Caledonian ophiolites, rift volcanics and the Laurentian margin: Constraining the context for subduction initiation in the Iapetus Ocean

About This Grant

Non-technical Abstract Earth’s solid outer layer is made up of either continental or oceanic crust. In Earth’s past, the continents had different arrangements to what we are familiar with today. The theory of plate tectonics, which emerged in the 1960s and was formalized in the 1970s, explains the changing face of Earth in terms of the opening and closing of oceans (i.e., creation and destruction of the oceanic crust that floors them). The coming and going of oceans act to fragment and separate continents then bring them back together in new formats. With over 50 years of scientific inquiry since the discovery of plate tectonics, geologists still do not understand the exact processes responsible for the birth and death of oceans. The Atlantic Ocean is 180 million years old and has not yet shown any signs that it is closing. Geological evidence suggests that a previous ocean existed in a similar region to the Atlantic of today; the Iapetus Ocean, named after the father of Atlas in Greek mythology. Geological constraints on the birth and death of the Iapetus Ocean suggest that it began closing less than 80 million years after it opened. Why the divergent fates of these two oceans? The answer to this question may lie in the respective processes that birthed the Atlantic and Iapetus. This project will test the idea that the process of ocean formation—the style of continental rifting—can impart differential susceptibility to subsequent closure of the ocean basin that forms. Research will proceed via geological, geochemical and geochronological study of fragments of the Iapetus Ocean preserved at five locations between western Scotland and Arctic Norway. Work will focus on the apparent spatial coincidence of records of Iapetus Ocean opening and closing, and implications for the setting in which the closing process was seeded. The project includes public outreach that will benefit local urban population in Baltimore. Summer research internships will provide opportunities for local high school students to get involved in hands-on research. The team will also lead field excursions to explore the “Baltimore Ophiolite’ – a rare sliver of ancient oceanic crust preserved in an urban setting. Technical Abstract The Wilson Cycle is a cherished concept in plate tectonics. However, an enduring issue with the model is the inherent strength of passive margins, which should resist the implied process of passive margin collapse to initiate subduction. Supra-subduction zone (SSZ) ophiolites of the Scottish and Norwegian Caledonides record the first instance of subduction initiation across approximately 2,000 km of the Wilson Cycle’s archetypal ocean, the Iapetus. Seeding work on these SSZ ophiolites suggests a potentially ubiquitous association between records of rifting to open the Iapetus Ocean and those of subduction initiation to close it. These associations provide tantalizing clues that margin structure/litho-architecture inherited from rifting may leave certain passive margins more susceptible to subduction initiation via passive margin collapse. This project will explore the idea of passive margin susceptibility to subduction initiation by testing two hypotheses: (1) Iapetan subduction initiation occurred in regions that also experienced Ediacaran rifting of Rodinia, and (2) Iapetan subduction initiation occurred in an oceanic setting proximal to the Laurentian margin. Testing Hypothesis 1 will involve geochemical characterization and zircon U-Pb dating of enigmatic metavolcanic packages adjacent to the SSZ ophiolites of Unst, Scotland, and Karmøy and Leka, Norway. Testing Hypothesis 2 will involve geochemical and detrital zircon characterization of metagraywackes from the cover sequences of the Karmøy, Leka and Lyngen SSZ Ophiolites, Norway. 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.