Aspen anomaly

Aspen anomaly is a geological structure in Colorado, United States. It consists of a low-seismic velocity anomaly in the mantle which underpins the highest sector of the Rocky Mountains.

Characteristics

The Aspen anomaly is a seismic velocity anomaly in the mantle beneath central Colorado (in the region of Aspen, Colorado^[1]),^[2] which appears to reach down into the upper mantle.^[3] Helium with isotope ratios indicative of mantle origin emanates from the terrain above the anomaly.^[4][5]

The Aspen anomaly coincides with the highest region of the Rocky Mountains (such as the San Juan Mountains and the Sawatch Range^[6]) and divergent drainages (Arkansas River, Colorado River and Gunnison River) which have cut deep gorges. This region underwent significant uplift during the Cenozoic^[3] starting from 10-5 million years ago and was subsequently eroded by the Colorado River.^[7] Ongoing present-day uplift of the San Juan Mountains may be linked to the Aspen anomaly.^[5]

River knickpoints in Gore Canyon and Black Canyon may mark the point at which the rivers pass through the edge of the region above the anomaly.^[8] The Colorado River may be influenced by the anomaly all the way to Lees Ferry, Arizona.^[9]

Hot springs and geysers above the anomaly are a major source of carbon dioxide and other gases, some linked to chemolithotrophic bacterial communities.^[4] Cenozoic volcanism is also associated with the anomaly,^[10] such as potentially the Twin Lakes pluton close to Leadville, Colorado.^[11]

Context

In seismic tomography images, the Aspen anomaly is characterized by a northwards tilted low seismic velocity anomaly.^[12] 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.^[2] Other structures that may be related to the Aspen anomaly are the Lester Mountain zone, the Colorado mineral belt and the Rio Grande Rift.^[13] The Aspen anomaly has been compared with the Yellowstone hotspot,^[3] but it lacks a volcanic caldera that Yellowstone has.^[5]

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.^[12]
• It may be a lithospheric melt zone, but the lithosphere is not thick enough to contain such a structure.^[14]
• It may be a Proterozoic structure, maybe the leftover of a subduction zone.^[13] Prolonged subduction would have enriched a segment of mantle with water and thus lowered its melting point.^[15]
• Shallower upwelling (such as asthenospheric upwelling), possibly linked with slab rollback or crustal delamination processes.^[10]
• Crustal weaknesses.^[10]