Collaborative Research: Investigating the Relationships Between Magmatic 'Flare-Ups', Crustal Rheology, and Arc Collapse

俯冲带，或海洋构造板块被拉入地球深处的区域，是我们星球上最具活力的地方之一。重要的是，它们促进了岩浆从地球内部地幔层向地表的转移。这种岩浆活动最清楚地表现为火山喷发，几条线索表明，这一过程是整个地球历史上大陆地壳形成的方式。然而，岩浆转移的速率并不是恒定的。有充分的证据表明，在俯冲带内有强烈的岩浆活动，被称为岩浆“爆发”。这些间断的事件代表了物质和热能向地壳的大量转移，描绘它们的原因和后果对于理解这些动态构造环境的长期演化非常重要。这项研究的重点是岩浆爆发和地壳变形之间的关系，通过比较发生在华盛顿州北喀斯喀特山脉的两个强烈岩浆活动时期，这两个时期分别发生在78 - 6000万年前和5000 - 4500万年前。这个区域是研究的理想场所，因为随后的构造事件，加上侵蚀，在两次爆发事件中，并置了代表各种地壳深度的岩石，并将这些岩石暴露在地球表面。研究人员将利用这个自然实验室创建一个详细的岩浆输入时间表，与地壳增厚和随后变薄、断裂和韧性弱地壳流动的区域证据进行比较。作为该项目的一部分，研究人员将制作一个科学播客，向公众传播这项研究的结果。该播客将突出女性地球科学家在该项目中的作用，其目标是在stem相关职业中增加对女性的招聘和保留。拟进行的研究将记录大陆弧地壳在岩浆爆发事件期间及其之间的流变演化。研究人员建议在华盛顿州北喀斯喀特地区开发一个暴露良好且研究相对充分的地壳剖面，该剖面保存了在白垩纪晚期和古近纪的两次爆发事件期间和之间的岩浆作用、变质作用和深部到上层地壳变形的记录。在部分较老的爆发过程中，冲断作用和沉积物沉积到弧的深部，而较年轻的爆发结束阶段与深弧地壳的发掘同时发生。研究人员将利用尖端的地球化学、地质年代学、热气压学和现场技术，研究弧内的火成岩侵入岩和变质沉积岩，包括那些暴露在主要断层系统中的岩石。本研究的一个关键组成部分将是利用铀铅锆石地/热年代学，创建两个爆发事件期间岩浆添加的高时间分辨率记录，可以直接与类似的变质、韧性变形和断裂记录进行比较，这些记录是由铀铅锆石、单氮石和钛矿地/热年代学组合确定的，新石英在石榴石地压计中的应用，以及传统的野外和微结构研究。这种方法将提供对岩浆爆发的整体理解，以及它们如何影响100万年尺度的俯冲带地壳流变。特别是，主要研究人员将解决：弱化的深层地壳与爆发的岩浆活动之间的时间关系；当弧围断系相对于爆发变得活跃时；变形是如何在不同的岩性和构造中划分的；如果地壳在爆发时发生融化；如果地壳厚度在爆发期间和爆发之间保持不变。更广泛的影响将包括研究生的培训，以及通过开发和广告播客系列来建立一个长期的公众宣传项目，该项目与拟议研究区域的路边地质有关，该研究区域位于罗斯湖国家游乐区。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Subduction zones, or areas where oceanic tectonic plates are pulled into the deep Earth, are one of the most dynamic locations on our planet. Importantly, they facilitate the transfer of magma from the Earth’s interior mantle layer to its surface. This magmatism is most clearly expressed as volcanic eruptions, and several lines of evidence suggest that this process is how continental crust has been made throughout Earth’s history. Nevertheless, the rate of magma transfer is not constant. There are well-documented periods of intense magmatism within subduction zones that are known as magmatic ‘flare-ups’. These punctuated events represent a large transfer of material and thermal energy into the Earth’s crust, and delineating their causes and consequences is important for understanding the long-term evolution of these dynamic tectonic settings. This study is focused on the relationship between magmatic flare-ups and deformation of the crust by comparing two periods of intense magmatism within the North Cascade Range, Washington, that occurred 78–60 and 50–45 million years ago. This area is ideal for the study because subsequent tectonic events, combined with erosion, have juxtaposed rocks that represent a variety of crustal depths during both flare-up events and have exposed these rocks at the Earth’s surface. The researchers will use this natural laboratory to create a detailed timeline of magmatic input compared to regional evidence for thickening and subsequent thinning of the Earth’s crust, faulting, and flow of ductile, weak crust. As part of this project the researchers will produce a scientific podcast that will communicate the results of this study to the general public. The podcast will highlight the role of women geoscientists in the project, with the goal of increasing recruitment to and retention of women in STEM-related careers. The proposed research will document the rheological evolution of continental arc crust during and between magmatic flare-up events. The researchers propose to exploit a well-exposed and relatively well-studied crustal section in the North Cascades, Washington, which preserves a record of magmatism, metamorphism, and deformation in the deep to upper crust during and between two flare-up events in the latest Cretaceous and Paleogene. Underthrusting and incorporation of sediment into the deep levels of the arc occurred during part of the older flare-up, and the end stage of the younger flare-up is coeval with exhumation of the deep arc crust. The researchers will utilize cutting-edge geochemical, geochronologic, thermobarometric, and field techniques on a combination of igneous intrusions and metasedimentary rocks within the arc, including those exposed within major fault systems. A key component of this study will be to create a high temporal resolution record of magma addition during the two flare-up events using uranium-lead zircon geo-/thermochronology that can be directly compared to a similar record of metamorphism, ductile deformation, and faulting determined by a combination of uranium-lead zircon, monazite, and titanite geo-/thermochronology, application of the new quartz in garnet geobarometer, and traditional field and microstructural study. This approach will provide a holistic understanding of magmatic flare-ups and how they affect the rheology of subduction zone crust on a 1-million-year-scale. In particular, the principal investigators will address: the temporal relationship between a weakened deep crust and flare-up magmatism; when the arc-bounding fault systems became active relative to flare-ups; how deformation was partitioned across different lithologies and structures; if and when crustal melting occurred during a flare-up; and if crustal thickness stayed constant during and between flare-ups. The broader impacts will include training of graduate students and creating a long-lasting program for public outreach through the development and advertisement of a podcast series tied to the roadside geology of the proposed research area, which is in the Ross Lake National Recreation Area.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.