72 Toe to Toe: Cordilleran Systems from Trench to Retroarc Domains (Posters)

This session highlights research across geoscience disciplines to advance understanding of the deformation, magmatism, seismicity, topography, and sedimentation processes governing trench to retroarc evolution during ocean-continent plate convergence.

Sunday, 19 October 2025
8:00 AM - 5:30 PM
HBGCC, Hall 1

(Micro)structure of Shear Zones in the Granite Wash Mountains (AZ, USA)

Paper number: 72-8
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

A debate remains on the Late Cretaceous-Eocene plate organization and geodynamics of the western North American margin. The Maria fold-and-thrust belt (MFTB) in SE California and west-central Arizona preserves multi-deformed rocks involved in Jurassic-Miocene tectonic events that affected the southwestern North American Cordillera. In this study, we investigated shear zones in the central MFTB to understand the spatio-temporal relationship between contraction and extension in the Cordilleran backarc during the Late Cretaceous-Eocene. We present field and microstructural analyses of metamorphic rocks in the Granite Wash Mountains (AZ) involved in Cretaceous shortening along the Hercules Thrust system and from shear zones associated with post-magmatic cooling of the Tank Pass pluton (ca. 78 Ma). Hercules-related shear zones dip N/NE with both top-N/NE and top-S/SW shear and are cross-cut by syn-to-post-kinematic dikes. Hercules Thrust samples show quartz grain boundary migration (GBM) and feldspar bulge nucleation (BLG) in thin sections. The Tank Pass pluton contains SE to SW dipping granite- and quartz vein-hosted mylonitic shear zones with variable quartz GBM, subgrain rotation, and BLG. The restored orientations of Tank Pass shear zones are more consistent with Late Cretaceous MFTB contraction than Late Cretaceous-Eocene or Miocene extension. Optical observations and EBSD analyses further constrain quartz crystallographic orientations and slip systems active during contractional top-SW deformation as dominantly prism <a> and prism <c> slip, while Tank Pass shear zones vary from basal <a> to prism <c> slip. We use these microstructural constraints to suggest targets for future titanite and apatite U-Pb petrochronology to further bracket the timing of mylonitization.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-412828

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