Download Tectonic Framework of the Willamette Valley (Josh Troyer)

ABSTRACT
The Cascade Volcanic Arc is the result of millions of years of tectonic
activity along the Cascadia Subduction Zone. The Cascadia Subduction Zone is
an oceanic-continental convergent plate margin where the Juan de Fuca plate is
being obliquely subducted beneath North America. Subduction and related
andesitic volcanism are roughly subdivided into five major episodes, beginning
about 35 Ma and continuing to the present. The evolution of the Cascade
Volcanic Arc has shaped the varied physiography of the present-day Pacific
Northwest.
The Willamette Valley is situated between the Cascade Range to the east
and the Coast Range to the west. The lowland is broadly interpreted as a
forearc basin associated with arc volcanism and accretionary uplift. Bedrock of
the Willamette Valley consists of oceanic basalts largely overlain by terrestrial
and marine sediments, related to episodic deltaic deposition. Volcanic rocks are
interspersed throughout the basin and the entire valley is overlain by Quaternary
surficial deposits.
Angular unconformities are common in the Willamette Valley owing to
regional tilting and localized faulting throughout the basin. Clockwise rotation of
fault blocks has resulted from a long history of oblique convergence and
accretionary tectonics.
Introduction: Physiography
The Willamette Valley extends
north from Eugene, OR roughly 120
km to approximately 30 km north of
Vancouver, WA. The width of the
basin ranges from about 30 km in
southern latitudes to about 60 km in
northern reaches.

The
Willamette Valley is situated
between two mountain ranges; the
Oregon Coast Range to the west
and the Cascade Range to the
east.
Willamette Valley owes much
of its present day physiography to
its association with the Cascadia
Subduction Zone.
Tectonic Framework of the Willamette Valley, Oregon
Prepared by:
Josh Troyer
Western Oregon University
The Tectonic Origin of the Willamette Valley
Near the end of the Cretaceous period (~66 Ma), a volcanic island arc formed
over a hot spot on the Kula and Juan de Fuca (Farallon) plates. As subduction
continued, this island arc collided with the North American plate and was
accreted to the continental margin. The basalts of the Siletz River Volcanics
and the Roseburg Formation make up the basement geology of the Oregon
Coast Range and the Willamette Valley.
After accretion, the subduction zone shifted westward and slow subsidence of
the volcanic block began.
Figure 1. Generalized geology and geographic features in the Willamette Valley.
Cascadia Subduction Zone
•Oceanic - Continental convergence along the Cascadia Subduction Zone has
been the primary driving force in the assemblage of the geology in the
Willamette Basin.
•Oceanic-Continental subduction of the Juan de Fuca plate under the North
American plate is responsible for Cascade volcanism and the accretion the
geology that comprises the Oregon Coast Range.
Figure 2. Cascadia Subduction Zone
 Major Structural Features
1) Faulting:
2) Folding:
 Corvallis Fault
 Coast Range Anticlinorium
 Eola Hills - Amity Hills
Normal Faults
 Willamette Valley Synclinorium
 Harrisburg Anticline
 Owl Creek Fault
As the newly accreted arc began to subside in a shallow marine environment,
sediment from the Klamath Mountains, and later the Idaho Batholith, fed into
large river systems that formed vast prograding deltaic complexes overtop the
basalt units. Such marine sedimentary units as the Tyee, Coaledo, Yamhill, and
Spencer formations comprise large portions of the Oregon Coast Range and
Willamette Valley.
 Mill Creek Fault
During the Miocene period, slow uplift and tilting of the Coast Range tectonic
block separated the Willamette Valley from the adjacent seaway.
 Yamhill - Sherwood
Structural Zone
 Jefferson Anticline
 Northern Willamette Downwarp
 Waldo Hills Range-Front
Fault
 Tualatin Basin
 Gales Creek - Mount
Angel Structural Zone
 Beaverton Fault Zone
 Elmonica Fault Zone
 Helvetia Fault Zone
Figure 3. Progradation of a delta. In
ancestral Oregon, clastic sediments
were deposited atop a volcanic
platform in a similar fashion.
The
http://or.water.usgs.gov/imgs/Markup/gifs/willgeol.gif
Structural Geology of the Willamette Valley
Figure 4. Structure Map
of the southern Willamette
Valley
The Cascade Volcanic Arc
 Cascade volcanism can be divided into five major episodes beginning
approximately 35 Ma and continuing through the present.
 Episodes
 Episode 1: 35 - 17 Ma
 Early Western Cascade Volcanism
 Characterized by local andesitic volcanos and voluminous
eruptions of tholeiitic lava and silicic pyroclastic rock.
 Episode 2: 16.9 - 7.5 Ma
 Late Western Cascade Volcanism
 Western Cascade volcanism slows from 18 -14 Ma.
 At 15.3 Ma, lavas of the Columbia River Basalt group flowed into
northwestern Oregon including the ancestral Willamette Valley.
 The Breitenbush anticline develops.
 Episode 3: 7.4 - 4.0 Ma
 Early High Cascade Volcanism
 Characterized by eruptions of basalt and basaltic andesite.
 Extensive uplift of the Western Cascades takes place during this
time period.
 The volcanic arc during this time period was most likely about 13
km wider than the modern High Cascade arc.
 Episode 4: 3.9 - 0.730 Ma
 Late High Cascade Volcanism
 Compositionally identical to episode 3 but the arc narrowed
dramatically during this time period.
 Episode 5: 0.730 - 0 Ma
 Continuation of High Cascade Volcanism
 The beginning of episode 5 is marked by the beginning of the
Brunhes normal magnetic polarity epoch.
Yeats et al, 1996
Conclusions
(1) The Willamette Valley is the direct product of arc volcanism and accretionary
tectonics associated with the Cascadia Subduction Zone.
(2) Changes in tectonic constraints led to five distinguishable episodes of
volcanism in the Cascade Range.
(3) The Oregon Coast Range consists of a volcanic arc platform overlain by
episodic deltaic deposits.
(4) The Willamette Valley’s proximity to an active subduction zone has resulted
in widespread structural deformation throughout the basin.
References Cited
Priest G.R., 1990, Volcanic and tectonic evolution of the Cascade Volcanic Arc,
central Oregon: Journal of Geophysical Research, v95 nB12, pp 19,583 19,599.
Yeats, R.S., Graven, E.P., Werner, K.S., Goldfinger, C., and Popowski, T., 1996,
Tectonics of the Willamette Valley,Oregon, in Rogers, A.M., et al., eds.,
Assessing earthquake hazards and reducing risk in the Pacific Northwest: U.S.
Geological Survey Professional Paper 1560, p. 183-222.