Collaborative Research: Glacial Erosion in the Patagonian Andes: Testing the Buzzsaw

摘要世界上许多活动性山脉的冰川平衡线高度（ELA）与峰顶高度之间的显著匹配使得人们认为冰川起着侵蚀电锯的作用。由于大多数观测到的冰川侵蚀速率非常高，它们被认为能够有效地通过ELA移除大部分构造抬升的地形。然而，冰川电锯的观点主要是基于间接证据。对这一过程的理论模拟定量预测只做了初步的尝试。本研究旨在利用现代技术和计算方法验证冰川电锯假说。由于抬升的地形通常是板块汇聚的结果，因此理解冰川侵蚀的重要性变得复杂，因此需要了解造山楔力学和地表过程之间的反馈。提出了一个耦合的地球动力学和地表过程模型，该模型将冰川侵蚀纳入活跃的临界造山带，以检验这些反馈。初步的模型预测表明，在活动造山带中，弱和强冰川压锯的隆升和侵蚀的时空模式与被动均衡作用下响应压锯的造山带的侵蚀模式有明显的对比。通过应用磷灰石(U-Th)/He组合和裂变径迹低温热年代学来评估巴塔哥尼亚安第斯山脉晚新生代冰川作用开始后侵蚀速率和空间侵蚀模式的变化，对模型预测进行测试，以判断冰川侵蚀是“强”还是“弱”的。巴塔哥尼亚安第斯山脉是首次发现ELA高度与峰顶高度匹配的造山带。巴塔哥尼亚安第斯山脉是一个特殊的自然实验室。除了拥有合适且有充分记录的构造和气候历史外，它们的南北范围还为研究空间变化的冰川历史提供了机会。两个不同的横断面将成为目标：一个在39S到41S（瓦尔迪维亚，普恩，巴里洛切），另一个在46S到51S（运河贝克，拉戈将军卡雷拉，南巴塔哥尼亚冰原）。这将包括近垂直和跨造山带样品剖面的收集。提出的工作将解决以下研究问题：1)冰川电锯假说是否解释了巴塔哥尼亚安第斯山脉的地表侵蚀行为？2)沿着巴塔哥尼亚安第斯山脉的长度，电锯是历时的还是同步的？3)造山带是接近稳定状态，还是仍然对ELA的下降有反应？4)在区域尺度和长时间内，目前的冰川侵蚀模型有多精确？更广泛的影响地球系统的长期行为依赖于气候、反照率、构造、造山带地形、风化和温室气体（如CO2）之间的动态相互作用。特别是，鉴于硅酸盐风化在减缓大气中二氧化碳浓度方面的重要作用，人们对造山带的侵蚀通量在减缓温室效应方面可能发挥的作用进行了许多推测。造山带地形影响降水的分布，如果地势足够高，也可以改变全球大气环流，就像目前在喜马拉雅和安第斯山脉中部的情况一样。因此，人们对可能影响造山地形演化或限制地形最大规模的因素仍有广泛的兴趣。这项研究将对了解高山冰川在控制山脉最大高度方面的作用作出重大贡献。这项建议的多学科方法将促进若干国家和国际教育和研究联系。来自路易斯安那州立大学、耶鲁大学、波茨坦GFZ和智利大学的学生和研究人员将参与该项目。来自两所美国院校的学生将有机会将实地研究与理论结合起来。拟议的研究有可能为美国国家科学基金会资助的地球科学联盟的校友提供研究支持，以加强少数民族参与地球科学（GAEMP）项目，通过联合pi JonathanTomkin参与路易斯安那州立大学的项目。研究结果将被整合到GAEMP暑期学校项目中，该项目强调正在进行的地球科学研究对少数民族本科生的价值。

ABSTRACTThe remarkable match between the glacial equilibrium line altitude (ELA) and summit elevations in many active mountain ranges of the world has led to the supposition that glaciers act as an erosional buzzsaw. Since most observed rates of glacial erosion are very high they are thought to be able to effectively remove most topography tectonically uplifted through the ELA. However, the glacial buzzsaw idea is based largely on circumstantial evidence. Only preliminary attempts have been made to theoretically simulate quantitative predictions of this process. This proposed research aims to test the glacial buzzsaw hypothesis using modern technical and computational methods. Understanding the importance of glacial erosion is complicated by the fact that elevated topographies are often the result of plate convergence, and so an appreciation of the feedbacks involved between orogenic wedge mechanics and surface processes is required. Development of a coupled geodynamic and surface process model that incorporates glacial erosion in an active critical orogenic wedge is proposed to examine these feedbacks. Preliminary model predictions indicate that the temporal and spatial patterns of uplift and erosion for both a weak and strong glacial buzzsaw in an active orogen contrast significantly with erosion patterns of an orogen that responds to the buzzsaw by passive isostasy. Testing the model predictions to judge whether glacial erosion acts as a 'strong' or 'weak' buzzsaw will be achieved by applying combined apatite (U-Th)/He and fission-track low temperature thermochronology to evaluate changes in erosion rates and spatial erosion patterns following the onset of late Cenozoic glaciation in the Patagonian Andes - the orogen where the match between the height of the ELA and summit elevation was first recognized. The Patagonian Andes are an exceptional natural laboratory. As well as having a suitable and well-documented tectonic and climatic history, their north-south range provides an opportunity to study a spatially varying glacial history. Two different transects will be targeted: one at 39S to 41S (Valdivia, Pucn, Bariloche), and another between 46S and 51S (Canal Baker, Lago General Carrera, South Patagonian Icefield). This will include the collection of both near-vertical and trans-orogen sample profiles. The proposed work will address the following research questions: 1) Does the glacial buzzsaw hypothesis explain the behavior of surface erosion in the Patagonian Andes? 2) Was the buzzsaw diachronous or synchronous along the length of the Patagonian Andes? 3) Is the orogen close to steady state, or still responding to the drop in ELA? and 4) How accurate are current models of glacial erosion over regional scales and long time periods?Broader ImpactThe long-term behavior of the earth system is dependent on dynamic interactions between climate, albedo, tectonics, orogenic topography, weathering, and greenhouse gases, such as CO2. In particular, there has been much speculation about the role that erosional fluxes from orogenic landscapes might play in moderating the greenhouse effect, given the important role of silicate weathering in moderating CO2 concentrations in the atmosphere. Orogenic topography as influences the distribution of precipitation, and can also change global atmospheric circulation, if the topography gets high enough, as is presently the case in the Himalaya and central Andes. Thus, there remains broad interest in the factors that might influence the evolution of orogenic topography, or limit the maximum size of the topography. This research will make a significant contribution towards understanding the role that alpine glaciation plays in controlling the maximum height of mountain ranges. The multidisciplinary approach of this proposal will foster a number of national and international educational and research links. Students and researchers from Louisiana State, Yale, GFZ Potsdam, the Universidad de Chile will be involved in this project. Students from two US institutions will have an opportunity to combine field research and theory. The proposed study has the potential to provide research support for an alumnus of the NSF funded Geoscience Alliance to Enhance Minority Participation in the Earth Sciences (GAEMP) project, through co-PI JonathanTomkin's participation in the program at LSU. The results of the research will be integrated into a GAEMP summer school program, which emphasizes the value of ongoing earth science research to minority undergraduates.