THE ROLE OF METASEDIMENTARY MYLONITES IN THE GEOMETRIC EVOLUTION OF THE BUCKSKIN DETACHMENT FAULT, WEST-CENTRAL ARIZONA

Metasedimentary mylonites in the footwall of the Buckskin detachment fault in west-central Arizona locally influenced the geometry and incisement history of the detachment fault system. These metasedimentary mylonites are most common in the southern portion of the Buckskin-Rawhide metamorphic core complex, where a package of marble and quartzite up to ~80 m thick parallels the brittle Buckskin detachment fault. Fabrics in these mylonites record middle to lower greenschist-facies deformation and top-to-the-NE-directed shear parallel to the upper plate transport direction. In the Battleship Peak area in the SW Buckskin Mountains, strain and paleo-flow stress recorded in metasedimentary mylonites increases with proximity to the Buckskin detachment fault. Recrystallized calcite and quartz grain sizes suggest peak flow stresses were 100-200 MPa right below the detachment fault. The footwall cataclasite zone along the detachment fault parallels the high-strain fabrics in the metamedimentary mylonites and is commonly <2 m thick. By contrast, in areas where crystalline mylonites are exposed directly below the detachment fault, the detachment cataclasite zone is commonly >10 m thick and truncates upper greenschist-facies fabrics. Postmylonitic brittle normal faulting within the metasedimentary rocks is relatively minor, whereas widespread brittle faulting within the crystalline mylonites accounts for up to ~30% extension. Altogether these observations suggest the Buckskin detachment fault localized along the weak metasedimentary rocks. During brittle incisement of the detachment fault system into crystalline portions of the lower plate at ~250-400˚C, the metasedimentary mylonites penetratively absorbed strain as a top-to-the-NE shear zone. The top of this shear zone evolved to a brittle detachment fault that accomplished relatively little subsequent incisement.