New oceanic crust is forged along Earth’s mid-ocean ridge system, a global chain of volcanic spreading centers whose segments are offset by perpendicular transform faults. Because these faults are not present during continental rifting, the first stage in the formation of a new ocean basin, but are pervasive features of newly minted seafloor, it remains unclear when they form and how their presence is perpetuated.
Seismic anisotropy, the directional variation in the velocity of seismic waves, is influenced by individual crystal alignment and is therefore a potentially powerful tool for deducing patterns of mantle flow along divergent plate boundaries. To date, however, the difficulties and expense of instrumenting the seafloor have limited the application of this technique. Now Eakin et al. have refined a method for conducting such measurements along mid-ocean ridges and have used the results to elucidate the role of transform faults in seafloor spreading.
By carefully eliminating seismic stations where the underlying mantle displays directionality, the team was able to measure the mantle properties beneath individual earthquake sources, rather than below each station, as is typical for this method. For the first time, this modification allowed the researchers to characterize mantle flow beneath active transform faults on a global scale.
The observed patterns of nearly vertically aligned anisotropy suggest that widespread upwelling of the mantle is occurring beneath oceanic transform faults. The results, which are consistent with geodynamic models, imply that mantle upwelling warms, and consequently weakens, transforms. These features thus appear to play an important role in localizing strain and ultimately helping to stabilize divergent plate boundaries.
In addition to representing a significant advance in how measurements of mantle anisotropy are conducted, this study is noteworthy for predicting the dominant pattern of mantle flow beneath transform faults. As more ocean bottom studies are conducted along mid-ocean ridge systems, these results offer a working hypothesis for other researchers to test. (Journal of Geophysical Research: Solid Earth, https://doi.org/10.1002/2017JB015176, 2018)
—Terri Cook, Freelance Writer