Dating deformation in the Alamo Mountain and Piru Creek shear zones: Insights into Late Cretaceous tectonics in southern California

ABSTRACT Paleomagnetic data from the Insular superterrane and related terranes in the western Canadian and northern U.S. Cordillera seem to indicate large-magnitude (~4000 km), northward translations along the western margin of the North American Cordillera in the Late Cretaceous (the Baja–British Columbia [Baja-BC] hypothesis). This model postulates that initial collision of the Insular superterrane occurred in southern California and/or northern Baja Mexico prior to dextral translation along the western North American margin from 85 to 55 Ma. A major unresolved problem with the Baja-BC hypothesis is that faults that could have accommodated large-magnitude translation are missing or obscured by later Cenozoic faulting and/or sedimentary cover. Here, we explored the deformation record of Late Cretaceous ductile shear zones in southern California with the goal of understanding the timing and kinematics of deformation at this time. We focused on the Alamo Mountain and Piru Creek shear zones, located within the central Transverse Ranges. The kinematics of these shear zones help to elucidate whether southern California was experiencing predominantly dextral or sinistral faulting during the time of postulated large-magnitude northward translation along the western margin of the North American Cordillera. This information in turn allows for assessment and refinement of the Baja-BC hypothesis. We report new field observations and 21 U-Pb laser ablation–inductively coupled plasma–mass spectrometry zircon ages from deformed and undeformed host rocks and dikes. We find that ductile shear zones in the Alamo Mountain region are localized at the boundary between Paleoproterozoic gneisses and Mesozoic plutons. High-strain rocks consist of quartzofeldspathic mylonites and phyllonites up to 750 m thick, and both shear zones record sinistral strike-slip to sinistral-normal motion with a predominantly top-to-the-NW sense of shear in their present-day orientations. Our data show that the Alamo Mountain and Piru Creek shear zones were active at ca. 76–72 Ma and possibly included an earlier phase of deformation. When Cenozoic block rotations are restored, we find that the Alamo Mountain and Piru Creek shear zones originated as NNW-SSE–striking, moderately ENE-dipping shear zones that formed at midcrustal conditions (500–600 °C and 4 kbar). Structural analysis of the shear zones indicates that the dominant component of motion was sinistral strike-slip and that the dip-slip component of motion was minor. The timing and kinematics of deformation in the Alamo Mountain and Piru Creek shear zones are similar to others in the Southern California Batholith, including the Nacimiento fault, the Mill Canyon window, and the Black Belt and Cucamonga shear zones (eastern San Gabriel Mountains), and the Tumamait shear zone (Pine Mountain block). The presence of this regionally extensive, sinistral shear zone system and the absence of dextral shear zones require reevaluation of the Baja-BC hypothesis in southern California during the Late Cretaceous.

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