Origins of cratonic mantle discontinuities: A view from petrology, geochemistry and thermodynamic models

Ancient Continental Lithosphere Dislocated Beneath Ocean Basins Along the Mid‐Lithosphere Discontinuity: A Hypothesis

“…The estimated thickness and averaged viscosities in these models are compatible with the available constraints on lithospheric discontinuities. The minimum MLD thickness required according to the models is ~25 km, consistent with the well‐documented 30–40 km MLD thickness in many cratons (Aulbach et al, ; Selway et al, ; Sodoudi et al, ; Wölbern et al, ; Yuan & Romanowicz, ). The model results suggest that the MLD viscosity constraints for the SW African case (10 19 –5*10 20 Pa s) are somewhat outside the formal viscosity range of mantle lithosphere by laboratory constraints (10 21 –5*10 24 Pa s) (Selway, ); factors such as water, partial melt, and crystal alignment and shear weakening all reduce viscosity.…”

“…In the initial stage (Stage 0, Figure a), cratonic lithosphere with a weak MLD is still stable because of insufficient weakness of the MLD, a large distance from a continental margin, or protection by surrounding orogenic belts. The lithosphere at this stage is likely similar to the lithospheres of the present Superior, Slave, Yilgarn, and western North China Cratons (Aulbach et al, ; Chen et al, ; Selway et al, ), where mid‐lithosphere discontinuities are imaged within the cratons but are probably not weak enough and protected by surrounding orogenic belts (e.g., Lenardic et al, ), so do not show offset of roots beneath oceans. For the cratons with sufficiently weak MLDs (low viscosity and large thickness), the cratonic roots would start to be displaced laterally during on‐craton rifting, making cratonic roots dislocated beneath the newly formed oceanic basins (Liao & Gerya, ; Liao et al, ).…”

“…Recently, a midlithospheric discontinuity (MLD, at depths between ~60 and ~160 km, and generally between ~80 and 100 km) has been detected in most cratonic areas, showing low seismic velocities and a distinct seismic anisotropy signature (Aulbach et al, ; Calò et al, ; Selway et al, ; Sodoudi et al, ; Thybo & Perchuć, ; Wölbern et al, ; Yuan & Romanowicz, ). Seismic velocity reductions across this layer are very large (3–10%) and greater than those measured across the lithosphere‐asthenosphere boundary (LAB) in continental areas (~1% or slightly larger) (Aulbach et al, ; Selway et al, , and references therein). The fast‐axis direction of azimuthal anisotropy in some cratonic areas (e.g., North America) changes sharply at MLD depths (Yuan & Romanowicz, ).…”

“…The fast‐axis direction of azimuthal anisotropy in some cratonic areas (e.g., North America) changes sharply at MLD depths (Yuan & Romanowicz, ). Proposed models for the nature of the MLD include enhanced partial melting or grain boundary sliding; enrichment in infiltrated frozen melts, pyroxenes, phlogopite, amphibole, or carbonates; and layering of minerals with distinct orientation or changes in deformation creep mechanisms (boundary between diffusion and dislocation creep) (Aulbach et al, ; Fei et al, ; Selway et al, ). The general view is that some MLDs are sufficiently weak layers in the mid‐lithosphere (e.g., Liao & Gerya, ; Liao et al, ; Thybo & Perchuć, , and references therein), where decoupling and shearing between the upper and lower part of the cratonic lithosphere will most likely occur (Liao & Gerya, ; Liao et al, ).…”

Craton Destruction 1: Cratonic Keel Delamination Along a Weak Midlithospheric Discontinuity Layer

“…We propose that potential intracratonic decoupling occurs along a seismologically revealed intracratonic layer named the midlithospheric discontinuity (MLD) that is seen at ~80–100 km (Abt et al, ; Aulbach et al, ; Chen et al, ; Hopper et al, ; Nita et al, ; Selway et al, ; Sodoudi et al, ). The MLD is a common but not ubiquitous seismic feature in cratonic regions.…”