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

点击翻译按钮获取中文摘要

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.