S-C mylonite, Windy Point in the Santa Catalina Mountains, Tucson, Arizona

This mylonite is beautifully exposed at Windy Point, a stop along the Mt. Lemmon Highway in the Santa Catalina Mountains. Among other things, the Windy Point area is a rock climber’s paradise. Before mylonitization, this was a 2-mica (muscovite and biotite) garnet-bearing granite, with an equagranular hypidiomorphic texture. Shear zone deformation accompanying the formation of the Catalina-Rincon metamorphic core complex transformed the granite to mylonite. This is a nice block of mylonite, with the two smooth ‘faceted’ sides reflecting jointing. The mylonitic fabric disclosed on these joint faces is classic S-C fabric The C-structure (slip planes) slants through the rock in the form of parallel planar subtle surfaces spaced at ~3 mm. These cut across the S-surfaces (flattening planes), best reflected by the average long dimension of the feldspar augen. Taken together, the intersection of the S and C fabrics disclose the sense of shear. The C surfaces are conventionally found to be oriented parallel to the shear zone boundaries overall. Lineation direction does not ‘jump out’ in this specimen, but it can be seen on the butt ends of the rock.

Windy Point in the Catalina Mountains.  This is among the perfect examples of S-C mylonite.

Age & Formation
Uncertain, but probably Paleocene or Eocene, ~55 Ma.