Colorado orogeny
Encyclopedia
The Colorado orogeny, or Colorado orogen, was an orogeny
in Colorado
and surrounding areas which was a part of the development of the ancestral Rockies. The Colorado orogeny, formerly called the Colorado province, is a >500-km-wide belt of oceanic
arc
rock (1.78-1.65 Ga) that extends southward into New Mexico
and composes a major part of the Proterozoic
provinces of southwestern United States
. This transcontinental collisional event occurred during the Paleoproterozoic
(Statherian
Period). The Wyoming
sector of the Colorado orogeny was formerly called the Medicine Bow orogeny. The eastern sector extends into the High Plains
and is called the Central Plains orogeny. The boundary between the Colorado orogeny and the Wyoming craton
is the Cheyenne belt
, a 5-km-wide mylonitic shear zone
that verges
northward. The Cheyenne belt transects and cuts off the south edge of the older Trans-Hudson orogeny
.
The Paleoproterozoic volcanic
and sedimentary rocks that resulted from the Colorado orogeny underwent metamorphism
followed by plastic folding
under moderate pressure and temperature (PT) conditions (temperature about 500 °C and pressures in excess of 1.2 GPa
). The metamorphism was accompanied by intrusion of intermediate calc-alkalic rocks, such as the granodiorite
s of the Boulder Creek batholith. The accompanying amphibolite
facies
metamorphism is characterized by sillimanite
and, locally, garnet
, andalusite
, and cordierite
. Contemporaneity of emplacement of the granodioritic rocks with folding is indicated by concordant plutonic boundaries and by conformity of the internal structure (of solid-state
recrystallization
) in the batholith with that in the supracrustal wall rocks. Comparable mineral facies in the country rocks and batholiths indicate that emplacement took place at moderate depths.
The Colorado orogeny exhibits differing structural deformation patterns throughout its range. Adjacent to the Cheyenne belt, and extending across a width of at least 150 km to the south, foliation and upright folds predominantly trend westward. In the northern Front Range
sector of this region, geologic mapping demonstrated three generations of northwest-trending folds that pre-date the ~1.4 Ga shear zones. Similar fold patterns are present in the northern Park Range
and Medicine Bow Mountains
. These structural fabrics
indicate shortening in a north-south direction and can be explained by collision, subsequent subduction, and continued convergence along the Cheyenne belt. Farther south, more distant from the Cheyenne belt, fold patterns differ materially from those in the northernmost part of the Colorado orogeny. In the north-central Front Range, west of Denver, in an area of >2000 km2 that has been mapped in detail the older regional folds mainly bear north-northeast; the folds range from broad open, upright folds to tight, upright folds that plunge gently to moderately northeast. In nearby areas to the east and west, however, the folds of the older orogenic event trend northwest; field observations indicate that these folds apparently are slightly older than the more prevalent north-trending folds, but both generations are part of the older gneiss
-forming episode inasmuch as they are cut by the Boulder Creek batholith. Both sets of these folds indicate shortening events resulting from regional stress patterns. Because of the consistency of these fold patterns over relatively large areas, evidence for folding resulting from forceful intrusion
of igneous rocks is generally lacking; intrusions of this orogenic event are synkinematic
, as exemplified by the much-studied Boulder Creek batholith.
Orogeny
Orogeny refers to forces and events leading to a severe structural deformation of the Earth's crust due to the engagement of tectonic plates. Response to such engagement results in the formation of long tracts of highly deformed rock called orogens or orogenic belts...
in Colorado
Colorado
Colorado is a U.S. state that encompasses much of the Rocky Mountains as well as the northeastern portion of the Colorado Plateau and the western edge of the Great Plains...
and surrounding areas which was a part of the development of the ancestral Rockies. The Colorado orogeny, formerly called the Colorado province, is a >500-km-wide belt of oceanic
Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...
arc
Volcanic arc
A volcanic arc is a chain of volcanoes positioned in an arc shape as seen from above. Offshore volcanoes form islands, resulting in a volcanic island arc. Generally they result from the subduction of an oceanic tectonic plate under another tectonic plate, and often parallel an oceanic trench...
rock (1.78-1.65 Ga) that extends southward into New Mexico
New Mexico
New Mexico is a state located in the southwest and western regions of the United States. New Mexico is also usually considered one of the Mountain States. With a population density of 16 per square mile, New Mexico is the sixth-most sparsely inhabited U.S...
and composes a major part of the Proterozoic
Proterozoic
The Proterozoic is a geological eon representing a period before the first abundant complex life on Earth. The name Proterozoic comes from the Greek "earlier life"...
provinces of southwestern United States
Southwestern United States
The Southwestern United States is a region defined in different ways by different sources. Broad definitions include nearly a quarter of the United States, including Arizona, California, Colorado, Nevada, New Mexico, Oklahoma, Texas and Utah...
. This transcontinental collisional event occurred during the Paleoproterozoic
Paleoproterozoic
The Paleoproterozoic is the first of the three sub-divisions of the Proterozoic occurring between . This is when the continents first stabilized...
(Statherian
Statherian
The Statherian is the final geologic period in the Paleoproterozoic Era and lasted from 1800 Mya to 1600 Mya . Instead of being based on stratigraphy, these dates are defined chronometrically....
Period). The Wyoming
Wyoming
Wyoming is a state in the mountain region of the Western United States. The western two thirds of the state is covered mostly with the mountain ranges and rangelands in the foothills of the Eastern Rocky Mountains, while the eastern third of the state is high elevation prairie known as the High...
sector of the Colorado orogeny was formerly called the Medicine Bow orogeny. The eastern sector extends into the High Plains
High Plains (United States)
The High Plains are a subregion of the Great Plains mostly in the Western United States, but also partly in the Midwest states of Nebraska, Kansas, and South Dakota, generally encompassing the western part of the Great Plains before the region reaches the Rocky Mountains...
and is called the Central Plains orogeny. The boundary between the Colorado orogeny and the Wyoming craton
Wyoming craton
The Wyoming craton is a craton located in the west-central United States and western Canada – more specifically, in Montana, Wyoming, southern Alberta, southern Saskatchewan, and parts of northern Utah...
is the Cheyenne belt
Cheyenne belt
The Cheyenne Belt is the tectonic suture zone between the Archean-age Wyoming craton to the north and the Paleoproterozoic-age Yavapai province to the south. In runs through the southeastern quadrant of the state of Wyoming, USA. It was formed during the Paleoproterozoic Medicine Bow orogeny...
, a 5-km-wide mylonitic shear zone
Shear zone
A shear zone is a very important structural discontinuity surface in the Earth's crust and upper mantle. It forms as a response to inhomogeneous deformation partitioning strain into planar or curviplanar high-strain zones. Intervening blocks stay relatively unaffected by the deformation...
that verges
Vergence (geology)
In structural geology, the vergence of a fold is the direction in which an antiform is inclined or overturned. The term vergence comes from the German vergenz, which means "overturn"....
northward. The Cheyenne belt transects and cuts off the south edge of the older Trans-Hudson orogeny
Trans-Hudson orogeny
The Trans-Hudson orogeny, Trans-Hudsonian orogeny, Trans-Hudson orogen , or Trans-Hudson Orogen Transect , , was the major mountain building event that formed the Precambrian Canadian Shield, the North American craton , and the...
.
The Paleoproterozoic volcanic
Volcanic rock
Volcanic rock is a rock formed from magma erupted from a volcano. In other words, it is an igneous rock of volcanic origin...
and sedimentary rocks that resulted from the Colorado orogeny underwent metamorphism
Metamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
followed by plastic folding
Fold (geology)
The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in...
under moderate pressure and temperature (PT) conditions (temperature about 500 °C and pressures in excess of 1.2 GPa
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...
). The metamorphism was accompanied by intrusion of intermediate calc-alkalic rocks, such as the granodiorite
Granodiorite
Granodiorite is an intrusive igneous rock similar to granite, but containing more plagioclase than orthoclase-type feldspar. Officially, it is defined as a phaneritic igneous rock with greater than 20% quartz by volume where at least 65% of the feldspar is plagioclase. It usually contains abundant...
s of the Boulder Creek batholith. The accompanying amphibolite
Amphibolite
Amphibolite is the name given to a rock consisting mainly of hornblende amphibole, the use of the term being restricted, however, to metamorphic rocks. The modern terminology for a holocrystalline plutonic igneous rocks composed primarily of hornblende amphibole is a hornblendite, which are...
facies
Metamorphic facies
The metamorphic facies are groups of mineral compositions in metamorphic rocks, that are typical for a certain field in pressure-temperature space...
metamorphism is characterized by sillimanite
Sillimanite
Sillimanite is an alumino-silicate mineral with the chemical formula Al2SiO5. Sillimanite is named after the American chemist Benjamin Silliman . It was first described in 1824 for an occurrence in Chester, Middlesex County, Connecticut, USA....
and, locally, garnet
Garnet
The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" may come from either the Middle English word gernet meaning 'dark red', or the Latin granatus , possibly a reference to the Punica granatum , a plant with red seeds...
, andalusite
Andalusite
Andalusite is an aluminium nesosilicate mineral with the chemical formula Al2SiO5.The variety chiastolite commonly contains dark inclusions of carbon or clay which form a checker-board pattern when shown in cross-section....
, and cordierite
Cordierite
Cordierite or iolite is a magnesium iron aluminium cyclosilicate. Iron is almost always present and a solid solution exists between Mg-rich cordierite and Fe-rich sekaninaite with a series formula: 2 to 2...
. Contemporaneity of emplacement of the granodioritic rocks with folding is indicated by concordant plutonic boundaries and by conformity of the internal structure (of solid-state
Solid
Solid is one of the three classical states of matter . It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a...
recrystallization
Crystallization
Crystallization is the process of formation of solid crystals precipitating from a solution, melt or more rarely deposited directly from a gas. Crystallization is also a chemical solid–liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid...
) in the batholith with that in the supracrustal wall rocks. Comparable mineral facies in the country rocks and batholiths indicate that emplacement took place at moderate depths.
The Colorado orogeny exhibits differing structural deformation patterns throughout its range. Adjacent to the Cheyenne belt, and extending across a width of at least 150 km to the south, foliation and upright folds predominantly trend westward. In the northern Front Range
Front Range
The Front Range is a mountain range of the Southern Rocky Mountains of North America located in the north-central portion of the U.S. State of Colorado and southeastern portion of the U.S. State of Wyoming. It is the first mountain range encountered moving west along the 40th parallel north across...
sector of this region, geologic mapping demonstrated three generations of northwest-trending folds that pre-date the ~1.4 Ga shear zones. Similar fold patterns are present in the northern Park Range
Park Range (Colorado)
The Park Range, elevation approximately , is a mountain range in the Rocky Mountains of northwestern Colorado in the United States. The range forms a relatively isolated part of the continental divide, extending north-to-south for approximately along the boundary between Jackson and Routt counties...
and Medicine Bow Mountains
Medicine Bow Mountains
The Medicine Bow Mountains are a mountain range in the Rocky Mountains that extend for from northern Colorado into southern Wyoming. The northern extent of this range is the sub-range the Snowy Range...
. These structural fabrics
Fabric (geology)
In geology, a rock's fabric describes the spatial and geometric configuration of all the elements that make it up.-Types of fabric:* Primary fabric — a fabric created during the original formation of the rock, e.g...
indicate shortening in a north-south direction and can be explained by collision, subsequent subduction, and continued convergence along the Cheyenne belt. Farther south, more distant from the Cheyenne belt, fold patterns differ materially from those in the northernmost part of the Colorado orogeny. In the north-central Front Range, west of Denver, in an area of >2000 km2 that has been mapped in detail the older regional folds mainly bear north-northeast; the folds range from broad open, upright folds to tight, upright folds that plunge gently to moderately northeast. In nearby areas to the east and west, however, the folds of the older orogenic event trend northwest; field observations indicate that these folds apparently are slightly older than the more prevalent north-trending folds, but both generations are part of the older gneiss
Gneiss
Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...
-forming episode inasmuch as they are cut by the Boulder Creek batholith. Both sets of these folds indicate shortening events resulting from regional stress patterns. Because of the consistency of these fold patterns over relatively large areas, evidence for folding resulting from forceful intrusion
Intrusion
An intrusion is liquid rock that forms under Earth's surface. Magma from under the surface is slowly pushed up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the rock slowly...
of igneous rocks is generally lacking; intrusions of this orogenic event are synkinematic
Kinematics
Kinematics is the branch of classical mechanics that describes the motion of bodies and systems without consideration of the forces that cause the motion....
, as exemplified by the much-studied Boulder Creek batholith.