5 The southern margin of the North American cordillera through time

This session deals with the geologic evolution of the southeastern Cordillera of the North American Plate, its role during ancient orogenic cycles, opening of the Gulf of Mexico, and its interaction with the Caribbean Plate.

Wednesday, 22 April 2026
8:00 AM - 12:00 PM
LMH, Meeting Room

New Timing Constraints on Mylonitic Deformation in the Sierra de Juárez Complex, Oaxaca: Implications for Mesozoic Tectonics in Southern Mexico

Paper number: 5-3
Program: Technical Sessions
Time: 8:50 AM – 9:10 AM

Authors

Abstract

Along the southern margin of the North American plate, particularly in southern Mexico, prolonged interactions with allochthonous terranes and crustal blocks produced pervasive deformation and metamorphism, preserving key information for reconstructing the tectonic evolution of southern Mexico, the Caribbean, and northern South America.

Several first-order tectonic models proposed for this region invoke major lithospheric-scale transfer zones, including long-lived fault systems such as the East Mexican Transform associated with the opening of the Gulf of Mexico or the North Oaxacan Transfer. However, direct temporal constraints on deformation related to these structures remain scarce. In this context, the Sierra de Juárez Complex (SJC) is characterized by regionally extensive mylonitic zones whose kinematics and timing provide critical constraints for evaluating plausible tectonic scenarios.

Here, we present new in-situ U–Pb SIMS ages obtained from simplectitic, µm-scale titanite crystals that grew in pressure shadows of rutile–ilmenite porphyroclasts within a mylonitized anorthosite in the central SJC. Unlike surrounding lithodems, the anorthosite developed a well-defined dextral NW–SE mylonitic shear zone, where most plagioclase is dynamically recrystallized. The U–Pb titanite age of 154.7 ± 7.8 Ma (Oxfordian–Kimmeridgian) is interpreted as directly dating deformation. Electron Backscatter Diffraction (EBSD) data reveal a consistent crystallographic preferred orientation shared by titanite and matrix phases, defining the mylonitic fabric and documenting syn-kinematic titanite growth with absent to low internal misorientation, indicating limited to no crystal-plastic deformation after crystallization.

In addition, Rb–Sr ages were obtained from white-mica fish using in-situ LA-ICP-MS/MS analyses, combined with whole-rock ID-TIMS data to constrain isochron intercepts. Preliminary results from the central SJC yield Middle Jurassic (Bathonian) ages between 168.3 ± 5.3 and 167.2 ± 7.8 Ma. In contrast, muscovite mica fish and whole-rock samples from the northern SJC analyzed by ID-TIMS yield Upper Jurassic (Callovian–Kimmeridgian) ages between 161.4 ± 6.0 and 151 ± 8.0 Ma. The Kimmeridgian age may be linked to deformation, consistent with the U–Pb titanite age, whereas Bathonian–Callovian ages reflect incomplete isotopic resetting of muscovite during deformation.

Together, these results provide robust temporal constraints on mylonitic deformation in the Sierra de Juárez and highlight its role as a key archive for evaluating Mesozoic tectonic models along the southern margin of the North American Cordillera.

Geological Society of America Abstracts with Programs. Vol. 58, No. 3

doi: 10.1130/abs/2026CD-410778

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