WHERE DID CRATONS COME FROM? NARROWING DOWN THE OPTIONS USING GEOLOGICAL CONSTRAINTS AND GEODYNAMICAL MODELS

Cratons are areas of continental lithosphere that exhibit long-term stability against deformation. One hypothesis for their formation invokes thrust stacking of a proto-cratonic lithosphere. In support of the hypothesis, seismology provides images of dipping reflectors within cratonic lithosphere interpreted as remnant subduction features. If valid, then proto-cratonic lithosphere must initiate as a deformable material, yet once formed must resist deformation henceforth to maintain long-term stability. We conducted simulations to test the viability of the thrust stacking craton formation hypothesis as well as to elucidate stabilization mechanisms. Combined with scaling analysis, we demonstrated that craton formation via thrust stacking is most feasible for buoyant and viscous proto-cratonic lithosphere. In addition, the lithosphere must be thin and/or possess low friction coefficient values such that it can yield in response to convection-generated stresses. Formation via thrust stacking is viable for lithosphere with chemical to thermal buoyancy ratios of B = 0.75-1.5, viscosity contrasts between the lithosphere and convective mantle of 100, and friction coefficients of 0.05-0.1. long-term stability These requirements allow us to make predictions about the nature of the proto-cratonic lithosphere. The required viscosity contrast can be achieved if the lithosphere is dehydrated of bound water, while the required friction coefficients are typical of lubricated faults. The buoyancy ratios can be provided with either continental arc lithosphere or buoyant oceanic lithosphere serving as the proto-cratonic lithosphere. However, craton formation via thrust stacking of buoyant oceanic lithosphere requires layered mantle convection during the time of formation; in addition, a thick oceanic crust is required to match Mg# constraints. Finally, preservation and long-term stability depends on the balance between the lithosphere's yield and convection-generated stresses and is promoted by the thickened cratonic root or increased friction coefficient values, due to pore-fluid dehydration.