Juan De Fuca Plate

Presently, the Juan de Fuca plate is vigorously subducting beneath southernmost British Columbia and the northwestern United States, yielding the Cascade-Garibaldi volcanic arc.

From: Encyclopedia of Geology (Second Edition) , 2021

Cascadia Volcanic Arc System

Joseph A. DiPietro , in Geology and Landscape Evolution (Second Edition), 2018

The Juan de Fuca Plate

The Juan de Fuca plate is separated into three semi-independent segments, two of which are shown in Fig. 19.1, the Juan de Fuca and Gorda segments. The third, the Explorer segment, is off the Canadian coast. The Cascadia trench, which marks the subduction zone of all three segments, is less than 100 miles from the coastline. Although it is the largest of the three segments, the Juan de Fuca segment is less than 275 miles wide measured from spreading ridge to subduction zone. The three segments subduct beneath North America at different rates. The Explorer segment may no longer be subducting. The average rate of convergence between the Juan de Fuca segment and North America over the past 5 million years is between 9.8 and 13.8 feet per 100 years and the direction is about 49 degree east of north, which implies oblique subduction. The much smaller Gorda segment is subducting below the Klamath Mountains at a similar angle, but possibly at a slower rate between about 6.5 and 9.8 feet/100 years, particularly in its southern part. The Gorda segment (and the Juan de Fuca plate) terminates southward at the Mendocino triple junction near Cape Mendocino, where the Mendocino transform fault intersects the Cascadia trench and San Andreas Fault (Fig. 19.1). The triple junction is not stable. It has been migrating northward for the past 29 million years at the expense of the Juan de Fuca plate, which has been getting progressively smaller. The present-day rate of northward migration is 16.4 feet per 100 years. In Chapter 20, we will discuss how the Juan de Fuca plate originated. For now, we can say that the Juan de Fuca plate is a shrinking remnant of the Farallon plate, which was introduced in Chapters 5 and 17 as the paleo-Pacific plate that underthrust the Cordillera to create the Laramide orogeny and then sank to form the ignimbrite flare-up (Fig. 5.13).

Figure 19.1. Landscape map that shows the US part of the Juan de Fuca plate. The Juan de Fuca and Gorda ridges mark the divergent plate boundary (the spreading ridge) with the Pacific plate. The Cascadia trench marks the subduction zone with the North American plate. The arrow shows the direction of convergence. Major Cascade volcanoes are labeled. The heavy line with double arrow along the Coast Range follows the crest of an anticlinal flexure. Area of Siletz River terrane is outlined in blue.

 From Wells et al., 2014.

Although geologically a trench, the Cascadia Trench does not have the topographic expression of a deep linear valley for two reasons. The first has to do with the age of the subducting plate. The oldest rocks on the Juan de Fuca and Gorda segments are less than 10 million years old. Tectonic plates cool, thicken, and become more dense as they age. The young age implies that the subducting lithosphere is warm and thin, and therefore isostatically buoyant. Seismic modeling suggests that the plate enters the subduction zone at a shallow initial angle of 10 to 15 degrees, which in turn, creates a shallow trench. The second reason has to do with the relatively slow rate of subduction in association with the Willapa, Columbia, Umpqua, Rogue, and other rivers that contribute copious amounts of sediment to the coastline. River sediment fills the shallow, slowly developing trench.

It was once thought that the absence of a trench coupled with the rarity of destructive historic earthquakes indicated that the Juan de Fuca plate was too warm to generate a great earthquake as it slipped below North America. Instead of snapping and fracturing, it was thought that the subducting plate was deforming like warm wax. This assumption has since been proven false. Key evidence for large destructive earthquakes was the discovery of rapidly drowned marshes and tree stands. Analysis of disrupted sediment suggests that as many as 12 powerful subduction-related earthquakes have occurred in the past 7700 years, about one every 642 years. The last great earthquake occurred more than 300 years ago on January 26, 1700. With an estimated magnitude of 8.7 to 9.2, this quake was on par with the famous New Madrid earthquakes. Smaller earthquakes, some large enough to be felt, occur on average every 5 years or so. Some of the more recent earthquakes include a magnitude 6.7 quake in 1949 and again in 1965, a magnitude 5.6 quake in 1996, and a magnitude 6.8 earthquake in 2001. These earthquakes were large enough to destroy buildings and cause injuries, yet they pale with respect to the 1700 earthquake, which released about 1000 times more energy.

The volcanic arc is a direct consequence of subduction of the Juan de Fuca plate. It is not surprising, therefore, that the northern terminus of the volcanic arc at Mt. Meager in British Columbia, and the southern terminus at Mount Lassen in northern California, correspond with the northern and southern termini of the Juan de Fuca plate. It is important to understand that direct melting of the subducting Juan de Fuca plate is not the cause of most of the volcanism. Instead, it is the partial melting of the solid upper mantle directly above the subducting plate. The location where partial melting occurs is shown in Fig. 19.2, which is a cross-section across the subduction zone in central Oregon. Although shallow at the trench, the angle of the subducting plate steepens to between 60 and 80 degrees within about 200 miles inland where it reaches a depth of 62 miles (100   km). Beyond 62 miles depth, the subducting plate becomes hot enough to undergo metamorphic reactions. These reactions release fluids (H2O and CO2) into the solid upper mantle above the subducting plate that disturb equilibrium conditions, causing mantle rock to melt.

Figure 19.2. Cross-section of the Cascadia Volcanic Arc System at Mt. Jefferson, Central Oregon.

The calamity caused by a major earthquake or large volcanic eruption is obvious. Less obvious is the potential damage resulting from landslides and avalanches caused by small eruptions. The problem with landslides and avalanches high on the mountaintop is that they could result in down-valley mudflows capable of destroying towns. Mt. Rainier is probably the most dangerous volcano in this respect because of the relatively large population in its lowland drainages. Landslides and avalanches off the slopes of Mt. Rainier have produced at least seven large mudflows in the past 5600 years, including the Osceola Mudflow 5600 years ago and the Electron Mudflow only 560 years ago. The Osceola Mudflow was by far the largest, extending all the way to the Seattle suburbs.

The overall topography shown in a Raisz landform map in Fig. 19.3, and in a shaded-relief elevation map in Fig. 19.4, is one of coastal mountains, an inland valley, and a tall, active volcanic, mountain range. This is a classic subduction-related tectonic landscape in which accretion has caused uplift along the coast, subduction has created a volcanic highland in the Cascade Mountains, and the intervening Puget Sound/Willamette Valley forms a basin known as the forearc basin. The relationship between topography and tectonics is well displayed in Fig. 19.2.

Figure 19.3. Landscape map of the Cascadia Volcanic Arc System.

Figure 19.4. USGS National Atlas digital shaded relief elevation map of the Cascadia Subduction complex. Nationalmap.gov/small_scale/atlasftp.html.

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Volume 3

Vincent S. Cronin , in Encyclopedia of Geology (Second Edition), 2021

Example 2: An FFT(b) Triple Junction

The Mendocino triple junction between the North American, Pacific, and Juan de Fuca Plates ( Figs. 1, #1 and 7A ) provides an example of one type of fault-fault-trench triple junction. The boundary between the Pacific and Juan de Fuca Plates at the PC-NA-JF triple junction is a right-lateral transform fault—the Mendocino Transform Fault—as is the boundary between the Pacific and North American Plates—the northern San Andreas Fault. The third boundary is a boundary along which the Juan de Fuca plate subducts under North America—the Cascadia Subduction Zone (Table 4) .

Fig. 7

Fig. 7. (A) One trench and two transform boundaries (black lines) converge at the Mendocino (PA-NA-JF) triple junction today, simplified from Bird (2003). Arrows indicate direction each plate is moving relative to the hotspot reference frame of Wang et al. (2017). CSZ, Cascadia Subduction Zone; MTF, Mendocino Transform Fault; SAF, San Andreas Fault. Both shortening and right-lateral strike slip occur along the MTF. (B) Current plate boundaries (colored lines) and predicted location of the triple junction (black lines) after 1 Myr of displacement. Base maps are from GeoMapApp.org.

Table 4. Input data for finite modeling of Mendocino triple junction.

Poles of rotation, HS4-EW-MORVEL56
Plate name Pole latitude (°N) Pole longitude (°E) Angular speed (°/Myr)
North America −   35.3 325.7 0.1679
Pacific −   59.3 96.1 0.7703
Juan de Fuca −   36.7 59.7 1.1191
Reference points
Description Latitude (°N) Longitude (°E)
Triple junction 40.311 −   124.416
On North America-Pacific transform 38.541 −   123.305
On Pacific-Juan de Fuca transform 40.422 −   126.869
On Juan de Fuca-North America trench 42.285 −   125.290

In this case, we will use a hotspot reference frame to define the motion of each plate, with plate velocities from the HS4-EW-MORVEL56 model published in 2017 by Chengzu Wang, Richard Gordon, and Tuo Zhang, with location data compiled by Peter Bird.

Earthquake focal mechanisms and locations, paleoseismic evidence, and geodetic measurements utilizing permanent GNSS (GPS) stations in the area support the interpretation that the North American and Juan de Fuca plates are deforming near the PC-NA-JF triple junction and its associated plate boundaries. The approach here is to gain a first-order impression of how the North American, Juan de Fuca, and Pacific plates might interact in the vicinity of the triple junction—a first impression that can be refined with the inclusion of additional kinematic or geodynamic data.

The western edge of the North American plate along the Cascadia Subduction Zone will continue to define the location of the North America-Juan de Fuca plate boundary at the triple junction. The Juan de Fuca plate's motion along this boundary does not affect the location of the boundary because it is subducting under North America. First-order modeling of the instantaneous motion of the Pacific and North American plates along the San Andreas Fault, using data from the MORVEL model of Charles DeMets, Richard Gordon, and Don Argus, indicates that motion is largely parallel to the fault trace (~50.8   mm/year) with some divergence perpendicular to the fault trace (~4.4   mm/year). Similar modeling along the Pacific-Juan de Fuca plate boundary suggests as much as ~18.1   mm/year convergence across the boundary and ~43.7   mm/year parallel to the boundary.

Modeling of 1   Myr of finite motion at the PC-NA-JF triple junction indicates growing boundary mismatches that are somewhat typical for triple junctions. For the purposes of a first-order kinematic model, assume that the future Pacific-Juan de Fuca plate boundary will be located half-way between the current boundaries as rotated over 1   Myr (Fig. 7B); in other words, assume that convergent deformation will be shared equally by the two plates along the boundary. (Note that no geological, geophysical, or geodynamic information is used in making this first-order assumption. Additional geoscience data and reasoning is likely to modify the resulting depiction of the boundary after 1   Myr of finite motion.) We can make the same sort of assumption to locate the future trace of the San Andreas Fault half-way between the slightly divergent edges of the North American and Pacific plates.

The general location of the PC-NA-JF triple junction shifts about 36   km to the northwest in 1   Myr, relative to the hotspot reference frame. In detail, the three plate boundaries at the PC-NA-JF triple junction do not converge at a common point after finite displacement in the first-order kinematic model, just as the boundaries did not converge at the NB-SM-AR triple junction. This sort of uncertainty in the exact location of a triple junction after finite displacement, based on kinematic reasoning alone, might be a characteristic of all such first-order kinematic models.

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International Handbook of Earthquake and Engineering Seismology, Part A

Kiyoshi Suyehiro , Kimihiro Mochizuki , in International Geophysics, 2002

3.1.3.1 Cascadia

Cascadia subduction zone is where young (4–8 Ma) Juan de Fuca plate subducts at a slow convergence rate (ENE-ward at 4 cm y −1) with high regional sedimentation rate beneath the North American continent (Fig. 1c). Large earthquakes are rare, but the potential for significant activity seems to exist. Onshore–offshore seismic experiment revealed the subducted oceanic crust subducting at a small angle into well beneath the continental margin. Forearc crustal structure changes significantly along strike (Trehu et al., 1994; Flueh et al., 1998a) (Fig. 4a–c).

FIGURE 4. Crustal structure models across the northeastern Pacific rim (see Fig. 1c, d, and e). (a) Cascadia forearc at about 47.5° N. (b) Cascadia forearc at about 46.7° N. (c) Cascadia forearc at about 45° N. Structure above the dashed line is modeled by onshore–offshore experiments. Open triangles are seismometers and filled triangles are shot points on land. RZ is reflective zone. (d) Nicaragua margin model. Here, the Cocos plate is subducting northwestward at about 9 cm y−1. (e) Costa Rica margin model. (f, g) Chile margin models at about 32.75° S and 33.5° S, respectively. (a), (b), (f), (g) copyright Elsevier Science; (c) reprinted with permission from Trehu et al., 1994, copyright American Association for the Advancement of Science; (d) copy with permission from Blackwell Science Ltd; (e) copyright by the American Geophysical Union.

 A. (From Flueh et al., 1998a.); B.(From Walther et al., 2000.); C. (From Trehu et al., 1994.); D.(From Christeson et al., 1999.); E.(From Flueh et al., 1998b.) Copyright © 1988

Flueh et al. (1998a) reported that the dip angle of the subducting plate is only a few degrees at about 47° N. The accretionary wedge is well developed and an increase of seismic velocity to > 4 km sec−1 seems to be due to dehydration, compaction, and diagenesis. Velocities > 5 km sec−1 may indicate igneous rocks that may sustain strain energy for seismic release.

Trehu et al. (1994) compiled an E–W cross section from previous results to show oceanic crust persisting beneath the coast with a larger dip angle than above (Fig. 4c). The Siletz terrane is exposed basement rock of the Cascadia forearc, which consists of accreted oceanic crust and seamounts about 50 Ma. Seismic velocities are much higher than observed by Flueh et al. (1998a). The terrane is bordered by lower seismic velocity subduction complex at about 100 km distance (75 km from deformation front). North American mantle starts from more than 40 km depth.

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Volcanic Landforms and Hazards☆

W.D. Huff , L.A. Owen , in Reference Module in Earth Systems and Environmental Sciences, 2015

Mount St. Helens

Mount St. Helens, situated in the Cascades in Washington State on the Northern American plate above the subducting Juan de Fuca plate is probably one of the best-studied stratovolcanoes. The catastrophic eruption on May 18, 1980, was the deadliest and most costly volcanic event in U.S. history, and resulted in 75 fatalities and the loss of 250 homes, 47 bridges, 24  km of railway, and   ∼   300   km of highway. Figure 25 shows the sequence of events on May 18, 1980. The massive debris avalanche (∼   3   km3 of debris) on the north flank of Mount St. Helens was triggered by a magnitude 5.1 earthquake due to a sudden surge of magma. The avalanche reduced the elevation of the mountain summit by   ∼   400   m and formed a 1.6   km-wide crater ( Figure 26 ). The magma within the volcano was released as a giant pyroclastic flow with a VEI of 5, flattening vegetation, and buildings over an area of   ∼   600   km2. The collapsing volcano mixed with ice, snow, and water to produce lahars that advanced down the Toutle and Cowlitz Rivers. Figure 27 shows a generalized map of the impacts and deposits from the 1980 eruption. Mount St. Helens continues to evolve as illustrated in Figure 28 .

Figure 25. Schematic cross sections through Mount St. Helens showing the succession of events during the 1980 climactic eruption. Reproduced from Lipman, P.W., Mullineaux, D.R. (Eds.), 1981. The 1980 Eruptions of Mount St. Helens, Washington. U.S. Geological Survey Professional Paper 1250, 844   pp. (a) The volcano in the early morning of May 18. The bulging of the north flank is clearly shown by the pre-1980 and precollapse profiles. (b), (c) Within about 30   s after the collapse, the debris avalanche, lateral blast, and vertical eruption begin as the cryptodome is exposed. The bulge block was the first to slide, followed by the graben block. (d) Within the next 30   s, the summit block had slid and the lateral blast had stopped, and the vertical eruption was in full fury.

Figure 26. Google image of the north side of Mount St. Helens showing the collapse of 1980 and the numerous volcanic landslides and debris flows (no vertical exaggeration).

Figure 27. Generalized map showing the impacts and deposits of the 1980 climactic eruption in the vicinity of Mount St. Helens.

 Reproduced from Tilling, R.I. Topinka, L., Swanson, D.A., 1984. Eruption of Mount St. Helens: past, present and future. USGS Special Interest Publication, US Government Printing Office, Washington DC, 56   pp.

Figure 28. (a) Topographic changes of Mount St. Helens measured from aerial photographs and laser imaging (LiDAR) showing rapid growth of a new lava dome, the whaleback-shape extrusion, and the deformed glacier. (b) Topographic profiles along a N–S axis through Mount St. Helens' crater showing the new lava dome relative to the south crater rim, 1980 crater floor, 1980–1986 lava dome, and 2000 glacier surface.

 Reproduced from Major, J., Scott, W.E., Driedger, C., Dzurisin, D., 2005. Mount St. Helens erupts again. USGS Fact sheet 2005–3036.

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Volume 3

Derek J. Thorkelson , in Encyclopedia of Geology (Second Edition), 2021

Volcanism of the Northern Cordilleran Slab Window

Volcanism associated with the Northern Cordilleran slab window varies profoundly from one side to the other. Presently, the Juan de Fuca plate is vigorously subducting beneath southernmost British Columbia and the northwestern United States, yielding the Cascade-Garibaldi volcanic arc. Far to the north, subduction of the Pacific plate beneath Alaska is generating the Aleutian volcanic arc. In the intervening region, above the Northern Cordilleran slab window, the volcanism is different, both chemically and physically ( Thorkelson et al., 2011). Whereas the two volcanic arcs consist mainly of explosive, steep-sided stratovolcanoes with mainly calc-alkaline compositions, the slab window volcanic field consists of more mafic, less explosive volcanoes with gentler slopes and more alkalic compositions. Prominent volcanic regions include Mount Edziza in northern British Columbia and the Chilcotin Group in southern British Columbia. The slab window volcanic field has an "intraplate" character, broadly similar to some continental rifts and some hotspot-generated ocean islands, and is sandwiched between normal volcanic arcs. In addition, the volcanic arc to slab window transition is marked by the presence of adakites, which are volcanic rocks with compositions consistent with melting of the subducted oceanic crust. Adakites are present in the Wrangel volcanic field above the eastern edge of the subducted Pacific plate, and in the Garibaldi field above the northern edge of the Juan de Fuca plate. This arrangement is remarkably consistent with the predictive view of adakite melt generation along the margins of slab windows as established in theoretical and empirical studies (Kay et al., 1993; Thorkelson and Breitsprecher, 2005) and with the geophysically imaged degradation of the Juan de Fuca plate.

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Problems and Solutions in Structural Geology and Tectonics

Beth Pratt-Sitaula , ... Donna Charlevoix , in Developments in Structural Geology and Tectonics, 2019

Solutions

Question 1

North velocity East velocity Vertical velocity
P402 9.0   mm/yr 13.4–14.0   mm/yr 2.0–3.0   mm/yr

Question 2

North velocity East velocity
PABH 11.2–11.7   mm/yr 14.8–15.2   mm/yr
P400 5.7–6.2   mm/yr 9.4–9.8   mm/yr

Unlabelled Image

Question 3

Magnitude Azimuth
P402 16.2–16.5   mm/yr 56–57 degrees
PABH 18.5–19.2   mm/yr 52–53 degrees
P400 11.0–11.6   mm/yr 58–59 degrees

Question 4

The coastal stations have higher velocities. The southern station (PABH) has a somewhat more northerly azimuth. The Juan de Fuca Plate off the coast of Washington is moving northeast compared to North America. The GPS stations seem to be affected by this northeast movement, even though they are on the North American Plate.

Question 5

Average of the three stations Horizontal velocity from N&E avg. Sketch
North velocity East velocity Magnitude Azimuth
8.6–9.0   mm/yr 12.5–13.0   mm/yr 15.2–15.8   mm/yr 55 degrees

Yes, it looks like a reasonable average of the other three. Translation is not important for determining earthquake hazard. It is internal deformation that leads to earthquake hazard.

Question 6

Residual N velocity Residual E velocity Horiz. velocity magnitude Azimuth
P402 0–0.4   mm/yr 0.9   mm/yr 0.9–1.0   mm/yr 68–88 degrees
PABH 2.6–2.8   mm/yr 2.2–2.3   mm/yr 3.4–3.6   mm/yr 39–41 degrees
P400 −   2.8 to −   2.9   mm/yr −   3.1 to −   3.2   mm/yr 4.2–4.3   mm/yr 227–229 degrees

Unlabelled Image

Question 7

The rotation is negative (clockwise). Rotation also does not involve internal accumulation of strain, so it is more relevant to understanding overall tectonic movements than earthquake hazard.

Question 8

Axis 1   = negative (contraction)—this is the axis in the NW-SE orientation; the contraction is actually very minor in this direction, so it would also be acceptable to say zero. Axis 2   = negative (contraction)—this is the axis in the NE-SW orientation.

Question 9

Yes! The (geologically) significant shortening in the NE-SW direction shows significant strain build-up that will need to be released in an earthquake.

Question 10

The shortening is on the order of 7–8   mm/yr. Over 500   years, that would be 3.5–4   m. We would expect that both stations would move southest in the next plate boundary earthquake. The coastal stations would be expected to move more than the inland ones.

Question 11

North velocity East velocity Horiz. velocity magnitude Azimuth
P240 23.7 −   19.0 30.4   mm/yr 321 degrees
P234 32.3 −   26.0 41.5   mm/yr 321 degrees
P243 11.2 −   12.0 16.4   mm/yr 313 degrees
Average of the three stations Horizontal velocity from N&E avg. Sketch
North velocity East velocity Magnitude Azimuth
22.4   mm/yr −   19.0   mm/yr 29.4   mm/yr 320 degrees
Residual N velocity Residual E velocity Horiz. velocity magnitude Azimuth
P240 1.3   mm/yr 0 1.3   mm/yr 0 degrees
P234 9.9   mm/yr −   7.0   mm/yr 12.1   mm/yr 325 degrees
P243 −   11.2   mm/yr 7.0   mm/yr 13.2   mm/yr 148 degrees

Unlabelled Image

Question 12

Rotation direction = negative (clockwise)

Strain ellipse extension (positive, zero, or negative): Axis 1 = positive Axis 2 = negative

Moving apart about 25   mm/yr. Answer should at least include some mention of San Andreas Fault system and movement between the Pacific and North American Plates. The Pacific Plate is moving northwest compared to North America. More specifically, P234 is west of the San Andreas Fault, and the other two stations are east of the main fault trace. Although the entire region is deforming, the relative motions decrease to the east, further from the Pacific Plate. In 100   years, the stations would be 2.5   m further apart.

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Forcing Agent

Joseph A. DiPietro , in Geology and Landscape Evolution (Second Edition), 2018

The Pacific Active Continental Margin

The western seaboard of the United States, as shown in Fig. 5.11 , is an active continental margin that involves three tectonic plates: the North American plate, the Pacific plate, and the Juan de Fuca plate. The boundary between the Pacific and North American plates in California is a transform plate boundary marked by the San Andreas strike-slip fault. Along this fault, the Pacific plate is moving northwestward relative to the North American plate at an approximate rate of 16.4 feet per 100 years (5  cm/year). A small piece of California, including Los Angeles, is west of the San Andreas Fault. San Francisco is east of the fault. If present-day plate motions continue, Los Angeles will slide northward and will reach the city of San Francisco in about 12 million years.

Figure 5.11. Map of western North America showing the plate tectonic configuration.

The San Andreas Fault extends from the vicinity of Cape Mendocino southward to the Salton Sea. Fig. 5.12 shows the southern termination. The San Andreas transform plate boundary continues southward as a series of transform fault segments within the Gulf of California that connect small divergent plate segments. The transforms have collectively forced the rifting (the separation) of Baja California from Mexico resulting in the opening of the Gulf of California. The Gulf began to open only 5–6 million years ago and is the most recent rifting event to affect the United States. Prior to 6 million years ago, Baja California was part of Mexico and the Gulf of California did not exist. Over time, the Gulf will continue to open and Baja California will slide northward with Los Angeles, eventually reaching San Francisco and beyond.

Figure 5.12. Map showing the tectonic setting of the Gulf of California. Based on Wallace (1990, p. 76).

A small tectonic plate, referred to as the Juan de Fuca plate, is present north of the San Andreas Fault off the northern California-Oregon-Washington coastline. This plate is moving northeastward relative to North America along a convergent plate boundary such that the Juan de Fuca plate is subducting beneath the North American plate at the Cascadia trench at an approximate rate between 9.8 and 13.8 feet per 100 years. Subduction of the Juan de Fuca plate causes melting and magma generation in the mantle, which rises to the surface to create the Cascade volcanoes.

As shown in Fig. 5.11, Juan de Fuca plate ends in southern Canada and the Cascadia trench is replaced by the Queen Charlotte transform fault. Much like the San Andreas Fault, the Queen Charlotte is a right-lateral transform that separates the North American and Pacific plates. However, unlike the San Andreas, it is located mostly offshore. It extends to Alaska where the Pacific-North American plate boundary makes a 90-degree turn allowing the Pacific plate to subduct beneath the North American plate at the Aleutian trench.

Notice that landscape along the Oregon-Washington coast shown in Fig. 5.11 (or Fig. 1.3) mimics the landscape shown in cross-section in Fig. 5.6B. Both consist of coastal mountains, an inland valley, and an inland volcanic mountain range. This landscape is a direct consequence of subduction of the Juan de Fuca plate. California has a similar landscape except that the inland mountain range (the Sierra Nevada) is not volcanic and the plate boundary is the San Andreas transform fault rather than a subduction zone. In Chapter 20 we will discover that the Juan de Fuca plate was once part of a larger plate known as the Farallon plate. For more than one hundred million years prior to initiation of the San Andreas Fault (at 29 million years ago), the Farallon plate underwent subduction beneath California. The California landscape is, in part, a relict of this ancient subduction zone setting.

Features that characterize an active continental margin include frequent earthquakes, recent volcanism, areas where folds and faults are actively forming, and rapid uplift/subsidence. Such features are expected to be present within about 200 miles inland of an active continental coastline. However, in the case of the United States, we find evidence to classify the entire Cordillera as an active tectonic landscape, a distance inland of more than 1000 miles. Every state in the Cordillera has experienced active faults, earthquakes, volcanism, and tectonic uplift/subsidence within the past 1 million years. There are several probable causes for active deformation so far inland. Prior to initiation of the San Andreas Fault, the Farallon plate was undergoing subduction beneath the North American plate along the entire US Pacific coastline. Beginning about 80 million years ago, rather than subducting directly into the mantle at a (normally) steep angle as shown in Fig. 5.6B, there is evidence that the Farallon plate subducted at such a shallow angle that it underplated the Cordilleran crust as depicted in Fig. 5.13. The underplated slab remained coherent all the way to Wyoming and Colorado. The forcing of this slab across the Cordillera was the likely impetus that elevated the Middle-Southern Rocky Mountains from sea level beginning 75 million years ago as discussed in Chapter 14. Additionally, the eventual sinking of this slab into the mantle was the likely impetus that created volcanism across the entire Cordillera as discussed in Chapter 17.

Figure 5.13. Schematic cross-section showing shallow subduction (underplating) of the Paleo-Pacific (Farallon) plate beneath the North American plate.

There are additional reasons for an active tectonic landscape across the Cordillera. We will discover in Chapter 18 that active normal faults are present, not only in the Basin and Range, but in all 12 of the Cordilleran provinces. One probable reason for normal fault activity is the present-day interaction of the Pacific and North American plates. Earlier, we noted that the North American plate is moving southwestward relative to the Pacific plate at an approximate rate of 8.5 feet per 100 years, and that the Pacific plate is moving northwestward relative to the North American plate at an approximate rate of 16.4 feet per 100 years. The result of this interaction is that the fast moving Pacific plate is diverging slightly from the North American plate. As the Pacific and North American plates diverge slightly, the remnants of the underplated Paleo-Pacific (Farallon) plate may be acting as a mechanical couple that helps drag the North American plate westward with the Pacific plate. This westward pull stretches the North American plate, thus creating normal faults. A contributing factor is volcanism associated with the sinking of the Farallon plate and with the Yellowstone hotspot. The heat from volcanic activity softens and weakens the interior Cordilleran lithosphere allowing it to deform more readily.

Even though it has been under active tectonic development for hundreds of millions of years, we will discover in Part II of this book that much of the present-day Cordilleran structural form and landscape has developed during only the past 40 million years, and in many places, within the past 17 million years. The modern Cascade volcanic arc, for example, began to develop only 48 million years ago, the San Andreas Fault initiated 29 million years ago, and widespread normal faulting in the Basin and Range, volcanism in the Columbia Plateau and Snake River Plain, and strong uplift of the Colorado Plateau and Middle-Southern Rocky Mountains all occurred within the past 17 million years. Tectonic change has been so complete that landscape that existed prior to 17 million years ago has mostly vanished from the Cordillera. Such recent tectonic change is in contrast to the rest of the country, which has seen little tectonic change over the past several hundred million years.

Prior to its present-day tectonic setup, the Cordillera experienced at least five orogenic events, and similar to the Appalachians, none of the events affected the entire Cordilleran Mountain system. The five orogenic events are the Late Devonian to Early Mississippian Antler orogeny (360–347   Ma), the Late Permian to Early Triassic Sonoma orogeny (260–247   Ma), the Late Jurassic Nevadan orogeny (164–145   Ma), the Late Cretaceous to Early Eocene Sevier orogeny (115–52   Ma), and the Late Cretaceous to Middle Eocene Laramide orogeny (75–45   Ma). To these events we can add Late Pennsylvanian-Early Permian (c.300   Ma) uplift of a series of mountains from Colorado to Texas known as the Ancestral Rocky Mountains. Let us also not forget the modern-day orogeny occurring right now along the west coast resulting from interaction of the North American, Pacific, and Juan de Fuca plates. The Cordillera, during this time, was involved in numerous terrane accretion events, but was never involved in major continent–continent collision of the magnitude seen in the Appalachians. A subduction zone and an ocean basin have existed continuously off the west coast since at least the Antler orogeny. The most recent large-scale terrane accretion in the United States ended along the Washington-Oregon coast about 49 million years ago as discussed in Chapter 19. Accreted terranes in the Cordillera (as well as in the Appalachians) were later dismembered and shuffled along strike-slip faults, thus complicating the collision history. In discussions that follow, we refer to the collection of orogenic events that created the Cordilleran Mountains as the Cordilleran orogeny.

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Cordilleran Sedimentary Basins of Western Canada Record 180 Million Years of Terrane Accretion

Brian D. Ricketts , in The Sedimentary Basins of the United States and Canada (Second Edition), 2019

The Modern Plate Boundary

The present relative motion of North America is towards the Pacific Ocean, relative to mantle hotspots (Riddihough and Hyndman, 1991); in the last 180   Ma the craton has moved through about 70° of longitude (Engebretson et al., 1985; Chapter 1, Fig. 24). Between northern California and Alaska, the modern North American plate margin is bounded by two major transform faults, two subduction zones, and two oceanic plates (Fig. 6 ). From the south, the dextral San Andreas fault separates the North American from the Pacific plate. The Mendocino triple junction marks the transition to the Juan de Fuca plate that extends north to about the southern tip of Queen Charlotte Islands; the Juan de Fuca Plate is moving eastward and is being subducted beneath North America at up to 46  mm/year relative to North America (Cascadia subduction zone). Deep reflection seismic shows that the top of the down-going Juan de Fuca plate is about 30   km beneath western Vancouver Island (Yorath et al., 1985; Hyndman et al., 1990; Cook et al., 1991). The Juan de Fuca plate is separated from the Pacific plate by en-echelon, northeast-trending spreading ridges (Juan de Fuca and Explorer ridges) and transform faults. A triple junction offshore Vancouver Island is hypothesized to be evolving at the junction of Juan de Fuca Ridge and Nootka fault (Rohr and Furlong, 1995). In this model, Explorer microplate, born about 5 Ma, accounts for strain partitioning between Juan de Fuca plate and the Queen Charlotte transform. North of the Vancouver Island, the Pacific plate and the much smaller Yukatat terrane have a strong northward component of motion, relative to North America, that is taken up by the Queen Charlotte-Fairweather dextral transform fault. North of the British Columbia-Alaska border, this motion is almost orthogonal such that Pacific crust is being subducted beneath Alaska at the Aleutian Trench.

Fig. 6

Fig. 6. The modern plate boundary of western Canada and northwestern United States.

 (Modified from Monger (1989) and Riddihough and Hyndman (1991).)

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TERTIARY TO PRESENT | Miocene

J.M. Theodor , in Encyclopedia of Geology, 2005

East Pacific Rise

Spreading at the East Pacific Rise continued during the Oligocene, causing the complete subduction of the Farallon Plate near Los Angeles by 30.0 Ma, leaving two remnants to the north and south still undergoing subduction. By the Early Miocene, most of the remaining Farallon Plate had been subducted under the North American Plate, bringing the Pacific Plate into contact with the North American Plate in southern California. The remaining two sections could no longer be considered a single plate by this time, and are henceforth regarded as independent plates with unique motions. The Juan de Fuca Plate is found to the north off the coast of Oregon and Washington and the Cocos Plate to the south off the coast of Mexico. Both continue to be subducted under the North American Plate. The contact of the Pacific, North American, and Juan de Fuca plates formed a triple junction at Mendocino, California by 20.0  Ma. During this transition, the subducting plate margin disappeared and the contact between the Pacific and North American plates developed into a transform fault system because the Pacific Plate is moving to the north, while the subducting Farallon Plate was moving more directly eastward. This transform fault system, which separates Baja California from the rest of California, runs under the continent between San Francisco and Los Angeles. The transform faulting formed along the Central Valley of California is known today as the San Andreas Fault Zone, and was in place by about 10.0   Ma.

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