Eocene-Mid Miocene sedimentary rocks and Olympic Mountains

Why is there a Bend at all?
Basin & Range Extention to the South
Big blocks are rotating - north-seeking magnetic
directions captured when Coast Range lavas
cooled now point eastward.
N
v
Coast Range basalt - similar to Hawaiian Islands - was
52 million-year million year-old old submarine
accreted to the continent about 50 million years ago;
pillow basalt, Trask River
then rotated clockwise.
v
Northwest Block
model for long-term
motion
• Sierra Nevada dragged
northward by Pacific
Plate
• Oregon (pink) pivots
clockwise to get out of
the way
• Washington (green) gets
squeezed against Canadahence
all the earthquakes
and faults.
• SE Oregon and Nevada
Stretch to fill in the gap
behind rotating block
Volcanoes, faults and EQ outline big blocks
OC
SN
From Wells et al., 1998, after Magill
et al., 1981, 1982; Guffanti and
Weaver, 1988; Sherrod and Smith,
1990
W. Oregon has been
rotating clockwise for
millions of years.
• Rotation rate of 1.2° per Million
years for at least 50 million years,
based on paleomagnetic study.
• Rotations are less to N, S, and E..
(pie-shaped wedges are uncertainties
about mean rotation from expected
north seeking direction; modified
from Gromme et al., 1986)
PBO GPS receivers installed in PNW (green dots) -
what are they going to measure?
• Plate and block
motions
• Deformation of crust
(its change in shape)
along plate
boundaries -
especially above the
Cascadia megathrust