Stress History of the Alpine Fault Using Rock Deformation Experiments and Numerical Modeling

利用岩石变形实验和数值模拟研究高山断层的应力历史

• 批准号: 1524602
• 负责人: Steven Kidder
• 金额: $ 25.93万
• 依托单位: CUNY City College
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2015
• 资助国家: 美国
• 起止时间: 2015-08-01 至 2021-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1524602&HistoricalAwards=false
• 关键词: Stress; History; Alpine; Fault; Using

The state of stress in the crust and its spatial and temporal variability is an active and longstanding research area in geodynamics, earthquake mechanics, structural geology and rock mechanics. Stress levels on plate boundary faults are the subject of ongoing debate and also a subject of particular societal importance because it is the buildup of stresses along these structures that is responsible for the most devastating earthquakes. This study examines stresses on the Alpine fault, New Zealand, using two complimentary approaches. First, a two-dimensional numerical model of the Alpine fault that incorporates shear heating will be developed to provide an independent constraint on long-term stress levels on the Alpine fault. Second, a set of rock deformation experiments will be carried out to explore the new idea that significant variations in the size of deformed grains in rock samples from the fault indicate that the large grains formed in the middle crust under low stress conditions and smaller grains formed during brief, seismically induced high-stress pulses. The project would advance desired societal outcomes through: (1) full participation of women and underrepresented minorities in STEM; (2) development of a diverse, globally competitive STEM workforce through training of graduate and undergraduate students from underrepresented groups in the Earth sciences and support of an early career scientist; and (3) increased partnerships through international collaboration with New Zealand scientists.Recrystallized quartz grain size piezometric data from Alpine fault mid-crustal rocks show two main features: (1) a significant lateral variation along strike of the fault such that peak stresses are significantly reduced in the central portion of the fault; and (2) grain size populations are strongly bimodal. When stresses based on recrystallized grain size are compared to available independent constraints on stress on the Alpine fault, a striking inconsistency emerges. Crustal strengths indicated by numerical models based on force balance, calculations based on potential energy, and earthquake focal mechanism data suggest integrated crustal strengths two to three times weaker than seemingly indicated by the grain size data. To reconcile these observations, this study investigates the hypothesis the coarse-grained quartz population formed at the brittle-ductile transition at a peak long-term stress of only ~50 MPa. The fine-grained quartz formed during brief, seismically induced high-stress pulse. The project delves deeper into the stress state(s) of the Alpine fault and the interpretation of quartz microstructures in potentially non-steady state settings using two complimentary approaches. First, a two-dimensional numerical model of the Alpine fault that incorporates shear heating will be developed. Two linked finite element models will be run together, one pertaining to the Southern Alpine fault where exhumation rates are low, and a second from the central Alpine fault where exhumation rates are extreme. The numerical model will be tested using an inversion against multiple thermal constraints such as metamorphic histories, thermochronologic data, and heat flow. As modeled stresses increase, it becomes impossible at a certain point to recreate observed thermal histories. The modeling thus provides an independent constraint on long-term stress levels on the Alpine fault and may provide some additional insights into its tectonic history. Second, a series of rock deformation experiments on quartzites in a Griggs rig will be carried out to explore the effects of stress variations on recrystallized grain size distributions and fabrics. Experiments will be conducted using both gradual stress increases (simulated exhumation paths toward the brittle ductile transition), and stress pulses followed by stress relaxation (simulated postseismic deformation). Fabrics from characteristic Alpine fault samples will be examined for purposes of comparison.

地壳应力状态及其时空变化是地球动力学、地震力学、构造地质学和岩石力学中一个长期活跃的研究领域。板块边界断层上的应力水平一直是争论的主题，也是一个具有特殊社会重要性的主题，因为沿着这些结构的应力积累是造成最具破坏性地震的原因。本研究考察了新西兰阿尔卑斯断层的应力，采用了两种互补的方法。首先，将建立一个包含剪切加热的高山断层二维数值模型，为高山断层的长期应力水平提供一个独立的约束。其次，将开展一系列岩石变形实验，以探索断层岩石样品中变形颗粒大小的显著变化表明中地壳在低应力条件下形成的大颗粒和在地震诱发的短暂高应力脉冲中形成的小颗粒的新思路。该项目将通过以下方式促进预期的社会成果：(1)妇女和代表性不足的少数民族充分参与STEM；(2)通过培训来自地球科学领域代表性不足群体的研究生和本科生，并支持早期职业科学家，发展一支多元化的、具有全球竞争力的STEM劳动力队伍；(3)通过与新西兰科学家的国际合作加强伙伴关系。阿尔卑斯断层中地壳岩石的重结晶石英粒度测压数据显示出两个主要特征：(1)沿断层走向有明显的横向变化，断层中部的峰值应力显著降低；(2)粒径居群呈强双峰分布。当基于再结晶粒度的应力与现有的对高山断层应力的独立约束进行比较时，出现了明显的不一致。基于力平衡的数值模型、基于势能的计算和震源机制数据表明，综合地壳强度比粒度数据表面上显示的弱2 - 3倍。为了与这些观察结果相一致，本研究研究了在峰值长期应力仅为~50 MPa时脆性-韧性转变时形成粗粒石英种群的假设。在短暂的地震诱发的高应力脉冲中形成的细粒石英。该项目深入研究了阿尔卑斯断层的应力状态，并使用两种互补的方法解释了潜在非稳态环境下的石英微观结构。首先，建立了考虑剪切加热的高寒断裂二维数值模型。两个相连的有限元模型将一起运行，一个与挖掘率低的南阿尔卑斯断层有关，另一个与挖掘率极端的阿尔卑斯中央断层有关。数值模型将通过对多种热约束条件（如变质史、热年代学数据和热流）的反演进行测试。随着模拟应力的增加，在某一点上不可能重现观测到的热历史。因此，该模型为阿尔卑斯断层的长期应力水平提供了一个独立的约束条件，并可能为其构造历史提供一些额外的见解。其次，在Griggs钻机上对石英岩进行一系列岩石变形实验，探讨应力变化对石英岩再结晶粒度分布和组构的影响。实验将使用逐渐的应力增加（模拟向脆-韧性转变的挖掘路径）和应力脉冲随后的应力松弛（模拟震后变形）进行。来自阿尔卑斯断层特征样品的织物将被检查以作比较。