Continental lithospheric mantle lateral flow during tectonic quiescence

In the last decades, most of the geodynamic numerical models have explored the evolution of the lithosphere during tectonic contraction or extension, while less attention has been given to periods of tectonic quiescence. As it is depicted by Western Gondwana Supercontinent, where at least 300 Ma have passed since its amalgamation until its onset of rifting, quiescence may last hundreds of millions of years. During this period, the interaction between the continental lithosphere with the asthenosphere can reshape the lithosphere-asthenosphere boundary, particularly in continental lithosphere where lateral variations in thickness are common. Examples of a thicker lithosphere (up to 300 km) are those represented by cratonic regions, whereas thinner lithosphere (~100-150 km) can be the result of younger tectonically active regions. Previous research has shown that the longevity and stability of the thicker cratonic lithosphere can be disturbed by edge-driven convection, considering that lithospheric lateral variations in thickness trigger small-scale convections. If not disturbed by its edges, cratonic roots can be influenced by upward asthenospheric flux or undergo delamination, the latter case being mainly caused by permanent compositional change. Based on numerical models we show that the roots of the thicker continental lithosphere can laterally flow towards regions of thinner lithosphere or sink into the asthenosphere in the form of drips. The compositional density contrast (Δρ) between the asthenosphere and continental lithospheric mantle seems to play a key role in governing either lateral or dripping behavior. Lateral flow towards thinner regions is favored when Δρ ≥ 48 kg/m3 while dripping erosion takes place otherwise. This Δρ threshold is valid when the thinner region has a width ≥ 400 km. For narrower regions, the threshold decreases with decreasing width. This “dynamism” during quiescence can reshape the rheological structure of the thinner lithosphere, changing its rigidity and strain accumulation pattern through time.