upon remote from a regional strike of ~319° along the Fair- side of Russell Fiord is indicated by an abrupt sensing techniques that augment available data weather fault to roughly east–west (270°) paral- increase in vertical uplift along the northeast- from published structural (e.g., Plafker, 1987; lel to the Chugach–Saint Elias and Bagley faults ern edge of the Fiord that occurred during the

Bruhn et al., 2004; Chapman et al., 2008) and (Fig. 2). This bend or “syntaxis” increases the Mw 8.1 earthquake of September 10, 1899 (Tarr thermochronology studies (O’Sullivan and Cur- ratio of convergence to strike-slip motion within and Martin, 1912; Bruhn et al., 2004; Plafker rie, 1996; Berger and Spotila, 2008; Berger et the central part of the Saint Elias orogen rela- and Thatcher, 2008). The Yakutat and Bancas– al., 2008a, 2008b; Enkelmann et al., 2009, 2010; tive to that along the Fairweather fault (e.g., see Esker Creek faults do not crop out, but are Spotila and Berger, 2010; Meigs et al., 2008). also ﬁ g. 7 of Sauber et al., 1993). The resulting inferred from dislocation modeling of shoreline Elevation data for analysis of the topography increase in transpressional strain drives uplift uplift during the September 10, 1899 earth- was obtained from the Ice, Sea and Land Satel- of the mountains creating some of the highest quake (Plafker and Thatcher; 2008). lite (ICESat; Schutz, 2001; Schutz et al., 2005), peaks in North America. Understanding the Bruhn et al. (2004) considered the Yakutat Airborne Terrain Mapper (ATM; Krabill et al., structural geology of the syntaxis is crucial to fault as a part of an asymmetrical or “one-sided”

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Mt. Saint Elias Chugach - St. Elias (5489 m) Thrust Fault Cascade Glacier Bagley Fault Thrust North Malaspina or Chaix Hills Fairweather terminus ‘Range Front’ Thrust Fault Thrust Upper Seward Glacier splay fault “Pop-up” in restraining fault bend Mt. Vancouver LSG (5090 m)

Malaspina Glacier

‘Bancas -Esker Creek ‘

i Dextral-ObliqueThrust’ c

a (concealed) l G

Yakutat ale V Bay BP Disenchantment Bay Pv’ 43 mm/aYakutat Fault Hubbard Glacier

Yakutat Thrust Fault Russell Fiord Hubbard Glacier Thrust Art Lewis Glacier Fault Fairweather Fairweather Transform Boundary Fault Fault Fault penetrates LSG: Lower Seward Glacier the lithosphere BP: Bancas Point (14 m coseismic Vertical Exaggeration: 5X uplift in 1899) 10 km Faults with known historical earthquake displacement are marked in dark red - maroon color.

Figure 5. Shaded relief image viewed toward the west of the structural syntaxis formed at the transition from dominantly strike-slip to intense transpression along the plate boundary in the central segment of the orogen. This transition is marked by two bends in the plate boundary. The boundary ﬁ rst bends ~10° westward at Disenchantment Bay where the terrain rises abruptly to the west. The second and more prominent bend is located beneath the upper Seward Glacier where the plate boundary rotates an additional 25° westward parallel to the Chugach–Saint Elias and Bagley faults. The shaded relief image is created from the ASTER Global Digital Elevation Model (GDEM). Refer to Figure 2 for locations of major geographic and structural features depicted in this scene.

fl ower structure that emanates from the side of of Mount Vancouver (Fig. 5). Evidence for con- level in 1899 (Tarr and Martin, 1912). The earth- the Fairweather fault because of transpressional temporary activity on the Art Lewis Glacier fault quake on September 10, 1899 was accompa- deformation (Fig. 5). The Fairweather boundary consists of two earthquake focal mechanisms nied by thrust faulting that caused contraction fault may be truncated at depth by the Yakutat that indicate dextral shearing parallel to the trace of the crust at a high angle to the Fairweather thrust fault as proposed by Bruhn et al. (2004). of the fault (Fig. 4). We do not know if the Hub- fault (Plafker and Thatcher, 2008). The 4 km to GPS displacements modeled by Elliott et al. bard Glacier thrust is currently active. 4.5 km dextral jog of the mountain front at the (2010) suggest that the mountain block bounded The uplifted shorelines surrounding Disen- terminus of the Valerie Glacier refl ects cumula- by the Fairweather and Fairweather boundary chantment Bay and Russell Fiord, together with tive displacement on the Fairweather fault dur- faults is a transpressional sliver, which is also an abrupt right-handed jog of the mountain front ing the last ca. 100 ka given slip rate estimates consistent with the interpretation of the struc- where the Fairweather fault extends beneath the between 43 mm/yr and 50 mm/yr (Fig. 6; Elliott tural geology. The Hubbard Glacier thrust crops terminus of the Valerie Glacier, provide geo- et al., 2010; Plafker et al., 1978). This amount of out along the mountain front where the Hubbard logical evidence for the role of faulting within displacement is ~1 km less than the lateral off- Glacier enters Disenchantment Bay, and the Art the syntaxis (Fig. 6). Rupturing on a combina- set and defl ection of a stream channel along the Lewis Glacier fault splays off the northeastern tion of the Yakutat, Bancas–Esker Creek, and Fairweather fault just south of the Hubbard Gla- side of the Fairweather fault and extends beneath Fairweather boundary fault systems uplifted cier, suggesting that some of Fairweather fault the Art Lewis Glacier into the mountains north the shoreline as much as 14 m relative to sea displacement is transferred onto the Fairweather

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contraction created by transpression east of the Fairweather fault. This thrust may emanate off the southern side of the Art Lewis Glacier fault forming a one-sided ﬂ ower structure. There is no geological evidence to demonstrate that the fault 10 km is active, but it does form a prominent mono- Contour Interval 15 m clinal ﬂ exure on the surface of the ice where it crosses beneath the Hubbard Glacier, and there ValerieVa Glacier le is a band of enhanced seismicity beneath the rie G mountains that form its hanging wall (Fig. 4). la ci (3) The Valerie Glacier trough necks down in er width between an elevation of 970 m and 1830 m where the Fairweather fault rotates several degrees counterclockwise between two restraining bends (Fig. 5; Supplemental Fig. 11). Nar- A rowing of the trough is caused by enhanced contraction because of the greater misalignment 1 of the fault with respect to plate motion. The 2 3 dome of bedrock that protrudes into the fault zone at 1830 m is then a transpressional “pop- C N up” structure. (4) The Fairweather fault extends DisenchantmeD Fjord

i s through the mountain pass at the head of the e HubbardHubbard

n B

Valerie Glacier and into a west-plunging glacial c GlacierGlacier h F a trough before terminating beneath the upper j n o r t m d Seward Glacier. The fault termination is marked

e nnt by a northwest-trending splay fault that extends t toward the southern ﬂ ank of Mount Logan (Ford Topographic Flexures in Glacier Surfaces Fairweather Fairweather et al., 2003) and by the Cascade Glacier thrust A. Ramp with cliff face on south side Valerie Glacier Fault Boundary Fault fault that lies beneath the outlet valley of the B. Projected location of Fairweather Fault fault lower Seward Glacier (Fig. 5). C. Steep flexure over lower Hubbard Glacier Thrust Marker Points for measuring The Fairweather and Bagley Faults apparent dextral displacement beneath the Upper Seward Glacier

Figure 6. Topographic map of the Hubbard and Valerie glaciers showing the concealed Geological information concerning the struc- traces of the Fairweather (B) and Fairweather boundary faults. Notable features include tural geology of this region relies on mapping (1) the south-facing cliff (A) at the confl uence of the Valerie and Hubbard glaciers created of outcrops in the surrounding mountains and by dextral displacement of ~2.5 km along the Fairweather fault (offset of stars 1 and 2), the nunataks that lie within the basin of the (2) 4.5 km dextral offset of the mountain front by displacement on the Fairweather fault upper Seward Glacier (Campbell and Dodds, between stars 1 and 3, and (3) a monoclinal fl exure on the surface of the Hubbard Glacier 1982; Richter et al., 2005). However, much of formed by a NE-dipping thrust fault that presumably intersects the Art Lewis Glacier fault the deformed terrain lies beneath glaciers that at depth (C). The topography is from Shuttle Radar Topographic Mission (SRTM) data. occupy alpine basins and valleys. Herein we attempt to locate geologic structures that lie beneath the glaciers based upon theoretical con- boundary and Bancas–Esker Creek faults within beneath the terminus of the Valerie Glacier (Fig. cepts and experimental observations that pre- the syntaxis. 6). The height of the cliff increases northward dict how ice fl ow on the surface of a glacier is The Fairweather fault is marked by several from a south-facing monocline on the surface perturbed by the topography and rheology at its signifi cant structural and geomorphic features of the Hubbard Glacier to a steep south-facing base (Gudmundsson, 2003; Bruhn et al., 2004). where it enters the syntaxis and cuts through the icefall on the lowermost Valerie Glacier that is The directions of the glacier velocity fi eld mountains into the basin of the upper Seward up to 200 m high (Fig. 6). (2) The northeast- determined by optical feature tracking of Land- Glacier. These include: (1) A 2.5-km-long south- dipping thrust fault that crosses the mouth of the sat V scenes acquired 1 yr apart (Fig. 7) are sim- facing cliff where the Fairweather fault extends Hubbard Glacier presumably represents crustal ilar to those reported by Ford et al. (2003) who

1Supplemental Figure 1. PDF fi le of supplement for Figure 6. Oblique view of the Fairweather fault where it lies beneath the trough of the Valerie Glacier. The fi gure illustrates the contractional bend in the fault trace where there is a “pop-up” ridge between 970 m and 1827 m. Note also that the active trace of the fault is located along the southern margin of the glacier valley, a common characteristic of large strike-slip fault zones. The fault terminates beneath the upper Seward Glacier in a northwest trending splay fault that we interpret to be partly extensional. The Cascade Glacier thrust may also be partly reactivated by enhanced contractional strain on the opposite side of the Fairweather termination, acting to partially close the outlet valley to the lower Seward Glacier, which feeds the Malaspina Glacier on the piedmont. If you are viewing the PDF of this paper or reading it offl ine, please visit http://dx.doi.org/10.1130/GES00807.S1 or the full-text article on www.gsapubs .org to view Supplemental Figure 1.

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Large blue arrows - 0029.30029.3 generalized direction of ice flow in various parts 00510051 of the basin. 01630163

IceIce FlowFlow RidgeRidge Divideide

MMt.t. SSaintaint EEliaslias

rd wa e r S ie r c e a w l o G L

Cascade Glacier Fairweather fault thrust fault Shallow ridge of bedrock B beneath the glacier

Fast glacier flow channel Accelerating flow on south side of basin in trough eroded into Fairweather terminal footwall of Fairweather Ice flow vector splay fault (normal-slip?) terminal splay (dilatational) (400 m/yr)

Figure 7. (A) Velocity of ice fl ow measured on the surface of the upper Seward Glacier by optical tracking of offset features using two Landsat images acquired 1 yr apart. The small arrows indicate direction and velocity of ice fl ow at points on a grid where the optical tracking algorithm is able to resolve displacement on the surface of the glacier. The thick blue lines with arrowheads represent generalized direction of fl ow in various parts of the basin summarized from the underlying optical tracking vectors. A subtle topographic welt or rise affects fl ow near the center of the glacier (7B). (B) Close-up of the ice velocity fi eld shown in A (dashed rectangle) focusing on the areas surrounding the ice-covered ridge of bedrock that affects ice fl ow in the western part of the basin (also see Supple- mental Figs. 2 and 3 [see footnotes 2 and 3], which show more details of deformed moraine bands and crevasses and several topographic profi les across the glacier’s basin).

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used 24 hr repeat-pass SAR scenes and inter- basin. Glacial plucking and abrasion of frac- late that a third fault (fault surface 1c) links ferometry techniques (InSAR). The velocity tured rock within the fault presumably provides the earthquake source faults 1a and 1b in the magnitudes obtained by optical feature track- a layer of weak, water saturated till within an subsurface. Key features include: (1) Faults 1a ing are signifi cantly larger, however, than those elongated trough that is excavated into the fault and 1b intersect at a depth of several kilome- reported by Ford et al. (2003); we hypothesize zone. Weak saturated till at the base of glacier ters near the ice fl ow divide that separates the that the higher rate is due to a glacier surge dur- enhances rates of fl ow (e.g., Boulton and Hind- Seward Glacier and Bagley Ice Valley. Fault ing the period of Landsat scene acquisition in marsh, 1987). surface 1a is part of the regional décollement 1986–1987 (Muskett et al., 2008). The surging The structural setting of the northern stream or plate boundary megathrust (Estabrook et al., state of the Seward Glacier caused large changes of slower fl owing ice in Figure 7 is more diffi - 1992) and presumably continues updip as part in fl ow speeds; fl ow direction, however, was not cult to interpret. The Columbus fault is mapped of the Malaspina and perhaps Bancas–Esker affected signifi cantly. along the northern fl ank of the Bagley Ice Val- Creek faults. (2) Fault 1b abuts the terminus West of the Fairweather fault ice fl ow veloci- ley and projects eastward beneath the northern of the Fairweather fault, providing a structural ties in the southern part of the upper Seward ice stream (Campbell and Dodds, 1982; Rich- linkage with the décollement (fault 1a) to the Glacier are 100 m/yr to 400 m/yr and signifi - ter et al., 2005). However, when one of us (T.L. west as originally proposed by Estabrook et al. cantly faster than in the northern part, where Pavlis) visited the fault exposures in 1998, he (1992). (3) Reactivation of uplift and exhuma- the fl ow is generally less than 100 m/yr. Rapid found that the feature is a compositional bound- tion at Mount Logan (O’Sullivan et al., 1996) ice fl ow along the southern side of the basin ary separating metamorphic rocks of different may be driven by reverse motion on faults 1b deforms rock debris that originates from cirque lithology rather than a fault. This observation and/or 1c, which dip northward as “fl ower- glaciers and rock slides into tightly curved and implies that the northern ice stream may fl ow structure” faults that emanate from the side narrow moraine bands that point downslope to along a lithological contact that predates col- of the Art Lewis Glacier strike-slip fault zone the east (Supplemental Figure 22). lision of the Yakutat Terrane and the rejuvena- (Figs. 2 and 5). Ice originating in the western part of the tion of uplift at Mount Logan (O’Sullivan and upper Seward Glacier fl ows eastward past an Currie, 1996). Alternatively, the glacier may The Bagley Fault within the Bagley Ice Valley ice-covered topographic rise that extends down fl ow above the outcrop of the reverse-oblique The Bagley Ice Valley is divided into eastern the center of the basin, separating more rapidly slip fault system that was activated during the and western arms that meet in a topographic

fl owing ice to the south from more slowly fl ow- Mw 7.4 St. Elias earthquake in 1979, an idea that saddle above the Bering piedmont lobe and the ing ice north of the rise. The two ice streams we explore further. Tana Glacier (Figs. 2 and 3). West of the Tana coalesce within a topographic low just below The St. Elias earthquake initiated on a gen- Glacier the northern fl ank of the ice valley has the rise, where fl ow is diverted toward the south- tly west-northwest–dipping thrust fault (Fig. 8, undergone younger uplift and exhumation than east along the splay fault at the terminus of the fault 1a) at a depth of ~22.4 ( ±3) km beneath in the region to the east, and the spatial pattern Fairweather fault (Fig. 7). The rise is ~10 km the mountains north of the eastern Bagley Ice of exhumation ages suggests that the structure is long, up to several km wide, and has ice-surface Valley (Estabrook et al., 1992). The rupture characterized by uplifted mountain blocks that relief of 25–50 m (Supplemental Figure 33). then propagated updip and to the east, initiating are bounded on one or more sides by reverse En echelon crevasses along the southern side slip at a depth of roughly 26 (±3) km on a dex- faults (Spotila and Berger, 2010; Berger et al., of the rise are presumably developed within a tral-oblique slip fault located beneath Mount 2008b). To the west of the Bering piedmont band that is experiencing high shear strain rate Logan (Fig. 8, fault 1b). Rupturing terminated lobe and Tana Glacier outlets the region is also (Supplemental Figure 2 [see footnote 2]). This updip along the north side of the Bagley fault marked by frequent earthquakes with a mixture ice-covered rise persists on topographic data and immediately west of the terminus of the of reverse and strike-slip focal mechanisms sets obtained over a period of years, and the en Fairweather fault. The fault map in Figure (Fig. 4; Doser et al., 2007). echelon crevasses are present in Landsat scenes 8 is constructed by locating the hypocenters Geology of the ice valley fl anks. Mountains acquired over several decades. and using fault plane orientations preferred by that fl ank the Bagley Ice Valley contain crystal- We interpret the rise as the surface expres- Estabrook et al. (table 3, 1992), with the fault line and sedimentary rocks that were incorpo- sion of an elongated ridge of rock that extends surfaces extended upwards to intersect beneath rated into the plate margin of southern Alaska down the center of the upper Seward Glacier. the upper Seward Glacier. Variability in the dip prior to collision of the Yakutat microplate (Fig. Presumably, one or more strands of the Bagley angles and directions of the faults are noted on 9; Plafker, 1987; Plafker et al., 1994; Gasser et fault lie beneath the stream of rapidly fl owing the fi gure based on estimates of uncertainly in al., 2011). These rocks were metamorphosed ca. ice that extends beneath the southern side of the the earthquake source parameters. We specu- 50 Ma, and subsequently thrust over the Yakutat

2Supplemental Figure 2. PDF fi le of supplement for Figure 7. Binary black and white segmentation image of a Landsat scene showing ice crevassing and fl ow- deformed traces of moraine on the surface of the upper Seward Glacier. Note en echelon crevasses along the southern fl ank of the ice-covered ridge of bedrock that is outlined in the dashed brown polygon. Red polygons are outcrops of bedrock that poke through the snow and ice on the southern and northern fl anks of the basin. The image is derived from a Landsat scene with bedrock mapped by band 7. The binary black and white image is constructed by segmentation of a false color composite image created with bands 7,4,1. Refer to Figure 7 for location of the image by comparing geographic features listed in each fi gure. The location of the fi gure is also indicated in Universal Transverse Mercator (UTM) coordinates that are marked around its borders. If you are viewing the PDF of this paper or reading it offl ine, please visit http://dx.doi.org/10.1130/GES00807.S2 or the full-text article on www.gsapubs.org to view Supplemental Figure 2.

3Supplemental Figure 3. PDF fi le of supplement for Figure 7. Three topographic profi les across the basin of the upper Seward Glacier extracted from ICESat laser profi ling tracks. Track locations are shown in the satellite image in the upper right corner of the fi gure, with the profi les marked in black on each track. The tracks are also marked and labeled by number on Figure 7. The location of a high-resolution laser data swath obtained by aircraft (ATM) that was used to verify features observed in Supplemental Figure 2 (see footnote 2) is marked in purple. If you are viewing the PDF of this paper or reading it offl ine, please visit http://dx.doi.org/10.1130/ GES00807.S3 or the full-text article on www.gsapubs.org to view Supplemental Figure 3.

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Terrane along the Chugach–Saint Elias fault The Bagley fault dips steeply where it cuts the Chugach–Saint Elias fault and east-trending (Figs. 1 and 2). The shallow southern edge of through the hanging wall of the Chugach–Saint foreland thrusts was dominantly thrust motion the Chugach–Saint Elias fault was subsequently Elias fault (Campbell et al., 1986), presumably with little or no strike-slip motion. This result uplifted and deformed as the Yakutat Terrane penetrates the deeper-seated part of the under- suggested that dextral shearing was possibly continued to accrete and deform into a broad thrust Yakutat Terrane, and abuts the décol- focused along the Bagley fault. foreland fold and thrust belt in the last 10 Ma lement that marks the subduction megathrust The steep slopes along the southern fl ank of to 20 Ma. The uppermost part of the Chugach– within the Yakutat lithosphere (e.g., Chapman the Bagley Ice Valley may refl ect tectonic uplift Saint Elias fault that underlies the mountains et al., 2008; Meigs et al., 2008; Wallace, 2008; that is keeping pace with erosion of the ice val- immediately south of the Bagley Ice Valley is Pavlis et al., 2012). The structural confi guration ley wall. Topographic relief of the mountains probably now an inactive imbricate fault that is of the Bagley fault and underlying subduction on both sides of the eastern Bagley Ice Valley truncated at depth by the basal décollement of thrust is ideal for partitioning oblique plate con- generally decreases to the west from Mount the foreland thrust belt (megathrust) and per- vergence into strike-slip and thrust-type dis- Logan and Mount Saint Elias, but the southern haps by reverse displacement along the southern placement (e.g., Haq and Davis, 2010). Struc- range fl ank is generally steeper and higher than side of the Bagley fault (Chapman et al., 2008; tural studies of foreland thrust kinematics by the northern fl ank, consistent with thermochro- Pavlis et al., 2012). Bruhn et al. (2004) demonstrated that slip along nology data that indicate more rapid rock uplift

Dip direction & angle of 1979 faults (degrees) Fault F1a: 300+/- 31, 15+/-9

Fault F1b: 061+/-6, 34+/-6

1a F1c 2 W.W Bagley . Bagley ML EasternE Bagley IceIce ValleyVall astern ey Bagley IceIce VValleyall ey F1b SewardSeward GlacierGlacier ArtA Lewis Glacier r F1a t Mid-basin ridge L BBeringering GGlacierlacier e MV w MI is MS G la c Fvg ie r

lt Fau ina CapeCape k rscho YakatagaYakataga AfterschockAfterschock7 GGulfulf ofof AAlaskalaska region (1979)(197(19 Malaspina GlacierGlaciac YakutatYakutat 50 Km BayBay

Legend Faults: F1a - Model fault for event 1a, 1979 (megathrust), F1b - Model fault for event 1b, 1979, F1c - possible fault link between F1a and F1b. Fvg - Valerie Glacier fault segment of Fairweather Fault. Earthquake locations: 1a - epicenter event 1a, 1979, 2 - epicenter of event 2 aftershock, 1979. Locations marked by red asterix in each case. Geographic Features (short hand): MS- Mt. Saint Elias, ML-Mount Logan, MV-Mt. Vancouver, MI-Mt. Irving nunatak in Seward Glacier Basin Coordinates: UTM Zone 7

Figure 8. Shaded relief map of the Saint Elias Mountains showing the mapped projections of faults 1a and 1b that ruptured during the

Mw 7.4 Saint Elias earthquake in 1979 (Estabrook et al., 1992). The main shock of the Saint Elias earthquake nucleated at the large red star in the northwest corner of fault 1a and the fault then ruptured updip toward the Bagley fault and along strike to the east, where rupturing then spread onto fault surface 1b. The aftershock zone of the 1979 earthquake spread southward from beneath the main range into the foreland beneath the Malaspina Glacier and Icy Bay (shaded orange polygon). Fault 1b projects upwards toward the east-west–trending mid-basin ridge of bedrock that affects ice ﬂ ow and is marked by the light brown polygon within the western part of the Seward Glacier Basin in Figure 7.

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Fault ‘beheaded’ Contour Interval: 25 m valleys

IceSAT 1279 IceSAT 416 Star3i DEM

KvmKvm Jefferies Glacierr r

e e

i i

c c

a a l MountMMo LoganLoLogano n G

20 1 e 0

0 S

1 t

Kvg n i 0 u Tov 0 16 Q N

0 TosTos 0

18 0 0 2

Tov 0 2

Ya 0

htsse G 0 e G TosTos lacci ier KvgKvg

10 km Chugach - Saint Bagley Ice Field Columbus - Upper Seward Glacier Elias Fault Trends of Cirque Valleys PRINCE WILLIAM TERRANE (southern flank) (Early Eocene - Late Paleocene) n = 20 Tos - turbidites, low metamorphic grade Tov - basalts, low metamorphic grade CHUGACH TERRANE Trend of Bagley Ice Valley (Late Cretaceous) Kv - Greenschist facies schist 2 4 Kvm - Amphibolite facies flysch Number of 6 Kvg - Gneiss and schist Occurrences

Thrust Fault (Barb on upper plate)

Figure 9. Topography and geology of the eastern Bagley Ice Valley and western part of the Seward Glacier basin. Glaciers are in grayscale and rocks are color-coded to demarcate units on the geologic map of the Wrangell–Saint Elias Park and Preserve (Richter et al., 2005). The rose diagram shows the trends of 20 cirque basins along the southern ﬂ ank of the Bagley Ice Valley, with the number of observations keyed to the lengths of each petal. IceSAT transects 1279 and 416 are marked by dashed white lines. The two thrust faults are the Chugach–Saint Elias fault along the margin of the Yahtse Glacier, and part of the Columbus fault located near the base of the Mount Logan massif. Topographic contours are in meters and the shaded-relief base map is constructed from the STAR3i DEM acquired by Intermap Technologies (1996). The dashed yellow rectangle is the location of the structural study area presented in Supplemental Figure 4 (see footnote 4).

rate with greater exhumation of the southern of the topography of other range fronts through- basins that are elongated and point slightly up mountain range (Figs. 10C-1 and 10D-1; Berger out the region. the topographic slope of the trunk valley glacier and Spotila, 2008; Berger et al., 2008b; Enkel- Small cirque basins that are elongated north to instead of normal or downslope as might be mann et al., 2010). Steepness of the mountain north-northeast embay the southern fl anks of the expected without a strong control of erosion by fl anks above the ice valley is expressed as the ice valley (Fig. 9). The geometry of the basins rock structure. average slope index (R/W), which is the ratio is partly if not wholly controlled by the struc- Three linear mountain front segments that of mountain fl ank relief above the glacier (R) ture of the underlying rock, which is character- are offset by right-handed jogs at the Quintino to mountain fl ank width (W) in Figure 10 (C-2 ized by east-trending folds that are overturned Sella and Jefferies glaciers form the northern and D-2). The steeper slopes along the southern toward the south, and cut by large fracture or margin of the eastern Bagley Ice Valley (Fig. range fl ank do not correlate with erosional resis- “joint zones” that dip steeply and strike north- 9). The ice valley widens from ~5 km to 7.5 km tance related to rock composition because the northeast across the fold axes (Supplemental on either side of the Quintino Sella Glacier, and rocks along the northern fl ank of the ice valley Figure 44). Failure along planar discontinuities then to ~10 km below the Jefferies Glacier. The are of equal, and in most areas higher, metamor- in the rock mass created by the structural geol- increased ice fl ux at the Quintino and Jefferies phic grade than those on the southern fl ank. The ogy creates rockslides that cascade northwards glaciers must enhance erosion along the north- azimuth or “aspect” of the fl ank is also poorly down the mountain front and onto the glaciers ern mountain fl ank, causing the valley to widen correlated with slope angle based on inspection below (Huggel, 2009). This leads to cirque below each tributary glacier. However, the linear

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mountain fronts and right-handed jogs are also features that refl ect ice fl ow over and around iary rather than a main trunk glacier fl ow chan- characteristic of dextral strike-slip fault systems, undulations at the base of the glacier. These nel. However, above the confl uence, the shear where the jogs mark pull-apart basins (Aydin features persist over decades and are observed strain rate boundary between left and right lat- and Nur, 1982). The ice valley is signifi cantly on images acquired by different sensors and on eral shearing remains along the southern side narrower west of the Tana and Bering Glacier elevation models and maps constructed by dif- of the ice valley, similar to the situation on the piedmont lobe outlets, where the northern edge ferent techniques. Bagley Glacier below the confl uence. Below of the ice valley steps left several kilometers North to north-northeast–trending ridges the confl uence with the Jefferies Glacier, the between the eastern and western arms. A left- and troughs are the most prominent features on rate of rapid ice fl ow is also concentrated along stepping jog in a dextral fault system is contrac- the surface of the glacier (Fig. 11A). The topo- the southern side of the ice valley and is then tional, which fi ts the structural map prepared graphic relief between ridges and troughs is on defl ected primarily into the head of the Bering by Berger and Spotila (2008) and Berger et al. the order of several tens of meters, and crest-to- piedmont lobe, rather than Tana Glacier. South- (2008b) to account for the younger exhumation crest spacing varies from roughly 1 km to 10 km ward curvature of the main trunk channel into ages obtained from the mountain block located (Fig. 11B). The ridge crests are curved or sig- the Bering Glacier piedmont lobe may refl ect on the western side of the Tana Glacier (see moidal in shape, several kilometers long, and deeper erosion along the major fault that lies Figs. 2 and 3). One other piece of circumstantial appear to be laterally offset at several localities beneath the top of the Bering Glacier piedmont evidence supports the dextral fault interpreta- (Fig. 11C). Glacier strain rates determined from lobe and the intersection with the Bagley fault tion: Several NW-trending glaciated valleys are the ice surface velocity fi eld in Figure 12A also (Fig. 2). Uplift of the mountains and the east- located within the mountain block between the show coincident north-northeast–oriented bands ward slope of the western arm of the Bagley Jefferies and Tana glaciers (Fig. 9). The longi- of longitudinal contraction downstream of the Ice Valley presumably refl ect movement on the tudinal axis of each valley projects out of the Jefferies confl uence (Fig. 12B), a strain-rate Bering Glacier fault and also crustal contrac- mountain front up and over the Bagley Ice Val- pattern that is expected where ice slows on the tion at the left-stepping jog in the northern wall ley as though the head of each valley is trun- upslope side of a basal ridge and then acceler- of the Bagley Ice Valley, which occurs across cated by faulting. ates over the downslope side of the ridge (e.g., the head of the Tana Glacier. Continuing defor- The glacier in the eastern Bagley Ice Val- Reeh, 1987; Gudmundsson, 2003). The west- mation beneath the western Bagley Ice Valley ley. The topography and dynamics of the gla- northwest–facing monocline that extends across and surrounding mountains is indicated by a cier in the eastern arm of the Bagley Ice Valley the glacier in the eastern arm of the Bagley Ice cluster of intense crustal seismicity (Doser et is evaluated using the STAR3i DEM (Fig. 11), Valley from just below the mouth of the Jeffer- al., 2007), and also by geodetic data which speckle tracking of SAR images (Fig. 12), and ies Glacier to near the head of the Bering Gla- indicates active deformation in this region ICESat topographic profi les (Fig. 13). The glacier is exceptional because of its continuity and (Savage and Lisowski, 1986, 1988; Sauber and cier slopes <1° west between its head and the relatively linear crest (Fig. 11A). These ridges Molnia, 2004; Elliott, 2011). topographic saddle where the eastern and west- and troughs are relatively stationary in location Additional details concerning the elevation of ern arms of the Bagley Ice Valley join together and persist over decades, thus implying irregu- the glacier’s surface were provided by ICESat (Figs. 2 and 3). The thickness of the ice is 800 m lar topography at the base of the glacier (Reeh, profi les that cross the eastern Bagley Ice Valley to 1000 m thick in the Bering–Tana region and 1987). The boundary between left-lateral (Fig. between its upper section and the Tana and Ber- most of the base of the Bering Glacier piedmont 12C, red pattern) and right-lateral (blue pattern) ing glaciers (Fig. 13). There is a broad rise or lobe is near sea-level elevation (Molnia and shear is also of interest. Rather than occurring “bulge” in the ice along the southern side of the Post, 1995; Conway et al., 2009). within the center of the glacier, the boundary glacier in the central area between Mount Saint Both persistent and transient features between the two dextral and sinistral shear strain Elias and Mount Miller, which is not present on mark the surface of the glacier in the eastern zones extends along the southern side of the gla- ICESat profi les that cross both higher and lower Bagley Ice Valley. Transient topographic fea- cier in the eastern ice valley, where the glacier is sections of the valley. The bulge may simply tures include circular to elliptical hillocks and presumably fl owing along an asymmetric valley refl ect additional snow and ice accumulation depressions above migrating englacial fl uids, that is deepest along its southern margin. along the southern edge of the glacier where the and “kinematic waves” that propagate along the Additional information concerning the mor- mountain crest is lower between Mount Saint length of the glacier in response to variations phology of the ice valley is provided by the Elias and Mount Miller, providing a pathway for in ice fl ux (e.g., Lingle et al., 1997; Lingle and dynamics of ice fl ow depicted in Figure 12. storms to move over the ice valley (Figs. 2 and Fatland, 2003; Fatland and Lingle, 2002). These The Quintino Sella Glacier fl ows into the ice 3). However, the bulge is also located where the transient features are related to ice dynam- valley as the primary ice stream, suggesting tectonic highland that is capped by the Guyot ics and not necessarily direct consequences of that the uppermost section of the Bagley Ice and Yahtse glaciers extends beneath and warps bed topography. Here, we focus on persistent Valley is a hanging valley, forming a subsid- the Chugach–Saint Elias fault upwards (Fig.

4Supplemental Figure 4. PDF fi le of supplement for Figure 9. Relationships between geologic structure, rock sliding, and the origin and shaping of cirque basins along the southern fl ank of the eastern Bagley Ice Valley. (A) Oblique view of part of the southern fl ank with prominent structural surfaces or “rock mass discontinuities” labeled on the fi gure. (B) Lower hemisphere stereographic projection showing poles to bedding on fold limbs (point clusters P1 and P2) and joints (cluster P3) within the rock mass. S1, S2, and S3 are the corresponding great circles to the average poles to bedding and joints. There is also a foliation that is axial planar to the folded bedding, and that dips to the north. Foliation and bedding strike subparallel to the trend of the ice valley, and axial surfaces of folds dip steeply towards the north. Joint set S3 intersects the foliation and bedding surfaces and cuts across the axial surfaces of the folds. Mass wasting by rock sliding is caused by failure along the north dipping foliation and bedding surfaces with joint set S3 providing the discontinuities that limit the lateral extent of slide masses. Topography and structural surfaces were extracted from the STAR3i digital elevation model (DEM) with the strike and dip of prominent structural surfaces determined by three-point solutions and fi eld reconnaissance. The area encompassed by the fi gure is marked by a yellow dashed rectangle in Figure 9. If you are viewing the PDF of this paper or reading it offl ine, please visit http://dx.doi.org/10.1130/GES00807.S4 or the full-text article on www.gsapubs.org to view Supplemental Figure 4.

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Downloaded from https://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/8/6/1384/3346159/1384.pdf by guest on 15 May 2019 Plate margin deformation, Saint Elias Orogen Mt. Logan Width ABWidth Mt. Saint Elias SewardSeward GlacierGlacier BBasinasin Relief IceIce North DivideDivide Ice Valley Geomorphic Index: 10 km y e r R/W = relief/width l e l ci a la V G Thermochronology Key e e c tsse I 0-1 Ma ah ap He y Y e l 1-2 Ma g a ap FT BagleyB Ice Valley 2-5 Ma r cie 5-10 Ma zr FT la B a G erin Tan g Gla 10-20 Ma cie r >20 Ma

6 Mt. Logan 5 Topography (North Wall) C-1 4 Ice Divide 3 2 Elevation (km) Elevation 1 Bagley Glacier Seward Glacier 4.2 4.4 4.6 4.8 5.0 5.2 5.4 x 105

N. Valley Wall Relief/Width Ratio C-2 0.6 R/W 0.4

0.2

4.2 4.4 4.6 4.8 5.0 5.2 5.4 x 105

6 5 Topography (South Wall) Mt. Saint Elias D-1 Ice Divide 4 Seward Glacier 3 Mt. Miller outlet valley 2 Elevation (km) Elevation 1 Bagley Glacier Seward Glacier 4.4 4.6 4.8 5.0 5.2 5.4 x 105

S. Valley Wall Relief/Width Ratio D-2 0.6 R/W 0.4

0.2

4.2 4.4 4.6 4.8 5.0 5.2 5.4 x 105 Easting (m)

Figure 10. (A) Perspective view of the eastern Bagley Ice Valley and Seward Glacier basin created by draping a false-color Landsat scene (bands 5,4,1) over the ASTER GDEM. (B) Schematic illustration of the mountainside relief (R) to width (W) geomorphic index, and symbols for thermochronology dates summarized by Enkelmann et al. (2010). The colored symbols indicate the age at which bedrock samples passed

below the thermal closing temperature (Tc) for apatite and zircon. Approximate closing temperatures associated with various minerals and methods are: (1) apatite - ap He (U-Th/He) = 60 °C, (2) apatite fi ssion track (ap FT) = 110 °C, and (3) zircon fi ssion track (Zr FT) = 250 °C. (C) and (D) Plots of the ridge-line topography on the north (C-1) and south (D-1) sides of the eastern Bagley Ice Valley, with associated plots of the geomorphic ratio R/W for each mountainside (C-2, D-2), respectively. The surface of the glacier is shown as a blue line on both C-1 and D-1 plots. Locations of bedrock samples collected for thermochronology analyses are indicated on profi les in C-1 and D-1, with the age color coded as indicated in B.

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Figure 11. (A) Shaded relief image of the residual topography on the surface of the eastern Bagley Ice Valley created by removing the regional surface slope on the glacier. Light source is directed toward the northwest. (B) Longitudinal proﬁ le along the center of the eastern Bagley Ice Valley showing the residual topography. Note that ridges with relief several tens of meters are separated by several kilometers. (C) Interpretation of the style of faulting at the base of the glacier that may cause the apparent offset in the surface topography (ridge crests) of the shaded relief image in A. The red dashed line denotes the inferred location of a major dextral fault or shear zone at the base of the glacier. Note that we infer dextral faulting of stair-step–like topography formed by erosion at the glacier’s base, as sketched in Figure 14.

2). The axis of this up-warp projects beneath thrust. We subsequently focus on the role of the sional strain at depth (Fig. 7; Ford et al., 2003). the mountains toward the interior of the eastern Bagley fault in accommodating plate boundary Consider a conceptual experiment, where one Bagley Ice Valley (Fig. 2). That is, the north- motion throughout the history of terrane accre- squeezes a tube of paste (Bancas–Esker Creek sloping surface may refl ect a structural up-warp tion, followed by a discussion of the seismic and Malaspina thrust faults driving mecha- beneath the northern side of the glacier. potential of the Bagley fault. nism—Figs. 5 and 7) and removes the lid from the tube (i.e., the Fairweather extensional splay DISCUSSION Structural Geology of Seward fault). The paste fl ows rapidly upwards and out Glacier Basin of the tube upon removing the cap. In the case The structure and tectonics of the collisional of the upper Seward Glacier, broad tectonic plate boundary is discussed proceeding from the Surprisingly, the most rapid upward advec- upwelling driven by transpression and thrust syntaxis in the east to the terminus of the Bagley tion and exhumation of deeply seated rock faulting at depth creates the topographic welt fault in the west. This format allows us to begin (rates up ~7 mm/yr) is located beneath the upper encircled by 5000+ m alpine peaks, but the cen- by describing the terminus of the Fairweather Seward Glacier rather than in the surrounding ter of the welt is tectonically denuded by crustal fault, where plate motion is best constrained by mountains where the exhumation rates are sig- dilatation at the end of the Fairweather fault. both geologic (Plafker et al., 1978; Plafker and nifi cantly less (≤4 mm/yr) (Enkelmann et al., This process creates the “tectonic aneurysm” Thatcher, 2008) and geodetic data (Elliott et al., 2009). Dilatational strain and splay faulting at (Koons, 1995) where upwelling rock is eroded 2010). The structural geology of the syntaxis the terminus of the Fairweather fault may play as quickly as it arrives at the base of the upper surrounding the Fairweather fault also provides an important role in tectonic exhumation by Seward Glacier. Subglacial rivers that extend insight into how deformation was transferred providing a local zone of extension into which beneath the lower Seward and Malaspina gla- onto the Bagley fault and the underlying mega- rocks are forced by thrust faulting and transpres- ciers transport the detritus out of the alpine basin

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N 20 km

Figure 12. Ice velocity and strain rate on the Bagley Icy Valley determined by SAR offset tracking using normalized cross correlation (GAMMA Software [Strozzi et al., 2002]) on two image pairs acquired between January and April, 2010. (A) Glacier fl ow velocity fi eld, speed indicated by color bar. (B) Glacier ice strain rates in the direction of ice fl ow. (C) Glacier ice shear strain rates showing vorticity or magnitude of the rotational component of the shear strain rate tensor. The Tana, Jefferies, and Quintino Sella glaciers are marked for geographic referencing to Figure 11.

and across the piedmont to the coast as proposed dred m/yr during surging (Fig. 7; Headley et a fundamental problem when attempting to by Enkelmann et al. (2009). al., 2007). Competition between contraction of understand the structural geology of the eastern The geometry of the splay fault within the the outlet valley by reactivation of the Cascade syntaxis (O’Sullivan and Currie, 1996; Spotila Seward Glacier’s basin is consistent with fault Glacier fault and widening of the valley by gla- and Berger, 2010). Geodynamic modeling by growth into a lobe of dilatational strain that cier erosion may account for the relatively nar- Hooks (2009) and Koons et al. (2010) implies develops adjacent to the tip of a strike-slip fault, row width given rapid glacier fl ow which must a south-dipping reverse fault or shear zone may or “mode II” shear crack in the parlance of frac- widen the valley by several millimeters to pos- lie beneath the northern side of the syntaxis that ture mechanics theory (e.g., Pollard and Segall, sibly more than 1 cm each year (e.g., Hallet, is conjugate to the thrust faults that dip north- 1987). Extensional splay faults or “wing cracks” 1979; Headley et al., 2007; Headley et al., 2008; ward beneath the southern side of the Saint Elias propagate outward at 70° from the surface of the Headley and Enkelmann, 2009). Displacement Mountains. However, this type of structure has primary strike-slip fault; the fault in the Seward along the Cascade Glacier fault may constrict not been mapped (Campbell and Dodds, 1982). Glacier basin splays at an angle of ~60°, con- the upper part of the outlet valley because it is Either the south-dipping fault or shear zone does sistent with dextral-oblique normal slip in the preferentially oriented for reactivation by thrust not exist, or it remains to be discovered amongst more complex natural strain fi eld that must exist faulting within the contractional strain lobe at the glaciers to the north of the Logan Massif. within the transpressional syntaxis (e.g., Koons the Fairweather fault’s terminus (Figs. 5 and Alternatively, the uplift of Mount Logan may et al., 2010). We also expect to fi nd a zone of 7). This process may be limited to the upper be driven by displacement on faults 1b and/or 1c intensifi ed contraction along the southern side reaches of the glacier valley where the thrust (Fig. 8) that dip to the northeast and splay off of of the Fairweather fault where the lower Seward fault is located. At lower elevation the Chaix the Art Lewis Glacier strike-slip fault. The Art Glacier descends the southern range front. Hills and Dome Pass thrust faults extend across Lewis Glacier fault occupies a linear ice-fi lled The persistence of the relatively narrow val- the valley of the lower Seward Glacier and are valley that branches off of the Fairweather fault ley of the lower Seward Glacier is puzzling to not truncated by younger faulting. and extends around the northeastern sides of glaciologists given that the ice fl ows into the The late stage uplift and exhumation of Mount Vancouver and projects toward the north- outlet valley at a rate in excess of several hun- Mount Logan within the last 5 Ma presents ern side of the Mount Logan massif (Figs. 2 and

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A the underlying bedrock. When this “step and 2400 tread” topography is offset by strike-slip fault- ICESat Track 1279 ing at the base of the glacier, the surface of the SSE NNW Bagley Ice Valley glacier will develop low-amplitude ridges and swales that are truncated along their axes (Figs. 14B and 14C). 2000 (3) Large strike-slip fault zones are character- Rise in topography ized by distinctive patterns of subsidiary fault- along southern side ing and related topography (Tchalenko, 1970). of ice valley Pull-apart basins and thrust-faulted ridges form

Elevation (m) 1600 at jogs and bends within the fault zone, and subsidiary strike-slip faults are inclined to the regional boundaries of the fault zone (Tcha- lenko, 1970). These subsidiary faults are clearly displayed in arid landscapes where rates of tec- 1200 tonic activity outpace erosion, but beneath tem- 0 5 10 km perate glaciers the smaller-scale features may be quickly removed by erosion, or mantled by till. Pull-apart basins and large thrust-faulted ridges that form at jogs and bends in a strike-slip fault B system persist because of greater original topo- 2400 graphic relief compared to lower-relief features ICESat Track 416 developed by lateral displacement along strike- SSE Bagley Ice Valley NNW slip faults. Features that may be partly preserved struc- Rise in topography 2000 tures associated with dextral faulting along the along southern side northern edge of the Bagley Glacier include of ice valley pull-apart depressions at the right-stepping jogs where the Quintino Sella and Jefferies glaciers

Elevation (m) enter the ice valley. Contractional jogs and 1600 bends occur (1) where the northern margin of the Bagley Ice Valley steps several kilometers south (left-handed step) across the head of the 0 5 10 km Tana Glacier, and (2) where the Bagley and 1200 Martin River Glacier faults join in a restraining fault bend (Fig. 2). Some dextral shearing along Figure 13. Topographic profi les across the eastern Bagley Ice Valley extracted from ICESat the Bagley Fault extends at least 40 km west tracks (A) 1279 and (B) 416 (see Fig. 9 for locations). Note higher terrain on the southern of this latter restraining bend, where the fault part of the glacier. lies beneath the linear Miles Glacier. The 4 km offset of the mountain front where the Miles Glacier enters the Copper River indicates dex- 4). The proposed structure is a one-sided fl ower (1) Topographic features at a glacier’s base tral shearing, and the pattern of rock exhumation structure where transpression is accommodated perturb the surface of the ice where they appear ages refl ects uplift along the southern side of the by a linked strike-slip and thrust fault system. as “muted” undulations that persist over time fault (e.g., Berger et al., 2008a, 2008b). Similar “one-sided” transpressional structures (Fowler, 1982; Gudmundsson, 2003). The sur- We interpret the low-amplitude sigmoidal occur elsewhere at the structural transition into face undulations may change in wavelength ridges on the surface of the glacier in the eastern the eastern syntaxis. One-sided fl ower structures and amplitude and shift laterally by a limited arm of the Bagley Ice Valley as a subdued topo- include the Hubbard Glacier thrust–Art Lewis amount because of temporal fl uctuations in ice graphic expression of dextrally faulted “step Glacier fault pair northeast of Disenchantment velocity, but the features persist on the surface and tread glacier valley” topography at the base Bay, and the Yakutat thrust–Fairweather bound- of the glacier over decades. of the glacier (Figs. 11 and 14). The sigmoidal ary fault–Fairweather fault system in Russell (2) Valley glaciers erode the bedrock into a shape of the ridges refl ects erosional smooth- Fiord and the coastal mountains (Fig. 5, also see repetitive series of steep down-glacier-facing ing of the bedrock and possibly clockwise rota- Bruhn et al., 2004). steps (cliffs) that are separated by longer tracts tions of the faulted blocks in the dextral shear of abraded and till-mantled bedrock that slope zone. Longitudinal ice strain rates derived from Evidence for Strike-Slip Faulting in gently either down or up-valley, e.g., large- ice fl ow velocity data confi rm the presence of the Bagley Ice Valley scale “roche moutonnee” landforms in degla- north-northeast elongated ridges at the glacier’s ciated terrain (Fig. 14A; Hooke, 1991). Cliffs base (Fig. 12B). The large northwest-facing Our interpretations of faulting beneath the are created by ice plucking out the bedrock as monoclinal fl exure on the surface of the glacier eastern arm of the Bagley Ice Valley are based it fl ows downslope, and the intervening treads located just below the inlet of the Jefferies tribu- on several observations. are created where the debris-laden ice abrades tary glacier may refl ect the presence of a large

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reverse-fault cored fold that trends obliquely studies that indicate much younger uplift and 2010). The region between the western Bagley across the dextral shear zone at the base of the exhumation along the southern sides of the fault Ice Valley and Martin River fault system has Bagley Ice Valley. zone between Mount Saint Elias and the Bering been uplifted and exhumed over a broader area, Uplift along the southern side of the Bagley Glacier, and also along the southern side of the including the mountains on both the northern fault is required during the last several Ma based Miles Glacier (e.g., Berger and Spotila, 2008; and southern side of the Bagley fault. Hypoth- upon the results of regional thermochronology Berger et al., 2008a, 2008b; Spotila and Berger, eses concerning the structure responsible for uplift of the southern mountain fl anks include a south-dipping reverse fault (Berger and Spotila, 2008; Chapman et al., 2008; Wallace, 2008), A up-warping of the mountains where active east-northeast–trending foreland thrust faults project beneath the mountains along the south- Direction of ice flow ern edge of the Bagley Ice Valley (Enkelmann et al., 2010), and uplift and “back-rotation” of Step the entire mountain block above a north-dipping Valley Wall Step fault ramp that connects foreland thrusts to the subduction zone beneath southern Alaska (Pav- 1 km Tread lis et al., 2012). Step Although our work on the Bagley fault and the geomorphology of its mountain fl anks does not distinguish between the various hypotheses for uplift along the southern side of the Bagley Step Ice Valley, we note that the southern fl ank of the eastern ice valley is steep and embayed by 5 km or a number of small cirque basins. This suggests B more that the southern fl ank is rejuvenated by tectonic activity rather than an old mountain front that Valley Wall is worn down by erosion. The most likely locus Step of reverse faulting is along the southern side of the Seward Glacier basin and eastern Bagley Step Tread Ice Valley, where the glaciers fl ow most rapidly Step leading us to infer the presence of a linear fault strand at depth. In this scenario, Pliocene and Step younger deformation is partitioned between dextral shearing beneath the central and north- Step ern edge of the ice valley, and uplift by reverse faulting along the southern edge. We postulate Step that the more complex pattern of deformation C and uplift surrounding the western part of the Bagley fault is related to (1) the intersection of the Bering Glacier fault, (2) crustal contraction and “pop-up” mountain blocks created by the left-stepping fault jog in the northern wall where the Tana Glacier exits the ice valley (Berger et al., 2008b), and (3) the restraining bend formed where the Bagley and Martin River Glacier faults meet (Fig. 2). This region is also one of the most seismically active areas within the Saint Elias orogen, which is consistent with this conclusion (Fig. 4; Doser et al., 2007). 10 km Role of Bagley Fault in Plate Figure 14. (A) Sketch of the step and tread topography that evolves by erosion at the base Boundary Deformation of valley glaciers (Hooke, 1991). (B) Illustration of offset in the step and tread topography caused by dextral strike-slip faulting. (C) Map view of part of the shaded relief Our analysis of remote sensing and geological image of residual topography on the surface of the lower Bagley Ice Valley in Figure 9. data provides a better foundation for integrating Note sigmoidal-shaped ridges on the surface of the glacier that may be caused by dextral the Bagley fault into the tectonic framework of shearing of bedrock beneath the ice. The illumination angle is from an azimuth of 150° the Saint Elias orogen. Structurally, the confi gu- and elevation of 20°. Northwest facing slopes are darker and southeast facing slopes are ration of the plate boundary created by the fore- brighter grayscale. land fold and thrust belt and the Bagley fault is

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ideal to partition oblique plate convergence into thrust décollement from the Kayak Island zone Bagley Ice Valley may mark the reverse fault thrust and strike-slip motion (Bruhn et al., 2004; to the eastern Pamplona zone and Malaspina fi rst proposed by Berger et al. (2008a). Haq and Davis, 2010). Bruhn et al. (2004) spe- fault from the Late Miocene to Late Pleistocene Cumulative strike-slip motion along the cifi cally searched for evidence of oblique thrust (Worthington et al., 2010). However, the Bagley Bagley fault is diffi cult to resolve because there faulting versus plate boundary slip-partitioning fault remained active since the Pliocene (e.g., are no unambiguous piercing points to match on during their structural study of the Chugach– Berger and Spotila, 2008; Chapman et al., 2008; either side of the fault zone. Restoring the sliver Saint Elias fault and east-trending foreland Spotila and Berger, 2010), although vertical of the relict plate boundary preserved at Ragged thrust faults in the central part of the Saint Elias motion may have become most important as the Mountain back to a location near the head of the orogen. They concluded that the requisite strike- leading edge of subduction migrated ~200 km Martin River Glacier requires roughly 50 km slip motion was accommodated along the Bagley eastward (e.g., Bruhn et al., 2004; Worthing- of dextral shearing along the Martin River and fault rather than by oblique thrust faulting (they ton et al., 2010). Doser et al. (2007) speculate Bagley faults (Fig. 15). This restoration follows used old Contact fault terminology) prior to that a linear south-dipping band of earthquakes the tectonic displacement pattern suggested by the progressive eastward steps of the mega- beneath the mountains on the south side of the Bruhn et al. (2004) and Pavlis et al. (2004) for

Mt. Tom White Mile T s Gla ana G cier l SSSS ac NSNS Bagleyier Ice Valley RMo RM1

r e i RM2 c a er l laci Bering G YahtseY Glacier G ahts ller (Growing( e G Tectonic Ste Gro lac win ier Highland)g T ecto Hig nic RM3 hla nd)

NNSS N 02550 Km

RMo, 1, 2, 3: Position of Ragged Mountain Restoration of as it moved ≥ 50 km from location RMo to RM3 valleys across the by dextral faulting and counter-clockwise rotation. Bagley Fault. SSSS See primary image Arrows trace out 50 km displacement for box on south along Bagley Fault system, including side (SS) and box path of Ragged Mountain (RM) and on north side (NS) offset of valleys in boxes NS and SS.

Figure 15. Landsat V false-color image of the west-central and western part of the Saint Elias orogen showing part of the foreland fold and thrust belt east of the Bering Glacier, and the multiple-phase deformed region of tectonic accretion and extrusion in the structural syntaxis west of the Bering Glacier. The two red dashed rectangles enclose remnants of NW-trending valleys that may have once been continuous across the Bagley fault. Restoration of the valleys by removing 50 km of dextral slip along the Bagley fault is illustrated in the lower right part of the ﬁ gure with each valley marked by a dashed line with the arrowhead pointing down-valley. Ragged Mount (RM) is a deformed sliver of the Alaskan plate margin that has been carried to the west and south by motion on the Bagley fault. Ragged Mountain is restored to a position near the head of the Martin River Glacier from its present position (RM3) through intermediate positions (RM2 → RM1 → RMo) by removing 50 km of dextral displacement on the Bagley fault. Note that locality RMo places Ragged Mountain adjacent to Mount Tom White and the upper Steller Glacier. The prominent peak of Mount Tom White rises abruptly from the restraining bend where the Bagley fault links to the Martin River Glacier fault. Bering Glacier fault is marked in red.

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deformation by tectonic extrusion and indenta- the northwestern end of the Fairweather fault. diffuse and spatially clustered seismicity with tion in the region west of the Bering Glacier. It Worthington et al. (2012) propose that driving focal mechanisms that indicate dextral shearing is interesting, albeit speculative, to note that this this wedge-shaped “door stopper” beneath the in some areas, but more complex deformation restoration aligns the several northwest-trending syntaxis may have triggered much of the uplift with both normal and reverse faulting in others glacier-fi lled valleys in the mountains along the and exhumation in the syntaxis. If so, then this (Fig. 4; Doser and Lomas, 2000; Doser et al., southeastern side of the Steller Glacier with the process overlapped in time with the ~20° clock- 2007; Ruppert, 2008); and the fault is favorably “headless” valleys located on the northern side wise rotation in relative motion between the oriented for reactivation in the contemporary of the Bagley Ice Valley between the Jefferies Pacifi c and North American plates, which trig- stress fi eld (e.g., see regional stress map of Rup- and Tana glaciers (Fig. 15, inset image). This gered oblique thrusting along the Queen Char- pert, 2008). The fault is capable of generating ≤ restored valley alignment could simply be for- lotte part of the transform boundary south of the Mw 8.0 earthquakes were it to rupture along tuitous given that glaciation can reset the land- Saint Elias region (Smith et al., 2003), and pre- the 125-km-long section extending between scape within a period of a few hundred thousand sumably increased the amount of transpression the upper Seward and Bering glaciers accord- years, but it is not outrageous given the long and in the Saint Elias orogen (Bruhn et al., 2004). ing to earthquake moment magnitude versus protracted history of oblique convergence along As noted by Bruhn et al. (2004), collision and surface rupture length equations of Wells and the plate boundary (Plafker et al., 1994). subduction of the Yakutat microplate into North Coppersmith (1994). This section of the fault The cause of the progressive eastward steps America provides a case study of terrane accre- zone is relatively linear and bounded by clus- in the megathrust décollement remains specula- tion that is the hallmark of mountain building ters of earthquake activity where the Malaspina tive, but the timing coincides with an increase worldwide. When studying ancient orogens, and Bering Glacier foreland thrust faults project in oblique transpression along the transform structural studies in the Saint Elias Orogen pro- toward and intersect the Bagley fault at depth. plate boundary created by an ~20° clockwise vide a number of caveats to consider when mak- Admittedly, the fault is a complex structural rotation of relative motion between the Pacifi c ing interpretations of plate motions and causes zone that contains several strike-slip and pre- and North America plates ca. 5 Ma (Smith et al., of deformation. Within the last several million sumably one or more reverse fault strands that 2003), The ratio of crustal shortening to dextral years deformation within the Saint Elias orogen may rupture independently to generate small to shearing increased within the orogen at that has been affected by climate-related erosion modest magnitude earthquakes, or alternatively, time, which would have inhibited lateral motion and deformation focusing, by plate boundary link together to form a complex seismic source and enhanced vertical motion along the Bagley geo metry, and perhaps lateral variations in the zone for larger earthquakes. fault. The proposed decrease in the rate of lat- crustal structure of the microplate and globally eral shearing together with initiation of reverse induced changes in relative motions between the CONCLUSIONS slip on the Bagley fault following plate motion interacting tectonic plates. In a more optimistic reorganization ca. 5 Ma is certainly consistent vein, our work, when integrated with that of (1) Collision and accretion of the Yakutat Ter- with plate margin structures and displacements others, provides considerable insight into how rane to North America provides a modern ana- inferred from geodynamic models of “fore-arc a major transform fault terminates and partially log for the geodynamics of mountain building slivers” presented by Haq and Davis (2010). controls tectonic uplift and rock exhumation in at cusp-like plate boundaries where deformation This change in fault behavior may be cap- a structural syntaxis. Additionally, we are able transitions from dominantly transform motion to tured by the thermochronology data, which indi- to better elucidate the underlying structure and more intense transpression within the orogen. In cates young uplift and rock exhumation along tectonic signifi cance of the Bagley Ice Valley, the Saint Elias orogen, the plate boundary cusp the southern side of the Bagley fault (Berger which is one of the most signifi cant geomorphic causes termination of the Fairweather transform et al., 2008a; Chapman et al., 2008), although features in the plate margin of southern Alaska. fault and formation of a tectonic crustal sliver the authors of those studies prefer a model in where the Bagley fault cuts through the south- which glacial erosion on the windward side of Seismic Potential of the Bagley Fault ern edge of the North American plate to inter- the mountain range triggered “back-thrusting” sect the megathrust at depth. This fault bounded along the Bagley fault. Estabrook et al. (1992) noted that the Bagley “sliver” has a complex history of strike-slip and The structural style and exhumation history fault is at the very least an important structural transpressional deformation created by tempo- (e.g., Enkelmann et al., 2009) of the region sur- and mechanical boundary in the orogen. The ral changes in plate motion and underthrusting rounding the upper Seward Glacier is broadly Bagley fault blocked updip rupture propaga- of microplate lithosphere with spatially variable consistent with geodynamic models of tectonic tion on faults 1a and 1b (Fig. 8) during the crustal thickness.

aneurysms developed in general by Koons Mw 7.4 Saint Elias earthquake, and also bounded (2) Rapid uplift and exhumation surrounding (1995) and Koons et al. (2010), and for the an offset or step in the hypocenter depths of after- the Seward Glacier is caused by crustal con- Saint Elias origin speciﬁ cally (Koons et al., shocks. We add that the Bagley fault also marks traction related to thrust faulting and transpres- 2010). That is, tectonic upwelling to create high the terminus of several active thrust faults that sional strain beneath the mountains and local- topography and rapid exhumation of rocks is a cut across the foreland of the Yakutat microplate ized extension high in the crust surrounding fundamental physical response of deformation and project beneath the western Seward Glacier the terminus of the Fairweather transform fault. in a tectonic “corner” like that formed where basin and Bagley Ice Valley (Figs. 2 and 16). Evidence for dextral shearing along the Bagley the plate boundary develops a cusp. In the case Although recent geodetic results across the fault includes a series of NNE-trending ridges of the Yakutat microplate the spatial variations Bagley Ice Valley suggest minimal strain accu- and swales on the surface of the Bagley Gla- in the thickness of the underthrust crust may mulation rates (Elliott, 2011), there is a strong cier that are offset in a dextral sense and right- also be in play (Worthington et al., 2012). The rationale for considering the Bagley fault handed jogs in the northern wall of the eastern crust of the microplate thickens to the south- capable of generating earthquakes. The fault is Bagley Ice Valley at the Quintino Sella and Jef- east, which in turn increases the buoyancy of structurally linked to major seismogenic faults feries glaciers. A tentative correlation of NW- the microplate approaching the syntaxis at in the orogen (Fig. 16); it is marked by both trending valleys that are truncated by the Bagley

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A MODIS satellite image of Saint Elias orogen with Bagley and other major faults

Bagley Fault

Martin River Yahtse & F1b Guyot Art Lewis Glacier Fault fault Glac. BG Ragged Mtn. uplift fault by duplexing EC Yakataga duplex faulting YF Malaspina Fairweather Yakutat Bay Fault N Pamplona fault system fold and fault zone 50 km Alsek River

B 3D Model of major faults that link structurally to the Bagley fault Bagley Fault Martin Bagley Fault River Fault (western) (eastern) Fault 1b Fairweather Art Lewis BeringBering - TanaTana Fault GlacierGlacier ooutletutlet Glacier Fault

Sewarrdd GGlaclacier

Megath BeringBering rust 0 GlacierGlacier faultfault 6.74 ) (BG)(BG) YakatagaYakataga Bancas-EskerBancas-Esker 6 10 6.70 duplexduplex faultingfaulting MalaspinaMalaspina CreekCreek faultfault (EC)(EC) Depth (km) faultfault systemsystem YakutatYakutat thrustthrust 6.66 faultfault (YF)(YF) 3.5 4.0 6.62 4.5 Easting (m x 10 5.0 thing (m x 10 5) 5.5 6.0 Nor Figure 16. Preliminary three-dimensional model of tectonically active faults in the Saint Elias orogen and their geometrical relationship to the Bagley fault, which is modeled as a vertical surface. The locations of the faults are shown in Figure 2. The strikes and dips of the faults are taken from the references noted in Tables 1 and 2. Note that the Bagley fault lies in the hanging wall of the megathrust décollement that accommodates subduction of the crystalline basement (crust and upper mantle) of the Yakutat microplate. The other faults are large imbricate thrust faults, or major strike-slip faults (e.g., Fairweather, Art Lewis Glacier, and Martin River faults).

fault indicates 50 km of dextral displacement, duction front migrated eastward to its present of fault zone between the upper Seward Glacier which is probably a minimum value. position near the eastern syntaxis, partly bypass- and the outlet of the Bering Glacier piedmont (3) The Bagley fault has a protracted history ing the slip-partitioned and structurally linked lobe, which is demarcated by clusters of earth- of deformation caused by collision of the Yaku- Chugach–Saint Elias–Bagley fault system. quake activity at its ends. tat microplate. This includes both dextral shear- (4) The Bagley fault is considered active ACKNOWLEDGMENTS ing as part of the slip-partitioned plate margin, and a potential earthquake source because it is and vertical uplift of the mountains along the directly linked to strike-slip and thrust faults that This research was supported by the National southern side of the Bagley Ice Valley. The have undergone Quaternary displacements and Aeronautics and Space Administration grant entitled vertical component of motion was presumably generated historical earthquakes. The largest “Geodetic imaging of glacio-seismotectonic pro- enhanced during the last ca. 5 Ma when the sub- earthquake of M ≤ 8.0 may occur on the section cesses in southern Alaska,” awarded to J. Sauber,

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R.L. Bruhn, and R.R. Forster. Geological fi eld plate, south central Alaska: Eos (Transactions, Ameri- Enkelmann, E., Zeitler, P.K., Pavlis, T.L., Garver, J.I., and observations and measurements cited in the paper can Geophysical Union), v. 67, p. 1195. Ridgway, K.D., 2009, Intense localized rock uplift and were done in part during the Saint Elias Erosion Campbell, R.B., and Dodds, C.J., 1982, Geology, Mt. St. erosion in the St. Elias orogen of Alaska: Nature Geo- and Tectonics Project, with funding through the Elias, Yukon Territory: Geological Survey Canada science, v. 2, p. 360–363, doi:10.1038/ngeo502. Open-File Report 830, scale 1:125,000, 1 sheet. Enkelmann, E., Zeitler, P.K., Garver, J.I., Pavlis, T.L., and National Science Foundation (NSF) Continental Chapman, J. B., Worthington, L.L., Pavlis, T.L., Bruhn, Hooks, B., 2010, The thermochronological record of Dynamics Program and Offi ce of Polar Programs, by R.L., and Gulick, S.P.S., 2011, The Suckling Hills tectonic and surface process interaction at the Yakutat– grants EAR0409009 and EAR0735402 to Pavlis and Fault, Kayak Island zone, and accretion of the Yaku- North American collision zone in southeast Alaska: EAR0408959 to Bruhn. tat microplate, Alaska: Tectonics, v. 30, TC6011, American Journal of Science, v. 310, p. 231–260, doi:10.1029/2011TC002945. doi:10.2475/04.2010.01. 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