Dependence of lithospheric slab buoyancy on
composition and convergence rate: insights from a thermally-coupled kinematic
model
K.
Boonma, A. Kumar, D. Garcia-Castellanos, I. Jimenez-Munt, M. Fernández
Imagine
a boat floating in the ocean. A single little hole will let water enter the
boat. Its buoyancy is now changing. When afloat, the boat has great buoyancy,
but as more and more water enters, the less buoyant the boat becomes – until it
eventually sunk!
Let’s keep that
buoyancy concept in mind and apply it to the case of the upper (lithospheric
mantle) and lower (asthenosphere) mantle.
At a
convergence zone, the cooler and less dense lithospheric mantle is subducting
into the hotter and denser asthenosphere (mineral physics viewpoint).
The subducted
portion of the lithosphere will experience hotter temperature and greater
pressure as it subducts – causing its average volumetric density to change
(denser = heavier). How does the density of this subducted lithosphere change?
Well, that depends on factors such as the convergence velocity or the type of
lithospheric mantle, both of which will be the key variables in this study.
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| Figure 1 Definition of mantle delamination |
This study focuses on one of the major factors which controls the
delamination (peeling of the lithospheric mantle from the crust) or subduction
process – the negative buoyancy force (Fbuoy) of the sinking
lithosphere. Our aim is to investigate the effect of the lithospheric mantle
type and the convergence velocity on the lithospheric mantle’s buoyancy force.
So, we used kinematic modelling approach to model the shortening process and
subduction of the lithospheric mantle.
We have 2 main group of test parameters:
(i) Types of lithospheric mantle: Continental Lithosphere:
Archon (Archean cratons, >2.5 Ga, highly depleted); Proton (Proterozoic
shields, 2.5-1.0 Ga, intermediately depleted); Tecton (Phanerozoic
mobile belts, < 1.0 Ga, mildly depleted), Oceanic Lithosphere:
30 Ma 120 Ma (short-lived, intrinsically denser than asthenosphere)
(ii) Convergence
velocities: 1, 4, 10, 20, 30, 40, and 80 mm/yr
Slow
convergence rate
At 4 mm/year,
Proton Fbuoy has a negative trend down to Fbuoy=-1.25e12
N/m (114 km shortening), after which the trend starts to increase (Figure 2a
and 3a). Tecton has a similar Fbuoy evolution as Proton, with a minimum Fbuoy=-9.51e11
N/m at around 103 km shortening (Figure 3b). This change of trend observed in
Proton and Tecton is because a low convergence rate allows time for the slab to
thermally re-equilibrate with the surrounding asthenosphere which increases the
diffusion rate and, therefore, causing the slab to become more buoyant.
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Figure 2 Example of effect of convergence velocity on Proton
lithosphere at:
(a) 4 mm/yr and (b) 80 mm/yr.
|
Fast convergence rate
At 70-80
mm/year, Proton Fbuoy evolution no longer has a minimum point but
instead continuously decreasing (Figure 2b and 3a). Tecton Fbuoy
also continuously decrease but at a lower rate than Proton (Figure 3b). The
fast convergence rate prevents the down-going slab from thermal
re-equilibration (low diffusion rate), so the colder material will get pushed
further down into the asthenosphere, compared to the cases at lower convergence
rate, and therefore maintaining the overall decreasing in total Fbuoy
(increasing in slab-pull force).
We concluded that:
1.
Thick and low averaged density
lithospheric mantle: not affected by the convergence rate and always stay
buoyant, implying that such mantle type does not favour lithosphere
delamination or subduction.
2.
Proton and Tecton exhibit
negative buoyancy force which show a tendency to, possibly, initiate
delamination.
3.
Both oceanic lithospheres
always subduct,
4.
Convergence velocity of ≥
60mm/yr is fast enough to hinder thermal re-equilibration and lead to negative
buoyancy – in the case of Proton and Tecton.
5.
Density contrast of < 50
kg/m3 across the LAB increase the slab’s ability to attain negative buoyancy.
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Figure 3 . (a)-(d) shows the effect of convergence velocity on the total
buoyancy force. (a)
Proton (b) Tecton (c) Archon (d) Oceanic – 30 Ma oceanic
(dash) and 120 Ma oceanic (solid)
lithosphere. (e) Effect of the convergence
rate on the maximum slab-pull force.
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