CAREER: Deformational Evolution and Serpentinization of the Mantle Wedge Corner in Subduction Zones

事业：俯冲带地幔楔角的变形演化和蛇纹石化

• 批准号: 1847612
• 负责人: Ikuko Wada
• 金额: $ 54.64万
• 依托单位: University of Minnesota-Twin Cities
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2019
• 资助国家: 美国
• 起止时间: 2019-09-01 至 2024-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1847612&HistoricalAwards=false
• 关键词: CAREER; Deformational; Evolution; Serpentinization; Mantle

Subduction zones are regions where one tectonic plate sinks into the mantle beneath another plate. In between the two plates, a wedge-shaped piece of the upper mantle experiences extensive physical and chemical transformations. While cooling down, the corner of the mantle wedge is sheared and hydrated by fluids that arise from the dehydrating sinking plate. As rocks become hydrated through a process called serpentinization, the wedge corner becomes weaker and lighter. Serpentinization also causes rock expansion and changes in permeability which in turn affects fluid migration in subduction zones. The resulting fluid distribution and weakening have strong implications for important geological processes, such as great earthquakes and volcanic eruptions in subduction-zone settings. To better constrain the physical properties of the wedge corner, the PI will quantify the spatial extent of serpentinization using numerical modeling. The models will account for geophysical observations collected in subduction zones in North America and Japan and incorporate experimental data on rock deformation and hydration. The project outcomes will further our understanding of subduction-zone dynamics. They will also help interpreting seismic observations and assessing geohazards near plate boundaries. This project will support an early-career female scientist, the training of two graduate students in Earth Science, and educational outreach toward K-12 students and teachers, and the public.Inferring the extent of serpentinization in the mantle-wedge corner from seismic velocities is challenging because of the strong elastic anisotropy of the mineral antigorite. Antigorite is the main product of serpentinization in the wedge corner. It develops crystal-preferred orientations (CPOs) through deformation and topotactic growth from its parent mineral olivine. This can make the wedge corner highly anisotropic, but antigorite CPOs in this region are not well understood. Fracture permeability in the wedge corner controls the extent of serpentinization. However, the effect of background and reaction-induced stresses on fracture permeability is also unclear. To quantify serpentinization in the wedge corner, the PI will use three distinct classes of numerical models: (1) a time-dependent thermo-mechanical model for mineral texture evolution, (2) a mechanical model for spatial variations in the stress state and (3) a hydraulic-chemical-mechanical model for the development of reaction-induced fractures. The models will account for seismological, geodetic, gravity and magnetic observations in the forearc regions of three subduction zones (Cascadia, Nankai, and NE Japan). The PI will also integrate in the modeling experimental results on the rheology and microstructures of the relevant minerals. Model outcomes will help to constrain CPOs in the mantle wedge corner and the degree of serpentinization. They will also improve our understanding of background and reaction-induced stresses and their effects on serpentinization. This project has direct implications for our understanding of subduction zone dynamics and broader implications for the assessment of geohazards near plate boundaries. It will support an early-career female scientist, two graduate students and educational outreaches toward K-12 students and teachers, and the public.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

俯冲带是指一个构造板块下沉到另一个板块之下的地幔的区域。在两个板块之间，上地幔的一块楔形部分经历了广泛的物理和化学变化。当降温时，地幔楔形的一角被来自脱水下沉板块的流体剪切和水化。随着岩石通过一种称为蛇纹岩化的过程变得水化，楔形角变得更弱、更轻。蛇纹岩作用还会引起岩石膨胀和渗透率的变化，进而影响俯冲带中的流体迁移。由此产生的流体分布和减弱对重要的地质过程有很强的影响，例如俯冲带背景下的大地震和火山喷发。为了更好地约束楔形角的物理特性，PI将使用数值建模来量化蛇纹化的空间程度。这些模型将考虑在北美和日本俯冲带收集的地球物理观测数据，并纳入关于岩石变形和水化的实验数据。项目成果将进一步加深我们对俯冲带动力学的理解。它们还将有助于解释地震观测和评估板块边界附近的地质灾害。这个项目将支持一名职业生涯早期的女科学家，两名地球科学研究生的培训，以及对K-12学生和教师以及公众的教育推广。由于矿物反辉石的强烈弹性各向异性，从地震速度推断地幔楔形角落的蛇纹化程度是具有挑战性的。安山岩是楔形拐角蛇纹岩的主要产物。它通过变形和局部定向生长，从其母矿物橄榄石中形成晶体择优取向(CPO)。这可能会使楔形角具有高度的各向异性，但该地区的反戈龙岩CPO还没有被很好地理解。楔形角的裂缝渗透率控制着蛇纹化的程度。然而，背景应力和反应诱导应力对裂缝渗透率的影响也不清楚。为了量化楔形角的蛇纹岩作用，PI将使用三类不同的数值模型：(1)矿物结构演化的时间相关热-力学模型，(2)应力状态空间变化的力学模型，以及(3)反应诱导裂缝发展的水力-化学-力学模型。这些模型将考虑三个俯冲带(卡斯卡迪亚、南开和日本东北部)弧前区的地震、大地测量、重力和磁力观测。PI还将整合到有关矿物的流变学和微观结构的模拟实验结果中。模型结果将有助于限制地幔楔形角的CPO和蛇纹岩化的程度。它们还将提高我们对背景和反应诱导的压力及其对蛇纹岩作用的影响的理解。该项目对我们理解俯冲带动力学有直接影响，对评估板块边界附近的地质灾害有更广泛的影响。它将支持一名职业生涯早期的女科学家、两名研究生和面向K-12学生和教师的教育外展活动，以及公众。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。