Cratons, the mantle, and time

Cratons contain the deepest records of time on the Earth and have potentially experienced the greatest variation in mantle dynamics. Yet, they have remained relatively unchanged and undeformed since their origin. Is this strictly due to their intrinsic characteristics or does the longevity of cratons imply anything about the evolution of the Earth's interior? The stability and longevity of cratons depends on their ability to resist deforming forces induced by the flowing and evolving mantle. Previous studies suggest that the combination of buoyancy, viscosity and finite strength provide cratons with sufficient stability to maintain a minimum lithospheric thickness atop a convecting mantle. However, geochemical observations also suggest that cratonic xenoliths originate from depths no greater than 250 km, which implies that a maximum craton thickness exists. What determines the maximum thickness that cratons? Does this thickness vary with time or mantle dynamics? We employ an analytical approach to relate the viscosity structure of the craton to flow within the underlying asthenosphere. In doing so, we show that as the net thickness of the chemical and underlying thermal boundary layers increases, the mantle-induced tractions on the combined structure will increase exponentially for non-Newtonian rheology. Thus, overly-thick lithosphere will be subjected to large stresses that will tend to weaken the thermal boundary layer, and diminish its role as a buffer between the flowing mantle and the chemically-distinct craton. This negative feedback prevents the cratonic lithosphere from exceeding some maximum value that depends on the viscosity structure of the thermal boundary layer. We present the initial estimates of the maximum thickness of the thermal buffer zone, which in turn controls the maximum thickness of cratonic chemical lithosphere and is required to maintain craton stability in the face of destabilizing mantle flow. In addition, we introduce preliminary scaling that suggests a non-dependence of maximum craton thickness on large-scale mantle dynamics.