Geology Reference
In-Depth Information
Backarc spreading as a model
for the origin of the Columbia
River lavas
duced the northeast-southwest trending Blue Moun-
tains anticline or arch. Where the Columbia River
bisects the central plateau, a number of surface folds
run east-west, corresponding to north-south compres-
sion. The Columbia River follows one of these wrinkles
as it traverses the axis of a major downwarp, the 160-
mile long combined The Dalles-Umatilla basins from
Wallula Gap to The Dalles. Further east, the Blue
Mountains anticline turns north at Meacham, Oregon,
to merge and run parallel to the Klamath-Blue Moun-
tains lineament. In Washington, the Columbia Hills
and hills of Horse Heaven anticlines follow The
Dalles-Umatilla syncline to intersect the Hite fault just
south of Milton-Freewater.
Beneath the plateau, major structural features
or lineaments converge. These extensive features can be
seen in aerial photographs, but they are not obvious at
ground level, and what caused them is uncertain. The
Klamath-Blue Mountains lineament runs across the
state in a southwest by northeast direction, intersecting
the Olympic-Wallowa lineament near Wallula Gap on
the Columbia River, while the Olympic-Wallowa
lineament runs southeast by northwest from Puget
Sound across Washington through the Oregon Blue
Mountains to the Idaho border. Cutting directly across
the Olympic-Wallowa lineament, the Hite fault system
trends southwest by northeast.
With phase two between 10 and 4 million years
ago, the spreading direction of the Pacific plate rotated
25 degrees clockwise to a more southwest by northeast
direction. At this time intense compression produced a
series of east-west wrinkles in southcentral Washington
and in the vicinity of the Columbia River. Most of the
visible folds in plateau lavas along the Columbia,
including the Yakima fold belt, Horse Heaven anti-
cline, and Columbia Hills anticline, are related to this
