Deformation in the mantle

Measurements of seismic anisotropy (Section 2.1.8) and the results of mineral physics experiments have been used to infer creep mechanisms and flow patterns in the mantle (Karato, 1998; Park & Levin, 2002; Bystricky, 2003). The deformation of mantle minerals, including olivine, by dislocation creep results in either a preferred orientation of crystal lattices or a preferred orientation of mineral shapes. This alignment affects how fast seismic waves propagate in different directions. Measurements of this directionality and other properties potentially allow investigators to image areas of the mantle that are deforming by dislocation creep (Section 2.10.3) and to determine whether the flow is mostly vertical or mostly horizontal. However, these interpretations are complicated by factors such as temperature, grain size, the presence of water and partial melt, and the amount of strain (Hirth & Kohlstedt, 2003; Faul et al., 2004).

Most authors view power-law (or dislocation) creep as the dominant deformation mechanism in the upper mantle. Experiments on olivine, structural evidence in mantle-derived nodules, and the presence of seismic anisotropy suggest that power-law creep occurs to a depth of at least 200 km. These results contrast with many studies of post-glacial isostatic rebound (Section 2.11.5), which tend to favor a diffusion creep mechanism for flow in the upper mantle. Karato & Wu (1993) resolved this apparent discrepancy by suggesting that a transition from power-law creep to diffusion creep occurs with depth in the upper mantle. Diffusion creep may become increasingly prominent with depth as pressure and temperature increase and stress differences decrease. A source of potential uncertainty in studies of mantle rheology using glacial rebound is the role of transient creep, where the strain rate varies with time under constant stress. Because the total strains associated with rebound are quite small (<10-3) compared to the large strains associated with mantle convection, transient creep may be important during post-glacial isostatic rebound (Ranalli, 2001).

In contrast to the upper mantle, much of the lower mantle is seismically isotropic, suggesting that diffusion creep is the dominant mechanism associated with mantle flow at great depths (Karato et al., 1995). Unlike dislocation creep, diffusion creep (and also superplastic creep) result in an isotropic crystal structure in lower mantle minerals, such as perovskite and magnesiowustite. Large uncertainties about lower mantle rheology exist because lower mantle materials are difficult to reproduce in the laboratory. Nevertheless, advances in high-pressure experimentation have allowed investigators to measure some of the physical properties of lower mantle minerals. Some measurements suggest that lower mantle rheology strongly depends on the occurrence and geometry of minor, very weak phases, such as magnesium oxide (Yamazaki & Karato, 2001). Murakami et al. (2004) demonstrated that at pressure and temperature conditions corresponding to those near the core-mantle boundary, MgSiO3 perovskite transforms to a high-pressure form that may influence the seismic characteristics of the mantle below the D" discontinuity (Section 12.8.4).

Unlike most of the lower mantle, observations at the base of the mesosphere, in the D" layer (Section 2.8.5), indicate the presence of seismic anisotropy (Panning & Romanowicz, 2004). The dominance of VSH polarization over VSV in shear waves implies large-scale horizontal flow, possibly analogous to that found in the upper 200 km of the mantle. The origin of the anisotropy, whether it is due to the alignment of crystal lattices or to the preferred orientation of mineral shapes, is uncertain. However, these observations suggest that D" is a mechanical boundary layer for mantle convection. Exceptions to the pattern of horizontal flow at the base of the lower mantle are equally interesting. Two exceptions occur at the bottom of extensive low velocity regions in the lower mantle beneath the central Pacific and southern Africa (Section 12.8.2) where anisotropy measurements indicate the onset of vertical upwelling (Panning & Romanowicz, 2004).

Another zone of seismic anisotropy and horizontal flow similar to that in the D" layer also may occur at the top of the lower mantle or mesosphere (Karato, 1998). However, this latter interpretation is highly controversial and awaits testing by continued investigation. If such a zone of horizontal flow does exist then convection in the mantle probably occurs in layers and does not involve the whole mantle (Section 12.5.3).

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