40Ar/39Ar, apatite fission-track, and apatite (U-Th)/He thermochronological techniques were used to determine the Neogene exhumation history of the topographically asymmetric eastern Alaska Range. Exhumation cooling ages range from ∼33 Ma to ∼18 Ma for 40Ar/39Ar biotite, ∼18 Ma to ∼6 Ma for K-feldspar minimum closure ages, and ∼15 Ma to ∼1 Ma for apatite fission-track ages, and apatite (U-Th)/He cooling ages range from ∼4 Ma to ∼1 Ma. There has been at least ∼11 km of exhumation adjacent to the north side of Denali fault during the Neogene inferred from biotite 40Ar/39Ar thermochronology. Variations in exhumation history along and across the strike of the fault are influenced by both far-field effects and local structural irregularities. We infer deformation and rapid exhumation have been occurring in the eastern Alaska Range since at least ∼22 Ma most likely related to the continued collision of the Yakutat microplate with the North American plate. The Nenana Mountain region is the late Pleistocene to Holocene (∼past 1 Ma) primary locus of tectonically driven exhumation in the eastern Alaska Range, possibly related to variations in fault geometry. During the Pliocene, a marked increase in climatic instability and related global cooling is temporally correlated with an increase in exhumation rates in the eastern Alaska Range north of the Denali fault system.
Regions of focused rapid (>0.5 mm/yr) exhumation are a common occurrence along major strike-slip fault systems such as the Alpine fault in New Zealand, the Denali fault in Alaska, and the San Andreas fault in California (e.g., Little et al., 2005; Fitzgerald et al., 1995; Spotila et al., 1998). However, the cause of rapid exhumation along these faults is often complex with near-field (<20 km) boundary conditions and far-field plate tectonic driving mechanisms contributing to exhumation patterns (e.g., Buscher and Spotila, 2007). Unexpectedly, the amount of exhumation along a strike-slip fault is not always a simple correlation with the degree of obliquity (of the plate motion vector with respect to the fault trace) or composition of the juxtaposed rocks (e.g., Roeske et al., 2007; Spotila et al., 2007). Climate change (Whipple, 2009), structural irregularities such as stepovers (Hilley and Arrowsmith, 2008), variations in master fault dip (Dair and Cooke, 2009), and changes in plate motion (e.g., Fitzgerald et al., 1995) all can play an important role in observed regional exhumation patterns. Given that many orogenic belts associated with strike-slip faults have heterogeneous exhumation patterns, there is a need for more case studies to examine how vertical motion is related to both near-field boundary conditions and far-field driving mechanisms (Buscher and Spotila, 2007).
The Denali fault, an active continental-scale strike-slip fault, has an asymmetric topographic signature (the Alaska Range), making the region an ideal location to examine the contribution of local and regional structures on strain partitioning and the timing of exhumation adjacent to the fault. We employ 40Ar/39Ar, apatite fission-track (AFT), and apatite (U-Th)/He (AHe) thermochronology to document cooling, and thus exhumation patterns along the Denali fault system in the eastern Alaska Range. We find that during the Neogene, rapid exhumation in the eastern Alaska Range was initiated by the early Miocene (∼22 Ma). A major contributing factor to rapid exhumation in the eastern Alaska Range during the Neogene is most likely the far-field effects of the continuing collision of the Yakutat microplate with southern Alaska (North American plate).
Our data show that exhumation is greatest in a narrow wedge (<10 km wide) proximal to the Denali fault and is partitioned across and along strike in relation to possible changes in fault geometry. Rapid ongoing exhumation in the eastern Alaska Range was possibly enhanced in the late Pliocene with the transition to more efficient glacial erosion with the onset of the Northern Hemisphere glaciation (∼3 Ma; Lisiecki and Raymo, 2005) and increased erosional forcing due to climatic instability (Clift, 2010).
The Denali fault system in central Alaska makes a broad arc through the extensively glaciated Alaska Range (Fig. 1; St. Amand, 1957). It is a major intracontinental, right-lateral strike-slip fault that is still active as demonstrated by the 2002 M7.9 Denali fault earthquake (Eberhart-Phillips et al., 2003). Neogene movement and deformation are driven by plate tectonic processes at the Alaska southern margin subduction zone 500 km to the south (e.g., Haeussler, 2008). Matmon et al. (2006) and Mériaux et al. (2009) dated late Pleistocene moraines offset by the Denali fault in the eastern Alaska Range and show that modern slip rates are higher to the east at ∼12 mm/yr, decreasing to the west at ∼7 mm/yr, which suggests that slip is predominately being partitioned between the Denali fault and the Northern Foothills thrust belt and being siphoned off the Denali fault system onto contractional structures south of the Denali fault (Figs. 1 and 2; Bemis and Wallace, 2007; Crone et al., 2004).
Uplift History of the Alaska Range
The ∼650-km-long Alaska Range follows the curve of the Denali fault system. Areas of high-peak elevation (e.g., 6194 m in the central Alaska Range and 4216 m in the eastern Alaska Range) are separated by broad regions of low topography (e.g., Broad Pass, ∼700 m, separating the central Alaska Range from the eastern Alaska Range). The majority of thermochronometric data from the Alaska Range are from the central Alaska Range (Plafker et al., 1992; Fitzgerald et al., 1993, 1995), including a 4 km vertical profile in the Mount McKinley region of the central Alaska Range (e.g., Fitzgerald et al., 1995) and from AFT work done in the western Alaska Range (Haeussler et al., 2008). These thermochronometric data sets indicate that the modern-day Alaska Range started to form ∼6 Ma, although the western Alaska Range data set shows evidence of an earlier phase of rapid exhumation ∼23 Ma.
With the available low-temperature data, it is logical to view the Alaska Range as a singular orogenic feature, which most likely has experienced a similar region-wide exhumation pulse at ∼6 Ma. However, in the western Alaska Range and central Alaska Range, all regions of high topography (e.g., Mount McKinley) are south of the Denali fault. This observation is in stark contrast to the eastern Alaska Range where all of the high peaks are located north of the Denali fault (Fig. 1). In addition, Ridgway et al. (2007), in an analysis of the Miocene Usibelli Group and Pliocene Nenana Gravel within the Tanana Basin, suggested development of significant topography in the eastern Alaska Range prior to the late Miocene. Thus, the spatial and temporal evolution of the Alaska Range may be more complex than the existing data suggest.
Physiography of the Eastern Alaska Range
Unlike the western and central Alaska Range, the high peaks of the eastern Alaska Range are sandwiched between the active McKinley strand of the Denali fault and the presumed-to-be inactive Hines Creek strand (Wahrhaftig et al., 1975). High topography (>3000 m) is limited to a narrow region (<20 km) along the north side of the Denali fault. There are numerous identified reverse and thrust faults in the greater eastern Alaska Range region with many located in the northern foothills fold and thrust belt to the north of the Hines Creek fault (Bemis and Wallace, 2007). There are few active and inactive structures identified in the ice-covered, high-peak region of the eastern Alaska Range, with the exception of the Susitna Glacier thrust fault, which was only recognized after it ruptured during the 2002 M7.9 Denali earthquake (Fig. 2; Crone et al., 2004).
Plutons north and south of the Susitna Glacier have ∼70 Ma U-Pb emplacement ages (Aleinikoff et al., 2000). Nenana Mountain, Schist Creek and Panorama plutons are thought to have ∼38 Ma emplacement ages based on K-Ar dating of biotite and hornblende (Csejtey et al., 1992). Both sides of the Denali fault in the high-peak region of the eastern Alaska Range experience the same climatic regime (precipitation of ∼1500 to ∼2000 mm/yr; Manley and Daly, 2005).
Sampling and Analytical Techniques
We applied 40Ar/39Ar, AFT, and AHe thermochronology to 30 bedrock samples along and across the strike of the eastern Alaska Range proximal to the Denali fault system in order to better constrain the timing and patterns of exhumation in the eastern Alaska Range. Sampling was focused along three transects north of the Denali fault: Nenana Mountain transect (NM), Mount Deborah transect (MD), and Mount Balchen transect (MB) (Fig. 2). One transect was collected south of the Denali fault in the Meteor Peak region (Fig. 3), which is the largest expanse of high topography south of the Denali fault in the eastern Alaska Range.
Eighteen samples were analyzed using 40Ar/39Ar thermochronology on potassium feldspar (K-spar) using either a laser or a resistance furnace (Supplemental Table 11). Six of the furnace-run samples were then modeled using a multi-domain diffusion modeling (MDD) approach (e.g., Lovera et al., 2002) to determine thermal histories and an associated closure temperature for the minimum isochron age grouping (K-spar minimum closure age). Unaltered biotite (16), muscovite (2), and hornblende (1) (Supplemental Table 22), when present, were also analyzed using 40Ar/39Ar laser step heating to supplement the time-temperature cooling trajectories of rocks in the Alaska Range.
The Geothermal Gradient and Closure Temperatures
Along the Denali fault, maximum aftershock depths of ∼11 km suggest the brittle-ductile transition and approximate a regional geothermal gradient to ∼30 °C/km (Fisher et al., 2004), which we use to estimate the depth to closure temperature (Tc) for the various thermochronologic systems. Approximately 30 °C/km is also the same standard regional geothermal gradient constrained by other thermochronology studies in Alaska (O'Sullivan and Currie, 1996; Haeussler et al., 2008; McAleer et al., 2009). For the purpose of constraining first-order estimates on exhumation during the Neogene, we assume that the gradient remained spatially and temporally uniform. Thus, we assume the same depth to closure isotherms across the region. We acknowledge that for the low-temperature AFT and AHe data, due to their lower closure temperatures, and hence shallower depths of closure, plus higher Pliocene–Quaternary exhumation rates, this assumption is a simplification and the resulting trends, an approximation. However, the majority of the data in this manuscript are from high-temperature thermochronometers with deep closure depths. These ages are less affected by high Quaternary exhumation rates and variation in isotherm depth related to surface topography (e.g., Braun et al., 2006). We first discern whether the higher temperature cooling ages are related to initial emplacement cooling or cooling related to exhumation before calculating depth of closure.
We use standard closure temperatures for 40Ar/39Ar dating of hornblende (550 °C), muscovite (Tc 400 °C), biotite (Tc 350 °C), K-spar (Tc 150 °C), and also for AFT (Tc 110 °C) and AHe (Tc 65 °C) cooling ages (Harrison et al., 2009; Reiners and Ehlers, 2005; e.g., McDougall and Harrison, 1999). Precise spatial differences in exhumation rates and/or trends are not critical because the variations are large enough to be significant and are not artifacts of variations in geothermal gradients. We also do not give quantitative rates of cooling, but instead discuss overall trends in cooling rates to avoid over interpreting time-averaged thermochronological constraints. In addition, the dip of the Denali fault at depth is poorly constrained (see discussion below), but we assume general subvertical rock trajectories based on available structural work done in the region (Sherwood and Craddock, 1979). These assumptions about temperature and, hence, depth of closure allow us to examine near-field exhumation trends and patterns during the Neogene in the eastern Alaska Range.
Most hornblende, muscovite, and biotite analyses from samples on both sides of the Denali fault show well-defined 40Ar/39Ar age plateaus with simple spectra with little or no argon loss (Figs. A1 to A19 in the Supplemental Figure File7, Supplemental Tables 2 and 5 [see footnotes 2 and 6]). This evidence of unaltered samples is demonstrated by the high concordance between integrated ages (whole gas) and plateau ages for the hornblende, muscovite, and biotite analyses (Fig. 4).
Along the Balchen transect, biotite cooling ages decrease southwards toward the Denali fault from ∼33.3 Ma to ∼18.4 Ma (Fig. 2). Proximal Black Rapids, Mount Deborah, and the far southeastern part of Nenana Mountain pluton 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 ages of ∼38 Ma. The Mount Deborah sample, which is most distal from the Denali fault, has a biotite age of ∼68 Ma.
K-spar minimum closure ages for Nenana Mountain, Mount Deborah, Mount Balchen, and the north side of the Black Rapids glacier regions range from ∼18.6 Ma to ∼6.0 Ma. K-spar minimum closure ages west of the Nenana Mountain region and also south of the Denali fault (Figs. 2 and 3) are distinctly older (∼28.6 to ∼30.8 Ma). K-spar MDD modeling of six samples from the region north of the Denali fault indicates that K-spar minimum ages represent cooling through the ∼150 °C isotherm. We interpret MDD models as indicating a diffuse pattern for the initiation of rapid cooling.
Approaching the Denali fault from the north, AFT ages generally decrease from ∼6.7 Ma to ∼1.2 Ma with the youngest ages in the Nenana Mountain region. AHe ages north of the Denali fault range from ∼4.0 Ma to ∼1.5 Ma. Near the south side of the Denali fault, we report an AFT age of ∼15.5 Ma and one AHe age of ∼4.5 Ma (Figs. 2 and 3).
North of the Denali Fault
Biotite ages of samples from the westernmost Nenana Mountain region have ages (∼38.3 Ma to ∼36.2 Ma; Fig. 2) concordant with the emplacement age of the pluton (∼38 Ma) indicating postemplacement rapid cooling and not exhumation-related cooling. Schist Creek and Panorama plutons have biotite ages of ∼38 Ma and are interpreted as being related to initial melt emplacement cooling as well.
Biotite ages of ∼32 Ma to ∼18 Ma from intrusions on the north side of the Black Rapids glacier and eastern edge of the Nenana Mountain pluton are interpreted as exhumation-related cooling ages based on the respective emplacement ages of the plutons (∼70 Ma; ∼38 Ma). Because the emplacement age of the Balchen intrusion is ∼70 Ma (Aleinikoff et al., 2000), the trend of younger biotite ages (∼33.3 Ma to ∼18.4 Ma) toward the Denali fault demonstrates that the biotite ages are cooling ages related to exhumation, with exhumation increasing toward the fault (Fig. 2). The Mount Deborah pluton, like the Balchen pluton, is assumed to have an ∼70 Ma emplacement age based on the upper sample having a biotite age of ∼68.4 Ma (Fig. 2). The Mount Deborah samples (proximal to the Denali fault) have biotite cooling ages that are related to exhumation (∼32 Ma to ∼28 Ma).
The K-spar minimum ages (∼30 Ma), the flat shape of the K-spar 40Ar/39Ar age spectra, and biotite 40Ar/39Ar ages (∼38 Ma) for the Panorama and Schist Creek plutons imply rapid postemplacement cooling for both plutons through the minimum closure temperature for K-spar (Figs. 5 and 6). The downward-stepping K-spar 40Ar/39Ar age spectra and MDD modeling of furnace runs of samples from Nenana Mountain, Mount Deborah, Mount Balchen transects, and the Black Rapids glacier regions north of the Denali fault confirm that the minimum closure ages indicate rapid cooling due to exhumation through ∼150 °C (Figs. 5, 7, and 8).
The MDD K-spar thermal models from north of the Denali fault show relatively diverse cooling histories with rapid cooling occurring in individual samples between ∼22 Ma and ∼6 Ma (Fig. 8). These north-side thermal models suggest heterogeneous exhumation along and across the strike of the Denali fault. AFT ages range from ∼6.7 Ma (Mount Balchen) to ∼1.2 Ma (Nenana Mountain region), and AHe ages range from ∼4 Ma to ∼1.5 Ma (Mount Balchen). Combined with the biotite and K-spar data, the AFT and AHe ages indicate that Neogene rapid exhumation has been a long-term (∼22 Ma to present) phenomenon along the north side of the Denali fault in the eastern Alaska Range.
South of the Denali Fault
In contrast to samples north of the fault, orthogneiss samples south of the Denali fault have flat, ∼30 Ma K-spar age spectra that are concordant with 40Ar/39Ar hornblende, muscovite, and biotite ages (∼35 Ma to ∼31 Ma) from the same samples (Figs. 5 and 9). We interpret this rapid cooling event as a short-lived metamorphic event because the samples have a penetrative fabric, and they have concordant 40Ar/39Ar cooling ages, which is in general agreement with past interpretations for the region (e.g., Nokleberg et al., 1992). Proximal to the Denali fault, ∼15.5 Ma AFT and ∼4.5 Ma AHe ages on the south side of the fault are older than AFT ∼1.2 Ma AFT and ∼1.5 Ma AHe ages on the north side of the fault. These low-temperature data combined with the aforementioned 40Ar/39Ar thermochronology results indicate a different exhumation history for the south side of the Denali fault than the north side.
Across-Strike Influence of the Denali Fault
The trend of cooling ages decreasing toward the Denali fault from the north is directly correlated with both elevation (Fig. 10A) and distance from the fault (Fig. 10B) for all four thermochronometric systems. This relationship indicates that exhumation just north of the Denali fault in the eastern Alaska Range has been focused there and persisted for at least the past ∼20 m.y. Although cooling age is correlated with both elevation and distance from the Denali fault, distance from the fault is more significant. A good example of this relationship is seen in the Mount Balchen transect K-spar minimum cooling age data where proximity to the Denali fault has a stronger effect on cooling age than elevation (Fig. 10).
Thermochronometric data from the south side of the Denali fault show no strong elevation or distance-from-fault trends. Muscovite, biotite, and K-spar minimum cooling ages are all concordant and suggest very rapid cooling at ∼30 Ma throughout the area (Figs. 2 and 3). No graphical representation is presented because the cooling ages would simply overlap—biotite ages are all ∼31 Ma; K-spar minimum ages are all ∼29 Ma.
Overall Depth of Exhumation Patterns in the Eastern Alaska Range
In the eastern Alaska Range, the depth of Neogene exhumation varies dramatically across and along the Denali fault allowing for general contouring of Neogene minimum or maximum exhumation depths (Fig. 11). The difference in cooling ages across the Denali fault defines a narrow (<∼10km) wedge of deep Neogene exhumation in the eastern Alaska Range. It is also clear that the region south of the Denali fault in the eastern Alaska Range experienced less total exhumation during the Neogene than the region juxtaposed to the north.
Samples south of the Denali fault have only been exhumed through the AFT closure system during the Neogene and experienced less than ∼5 km of exhumation since ∼28 Ma based on the assumed geothermal gradient of ∼30 °C/km, the assumed Tc of ∼150 °C for K-spar, and the concordant muscovite, biotite, and K-spar ages in the region. To the west, Panorama and Schist Creek plutons experienced less than ∼5 km of exhumation based on the 40Ar/39Ar biotite and 40Ar/39Ar K-spar systems being concordant. Because 40Ar/39Ar biotite ages (Tc ∼350 °C) from samples north of the Denali fault between the Black Rapids glacier and the southeast end of the Nenana Mountain pluton are exhumation-related closure ages, this region experienced significant enough unroofing to exhume rocks that have passed through the ∼350 °C isotherm. Therefore, the region has undergone a minimum of ∼11 km of exhumation. The region around the epicenter of the 23 October 2002 M6.7 Nenana Mountain earthquake has experienced at least ∼5 km of exhumation. This conclusion is based on K-spar minimum ages being exhumation-related closure ages, but the 40Ar/39Ar biotite ages of these samples are concordant with the emplacement age of the pluton. This relationship puts limits on the possible depth of exhumation for these samples at a maximum of ∼11 km. The Nenana Mountain region is located at the western end of the high topography in the eastern Alaska Range, where we found the youngest AFT age (1.2 Ma) and where young cooling ages transition to older cooling ages farther to the west (Fig. 2).
Modern earthquake behavior also varies along strike. The 23 October 2002 M6.7 Nenana Mountain earthquake had a westward-propagating rupture (Lu et al., 2003). However, the 2002 7.9M Denali fault earthquake had an eastward-propagating surface rupture (Eberhart-Phillips et. al., 2003) implying a possible change in the fault behavior from east to west along the fault (Ratchkovski et al., 2004) in the Nenana Mountain region. This is the same region where we see young AFT cooling ages in samples that have not been exhumed deeply enough to have exhumation-related biotite closure ages (Figs. 2 and 11) suggesting a possible Pleistocene increase in exhumation rates in the area.
Role of Denali Fault Dip in Driving Exhumation
Strike-slip faults by definition involve predominately lateral slip, along a vertical to high-angle fault plane (e.g., Sylvester, 1988). Asymmetric basin development as seen in the North Anatolian fault (Cormier et al., 2006) and variations in uplift rates along the San Andreas fault (Dair and Cooke, 2009) have recently been explained through models involving nonvertical fault planes. Variations in fault dip (both along strike and at depth) have been thoroughly documented for the Alpine fault of New Zealand (e.g., Little et al., 2005) and may be applicable to many strike-slip fault systems.
The dip of the Denali fault is near vertical along the Delta River corridor where the topography is low (Fisher et al., 2004). The measured surface dip of the Denali fault is ∼80° N in the region between the Balchen and Deborah transects (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 near-vertical (Fig. 12B) in areas of low elevation like Broad Pass, to south dipping in the central Alaska Range (Fig. 12A), then the change in fault geometry may partially or fully explain the variations in along- and across-strike variations in topography.
An alternative explanation for the high topography and/or deep exhumation north of the Denali fault in the eastern Alaska Range is the fact that there may be unmapped, ice-covered structures, splaying off the eastern Denali fault system that are the results of partitioned strain. The along-strike (west to east) change from deep exhumation to shallow exhumation west of the Nenana Mountain transect occurs along a section of the Denali fault that is straight. This relationship indicates that variations in obliquity of convergence are most likely not driving exhumation patterns in the eastern Alaska Range.
Climate and Exhumation Rate Trends in the Eastern Alaska Range
The increase in exhumation rates has been shown to correlate with climate change in tectonically active regions of Alaska (e.g., Berger et al., 2008a; McAleer et al., 2009) and in a variety of orogens globally (Clift, 2010). Our data show a temporal correlation with late Neogene climatic fluctuations; however, distribution is not sufficient to fully evaluate the role of climate on Alaska Range orogenesis. When we compare the global δ18O compilation (Zachos et al., 2001) with plots of closure temperature versus age for different thermochronometers from the eastern Alaska Range, we see a temporal link between an increase in bedrock cooling rates on the north side of the Denali fault (based on the kink in slope at ∼3 Ma) and the onset of Northern Hemisphere glaciation (Fig. 13; Lisiecki and Raymo, 2005; Zachos et al., 2001). Exhumation data from the south side of the Denali fault indicate a similar pattern, but at lower resolution, and with slower cooling and less rapid exhumation rates. This temporal correlation by no means proves that climate is forcing exhumation. However, the similarity in timing between climate change and exhumation in the eastern Alaska Range, combined with the vast number of other orogens showing similar linkages (Clift, 2010), suggests that climate may play a role in driving exhumation in the Alaskan interior.
A possible mechanism for a climate-related exhumation rate increase at ∼3 Ma is the addition of glaciers in the Alaska Range (i.e., more efficient erosion processes [Hallet et al., 1996; Westgate et al., 1990]). It has been shown that glaciers and climatic instability in a tectonically active region have a greater effect on denudation processes than simple isostatic response to tectonic forces in a quasi-stable climate setting (Clift, 2010; Berger et al., 2008a; Berger et al., 2008b; Molnar and England, 1990). It has also been shown that periods of climatic instability where glaciers advance and retreat enhance erosion by allowing the erosive power of glaciers and the mass-transport capabilities of fluvial systems to work in tandem (Owen et al., 2008; Shuster et al., 2005; Zhang et al., 2001). The Alaska Range has a documented Pleistocene glacial history of numerous advance and retreat cycles (e.g., Briner and Kaufman, 2008).
In addition, a large percentage of the glaciers along the Denali fault system are rare surge-type glaciers (Echelmeyer et al., 1987; Post, 1969; St. Amand, 1957). Surge-type glaciers experience surge events, involving large increases in velocity and advancement at a fairly consistent recurrence interval of 102 to 103 years (Woodward et al., 2002; Raymond, 1987; Kamb et al., 1985) and present a unique case of glacier retreat and advance. Surge events are accompanied by an exponential increase in erosion relative to steady-state surge quiescence (Humphrey and Raymond, 1994; Smith, 1990). In light of recent work by Thomson et al. (2010), we feel it is important to point out that the subpolar eastern Alaska Range has modern glaciers with known high basal velocities. Thomson et al. (2010) demonstrate that low-velocity glacial cover can actually limit erosion rates. We make no claims on how long there have been surge-type glaciers along the Denali fault, but it is clear that high-velocity, surge-type glaciers have a definitive spatial correlation to the Denali fault system and not simply the region's current climatic regime (Manley and Daly, 2005; Raymond, 1987; Post, 1969). Based on our thermochronological analysis (Fig. 13), the Denali fault system may have experienced climate-induced Pliocene exhumation rate increases. Thus we postulate that the combination of lateral fault-strain accumulation with highly efficient, episodic surge-type and nonsurge glacial erosion contribute to high exhumation rates adjacent to the Denali fault during the late Cenozoic.
Tectonic Driving Mechanisms of Neogene Exhumation in the Eastern Alaska Range
These new thermochronology data offer a long record of exhumation from the eastern Alaska Range. The first sign of rapid Neogene 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 data is consistent with the stratigraphic record of the Tanana basin, where Ridgway et al. (2007) interpret a transpressional foreland-basin system related to regional shortening of the Alaska Range, associated with strike-slip displacement along the Denali fault system. We also see the continuance of rapid exhumation from ∼22 Ma to the present. The absence of a region-wide exhumation rate change in the eastern Alaska Range at ∼6 Ma is curious considering the strong evidence that the central Alaska Range (Mount McKinley region) and the western Alaska Range experienced a change in exhumation rate at ∼6 Ma (Haeussler et al., 2008; Fitzgerald, et al., 1995). The central Alaska Range and western Alaska Range are located at the interface between the rotating southern Alaska block and the rotating Bering block (Cross and Freymueller, 2008; Fig. 1). The interactions between these tectonic entities likely have contributed to the complex patterns of exhumation occurring along the Alaska Range. More thermochronometric data with better spatial coverage across the entire Alaska Range will help constrain the near-field boundary conditions along the western Denali fault and test whether there are spatially and temporally complex exhumation patterns resulting from the interactions among these (micro) plates.
Our exhumation data from the Alaska Range correlate well with the initial timing of rapid Neogene exhumation in the eastern Chugach–St. Elias Mountains at ∼20 Ma based on an AFT vertical profile on Mount Logan (∼18 Ma; O'Sullivan and Currie, 1996) and a detrital zircon fission-track study on glacial outwash sands (Enkelmann et al., 2008, ∼20 Ma). Additionally, the still active Wrangell volcanic field first developed ∼23 Ma (Richter et al., 1990), suggesting that the early Miocene was an important time in the tectonic development of southern margin of Alaska.
The overall Neogene tectonic history of Alaska is not well constrained, but there is growing evidence from thermochronologic studies of exhumation events initiated at ∼23 Ma, ∼22 ma, ∼20 Ma, ∼16 Ma, ∼11 Ma, ∼6 Ma, 4 Ma, and ∼1 Ma (Spotila and Berger, 2010; Berger et al., 2008a; Berger et al., 2008b; Haeussler et al., 2008; Enklemann et al., 2008; O'Sullivan and Currie, 1996; Fitzgerald et al., 1995; this study). In addition, strata in regional basins suggest pulses of enhanced exhumation occurred from the Miocene to present (Finzel et al., 2009; Haeussler et al., 2008; Ridgway et al., 2007; Lagoe et al., 1993). Though Alaska exhumation events since ∼5 Ma have been correlated with climatic drivers by some researchers (e.g., Enkelmann et al., 2009; Berger et al. 2008a; Berger et al., 2008b), there is a general consensus that tectonic drivers have also been continual since the middle Miocene to present (e.g., Spotila and Berger, 2010).
The ∼22 Ma initiation of rapid exhumation in the eastern Alaska Range is roughly correlative with the arrival of the Yakutat microplate into the southern Alaska subduction zone (Plafker et al., 1994). The Yakutat microplate is thought to dominantly consist of mafic oceanic plateau rocks (Plafker, 1987) between 15 and 35 km thick (Christeson et al., 2010; Eberhart-Phillips et al., 2006; Ferris et al., 2003). Although the timing of the arrival of the Yakutat microplate and the change from normal subduction to flat slab subduction and/or collision is poorly constrained, the longevity (∼22 Ma to present) of rapid exhumation in the eastern Alaska Range and across south-central Alaska begs the question if the eastern Alaska Range long-term exhumation record is recording the timing of “collision” and the effects of continuing collision. This interpretation does not discount the influence of block rotation (Haeussler et al., 2008) or climatic influences on exhumation history (Berger et al., 2008a; Berger et al., 2008b). That said, both rotation of the southern Alaska block and quaternary tectonic/climate coupling are ultimately in the first order driven by tectonic processes at Alaska's southern margin subduction zone.
We applied 40Ar/39Ar (hornblende, muscovite, biotite, and K-spar), AFT, and AHe thermochronology to examine the asymmetric topographic signature of the Denali fault in the eastern Alaska Range. We identified spatial variations in the amount of exhumation along and across strike and temporal variations in focus of exhumation. Rapid exhumation has been occurring since ∼22 Ma with a zone of focused deep exhumation along, and on the north side, of the Denali fault. The youngest AFT ages (∼1 Ma) were identified in the Nenana Mountain region where total Neogene exhumation is less than elsewhere in the region. We postulate a segment of the Denali fault has a north-dipping fault plane and the bend is fixed onto the south side of the Denali fault with the deforming edge currently in the Nenana Mountain region.
An exhumation rate increase in the Pliocene and Quaternary may be related to climate change. The coincident timing between exhumation and climate is most obviously expressed on the north side of the Denali fault, whereas the south side of the Denali fault indicates a more suppressed Pliocene–Quaternary increase in exhumation. This apparent Pliocene–Quaternary exhumation gradient across the fault could be due to a lack of data on the south side or a weaker tectonic forcing. Although we discussed a unique glacial-tectonic exhumation feedback loop for ice-covered, strike-slip faults, more information is needed to conclusively link increases in Pliocene–Quaternary exhumation rates to climate in the Alaska Range.
The longevity of rapid exhumation in the eastern Alaska Range demands a long-term driving mechanism. We suggest that by at least ∼22 Ma, the Yakutat microplate was not undergoing standard subduction, but instead either flat-slab subduction or collision had initiated and continues today.
We would like to thank the Geological Society of America and the Geological Society of Alaska for supporting this research project through graduate research grants. We would also like to thank the American Alpine Club and Mugs Stump Climbing grant for field season funding. This manuscript benefited greatly from the constructive reviews of A. Berger, C. Sorlien, and S. Gulick. This research was also made possible by support from National Science Foundation grants EAR-0952793 and EAR-0952800. We thank W. Nokleberg for his work in the region and the numerous other Alaska-based research projects that were the foundation for this research. A special thanks to A. Sterns for field assistance.