• Introduction
• Stress in the Pacific Northwest
• Cretaceous Fold and Thrust Structures of the Pacific Northwest
• Early Tertiary Strike-Slip Structures of the Pacific Northwest
• Late Tertiary Compressional Structures of the Pacific Northwest
• 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 on response to physical force-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 they 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 early Tertiary time. It 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.

Early Tertiary 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. These faults include the famous 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. 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.

Late Tertiary 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, the Seattle Fault, and the anticlines and thrust faults of the Yakima Fold Belt.

Return to schedule

Glossary terms that appear on this page: metamorphic core complex; graben; 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