Tape
here
Tape
here
Page 4
Subduction
zone
backsteps
Seattle
Spokane
Quesnellia Terrane
Part of the
Intermontane
Superterrane
Farallon
Plate
Cut
along
dotted
line
Portland
Sedimentary Basins of the Eocene, Oligocene,
and Miocene
Tape
here
Tape
here
Wrangellia
Insular
Superterrane
Recap: How did we 5. Docking of Okanogan
Highlands
assemble pre-Eocene
(~170-180 m.y.)
Washington?
Aka: Insular Superterrane
Page 5
C-7 & C5
Major Terrane Groupings
of North Cascades
Skagit Crystalline Core
High Grade Met.
Seattle
Spokane
NW Cascades and
San Juan Thrust System
Medium Grade Met.
Portland
6. Docking of the North Cascades
100 – 60 Ma
Melanges
Low Grade Met.
Mount Stuart batholith - Stitching
Pluton
Nanaimo Formation:
Overlap Sequence
54–42? Ma
90 Ma
Overlap sequences sit
atop both terranes and the
rocks they dock
against—therefore they
can provide a minimum
age for both rock bodies
So called Stitching plutons also
provide minimum ages for terrane
docking since they are in contact
with both the terrane and the body
of rock it docked with.
1
Plutons shown in
red—note Mount
Stuart stitching
pluton at lower
right
Pickett Range
Page 6
Seattle
Spokane
Portland
7. Eocene to Miocene Sandstones and
Volcanics
(55 to 20 m.y.)
2
Photo donated by Larry McMIllan
Eocene Palm fossils in the
Chuckanut Sandstone near
Bellingham show a warmer climate
prevailed ~50 Ma
Northcraft
Formation volcanic
rocks, early
evidence of
Cascades
Volcanism ~37Ma
http://www.ci.tenino.wa.us/TeninoQuarry1_small.jpg
Eocene McIntosh Formation:
Tenino Sandstone—quartz
sandstones, but no evidence
of Cascades volcanism yet!
Pe Ell volcanic rocks near Rock Creek
Skookumchuck
Formation: view to south
in Centralia Coal Mine
Metasequoia fossil
3
Coal seam in the
Centralia Coal Mine:
Carina Henderson
points to a tonstein,
decomposed volcanic
ash
Eocene bivalve
Bivalve from outcrop on
Gold St. in Centralia
<= Portunites triangulum Crab,
Eocene, Wahiakum Co., Washington
http://www.geo-tools.com/fossils.htm
Outcrop of Lincoln Creek Formation at Galvin
http://orerockon.com/For_sale.htm
E-1
8. Docking of Olympic
Peninsula
(25 to 30 m.y.)
Seattle
Olympic Mountains, Willapa
Hills & Oregon Coast Range
Spokane
Olympic Subduction
Zone
Siletzia
Terrane
Portland
Tape
here
Cut
alon
g
dotte
d
line
4
Geology Map
http://geology.wr.usgs.gov
/parks/olym/geolmap.html
http://www.ess.washington.edu/SEIS/PNSN/H
AZARDS/CASCADIA/turbidite_record.html
http://www.geol.umd.edu/~jmerck/geol100/images
/32/turbidite.gif
Hoh Formation turbidite deposits (looking down on overturned beds)
20 my old Hoh Formation turbidite deposits (overturned) at
Beach #4
Miocene rocks at Ruby Beach
5
Major Rock Units of the Olympics
E-4
1. Peripheral Rocks
2. Core Rocks
Separated by reverse faults
Metasediments from the core of the Olympics at Ruby Beach
Peripheral Rocks:
E-4
Crescent Formation
<= from
http://www.kaleberg.com/images/goat/goat
big1.jpg
Mostly the Crescent Fm
Age:
Mostly Eocene (58–45 Myrs)
Rocks
Pillow Basalts
Some Vesicular Basalts
15-30 km thick
Limestone(Red)
w/deep water forams
Basalt of the Crescent Formation on Klahhane Ridge above Port
Angeles; from http://geology.wr.usgs.gov/parks/olym/olym5.html
E-4
Core Rocks
Core Rocks
Age:
Eocene-Miocene (40-20 Myrs)
Metamorphosed 14-12 Myrs
Rocks
Sandstones
w/graded beds
Shale
Metamorphism
East - Low Grade
(11 km burial)
West - Not Met.
Youngest
Oldest
Fig. 4. Thick beds of sandstone in the
Valhallas and geologists at work. (from:
http://geology.wr.usgs.gov/parks/olym/o
lym2.html)
Fig. 7. Sandstone beds with
thin shale interbeds on
Windfall Peak.
6
Olympic Structures
Core Rocks
E-3
Interpretation: Peripheral Rocks
E-4
Terrane (fault bounded)
Hot Spot Island (Yellowstone?)
Peripheral Rocks
Note faults place older rock over younger rock
Basalt Chemistry = Hot Spot & Divergent Zone
E-1
Modern Analogy for Crescent
basalts - Iceland
Possible Eocene Reconstruction
35 Myrs
55 Myrs
http://www.calstatela.edu/dept/geol
ogy/HotSpots.htm
http://www.answers.com/topic/geo
graphy-of-iceland
D-2
Plate Reconstruction: 55 Myrs
Supporting Evidence:
Rock Age
E-3
7
E-4
Interpretation: Core Rocks
Olympic Accretionary Wedge
Accretionary Wedge
Trench Sediments
E-4
But Why is the North “Wedge”
(Olympics) so much larger than the
South “Wedge” (Willapa Hills/Oregon
Coast Range)?
Olympic
History
Western Core rocks
accreted first &
were buried deepest
(explains why they
are oldest and most
metamorphosed)
Insular ST
1. Shoved in the Corner
Olympics
Coast
Range
But Why is the North “Wedge”
(Olympics) so much larger than the
South “Wedge” (Willapa Hills/Oregon
Coast Range)?
Washington
Oregon
Evidence for the Bend
1. NW Volcanoes
Steep Subduction
Shallow Subduction
100 Depth
Olympics
2. There is a bend in the
Juan de Fuca Plate
under the Olympics
Cross Section
North ------ South
N. America
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Juan de Fuca
8
Volcanoes, faults and EQ outline big blocks
Why is there a Bend at all?
Basin & Range Extention to the South
OC
SN
From Wells et al., 1998, after Magill
et al., 1981, 1982; Guffanti and
Weaver, 1988; Sherrod and Smith,
1990
Big blocks are rotating - north-seeking magnetic
directions captured when Coast Range lavas
cooled now point eastward.
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..
v
N
(pie-shaped wedges are uncertainties
about mean rotation from expected
north seeking direction; modified
from Gromme et al., 1986)
v
Coast Range basalt - similar to Hawaiian Islands - was
52 millionmillion-yearyear-old submarine
accreted
to the continent
about 50 million years ago;
pillow basalt, Trask River
then rotated
clockwise.
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
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
9
GPS velocities 1995-2005
Velocities rotate
clockwise (block
rotation)
• decrease away from
coast (compression
above SZ)
– Blue are campaign
sites, red are
continuous sites
(PBO)
– Many institutions:
RPI, UW, GSC,
USGS, OSU, NGS,
PANGA, PBO
•
McCaffrey, Qamar, King,
Wells, et al, s GJI, 2007
10