Gondwanan Flood Basalts Linked Seismically to Plume-Induced Lithosphere Delamination

Abstract

Delamination of continental lithospheric mantle is now well-recorded beneath several continents. However, the fate of delaminated continental lithosphere has been rarely noted, unlike subducted slabs that are reasonably well imaged in the upper and mid mantle. In the western US, the combination of high resolution USArray-based seismic imaging experiments and the rapid westward motion of the North American Plate allows us to recognize delaminated lithospheric fragments 400 km beneath where the ~17 Ma Columbia River Flood Basalt event took place, and ~500-600 km beneath where the ~75 Ma Western Wyoming Craton delamination event occurred. Beneath former Gondwana, recent high resolution seismic tomographic models indicate the presence of at least 6 horizontal fast-wavespeed anomalies at ~600 km depths that do not appear to be related to slab subduction, including fast structures in locations consistent with delamination associated with the Paraná Flood Basalt event at ~134 Ma and the Deccan Traps event at ~66 Ma. These fast-wavespeed anomalies often lie above broad slow seismic wavespeed trunks at 500-700 km depths beneath former Gondwana, with the slow wavespeed anomalies branching around them. Numerical experiments indicate that delaminated subcontinental lithosphere should tend to stagnate in the mid-mantle above a mantle plume where it shapes subsequent plume upwelling. For hot plumes, the melt volume generated during plume-influenced delamination can easily reach magnitudes of ~2-4×106 km3, consistent with the basalt eruption volume at the Deccan Traps. This seismic and numerical evidence suggests that observed high wavespeed mid-mantle anomalies beneath the locations of former flood basalts are fragments of delaminated former continental lithosphere, and that lithospheric delamination events in the presence of subcontinental plumes induced several of the continental flood basalts associated with the multiple breakup stages of Gondwanaland. Continued upwelling in these plumes can also have entrained subcontinental lithosphere in the mid-mantle to bring its distinctive geochemical signal to the modern mid-ocean spreading centers that surround southern and western Africa.