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

地壳应力状态及其时空变异性是地球动力学、地震力学、构造地质学和岩石力学中一个活跃而又由来已久的研究领域。板块边界断层上的应力水平是持续辩论的主题，也是一个具有特别社会重要性的主题，因为正是这些构造上的应力积累导致了最具破坏性的地震。这项研究使用两种互补的方法考察了新西兰阿尔卑斯山断层上的压力。首先，将开发一个包含剪切加热的阿尔卑斯山断层的二维数值模型，以提供对阿尔卑斯山断层长期应力水平的独立约束。其次，将进行一系列岩石变形实验，以探索新的观点，即断层岩石样品中变形颗粒尺寸的显著变化表明，在低应力条件下在中地壳形成的大颗粒，而在短暂的地震诱导的高应力脉冲中形成的较小颗粒。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中代表性不足群体的研究生和本科生以及对早期职业科学家的支持，发展一支多样化的、具有全球竞争力的STEM劳动力队伍；以及(3)通过与新西兰科学家的国际合作增加伙伴关系。来自阿尔卑斯山断裂中地壳岩石的重结晶石英颗粒尺寸测压数据显示了两个主要特征：(1)沿断层走向的显著横向变化，使得断层中部的峰值应力显著降低；以及(2)颗粒大小人口具有强烈的双峰性。当将基于再结晶晶粒度的应力与阿尔卑斯山断层上现有的独立应力约束进行比较时，出现了显著的不一致。基于力平衡、基于势能的计算和震源机制数据的数值模型所显示的地壳强度表明，综合地壳强度比表面上由粒度数据所显示的要弱两到三倍。为了与这些观察结果相一致，这项研究调查了粗晶石英组在脆韧性转变时形成的假说，其长期应力峰值仅为~50兆帕。细粒石英形成于地震诱发的短暂高应力脉冲期间。该项目使用两种互补的方法，更深入地研究了阿尔卑斯山断层的应力状态(S)和潜在非稳定状态下的石英微结构解释。首先，将开发一个包含剪切加热的阿尔卑斯山断层的二维数值模型。两个链接的有限元模型将一起运行，一个与南阿尔卑斯山断层有关，折返速率较低，另一个与中央阿尔卑斯山断层有关，折返速率极高。数值模型将使用多种热约束的反演进行测试，如变质历史、热年代学数据和热流。随着模拟应力的增加，在某个点上不可能重现观测到的热历史。因此，该模型对阿尔卑斯山断层的长期应力水平提供了一个独立的约束，并可能为了解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了一系列岩石变形实验，以探索应力变化对再结晶粒度分布和组构的影响。实验将使用逐渐增加的应力(模拟朝向脆性韧性转变的挖掘路径)和紧随其后的应力松弛(模拟震后变形)进行。出于比较的目的，将对阿尔卑斯山断层样品中的织物进行检验。

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.