Dynamics of continental lithosphere extension and passive continental rifting from numerical experiments of visco-elasto-plastic thermo-chemical convection in 2-D Cartesian geometry

Abstract

In contrast to active continental rifting, which is caused by the thermal erosion of active upwelling plumes, passive continental rifting is attributed to the extensional stress acting on the continental lithosphere due to either the basal drag force exerted by the passive mantle flow under the lithosphere or the boundary forces at the continental margins. Although the extension of continental lithosphere by passive continental rifting may cause the breakup of continental margins, such as the separation of Zealandia from Gondwanaland at ca. 100 Ma and the Japanese islands from Eurasia at ca. 25 Ma, the dynamics have not been fully understood from geophysical and geological perspectives. Here, a series of numerical experiments of visco-elasto-plastic thermo-chemical convection with a free surface is performed using 2-D Cartesian models to investigate the localization of strain in the extending continental lithosphere, with an initial depth of 200 km, and the behavior of continental rifting and breakup. Our results demonstrate that when extension rates at the continental margin were set between 1 and 4 cm yr−1, the time taken for seafloor subsidence and subsequent continental breakup under extensional stress is less than a few million years. During continental rifting, high-shear zones (i.e., high-strain-rate zones) develop under the base of the deforming continental lithosphere, with a strain rate of the order of 10−13 s−1, which is approximately two orders larger than the typical value of the mantle interior. Due to accumulating strain along the base of the deforming lithosphere, excess strain is not concentrated on the shallowest part of the underlying mantle; hence, the spreading ridge that breaks the thick continental lithosphere emerges as a locally weak plate boundary on the Earth's surface. This characteristic is more remarkable as the activation volume of the sublithospheric mantle rock and the resultant viscosity are low under hydrous conditions.