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

俯冲带断层孕育了地球上最大的地震，并产生了致命的海啸。在超过30公里的深度，这些断层不断变形，并在锁定的浅层带上积累应力，最终引发大地震。然而，由于缺乏实验约束，俯冲带深层活动的机制还没有被很好地理解。这项工作研究了蓝闪石的机械强度，蓝闪石是一种普遍存在的俯冲带矿物，它在很大程度上控制着这些韧性带的行为。麻省理工学院岩石变形实验室的一系列变形实验和华盛顿大学结构岩石学实验室的实验产品表征为蓝闪石强度提供了新的限制。通过将韧性变形置于俯冲带断层的更广泛框架内，这项工作的结果直接告诉我们对自然地震灾害的理解。这项研究资助了两名早期职业科学家，其中一人是女性，并将支持两名研究生，其中一人在STEM中是代表不足的少数民族。这项工作还将通过固体地球科学研究经验计划为未被充分代表的少数族裔本科生提供暑期研究机会、指导和资金。俯冲带是世界上最大的地质灾害的所在地，是地表物质和地球深处之间化学循环的主要途径。这些过程的速度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。该界面的流变演化是了解板块构造、俯冲带动力学、会聚速率和板块边界滑动行为的基础。对脆韧性过渡带和弧下深度之间的岩石记录的观察表明，这种流变性是由蓝片岩中的粘性蠕变控制的，变形在很大程度上是由苏打角闪石蓝闪石所调节的。然而，目前缺乏蓝闪石粘性变形的任何流变参数，因此蓝片岩相岩石不能纳入俯冲带的地质或地球动力学模型。这一认识差距是由于缺乏对蓝片岩相岩石蠕变的实验研究，部分是因为在实验室中获取含水矿物的粘性变形所带来的挑战。麻省理工学院岩石变形实验室的Griggs型高压高温变形装置能够通过主要利用单相集合体在蓝闪石中产生粘性蠕变。这些实验的力学结果为蓝片岩相岩石俯冲过程中的蠕变提供了关键的流变参数和流动规律。华盛顿大学结构岩石学实验室对实验产品进行的微观结构分析，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)联系起来，并在实验结果和岩石记录观察之间架起一座桥梁。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。

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.