EEPS Colloquium: Josh Russell

Ocean-bottom seismology: A window into mantle dynamics, convection, and grain size

Comprising ~71% of Earth’s surface, the ocean basins represent perhaps the most fundamental expression of plate tectonics. Yet, major outstanding questions remain regarding the physical state of the asthenosphere (temperature, grain size, viscosity), abundance of melt and volatiles, and degree to which these factors influence plate motion and the scale/vigor of mantle convection. Recent deployment of arrays of ocean-bottom seismometers (OBS) in the Pacific aim to fill these observational gaps by providing in-situ observations of upper mantle structure at lateral scales ≲500 km. Here, we explore ongoing work to map mantle flow and physical properties of the oceanic upper mantle beneath arrays of OBS, addressing the questions: What makes the asthenosphere weak, and what does this mean for mantle dynamics? Using laboratory models of anelasticity to fit our seismic observations, we find a zone of reduced grain size that corresponds with the low viscosity asthenosphere. Importantly, only a small amount of partial melt is required in the oceanic asthenosphere (<0.1%) and does not appear to contribute significantly to its weakness. These observations signify a relatively focused zone of active deformation and grain recrystallization within the low-viscosity asthenosphere, with new implications for the role of grain size.

Host: Doug Wiens

EEPS colloquia are made possible by the William C. Ferguson Fund