• Introduction
• What are some thrust faults and reverse faults of the Pacific Northwest?
  • The Cascadia Subduction Zone Mega-Thrust Fault
  • Reverse Faults of the Olympic Peninsula
  • The Seattle Fault
  • Accreted Terranes on Either Side of the North Cascades
  • Thrust Faults of the Yakima Fold Belt
• What are some anticlines and synclines of the Pacific Northwest?
  • The Olympic Mountains Antiform
  • The San Jan Islands Plunging Synform
  • Anticline Ridges of the Yakima Fold Belt
  • Anticlines of the Overthrust Belt and Fold and Thrust Belt
• What are some strike-slip faults of the Pacific Northwest?
  • Strike-slip Faults of the North Cascades
• What are some normal faults and grabens of the Pacific Northwest?
  • The Basin and Range
  • Grabens in Eastern Washington
  • The Methow Graben
• What are some metamorphic core complexes of the Pacific Northwest?
• Glossary Terms

Introduction

The crust of the Pacific Northwest contains a variety of geologic structures. These structures include folds and faults, grabens and metamorphic core complexes. Please refer to the Basics page on Geologic Structures for explanations of these structures.

Geologic structures provide key evidence of important stages in the geologic history of the Pacific Northwest. Such structures range in scale from folds so small you would need a microscope to see them to faults that extend the length of the Pacific Northwest coast. Geologic structures control the locations of ore deposits, influence the shape of the landscape, and determine which parts of the landscape are most at risk to earthquakes and landslides., When looking at the ongoing geologic activity in the region, geologists find that active structures provide ways for the earth to accommodate and release stress.

This page concentrates on examples of the larger and more obvious geologic structures in the Pacific Northwest. This catalog of structures seeks to illustrate the importance and variety of geologic structures in the region. It is not a complete list.

Thrust Faults and Reverse Faults

The Cascadia Subduction Zone Mega-Thrust Fault

The beginning of the Cascadia Subduction Zone, where the Juan de Fuca Plate bends beneath the edge of the North American Plate, is sometimes called a mega-thrust fault. It is the largest thrust fault in the Northwest. It is an active fault that generates powerful earthquakes. Subduction earthquakes-abrupt shifting of large masses of rock within the subducting lithosphere-occur beneath the continental crust and include the most powerful and deepest earthquakes in the world. The Cascadia Subduction Zone is a relatively small and slow-moving subduction zone compared to the other subduction zones in the world. It only generates the largest, most devastating type of subduction earthquakes every 200 to 600 years, with no way of predicting exactly when the next one will occur. The last devastating earthquake on the Cascadia mega-thrust occurred 300 years ago, according to geological evidence excavated along the Pacific Coast.

Reverse Faults of the Olympic Peninsula

The Olympic Mountains consist of large sheets of rock, up to more than 10 miles thick, each sheet separated from the next along a reverse fault. The rocks in the thrust sheets of the Olympic Peninsula are pillow basalts and sedimentary rocks from the ocean floor. These thrust sheets of oceanic crust have been accreted to the continent by slicing off the subducting plate onto the edge of North America along a reverse fault as the plate was being shoved beneath the leading edge of the plate North American Plate. An example of these large Olympic Peninsula faults is the Hurricane Ridge Fault, which runs through the Hurricane Ridge [link to VFS] area in Olympic National Park. Throughout the Olympic Mountains layers of rock from the ocean floor are tilted to steep angles, folded, and faulted.

The Seattle Fault

The Seattle Fault, sometimes referred to as the Seattle-Bremerton Fault, runs roughly east-west from the Issaquah area along the I-90 corridor. It runs south of downtown Seattle near the professional sports stadiums, across Elliott Bay and Puget Sound, across the southern end of Bainbridge Island, and onto the Kitsap Peninsula north of Bremerton. The Seattle Fault is a thrust fault that dips to the south. The rocks south of the fault, the hanging wall, have undergone thousands of feet along the fault and the rocks north of the fault, the footwall, have dropped down a similar distance along the fault.

There are several places south of the Seattle Fault where bedrock of the uplifted hanging wall is exposed at the surface of the earth. This includes the south end of Bainbridge Island, where fossil-bearing rocks of Oligocene age are exposed; Alki Point in West Seattle, where beds of the same formation of rock as the one on south Bainbridge Island can be seen on the beach at low tide; in the hill above I-5 near Boeing Field; and in the Newport Hills-Cougar Mountain region of Bellevue east of Seattle.

North of the Seattle fault, the older bedrock in the down-dropped footwall has become deeply buried beneath younger Pleistocene and Holocene sediments, which includes thick deposits of glacial drift from the Vashon glaciation of Puget Sound. So most of Seattle north of the fault sits on thick glacial sediments, while much of Seattle south of the fault sits on thin glacial sediments and bedrock just beneath the surface

The last great earthquake on the Seattle Fault took place approximately 1,100 years BP, according to carbon-14 ages from plants and trees that were apparently killed by the earthquake. The dead plant debris was found where marsh plants on the shore of Puget Sound were apparently buried by an inside-Puget-Sound tsunami, and where swaths of forest underwent slid into Lake Washington and other lakes in the area. Many small earthquakes, most too small to be felt, that occur along the fault today indicate that it is still active. It is possible that it may undergo a major earthquake again in the future.

Accreted Terranes on Either Side of the North Cascades

On either side of the crystalline core of the North Cascade Mountains in Washington accreted terranes in the form of thrust sheets, are stacked one on top of another along large thrust faults. The overlapping thrust sheets extend as far west as the San Juan Islands, which largely consist of accreted terranes. They occur throughout the mountains and hills on the west side of the North Cascades, including Mt. Shuksan near Mt. Baker and along Highway 20, the North Cascades Highway, from Mt. Vernon up to Marblemount. It is difficult to find exposures of the thrust faults that separate the thrust sheets from each other because rock weakened by sliding along the fault tends to be easily eroded and covered by streams, younger sediments, soil, and vegetation. The highway on the way to the Mt. Baker ski area crosses an exposure of one of the thrust faults, the Church Mountain Fault .

Thrust Faults of the Yakima Fold Belt

The Yakima Fold Belt is a system of ridges in the western Columbia Plateau, in the vicinity of Ellensburg, Yakima, and the Tri-Cities (Hanford, Pasco, and Richland). The crust in the area, including the thick stack of basalt flows known as the Columbia River Basalts, has been compressed into anticlines that show up as ridges at the earth's surface. The compression that formed the anticlines also created thrust faults beneath many of the anticlines. These structures provide an example of how anticlines can develop in the hanging wall as the rock is shoved up a thrust fault.

Anticlines and Synclines

The Pacific Northwest has its share of folds, most of which are plunging folds. Some related fold names are worth defining here. An antiform is an anticline-like (convex upward) fold in metamorphic rock, in rock where it is not known which layers are older, or in rock in which the beds do not follow the normal order of strata in an anticline. A synform is to a syncline what an antiform is to an anticline.

The Olympic Mountains Antiform

The Olympic Mountains as a whole have the appearance of an anticline that is plunging to the northeast. However, the beds are younger toward the axis, which violates the rules of anticlines. The reason the age sequence relative to the axis of the fold is backwards is because the beds are actually thrust fault sheets. Thrust faults put older rocks above younger rocks, disrupting the original stratigraphic sequence. So the Olympic Mountains look like an anticline in the way the beds (thrust sheets of oceanic crust) bend as seen on a map, but the structure involves a sequence of thrust sheets, so antiform is the better term. It is an accretionary complex with a bend in it, as if the sheets of oceanic crust were thrust into a corner, an indentation at the edge of the continent. Some geologists have suggested that this may be what actually happened.

The San Jan Islands Plunging Synform

Most of the bedrock of the San Juan Islands is a sequence of accreted terranes that were stacked on top of each other along thrust faults. The thrust sheets now bend in the pattern of a synform that plunges to the southeast. It has been suggested that the compression that folded the thrust sheets of the San Juan Islands into a synform be caused by the accretion of the nearby Olympic Mountains.

Anticline Ridges of the Yakima Fold Belt

The Yakima Fold Belt is defined by large anticlines that have folded the Columbia River Basalts up into ridges. These large ridges are a prominent part of the landscape and have been given names like the Saddle Mountains and Horse Heaven Hills. If you drive I-82 south from Ellensburg to Yakima, you cross through two of these ridges, Manastash Ridge, and Umtanum Ridge. We can infer that the folds developed after the basalts were erupted between 17 and 5 million years ago, because the basalt flows had to have been solid before they could have folded. This line of reasoning indicates that the crust in that area has undergone compression recently in geologic time.

Anticlines of the Fold and Thrust Belt

In the Rocky Mountains of Idaho, Montana and Wyoming, Alberta and eastern British Columbia, there are broad areas where Paleozoic and Mesozoic strata have been folded and thrust-faulted. This is often referred to as the Fold and Thrust Belt and sometimes referred to as the Overthrust Belt. The large size of the thrust-faulted area, the great numbers of thrust faults, and the geometry by which the faults are associated with folds in this region have attracted the interest of structural geologists from around the world.

Petroleum geologists have been especially interested in anticlines in the area. The anticlines have been a main target of oil exploration because they fold a variety of types of sedimentary strata. Some of these layers of sedimentary rock contain enough buried plankton (microscopic, floating sea life) to be sources of oil. The oil in these layers is confined in the pocket created by the overlying folded layers of less porous sedimentary rock, which form a barrier to the oil. Unless it is trapped underground by layers of impermeable rock, petroleum, which is less dense than water, will rise up out of the ground and evaporate.

The sheer scale of the Fold and Thrust Belt and its location far inland from the convergent plate boundary intrigues geologists. Can it be related to plate tectonics? What does it tell us about the way the crust behaves when compressed on such a large scale? These questions have not been completely answered. The timing of compression in the Fold and Thrust Belt is coincident with rapid subduction and accretion of the Insular Superterrane, so the two are linked by circumstantial evidence. Geologists continue to study the structures of the Fold and Thrust belt, attempting to move beyond description of the structures to an understanding of how they were formed.

Nearly all of Glacier National Park in northern Montana is a thrust sheet on top of a large thrust fault. The thrust fault beneath Glacier National Park is called the Lewis Thrust, and it places the Precambrian strata of the Belt Supergroup on top of Cretaceous shale and sandstone.

South of Glacier National Park, repeated thrust faults moved the rocks in the thrust sheets many miles sideways and thousands of feet upward from their original position. These thrust faults are exposed on the Sun River in Montana.

The faulting and folding of the Fold and Thrust Belt took place during an interval of time ranging from the Late Cretaceous period to the Eocene epoch, roughly 90 million to 45 million years ago.

Strike-slip Faults

Strike-slip Faults of the North Cascades

A series of strike-slip faults cuts through the rocks of the North Cascades Mountains. Most of these faults were active from approximately the end of the Cretaceous period through the Eocene epoch. The Ross Lake fault zone shows evidence of strike-slip motion and acts as the eastern boundary of the North Cascades crystalline core. The western boundary of the North Cascades crystalline core is the Straight Creek fault, which runs from the near the Canadian border south to the Snoqualmie Pass area. Rocks older than Tertiary age on the east side of the Straight Creek fault, east of Snoqualmie Pass, match rocks on the west side of the fault near Mount Baker and in Canada. Matching the rocks on either side of the fault and measuring the distance between them, allows geologists to estimate that the total amount of strike-slip motion on the Straight Creek Fault is in the range of 60 to 120 miles.

The strike slip faults in the North Cascades indicate that the area was subjected to a large amount of horizontal shear in the northward direction from the end of the Cretaceous through the Eocene. This shear may have been due to the Kula Plate moving northward alongside the edge of North America.

Normal Faults and Grabens

The Basin and Range

The Basin and Range region is defined by its geologic structures-its basins are grabens and half-grabens, and its ranges are horsts. A half-graben is a down-dropped block of rock with a normal fault along just one side, rather than along both sides as with a graben. The normal faults and related structures of the Basin and Range region have been forming for the last 25 million years or so, as the crust in the region has been stretched apart by tension.

There are no large population centers in the Pacific Northwest part of the Basin and Range. Klamath Falls in south central Oregon is in a region where the Basin and Range and Central Oregon Lava Plateau meet with the Cascade Range, and there are several active normal faults in the Klamath Falls area.

Grabens in Eastern Washington

During the Eocene epoch, several grabens formed in what is now eastern Washington. These grabens filled with sediments and volcanic rocks as they formed, and the layers of sedimentary and volcanic rock were preserved within their geologic structures, even as the mountains on either side of each graben underwent uplift and had their Eocene rocks eroded away.

The towns of Wenatchee and Republic in north central Washington are both in Eocene grabens, the Chiwaukum and Republic grabens, respectively. Gold mines have been active in the vicinity of each of the two towns, and in both cases some of the gold deposits were associated with Eocene volcanic rocks that erupted into the graben as it was forming. The gold mine near Wenatchee finished operations in the 1990s. Some mine activity near Republic continues, including exploration for possible new mines.

The Chiwaukum Graben is bordered by the Entiat Fault along its northeast side. The Entiat Fault is an oblique-slip fault. It has a large amount of strike-slip motion in its history, along with uplift of the Entiat Mountains on the northeast side and down-dropping of the Chiwaukum block on the southwest side. Some geologists have proposed that the Chiwaukum graben is actually a "pull-apart basin," a sort of graben with strike-slip motion along its borders as well as down-dropping of the central block. A pull-apart basin is thought to result from a combination of tension and shear of the crust.

The Methow Graben

The terranes of the Methow Valley in north central Washington, and the towns of Twisp and Winthrop, may be in a graben. The rocks are on a down-dropped fault block, with the mountains on either side that are uplifted along major faults. However, the faults include significant amounts of strike-slip motion, so they are properly classified as oblique-slip faults. Because a graben is defined as being bounded by normal faults, and because the main faults on either side of the Methow block have a history of oblique fault motion rather than just normal fault motion, some geologists do not call the Methow block a graben. Some refer to it as a pull-apart basin (see previous section on the Chiwaukum graben), and some think it has too complicated a geologic history to classify it.

Metamorphic Core Complexes

Metamorphic core complexes bring wide domes of metamorphic and intrusive rocks up from deep in the crust while sliding the rocks of the shallow crust off to the side along detachment faults. The Pacific Northwest is one of the places where metamorphic core complexes were first recognized and described. The Okanogan, Kettle, and Spokane metamorphic core complexes are in north central and northeastern Washington. The detachment fault along the west side of the Okanogan complex is called the Okanogan fault, and it runs along the Okanogan River valley north into Canada. Apparently the river valley formed where it is because the fault is located there. Faults weaken rocks and makes them easier to erode, influencing the shape of the developing landscape.

Idaho is home to several more metamorphic core complexes, including the Pioneer complex in the Pioneer Range in southeastern Idaho.

Glossary terms that appear on this page: thrust fault; subduction zone; pillow basalt; sedimentary rock; hanging wall; footwall; glacial drift; carbon-14; tsunami; accreted terrane; strata; accretionary complex; compression; shale; sandstone; tension; oblique fault; shear; intrusive rock; detachment faults

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