Olivine anisotropy suggests Gutenberg discontinuity is not the base of the lithosphere

Abstract

Tectonic plates are a key feature of Earth's structure, and their behavior and dynamics are fundamental drivers in a wide range of large-scale processes. The operation of plate tectonics, in general, depends intimately on the manner in which lithospheric plates couple to the convecting interior. Current debate centers on whether the transition from rigid lithosphere to flowing asthenosphere relates to increases in temperature or to changes in composition such as the presence of a small amount of melt or an increase in water content below a specified depth. Thus, the manner in which the rigid lithosphere couples to the flowing asthenosphere is currently unclear. Here we present results from laboratory-based torsion experiments on olivine aggregates with and without melt, yielding an improved database describing the crystallographic alignment of olivine grains. We combine this database with a flow model for oceanic upper mantle to predict the structure of the seismic anisotropy beneath ocean basins. Agreement between our model and seismological observations supports the view that the base of the lithosphere is thermally controlled. This model additionally supports the idea that discontinuities in velocity and anisotropy, often assumed to be the base of the lithosphere, are, instead, intralithospheric features reflecting a compositional boundary established at midocean ridges, not a rheological boundary.seismic anisotropy | lithosphere−asthenosphere boundary | upper mantle | geodynamics | crystallographic texture A lthough plate tectonics is the unifying paradigm in the Earth sciences, important questions remain regarding the physical nature of a tectonic plate. A cornerstone of plate tectonic theory states that plates translate across Earth's surface in a relatively rigid and coherent fashion, with deformation largely concentrated at plate boundaries. Restated, the plates are taken to have a high viscosity with a relatively sharp transition to the less viscous convecting mantle beneath. Referring to plates as the lithosphere and to the underlying rock as the asthenosphere [terminology which predates plate tectonic theory (1)] is now common. Partial decoupling of the asthenosphere from the lithosphere appears essential to plate-tectonic-like behavior (2), but whether a change in material properties at the base of the lithosphere arises from increasing temperature (3) or from transitions in melt (4-7) or water content (8, 9) remains unclear, even in the tectonically simple ocean basins.Recent seismological investigations of oceanic upper mantle indicate a change in composition at depths often associated with the lithosphere−asthenosphere boundary (LAB). Receiver-function studies and underside reflections of SS precursors detect a sharp velocity discontinuity (the Gutenberg discontinuity) at a depth of 40 to 150 km (4, 6, 10-12). The sharpness of this discontinuity may indicate the presence of a small amount of melt (4), and the lack of a strong dependence of the discontinuity depth on plate age (6, 10, 13) is roughl...

Crystallographic Preferred Orientation of Olivine in Sheared Partially Molten Rocks: The Source of the “a‐c Switch”

“…An olivine CPO very similar to that associated with the a-c switch was also reported in a plagioclase-bearing peridotite Geochemistry, Geophysics, Geosystems (Higgie & Tommasi, 2014). The seismic anisotropy predicted by the direct application of the results from microstructural analyses of our deformed melt-bearing samples has been presented in Hansen et al (2016, Figure 2). 2.…”

“…This explanation relies simply on the effect of melt on CPO without calling upon the elastic effects of aligned melt at the grain scale or larger (e.g., Holtzman & Kendall, 2010), trench-parallel flow (e.g., Hall et al, 2000;Kneller & Van Keken, 2007;Smith et al, 2001), high stress and water content (Jung & Karato, 2001), or 3-D flow (Mehl et al, 2003). This 10 misalignment may potentially be attributed to a 10 azimuthal anisotropy to the shear direction produced in a region composed of mixed melt-bearing and melt-free layers sheared to low strains (c 5), as exhibited in Figure 2 in Hansen et al (2016). This 10 misalignment may potentially be attributed to a 10 azimuthal anisotropy to the shear direction produced in a region composed of mixed melt-bearing and melt-free layers sheared to low strains (c 5), as exhibited in Figure 2 in Hansen et al (2016).…”

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

“…Lastly, we note that increased melt retention could explain mantle seismic anisotropy that is oblique to the paleospreading direction beneath the JdF plate interior (VanderBeek & Toomey, ). Laboratory studies on fabric development (Hansen et al, ; Qi et al, ) show that olivine aggregates deformed in the presence of partial melt may generate azimuthal seismic anisotropy that trends >60° from the shear direction. While our interpretation of the JdF age trend is speculative, we consider thermal anomalies or mantle alteration less likely.…”

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

“…In particular, deformation experiments conducted on olivine aggregates with melt present (e.g., Hansen et al, , 4% melt; Qi et al, , 7% melt) produce fabrics that appear to be a combination of CPO and shape‐preferred orientation (Hansen et al, , 2016; Holtzman et al, ; Qi et al, ). At the much lower melt fractions typical of mid‐ocean ridges, anisotropy due to this melt‐present fabric is still likely to be dominated by the CPO component aligned in the shear direction, but the strength of effective seismic anisotropy may be reduced (Hansen et al, ; Zhong et al, ). It is possible that this mechanism would impart vertical gradients in anisotropy if the percentage of melt present along a corner‐flow flow line decreased for progressively deeper flow lines.…”

“…Measurements of seismic anisotropy provide a means to learn about oceanic plate formation and mantle flow. This is possible because the evolution of lithospheric anisotropy is sensitive to a variety of factors including preexisting mantle fabric (Boneh & Skemer, ; Skemer et al, ), the amount of strain the lithosphere experiences (e.g., Hedjazian & Kaminski, ; Ribe, ; Zhang & Karato, ), the magnitude of shear strain relative to the rate of rotation of the strain axes (Kaminski & Ribe, ), and the presence of melt during deformation (Hansen et al, ; Qi et al, ). At the same time, this variety of sensitivities can make the interpretation of anisotropy difficult.…”