Hongliang Wang

A two-stage thickening and stabilization model for the formation of cratonic lithosphere

Hongliang Wang (UiB)

How Archaean tectonics was capable of producing virtually indestructible cratonic lithosphere remains a topic of considerable debate.  Recent geophysical and petrological studies refuelled the debate by suggesting thickening and associated vertical movement of the cratonic peridotite after its formation as an essential ingredient in the cratonization process.  This geodynamical study thus focuses on this thickening process during craton formation in a thermally evolving mantle.  Our numerical experiments demonstrate a cratonization process in which a depleted buoyant mantle layer (of around 30-130 km) forms a cratonic root that is thicker than 200 km within a few 100s of Myrs.  All successful models display a two-step craton-formation process, in which an initial phase of tectonic shortening is followed by a later phase of gravitational self-thickening. Substantial initial tectonic shortening of previously depleted mantle material is essential to initiate the cratonization process. Gravitational self-thickening takes place afterwards and contributes to a second thickening phase with an amount of thickening that is comparable to the initial tectonic phase. Our results further show that a combination of intrinsic compositional buoyancy of the cratonic root, rapid cooling of the root after shortening, and the long-term secular cooling of the mantle prevents a Rayleigh-Taylor type collapse, and stabilizes the thick cratonic root. This two- stage thickening model provides a geodynamically viable cratonization scenario that is consistent with petrological and geophysical constraints.