Mobilization and thinning of cratonic lithosphere by a lower mantle slab

Abstract

Abstract Continental cratons are characterized by thick lithospheric roots that remain intact for billions of years. However, some cratonic roots appear to have been thinned or completely removed, with the reasons for such thinning being debated. In this study, we obtain a high-resolution full-waveform seismic tomographic model for North America which newly illuminates ongoing craton-thinning. Extensive drip-like transport of lithosphere is imaged from the base of the craton beneath the central United States to the mantle transition zone. Geodynamical modeling suggests that such dripping may be mobilized by the sinking of the deep Farallon slab, whose associated mantle flow can drag material at the base of the craton from afar to the dripping location. There, lithospheric material can descend within the ambient downward mantle flow, even though the slab is presently in the lower mantle. Dripping lithosphere could be further facilitated by prior lithospheric weakening such as due to volatiles released from the slab. Our findings show how cratonic lithosphere can be altered by external forces, and that subduction can play a key role in craton mobilization and thinning even when slabs are at great depths in the mantle.