• Introduction
• What happened in ancient plate tectonics of the Pacific Northwest?
• How did the Pacific Northwest come to be at a convergent plate boundary?
• What is the role of the San Andreas Fault?
• What is the plate tectonic situation today in the Pacific Northwest?
• Web Links
• Glossary Terms

Introduction

Plate tectonics are the engine that drives the geology of the Pacific Northwest. The region's high risk of major earthquakes and volcanic eruptions is because of plate tectonics. The Cascade Mountains, Puget Sound lowlands and Willamette River Valley, and the coast ranges including the Olympic Mountains are all direct results of plate tectonics.

For about 200 million years the Pacific Northwest region has been at an active plate boundary, specifically an ocean-continent convergent plate boundary with a subduction zone. Subduction has created most of the volcanoes and mountain ranges of the Northwest. Plate convergence has also accreted exotic terranes to North America, adding most of the area of Oregon and Washington.

Since about 30 million years ago, the Juan de Fuca Plate has been slipping beneath North America. It continues to subduct, moving a couple of inches each year into the mantle beneath us. This rate of plate convergence is fast enough, and powerful enough, to continue shaping the geology of the whole region. A complete understanding of the geology of the Pacific Northwest requires close examination of the role that plate tectonics plays, and has played, in developing this part of the continent.

What happened in ancient plate tectonics of the Pacific Northwest?

During the Archean eon, there are strong geological signs that plate convergence and terrane accretion occurred, adding crust to the North American craton and causing orogenies. The new continental crust that was formed and added to North America during that time is on display in Archean rocks in the cores of mountain ranges in Montana, Wyoming, and Colorado.

In the Proterozoic eon, during deposition of the Belt Supergroup, now seen in the mountains of Glacier National Park and as far west as northeastern Washington state, the area was relatively stable. There is no evidence of a plate boundary in the region at that time.

Late in the Proterozoic eon, the crust rifted (split) apart, and a large section of the crust containing Belt Supergroup rocks was removed from the continent. This rifting event and the existence of a divergent plate boundary during the late Proterozoic are suggested by major normal fault and graben structures of late Proterozoic age at the western edge of the Belt rocks, and the presence of late Proterozoic basalt layers, which are typically erupted in association with rifting.

After the rifting event, new ocean floor developed in what is now the Pacific Ocean basin, and what is now western North America became a passive continental margin, with no plate boundary nearby. A passive continental margin is marked by widespread layers of sediment that accumulate on a gently sloping continental shelf and land at the edge of the continent. Widespread layers of continental shelf and continental lowland sediments from the Paleozoic era are found through much of the Rocky Mountain and Basin and Range regions.

In a few places throughout western North America, the long-lived, widespread passive margin of the Paleozoic was interrupted by at least two short-lived mountain-building events, or orogenies. These orogenies seem to be associated with temporary subduction zones that developed along the margin of the continent approximately where the Nevada-California border region is now. These short-lived, localized orogenies occurred during the Devonian and Pennsylvanian periods. They added accreted terranes to the continent, and folded and thrust-faulted some of the rocks that were already there.

After the Pennsylvanian period, North America became part of the supercontinent Pangaea. The Pacific Northwest remained a passive margin at the edge of the giant continent. This was the situation during the Permian period and for most of the Triassic period.

How did the Pacific Northwest come to be at a convergent plate boundary?

Late in the Triassic period the Pangaean supercontinent began to rift and break into separate continents drifting apart from each other. The North American continent began separating from the other parts of the supercontinent and drifting to the west, into what is now the Pacific Ocean basin. An ocean-continent convergent plate boundary formed along what is now the North American west coast, as the North American continent began moving across the ocean basin. Subduction began in the Pacific Northwest, accompanied by the geological processes typical of a subduction zone--eruption of composite cones in a volcanic arc, accretion of terranes and formation of mountain ranges.

What is the role of the San Andreas Fault?

The San Andreas Fault extends from north end of Gulf of California (inside of Baja) in Mexico to Cape Mendocino on the coast of northern California. This long transform fault is located beyond the Pacific Northwest, but its inception and growth plays a prominent role in the region's geologic history.

Beginning in the mid-Tertiary period, the story of the San Andreas Fault is a story of the changing relationship between three tectonic plates - the Farallon Plate, the Pacific Plate, and the North American Plate. A zig-zag mid-ocean spreading ridge separated the oceanic Farallon and Pacific Plates. Both oceanic plates lay to the west of the continental North American Plate, with the Pacific Plate on the outboard side. Prior to the mid-Tertiary period, the California coast lay along a convergent plate boundary between the Farallon Plate and the North American Plate. The oceanic plate was subducting beneath the continental plate. The results of this subduction are still apparent in Central California in the coast ranges, the forearc basin (now the great Central Valley of California), and the high mountain range composed mostly of granite batholiths batholiths (the Sierra Nevada).

During the Oligocene epoch, a large eastward indentation of the boundary between the Farallon Plate and the Pacific Plate subducted beneath California. This event brought the North American plate into contact with the Pacific Plate and the San Andreas Fault was born. It also signaled the demise of the Farallon Plate, leaving behind two remnants-the Juan de Fuca Plate to the north and the Cocos Plate to the south.

The new relationship between the Pacific Plate and the North American Plate brought about the San Andreas Fault. Unlike the Farallon Plate, which moved northeast relative to North America, the Pacific Plate moved northwest relative to North America (and continues to do so today). The difference in relative motion changed the geometry on the new plate boundary, replacing the convergent plate boundary with a transform plate boundary.

As the corner of the boundary between the Farallon and Pacific plates subducted farther, the contact zone between the North American and Pacific plates expanded, replacing more and more of the subduction zone with a transform plate boundary. The San Andreas Fault grew.

The switch from subduction to transform plate motion in California has had widespread effects. The extension of crust in the Basin and Range region began at approximately the time the San Andreas Fault began forming, and is probably related to the change from compression to shear along the coast to the west.

In the coastal Pacific Northwest, the effects of California being sheared and dragged alongside the northwest-moving Pacific Plate are adding stress. Movement along the San Andreas fault to the south is thought to be causing a combination of compression and rotation of blocks of crust along the Pacific Northwest coast. The Juan de Fuca Plate continues to subduct beneath the coastal Pacific Northwest, compressing the crust.

What is the plate tectonic situation today in the Pacific Northwest?

The leading edge of the Cascadia Subduction Zone, where the Juan de Fuca Plate bends down beneath the edge of North America, extends from Cape Mendocino in northern California to the northern end of Vancouver Island in British Columbia. The presence of fracture zones on the Juan de Fuca Plate leads some geologists to divide it into three smaller plates. From north to south these smaller plates are the Explorer Plate, the Juan de Fuca Plate and the Gorda Plate. In this class when we discuss the Juan de Fuca Plate we are referring to all three of these plates combined.

The accretionary complex of the Cascadia Subduction Zone is the Coast Ranges landscape region. This includes the mountains of Vancouver Island, the Olympic Peninsula and the coast ranges of Oregon and northern California.

The forearc basin of the Cascadia Subduction Zone is the Puget-Willamette Lowland.

The volcanic arc of the Cascadia Subduction Zone is the Cascade Range from Mount Meager in British Columbia to Lassen Peak in northern California. The Cascadia Subduction Zone is responsible for the volcanic eruptions of the Cascade Range and the risk of major earthquakes in the coastal region of the Pacific Northwest.

In the backarc region of the Cascadia Subduction Zone, the area east of the Cascade Range, some of the folding and faulting of the crust is probably due to compression by the Cascadia Subduction process. The stress from the Pacific Plate moving along the San Andreas Fault also plays a major role in the extension and rifting of the Basin and Range and rotation, folding and faulting of the rest of the Pacific Northwest.

Web Links

Visit the following Web links for plate tectonic maps and information on the Cascade volcanic arc and information on how earthquakes in the Pacific Northwest are related to plate tectonics

The Cascades Volcano Observatory, part of the U.S. Geological Survey, has some basic information on plate tectonics and the Pacific Northwest at
http://vulcan.wr.usgs.gov/Glossary/PlateTectonics/framework.html

A colored illustration of the Cascadia Subduction Zone is on the Cascades Volcano Observatory at
http://vulcan.wr.usgs.gov/Glossary/PlateTectonics/Maps/map_plate_tectonics_cascades.html

Glossary terms that appear on this page: plate tectonics; ocean-continent convergent plate boundary; subduction zone; exotic terrane; craton; orogeny; divergent plate boundary; normal fault; graben; basalt; passive continental margin; accreted terranes; Pangaea; volcanic arc; mid-ocean spreading ridge; forearc basin; granite; transform plate boundary; compression; shear; stress; accretionary complex