Collaborative Research: Origin of Long-lived Crustal Shear Zones as Transforms or Subduction Zones?

大规模断层（1000公里）是在地球活动板块和古板块边界观测到的一级特征。活动断层，如圣安德烈亚斯断层，可以容纳构造板块之间的相对运动。古老的板块边界断层现在在板块内部，比如密苏里州的新马德里断层，也可以继续活跃，大地震就证明了这一点。为什么一些板块边界断层在形成几亿年后仍然是地壳变形和隆起的场所，这一直是一个有争议的话题。这项研究将提供一些关于如何以及为什么在板块边界形成的一些主要断层在地壳中作为脆弱带持续存在，容易重新激活的想法。这些结果最终将为广大科学家提供关于控制地震活动、高热流和热液系统位置的深层地壳过程的信息，这些过程可能对了解地质灾害和资源有影响。除了项目的科学目标外，项目的重要社会相关成果将包括培养重要的STEM（科学、技术、工程和数学）学科的研究生和本科生。该项目将促进美国三个研究机构之间的合作研究，从而为科学基础设施提供支持。它将为两位早期职业科学家提供研究经费。该项目将有助于扩大弱势群体对STEM的参与。重要的是，该项目将促进美国和阿根廷科学家之间的国际合作与交流。研究结果将通过在专业学会会议上的报告和同行评议的科学出版物广泛传播。大陆地壳中主要断裂带的存在，显示了数亿年的再活动证据，这与大陆连续变形模型相违背，其中断层被视为响应地幔流动的被动特征。每个大陆都有大型断层，许多长期存在的大陆内断层系统记录了复杂的再激活历史，特别是局部收敛和走滑变形。这项研究将通过研究发掘出的古断裂带的中、下地壳剖面的构造历史，来检验关于持续大规模断层起源的竞争模型。端元模型预测，这类断裂带的起源可以是：1)不同强度块体之间的会聚碰撞边界，2)沿已存在的软弱带转换边界，或3)斜俯冲背景下弧前区域内的走滑边界。阿根廷西部的Valle Fertil断裂带是一条大约1200公里长的主要地壳线，记录了至少4亿年的间歇性变形，是测试上述模型的理想地点，因为地壳强度对比的绝佳地球物理约束，沿走向暴露的可变深度，构造演化的良好约束，以及用于确定断层内变形历史的理想矿物组合。Fertil断层的结果可以应用于其他大陆内断层，以解决哪些因素决定了复杂造山带内主要构造的起源，以及它们最终成为具有持续活化的复杂历史的大型断层的过程。

Large-scale faults (1000 kilometer) are first-order features observed at Earth's active and ancient plate boundaries. Active faults, such as the San Andreas, accommodate relative motion between tectonic plates. Ancient plate boundary faults now within plates, such as the New Madrid fault in Missouri, can also continue to be active, as demonstrated by major earthquakes on them. Why some plate boundary faults remain the locus of crustal deformation and uplift 100s of millions of years after they form has remained a topic of debate. This study will provide ideas on how and why some major faults that form at plate boundaries persist as zones of weakness in the crust, prone to reactivation. The results will ultimately inform a broad group of scientists on deep crustal processes that control the location of seismicity, high heat flow, and hydrothermal systems that may have implications for the understanding of geologic hazards and resources. In addition to the scientific goals of the project, important societal relevant outcomes of the project will include the training of graduate and undergraduate students in an important STEM (Science, Technology, Engineering and Mathematics) discipline. The project will facilitate collaborative research between three U.S. research institutions, thus contributing to support of scientific infrastructure. It will provide research funding for two early career gescientists. The project will contribute to the broadening of participation of underrepresented groups in STEM. Importantly, the project will foster international collaboration and exchange between U.S. and Argentine scientists. Results of the research will be broadly disseminated through presentations at professional society meetings and in peer-reviewed scientific publications. The presence of major fault zones within continental crust which show evidence for reactivation over 100s of millions of years defies models for continuum deformation of the continents, wherein faults are viewed as passive features responding to mantle flow. Large-scale faults occur on every continent and many long-lived intracontinental fault systems record complex histories of reactivation, in particular localizing convergent and strike-slip deformation. This research will test competing models for the origin of persistent large-scale faults by studying the tectonic history of exhumed middle and lower crust sections of an ancient fault zone. End member models predict that such fault zones originate either as: 1) convergent-collisional boundaries between blocks of different strength, 2) transform boundaries along pre-existing zones of weakness, or 3) strike-slip boundaries within the arc-forearc region of oblique subduction settings. The Valle Fertil fault zone of western Argentina is an approximately 1200-kilometer-long major crustal lineament that records at least 400 million years of intermittent deformation and is an ideal location to test the above models because of excellent geophysical constraints on crustal strength contrasts, variable depths of exposure along strike, a well constrained tectonic evolution, and ideal mineral assemblages for dating the history of deformation within the fault. The results from the Valle Fertil fault can be applied to other intracontinental faults to address what factors determine the origin of major structures within complex orogens and the processes by which they ultimately become large-scale faults with complex histories of continued reactivation.