Seismic full-waveform tomography of active cratonic thinning beneath North America consistent with slab-induced dripping

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 in the geological past. The mechanisms for thinning have been difficult to distinguish for these past events. Here we present a full-waveform seismic tomographic model for North America that allows the resolution of fine-scale structures and reveals an extensive craton-thinning event that is ongoing. The seismic images show extensive drip-like features that suggest the transport of lithospheric materials from the base of the craton beneath the central United States to the mantle transition zone, and thus active lithospheric thinning. Geodynamic modelling suggests that the dripping may be mobilized by large-scale mantle flow induced by the sinking of the Farallon slab that is currently in the lower mantle. Dripping lithosphere could be further facilitated by prior lithospheric weakening, for example by volatiles released from the slab. Our seismological observations of active extensive thinning of cratonic lithosphere support lithospheric removal could be a result of external mantle processes, which we hypothesize may include the deep mantle effects of subduction. Drip-like features imaged in a seismic tomography model are consistent with extensive ongoing thinning of cratonic lithosphere beneath North America.