Collaborative Research: A Geochemical Approach to Quantifying the Magnitude of Strain and Fluid Flow along the Subduction Interface

地球上最大的地震发生在俯冲带中分隔刚性构造板块的巨型逆冲(俯冲板块界面)上。沿这些板块界面的弹性应变的积累和释放取决于大地震后界面的“愈合”。这个项目将描述在两次地震之间控制这种愈合的化学和机械过程。对古代俯冲界面的观察，现在暴露在陆地上，将被用来提供滑移和流体流动的数值模型。总之，这些方法将使研究人员能够确定俯冲带过程如何影响地震的空间和时间格局--这是评估与活动俯冲带相关的危险的重要一步。这项研究包括对一名职业生涯早期的教师、一名博士生和九名本科生的支持。其中六名本科生将从为当地拉美裔社区服务的本科生组织--化身世界大学招收。该项目包括对两个挖掘出的地形进行结构和地球化学调查，保存与全球活动俯冲带板块界面相关的物理和化学条件的记录。日本的岛本带和阿拉斯加的科迪亚克增生杂岩结合在一起，形成了一个综合记录，深度从8到20公里，峰值温度在150到350˚C之间。对几次野外活动中收集的样品进行基于电子显微镜的元素制图，将能够量化矿物相及其随着递增应变的重新分布。结合阴极发光作图、电子背向散射衍射和流体包裹体显微测温，将约束古温度和有限应变，并识别变形机制。应变率的定量估计将结合应变级、剪切带厚度的现场观测和现今板块收敛速度的估计得出。这些测量和观测将被用来提供一个数值模型，该模型将地震滑动、流体流动和地震间再加强对板块界面上地震的时空分布的影响进行参数化。多学科工作计划旨在解决以下基本问题：Q1)与地震间变形有关的体积应变和剪切应变的大小是多少？Q2)沿着鳞片状组构的溶解和块体裂隙中矿物的沉淀之间是否存在平衡？Q3)二氧化硅的局部扩散传质与外部衍生流体流入溶解的二氧化硅相比，对石英脉形成的相对贡献是什么？问题4)从孕震带的上倾端到下倾端，变形机制的划分有何不同？这一奖项反映了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。

Earth’s largest earthquakes occur along the megathrusts (the subduction plate interfaces) that separate rigid tectonic plates in subduction zones. The accumulation and release of elastic strain along these plate interfaces is predicated on the “healing” of the interface after the large earthquakes. This project will characterize the chemical and mechanical processes that control this healing during the time between earthquakes. Observations of ancient subduction interfaces, now exposed on land, will be used to inform a numerical model for slip and fluid flow. Together, these approaches will allow the investigators to determine how subduction zone processes impact the spatial and temporal patterns of earthquakes—an important step in assessing the hazards associated with active subduction zones. The study includes support for an early-career faculty member, a Ph.D. student, and nine undergraduates. Six of the undergraduates will be recruited from University of the Incarnate Word, an undergraduate body that serves a local Hispanic community.This project comprises a structural and geochemical investigation of two exhumed terrains that preserve a record of the physical and chemical conditions relevant to the plate interface of the global array of active subduction zones. Combined, the Shimanto belt in Japan and the Kodiak Accretionary complex in Alaska, form a composite record spanning depths from 8 to 20 km and peak temperatures between 150 and 350˚C. Electron-microscope-based elemental mapping of samples collected during several field campaigns will enable quantification of mineral phases and their redistribution with progressive strain. Combined cathodoluminescence mapping, electron backscatter diffraction, and fluid-inclusion microthermometry will constrain paleotemperatures and finite strain, and identify deformation mechanisms. Quantitative estimates of strain rate will be derived by combining strain magnitudes with field observations of shear zone thickness and present-day estimates of plate convergence rate. These measurements and observations will be used to inform a numerical model that parameterizes the effects of seismic slip, fluid flow and interseismic restrengthening on the temporal and spatial distribution of earthquakes on the plate interface. The multidisciplinary workplan is designed to address the following fundamental questions: Q1) What are the magnitudes of volume strain and shear strain associated with interseismic deformation? Q2) Is there a balance between dissolution along the scaly fabric and precipitation of minerals within cracks in blocks? Q3) What are the relative contributions to quartz vein formation of local diffusive mass transfer of silica versus the influx of dissolved silica by an externally derived fluid? Q4) How does partitioning of deformation mechanisms vary from the updip to the downdip ends of the seismogenic zone?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.