10.6: Chapter Summary
- Page ID
- 21523
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10.1 Assembling a Continent
- The Klamath Mountains have historical significance in the development of the idea of terrane accretion.
- Terranes are composed of ancient island arcs.
- Melange forms through the accretion of sediments and other oceanic crustal rocks in an accretionary wedge.
- The Klamath Mountains are composed of several accreted terranes, with a general pattern of alternation between terrane and mélange.
- Large east-dipping thrust faults separate terranes.
- The sequence of terranes in the Klamath Mountains is similar to the sequence of terranes in the Sierra Nevada.
10.2 The McCloud Limestone: An Ancient Coral Reef
- The McCloud Limestone formed in a coral reef during the Permian Period.
- Permian coral reef communities were composed of many species that went extinct in the end-Permian mass extinction.
- Comparison of fossil species found in different geologic units can help geologists understand the paleoenvironment.
10.3 The Josephine Ophiolite: A Little Slice of the Mantle
- Obduction is the uplift of oceanic lithosphere onto continental lithosphere and it is less common by far than subduction.
- The Josephine Ophiolite formed in a back-arc basin and was obducted when a tectonic change resulted in the collapse of the back-arc basin and accretion of the island arc.
- Ophiolites themselves are rare, but the ophiolite sequence makes up most of the crust on earth (the oceanic crust).
- The ophiolite sequence forms as magma cools within a magma chamber and lava erupts on the ocean floor at mid ocean ridges (or back-arc spreading ridges).
- The ultramafic chemistry of ophiolites produce unique soils and correspondingly unique ecosystems.
10.4 Plutons of the Klamath Mountains
- The story of the Klamath mountains involves alternation of island arc volcanism and continental arc volcanism.
- Continental arc volcanism resulted in emplacement of plutons that cross-cut older accreted terranes.
- Dating of plutons in combination with cross cutting relationships allows for the dating of accretion events.
- Zoned plutons result from the slow cooling of magma chambers from outside inwards.
10.5 Uplifting a Mountain Range
- During the Cretaceous Period, when the Sierra Nevada volcanic arc was forming, the Klamath province was part of the forearc basin, much like the Great Valley.
- The topographic difference between the Klamath Mountains and the Great Valley is the result of Neogene uplift.
- The uplift of the current mountain range is a much younger event than the events involving the accretion of the bedrock.
- Normal faults in the southern Klamath, such as the La Grange Fault contributed to mountain uplift.
- The Weaverville Formation is composed of sedimentary fill formed in Neogene grabens and basins.
- Glacial erosional features formed during the Pleistocene Epoch.
- Small glaciers were still present in the Klamath Mountains at the start of the 21st century.