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Oregon backarc region resulting from plume
uplift and heating of the Farallon slab, leading
to volatile release and melting of the mantle
wedge; (4) coeval Oligocene adakite volca-
nism during thermomechanical erosion of the
slab and melting of oceanic crust; (5) slab rup-
ture resulting in adiabatic rise and melting of
a plume source component found in Miocene
tholeiitic flood basalts of the Columbia River
Basalt Group and the Nevada–Columbia
Basin Magmatic Belt; (6) active mantle
upwelling beneath the Nevadaplano that
may have aided in plateau collapse and
Miocene initiation of Basin-and-Range exten-
sion; (7) Miocene-to-recent bimodal volca-
nism and rhyolite migration along the
Yellowstone–Snake River Plain hotspot
Figure 3. Volcanic rocks younger than 10 Ma superimposed above sublithospheric track to the ENE; and (8) westward sub-
low-velocity surface of Wagner et al. (2010). Yellow lines are isochrons for rhyolite lithospheric flow of plume-modified man-
volcanism along the Yellowstone–Snake River Plain (YSRP) and High Lava Plains
(HLP). Red triangles are Holocene volcanoes of the high Cascades; yellow triangles tle, resulting in Miocene-to-recent bimodal
are areas of Quaternary volcanism in the Cascadia backarc region largely corre- volcanism with rhyolite migration along the
sponding with seismic low-velocity channels of plume-modified mantle flow; chan-
nel axes approximated by gray arrows. Modified from Camp (2019). Oregon High Lava Plains to the WNW.

ACKNOWLEDGMENTS
(2004), Wells and McCaffrey (2013) calcu- could reduce some, but not all, of the misfit, We thank Joe Colgan for constructive comments
lated that the High Lava Plains progression and the time required for the Yellowstone on an earlier draft of the manuscript, two anony-
migrated westward due to mantle counter- hotspot to escape from beneath the slab cur- mous journal reviewers for their helpful comments,
and Mihai Ducea for editorial handling.
flow at the slab subduction rate until it hit the tain also could have contributed substan-
slab at 5 Ma, when it slowed to the rollback tially to the delay. Jordan et al. (2004) sug- REFERENCES CITED
rate, driven by forearc rotation. Camp (2019) gested an additional mechanism, where Anders, M.H., DiVenere, V.J., Hemming, S.R., and
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8 GSA Today | January 2021