Rio Grande Rift

Advanced Tectonics
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.

MODIS overview of the San Luis Valley and surroundings.
Obtained and modified from NASA's Visible Earth.

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.

More about the San Luis Valley.

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.
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.
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.
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.
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.
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.
Turquoise mine in the San Luis Hills, an uplifted block of older Miocene crust in the center of the San Luis Valley. Highly prized "Manassa Green" turquoise was and still is produced from this mine (Bauer 2011). Turquoise is a product of hydrothermal metamorphism.
Rio Grande looking northward with Ute Mountain in the background, northern New Mexico (left). Ute Mountain is a young Pliocene volcanic feature at 2.7 million years old (Bauer 2011). The Rio Grande has eroded a deep canyon through thick lava flows of the Servilleta Basalt (right), which was erupted in the Taos Plateau between 4.8 and 2.8 million years ago.
Late Holocene lava flows in the Valley of Fires, New Mexico. Overview (left) and close-up view (right). Flows 1500-2000 years ago are the youngest volcanic eruptions in the Rio Grande Rift system.
Landsat false-color TM image of White Sands, New Mexico. This infrared (TM 457) image depicts the gypsum sand in blue and cyan colors.

Rio Grande Rift, West Texas

Chisos Mountains flanked by Tertiary sediments, Big Bend National Park, West Texas.
The Basin of the Chisos Mountains, a drainage basin surrounded by intrusive bodies. Intrusions and related volcanic eruptions took place during the mid-Tertiary.
Closeup view of intrusive rocks that form the sides of the Basin of the Chisos Mountains, Big Bend National Park, West Texas.
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.
Volcanoclastic deposits on Lost Mine Ridge, Chisos Mountains, Big Bend National Park, West Texas.
Boquillas Formation (upper Cretaceous) chalk exposed alongside the Rio Grande, Big Bend National Park, West Texas.
Massive lower Cretaceous limestones are uplifted along a fault at Boquillas Canyon of the Rio Grande.

Reference

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Notice: Advanced tectonics 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. All text and imagery © J.S. Aber (2016).