Publication Date:
2023-03-21
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.
Type:
Article
,
NonPeerReviewed
Format:
text
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