10: Klamath Mountains

Introduction to the Klamath Mountains

In the far northwest corner of California, just inland from the fog belt and its towering old growth redwood trees, the Klamath Mountains province is one of the most remote places in the state. Wild and scenic rivers cut deep valleys into steep hillsides covered in impenetrable brush, which give way to rocky peaks and alpine lakes at higher elevations. There are eleven designated wilderness areas in the Klamath Mountains, including the Trinity Alps, Marble Mountain and Siskiyou Wilderness areas.

The Klamath Mountains extend into southeast Oregon, with a little over half of the province existing on the California side and half on the Oregon side; geology doesn’t care about arbitrary state borders. The province is bounded by the Coast Ranges to the west, the Cascade Range to the east, and the Great Valley to the southeast (Figure \(\PageIndex{1}\)). The bedrock geology is also closely related to the geology of the Sierra Nevada, though the two provinces are not immediately adjacent. 

In their current configuration, the Klamath Mountains comprise part of the forearc mountain range at the southern end of the Cascadia subduction system. “Forearc” meaning that they lie between the subduction zone trench and the associated volcanic arc mountain range (in this case, the Cascade Range). The Klamath Mountains are considerably higher in elevation above sea level and relief compared to the Oregon Coast Range, which is the forearc range to the north. The Klamath Mountains produce a rain shadow effect (see California's Climate). The western part of the range is the one of the wettest regions of the state, with over 150 inches of average annual precipitation in some areas. 

Because forearc deformation and uplift processes are typically not associated with volcanism, forearc ranges such as the Klamath Mountains can provide a window into the geology that makes up the foundation of the continent. To the east, this basement rock is covered by younger volcanic and sedimentary rocks, but uplift without volcanism strips away the younger cover. In the 1960s and 1970s, coincident with the development of the modern Theory of Plate Tectonics, geologic mapping of the Klamath Mountains laid the foundations for our current understanding of how continents form and grow. The bedrock geology consists mainly of accreted terranes. Each terrane was once an island arc that developed offshore and was later added to the North American continent at the subduction zone. Each island arc was thrust up against and partially beneath an older, previously accreted terrane forming a sequence of terranes that get progressively younger to the west, separated by large eastward dipping thrust faults. Later, arc related volcanism intruded into these accreted terranes, leaving plutons that stitch together the quilt of terranes. These plutons have allowed geologists to use cross cutting relationships to determine the age of accretion related faulting.

The Klamath Mountains province was also important in the development of geologists' understanding of oceanic crust and the processes that form at mid-ocean ridges. In the process of accretion, sometimes bits of oceanic crust get caught up into the accreted terrane and part of the continental plate, forming ophiolites. There are several ophiolites of a variety of different ages within the Klamath Mountains, but in this chapter we have chosen to focus on one case-study example; the Josephine Ophiolite, one of the best exposed and most intact ophiolites in North America.

Although the general concept of terrane accretion is now fully accepted, geologists are still working to understand the details. Questions remain about where the terranes came from and why and when they accreted. Geologists are currently working to better comprehend exactly why the Klamath Mountains have anomalously higher modern elevations, compared to other forearc ranges. These questions and others continue to drive geologists into the Klamath Mountains, as this province remains key to understanding how our planet works.