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Rio Grande rift

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The Rio Grande rift is a north-trending continental rift zone. It separates the Colorado Plateau in the west from the interior of the North American craton on the east.[1] The rift extends from central Colorado in the north to the state of Chihuahua, Mexico, in the south.[2] The rift zone consists of four basins that have an average width of 50 kilometres (31 mi).[1] The rift can be observed on location at Rio Grande National Forest, White Sands National Park, Santa Fe National Forest, and Cibola National Forest, among other locations.

Locality map showing the Rio Grande rift extending from southern Colorado to Chihuahua, Mexico. The Rio Grande follows this rift for much of its course.

The Rio Grande rift has been an important site for humans for a long time, because it provides a north–south route that follows a major river. The Rio Grande follows the course of the rift from southern Colorado to El Paso, where it turns southeast and flows toward the Gulf of Mexico. Important cities, including Albuquerque, Santa Fe, Taos, Española, Las Cruces, El Paso, and Ciudad Juárez, lie within the rift.

Geology

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The Rio Grande rift represents the easternmost manifestation of widespread extension in the western U.S. during the past 35 million years. The rift consists of three major basins and many smaller basins, less than 100 square kilometres (39 sq mi). The three major basins (from northernmost to southernmost) are the San Luis, Española, and Albuquerque basins. The rift's northern extent is delineated by the upper Arkansas River basin between Leadville and Salida, Colorado. Further south, the rift is defined by a network of smaller, less topographically distinct alternating basins and ranges. The distinction between these smaller basins and those of the Basin and Range Province becomes blurred in northern Mexico.[3][4]

Basin size generally decreases to the north in the rift, though the Española covers approximately 120 kilometres (75 mi) north–south and 40 kilometres (25 mi) east–west, and the San Luis is roughly 120 by 80 kilometres (75 by 50 mi). These basins may contain smaller units within them, such as the Alamosa basin within the San Luis, which is bounded by the San Juan and Tusas mountains on the west and the Sangre de Cristo Mountains in the east.[5] The Albuquerque basin is the largest of the three basins, spanning 160 kilometres (99 mi) north–south and 86 kilometres (53 mi) east–west at its widest points. It is the oldest of the three major basins, and contains 7,350 metres (24,110 ft) of Paleogene clastic sediments deposited on Precambrian basement. The southernmost Albuquerque basin contains pre-rift volcanic deposits, while the central and northern portions contain volcanics erupted during rifting.[3]

 
A generalized cross section of the Albuquerque basin from east to west. Note the half-graben geometry, paleozoic and mesozoic sediments that existed pre-rift, and the large (up to 28%) amount of extension.[3]
 
A generalized cross section of the San Luis basin from east to west. Being further north, this basin has experienced less extension (up to 12%).[1] Also note the lack of pre-rift sediments and thinner profile.[5]

In cross-section, the geometry of the basins within the rift are asymmetrical half-grabens, with major fault boundaries on one side and a downward hinge on the other. Which side of the basin has the major fault or the hinge alternates along the rift. The alternation between these half-grabens occurs along transfer faults, which trend across the rift to connect the major basin-bounding faults and occur between basins or, in places, within basins. The Precambrian basement changes relief sharply in this area, from 8,700 metres (28,500 ft) below sea level at the bottom of the Albuquerque basin to 3,300 metres (10,800 ft) above sea level in the nearby Sandia Mountains, which flanks the Albuquerque basin to the east. Flanking mountains are generally taller along the east side of the rift (although some of this relief may be Laramide in origin).[1] The thickness of the crust increases to the north beneath the rift, where it may be as much as 5 kilometres (3.1 mi) thicker than it is in the south. The crustal thickness underneath the rift is on average 30–35 kilometres (19–22 mi), thinner by 10–15 kilometres (6.2–9.3 mi) than the Colorado Plateau on the west and the Great Plains to the east.[6]

 
Cañones Fault on southeastern margin of Colorado Plateau, near Abiquiú, New Mexico

Formation of the rift began with significant deformation and faulting with offsets of many kilometers starting about 35 Ma.[7] The largest-scale manifestation of rifting involves a pure-shear rifting mechanism, in which both sides of the rift pull apart evenly and slowly, with the lower crust and upper mantle (the lithosphere) stretching like taffy.[8][9][10] This extension is associated with very low seismic velocities in the upper mantle above approximately 400 kilometres (250 mi) depth associated with relatively hot mantle and low degrees of partial melting.[11] This intrusion of the asthenosphere into the lithosphere and continental crust is thought to be responsible for nearly all of the volcanism associated with the Rio Grande rift.

The sedimentary fill of the basins consists largely of alluvial fan and mafic volcanic flows. The most alkalic lavas erupted outside the rift.[12] The sediments that were deposited during rifting are commonly known as the Santa Fe Group. This group contains sandstones, conglomerates, and volcanics. Aeolian deposits are also present in some basins.[1][2]

The Rio Grande rift is intersected in northern New Mexico by the NE-SW trending Jemez Lineament which extends well into Arizona. The lineament is defined by aligned volcanic fields and several calderas in the area, including the Valles Caldera National Preserve in the Jemez Mountains. The Jemez Lineament is thought to be a hydrous subduction zone scar, separating Precambrian basement rock of the Yavapai-Mazatzal transition zone from the Mazaztl Province proper.[13][14] Also on the Colorado Plateau but further north lies the San Juan volcanic field in the San Juan Mountains of Colorado.

The youngest eruptions in the rift region are in the Valley of Fires, New Mexico, and are approximately 5,400 years old.[15][16] The Socorro, New Mexico, region of the central rift hosts an inflating mid-crustal sill-like magma body at a depth of 19 km that is responsible for anomalously high earthquake activity in the vicinity, including the largest rift-associated earthquakes in historic times (two events of approximately magnitude 5.8) in July and November 1906.[17][18][19] Earth and space-based geodetic measurements indicate ongoing surface uplift above the Socorro magma body[20] at approximately 2 mm/year.[21]

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    A generalized cross section of the Rio Grande Rift, showing lithospheric and asthenospheric structure. Note the magma lenses and volcanics caused by the welling up of the asthenosphere into the crust, and the thinning of the lithosphere[12]
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    Deep seismic image of the Rio Grande rift compiled from the seismic transect shown in the previous figure, showing inferred mantle flow and imaged crust-mantle (Moho) topography (after Wilson et al.)(2005).[8]\
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    A brief timeline showing extension and volcanism in the Rio Grande rift area. The end of the Laramide orogeny was followed by volcanism and then extension. Changes in lava chemistry are also found, resulting from changes in magmatic sources.[22]

Geologic history

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Seismic profile from the Rio Grande Rift Seismic Transect (RISTRA) experiment crossing the rift system, with Cenozoic extended terrain of the rift and southern Great Basin tectonic provinces indicated.

The Rio Grande rift's tectonic evolution is fairly complex. The fundamental change in the western margin of the North American plate from one of subduction to a transform boundary occurred during Cenozoic time. The Farallon plate continued to be subducted beneath western North America for at least 100 million years during Late Mesozoic and early Cenozoic time. Compressional and transpressional deformation incurred by the Laramide Orogeny lasted until about 40 Ma in New Mexico.[23][24][25] This deformation may have been a result of the coupling between the subducting Farallon plate and the overlying North American Plate. Crustal thickening occurred due to Laramide compression. After the Laramide Orogeny and until 20 Ma, a major period of volcanic activity occurred throughout the southwestern United States. Injection of hot magmas weakened the lithosphere and allowed for later extension of the region.[26]

Cenozoic extension started about 30 million years ago (Ma). There are two phases of extension observed: late Oligocene and middle Miocene.[27] The first period of extension produced broad, shallow basins bounded by low-angle faults. The crust may have been extended as much as 50% during this episode. Widespread magmatism in mid-Cenozoic time suggests that the lithosphere was hot, the brittle-ductile transition was relatively shallow.[26] There is evidence that the second period of extension began earlier in the central and northern Rio Grande rift than in the south.[1] A third period of extension may have begun in the early Pliocene.[28]

One theory is that the Colorado Plateau acts as a semi-independent microplate[29] and one way of explaining the creation of the Rio Grande rift is by the simple rotation of the Colorado Plateau 1-1.5° in a clockwise direction relative to the North American craton.[1] Other explanations that have been offered are that the extension is driven by mantle forces, such as large-scale mantle upwelling[30] or small-scale mantle convection at the edge of the stable craton;[31] collapse of over-thickened continental crust;[32] initiation of transform faulting along the western margin of the North American plate;[33] or detachment of a fragment of the Farallon plate beneath the Rio Grande region that enhanced asthenospheric upwelling in the slab window.[34]

See also

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References

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  1. ^ a b c d e f g Chapin, Charles E.; Cather, Steven M. (1994). "Tectonic setting of the axial basins of the northern and central Rio Grande rift". In Keller, G. Randy; Cather, Steven M. (eds.). Basins of the Rio Grande Rift: Structure, statigraphy, and tectonic setting. Geological Society of America Special Paper No. 291. Boulder, Colo.: Geological Society of America. pp. 1–3. ISBN 0-8137-2291-8.
  2. ^ a b Kluth, C.; Schaftenaar, C. (1994). "Depth and geometry of the northern Rio Grande rift in the San Luis basin, south-central Colorado". In Keller, G. Randy; Cather, Steven M. (eds.). Basins of the Rio Grande Rift: Structure, statigraphy, and tectonic setting. Geological Society of America Special Paper No. 291. Boulder, Colo.: Geological Society of America. pp. 27–37. ISBN 0-8137-2291-8.
  3. ^ a b c Russell, L.; Snelson, S. (1994). "Structure and tectonics of the Albuquerque basin segment of the Rio Grande Rift: Insights from reflection seismic data". In Keller, G. Randy; Cather, Steven M. (eds.). Basins of the Rio Grande Rift: Structure, statigraphy, and tectonic setting. Geological Society of America Special Paper No. 291. Boulder, Colo.: Geological Society of America. pp. 83–112. ISBN 0-8137-2291-8.
  4. ^ Keller, G. Randy; Baldridge, W. Scot (1999). "The Rio Grande Rift: a geological and geophysical overview". Rocky Mountain Geology. 34 (1): 121–130. Bibcode:1999RMGeo..34..121K. doi:10.2113/34.1.121.
  5. ^ a b Brister, B.; Gries, R. (1994). "Tertiary stratigraphy and development of the Alamosa basin (northern San Luis basin), Rio Grande Rift, south-central Colorado". In Keller, G. Randy; Cather, Steven M. (eds.). Basins of the Rio Grande Rift: Structure, statigraphy, and tectonic setting. Geological Society of America Special Paper No. 291. Boulder, Colo.: Geological Society of America. pp. 39–58. ISBN 0-8137-2291-8.
  6. ^ Perry, Frank; Baldridge, W.; DePaolo, D. (10 August 1987). "Role of Asthenosphere and Lithosphere in the Genesis of Late Cenozoic Basaltic rocks from the Rio Grande rift and adjacent regions of the Southwestern United States". Journal of Geophysical Research. 92 (B9): 9193–9213. Bibcode:1987JGR....92.9193P. doi:10.1029/JB092iB09p09193.
  7. ^ Grauch, V.J.S.; Hudson, Mark R. (2007). "Guides to understanding the aeromagnetic expression of faults in sedimentary basins: Lessons learned from the central Rio Grande rift, New Mexico". Geosphere. 3 (6): 596. Bibcode:2007Geosp...3..596G. doi:10.1130/GES00128.1.
  8. ^ a b Wilson, David; Aster, Richard; West, Michael; Ni, James; Grand, Steve; Gao, Wei; Baldridge, W. Scott; Semken, Steve; Pate, Paresh (24 February 2005). "Lithospheric structure of the Rio Grande rift". Nature. 433 (7028): 851–855. Bibcode:2005Natur.433..851W. doi:10.1038/nature03297. PMID 15729338. S2CID 4387908.
  9. ^ Fleck, John (24 February 2005). "Study Sheds Light on 30 Million Years of Rio Grande Valley Stretching". Albuquerque Journal.
  10. ^ Hill, Karl (25 February 2005). "The Rio Grande Rift: a continent "stretched like taffy"". NMSU News Release.
  11. ^ Gao, W.; Grand, S.; Baldridge, S.; Wilson, D.; West, M.; Ni, J.; Aster, R. (2004). "Upper mantle convection beneath the central Rio Grande rift imaged by P and S wave tomography". J. Geophys. Res. 109 (B3): B03305. Bibcode:2004JGRB..109.3305G. doi:10.1029/2003JB002743.
  12. ^ a b Baldridge, W.; Olsen, K.; Callender, J. (1984). "Rio Grande Rift: Problems and Perspectives". New Mexico Geological Society Guidebook, 35th field conference. pp. 1–11.
  13. ^ Aldrich Jr., M. J. (1986). "Tectonics of the Jemez Lineament in the Jemez Mountains and Rio Grande Rift". Journal of Geophysical Research. 91 (B2): 1753–1762. Bibcode:1986JGR....91.1753A. doi:10.1029/JB091iB02p01753.
  14. ^ Whitmeyer, Steven; Karlstrom, Karl E. (2007). "Tectonic model for the Proterozoic growth of North America". Geosphere. 3 (4): 220. doi:10.1130/GES00055.1.
  15. ^ Aber, James S. "Rio Grande Rift". GO 568 Structural Geology. Archived from the original on 29 January 2006. Retrieved 8 February 2006.
  16. ^ Veatch, Steven Wade (20 March 1998). "The Rio Grande Rift". Archived from the original on 20 December 2005. Retrieved 8 February 2006.
  17. ^ Reid, H.G. (1911). "Remarkable earthquakes in central New Mexico in 1906 and 1907". Bull. Seismol. Soc. Am. 1 (1): 10–16. Bibcode:1911BuSSA...1...10R. doi:10.1785/BSSA0010010010.
  18. ^ Sanford, A.R.; Balch, R.S.; Lin, K.W. (1995). "A seismic anomaly in the Rio Grande Rift near Socorro, New Mexico". 78. Socorro, NM: New Mexico Institute of Mining and Technology Geophysics Open-File Report: 17. {{cite journal}}: Cite journal requires |journal= (help) Abstract at: "Annual Meeting". Seismological Research Letters. 66 (2). page 44 of 15–61. 1995. doi:10.1785/gssrl.66.2.15.
  19. ^ Schlue, J.; Aster, R.; Meyer, R. (1996). "A lower-crustal extension to a mid-crustal magma body in the Rio Grande Rift, New Mexico". J. Geophys. Res. 101 (B11): 25, 283–25, 291. Bibcode:1996JGR...10125283S. doi:10.1029/96JB02464.
  20. ^ "Socorro Magma Body". Archived from the original on 15 June 2010.
  21. ^ Fialko, Yuri; Simons, Mark (2001). "Evidence for on-going inflation of the Socorro magma body, New Mexico, from interferometric synthetic aperture radar imaging". Geophysical Research Letters. 28 (18): 3549–3552. Bibcode:2001GeoRL..28.3549F. doi:10.1029/2001GL013318. (Doi fails redirect.)
  22. ^ Morgan, P.; Golombek, M. (1984). "Factors controlling the phases and styles of extension in the northern Rio Grande rift". New Mexico Geological Society Guidebook, 35th field conference. pp. 13–20.
  23. ^ Seager, W.R.; Mack, G.H. (1986). "Memoir 41: Laramide Paleotectonics in southern New Mexico". Paleotectonics and Sedimentation in the Rocky Mountain Region: Tulsa, Oklahoma. 155. American Association of Petroleum Geologists: 669–685.
  24. ^ Chapin, C.E.; Cather, S.M. "Eocene tectonics and sedimentation in the Colorado Plateau-Rocky Mountain area". Arizona Geological Digest. 4: 173–198.
  25. ^ Karlstrom, K.E.; Daniel, C. G.; Taira, Asahiko (1993). "Restoration of Laramide right-lateral strike-slip in northern New Mexico by using Proterozoic piercing points". Geology. 21 (2): 188–211. Bibcode:1993Geo....21..188C. doi:10.1130/0091-7613(1993)021<0188:TPUOHH>2.3.CO;2.
  26. ^ a b Morgan, P.; Sedger, W.R.; Golombek, M.P. (1986). "Cenozoic thermal, mechanical, and tectonic evolution of the Rio Grande rift". Journal of Geophysical Research. 91: 6263–6276. Bibcode:1986JGR....91.6263M. doi:10.1029/JB091iB06p06263.
  27. ^ Seager, W. R.; Shafiqullah, M.; Hawley, J. W.; Marvin, R. F. (1984). "New K-Ar dates from basalts and the evolution of the southern Rio Grande Rift". Geological Society of America Bulletin. 95 (1): 87–99. Bibcode:1984GSAB...95...87S. doi:10.1130/0016-7606(1984)95<87:nkdfba>2.0.co;2.
  28. ^ GOLOMBEK, MATTHEW P. (1983). "Geology, structure, and tectonics of the Pajarito fault zone in the Española basin of the Rio Grande rift, New Mexico". Geological Society of America Bulletin. 94 (2): 192. Bibcode:1983GSAB...94..192G. doi:10.1130/0016-7606(1983)94<192:gsatot>2.0.co;2.
  29. ^ Steiner, M. B. (1988). "Paleomagnetism of the late Pennsylvanian and Permian: A test of the rotation of the Colorado Plateau". Journal of Geophysical Research. 93 (B3): 2201–2215. Bibcode:1988JGR....93.2201S. doi:10.1029/JB093iB03p02201.
  30. ^ Moucha, Robert; Forte, Alessandro M.; Rowley, David B.; Mitrovica, Jerry X.; Simmons, Nathan A.; Grand, Stephen P. (2008). "Mantle convection and the recent evolution of the Colorado Plateau and the Rio Grande Rift valley". Geology. 36 (6): 439. Bibcode:2008Geo....36..439M. doi:10.1130/G24577A.1.
  31. ^ van Wijk, J.; van Hunen, J.; Goes, S. (2008). "Small-scale convection during continental rifting: Evidence from the Rio Grande rift". Geology. 36 (7): 575. Bibcode:2008Geo....36..575V. doi:10.1130/G24691A.1.
  32. ^ Eaton, Gordon P. (December 1986). "A tectonic redefinition of the Southern Rocky Mountains". Tectonophysics. 132 (1–3): 163–193. Bibcode:1986Tectp.132..163E. doi:10.1016/0040-1951(86)90031-4.
  33. ^ Dickinson, William R.; Snyder, Walter S. (November 1979). "Geometry of Subducted Slabs Related to San Andreas Transform". The Journal of Geology. 87 (6): 609–627. Bibcode:1979JG.....87..609D. doi:10.1086/628456. S2CID 129169733.
  34. ^ Ricketts, Jason W.; Kelley, Shari A.; Karlstrom, Karl E.; Schmandt, Brandon; Donahue, Magdalena S.; van Wijk, Jolante (March 2016). "Synchronous opening of the Rio Grande rift along its entire length at 25–10 Ma supported by apatite (U-Th)/He and fission-track thermochronology, and evaluation of possible driving mechanisms". Geological Society of America Bulletin. 128 (3–4): 397–424. Bibcode:2016GSAB..128..397R. doi:10.1130/B31223.1.

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