Sierra Nevada Batholith
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The Sierra Nevada Batholith is a large batholith that is approximately 400 miles long and 60-80 miles wide which forms the core of the
Sierra Nevada The Sierra Nevada ( ) is a mountain range in the Western United States, between the Central Valley of California and the Great Basin. The vast majority of the range lies in the state of California, although the Carson Range spur lies primari ...
mountain range in
California California () is a U.S. state, state in the Western United States that lies on the West Coast of the United States, Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares Mexico–United States border, an ...
, exposed at the surface as
granite Granite ( ) is a coarse-grained (phanerite, phaneritic) intrusive rock, intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly coo ...
. The batholith is composed of many individual masses of rock called '' plutons'', which formed deep underground during separate episodes of
magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...
intrusion, millions of years before the Sierra itself first began to rise. The extremely hot, relatively buoyant plutons, also called ''plutonic diapirs'', intruded through denser, native country rock and sediments, never reaching the surface. At the same time, some magma managed to reach the surface as volcanic lava flows, but most of it cooled and hardened below the surface and remained buried for millions of years. The batholith – the combined mass of subsurface plutons – became exposed as
tectonic Tectonics ( via Latin ) are the processes that result in the structure and properties of the Earth's crust and its evolution through time. The field of ''planetary tectonics'' extends the concept to other planets and moons. These processes ...
forces initiated the formation of the Basin and Range geologic province, including the Sierra Nevada. As the mountains rose, the forces of
erosion Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust and then sediment transport, tran ...
eventually wore down the material which had covered the batholith for millions of years. The exposed portions of the batholith became the granite peaks of the High Sierra, including Mount Whitney, Half Dome and El Capitan. Most of the batholith, however, remains below the surface.


Origins

The Sierra batholith was formed when the Farallon Plate subducted below the North American Plate. The resultant molten rock rose through the Earth's crust over the span of 100 Ma, forming several plutons, or a chain of volcanoes if the
magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...
reached the surface. Most of the granitic rocks formed between 105 and 85 Ma, during the
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...
, with pluton formation ending around about 70 Ma.
Erosion Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust and then sediment transport, tran ...
from 85 until 15 Ma removed the volcanic rocks and exposed the granitic core.


Cooling and Uplift

Around 80-76 million years ago, subduction beneath the Sierra Nevada batholith transitioned from steep-angle to shallow-angle. This shut down arc magmatism, moving the volcanic arc westward and leaving the Sierra Nevada block in a forearc setting.
Apatite Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of Hydroxide, OH−, Fluoride, F− and Chloride, Cl− ion, respectively, in the crystal. The formula of the admixture of ...
and Sphene fission track thermochronology done by Dumitru (1990) revealed a period of rapid decrease in geothermal gradient (>270 °C to <70 °C from 80Ma to 60-50Ma) as the block cooled, followed by a relatively stable period of subnormal geothermal gradients (5-15 °C/km) throughout the Cenozoic. Modeling of the rapid decrease in geothermal gradient returned a crude estimate of depth to the subducting plate of about 35–50 km, with a hard upper limit of 60 km. This is much shallower than the more typical ~120 km depth to the subducting plate in volcanic arc regimes. Using data from his thermochronology analysis, Dumitru (1990) also constrained ages for the beginning of unroofing and uplift of the Sierra Nevada block to approximately 30-15Ma. Fission tracks – destructive remnants of
radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...
in
Uranium Uranium is a chemical element; it has chemical symbol, symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Ura ...
-bearing minerals – were shorter than expected in samples taken from several Sierra Nevada plutons. This implied a late-Cenozoic residency at depth, meaning the unroofing and uplift of the Sierra Nevada block happened rapidly near the end of the Cenozoic. Geologic evidence in the form of
erosion Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust and then sediment transport, tran ...
surfaces, paleo-canyons, and related deposits suggests the majority of the uplift was achieved prior to 4-10Ma.


Basement Units

The plutons associated with the Sierra Nevada Batholith intruded into pre-existing rocks on the North American Continent. As the plutons intruded into these rocks, many were altered or metamorphosed. The "host rocks" include passive margin sequence units, deep water sediments, and shallow-water passive margin units. There are several locations that the contact between the granitic intrusions and the now metamorphosed sedimentary units can be seen. These unique contacts are called roof pendants.


See also

* Geologic timeline of Western North America * Bald Rock, California * Sierra Nevada–Great Valley Block * Sierran Arc * Tuolumne Intrusive Suite * Sentinel granodiorite * Fine Gold Intrusive Suite * Intrusive Suite of Yosemite Valley


References


External links

* * {{Authority control Sierra Nevada (United States) Batholiths of North America Igneous petrology of California Mesozoic magmatism