Rio Grande Rift

GO 568 Structural Geology
James S. Aber


Tectonic overview

The Rio Grande Rift system is a series of grabens (fault-bounded basins) that extend from central Colorado southward through New Mexico and into western Texas and Mexico. The grabens were created primarily during the Oligocene and Miocene. Continental rifting was associated with crustal stretching and uplift of the southwestern United States. Grabens dropped down thousands of meters relative to adjacent uplifts, and alluvial sediment accumulated to great thickness in the basins. Meanwhile, intrusions and volcanic eruptions took place within the rift valleys and throughout the surrounding region. The San Juan Mountains, Spanish Peaks, Raton Mesas and Chisos Mountains are examples of the widespread igneous activity associated with the Rio Grande Rift.

Opening of the Rio Grande Rift system represented a reversal of tectonic stress from the previous Larimide Orogeny. During the late Cretaceous and early Tertiary, the region was subjected to crustal compression that led to deformation and uplift of the Sangre de Cristo Mountains. The culmination of this orogeny was in the Eocene, when sediments were eroded from the mountains and deposited in the Raton Basin in massive amounts. However, by the late Oligocene, crustal tension had become pervasive, and the rift system began to develop. The Rio Grande Rift was similar in its active development to the East African Rift system of today. However, the Rio Grande Rift has become largely dormant in the late Cenozoic. There are no active volcanoes now, and earthquakes are not common. This suggests a "failed" continental rift system.

Rio Grande Rift, Colorado and New Mexico

East and West Spanish Peaks with Goemmer Butte in the foreground. Spanish Peaks represent the eroded remains of granite-syenite intrusions, and Goemmer Butte is the neck of a small volcano. These intrusions took place within the Cuchara Formation (Tertiary) of the Raton Basin. Kite aerial photograph, date 3/99, © J.S. Aber.
East and West Spanish Peaks seen from Goemmer Butte. The east peak (left) is 12,683 feet (3866 m), and the west peak is 13,626 feet (4183 m). Notice the rock walls (dikes) that radiate like spokes of a wheel from West Spanish Peak. Photo date 6/99, © J.S. Aber.
The dike of Devils Stairsteps seen here from the side. Erosion of softer surrounding sediments (Cuchara Fm.) has left the dike standing like a wall across the landscape. Photo date 6/99, © J.S. Aber.
Closeup view of the igneous rock of Goemmer Butte. The butte consists of latite and trachyte, which are aphanitic equivalents of monzonite and syenite. Continental rift systems are the tectonic settings for such rocks that have a low silica content. Photo date 4/99, © J.S. Aber.
Roadside exposure of volcanic strata of the San Luis Valley. Massive volcanic eruptions took place during mid-Tertiary rifting throughout the valley, which is part of the Rio Grande rift system. Photo date 6/00, © J.S. Aber.
Gold mine at Cripple Creek, Colorado. Gold mineralization was associated with explosive volcanic eruptions during the mid-Tertiary, possibly in connection with Rio Grande rifting, widespread volcanism, and hydrothermal metamorphism. Photo date 3/00, © J.S. Aber.
Students take a lunch break at an abandonned mine on the side of Trinchera Peak, Culebra Range, southern Colorado. Epidote mineralization indicates hydrothermal alteration of rocks in this vicinity. Photo date 3/00, © J.S. Aber.
Mount Capulin, northeastern New Mexico. A well-preserved volcanic cinder cone about 60,000 years old. Mount Capulin is situated at the eastern end of the Raton Mesas. Kite aerial photograph, date 3/98, © J.S. Aber.
Late Holocene lava flows in the Valley of Fires, New Mexico. Flows 1500-2000 years ago are the youngest volcanic eruptions in the Rio Grande Rift system. Photo date 3/98, © J.S. Aber.
Closeup view of lava flows in the Valley of Fires, New Mexico. Photo date 3/98, © J.S. Aber.
Landsat false-color TM image of White Sands, New Mexico. This infrared (TM 457) image depicts the gypsum sand in blue and cyan colors. Photo date 3/99, © J.S. Aber.

Rio Grande Rift, West Texas

Chisos Mountains flanked by Tertiary sediments, Big Bend National Park, West Texas. Photo date 11/75, © J.S. Aber.
The Basin of the Chisos Mountains, a drainage basin surrounded by intrusive bodies. Intrusions and related volcanic eruptions took place during the mid-Tertiary. Photo date 3/80, © J.S. Aber.
Closeup view of intrusive rocks that form the sides of the Basin of the Chisos Mountains, Big Bend National Park, West Texas. Photo date 3/80, © J.S. Aber.
Casa Grande, a pile of volcanic strata capped by a thick lava flow. These eruptions took place in connection with underlying intrusions, Chisos Mountains, Big Bend National Park, West Texas. Photo date 1/78, © J.S. Aber.
Volcanoclastic deposits on Lost Mine Ridge, Chisos Mountains, Big Bend National Park, West Texas. Photo date 1/78, © J.S. Aber.
Boquillas Formation (upper Cretaceous) chalk exposed alongside the Rio Grande, Big Bend National Park, West Texas. Photo date 3/80, © J.S. Aber.
Massive lower Cretaceous limestones are uplifted along a fault at Boquillas Canyon of the Rio Grande. Photo date 3/80, © J.S. Aber.

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Notice: Structural Geology is presented for the use and benefit of students enrolled at Emporia State University. Others are welcome to view the course webpages. Any other use of text, imagery or curriculum materials is prohibited without permission of the instructor. GO 568 © J.S. Aber (2002).