pp iii Archean cratons and mantle dynamics [rapid communication]

The apparent stability of Archean cratons and cratonic keels for billions of years is a difficult observation for geodynamic modeling to explain. While it may be straight-forward to assert that chemical buoyancy and high viscosity are needed to stabilize cratons, there are many questions regarding craton formation, variability (both in space and time), and evolution that remain unanswered. In numerical studies, strong and buoyant cratonic keels survive relatively undeformed for several mantle overturn times (the equivalent of several hundred million years); extending this to several billion years remains a challenge. The strength required to stabilize keels in some of these numerical experiments exceeds reasonable estimates of the laboratory measurements of strength of mantle materials (including both the effects of temperature and melt-depletion). In addition, the most common explanation of keel formation, vertical stacking of subducted plate, requires the keel material to be deformable at the time of formation and soon afterward the keel material becomes strong enough to resist shearing. The extent to which cratonic keels interact with and influence the pattern of mantle convection, by nucleating small-scale edge-driven convection or by coupling plate motions to deeper mantle flow, remains an open question.