Santa Catalina Mtns | Pima County, Arizona. Mylonitized Oracle Granite from the lower plate of the Santa Catalina detachment fault exposed in the Lower Tanque Verde Falls. The porphyroclast - large deformed spherical shape with an obvious tail - provides a sense of shear. (Photo by Jessica Rudd)
Mylonite - a metamorphic rock formed by ductile deformation that accompanies intense shearing encountered during folding and faulting, a process termed cataclastic or dynamic metamorphism. In this case the Oracle granite was ductily sheared during extension that moved the rock ~20 miles northward and 2-3 miles surfaceward. The parent granite was severely pulverized, obliterating the original minerals which recrystallise as smaller, tightly intergrown grains, forming a dense, hard rock. As a result of the shearing encountered during formation, recrystallised minerals grow preferentially along planes of foliation parallel to the direction of shear.
For a geologist's guide to the rocks and structures of the Santa Catalina Mountain core complex, see Jon Spencer's 38-pg illustrated report:
https://library.azgs.arizona.edu/item/AOFR-1552429673312-319
Photographer: Jessica Rudd
Photo Date: 2013