The Rio Grande Rift: Albuquerque Basin



Sandia Mountains, Albuquerque, New Mexico. Image courtesy of
http://www.itsnature.org


Prepared by Molly Reardon

ES 767 Global Tectonics Emporia State University

Spring 2012


Table of Contents
Introduction
Tectonic History
Geology
Conclusion
References

Introduction

The Rio Grande River is the major water feature of central New Mexico. It is easy to assume that this river eroded and shaped the high desert landscape that it flows through. However, the reverse is true. The Rio Grande River exists because of the divergent rift which creates a low lying area and a natural drainage basin through central New Mexico. The Rio Grande Rift is a continental rift zone which starts in the San Luis Basin in Colorado and extends south to Cuidad Juarez, Mexico. Distinct geologic basins comprise the rift system and this report will focus on the Middle Rio Grande Basin, also known as the Albuquerque Basin in central New Mexico. This basin is the largest and spans approximately 160km north-south and 86km east-west. Several of the rift's significant geologic features are visible from most locations in Albuquerque, NM.



Figure 1: Major Basins of the Rio Grande Rift. Image courtesy of USGS.



Return to: Table of Contents



Tectonic History

The tectonic history of this region is a diverse and complicated story. To better understand the tectonics, it is helpful to understand what was occurring in North America during the early Cenozoic time. North America's western margin was transitioning from a subduction zone to a transform boundary, as the Farallon plate slowly ceased subducting under the North American plate. Next, the Laramide Oregeny introduced a major period of mountain building from approximately 70 to 40 million years ago throughout the western US. Scientists believe that a coupling between the subducting Farallon plate and the North American plate may have caused a compressional and transpressional deformation zone in the vicinity of the modern day Rio Grande Rift, setting the stage for future tectonic activity in central New Mexico. Starting approximately 40 million years ago, a major period of volcanism occurred throughout the southwestern United States. The lithosphere was weakened by the injection of hot magmas through this zone and volcanic activity prevailed.

The Rio Grande Rift began forming between 35 and 29 million years ago when the Earth's lithosphere began to spread apart. The early phase extension occurred first and was marked by broad, shallow basins bound by low-angle faults through southern portions of the rift. The second phase occurred approximately 15 million years ago and was marked by the opening of basins in the northern and central portions of the rift and included mafic extrusive igneous rocks. As the Earth's crust spread apart, in the Albuquerque area, a block of land approximately 86 km wide began to sink. Today, the Rio Grande River runs though this low-lying area. Land on both the eastern and western side of this rift is undergoing uplift, which has produced the Sandia Mountain Range to the east of Albuquerque and a boundary marked by the Rio Puerco to the west. This entire low laying feature is called a graben and it sits thousands of meters lower than the surrounding terrain.



Figure 2: Tectonic timeline of Rio Grande Rift.




Return to: Table of Contents

Geology

The Sandia Mountains to the east of Albuquerque are a result of the uplift that occurred as the Rio Grande Rift was forming approximately 10 million years ago. The Sandia's consist primarily of Pre-Cambrian granite and Pennsylvanian limestone and total vertical movement along the fault is possibly over 5 miles from its original position. The composition of the Sandia Mountain peaks are identical to the geologic composition found 15,000 feet below Albuquerque in the heart of the Rio Grande Rift. This is because as the Sandia Mountains rose, they continued to erode. Sand, silt, water, and mud were carried downstream by a series of mountain streams and arroyos, and continue to fill the low lying graben. Alluvial fans of debris can also be found in the foothills of the Sandia Mountains. Today, the Rio Grande River erodes and slightly shapes the soft sediment landscape in which it flows through.

The innermost valley of the Rio Grande Rift is the conduit for the Rio Grande River. This area and flood plain consist of sediment that has been eroded from the Sandia Mountains over thousands of years. Thousands of feet of sediment lay under Albuquerque, NM, providing a porous space for water to accumulate. In recent years, the hydrology community has gained a better understanding of the capiblity for water to accumulate and be stored in this area. Unfortunately, discoveries are showing that not as much water resides under the city as originally thought. This may be due to several factor such as a drastic increase in human consumption and an underground environment that allows water to penetrate to unreachable depths.

Approximately 150,000 years ago, hot magma began to make its way upward along faults in the Rio Grand Rift to the wast of Albuquerque. Volcanoes formed and are believed to have been active up until 70,000 years ago. Today, 3 significant volcanic features dot the landscape to the west and are known as the Albuquerque Volcanoes. These particular volcanos are a result of fissure eruptions and this explains why the volcanos are aligned in a row of eruption craters which were all active at the same time. The basaltic lava flow from these volcanoes extends eastward toward the city and today stands as a lava-covered plateau, known as West Mesa. Scientists believe that there were six major lava flows associated with the volcanos, and the first two flows comprise the West Mesa. Additionally, an escarpment consisting of boulder and debris from the flows borders the eastern edge of the West Mesa.

Figure 3: Geologic cross section of Rio Grande Rift through Albuquerque, NM. Image courtesy of
http://www.abqenvironmentalstory.org.



Return to: Table of Contents

Conclusion

The Rio Grande Rift is an extensive and intricate geologic event which marks the central New Mexico landscape. The Sandia Mountains, the Rio Grande River, and the Albuquerque Volcanos are a direct result of the rifting that has occurred over the past 30 million years. Each geologic feature is interesting and significant on its own, however, one cannot truly understand the dynamic of the Rio Grande Rift through central New Mexico, unless all associated geologic features are taken into consideration.

References

Albuquerque's Environmental Story: Educating for a Sustainable Community. World Wide Web homepage <http://www.abqenvironmentalstory.org/nature/geology/s1picgeo.html> [retrieved March 2012]

It's Nature: New's, Videos, and Pictures from the Natural World. World Wide Web homepage<http://www.itsnature.org>[retrieved April 2012]

National Park Service: New Mexico Bureau. World Wide Web homepage <http://www.nps.gov/petr/planyourvisit/volcanoes.htm> [retrieved April 2012]

USGS: New Mexico Water Science Center. World Wide Web homepage <http://nm.water.usgs.gov> [retrieved April 2012]

Utah State University: Rio Grande Rift FAQs. World Wide Web homepage <http://aconcagua.geol.usu.edu/~arlowry/RGR/faq.html#WHAT"> [retrieved April 2012]

Return to: Table of Contents

Last Updated: April 18, 2012