Rio Grande Rift Valley

Rio Grande Rift Valley

An Overview of Area Resources, Stratigraphy and Tectonics

Jacob Cartwright


Rio Grande Rift Valley:

An Overview of Area Resources, Stratigraphy and Tectonics

Introduction

The Rio Grande Rift Valley extends 1000 km (Chapin, 1972) from Southern Colorado to Northern Mexico. The rift has created a series of basins with an average width of 50 km (Chapin, 1972). Basins in the north are larger, but the rift widens towards the south where smaller basins occupy a larger area similar in structure as the Basin and Range Provinces west of the rift. The basins of the Rio Grande Rift serve as important areas for mineral deposits (MASE, 2011), water resources (Hawley, 2003; Plummer, 2001; Ulmer-Scholle, 2008; & USGS, 2012) and have renewable energy potential in geothermal (U.S. DOE, 2012).

    The rift exposes rocks that range from Late Proterozoic to Cenozoic ages (Lindsey, 2010). The stratigraphy of the area is one resembling a long-standing passive margin illustrated by the depositional history of the area controlled by processes of transgression, regression and erosion of the North American Craton and Ancestral Rockies (McCalpin, 1994). Later depositional history includes volcanics and magmatic intrusion attributed to the subduction of the Farrollon Plate beneath the North American Plate (McCalpin, 1994). Final deposition sequences are related to alluvial rift fill (Lindsey, 2010).

    The tectonics of the Rio Grande Rift Valley represents extensional tectonics of the Colorado Plateau and North American Craton rifting from one another and is controlled by the subduction of the Farrollon Plate under the North American Plate and possible interaction of shear stress from the transform boundary along the San Andreas Fault Zone (Eaton, 1979). The area has undergone deformation and faulting from the Laramide Orogeny occurring 80 million years ago from shallow subduction (Lindsey, 2010). About 50 million years later, steepening subduction of the Farrollon Plate led to convection cells in the asthenosphere driving rifting of the area (Eaton, 1979). The rift can be divided into three distinct phases of volcanic and magmatic dominated terrains (Chapin, 1979).

Basins

    The Rio Grande Rift Valley contains several basins seen in figure 1. These basins contain important mineral deposits, aquifers and have the potential for renewable energy generation making the area economically significant. These basins can be divided into three regions: the Northern Basin, the Central Basins and the Southern Basins.

Northern Basin

    The northern topography of the Rio Grande Rift is dominated by the San Luis Basin. The San Luis Basin is the northern most exposed topography of the Rio Grande Rift Valley and is fault bounded by the Sangre de Christo Mountains east of the basin and volcanic rocks to the west (Tweto, 1979). Formation of the valley began during the late Laramide Orogeny, where deformation caused faulting west of the Sangre de Christo volcanic arc (McCalpin, 1994). During the Cenozoic, extensional tectonics reactivated these faults opening the valley west of the mountains (Tweeto, 1979). The larger basin can be subdivided into smaller basins and areas. In the north, a Pleistocene lake deposited strata of clay and sandstone and is currently being investigated by oil and gas companies and public works for sites as potential plays and aquifer systems (USGS, 2012). The central area of the basin is composed of volcanic and sedimentary material that has been a proven aquifer system (USGS, 2012). The southern basin is dominated by a basaltic plateau (USGS, 2012) and is limited in resource potential.

Central Basins

    The central basins are located in northern and central New Mexico and consist of the Espanola and Middle Rio Grande Basins and underwent the same extensional tectonics demonstrated in the San Luis Basin (McCalpin, 1994). The Espanola Basin is further north and contains known aquifers supplying Taos and Albuquerque populations (Hawley, 2003). This very large aquifer is also the close study of degraded quality from uranium mining conducted to the west of the aquifer (Ulmer-Scholle, 2008). This has led to the allocation of superfund status in the area seen in figure 2 (MASE, 2011). The area is also productive for hydrocarbons to the north (MASE, 2011). The Middle Rio Grande Basin widens more than the Espanola and has a large aquifer underlying the area seen in cross-section in figure 3 (Plummer, 2001). .


Figure 3: from Hawley, J. W., & Cook, C.W. 2003 “Hydrogeology of the Santa Fe Group Aquifer system, southern Espanola Basin, New Mexico”. U.S. Geological Survey Open-File Report; 15.

Southern Basins

    The southern basins begin with the Socorro Basin and continue southward into Texas and northern Mexico in a series of smaller basins. The Socorro Basin is rich in Uranium deposits and is currently being mined, refer to figure 2 (MASE, 2011). The southern smaller basins contain natural gas deposits. The entire Southern Basins are currently being considered for their geothermal potential as hot springs are common in the area (MASE, 2011). A map of geothermal resources has been provided in figure 4.


Figure 4: from US Department of Energy. Date accessed 12/03/2012. http://www1.eere.energy.gov/geothermal/maps.html.

Stratigraphy    

The Rio Grande Rift exposes a series of sedimentary rocks deposited unconformably on Late Proterozoic basement granite gneiss (Lindsey, 2010) Exposed late Proterozoic basement rock formed around 1.8-1.7 Ga and consists of light colored gneiss and granitic plutonic intrusion with gabbro dikes (Lindsey, 2010). Subsequent stratigraphy is attributed to periods of transgression and regression controlling the deposition of sediments. Ordovician deposition of thin dolomites and sandstones (McCalpin, 1994) are deposited indicating a shallow marine environment. An unconformity follows, with resuming deposition in the Devonian from about 360-326 Ma with shaley and coaly, gray limestone and sandstone deposition (Lindsey, 2010) suggesting a deeper water environment. This is followed by another unconformity with arkosic sandstone fining from north to south in alluvium deposition (McClapin, 1994) with carbonate deposition (Lindsey, 2010) further south during the Pennsylvanian.

The arkosic sandstone in the north during the Pennsylvanian is attributed to the formation of the Ancestral Rockies (McClapin, 1994). Shallow seas surrounded the Ancestral Rockies created by glacial runoff from Gondwanaland during the formation of Pangea (Lindsey, 2010). Erosion of the Uncompahgre Range that shed sediments southward into basins at the same time of the glacial runoff resulted in the oxidation of iron (Lindsey, 2010) in the sandstone creating red beds into the Permian. About 80 million years ago the Laramide orogeny would further deform the area (Lindsey, 2010). This led to volcanics, intrusion and uplift of the area. Evidences of volcanics include tuff and rhylitic and andesitic basalts (Lindsey, 2010). The uplift from the sub-ducting Farrolon Plate that was forming the Rocky Mountains created compressional stress that caused faulting found throughout the area (McCalpin,1994).


Figure 5: from Collins, Liz. 2012. “New Mexico in the Beginning”.

Tectonics

    The further subduction of the Farrolon Plate would set in motion the extensional tectonics that would shape the rift. Mantle upwelling has been measured and is calculated to intrude 60 km into the lithosphere (Baldridge,1984). This process is caused by convectional heat transfer that began thinning the crust through melting. As the lithosphere thinned and an epierogenic episode lifted the area rifting the continent and a series of half-grabens and horsts developed along the rift (Eaton, 1979). The rift can be separated into three segments running from the south in northern Mexico to the north in southern Colorado and possibly further north. These were described by Chapin in 1979 and include the following presented oldest to youngest: (1) South, (2) Central and (3) North. A table has been prepared from his data and is shown in table 1.

Rift Area

Age (Ma)

Trend

Tectonics

Basins

(1) South

32

South – North

Volcanism and magmatism

Several wider and less distinct basins

(2) Central

30

North-North East

High volcanism

Socorro and Albuquerque

(3) North

27

North-North West

Volcanism

San Luis and

Espaňola

In addition to the 3 distinct segments are three pulses described by Eaton in 1979 that explain the structure and tectonics of the developing rift. These pulses can be separated by age and are as follows: (1) ~30-20 Ma, (2) 20-10 Ma and (3) 10-5 Ma, and are presented in table 2 accompanied by maps prepared by the University of New Mexico in figure 6.

Pulse

Tectonics

Faulting

Material

30-20 Ma

Crustal Extension

Normal

Basaltic andesites (volcanism)

20-10 Ma

Rifting

Horst-half-graben

Basaltic flows, eruption and erosion

10-5 Ma

Uplift

Block

Further erosion and volcanism


Figure 6: after Smith, Gary. 2003. “Middle to Late Cenozoic Development of the Rio Grande Rift and Adjacent Regions in Northern New Mexico”. Geology of New Mexico. New Mexico Geological Society Special Publication.

The tectonics and several episodes of compression, extension and volcanism created the dynamic area of the RGRV that has been further modified by erosion from the Rio Grande finding its way into the basin. A geologic map and cross-sections from the University of New Mexico has been provided to show the current geology of the northern New Mexico region of the RGRV in figure 7. Studies by the USGS in collaboration with several universities show current spreading at an average of 5mm/year with the fastest movement to the south and slowest to the north; this information was obtained from GPS data stretching 600 mi along the rift measured at 24 sites (Berglund, 2012).


Figure 7: after Smith, Gary. 2003. “Middle to Late Cenozoic Development of the Rio Grande Rift and Adjacent Regions in Northern New Mexico”. Geology of New Mexico. New Mexico Geological Society Special Publication.

Conclusion

    The Rio Grande Rift Valley has been widely studied extensively and continues to be an area of intense study by professionals, universities and private interests to gain a better understanding of the economic importance of the basins, the stratigraphy of the area and the active tectonics shaping the rift today. The basins of the rift include important mineral resources, water resources and have the potential for geothermal energy generation. The stratigraphy of the rift will aide in the exploration of metal and hydrocarbon deposits and aquifer systems. The tectonics controlled by upwelling mantle dynamics can aide in the further study of geothermal potential the rift holds for future energy needs.

List of Figures

Figure 1: from US Geological Survey. 2012. “Geophysics of Rio Grande Basins”. http://crustal.usgs.gov/projects/rgb/overview.html

Figure 2: from Multicultural Alliance for a Safe Environment. 2011. “Mining on Navajo Land”. http://masecoalition.org/navajonation/.

Figure 3: from Hawley, J. W., & Cook, C.W. 2003 “Hydrogeology of the Santa Fe Group Aquifer system, southern Espanola Basin, New Mexico”. U.S. Geological Survey Open-File Report; 15.

Figure 4: from US Department of Energy. Date accessed 12/03/2012. http://www1.eere.energy.gov/geothermal/maps.html.

Figure 5: from Collins, Liz. 2012. “New Mexico in the Beginning”. http://www.lizcollinshistoryclasses.com/new-mexico-in-the-beginning.html.

Figure 6: after Smith, Gary. 2003. “Middle to Late Cenozoic Development of the Rio Grande Rift and Adjacent Regions in Northern New Mexico”. Geology of New Mexico. New Mexico Geological Society Special Publication.

Figure 7: after Smith, Gary. 2003. “Middle to Late Cenozoic Development of the Rio Grande Rift and Adjacent Regions in Northern New Mexico”. Geology of New Mexico. New Mexico Geological Society Special Publication.

List of Tables

Table 1: Chapin, C.E.. 1979. “Evolution of the Rio Grande Rift”. Rio Grande Rift: Tectonics and Magmatism. 1: 1-5.

Table 2: Eaton, Gordon. 1979. “The Plate Tectonic Model for late Cenozoic Spreading in the Western United States”. Rio Grande Rift: Tectonics and Magmatism. 1: 7-27.

References

Baldridge, W.; Olsen, K.; Callender, J.. 1984. “Rio Grande Rift: Problems and Perspectives”. 1-11.

Berglund, H.T; Sheehan, A.F.; Murray, M. H.; et al. 2012. “Distributed deformation across the Rio Grande Rift, Great Plains and Colorado Plateau”. Geology. 40: 23-36.

Chapin, C.E.. 1972. “The Rio Grande Rift, Part I: Modifications and Additions” Rio Grande Rift: Tectonics and Magmatism.1: 45.

Chapin, C.E.. 1979. “Evolution of the Rio Grande Rift”. Rio Grande Rift: Tectonics and Magmatism. 1: 1-5.

Eaton, Gordon. 1979. “The Plate Tectonic Model for late Cenozoic Spreading in the Western United States”. Rio Grande Rift: Tectonics and Magmatism. 1: 7-27.

Hawley, J. W., & Cook, C.W. 2003 “Hydrogeology of the Santa Fe Group Aquifer system, southern Espanola Basin, New Mexico”. U.S. Geological Survey Open-File Report; 15.

Lindsey, David. 2010. “The Geologic Story of Colorado’s Sangre de Cristo Range”. United States Geological Survey and the Department of the Interior.

McCalpin, James. 1994. “General Geology of the Northern San Luis Valley, Colorado”. Geo-Haz Consulting.

Plummer, L. Niel & Sanford, Ward E..2001. “Case Study: Middle Rio Grande Basin, New Mexico, USA”. US Geological Survey.

Tweto, Ogden. 1979. “The Rio Grande Rift System in Colorado”. Rio Grande Rift: Tectonics and Magmatism. 1: 33.

Ulmer-Scholle, Dana S. 2008. “Uranium: Where is it?”. New Mexico Tech. date accessed 11/18/2012. http://geoinfo.nmt.edu/resources/uranium/where.html.

US Geological Survey. date accessed 12/03/2012 http://crustal.usgs.gov/projects/rgb/SanLuisBasin/index.html.

US Department of Energy. Date accessed 12/03/2012. http://www1.eere.energy.gov/geothermal/maps.html.

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