Abstract:
K-Ar and ⁴⁰Ar-³⁹Ar geochronologic
data reveal the Paleocene to Eocene eruptive
history of volcanic centers which produced the
basaltic basement rocks of the Coast Range of
Oregon and Washington. Volcanism was short
lived at each center and migrated with time from
the northern and southern ends toward the
center, near the present mouth of the Columbia River. A plot of crystallization ages against
geographic location produces a distinct V-shaped
pattern. An origin by hot spot volcanism centered
on or near a spreading ridge in early Tertiary
time best explains the observed age distribution.
Limited geochemical data support
this model, as the basalts exhibit attributes of both spreading ridge and hot spot-related volcanism.
Icelandic volcanism may be a present-day
analog. Absolute motion modeling shows that
the Yellowstone hot spot could have generated an
island chain with approximately the correct
paleoazimuth and volcanic propagation rate.
Shortly after formation, this oceanic volcanic
lineament collided with North America and rotated into its present marginal setting. The
western edge of North America subsequently
crossed over the hot spot, perhaps triggering
late Eocene to Oligocene volcanism in the Coast
Range, the Miocene Columbia River basalts in
eastern Washington and Oregon and most recently,
the basalts of the Snake River Plain and the
Yellowstone area.
