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Basal drag of lithospheric keels
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|On the relation between cratonic lithosphere thickness,
plate motions, and basal drag
Seismic, thermal, and petrological data indicate that the local maximum
thickness of Precambrian lithosphere is highly variable (140-350 km),
with a bimodal distribution for Archean cratons (200-220 km and 300-350
km). We discuss the origin of such large differences in lithospheric
thickness, and propose that the lithospheric base can have large depth
variations over short distances. We find a linear correlation between the
horizontal and vertical dimensions of Archean cratons: larger cratons
have thicker lithosphere with the “critical” surface area of >6-8 x106 km2
for cratons to have deep (>300 km) roots.
We evaluate the basal drag model as a possible mechanism that may
thin the cratonic lithosphere. Inverse correlations are found between
effective ridge length. In agreement with the basal drag model, we find
that lithospheric thickness of Archean cratons is proportional to the
square root of the ratio of the craton length (along the direction of plate
motion) to the plate velocity. Large cratons with thick keels and low plate
velocities are less eroded by basal drag than small fast-moving cratons.
|Artemieva I. M. and
Tectonophysics, v. 358,
Click on images to enlarge them
Vertical versus lateral dimensions of the cratons.
between cratonic size, plate velocities, and lithospheric thickness. The results suggest that the horizontal and
vertical dimensions of the Archean cratons are well correlated: larger Archean cratons have thicker lithosphere.
trend for cratonic size versus lithospheric thickness to the total size of all present-day Archean cratons
(hypothesized to have formed an early Archean supercontinent) suggests that the ancient (~4.0 Ga) lithosphere
may have been ~450 km thick.
|The ‘‘Bryce Canyon’’ model of the lithospheric structure. The lithospheric base is not a flat
sub-horizontal boundary as usually assumed, but an undulating boundary due to a
heterogeneous erosion of the lithospheric base by an interaction with mantle plumes and
its nonuniform destruction by convection flow due to compositional inhomogeneities.
inhomogeneities. Xenolith data sample the most shallow parts of the lithospheric base
(200-250 km), where pressures are low enough to initiate low-percentage melting and
produce kimberlite-type magmatism. Seismic tomography samples the top of the
convective mantle, i.e. the lowermost parts of the lithospheric base. A diffuse character of
the seismic lithospheric base, especially based on surface waves, supports the model.
Thermal data provide a smoothed integrated picture of the lithospheric structure and thus
give values of the thickness of cratonic keels between xenolith and seismic estimates.
|Lithosphere thermal thickness in the Archean cratons versus craton area. The extension of the
Archean. Assuming that the area of the oldest (e.g. ~4.0 Ga) hypothesized Archean
supercontinent was equal to the total area of the present Archean cratons, its lithospheric
thickness could have been 350-450 km. Similarly, lithospheric thickness at ~550-500 Ma (when
Gondwanaland was formed) is estimated to be about 280-400 km. Black dot shows estimated
lithospheric thickness in the Slave Craton at the time of Gondwanaland (Pokhilenko et al., 2001).
Cartoon depicting the process of erosion of cratonic lithosphere.
An early Archean supercontinent with a keel down to the mantle transition zone (400-450 km) is split by a mantle plume into two parts with non-equal
dimensions. The fate of these supercontinent fragments is determined by their lateral dimensions. The larger craton diverts the mantle heat from its
base (Ballard and Pollack, 1987) and is mostly affected by secondary convection on its margins (Doin et al., 1997). This process promotes the
preservation of a thick keel (~350 km) in the interior of the craton, but leads to erosion of cratonic margins and thus a gradual decrease of the
When the area of the larger craton is reduced to a critical value of about 6-8 x106 km2 (Fig. 4A), it will start to evolve as a smaller craton. The smaller
craton is not effective in diverting the heat because of its smaller lateral dimension and is more subject to erosion from below by mantle convection
until an equilibrium lithospheric thickness of ~220 km is reached.
Basal drag model: correlation of plate velocities with lithospheric thickness and cratonic area.
We evaluate the basal drag model, whereby the lithosphere is eroded due to its relative movement with respect to the underlying mantle (Sleep,
2001), and find that for Archean cratons this model is in excellent agreement with observed data: lithospheric thickness is proportional to the square
root of the ratio of the craton length (along the direction of plate motion) to the plate velocity. Large cratons with thick keels and low plate velocities
(e.g., Eurasia and North America) are less eroded by basal drag than fast-moving small cratons (e.g., India and Australia). This means that earlier
studies (Forsyth and Uyeda, 1975; Stoddard and Abbott, 1996) have addressed only one aspect of a more complicated relationship, whereby both
craton size and plate velocity correlate with the lithospheric thickness: large Archean cratons tend to have thick lithospheric keels and very slow plate
velocities. We emphasize that these correlations hold only for the Archean cratons, not for middle-late Proterozoic cratons; for early Proterozoic
cratons the correlation is very weak.
Preservation of the lithospheric keels.
long time (i.e. 3-4 Ga). Cratons with large sizes are also more stable with respect to basal erosion by mantle convection due to an efficient deflection of
heat from the deep mantle (Ballard and Pollack, 1987; Lenardic and Moresi, 2001). Their stability is further maintained by the depleted (e.g. Jordan,
1988) and dry (e.g. Pollack, 1986) composition of the Archean lithosphere. However, very thick (~400 km) lithospheric keels could have survived until
present probably only locally, an observation supported by the fact that Archean cratonic lithosphere thickness rarely exceeds 300-350 km.
Relations between lithospheric thickness, slab pull, and ridge
The analysis shows an inverse correlations between lithospheric thickness
and: (a) fractional subduction length; and (b) the effective ridge length.
These results indicate that lithosphere erosion by mantle drag is
proportional to the plate velocity.
Reworking of Archean keels in Proterozoic.
When erosion due to secondary convection at the margins of a thick (~350 km), large craton reduces its lateral dimension to a critical value of ~6-8
x106 km2, the keel fails to divert the basal heat efficiently. In this case, the Archean keel is thinned by mantle convection to an equilibrium thickness of ~
is a candidate for the geologically known reworking into Proterozoic lithosphere.
Due to the viscosity-depth structure of the upper mantle, thinning of the Archean lithospheric keel will reduce basal drag and therefore resistance to
plate motion. This will permit faster movement of the craton with respect to the underlying mantle, which, in turn, will enhance lithosphere erosion by the
basal drag. If an Archean keel is thinned to significantly less than 200 km, the remaining lithospheric column will be a candidate for strong deformation in
a collisional environment and for modification by metasomatism (i.e. invasion by volatiles and relatively enriched mantle magmas).
Secular cooling of the mantle and mantle drag.
We recognize that the basal drag model suggests a long-term preservation of thick (300-350 km) Archean keels only if the cratons have never
experienced a period of high plate velocity. In view of the long (~4 Ga) existence of the keels, this scenario seems unlikely. Though it has been
suggested that plate velocities in Archean could have been even slower than at present, it is difficult to imagine that the Archean cratons of West Africa,
Baltica, and Siberia were never a part of a fast-moving plate. However, if the viscosity of the mantle to a depth of ~450 km was one or two orders of
magnitude lower during the Archean, corresponding to mantle temperature some 100-200oC higher than today, the basal drag, and along with it, basal
erosion would have been much smaller than today. Thus, the thick keels of Archean cratons would have been preserved, even if Archean plate
velocities were high.
Irina M. Artemieva
Geology Section, IGN
University of Copenhagen
Øster Voldgade 10