Owyhee Plateau-Snake River Plain-Yellowstone Plateau

by Dr. Paul F. Ciesielski, University of Florida

Visit:

• Yellowstone National Park

• Yellowstone Hotspot

I. WHAT AND WHERE ARE THEY?

• The Owyhee Plateau (OP) is a volcanic plateau centered on the joint boundaries of Oregon, Idaho, and Nevada. The OP lies to the southwest of the Snake River Plain.
• The Snake River Plain (SRP) is an arcuate volcanic plateau located in southern Idaho. The SRP cuts through extensional Basin and Range like structures of the northern Basin and Range.
• The Yellowstone Plateau is located at the northeastern end-point of the SRP, in the northwest corner of Wyoming.

II. CENOZOIC GEOLOGIC HISTORY

A. OWYHEE PLATEAU

• The Owyhee volcanic plateau was formed by rhyolitic volcanic events over a short period of time during the Miocene.
• Volcanism here occurred at approximately 16 Ma, about 1 m.y. after the start of the main phase of Columbia Plateau volcanism.

B. SNAKE RIVER PLAIN

• All volcanism occurred during the late Miocene-Pleistocene, decreasing in age to east.
• A linear late Cenozoic volcanic province characterized by thick successions of rhyolitic tuffs, intercalated alluvial and lacustrine sedimentary rocks and overlying basalt.
• Trend N56E, N. American plate vector S56W at 24mm/yr.
• Cause: migration of N.A. plate over mantle plume
• Extension and basin formation 125 km in advance of plume.
• Extension lasting 4 m.y.
• Partial filling of basins with siliciclastic sedimentary rocks and grading upward into volcanic clastic and/or ash-flow tuff with arrival of plume volcanism.
• Basalt injection after plume passage.
• Plume related to development of Northern Basin and Range.

C. YELLOWSTONE PLATEAU

• The Yellowstone is largely a huge, multiple-eruption caldera.
• The caldera is about 1500 square miles (40 by 30 miles)
• Three volcanic cycles are noted from study of the caldera rocks
1. cycle one occurred between 2.2 and 1.6 Ma (2500 km³ ejecta). Caldera collapse occurred about 2.0 m.y., after gases escape through magma chamber tapping ring fractures, causing major pyroclastic flows.
2. cycle two, short and peaked at about 1.3 Ma. Explosive eruptions and caldera collapse (again)
3. the final cycle of explosions began about 1.2 Ma and came to a climax at about about 630,000 y.a. (1000 km³ eject) forming the present caldera. Later viscous lava flows filled caldera floor.
• about 150,000 yr ago caldera began buldging up again.
• glacial modification
• the plateau has uplifted during the Holocene. High heat flow, frequent shallow earthquakes, hot shallow, low-density material. .. new cycle in the offering?

INTERESTING FACTS ABOUT THE YELLOWSTONE PLATEAU AND NATIONAL PARK

• Yellowstone heat flow 30x global average
• Yellowstone Plateau ~8000'
• most is a huge collapsed caldera
• Yellowstone Caldera is 1500 sq. miles
• 3780 cubic kms of ejecta (Mt. St. Helens=1.2 cubic km)
• 6000' of extrusives
• geysers (62% of all known),
• hot springs still active
• succession of 27 Eocene forests buried by volcanics on Specimen Ridge
• lowland, subtropical vegetation in Eocene!! (see GSA Bull, 1990, 18:1138-1141)
• oldest national park
  

III. HYPOTHESIS OF PLUME ORIGIN FOR COLUMBIA PLATEAU, SNAKE RIVER PLAIN, AND YELLOWSTONE PLATEAU

• A North American plate overriding the Yellowstone mantle plume has been suggested to be responsible for various northwestern volcanic provinces: Columbia Plateau, Owyhee Plateau, Snake River Plain, and Yellowstone Plateau.
1. the fundamental problem with such a hot spot being responsible for all these features is the discrepancies in their temporal and spacial distribution.
2. a model must account for earliest volcanism on the Columbia Plateau and proceding to the OP, SRP, and Yellowstone Plateau.
3. a model must also account for passage of the plume under the North American plate (NAP) without an incompatible change in plate motion or plume direction.
• Such a model (Geist and Richards, 1993) has been proposed which we will discuss and illustrate in class. Basics of the model are as follows:
1. During the late Eocene, the NAP moved over the Yellowstone mantle plume.
2. From the late Eocene to middle Miocene, the plume was shielded from the surface by the suducting Farallon plate and deflected to the NE beneath the plate.
3. By 17.5 Ma the plume broke through the Farallon plate, the rising stored head of the plume formed the CRP.
4. The plume then readjusted to a vertical profile. The recovery phase may have involved the OP and the western curved portion of the SRP.
5. Once readjusted the plume passed under the S56W moving NAP, forming the eastern SRP and Yellowstone Plateau.