Petrographic and Kinematic Investigation of the Volcaniclastic and Plutonic Rocks of the Northern Alisitos Arc, Baja California, Mexico

The Alisitos arc segment forms part of the western zone of Jura-Cretaceous Peninsular Ranges batholith of Baja California. It extends south from the ancestral Agua Blanca Fault to the state boundary between Baja Norte and Sur. The study area is located within a fold and thrust belt intruded by a number of plutons that were emplaced during and after the deformational event. The northern end of the Alisitos arc is characterized by subvertical tight to isoclinal folds and high-angle reverse faults that define a northwest trending, southwest vergent fold and thrust belt. The aABF defines the northern limit to the Alisitos arc segment. In this study we present the results of a petrographic study of igneous rocks in order to determine the relative timing and the distribution of deformation within the northern Alisitos arc segment. The study includes samples of the mylonitic shear zone of the aABF, and plutonic samples from intrusions proximal to the aABF emplaced later during regional deformation. These samples were investigated in order to characterize the distribution of the subsolidus strain in grain scale and the sense of shear during later phases of deformation in the northern Alisitos arc. The results are presented and discussed based on the mineralogical and textural observations from the Balbuena pluton, the Piedra Rodada pluton, and volcaniclastics that were deformed within the aABF. The Balbuena pluton, emplaced at ~ 108 Ma after the surrounding country rocks had already been folded, exhibits little if any evidence for subsolidus deformation. In contrast, the Piedra Rodada pluton, emplaced at ~ 105 Ma just to the southwest of the aABF, exhibits a strong magmatic fabric overprinted by a moderate subsolidus fabric to the southwest that grades into a strong subsolidus fabric with proximity to the fault. Kinematics observed from lineation parallel-foliation normal sections exhibit consistent top-to-the southwest sense of shear