2018-19 EARTH SCIENCES SEMINAR SERIES

THURSDAY Apr 11| 12:00pm | Seminar Room

Coffee will be served!

Cratonic mantle lithosphere is generally considered to be tectonically stable due to its neutral

buoyancy and large mechanical strength that allow it to have survived destruction over billions

of years at the Earth’s surface. However, many recent observations suggest that cratonic

lithosphere has complex internal structures and has clearly evolved over time. Based on a

combination of mantle flow modeling, seismic tomography image, paleogeographic and geologic

data, we show that the cratons in Brazil and central-southern Africa experienced late-Mesozoic

delamination of their lower-lithosphere that was subsequently reestablished thermally by the

present. Another important implication is a stratified density structure of the cratonic mantle

with the lower lithosphere being denser than the upper lithosphere. In an ongoing study, we

further look at the absolute density (or buoyancy) of all continental lithospheres by evaluating

their residual topography on a global scale and comparing it with dynamic topography induced by

the convecting mantle. We find that most cratonic lithospheres possess a net negative buoyancy

largely equivalent to that of a purely thermal boundary layer of the same thickness. These results

suggest that cratonic lithospheres are not unconditionally stable and that the dense lowermost

lithosphere could delaminate when sufficiently perturbed. Therefore, it is the low-density upper

lithosphere that helps to maintain the cratonic properties of the crust. We suggest that this

process should represent a common mechanism for craton evolution.

University of Illinois Urbana-Champaign

Lijun Liu

How stable are continental

cratons? Insight from the western

Gondwana