They Don't Make 'Em Like That Anymore

Cratons, ancient domains of lithosphere, have remained largely undisturbed since their final amalgamation, ~1.8 Ga. The lithosphere beneath cratons are distinctly different (thicker, colder, more depleted and buoyant) from younger, Phanerozoic lithosphere. Their longevity testifies to their strength, but cratons are not indestructible, and examples of recent craton destruction include North China, Wyoming, and Tanzania. Given the significant debate about the governing tectonics in the Archaean (i.e. When did plate tectonics start?), the boundary conditions and geodynamics of craton formation remain uncertain. In addition, significant debate continues around why and how cratonic keels are destroyed. Here, we present some recent numerical modelling results that can shed new light on the processes that formed and destroyed cratons.

We propose a two-stage craton formation process, in which forced shortening (in whichever tectonic regime was operating at the time) is followed by gravitational self-thickening. The final stage of craton amalgamation is marked by a shortening, compressive regime. We show that a combination of intrinsic compositional buoyancy of the cratonic root, prolonged cooling of the root after shortening, and the long-term secular cooling of the mantle will stabilize the thick cratonic root for future preservation. This two-stage thickening model provides a geodynamically viable cratonization scenario that is consistent with petrological and geophysical constraints.

Destroying a craton that has survived billions of years of mantle convection is not straightforward, and increased convective vigour (e.g. impingement of a mantle plume) is unlikely to be sufficient. Fluid infiltration from stagnant slabs underneath old cratons has been proposed for some cratons (e.g. NE China), but such scenario cannot be invoked for cratons in e.g. Southern Africa. Another method is the metasomatic weakening of cratonic keels by impregnation with small-degree melts, and we investigate this process numerically. Results show that a series of small-degree melting events can weaken cratonic roots sufficiently to cause collapse.

These results constrain the physical and geodynamical properties of cratons and their environment, and provide new insight in their formation and destruction dynamics.