Rio Grande Rift

A History of the Rio Grande Rift

 

Introduction

The Rio Grande Rift extends from southern Colorado into New Mexico, southern Texas and Mexico. Young volcanic fields and lava flows can be sited in New Mexico. While studying the vast Rio Grande Rift structure that shaped the topography of the land, rocks from the Precambrian through the Cenozoic are present due to periods are subduction, compression, uplift, extension, creation of basins, and periods of magmatism. The diverse region consists of a range of igneous and metamorphic rock compositions, as well as mantle xenoliths that can be found in southwestern Texas and Mexico. This webpage will give a brief overview of significant markers found along the large rift in southwestern United States. Little information is know about what happens to the rift in Mexico.

 

Geology

Nearly 1.6-1.0 billion years ago new continental crust was forming as subduction under the micro-continent Laurentia was occuring. Two major orogenies affecting the rifted region during the Precambrian are significant events dicussed later, which include I. the Mazatzal orogeny that marks lineament of the Jemez Mountians and II. an unnamed orogeny but was the same conditions found at the Laramide orogeny. The Cambrian gave rise to the Great Unconformity where old, Precambrian sedimentary rocks were overlayen ontop of younger granite composition. As Pangea broke up, the Rio Grande Rift was in the initial stages of forming (Weaver, 2012).

 

The Farallon plate subducted below the North American plate on the western margin of North America at a very shallow depth and horizontal angle. Beginning of subduction is noted at 200 million years ago, but not until 80 million years ago were the results seen as the plate moved further under the continental crust with extremely high heat flow and shallow subduction  this allowed water to stay within the plate (Weaver, 2012). Evidence of the high heat flow is referred to as the Laramide orogeny and includes; uplift of mountins like the Rocky Mountains, normal faulting, and compression (Baldridge et. al, 1995). Near the end of the Laramide orogeny the regional structure changed to a strike-slip system as subduction of the Farallon plate ended . Shallow, hot heat flow was still present and allow for extension below the crust. The region extended on average 40% but much larger extension occured in some areas and is marked by igenous activity that is more extensive in the central and southern region of the Rio Grande Rift. The most northern portion, referred to as the Colorado Plateau, remained relatively stable and did not compress or extend compared to the other regions (Baldridge et al., 1995; Keller and Baldridge, 1999).

 

Igneous activity was found in two pulses in the Oligoncene-Miocene and Micocene-Holocene, which also coincide with phases of extension that created basins with high and low angle normal faults. Sedimentary fill of limestones and carbonates are infilled in the basins to the south, but larger basins like the Espanola Basin north of Alburquerque,New Mexico are infilled with igneous deposits. Volcanic activity was marked in the Oligocene-Micocene and in quiecence until the Micocene-Holocene about 14 million years ago. Activity was most notable in the Jemez Volcanic Field that is located north of Alburquerque, New Mexico. The Jemez Volcanic Field follows the fault planes along the Jemez Mountains that run southwest to northest. The field formed as the rift extended in the south as the northern part of the rift remained stable (Baldridge et al., 1995; Keller and Baldridge, 1999).

 

Valles Caldera is a famous landmark that is part of the Jemez field. The Jemez field began erupting 14 million years ago with basalt deposits, but geologist have found that as the field continued to erupt the magma continued to transition into andesite and dacite until Valles formed a dome and erupted dacite and rhyolite through faults, most notably the Bandlier Tuff (Weaver, 2012).  It is also noted that there was signifcant crustal contamination as peridotite and quartz were found in the rocks (Baldridge et al., 1995). Near Valles Caldera is Soda Dam that is a built of travertine and supplied by geothermal fluids from the caldera as the heat source and cold waters from the Jemez River. 

 

Zuni-Bandera is located along the Jemez lineament north of Alburquerque, New Mexico. There are 74 vents that erupted depleted mid-ocean ridge basalts of alkali  and tholeiitic basalts composition (Ander and Huestis, 1982;Menzies et al.,1991; Peters et al., 2008). Some of the most recent eruptions from the Pliocene to Pliestocene are the Pahoehoe lava flows that are easily accessible near Grant, New Mexico (Nicols, 1946). Bandera Crater is a larger crater created from a cinder cone eruption.Cinder cones and shield volcanoes are the primarly features, but where Bandera Crater is located is a tourist attraction now. Ice caves and lava tubes are found near around the crater (Laughlin et al., 1971).

 

The southern portion of the Rio Grande Rift, on the border of Texas and Mexico, the Potrillo Maar and Kilbourne Hole Volcanic Field are located. These fields are known for xenoliths. The volcanic fields are aligned north-south and erupted during the Quaternary. The fields were covered with sedimentary deposits before the eruptions, which were large violent eruptions that created caldera-like structures. Kilbourne Hole has deformed sedimentary layers and bomb sags on the outter rim of where the eruption occured. Crust and mantle xenoliths are found. Peridoitite and garnet are the predominate compositions found, but quartzite, sandstone, limestone, and anorthosite are common in many samples. There is alot of crustal contamination in the mantle xenoliths (Weaver, 2012).

 

Summary

The Rio Grande Rift extends through a large portion of New Mexico and parts of Texas, Colorado, and Mexico. It went through phases of compression, uplift, and extension.Volcanic activity was high in the central and southern regions extending into Mexico. Deposits were predominatly basalt and even in some volcanic fields, such as the Cerros del Rio, form columnar joints. Andesite, dacite, and rhyolite are found in various forms, indicating how the volcanic fields and magma chambers transitioned over time. The eruptive styles are also very different in each field, where some rocks are very-welded with large phenocrysts and others are composed of mostly metamorphic facies. The website highlights some of the facinating features located in the Rio Grande Rift and the general history, however, the rift is vast and has more sites,information, and history.

 

 

 

Works Cited

 

Ander, M. E. and S. P. Huestis (1982). "Mafic intrusion beneath the Zuni-Bandera volcanic field, New Mexico." Geological Society of America Bulletin 93: 1142-1144.

Baldridge, W. S.,Keller, G.R., Haak, V., Wendlandt, E., Jiracek, G.R., and Olsen, K.H., 1995. The Rio Grande Rift. In Olsen, K.H. (ed.) Continental Rifts: Evolution, Structure, Tectonics. Elvsevier, Developments in Tectonics, v. 25, p. 233-275.

Keller, G. R. and Baldridge W.S., 1999. The Rio Grande Rift: A Geological and Geophyical Overview. Rocky Mountain Geology, v 34, p. 121-130.

Laughlin, A. W., D. G. Brookins, et al. (1971). "Chemical and strontium isotopic investigations of ultramafic inclusions and basalt, Bandera Crater, New Mexico." Geochimica et Cosmochimica Acta 35: 107-113.

Menzies, M. A., P. R. Kyle, et al. (1991). "Enriched and Depleted Source Components for Tholeiitic and Alkaline Lavas from Zuni-Bandera, New Mexico: Inferences About Intraplate Processes and Stratified Lithosphere." Journal of Geophysical Research 96(B8): 13,645-613,671.

Nichols, R. L. (1946). "McCartys Basalt Flow, Valencia County, New Mexico." Bullentin of the Geological Society of America 57: 1049-1058.

Peters, T. J., M. Menzies, et al. (2008). "Zuni-Bandera volcanism, Rio Grande, USA-Melt formation in garnet- and spinel-facies mantle straddling the asthenosphere-lithosphere boundary." Science Direct: 295-315.

Weaver, B., 2012. Igneous and Metamorphic Petrology: New Mexico Field Trip Spring Break. Geology and Geophysics, University of Oklahoma.