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Lithosphere depletion from gravity and T
THE CONTINENTAL LITHOSPHERE
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|Artemieva I.M., 2011.
Cambridge University Press,
794 pp., ISBN 9780521843966.
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Density of continental roots based on gravity data:
separating compositional and thermal effects
Artemieva I.M., and
Earth Planet. Sci. Lett.,
v. 209, 53-69, 2003.
The origin and evolution of cratonic roots has been debated for many years. Precambrian cratons are underlain
by cold lithospheric roots that are chemically depleted. Thermal and petrologic data indicate that Archean roots
are colder and more chemically depleted than Proterozoic roots. This observation Here we test this hypothesis
using gravity, thermal, petrologic, and seismic data to quantify differences in the density of cratonic roots globally.
In the first step in our analysis we use a global crustal model to remove the crustal contribution to the observed
gravity. The result is the mantle gravity anomaly field, which varies over cratonic areas from 3100 to +100 mGal.
Positive mantle gravity anomalies are observed for cratons in the northern hemisphere: the Baltic shield, East
European Platform, and the Siberian Platform. Negative anomalies are observed over cratons in the southern
hemisphere: Western Australia, South America, the Indian shield, and Southern Africa. This indicates that there
are significant differences in the density of cratonic roots, even for those of similar age.
Root density depends on temperature and chemical depletion. In order to separate these effects we apply a
lithospheric temperature correction using thermal estimates from a combination of geothermal modeling and
global seismic tomography models. Gravity anomalies induced by temperature variations in the uppermost mantle
range from 3200 to +300 mGal, with the strongest negative anomalies associated with mid-ocean ridges and the
strongest positive anomalies associated with cratons.
After correcting for thermal effects, we obtain a map of density variations due to lithospheric compositional
variations. These maps indicate that the average density decrease due to the chemical depletion within cratonic
roots varies from 1.1% to 1.5%, assuming the chemical boundary layer has the same thickness as the thermal
boundary layer. The maximal values of the density drop are in the range 1.7^2.5%, and correspond to the
Archean portion of each craton. Temperatures within cratonic roots vary strongly, and our analysis indicates that
density variations in the roots due to temperature are larger than the variations due to chemical differences.
|Fig. 11. Gravity anomalies (in mGal) due to the
compositional variations in the mantle obtained by
subtracting from mantle gravity anomalies:
(a) gravity effects of temperature-induced density
variations in the lithosphere (Fig. 9) and
(b) sublithospheric gravity signals. Here we do not use
the cooling lithosphere model, but the entire effect of
the oceanic mantle is based on a conversion of the S20
The field is truncated after degree/order 20.
The white isolines mark anomalies exceeding 100
mGal, assumed to be the maximum error in the crustal
and temperature reductions of the gravity field.
|Fig. 12. Compositional density anomalies (in kg/m3) in the subcrustal layer of the cratons as estimated from an inversion
of the compositional gravity anomalies (Fig. 11).
For inversion, the thickness of CBL is assumed to be:
- (left): equal to the lithosphere thermal thickness [Artemieva & Mooney, 2001]
- (right): constant with its base 200 km below Moho.
We calculate the gravity effect of compositional variations in the
lithosphere by subtracting temperature-induced gravity anomalies from the
mantle gravity anomalies (Figs. 12-13). These compositional gravity anomalies
vary from –300 mGal to +220 mGal. The cratonic areas are characterised by
pronounced gravity lows, typically within the range –150 to –250 mGal, implying
corresponding compositional changes. Large positive compositional gravity
anomalies are found in two distinct regions: (1) near ocean-continent and
continent-continent subduction zones, and (2) within some continental interiors,
e.g. in the southern part of North America. The origin of the latter positive
anomalies is uncertain.
We produce a map of compositional density anomalies in the cratonic
lithosphere and compare the degree of depletion between different continental
roots (Fig. 14). The average depletion for the individual cratons varies only
slightly, between 1.1% to 1.5%, assuming that the thickness of the chemical
boundary layer is proportional to the thermal boundary layer thickness. These
values depend to some extent on the ratio between Archean and Proterozoic
lithosphere within each of the cratons. The maximal values of depletion are
within the interval 1.7-2.5 %, and should characterize the Archean portion of
each area. This result is in excellent agreement with petrological studies.
If we assume that the thickness of the CBL is constant for all the cratons (Fig.
13), the obtained composition density anomalies vary much more between the
individual roots. For a 200 km thick CBL, the values of depletion averaged over
each craton are in between 0.6-1.5 %, with peaks from 1.2% to 2.4 %.