10-Be Surface Exposure Ages along the Blue Cut Fault in Joshua Tree National Park, California

To better evaluate the late Pleistocene slip rate history of the Blue Cut Fault in Joshua Tree National Park, we constrain the ages of six depositional surfaces that are correlated and offset along the fault. We collect 38 Beryllium-10 surface exposure samples, using a mixture of surface clast types on each mapped surface unit, including sampling boulder tops, cobbles, and amalgamated pebble clasts. Age agreement between sample types indicates that effects of these processes are negligible, because sample types are differentially affected by inheritance/exhumation. We crush whole rock material, remove all minerals but quartz grains, digest the quartz and proceed to extract the element Beryllium in order to measure the 10-Be/9-Be ratio in the Center for Accelerator Mass Spectrometry at Lawrence Livermore National Laboratory. Seven of our samples were processed at Arizona State University, while the rest were processed jointly at San José State University and Lawrence Livermore National Laboratory, following the procedures of Ditchburn & Whitehead (1994). 10-Be geochronological results, in combination with geomorphic mapping indicate multiple ages of offset alluvial surfaces that range from the oldest mapped Qf1a surface of >129 ka, to the youngest offset Qf3b surface of 13.2 ± 1 ka (2𝞂). In addition, we find good agreement between ages of different sample types for each surface. See Guns et al. (2022) for further information.

为更精确地评估约书亚树国家公园内蓝割断层全新世晚期滑动速率历史，本研究对沿断层相关并错位的六个沉积表面进行了年代测定。我们收集了38个铍-10地表暴露样品，采用各测绘单元表面碎屑类型的混合采样方法，包括采样巨石顶部、砾石和混合鹅卵石碎屑。样本类型之间的年龄一致性表明，这些过程的影响可忽略不计，因为样本类型受到继承/剥露的影响存在差异。我们将整块岩石材料进行破碎，去除所有矿物但保留石英颗粒，消化石英并前往劳伦斯利弗莫尔国家实验室的加速器质谱中心提取铍元素，以测量10-Be/9-Be比率。其中七个样品在亚利桑那州立大学进行处理，其余样品则在圣何塞州立大学和劳伦斯利弗莫尔国家实验室联合处理，遵循Ditchburn & Whitehead (1994)的程序。10-Be地质年代学结果与地貌制图相结合，表明错位冲积表面存在多个年龄，从最古老的测绘Qf1a表面（>129 ka）到最年轻的错位Qf3b表面（13.2 ± 1 ka，2𝞂）。此外，我们发现每个表面不同样本类型的年龄之间存在良好的一致性。有关更多信息，请参阅Guns et al. (2022)。