Modeling of Seismic Anisotropy Observations Reveals Plausible Lowermost Mantle Flow Directions Beneath Siberia

Abstract

Observations of seismic anisotropy are a powerful tool for mapping deformation within the Earth (e.g., Long & Becker, 2010), and are often used to study deformation and flow in the upper mantle (e.g., Skemer & Hansen, 2016). The lowermost mantle, also known as the D″ layer, also clearly exhibits seismic anisotropy (e.g., Garnero & Lay, 1997;Silver, 1996, and references within Nowacki et al., 2011;Romanowicz & Wenk, 2017), and seismic anisotropy observations can be used to map deformation at the base of the mantle. However, this requires thorough knowledge of the mechanism for D″ seismic anisotropy and the relationship between deformation and strain, which can be established by experiments and theoretical modeling. There are several proposed mechanisms for D″ seismic anisotropy, including the shape preferred orientation (SPO) of melt inclusions (or other elastically distinct material) and the crystallographic preferred orientation (CPO) of bridgmanite (bm), ferropericlase (fp), post-perovskite (ppv), or some mixture of these minerals (e.g., Nowacki et al., 2011).A major obstacle in the interpretation of D″ seismic anisotropy measurements is our imprecise knowledge of the mechanism responsible, along with the non-uniqueness of data sets that are based on a limited number of measurements in a given region. A recent synthetic modeling study (Creasy et al., 2019) demonstrated that tighter constraints on seismic anisotropy at the base of the mantle can be obtained by combining different types of observations of body wave anisotropy than by a single type of data alone. Specifically, Creasy et al. ( 2019) examined reflection polarity measurements (P or S waves that reflect off the D″ discontinuity-PdP and SdS) and shear wave splitting (seismic wave birefringence). In this study, we apply the insights gained from the synthetic modeling of Creasy et al. (2019) and combine observations of shear wave splitting (for both ScS and PKS phases) due to D″ seismic anisotropy with observations of PdP/SdS polarities (and their variation with direction) to obtain tight constraints on geometry in a single target region. We apply a novel modeling approach that is based on previous work (Creasy et al., 2017;Ford et al., 2015) and has been

D" Reflection Polarities Inform Lowermost Mantle Mineralogy

“…(2019), and Creasy et al. (2021, 2019) show that polarity variations, especially of P waves reflected at the D" layer, can also be generated by anisotropy of the D" minerals (ppv, bm, and even ferropericlase), even over small azimuth ranges. The anisotropy, generated by the flow of anisotropic minerals in D", will, however, cause polarity observations that vary with azimuth as shown in Pisconti et al.…”

“…In addition, when anisotropy is present in the D" region, ScS‐waves will experience directionally dependent splitting (Nowacki et al., 2011). It has been shown that a combination of splitting measurements together with PdP and SdS polarity observations (Pisconti et al., 2019), and including discrepant SKS and SKKS splitting measurements (e.g., Creasy et al., 2021) can further constrain mineralogy in the D" region, but the observations vary with azimuth due to the directional velocity variations of the deformed minerals. Therefore, a mapping of regions such as beneath the Caribbean or the western Pacific with crossing paths and taking shear wave splitting of ScS waves into account would help to discriminate between the alternative hypotheses of anisotropy vs. large R ‐values.…”

“…While we are focusing here on isotropic minerals and combinations of minerals above and below the D" reflector to explain polarities, we have to keep in mind that anisotropy is another mechanism that causes polarity changes in seismic waves. For example, C. Thomas et al (2011), and Creasy et al (2021, 2019 show that polarity variations, especially of P waves reflected at the D" layer, can also be generated by anisotropy of the D" minerals (ppv, bm, and even ferropericlase), even over small azimuth ranges. The anisotropy, generated by the flow of anisotropic minerals in D", will, however, cause polarity observations that vary with azimuth as shown in Pisconti et al (2019); these are unlike the results here, where polarity variations are isotropic.…”

Observations of mantle seismic anisotropy using array techniques: shear-wave splitting of beamformed SmKS phases

“…Lowermost mantle anisotropy is thought to be particularly strong at the edges of the two antipodal large-low velocity provinces (LLVPs) atop the core-mantle boundary (e.g., Wang & Wen, 2004;Cottaar & Romanowicz, 2013;Deng et al, 2017;Reiss et -3-manuscript submitted to JGR: Solid Earth al., 2019). Anisotropy at the base of the mantle has also been connected to slab-driven flow in lowermost mantle regions with faster than average seismic velocities (e.g., Nowacki et al, 2010;Asplet et al, 2020;Creasy et al, 2021;Wolf & Long, 2022) and to upwelling flow in the deep mantle at the base of plumes (e.g., Ford et al, 2015;Wolf et al, 2019).…”

On the measurement of Sdiff splitting caused by lowermost mantle anisotropy

“…Therefore, it is not completely straightforward to infer deformation geometry from measured shear wave splitting parameters (fast polarization directions and delay times). One strategy is to assume a plausible lowermost mantle composition based on the likely temperature conditions and seismic velocities of a certain region 2 and carry out forward modelling to make predictions that can be compared to observations (e.g., Nowacki et al, 2010;Ford et al, 2015;Creasy et al, 2021;Wolf and Long, 2022).…”