Collaborative Research: Formation of the Dry Valleys, Antarctica: Linking Thermochronometric (U-Th/He) and Cosmogenic Constraints on Landscape Development

该奖项由极地项目办公室的南极地质和地球物理项目提供，支持对横贯南极山脉干谷部分的隆起和景观演变的研究。山脉作为地形特征形成的时间是理解它们作为地壳变形引力驱动力的作用以及评估它们对气候变化的影响或依赖的关键因素。然而，造山作用的这一方面仍然缺乏文献记载，因为与地表海拔升高有关的地貌表达或沉积记录可能被后来的侵蚀所改变。同样，用于跟踪侵蚀和地貌发展的地球化学工具（例如，地表暴露测年）很少用于此目的，因为它们通常只提供最近（100万年）历史的信息。因此，我们对山带的时间和地理/构造演化的理解仍然存在很大的差距。由于对不断演变的山脉的地貌没有直接的限制，从磷灰石裂变径迹热时计等技术推断出的快速冷却事件通常归因于山带的地形上升。这种做法假设，从冷却数据推断出的挖掘（岩石相对于地球表面向上移动）和基岩隆起（岩石相对于大地水准面向上移动）代表了地球表面高度（和起伏）的等效增加，尽管工作人员已经表明，这种相关性并不总是成立。解决长期景观演变问题的一种方法是将低温温度测年法与地表暴露测年相结合。横贯南极山脉的干谷地区是进行此类试验的理想地点，因为该地区的许多现代陆地表面可能已有15亿年的历史，显然中新世中期以来的侵蚀率非常低。磷灰石裂变痕迹表明，干谷中的岩石冷却了大约105摄氏度，距今约45 Ma，在我们对该山脉演化的认识中留下了约30 Ma的空白。应用新开发的磷灰石(U-Th)/He温度计（封闭温度为70℃）可以进一步缩小这一差距，因为该技术对更低的温度很敏感，更重要的是磷灰石氦年龄可以提供地形起伏形成时间的独特敏感记录。该项目是一项紧密聚焦的研究，旨在将河谷切口的热时学指示与宇宙射线暴露年龄相结合，以更好地约束干谷地区的形成和地貌演化。收集的磷灰石(U-Th)/He和暴露年龄数据将在大量已有的基岩隆起和挖掘磷灰石裂变径迹研究数据与景观演化之间提供急需的联系。特别是，通过限制干谷的形成年龄，该项目将确定横贯南极山脉作为地形特征的形成时间。这项工作的结果将对该地区的地球动力学演化具有普遍意义，并将有助于关于新生代古气候变化影响南极东部冰盖生长和稳定的争论。

9909435FarleyThis award, provided by the Antarctic Geology and Geophysics Program of the Office of Polar Programs, supports an investigation of uplift and landscape evolution of the Dry Valleys segment of the Transantarctic Mountains. The time when mountains form as topographic features is a crucial factor for understanding their role as gravitational driving forces for crustal deformation, as well as for assessing their influence or dependence on climate variations. Yet, this aspect of orogenesis remains poorly documented because the geomorphologic expressions or sedimentation record associated with increasing surface elevations may be modified by later erosion. Similarly, geochemical tools used for tracking erosion and landform development (e.g., surface exposure dating) are seldom useful for this purpose because they usually provide information on only the most recent (1 million year (Ma)) history. As a result, a large gap in our understanding of the temporal and physiographic/tectonic evolution of mountain belts remains. In the absence of direct constraints on the geomorphology of evolving mountain ranges, episodes of rapid cooling deduced from techniques like apatite fission track thermochronometry are often attributed to the topographic rise of mountain belts. This practice assumes that exhumation (movement of a rock upward with respect to the earth's surface) and bedrock uplift (movement of the rock upward with respect to the geoid) inferred from cooling data represent an equivalent increase in elevation at the earth's surface (and relief), even though workers have shown that such a correlation does not always hold. One way to address the problem of long-term landscape evolution is to combine low-temperature thermochronometry with surface exposure dating. The Dry Valleys region of the Transantarctic Mountains is an ideal place for such an exercise because many of the modern land surfaces in the region may be as old as 15 Ma, clearly requiring very low erosion rates since the mid-Miocene. Apatite fission tracks indicate that rocks in the Dry Valleys cooled through approximately 105 degrees C as recently as about 45 Ma, leaving a gap of about 30 Ma in our knowledge of the evolution of this mountain range. Application of the newly developed apatite (U-Th)/He thermochronometer (closure temperature of 70 degrees C) can further close this gap because the technique is sensitive to even lower temperatures and more importantly apatite helium ages can provide a uniquely sensitive record of the time at which topographic relief develops. This project is a tightly focused study designed to combine thermochronometric indications of river valley incision with cosmic-ray exposure ages to better constrain the formation and geomorphologic evolution of the Dry Valleys region. The apatite (U-Th)/He and exposure age data that will be collected will provide a much-needed link between the large amounts of data already available from apatite fission track studies on bedrock uplift and exhumation on the one hand, and landscape evolution on the other. In particular, by constraining the formation age of the Dry Valleys, this project will determine when the Transantarctic Mountains formed as a topographic feature. The results of this work should have general implications for the geodynamic evolution of the region, and will contribute to the debate regarding Cenozoic paleoclimate changes influencing the growth and stability of the East Antarctic ice sheet.