SUBDUCTION DYNAMICS, CONTINENTAL COLLISION AND THE FORMATION OF CRATONS

In the modern Earth, accretion and continental collision are driven by the combined buoyancy forces along the length of subduction zone acting to overcome the resistance in the congested section [Moresi et al, 2014; Betts et al, 2015]. This is a way that we can amplify the typical stresses available to drive accretion and orogeny if we only consider a vertical cross section of the subduction zone and this can be helpful in understanding the dynamics interaction of plate tectonics, mantle flow and continental deformation.

We consider whether this amplification would be possible in the early Earth and if it would be a way to supply the high stresses needed to construct a long-lived craton. Tectonic origins for the cratonic lithosphere have been favoured in recent discussions [e.g. Lee et al, (2011), McKenzie & Priestley, (2016)] but there are reasons to suspect accretion and crustal thickening would have been more difficult under Archean conditions. Convective stresses were lower in the early Earth which had internal temperatures that were higher and lower viscosities. The viability of subduction itself in the Archean Earth is still a subject of debate because thinner cold-boundary layers faced a thicker oceanic crust formed at high temperatures. Numerical models predict subduction would have been viable in the Archean but slabs would have been weaker and more prone to break-off [van Hunen & Moyen, 2012].

Early in Earth’s evolution, a heat-pipe mode of convection might have been active instead — this is a form of stagnant lid convection where internal heat is lost by magmatic pipes by-passing the lithosphere. A stagnant lid with a combination of intrusive and extrusive magmatism can satisfy the thermal and petrological constraints for the early Archean Earth. [Moore and Webb (2013), Rozel et al, (2017)]. These models naturally become unstable with respect to mobile-lid convection models as the level of internal heat production decreases.

We show that the transition phase between stagnant lid and a mobile-lid or plate-tectonic style of convection is capable of building or assembling the cratonic lithosphere and not just destroying the stagnant lithosphere. The stresses during the collapse of the thick, stagnant lid can be significantly higher than the convective stresses in either the steady, stagnant lid or the mobile-lid convective regimes (figure 2 B). We discuss these models and compare the deep lithospheric structure with that which occurs when growth is driven by lateral accretion. This builds upon previous studies by Beall at al (2017).