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sill-like

Rio Grande rift


The Rio Grande runs a north-south course, in New Mexico and Colorado, along the Rio Grande rift. Southern boundary line of NM added for clarity
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The Rio Grande rift is a rift valley extending north from Mexico, near El Paso, Texas through New Mexico into central Colorado. The upper Rio Grande flows south down the rift valley, but did not incise the rift valley. The Rio Grande Rift is generally considered to be an aulacogen, a "failed" rift valley which was once more active than it is today.

The rift is presently nearly tectonically quiescent, but significant deformation and faulting with offsets of many km was responsible for the formation of the rift starting about 35 million years ago. 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 associated with partial melting. The upper crustal manifestation of the rift is a sequence of asymmetric half-grabens that accommodate deep basins that are substantially filled with alluvium.

After the Farallon Plate subduction-associated compressive forces of the Laramide orogeny ended during the Eocene Epoch (which had produced the Sangre de Cristo Mountains), erosion of these uplands filled the area of the Raton Basin with abundant sediments. In the late Oligocene Epoch regional tensional forces became dominant and rifting was initiated as the crust began extending. The rifting produced fault zone-bounded valleys (grabens or half-grabens). A graben consists of normal faulting on each flank with the central portion downdropped. Igneous intrusions moved into the zones of weakness produced by this faulting and reached the surface in many areas as extensive volcanism along the margins of the rift. The most active rifting and associated volcanism came to an end in the late Cenozoic. The youngest eruptions are in the Valley of Fires, New Mexico and are 1500-2000 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. Earth and space-based geodetic measurements indicate ongoing surface uplift above the Socorro Magma Body at approximately 2 mm/year.

The Sangre de Cristo Range lies to the east of the north portion of the rift. The Valles Caldera National Preserve is dominated by a huge caldera in the Jemez Mountains, located where the Rio Grande Rift intersects the Jemez Lineament, a linear crustal feature in the southwestern United States that may represent a suture zone from the Proterozoic accretion of North America. However, the Jemez Mountains themselves are not primarily a tectonic feature of the rift; rather, they partially overlie a range on the west side of the graben, the lower and less well-known Nacimiento Mountains. The Colorado Plateau, to the west, includes the San Juan Volcanic Field and the San Juan Mountains. The city of Albuquerque lies within the rift. The Rio Grande follows the course of the rift from southern Colorado to El Paso; it starts a southeast course out of New Mexico at the pass between the Franklin Mountains and the Sierra Juarez, which ends at the Gulf of Mexico.

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