Azimuthal Seismic Anisotropy of 70‐Ma Pacific‐Plate Upper Mantle

Abstract

Plate formation and evolution processes are predicted to generate upper mantle seismic anisotropy and negative vertical velocity gradients in oceanic lithosphere. However, predictions for upper mantle seismic velocity structure do not fully agree with the results of seismic experiments. The strength of anisotropy observed in the upper mantle varies widely. Further, many refraction studies observe a fast direction of anisotropy rotated several degrees with respect to the paleospreading direction, suggesting that upper mantle anisotropy records processes other than 2‐D corner flow and plate‐driven shear near mid‐ocean ridges. We measure 6.0 ± 0.3% anisotropy at the Moho in 70‐Ma lithosphere in the central Pacific with a fast direction parallel to paleospreading, consistent with mineral alignment by 2‐D mantle flow near a mid‐ocean ridge. We also find an increase in the strength of anisotropy with depth, with vertical velocity gradients estimated at 0.02 km/s/km in the fast direction and 0 km/s/km in the slow direction. The increase in anisotropy with depth can be explained by mechanisms for producing anisotropy other than intrinsic effects from mineral fabric, such as aligned cracks or other structures. This measurement of seismic anisotropy and gradients reflects the effects of both plate formation and evolution processes on seismic velocity structure in mature oceanic lithosphere, and can serve as a reference for future studies to investigate the processes involved in lithospheric formation and evolution.

Pn Tomography of the Juan de Fuca and Gorda Plates: Implications for Mantle Deformation and Hydration in the Oceanic Lithosphere

“…We note that while our data set cannot constrain the 4 θ parameters, this does not preclude their existence. Mineralogically, the 4 θ parameters are required to describe the hexagonal symmetry that characterizes mantle anisotropy (Backus, ; Becker et al, ) and higher resolution studies (e.g., Mark et al, ) have reliably measured these higher order terms.…”

“…Our tomographic algorithm solves only for the coefficients that modulate the 2 θ variations (see Dunn et al, ). This simplification is warranted considering that the 2 θ terms typically dominate azimuthal velocity variations observed within the oceanic crust (Dunn & Toomey, ; Shearer & Orcutt, ; Weekly et al, ) and upper mantle (Becker et al, ; Mark et al, ; Morris et al, ; Russell et al, ; Shearer & Orcutt, ; VanderBeek & Toomey, ). Furthermore, we do not find significant 4 θ terms in our data set (see section ).…”

Crustal Cracks and Frozen Flow in Oceanic Lithosphere Inferred From Electrical Anisotropy

“…Alignment of olivine by mantle flow is widely held as the mechanism by which oceanic lithospheric mantle inherits seismic anisotropy (Gaherty et al, ; Kodaira et al, ; Mark et al, ; Russell et al, ; Shearer & Orcutt, ; Vanderbeek & Toomey, ). Such seismic anisotropy studies have determined that the fast a axis of olivine is aligned with the flow direction in the uppermost mantle (Karato et al, ), although the degree of this anisotropy is variable and seems to correlate with spreading rate.…”

“…Even so, the remarkable similarity of our preferred model's uppermost mantle anisotropy with lab results on sheared olivine strongly suggests that shearing plays a dominant role in creating the electrical character of the uppermost mantle and may define the electrical response of the Moho. Alignment of olivine by mantle flow is widely held as the mechanism by which oceanic lithospheric mantle inherits seismic anisotropy (Gaherty et al, 2004;Kodaira et al, 2014;Mark et al, 2019;Russell et al, 2018;Shearer & Orcutt, 1985;Vanderbeek & Toomey, 2017). Such seismic anisotropy studies have determined that the fast a axis of olivine is aligned with the flow direction in the uppermost mantle (Karato et al, 2008), although the degree of this anisotropy is variable and seems to correlate with spreading rate.…”

“…Anisotropy, which denotes a directional variation in a material property, is becoming a standard feature in geophysical characterizations of oceanic plates because it provides a window into lithospheric development and evolution. Seismic studies have illuminated and interpreted anisotropy in the lithospheric mantle using its magnitude and direction to infer mantle flow (Gaherty et al, ; Kodaira et al, ; Mark et al, ; Russell et al, ; Shearer & Orcutt, ; Vanderbeek & Toomey, ). Electrical anisotropy, which can arise from a crystal's intrinsic properties (Yang et al, ), mineral‐scale fabrics that produce a lattice or crystal‐preferred orientation (Karato et al, ), or macroscopic features such as fault‐enhanced hydration pathways (Key et al, ; Naif et al, ), offers complementary insight into crust‐mantle processes and the history of lithospheric development.…”