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Density of continental roots:
Compositional and thermal effects.
Compositional and thermal effects.
We use jointly gravity, topography, thermal, and seismic data to
investigate how the compositional component of density of the
lithospheric roots varies beneath continents. Our analysis is based on
the interpretation of residual topography and mantle gravity
anomalies, calculated by subtracting the crustal effects from the
observed field, and on the upper mantle temperature variations
estimated from heat flow and seismic tomography data.
1. We find that the mantle gravity anomalies (Fig. 1) vary globally
from –250 mGal to +150 mGal, with the largest negative anomalies,
indicating a low-density lithosphere, being associated with vast
Cenozoic regions of plume-lithosphere interaction: the East-African
Rift, and the Basin and Range Province of the western USA. The
largest positive anomalies are associated over the continents with the
Andes, the East European Platform, the Alpine-Mediterranean fold
belt and the central - south-eastern part of North America. In the
oceanic regions there are positive anomalies in parts of the western
Pacific. The residual topography variations (Fig. 2), which besides
a dynamically supported component indicate mass excess or mass
deficiency in the upper mantle required for isostatic equilibrium, are
inversely correlated with the residual gravity anomalies.
2. For cratonic areas, we deduce from the gravity and topography
data a large range of density anomalies in the subcrustal
lithosphere, produced by both temperature and compositional
variations (Fig. 11). The cratonic areas fall into two main groups. The
largest positive residual gravity anomalies and the most significant
negative residual topography are observed over Precambrian
Eurasia (the Baltic Shield, the East European Platform, the Ukrainian
Shield, and the Siberian craton). Cratons of the Southern hemisphere
(Western Australia, the South American Craton, the Indian Shield and
South Africa) reveal negative mantle gravity anomalies and positive
residual topography, with the most pronounced anomalies found for
South Africa. The Canadian Shield and West Africa have an
intermediate position between these two groups. The cratons of the
first group have a dense lithospheric mantle, while for the cratons of
the second group the average lithospheric density is less than upper
mantle density beneath old ocean, which is here taken as the
reference lithosphere.
3. We account for the thermal state of the lithosphere to separate the
effects of composition and thermal expansion on density
anomalies (Fig. 12). Our analysis is based on the recent calculations
of the thermal regime of stable continental lithosphere [Artemieva &
Mooney, 2001]. Gravity anomalies induced by temperature variations
in the cratonic lithosphere are typically greater than 100 mGal and in
some regions (the Baltic Shield, the Siberian craton, and the West
African craton) reach 250 mGal. We found that the temperature
induced gravity anomalies under cratons are well correlated with
mantle gravity anomalies but the total amplitude of the latter is 1.5
times smaller than the range of pure temperature-induced gravity
anomalies. This means that the density variations due to temperature
are only partly (about 40%) compensated by density variations due to
compositional differences. This conclusion contradicts the classical
isopycnic hypothesis of Jordan that predicts a complete balance
between thermal and compositional buoyancy anomalies in the
cratonic lithosphere. A plausible explanation is variation in the amount
of compensation between thermal and compositional density changes
with depth, and this is supported by recent petrological studies.
4. 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.
5. 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 %.
investigate how the compositional component of density of the
lithospheric roots varies beneath continents. Our analysis is based on
the interpretation of residual topography and mantle gravity
anomalies, calculated by subtracting the crustal effects from the
observed field, and on the upper mantle temperature variations
estimated from heat flow and seismic tomography data.
1. We find that the mantle gravity anomalies (Fig. 1) vary globally
from –250 mGal to +150 mGal, with the largest negative anomalies,
indicating a low-density lithosphere, being associated with vast
Cenozoic regions of plume-lithosphere interaction: the East-African
Rift, and the Basin and Range Province of the western USA. The
largest positive anomalies are associated over the continents with the
Andes, the East European Platform, the Alpine-Mediterranean fold
belt and the central - south-eastern part of North America. In the
oceanic regions there are positive anomalies in parts of the western
Pacific. The residual topography variations (Fig. 2), which besides
a dynamically supported component indicate mass excess or mass
deficiency in the upper mantle required for isostatic equilibrium, are
inversely correlated with the residual gravity anomalies.
2. For cratonic areas, we deduce from the gravity and topography
data a large range of density anomalies in the subcrustal
lithosphere, produced by both temperature and compositional
variations (Fig. 11). The cratonic areas fall into two main groups. The
largest positive residual gravity anomalies and the most significant
negative residual topography are observed over Precambrian
Eurasia (the Baltic Shield, the East European Platform, the Ukrainian
Shield, and the Siberian craton). Cratons of the Southern hemisphere
(Western Australia, the South American Craton, the Indian Shield and
South Africa) reveal negative mantle gravity anomalies and positive
residual topography, with the most pronounced anomalies found for
South Africa. The Canadian Shield and West Africa have an
intermediate position between these two groups. The cratons of the
first group have a dense lithospheric mantle, while for the cratons of
the second group the average lithospheric density is less than upper
mantle density beneath old ocean, which is here taken as the
reference lithosphere.
3. We account for the thermal state of the lithosphere to separate the
effects of composition and thermal expansion on density
anomalies (Fig. 12). Our analysis is based on the recent calculations
of the thermal regime of stable continental lithosphere [Artemieva &
Mooney, 2001]. Gravity anomalies induced by temperature variations
in the cratonic lithosphere are typically greater than 100 mGal and in
some regions (the Baltic Shield, the Siberian craton, and the West
African craton) reach 250 mGal. We found that the temperature
induced gravity anomalies under cratons are well correlated with
mantle gravity anomalies but the total amplitude of the latter is 1.5
times smaller than the range of pure temperature-induced gravity
anomalies. This means that the density variations due to temperature
are only partly (about 40%) compensated by density variations due to
compositional differences. This conclusion contradicts the classical
isopycnic hypothesis of Jordan that predicts a complete balance
between thermal and compositional buoyancy anomalies in the
cratonic lithosphere. A plausible explanation is variation in the amount
of compensation between thermal and compositional density changes
with depth, and this is supported by recent petrological studies.
4. 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.
5. 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 %.
| Last modified June 24, 2006, irina@geol.ku.dk |
Kaban M.K., Schwintzer P., Artemieva I.M., and Mooney W.D.
Earth Planet. Sci. Lett., v. 209, 53-69, 2003.
Earth Planet. Sci. Lett., v. 209, 53-69, 2003.
- THE CONTINENTAL LITHOSPHERE
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- THE CONTINENTAL LITHOSPHERE
| THERMAL REGIME, STRUCTURE, AND EVOLUTION OF |
| Fig. 1. Mantle gravity anomalies (in mGal), calculated by subtracting from observed gravity: (a) globally - the crustal gravity effect including topography and bathymetry and (b) for the oceans – the gravity effect of the cooling oceanic lithosphere (plate model) estimated from ocean floor age data. The anomalies are truncated after degree/order 20 and centred by subtracting the mean value. |
| Fig. 2. Residual topography (in km), calculated in spherical harmonics up to degree/order 20 by removing isostatic compensation masses produced by the crustal density structure and by the oceanic lithosphere (for the model of a cooling plate in accordance with the ocean floor age data) from observed topography. Zero level corresponds to 180 Ma old standard oceanic lithosphere (cooling plate). |
| 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 model. 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. For inversion, the thickness of CBL is assumed to be equal to the lithosphere thermal thickness [Artemieva & Mooney, 2001]. |
| Fig. 13. 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 constant with its base 200 km below Moho |
| Fig. 14. Average and maximal values of depletion for the individual cratons. The depletion can be due to a variety of processes (e.g. removal of basaltic component, melt extraction), which cannot be distinguished from gravity data. We are using this term to refer to the lithospheric density reduction of any chemical origin. Legend: CS – Canadian shield, AUS – western Australia, BS – Baltic Shield and East European platform, SIB – Siberian Platform, SAF – South African craton, IND – Indian Shield, SAM - South American craton, WAF - West African craton. |