Geology Reference
In-Depth Information
ago, the Farallon slab rose from a very steep angle to
a relatively shallow position transferring volcanic
activity in the Columbia Plateau and Basin and Range
from west to east. About 30 million years ago, during
the Oligocene, with decelerating convergence, the slab
began to steepen. Then at 17 million years ago the
subducting slab was detached and heat broke through
at the rupture to initiate backarc spreading and issue
as the Steens Mountain and Columbia River lavas. For
the past 5 million years extensional spreading forces
have intensified to create crustal thinning faults, and
volcanism in the Basin and Range.
The most striking aspect of the Basin and
Range volcanic activity is that eruptions of silica-rich
rhyolitic lavas are progressively younger in a line
running northwest across the province. In the eastern
margin of the area during the late Miocene, volcanic
activity migrated rapidly westward at a rate of 1 1/2
inches per year. However, the volcanic progression
slowed to approximately 1/2 inch per year in the
western section where volcanism is younger. While this
migration of volcanic centers resembles a "hot spot",
both the trend and direction are inconsistent with the
steady westward movement of the North American
Stages in the extensional stretching of the northern
Basin and Range as well as movement of the
North American plate westward over the Yellow-
stone hot spot. In this model the subducting slab
steepens sending the volcanic front back to the
west. Simultaneously the direction of extension in
the basin shifts clockwise from northeast-southwest
to a more southeast-northwest orientation (after
Zoback, Anderson, and Thompson,
1981).
plate during the last 10 million years of geologic time.
It seems probable that after the subducting Farallon
slab became detached, collision slowed considerably to
steepen the slab and drive a volcanic wave gradually
back to the west.
A similar pattern of volcanic progression
extends from Steens Mountain in the southwest all the
way across Idaho, through the Snake River downwarp,
