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
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:

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:
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:

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).

Basins along the Rio Grande Rift

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).