10.4: Plutons of the Klamath Mountains
- Page ID
- 21521
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)The Stitching in the Quilt
Drivers along Interstate-5 between the Oregon border and the Great Valley have the pleasure of excellent views on both sides of the highway. The freeway here passes right through the intersection of the Klamath Mountains and the Cascade Range. To the east, Mount Shasta presents an imposing view of an active volcano, and to the west, the 160 Ma granite mountain of Castle Crags provides a view of a volcano long gone. The Castle Crags Pluton is one of many such plutons in the Klamath Mountains province, but it is the most famous plutonic outcropping on account of its visibility to travelers. Today, most travelers merely glimpse the crags as they speed by at 70 miles per hour, but Castle Crags has been an important cultural landmark since prehistoric times. The ancient Siskiyou Trail, which was the main route between the Great Valley and the Willamette Valley, passed near Castle Crags. Today Castle Crags is designated as a state park.
Plutonic rocks in the Klamath Mountains do not necessarily follow the same pattern of younging to the west that is seen in other rock types. For example, in the eastern part of the province, within the Redding subterrane, two plutons can be found in close proximity, with widely varying ages. The Mule Mountain Pluton is approximately 400 Ma, consistent with the age of the terrane, but the nearby Shasta Bally Batholith is a mere 136 Ma, younger than any of the Klamath terranes. The Mule Mountain Pluton formed within the island arc prior to accretion, whereas the Shasta Bally Batholith represents a much younger episode of volcanism that cross cuts the geology of the older terrane. During episodes of Andean type subduction, arc volcanism occurs within the continent and the earlier accreted terranes become the basement onto which volcanoes are erupted and into which plutons are intruded. Though the volcanoes have been eroded, the plutons remain as evidence that a volcanic range once existed. Shasta Bally is part of the youngest of these episodes preserved in the Klamath Mountains. Castle Crags formed during an earlier episode that corresponds to the formation of the accretionary wedge mélange that would eventually become the Rattlesnake Creek terrane. Because igneous intrusions can be easily radiometrically dated (see 5.4: Absolute Dating of Geologic Materials), the plutons in the Klamath Range have helped geologists determine the timing of faulting associated with accretion. You can practice a simplified version of the relative dating required in Query \(\PageIndex{1}\).
You may find review of 5.1: Relative Dating helpful in completing this activity.
Castle Crags, like many plutons in the Klamath Mountains, is a zoned pluton. As the magma initially intruded into the ultramafic country rock, the outside of the chamber cooled more quickly, producing a finer-grained texture, though still coarse enough to be considered phaneritic. This forms a solidified but still warm layer at the edge of the pluton, which insulates the interior and allows it to cool more slowly, producing granitic rock with large mineral crystals near the interior. There are also compositional differences between the various zones of the pluton as the magma composition changed over time (see 7.1: The Cascadia Subduction Zone and the Cascade Continental Volcanic Arc).
Like the plutonic rocks of the Sierra Nevada (see 9.3: The Sierra Nevada Batholith), Castle has gone from being a magma chamber deep within the crust to a prominent, craggy outcrop due to differential erosion. The granitic rock of the pluton is more resistant to weathering and erosion than the surrounding ultramafic rocks of the Eastern Klamath terrane. Also much like the Sierra Nevada, the pluton was carved into spires by glacial erosion during the Pleistocene Epoch (see 9.6: Ice Shapes the Landscape).
References
- Irwin, W. P., & Wooden, J. L. (1999). Plutons and accretionary episodes of the Klamath Mountains, California and Oregon. US Geological Survey. https://pubs.usgs.gov/publication/ofr99374
- Michael Wopat. (2015). Castle Crags State Park, Geological Gems of California State Parks, GeoGem Note 23. In Syd Brown, Mike Fuller, Bill Short, & Chris Wills (Eds.), California Geological Survey Special Report 230 under Interagency Agreement C01718011 with California State Parks. https://www.parks.ca.gov/pages/734/files/GeoGem%20Note%2023%20Castle%20Crags%20State%20Park.pdf