The stability of lithospheric steps: The Cordillera-craton transition in western North America

Abstract:

Lateral variations in lithosphere thickness are observed in many continental regions. For some places, such as the Colorado Plateau, steps in thickness are gravitationally unstable, resulting in a gradational transition. In other places, sharp thickness variations appear to be stable over geological time. One example is in western Canada, at the boundary between Proterozoic/ Archean cratonic lithosphere and the accreted terranes of the Canadian Cordillera. This boundary is marked by an eastward decrease in surface heat flow and increase in elastic thickness and mantle seismic velocity. Together, these indicate that lithosphere thickness increases from ~60 km for the Cordillera to >150 km for the craton. High-resolution seismic images show that the transition occurs over a horizontal distance of 50-100 km and reconstructions show that the boundary has persisted for >50 Ma.

In this study, thermal-mechanical numerical models are used to address the dynamics of lithospheric steps on timescales of 50-100 Ma. Models start with the juxtaposition of 60 km thick lithosphere (“Cordillera”) and 160 km thick lithosphere (“craton”). The density and rheology of the craton are varied to simulate changes in composition and water content. For all densities, a steep transition (>50°) is maintained only if the craton is stronger than a dry olivine rheology. As water content increases, the transition in thickness evolves to a lower angle, as the craton undergoes gravitational instability. If the compositional density of the craton and underlying mantle are similar, the step is eroded through downwellings, and the boundary migrates below the craton. Conversely, a compositionally buoyant craton destabilizes through upwellings that underplate the Cordillera. The models demonstrate that the observed sharp Cordillera-craton transition in western Canada requires a dry craton lithosphere. The results also have implications for the long-term stability of cratonic lithosphere, as even a small amount of water enables convective erosion.