2. "Metamorphic Core Complexes" and Detachment Faulting:

The Catalina-Rincon Mountains Complex and the Pinalenos Complex

Above: a portion of the western Catalina Mountains complex (photo taken from Oracle Road northwest of Tucson during the July 2003 fire). The Pirate Fault, which raised this part of the Catalina Core Complex several thousand feet well after its intrusion, runs along this front.

[In the following discussion, to place these processes in their wider geohistorical context, see Meader: Geological History of our Area.]

A. Regional Overview:

A Time of Plutonic Intrusions and Pyroclastic Volcanism in our Area

In our Geology Walks (on which this treatment is largely based) Bob Scarborough pointed out some very interesting patterns for our own immediate area that suggest a major large-scale process of a very unusual kind. He warned us that some of these models are much oversimplified, problematic, and uncertain, but they're worth bearing in mind as we consider the geology of this area.

First, two great metamorphic Core Complexes -- shown standing in a roughly east-west opposition to each other at a distance in the diagram below, rendered in dark gray color: the Catalina-Rincon Core Complex and the Pinaleno Core Complex, both of which contain huge masses (plutons) of "youthful" granite -- from 20, 30, and 50 Million years old -- that have "intruded" to the surface, risen from great depths (5 to 8 miles) like two uplifted pimples. (Click on each of these images to enlarge it.)


These two Core Complexes, here colored in grays, rose to the surface beginning around 30 Million Years ago.

Second, two other opposing features (shown mainly in greens and blues) appear that run between the Core Complexes from northwest to southeast: to the southeast, the Chiricahua Mountain range was formed largely out of rocks that erupted from a huge rhyolite pyroclastic caldera volcano (known as the Turkey Creek Caldera, see the map above) about 27-25 MYa, which erupted some 500-1000 kilometers of Rhyolite Canyon Tuff. To the northwest of the Galiuro range, a second huge rhyolite volcano, now forming part of the Superstition Mountain range, erupted around 24-22 Mya, about the same time as the two core complexes were erupting. A third caldera appears to lie between them in the Galiuros, up near Copper Creek. Most of the explosive, pyroclastic Galiuro Volcanics (a major geological Formation in our immediate area) appears to have been blown out of long fissures (rhyolites, from 30-20 MYa, shown in the elongated green rectangle, marked with long lines of dikes through our area) which are indicated -- though not very clearly here -- on the map above (sadly, taken with a video camera).

So looking at the whole, you can see a giant oval area, with two distinct events occurring at the same general time -- two huge masses of granite rising to the east and west (to become layered Gneisses), and explosive rhyolite volcanism occurring to the northwest, to the southeast, and in the center (and also further to the east in the Gila Mountains and New Mexico. The latter seems to suggest northwest-to-southeast "rifting", splitting apart of the crust.

Finally, toward the end of this period -- generally labeled as the "Mid-Tertiary Orogeny", massive erosion occurred in our area between 23 and 18 Million years ago, carrying volcanic ash and rocks, and notably ancient plutonic granodiorite clasts into the basins produced by the process just described. These deposits we now call the San Manuel Formation.

B. Core Complexes and Detachment Faults: the San Pedro Valley

Before the mass of the Catalina-Rincon Mountains began to rise, a huge, west-dipping fault formed along the western front of the Galiuro Mountains (for images of these mountains, see this link). As the rock mass above the fault slid southwestward, the Catalina-Rincon Mountains arched upward, bending the fault and eventually exposing it at the surface along their western flank.

This kind of fault appears in a broad belt along the eastern portion of the Basin-Range Province all the way to Canada, and this one was probably fairly flat when it formed about 30-25 MYa. If you look at the diagram near the top of the page (drawn by Bob Scarborough), you can see that both the western part of the Catalinas and the eastern part of the Pinalenos show gneiss with "lineations", the lighter gray rock being streaked with lines; these represent the "landsliding" effect of detachment as the area started to pull apart. Geologists call rocks that display these lineations "mylonitic", a term derived from ancient Greek that refers to the grinding action of millstones, in other words, rocks heated by friction and ground by the movement across one another. "Mylonitic gneiss" is a metamorphosed granite, showing dark streaks in lighter stone produced by this process.

Detachment Faults are an important part of our picture, so we need a more detailed look at them.

The diagram below (taken from a publication by Jon Spencer and Steve Reynolds, adapted by Mick Meader to our area) shows several details of the process in a three-part sequence. Note that in contrast to that above, in this series west is shown to the right. The initial major detachment fault, which occurred somewhere in our area, began to form prior to the uplift of the Core Complex "pimple", generating a series of breakaway faults. As the Santa-Catalina mountain complex rose, the San Pedro Valley part of the fault became cut off ("fossilized"), then formed another set of lesser detachments. One of these detachment faults is the Soza Mesa Fault (visible in our own Hot Springs Canyon, see further below). [Note here the reference to "mylonitization", referring to the grinding process.)

A previous diagram now more detailed below (thanks again to Mick Meader) shows how, as the older rocks slid off the top of the Catalina-Rincon complex during detachment, this removed the overburden and allowed the hot underlying pluton to rise even higher. The Rincon-Catalina pluton as shown here was composed of granitic crystalline rocks, which -- as the detached rock overburden slid away -- were deformed under this grinding pressure into "mylonitic gneisses", shown as the "lineations" indicated by Bob Scarborough for the southwestern flanks of the Catalina-Rincon Complex (and also for the east flanks of the Pinalenos).

Note here how the "breakaway fault" that marks the western margin of the Galiuro Mountains suggests a rift line for the San Pedro Valley. The entire process described above (including both the Catalina-Rincon Core Complex and the Galiuro Volcanics) is also labeled the "Mid-tertiary Taphrogeny", this latter term denoting formation of large-scale geological structures by high-angle or block faulting, typically due to tensional (as opposed to compressional) forces in the crust. In the next section we discuss the Basin-Range Faulting process, which also involves this extensional rifting, but it appears that the San Pedro Valley rift has a much older base than the time of the Basin-Range deformations.

The Soza Mesa Fault: a detail from our own area

In Hot Springs Canyon, we find what is apparently an example of this Detachment-faulting process affecting our part of the Galiuro mountains complex. This fault -- the Soza Mesa Fault, a mid-Tertiary (less than 24 MYa) low-angle fault, is located as shown on the map below, provided by Mick Meader. It lies in the upper-left corner of the map, near what Saguaro Juniper members call the Red Trail. The Hot Springs Canyon Windmill (one of our main orientation centers in the Canyon) is shown by a circle at the lower left, while another fault, the Teran Wash Fault (discussed elswhere, in the Basin-Range page) lies in the middle.

Below, a view of the seminar group led by Robert Scarborough & Mick Meader in March 17, 2001, seated on the surface of the fault, with dark gray Galiuro Volcanics Formation above and the (much older) reddish Willow Canyon Formation below. The detachment proceded from upper right to lower left, as Galiuro Volcanics slid over Willow Canyon, deforming the latter at the points of contact. As Bob Scarborough remarked while we were sitting on the reddish rock, "this surface is all messed up", that is by the movement of the volcanic rocks sliding downhill past it, both heating and grinding both contact surfaces.

Below: portion of the Rincon and Little Rincon side of the Core Complex, taken from near Pomerene looking west in February 2007. The Martinez Ranch Fault, which raised the Core Complex several thousand feet long after its initial intrusion, runs from left to right along the vertical flanks of Rincon Peak at upper left of the image.

Below: the southernmost end of the Catalina-Rincon Metamorphic Core Complex, viewed from Interstate-10 looking northward. Rincon Peak is just left of upper-center in this photograph, taken in February 2007.