Siletzia's origin along an oceanic spreading center: What's Bremerton got to do with it?

October 24, 2017, Geological Society of America
Credit: CC0 Public Domain

Fifty million years ago, Bremerton, Washington, may have looked a lot like Iceland: hot new land atop an oceanic spreading center. That land was part of the Siletzia terrane, a thick wedge of basaltic crust that extends from Oregon to British Columbia.

Siletzia has been recognized and studied for decades (by Ray Wells at the USGS, GSA Annual meeting abstract 321-2, and many others). The wedge of oceanic material that forms this terrane is thick, as much as 32 kilometers, and accreted onto North America 50 million years ago.

Yet big questions remain about Siletzia's magmatic and tectonic history and its role in the tectonic evolution of the Pacific Northwest and the Western Cordillera as a whole. For example, what was the source of all that magma? Was Siletzia a series of oceanic plateaus along a spreading center, or the product of a mantle plume? What was the precise age of basalt and how did it relate to ongoing subduction of the Farallon plate under North America?

On Wednesday, 25 October, at the Geological Society of America's 2017 Annual Meeting, Michael Eddy will present new data that sheds light on these questions. Eddy's work supports the hypothesis that Siletzia was an oceanic plateau or island chain built along a northeast-trending spreading center. His high-precision age dates show that oceanic rocks to the northwest in British Columbia are 51 million years old, while similar rocks near Bremerton are a million years younger. "If my timing for the collision between Siletzia and North America is correct, then the rocks in the Bremerton area represent the location of the spreading center during collision," Eddy says.

In this case, that timing was everything. At the same time that hot new crust was building Siletzia near Bremerton, active subduction worked to pull that new crust beneath North America. Eddy's data adds credence to the model that this young crust was still too hot and buoyant to subduct. Instead, it "jammed" the subduction zone and stuck to North America.

Like a pile-up on a highway, Siletzia's jamming of the and the subsequent collision likely had a tectonic ripple effect. Eddy's work supports the scenario in which that collision coincides with the deformation of the Cascades, caused crunching of central Washington, impacted the direction of subduction, and may help explain why the Farallon plate broke off on its way under North America.

Explore further: Data mining finds more than expected beneath Andean Plateau

Related Stories

Data mining finds more than expected beneath Andean Plateau

August 23, 2017

Seismologists investigating how Earth forms new continental crust have compiled more than 20 years of seismic data from a wide swath of South America's Andean Plateau and determined that processes there have produced far ...

The North American Cordillera: Constructive collisions

April 3, 2013

The mountain ranges of the North American Cordillera are made up of dozens of distinct crustal blocks. A new study clarifies their mode of origin and identifies a previously unknown oceanic plate that contributed to their ...

The Atlantic 'resting' -- for now

May 23, 2011

Geophysicists have simulated when the continents around the Atlantic develop active continental margins with earthquakes and volcanoes. According to the model, ‘real’ fully active subduction zones will not form ...

Studying ancient Earth's geochemistry

January 18, 2013

Researchers still have much to learn about the volcanism that shaped our planet's early history. New evidence from a team led by Carnegie's Frances Jenner demonstrates that some of the tectonic processes driving volcanic ...

New timeline proposed for plate tectonics

May 11, 2017

Earth's history should include 'pre-plate tectonic' and 'plate tectonic' phases beginning less than a billion years ago, according to a team of geoscientists in the journal Geology.

Recommended for you

Study: With Twitter, race of the messenger matters

February 23, 2019

When NFL player Colin Kaepernick took a knee during the national anthem to protest police brutality and racial injustice, the ensuing debate took traditional and social media by storm. University of Kansas researchers have ...

After a reset, Сuriosity is operating normally

February 23, 2019

NASA's Curiosity rover is busy making new discoveries on Mars. The rover has been climbing Mount Sharp since 2014 and recently reached a clay region that may offer new clues about the ancient Martian environment's potential ...

Researchers engineer a tougher fiber

February 22, 2019

North Carolina State University researchers have developed a fiber that combines the elasticity of rubber with the strength of a metal, resulting in a tougher material that could be incorporated into soft robotics, packaging ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.