Exhumation in the Eastern Alaska Range Constrained by Stacked Thermochronology

Home , Alaska Range, Mount Deborah, Mount Kimball (Alaska)

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Persistent long-term (c. 24 Ma) exhumation in the Eastern Alaska Range constrained by stacked thermochronology

JEFF A. BENOWITZ1*, PAUL W. LAYER2 & SAM VANLANINGHAM3 1Geophysical Institute, University of Alaska Fairbanks, PO Box 755940, Fairbanks, AK 99775, USA 2College of Natural Science and Mathematics, University of Alaska Fairbanks, PO Box 755940, Fairbanks, AK 99775, USA 3Institute of Marine Science, University of Alaska Fairbanks, Fairbanks, AK 99775, USA *Corresponding author (e-mail: [email protected])

Abstract: To address Miocene–present episodic v. persistent exhumation, we utilize a simple graphical procedure that vertically stacks spatially diverse K-feldspar 40Ar/39Ar multi-domain diffusion (MDD) models from the length of the approximately 100 km-long high-peak region of the Eastern Alaska Range. We supply additional constraints with 40Ar/39Ar mica dating because the higher closure-temperature-window places limits on the initiation of rapid Eastern Alaska Range exhumation. We also provide a broad 40Ar/39Ar K-feldspar minimum closure age data set to add more detail on spatial patterns in the regional exhumation history for the Eastern Alaska Range. We ﬁnd that rapid and persistent exhumation has occurred in the Eastern Alaska Range since about 24 Ma at a long-term rate of approximately 0.9 km/Ma, but that this rapid exhumation is spatially variable through time. Onset of rapid Eastern Alaska Range exhumation is coincident with the initiation of rapid exhumation in SW Alaska, the Western Alaska Range and the Chugach– Saint Elias Range at around 25 Ma, implying a region-wide deformational response to a change in tectonic forcing. The initiation of highly coupled ﬂat-slab subduction of the Yakutat microplate is probably responsible for this prolonged period of rapid exhumation in southern Alaska.

Supplementary material: Sample locations from the Eastern Alaska Range, and 40Ar/39Ar data tables and age spectrum ﬁgures are available at www.geolsoc.org.uk/SUP18603.

Gold Open Access: This article is published under the terms of the CC-BY 3.0 license.

Thermochronology has now been used for over 2005). In addition, along-strike variations in the 30 years (see Reiners et al. 2005) to study orogenic focus of exhumation are seen across the world (e.g. development. Many data interpretation methods Little et al. 2005; Yin 2006; Tricart et al. 2007; regarding orogenic tectonothermal histories have Seeber et al. 2010). Thus, orogen-scale inter- been standardized; including the time-averaged pretations of tectonic processes based on thermo- time–temperature (T–t) plots of one sample chronological analyses of a single vertical transect (McAleer et al. 2009) or numerous samples from (Fitzgerald et al. 1993; O’Sullivan & Currie 1996; one vertical mountain transect (Fitzgerald et al. Haeussler et al. 2008) or a single rock sample 1993). Typically, any significant changes in time- (Richter et al. 1990) may not capture the spatial and averaged cooling rates (i.e. breaks in slope in T–t temporal variability inherent to broad-scale oro- plots) based on these results combined with other genesis. Although spatially restricted sampling for geological evidence of unroofing are interpreted to thermochronology can provide first-order infor- indicate changes in exhumation rate (Batt et al. mation on the timing of orogenesis, can the total 2004). These changes in exhumation rate, also exhumation history of an orogen be constrained by known as ‘events’ or ‘episodes’, are usually inter- such a practice? In addition, a single traverse, verti- preted to reflect either large-scale changes in tec- cal profile or sample may contain biased or no infor- tonic forcing or variations in near-field (local) mation regarding orogenesis due to one or more conditions that may influence exhumation (e.g. cli- thermochronometers not being in record mode mate, fault geometry, lithology: Buscher & Spotila during crustal movement. 2007). Yet, tectonic processes (e.g. continental col- One approach for capturing along- and across- lision) produce exhumation that can occur over strike variations in deformation response in com- long timescales (10–50 Ma) and great distances plicated tectonic settings is to apply a ‘shotgun’ (hundreds to thousands of km: e.g. Zhu et al. sampling strategy (Spotila 2005). Interpreting

From:Jourdan, F., Mark,D.F.&Verati, C. (eds) 2014. Advances in 40Ar/39Ar Dating: from Archaeology to Planetary Sciences. Geological Society, London, Special Publications, 378, 225–243. First published online June 20, 2013, http://dx.doi.org/10.1144/SP378.12 # 2014 The Authors. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

226 J. A. BENOWITZ ET AL. cooling age data from shotgun sampling strategy 2000; Ridgway et al. 2007; Haeussler et al. 2008; in terms of episodic or persistent exhumation is Finzel et al. 2009). What is still not clear is whether critical to correlating an orogen’s exhumation the Neogene exhumation history of southern Alaska history to far-field tectonic processes. Thus, a need is a series of discrete events or part of a contin- remains for developing approaches that capture uum related to on-going long-term tectonic pro- the ‘big-picture’ tectonic development of orogens cesses such as the progressive highly coupled that have experienced subregional-varying exhuma- flat-slab subduction of the Yakutat microplate into tion histories. the North American plate (Fig. 1). We have investigated these orogenic devel- To address the continuous v. episodic nature opment/plate-tectonic reconstruction difficulties of exhumation in southern Alaska, we employ along the continental-scale transpressional Denali 40Ar/39Ar K-feldspar (K-spar) thermochronology Fault in the topographically high Eastern Alaska combined with 40Ar/39Ar muscovite and 40Ar/39Ar Range (Figs 1 & 2), and use a simple graphical pro- biotite analysis of 50 samples collected across and cedure to view an orogen’s exhumation history along the Eastern Alaska Range. These thermo- through time and space. Thermochronological chronometers span a closure temperature range of research on the Neogene tectonic history of south- approximately150–450 8C.Thus,muscovite,biotite ern Alaska has led to the interpretation of exhuma- and K-spar record exhumation from a broad range tion pulses at approximately 25, 23, 20, 18, 16, 11, 6, of crustal depths, thereby providing a large amount 4 and 1 Ma (Plafker et al. 1992; Fitzgerald et al. of T–t information to allow us to assess whether 1995; O’Sullivan & Currie 1996; Berger et al. orogenesis was persistent or episodic. We vertically 2008; Enkelmann et al. 2008; Haeussler et al. 2008; stack numerous best-fit K-spar MDD thermal Spotila & Berger 2010). Palaeo-environmental models (Lovera et al. 2002) (herein referred to as analysis of Alaska’s sedimentary basins also sug- ‘vertical stacking’ or ‘stacked thermochronology’) gests that pulses of exhumation occurred during to account for spatial variations in the focus of exhu- the Miocene–present (Lagoe et al. 1993; Thoms mation of an orogen.

Fig. 1. Tectonic map of southern Alaska (modified from Haeussler et al. 2000). Major faults, tectonic plates and tectonic blocks are labelled. Yakutat–North America plate motion (solid arrow) is from Fletcher & Freymueller (2003). The present location of the subducted Yakutat slab beneath the continental margin is denoted (after Eberhart-Phillips et al. 2006). A detailed digital elevation model of the segmented Alaska Range flooded to 1000 m to emphasize topography. The main topography of the Western and Central Alaska Range is south of the Denali Fault. The main topography in the Eastern Alaska Range is 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; NR, Nenana River; DR, Delta River; TR, Tok River; WVF, Wrangell volcanic field. Mount McKinley (Denali) and Mount Kimball locations are noted. The small digital elevation model includes all of Alaska. SW, SW Alaska samples; CW, Cottonwood Metamorphic Complex. Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

EASTERN ALASKA RANGE EXHUMATION 227

Fig. 2. Colour digital elevation map of the Eastern Alaska Range showing labelled sample locations and 40Ar/39Ar K-feldspar minimum ages. The red age labels are new data, black age labels are from Benowitz et al. (2011a) and blue age labels are from Benowitz et al. (2011b). The shaded area is an approximately 20 km-wide wedge of Miocene 40Ar/39Ar K-feldspar minimum ages with the Denali Fault as the southern boundary. The red cross shows the epicentre of the 2 November 2002 M7.9 Denali Fault earthquake. TB, Tanana Basin; NFTB, northern foothills thrust belt; HCF, Hines Creek Fault; NG, Nenana Glacier; BRG, Black Rapids Glacier; NR, Nenana River; DR, Delta River. We do not have any 40Ar/39Ar K-feldspar data for sample 28RAP (Mica, Fig. 4). The solid red lines are Quaternary fault traces from Koehler et al. (2012). The dashed black line is the Hines Creek fault trace from Wahrhaftig et al. (1975).

We show that rapid exhumation in the Eastern 2008). A variety of studies have used thermochro- Alaska Range began by the late Oligocene and nology to examine the possibility of these as the that it continues to the present. At the regional drivers of Alaska Range exhumation, and we sum- scale, our data indicate that, at any snapshot in marize them to provide context for the debate time, rapid exhumation was occurring somewhere about continuous v. episodic exhumation. We dis- in the orogen throughout the Neogene at a relatively cuss them in terms of the western, central and consistent rate of about 0.9 km/Ma. However, at eastern parts of the Alaska Range (Fig. 1). the local scale, the focus of rapid exhumation varied in timing depending on the location. We infer that deformation and exhumation have been Western Alaska Range occurring in southern Alaska for at least about The Tordrillo Mountains contain the area of high 25 Ma, and are related to the progressive collision peaks in the Western Alaska Range (Fig. 1), and via highly coupled flat-slab subduction of the the rocks preserve evidence of rapid exhumation Yakutat microplate with North America. at approximately 23 and 6 Ma based on apatite fission track (AFT) thermochronology and an earlier Eocene exhumation event based on 40Ar/39Ar Tectonics and exhumation in K-feldspar thermochronology (Haeussler et al. 2008; Benowitz et al. 2012a). The approximately southern Alaska 23 Ma exhumation pulse is thought to be controlled The Alaska Range lies along the continental-scale by regional uplift and is corroborated by the high- dextral strike-slip Denali Fault (Figs 1 & 2). This energy depositional environment of the early Mio- intraplate range is located approximately 500 km cene Tyonek Formation of Cook Inlet (Stricker & inboard from the active subduction zone of south- Flores 1996). The approximately 6 Ma exhumation ern Alaska. Suggested drivers for Neogene defor- pulse is also thought to be controlled by regional mation in the Alaska Range are changes in Pacific uplift and is corroborated by the clastic Pliocene plate motion relative to stable North America (Fitz- Sterling Formation (Haeussler et al. 2008). gerald et al. 1995; Enkelmann et al. 2008), Yakutat microplate collision in the Gulf of Alaska (Plafker Central Alaska Range et al. 1992) and block rotation associated with the Yakutat microplate flat-slab subduction/collision The Central Alaska Range is defined by the Mt (Fig. 1) (Cross & Freymueller 2008; Haeussler McKinley region (Fig. 1) and is the site of one Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

228 J. A. BENOWITZ ET AL. of the landmark vertical-transect AFT studies (Fitz- Methods and stacked thermochronology gerald et al. 1993). The results clearly indicate a approach change in exhumation rate at approximately 6 Ma, which is interpreted as the beginning of the uplift Analytical and sampling methods of the Central Alaska Range. The exhumation 40 39 event is correlated with the inferred depositional We undertook Ar/ Ar dating of 30 bedrock age of the 1000 m-thick Nenana Gravel of the samples of granitoid rocks from granitic plutons Tanana Basin, located about 150 km to the east along and across the strike of the Eastern Alaska (Wahrhaftig et al. 1994). The question remains Range, proximal to the north side of the Denali whether the limited spatial scope of the Fitzgerald fault system, in order to better constrain the initi- et al. (1993) study (restricted to Mt McKinley, ation of rapid Neogene exhumation, the long-term with no samples dated from the vicinity of the exhumation history in the Eastern Alaska Range Denali Fault) was sufficient to capture the full and the spatial pattern of exhumation (Fig. 2). exhumation history of the region considering the These samples combined with 20 previously 40Ar/ 39 large variation in exhumation patterns found in Ar K-spar geochronologically dated samples other major orogens (e.g. Little et al. 2005; Yin (Benowitz et al. 2011a, b) are predominately from 2006; Tricart et al. 2007). the Mount Kimball (KIM) College Glacier (COL) the Black Rapids Glacier (RAP), the Mount Balchen (BAL) and Mount Deborah (DEB) and Nenana Glacier (NEN) regions (Fig. 2). A compi- Eastern Alaska Range lation of K-spar minimum age 40Ar/39Ar results The Eastern Alaska Range as defined here spans are given in Figure 2. A summary of the three biotite (integrated ages) and two white mica 40Ar/ the area between the Nenana River in the west 39 and the Tok River valley in the east (Figs 1 & 2). Ar results (integrated and plateau ages) are given in Figures 3 and 4, with all ages quoted to the Uranium–lead zircon emplacement ages of 40 39 approximately 70 Ma were obtained from plutons +1s level. All Ar/ Ar age determinations were in the Black Rapids Glacier, which is part of the calculated using the constants of Steiger & Ja¨ger et al. (1977). Eastern Alaska Range (Aleinikoff 2000). 40 39 Plutons in the Mount Nenana region of the west- For Ar/ Ar analysis, samples were processed ern part of the Eastern Alaska Range are thought at the geochronology laboratory at the University to have approximately 38 Ma emplacement ages of Alaska Fairbanks (UAF), where samples were based on K–Ar and 40Ar/39Ar dating of biotite crushed, sieved for the 250–1000 mm fraction, and hornblende (Csejtey et al. 1992; Benowitz washed, paper-shook and hand-picked for mica et al. 2011a). The plutons around Mount Kimball mineral phases. A Franz magnetic separator and a along the easternmost edge of the Eastern Alaska variable density liquid (sodium-polytungstate and Range have inferred emplacement ages of about deionized water) were used to separate out K-spar 100 Ma based on K–Ar dating of hornblende (Nok- grains. Aliquots of K-spar separates derived from leberg et al. 1992). Knowing the age of pluton the heavy liquid separation were analysed at UAF emplacement in the Eastern Alaska Range allows using a Panalytical wavelength dispersive Axios distinction between post-emplacement cooling and cooling related to exhumation. Less than about 3 Ma AFT and (U–Th/He) apatite (AHe) ages in the Eastern Alaska Range (Armstrong et al. 2007; Benowitz et al. 2011a), as well as active seismicity including the 2002 M7.9 Denali Fault earthquake (Eberhart-Phillips et al. 2003), imply that the region is tectonically active. Deformation and uplift of the Eastern Alaska Range has recently been correlated with the Neogene Usibelli Group of the Tanana Basin, which is interpreted to contain a long-term record of a transpressional foreland basin system related to regional shortening in the Alaska Range along the Denali fault system (Ridgway et al. 2007). This Neogene basin association makes the region a prime location to investigate long-term exhumation patterns and, thus, the far-field drivers of exhuma- Fig. 3. Biotite 40Ar/39Ar age spectrum for sample tion in southern Alaska. 01KIM. Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

EASTERN ALASKA RANGE EXHUMATION 229

28RAP Muscovite Integrated Age: 23.4 ± 0.1 Ma Plateau Age: 23.4 ± 0.1 Ma 30 Fractions: 5 (62.8%) MSWD: 2.16 P: 0.075

Age (Ma) 28RAP Biotite Integrated Age: 19.7 ± 0.1 Ma

10 Age Gap: 3.7 Ma

0 0.0 0.2 0.4 0.6 0.8 1.0 Fraction of 39Ar Released 40

26Rap Muscovite Integrated Age: 24.3 ± 0.1 Ma Plateau Age: 24.3 ± 0.2 Ma Fractions: 5 (99.9%) 30 MSWD: 1.19 P: 0.64

20 Age (Ma) 26RAP Biotite Integrated Age: 19.5 ± 0.2 Ma

10 Age Gap: 4.8 Ma

0 0.0 0.2 0.4 0.6 0.8 1.0 Fraction of 39Ar Released

Fig. 4. Muscovite/biotite 40Ar/39Ar age spectra pairs for samples 26 and 28 RAP. The ‘age gap’ represents the time between Ar closure of the two mineral phases. Steps ﬁlled in grey (top line) were used for muscovite plateau determination.

X-ray fluorometer (XRF) to confirm mineral identi- were irradiated in position 5c of the uranium- fication and purity. enriched research reactor of McMaster University The mineral standard MMhb-1 (Samson & Alex- in Hamilton, Ontario, Canada for 30 MWh. Upon ander 1987), with an age of 513.9 Ma (Lanphere & their return from the reactor, the samples and moni- Dalrymple 2000), was used to monitor neutron tors were loaded into 2 mm-diameter holes in a fluence and calculate the irradiation parameter (J ). copper tray that was then loaded in an ultra-high The samples and standards were wrapped in alu- vacuum extraction line. minium foil and loaded into aluminium cans of The monitors were fused and samples were 2.5 cm diameter and 6 cm height. The samples step-heated using a Coherent Innova 300w 6W Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

230 J. A. BENOWITZ ET AL. argon-ion laser, following the technique described age is the age given by the total gas measured and in Layer et al. (1987) and Layer (2000). Argon puri- is analogous to a K–Ar age. The spectrum pro- fication was achieved using a liquid nitrogen cold vides a plateau age if three or more consecutive trap and two SAES Zr–Al getters at 400 8C and gas fractions represent at least 50% of the total gas room temperature. The samples were analysed release and are within two standard deviations with a fully automated VG-3600 mass spectrometer of each other, with a mean-square weighted devia- at the UAF Geophysical Institute, controlled by an tion (MSWD) equivalent to a probability of greater in-house Visual Basic control program. The UAF than 5%. instrument has a Neir-type source operating with Bulk furnace-run samples consisting of about a trap current of 200 mA, and a 4.5 kV accelerat- five K-spar crystals were reloaded in aluminium ing voltage producing an effective sensitivity of packets and into the fingers of a glass storage tree 1.5 × 10214 mol/V. The instrument has both an and which was then attached to the top of the Modi- off-axis Faraday (with a 10211 ohm resistor) and a fications Ltd low-blank furnace connected on-line Daly detector, and operates in single-collector to the mass spectrometer. Samples were step-heated mode. Data for the five argon isotopes were col- after being dropped into the furnace tantalum cruci- lected primarily using the Daly in peak-hopping ble. The furnace is controlled using a Eurotherm mode (5 s integration time on each peak), with thyristor and controller. Temperature was moni- baseline values collected at ‘masses’ 36.5 and 39.5. tored by means of a thermocouple positioned in Nine scans were performed over a period of a pit at the base of the crucible and a maximum approximately 20 min, and then linearly regressed temperature in excess of 1600 8C is achievable. A to determine the isotopic value and error at the molybdenum liner was not used in order to: (a) time of sample introduction. The 40Ar was measured increase accuracy of the diffusion experiment on each detector for each step to determine the recorded temperature by lessening the distance relative gain between them (c. 70). For fractions from the thermocouple to the degassing sample; that exceeded the range of the Daly Multiplier, (b) lessen thermal mass of the unit to decrease the gain determined from other analyses from the heating time and decrease cooling off time; and run was used to calibrate the 40Ar measured on (c) reduce the cost. the Faraday detector. Error in this gain calculation The furnace was calibrated by both the colour was included in the determination of the isotopic temperature correlation assuming a black body values for isotopes measured on the Faraday. (Davis 1931), by the temperature aluminium foil The argon isotopes measured were corrected melts at (660 8C) and collaborated by the break- for system blank and mass discrimination, as well down of volume diffusion behaviour in K-spar as calcium and potassium interference reactions above 1150 8C (Lovera et al. 1991) demonstrated following procedures outlined in McDougall & by irregular higher temperature age spectra (Fig. Harrison (1999). Typical full-system 8 min laser 5). Temperature was recorded every 30 s, and blank values (in moles) were generally 2 × 10216 averaged to mitigate and take into account heat mol 40Ar, 3 × 10218 mol 39Ar, 9 × 10218 mol 38Ar up time, cool off time and any slight overshot of and 2 × 10218 mol 36Ar, which are 10–50 times set temperature. Recorded temperature is estimated smaller than the sample/standard volume fractions. to have an error of +5 8C with a limited affect on Correction factors for nucleogenic interferences thermal models based on numerous diffusion during irradiation were determined from irradiated experiments on the same sample (Lovera et al. 39 37 CaF2 and K2SO4 as follows: ( Ar/ Ar)Ca ¼ 2002). 24 36 37 24 7.06 × 10 ,(Ar/ Ar)Ca ¼ 2.79 × 10 and Approximately 31 duplicated isothermal step- 40 39 ( Ar/ Ar)K ¼ 0.0297. heating schedules were conducted on the K-spar Mass discrimination was monitored by run- separates in order to retrieve 39Ar diffusion charac- ning calibrated air shots. The mass discrimination teristics, to apply diffusion models and to calculate during the 5 year period of this research varied model thermal histories (Fig. 5) (Lovera et al. due to filament replacement, ranging from 0.2 to 1993; Harrison et al. 1994). About three high- 1.3% per mass unit. While doing our experiments, temperature step-heats (c. .1150) were run to calibration measurements were made on a weekly– fully degas the furnace before the next analysis monthly basis to check for changes in mass dis- and were not input for MDD thermochronology, crimination with no significant variation seen with 12 min blanks run at room temperature (cold during these intervals, other than during filament blank), and at approximately 588 and 980 8C. replacement. Typical full-system cold 12 min furnace blank Plateau and integrated ages were determined values were generally 3 × 10215 mol 40Ar, 1 × from the isotopic ratios for each step determined 10218 mol 39Ar, 1 × 10217 mol 38Ar and 1 × from these corrected measurements weighted by 10217 mol 36Ar. Blanks at about 588 and 980 8C the amount of 39Ar released per step. The integrated were generally the same, at 5 × 10215 mol 40Ar, Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

EASTERN ALASKA RANGE EXHUMATION 231

MDD thermochronology is also useful because the depth for closure of the Ar system minimizes the effect of topography and advection on the temperature distribution in the upper crust (Ehlers 2005). MDD thermal models were created using soft- ware developed by Lovera et al. (1993). Low- temperature steps were adjusted to account for the likely presence of fluid-inclusion hosted excess Ar, associated with Cl, resulting in older apparent ages. In many cases, the first step of an isothermal duplicate yielded a significantly older age than the second step, consistent with the presence of fluid- inclusion hosted excess Ar (Harrison et al. 1994). However, corrections using the equations from Harrison et al. (1994) did not yield usable results, as also found by Sanders et al. (2006). We used the isothermal correction technique outlined by San- ders et al. (2006), in which they took the average age of the step before and the step after an apparent old age as an estimate of the excess Ar correction. We base our calculation of the initiation of rapid cooling and rapid cooling rate determination on the slope of the bottom line of the MDD best- fit thermal history (90% confidence interval of the media) for each sample from the core of the Range (Fig. 6). We used the 10 8C/Ma knick-points of the lower slope, based on constraints on regional background exhumation rates, to quantify when rapid cooling began and ended (Fig. 6). This pro- vides an estimate of the rapid cooling rate for each sample, allowing us to discuss overall variations in cooling rates for one or multiple samples. The minimum K-spar age method is based on the work of Copeland & Harrison (1990) using minimum age steps and Valli et al. (2007) using minimum age isochron populations. The minimum age determination of an 40Ar/39Ar K-spar analysis is associated with lower Ar closure temperature for K-spar (c. 150 8C). In this paper we use minimum age steps to determine the minimum K-spar age for each sample because in 40Ar/39Ar dating there is often a component of trapped initial 40Ar in the low temperature release steps. These steps may provide information for regressing back to the initial 40Ar/36Ar ratio but these steps often have limited use in determining a plateau age based on their small radiogenic 40Ar component (e.g. Fig. 5. Complex K-spar 40Ar/39Ar age spectra from samples 01KIM, 26RAP and 19BAL. Steps filled in red Kuiper 2002). Hence, we often used more steps to (grey in print version) were used for MDD modelling. calculate an isochron age for an individual sample than to calculate a corresponding plateau age. We use the same general geothermal gradient, 3 × 10218 mol 39Ar, 1 × 10217 mol 38Ar and approximately 30 8C/km, used in many of the exhu- 2 × 10217 mol 36Ar. mation studies performed in southern Alaska to cal- MDD thermochronology has proven to be a culate exhumation rate (O’Sullivan & Currie 1996; useful tool in examining orogenic development Haeussler et al. 2008; McAleer et al. 2009; Beno- because of the wide Ar closure-temperature witz et al. 2011a). This allows direct comparison window (c. 150–350 8C) of the system (McDougall of exhumation rates between southern Alaska & Harrison 1999 and references therein). K-spar regional exhumation studies. In addition, based on Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

232 J. A. BENOWITZ ET AL. Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

EASTERN ALASKA RANGE EXHUMATION 233 the maximum depth of aftershocks (c. 11 km), the known to yield flat 40Ar/39Ar age spectra when geothermal gradient for the Denali Fault region is rapidly cooled (e.g. Batt et al. 2004). inferred to presently be approximately 30 8C/km We step-heated the biotite separates of samples (Fisher et al. 2004). We acknowledge that we 26RAP, 28RAP and 01KIM to check for evidence have no actual constraints on the Denali Fault of gross alteration. We used the integrated (total gas zone palaeo-geothermal gradient, nor temporal and age)ofthebiotite 40Ar/39Aranalysisasouragedeter- spatial variations in palaeo-geothermal gradient. mination because of the aforementioned potential However, K-spar has an Ar closure tempera- for instability during biotite heating in vacuo. ture window of approximately 150–350 8C. This crustal temperature zone is less affected by both heat advection and variations in isotherm depth Stacked multi-domain diffusion modelling linked to variations in surface topography (e.g. We offer a simple approach to reconstructing the Braun et al. 2006). In addition, there is no known exhumation history of an orogen, building on the history of extensional events, magmatic events or previous work of Batt et al. (2004) by vertically hot spring activity along the Denali Fault during stacking the best-fit MDD thermal histories of the Neogene that would dramatically alter the eight K-spar-bearing bedrock samples collected regional geothermal gradients (Plafker et al. 1994). along and across the strike of an orogen (Figs 2 & 6), including two new samples (26RAP and 19BAL) Background rock cooling rate and indicators and six others from Benowitz et al. (2011a). The amount of cooling (inferred in terms of exhumation) of deeper crustal exhumation recorded in each sample is complicated by differ- ences in palaeo-depth, present-day elevation and K-spar MDD modelling of sample 01KIM was used the location of each individual sample in relation to examine the background exhumation rate adja- to structures (e.g. the Denali Fault). Stacking each cent to the Denali Fault at the SE edge of the MDD best-fit thermal history using a y-axis with region of high topography in the Eastern Alaska no physical meaning allows construction of a com- Range (Figs 1, 2 & 7). Unaltered biotite from sam- 40 39 posite mosaic that shows slopes and trends in the ple 01KIM was also dated using Ar/ Ar laser thermal history that reflect the long-term spatial step heating to confirm the previous K–Ar emplace- variations in the rate of bedrock cooling along and ment age of 103.6 + 3.1 Ma (Hornblende K–Ar) across the strike of an orogen (Fig. 8). We stack determined in the region (Fig. 3) (Nokleberg et al. 40 39 the cooling histories in descending order of age of 1992). Ar/ Ar laser step heating of muscovite– initiation of rapid cooling (i.e. using the ages of the biotite pairs from samples 26RAP and 28RAP 10 8C/Ma knick-points in Fig. 6). Thus, by stacking (Figs 2 & 4) were used to constrain the exhumation the cooling histories of numerous samples (Fig. 8), rates above the K-spar closure temperature in the breaks in slope between samples reveal informa- high peak region of the Eastern Alaska Range. tion about whether rapid exhumation within an Age spectra from biotite have been shown to be entire orogen is episodic or relatively constant. a potentially unreliable means to evaluate Ar con- centration gradients because of instability during heating in vacuo (Gaber et al. 1988). However, biotite 40Ar/39Ar analysis can provide useful cooling- Results rate information if ages are reported as integrated Mica 40Ar/39Ar ages ages and the range of published possible closure temperatures (Tc; c. 350–400 8C) (e.g. Reiners Biotite is a hydrous mineral that is unstable during et al. 2005) is taken into consideration and it can in vacuo laser heating (Gaber et al. 1988), and thus be compared to muscovite, which has a better- may not produce geologically meaningful release constrained closure temperature and is stable dur- patterns that reflect spatial Ar isotopic gradients ing heating in vacuo (Harrison et al. 2009). We (e.g. Brownlee & Renne 2010). Although a flat assume an Ar closure temperature of approxi- age spectrum does not definitively indicate limited mately 450 8C for muscovite (500 mm grain size) 40Ar loss, conversely there are numerous examples (Harrison et al. 2009), which is a reasonable ap- in the literature where concave biotite age spec- proximation for our samples because muscovite is tra are interpreted as indicative of loss or slow

Fig. 6. MDD thermal models generated from K-spar samples 26RAP and 19BAL (new data), and 26BAL, 03RAP, 03BAL, 18BAL, 22DEB and 32NEN data are from Benowitz et al. (2011a). The MDD magenta band is the 90% conﬁdence interval of the mean and the purple band is the 90% conﬁdence of the distribution. The black lines mark the exhumation rate knick-points. The error bars for the micas for sample 26RAP are smaller than the symbols used. Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

234 J. A. BENOWITZ ET AL.

Fig. 7. MDD thermal models generated from K-spar and the biotite age from sample 01KIM. The MDD magenta band is the 90% confidence interval of the mean and the purple band is the 90% confidence of the distribution. The blue line is a projection of the long-term cooling rate past the closure temperature of K-spar to the 0 8C intercept. cooling (Hacker et al. 2009). Based on the general interpretation is not unequivocal. In addition, sequential closure of mineral phases from high to based on the general sequential closure of mineral low nominal closure temperatures, we infer that a phases from high to low nominal closure tempera- flat biotite age spectrum is evidence of minimal tures, we infer that the biotite 40Ar/39Ar age deter- 40Ar loss, although we are aware that this minations are not affected by excess 40Ar.

(a)(b) C) C) o o 350

350 03RAP 21o C/Ma 03BAL 350 o 150 18BAL 24 C/Ma Temperature ( Temperature ( 350 o 150 22DEB 31 C/Ma

150 29o C/Ma 150

0 102030400 10203040 Age (Ma) Age (Ma)

Fig. 8. (a) Examples of stacked MDD plots, using the 90% conﬁdence interval of the mean from samples 03RAP, 03BAL, 18BAL and 22DEB from the high peak region of the Eastern Alaska Range with the temperature window shown for each sample. (b) Stacked MDD 90% conﬁdence interval of the mean of all the samples from the high peak region of the Eastern Alaska Range (26RAP, 19BAL, 26BAL, 03RAP, 03BAL, 18BAL, 22DEB and 32NEN). The average rapid cooling rate is labelled. Temperature windows have been removed for clarity. Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

EASTERN ALASKA RANGE EXHUMATION 235

Biotite from sample 01KIM yields an integrated older (c. 64 Ma) than the K-spar minimum ages (total fusion) age of 94.8 + 0.5 Ma (Fig. 3). Mus- proximal to the Denali Fault. In addition, one covite from samples 26RAP and 28RAP show sample distal to the north side of the Denali Fault simple spectra with little evidence of argon loss east of the Hines Creek intersect has an old age of (Fig. 4). The individual integrated and plateau about 58 Ma (Fig. 2). ages are within error for both muscovite samples and yielded flat age spectra. Muscovite from sam- 40 39 ple 26RAP has a plateau age of 24.3 + 0.2 Ma K-spar Ar/ Ar MDD thermal models (Fig. 4). Biotite from sample 26RAP has an inte- All three new bulk K-spar furnace step-heating grated age of 19.5 + 0.2 Ma (Fig. 4). The time experiments yielded K-spar age spectra that dis- between Ar closure of the two distinct mineral play complex thermal histories that reflect pro- phases is approximately 4.8 Ma. Muscovite from longed post-emplacement cooling (Fig. 5). In sample 28RAP has a plateau age of 23.4 + 0.1 Ma addition, all three new bulk K-spar furnace step- (Fig. 4), while biotite from 28RAP has an inte- heating experiments yielded diffusion patterns grated age of 19.7 + 0.2 Ma (Fig. 4). The time compatible with MDD modelling. The step-heating between Ar closure of the two distinct mineral results yielded an activation energy (E)of phases is about 3.7 Ma. 48.52 kcal/mol (203.14 kJ mol21) and a frequency 2 21 factor (Do) ¼ 7.97 cm s for 01KIM, E ¼ 40 39 21 K-spar Ar/ Ar minimum age constraints 47.55 kcal/mol (199.08 kJ mol ) and Do ¼ 7.97 cm2 s21 for 26RAP, and E ¼ 47.45 kcal/mol 40 39 21 2 21 The majority of the new Ar/ Ar analysis of the (198.66 kJ mol ) and Do ¼ 6.68 cm s for K-spar samples (20 out of 29) did not provide iso- 19BAL. Results are all within the ranges expected chron age determinations because of the generally for K-spars (Lovera et al. 1997). The thermal homogenous radiogenic content of the minimum model from sample 01KIM demonstrates slow pro- age step release (Kuiper 2002). The nine samples longed cooling starting at about 68 Ma at a long- that did provide isochron age determinations were term rate of approximately 6 8C/Ma until around within error of the minimum age step(s) determi- 52 Ma when cooling is slightly slower at 3.4 8C/ nation and did not demonstrate the presence of Ma until Ar closure (Fig. 7). MDD models (Fig. 6) excess 40Ar. For this study, we use the minimum from samples 26RAP and 19BAL show initiation age steps(s) determination for the new data. of rapid cooling at about 12 Ma. This falls in the middle of the range of rapid cooling initiation docu- Samples proximal to the north side of the Denali mented in previous MDD models from the high Fault south of the Hines Creek Fault. The youngest relief region of the Eastern Alaska Range Rapid (Miocene) K-spar minimum ages are located in a (samples 03RAP, 03BAL, 18BAL, 22DEB and tight wedge within about 15 km of the north side 32NEN in Fig. 6) (Benowitz et al. 2011a). of the Denali Fault and south of the Hines Creek Fault (shaded area Figs 2 & 9). There is a general trend of younging of K-spar minimum ages Stacked K-spar 40Ar/39Ar MDD thermal towards the Denali Fault, as was identified in Beno- models witz et al. (2011a). The youngest K-spar minimum ages are approximately 8–6 Ma at the apex of the Most of the models (Fig. 6) show non-systematic Denali Fault. short-term maximum cooling rates (c. 1 Ma) between approximately 30 and 45 8C/Ma. Analysis Samples south of the Denali Fault. K-spar minimum of such short timescales is not relevant to our goal ages are significantly older south of the Denali of understanding long-term cooling patterns and Fault, with the youngest recorded ages at about may be overinterpreting the T–t constraints pro- 26 Ma. There is a general trend of younger ages at vided by the MDD modelling technique. It has lower elevations and closer to the Denali Fault also been shown that long-term histories of cool- (Figs 2 & 9). ing rate better reflect the effects of far-field driving Samples on the eastern and western edge of the forces than short-term, spatially limited thermal Range front. K-spar minimum ages are older than variations (Bernet et al. 2009). The 90% confidence ages at the centre of the range in the high peak level of the median (best-fit) of K-spar thermal region, with the youngest recorded ages at approxi- models from samples 26RAP and 19BAL were mately 28 Ma (Fig. 2). stacked with six previously published K-spar best- fit thermal models (Benowitz et al. 2011a) from Samples near or north of the Hines Creek Fault. the Eastern Alaska Range (Fig. 8b). The eight K-spar minimum ages near and across the Hines K-spar samples all record periods of rapid cooling, Creek Fault at the core of the range are significantly with an average rate of approximately 26 8C/Ma Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

236 J. A. BENOWITZ ET AL.

Wedge of Miocene ages North of the Denali fault N= 37

Denali fault South of the Denali fault N= 11 Age Probability

0 20406080100 Age (Ma)

Fig. 9. Normalized probability density function plot of all K-spar minimum ages north and south of the Denali Fault. Miocene cooling ages are only present in a narrow wedge (c. 15 km wide) north of the Denali Fault and south of the Hines Creek Fault.

(Fig. 8b). No single K-spar sample records rapid the 103.6 + 3.1 Ma hornblende age of plutons in the cooling for more than about 4 Ma but, if the results Kimball region (Nokleberg et al. 1992). We infer are viewed collectively, rapid cooling occurred that the emplacement age of the Kimball pluton somewhere within the region throughout the entire is slightly older than about 104 Ma based on the time between approximately 21 and 6 Ma (Fig. 8b). concordance of these two thermochronometers and extrapolation to a pluton emplacement temperature of approximately 700 8C. The average rate of Discussion cooling between about 90 and 68 Ma, based on K-spar from sample 01KIM, is approximately We will: (1) constrain the background rock-cooling 0.5 8C/Ma (Fig. 7). From around 68 to 36 Ma, the rate (inferred due to exhumation) for the Eastern long-term average rate of cooling for sample Alaska Range; (2) demonstrate the minimum age 01KIM increases to about 4 8C/Ma. There is a for the start of Neogene rapid exhumation in the slightly higher (c.68C/Ma) long-term cooling region; (3) examine the spatial pattern of Miocene rate between about 68 and 52 Ma. With that in exhumation in the Eastern Alaska Range based on mind, we interpret the K-spar MDD model for the collective K-spar minimum age data set; (4) sample 01KIM (Fig. 7) to reflect slow 0.5 8C/Ma apply (and thus evaluate) the stacked thermochro- post-emplacement cooling until a change in tec- nology approach to eight best-fit MDD thermal tonic environment, at around 68 Ma, led to an models from the high peak region to discern increase in the cooling rate to 6 8C/Ma. Intrusion whether exhumation has been episodic or persistent of alkaline dykes occurred in the Kimball region in the core of the orogen since the initiation of rapid at about 68 Ma (Foley 1985) and may have been exhumation; and, finally, (5) discuss the large-scale related to a short-lived extensional event. The time regional tectonic implications of applying our of the change in cooling rate is also coincident stacked approach to thermochronology. with an interpreted period of movement along the western Denali Fault at around 66 Ma (Miller et al. Background cooling rate 2002). Thus, we interpret that the approximately 6 8C/Ma cooling rate at about 68 Ma derived from The biotite age of 01KIM (40Ar/39Ar age ¼ sample 01KIM may be related to the onset of move- 94.8 + 0.5 Ma, Fig. 3) is roughly concordant with ment of the Denali Fault in the region. Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

EASTERN ALASKA RANGE EXHUMATION 237

If the average cooling rate of sample 01KIM is rapid exhumation in the Eastern Alaska Range, projected past the closure age of K-spar to the which began by about 24 Ma. 0 8C/Ma intercept, the derived cooling rate of approximately 4 8C/Ma is the same average Miocene K-spar minimum ages cooling rate (c. 3.4 8C/Ma) demonstrated between about 52 and 36 Ma in the MDD modelling from Miocene K-spar minimum cooling ages are only sample 01KIM. Fitzgerald et al. (1995) inferred a located between the Denali Fault and the Hines similar background cooling rate of approximately Creek Fault at the apex of the Denali Fault (Figs 2 3 8C/Ma between about 20 and 6 Ma for the & 9). The youngest ages are also proximal (within Central Alaska Range (i.e. Mount McKinley). We c. 10 km) of the Denali Fault itself. The pattern of therefore assume any documented well-constrained the youngest K-spar minimum cooling ages being best-fit MDD thermal model cooling rate change located along the Denali Fault is a common from slow (c.58C/Ma) to rapid (c. .10 8C/Ma) pattern along transpressive strike-slip faults (e.g. represents a definitive increase in exhumation- Little et al. 2005). This distinct pattern is most prob- related cooling (e.g. Fig. 6). We choose a rate of ably related to differential unroofing adjacent to about 10 8C/Ma to avoid rate changes that are the master strand of the Denali Fault caused by a simply a reflection of minor thermal perturbations combination of a non-vertical fault dip, as discussed along the Denali fault zone and to take into in detail in Benowitz et al. (2011a), and the fault account the constrained Tertiary background exhu- zone being a region of erosional weakness (Fig. 2). mation rate along the long-lived (c. 85 Ma: Miller K-spar minimum cooling ages south of the et al. 2002) Denali Fault strike-slip fault system. Denali Fault (Fig. 9) and to the east of the Hines The use of a standard rapid-rate-change limit (c. Creek–Denali fault intersection are significantly .10 8C/Ma) significantly greater than the demon- older (Fig. 2), as are the K-spar minimum cooling strated background cooling rate allows us to ages near or north of the Hines Creek Fault and to compare cooling rate trends between samples and the west of the Denali Fault apex (Fig. 2). We the sample set as a whole from the high peak discuss a geodynamical model for this large-scale region of the Eastern Alaska Range. After each pattern of focused Miocene deformation in the sample (Fig. 6) starts cooling at a rate greater than Eastern Alaska Range in the subsection ‘Alaska about 10 8C/Ma most of the individual samples Range deformation in response to the Yakutat col- continue to cool at this rate or higher through the lision’ later in this paper. low-temperature K-spar Ar closure temperature (c. 150 8C). Samples 18BAL and 26RAP are excep- Vertically stacked approach tions where after a period of rapid cooling the samples records period of slow cooling (c. The stacked approach presented here (Fig. 8) reveals ,10 8C/Ma) (Fig. 6). that, although the location of rapid exhumation varies over time, orogenesis has been occurring Initiation of rapid exhumation-related cooling within the region quite persistently since about 24 Ma (K-spar and mica analysis combined). From Based on the emplacement age (c. 70 Ma) of plutons these findings, we now believe that our previous in the Black Rapids Glacier region (Nokleberg et al. work (Layer & Benowitz 2008) misinterpreted the 1992), the Ar closure ages of about 20–24 Ma for lack of evidence for exhumation to conclude that muscovite and biotite from samples 26RAP and no exhumation had occurred. We believe that the 28RAP (Figs 4 & 6) reflect cooling related to exhu- stacked approach reveals a more complete and mation and not cooling related to initial pluton coherent story, which indicates that the focus of emplacement. Assuming an Ar closure temperature rapid exhumation in the Eastern Alaska Range of around 450 8C for muscovite (Harrison et al. varied spatially through time and reflects a long 2009) and about 350–400 8C for biotite (Reiners history of persistent deformation. We also believe et al. 2005), the difference in muscovite and that the stacked approach better constrains the biotite closure ages for sample 26RAP (c. 4.8 Ma) initial timing of rapid exhumation in the Eastern indicates a cooling rate of between approximately Alaska Range and makes the case that there is no 10 and 21 8C/Ma. The difference in muscovite single all-encompassing uplift age for the entire and biotite closure ages for sample 28RAP (c. region. 3.7 Ma) reflects a cooling rate of between about 14 and 27 8C/Ma. Regardless of which biotite closure A new view of Eastern Alaska Range temperature is used for samples 26RAP and exhumation rates 28RAP, the short age span between the mica pairs from these samples demonstrate rapid cooling. We Assuming a geothermal gradient of about 30 8C/km infer that the cooling is related to rock uplift and (Fisher et al. 2004), as discussed in the Methods Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

238 J. A. BENOWITZ ET AL. section of this paper, when the sample set is viewed south-central Alaska is an obvious candidate in its entirety a rapid exhumation has occurred (Plafker 1987; Spotila & Berger 2010). We define within the Eastern Alaska Range at a persistent ‘collision’ in this case as highly coupled flat-slab rate of approximately 0.9 km/Ma between about subduction at the subduction margin. The Yakutat 24 and 6 Ma (Fig. 10). This is roughly the same microplate is composed of crystalline crust that is approximate average of 1.0 km/Ma exhumation 15–30 km thick and is inferred to have an oceanic rate from about 6 Ma to present as determined by plateau origin (Christenson et al. 2010; Worthington apatite fission track work in the Nenana Glacier et al. 2012). Based on tomographical studies, the region (Perry et al. 2010) if calculated using the Yakutat microplate is currently undergoing flat-slab same 30 8C/km geothermal gradient applied in this subduction beneath the Chugach–Saint Elias study. Thus, there is no evidence of a region-wide Mountains, with a northern edge 500 km inboard increase in tectonically driven exhumation since of the subduction zone at a depth of about 100 km about 24 Ma–present in the Eastern Alaska Range. (Eberhart-Phillips et al. 2006). The arrival time of the Yakutat microplate into the southern Alaska subduction zone is thought to have been around Alaska Range deformation in response 25 Ma (Plafker et al. 1994), yet it remains unclear to the Yakutat collision when collision began (e.g. Spotila & Berger 2010). The 40Ar/39Ar thermochronology data pre- The long-term occurrence of rapid exhumation in sented here demonstrate an increase in the exhuma- the Eastern Alaska Range demands a long-term, tion rate at about 24 Ma in the Eastern Alaska continuous tectonic driving mechanism. The pro- Range, potentially in response to Yakutat flat-slab gressive ‘collision’ of the Yakutat microplate with subduction.

Start of movement Minimum age of initial Slow background along the Eastern Emplacement Yakutat microplate highly exhumation rate Denali fault? age of coupled flat slab subduction Kimball pluton

0 10 20 30 40 50 60 70 80 90 100 Denali Fault

01KIM 3 6 <1

03RAP 21

26BAL 15 Rapid cooling Key 26 N 03BAL 24 indicated by K-spar

18BAL 25 MDD models 26 = average 26RAP 35 cooling rate (°C/Ma) Biotite 19BAL 31 Biotite from Benowitz et al. [2011] 22DEB 29 Muscovite

32NEN 25

Hines Creek Fault W 148° 0 10 20 30 40 50 60 70 80 90 100

N 63°30´ Rapid exhumation Emplacement age 0 25 50 km of the Eastern of plutons in the high Alaska Range peak region of the Eastern Alaska Range

Fig. 10. Summary figure of K-spar MDD thermal models and mica closure ages for the Eastern Alaska Range. The base map is a detailed digital elevation model of the Eastern Alaska Range flooded to 1000 m to emphasize topography. The thermal history of 01KIM on the SE edge of the high peak region indicates slow post-pluton emplacement cooling followed by slower background exhumation related to inferred movement along the Denali Fault. The initiation of rapid cooling of samples from the high peak region is based on the 40Ar/39Ar K-spar MDD modelling of each sample (Fig. 6) at the initial point where cooling is .10 8C/Ma. The average rapid cooling rate (8C/Ma) is a minimum time-averaged long-term rapid cooling rate of .10 8C/Ma. The bold sample labels indicate close proximity to the Denali Fault and the grey filled triangles are biotite cooling ages from Benowitz et al. (2011a). The error bars for the micas are smaller than the symbols used. The figure was inspired by Batt et al. (2004). Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

EASTERN ALASKA RANGE EXHUMATION 239

Other regions of Alaska also show evidence of a Tibet region, which began soon after collision, was late Oligocene–early Miocene exhumation rate caused by an indenting boundary concentrating change in regional tectonic forcing. Ridgway et al. strain along the southern edge of the strong Tarim (2007, 2012) suggested that the strata of the Tanana Basin region (Dayem et al. 2009). Similarly, the Basin encompass the entire Neogene and showed deepest Neogene exhumation recorded in the that the basin is genetically related to the formation Alaska Range is located at the apex of the curve in of the Alaska Range (Fig. 2). Trop et al. (2012) cor- the Denali Fault. The Hines Creek Fault zone related Miocene basin development in the Wran- (Figs 2 & 10), which is a major tectonostratigraphi- gell volcanic field with flat-slab subduction of the cal boundary and offsets the Moho, is possibly Yakutat microplate. Thermochronological work acting as a strong backstop that concentrates strain in the Tordrillo Mountains (Haeussler et al. 2008), from the Yakutat microplate collision (Figs 1, 2 & approximately 50 km WNW of Anchorage, Alaska 10) (Veenstra et al. 2006), with deformation being (Fig. 1), indicates that rapid exhumation began focused along the weak zone of the north-dipping there at around 23 Ma. Detrital zircon fission track Denali Fault (Freed et al. 2006; Benowitz et al. work in the Chugach–Saint Elias Range at Alaska’s 2011a). southern margin (Fig. 1) (Enkelmann et al. 2008) Geophysically plausible kinematic mechanisms shows a pulse of exhumation beginning at about have been proposed to drive coeval orogenesis 25 Ma. Recent AFT work in SW Alaska indicates in the Chugach–Saint Elias and Alaska ranges. rapid cooling in both plutonic and meta-sedimentary During the Neogene, basin formation and regional samples between approximately 24 and 20 Ma, magmatism also occurred concurrently in southern implying regional cooling due to exhumation dur- Alaska. Although we cannot definitively constrain ing this time period (Fig. 1) (O’Sullivan et al. 2010; the timing of the initial collision of the Yakutat Benowitz et al. 2012a). Deformation and meta- microplate, we add the results of this paper to the morphism was occurring on part of the eastern growing evidence that flat-slab subduction was Denali Fault by about 25 Ma at the Cottonwood underway by about 25 Ma (Fig. 10). complex (Fig. 1) (Richter 1976; Benowitz et al. 2011c, 2012b). Fold-and-thrust-style deformation began in the St Elias Range by the early Miocene Conclusions (Chapman et al. 2012). In addition, the Wrangell volcano field became active at around 26 Ma and In this paper we investigated the spatially variable is associated with convergence of the Yakutat exhumation history of the Eastern Alaska Range microplate (Richter et al. 1990). Furthermore, and the far-field driving mechanism for regional AFT data from a core in Cook Inlet were used to Neogene deformation in southern Alaska using a document about 4 km of exhumation at approxi- simple graphical approach for thermochronological mately 25 Ma associated with a regional unconfor- analysis combined with extensive 40Ar/39Ar K-spar mity (Murphy & Clough 1999). With the compiled minimum cooling ages, and 40Ar/39Ar muscovite regional evidence and this study, it is clear that sig- and biotite cooling ages. nificant deformation across southern Alaska was Based on stacked K-spar MDD thermal models initiated by about 25 Ma and continues presently. and mica 40Ar/39Ar thermochronology of samples In terms of kinematics, Koons et al. (2010) used collected over an approximately 100 km transect a three-dimensional thermal–mechanical numerical from the Eastern Alaska Range, we show that sub- model to show that both near-field (i.e. Chugach– orogen-scale regions have cooled rapidly, at an Saint Elias) and far-field (i.e. the Alaska Range) average rate of approximately 26 8C/Ma, between mountain ranges could have been created by the about 24 Ma and the present. Although the focus Yakutat microplate collision. An alternative model of exhumation varied over time, continual exhu- (Soofi & Wu 2008) used a thin-viscous-sheet mation at a rate of approximately 0.9 km/Ma has model to investigate the effect of the Yakutat micro- occurred somewhere within the Eastern Alaska plate colliding with the North American plate. The Range since about 24 Ma. Neogene exhumation model predicted topographical development where north of the Denali Fault in the Eastern Alaska the Alaska Range is located and demonstrated that Range has been constant, not episodic. lateral strength heterogeneities in the crust play a The preponderance of evidence from thermo- role in the location of deformation. The timing of chronology and basin analysis data shows initiation deformation predicted by the models differs from of rapid exhumation by around 25 Ma in southern our new geological constraints but the general con- Alaska that continues to the present. Collision cepts of the models are still applicable. (highly coupled flat-slab subduction) of the Yakutat An analogue for the Yakutat–Southern Alaska microplate is the most plausible far-field driving collision is the India–Asia collision, where model- mechanism and we suggest that it began by about ling predicts that the intraplate deformation in the 25 Ma, as Plafker et al. (1994) first broadly proposed. Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

240 J. A. BENOWITZ ET AL.

We thank P. Haeussler, P. Fitzgerald, S. Perry and of a convergent orogenic wedge: a thermochronologi- S. Roeske for constructive comments during the manu- cal investigation of tectonic-climatic interactions in script development. This project would not have been the central St. Elias orogen, Alaska. Earth and Plane- possible without the field-assistance from S. Herreid, tary Science Letters, 270, 13–24, http://dx.doi.org/ A. Liljedahl, A. Sterns and R. Wing. The manuscript bene- 10.1016/j.epsl.2008.02.034 fited from thorough reviews by G. Batt and an anonymous Bernet , M., Brandon, M., Garver, J., Balestieri, reviewer on an earlier version of this manuscript. Two M. L., Ventura,B.&Zattin, M. 2009. Exhuming other anonymous reviewers offered insightful direction the Alps through time: clues from detrital zircon for the manuscript in its current form. Support was pro- fission-track thermochronology. Basin Research, 21, vided by the National Science Foundation through grant 781–798. EAR #0952793. Braun, J. P., van der Beek,&Batt, G. 2006. Quantitat- ive Thermochronology: Numerical Methods for the Interpretation of Thermochronological Data. Cam- References bridge University Press, Cambridge. Brownlee,S.J.&Renne, P. R. 2010. Thermal history Aleinikoff, J. N., Farmer, G. L., Rye,R.O.&Nokle- of the Ecstall pluton from 40Ar/39Ar geochronology berg, W. J. 2000. Isotopic evidence for the sources and thermal modeling. Geochimica et Cosmochimica of Cretaceous and Tertiary granitic rocks, east-central Acta, 74, 4375–4391. Alaska: implications for the tectonic evolution of the Buscher,J.T.&Spotila, J. A. 2007. Near-field response Yukon–Tanana terrane. Canadian Journal of Earth to transpression along the southern San Andreas Fault, Sciences, 37, 945–956. based on exhumation of the northern San Gabriel Armstrong, P., Haeussler,P.&Arkle, J. 2007. Rapid Mountains, southern California. Tectonics, 26, Quaternary exhumation of the Eastern Alaska Range. 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