Geology Reference
In-Depth Information
Block diagram across the Coast Range, continental
shelf, and slope at Siletz Bay
has been built up by multiple turbidity currents of mud,
clay, and sand from the Astoria and Willapa canyons.
Pressure along the subduction zone produces
a series of oblique faults and broad folds which contin-
ue from the lower to upper continental slope. Miocene
and younger oceanic basalts as well as the veneer of
upper Miocene, Pleistocene, and Holocene clays, silts,
and sands here are deformed into a wedge or an
accretionary melange. Sediments of the descending
Juan de Fuca plate have been scraped off to pile up on
the leading edge of the North American plate.
Across the abyssal plain to the west, the major
boundaries between the subducting plates or tectonic
blocks are the Juan de Fuca and Gorda spreading
ridges and the Blanco and Mendocino fracture zones.
In harmony with the Coast Range block, this entire
network of spreading ridges and fracture zones shows
evidence of having been rotated as much as 20 degrees
clockwise in the past 10 million years, since late
