July 17, 2024

Geology Notes

Recently I got together with a good friend of mine who has been a life long geologist. We spent several hours talking about geology topics, and I took some notes. Here they are with some augmentation from memory.

Tucson Mountains

We didn't even talk about the idea that they slid off of the Catalinas, that idea had a short period when it was in vogue, but is no longer given serious consideration. Many people talk about the Tucson Mountains as a caldera. My friend points out that the rock structures can also be interpreted as coming from a linear vent at the north end of the range.

Core complexes

There is no end of controversy. Maybe the most fundamental question is whether they formed in a tensional or compressional regime. My friend favors compression, with the entire state being buckled under a SW to NE squeeze. This predated the tensional basin and range block faulting.

Equally interesting is that the nature of the Wilderness granite has various interpretations. Those who call it the Wilderness granite advocate for an intrusive origin. The other school calls it the Catalina Gneiss and views it entirely as a metamorphic rock Those in the "gneiss school" are taking a view more in line with tradition and history.

In the big picture, one can view the entire south part of the state as having two layers. We have the upper plate, that behaves rigidly and is broken by faults. We also have the lower plate, that behaves in a plastic manner, with folds and metamorphic and/or cataclastic textures. Looking at a given mountain range, the question then becomes whether is is upper plate (like the Tucson Mountains or Galuiros), or whether is is lower plate (like the various core complexes).

Orocopia Schist, Plomosa Mountains

The Plomosa Mountains are NE of Quartsite, north of I-10 in western Arizona. They have been viewed as a core complex, but this may be controversial. They do have a rock unit called the Orocopia Schist. This unit has long been viewed as Precambrian metamorphic terrain. They are now being viewed as much younger rocks. This is true of most core complexes, they were viewed by previous geologists as ancient Precambrian rock, but are now viewed asTertiary This schists and gneisses in what are today considered Tertiary core complex
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Tom's Geology Info / [email protected]