Earth scientists explain almost all dynamic activity of the surface of the planet -- earthquakes, volcanoes, mountain building, erosion -- as a direct consequence of plate tectonics, the notion that the outer surface of Earth consists of rigid shell-like pieces (plates) that are constantly moving relative to each other.  Scientists also have long envisioned that the plates' motion is driven by convection in the Earth's rocky mantle -- the never-ending cycle of hot, buoyant rock rising from deep in the Earth's interior, and cold, dense rock sinking from the cool surface of the planet, which pushes the plates about like rafts on the open sea.  One of the primary mechanisms that Earth scientists use to characterize convection is via the fabric, or coherent crystal alignment in minerals, that forms in rocks when they deform.  For understanding convection that drives the plates, structural geologists can directly quantify crystalline fabric in olivine samples from the mantle, but such samples are spatially limited and very rare.  For a more general understanding of rock fabric and mantle convection on larger scales, scientists exploit observations of seismic anisotropy -- the property that seismic waves traveling though rock with a coherent crystalline fabric will exhibit different speeds depending on their polarization and their direction of travel.

In this study, we utilize the unique NoMelt geophysical experiment from the center of the Pacific basin to develop a high-resolution portrait of seismic anisotropy produced by the spreading of oceanic plates.  The ocean basins represent an excellent environment to study mantle convection, because it avoids the complicated geological structure imposed by continental crust. By combining direct constraints on anisotropy from both compressional and shear seismic waves, we develop a nearly complete quantification of seismic anisotropy over seismic length scales (~500 km), and directly compare the resulting anisotropy to that predicted from rock samples from the oceanic upper mantle. We then place the anisotropy into context with several other geophysical observations, improving our understanding of the underlying processes (temperature, partial melt, water or other volatiles) that control the strength, and thus deformation, of the plate and the underlying mantle.

Intellectual merit:  We find that mantle seismic anisotropy in old oceanic lithosphere is consistent with fabric formed by seafloor spreading at the mid-ocean ridge.
The in situ mantle anisotropy is in excellent quantitative agreement with rock samples from oceanic lithosphere, bridging the gap between outcrop and seismic length scales.  The observed anisotropy directly constrains the specific type of deformation occurring in the olivine crystals, and comparison to laboratory experiments allows us to quantify how much deformation occurred near the ridge.  Furthermore, for the first time, strong anisotropy is identified in oceanic crust, providing new evidence and constraints on previous models of horizontal layering and/or shearing during crustal accretion. 

The overall geophysical character of the oceanic mantle beneath old oceanic plates suggests that removal of dissolved water and other volatiles from the mantle during volcanic melting at the mid-ocean ridge establishes the primary rheological (strength) framework of oceanic plates and the underlying convecting asthenosphere.  Relatively abrupt transitions with depth of electrical conductivity, seismic velocity, seismic attenuation, and azimuthal seismic anisotropy all occur in the 60-80 km depth interval and are consistent in a transition from a dry lithosphere to a damp asthenosphere.

The scientific results have been shared through publication in major research journals. 

Broader impacts.  This project provides new constraints on the nature of oceanic lithosphere, which directly impacts the behavior of subduction-zone faults that represent Earth's greatest earthquake and tsunami hazard. It also provides fundamental new constraints on the mantle-convection processes that drive surface deformation, including faulting, earthquakes, and volcanoes.  It has provided exceptional training in collaborative, interdisciplinary scientific study for three young scientists (undergraduate and graduate students and a post-doc).  The project supported an undergraduate senior thesis, and a significant portion of a PhD thesis. Finally, it promotes the broader use of seafloor geophysical data, a rich resource previously invested in by NSF that is now generally available for wide use.

Last Modified: 12/12/2019
Modified by: James B Gaherty