Geology of the Pacific Northwest

Week 3:  TERRANES

Landmass Accretion |  Terranes |  Exotic Terranes |  Resources | 

Landmass Accretion

Eight hundred million years ago, the North American continent's western border included the southeastern part of California but not Washington or Oregon. Geologists attribute the accretion (addition) of land, such as Washington and Oregon, to various phenomena:

• Subduction zones:
• Volcanism (lava and ash) supply accretionary wedges.
• Sediments on the subducted plate are scraped off onto the upper plate.
• Hot spots that erupt near a coast:
• However, this is not as likely.
• Collision of materials:
• Plate tectonics:  North America's plate is moving westward and enlarging as the boundary of its plate connects with other floating plates. The Atlantic Ocean is widening, as well.
• Terranes:  Fault-boundaried groups of rocks with a common origin, age, and history.

Terranes

In the 1980s a theory was proposed, based on Alaska's pattern of rocks, that terranes are responsible for areas in which a particular formation or group of predominant rocks is different from those that are native to the area. Terranes are bordered by fractures in the crust (fault lines) and are made up of rocks different than those in the area to which the terrane has become accreted.

Terranes that have accreted to North America may have arrived from Asia or Australia by way of island microcontinents in the Pacific Ocean.

Exotic Terranes

A terrane is said to be exotic if it has been transported into its present location from another place a great distance away. It originates somewhere else and is transported by plate movement and added on to a continent's coast when it collides with it. Two ways geologists determine where terranes came from are:

1. paleobotany – the study of fossils. When geologists identify a fossil on a particular terrane and discover where and when that plant or animal lived, they can deduce that the terrane came from the same place.
2. paleomagnetics – the study of the earth's magnetic fields, expressed as "force fields" around the earth. Geologists measure magnetic grains in the rock.
1. If the grains are horizontal, the terrane came from or near the equator.
2. If the grains are vertical, the terrane came from or near the north pole.
3. If the grains are tilted (i.e., at a 45-degree angle), the terrane used to be 45 degrees north or 45 degrees south of the equator.

When Accreted

Terranes were added to the continent either separately or as an amalgamation of other terranes (a super-terrane). Geologists use "overlap sequences" and "relative" ages to figure out the story.

RESOURCES

Send comments to Rob Viens (e-mail: rviens@bcc.ctc.edu) or call him at his Bellevue Community College office at (425) 564-3158. Office hours are by appointment.