The Catalina detachment fault in southern Arizona brought a series of metamorphic core complexes to the surface, including the Rincon Mountains, the Santa Catalina Mountains, and the Picacho Mountains. Both brittle and ductile deformation accommodated slip along the detachment fault in response to extreme extension (perhaps as much as 100%) during the Middle Tertiary.

At East Saguaro National Park in the Rincon Mountains, our Advanced Structural Geology class examined the ductile and brittle structures associated with the Catalina detachment fault. In this poster, I will focus on the brittle deformation along the fault. My purpose is to provide a preliminary view of the character and the spatial variation of brittle behavior exhibited by rocks above and below the detachment fault.


In the area we examined, the lower plate rocks of the detachment fault consist of a mylonitic injection gneiss displaying a strong compositional banding, and the upper plate rocks consist of Paleozoic carbonates (Photo #0) and (Photo #2). Within tens of meters of the base of the detachment fault, the mylonitic gneiss shows evidence of brittle deformation in the form of large angular feldspar grains (Photo #2). The ductile S-C fabric of the gneiss within the same zone below the fault is clearly overprinted by fracturing and brecciation (Photo #1), suggesting that brittle deformation followed ductile deformation.

The mylonitic gneiss is highly fractured and faulted in a zone 10-15 meters below the fault; in addition, there are smaller angular grains (<1 centimeter) and a much greater number of grains (spaced only centimeters apart) (Photo #3). The gneiss has a green tint, probably due to chlorite alteration (Photo #3), and zones of significant chlorite alteration can be found at this same structural level below the fault (Photo #4). Extensive fracturing is also found in the zones of chlorite alteration and may have provided conduits for fluid flow (Photo #4).

A 3-5 meter thick, planar zone of chlorite breccia lies below the microbreccia ledge (Photo #5). This zone is discontinuous, having pods of unaltered but brecciated material. The grain size of the breccia grains varies from >1 centimeter to microscopic but is generally smaller than sizes in the brecciated zones below. The fracturing is intense, especially in altered zones (Photo #5).

A 1-2 meter thick, brown, resistant ledge of microbreccia marks the location of the detachment fault (Photo #0) and (Photo #6). The top of the microbreccia is generally considered to be the detachment fault surface because it has lineations, such as grooves, that demonstrate that motion took place along that surface. In general, grains cannot be seen in the microbreccia because of the extreme fracturing, shearing, and (or) fragmentation; however, in places, angular grains are still present even though the original gneiss is no longer recognizable (Photo #7).

In the Paleozoic carbonates above the detachment fault, there are extensive fractures filled with quartz and calcite. I do not know if these features are related to movement along the detachment fault, but it is quite possible because quartz and calcite deposition are associated with the South Mountains detachment fault (Champine 1982).


1. The brittle deformation came after the ductile deformation. Champine (1982) provided much more support for this conclusion at the South Mountains detachment fault; for example, microscopic fragments of mylonite are found in the microbreccia there.

2. The degree of brittle deformation increases structurally upwards toward the detachment fault as reflected by:

  • the increase in the number of the angular grains
  • the decrease in the size of the grains
  • the increase in the number of the fractures
    Champine (1982) reached the same conclusion at the South Mountains detachment fault.

    3. The degree of alteration increases structurally upwards toward the detachment fault as reflected by:

  • the increase in the green color (chlorite content)
    Champine (1982) demonstrated that the abundance of alteration products besides chlorite, including sericite, epidote, and calcite, also increase toward the detachment fault, culminating in the microbreccia which commonly contains 80% of these minerals.

    4. The degree of brittle deformation and alteration also vary in space as shown by:

  • the variation in grain size at the same structural level
  • the discontinuous zones of chlorite alteration at the same structural level

    5. The mylonitic gneiss is still recognizable in most of the brecciated rocks (except the microbreccia), so it appears to have accommodated most of the brittle deformation (rather than the Paleozoic carbonates or other upper plate rocks, for example). Champine (1982) showed using microscopic evidence that the microbreccia at the South Mountains probably formed from mylonitic rocks, so this conclusion seems reasonable.


    This reconnaissance work in the Rincon Mountains raises a series of interesting and important questions about the nature of brittle deformation at detachment faults.

    1. Why did the behavior of the rocks accommodating the slip change from ductile to brittle?

  • the introduction of the fluids that caused the alteration?
  • the rising of the lower plate to more shallow crustal depths (decrease pressure)?
  • the loss of heat--could be related to fluids or ?--(decrease temperature)?

    2. Why was the brecciation and alteration discontinuous in space?

  • original inhomogeneities in the mylonitic gneiss? if so, what kind (e.g., composition or fractures)?
  • local stress fields due to inhomogeneities in the fault surface?

    3. How much strain did each brittle 'zone' accommodate?

  • for example, did the microbreccia experience the most strain or was it more of a distributed deformation?

    4. Why did most of the brittle deformation occur in the mylonitic gneiss (in other words, why didn't it move into the upper plate rocks more?)

  • were the mylonitic rocks simply weaker because of their smaller grain size?
  • did the zone of alteration minerals provide a weak slip plane?
  • were the fluids causing alteration still present during slip, reducing the friction of that zone significantly?

    5. What are the dominant mechanisms of brittle deformation?

  • shearing?
  • fracturing?
  • does alteration play a role?

    6. How much slip did brittle deformation allow relative to ductile deformation at the detachment fault?

    These are just my own questions and thoughts on the issues, and probably many of them have been worked out by other researchers that I am not aware of (that's just my little caveat for this poster).


    Champine, A.L., 1982, The microstructures and petrofabrics of the mylonite-chlorite breccia transition,
    South Mountains, Phoenix, Arizona [M.S. Thesis]: Arizona State University, Tempe, Arizona.


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