• Introduction
• Stress in the Pacific Northwest
• Cretaceous Fold and Thrust Structures of the Pacific Northwest
• Paleogene Strike-Slip Structures of the Pacific Northwest
• Paleogene Tensional Structures of the Pacific Northwest
• Neogene Compressional Structures of the Pacific Northwest
• Neogene Tensional Structures of the Basin and Range
• Glossary Terms

Related Basics Pages: Geologic Structures
Related Focus Pages: #9--Geologic Structures of the Pacific Northwest

Introduction

Welcome to Week 6 of Pacific Northwest Geology. The topic of this week's lecture is geologic structures, including faults, folds, metamorphic core complexes, and grabens. Geologic structures develop in response to physical force, known as stress, applied to the earth's crust. For the last 200 million years or so, most of the stress in the crust of the Pacific Northwest has been a result of plate tectonics.

Stress in the Pacific Northwest

At convergent plate boundaries the crust undergoes compression. The predominant structures created by compression are anticlines, synclines, reverse faults, and thrust faults. Most of the anticlines, synclines, reverse faults, and thrust faults in the Pacific Northwest can be linked by the geologic evidence to plate convergence and terrane accretion.

At a transform plate boundary the crust undergoes horizontal shearing. The predominant geologic structure that forms as a result of horizontal shear is the strike-slip fault. There are some major strike-slip faults in the Pacific Northwest, but most of them are now inactive. They probably formed at a time when horizontal shear was the major force on the region.

The interior of the Pacific Northwest is not thought to have been the location of a divergent plate boundary. Nonetheless, there are some major geologic structures east of the North Cascades that formed as a result of tension-pulling apart-of the crust. These structures are normal faults, grabens, and metamorphic core complexes.

By looking at the intervals of geologic time when the major geologic structures of the Pacific Northwest formed, we can get a sense of how stress on the crust in the region has changed over time, and see if we can link those changes to changes in large-scale processes such as plate tectonics.

Cretaceous Fold and Thrust Structures of the Pacific Northwest

The Fold and Thrust belt developed from Cretaceous to Neogene time. The Fold and Thrust belt is a broad zone of anticlines, synclines, and thrust faults in the Rocky Mountains. It indicates forceful compression of the crust took place at that time. This time of unusually intense compression may be due to rapid subduction and accretion of large terranes, or a superterrane, along the coast at that time.

Also during the Cretaceous Period, mainly between 110 and 80 million years ago, terranes were thrust on top of each other and stacked along major faults in the Methow Valley, in the western North Cascades, and in the San Juan Island areas of Washington State. It is tempting to draw connections between the compression of this region around the North Cascades and the Fold and Thrust Belt that is located inland to the east. However, the Fold and Thrust Belt continued forming into Tertiary Time after the terranes near the North Cascades were done thrusting. Furthermore, some of the terranes adjacent to the North Cascades seem to have moved northward at least 800 km since the middle Cretaceous Period, according to their paleomagnetism. Therefore, connections between the thrusting and folding of the terranes around the North Cascades and the inland Fold and Thrust Belt in the Rocky Mountains may not be so simple.

Paleogene Strike-Slip Structures of the Pacific Northwest

Large strike-slip faults that cut and offset the rocks of the North Cascades were active during the early or middle Tertiary Period, mainly during the Eocene epoch. These faults include the Straight Creek Fault and the Ross Lake Fault. It may be that at that time the Kula Plate was converging at such a low angle along the coast that it sheared the rocks northward at least as much as it compressed them inland.

During the Eocene epoch, large grabens and metamorphic core complexes formed in the inland Northwest. One of the grabens is the Chiwaukum graben, in which the town of Wenatchee is located, in central Washington state. Detailed study of the faults along the boundaries of the Chiwaukum graben indicates that they are not simple normal faults. Besides sliding up on one side and down on the other, the rock also moved sideways along the faults. This indicates that the crust was sheared horizontally at the same time as it underwent tension and stretched apart. This may have been due to the Kula plate moving northward alongside the coast.

Paleogene Tensional Structures of the Pacific Northwest

During the Paleogene period, stress on the Pacific Northwest shifted from mostly compressional to a combination of shear and tension. Tension means pulling apart, also called extension, of the crust. The Paleogene stress culminated in the creation of two types of structures that are prominent in the geology of the region: grabens and metamorphic core complexes.

Grabens are created by pure tension and have normal faults defining their boundaries. Some of the structures called grabens in the Pacific Northwest are bordered by faults that slipped sideways (strike-slip faulting) as well as slid downward (normal faulting). These not-so-simple grabens include the Chiwaukum graben in the Wenatchee Valley and the Methow graben in the Methow Valley. Some geologists refer to these structures as "transtensional pull-apart basins" to recognize the fact that they appear to have partly pulled apart (tension) and partly been offset in a transform manner (sideways shearing). There is strong evidence that along the Pacific Northwest coast at that time, previously a normal subduction zone, had changed into a plate margin with the offshore plate moving northward and translating terranes northward with it along the coast.

Also during the Eocene epoch, the Okanogan, Kettle, Priest River, and Spokane metamorphic core complexes formed in northeastern Washington and nearby Idaho. The tensional stress caused deep crustal rocks, including gneiss and schist to bulge upward into a dome. As the deep crustal rocks were being uplifted, they were also undergoing partial melting and intrusion by granitic plutons. As the gneiss, schist, and granite bulged upward from deep in the crust, overlying sedimentary and volcanic rocks slid off to the sides on low-angle normal faults known as detachment faults. The Okanogan fault is one of those detachment faults, bordering the west side of the Okanogan metamorphic core complex.

Neogene Compressional Structures of the Pacific Northwest

Since the Eocene epoch, the large-scale structures that have developed are a combination of anticlines, reverse faults, and thrust faults. Such structures result from regional compression of the crust. These post-Eocene structures are consistent with subduction in the usual sense, with the oceanic plate coming in at a high angle against the edge of the continent. Specific examples of large compressional structures formed after the Eocene include the reverse faults of the Olympic Mountains (an accretionary complex), the anticlines and thrust faults of the Yakima Fold Belt, and the Seattle fault.

Neogene Tensional Structures in the Basin and Range

In the Basin and Range landscape region, tension stress has dominated the crust during the Neogene period. This is thought to be because the subduction zone along the coast of California, which was a source of strong compressional stress across the region, has been replaced by the San Andreas fault, a transform fault which is shearing western California to the north. This has relaxed the compression in the crust east of California and caused the crust and upper mantle there to undergo tension, stretching and thinning in a roughly east-west orientation. The result is that the mountains and valleys that give the Basin and Range its name consist largely of horsts and grabens, geologic structures bordered by normal faults and produced by stretching of the crust. In addition, many metamorphic core complexes have developed in the Basin and Range region during the Neogene period as well, from southern Idaho to northern Mexico. As we have seen, metamorphic core complexes, like grabens, are products of tension in the crust. In other words, normal faults, grabens, and metamorphic core complexes are the result of the crust stretching apart.

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Glossary terms that appear on this page: fault; fold; metamorphic core complex; graben; stress; plate tectonics; convergent plate boundary; anticline; syncline; reverse fault; thrust fault; transform plate boundary; strike-slip fault; divergent plate boundary; normal fault; subduction; accretion (or accreted terrane); paleomagnetism; compression; shear; tension; hanging wall; footwall; accretionary complex; horst