PNW Subduction

The Geology of the Pacific North West is largely dominated by the Cascadia Subduction Zone (CSZ), which extends from Northern California up into British Columbia.  The Cascade Range, where the CSZ got its name, was formd as the Farallon plate subducted beneath the North American Plate.  The Cascade Range extends from southern British Columbia to northern California. The highest peaks are Mt. Rainier (14,410') of Washington and Mount Shasta (14,162') in northern California.

Plate Description

The Juan de Fuca plate, whose subduction beneath the North American plate results in the formation of the Cascade Range, is further subdivided. If we consider the segments as one, the Juan de Fuca plate is located between the Queen Charlotte Sound in British Columbia and Cape Mendocino in northern California. The central, largest section of the Juan de Fuca plate is off the coasts of Washington and Oregon. Its western diverging boundary is the Juan de Fuca Ridge which is 500 km long. This ridge is composed of four segments which are named, from north to south, the Endeavor, Cobb, Vance, and Cleft Segments. The northern boundary of the central Juan de Fuca plate is the Nootka fracture zone which separates it from the northern Explorer segment. The Explorer segment is occasionally referred to as a plate. To the south of the main segment is the Gorda plate. Numerous references to this segment refer to it as a plate, but sometimes it is not so differentiated from the Juan de Fuca plate. The northern boundary of the Gorda plate is the Blanco Fracture Zone. The western boundary is the Gorda Ridge which is further divided into five segments. The southern boundary is the Mendocino Fracture Zone. The eastern border of all these segments is the zone of subduction beneath the North American plate. This zone is commonly referred to as the Cascadia Subduction Zone (CSZ) or the Cascadia Megathrust. The Mendocino Fracture Zone and the CSZ join the San Andreas fault in a triple junction at the southeasternmost corner of the plate. Queen Charlotte fault is north of the CSZ.

The North American plate is moving westward at an approximate rate of 28 mm per year. The Juan de Fuca plates are moving northeastward at rates varying between 20-60 mm per year. The Pacific plate is moving to the northwest about 60 mm per year in the vicinity of the Juan de Fuca plates. The subduction zone dips 10-25° in Washington, about 25° in Oregon, and 5-15° in northern California as the plates first subduct beneath the North American continent (Hammond, 1989; Trehu, 1996). Below 50 km the angle of subduction increases (Hammond, 1989). The CSZ is currently subducting at a rate of around 40 mm per year; the rate has continued slowing during the last 7 Ma (Swanson, et. al., 1989).

Plate History During the Jurassic, about 200 million years ago, the supercontinent of Pangaea began to rift apart, probably along the seams of a former collision.  This continental rifting resulted in crustal extension and thinning which formed grabens and allowed mantle material to rise due to decreased pressure. Eventually, the continental rift zone subsided below sea level, the land ripped apart, and a new ocean, the Atlantic, was born. The seafloor spreading which occurred along the Mid-Atlantic Ridge resulted in the movement of the North American plate (Laurasia at the time) toward the west. This movement in turn resulted in subduction of the former Kula and Farallon plates beneath the North American continent. Beginning in the Late Jurassic and ending in the Eocene epoch of the Tertiary period the Cordilleran Orogeny formed the Rockies as a result of shallow subduction. During this time twenty or more major terranes were accreted to the western margin of North America. One of these, Siletzia, was possibly formed over the Yellowstone hotspot in its earliest years. When the central portion of the Farallon plate completely subducted beneath the continental plate during the Oligocene 29 mya, the San Andreas fault came into existence. The northern remnant of the Farallon plate is called the Juan de Fuca plate and is composed of three major segments, the Explorer, Juan de Fuca, and Gorda plates. The southern sections of the Farallon plate exist as the Rivera platelet south of Baja California, the Cocos plate to the west of Central America, and the Nazca plate west of South America.