Role of Silica Redistribution in the Evolution of Subduction Megathrusts, Shimanto Belt Japan

二氧化硅重新分布在日本四万十带俯冲逆冲断层演化中的作用

• 批准号: 1524530
• 负责人: Donald Fisher
• 金额: $ 32.49万
• 依托单位: Pennsylvania State Univ University Park
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2015
• 资助国家: 美国
• 起止时间: 2015-07-01 至 2021-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1524530&HistoricalAwards=false
• 关键词: Role; Silica; Redistribution; Evolution; Subduction

The last decade has seen two of the most destructive earthquakes ever recorded, with nearly 300,000 lives lost due to ground shaking and inundation of coastal areas by tsunami. In the case of the 2011 Tohoku earthquake and several other large recent earthquakes, the seismic event was preceded by, and therefore potentially triggered by, a type of fault behavior known as "slow slip" -- a behavior that was unknown two decades ago. Therefore, the old paradigm that faults along plate boundaries either "creep" or experience earthquakes due to "stick-slip" behavior does not explain the full range of fault behavior along active subduction zones-- behavior that includes periodic "slow earthquakes" and microseismicity that coincides with tremor. Recognition of this diversity of slip behavior demands a new paradigm that explains the role that these various behaviors may play in triggering earthquakes. The central tenet of this study is that information about heterogeneity in plate boundary slip behavior is not only recorded by seismicity, but also in the distributions and textures of veins, or mineralized cracks, that we can observe today in exhumed ancient subduction boundaries. This project is specifically designed to investigate the roles of natural hydrofracking and local redistribution of calcium carbonate and silica for the evolution of slip instabilities associated with microseismicity, slow slip, and the buildup and release of elastic strain in earthquakes. Vein systems will be examined in ancient plate boundary fault zones in Japan that record a range of conditions that reflect the depths and temperatures at which earthquakes are generated. The principal investigator and his colleagues have developed an over-arching hypothesis that relates silica redistribution in fault zones - as exemplified in vein textures and mineralogies - to earthquake dynamics. If the hypothesis is correct, models of subduction zone behavior must consider not just the frictional behavior of the fault but also the role of footwall hydrofracturing (i.e., a predictable function of fluid sources and permeability) and silica redistribution (i.e., a thermally activated process) as an explanation for the heterogeneity in plate boundary behavior in subduction zones. In addition to the research goals of the project, this award provides support for the training of an Hispanic female graduate student at Penn State thus contributing to broadening of underrepresented groups in a Science, Technologoy, Engineering and Math (STEM) discipline, as well as providing opportunities for the participation of an undergraduate student who will complete research-heavy independent senior thesis during year two of the project. Students will be trained in the use of lower temperature models for crack sealing that have been used along passive margins with application to the oil industry while bridging these models toward application to higher temperature rocks from subduction plate boundaries. The graduate student will also be involved in an international collaboration with Japanese scientists, and this study of exposed ancient rocks will complement the ongoing NantroSEIZE offshore drilling experiment designed to evaluate in situ the processes that characterize the plate boundary at depths where earthquakes are generated. Because subduction zones have significant potential for large magnitude earthquakes, studies of fault zone behavior in these settings have potentially significant implications for the health and economic well being of society.This study is designed to investigate the hydrofracturing and subsequent healing recorded by mineralized veins and fabrics that develop in the footwall of subduction interfaces. It is hypothesized that the range of textures observed in veins is a manifestation of the range of heterogeneous plate boundary slip behavior that is observed along convergent margins. The healing of the fractures within the underthrusting sediments adjacent to the plate interface can occur in times that overlap with earthquake recurrence intervals at the temperatures of the seismogenic zone. Open fluid-filled cracks could impact the effective stress and the strength/elastic properties of the rocks that store and release elastic strain energy, so the healing of cracks could be fundamental to the locking behavior of the seismogenic zone. Vein systems and related scaly fabrics from six regionally extensive shear zones within the Shimanto Belt in Japan that formed during subduction and contain pervasive quartz veins representing the full range of temperatures within the seismogenic zone will be studies. The principal investigators will: 1) characterize the vein systematics on the outcrop both as a function of lithology and position relative to major faults, 2) evaluate vein microstructures petrographically and with cathodoluminescence, and 3) develop elemental maps of potential silica sources adjacent to veins, including scaly fabrics. To evaluate the role of local diffusion of silica, we will map major element concentrations in areas that define potential silica sources (e.g., scaly fabrics and wall rock with cleavage adjacent to fractures). They will also determine the aperture of cracks and the degree to which fractures are sealed based on microstructures. Microstructural information will be used in conjunction with scanline surveys of vein spacings, thicknesses, and lengths to put constraints on the spacings of open fractures and the times needed to seal fractures. This study will enable us to address fundamental questions about quartz veins adjacent to the subduction interface: What are the sources of silica within veins? Can we identify silica depletion zones typical of local silica redistribution? How does crack aperture and crack spacing vary as a function of rock type and temperature? Is there a systematic variation in vein textures with increasing temperature and depth within the seismogenic zone? The Shimanto belt of Japan is ideal for this study as it exposes regional fault zones that are interpreted as paleo-decollements (active plate boundary damage zones in the underthrusting footwall) and for comparison, examples of out-of sequence splays that accommodated seismic slip and juxtaposed rocks of the existing accretionary prism.

在过去的十年中，有史以来记录了两次最具破坏性的地震，海啸被海岸地区震动和沿海地区淹没，造成了近30万人的生命。 在2011年Tohoku地震和其他几次大地震中，地震事件是在此之前的，因此可能触发的是一种被称为“慢滑”的断层行为，这种行为是二十年前未知的行为。 因此，沿板边界断层的旧范式要么“蠕变”或由于“粘跑”行为而遭受地震并不能解释沿主动俯冲区域的全部断层行为 - 包括周期性“缓慢地震”和Microseasicities的行为这与震颤相吻合。 对这种滑移行为多样性的认识需要一种新的范式，该范式解释了这些各种行为在触发地震中可能起的作用。 这项研究的核心宗旨是，关于板边界滑移行为中异质性的信息不仅是通过地震性记录的，而且还记录在静脉或矿化裂纹的分布和质地中，我们今天可以在灌输的古代俯冲边界中观察到这些信息。 该项目的专门设计用于研究碳酸钙和二氧化硅的天然水力缩放和局部再分配的作用，以进化与微震震性，慢速滑移以及地震中弹性菌株的堆积和释放相关的作用。 将在日本的古代板块边界断层区域中检查静脉系统，该区域记录了一系列反映地震的深度和温度的条件。 首席研究员及其同事已经开发了一个超大的假设，该假设将断层区域的二氧化硅重新分布（如静脉纹理和矿物学中所示）与地震动态有关。 如果假设是正确的，则俯冲带行为的模型不仅必须考虑断层的摩擦行为，而且还必须考虑脚壁液压骨压裂的作用（即流体源和渗透性的可预测功能）和硅胶再分配（即，热激活过程）作为俯冲区板边界行为异质性的解释。除了该项目的研究目标外，该奖项还为培训宾夕法尼亚州立大学的西班牙裔女性研究生的培训提供了支持，从而有助于扩大科学，技术，工程和数学（STEM）纪律中代表性不足的群体，以及为一名本科生的参与提供了机会，该学生将在项目的第二年完成研究成熟的独立高级论文。将对学生使用较低温度模型进行裂纹密封的培训，这些裂纹密封件已沿着被动边缘使用，并应用于石油行业，同时将这些模型桥接到俯冲板边界上的较高温度岩石上。该研究生还将与日本科学家进行国际合作，这项对裸露的古代岩石的研究将补充持续的鼻孔海上钻井实验，旨在原位评估在产生地震的深度处的板块边界的过程。由于俯冲带在大范围地震中具有巨大的潜力，因此在这些环境中对断层区行为的研究可能对社会的健康和经济健康具有重要意义。这项研究旨在调查矿化静脉和织物记录的水反应和随后的愈合在俯冲界面的脚壁上发展。假设在静脉中观察到的纹理范围是沿收敛缘观察到的异质板边界滑移行为范围的表现。 在地震区的温度下，与地震复发间隔重叠的时间可能会发生与板界面相邻的裂缝骨折的愈合。开放的液体裂缝可能会影响储存和释放弹性应变能的岩石的有效应力以及强度/弹性特性，因此裂纹的愈合可能是地震生成区的锁定行为至关重要的。来自日本Shimanto带内六个区域广泛的剪切区的静脉系统和相关的鳞片状织物将在俯冲期间形成，并包含代表地震区内温度全范围的普遍性石英静脉。 首席研究人员将：1）在露头上表征静脉系统学是岩性和相对于主要断层的位置的函数，2）评估静脉微结构在岩石学上和阴极发光上，以及3）发展与静脉相邻的元素图的元素图，包括鳞片织物。 为了评估二氧化硅局部扩散的作用，我们将在定义潜在二氧化硅来源的区域（例如，鳞片织物和壁岩石岩石裂解与裂缝相邻）的区域中绘制主要元素浓度。 他们还将确定裂缝的光圈以及根据微观结构密封断裂的程度。 微观结构信息将与静脉间距，厚度和长度的扫描线调查一起使用，以在开放裂缝的间距以及密封裂缝所需的时间上放置约束。 这项研究将使我们能够解决有关俯冲界面附近的石英静脉的基本问题：静脉内二氧化硅的来源是什么？我们可以确定二氧化硅耗尽区域的典型二氧化硅再分配吗？ 裂纹孔径和裂纹间距如何随岩石类型和温度的函数而变化？在地震区内温度和深度升高时，是否存在系统的静脉纹理变化？ 日本的Shimanto带非常适合这项研究，因为它揭示了被解释为古票的区域性断层区（主动板边界损坏区域中的易于伪造的脚壁）和比较，序列外splays的示例可容纳地震滑移和现有增生棱镜的岩石并列。