Introduction
A major break in the Earth’s crust – the Rio Grande rift – starts in central Colorado’s Rocky Mountains and runs southward through Colorado and New Mexico into the Mexican State of Chihuahua. The rift is formed where a section of the Earth’s crust arched, weakened, and spread apart due to heat from basaltic magma welling up from the mantle 29 million years ago.
The stretched and brittle crust in the rift thinned and fractured. As tensional forces pulled apart the crust, sections subsided along north-south faults. Some sections dropped more than 8,000 meters. The rifting process resulted in a long rift valley dominated by four connected closed basins in which lava, volcanic ash, and sediments accumulated.
Twenty million years ago, as the North American plate continued to scrape along the east edge of the Pacific plate, crustal stresses resulted in a period of regional uplift, raising much of the southwestern United states to its present elevation. Colorado and adjacent states rose 1,500 meters higher than before the uplift (Chapin and Cather, 1994). During this time of uplift rivers, including the Rio Grande, began flowing into closed basins along the rift. As sand, gravel, and volcanic deposits filled the basins and valleys, the streams ultimately came together forming the Rio Grande River.
Beginning of Rifting
The Rio Grande rift is geologically young, starting 29 million years ago in the Leadville area. Two major episodes of extension have formed the Rio Grande rift. The fist occurred in late Oligocene to early Miocene time beginning 29 million years ago and lasting 10 to 12 million years. Strain rates were less during this early phase of rifting, except where volcanism and high heat flow caused local concentration of extensional strain.
Rifting began to separate the Colorado Plateau from the Great Plains. West of the rift the crust of the Colorado Plateau is approximately 45 kilometers thick; east of the rift the crust beneath the Great Plains is 50 kilometers thick; beneath the rift zone the crust is only 35 kilometers thick (Chapin and Cather, 1994).
Faulting began with the onset of rifting, causing earthquakes along certain areas of the rift zone. Several of New Mexico’s early pueblos were partially destroyed by earthquakes caused by the Rio Grande rift. Faulting and associated earthquakes continue today as the Rio Grande rift continues to widen.
The second episode of extension, the mid-Miocene to Quaternary phase, began about 17 million years ago and continues today. The most rapid phase of regional extension was during this period, from middle to late Miocene time. Evidence of continuing rifting includes young fault scarps, seismiscity, high heat flow, and ongoing uplift as established by geodetic measurements.
Rift Volcanism
The onset of rifting coincided with a period of intense volcanism associated with the early rift basins - as is common in other rift valleys of the world. As extensional forces continued and the crust beneath the widening rift zone was thinned, magma surged to the surface. Most of the volcanism was concentrated on the western side of the Rio Grande rift, where immense eruptions spewed ash over the region. In some areas magma flowed onto the surface and created vast lava fields. Some lavas flowed quietly from vents, forming broad flows that accumulated in layers to form expansive, gently sloping shield volcanoes. In other areas thick lava flows eventually built up to form major mountains. Huge calderas were created when the volcanoes collapsed into vacated magma chambers beneath the volcanoes. Much of the lava is basaltic, evidence that the faults along the rift reach down to the mantle, the source of most basaltic magma (Lipman and Mehnert, 1975).
Smaller volcanic eruptions, some of them within the last few thousand years, have added cinder cones, flows of dark basalt, squeeze-ups, and lava tunnels to the area. These volcanic episodes may have been witnessed by some of North America’s early human inhabitants – Clovis and Folsom Man. It is thought that early man may have witnessed the eruption that formed the Capulin cinder cone at what is now the Capulin Mountain National Monument in northern New Mexico. Capulin Mountain is near the center of the Raton-Clayton volcanic field, which is related to the Rio Grande rift (Stormer, 1987).
The intense volcanic activity along the Rio Grande rift fractured the nearby rocks and permitted the movement of superheated, mineral-rich solutions that formed hydrothermal deposits. These deposits brought in a later group of human inhabitants – the prospectors and miners. Most of New Mexico’s mining districts with their deposits of gold, silver, lead, copper, zinc, molybdenum, fluorite, and barite are concentrated along this mineralized trend near the edges of the Rio Grande rift.
Basins
The Rio Grande rift first developed as a chain of closed basins or half grabens (trench-like features formed by down-dropped blocks of crustal rocks), which gradually filled with lava, ash flows, and sediments that washed in from nearby mountain ranges. Large amounts of sand, gravel, lava, and volcanic ash fill the rift basins to a depth as great as 7.3 kilometers. Sedimentation began in most rift basins in late Oligocene to early Miocene time; however, the basin fill deposits of middle to late Miocene age are dominant in volume. The sedimentary and volcanic deposits of the Rio Grande rift are collectively known as the Santa Fe Group. The Santa Fe Group includes fine to coarse-grained sandstone interbedded with siltsone, conglomerate, and volcanic material. The sediments are generally soft and easily eroded.
Today, starting near Leadville, Colorado and southward to Soccorro, New Mexico, a distance of 550 kilometers, the north-trending rift consists of a series of four en echelon (staggered) basins that join. These basins range from 80 to 240 kilometers in length and from 5 to 95 kilometers in width. The average basin width is 50 kilometers (Chapin and Cather, 1994).
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| Satellite Image of northern extension of the Rio Grande Rift starting near Leadville, Colorado.Image courtesey of NASA. |
The San Luis Basin is 75 kilometers wide and 160 kilometers long, with the deepest part of the rift just northwest of the Great Sand Dunes National Monument in south central Colorado. The basin is filled with 6.4 kilometers of sediments, mostly Oligocene in age or younger, from the Sangre de Cristo Mountains. The Alamosa basin is the northern part of the San Luis Basin, and is bordered by the San Juan Mountains to the west and the San Luis Hills to the east. The San Luis Hills are erosional remnants of a once extensive volcanic field.
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| Satellite image of San Luis Valley. Image courtesy of NASA. |
South of the Colorado-New Mexico state line is the Taos Plateau, the southern subdivision of the San Luis Basin. This region of broad plains is underlain by basalt. The mountains and hills on the plateau are volcanic features. The Taos Plateau is trenched by gorges of the Rio Grande. Where the Rio Grande Bridge crosses U.S. 64, the Rio Grande is entrenched 185 meters below the bridge and the gorge is 370 meters wide. The gorge cuts into the Serrvilleta Formation (Pliocene), a sequence of coarse-grained and vuggy basalt flows that erupted as highly fluid pahoehoe lava that traveled many kilometers and formed individual units only a few meters thick.
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| Satellite image of the Taos Plateau. Note extinct volcanic cinder cones at lower end of the image. Image courtesy of NASA. |
The Albuquerque Basin is one of the largest and deepest basins of the Rio Grande rift. The width varies from about 20 kilometers in the north to 60 kilometers of sediments in the central region. The deepest part of the basin is filled by 7.3 kilometers of sediments.
South of the Albuquerque Basin the rift branches out and widens into a pattern of tilted ranges and parallel basin that resembles the Basin and Range province. Some researchers consider the rift south of the Albuquerque Basin to be part of the Basin and Range province. Others feel the Rio Grande rift should be distinguished from the adjacent Basin and Range province on the basis of its high heat flow, more frequent Pliocene and Quaternary faulting, deep basins, and late Quaternary volcanism (Chapin and Cather, 1994).
Summary
The Rio Grande Valley is not a usual valley – it was not cut by a river and does not branch upstream, as do most river valleys. The Rio Grande, instead, followed a pre-established and partly filled rift valley. The Rio Grande rift is geologically young and resulted from a process of regional extension and mantle upwelling in Neogene times. The Rio Grande rift continues to widen today, and ongoing geologic activity is evident through high heat flow, hot springs, continued siesmicity, geodetic observations, and some of North America's most recent lava flows.
Acknowledgments
Donald A. Coates provided critical review, which improved this paper.
References Cited:
Chapin, Charles E., and Cather, Steven M., 1994, Tectonic setting of the axial basins of the northern and central Rio Grande rift, Geological Society of America Special Paper 291
Halley, J.W., 1978, Guidebook to Rio Grande rift in New Mexico and Colorado, New Mexico Bureau of Mines and Mineral Resources, Circular 163
Lipman, P.W. and Mehnert, H.H., 1975, Late Cenozoic basaltic volcanism and development of the Rio Grande depression in the southern Rocky Mountains: Geological Society of America, Mem. 14
Stormer, John C. Jr., Capulin Mountain volcano and the Raton-Clayton volcanic field, northeastern New Mexico: Geological Society of America Field Guide – Rocky Mountain Section