The continental lithosphere

Global correlations of seismic elastic and anelastic
tomography and mantle temperatures

Summary

Seismic velocity and attenuation anomalies in the mantle are commonly interpreted
in terms of temperature variations on the basis of laboratory studies of elastic and
anelastic properties of rocks. In order to evaluate the relative contributions of
thermal and non-thermal effects into anomalies of attenuation of seismic shear
waves, 1/Qs, and seismic velocity, Vs, we compare global maps of the thermal
structure of the continental upper mantle with global 1/Qs and Vs maps as
determined from Rayleigh waves at periods between 40 and 150 sec. We limit the
comparison to three continental mantle depths (50, 100, and 150 km), where
model resolution is relatively high.

The available dataset does not indicate that, at a global scale, seismic anomalies in
the upper mantle are controlled solely by temperature variations. Continental maps
have correlation coefficients of <0.56 between Vs and T and of <0.47 between Qs
and T at any depth. Such low correlation coefficients can partially be attributed to
modeling artifacts; however, they also suggest that not all of Vs and Qs anomalies
in the continental upper mantle can be explained by T variations.

Global maps show that, by the sign of the anomaly, Vs and Qs usually inversely
correlate with lithospheric temperatures: most cratonic regions show high Vs and
Qs and low T, while most of active regions have seismic and thermal anomalies of
the opposite sign. The strongest inverse correlation is found at a depth of 100 km,
where the attenuation model is best resolved. Significantly, at this depth, the
contours of near-zero Qs anomalies approximately correspond to the 1000o C
isotherm, in agreement with laboratory measurements that show a pronounced
increase in seismic attenuation in upper mantle rocks at 1000o-1100o C. East-west
profiles of Vs, Qs, and T where continental data coverage is best (50o N latitude
for North America and 60o N latitude for Eurasia) further demonstrate that
temperature plays a dominant, but non-unique, role in determining the value of
lithospheric Vs and Qs.

At 100 km depth, where the resolution of seismic models is the highest, we
compare observed seismic Vs and Qs with theoretical Vs(T) and Qs(T) values
that are calculated solely from temperature anomalies and constrained by
experimental data on temperature dependences of velocity and attenuation. This
comparison shows that temperature variations alone are sufficient to explain
seismic Vs and Qs in ca. 50% of continental regions. We hypothesize that
compositional anomalies due to Fe-depletion can explain the misfit between
seismic and theoretical Vs in cratonic lithosphere. In regions of active tectonics,
temperature effects alone cannot explain seismic Vs and Qs in the lithosphere. It is
likely that partial melts and/or fluids may affect seismic parameters in these
regions.

This study demonstrates that lithospheric temperature plays the dominant role in
controlling Vs and Qs anomalies, but other physical parameters, such as
compositional variations, fluids, partial melting, and scattering may also play a
significant role in determining Vs and Qs variations in the continental mantle.

Keywords: temperature, heat flow, Rayleigh waves, seismic inversion, lithosphere,
composition, partial melts, cratons, continents

Experimental data on temperature dependence of Q, Vp, and Vs
At high temperatures, the effect of T on Qs and Vs is different: a
sharp increase in Qs occurs at lower temperature than a drop in Vs
associated with partial melting.

Qualitative comparison of Vs, Qs , and T anomalies
in the upper mantle
A comparison of the three maps reveals the existence of a strong
overall qualitative correlation between the signs of Vs, Qs, and T
anomalies for most of the continental lithosphere at depths of 50-150
km. There are two types of regions where the signs of all three
anomalies correlate throughout the entire lithosphere, which suggests
a primarily thermal origin of most of the seismic anomalies.
Globally, high Vs, high Qs, and low T correlate with cratons. Thus,
in agreement with previous work, we infer that positive seismic
anomalies in the cratonic lithosphere are primarily of thermal (low-T)
origin. Similarly, we find the expected inverse correlations (low Vs,
low Qs, and high T) for many tectonically active regions. We
hypothesize that partial melts and/or fluids in the upper mantle of
active regions weaken global Vs-T and Qs-T correlations.

Quantitative comparison of Vs, Qs , and T anomalies
in the upper mantle
The calculated correlation coefficients, r, between Vs, Qs, and T
anomalies at 50, 100, and 150 km depths do not show a global strong
correlation between any pair of the parameters. At 100 km depth
(where both Vs and Qs models have high resolution), r=0.66 for Qs–
Vs, r=–0.56 for Vs–T, and r=–0.33 for Qs–T. We discuss different
physical mechanisms, as well as data modeling artifacts, that can be
responsible for a deterioration of global correlations from strong
dependencies expected from laboratory measurements.

Which part of observed variations in seismic Qs and Vs cannot
be predicted from thermal anomalies alone?
Using laboratory data on the temperature dependence of Vs and Qs in
olivine-rich rocks, we calculate theoretical Qs(T) and Vs(T) at z=100
km from known temperatures. A comparison of observed and
theoretical values provides an estimate of a part of seismic anomalies
that cannot be predicted from temperature variations alone.
In ca. 50% of the continents seismic attenuation and velocity
anomalies can be explained by thermal effects alone. However, these
regions do not show any systematic correlation either with the
tectonic setting, nor with geological age.

Qs<Qs(T) and Vs<Vs(T) anomalies are observed chiefly in
tectonically active regions with high mantle temperatures, where
presence of partial melts/ fluids may be the cause of very low Vs and
Qs values. However, with few exceptions, regions with Qs<Qs(T)
and Vs<Vs(T) anomalies do not overlap, suggesting that no single
mechanism gives rise to these anomalies.
Regions with Qs>Qs(T) and Vs>Vs(T) anomalies include mostly
cratons, for which we favor compositional origin of seismic anomalies