Geology Reference
In-Depth Information
A gravity map of Nevada and southern Oregon
clearly displays a twinned image east and west of
the 116 degree meridian which supports a spread-
ing or stretching hypothesis of the Great Basin.
Smaller map for comparison is a mirror-image of
the eastern half of the area (after Eaton, 1984).
southeastern Oregon border. Here metamorphic rocks,
which include phyllites and greenstones as well as some
granitic intrusions, are tentatively dated as Jurassic and
older. Just over the border in Nevada identical rocks
have been dated with some confidence as late Triassic
and Paleozoic suggesting that the foundation of the
Oregon Basin and Range could be part of a composite
Paleozoic accreted terrane block that considerably
predates terranes in the Klamaths and Blue Mountains.
and up to very young volcanics at Yellowstone in
northwest Wyoming. The rapid eastward movement of
the volcanic centers, which is twice as fast as that
across Oregon, is due to a combined effect of plate
movement and extension of the Basin and Range. The
hot spot presently beneath Yellowstone may have
generated a large volume of Eocene basalt as it passed
beneath the area of the Oregon Coast Range.
Volcanic "hot spots" are known from dozens of
sites around the globe. Recognition of these volcanic
centers is a simple matter of tracing a chain of volca-
noes back to its source or youngest vent. As a conti-
nental or oceanic plate moves across such a hot spot,
intermittent volcanic activity leaves a plume of volca-
noes as is the case with the Hawaiian Islands. The
more speculative geologists have suggested that these
hot spots may represent locally weak places in the
earth's upper mantle where incoming meteorites
punched through millions of years in the past.
Cenozoic
Early Cenozoic rocks are apparently missing,
but the Miocene, Pliocene, and Pleistocene epochs are
represented in the Basin and Range by basalts and tuffs
from the extensive and fairly continuous volcanic
activity. Coinciding with the initial stretching and
extension of the Basin and Range 16 million years ago,
the most productive volcanic event in the province was
the eruption that centered in Steens Mountain when
fluid basaltic lavas spread across southeast Oregon
from a large, low profile shield volcano. The uneven
topography of the surface covered by the liquid flows
produced wide variations in thickness of the layers.
Because of glacial erosion, faulting, and younger lava
flows, the maximum thickness of the basalt can not be
determined. However, the flows average 3,000 feet
thick and cover 6,000 square miles for a total volume
of over 3,000 cubic miles. Although the volume of the
Steens basalt is less than 10% of that of the Columbia
plateau lavas, Steens lavas are remarkable in that they
Mesozoic
Because of the two-mile thick mantle of late
Tertiary volcanics and sediments, exposures of Mesozic
and older rocks are very limited in the Basin and
Range. It is necessary to step over into Nevada and
compare rocks from there with those in Oregon in
order to reach conclusions about the preCenozoic
history of this province. Older Mesozoic rocks occur in
the Pueblo and Trout Creek mountains on the extreme
