Low-temperature thermochronology of the northern Rocky Mountains, western U.S.A.
Low-temperature thermochronology of the northern Rocky Mountains, western U.S.A.
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美国西部落基山脉北部的低温热年代学
DOI：
10.2475/02.2012.04
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发表时间：
2012
影响因子：
2.9
通讯作者：
P. DeCelles
中科院分区：
文献类型：
--
作者：
S. L. Peyton;P. Reiners;B. Carrapa;P. DeCelles
我们使用磷灰石（U-Th）/He系统和11个样品使用磷灰石裂变径迹系统（风河和大角山脉），从基底芯Laramide隆起的北方落基山脉前陆（风河，Beartooth，大角和拉勒米山脉）的86个钻孔和表面样品。磷灰石（U-Th）/He年龄一般随地下深度的增加（海拔的降低）而降低，在地表101 km范围内，年龄一般从100 ~ 50 Ma（白垩纪至始新世），到1020 Ma（中新世），在深度大于102 ~ 2.5 km时年龄更小。大多数样品显示（U-Th）/He年龄分散范围从几十到几百Ma，对于一些样品，我们发现年龄比相应的裂变径迹年龄。每个范围至少有一个样品显示磷灰石（U-Th）/He年龄和晶体的有效U浓度（eU = [U] + 0.235[Th]）之间的相关性，表明辐射损伤影响了He扩散率，因此（U-Th）/He年龄。使用辐射损伤扩散模型对简单Laramide型热历史进行正演模拟，预测：（1）相似海拔范围内的化石磷灰石裂变径迹部分退火和磷灰石（U-Th）/He部分保留带;（2）化石部分保留带内的（U-Th）/He年龄离散达数百Ma;（3）eU ≤ 20 ppm时，（U-Th）/He年龄大于化石部分保留带的裂变径迹年龄。我们在大角山脉和拉勒米山脉的数据中观察到了这些特征。然而，我们的大多数样品，不显示（U-Th）/He年龄和eU辐射损伤扩散模型预测之间的相关性。这些样品的年龄分散性可能是由于晶粒尺寸和eU含量的影响，或者由于磷灰石晶体周围的高U或Th次级边缘。（U-Th）/He年龄比风河山脉甘尼特峰和弗里蒙特峰的裂变径迹年龄更老，一些样品来自Beartooth山脉，最有可能是来自高eU次级边缘的He注入的结果。从（U-Th）/He年龄-eU对的逆建模得到的最佳时间-温度路径，当外推到其他海拔以创建模型年龄-海拔图时，再现了大角岩（U-Th）/He年龄的一般分布和分散，熊牙山脉和风河山脉，并表明在拉腊米德省的快速挖掘可能在大角山脉更早开始熊牙岭（Beartooth Range）（距今158 Ma）。对拉勒米山脉北方端的钻孔数据进行的逆向建模表明，该井在深度处穿透了一个断层带。从这些数据中无法确定Laramide事件后埋葬和挖掘的数量和时间。
We dated 86 borehole and surface samples from basement-cored Laramide uplifts of the northern Rocky Mountain foreland (Wind River, Beartooth, Bighorn and Laramie Ranges) using the apatite (U-Th)/He system, and eleven samples using the apatite fission-track system (Wind River and Bighorn Ranges). Apatite (U-Th)/He ages generally decrease with increasing subsurface depth (decreasing elevation), and typically range from ∼100 to 50 Ma (Cretaceous to Eocene) within ∼1 km of the surface, to ∼20 Ma (Miocene) and younger ages at depths greater than ∼2 to 2.5 km. Most samples display (U-Th)/He age dispersion ranging from tens to hundreds of Ma, and for some samples we find ages that are older than corresponding fission-track ages. At least one sample per range shows a correlation between apatite (U-Th)/He age and effective U concentration (eU = [U] + 0.235[Th]) of the crystal, indicating that radiation damage has affected He diffusivity, and hence (U-Th)/He age. Forward modeling of simple Laramide-type thermal histories using a radiation damage diffusion model predicts: 1) fossil apatite fission-track partial annealing and apatite (U-Th)/He partial retention zones over similar elevation ranges, 2) (U-Th)/He age dispersion within a fossil partial retention zone up to hundreds of Ma, and 3) (U-Th)/He ages older than fission-track ages within a fossil partial retention zone if eU ≳ 20 ppm. We observe these features in our data from the Bighorn and Laramie Ranges. Most of our samples, however, do not show the correlation between (U-Th)/He age and eU predicted by radiation damage diffusion models. The age dispersion of these samples could be due to the influence of both grain size and eU content, or alternatively due to high U or Th secondary rims around the apatite crystals. (U-Th)/He ages that are older than fission-track ages from Gannett Peak and Fremont Peak in the Wind River Range, and some samples from the Beartooth Range, are most likely the result of He implantation from high eU secondary rims. Best-fit time-temperature paths from inverse modeling of (U-Th)/He age-eU pairs, when extrapolated to other elevations to create model age-elevation plots, reproduce the general distribution and dispersion of (U-Th)/He ages from the Bighorn, Beartooth and Wind River Ranges and suggest that rapid exhumation within the Laramide province likely began earlier in the Bighorn Range (before ∼71 Ma) than the Beartooth Range (before ∼58 Ma). Inverse modeling of borehole data at the northern end of the Laramie Range suggests that the well penetrated a fault sliver at depth. The amount and timing of post-Laramide burial and exhumation cannot be determined from these data.