Collaborative Research: Blueschist rheology: experimental constraints on glaucophane strength and deformation mechanisms

俯冲带断层是地球上最大的地震和致命海啸的发源地。在深度超过30公里的地方，这些断层不断变形，并在锁定的较浅区域积聚应力，最终导致大地震。然而，由于缺乏实验的限制，俯冲带深部的行为机制还没有得到很好的理解。这项工作调查蓝闪石，一个无处不在的俯冲带矿物，在很大程度上控制这些韧性带的行为的机械强度。麻省理工学院岩石变形实验室的一组变形实验和华盛顿大学结构岩石学实验室实验产品的表征提供了蓝闪石强度的新约束。从这项工作的结果直接告知我们的理解自然地震灾害的范围内的俯冲带断层的更广泛的框架内放置韧性变形。这项研究资助了两名早期职业科学家，其中一名是女性，并将支持两名研究生，其中一名是STEM中代表性不足的少数民族。这项工作还将通过固体地球科学研究经验计划为代表性不足的少数民族本科生提供夏季研究机会，导师和资金。俯冲带是世界上最大的地质灾害的所在地，是地表物质与地球深部之间化学循环的主要途径。这些过程的克里思和发生直接受到俯冲界面的力学行为和强度的影响，俯冲界面是容纳俯冲变形和运动的薄板状断层和剪切带。这个界面的流变演化是理解板块构造、俯冲带动力学、收敛速率和板块边界滑动行为的基础。脆韧性过渡带和弧下深度之间的岩石记录的观察表明，这种流变学控制的蓝片岩的粘性蠕变和变形是容纳在很大程度上由蓝闪石，钠质角闪石。然而，目前缺乏蓝闪石的粘性变形的任何流变参数，因此蓝片岩相岩石不能被纳入俯冲带的地质或地球动力学模型。这种知识差距是由于缺乏蓝片岩相岩石蠕变的实验研究，部分原因是在实验室中访问含水矿物的粘性变形相关的挑战。麻省理工学院岩石变形实验室的格里格型高压高温变形装置能够通过主要利用单相聚集体在蓝闪石中产生粘性蠕变。这些实验的力学结果提供了蓝片岩相岩石在俯冲过程中蠕变的关键流变参数和流动规律。在华盛顿大学结构岩石学实验室进行的实验产品的显微结构分析，包括高分辨率电子束成像和电子背散射衍射（EBSD），将这些力学结果与变形机制（例如，位错蠕变或扩散蠕变），并提供了一个桥梁之间的实验结果和观察岩石记录。这个奖项反映了NSF的法定使命，并已被认为是值得支持的，通过评估使用基金会的知识价值和更广泛的影响审查标准。

Subduction zone faults host the largest earthquakes on earth and produce deadly tsunamis. At depths greater than about 30 kilometers, these faults deform continuously and build up stress on the locked shallower zones, ultimately causing major earthquakes. However, because of a lack of experimental constraints, the mechanisms by which deep portions of subduction zones behave is not well understood. This work investigates the mechanical strength of glaucophane, a ubiquitous subduction zone mineral, which largely controls the behavior of these ductile zones. A set of deformation experiments at MIT’s Rock Deformation Laboratory and the characterization of experimental products at the University of Washington’s Structural Petrology Laboratory provide new constraints on glaucophane strength. Results from this work directly inform our understanding of natural earthquake hazards by placing ductile deformation within the broader framework of subduction zone faults. This research funds two early career scientists, one of whom is a woman, and will support two graduate students, one of whom is an underrepresented minority in STEM. This work will also provide a summer research opportunity, mentorship, and funding for an underrepresented minority undergraduate student through the Research Experiences in Solid Earth Science program. Subduction zones are the loci of the world’s largest geologic hazards and constitute the main avenue for chemical recycling between surface material and the deep earth. The tempo and occurrence of these processes are directly influenced by the mechanical behavior and strength of the subduction interface, the thin tabular fault and shear zone that accommodates subduction deformation and motion. The rheological evolution of this interface is fundamental to understanding plate tectonics, subduction zone dynamics, convergence rates, and plate boundary slip behaviors. Observations of the rock record from between the brittle-ductile transition zone and sub-arc depths suggest that this rheology is controlled by viscous creep in blueschists and that deformation is accommodated in large part by glaucophane, a sodic amphibole. However, there is currently a lack of any rheological parameters for the viscous deformation of glaucophane, and thus blueschist facies rocks cannot be incorporated into geologic or geodynamic models of subduction zones. This knowledge gap is due to a dearth of experimental studies on creep in blueschist facies rocks and exists in part because of the challenges associated with accessing viscous deformation in hydrous minerals in the laboratory. A Griggs-type high pressure high temperature deformation apparatus at MIT’s Rock Deformation Laboratory is capable of producing viscous creep in glaucophane by utilizing predominantly single-phase aggregates. The mechanical results of these experiments provide key rheological parameters and a flow law for creep in blueschist facies rocks during subduction. Microstructural analyses of experimental products, done at the University of Washington’s Structural Petrology Laboratory, including high-resolution electron beam imaging and electron backscattered diffraction (EBSD), link these mechanical results to deformation mechanisms (e.g., dislocation creep or diffusion creep) and provide a bridge between the experimental results and observations from the rock record.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.