Description: Stable cratons with a thick (〉 200 km) and cold lithosphere form rheologically strong plates that move atop a ductile asthenospheric mantle. Various types of seismic observations show the presence of a potentially rheologically weak zone at depths of ca. 80 – 150 km termed the Mid-Lithosphere Discontinuity (MLD). While various mechanisms may explain the MLD, the dynamic processes leading to the seismic observations are unclear. We propose that the MLD can be caused by channel flow in the lower lithosphere, triggered by negative Rayleigh-Taylor instabilities at cratonic margins in the Archean, when the mantle was hotter than at present. Presence of a chemically distinct, low-density cratonic lithospheric root is required to initiate the process. Numerical modeling shows that the top of the channel flow creates a shear zone at a depth comparable to the globally observed seismic MLD. Grain size reduction in the shear zone and accumulation of percolated melts or fluids along the channel top may reduce seismic wave speeds as observed in the MLD, while the channel flow itself may explain radial anisotropy of seismic wave speeds. Secular cooling of the Earth deepens the top of the channel flow on a 1 Gyr scale, and early-stage large-scale (1000’s km long) channel flow deformation switches to a different deformation style with a smaller (100’s km) wavelength. These different flow patterns may explain the different seismic response of the MLD and the lithosphere base.

Description: Formation of new oceans by continental break-up is understood as a continuous evolution from rifting to ocean spreading. The Red Sea is one of few locations on Earth where a new plate boundary presently forms. Its evolution provides key information on how the plate tectonics operates and how the plate boundaries form and evolve in time. While the new plate boundary has already been formed in the southern Red Sea where ocean spreading is active, the north-central segment still experiences continental rifting. The region also has west-east asymmetry: in the north-central Red Sea the rift-related magmatism is not located beneath the rift axis, as conventional models predict, but instead is offset by ca 300 km into Arabia. We propose a new geodynamic model which explains the enigmatic asymmetry of the Red Sea region and is fully consistent with various types of geological and geophysical observations. We demonstrate that the north-central rift is a transient feature that will not develop into coincident ocean spreading. Instead, the new plate boundary forms across Arabia. Our numerical experiments, supported by geological, seismic and gravity observations, predict that in 1–5 Myr the north-central extensional axis will jump ~300 km eastward into Arabia. The Ad Damm strike-slip fault, normal to the central Red Sea rift axis, will evolve into a transform fault between the on-going ocean spreading in the southern Red Sea and the future spreading in north-central Arabia. We demonstrate that crustal-scale weakness zones control lithosphere extension and lead to long-distance jumps of extensional axes in continental lithosphere not affected by hotspots. Therefore, our model also provides theoretical basis for understanding dynamics and mechanisms of the transition from rifting to continental break-up at passive continental margins not affected by hotspots.

Description: Global geophysical observations show the presence of the enigmatic mid‐lithospheric discontinuity (MLD) at depths of ca. 80–150 km which may question the stability and internal structure of the continental lithosphere. While various mechanisms may explain the MLD, the dynamic processes leading to the seismic observations are unclear. Here we present a physical mechanism for the origin of MLD by channel flow in the cratonic mantle lithosphere, triggered by convective instabilities at cratonic margins in the Archean when the mantle was hot. Our numerical modeling shows that the top of the frozen‐in channel flow creates a shear zone at a depth comparable to the globally observed seismic MLD. Grain size reduction in the shear zone and accumulation of percolated melts or fluids along the channel top may reduce seismic wave speeds as observed below the MLD, while the channel flow itself may explain radial anisotropy of seismic wave speeds and change in direction of the seismic anisotropic fast axis. The proposed mechanism is valid for a broad range of physically realistic parameters and that MLD may have been preserved since its formation in the Archean. The intensity of the channel flow ceased with time due to secular cooling of the Earth's interior. The new mechanism may reshape our understanding of the evolution and stability of cratonic lithosphere.