Siletzia is a geologic terrane in western Oregon and Northern California. Terranes are fragments of Earth's crust that have been transported to their present position by the movements of tectonic plates. Siletzia is thought to have originated as an oceanic island arc near a spreading center between the Farallon Plate and the Kula Plate. The spreading center was located near present-day Bremerton, Washington.

As the Farallon Plate and Kula Plate moved apart, Siletzia was carried away from the spreading center. The Kula Plate eventually subducted beneath the North American Plate, and Siletzia collided with the North American Plate about 150 million years ago. This collision caused the uplift of the Coast Range Mountains in Oregon and Northern California.

Evidence for Siletzia's origin along an oceanic spreading center includes the presence of volcanic rocks and gabbros that are typically found in oceanic crust. Additionally, the rocks in Siletzia are younger than the rocks in the North American Plate, indicating that they were formed after Siletzia separated from the North American Plate.

The relationship between Siletzia and Bremerton is important because it provides evidence for the past movements of tectonic plates in the Pacific Northwest. The presence of Siletzia in its current location is the result of the subduction of the Kula Plate beneath the North American Plate. This subduction also led to the uplift of the Coast Range Mountains.