3. Metamorphic Core Complexes and their Regional Contexts
Above: a portion of the Catalina Mountains complex (taken from Oracle Road during the July 2003 fire)
[In the following discussion, you may want to refer to an authoritative geological time scale; to see one adapted from the work of the University of California Berkeley Museum of Paleontology (cited in Credits & Bibliography) click here.]
For our own immediate area, Bob pointed out some very interesting patterns 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::
Keys:
(Click on each of these images to enlarge it.)
In this rather complex diagram, the Metamorphic Core Complexes -- the two very large granitic and metamorphic mountain complexes shown in dark gray -- are unusual. The Catalina-Rincon Core Complex, and the Pinaleno Core Complex, contain huge masses (plutons) of "youthful" granite -- 20, 30, 50 MYa -- that have "intruded" to the surface, risen from great depths (5 to 8 miles) like two uplifted pimples. These uplifts occurred just before the Basin-Range times. Note how they stand in opposition to each other.
There are also two other opposing features: to the southeast, the Chiricahua range contains rocks that erupted from a huge rhyolite pyroclastic caldera volcano, about 25-26 MYa. To the northwest of the Galiuro range, a second huge rhyolite volcano, now forming part of the Superstition range, erupted around 24-22 Mya, about the same time as these 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 appears to have been blown out of long fissures (rhyolites, from 30-20 MYa, shown in green, 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 -- huge masses of granite rising to the east and west (to become layered Gneisses), and explosive rhyolite volcanism occurring to the North, to the south, and also to the East in the Gila Mountains and New Mexico (see below).
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 (see the link numbered 4 below). 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 (see the diagram above). This fault was probably fairly flat when it formed about 25 MYa, and this kind of fault reappears in a broad belt along the eastern portion of the Basin-Range Province all the way to Canada. If you look at the diagram above, you can see that the western part of the Catalinas, and the eastern part of the Pinalenos, shows gneiss with "lineations", the lighter gray rock being streaked with lines; these represent the "landsliding" effect of a detachment as the area started to pull apart (though the Basin-Range "shattering" comes later). Detachment Faults are an important part of our picture -- for more details on the process, see 4. Detachment Faults and the San Pedro Valley.
In the photograph below, taken in Hot Springs Canyon at the site of the Red Trail, the people are milling at the location of the Soza Mesa Fault, the eastern surface extension of the main detachment fault associated with the Catalina-Rincon core complex, which cuts left-right across the Canyon at that point (to the right, the white-barked Sycamores marking Coati Terrace). Galiuro Volcanics are prominent in the background, but beyond this point upstream (behind the photographer), the main formation is the Willow Canyon Formation. The Soza Mesa fault dips away from the viewer and underlies the Galiuro Volcanics shown in this downstream view.
For a summary placing the Metamorphic Core Complex process in its wider historical context, see Meader: Geological History of our Area. (Below: a portion of the Rincon and Little Rincon complex, taken from near Pomerene looking west in February 2007.)
