A Window into a Fossil Subduction Zone

Authored by Carolyn Tewksbury-Christle.

Whitney Behr and Mark Helper, both fresh off several weeks of field work in Alaska, joined me in another field season exploring the geology of the Condrey Mountain Window. Straddling the California-Oregon border, the Condrey Mountain Window is a part of the Klamath Mountains and belongs to both the Rogue Valley and Klamath National Forests. This spectacular wilderness area is sparsely populated, and we typically only get to talk about the geology with the occasional hikers on the Pacific Crest Trail, which cuts across the Window, and with the cows that roam the high meadows.

A view from White Mountain looking north at the Klamaths with Whitney for scale (small shape on the slope in the bottom center of the photo).
A typical traffic jam in the Condrey Mountain Window.

The Klamath Mountains consist primarily of fringing island arcs and affiliated rocks that have been tacked onto the western margin of North America. The Condrey Mountain rocks, on the other hand, occupy a window through the Klamath Mountains that exposes younger rocks that started out life on a subducting plate. These rocks subducted along the interface between the overriding and subducting plates to a depth of 25-30 km before being brought back to the surface. By looking at the deformation recorded in these rocks, we hope to better understand processes in modern subduction zones, including – but not limited to – earthquakes and related seismic phenomena, recycling to the Earth’s deep interior, and subduction zone evolution. And these rocks have lots of deformation to look at!

In previous years, wildfires have been common near (or in) the Window to the point where smoke in the air was ever-present, and we actually had to leave early because of road closures due to fires in 2017. The lack of fires this year was so unusual that we woke one morning to a campsite socked in with fog and immediately thought that we were going to have to evacuate. The fog persisted well into the morning, piling up against the east-west ridge that the Pacific Crest Trail follows through the Window. Once we got up high enough, we were treated to beautiful views looking down on the clouds and across the Klamaths to see Mount Shasta in the distance.

View from the top of White Mountain with Mount Shasta in the background. All the rocks that we wanted to look at are down the slope to the left in the fog!

We braved a third ascent up Scraggy Peak – a short but tough hike thanks to steep, deadfall-covered slopes interspersed with cliffy outcrops and dense scrub that is wholly determined to throw you off the side of the mountain if given the chance. Scraggy Peak is topped with epidote blueschists that show off some truly spectacular folds, so the hike was well worth it!

Smiles post Scraggy Peak (rocky fins in the upper right corner of the photo).

We came home with 6 five-gallon buckets of rocks, practically more structural measurements than we know what to do with, and a really interesting tectonic history for the rocks of the Condrey Mountain Window. Now it’s time to write up our conclusions and dream of steep slopes and jagged rocks.

Steep, rocky exposures on the flanks of Scraggy Peak with me for scale (bottom center of the picture).

We’d like to thank the Rogue-Siskyou Forest Service and, in particular, the Star Ranger Station for their support of our field work. See you next year!

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