TECTONITES
Rock structurally beneath the Santa Catalina Fault on the northwest flank of Tanque Verde anticline exhibits semi-ductile and brittle strain, reflecting the progressive uplift and cooling of the core-rock. Semi-ductile mylonitic deformation of precambrian quartz monzonite augen gneiss is followed by further cataclastic deformation, exhibited in chlorite breccia, cataclastic mylonitic gneiss, pseudotachylite, and brecciated mylonitic gneiss.
The core of the CMCC is largely comprised of two intrusive granitic suites. The first intrusive bodies are part of the mid-Proterozoic accumulation of the Central Province from 1.6 to 1.4 billion years ago. Proterozoic granites in the CMCC region are the Johnny Lyon Granodiorite (~1.6 Ga), Rincon Valley Granodiorite (mesoproterozoic), and Oracle Quartz Monzonite (1.44 Ga). The most significant local continental crust-forming granite being the Oracle suite. These are porphyrytic to hypidiomorphic, biotite-bearing granodioritic to quartz monzonitic, including orthoclase, lesser microcline and plagioclase, and 5-15% biotite. Oracle granite is considered to be laterally most extensive and part of a larger accretionary event in the CMCC study area.
The other major intrusion occurred 44 to 47 million years ago. It is a suite of leucocratic fine-grained, equigranular, two-mica, garnet-bearing granite to quartz monzonite called the Wilderness Suite. This composite of granites varies in texture and composition including zones that are garnet-rich, or muscovite-rich, or aplitic. This Eocene intrusion was sufficiently large enough to re-heat portions of the proterozoic granite. It has been determined that both bodies cooled 25 Ma ago.
In terms of their metamorphic character, the Proterozoic granite (Yom) is a darker, medium to course-grained biotite-rich mylonitic augen gneiss, and the Eocene granite (Ewm) is a leucocratic, medium to fine-grained two mica, garnet-bearing equigranular augen gneiss. Although there are other intrusives, such as the Leatherwood Quartz Diorite, Rice Peak Granodiorite Porphyry, and the Catalina granite, the majority of the core is comprised of Oracle and Wilderness granitoids.
Late Cretaceous through Tertiary intrusions heated mid-crustal granitoids, and these bodies were thinned and extended. Ductile to semi-ductile flow occurred during flatenning and extension of the crust, imposing mylonitic strain on the gneisses. At the interface of the proterozoic and early tertiary granite, the two granitoids demonstrated contrasting properties of plasticity which created a shear zone that produced mylonite schist (ultramylonite). At upper-crustal levels, where plastic deformation yields to brittle deformation, a dislocation surface (the pre-existing thrust plane in my opinion) marks the boundary where listric-normal faulting occurs, rather than semi-ductile flow. Progressive rising of the batholithic suites and magmatic heat dissipation caused brecciation and cataclastic deformation to become the strain pattern. This strain is most conspicious in the vicinity of the detachment surface where relative displacement was greatest during the last stages of extensional tectonism. Brittle deformation of mylonite gneiss core rock intensifies in a zone just beneath the Santa-Catalina Detachment Fault.
In order to compare tectonites of the detachment locality, a discussion should first describe characteristics of a ‘typical’ mylonite gneiss. Mylonite gneiss in the CMCC is comprised mostly of Proterozoic granites. In general, the phyllosilicates in proterozoic granodiorite and quartz monzonite is mainly biotite with porphyroblastic muscovite, whereas Tertiary granite has muscovite and rare biotite. A rule of thumb: the darker mylonite gneiss is proterozoic granodiorite to quartz monzonite and the lighter slightly-mylonitic gneiss is Tertiary leucogranites. Mylonite gneiss 'layers' are not of uniform in thickness but they can be traced for some distance. Relict gneissic fabric is deformed by recumbent, isoclinal, flexural-slip folds. Although layers lack uniformity even in hand-specimen scale, mylonite gneiss attitudes are sufficiently coherent for characterization on map scale. Observe photos of a mylonite gneiss outcropping found next to Cactus Loop Drive, Saguaro Monument East.
Exception to the above generalized explanation are cases of possible Oligocene Catalina granite intrusions (or extremely-late Wilderness leucogranite) into the mylonite gneiss. There are instances where granites intrude, and sometimes dilate, mylonite gneiss along the planes of weakness, i. e., parallel to mylonitic fabric, mimicking mylonite gneiss by interfingering with gneissic banding of the host. In addition to dilation, these intrusive, anomalous granite bodies do not exhibit gneissic fabric, and rarely show weak mylonitic deformation. Keep in mind as well that there are large (and small) aplite veins throughout the entire complex. Also note that I don't know everything about the CMCC and I am still perplexed by some things.
ULTRAMYLONITE
One particular tectonite that sparked debate when research on the CMCC intensified during the '70's and '80's is the ultramylonite, or mylonite schist. One occurrence found just outside and east of Cactus Loop was originally mapped as Pinal Schist, the Proterozoic submarine turbidite that hosted continental granitoid suites. Ultramylonite can be found to occupy the contact between Proterozoic quartz monzonite and Tertiary granite for several kilometers on the southwest flank of the Rincon Mountains. The ultramylonite is a distinctive, black to gray tectonite formed by shearing of core rock at the contact of the two intrusive bodies. Porphyroblastic muscovite formed during later stages of mylonitization is oriented parallel to normal-slip extensional shear.
CATACLASITES
One of the most intriguing and enigmatic things in the complex is a zone of cataclasite found just below the detachment called the "decollement zone". The core gneiss and sedimentary units that comprise this zone were subject to deviatoric stress and shear at temperatures below the plastic limit of deformation. The upper boundary of the decollement zone is the Catalina Detachment Fault surface itself, above which lies the unmetamorphosed cover rock. The lower boundary of the decollement zone is a sharp contact with 'typical' mylonite gneiss (I never actually found this lower boundary but the difference in core rock is obvious in some locations if you are armed with the G. H. Davis map). The cataclasis rock of this zone can be found along the southwestern flank of the complex, from the Catalina foothills north of Tucson, to the southern Rincon Mountains, southeast of Saguaro Monument East. This cataclasite zone does not preferentially effect any particular unit and can include a portions of units at the bottom of the allochthon.
Cataclasites of the decollement can be described as "brecciated", "mildly brecciated", "strongly brecciated", and "extremely brecciated", per Di Tuillo. Personally, I found it difficult to find the boundaries he described, so rather than commit errors of interpretation, I shall try to explain the cataclasites of the decollement zone through the use of photos so that the reader can capitalize on the striking visual characteristics of this interesting zone.
The lower levels of the decollement include green to bluish-green vein-like bodies with tiny angular bits of felsic components of the core. G. H. Davis termed these chloritic microbreccia. These vein-like bodies resemble the character of the pervasive microbrecciated gneiss of the decollement zone. My interpretation of the work published by the experts is that these are 'fluidized' segments of mylonite gneiss. This does not mean that the rock was melted, but rather, due to non-uniform (deviatoric) stress and low temperature, mylonite gneiss was shattered into small fragments to accommodate stress.
Within the decollement zone mylonite gneiss is brecciated to varying degrees in a seemingly random pattern. Di Tuillo mapped these differences. The map shows extremely brecciated mylonite gneiss is found at the periphery of the decollement zone and less brecciation in the middle and north of the depiction. Brecciation patterns in map view may appear random but they are not. The decollement is comprised of many relatively small fault blocks with innumerable internal off-sets of 'fluidized' solids that weathered at different rates. Sub-blocks have down-dip anastomosing (sub-block shapes resemble triangles pointing down the fault) patterns that leave seemingly random juxtaposition of different degrees of brecciation. Extremely brecciated rock is more susceptible to weathering and the remaining brecciated rock the in the middle (in terms of a map view) of the decollement zone has survived because it is more resistant and also occupies a lower topographic gradient. Presumably, extremely brecciated rock was shielded from weathering by the cover rock above the detachment fault.
There is extremely brecciated rock, presumably cataclastic mylonite gneiss, that can be found at the top of the decollement zone. Cataclasis has obliterated any recognizable gneissic banding or mylonitic fabric. The photos included here were found near the detachment fault in the upper part of the decollement zone.
Just beneath the detachment fault, within the decollement zone is an outcropping of pulverized rock. The term "rock" is used here because I am not completely sure what it is. It could be a large cataclastic aplite or felsite dike. Drewes' 1977 map shows that there is an allochthonous Tertiary rhyolite intrusive in the vicinity, but this rock, in my opinion, is quite brecciated whatever it is. This could be a fragment of Tertiary hypabyssal rhyolite/dacite that was caught beneath the detachment and pulverized in the decollement. Paleozoic sedimentary rock is common to the cover rock above the detachment in this area, but I know of no instance where the core and cover coincide where juxtaposed.