Imbolc                                                Black Mountain Moon

Into the Colorado School of Mines last night, its Museum of Colorado Geology, for a second lecture to the Friends of the Museum. This one: Whither the Rio Grande Rift?

The significance of the title escaped me until Vince Matthews, former Colorado State Geologist, explained that the rift was a spreading of the earth’s crust, a spreading that thins the mantle and increases volcanism and creates faults. Then it hit me. Oh, a rift. Like the Olduvai Gorge in the horn of Africa.

There are three faults within the Colorado portion of the Rio Grande Rift that made it onto the USGS hazards map, one believed capable of producing a 7.5 magnitude quake and another of producing a 7.0 quake. Logarithmic scale. Those would be powerful and they would come in the middle of Colorado, toward the New Mexico border.

He used two terms in this dense, finely argued lecture that were completely new to me: graben and lineament.

Graben: In geology, a graben is a depressed block of land bordered by parallel faults. Graben is German for ditch or trench.

 

Lineament: A lineament is a linear feature in a landscape which is an expression of an underlying geological structure such as a fault. Typically a lineament will comprise a fault-aligned valley, a series of fault or fold-aligned hills, a straight coastline or indeed a combination of these features.

The focus of his presentation was the true northern extent of the Rio Grande Rift. Here’s a map that shows its extension in the consensus view (more or less). In this map you can see the Rio Grande rising in southwestern Colorado, then flowing through the San Luis basin into New Mexico and then onto its more familiar location as a major boundary feature between the US and Mexico.

Vince said that current thinking took the Rio Grande Rift as far as Leadville.

 

Leadville in this map is the first black lettered city above the C in Colorado. I use this map to show you the San Luis Basin (the light tan opening to the left of Highway 25 and starting at the New Mexico border. The San Luis Basin is a major feature of the Rio Grande Rift as it comes north out of New Mexico.

Matthew’s argument extended the Rio Grande Rift considerably further north and then hypothesized a turn from its primarily north/south axis to an east/west one. This map of the Colorado Plateau can be used to illustrate his argument:

 

Matthews extended the boundary of the Colorado Plateau east to include the Rio Grande Rift, then proposed that the rift extended east/west toward the area here marked as the White River Plateau. He based his argument on indicators of a rift zone (which I won’t go into here) and on an experiment on a clay model of the Colorado Plateau.

In essence he argues that the Colorado Plateau is a tectonic feature that has been rotated clockwise. When asked how that could have happened, he said, “I don’t know.” But, if you imagine the Plateau as a piece of the earth’s crust that has physical integrity, then a motion pushing up on its southwestern edge would turn it clockwise. One of the other geologists in the room proposed the San Andreas Fault as it developed. (I got lost right here, but I followed the argument up to this point.)

Very interesting. These lectures are helping me orient myself to the unusual topography of Colorado and some of forces that shaped it.

BTW: I loved Matthew’s description of two cinder cones as “very young.” They were only 640,000 years old. Puts 68 in a very satisfying context.