• Introduction
• Accretionary Processes
• Accretionary History of the Pacific Northwest
• Intermontane Superterrane
• Insular Superterrane
• Transforming and Rotating
• Latest Arrivals
• Web Links
• Glossary Terms

Related Basics Pages: Accreted Terranes
Related Focus Pages: #7--Accreted Terranes of the Pacific Northwest

Introduction

Welcome to Week 5 of Pacific Northwest Geology. The topic of this week's lecture is accreted terranes. Accreted terranes are a major of component of the land on which we live. If you live in Seattle, deep in the crust beneath you is the Crescent Terrane, made mostly of pillow basalt that erupted onto the floor of an ocean. If you look at the Olympic Mountains from the Puget Sound area most of the rocks you see there are of the Crescent Terrane. If you live around Wenatchee, you can drive north five miles to Rocky Reach Dam and be in the Swakane Terrane, an accreted terrane with some minerals that are among the oldest in the North Cascades.

It is common for accreted terranes to be altered by intrusion, metamorphism, folds, and faults. The folds and faults occur as a result of the tectonic forces that shove the terranes into their new homeland. The intrusion and metamorphism result from the subduction processes that add the terranes to the continent.

For example, in Okanogan County, many of the rocks in the Okanogan Highlands are accreted terranes that formed far away and long ago. Since the terranes in the Okanogan Highlands arrived in North America they have been intruded by magmas and metamorphosed, and they have been folded and faulted. The changes to these terranes make it harder to distinguish where the terranes came from and what kinds of rocks they used to be. However, it is clear that some of the accreted rock in the Okanogan Highlands formed on a shallow floor of a warm ocean, where sediments accumulated from the remains of dead plankton and marine invertebrates which eventually was lithified into limestone and dolostone and then metamorphosed into marble. Nearby in the Okanogan Highlands are metamorphic rocks that used to be "dirty sandstone" (also called graywacke), andesite, and diorite, suggesting that the rocks originated in an island arc. Other rock in the Okanogan Highlands formed as new oceanic plate at an ancient mid-ocean spreading ridge. Over the course of millions of years, a moving tectonic plate carried these rocks far from where they originated, eventually causing them to collide with the edge of North America and be added to the continent.

In spite of the challenges of unraveling the faults and folds and seeing through the metamorphism, geologists have been able to reconstruct key parts of the geologic history and put together the general story of the accreted terranes of the Pacific Northwest.

Accretionary Processes in the Pacific Northwest

Application of the theory of plate tectonics to a variety of geologic evidence leads to the conclusion that the terranes of the Pacific Northwest were transported on one or more plates carrying oceanic crust, mainly the Farallon Plate.

Oceanic crust consists mostly of gabbro, basalt, and ocean-floor sedimentary rock. In addition to standard oceanic crust, large oceanic plates such as the Farallon plate also contain oceanic plateaus (extra-thick platforms of oceanic crust that contain more volcanic and sedimentary rock than standard oceanic crust), island arcs formed at ocean-ocean convergent plate margins, and ocean islands that formed at hotspots similar to the Hawaiian Islands of today. From the moving oceanic plate, terranes consisting of these various types of rock from oceans and islands, plateaus and archipelagos, were added to North America at a subduction zone.

Transform faulting along the margin of the continent also appears to have occurred during part of the terrane accretion sequence. The major strike-slip faults moved terranes sideways (mainly to the north) along the edge of North America and caused them to rotate (sort of like ball bearings or disks caught between larger moving plates).

Accretionary History of the Pacific Northwest

Prior to the late Triassic Period, the coast of the Pacific Northwest was not a subduction zone and no terrane accretion was occurring. North America was part of the supercontinent Pangaea. The coast of the Pacific Northwest, the edge of the old, established continent, was a passive margin located running roughly through Idaho and part way into what is now Washington state north and south of the city of Spokane.

Pangaea began breaking apart and the continents began drifting their separate ways about 225 to 200 million years ago. The North American continent began moving westward, carried on the back of the North American plate. The oceanic plate to the west began subducting beneath the western margin of North America. Volcanoes started forming, magmas started intruding, faults starting moving, folds started developing, mountain ranges started building, and terranes started accreting. The old, passive margin of the continent was replaced by an active margin. The continental margin continues to be active, as seen in the active volcanoes, earthquakes, uplifting mountain ranges, and ongoing accretion of the Northwest today.

Intermontane superterrane

There appear to have been two peak intervals of terrane accretion in the Pacific Northwest. In each peak interval, several terranes that had already been joined together were added as a group to the edge of the continent. These two groups of terranes are the Intermontane Superterrane and the Insular Superterrane. The superterranes are larger than any individual terrane.

The Intermontane superterrane was being accreted to North America by the end of the Jurassic Period, as North America moved west over the subducting oceanic plate. The Intermontane terranes are a major part of the Okanogan Highlands in Washington and the interior of British Columbia in Canada.

At the same time as the Intermontane superterrane was docking along the western edge of North America, the rocks to the east were being thoroughly intruded by magma and metamorphosed by heat and pressure. This zone of igneous intrusion and regional metamorphism, mainly of Jurassic age, is known as the Omineca Belt, which runs from eastern British Columbia south to the Spokane and Idaho Panhandle region.

Once the Intermontane superterrane had been added to the continent, the location of the subduction zone shifted west, to the new outer edge of the continent along the west side of the Intermontane Superterrane. This shifted the zone of intrusion and metamorphism into the recently added Intermontane superterrane itself.

Insular superterrane

During the Cretaceous Period the Insular superterrane was added to the west coast. Rocks of the Insular superterrane extend along the coast from the Pacific Northwest up to Alaska. The Wrangellia terrane is a large component of the Insular superterrane. Geologists studying Wrangellia in the 1970s developed some of the first ideas about terrane accretion in the Pacific Northwest.

As happened with the Intermontane superterrane, the zone along the east side of the Insular superterrane was subjected to igneous intrusions and intensive metamorphism at the same time as the Insular superterrane was accreting to North America. This created the North Cascades Crystalline Core and the crystalline core of the coast mountains of British Columbia. Once the Insular superterrane had been added to the continent, the location of the subduction zone again shifted west, to the new outer edge of the continent along the west side of the newly accreted superterrane.

Transforming and Rotating

Paleomagnetic evidence indicates that the original rocks of the Insular superterrane moved as much as 1000 miles north from where they formed. The evidence suggests that the terranes moved north after they had become part of the active margin of the continent. Geologists have not yet identified all the major strike-slip faults that would have had to be active to shift the terranes so far north along the coast.

Paleomagnetic evidence also indicates that many of the terranes of the Pacific Northwest have been rotated. The rotation is consistent with the idea that the terranes were caught up between major transform or strike-slip fault zones along which terranes farther to the west were moving north.

In sum, the paleomagnetic evidence strongly suggests that terranes carried on eastward-subducting oceanic plates were also shifted northward along the margin of the continent during or after accretion.

Latest Arrivals

Since the Insular superterrane was added to the Pacific Northwest during the Cretaceous period, the modern Cascadia Subduction Zone has become established and terranes have continued to be added. The largest terrane accreted during the Cenozoic era is the Crescent Terrane, as it is called in Washington state, where it composes much of the northern and eastern parts of the Olympic Peninsula. The similar Siletzia Terrane in the Oregon Coast Ranges is considered by most geologists to be more of the same terrane. Therefore, the Crescent Terrane is a name that is no longer used, having been subsumed by the name Siletzia Terrane.

The Siletzia Terrane is a huge body of ocean island basalt and oceanic sediments, including lots of seafloor-erupted pillow basalt. In the Olympic Mountains, the seafloor sedimentary rock - mudstone and sandstone - has been slightly metamorposed.

The accretion of the Siletzia Terrane was centered around 50 million years ago, with some parts of the terrane hitting the coast a little bit earlier. The accretion of the Siletzia Terrane continued for a few million years until the whole terrane was shoved thoroughly into North America. The accretion of this large terrane grew the Pacific Norhwest westward. The earlier subduction zone was blocked by the accreted terrane. The previously subducted part of the Farallon Plate broke free and continued its slow descent into the mantle. A new subduction zone was then established west of the Siletzia Terrane, marked by the start-up of the Cascades Volcanic Arc around 40-44 Ma.

After the Crescent-Siletzia terranes, more pieces of the ocean floor have continued to be shoved into the Coast Ranges along the leading edge of the subduction zone, and the process continues today.

Web Links

For a review of the accreted terrane concept and the role of accreted terranes in western North America, see the USGS Web page http://pubs.usgs.gov/publications/text/Pangaea.html

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Glossary terms that appear on this page: accreted terrane; pillow basalt; intrusion; metamorphism; fold; fault; subduction; oceanic crust; strike-slip fault; Pangaea; passive margin; active margin; paleomagnetism