Something Completely Different - The Rio Grande Rift

Yes, I love Monty Python. And now, time for something completely different! 

During this lockdown, I've been attending geology livestreams offered up by Nick Zentner, a geology professor at Central Washington University in Ellensburg, WA. I highly recommend them for anyone with an interest in geology. The first batch of livestreams started in March and ended in June. The "2nd semester", which was exclusively on exotic terranes of the west coast, ran from Sept to December (26 weeks). I have learned a lot and so, I've ventured to put together this little blurb on the Rio Grande Rift all from readily available information in books, papers, and websites. Here goes...

The Rio Grande Rift is a major break in the Earth's crust. It's a north-south trending zone of east-west crustal extension that has deformed areas from the Mexican state of Chihuahua to as far north as central Colorado, Leadville, totally bisecting the state of New Mexico. This crustal extension is due to the Colorado Plateau pulling away from the High Plains, which is part of the craton of North America, causing the crust to be pulled thin. The northern section of the rift is relatively narrow, consisting of a series of westward stepping basins flanked by rugged mountains. The rift broadens to about 30 miles (48km) at Albuquerque, from the rift's eastern margin - the western face of Sandia Mountain - to the rift's western margin near the Rio Puerco.

South of Socorro, New Mexico, the rift broadens considerably and the southern portion of the rift and its associated ranges blends into and with the Basin and Range province in southwestern New Mexico. Here the basins are in the form of half-grabens that are tilted strongly toward the east or west depending on the location of the master fault system on the margins of each basin.

Fault movement along the Rio Grande rift in the Albuquerque basin area started about 30 Ma (million years ago) and continues today. Sandia Mountain, the eastern margin of the rift, is a single east tilted fault block with a 1.4 billion year old Precambrian igneous and metamorphic core topped by 300 Ma sedimentary rocks. This fault block rises nearly 11,000 feet (3350 meters) above sea level, and the sedimentary rocks on the summit are nearly 26,000 feet (6100 meters) above the corresponding layer of rocks beneath the valley floor. That's nearly 5 miles (8 km) of fault movement!

2 of 3 Albuquerque Volcanoes

Volcanism is common along this rift. In Albuquerque, at the Volcanoes Day Use Area (part of Petroglyphs National Monument), there are five cones, which erupted along a north-south oriented fissure some 150,000 years ago. According to the National Park Service, these cones, locally called The Three Sisters, "are a classic and rare example of a fissure eruption." Fissure eruptions can produce spatter cones, steep sided cones made from hardened blobs of erupted basalt. There were several eruptions along the 5 mile (8 km) long fissure that produced both aa and pahoehoe flows of basalt. These cones are very near the middle of the Rio Grande rift. South of the Three Sisters, near Isleta Pueblo, the interstate, I-25, cuts through a lava field exposing columnar jointing. Other volcanic areas of note along the rift include: Taos volcanic field, Jemez volcanic field, Isleta volcanic field, Socorro magma body, and Portrillo volcanic field.

The Jemez volcanic field is dominated by the Valles Caldera, which is located at the junction of the Rio Grande rift and the Jemez Lineament (JL), both of which are major crustal features. The JL is a southwest to northeast trending line of volcanoes along what is now interpreted as a subduction zone scar. This zone runs from Springerville and White Mountains volcanic fields in Arizona to the Raton-Clayton volcanic field in northeastern New Mexico. The JL is 375 miles (600 km) long and was formed about 1.7 billion years ago along the southern edge of what was then to become the North American continent.

Hmmm....  It seems New Mexico has some exotic terrane story from giga years ago.

Later gators!  Yours in New Mexico...