Background and Imagery of Rio Grande Rift, northern New Mexico
Jet Tilton
April 22, 2001
This project is a partial requirement for ES 767, Global Tectonics, which is taught by Dr. James Aber of Emporia State University, Emporia, Kansas.


Rio Grande Gorge, Taos, New Mexico
(Photo courtesy of Dave Clark at http://rockymountainscenery.com)

Introduction

The Rio Grande rift is one of the major late Cenozoic continental rifts, sharing most geophysical, geological, and geochemical characteristics with other rifts of the world. The Rio Grande rift, along with the Rhinegraben, Baikal rift, and East African rift, have the following characteristics:

• high heat flow
• predominantly basaltic and associated silicic volcanism (bimodal volcanism)
• lithospheric attenuation in both the crust and the mantle
• regional high elevation
• rise of aesthenosphere beneath the attenuated lithosphere
• a layer of low seismic velocity and high electrical conductivity in the middle crust
• normal faulting in the crust, with associated basin formation

It is theorized that the Rio Grande rift is related to regional extension in the western United States, which began in the middle Cenozoic. The physiographic expression of continental extension includes the horst blocks, grabens, half grabens, and tilted ranges of the Basin and Range province, of which the Rio Grande rift is the easternmost expression. The continental lithosphere here is currently extending, probably at slower rates than during the late Cenozoic (Ingersoll and others, 1990).


Chronologic Summary of Crustal Extension in western North America

Cenozoic crustal spreading in space and time in the western North American plate can be summarized as follows:

• 30 - 20 million years ago - Characterized by rapid spreading, closely-spaced normal faults and fault block rotation, eruption of basaltic andesites and high-silica rhyolites. Took place in Rio Grande rift at shallow crustal levels. Block faulting of the shallow crust and regional ductile extension of the middle crust occurred in the Sonoran Desert section of the southern Basin and Range province.
  
  
• 20 - 10 million years ago - Crustal rifting and normal faulting occurred throughout most of the basin and range and Columbia Plateau provinces, and locally it was accompanied by the voluminous eruption of basalt. Horst and graben structures developed in the Rio Grande rift area with little rotation and a slow spreading rate. A lull in magmatism that occurred throughout most of the west in the first part of this period is possibly indicative of this slowed spreading rate.
  
  
• 10 million years ago to present - In the Basin and Range province, there was a restriction of extensional spreading to the Great Basin section alone, and extensional deformation continued in the Rio Grande rift. There was a pronounced pulse of uplift and block faulting during the middle of this period. Northwest extension in the Humboldt zone of the northern Great Basin also occurred during this time (Eaton, 1979).

Evolution of the Rio Grande Rift

The Rio Grande rift extends as a series of asymmetrical grabens from Leadville, Colorado to Chihuahua, Mexico, which is a distance of more than 1000 kilometers. Each major graben forms a tectonic basin in which late Cenozoic volcanic and sedimentary rocks have accumulated (Wells and Menges, 1987).

The main portion of the rift can be divided into three segments, each with its own structural style and history, but with a common thread of timing and major events (Chapin, 1979). These include:

• Northern segment - From Leadville, Colorado to Alamosa, New Mexico, which began to open around 27 million years ago and is characterized by a north-northwest trend parallel to the late Paleozoic and Laramide structural grain. Has a near absence of syn-rift volcanism in the axial basins and a shift in extension away from the axial grabens into a broad belt along the east shoulder of the rift.
  
  
• Central segment - From Alamosa to Socorro, New Mexico, is characterized by a north-northeast-trending series of "en echelon" basins separated by complex transverse structures. Except near Taos, early-rift magmatism was sparse within this segment, but "late rift" bimodal volcanism was substantial, with the Jemez lineament and the Taos Plateau being very active during the past 5 million years.
  
  
• Southern segment - From Socorro, New Mexico to El Paso, Texas, this segment has undergone the most extension as evidenced by the widening of the rift near Socorro into a north-trending series of parallel basins and ranges with a total width around 2.5 times that of the single, large basins to the north.

It is believed that rifting began between 27 and 32 million years ago when regional extension reactivated the southern Rocky Mountains, which are a major north-trending zone of weakness that had developed during late Paleozoic and late Cretaceous-early Tertiary orogenies. By around 26 million years ago, the crust along the developing rift had sagged sufficiently to form shallow, broad basins where volcanic ash beds and mafic flows were intercalated with alluvial fill (Chapin, 1979). Regional doming as an early rift process can possibly be eliminated because of the preservation of pre-rift volcanic sections and early rift volcanic-alluvial sections within rift basins. As the rift opened, it broke "en echelon" over a series of northeast and west-northwest-trending flaws which developed into lineaments in the basement terrane of the southern Rocky Mountains. The lineaments that connected the ends of staggered basins were subjected to "scissor-like" torque in the near surface rocks and to a transverse shear at depth, and these deeply-penetrating transverse structures have tended to leak magma and to be zones of high heat flow and geothermal activity (Chapin, 1979).

Volcanism along the rift increased slowly after the middle Miocene lull, which occurred around 20 to 13 million years ago. Most of the activity at this time was concentrated initially in the Jemez Mountains and in the Socorro area, where the rift transects major northeast trending lineaments. The southern Rocky Mountains and adjacent areas were strongly uplifted between about 7 and 4 million years ago, and total uplift since the middle Miocene has been about 1100 meters, with much of this uplift occurring during this brief interval (Chapin, 1979).

Increased runoff from newly elevated alpine regions combined the drainage to form the ancestral Rio Grande prior to 4 million years ago. Widespread geomorphic surfaces, graded to the ancestral Rio Grande and the associated tributaries, were mantled with basalt flows following sharp acceleration of basaltic volcanism around 5 million years ago. A lowering of base levels following capture of the Rio Grande at El Paso during middle Pleistocene time resulted in dissection of basins along the Rio Grande drainage and isolation of basalt-capped mesas. Uplift occurs today, but at a reduced rate, and rifting continues, as evidenced by abundant fault scarps which cut Pleistocene deposits. High heat flow, modern magma bodies, modern elevation changes, and geophysical evidence for anomalous crust and upper mantle beneath the rift are also indications that uplift is still occurring (Chapin, 1979).



General geologic map of the Espanola basin, New Mexico.
Source: Manley, 1979, "Copyright [1979] American Geophysical Union. Reproduced/modified by permission of American Geophysical Union. Further reproduction or electronic distribution is not permitted."


Major Basins of the Rio Grande rift

Espanola Basin

The Espanola basin is located south of the San Luis Basin, and north of the Albuquerque basin (See Figure below). It is 40 kilometers long and 65 kilometers wide. This basin was probably a shallow depression between the eastward tilting Nacimiento uplift and the westward-tilting Sangre de Cristo Mountains. To the north, it is separated from the Taos Plateau and the San Luis basin by a basement ridge, which extends northwest from the Picuris Range through an isolated Precambrian rock exposure at Cerro Azul to the La Madera area. The basin is bounded by the Sangre de Cristo Mountains to the east, and the Precambrian-cored Nacimiento uplift to the west (Manley, 1979).

The western half of the basin is filled with volcanic rocks of the Jemez Mountains, which represent the western edge of the topographic basin. To the north and south, the basin terminates near basalts of the Servilleta Formation and Cerros del Rio volcanic field. Tertiary sedimentary rocks and alluvial deposits of Plio-Pleistocene age are preserved in the central basin and the Picuris and Abiquiu reentrants (Manley, 1979).



Regional map of New Mexico and southern Colorado, showing basins mentioned in discussion.
Source: Manley, 1979, "Copyright [1979] American Geophysical Union. Reproduced/modified by permission of American Geophysical Union. Further reproduction or electronic distribution is not permitted."


The San Luis Basin

The San Luis basin is approximately 240 kilometers long and may be as wide as 90 kilometers. The basin is tilted to the east in New Mexico, with major tectonic uplift along the eastern margin with the Sangre de Cristo Mountains. The San Luis basin also encompasses the Taos Plateau (Wells and Menges, 1987).