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]

References

1. Morgan, Lisa A.; Quane, Steven L. (2010-01-01). Through the Generations: Geologic and Anthropogenic Field Excursions in the Rocky Mountains from Modern to Ancient. Geological Society of America. p. 24. ISBN 9780813700182.
2. Dueker, Yuan & Zurek 2001, p. 6.
3. Coblentz, D.D.; van Wijk, J. (December 2007). "Mechanisms of Topographic Uplift for the Southern Rocky Mountains". AGU Fall Meeting Abstracts. 2007: T11C–0723. Bibcode:2007AGUFM.T11C0723C.
4. "CO2-RICH SPRINGS AND TRAVERTINES OF THE ROCKY MOUNTAIN REGION: MANTLE HELIUM ASSOCIATED WITH THE ASPEN ANOMALY AND GEOMICROBIOLOGY OF "CONTINENTAL SMOKERS"". gsa.confex.com. Retrieved 2018-04-08.
5. Blair, Rob; Bracksieck, George (2011-09-01). The Eastern San Juan Mountains: Their Ecology, Geology, and Human History. University Press of Colorado. p. 12. ISBN 9781607320852.
6. Reiter, Marshall (1 March 2008). "Geothermal anomalies in the crust and upper mantle along Southern Rocky Mountain transitions". GSA Bulletin. 120 (3–4): 439. doi:10.1130/B26198.1. ISSN 0016-7606.
7. E., Karlstrom, K.; D., Coblentz, D.; B., Ouimet, W.; E., Kirby; W., van Wijk, J.; B., Schmandt; J., Crossey, L.; R., Crow; S., Kelley (December 2009). "Dynamic uplift of the Colorado Rockies and western Colorado Plateau in the last 6 Ma driven by mantle flow and buoyancy: Evidence from the Colorado River region". AGU Fall Meeting Abstracts. 2009: T51F–04. Bibcode:2009AGUFM.T51F..04K.
8. A., Darling; K., Karlstrom; E., Kirby; W., Ouimet; D., Coblentz; A., Aslan (2008). "Evaluating Neogene Uplift and Denudational History of the Colorado Rockies Using River Profiles and Incision Records". AGU Fall Meeting Abstracts. 2008: T11C–1893. Bibcode:2008AGUFM.T11C1893D.
9. M., Sandoval; D., Coblentz; K., Karlstrom; A., Sussman (2005). "Connecting Topographic Analysis With Colorado River Incision History: Sensitive Gauges of Neotectonics in the Rocky Mountains". AGU Fall Meeting Abstracts. 2005: T23C–0580. Bibcode:2005AGUFM.T23C0580S.
10. K., MacCarthy, J.; C., Aster, R.; M., Hansen, S.; C., Stachnik, J.; G., Dueker, K.; E., Karlstrom, K. (2009). "Joint inversion of teleseismic body wave residuals and Joint Inversion of Teleseismic Body Wave Residuals and Bouguer Gravity Data to Constrain the Origin of the Colorado Rockies". AGU Fall Meeting Abstracts. 2009: S13B–1745. Bibcode:2009AGUFM.S13B1745M.
11. "GEO AND THERMOCHRONOLOGICAL EVICENDE FOR THE EMPLACEMENT AND EXHUMATIONAL HISTORY OF THE TWIN LAKES BATHOLITH: IMPLICATIONS FOR THE LARAMIDE OROGENY". gsa.confex.com. Retrieved 2018-04-08.
12. Dueker, Yuan & Zurek 2001, p. 7.
13. Karlstrom et al. 2005, p. 425.
14. Dueker, Yuan & Zurek 2001, p. 8.
15. Karlstrom et al. 2005, p. 429.

Sources

• Dueker, Ken; Yuan, Huaiyu; Zurek, Brian (2001). "Thick-Structured Proterozoic Lithosphere of the Rocky Mountain Region". GSA Today. 11 (12): 4. doi:10.1130/1052-5173(2001)011<0004:TSPLOT>2.0.CO;2.
• Karlstrom, Karl E.; Whitmeyer, Steven J.; Dueker, Ken; Williams, Michael L.; Bowring, Samuel A.; Levander, Alan R.; Humphreys, E. D.; Keller, G. Randy; Group, CD-ROM Working (2005). "Synthesis of results from the CD-ROM Experiment: 4-D image of the lithosphere beneath the Rocky Mountains and implications for understanding the evolution of continental lithosphere". The Rocky Mountain Region: An Evolving Lithosphere Tectonics, Geochemistry, and Geophysics. Geophysical Monograph Series. Vol. 154. American Geophysical Union (AGU). pp. 421–441. doi:10.1029/154gm31. ISBN 978-0-87590-419-1. {{cite book}}: |last9= has generic name (help)