Aspen anomaly is a geological structure in

Colorado, United States Colorado (, other variants) is a state in the Mountain states, Mountain West subregion of the Western United States. It encompasses most of the Southern Rocky Mountains, as well as the northeastern portion of the Colorado Plateau and the wes ...

. It consists of a low- seismic velocity anomaly in the

mantle A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that. Mantle may refer to: *Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear **Mantle (vesture), an Eastern Orthodox ve ...

which underpins the highest sector of the Rocky Mountains.

Characteristics

The Aspen anomaly is a seismic velocity anomaly in the

beneath central Colorado (in the region of Aspen, Colorado), which appears to reach down into the upper mantle. Helium with

isotope ratio The term stable isotope has a meaning similar to stable nuclide, but is preferably used when speaking of nuclides of a specific element. Hence, the plural form stable isotopes usually refers to isotopes of the same element. The relative abundanc ...

s indicative of mantle origin emanates from the terrain above the anomaly. The Aspen anomaly coincides with the highest region of the Rocky Mountains (such as the San Juan Mountains and the Sawatch Range) and divergent drainages ( Arkansas River, Colorado River and Gunnison River) which have cut deep gorges. This region underwent significant uplift during the

Cenozoic The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configura ...

starting from 10-5 million years ago and was subsequently eroded by the Colorado River. Ongoing present-day uplift of the San Juan Mountains may be linked to the Aspen anomaly. River knickpoints in

Gore Canyon Gore Canyon, elevation , is a short isolated canyon on the upper Colorado River in southwestern Grand County, Colorado in the United States. Steep and rugged, the approximately 3 mile (5 km) long gorge was carved by the river as it passed ...

and Black Canyon may mark the point at which the rivers pass through the edge of the region above the anomaly. The Colorado River may be influenced by the anomaly all the way to Lees Ferry, Arizona. Hot springs and

geyser A geyser (, ) is a spring characterized by an intermittent discharge of water ejected turbulently and accompanied by steam. As a fairly rare phenomenon, the formation of geysers is due to particular hydrogeological conditions that exist only in ...

s above the anomaly are a major source of carbon dioxide and other gases, some linked to

chemolithotrophic Lithotrophs are a diverse group of organisms using an inorganic substrate (usually of mineral origin) to obtain reducing equivalents for use in biosynthesis (e.g., carbon dioxide fixation) or energy conservation (i.e., ATP production) via aerobi ...

bacterial communities.

volcanism is also associated with the anomaly, such as potentially the Twin Lakes

pluton In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

close to

Leadville, Colorado The City of Leadville is a List of municipalities in Colorado#Statutory city, statutory city that is the county seat, the most populous community, and the only List of municipalities in Colorado, incorporated municipality in Lake County, Colorad ...

Context

seismic tomography Seismic tomography or seismotomography is a technique for imaging the subsurface of the Earth with seismic waves produced by earthquakes or explosions. P-, S-, and surface waves can be used for tomographic models of different resolutions based on ...

images, the Aspen anomaly is characterized by a northwards tilted low seismic velocity anomaly. The anomaly is one among several low velocity anomalies beneath the western United States, although unlike the others known as the Jemez, Yellowstone and St. George it does not have a northeastward throw. Other structures that may be related to the Aspen anomaly are the Lester Mountain zone, the Colorado mineral belt and the Rio Grande Rift. The Aspen anomaly has been compared with the Yellowstone hotspot, but it lacks a volcanic

caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is ...

that Yellowstone has.

Origin

The Aspen anomaly has been interpreted in several ways. * It may be a mantle plume, but the steep tilt angle is unusual for a mantle plume. * It may be a lithospheric melt zone, but the lithosphere is not thick enough to contain such a structure. * It may be a

Proterozoic The Proterozoic () is a geological eon spanning the time interval from 2500 to 538.8million years ago. It is the most recent part of the Precambrian "supereon". It is also the longest eon of the Earth's geologic time scale, and it is subdivided ...

structure, maybe the leftover of a

subduction Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...

zone. Prolonged subduction would have enriched a segment of mantle with water and thus lowered its melting point. * Shallower upwelling (such as

asthenospheric The asthenosphere () is the mechanically weak and ductile region of the upper mantle of Earth. It lies below the lithosphere, at a depth between ~ below the surface, and extends as deep as . However, the lower boundary of the asthenosphere is not ...

upwelling), possibly linked with slab rollback or crustal delamination processes. * Crustal weaknesses.

References

Sources

* * {{refend Geology of Colorado Rocky Mountains