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

根据板块构造学，大陆在地质时间尺度上移动。这些重新排列需要广泛的板块边界的诞生和死亡，大陆在这些边界上分离或聚集（聚合）。会聚边缘是在海洋地壳开始下降和再循环回到地球内部（俯冲）的过程之后在海洋中形成的。拟议的工作将侧重于沿着阿巴拉契亚山脉系统及其欧洲延续部分（加里东）保存的海洋岩石，并记录古代海洋（Iapetus）的俯冲起始。它将使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，包括俯冲如何开始以及它在整个海洋中传播的速度。与该提案相关的研究活动包括在挪威中部、设得兰群岛、纽芬兰和魁北克的偏远地区开展工作，以及在马里兰州的巴尔的摩的城市化地区开展工作。拟议的研究与教育推广活动合作，涉及教育推广中心，约翰霍普金斯大学和Y在中央马里兰州的夏季教育计划。这些活动的目的是建立一个以巴尔的摩杰出的城市地质为中心的长期实地和实验室社区教育计划。推广活动力求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，最终目标是增加代表性不足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，构造会聚是在海洋盆地中产生的。然而，这一进程的细节仍不得而知。俯冲到底是如何开始的？聚合板块边界出现的速度有多快？它们的发展与全球规模的构造重组有何关系？要解决这些问题，需要对俯冲开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目标是有限的;回答这些问题的关键是蛇行。为了追踪Iapetus洋内1000多公里的俯冲起始和传播，一项野外工作、地球化学和高精度地质年代学方案将侧重于从马里兰州的巴尔的摩到挪威的莱卡沿沿着~4800公里的阿巴拉契亚-喀里多尼亚系分布的6个寒武-奥陶纪“蛇绿岩”。对伊豆-小笠原-马里亚纳系统的广泛研究表明，可以根据地球化学区分俯冲带扩张与（新生）弧活动的产物。拟议的研究是围绕高精度U-Pb测年，以评估其全岩地球化学约束地球动力学背景的单位的时间。它将检验两个假设：（1）SSZ扩展先于（新生）弧活动，（2）俯冲起始在整个土卫八的传播发生在“板块速率”（≤ 25 cm y-1）。假设1的检验将揭示六条蛇绿岩的俯冲启动模式--“自发”（密度驱动）与“诱导”（横向应力驱动）。假设2的检验将限制新的会聚板块边界出现的速度。该项目的资金由NSF地质构造和岩石地球化学计划提供。该奖项反映了NSF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。

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.