The Role of Oceanic Transform Faults in Seafloor Spreading: A Global Perspective From Seismic Anisotropy

Abstract

Mantle anisotropy beneath mid‐ocean ridges and oceanic transforms is key to our understanding of seafloor spreading and underlying dynamics of divergent plate boundaries. Observations are sparse, however, given the remoteness of the oceans and the difficulties of seismic instrumentation. To overcome this, we utilize the global distribution of seismicity along transform faults to measure shear wave splitting of over 550 direct S phases recorded at 56 carefully selected seismic stations worldwide. Applying this source‐side splitting technique allows for characterization of the upper mantle seismic anisotropy, and therefore the pattern of mantle flow, directly beneath seismically active transform faults. The majority of the results (60%) return nulls (no splitting), while the non‐null measurements display clear azimuthal dependency. This is best simply explained by anisotropy with a near vertical symmetry axis, consistent with mantle upwelling beneath oceanic transforms as suggested by numerical models. It appears therefore that the long‐term stability of seafloor spreading may be associated with widespread mantle upwelling beneath the transforms creating warm and weak faults that localize strain to the plate boundary.

Upper Mantle Anisotropic Shear Velocity Structure at the Equatorial Mid‐Atlantic Ridge Constrained by Rayleigh Wave Group Velocity Analysis From the PI‐LAB Experiment

“…SKS splitting results near the ridge also find evidence for up to 3 s splitting in the ridge parallel direction (Kendall et al., 2019), which could reflect SPO from melt bands (Kendall, 1994; Kendall et al., 2019). Source side splitting measurements also suggest both strong (>2 s) ridge subparallel fast direction splitting near the ridge axis, as well as spreading direction fast axes and nulls, from different back azimuths, also suggesting complexity possibly due to multiple layers of anisotropy caused by 3D flow (Eakin et al., 2018; Nowacki et al., 2012). In summary, considering map‐view and the independent constraints from shear‐wave splitting, the just‐off ridge‐parallel surface wave fast directions can be explained by an average of ridge parallel directions on‐axis and the influence of one of the other mechanisms off‐axis and averaged with the SPO from melt bands by the broad surface wave sensitivity.…”

Spatial Variations in Crustal and Mantle Anisotropy Across the North American‐Caribbean Boundary on Haiti

“…The lithospheric shear zone is likely to have a fast axis that is horizontal, whereas any fabric induced by the subducting slab should be dipping with the slab, likely 20–30°. However, the difference in splitting parameters between a flat versus shallowly dipping layer would be expected to be small (e.g., Eakin et al, 2018) and would require a wide range of backazimuths, from 0–360°, to be able to distinguish between the two. With a limited number of results from intermediate earthquakes, we therefore lack the range of measurements required to explore the parameter space necessary to discriminate between a flat anisotropic layer at the base of the lithosphere versus a dipping layer of anisotropy just above the slab.…”

SKS Splitting Beneath Mount St. Helens: Constraints on Subslab Mantle Entrainment

“…The top of the Juan de Fuca slab, however, is only 68‐km deep beneath Mount St. Helens (Mann et al, ), meaning that any serpentinized layer can only be a few tens of kilometers thick. This is unlikely to be detected by SKS splitting, which has a minimal resolution of 0.5 s (~50 km for 4% anisotropy) for a typical characteristic frequency of 0.1 Hz (Eakin et al, ). This explains why we do not see any variability in shear wave splitting across the iMUSH array despite the possible presence of a serpentinized mantle wedge toward the west (see Figure S6).…”

“…Henceforth, we choose to discuss the results from the RC method, unless otherwise stated, as it is independent of the initial polarization (i.e., the back azimuth for SKS phases). Any misalignment in the station orientation, or deviation of the incoming SKS polarization, will impact the quality and accuracy of splitting measurements made via the SC method (Eakin et al, ; Liu & Gao, ), leading to greater variability. It is worth noting that the RC method is known to produce a systematic 45° deviation at near‐null back azimuths (Wüstefeld & Bokelmann, ).…”