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Geology of the Pacific Northwest

Focus Page #7 -- Accreted Terranes of the Pacific Northwest

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 accreted terranes in the Pacific Northwest

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

Ophiolites

There are several ophiolites in the Pacific Northwest, including:

These ophiolites were all thrust onto or faulted against other accreted terranes, showing that the plate tectonic processes were powerful enough to move whole sections of oceanic lithosphere, including parts of the mantle, onto continents.

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.

Overlap formations and stitching plutons

As described in the Basics page on accreted terranes, stitching plutons and overlap formations allow us to estimate the geologic age when a terrane must have been accreted to the continent. For example, several of the terranes in the San Juan Islands are overlapped by the sedimentary strata of the Namaimo group. The Nanaimo group also overlaps rocks that were already part of North America.

An example of a stitching pluton is the Mount Stuart batholith, near the town of Leavenworth in the central Washington state Cascade Range. The Mount Stuart batholith is about 92 million years old, and intruded both the Ingalls ophiolite terrane and the adjacent Nason terrane, showing that those two terranes were already thrust-faulted next to each other by 92 Ma.

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

This section proceeds approximately from east to west, from the edge of the craton (old, thick, stabilized continent) of North America out to the Pacific Coast, approximately in the order that the terranes or groups of terranes accreted.

The list below of some of the more prominent terranes aids in seeing where a specific terrane fits into the larger picture. Keep in mind that the names of terranes change over the years as new evidence is discovered and new geologic maps are published.

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 edge of western North America. 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 state north into the interior of British Columbia in Canada.

The Kootenay Arc is a zone where layers of sedimentary rock from the continental shelf of the old North American continent were shoved up and inland on the margin of the continent. The layers of rock were thrust-faulted and folded by tectonic compression. The rocks were also intruded by granites of the type that are common beneath volcanic arcs related to subduction zones. The sedimentary rocks that were tilted, faulted, folded, and intruded in the Kootenay Arc are mainly Paleozoic in age, ranging from the Proterozoic to the Triassic.

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 and were apparently thrust-faulted from not very far away. The Kootenay rocks have been shoved eastward, from where they formed offshore, onto the continental margin by compressional tectonic processes which most geologists interpret as the result of the Farallon plate converging and subducting beneath that area. 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 follow.

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, folds, and intrusions. These rocks add 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.

By the Jurassic period, the oceanic plate to the west of North America, the Farallon plate, had begun subducting beneath the edge of the continent. Farther out to sea lay a variety of terranes, several in the form of island arcs, along with oceanic plateaus and ocean islands, some eroded down to submarine plateaus with extensive reefs. These island arcs, oceanic plateaus, coral reefs, and the piles of sediment eroded from them and deposited on the adjacent ocean floor, 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 encompasses a large swath of terranes. These terranes apparently already accreted with each other at a subduction zone farther out in the ocean basin before they accreted against North America as a group.

The Intermontane superterrane includes the following specific terranes, named after locations in British Columbia: the Slide Mountan, Quesnellia, Cache Creek, and Stikinia terrranes. The Stikinia and Quesnellia terranes are composed mainly of island arc crust. 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.

The Slide Mountain and Cache Creek terranes are composed mainly of oceanic crust. The Slide Mountain terrane is interpreted by geologists to represent the ocean basin which was between the Quesnellia islands/oceanic plateau and North America. The Cache Creek terrane is thought to represent the ocean floor that was farther outboard between Quesnellia and the Stikinia island arc. The ocean basins. and the ocean-ocean subduction zones where extensive island arcs formed in the ocean, were all were telescoped together and shoved into North America by convergence of the Farallon plate against 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 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 the interior of British Columbia. They extend all the way to southeastern Alaska.

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.

The Blue-Wallowa terranes formed predominantly 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 90-118 million-year age intruded several of the terranes simultaneously, stitching them together and showing that they had accreted with each other by then. Not only that, but some of the 90-118 Ma plutons intrude the terranes and the edge of older North America, in the "suture zone" along which the terranes were accreted to the continent. This leads to the conclusion that the final accretion of the Wallowa-Blue Mountains terranes occurred during the Cretaceous period, starting at approximately 118 Ma.

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 Valley terranes
(accreted by end of middle Cretaceous)

A sequence of rocks that form the terranes of the Methow Valley in north central Washington also extends into 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.

A second fault-bounded terrane in the Methow Valley is called the Hozomeen terrane. It includes lots of pillow basalt, deep-sea shale and silststone, and many beds of chert from deep sea deposits. Chert is a very resistant rock, and pieces of Hozomeen chert were deposited in sediments east of the terrane, along what was then the coast of North America near the west side of the Okanogan Highlands. The chert derived from the Hozomeent terrane and shed to the east is thought to mark a stage of the closing of the Methow Ocean, when the Hozomeen terrane was being thrust faulted towards the continent so it was undergoing uplift and erosion nearby.

Some of the rocks in the Methow terrane show paleomagnetic evidence of having formed somewhere between 500 and 1,500 miles south of where they are now. Some geologists think that the rocks had accreted to the continent by 80 Ma, and then moved northward after accretion, in a piece of the continent which included the terrane, arriving where it is now by some time in the Paleogene period, perhaps between 70 and 55 Ma. Other geologists are not so sure, and point to what they interpret as a Late Cretaceous overlap terrane in the Methow Valley, the Pipestone Canyon Formation, as showing that the Methow Terrane was where it is now earlier than the paleomagneism seems to allow. This contradiction has yet to be resolved.

Sedimentary rocks deposited by streams above sea level on the coastal plain of wester North America accumulated on the eastern side of the "Methow Seaway" or "Methow Ocean" near the end of its existence. This is one of the few places in Washington state where there is a chance that dinosaur fossils may someday be discovered.

The North Cascades Crystalline Core and Its Terranes
(accreted 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 rocks 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 - extensive instrusions of diorite, granodiorite, and granite interwoven with large areas of gneiss and schist - compose much of the British Columbia Coast Range, which extends over a thousand miles north into Alaska. The British Columbia Coast Range (which is different from the Coast Ranges landscape region in the Pacific Northwest) is one of the largest regions of granitic rock and gneiss in the world. The North Cascades are at the southern end of this massive orogen.

Much of the North Cascades consists of the Skagit Gneiss complex. The Skagit Gneiss includes metamorphosed oceanic lithosphere and island arc crust. These accreted rocks are classified as part of the Chelan Mountains terrane. In addition to its accreted terrane material, the Skagit Gneiss also contains large volumes of plutonic rock which intruded the terranes after they had accreted. The plutonic rock itself was then metamorphosed into gneiss.

In the eastern part of the crystalline core of the Cascades, there is an unusual terrane, The Swakane terrane. It runs from the north end of the city of Wenatchee to the towns of Entiat and Orondo in north central Washington. The Swakane terrane consists largely of gneiss with lots of biotite in it, hence its official name as a geologic formation, the Swakane biotite gneiss.

The Swakane terrane also contains tiny zircons, a type of mineral that incorporates some uranum in it when it first crystallized. The uranium undergoes radioactive decay into lead, allowing the absolute age of the zircon to be measured. The cores of some of the zircons in the Swakane terrane have a Proterozoic radiometric age, going back to about 1.6 Ga. It may be that the zircons originated in an old granite which eroded into grains of sand, and the sandstone was later metamorposed into the Swakane biotite gniess. Given the 1.6 Ga zircon cores, the Swakane terrane seems to be a terrane derived from a continent, a terrane that got separated by faulting and moved by plate tectonics to the North Cascades. It might be that the Swakane terrane was part of North America somewhere in southern California or northwestern Mexico, brought northward by strike-slip faulting due to oblique convergence and possible transform tectonics along the western margin of North America.

The terranes in the North Cascades crystalline core were subjected to highly variable amounts of deep burial, tectonic stress, heat, and regional metamorphism.

Rocks in the terranes of the North Cascades crystalline core, like in the Methow terrane and Insular superterrane, show paleomagnetic evidence of having moved northward along the continental margin for a distance of several hundred to possibly over a thousand miles.

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. The Western Cascades-San Juan Islands terranes also contain parts of at least one ophiolite, the Fidalgo ophiolite on Fidalgo Island and parts of adjacent islands.

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 105 and 90 million years ago. As with the Methow terrane, there is paleomagnetic evidence that rocks in the North Cascades-San Juan Islands terranes were moved northward as much as 1000 miles since they formed, perhaps after they had accreted to North America. This aspect of how the North Cascades-San Juan Islands terrane mosaic was put together - how there could have been as much as 1000 miles of northward translation of the terranes after they had arrived in the vicinity of North America - has not yet been completely worked out. Nevertheless, if the paleomagnetic evidence is correct, the northward shift of the location of the rocks must have occurred after they had formed, and probably after they were part of North America.

Layers of sedimentary rock of Late Cretaceous age overlie both the accreted terranes of the San Juan Islands and adjacent, "native" bedrock of North America. The terranes must have accreted to North America by the time the sediments were deposited. The native Late Cretaceous sedimentary rocks, called the Nanaimio Group, crop out on southern Vancouver Island and on the periphery of the San Juan Islands themselves. The presence of the Nanaimo Group on top of the terranes, and on top of nearby bedrock of North American, pins the terranes to North America by Late Cretaceous time.

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.

Terranes of the Coast Ranges
(accreted Paleogene to present)

Since the Insular superterrane was added to the edge of North America, subduction has continued and yet more terranes have arrived and accreted to the Pacific Northwest. These terranes, which docked with North America during the Paleogene and early Neogene periods, 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. Much of the volume of the Crescent terrane is 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.

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

Web Links

The National Park Service and United States Geological Survey combined to produce a web page about the major terranes in the North Cascades
http://www.nature.nps.gov/geology/usgsnps/noca/nocageol5.html


Geology of the Pacific Northwest
Focus Page #7--Accreted Terranes of the Pacific Northwest
© 2001 Ralph L. Dawes, Ph.D. and Cheryl D. Dawes
updated: 7/18/13