ICTJA PhD presentation award 2018 - Kittiphon Boonma (Oral Presentation)

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.

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 (F_buoy) 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 F_buoy has a negative trend down to F_buoy=-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 F_buoy=-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.

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 F_buoy evolution no longer has a minimum point but instead continuously decreasing (Figure 2b and 3a). Tecton F_buoy 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 F_buoy (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.  

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.