CAREER: Subduction initiation in the Iapetus Ocean: How, and how fast?

根据板块构造，大陆在地质时间尺度上运动。这些重新安排需要广泛的板块边界的诞生和死亡，大陆在这些边界上分开或汇聚(汇合)。在洋壳开始下降和再循环回到地球内部(俯冲)之后，在海洋中建立了会聚边缘。拟议的工作将集中在阿巴拉契亚山脉及其欧洲延续(喀里多尼德山脉)沿线保存的海洋岩石，并记录古代海洋(伊阿普图斯)的俯冲起源。它将利用野外工作和现代实验室技术来解决板块构造中的关键未知问题，包括俯冲如何启动以及它在整个海洋中传播的速度有多快。与这项提议相关的研究活动包括在挪威中部偏远地区、设得兰群岛、纽芬兰和魁北克开展工作，但也在马里兰州巴尔的摩的城市化地区开展工作。这项拟议的研究是与教育外联中心、约翰霍普金斯大学和马里兰州中部夏季教育规划中的Y的教育外联活动合作进行的。这些活动旨在以巴尔的摩杰出的城市地质为中心，建立一个以实地和实验室为基础的社区教育的长期计划。外展活动旨在通过注重可获得性和所教授概念的当地文化/环境意义来丰富教育经验，最终目标是增加未被充分代表的少数群体对地质学的参与。板块构造理论的一个中心假设是，构造汇聚是在大洋盆地播种的。然而，这一过程的细节仍不得而知。潜水到底是如何开始的？收敛的板块边界出现的速度有多快？它们的发展与全球范围的构造重组有何关系？要解决这些问题，需要对俯冲起始和传播的运动学进行约束，但现代海洋记录中的实际研究对象有限；回答这些问题的关键是蛇绿岩。为了追踪伊阿普图斯海洋内数千公里范围内俯冲的开始和传播，一个野外工作、地球化学和高精度地质年代学项目将重点关注分布在阿巴拉契亚-加里多尼亚系统约4800公里处的六个寒武纪-奥陶纪蛇绿岩，从马里兰州的巴尔的摩到挪威的莱卡。对Izu-Bonin-Marianas系统的广泛研究表明，可以根据地球化学来区分超俯冲带(SSZ)扩张和(新生)弧活动的产物。拟议的研究以高精度U-Pb测年为中心，以评估其全岩地球化学制约地球动力学背景的单元的时间。它将检验两个假设：(1)在六个蛇绿岩中，SSZ的扩张先于(新生的)弧形活动，(2)俯冲开始在整个土卫八的传播以‘板块速率’(≤25 cm y-1)发生。假设1的检验将揭示六种蛇绿岩的俯冲启动模式--“自发”(密度驱动)和“诱导”(侧向应力驱动)。假设2的检验将限制新的会聚板块边界的出现速度。该项目的资金由NSF EAR构造和岩石学与地球化学项目提供。该奖项反映了NSF的法定使命，并已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。

According to plate tectonics, the continents move on geologic time scales. These rearrangements require the birth and death of extensive plate boundaries across which continents separate or come together (converge). Convergent margins are established in the oceans following initiation of a process of descent and recycling of oceanic crust back into the Earth’s interior (subduction). The proposed work will focus on oceanic rocks that are preserved along the Appalachian mountain system and its European continuation (the Caledonides), and record subduction initiation in an ancient ocean (the Iapetus). It will use fieldwork and modern laboratory techniques to address key unknowns in plate tectonics, including how subduction initiates and how quickly it propagates throughout an ocean. Research activities associated with this proposal include work in remote areas of central Norway, the Shetlands, Newfoundland and Quebec, but also in urbanized parts of Baltimore, Maryland. The proposed research is partnered with educational outreach activities involving the Center for Educational Outreach, Johns Hopkins University, and the Y in Central Maryland’s summer education programming. These activities are aimed at establishing a long-term program of field- and laboratory-based community education centered around the outstanding urban geology of Baltimore. The outreach activities seek to enrich the educational experience by focusing on accessibility and the local cultural/environmental significance of concepts being taught, and have an ultimate goal of increasing participation in geology of underrepresented minority groups.A central postulate of the theory of plate tectonics is that tectonic convergence is seeded in oceanic basins. However, details of the process remain unknown. How exactly does subduction initiate? How quickly do convergent plate boundaries emerge? How does their development relate to global-scale tectonic reorganizations? Constraints on the kinematics of subduction initiation and propagation are required to address these questions, but practical research targets in the modern oceanic record are limited; key to answering these questions are ophiolites. To track subduction initiation and propagation over 1000s of km within the Iapetus Ocean, a program of fieldwork, geochemistry and high-precision geochronology will focus on six Cambro–Ordovician ‘ophiolites’ distributed along ~4800 km of the Appalachian–Caledonian system, from Baltimore, Maryland to Leka, Norway. Extensive work on the Izu–Bonin–Marianas system has demonstrated that products of suprasubduction-zone (SSZ) spreading v. (nascent) arc activity can be distinguished on the basis of geochemistry. The proposed research is centered around high-precision U–Pb dating to assess the timing of units whose whole-rock geochemistry constrains geodynamic context. It will test two hypotheses: (1) SSZ spreading preceded (nascent) arc activity in each of the six ophiolites, and (2) propagation of subduction initiation throughout the Iapetus occurred at ‘plate rates’ (≤ 25 cm y-1). Testing of Hypothesis 1 will reveal mode of subduction initiation—‘spontaneous’ (density-driven) v. ‘induced’ (lateral stress-driven)—for the six ophiolites. Testing of Hypothesis 2 will constrain rates of emergence of a new convergent plate boundary.Funding for this project is provided by NSF EAR Tectonics and Petrology & Geochemistry Programs.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.