How does the Rogue Valley fit into the larger tectonic picture?

In the last three posts, we explored the geologic makeup of the Rogue Valley region. Most people know that there is a subduction zone off the coast of the Pacific NW and you might wonder: how does the Rogue Valley region fit into this larger picture? We will explore the tectonic setting in this post.

This diagram shows the present plate tectonic setting of the Pacific NW. The Juan de Fuca Ridge is the boundary between the Pacific and Juan de Fuca tectonic plates. At the ridge, the plates are diverging (i.e., moving AWAY from each other) and causing new oceanic lithosphere to be created. The ridge is 300-500 km (180–300 miles) offshore from the coastline.

Closer to the coastline, the Juan de Fuca also has a boundary with the North American plate. Here, the plates are converging (i.e., moving TOWARD each other) and causing the denser oceanic (subducting) plate to descend beneath the continental plate. The oceanic plate’s descent has created a trench-like feature parallel to the coast—this particular plate boundary is referred to as the Cascadia subduction zone. On land, subduction zones can be recognized by chains of volcanoes. In the Pacific NW, this chain is called the Cascades. These types of volcanoes are created because water-rich sediments on the descending plate lower the melting temperature of the sub-surface rock (hydrous melting zone on the diagram) and the melted rock (i.e., magma) rises up through the continent and eventually some magma even reaches the Earth’s surface to be erupted from volcanoes. The diagram above is from EARTH magazine (see references below).

Subduction zones are where the largest earthquakes on Earth occur. The largest recorded earthquake—magnitude 9.5—occurred in 1960 in the subduction zone off the coast of Chile. The second largest recorded earthquake occurred in 1964 in the subduction zone off the coast of Alaska.

This map shows the distribution of historical earthquakes in the Cascadia subduction zone. Earthquake magnitude is indicated by the diameter of the yellow circles. Notably absent are any earthquakes greater than magnitude 8; in fact, most are less then magnitude 7. However, there is abundant geologic and cultural records of very-large-magnitude earthquakes that occurred prior to European colonization of the Pacific NW (see next diagram below).

There is a concentration of earthquakes in the Seattle region where there are many crustal faults (explained in next figure). Another earthquake concentration in northern California, including the 1992 Petrolia earthquake (magnitude 7.1), is where the San Andreas fault ends and the subduction zone begins—it is a place with localized compression that creates faults and uplift.

The Rogue Valley region is mostly earthquake free. The 1993 earthquakes (magnitude 6.0) occurred near Klamath Falls directly east of the Rogue Valley. These earthquakes occur on faults that are the western edge of the Basin and Range Province where the continental crust is extending (i.e., being pulled apart). This province continues throughout eastern Oregon and most of Nevada.

Because the Juan de Fuca plate has been mostly consumed beneath North America, it has begun to split into pieces—the Explorer and Gorda plates. The active Cascade volcanoes are shown in orange with red triangles. The circled “36” indicates how fast the Juan de Fuca plate is converging with North America—36 mm/yr, on average. The diagram above is from a USGS site (see references below).
This 3-D diagram shows the difference between crustal earthquakes and subduction zone earthquakes. The yellow circles are crustal earthquakes that occur on faults within the continental crust. In contrast, subduction zone earthquakes occur at the interface between the down-going oceanic (Juan de Fuca) plate and the overlying continental (North American) plate—these are also referred to as “mega-thrust” earthquakes. The latest one was in 1700 (red circle) and is estimated to have been about a magnitude 9. There were no instruments to record the event, but written records in Japan, which experienced tsunami damage, and geologic evidence along the coast of the Pacific NW, have indicated it was a very-large-magnitude event.

The subduction zone is now firmly locked (by friction) and another magnitude 9 earthquake will occur when the compressive forces exceed the frictional strength between the plates. Geologic evidence indicates a recurrence interval of 300–600 years for the very-large-magnitude earthquakes. There are also deep earthquakes (in pink on the diagram) in the Juan de Fuca plate, but they are not strongly felt because of their depth. The diagram above is from a PNSN web site (see references below).
This map shows the really big picture! Orange arrows indicate the direction each plate (or micro-plate) is moving and the circled numbers are the velocities of the movement, in mm/yr. Keep in mind that velocities are yearly averages—in reality, most movement occurs during earthquakes and little movement occurs between earthquakes. The Pacific plate is moving ~parallel to the continental edge; it has already captured the western part of California (everything west of the San Andreas fault, which is the small yellow area along the coast of California) and it is contributing to the extension (pulling apart) of eastern Oregon and Nevada (large light yellow area called the Basin and Range Province—BRP). The western edge of the BRP extends along the eastern side of the Cascade Range and the Sierra Nevada.

The purple block (translating Sierra Nevada block) is a micro-plate that has begun to move northward—it will probably eventually be captured and become part of the Pacific plate just as the area west of the San Andreas fault has been. Western Oregon is part of the OC (Oregon Coast block) that is rotating clockwise relative to the other crustal blocks. This rotation is causing northward compression into the green block around Seattle and causing N-S shortening. The multitude of crustal faults around Seattle are a result of these compressive forces. All of these blocks, with different motions, are a result of the complexity of so many interacting tectonic plates. The diagram above is from a USGS site (see references below).

In summary, there appears to be little danger from crustal faults in the Rogue Valley region, although moderate-sized earthquakes can be expected to the east near Klamath Falls. And when the Cascadia subduction zone moves (the “big one”), it will definitely be felt in the region, although without the impact of a tsunami. It is likely that structural damage will be less in this region than along the coast, although damage is highly dependent on local geologic conditions and specific building construction. During the last week of April (2020), people were surprised by shaking associated with several magnitude 3 earthquakes that were located ~20 km SW of Ashland—for approximate location, see blue star in the map below. It is not known which crustal fault caused these shallow-depth earthquakes.

Back to the Rogue Valley region: this map shows the regional geology with the major rock units in different colors. Remember that all of the units on this map were tilted to the NE before today’s active Cascade volcanoes were established farther east. This tilting implies that the region was subjected to compressive forces that folded and uplifted all of the rock units. The blue symbols show the direction of tilt—to the NE and (probably) to the SW, the other side of this large-scale fold structure. There are only very old Klamath terrane rocks to the west, so the SW tilt is difficult to discern.

Why did tilting in the Rogue Valley region occur? Geologists remain uncertain, but we can explore some possibilities. One thing we know is that a variety of changes in the subduction zone can affect the overlying continental plate. For example, the oceanic plate can descend at a steeper angle or at a shallower angle. Influences on the angle of subduction include the age of the oceanic lithosphere and “bumps” such as extinct volcanoes on top of the down-going oceanic lithosphere that interfere with the subduction process.

This diagram shows how the descending oceanic plate can influence what happens in the overlying continental plate. With steep subduction (bottom image) the oceanic plate reaches deep levels, with very hot temperatures, close to the coast and forms the typical configuration of coast-parallel volcanoes. In contrast, with shallow subduction, the oceanic plate remains at a shallow depth far inland from the coast, which extinguishes the chain of coast-parallel volcanoes and pushes the effects of subduction farther inland. From ~80–60 million years ago, the subduction angle was extremely shallow and the effects went all of the way to the Rocky Mountains. Because the oceanic plate was so shallow, it was more coupled to the overlying continental plate, and compressive forces caused extensive folding and uplift in the overlying continent. This may be the reason for the missing geologic record, from ~70–50 million years ago, between the Hornbrook and Payne Cliffs Formations, since missing record usually means the land was being uplifted and eroded away. The diagram above is from the Digital Geology of Idaho web site (see references below).

It could also have been a change in the subducting angle that caused the younger tilting event around 10 million years ago. The Western Cascade volcanoes turned off and then the younger (currently active) volcanoes turned on farther east. A small change in the subducting angle could cause compression (instigating folding and tilting) across the region and a more eastward location for melting and volcano production.

This diagram shows another interesting aspect of the Rogue Valley region. The Klamath terrane, shown in blue and purple colors, appears not only in SW Oregon and NW California, but also in the western foothills of the Sierra Nevada in California and in the Blue Mountains in eastern Oregon. These areas were once continuous but have now been rearranged by subsequent tectonic activity. Also shown is the eastern extent of the shallow-subduction event ~80–60 million years ago (Cretaceous fold & thrust belt) and the extent of the extensional Basin & Range province.
The bottom line is that the Rogue Valley region is near plate boundaries, where most tectonic action occurs. Plates interact at their boundaries, where one is likely to see mountains and beautiful scenery. Natural hazards, mainly volcanic eruptions and earthquakes, are a risk of life “on the edge”, but the price many are willing to pay to live in gorgeous places. The photo above is one of these gorgeous places—Crater Lake (actually a caldera) that was produced when the Cascade volcano named Mazama blew up ~7,000 years ago. Wizard Island is where the volcano has begun to built up its cone again. The edges of the lake are the former flanks of a volcano that would have projected up to much higher elevations prior to the explosion.

Important references

Unlocking the Cascadia Subduction Zone’s secrets: Peering into recent research and findings, by Andrea Watts, 2014, EARTH magazine: https://www.earthmagazine.org/article/unlocking-cascadia-subduction-zones-secrets-peering-recent-research-and-findings

PNSN (Pacific Northwest Seismic Network) web site about the Cascadia Subdution Zone: https://pnsn.org/outreach/earthquakesources/csz

USGS (U.S. Geological Survey) site about Cascadia (this web page no longer exists but the link goes to USGS publications): https://geomaps.wr.usgs.gov/pacnw/rescasp1.html

Elliot, Bill, 2007, Field trip guide to the Upper Cretaceous Hornbrook Formation and Cenozoic rocks of southern Oregon and northern CA.

Digital Geology of Idaho web site about the Challis Magmatic Episode: http://geology.isu.edu/Digital_Geology_Idaho/Module8/mod8.htm

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