ABSTRACTThe 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.

The Willamette Valley owes much of its present day physiography to its association with the Cascadia Subduction Zone.

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.

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.

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.

Figure 1. Generalized geology and geographic features in the Willamette Valley.http://or.water.usgs.gov/imgs/Markup/gifs/willgeol.gif

Tectonic Framework of the Willamette Valley, OregonPrepared 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.

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.

During the Miocene period, slow uplift and tilting of the Coast Range tectonic block separated the Willamette Valley from the adjacent seaway.

Figure 3. Progradation of a delta. In ancestral Oregon, clastic sediments were deposited atop a volcanic platform in a similar fashion.

Figure 2. Cascadia Subduction Zone

Structural Geology of the Willamette Valley

Major Structural Features

1) Faulting:

Corvallis Fault

Eola Hills - Amity Hills Normal Faults

Owl Creek Fault

Mill Creek Fault

Waldo Hills Range-Front Fault

Gales Creek - Mount Angel Structural Zone

Yamhill - Sherwood Structural Zone

Beaverton Fault Zone

Elmonica Fault Zone

Helvetia Fault Zone

2) Folding:

Coast Range Anticlinorium

Willamette Valley Synclinorium

Harrisburg Anticline

Jefferson Anticline

Northern Willamette Downwarp

Tualatin Basin

Figure 4. Structure Map of the southern Willamette Valley

Yeats et al, 1996