My last post examined some structures and petrology of a Metamorphic Core Complex (MCC), whereas the previous one discussed the geology of the Central Highlands of Arizona. I mentioned several times that these geologic provinces were but two manifestations of the profound tectonic change associated with uplift of the the Colorado Plateau.
Today’s post is from the Phoenix Mountains, a municipal park inside the city limits (Fig. 2). My geological interpretations and dates, etc, come from a report by the Arizona State Geological Survey.
To the west (left of Squaw Peak in Fig. 3), a deep fault has been identified, which thrust the metasedimentary rocks comprising the eastern part of the range into juxtaposition with the metavolcanic rocks of Stoney Mtn and other outcrops to the west (Fig. 4).
Today’s post is focused on the circled area in Fig. 4 which, if we look back to Fig. 3, is the highest peak within the Phoenix Mountains. I was intrigued by the view from the parking lot (Fig. 1), and compelled to explore this fault-block in person. Note that the area discussed in this post in contained within the circle in Fig. 4.
The geology of Piestewa Peak is relatively simple. Schist. In this case, the metamorphic grade isn’t too high and the rocks preserve much of their original thin-bedded layering. However, they are standing on end (Fig. 5).
There isn’t much to say about this post after my road trip to Prescott, and then a hike in the White Tank mountains. The first thing I can say with confidence, however, is that Squaw Peak (aka Piestewa Peak) is squarely located within the Basin and Range, defined by faults that have brought disparate rocks into juxtaposition, but only in a small area. The Phoenix Mountains are nothing like the vast, overlapping fault-bounded mountains of the Central Highlands, but instead they are isolated in a sea of sand and gravel, sediment eroded from the long-gone rocks that encased them for eons. There was no superimposed shear evident in these rocks as in the White Tank mountains. They just rose from the earth’s bowels along nearly vertical faults.
These rocks aren’t as old as those we encountered in the Central Highlands — by about a billion years. Nevertheless, they suggest that plate tectonics determined the history of central Arizona, even so long ago. Because of the lack of suitable rocks, no plate reconstruction can be attempted for the Precambrian (neither a geologic period, era, or eon); thus, we can only assume that things were the same but different — upper mantle processes dragging crustal plates around, but without plants, oxygen, or animals to intervene in surface erosion.
We don’t know what happened that long ago, not to mention the billion years between the creation of these sedimentary/metamorphic rocks and the emergence of multicellular life. We can only view the rocks we’ve seen in Arizona through a glass darkly…