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Intense Localized Rock Uplift and Erosion in the St Elias Orogen of Alaska E

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Intense Localized Rock Uplift and Erosion in the St Elias Orogen of Alaska E LETTERS PUBLISHED ONLINE: 26 APRIL 2009 | DOI: 10.1038/NGEO502 Intense localized rock uplift and erosion in the St Elias orogen of Alaska E. Enkelmann1*, P. K. Zeitler1, T. L. Pavlis2, J. I. Garver3 and K. D. Ridgway4 Alaska The timing and role of exhumation in the St Elias orogen, Denali F. Yukon the world’s highest coastal mountain range, has been unclear. Alaska Range Alaska Sampling is limited to high mountain ridges that tower over Wrangell Mtns Border widespread ice fields that sit in deeply eroded parts of Anchorage Range F. 1–3 the orogen. Existing bedrock studies in the region are Mt Logan 5,959 m therefore prone to bias. Here we analyse detrital material of Kenai Contact F 60° N Penninsula active sediment systems in the St Elias orogen to obtain age Fig. 2 information from the inaccessible ice-covered valley bottoms. Yakutat We present 1,674 detrital zircon fission-track ages from terrane Transition44 faul mm yr¬1 modern rivers that drain the glaciers. We find a population of t Fairweather very young ages of less than 3 Myr from the Seward–Malaspina Kodiak Is. glacier systems that is sharply localized in the area of the ¬1 Aleutian trench 52 mm yr orogen’s highest relief, highest seismicity and at the transition F. from transform to subduction tectonics. Our data provide 55° N evidence for intense localized exhumation that is driven by 150° W 145° W 140° W 135° W coupling between erosion and active tectonic rock uplift. The St Elias mountain belt originates from the collision of the Figure 1 Tectonic map of southern Alaska. Plate motion vectors are Yakutat terrane with North America, at the corner formed by from ref.|29. the dextral Fairweather transform and the Aleutian subduction zone (Fig. 1). Initiation of the Fairweather fault and northward ages of these young exhumed parts of the orogen (Fig. 2). This transport of the Yakutat terrane started 30 Myr ago, but collision approach was successful in a study of samples collected from the began at 10–5 Myr as the thickened crust∼ of the Yakutat terrane area north of the Bagley Ice field, where the previously unknown accreted to the Aleutian trench4,5, stripping sedimentary cover from older exhumation history was revealed14 (Fig. 2). We applied the basement to construct a foreland fold and thrust belt4,6 (Fig. 2). same approach to the glacial drainages along the southern side of Along the orogenic belt, the youngest (5–0.5 Myr) low-temperature the orogen (Fig. 2; Supplementary Table S1). cooling ages of bedrock (60–110 ◦C closure temperatures (Tc) for Detrital fission-track age distributions from single zircons are apatite (U–Th)/He (ref. 7) and fission track8) are strongly correlated shown in pie charts (Fig. 2). The key finding is that samples from with those areas with the highest precipitation along the southern, the Seward–Malaspina glacier have a significant fraction of ZFT seaward flanks of the orogen. Bedrock cooling ages are oldest ages of <3 Myr, whereas samples farther west have increasingly (40–8 Myr) in the drier northern side1,2 (Fig. 2). Bedrock zircon larger percentages of older ages (>6 Myr). Analysis of these data 3,10 15 fission-track (ZFT) ages (Tc 250 ◦C; ref. 9) give >10 Myr ages using binomial peak-fitting routines yields age populations for (Fig. 2), or are non-reset in∼ the fold–thrust belt, because lateral each sample. A remarkable aspect of the cooling ages in the transport of material into the orogenic wedge results in exhumation Seward–Malaspina samples is that single-grain ZFT ages are as restricted to the upper 5 km (ref. 10). young as 0.4 Myr and define an age-population peak at 2–3 Myr that Similar to other active orogenic belts with high erosion rates, comprises 25–41% of the dated grains. Laser-ablation inductively the St Elias range seems to have developed localized feedback coupled plasma mass spectrometry U/Pb ages of all detrital zircons between erosion and crustal strain11–13. Thus, it is puzzling that no that yielded ZFT ages of <15 Myr (samples I3 and I4) show that evidence has emerged for locally enhanced exhumation and erosion these zircons crystallized at 100–50 Myr, indicating that the ZFT in the form of localized young, higher-temperature cooling ages. ages are the result of cooling due to exhumation and not local However, the St Elias orogen is unique among active orogens in that volcanic activity (Fig. 3; Supplementary Table S2). more than 50% of its area is covered by glaciers hundreds of metres The young detrital zircons were transported by an unusual in thickness (Fig. 2). Besides reducing bedrock exposures in general, glacial system that begins in the core of the highest part of the this glaciation also prohibits direct sampling of the low-elevation St Elias orogen (red shaded area in Fig. 2). The upper part of intensely glaciated valley bottoms where the most recently exhumed this system is the Seward Glacier, which flows seaward through rocks and youngest cooling ages would be expected. a narrow (3–4 km), 2–3-km-deep gorge with fast-moving ice16 1 To overcome this sampling obstacle, we analysed detrital zircons (1,600 m yr− ), known as the Seward throat. When the ice emerges from rivers draining the main glacial systems to evaluate the cooling from the Seward throat, it spreads onto a flat plain to form 1Earth and Environmental Science, Lehigh University, 31 Williams Dr., Bethlehem, Pennsylvania 18015, USA, 2Department of Geological Sciences, University of Texas at El Paso, 500 West University Ave., El Paso, Texas 79968, USA, 3Department of Geology, Union College, 807 Union St., Schenectady, New York 12308-2311, USA, 4Department of Earth and Atmospheric Sciences, Purdue University, 550 Stadium Mall Dr., West Lafayette, Indiana 47907, USA. *e-mail: [email protected]. NATURE GEOSCIENCE ADVANCE ONLINE PUBLICATION www.nature.com/naturegeoscience 1 | | © 2009 Macmillan Publishers Limited. All rights reserved. LETTERS NATURE GEOSCIENCE DOI: 10.1038/NGEO502 Age contours of apatite bedrock (U¬Th)/He data (from ref. 2) Chugach Yukon Border Range fault Alaska 1 Myr terrane >24 <3 0.75 Myr (CH46) 7¬27 AHe 5¬40 AFT Precipitation rate A/Z FT 12¬24 3¬6 Tana Gl. 26 ZHe 5,924 m 40/44 Myr a 34¬48 ZFT ¬1¬1 1,783 m 3.2/41 Myr Mt Logan Gl <1.<1 55m m yryr 6¬12 CSEFContact F. 5,959 m Bagley Ice field <1.5 m yryr¬¬1 Z FT age (Myr) 3¬5 m yr ¬11 Mt Vancouver CSEF ¬1 >7m> m yry Seward Glacier Border St Elias 4,812 m Bering Glacier Y7 5,489 m Ran ge fault MtM Cook Y3 Yakutat IB7 Group 4,196 m IB6 Fairweather F. Malaspina fault Y11 3¬5 m yr¬1 60 N Y12 Y1 Agassiz G. ° Icy Bay Y9 Y8 Y10 Y2 MalaspinaMalaspina Y4 IB5 Y6 Y5 IB3IBIB IB2 IB1 Kayak Island zone Yakutat IB4 Pacific Ocean Bay 20 km Pamplona zone 143° W 142° W 141° W 140° W 139° W Figure 2 Sample locations and main tectonic structures. Samples Y1, Y2, Y3 and CH46 were published in Enkelmann et al.14. CSEF: Chugach/St Elias fault, separating| the Yakutat terrane from the Prince William terrane. The Contact fault sutures the Prince William from the Chugach terrane, and the Border Range fault sutures the Chugach (south) from the Wrangellia terrane (north). The drainage area of the Seward–Malaspina glacier is outlined in red. Thermochronological ages from bedrock studies are shown1–3,10. AHe: apatite (U–Th)/He, AFT: apatite fission track, ZHe: zircon (U–Th)/He, ZFT: zircon fission-track. the Malaspina Glacier. Sediments in a glacial drainage can be IB4 IB3 transported parallel to each other and mix less than in fluvial systems. In addition, cooling ages from a detrital sample give an Chugach average of mainly the low-elevation part of the drainage basin that is (CH46) covered by ice. This feature allows much greater insight into specific source rocks. The samples with the young ZFT ages (IB1 to IB4) Yakutat Group lie within the part of the Malaspina lobe that originates from the Seward Glacier catchment, indicating that all of the young ages originate from the Seward Glacier, the Seward throat, or both. On the other hand, sample IB5 (Fig. 2) is from the Malaspina Glacier, density Probability but the ice mass feeding that specific site is the Agassiz Glacier, which originates from the south face of Mount St Elias. In this case, the lack of very young ZFT ages indicates that rapidly exhuming rocks do not occur in this part of the orogen. This finding is crucial because all young (<2 Myr) bedrock apatite (U–Th)/He ages seen 0 25 50 75 100 125 150 175 200 225 1,2 so far are from this zone (Fig. 2). U/Pb age (Myr) To further refine the source rock identity for the young detrital ZFT ages, we compared the U/Pb zircon crystallization ages of Figure 3 Comparison of U/Pb zircon ages. Probability density plots of samples IB3 and IB4 with samples from potential source areas. The U/Pb zircon| ages of samples IB3 and IB4 compared with the zircon U/Pb U/Pb ages of sample IB4 show a distinct population of 60–50 Myr ages from sample CH46 (ref. 14) representing material of the Chugach similar to the detrital grains from sediments of the Tana Glacier14 metamorphic complex that was transported by the Bagley Ice field/Tana draining the Bagley Ice field towards the northwest (Figs 2 and 3).
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