Mantle Convection and Surface Expressions. Группа авторов

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СКАЧАТЬ (Figure 1.5) than any of the other cases, showing an increase in the slope of the power spectrum below the base of the transition zone, similar to the feature observed in SEISGLOB2 (Durand et al., 2017). The key parameter that distinguishes this model from the others is the inclusion of the low‐viscosity channel, which can have a “lubrication” effect on slabs, allowing them to move laterally below the base of the transition zone. Among the other cases, we can see that there is limited sensitivity of the power spectral slope to whether viscosity is increased at 660 km or 1,000 km depth. Indeed, in Cases 18 (viscosity increase at 1,000 km) and 9 (viscosity increase at 660 km depth), the most significant change in spectral slope is at a depth of 660 km, coincident with the included phase transition. We note that Case 9 has the best overall correlation with the tomographic model due to high values of correlation throughout much of the lower mantle, but does not reproduce structure in the transition zone or shallow lower mantle as well as some of the other models.

      In previous work (Rudolph et al., 2015), we presented evidence for an increase in viscosity in the mid‐mantle based on inversions constrained by the long‐wavelength geoid. The viscosity inversions shown in Figure 1.6 are quite similar to what we found previously, despite different choices in parameterization (piecewise linear variation of viscosity vs. piecewise constant), and the use of a different tomographic model (the density model ME16‐160, for which results are shown in Figure 1.6b). There are key differences in the parameterizations of SEMUCB‐WM1 versus the density model ME16‐160, especially near the transition zone. SEMUCB‐WM1 uses a continuous parameterization in the radial direction using splines, whereas ME16‐160, which adopts the same radial parameterization as S362ANI and S362ANI+M (e.g., Kustowski et al., 2008; Moulik and Ekström, 2014), allows a discontinuity in the parameterization at 650 km depth. Moreover, S362ANI+M includes data particularly sensitive to these depths such as normal modes and the precursors to the body wave phase SS that reflect off transition‐zone discontinuities. As a result, the change in the pattern of heterogeneity from the transition zone to the lower mantle across the 650‐km discontinuity is more abrupt in ME16‐160 compared to SEMUCB‐WM1. The depth and abruptness of changes in structure are exactly the features reflected in the plots of the radial correlation function in Figure 1.3. SEMUCB‐WM1 shows a clear decorrelation at 1,000 km depth and a minimum in correlation length at 650 km. On the other hand, S362ANI+M and GLAD‐M15 show the most substantial change in correlation structure at 650 km depth and a minimum in correlation at shallower depths in the upper mantle. Given the differences in the depths at which major changes in lateral structure occur in SEMUCB‐WM1 vs. ME16‐160, one might expect to recover a somewhat different preferred depth of viscosity increase between the upper mantle and lower mantle, because the preferred depth of the viscosity increase is typically very close to the crossover depth from positive to negative sensitivity in the geoid kernel. The fact that viscosity inversions with both tomographic models yield a viscosity increase substantially deeper than 650 km and closer to 1,000 km may therefore be significant.

Some of the inferred viscosity profiles contain a region with reduced viscosity below the 650 km phase transition (Figure 1.6b). The low‐viscosity channel emerges as a feature in our ensemble solutions as additional data constraints are added to the inversion, justifying more complex solutions. The low‐viscosity region is a pronounced feature in the viscosity profiles based on ME16‐160 and there also appears to be a more subtle expression of this feature in the viscosity profiles based on SEMUCB‐WM1. Such a feature has been suggested on the basis of several lines of evidence. First, the transition from ringwoodite to bridgmanite plus ferropericlase involves complete recrystallization of the dominant phases present, and multiple mechanisms associated with the phase transition could modify the viscosity. In convective downwellings, the phase transition could be accompanied by a dramatic reduction in grain size to ∼μm size (Solomatov and Reese, 2008). On theoretical grounds, it might be expected that transformational superplasticity could reduce viscosity by two to three orders of magnitude within 1.5 km of the 650 km phase transition (Panasyuk and Hager, 1998). Second, inversions for the viscosity profile constrained by the global long‐wavelength geoid allowed for the presence of a low‐viscosity channel at the base of the upper mantle (Forte et al., 1993), and a similar “second asthenosphere” was recovered in inversions constrained by regional (oceanic) geoid anomalies (Kido et al., 1998). The effect of the low‐viscosity channel can modify predictions associated with GIA observables Milne et al. (1998), and in joint inversions of GIA, misfits can be significantly reduced for models that include such a low‐viscosity notch (Mitrovica and Forte, 2004). Finally, in global geodynamic models with prescribed plate motions, the behavior of slabs is broadly consistent with observations of stagnation when such a feature is included (Mao and Zhong, 2018; Lourenço and Rudolph, in review).

      An increase in viscosity in the mid‐mantle or viscosity “hill,” which is a feature common to all of our viscosity inversions, has been suggested on the basis of geophysical inversions, and several potential mechanisms exist to explain such a feature. An increase in viscosity below 650 km depth has been recovered in many inversions constrained by the long wavelength geoid and GIA observables (e.g., King and Masters, 1992; Mitrovica and Forte, 1997; Forte and Mitrovica, 2001; Rudolph et al., 2015). An increase in viscosity would be expected to slow sinking slabs (Morra et al., 2010) and affect the dynamics of plumes. The correlation between subduction history and tomographic models has been used to test whether slabs sink at a uniform rate in the lower mantle. A recent study of the similarity between convergence patterns in plate reconstructions and patterns of mantle lateral heterogeneity from an average of V_S tomographic models suggests that the data can neither confirm nor reject the possibility of a change in viscosity below 600 km (Domeier et al., 2016). On the other hand, an analysis of a catalog that relates imaged fast anomalies to specific subduction events does find evidence that the rate of slab sinking decreases across a “slab deceleration zone” between 650–1500 km (van der Meer et al., 2018); one explanation for such a deceleration zone is the increase in viscosity in the shallow lower mantle seen in all of our inverted viscosity profiles.

      Several mechanisms exist that could produce an increase in viscosity in the mid‐mantle. Marquardt and Miyagi (2015) measured the strength of ferropericlase at pressures of 20–60 GPa (600–1,000 km) and observed an increase in strength across this range of pressures. Though ferropericlase is a minor modal component of the lower mantle, it could become rheologically limiting if organized into sheets within rapidly deforming regions, an idea supported by experiments with two‐phase analog materials (Kaercher et al., 2016) and with bridgmanite‐magnesiowüstite mixtures (Girard et al., 2016). If the lower mantle rheology is determined by the arrangement of distinct mineral phases, we expect history‐dependence and anisotropy of viscosity (Thielmann et al., 2020), further confounding our interpretations of viscosity in inversions. An increase in the viscosity of ferropericlase is also supported by experimental determinations of the melting temperature at mantle pressures (Deng and Lee, 2017), which show a local maximum in melting temperature for pressures near 40 GPa (1,000 km). Changes in the proportionation of iron could also alter the viscosity of bridgmanite across a depth range consistent with the inferred mid‐mantle viscosity increase. Shim et al. (2017) suggested that at depths of 1,100–1,700 km, an increase in the proportionation of iron into ferropericlase could depress the melting point of bridgmanite, increasing the viscosity predicted using homologous temperature scaling. These various mechanisms are not mutually exclusive and could operate in concert to produce an increase in viscosity near 1,000 km. Finally, we note that the deformation mechanisms of even single phases within the lower mantle remain uncertain. While the lower mantle has long been thought to deform by diffusion creep due to absence of seismic anisotropy at most lower mantle depths, recent calculations suggest that another deformation mechanism – pure climb creep, which is insensitive to grain size and produces no seismic anisotropy – may be active in bridgmanite at lower mantle conditions (Boioli et al., 2017).

1.5 СКАЧАТЬ