Abstract:
The High Lava Plains province (HLP) is a late Cenozoic bimodal volcanic field at
the northern margin of the Basin and Range province in southeastern Oregon that hosts
a westward younging trend of silicic volcanism that crudely mirrors northeastward
migration of silicic volcanism along the Yellowstone–Snake River Plain (YSRP) trend.
We present ⁴⁰Ar/³⁹Ar ages for 19 rhyolite domes, 5 rhyolite ash flow tuffs, and
34 basaltic lavas from the HLP. The previously identified trend of westward migration
of HLP rhyolites is confirmed. The rate of propagation is ~33 km/m.y. from 10 to 5 Ma,
slowing to _13 km/m.y. after 5 Ma. The duration of silicic volcanism at any locus is
~2 m.y. Three older HLP dacite domes yielded ages of ~15.5 Ma. Basalts are not
age progressive. We identify several episodes of increased basaltic activity at 7.5–7.8,
5.3–5.9, and 2–3 Ma, with the younger episode likely continuing into the Recent. The
HLP and YSRP trends emerged from the axis of middle Miocene basaltic volcanism of
the Columbia River and Steens basalts. We propose a model in which (1) Miocene
flood basalts and widespread silicic rocks are the result of emplacement of a plume head
near the craton margin, enhanced by flow up a topographic gradient along the base of the
lithosphere at the craton margin; (2) the HLP trend is the result of westward flow
originating at the craton margin; and (3) the YSRP trend is the trace of the motion of the
North American plate over the tail of the plume.
