Deformation of mylonites in Palm Canyon, California, based on xenolith geometry

An analysis of xenoliths in granodiorite and mylonites of the Peninsular Ranges batholith in southern California documents large deformation with Von Mises equivalent strains ranging from 1 to 4. From the geometry of xenoliths in mylonites it is concluded that much of the deformation occurred by flattening. If deformation had been homogeneous, the 800 m thick mylonite sequence would have extended originally over more than 10 km. Contrary to xenoliths associated with ballooning diapirs, these in Santa Rosa mylonites show much larger strains and are elongated in the lineation direction. It is proposed that the overall deformation pattern is consistent with a top to the west shear displacement, but most of the strain in the Santa Rosa mylonites was produced in flattening, during emplacement of the partially solidified batholith. The large deformation may have produced folds that have coalesced with the foliation to accommodate the strain.