Rio Grande rift

Geology

The Rio Grande rift represents the easternmost manifestation of widespread extension in the western U.S. during the past 35 million years. The rift consists of three major basins and many smaller basins, less than 100 km2. The three major basins (from northernmost to southernmost) are the San Luis, Española, and Albuquerque basins. The rift's northern extent is delineated by the upper Arkansas River basin between Leadville and Salida, Colorado. Further south, the rift is defined by a network of smaller, less topographically distinct alternating basins and ranges. The distinction between these smaller basins and those of the Basin and Range Province becomes blurred in northern Mexico.

Basin size generally decreases to the north in the rift, though the Española covers approximately 120 km north–south and 40 km east–west, and the San Luis is roughly 120 by. These basins may contain smaller units within them, such as the Alamosa basin within the San Luis, which is bounded by the San Juan and Tusas mountains on the west and the Sangre de Cristo Mountains in the east. The Albuquerque basin is the largest of the three basins, spanning 160 km north–south and 86 km east–west at its widest points. It is the oldest of the three major basins, and contains 7,350 m of Paleogene clastic sediments deposited on Precambrian basement. The southernmost Albuquerque basin contains pre-rift volcanic deposits, while the central and northern portions contain volcanics erupted during rifting.

In cross-section, the geometry of the basins within the rift are asymmetrical half-grabens, with major fault boundaries on one side and a downward hinge on the other. Which side of the basin has the major fault or the hinge alternates along the rift. The alternation between these half-grabens occurs along transfer faults, which trend across the rift to connect the major basin-bounding faults and occur between basins or, in places, within basins. The Precambrian basement changes relief sharply in this area, from 8700 m below sea level at the bottom of the Albuquerque basin to 3300 m above sea level in the nearby Sandia Mountains, which flanks the Albuquerque basin to the east. Flanking mountains are generally taller along the east side of the rift (although some of this relief may be Laramide in origin).

Formation of the rift began with significant deformation and faulting with offsets of many kilometers starting about 35 Ma. The largest-scale manifestation of rifting involves a pure-shear rifting mechanism, in which both sides of the rift pull apart evenly and slowly, with the lower crust and upper mantle (the lithosphere) stretching like taffy. This extension is associated with very low seismic velocities in the upper mantle above approximately 400 km depth associated with relatively hot mantle and low degrees of partial melting. This intrusion of the asthenosphere into the lithosphere and continental crust is thought to be responsible for nearly all of the volcanism associated with the Rio Grande rift.

The sedimentary fill of the basins consists largely of alluvial fan and mafic volcanic flows. The most alkalic lavas erupted outside the rift. The sediments that were deposited during rifting are commonly known as the Santa Fe Group. This group contains sandstones, conglomerates, and volcanics. Aeolian deposits are also present in some basins.

The Rio Grande rift is intersected in northern New Mexico by the NE-SW trending Jemez Lineament which extends well into Arizona. The lineament is defined by aligned volcanic fields and several calderas in the area, including the Valles Caldera National Preserve in the Jemez Mountains. The Jemez Lineament is thought to be a hydrous subduction zone scar, separating Precambrian basement rock of the Yavapai-Mazatzal transition zone from the Mazaztl Province proper. Also on the Colorado Plateau but further north lies the San Juan volcanic field in the San Juan Mountains of Colorado.

The youngest eruptions in the rift region are in the Valley of Fires, New Mexico, and are approximately 5,400 years old. The Socorro, New Mexico, region of the central rift hosts an inflating mid-crustal sill-like magma body at a depth of 19 km that is responsible for anomalously high earthquake activity in the vicinity, including the largest rift-associated earthquakes in historic times (two events of approximately magnitude 5.8) in July and November 1906. Abstract at: Earth and space-based geodetic measurements indicate ongoing surface uplift above the Socorro magma body at approximately 2 mm/year.

File:Riogranderift deepXsection.png|alt=Generalized cross section of the Rio Grande Rift, showing lithospheric and asthenospheric structure.|A generalized cross section of the Rio Grande Rift, showing lithospheric and asthenospheric structure. Note the magma lenses and volcanics caused by the welling up of the asthenosphere into the crust, and the thinning of the lithosphere

Image:RioGrande Rift RISTRA Seismic Image.jpg|Deep seismic image of the Rio Grande rift compiled from the seismic transect shown in the previous figure, showing inferred mantle flow and imaged crust-mantle (Moho) topography (after Wilson et al.)(2005).\

File:Riogranderift timeline.png|alt=Timeline for extension and volcanism in the area of the Rio Grande rift.|A brief timeline showing extension and volcanism in the Rio Grande rift area. The end of the Laramide orogeny was followed by volcanism and then extension. Changes in lava chemistry are also found, resulting from changes in magmatic sources.

Geologic history

The Rio Grande rift's tectonic evolution is fairly complex. The fundamental change in the western margin of the North American plate from one of subduction to a transform boundary occurred during Cenozoic time. The Farallon plate continued to be subducted beneath western North America for at least 100 million years during Late Mesozoic and early Cenozoic time. Compressional and transpressional deformation incurred by the Laramide Orogeny lasted until about 40 Ma in New Mexico. This deformation may have been a result of the coupling between the subducting Farallon plate and the overlying North American Plate. Crustal thickening occurred due to Laramide compression. After the Laramide Orogeny and until 20 Ma, a major period of volcanic activity occurred throughout the southwestern United States. Injection of hot magmas weakened the lithosphere and allowed for later extension of the region.

Cenozoic extension started about 30 million years ago (Ma). There are two phases of extension observed: late Oligocene and middle Miocene. The first period of extension produced broad, shallow basins bounded by low-angle faults. The crust may have been extended as much as 50% during this episode. Widespread magmatism in mid-Cenozoic time suggests that the lithosphere was hot, the brittle-ductile transition was relatively shallow.

One theory is that the Colorado Plateau acts as a semi-independent microplate and one way of explaining the creation of the Rio Grande rift is by the simple rotation of the Colorado Plateau 1-1.5° in a clockwise direction relative to the North American craton. or small-scale mantle convection at the edge of the stable craton; collapse of over-thickened continental crust; initiation of transform faulting along the western margin of the North American plate; or detachment of a fragment of the Farallon plate beneath the Rio Grande region that enhanced asthenospheric upwelling in the slab window.

References

References

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