The research supported by this proposal is an international collaboration between U.S. scientists from the University of Wisconsin and the University of Alabama-Tuscaloosa and their Icelandic colleagues that is aimed at quantifying the role of magmatism in accommodating spreading at mid-oceanic ridges. The investigators are performing an impulse-response experiment, in which the impulse is the injection of magma at the rift, and the response is the deformation recorded by geodesic and seismological methods. The research focuses on extension related to an intrusive event from the northern volcanic zone of Krafla in Iceland in 1975-1984, where the width of the axial valley was increased by several meters due to the intrusion of dikes, resulting in a century?s worth of spreading at the long-term average rate of about two centimeters per year. After the magma supply diminishes, the extension continues over years or decades, and finally slows as the stresses relax. The research is addressing the following questions: (1) What is the geometry of the magmatic plumbing? (2) How does the magmatic pressure in the plumbing evolve with time? (3) What are the material properties of the rift structure? Providing quantitative answers to these questions requires answering a more fundamental question: which constitutive relation (rheology) best describes the rock below the lithosphere? Accordingly, the primary goal of the research is determining the constitutive relations and rheologic properties governing rifting. The research is contributing to the understanding of the mechanics of rifting at mid-ocean ridges, a fundamental process in plate tectonics. The research is fostering the training of a doctoral graduate student and is supporting the research efforts of an early career scientist. Research results and the modeling protocols developed during this study will be disseminated among the geological community. From a societal standpoint, the research is leading to a broader understanding of magmatic and seismological hazards associated with rifting events. The research is supported by the Geophysics Program and Marine Geology and Geophysics Programs.
The completed research activity has quantified the role of magmatism in accommodating spreading at mid-oceanic ridges by determining the constitutive laws and rheological properties of divergent plate boundaries. To do so, the research activity has performed an impulse-response experiment. The impulse is the injection of magma at the rift. The response is the deformation recorded by geodesy and seismology. To date, three such rifting episodes have been recorded on dry land: Krafla in Iceland (1975-1980), Asal in Djibouti (1978), and Dabbahu in Ethiopia (2005). Each of these events increased the width of the axial valley by several meters, or over a century’s worth of spreading at the long-term average rate of approximately 2 cm/yr. Such rifting events are caused by magma intruding as dikes. After the magma supply diminishes, the extension continues over years or decades, and finally slows as the stresses relax. The research activity has focused on measuring, modeling, and interpreting the post-rifting deformation in Northern Volcanic Zone (NVZ) in Iceland. The activity combined detailed observations with sophisticated modeling in a rheological impulse-response experiment. Iceland provides an ideal natural laboratory for this experiment because the mid-oceanic ridge is exposed on dry land there. The measurements include geodetic time series from: (1) interferometry using satellite radar images (INSAR) and (2) Global Positioning System (GPS) surveys. The modeling employs 3-dimensional finite element modeling (FEM) to account for: (1) the timing and geometry of the magma chambers, dikes and faults; (2) the spatial distribution of material properties; and (3) the constitutive (rheological) relations between stress and strain. By optimizing the model calculations to match the measured quantities, the proposed research has increased geophysical understanding of: (1) magmatic and faulting sources, including their location, shape, and timing; (2) rheology of rocks in a rift zone, including fissures, pore fluids and partial melt; and (3) effective material properties for these rocks, including density, rigidity and viscosity. Broader impact of the research activity: The investigators, in collaboration with colleagues in Iceland, have fostered the careers of two graduate students and a post-doctoral researcher in two techniques of modern geophysics (INSAR and FEM) with applications beyond the university setting. The modeling protocol and geophysical results have been disseminated among the scientific community for application to other plate boundaries. The research activity facilitated a short course on how to design and construct 3D FEMs of quasi-static deformation processes (magma chamber expansion; dike inflation; viscoelastic, thermoelastic, and poroelastic effects; fault slip; and surface loading) using Abaqus software. The purpose of this course is to display the powerful capabilities of FEMs and make them directly available to the geophysical community via a user-friendly interface. Upon completion of the course, students will be able to design, construct, and implement FEMs that can be used in forward and inverse models of deformation. The four-day short course was held in facilities operated by the Nordic Volcanological Center, University of Iceland, Reykjavik, Iceland. It followed the existing Summer School on Geodynamics and Magmatic Processes operated by the Nordic Volcanological Center. The course included a bound manual of notes, appendices, references, and other useful information. In addition, Simulia, Inc., the producers of Abaqus, agreed to provide a copy of Abaqus SE to each student at no cost. All data used in publications are archived at UW-Madison as specified by the NSF/EAR data policy. The results are published in the international, peer-reviewed literature.
