The Denali Fault of the Eastern Alaska Range

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

Spatial variations in focused exhumation along a continental-scale strike-slip fault: The Denali fault of the eastern Alaska Range

Jeff A. Benowitz1, Paul W. Layer1, Phil Armstrong2, Stephanie E. Perry3, Peter J. Haeussler4, Paul G. Fitzgerald3, and Sam VanLaningham5 1Geophysical Institute, University of Alaska Fairbanks, Fairbanks, Alaska 99775, USA 2Department of Geological Sciences, California State University, Fullerton, 800 N. State College Boulevard, Fullerton, California 92831, USA 3Department of Earth Sciences, Syracuse University, Syracuse, New York 13244, USA 4U.S. Geological Survey, 4200 University Drive, Anchorage, Alaska 99508, USA 5Institute of Marine Science, University of Alaska Fairbanks, Fairbanks, Alaska 99775, USA

ABSTRACT in New Zealand, the Denali fault in Alaska, and early Miocene (~22 Ma). A major contributing the San Andreas fault in California (e.g., Little factor to rapid exhumation in the eastern Alaska 40Ar/39Ar, apatite fi ssion-track, and apa- et al., 2005; Fitzgerald et al., 1995; Spotila Range during the Neogene is most likely the tite (U-Th)/He thermochronological tech- et al., 1998). However, the cause of rapid exhu- far-fi eld effects of the continuing collision of niques were used to determine the Neogene mation along these faults is often complex with the Yakutat microplate with southern Alaska exhumation history of the topographically near-fi eld (<20 km) boundary conditions and (North American plate). asymmetric eastern Alaska Range. Exhu- far-fi eld plate tectonic driving mechanisms con- Our data show that exhumation is greatest in mation cooling ages range from ~33 Ma to tributing to exhumation patterns (e.g., Buscher a narrow wedge (<10 km wide) proximal to the ~18 Ma for 40Ar/39Ar biotite, ~18 Ma to ~6 Ma and Spotila , 2007). Unexpectedly, the amount Denali fault and is partitioned across and along for K-feldspar minimum closure ages, and of exhumation along a strike-slip fault is not strike in relation to possible changes in fault ~15 Ma to ~1 Ma for apatite fi ssion-track always a simple correlation with the degree geometry. Rapid ongoing exhumation in the ages, and apatite (U-Th)/He cooling ages of obliquity (of the plate motion vector with eastern Alaska Range was possibly enhanced in range from ~4 Ma to ~1 Ma. There has been respect to the fault trace) or composition of the late Pliocene with the transition to more effi - at least ~11 km of exhumation adjacent to the juxtaposed rocks (e.g., Roeske et al., 2007; cient glacial erosion with the onset of the North- the north side of Denali fault during the Neo- Spotila et al., 2007). Climate change (Whipple, ern Hemisphere glaciation (~3 Ma; Lisiecki and gene inferred from biotite 40Ar/39Ar thermo- 2009), structural irregularities such as stepovers Raymo, 2005) and increased erosional forcing chronology. Variations in exhumation history (Hilley and Arrowsmith, 2008), variations in due to climatic instability (Clift, 2010). along and across the strike of the fault are master fault dip (Dair and Cooke, 2009), and infl uenced by both far-fi eld effects and local changes in plate motion (e.g., Fitzgerald et al., GEOLOGICAL BACKGROUND structural irregularities. We infer deforma- 1995) all can play an important role in observed tion and rapid exhumation have been occur- regional exhumation patterns. Given that many Denali Fault ring in the eastern Alaska Range since at least orogenic belts associated with strike-slip faults ~22 Ma most likely related to the continued have hetero geneous exhumation patterns, there The Denali fault system in central Alaska collision of the Yakutat microplate with the is a need for more case studies to examine how makes a broad arc through the extensively glaci- North American plate. The Nenana Moun- vertical motion is related to both near-fi eld ated Alaska Range (Fig. 1; St. Amand, 1957). It tain region is the late Pleistocene to Holocene boundary conditions and far-fi eld driving mech- is a major intracontinental, right-lateral strike- (~past 1 Ma) primary locus of tectonically anisms (Buscher and Spotila, 2007). slip fault that is still active as demonstrated by driven exhumation in the eastern Alaska The Denali fault, an active continental-scale the 2002 M7.9 Denali fault earthquake (Eber- Range, possibly related to variations in fault strike-slip fault, has an asymmetric topographic hart-Phillips et al., 2003). Neogene movement geometry. During the Pliocene, a marked signature (the Alaska Range), making the and deformation are driven by plate tectonic increase in climatic instability and related region an ideal location to examine the contri- processes at the Alaska southern margin subduc- global cooling is temporally correlated with bution of local and regional structures on strain tion zone 500 km to the south (e.g., Haeussler, an increase in exhumation rates in the eastern partitioning and the timing of exhumation adja- 2008). Matmon et al. (2006) and Mériaux et al. Alaska Range north of the Denali fault system. cent to the fault. We employ 40Ar/39Ar, apatite (2009) dated late Pleistocene moraines offset fi ssion-track (AFT), and apatite (U-Th)/He by the Denali fault in the eastern Alaska Range INTRODUCTION (AHe) thermochronology to document cooling, and show that modern slip rates are higher to and thus exhumation patterns along the Denali the east at ~12 mm/yr, decreasing to the west at Regions of focused rapid (>0.5 mm/yr) exhu- fault system in the eastern Alaska Range. We ~7 mm/yr, which suggests that slip is predomi- mation are a common occurrence along major fi nd that during the Neogene, rapid exhumation nately being partitioned between the Denali strike-slip fault systems such as the Alpine fault in the eastern Alaska Range was initiated by the fault and the Northern Foothills thrust belt and

Geosphere; April 2011; v. 7; no. 2; p. 455–467; doi: 10.1130/GES00589.1; 13 fi gures; 1 supplemental fi gure fi le; 1 supplemental text fi le; 5 supplemental tables.

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Tanana Basin NR N Mount Fig. 2 McKinley

CAR EAR DF DF Broad DR 62°°N Pass Alaska Mt. McKinley mm/yr Canada 7 12 U.S. Elevation Denali mm/yr Quaternary faults (m) Fault ? Neogene faults WAR ? 62° Fold axes 6000 To Bering WRANGELLS. ALASKA tschunda Faul 5000 BlockCook BLOCK NORTH Inlet 4000 t AMERICAN 60° 3000 Chugach/St. Elias PLATE Seldovia 2000 Arch 1000 4.650 mm/yrcm/yr Stevenson Basin 050100km 58° t s Gulf of u r Alaska h Kodiak t F a Island g a e 54 mm/yr irw M 5.4 cm/yr Yakutat YAKUTAT F n ° a au e 56 t i ath e u PACIFIC MicroplateBLOCK A l lt PLATE er 50 0 100 200 300 km 135° 153°°W 150° 145° 140°

Figure 1. Tectonic map of southern Alaska, modifi ed from Haeussler et al. (2000), showing major faults, tectonic plates, and tectonic blocks. Decreasing westward Pleistocene Denali fault slip rates are from Matmon et al. (2006) and Mériaux et al. (2009). Yakutat– North America plate motion (solid arrow) is from Elliot et al. (2010). The open arrows show the inferred movement of the southern Alaska block with respect to North America. The western boundary of the southern Alaska block is poorly defi ned. The gray dashed line denotes the modern- day location of the subducted Yakutat slab beneath the continental margin (after Eberhart-Phillips et al., 2006). Detailed digital elevation model of segmented Alaska Range fl ooded to 1000 m elevation to emphasize higher topography. Most of the western and central Alaska Range are south of the Denali fault. The eastern Alaska Range is mostly north of the Denali fault. The rectangle delineates the study area in the eastern Alaska Range along the Denali fault shown in Figure 2.WAR—Western Alaska Range; CAR—Central Alaska Range; EAR— Eastern Alaska Range; DF—Denali Fault; DR—Delta River; NR—Nenana River.

being siphoned off the Denali fault system onto topography (e.g., Broad Pass, ~700 m, separat- ~6 Ma, although the western Alaska Range data contractional structures south of the Denali fault ing the central Alaska Range from the eastern set shows evidence of an earlier phase of rapid (Figs. 1 and 2; Bemis and Wallace, 2007; Crone Alaska Range). The majority of thermochrono- exhumation ~23 Ma. et al., 2004). metric data from the Alaska Range are from With the available low-temperature data, it the central Alaska Range (Plafker et al., 1992; is logical to view the Alaska Range as a sin- Uplift History of the Alaska Range Fitzgerald et al., 1993, 1995), including a 4 km gular orogenic feature, which most likely has vertical proﬁ le in the Mount McKinley region of experienced a similar region-wide exhumation The ~650-km-long Alaska Range follows the central Alaska Range (e.g., Fitzgerald et al., pulse at ~6 Ma. However, in the western Alaska the curve of the Denali fault system. Areas of 1995) and from AFT work done in the western Range and central Alaska Range, all regions of high-peak elevation (e.g., 6194 m in the central Alaska Range (Haeussler et al., 2008). These high topography (e.g., Mount McKinley) are Alaska Range and 4216 m in the eastern Alaska thermochronometric data sets indicate that south of the Denali fault. This observation is Range) are separated by broad regions of low the modern-day Alaska Range started to form in stark contrast to the eastern Alaska Range

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N TB -- -- TB 63.8°N NFTB -- -- HCF -- 9.7 ------Elevation 28.5 MB -- 3.5 -- -- in meters MD ------8.5 21.3 -- -- 3.7 NM -- 6.7 -- 22.5 -- 4000 -- -- 5.1 -- -- 12.8 -- 3500 -- 10.7 14.8 -- -- 36.2 ------33.3 -- 10.9 68.4 3000 -- 12.5 --

Nenana River -- 32.5 2500 SC 38.3 37.9 -- River Delta PP 1.2 ------2000 ------18.4 -- 30.0 -- 18.6 1500 30.0 37.9 32.5 1000 36.9 -- BRG -- 2.7 -- 500 -- -- 3.3 -- 1.4 4.5 – 1.4 ------Denali Faul AHE 6.0 -- 2.8 15.5 29.9 2.8 AFT 6.5 -- 28.6 ------34.6 K-Spar 6.5 30.2 -- 32.5 t Min. Age Biotite -- -- 30.2 18.2 Fig. 3 0 10 20 km Italics = K/Arb iotite 148°W

Figure 2. Colored digital elevation map of the eastern Alaska Range showing sample locations and major faults. Sample labels show apatite helium (AHe), apatite ﬁ ssion-track (AFT), potassium feldspar minimum (K-spar), and biotite closure ages in descending order. Two hyphens (--) indicate no data. Italics refer to previous K-Ar work from Csejtey et al. (1992) or Nokleberg et al. (1992). The red cross shows the epicenter of the 2 November 2002 M7.9 Denali fault earthquake; the yellow cross shows the epicenter of the 23 October 2002 M6.7 Nenana Mountain earthquake. NFTB—Northern Foothills thrust belt; HCF—Hines Creek fault; SGTF—Susitna Glacier thrust fault; DF—Denali fault; PP—Panorama Peak; SC—Schist Creek pluton; NM—Nenana Mountain transect; MD—Mount Deborah transect; MB—Mount Balchen transect; BRG—Black Rapids glacier; MET—Meteor Peak region. The rectangle delineates the study area in the eastern Alaska Range along the Denali fault shown in Figure 3.

where all of the high peaks are located north Plutons north and south of the Susitna Glacier furnace (Supplemental Table 11). Six of the of the Denali fault (Fig. 1). In addition, Ridg- have ~70 Ma U-Pb emplacement ages (Aleini- furnace-run samples were then modeled using way et al. (2007), in an analysis of the Miocene koff et al., 2000). Nenana Mountain, Schist a multi-domain diffusion modeling (MDD) Usibelli Group and Pliocene Nenana Gravel Creek and Panorama plutons are thought to have approach (e.g., Lovera et al., 2002) to determine within the Tanana Basin, suggested develop- ~38 Ma emplacement ages based on K-Ar dating thermal histories and an associated closure tem- ment of signifi cant topography in the eastern of biotite and hornblende (Csejtey et al., 1992). perature for the minimum isochron age group- Alaska Range prior to the late Miocene. Thus, Both sides of the Denali fault in the high-peak ing (K-spar minimum closure age). Unaltered the spatial and temporal evolution of the Alaska region of the eastern Alaska Range experience biotite (16), muscovite (2), and hornblende (1) Range may be more complex than the existing the same climatic regime (precipitation of ~1500 (Supplemental Table 22), when present, were data suggest. to ~2000 mm/yr; Manley and Daly, 2005). also analyzed using 40Ar/39Ar laser step heating to supplement the time-temperature cooling tra- Physiography of the Eastern Alaska Range METHODS jectories of rocks in the Alaska Range. Eight new AFT (Supplemental Table 33) and Unlike the western and central Alaska Range, Sampling and Analytical Techniques three new AHe ages (Supplemental Table 44) are the high peaks of the eastern Alaska Range 1 40 39 are sandwiched between the active McKinley We applied 40Ar/39Ar, AFT, and AHe thermo- Supplemental Table 1. Word fi le of Ar/ Ar data from potassium feldspar. If you are viewing the PDF of strand of the Denali fault and the presumed- chronology to 30 bedrock samples along and this paper or reading it offl ine, please visit http://dx.doi to-be inactive Hines Creek strand (Wahrhaftig across the strike of the eastern Alaska Range .org/10.1130/GES00589.S1 or the full-text article on et al., 1975). High topography (>3000 m) is proximal to the Denali fault system in order to www.gsapubs.org to view Supplemental Table 1. limited to a narrow region (<20 km) along the better constrain the timing and patterns of exhu- 2Supplemental Table 2. Word fi le of 40Ar/39Ar and K-Ar data from biotite, muscovite, and hornblende. If north side of the Denali fault. There are numer- mation in the eastern Alaska Range. Sampling you are viewing the PDF of this paper or reading it off- ous identifi ed reverse and thrust faults in the was focused along three transects north of the line, please visit http://dx.doi.org/10.1130/GES00589 greater eastern Alaska Range region with many Denali fault: Nenana Mountain transect (NM), .S2 or the full-text article on www.gsapubs.org to located in the northern foothills fold and thrust Mount Deborah transect (MD), and Mount view Supplemental Table 2. 3 belt to the north of the Hines Creek fault (Bemis Balchen transect (MB) (Fig. 2). One transect Supplemental Table 3. Word file of apatite fi ssion-track analysis. If you are viewing the PDF of and Wallace, 2007). There are few active and was collected south of the Denali fault in the this paper or reading it offl ine, please visit http://dx.doi inactive structures identifi ed in the ice-covered, Meteor Peak region (Fig. 3), which is the largest .org/10.1130/GES00589.S3 or the full-text article on high-peak region of the eastern Alaska Range, expanse of high topography south of the Denali www.gsapubs.org to view Supplemental Table 3. with the exception of the Susitna Glacier thrust fault in the eastern Alaska Range. 4Supplemental Table 4. Word file of apatite U-Th/He data. If you are viewing the PDF of this fault, which was only recognized after it rup- Eighteen samples were analyzed using paper or reading it offl ine, please visit http://dx.doi 40 39 tured during the 2002 M7.9 Denali earthquake Ar/ Ar thermochronology on potassium feld- .org/10.1130/GES00589.S4 or the full-text article on (Fig. 2; Crone et al., 2004). spar (K-spar) using either a laser or a resistance www.gsapubs.org to view Supplemental Table 4.

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Denali Fault -- N -- 28.2 Elevation 1.5 -- 15.5 -- in meters ------4000 -- -- 3500 -- 29.9 BRG -- 34.6 -- 3000 -- -- 2500 35.4 28.6 32.5 2000 -- -- 31.6 1500 ------29.0 1000 28.7 28.8 -- 500 31.9 31.4 -- Meteor 32.6 ------63.40°N --

AHE 1.4 2.8 AFT K-Spar 6.5 Min. Age 30.2 Biotite 30.4 Muscovite 146.90°W 34.4 Ho 0 05 10 km

Figure 3. Colored digital elevation map of the eastern Alaska Range showing sample locations south of the Denali fault. Sample labels show apatite helium (AHe), apatite ﬁ ssion-track (AFT), K-spar minimum (K-spar), biotite, muscovite, and hornblende closure ages in descending order. Two hyphens (--) refer to no data. BRG—Black Rapids glacier; Meteor—Meteor Peak region.

presented for the eastern Alaska Range. Further constrained by other thermochronology stud- (Tc 150 °C), and also for AFT (Tc 110 °C) details of the 40Ar/39Ar, AFT, and AHe methods ies in Alaska (O’Sullivan and Currie, 1996; and AHe (Tc 65 °C) cooling ages (Harrison and closure temperature and data are presented Haeussler et al., 2008; McAleer et al., 2009). et al., 2009; Reiners and Ehlers, 2005; e.g., in the Supplemental Text File5 and Supplemen- For the purpose of constraining fi rst-order esti- McDougall and Harrison, 1999). Precise spatial tal Table 56. mates on exhumation during the Neogene, we differences in exhumation rates and/or trends assume that the gradient remained spatially and are not critical because the variations are large The Geothermal Gradient and temporally uniform. Thus, we assume the same enough to be signifi cant and are not artifacts Closure Temperatures depth to closure isotherms across the region. We of variations in geothermal gradients. We also acknowledge that for the low-temperature AFT do not give quantitative rates of cooling, but Along the Denali fault, maximum aftershock and AHe data, due to their lower closure temper- instead discuss overall trends in cooling rates to depths of ~11 km suggest the brittle-ductile atures, and hence shallower depths of closure, avoid over interpreting time-averaged thermo- transition and approximate a regional geother- plus higher Pliocene–Quaternary exhumation chronological constraints. In addition, the dip mal gradient to ~30 °C/km (Fisher et al., 2004), rates, this assumption is a simplifi cation and the of the Denali fault at depth is poorly constrained which we use to estimate the depth to closure resulting trends, an approximation. However, (see discussion below), but we assume general temperature (Tc) for the various thermochrono- the majority of the data in this manuscript are subvertical rock trajectories based on available logic systems. Approximately 30 °C/km is also from high-temperature thermochronometers structural work done in the region (Sherwood the same standard regional geothermal gradient with deep closure depths. These ages are less and Craddock, 1979). These assumptions about affected by high Quaternary exhumation rates temperature and, hence, depth of closure allow 5Supplemental Text File. PDF fi le of supplemental and variation in isotherm depth related to sur- us to examine near-fi eld exhumation trends text. If you are viewing the PDF of this paper or read- face topography (e.g., Braun et al., 2006). We and patterns during the Neogene in the eastern ing it offl ine, please visit http://dx.doi.org/10.1130/ GES00589.S5 or the full-text article on www.gsapubs fi rst discern whether the higher temperature Alaska Range. .org to view the Supplemental Text File. cooling ages are related to initial emplacement 6Supplemental Table 5. Word fi le of 40Ar/39Ar data cooling or cooling related to exhumation before RESULTS for biotite, muscovite, hornblende, and K-spar. If you calculating depth of closure. are viewing the PDF of this paper or reading it offl ine, please visit http://dx.doi.org/10.1130/GES00589.S6 We use standard closure temperatures for Most hornblende, muscovite, and biotite 40 39 or the full-text article on www.gsapubs.org to view Ar/ Ar dating of hornblende (550 °C), musco- analyses from samples on both sides of the Supplemental Table 5. vite (Tc 400 °C), biotite (Tc 350 °C), K-spar Denali fault show well-defi ned 40Ar/39Ar age

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plateaus with simple spectra with little or no 70 argon loss (Figs. A1 to A19 in the Supplemental 60 2 Figure File7, Supplemental Tables 2 and 5 [see R = 1.00 footnotes 2 and 6]). This evidence of unaltered Figure 4. Integrated age ver- 50 samples is demonstrated by the high concor- sus plateau age for the entire 40 39 40 dance between integrated ages (whole gas) and Ar/ Ar hornblende, muscovite, and biotite data set. The plateau ages for the hornblende, muscovite, 30 similarity between the two age N = 20 and biotite analyses (Fig. 4). Biotite (16) determinations demonstrates Integrated age (Ma) 20 Muscovite (2) Along the Balchen transect, biotite cooling Horblende (1) little or no argon loss for the ages decrease southwards toward the Denali 10 fault from ~33.3 Ma to ~18.4 Ma (Fig. 2). Proxi- sample analyses. 0 mal Black Rapids, Mount Deborah, and the far 0 10203040506070 southeastern part of Nenana Mountain pluton Plateau age (Ma) have biotite cooling ages of ~32 Ma to ~18 Ma. The west part of the Nenana Mountain region, Schist Creek, and Panorama plutons have biotite Biotite ages of ~32 Ma to ~18 Ma from intru- and AHe ages indicate that Neogene rapid exhu- ages of ~38 Ma. The Mount Deborah sample, sions on the north side of the Black Rapids gla- mation has been a long-term (~22 Ma to pres- which is most distal from the Denali fault, has a cier and eastern edge of the Nenana Mountain ent) phenomenon along the north side of the biotite age of ~68 Ma. pluton are interpreted as exhumation-related Denali fault in the eastern Alaska Range. K-spar minimum closure ages for Nenana cooling ages based on the respective emplace- Mountain, Mount Deborah, Mount Balchen, and ment ages of the plutons (~70 Ma; ~38 Ma). South of the Denali Fault the north side of the Black Rapids glacier regions Because the emplacement age of the Balchen In contrast to samples north of the fault, ortho- range from ~18.6 Ma to ~6.0 Ma. K-spar mini- intrusion is ~70 Ma (Aleinikoff et al., 2000), gneiss samples south of the Denali fault have mum closure ages west of the Nenana Mountain the trend of younger biotite ages (~33.3 Ma fl at, ~30 Ma K-spar age spectra that are con- region and also south of the Denali fault (Figs. 2 to ~18.4 Ma) toward the Denali fault dem- cordant with 40Ar/39Ar hornblende, muscovite, and 3) are distinctly older (~28.6 to ~30.8 Ma). onstrates that the biotite ages are cooling and biotite ages (~35 Ma to ~31 Ma) from the K-spar MDD modeling of six samples from ages related to exhumation, with exhumation same samples (Figs. 5 and 9). We interpret this the region north of the Denali fault indicates increasing toward the fault (Fig. 2). The Mount rapid cooling event as a short-lived metamor- that K-spar minimum ages represent cooling Deborah pluton, like the Balchen pluton, is phic event because the samples have a penetra- through the ~150 °C isotherm. We interpret assumed to have an ~70 Ma emplacement age tive fabric, and they have concordant 40Ar/39Ar MDD models as indicating a diffuse pattern for based on the upper sample having a biotite age of cooling ages, which is in general agreement the initiation of rapid cooling. ~68.4 Ma (Fig. 2). The Mount Deborah sam- with past interpretations for the region (e.g., Approaching the Denali fault from the north, ples (proximal to the Denali fault) have biotite Nokleberg et al., 1992). Proximal to the Denali AFT ages generally decrease from ~6.7 Ma to cooling ages that are related to exhumation fault, ~15.5 Ma AFT and ~4.5 Ma AHe ages on ~1.2 Ma with the youngest ages in the Nenana (~32 Ma to ~28 Ma). the south side of the fault are older than AFT Mountain region. AHe ages north of the Denali The K-spar minimum ages (~30 Ma), the ~1.2 Ma AFT and ~1.5 Ma AHe ages on the fault range from ~4.0 Ma to ~1.5 Ma. Near the fl at shape of the K-spar 40Ar/39Ar age spectra, north side of the fault. These low-temperature south side of the Denali fault, we report an AFT and biotite 40Ar/39Ar ages (~38 Ma) for the Pan- data combined with the aforementioned 40Ar/39Ar age of ~15.5 Ma and one AHe age of ~4.5 Ma orama and Schist Creek plutons imply rapid thermochronology results indicate a different (Figs. 2 and 3). postemplacement cooling for both plutons exhumation history for the south side of the through the minimum closure temperature for Denali fault than the north side. DISCUSSION K-spar (Figs. 5 and 6). The downward-stepping K-spar 40Ar/39Ar age spectra and MDD model- Across-Strike Infl uence of the Denali Fault Along-Strike Exhumation ing of furnace runs of samples from Nenana Mountain, Mount Deborah, Mount Balchen The trend of cooling ages decreasing toward North of the Denali Fault transects, and the Black Rapids glacier regions the Denali fault from the north is directly cor- Biotite ages of samples from the westernmost north of the Denali fault confi rm that the mini- related with both elevation (Fig. 10A) and Nenana Mountain region have ages (~38.3 Ma mum closure ages indicate rapid cooling due to distance from the fault (Fig. 10B) for all four to ~36.2 Ma; Fig. 2) concordant with the exhumation through ~150 °C (Figs. 5, 7, and 8). thermochronometric systems. This relation- emplacement age of the pluton (~38 Ma) indi- The MDD K-spar thermal models from north ship indicates that exhumation just north of the cating postemplacement rapid cooling and not of the Denali fault show relatively diverse cool- Denali fault in the eastern Alaska Range has exhumation-related cooling. Schist Creek and ing histories with rapid cooling occurring in been focused there and persisted for at least the Panorama plutons have biotite ages of ~38 Ma individual samples between ~22 Ma and ~6 Ma past ~20 m.y. Although cooling age is correlated and are interpreted as being related to initial (Fig. 8). These north-side thermal models sug- with both elevation and distance from the Denali melt emplacement cooling as well. gest heterogeneous exhumation along and across fault, distance from the fault is more signifi cant. the strike of the Denali fault. AFT ages range A good example of this relationship is seen in 7Supplemental Figure File. PDF fi le of 43 images. from ~6.7 Ma (Mount Balchen) to ~1.2 Ma the Mount Balchen transect K-spar minimum If you are viewing the PDF of this paper or reading it offl ine, please visit http://dx.doi.org/10.1130/ (Nenana Mountain region), and AHe ages range cooling age data where proximity to the Denali GES00589.S7 or the full-text article on www from ~4 Ma to ~1.5 Ma (Mount Balchen). Com- fault has a stronger effect on cooling age than .gsapubs.org to view the Supplemental Figure File. bined with the biotite and K-spar data, the AFT elevation (Fig. 10).

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TB N 63.8°N NFTB TB Elevation HCF 26 BAL in meters 18 BAL 4000 3500 03 BAL 3000

Nenana

River 2500 Delta River Delta SC 2000 PP 1500 322 AST 03 RAP 01PAN BRG 1000 500 32 NEN 22 DEB 45 RAP Denali Fault 311 AST

0 10 20 km 148°W

Figure 5. Colored digital elevation map of the eastern Alaska Range showing major faults and sample locations for those in Figures 6, 7, 8, and 9. The red cross shows the epicenter of the 2 November 2002 M7.9 Denali fault earthquake; the yellow cross indicates the epicenter of the 23 October 2002 M6.7 Nenana Mountain earthquake. NFTB—Northern Foothills thrust belt; HCF—Hines Creek fault; SGTF—Susitna Glacier thrust fault (shown in blue); PP—Panorama Peak; SC—Schist Creek pluton; BRG—Black Rapids glacier.

22DEB FS#F1 age spectrum 01PAN FS#L2 age spectrum 322AST FS#L1 age spectrum 40 40 40

30 30 30

20 20 Ma) 20

Age (Ma)

Age ( Age (Ma) 10 10 10

0 0 0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Fraction of 39Ar released Fraction of 39Ar released Fraction of 39Ar released Figure 7. 40Ar/39Ar age spectra for K-spar Figure 6. 40Ar/39Ar age spectra for K-spar from sample 01PAN from Mount Panorama and from sample 22DEB from the Mount Deborah sample 322AST from Schist Creek pluton. The general fl at shape of the age spectra implies transect. The downstepping nature of the rapid postemplacement cooling. Sample locations are shown on Figure 5. release indicates a prolonged and compli- cated thermal history. Sample location is shown on Figure 5. See Figs. A22–A31 in the Thermochronometric data from the south exhumation depths (Fig. 11). The difference Supplemental Figure File (see footnote 7) side of the Denali fault show no strong eleva- in cooling ages across the Denali fault defi nes for other samples. tion or distance-from-fault trends. Muscovite, a narrow (<~10km) wedge of deep Neogene biotite, and K-spar minimum cooling ages are exhumation in the eastern Alaska Range. It is all concordant and suggest very rapid cooling at also clear that the region south of the Denali ~30 Ma throughout the area (Figs. 2 and 3). No fault in the eastern Alaska Range experienced 40Ar/39Ar K-spar systems being concordant. graphical representation is presented because less total exhumation during the Neogene than Because 40Ar/39Ar biotite ages (Tc ~350 °C) the cooling ages would simply overlap—biotite the region juxtaposed to the north. from samples north of the Denali fault between ages are all ~31 Ma; K-spar minimum ages are Samples south of the Denali fault have only the Black Rapids glacier and the southeast end all ~29 Ma. been exhumed through the AFT closure system of the Nenana Mountain pluton are exhumation- during the Neogene and experienced less than related closure ages, this region experienced Overall Depth of Exhumation Patterns in ~5 km of exhumation since ~28 Ma based on signifi cant enough unroofi ng to exhume rocks the Eastern Alaska Range the assumed geothermal gradient of ~30 °C/km, that have passed through the ~350 °C isotherm. the assumed Tc of ~150 °C for K-spar, and the Therefore, the region has undergone a minimum In the eastern Alaska Range, the depth of concordant muscovite, biotite, and K-spar ages of ~11 km of exhumation. The region around Neogene exhumation varies dramatically across in the region. To the west, Panorama and Schist the epicenter of the 23 October 2002 M6.7 and along the Denali fault allowing for general Creek plutons experienced less than ~5 km of Nenana Mountain earthquake has experienced contouring of Neogene minimum or maximum exhumation based on the 40Ar/39Ar biotite and at least ~5 km of exhumation. This conclusion

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26BAL cooling history 03RAP cooling history

400 400

300 300

200 200

Temperature (°C) 100 100

0 0 0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 30 35 40 Age (Ma) Age (Ma)

03BAL cooling history 18BAL cooling history

400 400

300 300

200 200

100 Temperature (°C) 100

0 0 0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 30 35 40 Age (Ma) Age (Ma)

22DEB cooling history 32NEN cooling history

400 400

300 300

200 200

Temperature (°C) 100 100

0 0 0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 30 35 40 Age (Ma) Age (Ma)

Figure 8. Monotonic multiple diffusion domain (MDD) thermal models for K-spar from samples 26BAL, 03RAP, 03BAL, 18BAL, 22DEB, and 32NEN. The MDD magenta band is the 90% conﬁ dence interval of the mean, and the purple band is the 90% conﬁ dence of the distribution. Sample locations are shown on Figures 5.

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311AST FS#F1 age spectrum 45RAP FS#L1 age spectrum 40 40 Modern earthquake behavior also varies along strike. The 23 October 2002 M6.7 Nenana Mountain earthquake had a westward-propagat- 30 30 ing rupture (Lu et al., 2003). However, the 2002 7.9M Denali fault earthquake had an eastward- propagating surface rupture (Eberhart-Phillips 20 (Ma) 20 et. al., 2003) implying a possible change in the Age (Ma) Age fault behavior from east to west along the fault 10 10 (Ratchkovski et al., 2004) in the Nenana Moun- tain region. This is the same region where we see young AFT cooling ages in samples that 0 0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 have not been exhumed deeply enough to have Fraction of 39Ar released Fraction of 39Ar released exhumation-related biotite closure ages (Figs. Figure 9. 40Ar/39Ar age spectra for K-spar for samples 311AST and 45RAP from south of the 2 and 11) suggesting a possible Pleistocene Denali fault. The ﬂ at shape of the age spectra implies rapid cooling, which we interpret as a increase in exhumation rates in the area. short-lived metamorphic event based on rock fabric. Sample locations are shown on Figure 5. Role of Denali Fault Dip in Driving Exhumation is based on K-spar minimum ages being exhu- of ~11 km. The Nenana Mountain region is mation-related closure ages, but the 40Ar/39Ar located at the western end of the high topog- Strike-slip faults by deﬁ nition involve pre- biotite ages of these samples are concordant raphy in the eastern Alaska Range, where we dominately lateral slip, along a vertical to with the emplacement age of the pluton. This found the youngest AFT age (1.2 Ma) and high-angle fault plane (e.g., Sylvester, 1988). relationship puts limits on the possible depth of where young cooling ages transition to older Asymmetric basin development as seen in the exhumation for these samples at a maximum cooling ages farther to the west (Fig. 2). North Anatolian fault (Cormier et al., 2006) and

Nenana Transect Deborah Transect Balchen Transect 3000

R2 = 0.58 R2 = 0.98 R2 = 0.73 R2 = 0.67 R2 = 0.02

2000 2

Elevation (m) R = 0.92

R2 = 0.86 B 1000

12000 R2 = 0.96 R2 = 0.75 R2 = 0.74 R2 = 0.57

8000 R2 = 0.66

AHe R2 = 0.94 4000 AFT

Distance from Denali Fault (m) K-spar A BI R2 = 0.97 0 0 102030400 10 20 30 40 50 60 70 0 10203040 Age (Ma) Age (Ma) Age (Ma) Figure 10. Exhumation cooling ages versus (A) distance from the Denali fault and (B) elevation above sea level for all samples from the three transects shown in Figure 2. The Denali fault is interpreted as lying in the center of each glacier in the study area. Lines show best linear fi t to data points of same thermochronology method. Correlation coeffi cient is shown next to the best-fi t lines. Because there were only two AHe samples from near Mount Balchen, these AHe ages are shown without a trend line. Although cooling age in all systems is overall correlated with both elevation and distance from the Denali fault, distance from the fault is more signifi cant. A good example of this relationship is seen in the Mount Balchen transect K-spar minimum age data where proximity to the Denali fault has a stronger effect on cooling age than elevation.

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TB N 63.8°N NFTB TB HCF Elevation (m) 4000 Min 5 km 3500 Max 11 km 3000

Nenana NM

River 2500 Delta River Delta Panorama Min 11 km 2000 1500 Max 5 km BRG 1000 500 Denali Fault Max 5 km

0 10 20 km 148°W

Figure 11. Regions (contours) of varying exhumation depth along the Denali fault based on the current set of high temperature hornblende, muscovite, biotite, and K-spar minimum closure ages. The red cross shows the epicenter of the 2 November 2002 M7.9 Denali fault earthquake; the yellow cross indicates the epicenter of the 23 October 2002 M6.7 Nenana Mountain earthquake. NFTB—Northern Foothills thrust belt; HCF—Hines Creek fault; SGTF—Susitna Glacier thrust fault; BRG—Black Rapids glacier region; NM—Nenana Mountain region; Panorama—Panorama and Schist Creek plutons. The current locus of rapid exhumation is inferred to be near the epicenter (yellow cross) of the 23 October 2002 M6.7 Nenana Mountain earthquake and related to the edge of a segment of north-dipping fault plane.

variations in uplift rates along the San Andreas fault (Dair and Cooke, 2009) have recently been Depth Figure 12. Schematic model of variations in Mts. Pass Mts. explained through models involving nonverti- 0 km cal fault planes. Variations in fault dip (both the dip of the Denali fault plane, at depth, S N S N S N along strike and at depth) have been thoroughly which may contribute to asymmetric topo- documented for the Alpine fault of New Zealand graphic development along the Denali fault. (e.g., Little et al., 2005) and may be applicable View is along the fault plane looking west- to many strike-slip fault systems. ward. Note, the Denali fault in the central The dip of the Denali fault is near vertical Alaska Range may dip south (A), the broad along the Delta River corridor where the topog- pass region may be vertical (B), and the raphy is low (Fisher et al., 2004). The measured Denali fault in the eastern Alaska Range surface dip of the Denali fault is ~80° N in the may dip to the north (C), resulting in asym- 30 km region between the Balchen and Deborah tran- metric topographic development. A B C sects (Haeussler et al., 2004), but it is not well constrained beneath the glaciers or at depth. If the dip of the Denali fault plane (Fig. 12) varies along strike (east to west) from north dipping in parts of the eastern Alaska Range (Fig. 12C), to a section of the Denali fault that is straight. This mate on Alaska Range orogenesis. When we near-vertical (Fig. 12B) in areas of low elevation relationship indicates that variations in obliquity compare the global δ18O compilation (Zachos like Broad Pass, to south dipping in the central of convergence are most likely not driving exhu- et al., 2001) with plots of closure temperature Alaska Range (Fig. 12A), then the change in mation patterns in the eastern Alaska Range. versus age for different thermochronometers fault geometry may partially or fully explain the from the eastern Alaska Range, we see a tem- variations in along- and across-strike variations Climate and Exhumation Rate Trends in poral link between an increase in bedrock cool- in topography. the Eastern Alaska Range ing rates on the north side of the Denali fault An alternative explanation for the high topog- (based on the kink in slope at ~3 Ma) and the raphy and/or deep exhumation north of the The increase in exhumation rates has been onset of Northern Hemisphere glaciation (Fig. Denali fault in the eastern Alaska Range is the shown to correlate with climate change in tec- 13; Lisiecki and Raymo, 2005; Zachos et al., fact that there may be unmapped, ice-covered tonically active regions of Alaska (e.g., Berger 2001). Exhumation data from the south side of structures, splaying off the eastern Denali fault et al., 2008a; McAleer et al., 2009) and in a vari- the Denali fault indicate a similar pattern, but system that are the results of partitioned strain. ety of orogens globally (Clift, 2010). Our data at lower resolution, and with slower cooling The along-strike (west to east) change from show a temporal correlation with late Neogene and less rapid exhumation rates. This temporal deep exhumation to shallow exhumation west climatic ﬂ uctuations; however, distribution is correlation by no means proves that climate is of the Nenana Mountain transect occurs along not sufﬁ cient to fully evaluate the role of cli- forcing exhumation. However, the similarity in

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500 Shuster et al., 2005; Zhang et al., 2001). The A North side of Denali Fault Bi Alaska Range has a documented Pleistocene K-spar glacial history of numerous advance and retreat AFT 400 cycles (e.g., Briner and Kaufman, 2008). AHe In addition, a large percentage of the glaciers Start of Northern along the Denali fault system are rare surge-type Hemisphere glaciation 1.0 and increase in glaciers (Echelmeyer et al., 1987; Post, 1969; 300 exhumation rate 1.5 St. Amand, 1957). Surge-type glaciers experience surge events, involving large increases in 2.0

δ velocity and advancement at a fairly consistent 18 recurrence interval of 102 to 103 years (Wood- 200 2.5 O (‰) ward et al., 2002; Raymond, 1987; Kamb et al., Temperature (°C) 3.0 1985) and present a unique case of glacier retreat colder and advance. Surge events are accompanied by 100 3.5 climate an exponential increase in erosion relative to 4.0 steady-state surge quiescence (Humphrey and Raymond, 1994; Smith, 1990). In light of recent 0 4.5 work by Thomson et al. (2010), we feel it is B South side of Denali Fault Mu important to point out that the subpolar eastern Bi Alaska Range has modern glaciers with known K-spar 400 high basal velocities. Thomson et al. (2010) AFT demonstrate that low-velocity glacial cover can Start of Northern AHe Hemisphere glaciation actually limit erosion rates. We make no claims and increase in 1.0 on how long there have been surge-type glaciers 300 exhumation rate 1.5 along the Denali fault, but it is clear that high- velocity, surge-type glaciers have a deﬁ nitive 2.0

δ spatial correlation to the Denali fault system and 200 18 not simply the region’s current climatic regime 2.5 O (‰) (Manley and Daly, 2005; Raymond, 1987; Post, Temperature (°C) 3.0 1969). Based on our thermochronological analy- colder sis (Fig. 13), the Denali fault system may have 100 climate 3.5 experienced climate-induced Pliocene exhuma- 4.0 tion rate increases. Thus we postulate that the combination of lateral fault-strain accumula- 0 4.5 tion with highly effi cient, episodic surge-type 0 5 10 15 20 25 30 35 40 Age (Ma) and nonsurge glacial erosion contribute to high exhumation rates adjacent to the Denali fault Figure 13. Time-averaged temperature paths for individual samples from north (A) and during the late Cenozoic. south (B) of the Denali fault, based on multiple thermochronometers. Data include hornblende (550 °C; off graph), muscovite (Tc 400 °C; Mu) biotite (Tc 350 °C; Bi), K-spar Tectonic Driving Mechanisms of Neogene (Tc 150 °C), AFT (Tc 110 °C), and AHE (Tc 65 °C) ages (Reiners and Ehlers, 2005). Samples Exhumation in the Eastern Alaska Range with multiple cooling ages are shown. All samples are within ~10 km of the Denali fault. Cooling paths are assumed to be linear in between data points. δ18O is modifi ed from Zachos These new thermochronology data offer a et al. (2001) and used as a proxy for world temperature trends. Samples north of the Denali long record of exhumation from the eastern fault show a marked increase in cooling rate at ~3 Ma. Limited data from samples south of Alaska Range. The fi rst sign of rapid Neogene the Denali fault show a marked increase in cooling rate at ~4–5 Ma. exhumation begins ~22 Ma and is consistent with early exhumation events in the western Alaska Range (~23 Ma; Haeussler et al., 2008). Our interpretation of the thermochronological timing between climate change and exhumation glaciers and climatic instability in a tectonically data is consistent with the stratigraphic record of in the eastern Alaska Range, combined with the active region have a greater effect on denuda- the Tanana basin, where Ridgway et al. (2007) vast number of other orogens showing similar tion processes than simple isostatic response to interpret a transpressional foreland-basin sys- linkages (Clift, 2010), suggests that climate may tectonic forces in a quasi-stable climate setting tem related to regional shortening of the Alaska play a role in driving exhumation in the Alaskan (Clift, 2010; Berger et al., 2008a; Berger et al., Range, associated with strike-slip displacement interior. 2008b; Molnar and England, 1990). It has also along the Denali fault system. We also see the A possible mechanism for a climate-related been shown that periods of climatic instability continuance of rapid exhumation from ~22 Ma exhumation rate increase at ~3 Ma is the addi- where glaciers advance and retreat enhance ero- to the present. The absence of a region-wide tion of glaciers in the Alaska Range (i.e., more sion by allowing the erosive power of glaciers exhumation rate change in the eastern Alaska effi cient erosion processes [Hallet et al., 1996; and the mass-transport capabilities of fl uvial Range at ~6 Ma is curious considering the Westgate et al., 1990]). It has been shown that systems to work in tandem (Owen et al., 2008; strong evidence that the central Alaska Range

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(Mount McKinley region) and the western et al., 2006; Ferris et al., 2003). Although the going standard subduction, but instead either Alaska Range experienced a change in exhuma- timing of the arrival of the Yakutat microplate fl at-slab subduction or collision had initiated tion rate at ~6 Ma (Haeussler et al., 2008; Fitz- and the change from normal subduction to fl at and continues today. gerald, et al., 1995). The central Alaska Range slab subduction and/or collision is poorly con- and western Alaska Range are located at the strained, the longevity (~22 Ma to present) of ACKNOWLEDGMENTS interface between the rotating southern Alaska rapid exhumation in the eastern Alaska Range We would like to thank the Geological Society block and the rotating Bering block (Cross and and across south-central Alaska begs the ques- of America and the Geological Society of Alaska Freymueller, 2008; Fig. 1). The inter actions tion if the eastern Alaska Range long-term for supporting this research project through gradu- between these tectonic entities likely have con- exhumation record is recording the timing of ate research grants. We would also like to thank the American Alpine Club and Mugs Stump Climb- tributed to the complex patterns of exhuma- “collision” and the effects of continuing colli- ing grant for fi eld season funding. This manuscript tion occurring along the Alaska Range. More sion. This interpretation does not discount the benefi ted greatly from the constructive reviews of thermo chronometric data with better spatial infl uence of block rotation (Haeussler et al., A. Berger, C. Sorlien, and S. Gulick. This research coverage across the entire Alaska Range will 2008) or climatic infl uences on exhumation his- was also made possible by support from National help constrain the near-fi eld boundary condi- tory (Berger et al., 2008a; Berger et al., 2008b). Science Foundation grants EAR-0952793 and EAR- 0952800. We thank W. Nokleberg for his work in the tions along the western Denali fault and test That said, both rotation of the southern Alaska region and the numerous other Alaska-based research whether there are spatially and temporally com- block and quaternary tectonic/climate coupling projects that were the foundation for this research. A plex exhumation patterns resulting from the are ultimately in the fi rst order driven by tec- special thanks to A. Sterns for fi eld assistance. interactions among these (micro) plates. tonic processes at Alaska’s southern margin REFERENCES CITED Our exhumation data from the Alaska Range subduction zone. correlate well with the initial timing of rapid Aleinikoff, J.N., Farmer, G.L., Rye, R.O., and Nokleberg, Neogene exhumation in the eastern Chugach– CONCLUSIONS W.J., 2000, Isotopic evidence for the sources of Creta- ceous and Tertiary granitic rocks, east-central Alaska: St. Elias Mountains at ~20 Ma based on an Implications for the tectonic evolution of the Yukon- AFT vertical profi le on Mount Logan (~18 Ma; We applied 40Ar/39Ar (hornblende, muscovite, Tanana terrane: Canadian Journal of Earth Sciences, O’Sullivan and Currie, 1996) and a detrital zir- biotite, and K-spar), AFT, and AHe thermo- v. 37, p. 945–956, doi: 10.1139/cjes-37-6-945. Bemis, S.P., and Wallace, W., 2007, Neotectonic framework con fi ssion-track study on glacial outwash sands chronol ogy to examine the asymmetric topo- of the north-central Alaska Range foothills, in Ridg- (Enkelmann et al., 2008, ~20 Ma). Addition- graphic signature of the Denali fault in the way, K.D., Trop, J.M., Glen, J.M.G., and O’Neill, J.M., eds., Tectonic Growth of a Collisional Conti- ally, the still active Wrangell volcanic fi eld fi rst eastern Alaska Range. We identifi ed spatial vari- nental Margin: Crustal Evolution of Southern Alaska: developed ~23 Ma (Richter et al., 1990), sug- ations in the amount of exhumation along and Geological Society of America Special Paper 431, gesting that the early Miocene was an important across strike and temporal variations in focus of p. 549–572, doi: 10.1130/2007.2431(21). Berger, A.L., Gulick, S.P.S., Spotila, J.A., Upton, P., Jaeger, time in the tectonic development of southern exhumation. Rapid exhumation has been occur- J.M., Chapman, J.B., Worthington, L.A., Pavlis, T.L., margin of Alaska. ring since ~22 Ma with a zone of focused deep Ridgway, K.D., Willems, B.A., and McAleer, R.J., The overall Neogene tectonic history of exhumation along, and on the north side, of the 2008a, Quaternary tectonic response to intensifi ed glacial erosion in an orogenic wedge: Nature Geoscience, Alaska is not well constrained, but there is Denali fault. The youngest AFT ages (~1 Ma) v. 1, p. 793–799, doi: 10.1038/ngeo334. growing evidence from thermochronologic were identifi ed in the Nenana Mountain region Berger, A.L., Spotila, J.A., Chapman, J., Pavlis, T.L., Enkelmann, E., Ruppert, N.A., and Buscher, J.T., studies of exhumation events initiated at where total Neogene exhumation is less than 2008b, Architecture, kinematics, and exhumation of ~23 Ma, ~22 ma, ~20 Ma, ~16 Ma, ~11 Ma, elsewhere in the region. We postulate a segment a convergent orogenic wedge: A thermochronological ~6 Ma, 4 Ma, and ~1 Ma (Spotila and Berger, of the Denali fault has a north-dipping fault investigation of tectonic-climatic interactions within the central St. Elias orogen, Alaska: Earth and Plan- 2010; Berger et al., 2008a; Berger et al., 2008b; plane and the bend is fi xed onto the south side etary Science Letters, v. 270, p. 13–24, doi: 10.1016/ Haeussler et al., 2008; Enklemann et al., 2008; of the Denali fault with the deforming edge cur- j.epsl.2008.02.034. O’Sullivan and Currie, 1996; Fitzgerald et al., rently in the Nenana Mountain region. Braun, J., van der Beek, P., and Batt, G., 2006, Quantitative Thermochronology: Cambridge, Cambridge Univer- 1995; this study). In addition, strata in regional An exhumation rate increase in the Plio- sity Press. basins suggest pulses of enhanced exhumation cene and Quaternary may be related to climate Briner, J.P., and Kaufman, D.S., 2008, Late Pleistocene mountain glaciation in Alaska: Key chronologies: occurred from the Miocene to present (Finzel change. The coincident timing between exhu- Journal of Quaternary Science, v. 23, p. 659–670, doi: et al., 2009; Haeussler et al., 2008; Ridgway mation and climate is most obviously expressed 10.1002/jqs.1196. et al., 2007; Lagoe et al., 1993). Though Alaska on the north side of the Denali fault, whereas Buscher, J.T., and Spotila, J.A., 2007, Near-fi eld response to transpression along the southern San Andreas fault, exhumation events since ~5 Ma have been corre- the south side of the Denali fault indicates a based on exhumation of the northern San Gabriel lated with climatic drivers by some researchers more suppressed Pliocene–Quaternary increase Mountains, southern California: Tectonics, v. 26, (e.g., Enkelmann et al., 2009; Berger et al. in exhumation. This apparent Pliocene– p. TC5004, doi: 10.1029/2006TC002017. Christeson, G.L., Gulick, S.P.S., van Avendonk, H.J.A., 2008a; Berger et al., 2008b), there is a general Quaternary exhumation gradient across the Worthington, L., Reece, R.S., and Pavlis, T.L., 2010, consensus that tectonic drivers have also been fault could be due to a lack of data on the south The Yakutat terrane: Dramatic change in crustal thick- ness across the Transition fault, Alaska: Geology, v. 38, continual since the middle Miocene to present side or a weaker tectonic forcing. Although we p. 895–898, doi: 10.1130/G31170.1. (e.g., Spotila and Berger, 2010). discussed a unique glacial-tectonic exhumation Clift, P.D., 2010, Enhanced global continental erosion and The ~22 Ma initiation of rapid exhumation in feedback loop for ice-covered, strike-slip faults, exhumation driven by Oligo-Miocene climate change: Geophysical Research Letters, v. 37, p. L09402, doi: the eastern Alaska Range is roughly correlative more information is needed to conclusively link 10.1029/2010GL043067. with the arrival of the Yakutat microplate into increases in Pliocene–Quaternary exhumation Cormier, M.H., Seeber, L., McHugh, C.M.G., Polonia, A., the southern Alaska subduction zone (Plafker rates to climate in the Alaska Range. Cagatay, N., Emre, O., Gasperini, L., Gorur, N., Borto- luzzi, G., Bonatti, E., Ryan, W.B.F., and Newman, et al., 1994). The Yakutat microplate is thought The longevity of rapid exhumation in the K.R., 2006, North Anatolian fault in the Gulf of Izmit to dominantly consist of mafi c oceanic plateau eastern Alaska Range demands a long-term (Turkey): Rapid vertical motion in response to minor bends of a nonvertical continental transform: Journal rocks (Plafker, 1987) between 15 and 35 km driving mechanism. We suggest that by at least of Geophysical Research, v. 111, B4, p. B04102, doi: thick (Christeson et al., 2010; Eberhart-Phillips ~22 Ma, the Yakutat microplate was not under- 10.1029/2005JB003633.

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Crone, A., Personius, S., Craw, P., Haeussler, P., and Staft, parameters: Journal of Geophysical Research, v. 100, eastern Alaska, based on low-temperature thermo- L., 2004, The Susitna Glacier thrust fault—Character- p. 20,175–20,191, doi: 10.1029/95JB02150. chronometry: Tectonics, v. 28, p. TC1007, doi: 10.1029/ istics of ruptures that initiated the 2002 Denali fault Haeussler, P.J., 2008, An overview of the neotectonics of 2007TC002240. earthquake: Bulletin of the Seismological Society interior Alaska, far-fi eld deformation from the Yakutat McDougall, I., and Harrison, T.M., 1999, Geochronology of America, v. 94, no. 6B, p. S5–S22, doi: 10.1785/ Microplate collision: Geophysical Monograph, v. 179, and Thermochronology by the 40Ar/39Ar Method (2nd 0120040619. p. 83–108. edition): New York, Oxford University Press, 269 p. Cross, R., and Freymueller, J., 2008, Evidence for and implica- Haeussler, P.J., Bruhn, R.L., and Pratt, T.L., 2000, Poten- Mériaux, A.-S., Sieh, K., Finkel, R.C., Rubin, C.M., Taylor , tions of a Bering plate based on geodetic measure ments tial seismic hazards and tectonics of the upper Cook M.H., Meltzner, A.J., and Ryerson, F.J., 2009, Kine- from the Aleutians and western Alaska: Journal of Inlet basin, Alaska, based on analysis of Pliocene and matic behavior of southern Alaska constrained by Geophysical Research, v. 113, B07405, doi: 10.1029/ younger deformation: Geological Society of America westward decreasing postglacial slip rates on the 2007JB005136. Bulletin, v. 112, p. 1414–1429. Denali fault, Alaska: Journal of Geophysical Research, Csejtey, B., Jr., Mullen, M.W., Cox, D.P., and Stricker, G.D., Haeussler, P.J., Schwartz, D.P., Dawson, T.E., Stenner, H.D., v. 114, p. B03404, doi: 10.1029/2007JB005053. 1992, Geology and geochronology of the Healy quad- Lienkaemper, J.J., Cinti, F., Montone, P., Sherrod, Molnar, P., and England, P., 1990, Late Cenozoic uplift of rangle, south-central Alaska: U.S. Geological Survey B., and Craw, P., 2004, Surface rupture of the 2002 mountain ranges and global climate change: Chicken or Miscellaneous Investigations Map I-1961, scale Denali Fault, Alaska, Earthquake and comparison with egg?: Nature, v. 346, p. 29–34, doi: 10.1038/346029a0. 1:250,000. other strike-slip ruptures: Earthquake Spectra, v. 20, Nokleberg, W.J., Aleinikoff, J.N., Dutro, J.T., Jr., Lanphere, Dair, L., and Cooke, M.L., 2009, San Andreas fault geom- p. 565–578. M.A., Silberling, N.J., Silva, S.R., Smith, T.E., and etry through the San Gorgonio Pass, California: Geol- Haeussler, P.J., O’Sullivan, P., Berger, A.L., and Spotila, J.A., Turner, D.L., 1992, Map, tables, and summary of fos- ogy, v. 37, p. 119–122, doi: 10.1130/G25101A.1. 2008, Neogene exhumation of the Tordrillo Mountains, sil and isotopic age data, Mount Hayes quadrangle, Eberhart-Phillips, D., Haeussler, P.J., Freymueller, J.T., Alaska, and correlations with Denali (Mt. McKinley), eastern Alaska Range, Alaska: U.S. Geological Sur- Frankel, A.D., Rubin, C.M., Craw, P., Ratchkovski, in Freymueller, J.J., Haeussler, P.J., Wesson, R., and vey Miscellaneous Field Studies Map 1996-D, scale N.A., Anderson, G., Carver, G.A., Crone, A.J., Daw- Ekstrom, G., Active Tectonics and Seismic Potential 1:250,000. son, T.E., Fletcher, H., Hansen, R., Harp, E.L., Harris, of Alaska: Washington, D.C., American Geophysical O’Sullivan, P., and Currie, L., 1996, Thermotectonic history R.A., Hill, D.P., Hreinsdottir, S., Jibson, R.W., Jones, Union Monograph 179, p. 269–285. of Mt. Logan, Yukon Territory, Canada: Implications L.M., Kayen, R.E., Keefer, D.K., Larsen, C.F., Moran, Hallet, B., Hunter, L., and Bogen, J., 1996, Rates of ero- of multiple episodes of middle to late Cenozoic denu- S.C., Personius, S.F., Plafker, G., Sherrod, B.L., Sieh, sion and sediment evacuation by glaciers: A review dation: Earth and Planetary Science Letters, v. 144, K., Sitar, N., and Wallace, W.K., 2003, The 2002 of field data and their implications: Global and p. 251–261, doi: 10.1016/0012-821X(96)00161-6. Denali fault earthquake, Alaska: A large magnitude Planetary Change, v. 12, p. 213–235, doi: 10.1016/ Owen, L.A., Thackray, G., Anderson, R.S., Briner, J.P., slip-partitioned event: Science, v. 300, p. 1113–1118, 0921-8181(95)00021-6. Kaufman, D., Roe, G., Pfeffer, W., and Yi, C., 2008, doi: 10.1126/science.1082703. Harrison, T.M., Celerier, J., Aikman, A.B., Hermann, J., and Integrated mountain glacier research: Current status, Eberhart-Phillips, D., Christensen, D.H., Brocher, T.M., Heizler, M.T., 2009, Diffusion of 40Ar in muscovite: priorities, and future prospects: Geomorphology, v. 103, Hansen, R., Ruppert, N.A., Haeussler, P.J., and Abers, Geochimica et Cosmochimica Acta, v. 73, p. 1039– p. 158–171, doi: 10.1016/j.geomorph.2008.04.019. G.A., 2006, Imaging the transition from Aleutian 1051, doi: 10.1016/j.gca.2008.09.038. Plafker, G., 1987, Regional geology and petroleum poten- subduction to Yakutat collision in central Alaska, Harrison, W.D., and Post, A., 2003, How much do we really tial of the northern Gulf of Alaska continental margin, with local earthquakes and active source data: Jour- know about glacier surges?: Annals of Glaciology, in Scholl, D.W., et al., eds., Geology and Resource nal of Geophysical Research, v. 111, p. B11303, doi: v. 36, p. 1–6, doi: 10.3189/172756403781816185. Potential of the Continental Margin of Western North 10.1029/2005JB004240. Hilley, G., and Arrowsmith, J., 2008, Geomorphic response America and Adjacent Ocean Basins: Circum-Pacifi c Echelmeyer, K., Butterfi eld, R., and Cuillard, D., 1987, to uplift along the Dragon’s Back pressure ridge, Car- Council for Energy and Mineral Resources Earth Sci- Some observations on a recent surge of Peters glacier, rizo Plain: California Geology, v. 36, doi: 10.1130/ ence Series, v. 6, p. 229–268. Alaska, U.S.A: Journal of Glaciology, v. 33, no. 115, G24517A.1. Plafker, G., Naeser, C.W., Zimmermann, R.A., Lull, J.S., p. 341–345. Humphrey, N.F., and Raymond, C.F., 1994, Hydrology, and Hudson, T., 1992, Cenozoic uplift history of the Elliot, J.L., Larsen, C.F., Freymueller, J.T., and Motyka, erosion and sediment production in a surging glacier: Mount McKinley area in the central Alaska Range R.J., 2010, Tectonic block motion and glacial iso- Variegated glacier, Alaska, 1982–1983: Journal of Gla- based on fi ssion-track dating: U.S. Geological Survey static adjustment in southeast Alaska and adjacent ciology, v. 40, no. 136, p. 539–552. Bulletin 2041, p. 202–212. Canada constrained by GPS measurements: Jour- Kamb, B., Raymond, C., Harrison, W., Engelhardt, H., Plafker, G., Moore, J.C., and Winkler, G.R., 1994, Geology nal of Geophysical Research, v. 115, doi: 10.1029/ Echel meyer, K., Humphrey, N., Brugman, M., and of the southern Alaska margin, in Plafker, G., and Berg, 2009JB007139. Pfeffer, T., 1985, Glacier surge mechanism: 1982–1983 H.C., eds., The Geology of Alaska: Boulder, Colorado, Enkelmann, E., Garver, J.I., and Pavlis, T.L., 2008, Rapid surge of Variegated glacier, Alaska: Science, v. 227, Geological Society of America, Geology of North exhumation of ice-covered rocks of the Chugach–St. p. 469–479, doi: 10.1126/science.227.4686.469. America, v. G-1, p. 389–449. Elias orogen, Southeast Alaska: Geology, v. 36, no. 12, Lagoe, M.B., Eyles, C.H., Eyles, N., and Hale, C., 1993, Post, A., 1969, Distribution of surging glaciers in western p. 915–918, doi: 10.1130/G2252A.1. Timing of late Cenozoic tidewater glaciation in the far North America: Journal of Glaciology, v. 8, p. 229–240. Enkelmann, E., Zeitler, P.K., Pavlis, T.L., Garver, J.I., and North Pacifi c: Geological Society of America Bulletin, Ratchkovski, N.A., Wiemer, S., and Hansen, R.A., 2004, Ridgway, K.D., 2009, Intense localized rock uplift and v. 105, p. 1542–1560, doi: 10.1130/0016-7606. Seismotectonics of the Central Denalis Fault, Alaska, erosion in the St. Elias orogen of Alaska: Nature Geo- Lisiecki, L.E., and Raymo, M.E., 2005, A Pliocene–Pleisto- and the 2002 Denali Fault Earthquake Sequence: Bul- science, v. 2, p. 360–363, doi: 10.1038/ngeo502. cene stack of 57 globally distributed benthic δ18O letin of the Seismological Society of America, v. 94, Ferris, A., Abers, G.A., Christensen, D.H., and Veenstra, E., records: Paleoceanography, v. 20, p. PA1003, doi: p. S156–S174, doi:10.1785/0120040621. 2003, High resolution image of the subducted Pacifi c 10.1029/2004PA001071. Raymond, C.F., 1987, How do glaciers surge?: A review: plate beneath central Alaska, 50–150 km depth: Earth Little, T.A., Cox, S., Vry, J.K., and Batt, G., 2005, Variations Journal of Geophysical Research, v. 92, no. B9, and Planetary Science Letters, v. 214, no. 3–4, p. 575– in exhumation level and uplift rate along the oblique- p. 9121–9134, doi: 10.1029/JB092iB09p09121. 588, doi: 10.1016/S0012-821X(03)00403-5. slip Alpine fault, central Southern Alps, New Zea- Reiners, P.W., and Ehlers, T.A., eds., 2005, Low-Tempera- Finzel, E., Ridgway, K., Reifenstuhl, R., Blodgett, R., White, land: Geological Society of America Bulletin, v. 117, ture Thermochronology: Techniques, Interpretations, J., and Decker, P., 2009, Stratigraphic framework and p. 707–723, doi: 10.1130/B25500.1. Applications, Volume 58: Reviews in Mineralogy and estuarine depositional environments of the Miocene Lovera, O., Grove, M., and Harrison, T., 2002, Systematic Geochemistry: Chantilly, Virginia, Mineralogical Soci- Bear Lake Formation, Bristol Bay Basin, Alaska: analysis of K-feldspar 40Ar/39Ar step-heating results II: ety of America, Geochemical Society, 622 p. Onshore equivalents to potential reservoir strata in a Relevance of laboratory argon diffusion properties to Richter, D.H., Smith, J.B., Lanphere, M.A., Dalrymple, frontier gas-rich basin: American Association of Petro- nature: Geochimica et Cosmochimica Acta, v. 66, no. 7, G.B., Reed, B.L., and Shew, N., 1990, Age and pro- leum Geologists Bulletin, v. 93, no. 3, p. 379–405, doi: p. 1237–1255, doi: 10.1016/S0016-7037(01)00846-8. gression of volcanism, Wrangell volcanic fi eld, Alaska: 10.1306/10010808030. Lu, Z., Wright, T., and Wicks, C., 2003, Deformation of the Bulletin of Volcanology, v. 53, p. 29–44. Fisher, M.A., Ratchkovski, N.A., Nokleberg, W.J., Pellerin, 2002 Denali fault earthquakes mapped by Radarsat-1 Ridgway, K., Thoms, E., Layer, P., Lesh, M., White, J., and L., and Glen, J.M.G., 2004, Geophysical data reveal interferometry: Eos (Transactions, American Geophysi- Smith, S., 2007, Neogene transpressional foreland the crustal structure of the Alaska Range orogen within cal Union), v. 84, no. 41, doi: 10.1029/2003EO410002. basin development on the north side of the central the aftershock zone of the Mw 7.9 Denali fault earth- Manley, W.F., and Daly, C., 2005, Alaska geospatial climate Alaska Range, Usibelli Group and Nenana Gravel, quake: Bulletin of the Seismological Society of Amer- animations of monthly temperature and precipitation: Tanana basin, in Ridgway, K.D., et al., Tectonic ica, v. 94, p. S107–S131, doi: 10.1785/0120040613. Institute of Arctic and Alpine Research, University of Growth of a Collisional Continental Margin: Crustal Fitzgerald, P.G., Stump, E., and Redfi eld, T.F., 1993, Late Colorado, http://instaar.colorado.edu/QGISL/AGCA. Evolution of Southern Alaska: Geological Society of Cenozoic uplift of Denali and its relation to relative Matmon, A., Schwartz, D., Haeussler, P., Finkel, R., Lien- America Special Paper 431, p. 507–547, doi: 10.1130/ plate motion and fault morphology: Science, v. 259, kaemper, J., Stenner, H., and Dawson, T., 2006, Denali 2007.2431(20). p. 497–499. fault slip rates and Holocene–late Pleistocene kine- Roeske, S., Till, A., Foster, D., and Sample, J., 2007, Intro- Fitzgerald, P.G., Sorkhabi, R., Redfi eld, T., and Stump, matics of central Alaska: Geology, v. 34, no. 8, p. 645– duction, in Till, A., Roeske, S., Foster, D., and Sample, E., 1995, Uplift and denudation of the central Alaska 648, doi: 10.1130/G22361.1. J., eds., Uplift and Extension along Continental Strike- Range: A case study in the use of apatite fission- McAleer, R., Spotila, J., Enkelmann, E., and Berger, A., Slip Faults: Geological Society of America Special track thermochronology to determine absolute uplift 2009, Exhumation along the Fairweather fault, south- Paper 434, p. 15–33.

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/2/455/3715213/455.pdf by guest on 27 September 2021 The Denali fault of the eastern Alaska Range

Sherwood, K.L., and Craddock, C., 1979, General geology Spotila, J.A., Niemi, N., Brady, R., House, M., and Buscher, Geology, v. 18, p. 858–861, doi: 10.1130/0091-7613 of the central Alaska Range between the Nenana River J., 2007, Long-term continental deformation associ- (1990)018<0858:AMYROP>2.3.CO;2. and Mount Deborah: Alaska Division of Geological ated with transpressive plate motion: The San Andreas Whipple, K., 2009, The infl uence of climate on the tectonic and Geophysical Surveys Open-File Report AOF-116, fault: Geology, v. 35, p. 967–970, doi: 10.1130/ evolution of mountain belts: Nature Geoscience, v. 2, scale 1:63,360, 3 sheets. G23816A.1. p. 97–104, doi: 10.1038/ngeo413. Shuster, D.L., Ehlers, T.A., Rusmore, M.E., and Farley, St. Amand, P., 1957, Geological and geophysical synthesis Woodward, J., Carver, S., Kunzendorf, H., and Bennike, O., K.A., 2005, Rapid glacial erosion at 1.8 Ma revealed of the tectonics of portions of British Columbia, the 2002, Observations of surge periodicity in East Green- by 4He/3He thermochronometry: Science, v. 310, Yukon Territory, and Alaska: Geological Society of land using molybdenum records from marine sediment p. 1668–1670, doi: 10.1126/science.1118519. America Bulletin, v. 68, p. 1343–1370, doi: 10.1130/ cores, in Dowdeswell, J.A., and Ó Cofaigh, C., eds., Smith, N.D., 1990, The effects of glacial surging on sedi- 0016-7606(1957)68[1343:GAGSOT]2.0.CO;2. Glacier-Infl uenced Sedimentation on High-Latitude mentation in a modern ice-contact lake, Alaska: Sylvester, A.G., 1988, Strike-slip faults: Geological Soci- Continental Margins: The Geological Society of Lon- Geological Society of America Bulletin, v. 102, ety of America Bulletin, v. 100, p. 1666–1703, doi: don Special Publication 203, p. 367–373. p. 1393–1403, doi: 10.1130/0016-7606(1990)102 10.1130/0016-7606(1988)100<1666:SSF>2.3.CO;2. Zachos, J., Pagani, M., Sloan, L., Thomas, E., and Billups, <1393:TEOGSO>2.3.CO;2. Thomson, S.N., Brandon, M.T., Tomkin, J.H., Reiners, P.W., K., 2001, Trends, rhythms, and aberrations in global Spotila, J., and Berger, A., 2010, Exhumation at orogenic Vasquez, C., and Wilson, N.J., 2010, Glaciation as a climate 65 Ma to present: Science, v. 292, p. 686–693, indentor corners under long-term glacial conditions: destructive and constructive control on mountain build- doi: 10.1126/science.1059412. Example of the St. Elias orogen, southern Alaska: ing: Nature, v. 467, p. 313–317. Zhang, P.Z., Molnar, P., and Downs, W.R., 2001, Increased Tectonophysics, v. 490, p. 241–256, doi: 10.1016/ Wahrhaftig, C., Turner, D., Weber, F., and Smith, T., 1975, sedimentation rates and grain sizes 2–4 Myr ago due j.tecto.2010.05.015. Nature and timing of movement on Hines Creek to the infl uence of climate change on erosion rates: Spotila, J.A., Farley, K.A., and Sieh, K., 1998, Uplift and ero- strand of Denali fault system, Alaska: Geology, v. 3, Nature, v. 410, p. 891–897, doi: 10.1038/35073504. sion of the San Bernardino Mountains associated with no. 8, p. 463–466, doi: 10.1130/0091-7613(1975)3 transpression along the San Andreas fault, California, <463:NATOMO>2.0.CO;2. MANUSCRIPT RECEIVED 11 FEBRUARY 2010 as constrained by radiogenic helium thermochronology: Westgate, J.A., Stemper, B.A., and Pewe, T.L., 1990, A 3 m.y. REVISED MANUSCRIPT RECEIVED 23 SEPTEMBER 2010 Tectonics, v. 17, p. 360–378, doi: 10.1029/98TC00378. record of Pliocene–Pleistocene loess in interior Alaska: MANUSCRIPT ACCEPTED 17 OCTOBER 2010

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