Thermal constraints on the role of hydrated oceanic mantle lithosphere in the genesis of intermediate-depth seismicity

Carnegie Institution Of Washington, Washington DC

Investigators

Abstract

While most of the Earth’s earthquakes happen near the surface, a significant subset occurs at greater depths. These earthquakes are generated within tectonic plates that are sinking into the Earth’s mantle through a process called subduction. Subduction plays an important role in the evolution of our planet by cycling volatile materials like water and carbon back into the planet’s interior. Some of these volatiles are released from the slab in time to be returned to the surface through volcanoes, but some fraction are subducted to much greater depths. What we don’t know is what those relative fractions are. Part of the difficulty is in tracing where volatiles are released. The investigators hypothesize that earthquakes that happen in downgoing plates at “intermediate depths” (or between about 70 and 300 km below the Earth’s surface) are caused by the release of water from deep within the subducting plates. Water brought down in the crust of the downgoing plate is probably released at shallower depths, but the water the team is studying, located below the crust of the downgoing plate (known as the “mantle lithosphere”) has the potential to be subducted to much greater depths given the correct conditions. The investigators have found a geological location where they can test whether or not water in the mantle lithosphere is being released during intermediate depth earthquakes. In this project, they will develop thermal models for the complex subduction zones that exist in the selected test areas in order to examine whether the conditions are right for the release of water at the locations where earthquakes are seen. If so, then it will show that these volatiles do not get subducted to greater depths in the Earth, but rather are likely to eventually be returned to the Earth’s surface. This is important for our understanding of the chemical evolution of our planet and its ability to maintain a volatile-rich atmosphere over geologic time. The thermal modeling approach the team develops will be made public to the scientific community. Other scientists can use it to study the thermal properties of complex slabs elsewhere to further our understanding on a broader scale. This project will support an early-career scientist, and will also engage undergraduate summer interns in research visualization efforts. The occurrence of intermediate-depth seismicity in subduction zones is commonly attributed to the metamorphic dehydration of mineral phases within the downgoing oceanic plate. Water is introduced to the plate upon its formation at the ridge and by outer-rise faulting just before subduction, yet the amount of water introduced and its role in intermediate-depth seismicity remains uncertain. Two flat segments in the South American subduction zone are characterized by strong variations in slab geometry and convergence both in space and in time. The investigators hypothesize that these variations lead to temperature variations at depth that control the variable seismicity at intermediate depths. They will test this hypothesis by predicting the thermal structure of the subducted lithosphere through high-resolution 3D and time-dependent finite element models from which they can predict the metamorphic conditions and where dehydration takes place. The full thermal models and open-source modeling capability will be made available in multiple forms so that they can be used by a wide range of researchers ranging from graduate students in petrology and geochemistry to specialists in geodynamical modeling. This project will therefore contribute to software infrastructure and leverage significant investments by the National Science Foundation in subduction zone research. 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.