STRAIN ANALYSIS ACROSS A DETACHMENT SHEAR ZONE, PICACHO PEAK, AZ

The Picacho Mountains form the western part of an extensive Miocene metamorphic core complex, including the Tortolita, Catalina, and Rincon Mountains in southern Arizona. The study area of Picacho Peak is located on the south end of a north-south running mountain range, 40 miles NW of Tucson. The Picacho Peak detachment shear zone is divided into three levels: (1) a lower plate, consisting of undeformed to mylonitized quartz-feldspatic Oracle granite, (2) a middle plate is made of altered and fractured Oracle granite, (3) an upper plate entails Miocene allochtonous volcanic and sedimentary rocks. The three plates are separated with detachment zones associated with chloritic breccia. The rocks in this area have undergone middle Tertiary mylonitic deformation.

In the lower plate, the quartz grains display regime 2 to 3 microstructures and shows extensive recrystallisation by subgrain rotation and grain boundary migration. The recrystallized grain size ranges between 20 and 50 microns in all samples. Quartz crystallographic preferred orientation measured using EBSD (9 samples) shows that recrystallization was accommodated by dominant prism <a> and minor rhomb <a> slip, suggesting deformation temperature ranging from 450°C to 550°C, compatible with previously published data. In contrast, the middle plate exhibits numerous evidence of cataclastic flow, with fractured, sutured, and serrated grains of quartz and feldspar, indicating limited crystal plasticity.

3D strain analyses results of plagioclase porphyroblasts show that strain is homogeneous throughout the lower and middle plates with bulk strain, Rs ranging from 1.6 to 1.8. The data also shows that axial ratios, Rf, and angular orientations, χ, are consistent throughout the samples, indicating that strain deformation is dominant over original grain shape.

These preliminary results suggest that the detachment shear zone evolved at its peak strength, close to the dislocation creep/exponential creep transition, where mechanical instabilities caused strain hardening, embrittlement, and eventually seismic failure.