Global Anisotropy and the Thickness of Continents

Yuancheng Gung, Mark Panning and Barbara Romanowicz

Introduction

For decades there has been a vigorous debate about the depth extent of continental roots (Jordan, 1975) The analysis of heat flow (Jaupart et al., 1998), mantle xenoliths (Rudnick et al, 1998) and electrical conductivity (Hirth, 2000) indicate that the coherent, conductive part of continental roots is not much thicker than 200-250 km. Some global seismic tomographic models agree with this estimate but others indicate much thicker zone of fast velocities under continental shields, reaching at least 400km in depth. This is manifested by a drop in correlation between some models from $\sim$0.80 at 100km to less than 0.45 at 300 km depth (Figure 33.1a), which casts some doubt on the ability of global tomography to accurately resolve upper mantle structure.

However, although global $V_S$ models differ from each other significantly in the depth range 200-400km under the main continental shields, these differences are consistent when they are classified into three categories, depending on the type of data used to derive them: $SV$ (mostly vertical or longitudinal component data, dominated by Rayleigh waves in the upper mantle), $SH$ (mostly transverse component data, dominated by Love waves), and (3)hybrid (three component data). $SH$ and hybrid models are better correlated with each other than with $SV$ models. This difference is accentuated when the correlation is computed only across continental areas, as shown in Figure 33.1b. The reduced correlation in the depth range 250-400 km between $SH$ and hybrid models and $SV$ models is strongly accentuated over continents.

On the other hand, global tomographic studies that account for seismic anisotropy, either by inverting three component data for $V_{SV}$ and $V_{SH}$ using isotropic kernels (Ekström and Dziewonski, 1998), or in the framework of more general anisotropic theory (Montagner and Tanimoto, 1991), have documented significant lateral variations in the anisotropic parameter $\xi$ ( $= {(V_{SH}/V_{SV})}^2$) on the global scale. Until now, attention has mostly focused on the strong positive $\delta ln \xi (=2(\delta ln V_{SH} - \delta ln V_{SV}))$ observed in the central part of the Pacific Ocean in the depth range 80-200 km. The presence of this anisotropy has been related to shear flow in the asthenosphere, with a significant horizontal component. Deeper anisotropy was suggested, but not well resolved in these studies, either because the dataset was limited to fundamental mode surface waves, or because of the use of inaccurate depth sensitivity kernels. In particular, it is important to verify that any differences in $V_{SV}$ and $V_{SH}$ observed below 200km depth are not an artifact of simplified theoretical assumptions, which ignore the influence of radial anisotropy on depth sensitivity kernels.

Figure 33.1: Correlation coefficient as a function of depth between model SAW24B16 (Mégnin and Romanowicz, 1999), an $SH$ model, and other global tomographic $S$ velocity models. (a) over the whole globe; (b)over continental areas only. S20ASH (Ekström and Dziewonski, 1998) is an $SH$ model, SB 4L18 (Masters et al., 1996) is a hybrid model and S20ASV (Ekström and Dziewonski, 1998) and S20RTS (Ritsema et al., 1999) are both $SV$ models.

Results

We have developed an inversion procedure for transverse isotropy using three component surface and body waveform data, in the framework of normal mode asymptotic coupling theory (Li and Romanowicz, 1995), which in particular, involves the use of 2D broadband anisotropic sensitivity kernels appropriate for higher modes and body waves.

Figure 33.2 shows the distributions of $\delta ln\xi$ in the resulting degree 16 anisotropic model SAW16AN. At 175 km depth, the global distribution of $\delta ln\xi$ confirms features found in previous studies, and is dominated by the striking positive $\delta ln \xi > 0$ ( $V_{SH} > V_{SV}$) anomaly in the central Pacific and a similar one in the Indian Ocean. However, at depths greater than 250 km, the character of the distribution changes: positive $\delta ln\xi$ emerges under the Canadian Shield, Siberian Platform, Baltic Shield, southern Africa, Amazonian and Australian cratons, while the positive $\delta ln\xi$ fades out under the Pacific and Indian oceans. At 300 km depth, the roots of most cratons are characterized by positive $\delta ln\xi$, which extend down to about 400 km. These features are emphasized in depth cross sections across major continental shields (Figure 33.3), where we compare $V_{SH}$ and $V_{SV}$ distributions, consistently showing deeper continental roots in $V_{SH}$. Interestingly, the East Pacific Rise has a signature with $\delta ln\xi < 0$ down to 300km, indicative of a significant component of vertical flow. At 400km depth, we also note the negative $\delta ln\xi$ around the Pacific ring, consistent with quasi-vertical flow in the subduction zone regions in the western Pacific and south America.

Discussions

Temperatures in the 250-400 km depth range exceed $1000^{\circ}C$, and are therefore too high to allow sustained frozen anisotropy in a mechanically coherent lithospheric lid on geologically relevant time scales (Vinnik et al, 1992). Therefore we infer that the $V_{SH} > V_{SV}$ anisotropy under continental roots we describe here must be related to present day flow-induced shear, with a significant horizontal component.

We note the similarity of the character of $V_{SH} > V_{SV}$ anisotropy, in the depth range 200-400km under cratons, and 80-200km under ocean basins, and we suggest that both are related to shear in the asthenosphere, the difference in depth simply reflecting the varying depth of the asthenospheric channel. Although our inference is indirect, it reconciles tomographic studies with other geophysical observations of lithospheric thickness based on heat flow, xenoliths and mantle electrical conductivity.

Another contentious issue is the nature of the Lehmann discontinuity ($L$), and in particular the puzzling observation that it is not a consistent global feature, but is observed primarily in stable continental areas and not under oceans (Gu et al., 2001). Since the $V_{SH} > V_{SV}$ anisotropy under continental cratons is found deeper than 200 km, we propose that $L$ actually marks the top of the asthenospheric layer, a transition from weak anisotropic lowermost continental lithosphere to anisotropic asthenosphere. Under oceans, the lithosphere is much thinner, and the lithosphere/asthenosphere boundary occurs at much shallower depths. There is no consistently observed discontinuity around 200-250 km depth. On the other hand, a shallower discontinuity, the Gutenberg discontinuity ($G$), is often reported under oceans and appears as a negative impedance reflector (Revenaugh and Jordan, 1991). The difference in depth of the observed $\delta ln \xi > 0$ anisotropy between continents and oceans is consistent with an interpretation of $L$ and $G$ as both marking the bottom of the mechanically coherent lithosphere, in areas where it is quasi-horizontal (Figure 33.4).

Conclusion

Thus, the inspection of radial anisotropy in the depth range 200-400 km allows us to infer that continental roots do not extend much beyond 250km depth, in agreement with other geophysical observations. The part of the mantle under old continents that translates coherently with plate motions need not be thicker than 200-250km. Tomographic models reveal the varying depth of the top of the anisotropic asthenospheric channel, marked by a detectable seismic discontinuity called $L$ under continents (about 200-250km depth), and $G$ under oceans (about 60-80km depth) . Finally, seemingly incompatible tomographic models obtained by different researchers can thus also be reconciled: the relatively poor correlation between different models in the depth range 250-400 km is not due to a lack of resolution of the tomographic approach, but rather to the different sensitivity to anisotropy of different types of data.

Acknowledgements

We thank the National Science Foundation for support of this research.

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Figure 33.2: Maps of relative lateral variations in $\xi$ of model SAW16AN at 3 depths in the upper mantle. L ateral variations are referred to reference model PREM, which is isotropic below 220km depth, but has sig nificant $\delta ln \xi > 0$ at 175km depth.

Figure 33.3: Depths cross-sections through 3 continents showing the $SH$ (left) and $SV$ (right) components of anisotropic model SAW16AN. The $SH$ sections consistently indicate fast velocities extending to depths in excess of 220 km, whereas the $SV$ sections do not.

Figure 33.4: Sketch illustrating our interpretation of the observed anisotropy in relation to lithospheric thickness, and its relationship to Lehmann ($L$) and Gutenberg ($G$) discontinuities. The Hales discontinuity ($H$) is also shown. H is generally observed as a positive impedance embedded within the continental lithosphere in the depth range 60-80km. H and G may not be related.