Localized Anisotropy in the Mantle Transition Zone Due to Flow Through Slab Gaps. Issue 10 (24th May 2021)

• Record Type: Journal Article
• Authors: Zhang, Han; Schmandt, Brandon; Zhang, Jin S.
• Is Part Of: Geophysical research letters. Volume 48:Issue 10(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-05-24
• Subjects: anisotropy -- mantle flow wadsleyite -- mantle transition zone -- shear wave splitting -- slab gap; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2021GL092712
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: Measurement of anisotropy advances our understanding of mantle dynamics by linking remote seismic observations to local deformation state through constraints from mineral physics. The Pacific Northwest records the largest depth‐integrated anisotropic signals across the western United States but the depths contributing to the total signal are unclear. We used the amplitudes of orthogonally polarized P‐to‐S converted phases from the mantle transition zone boundaries to identify anisotropy within the ∼400–700 km deep layer. Significant anisotropy is found near slab gaps imaged by prior tomography. Focusing of mantle flow through slab gaps may lead to locally elevated stress that enhances lattice preferred orientation of anisotropic minerals within the transition zone, such as wadsleyite. Plain Language Summary: Earth's mantle convects like a fluid over geological time and it organizes mineral fabrics resulting in directional dependence of seismic velocities, that is seismic anisotropy. There is abundant evidence for flow‐induced seismic anisotropy at depths above about 400 km, but it is less clear if anisotropy is developed in the mantle transition zone at about 400–700 km deep. Here, we use seismic waves generated from the bottom and top of the transition zone to constrain anisotropy within the layer. Localized evidence of strong anisotropy is found beneath the Pacific Northwest near locations where prior imaging studies show gaps between subducted oceanic plate fragments. We propose that focused flow through constrictions like slab gaps may cause seismic anisotropy in the mantle transition zone. Key Points: PSH /PSV amplitude ratios between P660s and P410s are useful for constraining transition zone anisotropy Evidence for up to 2% mantle transition zone anisotropy is found beneath the Pacific Northwest of the US Mantle flow between slab fragments may enhance development of transition zone anisotropy