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.
Highlights
Most of Earth’s crust, both present and in the past, was formed along the global mid-ocean ridge (MOR) system where two oceanic plates are pulled apart
Mantle anisotropy beneath mid-ocean ridges and oceanic transforms is key to our understanding of seafloor spreading and underlying dynamics of divergent plate boundaries
Applying this source-side splitting technique allows for characterization of the upper mantle seismic anisotropy, and the pattern of mantle flow, directly beneath seismically active transform faults
Summary
Most of Earth’s crust, both present and in the past, was formed along the global mid-ocean ridge (MOR) system where two oceanic plates are pulled apart. A fundamental feature of this seafloor spreading is the formation of transform faults of varying length that offset the ridge segments at 90°. This characteristic ridge-transform geometry is a key component of plate tectonics and governs the creation of new seafloor (Wilson, 1965). Despite the fundamental role of oceanic transform faults, tight constraints on the underlying dynamics have proven challenging due to the inaccessibility of the oceans. Elevated levels of aseismic slip, or rather a seismic deficit, points toward weak faults (Abercrombie & Ekstrom, 2001)
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