The Smartville Block, also called the Smartville Ophiolite, Smartville Complex, or Smartville Intrusive Complex, is a geologic
terrane
In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust (geology), crust fragment formed on a tectonic plate (or broken off from it) and Accretion (geology), accreted or "Suture (geology), sutured" to crust lying on another pla ...
formed in the ocean from a volcanic
island arc
Island arcs are long chains of active volcanoes with intense seismic activity found along convergent tectonic plate boundaries. Most island arcs originate on oceanic crust and have resulted from the descent of the lithosphere into the mantle alon ...
that was accreted onto the
North American Plate
The North American Plate is a tectonic plate covering most of North America, Cuba, the Bahamas, extreme northeastern Asia, and parts of Iceland and the Azores. With an area of , it is the Earth's second largest tectonic plate, behind the Pacif ...
during the late
Jurassic
The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...
(~160–150 million years ago). The collision created sufficient crustal heating to drive mineral-laden water up through numerous fissures along the contact zone. When these cooled, among the precipitating minerals was
gold
Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile me ...
. Associated with the Western Metamorphic Belt of the Sierra Nevada foothills it extends from the central
Sierra Nevada mountain range, due west, under a section of the
Central Valley and
California Coast Ranges
The Coast Ranges of California span from Del Norte or Humboldt County, California, south to Santa Barbara County. The other three coastal California mountain ranges are the Transverse Ranges, Peninsular Ranges and the Klamath Mountains.
P ...
, in northern California. The ophiolitic sequence found in this terrane is one of several major ophiolites found in California.
Ophiolites
An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crustal rocks.
The Greek word ὄφις, ''ophis'' (''snake'') is found i ...
are crustal and upper-mantle rocks from the ocean floor that have been moved on land. Ophiolites have been studied extensively regarding the movement of crustal rocks by
plate tectonics
Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large ...
.
Gold
Gold was discovered in California in the mid 1800s. The Smartville Block is named for the small Gold Rush town of
Smartville in
Yuba County
Yuba County (; Maidu: ''Yubu'') is a county in the U.S. state of California. As of the 2020 U.S. Census, the population was 81,575. The county seat is Marysville. Yuba County is included in the Yuba City, California Metropolitan Statistical A ...
, California. Gold found in the Smartville Block and within the metamorphic belts of the Sierra Nevada foothills were created through hydrothermal processes. Mineral rich groundwater is heated geothermally and injected towards the surface through fractures in the rock created from faulting. Gold and other minerals are precipitated from this fluid and have accumulated in the Sierra Nevadan metamorphic belt region in features called
veins
Veins are blood vessels in humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated b ...
.
The Smartville Block's eastern margin is the
Rescue Lineament-Bear Mountains fault zone, and generally defines the range of gold-bearing veins of the
Mother Lode region, important in the history of the
California Gold Rush.
Geology
The Smartville Ophiolite is divided into sections based on
lithology that describe the origin of the rocks it is made up of. In order of increasing depth: the top most unit is called the upper volcanic unit, then the lower volcanic unit, followed by the dike complex, and the plutonic suite at the bottom. The upper volcanic unit consists mainly of
pillow lava
Pillow lavas are lavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava underwater, or ''subaqueous extrusion''. Pillow lavas in volcanic rock are characterized by thick sequences of discont ...
and massive lavas with
turbidites
A turbidite is the geologic deposit of a turbidity current, which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean.
Sequencing
Turbidites were ...
also being common. The turbidites of the upper volcanic unit are made of a
siliceous rock
Siliceous rocks are sedimentary rocks that have silica (SiO2) as the principal constituent. The most common siliceous rock is chert; other types include diatomite. They commonly form from silica-secreting organisms such as radiolarians, diatoms
...
type and have
sandstones
Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.
Most sandstone is composed of quartz or feldspar (both silicates) b ...
and
conglomerate tops. Next is the lower volcanic unit, which is mostly pillow lava,
breccia
Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix.
The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of ...
, and intrusive rocks. Metasediments of mostly
mafic
A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks incl ...
and intermediate
tuffaceous
Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock c ...
composition are cut by dikes (intrusive rock). Below the lower volcanic unit is the dike complex. The dike complex has
dacite
Dacite () is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite ...
and
rhyolite dikes that cut into the rocks of the lower volcanic unit. At the base is the plutonic suite that are made up of
gabbro
Gabbro () is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is ch ...
,
diorite
Diorite ( ) is an intrusive igneous rock formed by the slow cooling underground of magma (molten rock) that has a moderate content of silica and a relatively low content of alkali metals. It is intermediate in composition between low-sili ...
, and other plutonic rocks that are intruded by dikes as well as granitic plutons from the
Nevadan Orogeny
The Nevadan orogeny occurred along the western margin of North America during the Middle Jurassic to Early Cretaceous time which is approximately from 155 Ma to 145 Ma. Throughout the duration of this orogeny there were at least two different kin ...
.
See also
*
*
References
Additional Reading
* ''Assembling California'',
John McPhee
John Angus McPhee (born March 8, 1931) is an American writer. He is considered one of the pioneers of creative nonfiction. He is a four-time finalist for the Pulitzer Prize in the category General Nonfiction, and he won that award on the four ...
, 1993. Farrar, Straus and Giroux, New York.
* Day, Howard W., E. M. Moores, and A. C. Tuminas. "Structure and tectonics of the northern Sierra Nevada." ''Geological Society of America Bulletin'' 96.4 (1985): 436–50.
* Dilek, Yildirim. “Tectonic Significance of Post-Accretion Rifting of a Mesozoic Island-Arc Terrane in the Northern Sierra Nevada, California.” ''The Journal of Geology'', vol. 97, no. 4, 1989, pp. 503–518.
* Dilek, Yildirim. "Structure and tectonics of an Early Mesozoic oceanic basement in the northern Sierra Nevada metamorphic belt, California: Evidence for transform faulting and ensimatic arc evolution." ''Tectonics'' 8.5 (1989): 999–1014.
* Menzies, Martin, Douglas Blanchard, and Costas Xenophontos. "Genesis of the Smartville arc-ophiolite, Sierra Nevada foothills, California." ''American Journal of Science'' 280 (1980): 329–44.
* Saleeby, Jason B. "Polygenetic ophiolite belt of the California Sierra Nevada: Geochronological and tectonostratigraphic development." ''Journal of Geophysical Research: Solid Earth'' 87.B3 (1982): 1803–24.
* Taylor, Ryan D. (2015). ''Orogenic gold formation and tectonic evolution of the Grass Valley gold district and temporal correlations of gold deposits in California''. Diss. Colorado School of Mines. Arthur Lakes Library.
* Xenophontos, Costas, and Gerard C. Bond. "Petrology, sedimentation and paleogeography of the Smartville terrane (Jurassic)-bearing on the genesis of the Smartville ophiolite." (1978): 291–302.
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Terranes
Geology of California
California Gold Rush
Sierra Nevada (United States)
Geography of Amador County, California
Geography of Calaveras County, California
Geography of El Dorado County, California
Geography of Mariposa County, California
Geography of Tuolumne County, California
Geography of Yuba County, California
Seismic faults of California