Region 5: The Pacific Border

The Pacific Border can be divided into two major areas based entirely on dominant plate tectonic forces. A transform (sideways-moving) boundary between the Pacific plate and North America extends through California. Meanwhile, a convergent boundary subducts off the coast of Washington and Oregon, between North America and the remnants of the ancient oceanic Farallon plate. These two areas are separated by the great Mendocino fracture zone off the coast of northernmost California.

Before the formation of the San Andreas Fault system, the area south of the Mendocino fracture zone was a subduction zone. The features of subduction zones can vary, depending on how fast the plates are moving and the angle at which the subducting oceanic crust is traveling. Where the descending plate subducts at a relatively shallow angle, a forearc basin may form oceanward of the volcanic arc (Figure 4.12). The Central Valley of California is the forearc basin associated with the volcanic arc that became the Sierra Nevada, and it formed partly due to the weight of the growing mountains on the crust. West of this basin, a pile of sediments and even ocean crust, known as an accretionary prism (Figure 4.12), was scraped off the subducting oceanic plate and piled higher and higher onto the overlying continental crust. The Coast Ranges of California formed in this way.

Figure 4.12: Some of the features associated with subduction zones.

About 30 million years ago, a mid-ocean ridge called the East Pacific Rise collided with and subducted beneath North America. As this ridge subducted, the plate boundary adjacent to California gradually changed, becoming the transform boundary that formed the San Andreas Fault system. At this time, the accretionary prism that formed the Coast Ranges stopped growing. Since the fault is not a straight line but rather a system of faults, a lot of tectonic jostling, pushing, and pulling occurs in this part of California. Movement along these faults is responsible for the formation of hills and valleys, and sometimes of broad basins such as the western part of the Mojave Desert. The Transverse Ranges, Tehachapi Mountains, and southern Coast Ranges all owe much of their topography to movement along the San Andreas Fault system. In fact, the Transverse Ranges have been rotated as much as 110 degrees from their original position! The southern end of the fault system lies just north of the Gulf of California, where it terminates in a rift zone. The rifting is responsible for the formation of the Imperial Valley, one of the lowest spots in North America. Despite the overall translational motion of the San Andreas Fault system, there is a small compressional component, and this has resulted in uplift along parts of the coastline during the last three million years. This uplift can be seen in a series of wave-cut terraces that now lie well above the modern shoreline (Figure 4.13).

The San Andreas Fault travels off the coastline north of San Francisco, eventually meeting up with the Mendocino fracture zone. The Klamath Mountains, which lie directly east of the fracture zone, are part of an accretionary wedge that has been substantially uplifted.

The subduction zone north of the Mendocino fracture zone is responsible for the formation of the Oregon Coast Range and the Olympic Mountains. These mountains are part of a 66-million-year-old accretionary wedge associated with the subduction zone. In Oregon, the wedge is composed largely of an ancient volcanic arc, which was carried to the coast of Oregon on an older subducting slab. The Oregon Coast Range is lower in altitude than the coastal ranges north and south, partly because of slower uplift and partly because the wet climate of western Oregon quickly erodes the rock. Despite these conditions, marine terraces similar to those in California are found in this area. The Olympics were uplifted during the Miocene, around the same time as the most recent uplift in the northern Cascades.

Like the Central Valley of California, the Willamette Valley in Oregon and Puget Sound in Washington are parts of a forearc basin, but in this case they are associated with an active volcanic arc—the Cascades.

Figure 4.13: Marine terraces north of Santa Cruz, California. The third terrace is the oldest; the first is the youngest. These terraces indicate that the coastline has continued to rise, even though the predominant plate tectonic motion is translational.