BORN STRONG IS NOT ENOUGH: THE MAKING AND BREAKING OF CRATONS

Geophysical evidence suggests that some cratons (for example, Wyoming) have had their cratonic lithosphere modified and partially destroyed, whereas others survive undeformed (such as Pilbara). Because geophysical imaging provides only a present-day snapshot of lithospheric structure, we must rely on geological evidence to identify the characteristics that promote strong cratonic lithosphere and those processes that destroy them.

Three geological characteristics promote development of a strong craton. The first is early crust formation. Cratonic lithosphere that formed very early in Earth history when heat flow was greater may have undergone greater degrees of melt depletion and therefore have greater compositional buoyancy and strength. Evidence of Hadean and Eoarchean craton formation is preserved by Pb isotopic evidence (so-called “high mu” character) and ancient xenocrystic zircon. A second characteristic that may be associated with craton stability is craton size and shape, since a large, equant-shaped craton may be less susceptible to destruction by processes that work at the edges of continental blocks, such as convective removal or rheological weakening. A third characteristic is the time of cratonization, following which cratonic lithosphere continues to cool and strengthen.

The Wyoming craton has all three of these qualities: high mu signature, detrital zircon up to 4 Ga and xenocrystic zircon up to 3.8 Ga, craton-wide Paleoarchean to Neoarchean crust, and no penetrative deformation/metamorphism since 2.5 Ga. By comparison, Pilbara lacks the high mu character and has limited evidence of Eoarchean origins, but the craton was completely formed in the Paleo- and Mesoarchean and cratonized by 3.0 Ga. Both Wyoming and Pilbara appear to have been “born strong,” yet only one remains intact.

This suggests that tectonic processes subsequent to craton formation are critical. The Pilbara craton has been shielded in a continental interior since the Paleoproterozoic, whereas the Wyoming craton has been affected by Farallon flat-slab subduction and rollback, Rio Grande and Basin and Range extension, and the Yellowstone hotspot, all of which appear to have eroded and weakened the cratonic lithosphere. Being “born strong” does not guarantee survival.