Origin of Columbia River flood basalt controlled by propagating rupture of the Farallon slab

A model of subduction that reveals a long tear under Oregon and Nevada provides a new mechanism for the origin of Columbia River flood basalt, resolving previous hypotheses. Origins of Columbia River flood basalts A flood basalt is the product of a volcanic eruption that covers large areas with basalt lava. The origin of the Steens–Columbia River flood basalts, thought to be associated with the onset of volcanism within the Yellowstone hotspot, has remained controversial. Using geodynamic modelling, Lijun Liu and Dave Stegman show that an episode of slab tearing about 17 million years ago under eastern Oregon quickly ruptured north and south, covering a distance of about 900 kilometres along eastern Oregon and northern Nevada. This is consistent both in space and time with the Steens–Columbia River–Northern Nevada Rift flood basalt event. The model also predicts the sequence of flood basalt composition, which should help to reconcile controversies regarding the origin of these flood basalts and involvement of the putative Yellowstone plume. The origin of the Steens–Columbia River (SCR) flood basalts, which is presumed to be the onset of Yellowstone volcanism, has remained controversial, with the proposed conceptual models involving either a mantle plume 1 , 2 , 3 , 4 , 5 or back-arc processes 6 , 7 , 8 . Recent tomographic inversions based on the USArray data reveal unprecedented detail of upper-mantle structures of the western USA 9 and tightly constrain geodynamic models simulating Farallon subduction, which has been proposed to influence the Yellowstone volcanism 5 , 6 . Here we show that the best-fitting geodynamic model 10 depicts an episode of slab tearing about 17 million years ago under eastern Oregon, where an associated sub-slab asthenospheric upwelling thermally erodes the Farallon slab, leading to formation of a slab gap at shallow depth. Driven by a gradient of dynamic pressure, the tear ruptured quickly north and south and within about two million years covering a distance of around 900 kilometres along all of eastern Oregon and northern Nevada. This tear would be consistent with the occurrence of major volcanic dikes during the SCR–Northern Nevada Rift flood basalt event both in space and time. The model predicts a petrogenetic sequence for the flood basalt with sources of melt starting from the base of the slab, at first remelting oceanic lithosphere and then evolving upwards, ending with remelting of oceanic crust. Such a progression helps