Continental lithospheric layering beneath stable, modified, and destroyed cratons from seismic daylight imaging

The origin of the midlithospheric discontinuity (MLD) is still controversial and enigmatic, but has significant implications for cratonic formation and long-term continental evolution. Here we use the novel seismic daylight imaging (SDI) approach to interrogate fine-scale structure in the lithosphere beneath two Archean cratons: the stable West Australian craton (WAC), and the modified and partially destroyed North China craton (NCC). Both synthetic and real-data SDI results illustrate that the SDI procedure can delineate vertical heterogeneities as fine as 2 km in the mantle lithosphere. A quasi-laminate heterogeneous model is able to well represent and reconcile various observations associated with the MLD. Beneath the NCC the MLD ranges in depth from 80 to 120 km with a good match to the base of the thermal lithosphere, indicating that the MLD is likely to be thermally controlled within the seismological lithosphere. Multiple MLDs are observed beneath the WAC. The upper MLD is rather shallow at 68-82 km, probably formed prior to WAC assembly (~1.8 Ga). The lower MLD at depths of ~90-110 km correlates with the top of the lithosphere-asthenosphere transition (LAT). The thick LAT suggests the lithosphere is deeply coupled to the underlying asthenosphere. Further, the MLD as the top bound of the LAT might be formed via the metasomatism of the mantle lithosphere.