Structural evolution of mid-crustal shear zones: Integrated field and thermochronologic studies of the Sierra Mazatan metamorphic core complex, Sonora, Mexico and the Proto-Kern Canyon dextral shear zone, southern Sierra Nevada, California

New geologic mapping, structural analysis, petrographic observations, and 40Ar/39Ar geochronology and thermochronology reveals the geologic evolution of ductile shear zones at the Sierra Mazatan metamorphic core complex, Sonora, Mexico and the Proto-Kern Canyon shear zone, southern Sierra Nevada, California. The Sierra Mazatan records large-magnitude crustal extension in northwestern Mexico along a presently low-angle shear zone that evolved from ductile to brittle. The Sierra Mazatan is a critical site for understanding the forces that drove crustal extension in northwestern Mexico as well as how large-magnitude extension is accommodated. This study demonstrates that tectonic unroofing of the core complex was a poly-phase process with early slip from 25-23 Ma and major slip from 21-15 Ma. Total fault slip was 15-20 km and occurred at average rates of 3-4 mm/yr. A significant amount of extension in northwestern Mexico occurred during subduction and was not directly related to tectonic change to transtension that developed after 17-12 Ma. The presently low-angle normal fault at Sierra Mazatan initiated at a steep initial dip (50-60°) and was rotated to shallower angles as slip proceeded, suggesting that at least some metamorphic core complexes need not be unique modes of crustal extension but rather may be end-member examples of typical extensional processes active today.; The Proto-Kern Canyon shear zone accommodated ductile strain in the southern part of the Sierra Nevada magmatic arc. New geologic and structural mapping, petrologic observations and 40Ar/39Ar thermochronology demonstrate that the shear zone accommodated dextral transpression from ca. 90-85 Ma until 75 Ma. Bulk transpression within the shear zone was partitioned into zones of dominantly reverse shear and zones of dextral shear, although the shear zone may also have evolved from reverse to dextral motion. Transpression in the Proto-Kern Canyon shear zone occurred during the waning stages of magmatic arc construction in the Sierra Nevada and was broadly synchronous with an increase in the obliquity of subduction and a decrease in the dip of the subducting slab, suggesting that intra-arc deformation within the Sierra Nevada was coupled to changes in the subduction parameters. Similar timing and kinematics of intra-arc shear zones exposed in the rest of the Sierra Nevada arc support this conclusion.