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- Title
The Mid‐Lithospheric Discontinuity Caused by Channel Flow in Proto‐Cratonic Mantle.
- Authors
Yang, Haibin; Artemieva, Irina M.; Thybo, Hans
- Abstract
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. Plain Language Summary: Stable cratons form rheologically strong plates 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‐lithospheric discontinuity (MLD). While several mechanisms may explain the present‐day MLD seismic structure, the dynamic processes that created such structure are unclear. We propose that the MLD was created by channel flow in the lower cratonic lithosphere, triggered by convective instabilities at cratonic margins in the Archean when the mantle was hot. The presence of a chemically distinct, low‐density cratonic lithospheric root is required to initiate the process. We test our proposed model by numerical modeling and demonstrate that the proposed mechanism is valid (a) for a broad range of physically possible parameters in nature and (b) that MLD may have been preserved since its formation in the Archean. Key Points: The MLD may be caused by channel flow in the cratonic mantle lithosphere in the Archean when the mantle was hotThe top of the channel flow can cause radial anisotropy and reduce seismic wave velocitySecular cooling of the earth can preserve the top of the shear zone caused by the channel flow
- Subjects
CHANNEL flow; CRATONS; INTERNAL structure of the Earth; SEISMIC wave velocity; GEOPHYSICAL observations; THERMAL instability
- Publication
Journal of Geophysical Research. Solid Earth, 2023, Vol 128, Issue 4, p1
- ISSN
2169-9313
- Publication type
Article
- DOI
10.1029/2022JB026202