Observations of active cratonic thinning beneath North America consistent with Farallon slab-induced dripping

Continental cratons are characterized by thick lithospheric roots that remain intact for billions of years. However, in the geological past, some cratonic roots appear to be 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 uniquely illuminates an 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 is possibly 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 may descend within the ambient downward mantle flow, even though the slab is presently in the lower mantle. Dripping lithosphere could also be facilitated by prior lithospheric weakening such as due to volatiles released from the slab. Our findings show how cratonic lithosphere could be altered by external forces, and that subduction may play a key role in craton mobilization and thinning even when slabs are at great depths in the mantle.