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.

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.

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).

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).

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.

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.

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.


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
Wow Karen, you really put a lot of work into these posts! What a labour of love!!!
It feels so important to present science and analysis based on evidence, especially in these times we live in. Thanks for reading.
Terrific post, Karen! I would love to use some of these images for my National Parks class, if that is ok!
Yes of course Julie!