• Introduction
• Evidence for exotic terranes in the Pacific Northwest
  • Ophiolites
  • Fossils
  • Paleomagnetism
  • Isotopes
• What are the major terranes of the Pacific Northwest, where are they, and when did they arrive?
  • The Kootenay Arc--Not So Exotic
  • The Intermontane Superterrane-Quesnellia and Associates
    • The Omineca Belt
  • Terranes of the Blue-Wallowa Mountains Region
  • Methow Terrane
  • Terranes of the Western North Cascades-San Juan Islands
  • The Insular Superterrane
    • The North Cascades Crystalline Core
  • Terranes of the Coast Ranges
• Glossary Terms

Introduction

The process of terrane accretion has profoundly marked the geologic history of the Pacific Northwest. Much of the real estate of Washington and Oregon consists of rocks that formed someplace else and were added to the continent by accretion.

For 200 million years, since the beginning of the Jurassic period, the Farallon Plate, an oceanic plate the width of today's Pacific Ocean, subducted beneath western North America. For a while in the early Tertiary period, the Farallon Plate was accompanied by another tectonic plate, the Kula Plate. Rather than undergoing straightforward subduction, the Kula plate approached the Pacific Northwest coast at a low angle. The low-angle approach of the Kula Plate may have caused some terranes to rotate and slide northward along the west coast of North America. Late in the Tertiary period the Farallon Plate became two smaller plates: the Cocos Plate, suducting beneath Central America, and the Juan de Fuca Plate, suducting beneath the Pacific Northwest. As these plates have subducted, one terrane after another has been added to the edge of the continent.

Evidence for exotic terranes in the Pacific Northwest

(Note: refer to the Basics page on Exotic Terranes for a review of how the following types of evidence indicate exotic terranes.)

Ophiolites

Ophiolites are stratified sequences of rock that include, in their complete form, from the top down:

• oceanic sediments
• pillow basalt
• sheeted dikes (dikes intruding dikes)
• gabbro
• layered gabbro
• ultramafic rock such as peridotite, dunite, or serpentinite

The oceanic sediments may include deep-ocean layers of chert or shale, shallower-ocean layers of limestone, or possibly layers of sandstone that were deposited off a continental convergent margin.

The ultramafic rocks are inferred to come from the upper mantle. Serpentinite is what olivine-rich rocks such as peridotite or dunite metamorphose into in the presence of moderate heat and lots of hot water in the Earth.

For a long time the origin of ophiolites was something of a mystery to geologists, because although they clearly had oceanic affinities, they were commonly found in mountain ranges such as the Alps in Europe or the Coast Range in northern California.

With the establishment of plate tectonics theory in the 1960s, an explanation of ophiolites was developed.

Ophiolites are likely to have been formed at divergent plate boundaries where upwelling mantle rock melts to produce bodies of magma beneath the oceanic crust. The magma settles out crystals at the base of the magma chambers, forming layered gabbro as it slowly cools. The top of the magma body forces molten rock through cracks in the spreading-apart crust up above, forming sheeting dikes. On the floor of the ocean the magma erupts from fissures and quenches in blobs on the ocean floor, forming pillow basalts. Divergent plate boundaries are like factories which keep producing these rocks, which spread away from the mid-ocean ridge as if on conveyor belts. As the newly-formed oceanic lithosphere moves away from the spreading center and ages, oceanic sediments accumulate on top of it.

Ophiolites in mountain ranges are thought to have been acrreted to the edges of continents along subduction zones, with continued accretion and mountain building eventually making the accreted ophiolites parts of orogenic belts -- major mountain ranges -- at convergent plate margins.

Fossils

Several terranes in the Pacific Northwest have fossils of fusulinids, a type of primitive, single-celled animal that floated in ocean water. Fusulinids have intricately shaped shells that make different species distinguishable under a microscope. They flourished in tropical oceans during the late Paleozoic era, Mississippian through Permian periods. The fusulinid fossils in terranes of the Northwest are called tethyan fusulinids because they are types that existed in the large sea known as the Tethys Sea on the east side of the supercontinent Pangaea. The tethyan fusulinid fossils suggest that the terranes came most of the way across the Pacific Ocean basin, from tropical latitudes, and were then accreted to North America.

Paleomagnetism

Paleomagnetic measurements in terranes of the North Cascades, British Columbia Coast Range, and San Juan Islands indicate that they formed far to the south, perhaps as much as 1,000 miles to the south. The paleomagnetism also indicates that the terranes have been rotated. The rotation is clockwise, as if the terranes rolled like ball bearings or disks between the North American continent and other terranes moving northward farther out along the coast, along strike-slip faults.

Isotopes

Measurements of strontium-87 have allowed geologists to locate where the edge of the old North American continent makes a transition to accreted terranes. The boundary runs very roughly through northeastern Washington and then approximately along the border of Washington and Idaho and the border of Oregon and Idaho. The strontium-87 line indicates that if you live in Missoula, Montana, or Coeur d'Alene, Idaho, then deep beneath you the crust has been part of the North American continent since Precambrian time. However, if you live in Oregon, or in Washington west of Spokane, then the crust beneath you was added to North America in the last 200 million years or less.

What are the major terranes of the Pacific Northwest, where are they, and when did they arrive?

The Kootenay Arc--Not So Exotic
(accreted early Jurassic)

By early Jurassic Time, approximately 200 million years ago, subduction had begun to occur along the Cordilleran margin, the margin of what is now western North America. Some evidence suggests that subduction along the coast may have started during late Triassic time, but the evidence is conclusive that it was ongoing by the early Jurassic. In the Pacific Northwest, the first clear evidence of this appears in the Kootenay Arc, which runs for 250 miles from northwest of Spokane in Washington north into interior British Columbia.

The Kootenay Arc is a zone where sedimentary rocks from the continental shelf of the old continent were shoved up onto the edge of the continent, sheared and faulted, and intruded by granites of the type that are common beneath volcanic arcs related to subduction zones. The sedimentary rocks that were subsequently tilted, faulted, sheared, and intruded in the Kootenay Arc range in age from Proterozoic to Triassic. These rocks show some similarities to rocks that formed on the continent during that time, except the Kootenay Arc rocks formed in deeper water farther out from shore.

The sedimentary rocks of the Kootenay Arc do not qualify as an exotic terrane because they share some geologic history with the native rocks of the continent to the east. However, the Kootenay rocks have been shoved eastward, from where they formed offshore, onto the continental margin by convergent plate processes. The Kootenay Arc provides the first clear evidence of convergent plate boundary processes in the Jurassic, so it is mentioned here along with the truly exotic terranes that followed.

The southern end of the Kootenay Arc disappears under the much younger Columbia River basalts. Far to the south, on the east side of the Sierra Nevada Mountains, in California, there are similar rocks with similar faults, shears, and intrusions. These rocks support the idea that most of the entire western edge of the continent switched from a passive margin to a convergent plate boundary after North America rifted from the rest of Pangaea and started drifting to the west late in the Triassic period.

During the late Triassic, the oceanic plate to the west of North America subducted beneath the edge of the continent. Farther out to sea lay a variety of terranes, mainly in the form of island arcs--some active, some eroded down to plateaus with extensive reefs. These island arcs, oceanic plateaus, coral reefs, and the thick piles of sediment eroded from the islands, would eventually accrete to North America.

The Intermontane Superterrane-Quesnellia and Associates
(accreted middle to late Jurassic)

After the Kootenay Arc, the next group of accreted terranes to dock with North America is referred to as the Intermontane superterrane. This "superterrane" is truly exotic and encompass a large swath of terranes, which had apparently already accreted with each other at a subduction zone farther out in the ocean basin, and then accreted against North America as a group.

The Intermontane superterrane includes the following specific terranes, named after locations in British Columbia: Stikinia, Cache Creek, Slide Mountain, and Quesnellia, which is the largest?. All of these terranes originated as island arcs that erupted volcanic rocks and were intruded by granitic rocks. Thick sequences of sediments built up next to the volcanic islands as they eroded. Reefs developed in the shallow waters around the islands. Some reefs developed on top of the plateaus that formed where older islands eroded below sea level. Tethyan fusulinids, distinctive fossils from the Tethys Sea region near what is now southeastern Asia, have been found in some Intermontane terranes. Along with the coral reefs that require warm tropical oceans to form, the tethyan fusulinids show how far the Intermontane terranes traveled before accreting with North America.

Rocks in the Intermontane superterrane were forming up to middle Jurassic time as they approached North America. Accretion of the superterrane with North America occurred right afterward, in middle to late Jurassic time, starting perhaps 175 million years ago. The Intermontane terranes are exposed in the Pacific Northwest in the Okanogan Highlands of Washington, west of the Kootenay Arc. North of the Okanogan Highlands exposure, the Intermontane terranes make up thousands of square miles in a large plateau region in British Columbia. They extend all the way to southeastern Alaska.

The Omineca Belt
[formed middle to late Jurassic]

The subduction process that brought in the Intermontane terranes, and the accretion of the terranes themselves, caused the edge of the continent to the east to undergo extensive folding, faulting, metamorphism, and widespread intrusion by granites. This created a broad zone that consists largely of granite and gneiss. In British Columbia this zone is called the Omineca belt, and has been referred to as a "welt" produced by the suturing of the Intermontane terranes with the continental margin. The Omineca belt extends south into Washington in the vicinity of Spokane and into the northern Idaho panhandle northeast of Spokane.

Terranes of the Blue-Wallowa Mountains Region
(accreted middle Cretaceous)

Several accreted terranes make up the eastern Blue Mountains and the Wallowa Mountains of northeastern Oregon and some of them extend across the Snake River into the Seven Devils Mountains of Idaho. The names given these terranes have changed several times in the last 20 years, as geologists try to sort out the history of each terrane and identify the faults that separate them. Igneous intrusions have obscured some of the geological relations, adding to the challenge of sorting out the terrane puzzle.

Names recently applied to these terranes include Wallowa, Baker, and Eastern Arc terranes. The Eastern Arc terrane includes terranes that have been called the Olds Ferry and Izee terranes. The northeastern Wallowa terrane includes what was formerly called the Seven Devils terrane.

All of these terranes formed as island arcs with thick sequences of sediment eroded from the island arcs. Some of the sediments were deposited in oceanic trenches and some were deposited in shallow water. Some of the rocks are limestone that formed from coral reefs in a marine environment, and some are rocks formed from swamp and floodplain deposits on land. There are also volcanic rocks and numerous diorite, granodiorite, and granite bodies that intruded into the island arcs. One or two ophiolite sequences are associated with the terranes, and are seen in the mountains south of Baker in eastern Oregon.

The terranes accreted in middle Cretaceous time, approximately 100 million years ago. Plutons of 90-100 million-year age intruded several of the terranes simultaneously, stitching them together and showing that they had accreted with each other by then.

Part of the Wallowa terrane shares the same rock types and geologic history as a terrane in western British Columbia and southeastern Alaska called the Wrangellia terrane. Many geologists consider them to be the same terrane, which was accreted at two locations along the old coast of the continent.

Methow Terrane
(accreted by end of middle Cretaceous)

A sequence of rocks that form the Methow Terrane is located in the Methow Valley region of north central Washington and the Tyaughton region across the border in British Columbia. The basement of the Methow terrane is oceanic and island arc rock that may go back to Triassic age. On top of the Triassic-Jurassic basement is a series of sedimentary layers of early to middle Cretaceous age. This sedimentary sequence in the Methow formed in an ocean basin west of the margin of North America and east of an island arc that was offshore of North America. By middle Cretaceous time the sediments were being deposited on the deltas and floodplains of rivers on the island arc that were emptying into the ocean basin and had partly filled it. By the end of the middle Cretaceous, between 100 and 80 million years ago, the Methow Terrane had been thrust-faulted and accreted onto the edge of North America

Terranes of the Western North Cascades-San Juan Islands
(accreted middle Cretaceous)

Many terranes have been stacked and juxtaposed along thrust faults in the western North Cascade Mountains and San Juan Islands of Washington. As with the other accreted terranes discussed so far, most of these terranes originated as island arcs and sections of oceanic crust.

However, there is an unusual terrane located between Wenatchee and the towns of Entiat and Orondo in north central Washington. It is called the Swakane Biotite Gneiss and contains minerals with a Proterozoic radiometric age. It seems to be an old piece of a continent that got caught up in the terrane shuffle and added to what is now Washington.

The North Cascades-San Juan Islands terranes also contain two ophiolites, the Ingalls ophiolite south and east of Mt. Stuart in the Alpine Lakes Wilderness, and the Fidalgo ophiolite west of Anacortes on the edge of the San Juan Islands.

The terranes in the North Cascades-San Juan Islands underwent highly variable amounts of deep burial, tectonic stress, heat, and regional metamorphism. The rocks in the San Juan Islands were slightly heated and subjected to moderate pressure, resulting in low-grade metamorphism that hardly changed the appearance of many of the rocks and preserved many of the fossils. The Easton Terrane in the western North Cascades contains a mixture of metamorphic rock that includes blueschist. Blueschist is rock that forms from ocean-floor basalt that goes part way down a subduction zone and is metamorphosed at unusually high pressure and low temperature. This rock is the Shuksan blueschist, which makes up most of Mt. Shuksan near Mt. Baker.

The Western North Cascades-San Juan Islands terranes all appear to have accreted to North America between 100 and 90 million years ago.

Some terranes of the western North Cascades are discussed on the following Web page:
http://www.nature.nps.gov/geology/usgsnps/noca/nocageol5.html

The Insular Superterrane
(accreted middle Cretaceous)

The Insular superterrane, which lies northwest of the Western North Cascades-San Juan Islands and stretches from Vancouver Island north to Alaska, probably accreted to North America in the middle Cretaceous, about 90 million years ago. Wrangellia is the most well-know terrane in the Insular group. Wrangellia formed as a sequence of island arcs and oceanic plateaus with coral reefs, starting late in the Paleozoic era. .

The North Cascades Crystalline Core
[formed in the middle Cretaceous]

Like the Intermontane superterrane, the terranes of the Insular superterrane collided and accreted with each other offshore and then accreted with the coast of North America as a group. Also like the Intermontane superterrane, the accretion of the Insular superterrane is associated with widespread regional metamorphism and igneous intrusion of the pre-existing rocks east of the accretion zone, along the margin of the continent. This created granitic batholiths and large areas of gneiss and schist that formed at high temperatures and pressures within the crust.

An example of this zone of middle Cretaceous granitic batholiths and widespread gneiss and schist is seen in the crystalline core of the North Cascades in Washington. Rocks of the crystalline core are visible along State Route 20, the North Cascades Highway, and U.S. 2, the Stevens Pass Highway. Similar rocks-batholiths of diorite, granodiorite, and granite interwoven with areas of gneiss and schist-compose much of the British Columbia Coast Range, which extends over a thousand miles north into southeast Alaska. It is one of the largest regions of granitic rock and gneiss in the world.

Terranes of the Coast Ranges
(accreted Tertiary to present)

Since the Insular superterrane was added, subduction has continued and yet more terranes have arrived and accreted to the Pacific Northwest. These terranes, which docked with North America during the Tertiary period, make up much of the geology of the Coast Ranges.

However, the Coast Ranges, which include the Olympic Mountains, Black Hills and Willapa Hills in Washington and the Coast Ranges of Oregon, consist of more than exotic terranes. Along with accreted terranes, the Coast Ranges contain layers of sedimentary rock that formed along the edge of the continent, about where they are now. These native sedimentary formations have been tilted and folded by the accretion process. Volcanic eruptions and igneous intrusions in the Oregon Coast Ranges have also occurred since the terranes accreted to North America.

The largest accreted terrane in the Coast Ranges is the Crescent Terrane. It is mostly a thick sequence of basalt and gabbro. Much of the basalt is in the form of pillows, created by erupting onto the ocean floor. However, the Crescent basalts make a much thicker pile than normal oceanic crust, and in some places it is clear that these basalts built up into ocean islands. This island build up may have been due to hot spot activity, similar to the source of the Hawaiian Islands. The age of the Crescent basalts (Paleocene-Eocene) and reconstructions of the rates of plate motion suggest that the hot spot that now underlies Yellowstone National Park may be responsible. for creating the large volume of basalt. Before the North American continent moved westward across the Yellowstone hot spot, it would have been located about where the large volumes of Crescent basalts erupted.

It is also considered likely, based on paleomagnetic evidence, that the Kula plate was off the Pacific Northwest coast during Paleocene and Eocene time, and may have been sliding north along the coast at a transform plate boundary. If so, there would have been a junction between a mid-ocean spreading ridge and a transform plate boundary, which could produce higher than normal amounts of seafloor eruptions. It is possible that both a hot spot and a ridge-transform plate boundary intersection were off the Northwest coast at the time.

Within a few million years of erupting onto the seafloor, the Crescent terrane thrust beneath the edge of North America and accreted, scraping off the rocks that were added to the continent. The rocks of the Crescent terrane are seen as far north as southern Vancouver Island and extend as far south as the southern Oregon Coast Ranges. They are particularly prominent in the Olympic Mountains, where they wrap around the north and east sides of the mountain range. Most of the high peaks of the Olympics that are visible from Seattle, Tacoma, and Everett consist of the Crescent terrane.

The central and western Olympic Mountains contain other accreted terranes. Unlike the basalt-rich Crescent terrane, these other terranes consist mainly of sedimentary rock from the ocean floor. The rocks in these terranes probably formed relatively close to the coast before being shoved into the continent and accreted. The fossils in the rocks indicate their close proximity. Also, the rocks are not much older than when they accreted, giving them little time, geologically speaking, to ride very far on the moving oceanic plate.

Ocean-floor sediments continue to be faulted and uplifted into the leading edge of the North American continent. There are no large terranes or superterranes docking with the continent these days, but slivers of oceanic crust continue to accrete.

Glossary terms that appear on this page: subduction; accreted terrane; Tethys Sea; Pangaea; strike-slip fault; sedimentary rock; granite; exotic terrane; intrusion; passive margin; convergent plate boundary; island arc; gneiss; limestone; diorite; granodiorite; ophiolite; thrust fault; regional metamorphism; metamorphic rock; blueschist; basalt; batholith; schist; gabbro; oceanic crust; hot spot; paleomagnetism; transform plate boundary