Ocean-Bottom Seismology: A window into mantle convection and mid-ocean ridge dynamics

Presented by Dr. Joshua Russell: Syracuse University

Comprising ~71% of Earth’s surface, the ocean basins represent perhaps the most fundamental expression of plate tectonics. Yet, major questions remain unanswered regarding the structure and evolution of oceanic lithosphere, physical state of the asthenosphere (temperature, grain size,  viscosity), abundance of melt and volatiles (H2O, CO2), 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 recent and ongoing work to map mantle flow and physical properties of the oceanic upper mantle beneath arrays of OBS. The first part of the talk will focus on in-situ observations of seismic anisotropy in oceanic lithosphere, which provide insight into near-ridge deformation prior to plate cooling. By integrating seismic observations with laboratory-sheared olivine data and natural peridotite fabrics from ophiolites, we quantify the degree of strain accumulated in the lithosphere and find new evidence that deformation was largely accommodated by grain-boundary sliding rather than pure dislocation creep, as is typically assumed. The second part of the talk looks below the plate, focusing on the oceanic asthenosphere to answer the questions: What makes the asthenosphere weak, and what does this mean for mantle dynamics? We see significant heterogeneity of seismic properties between regions and observe among the highest seismic attenuation ever found beneath a typical oceanic plate. Using laboratory-derived 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 present in the oceanic asthenosphere (<0.1%) and does not appear to contribute significantly to its weakness. These observations are consistent with active shear deformation and grain recrystallization within a low-viscosity asthenosphere that, at the scale of the upper mantle, represents a focused zone of horizontal and/or vertical flow, perhaps associated with presence of small-scale convection.