• Introduction
• What are the Pre-Mesozoic orogenies of the Pacific Northwest?
• What are the Mesozoic-Cenozoic orogenies of the Pacific Northwest?
  • The Cordilleran Orogeny
  • The Sevier Orogeny
  • The Laramide Orogeny
  • The Coast Range Orogeny
  • The Cascadia Orogeny
• Glossary Terms

Introduction

An orogeny is a major mountain-building event, beyond the scale of a single volcano or a single, small range. The Pacific Northwest has been the site of ongoing orogenies since North America rifted from the rest of Pangaea approximately 200 million years ago. Prior to that, earlier orogenies affected the region east of Washington and Oregon. For more information on orogenies, see the page Basics page on orogenies.

What are the Pre-Mesozoic orogenies of the Pacific Northwest?

During Archean time orogenies occurred in the part of the North American craton that is now the Rocky Mountains. Evidence of these long-ago orogenies includes the granites that formed in cores of these ancient mountains, the metamorphosed volcanic rocks that they erupted, and the sedimentary rocks that formed as they eroded. The North American craton was flat and stable after the Archean mountains were leveled by erosion. During much of the Paleozoic era, the edge of the craton in the Pacific Northwest was a passive continental margin, with no plate boundaries nearby and no mountain building. However, there were a few exceptions to this passive plate margin during the Paleozoic era and the geologic record indicates some mountain-building activity at the western edge of the craton. These Paleozoic orogenies seem to have involved temporary subduction zones along the coast where terranes accreted and thrust inland onto the continent. One of these was the Antler Orogeny, which affected parts of Nevada, Utah, and Idaho in the Pennsylvanian period.

What are the Mesozoic-Cenozoic orogenies of the Pacific Northwest?

The Cordilleran Orogeny

The Cordilleran orogeny includes all the mountain-building events that have been taking place in the west since that edge of North America became predominantly a convergent plate boundary in late Triassic or early Jurassic time. The Cordilleran orogeny can be broken down into several individual events each named according to either the specific region within the west in which it occurred, the specific time interval in which it occurred, or the specific set of geologic structures associated with that event.

The Sevier Orogeny

The Sevier orogeny took place from either the Late Jurassic or Early Cretaceous through the Cretaceous period. Some geologists think it extended into the Tertiary period. The Sevier orogeny is distinguished on the basis of fold and thrust structures that affected the upper continental crust throughout the Rocky Mountain region.

The Laramide Orogeny

The Laramide orogeny is generally said to have taken place from the Cretaceous period up until the Eocene epoch, so it overlaps in time with the Sevier orogeny. The Laramide Orogeny is characterized by uplift of mountain ranges along steep faults from deep in the crust. Most of the major, high ranges of the northern Rocky Mountains are Laramide ranges, including the Beartooth and Bighorn ranges in Montana, the Wind River and Laramie ranges in Wyoming, and the Uintah range in northeastern Utah. Ancient rocks that formed deep in the continental crust have been revealed in the cores of these ranges, as a consequence of Laramide uplift and subsequent erosion.

The Coast Range Orogeny

The Coast Range orogeny is named after the Coast Range of British Columbia, which is not the same thing as the accretionary complex of coast ranges in coastal Washington, Oregon and northern California. The Coast Range of British Columbia is an uplifted region of rock consisting mainly of granite and similar intrusive rock along with regional metamorphic rocks. This Canadian definition of Coast Range extends into Washington state in the crystalline core of the North Cascade Mountains. The rocks in the Coast Range orogeny were intruded and metamorphosed mainly in mid-Cretaceous to Late Cretaceous or Early Tertiary time, although in the British Columbia Coast Range there are also older intrusive rocks that go back to the Early Cretaceous period. Paleomagnetic studies of rocks in the Coast Range orogen indicate that the rocks formed 1,000 miles or so to the south of where they are now. This southern origin applies to granites that intrude several accreted terranes, and volcanic rocks that erupted on top of the range. If the paleomagnetic evidence is correct, then the Coast Range has moved a great distance north along the west coast of North America since the peak stage of intrusion and metamorphism in the Late Cretaceous period. Horizontal movement of pieces of the crust along strike-slip faults could make such movement possible. However, geologists have not yet identified sufficient strike-slip faults or other geologic structures that could account for the Coast Range rocks moving so far north along the coast. The source location of the Coast Range orogen and its movement is a topic of ongoing research.

The Cascadia Orogeny

The Cascadia orogeny is occurring today in association with the Cascadia subduction zone. The volcanoes of the Cascades, the unseen magmas that are intruding and solidifying in the crust beneath the volcanoes, and the accretionary complex of the Coast Ranges, including the Olympic Mountains, are all part of the Cascadia orogeny. The Cascadia orogeny began either during or shortly after the Eocene epoch. Volcanic and plutonic rocks of Eocene age appear in what is called the Western Cascades, west of the main Cascade Range today. These Eocene rocks may represent movement of the Cascadia subduction zone, which led to formation of an early version of the Cascade volcanic arc. The modern Cascade volcanic arc, west of the early version, became established in Oligocene or Miocene time. This volcanism which began about 25 million years ago or so continues to this day.

Glossary terms that appear on this page: Pangaea; craton; passive margin; subduction zone; paleomagnetism; strike-slip fault; accretionary complex

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